WO1993002710A2 - Use of persistent free radicals in magnetic resonance imaging - Google Patents

Abstract

The present invention provides the use of a persistent aryloxy or arylthio free radicals, other than perhalo radicals, for the manufacture of a contrast medium for use in magnetic resonance imaging. Also provided are magnetic resonance imaging contrast medium compositions and methods containing or using such radicals.

Description

Use of persistent free radicals

in magnetic resonance imaging The present invention relates to the use of

persistent free radicals, in particular persistent aryloxy and arylthio free radicals, as image enhancing agents in magnetic resonance imaging (MRI) as well as to contrast media containing such radicals and to the use of such radicals and their non-radical precursors in the manufacture of MRI contrast media.

MRI is a diagnostic technique that has become particularly attractive to physicians as it is non- invasive and does not involve exposing the patient under study to potentially harmful radiation, such as for example the X-radiation of conventional radiography.

This technique, however suffers from several serious drawbacks, including in particular the expense of manufacture and operation of the MRI apparatus, the relatively long scanning time require to produce an image of acceptable spatial resolution, and the problem of achieving contrast in the magnetic resonance (MR) images between tissue types having the same or closely similar imaging parameters, for example in order to cause a tissue abnormality to show up clearly in the images.

The expense of manufacture and operation of an MRI apparatus is closely associated with the strength of the magnetic field that the primary magnet in the apparatus is required to generate in order to produce images of acceptable spatial resolution in an acceptable time.

MR images are generated by manipulation of the MR signals detected from the sample, for example a human or animal body, placed in a magnetic field and exposed to pulses of radiation of a frequency (typically

radiofrequency (RF)) selected to excite MR transitions in selected non-zero spin nuclei (the "imaging nuclei", which are generally water protons in body fluids) in the sample.

The amplitude of the induced MR signals is

dependent upon various factors such as the strength of the magnetic field experienced by the sample, the temperature of the sample, the density of the imaging nuclei within the sample, the isotopic nature and chemical environment of the imaging nuclei and the local inhomogeneities in magnetic field experienced by the imaging nuclei.

Thus many techniques have been proposed for

enhancing MR image quality, for example by increasing MR signal amplitude or by increasing the difference in MR signal amplitude between different tissue types.

The imaging parameters (nuclear density, T₁ and T₂) for tissues of interest may be altered and many

proposals have been made for doing this by the

administration of magnetically responsive materials into patients under study (see for example EP-A-71564

(Schering), EP-A-133674 (Schering) and WO-A-85/04330 (Jacobsen)). Where such materials, generally referred to as MRI contrast agents, are paramagnetic they produce significant reduction in the T₁ of the water protons in the body zones into which they are administered or at which they congregate, and where the materials are ferromagnetic or superparamagnetic (for example as suggested by Jacobsen) they produce a significant

reduction in the T₂ of the water protons. In either case the result is enhanced (positive or negative) contrast in the MR images of such zones.

The contrast enhancement achievable by such agents in conventional MRI is relatively limited and it is generally not such as to allow a reduction in the image acquisition period or in the field strength of the primary magnet.

Utilisation of the spin transition coupling

phenomenon known as dynamic nuclear polarisation or as the Overhauser effect to amplify the population

difference between the ground and excited spin states of the imaging nuclei by the excitation of a coupled ESR transition in a paramagnetic species present in the sample being imaged has been described by Hafslund

Nycomed Innovation AB in WO-A-88/10419.

This new technique for generating a MR image of the sample, which is hereinafter termed electron spin resonance enhanced magnetic resonance imaging (ESREMRI) or Overhauser MRI (OMRI), involves exposing the sample to a first radiation of a frequency selected to excite nuclear spin transitions in selected nuclei in the sample (radiation which is generally of radiofrequency or thereabouts and thus for convenience will be referred to hereinafter as RF radiation) and also exposing the sample to a second radiation of a frequency selected to excite electron spin transitions coupled to nuclear spin transitions for at least some of the selected nuclei (radiation which is generally of microwave frequency or thereabouts and thus for convenience is referred to hereinafter as MW or UHF radiation), the MR images being generated from the resulting amplified MR signals (free induction decay signals) emitted by the sample.

The paramagnetic substance which possesses the ESR transition which couples with the NMR transition of the imaging nuclei may be naturally present within the imaging sample or more usually may be administered as an OMRI contrast agent.

In WO-A-88/10419 various OMRI contrast agents were proposed, for the most part these being nitroxide stable free radicals, although the use of the chloranil

semiquinone radical and of Fremy's salt was also

proposed.

In WO-A-90/00904 Hafslund Nycomed Innovation AB proposed the use of deuterated stable free radicals, in particular deuterated nitroxide stable free radicals, as OMRI contrast agents. Organic free radicals however frequently have properties which render them unsuitable for use as OMRI contrast agents. Thus free radicals commonly are unstable in physiological conditions, or have very short half-lives leading to toxicity problems. A further drawback is the low relaxivity exhibited by many free radicals, which results in poor coupling of the electron and nuclear spin transitions and thus a poor enhancement of the magnetic resonance signa. A need therefore exists for improved free radical OMRI contrast agents and in WO-A-91/12024 Hafslund Nycomed Innovation AB proposed the use of carbon free radicals, and in

particular various triarylmethyl radicals. The

disclosure of WO-A-91/12024 is incorporated herein by reference.

For such free radicals to be effective, they should be relatively long lived and to distinguish from free radicals which have a momentary existence, those usable as OMRI contrast agents will be referred to herein as being "persistent" free radicals, that is having a half life of at least one minute at ambient temperature.

We have now found that other radical structures are useful as OMRI contrast agents and viewed from one aspect the present invention provides the use of

persistent aryloxy or arylthio radicals other than perhalo radicals for the manufacture of a contrast medium for use in MRI, and especially for use in OMRI, said radical preferably having an inherent linewidth for the peaks in its esr spectrum of less than 500 mG, especially less than 100 mG, and most especially no more than 50 mG.

Since it is generally preferred for OMRI contrast agents that their esr spectra should contain as few lines as possible, it is especially preferred that the number of non-zero spin nuclei in the proximity of high free electron density sites within the radical should be as low as possible. Accordingly proton (¹H) substitution of the atoms of the aryl moiety should be minimized and while halogen atoms such as chlorines may (by virtue of their vacant d orbitals) participate in the aryl π- electron system and so enhance radical stability their presence as substituents is generally to be avoided.

Examples of suitable radicals usable according to the invention thus include the following:

Ar - 0º

and Ar - Sº where Ar is a 5-7 membered carbo- or heterocyclic

aromatic ring optionally carrying one or more

(preferably 0, 1, 2 or 3) fused carbocyclic or

heterocyclic aromatic rings, the resultant aryl ring structure preferably containing 0, 1 or 2 heteroatoms selected from O, N and S and optionally being

substituted by one or more steric hindrance, electron withdrawing, electron donor or water solubilizing

groups. More explicit examples of radical skeletons include

(phenoxy)

(indolizinyl)

(semiquinone anion)

In the skeletal structures indicated above, -0º and -Sº moieties are generally interchangeable and fused aryl rings may be added on if desired, subject of course to a general preference that the π-system should preferably contain no more than 4, especially no more than 3, fused rings.

In order that the radicals should perform most effectively as MRI contrast agents it will generally be preferred that the atoms of the aryl moiety be

substituted. In this regard substitution is intended to fulfil a dual or treble function - to stabilize the radical and to reduce esr linewidths and/or reduce the number of lines in the esr spectrum. Of course for many structures or substitution sites one or more of these functions can be achieved by the same manner of

substitution.

Thus as mentioned above, substitution should

generally be designed to minimize the occurrence of nonzero spin nuclei (especially hydrogen (¹H)) at or even closely adjacent sites of high free electron density.

Above and beyond this however substitution should

generally be such as to block off or sterically hinder approach to atoms having high free electron density, so reducing radical reactivity and increasing stability, and also to provide electron withdrawing or electron donating substituents at sites where such effects serve to enhance stability. Generally speaking, electron donor or withdrawing substituents should preferably be selected to minimize esr line broadening or line

splitting effects and sterically hindering or blocking groups should be selected to achieve their steric effect of hindering intermolecular approach with minimal deformation of the delocalizing π-system as such

deformation reduces the radical stabilizing efficacy of the system.

Although discussed further below, steric hindrance of neighbouring ring sites is preferably effected by substitution with t-butyl-thio, t-butoxy or t-butyl groups or by substitution of ortho and meta positions by bridging groups of formula -X⁷-CR⁷ ₂-X⁷-, where each X⁷, which may be the same or different is O, S, C=O or SO₂ (both X⁷ preferably being O or CO) and R⁷ is a hydrogen atom or a C_1-6alkyl group optionally substituted by hydroxyl, C_1-6alkoxy or carboxyl groups or amides, esters or salts thereof, e.g. a -O-C(CH₃)₂-O- group.

Among electron withdrawing groups for substitution of the radical skeleton, nitrile and, more preferably, carboxyl groups (and esters, amides and salts thereof) are especially preferred. Within any one aryl ring however, generally only one or at most two such electron withdrawing groups will be desired.

For electron donor groups, especially those at a para (or δ ) position to a radical centre -Oº or -Sº group, groups of formula R²O, R²S, R²SO₂, R²OCOSO₂ and R² ₂NCOSO₂ are especially preferred where R² is hydrogen or C_1-6alkyl optionally substituted by hydroxyl, or C_{1- 6}alkoxy, amine, C_1-6alkyl or dialkyl amine, carboxyl (and amides and esters thereof) etc.

Although many persistent aryloxy and arylthio radicals are known, those having -X⁷-CR⁷ ₂-X⁷- steric hindrance groups substituted on neighbouring carbon atoms of the ring systems and those having SO₂R³ (where R³ is R², CO₂R² or CONR₂ ²) solubilizing and/or stabilizing groups are novel and particularly suited for use

according to the invention and thus form a further aspect of the invention.

Viewed from a still further aspect, the invention also provides a method of magnetic resonance

investigation of a sample, said method comprising introducing into said sample a persistent aryloxy or arylthio radical as discussed above, exposing said sample to a first radiation of a frequency selected to excite electron spin transitions in said free radical, exposing said sample to a second radiation of a

frequency selected to excite nuclear spin transitions in selected nuclei in said sample, detecting free induction decay signals from said sample, and, optionally, generating an image or dynamic flow data from said detected signals.

Viewed from another aspect, the invention also provides a magnetic resonance imaging contrast medium comprising a physiologically tolerable persistent aryloxy or arylthio free radical together with at least one pharmacologically acceptable carrier or excipient.

For in vivo imaging, the free radical should of course preferably be a physiologically tolerable

radical, or one presented in a physiologically

tolerable, e.g. encapsulated, form.

Preferred free radicals for use according to the invention exhibit high stability to oxygen, to pH, for example in the range pH 5-9, and in aqueous solution, particularly stability up to a concentration of 300 mM. Further desirable characteristics include reduced tendency to dimerization, long half-life, preferably greater than 1 minute, particularly preferably greater than 1 hour and especially preferably 1 year, long relaxation times, both T_1e and T_2e preferably being greater than 1 μsec, high relaxivity, for example

greater than 0.3 mM^-1sec^-1 and a small number of esr transition lines.

As indicated above, the possibility exists to optimize different characteristics, e.g. solubility, stability and line broadening, of the overall radical by appropriate combinations of different substituents on the radical skeleton. Combinations, where one or more substituent is selected to optimize stability and line broadening, and one or more substituerit is selected to optimize solubility are considered particularly

interesting.

In order to optimize the above-mentioned desirable properties, a number of criteria need to be borne in mind in selecting or constructing radicals for use according to the invention.

Thus, the aromatic rings of the radicals

advantageously are substituted and the nuclear

identities of nuclei in all substituents and their positions within the molecule should be selected so as to minimise their effect (line splitting or broadening) on the esr transitions. Substitution of ortho and para and equivalent carbons is desirable in order to minimise dimerisation and oxygen attack on the molecule. Carbons in the orthoposition are preferably substituted by bulky substituents to minimise attack by oxygen and

substitution of paracarbons by electron withdrawing and/or water solubilizing groups is also particularly preferred. Such substituents preferably have no

magnetic moment, or have a very low effective spin density. Alternatively, in order to minimise their effect on the esr transition, the substituents should be bonded in such a manner that they are capable of free rotation.

In the radicals used according to the invention, the carbons of the aryl moiety preferably carry

substituents other than protons (¹H) and indeed it is preferred that only one such carbon at most is

unsubstituted. Suitable substituents include groups R¹ which may be the same or different, and independently represent alkyl groups or groups of formula -M, -X³M, - X³Ar² where M represents a water solubilizing group, each group X³, which may be the same or different, represents an oxygen or sulphur atom or a NH, CH₂, CO or SO₂ group; Ar² represents a 5 to 10 membered aromatic ring

optionally substituted by a solubilizing group M;

or R¹ groups on different or adjacent R¹ groups

(preferably groups at the ortho and meta positions) together with the two intervening carbon atoms may represent groups of formula

where R⁶ represents a hydrogen atom, a hydroxyl group, an optionally alkoxylated, optionally hydroxylated acyloxy or alkyl group or a solubilising group M; Z represents an oxygen or sulphur atom or a group NR⁵, CR⁷ ₂, or SiR⁷ ₂; R⁵ represents a hydrogen atom or an optionally

hydroxylated, optionally aminated, optionally

alkoxylated, optionally carboxylated alkyl, oxo-alkyl, alkenyl or alkaryl group; each R⁷, which may be the same or different, represents a hydrogen atom, an alkyl, hydroxyalkyl, alkoxycarbonyl or carbamoyl group or two groups R⁷ together with the atom to which they are bound represent a carbonyl group or a 5 to 8 membered

cycloalkylidene, mono- or di-oxacycloalkylidene, mono- or di-azacycloalkylidene or mono- or di- thiacycloalkylidene group optionally with the ring attachment carbon replaced by a silicon atom (preferably however in any spiro structure the ring linking atom will be bonded to no more than three heteroatoms) and R⁷ where it is other than hydrogen, is optionally

substituted by a group R⁶. Certain of the radicals substituted in this fashion are new and they, their salts and their non-radical precursors (e.g. compounds having a structural unit ArOX⁴ or ArSX⁴ where X⁴ is a leaving group, e.g. hydrogen, hydroxyl, halogen, carboxyl, CO₂OCO. C(Ar)₃ or NNC(Ar)₃) form further aspects of the present invention.

In the radicals used according to the invention the solubilizing groups M may be any of the solubilizing groups conventionally used in diagnostic and

pharmaceutical products. Particularly preferred

solubilizing groups M include optionally hydroxylated, optionally alkoxylated alkyl or oxo-alkyl groups and groups of formulae R⁵, COOR⁵, OCOR⁵, CHO, CN, CH₂S(O)R⁵, CONR⁵ ₂, NR⁵COR⁵, NR⁵ ₂, SO₂NR⁵ ₂, OR⁵, PO₃ ^2" , SOR⁵, SO₂R⁵, SO₃M¹, COOM¹ (where M¹ is one equivalent of a physiologically tolerable cation, for example an alkali or alkaline earth metal cation, an ammonium ion or an organic amine cation, for example a meglumine ion), -(O(CH₂)_p)_mOR⁵

(where p is an integer having a value of from 1 to 3 and m is an integer having a value of from 1 to 5), - CX³(CHR⁵)_pX³ or CH₂R⁸ (where R⁸ is a hydrophilic R⁵ group) or SR¹⁰ or S0₂R¹⁰ where R¹⁰ is a group R⁵ or an alkyl group optionally substituted by one or more, especially two or three groups COOR⁵, OCOR⁵, CHO, CN, CONR⁵ ₂, NR⁵COR⁵, NR⁵ ₂, SO₂NR⁵ ₂, OR⁵, PO₃ ^2-, SOR⁵, SO₂R⁵, SO₃M¹, COOM¹, or - (O(CH₂)_n)_mOR⁵.

Especially preferred as solubilizing groups M are groups or formula C(H) _3-p(CH₂OH) , R⁹, COR⁹, SR⁹, SOR⁹, SO₂R⁹, CON(R⁹)₂, NR⁹ ₂, NHR⁹ and CONHR⁹ [where R⁹ may

represent a C_1-5alkyl group optionally substituted by hydroxyl, alkoxy or amino groups or carboxyl groups or esters or amides thereof, e.g. groups

(although any R⁹ group attached to a sulphur, nitrogen or oxygen atom is preferably not hydroxylated at the α carbon)], and groups of formula SR¹² where R¹² is a group CH₂COOR¹³, CH(COOR¹³)₂, CH₂CONHR⁹, CH₂CONR⁹ ₂, CR⁵ (COOR¹³) ₂, CH(CN)CO₂R¹³, (CH₂)_pSO₃-M¹, (CH₂)_pCOR⁹, CH(COR⁹) CH₂COR⁹ and CH(R⁵)COR⁹ where p, M¹ and R⁵ are as earlier defined and R¹³ is a hydrogen atom, an alkyl group or a group M¹ or R⁹.

Further especially preferred solubilising groups M or X³M include groups of formula X⁵C((CH₂) COOR¹³)₂R¹⁴, X⁵C((CH₂)_pCCOR¹³)₃ and X⁵C((CH₂)_pCOOR¹³)R¹⁴ ₂, where R¹³ is as defined above, p is an integer from 1 to 3, X⁵ is an oxygen or sulphur atom, and R¹⁴ is a hydroxyalkyl group such as a group R⁹ as earlier defined. Other examples of preferred R¹ groups include for example the following structures

-S-(CH₂CH₂O)_p, R¹⁹ where p' is 0, 1 or 2 and R¹⁹

is hydrogen or C_1-4alkyl

-S-(CH₂)_p,-CO-R²³ where R²³ is C_1-4alkyl (e.g.

methyl, ethyl or t-butyl) ,

NR₂ ²¹ or OR²¹ and R²¹ is C_1-4

alkyl

-COR²² where R²² is hydrogen,

hydroxyl, R²³, or COOR²¹

-CH₂O[CH₂CH₂O]_p,CH₃

-CH₂OCOR²¹

and -CHX³-CR⁵ ₂CR⁵ ₂-X³ where X³ is oxygen or sulphur.

Where M represents a group containing a moiety NR⁵ ₂, this may also represent an optionally substituted nitrogen-attached 5 to 7 membered heterocyclic ring optionally containing at least one further ring

heteroatom, e.g. N or O, for example a group of formula

In the substituents on the radicals used according to the invention, any alkyl or alkenyl moiety

conveniently will contain up to 6, especially up to 4, carbon atoms and any aryl moiety will preferably contain 5 to 7 ring atoms in the or any aromatic ring and especially preferably will comprise an aromatic ring with 0, 1 or 2 further aromatic rings fused directly or indirectly thereto.

Preferred structures for the radicals include those in which at least one pair of adjacent ring carbons of the aryl moiety carries a fused ring of formula

where X³ and Z are as defined before, especially rings of formulae

where X³ is oxygen, sulphur, carbonyl or SO₂ and R⁷ is hydrogen or optionally hydroxylated methyl.

As has been discussed above, the substituents on the aryl skeleton serve primarily to achieve one or more of the functions of i) steric hindrance (blocking), ii) electron withdrawing (from the π-system), iii) electron donating (into the π-system) and iv) enhancing the water solubility of the overall radical. The preferred

electron donating blocking groups are t-butoxy, t- butylthio, NR⁷⁰ ₂ (where R⁷⁰ is as described below), and the -X⁷-CR⁷ ₂-X⁷- (where X⁷ is O or S) bridging groups.

The preferred electron withdrawing blocking groups include -X⁷-CR⁷ ₂-X⁷- (where at least one X⁷ is SO or SO₂) bridging groups, CHO, CONR⁷⁰ ₂, COOR⁷⁰, OCOR⁷⁰, SO₂NR⁷⁰ ₂, SO₂CR⁷⁰ ₃, NR⁷⁰COR⁷⁰, NR⁷⁰COOR⁷⁰, OCONR⁷⁰ ₂, NR⁷⁰SO₂R⁷⁰,

NR⁷⁰CONR⁷⁰ ₂, NR⁷⁰SO₂NR⁷⁰ ₂, COCR⁷⁰ ₃, COCOR⁷⁰, SO₂R⁷⁰, COCOOR⁷⁰, CN, COSR⁷⁰, SOCR⁷⁰ and CR⁷⁰=NOR⁷⁰ where R⁷⁰ is hydrogen or alkyl or cycloalkyl (preferably C_1-4alkyl or C_{5- 6}cycloalkyl) optionally substituted by one or more groups selected from OH, NH₂, CONR⁷¹ ₂ and COOR⁷¹ (preferably 1, 2 or 3 hydroxy groups) and R⁷¹ is hydrogen or optionally hydroxylated C_1.3alkyl. Preferably R⁷⁰ is C_1-4hydroxyalkyl (e.g. CH₂OH, CH₂CH₂OH, CH₂CH₂CHOHCH₂OH, CH₂CHOHCHOHCH₂OH, CH₂CHOHCH₂OH, and C(CH₂OH)₃) or 2,3- dihydroxycyclopentyl or 2,3-dihydroxycyclohexyl.

Thus taking for illustrative purposes the phenoxy, indolizinyl, indolyl and semiquinone structures,

examples of preferred substitution include those

disclosed below:

Phenoxy:

where each R³² which may be the same or different

represents a hydrogen atom, a group R³¹ or a solubilizing group, e.g. a group M; R³³ represents a group M²⁰ or, less preferably, R³¹; each R³¹, which may be the same or different, represents a steric hindrance group, e.g. t- butyl or more preferably a -O-t-butyl or -S-t-butyl group, or two groups R³¹ on adjacent carbons together represent a steric hindrance bridging group e.g. a group -X⁷-CR⁷ ₂-X⁷-, or X⁷-NR⁵-X⁷- it being particularly

convenient that both sets of R³¹ and R³² groups represent such bridging groups; M²⁰ represents an electron donor group, e.g. a group OR⁹, SR⁹ (where R⁹ is preferably methyl), -CR³⁶=CR³⁴R³⁵ (where R³⁴ and R³⁵ are hydrogen, cyano, alkyl, aryl, or carboxyl or an amide or ester thereof and R³⁶ is hydrogen or alkyl), or -CR³⁶=N-R³⁷

(where R³⁷ is alkyl), preferably a group capable of lying in the plane of the phenyl ring. Examples of suitable steric hindrance R³¹ groups include Ar-O-, Ar-S-, Ar-SO₂-, Ar-CO-, alkyl-CO-, and other carbon or nitrogen attached homo or heterocyclic rings (preferably 5-7 membered, especially 5-membered and particularly preferably dithiacyclopentanes and derivatives thereof), e.g. P

D

-

Thus exemplary phenoxy structures include the following

Indolizinyl:

where R⁵² is an electron withdrawing group (e.g. a cyano or carboxyl group or an amide or ester thereof, e.g. a group COOR⁵⁴ or CONR₂ ⁵⁴ where R⁵⁴ is hydrogen or optionally hydroxylated, alkoxylated or aminated alkyl) or, less preferably, a steric hindrance or solubilizing group, e.g. R³¹ or M;

each of R⁴⁸, R⁴⁹, R⁵⁰, R⁵¹ and R⁵³ is a hydrogen or a steric hindrance or solubilizing roup (e.g. R³¹ or M), R⁵⁰ preferably being hydrogen and the remaining preferably being other than hydrogen, especially R⁴⁸ and R⁴⁹ which particularly preferably represent steric hindrance groups such as -S-tBu, -O-tBu etc.

In a preferred embodiment each of the groups R⁵⁰, R⁵¹ and R⁵³, which may be the same or different,

independently represents a hydrogen atom, a hydroxy group or an optionally hydroxylated optionally

alkoxylated alkyl, alkoxy, alkylthio or acyloxy group or a water solubilising group M; R⁵² represents an electron withdrawing group or a group as defined for R⁵⁰ with the exception of hydrogen;

each of the groups R⁴⁸ and R⁴⁹ independently represents a hydrogen atom, a water solubilising group M or an alkyl, alkoxy, alkylthio, acyloxy or aryl group optionally substituted by alkyl, hydroxy, mercapto, alkoxy or optionally alkoxylated, optionally hydroxylated acyloxy groups, or by a water solubilising group M;

or adjacent groups R⁴⁸ and R⁴⁹, R⁵⁰ and R⁵¹, R⁵¹ and R⁵² and/or R⁵² and R⁵³, together with the two intervening carbon atoms may represent groups of formula

where R⁷ represents a hydrogen atom, a hydroxy, or optionally hydroxylated, optionally alkoxylated acyloxy group or a water solubilising group M.

Preferred indolizinyl radicals include those wherein R⁵² is an electron withdrawing group, especially an ester or amide or a carboxy group or a salt thereof. Preferably also R⁴⁸ and R⁴⁹ are identical, and

particularly preferably R⁴⁸ and R⁴⁹ are both solubilizing groups M or optionally substituted alkoxy or alkylthio groups.

More preferably R⁵² and one of R⁵⁰, R⁵¹ and R⁵³ are alkoxy groups or a group

-COOR⁵⁴, -OCOR⁵⁴, -CONHR⁵⁴ or -CONR⁵⁴ ₂, e.g. -CON(CH₂CH₂OH)₂.

Examples of particularly preferred identities for R⁴⁸ to R⁵³ are as follows:

for R⁵³: hydrogen, methoxy and carboxy and salts, esters and amides thereof

for R⁵²: cyano, carboxy and salts, esters and amides thereof

for R⁵¹: hydrogen, methoxy and carboxy and salts, esters and amides thereof

for R⁵⁰: hydrogen, methoxy, tri (hydroxymethyl)- methylthio and carboxy and salts, esters and amides thereof

for R⁵⁰ and R⁵¹ together: dimethyl methylenedioxy and di(hydroxymethyl)methylenedioxy

for R⁴⁸ and R⁴⁹: phenyl, t-butoxy, t-butylthio, carboxymethylthio, 3,4-dihydroxybutanoyloxy, 2,3- dihydroxypropoxycarbonyl, 2-sulphoethylthio,

tri (hydroxymethyl)methyl, bis 2-hydroxyethyl carbamoyl and bis (2,3-dihydroxypropyl) carbamoyl.

for R⁴⁸ and R⁴⁹ together: dimethylmethylenedioxy and di (hydroxymethyl) methylenedioxy.

Particularly preferred indolizinyl radicals for use in accordance with the invention include

2 , 3-di-t-butoxy-6 , 7-dicarboxy-1-indolizinyl radical

2,3-di-t-butoxy-6,7,8-tricarboxy-1-indolizinyl radical More preferred indolizinyl radicals include:

2,3-di-carboxymethylthio-6,7-dicarboxy-1-indolizinyl radical

2,3-dibutylthio-6,7,8-tricarboxy-1-indolizinyl radical

2,3-di-carboxymethylthio-7-carboxy-6,8 dimethoxy-1- indolizinyl radical

2,3-di[3,4-dihydroxybutanoyloxy]-6,7,8-tricarboxy-1- indolizinyl radical

2,3-di[3,4-dihydroxybutanoyloxy]-6,7,8-tri[2- hydroxyethoxycarbonyl]-1-indolizinyl radical

2,3-di-[3,4-dihydroxybutanoyloxy]-6,7,8-tri[di-(2- hydroxyethyl) amino carbonyl]-1-indolizinyl radical

2,3-di[2,3-dihydroxypropoxycarbonyl)-7-carboxy-6,8- dimethoxy-1-indolizinyl radical

2,3-di[3-hydroxypropanyloxy]-6,7,8-tricarboxy-1- indolizinyl radical

2,3-di[2-hydroxyethoxycarbonyl]7-carboxy-6,8-dimethoxy- 1-indolizinyl radical

2,3-di[2-sulphoethylthio]-6,7,8-tricarboxy-1-indolizinyl radical

2,3-di[tri-hydroxymethyl)methyl-7-carboxy-6,8-dimethoxy- 1-indolizinyl radical

2,3-di[di-(2-hydroxyethyl)-aminocarbonyl]-6,7,8- tricarboxy-1-indolizinyl radical

2,3-di-[di(2,3-dihydroxypropyl)amino carbonyl]-6,7,8- tricarboxy-1-indolizinyl radical

as well as radicals of the general formulae

Indolizinyl radicals wherein R⁵³ and R⁵² are carboxy groups and R⁵⁰ and R⁵¹ together are dimethylmethylenedioxy or di(hydroxymehtyl)methylenedioxy groups or where R⁵³ and R⁵¹ are methoxy groups, R⁵² is a carboxy group and R⁵⁰ is a trihydroxymehtyl methylthio group are also

preferred.

Examples of indolizinyl radicals include

R⁵⁶ = H, CN

R⁵⁴ = H, CH₃ , CH₂CH₃ ,

CH₂CH₂OH, CH₂CHOHCH₂OH

one R⁵⁷ = H the other = CN X = -Sº or -Oº

one of R⁵⁹ = R⁵⁸ the other

= H

R⁵⁸ = COOH (or NEt₃H⁺ salt thereof) , or CONR⁵⁴ ₂ X = -Sº or -Oº

R⁵⁶ = H, CN

R⁶¹ = COOH, CH₃

R⁶² = alkyl, phenyl, alkoxy, alkylthio

Most of the persistent indolizinyl radicals

discussed above are themselves novel and they, their salts, and their non-radical precursors form further aspects of the invention. In particular the water- soluble compounds are all novel.

In particular the novel indolizinyl radicals include compounds wherein R⁴⁸ to R⁵³ are as hereinbefore defined

with the proviso that where either one of R⁵³, R⁵² or R⁵¹ is cyano, or R⁵² is -CHO, -CO₂CH₃, -CONH₂, or -COOH₃, and the remaining substituents R⁵⁰, R⁵¹, R⁵², R⁵³ are hydrogen, at least one of R⁴⁸ and R⁴⁹ is other than a substituted or unsubstituted phenyl group, and that where R⁵² is cyano, and R⁵⁰, R⁵¹, and R⁵³ are hydrogen, at least one of R⁴⁸ and R⁴⁹ is other than n-C₃H₇.

Semiquinone:

where R⁶⁹ to R⁷² which may be the same or different represent steric hindrance and/or solubilizing groups or more preferably R⁶⁹ and R⁷⁰ and/or R⁷¹ and R⁷², together with the intervening carbons form fused aryl rings, preferably 5-7 membered rings, which optionally but preferably themselves carry steric hindrance and/or solubilizing (e.g. R³¹ and M) groups. Particularly preferably, the mesomeric forms of the semiquinone anion radicals, i.e. ºO-B-O- and -O-B-Oº (where B is used to represent the body of the molecule) are identical.

Examples of semiquinone anion radicals thus include

Generally speaking, substitution to enhance radical stability should be at or adjacent sites in the aryl system which have high spin density. Substitution at high spin density sites should generally be with

unreactive groups and frequently electron withdrawing or electron donor substituents will be preferred.

Substitution at neighbouring sites should generally be by bulky steric hindrance groups which serve to prevent the radical from reacting with other molecules or radicals. The steric hindrance groups can also serve to enhance water solubility of the radical; alternatively separate solubilizing substituents may be included.

The particularly preferred substituent groups for the radicals for use according to the invention include the following -tBu, -O-tBu, -S-tBu, -OC(CH₃)₂-O-, I, -CO- CR⁷ ₂-CO-, -CO-NR⁵-CO-, -SO₃Na, -COOR², -S-R², -SO₂R²,

SO₂NR² ₂.

Persistent aryloxy and arylthio radicals are widely known from the literature and ones suitable for -use according to the invention may be prepared by the

methods described in the literature. Substitution along the lines discussed above may be achieved using methods known from the literature or by using methods analogous to those discussed in PCT/EP91/00285. Examples of relevant literature references include Forrester et al "Organic chemistry of stable free radicals" Academic Press, London 1968, Tetrahedron 18 : 61 (19..), Berichte (1957) page 1634, Angew Chem. Int. (1984) page 447,

Helvetica (1988) page 1665, JACS (1957) page 4439,

Rosenblatt JACS 62:1092 (1940), Taube et al. Berichte

86:1036 (1953), Weygand et al. Berichte 90:1879 (1957), Dann et al. Berichte 93:2829 (1960), Sziki Berichte

62:1373 (1929), Moore J Org Chem 33:4019 (1968), Fieser et al. JACS 7(3:3165 (1948), Reynolds et al. Org.

Synthesis 34:1 (1954); Fujita Tet. Lett (1975) page 1695, Akita J Pharm Soc. Japan 82:91 (1962), Graebbe J. fur Praktische Chemie 62:32 (1900), Helferich et al. Annalen 551:235 (1942), Indian J. Chem. 12:893 (1974), Ramirez et al. JACS 81:4338 (1959), Ramirez et al. JOC 11:778 (1958), Stock et al. JACS 86.1761 (1964), Ramirez et al. JOC 11:20 (1968), Methoden der Organischen Chemie - Houben Weyl page 464-5, No. VII/3a (1977), Can J Chem 40:1235 (1962), Chem Lett (1984) page 341, JCS Perkins II (1989) page 1349, JACS (1960) page 6208, J Chem Phys (1965) page 308, JOC (1988) page 5770, McNab et al. JCS Perkins II (1988) page 759, Russell et al. JACS (1970) page 2762, Weiser et al. Tet. Lett. 30:6161, J Phys Chem 71:68 (1967), Dimroth et al. Liebigs Annalen 624:51 (1959), Miura et al. JOC 58:5770 (1988), Ata et al. Chem Lett (1989) 341-344, Solar JOC 28:2911 (1963), JOC

51:4639 (1986), JOC 54:3652 (1989), Theophil. Eicher and Josef L Weber "Structure and Reactivity of

Cyclopropenones and Triafulvenes" in Topics in Current Chemistry vol 57, Springer Verlag pages 1-109,

Comprehensive Heterocyclic Chemistry Vol 4 part 3

Pergamon 1984 London, ISBN 0-08-030704-3, Chapter 3/08 Pyrroles with Fused Six-membered Heterocyclic Rings: (i) a-Fused, Pages 443-495; W Flitsch Methods for the construction of the Indolizine Nucleus; Takane Uchida, Synthesis pages 209-236; Moria L Bode and Perry T Kaye: A New Synthesis of Indolizines via Thermal Cyclisation of 2-Pyridyl derivatives. JCS PERKIN TRANS I (1990) 2612-2613; K Matsumoto and T Uchida Synthesis (1978) 207-208; Esko Pohjala. Acta Chem Scand. B 28 (1974) P582-583, B 29 (1975) 1079-1084, B 10 (1976) 198-202, B 31 (1977) 321-324; Heterocycles (1974) 585-588;

Heterocycles (1975) 615-618; J Heterocyclic Chem (1977) 273-279; J Heterocyclic Chem (1978) 955-960; D H Wadsworth et al J. Org Chem (1989) 3660-3664; Tet Lett. (1981) 3569-3572; J. Org Chem (1986) 4639-4644; J Org Chem (1989) 3652-3660; L Cardellini et al. JCS PERKIN TRANS II (1990) 2177-2121; Tominaga et al. Heterocycles J Het Chem (1989) page 477; JACS (1990) p 8100; Tet. Lett. (1990) pp. 5689, 7109 and 6949; JCS Perkin I

(1990) p. 2612; J Het. Chem (1990) p 263; JCS Perkin I (1989) p. 1547; Bordwell JACS 113:3495 (1991); Chem Ber 93:2649 (1960); Chem Ber 87:922 (1954); Acta Chem Scand. 23:751 (1969); Chem Ber 42:2539 (1909); Becker et al. New J Chem 12:875 (1988).

Persistent free radicals which have relatively few transitions, e.g. less than 15, preferably less than 10, in their esr spectra and radicals having narrow

linewidth esr transitions, e.g. up to 500 mG, preferably less than 150 mG, especially less than 60 mG and

particularly less than 25 mG, are especially preferred for use as OMRI contrast agents. (The linewidths referred to are conveniently the intrinsic linewidths (full width at half maximum in the absorption spectrum) at ambient conditions).

Whilst low numbers of esr transition lines are generally preferred to obtain more effective coupling of the esr and NMR transitions, we have found that

surprisingly good coupling, and therefore enhancement of the MR signal, may also be achieved with radicals showing a large number of ESR transitions.

Where the radicals have a multiplicity of esr transitions, the hyperfine splitting constant is

preferably very small. In this connection radicals having as few as possible non-zero spin nuclei,

positioned as far away as possible from the paramagnetic centire are thus especially preferred.

The novel radicals of the invention include

radicals which surprisingly are stable at physiological pH, have long half lives (at least one minute, and preferably at least one hour), long relaxation times, and exhibit surprisingly good relaxivity. Water-soluble radicals are a particularly important aspect of the invention.

The radicals may be coupled to further molecules for example to lipophilic moieties such as long chain fatty acids or to macromolecules, such as polymers, proteins, polysaccharides (e.g. dextrans), polypeptides and polyethyleneimines. The macromolecule may be a tissue-specific biomolecule such as an antibody or a backbone polymer such as polylysine capable of carrying a number of independent radical groups which may itself be attached to a further macromolecule. Coupling to lipophilic groups is particularly useful since it may enhance the relaxivity of the radicals in certain systems such as blood. Such lipophilic and

macromolecular derivatives of the radicals and salts thereof form a further aspect of the present invention.

The linkage of a radical to the further molecule may be effected by any of the conventional methods such as the carbodiimide method, the mixed anhydride

procedure of Krejcarek et al. (see Biochemical and

Biophysical Research Communications 77:581 (1977)), the cyclic anhydride method of Hnatowich et al. (see Science 220: 613 (1983) and elsewhere), the backbone conjugation techniques of Meares et al. (see Anal. Biochem. 142:68 (1984) and elsewhere) and Schering (see EP-A-331616 for example) and by the use of linker molecules as described for example by Nycomed in WO-A-89/06979.

In view of their surprisingly beneficial

properties, the novel radicals of the invention may also be used as esr spin labels in esr imaging or in

magnetometry.

The radicals may be prepared from their non-radical precursor compounds by conventional radical generation, methods for example comproportionation, oxidation, reduction or any of the other methods known from the literature or described in PCT/EP91/00285. Thus in a further aspect the invention provides a process for the preparation of the novel radicals of the invention which comprises subjecting a radical precursor therefor to a radical generation step and optionally subsequently modifying the substitution on the aryl moieties, e.g. by oxidation or reduction. By such modification for example, sulphide substituents (e.g. - SCH₃ or -SCH₂COOEt) may be oxidized to the corresponding sulphones so avoiding problems of acidic hydrogens prior to radical formulation. Similarly lipophilic

substituents (such as -SCH₂COOEt) may be reduced to corresponding hydrophilic substituents (e.g. -SCH₂CH₂OH).

The non-radical precursors may themselves be prepared by methods conventional in the art or analogous to those described in PCT/EP91/00285.

While radicals with long half lives in aqueous solution, for example at least one hour, preferably ten days, more preferably fifty days and especially

preferably at least one year are clearly particularly desirable for use in in vivo imaging, shorter lived inert free radicals may still be utilised in imaging (e.g. of inanimate samples) and these may particularly conveniently be prepared immediately pre-administration.

Taking as another illustrative example the

indolizinyl radicals, these radicals may be generated from the corresponding indolizinols by oxidation under air or oxygen, or by using a chemical oxidant such as benzoquinone, iodine or chloranil. Oxidation under air or oxygen is preferred.

Oxidation may conveniently be effected during cyclization to form the indolizinyl skeleton, during work-up or even before or during administration.

The non-radical indolizinyl precursors may

themselves be prepared by methods conventional in the art. Thus to form an indolizinol, a suitable

cyclopropenone is conveniently reacted with an

appropriately substituted pyridine, following for example the procedures described by Wadsworth et al in Tetrahedron lett. 22:3569 (1981) and J. Org. Chem

51:4639 (1986).

Further processes for the preparation of

oxoindolizine and oxoindilizinium compounds, i.e.

derivatives in the keto as opposed to enol form, which may be used as non-radical precursors are described in EP-A-68880 and US-A-4446223.

Thus indolizinyl free radicals according to the invention may be prepared by following reaction schemes such as those suggested below:-

For the preparation of the non-radical precursors for indolizinyl radicals for use according to the invention, the literature contains many further useful guidelines. Thus one suitable approach for the

production of nitro substituted precursors is described by Tominaya et al in J Heterocyclic Chem (1989) p. 477

The nitro group can then be transformed into an oxygen radical , e. g. folowing the sequence:

C-NO₂ → C-NH₂ → C- N₂₊

OH- → C-OH → C-O*

Hydrogenated indolizinyls, for instance indolizinyl alkaloids like castanospermine or similar substances also represent useful reagents in the synthesis of the indolizinyl radicals. These hydrogenated substances can be dehydrogenated and/or dehydrated to the

indolizinols/indolizinyls. (See J.A.C.S. 1990, 8100; Tet Lett 1990, 5689; Tet Lett 1990, 7109; Tet Lett 1990, 6949).

More specific routes to indolizinyl radicals include the following:

R^2'=CN, CCR^3'

R^3'=alkyl, S-alkyl, O-alkyl or = R^4'

R^4'=electron-withdrawing group,

e.g. ester, cyano, ketone, sulfone, sulfonamide

Water-solubilizing groups on R^2', R^3' and/or R^{4 '} .

For example

The preparation of semiquinone anion radicals is widely described in the literature. However, by way of illustration, aryloxy and semiquinone radicals can be prepared from quinones/hydroquinones according to the following general schemes:

R*X

------------→

Base

R*X = suitable

alkylating agent,

e.g. t-BuCl

If several products are formed, they can be separated by chromatography or crystallization, or by a combination of these techniques.

This group of alkoxyphenoxyl radicals is thus clearly related to the semiquinone anion radicals, the only difference being the R* instead of the minus charge, i.e.

Where a monoalkylated product is desired, in order to generate phenoxyl rather than semiquinone anion radicals, a quinone starting material should be reduced to the hydroquinone form before the alkylation is

effected. Suitable reduction techniques are described for example by E F Rosenblatt JACS 62, 1940 p 1092; H J Taube et al. Berichte 86, 1953, p 1036; F W Weygand et al. Berichte 90, 1957, p 1879; O Daun et al. Berichte 93, 1960, p 2829; T Sziki, Berichte 62, 1929, p 1373; H W Moore, J. Org. Chem. 33, 1968, p 4019; L Feiser et al. JACS 70, 1948, p 3165; G A Reynolds et al., Organic Synthesis 3_4, 1954, p 1; T Akita, J. Pharm. Soc. Jpn. 82, 1962, p 91; S Fujita et al., Tet Lett, 1975, p 1965; and C Graebbe, Journal fur Praktische Chemie [2], 62, 1900, p 32.

Moreover using sodium borohydride, a whole range of quinones may be reduced to semiquinone anion radicals and, with more than one equivalent of H-, further

reduction to hydroquinones is observed. An example is given below.

Other examples of quinone reductions useful for the preparation of radical precursors include

(In these formulae the R groups will generally be identical to the specifically identified substituents at the 2- positions).

General methods for alkylation of

phenols/hydroquinones can be found in Compendium of Organic Synthetic Methods Vol. I-V by Harrison and

Harrison and later by Hegedus and Wade, Wiley

Interscience. Compounds of formula

may be made either from diacylated hydroquinone by mild hydrolysis of one acyl group or by selective

monoacylations.

In general, phenoxy radical precursors of formulae

(where M³ represents a group which makes the molecule water soluble) are desirable and may be made in this fashion, for example according to a scheme such as:

Other phenol/quinone substitutions are described for example in:

F Ramirez et al JACS 81, 1959, p 4338;

F Ramirez et al JOC 23 , 1958, p 778;

G Stork et al JACS 86, 1964, p 1761; and

F Ramirez et al JOC 33, 1968, p 20.

In synthesising substituted radicals, the

substituents may be introduced onto individual component substructures before they are put together to form the radical precursor compounds, or they may be introduced directly onto the precursor compound or the actual radical itself. It is also possible to effect the substitution and radical construction steps

simultaneously in a "one-pot" reaction.

For use in OMRI, the radicals are conveniently formulated into contrast media together with

conventional pharmaceutical carriers or excipients.

Contrast media manufactured or used according to this invention may contain, besides the radicals (or the non- radical precursor where radical formation is to be effected immediately before administration), formulation aids such as are conventional for therapeutic and diagnostic compositions in human or veterinary medicine. Thus the media may for example include solubilizing agents, emulsifiers, viscosity enhancers, buffers, etc. The media may be in forms suitable for parenteral (e.g. intravenous) or enteral (e.g. oral) application, for example for application directly into body cavities having external voidance ducts (such as the

gastrointestinal tract, the bladder and the uterus), or for injection or infusion into the systemic vasculature. However, solutions, suspensions and dispersions in physiologically tolerable media will generally be

preferred. Free radicals which are relatively unstable or insoluble in the sample environment may be encapsulated, e.g. in gastric juice resistant capsules containing a medium in which they are stable. Alternatively, the radicals may be presented as an encapsulated freeze dried powder in a soluble capsule. Such formulations might conveniently be dissolved shortly before in vivo use.

For use in in vivo diagnostic imaging, the medium, which preferably will be substantially isotonic, may conveniently be administered at a concentration

sufficient to yield a 1 micromolar to 10 mM

concentration of the free radical in the imaging zone; however the precise concentration and dosage will of course depend upon a range of factors such as toxicity, the organ targetting ability of the contrast agent, and the administration route. The optimum concentration for the free radical represents a balance between various factors. In general, optimum concentrations would in most cases lie in the range 0.1 to 100 mM, especially 0.2 to 10 mM, more especially 0.5 to 5 mM. Compositions for intravenous administration would preferably contain the free radical in concentrations of 10 to 1000 mM especially 50 to 500 mM. For ionic materials, the concentration will particularly preferably be in the range 50 to 200 mM, especially 130 to 170 mM and for non-ionic materials 200 to 400 mM, especially 290 to 330 mM. For imaging of the urinary tract or the renal or biliary system however, compositions may perhaps be used having concentrations of for example 10 to 100 mM for ionic or 20 to 200 mM for non-ionic materials. Moreover for bolus injection the concentration may conveniently be 0.1 to 100 mM, preferably 5 to 25 mM, especially preferably 6 to 15 mM.

The present invention will now be further

illustrated by the following non-limiting Examples

(percentages, parts and ratios are by weight and temperatures are in degrees Celsius unless otherwise stated).

Example 1

2,3-Diphenyl-1-hydroxyindolizine-7-carboxylate

triethylammonium salt

Diphenylcyclopropenone (Aldrich 17,737-7) (0.5000 g 2.424 * 10^-3 mole) and isonicotinic acid (Aldrich I- 1,750-8) (0.2985 g 2.424 * 10^-3 mole) were added in solid form to a carefully dried reaction flask. The flask was equipped with a septum and the flask was evacuated three times with addition of nitrogen after each evacuation. Chlorobenzene (Aldrich 27,064-4) (5 ml) was added with a gastight syringe. The stirred mixture was cooled to 0°C. Triethylamine (Aldrich 23,962-3) (0.3379 ml, 2.42 * 10^-3 mole) was added dropwise with a gastight syringe. The resulting mixture was stirred at ambient temperature for 2 days. The colour of the mixture changed to yellow and then to green. The solvent was removed on a

rotavap, and the resulting semisolid was redissolved in hot ethanol and water. After cooling to ambient

temperature, the product was filtered and washed with diethylether and dried in vacuum. All these operations were performed under an atmosphere of N₂. Yield: 0.653 g (1.517 * 10^-3 mole) = 62.6% of theory

¹H NMR (250 MHz) (DMSO_d6/D₂O with sodium hydrosulphite (Aldrich 15,795-3) present)) (water resonance at 4.60 ppm as reference) δ : 1.10 (t, 9H), 2.98 (q, 6H), 6.70 (d, 1H, H6, J_H6._H5, 7.56 Hz), 7.1-7.2 (m) and 7.25-7.35 (m) (total 10 H, 2 Ph), 7.70 (d, 1H, H₅, J_H5-H6, 7.56 Hz),

8.04 (bs, 1H, H8) .

MS (DEI) (DCI probe and electron impact ionization) M/Z: 329 (10%), 178 (8%), 86 (100%)

Example 2

2,3-Diphenyl-1-hydroxyindolizine-6,7-dicarboxylate di-triethylammonium salt

Diphenylcyclopropenone (0.5000 g, 2.424 * 10^-3 mole) and pyridine-3,4-dicarboxylic acid (0.4051 g, 2.424 * 10^-3 mole) were added to a carefully dried reaction flask. The flask was equipped with a septum and evacuated three times with addition of nitrogen after each evacuation. Methanol (10 ml) (degassed with N₂) was added and the stirred slurry cooled to 0°C. Triethylamine (0.666 ml, 6.20 * 10^-3 mole) was then added with a syringe. The reaction mixture was stirred for 3 days at ambient temperature. (Thin layer chromatography showed complete conversion after one day). The yellowish product was filtered (under nitrogen to prevent radical formation) and washed with diethyl ether and dried at high vacuum. 1H NMR in D₂O/DMSO_d6 showed only ethyl resonances, upon addition of sodium hydrosulphite the resonances from the heterocycle appeared.

Yield: 0.762 g (1.3235 * 10^-3 mole) = 54.6% of theory

¹H NMR (250 MHz) (D₂O/DMSO _d6, sodium hydrosulphite present) (water resonance at 4.60 ppm as reference) d : 3.12 (t, 18H), 4.99 (q, 12H) 9.16-9.30 (m) and 9.34-9.46 (m) (total 10 H, 2 Ph), 10.42 (s, 1H, H5) and 10.59 (s, 1H, H8).

MS (DEI) M/Z 373 (5%), 355 (47%), 329 (100%)

Example 3

2,3-Diphenyl-1-hydroxyindolizine-6,7-dicarboxylate triethanolammonium salt

Diphenylcyclopropenone (0.5000 g, 2.424 * 10^-3 mole) and pyridine-3,4-dicarboxylic acid (0.4051 g, 2.424 * 10^-3 mole) were added to a carefully dried reaction flask. The flask was equipped with a septum and evacuated three times with addition of nitrogen after each evacuation. Methanol (10 ml) (degassed with N₂) was added with a gastight syringe and the stirred suspension cooled to 0°C. Triethanolamine (0.3217 ml, 2.424 * 10^-3 mole) was added dropwise with a gastight syringe. The mixture was stirred for 48 hours at ambient temperature, cooled to about + 10°C and the product was isolated by filtration under N₂. The product was washed with a little cold methanol and ether on the filter and dried in vacuum.

Yield 0.548 g (1.0487 * 10^-3 mole) = 43% of theory

¹H NMR (250 MHz) (DMSO _d6/D₂O with sodium hydrosulphite present) (water resonance at 4.60 ppm as reference) d : 3.36 (t, CH₂, 6H), 3.82 (t, CH₂, 6H), 7.15-7.35 (m) and 7.40-7.50 (m) (total 10H, 2-Ph) , 8.37 (s, 1H, H5) and 8.58 (s, 1H, H8).

MS (DEI) M/Z: 373 (11%), 355 (48%), 329 (100%)

MS (Thermospray after RP 18 column , MeOH : H₂O 3 : 1 0 . 2 M NH₄OAc) M/Z : 390 (M + 18 , 1% ) , 374 (M + 1 , 1% ) , 344 ( 17%) , 330 ( 34% ) .

Example 4

2,3-Diphenyl-1-hydroxyindolizine-7-carboxylate

triethanolammonium salt

Diphenylcyclopropenone (0.6249 g, 3.03 * 10^-3 mole) and isonicotinic acid (0.2985 g, 2.424 * 10^-3 mole) were added to a carefully dried reaction flask. The flask was equipped with a septum and the flask was evacuated three times with addition of nitrogen after each evacuation. Methanol (19 ml) was added with a gastight syringe. The stirred suspension (slightly yellowish) was cooled to 0°C. Triethanolamine (Aldrich T5, 830-0) (0.3217 ml, 2.424 * 10^-3 mole) was added dropwise with a gastight syringe. The suspension went into solution immediately and an orange colour appeared. The reaction mixture was stirred at ambient temperature for 2.5 hours, while the title compound precipitated. The mixture was cooled to about 0°C and the product isolated by filtration under N₂. The product was washed with minute amounts of methanol and some diethylether and dried.

Yield: 0.246g (5.151 * 10^-4 mole) = 17% of theory

The product was identified by mass spectrometry; DCI probe and electron impact conditions identified the heterocyclic part and ¹H NMR identified the ammonium part. The product was further characterized by ESR and OMRI, measurements of the corresponding radical which was generated by treatment with oxygen.

MS (DEI) M/Z: 329 (97%), 178 (100%)

Example 5

THIS PAGE WAS NOT FURNISHED UPON FILING THE INTERNATIONAL APPLICATION

2,3-Diphenyl-1-hydroxyindolizin-6,7-dicarboxylate di- dipropan-2,3-diol ammonium radical salt. In situ formation of the radical

3,4-Pyridinedicarboxylic acid (2.424 * 10^-3 mole, 0.4051 g), diphenylcyclopropenone (2.424 * 10^-3 mole, 0.5000 g) and di(propane-2,3-diol) amine (4.848 * 10^-3 mole, 0.8008 g) were stirred in methanol (10 ml), under an atmosphere of air for 24 hours at ambient temperature. Thin layer chromatography revealed complete consumption of the cyclopropenone and the solvent was removed on high vacuum, yielding the product as a foam.

The radical was identified by mass spectrometry (DCI-EI and thermospray) and by the ESR spectrum and the OMRI effect in a water solution (buffer pH 7.4).

Example 7

2,3-Diphenyl-1-hydroxyindclizin-6,7-dicarboxylate di-N- methylglucammonium salt

3,4-Pyridinedicarboxylic acid (2.424 * 10^-3 mole, 0.4051 g), diphenylcyclopropenone (2.424 * 10^-3 mole, 0.5000 g) and N-methylglucamine (4.848 * 10^-3 mole, 0.9404 g) were stirred in a mixture of tetrahydrofuran (10 ml, degassed with helium) and methanol (3 ml, degassed with helium) at ambient temperature for 24 hours. The solvent was removed and the product triturated with diethyl ether and methanol and dried.

Yield: 0.870 g (1.139 * 10^-3 mole) = 47% of theory

MS (DCI) M/Z: 373 (5%), 329 (100%), 178 (71%)

Example 8

Radical formation

The compounds of Examples 1 to 5 and 7 are converted to their radicals by oxidation in air or with benioquinone.

Example 9

1-Hydroxy-2,3-diphenyl-7-cyanoindolizine

The title compound was synthesized according to the procedure of D H Wadsworth , J . Org . Chem . , 1986 , 51 , 4639

Yield 0.184 g (0.59 mmol, 49%)

¹H NMR (300 MHz) (Acetone D₆) d : 6.46 (dd, CH, 1H), 7.50-7.20 (m, 2-Ph, 10H), 7.90 (dd, CH, 1H), 8.01 (dd, CH, 1H)

MS (Thermospray via loop) M/Z : 310 (M+, 100%) Example 10

1-Oxy-2,3-dipehnyl-7-cyanoindolizinyl

The title compound was synthesized from the product of Example 9 according to the procedure of D H Wadsworth, J. Org. Chem., 1989, 54, 3652. The isolated green to black precipitate was analyzed by HPLC and the radical content was determined to be 20%.

OMRI signal enhancement at 5 Watts = 60

Example 11

1-Hydroxy-2,3-diphenyl-6,7-diamidoindolizine

Diphenylcylcopropenone (0.250 g, 1.21 mmol) and 3,4- diamidopyridine (0.200 g, 1.21 mmol) were mixed in a dry, argon filled reaction flask. Chlorobenzene (2.5 mL) (oxygen free) was added, and the reaction was heated to 130ºC. After 2 h the heating was stopped and the reaction was allowed to reach room temperature.

Petroleum ether 40-60ºC (2.5 mL) was added in order to obtain a complete precipitate of the product. The solvent was filtered off and the precipitate was washed with petroleum ether. Acetone (30 mL) was added to the crude product, and the mixture was stirred for lh. The dark acetone solution was filtered off leaving a yellow precipitate. The precipitate was analyzed by HPLC

(Kromasil C8, CH₃CN/H₂O). Two peaks were found with a ratio of 2:1. HPLC-MS showed that the larger peak was the desired product.

Yield 0.155 g (0.418 mmol, 34%)

MS (Thermospray after HPLC C18) M/Z : 371 (M+, 12%), 356 (14%), 344 (17%), 326 (100%).

Example 12

1-Oxy-2,3-diphenyl-6,7-diamidoindolizinyl

1-Hydroxy-2,3-diphenyl-6,7-diamidoindolizine (Example 11) was dissolved in THF and 4-benzoquinone was added. The reaction was stirred for 15 min at 50ºC. The colour changed during the reaction from yellow to dark red. The product was analyzed and the formation of the radical was determined by an OMRI experiment.

OMRI signal enhancement at 5 Watts = 70.

Example 13

1-Hydroxy-2,3-diphenyl-6,7-dicyanoindolizine

Diphenylcyclopropenone (0.319 g, 1.55 mmol) and 3,4- dicyanopyridine (0.200 g, 1.55 mmol) were mixed in a dry, argon filled reaction flask. Chlorobenzene (2.5 mL) (oxygen free) was added, and the reaction was heated to 130ºC. After 2 h the heating was stopped and. the reaction was allowed to reach room temperature.

Petroleum ether 40-60ºC (2.5 mL) was added in order to obtain a complete precipitate of the product. The solvent was filtered off and the precipitate was washed with petroleum ether. The crude product was stirred with chloroform (30 mL) for lh. The dark chloroform solution was filtered off leaving the title product as a yellow precipitate.

Yield 0.100g (0.298 mmol, 19%) ¹H NMR (300 MHz) (DMSO D₆) a : 7.50-7.20 (m, 2-Ph, 10H), 8.29 (CH, 1H), 8.48 (CH, 1H) MS (Thermospray via loop) M/Z 359 (30%), 353 (45%) 337 (100%).

Example 14

1-Oxy-2,3-diphenyl-6,7-dicyanoindolizinyl

1-Hydroxy-2,3-diphenyl-6,7-dicyanoindolizine (Example 13) (10 mg, 0.03 mmol) was dissolved in DMSO (5 mL) and 4-benzoquinone (13.0 mg, 0.12 mmol) was added. The reaction was stirred for 15 min at 700. The colour of the reaction became dark. The product was analyzed and the formation of the radical was determined by an OMRI experiment.

OMRI signal enhancement (5 Watts) 80.

Example 15

1-Mercapto-2,3-di-t-butylthio-7,8-dicyanoindolizine and 1-Mercapto-2,3-di-t-butylthio-6,7-dicyanoindolizine

The title compounds are prepared according to the following reaction scheme

a) Bis(t-butylthio)cyclopropenethione

Silver tetrafluoroborate (21.8 g, 112 mmol) was

dissolved in dry acetonitrile (50 mL) in a dry, argon filled reaction flask. The solution was cooled to -20ºC and tetrachlorocyclopropene (19.8 g, 11.0 mmol)

dissolved in dry acetonitrile (25 mL) was added drop- wise. When all was added the reaction was stirred for 0.5 h at -15C. The temperature was lowered to -20ºC and t-BuOH (50.0 mL, 444.0 mmol) dissolved in dry acetonitrile was added. The reaction was allowed to reach room temperature and was stirred over night. The precipitated AgCl was filtered off and the filtrate was concentrated, almost to dryness. Chloroform and water were added, and after vigorous shaking, the water phase was discarded. The organic phase was dried over Na₂SO₄, filtered and evaporated. To the remaining crude product was added EtOAc (10 mL), and the mixture was stirred for 2h. The dark oil transformed into yellow crystals

(tris (t-butylthio) cyclopropenium tetrafluoroborate). The crystals were collected by filtration and were dissolved in a mixture of hydrochloric acid (50 mL, 2N) and THF (50 mL). The solution was refluxed for 4 h.

After cooling to room temperature, chloroform (200 mL) was added. The organic phase was separated, washed once with water and dried over Na₂SO₄. The title compound was purified by flash-chromatography (DCM : Petroleum ether 40-600 1:1).

Yield 7.92 g (32.2 mmol, 29%)

¹H NMR (300 MHz) (CDCl₃) a : 1.67 (s)

¹³C NMR (75 MHz) (CDCl₃) a : 169.7, 154.9, 50.8, 32.2 MS (Electron impact ionization) M/Z : 247 (M+1, 27%), 190 (35%), 134 (60%), 102 (14%), 59 (100%). b) 1,1'-(2,2',3,3'-tetra-t-butylthio-7,7',8,8'- tetracyano-diindolizine)-disulfide and 1,1'-(2,2-',3,3'- tetra-t-butylthio-6,6',7,7'-tetracyano-diindolizine)- disulfide

Bis (t-butylthio)cyclopropenethione (0.382 g, 1.55 mmol) and 3,4-dicyanopyridine (0.200 g, 1.55 mmol) were mixed in a dry, argon filled reaction flask. Chlorobenzene (25 mL) (oxygen free) was added, and the reaction

mixture was heated to 130ºC for 70 h. The reaction was stopped and the crude product was purified by flash- chromatography (DCM : petroleum ether 40-60ºC 75:25). A mixture of the title homodimer compounds and the heterodimer disulfide was obtained (91 mg). On TLC all three appeared in the same spot (Rf. 0.21/ DCM :

petroleum ether 40-60ºC 75:25). The isomers were separated on HPLC (Kromasil KR100-10-C8, 250 x 10 mm, CH₃CN : H₂O 80:20).

Yield: 7,7',8,8' dimer 0.027 g (0.036 mmol, 4.6%)

6,6',7,7' dimer 0.009 g (0.012 mmol, 1.5%) hybrid dimer 0.028 g (0.036 mmol, 4.6%)

¹H NMR (300 MHz) (CDCl₃) a : (7,7',8,8' dimer) : 8.98 (d, ArH, 1H) 6.86 (d, ArH, 1H), 1.30 (s, t-Bu, 9H), 1.20 (s, t-Bu, 9H). (6,6',7,7' dimer) : 9.13 (d, ArH, 1H 7.88 (s, ArH, 1H), 1.34 (s, t-Bu, 9H), 1.15 (s, t-Bu, 9H)

MS (Thermospray after HPLC C18) (7,7',8,8') : M/Z 767 (M+19) (100%). (6,6',7,7') : M/Z : 767 (M+19) (100%). c) 1-Mercapto-2.3-di-t-butylthio-7,8-dicyanoindolizine

1,1'-(2,2',3,3'-tetra-t-butylthio-6,6',7,7'-tetracyano- diindolizine)-disulfide is treated with a reducing agent in an appropriate solvent until all disulfide is

consumed. The reaction is stopped and the product is isolated by chromatography or recrystallization, or by a combination thereof. The radical is produced by

conventional techniques. d) 1-Mercapto-2.3-di-t-butylthio-6,7-dicyanoindolizine

1,1'-(2,2',3,3'-tetra-t-butylthio-7,7',8,8'-tetracyano- diindolizine)-disulfide is treated with a reducing agent in an appropriate solvent until all disulfide is

consumed. The reaction is stopped and the product is isolated by chromatography or recrystallization, or by a combination thereof. The radical is produced by conventional techniques .

Example 16

1-Mercapto-2,3-di-t-butylthio-7,8-diamidoindolizine

The title product and the resulting radical are synthesized analogously to Example 15.

Example 17

1-Mercapto-2,3-di-t-butylthio-6,7-diamidoindolizine

The title product and the resulting radical are synthesized analogously to Example 15. Example 18

1-Mercapto-2,3-di-t-butylthio-7,8-di(triethylammonium carboxylate) indolizine

Bis (t-butylthio) cyclopropenethione and pyridine-3,4- dicarboxylic acid are mixed in a dried, argon filled reaction flask. A dry degassed solvent is added. To the mixture is added triethylamine. The reaction is stirred until no more product is obtained. The product is isolated either by chromatography or by

recrystallization, or by a combination thereof. The radical is generated by conventional techniques.

Example 19

1-Mercapto-2,3-di-t-butylthio-6,7-di(triethylammonium carboxylate) indolizine

Bis (t-butylthio) cyclopropenethione and pyridine 3,4- dicarboxylic acid are mixed in a dried, argon filled reaction flask. A dry degassed solvent is added. To the mixture is added triethylamine. The reaction is stirred until no more product is obtained. The product is isolated either by chromatography or by

recrystallization, or by a combination thereof. The radical is generated by conventional techniques.

Example 20

1-Hydroxy-2,3-di-t-butoxy-6,7-dicyanoindolizine

The title compound is prepared by the following reaction scheme

a) 1,2-Di-tert-butoxycyclobutenedione

3,4-Dihydroxy-3-cyclobutene-1,2-dione (5.0 g, 43.8 mmol) was dissolved in water (230 mL). While stirring the solution NaOH (87.7 mL, 1M, 87.7 mmol) was added

dropwise. AgNO₃ (14.9 g, 87.7 mmol) dissolved in water (90 mL) was then slowly added to the solution. A yellow to green precipitate was formed. The suspension was stirred for lh. The precipitated silver salt was collected by filtration. It was washed with water, acetone and ether and was dried in vacum overnight. In a dry reaction flask the silver salt and dry ether (50 mL) were mixed. While stirring the suspension, t-butyl chloride (40.4 mL, 367 mmol) was added. After 48h the reaction was stopped. The silver chloride formed was filtered off and washed with ether. The organic phases were washed with diluted NaHCO₃ and with water, dried over Na₂SO₄ and the solvent was evaporated.

Yield: 3.33g (14.7 mmol, 34%)

¹H NMR (300 MHz) (CDCl₃) a : 1.61 (s, t-Bu).

1³C NMR (75 MHz) (CDCl₃) a : 188.6, 186.2, 87.0, -28.6 MS (Thermospray after HPLC C18) : M/Z : 228 (M+2) (24%), 173 (100%), 157 (37%), 117 (44%). b) 2,3-Di-tert-butoxycyclopropenone

1,2-Di-tert-butoxycyclobutenedione is dissolved in ether and photolyzed under nitrogen by a mercury high pressure lamp through quartz glass for 2-8h depending on the quality of the mercury lamp. The title compound

produced is purified by HPLC-RP, recrystallization or by distillation at low pressure, or by a combination of this techniques. (See E V Dehmlow, Chem. Ber. 121, 569, 1988). c) 1-Hydroxy-2,3-di-t-butoxy-6,7-dicyanoindolizine

2,3-Di-tert-butoxycyclopropenone and 3,4-dicyanopyridine are mixed in a dry, argon filled flask. A solvent such as chlorobenzene (oxygen free) is used. After the completion of the reaction the product is purified by chromatography or recrystallization, or by a combination of these techniques. The radical is then generated by conventional techniques.

Example 21

1-Oxy-2,3-di(t-butylthiol)-7,8-dicarboxylic acid- indolizinyl

The title compound was prepared by the following

reaction scheme

a) Bis (t-butylthio) cyclopropenone

In a dry, argon filled reaction flask was placed bis(t- butylthio) cyclopropenethione (0.200 g, 0.81 mmol).

Thionychloride (1.0 mL, 5.12 mmol) was added dropwise with stirring at room temperature. A yellow precipitate was formed. After lh excess of thionylchloride was removed under reduced pressure, using a Rotary

evaporator connected to an oil pump and an ethanol-:

carbon dioxide trap. By adding CH₂Cl₂ (5 mL) to the residual material a red solution was formed leaving a white precipitate. The solution was cooled to 0°C, washed with cold NaHCO₃ (5%) and dried over Na₂SO₄. The solvent was removed under reduced pressure. The product was filtered through a column of microcrystalline cellulose using petroleum ether as eluent and purified by recrystallisation from petroleum ether.

Yield: 122 mg (0.53 mmol, 66%)

¹H NMR (300 MHz) (CDCl₃) δ : 1.55 (s, t-bu)

1C NMR (75 MHz) (CDCl₃) δ : 152.2, 143.0, 48.8, 31.6

MS (El): M/Z: 230 (M⁺) (25%), 202 (62%), 173 (65%), 146 (100%). b) 1-Hydroxy-2,3-di(t-butylthio)-7,8-dicarboxylic acid-indolizine

3,4-Pyridinedicarboxylic acid (1.31 g, 7.83 mmol) and triethyl amine (1.58 g, 15.7 mmol) were dissolved in chloroform (5.0 mL) (oxygen free) in a dry, argon filled reaction flask. Bis (t-butylthio) cyclopropenone (0.30 g, 1.30 mmol) was added. The reaction was stirred at 35°C for 48h. The reaction was terminated and the product was purified by preparative HPLC (Kromasil C18, 250 x 20 mm, CH₃CN: H₂O, NH₄OAc pH=5). The product was unstable in the water-acetonitrile solution. It was therefore impossible to evaporate the solution. However, by allowing the fraction with pure product to stand in the freezer overnight the water and the acetonitrile were separated into two phases. The organic phase was separated and stored in the freezer. The product, which was dissolved in acetonitrile was stable in the freezer for months.

Yield 0.052 g (0.130 mmol, 10%)

¹H NMR (300 MHz) (CDCl₃) δ : 8.54 (d, ArH, 1H), 7.15 (d, ArH, 1H), 3.2 (q, CH₂), 1.4 (t, CH₃), 1.29 (s, t-bu, 9H), 1.25 (s, t-Bu, 9H).

MS (Plasma spray): M/Z: 398 (M+1) (4%), 352 (17%), 312 (100%), 256 (33%) . c) 1-Oxy-2,3-di(t-butylthio)-7,8-dicarboxylic acid- indolizinyl

1-Hydroxy-2,3-di(t-butylthio)-7,8-dicarboxylic acid- indolizine (0.014 g, 7.83 mmol) was dissolved in sodium phosphate buffer (2.5 mL, pH=8). The solution was purged for 15sec. with air. The colour of the resulting solution was brown-green.

ESR (water, 1.23 mM, 200G) : doublet, a_H=1.95 G, linewidth 73 mG.

Overhauser enhancement (water, 1.23 mM) : 144 at 16W microwave power. Example 22

1-Hydroxy-2,3-di-(8-methylthio-2,2,6,6- tetramethylbenzo[1,2-d:4,5-d']bisd,3)dioxole-4-yl)-6,7- di-(trialkylammonium carboxylate) indolizine

The title compound is prepared according to the

following reaction scheme

a) 4-Hydroxymethyl-8-methylthio-2,2,6,6- tetramethylbenzo[1,2-d:4,5-d']bis(1,3)dioxole

4-Hydroxymethyl-8-methylthio-2,2,6,6- tetramethylbenzo[1,2-d:4,5-d']bis(1,3)dioxole (see

PCT/EP91/00285) is dissolved with stirring in dry THF in a dry, argon filled reaction flask. The solution is cooled to (-25) - (-30) ºC. Butyllithium in hexane is added dropwise with a syringe. The reaction is stirred for 0.5 h. In another reaction flask, a large excess of paraformaldehyde is depolymerized by heating. The formaldehyde formed is distilled, by means of an argon stream, into the reaction via a glass tube. When the reaction is complete, the product is hydrolyzed. The crude product is collected and is purified by

recrystallization or chromatography, or by a combination of these techniques. b) 4-Bromomethyl-8-methylthio-2,2,6,6- tetramethylbenzo[1,2-d:4,5-d']bis(1,3)dioxole

4-Hydroxymethyl-8-methylthio-2,2,6,6- tetramethylbenzo [1,2-d:4,5-d']bis (1,3) dioxole is

dissolved in pyridine. The solution is chilled and triphenylphosphine followed by carbontetrabromide are added. the reaction is stirred for an appropriate time. Methanol is added and the product is isolated by a suitable method. c) 1,3-Bis(8-methylthio-2,2,6,6-tetramethylbenzo[1,2- d:4,5-d']bis(1,3)dioxole-4-yl) acetone

4-Bromomethyl-8-methylthio-2,2,6,6-tetramethylbenzo [1,2- d: 4,5-d']bis (1, 3) dioxole is dissolved in dry ether in a dry, argon filled reaction vessel. The solution is cooled with a dry ice ethanol bath. With stirring, butyllithium in hexane is added. After the completion of the halogen metal exchange reaction ethyl (N,N- dimethyl) carbamate dissolved in dry ether is added. The reaction is hydrolyzed and the product is purified by chromatography or recrystallization, or by a

combination of these techniques. d) 1,1-Dibromo-1,3-bis(8-methylthio-2,2,6,6- tetramethylbenzol[1,2-d:4,5-d']bis(1,3)dioxole-4-yl) acetone

1,3-Bis (8-methylthio-2,2,6,6-tetramethylbenzo [1,2-d:4,5- d']bis(1,3)dioxole-4-yl) acetone is dissolved in

solvent. In the presence of base, bromine is added. After workup, the product is purified by chromatography or recrystallization, or by a combination of these techniques. e) 2,3-Di(8-methylthio-2,2,6,6-tetramethylbenzo[1,2- d:4,5-d']bis(1,3)dioxole-4-yl)cyclopropenone

Triethylamine is dissolved in CH₂Cl₂ with stirring. 1,1- Dibromo-1,3-bis (8-methylthio-2,2,6,6- tetramethylbenzol [1,2-d:4,5-d']bis (1,3) dioxole-4-yl) acetone in CH₂Cl₂ is slowly added. After completion, the reaction mixture is worked up. The product is isolated by ctiromatography or recrystallization, or by a

combination of these techniques.

f) 1-hydroxy-2,3-di (8-methylthio-2,2,6,6- tetramethylbenzo[1,2-d:4,5-d']bis(1,3)dioxole-4-yl)-6,7- di (triethylammonium carboxylate) indolizine

2,3-Di(8-methylthio-2,2,6,6-tetramethylbenzo [1,2-d:4,5- d']bis (1,3) dioxole-4-yl) cyclopropenone and pyridine-3,4- dicarboxylic acid are mixed in a dry, argon filled reaction flask. A dry, degassed solvent and

triethylamine are added. The reaction mixture is stirred until no more product is formed. The product is isolated either by chromatography or by

recrystallization, or by a combination of these

techniques. The radical is generated by conventional techniques.

Example 23

8-Oxyquinolinyl radical

8-Hydroxyquinoline (0.145 g, 1 mmol) was dissolved in a mixture of acetonitrile (20 ml) and DMSO (10 ml).

Sodium hydroxide (1 ml of a 1 M aqueous solution) was added. p-Benzoquinone (0.43 g, 4 mmol) was dissolved in acetonitrile (20 ml). Both solutions were purged with argon for 30 minutes and then mixed. An instant colour change from yellow to dark green was observed. The formation of the radical was verified by ESR

measurements.

Example 24

8-Thiomethyl-2,2,6,6-tetramethylbenzo[1,2-d:4,5- d']bis (1,3) dioxole-4-oxy radical

Sodium hydroxide (3.2 g, 80 mmol) and potassium

ferricyanide (250 mg, 0.76 mmol) were dissolved in water (80 ml). 4-Hydroxy-8-thiomethyl-2,2,6,6- tetramethylbenzo[1,2-d:4,5-d']bis(1,3)dioxole (100 mg, 0.35 mmol) was then added and the solution was heated to 80ºC for 2 hours. A colour change from orange to pale green was observed. the formation of the radical was verified by ESR measurements.

ESR frequency 548.9 MHz.

5 lines with a_H = 106 mg, LW=53 mg.

Example 25

Phenol (502.1 mg, 5.335 mmol) was dissolved in DMF (4 mL, dry Aldrich sureseal). Sodium hydride (159.9 mg, 5.330 mmol, 80% in white oil) was washed twice with dry petroleum ether (decanting most of the petroleum ether after settling of the NaH), dired with argon gas and added to the phenolic solution. The resulting solution was stirred under argon while hydrogen evolved. When the gas evolution had ceased, tetrafluoroquinone (199.0 mg, 1.105 mmol) was added in portions, while cooling the mixture with an ice-water bath. The resulting solution was stirred 48 h, acidified with dilute HCl and

evaporated. Water was added and the product was extracted with CHCl₃ (3 x 50 mL). The organic phase was washed with water (25 mL), dried (Na₂SO₄), filtered and evaporated yielding 0.7 g crude product.

The pure product was obtained by flash chromatography on silica gel eluting with CHCl₃. Yield 250 mg (47%). The product was identified by ¹HNMR- and ¹³CNMR spectroscopy. 1HNMR (CDCl₃, 300 MHz) a : 7.17 (m, 8H, Ar), 7.01 (m, 4H, Ar), 6.86 (m, 8H, Ar) ¹³CNMR (CDCl₃, 75 MHz) a : 171.48, 156.37, 142.50, 129.46, 123.91, 116.80.

Small amounts of a different product was also obtained in the chromatographic separation. Using MS and NMR data, this, product was identified as:

The semiquinone anion radicals are generated by

convnetion techniques from the product of the Example.

Examples 26 and 27

The following products were synthesized in the same way as described in Example 25 (yield: 39%, and 45%

respectively).

The corresponding radicals are generated using conventional techniques.

Example 28

This semiquinone was made according to: Methoden der

Organischen Chemie - Houben Weyl pp 464-465 number VII/3a 1977. The product was crystallized from hot EtOH. The radical is generated using conventional techniques.

Example 29

h

PhSO₂Na (1.6579 g., 0.0101 mol) was dissolved in water (100 mL), while keeping an atmosphere of N₂. HCl (12 M, 0.84 mL, 0.0101 mol) was added in order to produce PhSO₂H.

Benzoquinone (0.01 mol, 1.081 g) was added while flushing with N₂. A white to grey precipitate was formed

immediately. The solution was stirred for 5 min, filtered (glass sinter no. 3) under N₂, washed with distilled water (-20 mL) and dried under vacuum (+20ºC) over night. Yield 2.08 g. The product was identified by ¹H- and ¹³CNMR

spectroscopy.

¹H NMR (CDCl₃, 300 MHz) a : 6.82 (d, 1H), 7.01 (dd, 1H), 7.24 (d, 1H), 7.58-7.73 (m, 3H, Ph), 8.00 (m, 2H, Ph) 7.4- 7.7 (b, OH, 2H)

¹³CNMR (CDCI₃, 75 MHZ) a : 150.72, 148.97, 141.91, 134.20, 129.83, 127.70, 125.43, 124.24, 119.88, 117.80, 114.47 b)

Phenylsulfonylhydroquinone (0.0250 g, 0.1 mmol) was dissolved in CH₂Cl₂ (4 mL). Silicagel (0.5 g) and NalO₄ (0.65 M in H₂O, 0.5 mL) were added. The clear solution turned yellow quickly and the solution was filtered throug a short plug of silica after 15 min stirring. The product was eluted with CH₂Cl₂. Yield 0.0218 g.

The product was identified by ¹H and ¹³CNMR spectroscopy.

¹H NMR (CDCl₃, 300 MHz) 3 : 6.75 (d, 1H, J=10.2 HZ,), 6.86 (q, 1H, J_a=10.2 Hz, J_b=2.3 Hz) 7.62 (d, 1H, J=2.3 Hz), 7.55- 7.62 (m, 2H, arom.H), 7.66-7.73 (m, 1H, arom.H), 8.07-8.12 (m, 2H, arom.H)

¹³CNMR (CDCl₃, 75 MHz) a : 185.79, 180.82, 138.13, 137.07, 136.90, 136.84, 134.77, 129.67, 129.31.

The radical is generated using conventional techniques.

Example 30

NaSO₂Ph (0.829 g, 5.05 mmol) was dissolved in H₂O (50 mL) under N₂. HCl (0.42 mL, cone.) was added, followed by the monopenylsulfonylquinone (1.2413 g, 5 mmol). The quinone did not dissolve, and consequently, THF (50 mL) was added with concomitant dissolution of the substrate. A red colour appeared, which changed into brown within 15 min. HCl (2 drops, cone.) were added and the solution became clearer. pH was measured to be -5. TLC analysis indicated a new lipophilic product. The THF was evaporated off and the water phase was extracted with EtOAc (3 x 100 mL). In the first extraction some difficulties to separate the phases were observed. Addition of some saturated NaCl solution forced the phases apart. The combined EtOAc phase was washed once with saturated NaCl, dried (Na₂SO₄), filtered and evaporated. The product was dissolved in EtOAc and filtered through a short silica column. Evaporation yielded a grey powder. Yield 30%.

The product was identified by ¹H NMR spectroscopy.

The radical is generated using conventional techniques.

Example 31

The hydroquinone is synthesized according to the procedure of Can. J. Chem. 1962, 40, page 1235. If desired the solvent may be changed to DMF and the reaction may be run at a higher temperature. The radical may be generated by conventional techniques. Example 32

Sodium hydride (1.98 g, 0.066 mol, 80% in mineral oil), previously washed with dry petroleum ether (2 x 5 mL) and dried under a stream of N₂ was added to a solution of EtSH (4.1006 g, 0.66 mol) in DMR (55 mL) at 0ºC. The resulting thick slurry was transferred to a dropping funnel and added gradually to a stirred (+10ºC) solution of chloranil (3.6882 g, 0.015 mol) in benzene (100 mL) over a period of 40 min. The reaction mixture was allowed to warm up to room temperature and stirred for 24 hours. Dilute HCl (ca 1 M) was added to pH 6. The solution was evaporated at ≤40ºC/4 mm Hg. The resulting black oil was partitioned between CHCl₃ adn water (with a little dilute HCl added to ensure a low pH). The water phase was extracted with CHCl₃ (4 x100 mL). The combined CHCl₃ phases were washed with water (1 x 100 mL) and dried (Na₂SO₄). Evaporation gave a black oil, which crystallized in a water/ethanol mixture (dissolved in hot EtOH and hot water added until cloudiness appeared. The mixture was heated again and scratched to induce crystallization).

The product was isolated in a yield of 300 mg as yellow crystals. The identification and verification were done with the help of ¹H NMR and IR spectroscopy and MS. ¹HNMR (CDCl₃, 300 MHz) a : 7.37 (s, OH, 2H), 2.92 (q,

CH₂, 8H), 1.21 (, CH₃, 12H)

¹³CNMR (CDCl₃, 75 MHz) a : 152.42, 125.07, 29.73, 14.71

The use of a larger amount of EtSH resulted in a higher yield.

The radical may be generated by conventional techniques.

Example 33

This reaction was perfomed analogously to Example 32. The product was identified by mass spectrometry.

¹HNMR (CDCl₃, 300 MHz) a : 7.69 (s, OH, 2H), 1.31 (s,

CH₃, 27H),

¹³CNMR (CDCl₃, 75 MHz) a : 155.61, 128.03, 50.42, 31.59

The radical may be generated by conventional techniques.

Example 34

Tetraphenoxy benzoquinone is reduced with Na₂S₂O₄ to the tetraphehoxy hydroquinone, as described. by L Feiser et al., JACS 70, 1948, p 3165.

The product is purified by crystallization or

chromatography, or by a combination of these techniques. Example 35

Tetraphenoxy benzoquinone is reduced with excess NaBH₄ in a mixture of EtOH and water. The product is purified by extractions and chromatography, or by a combination of these techniques.

The product is then monoalkylated or monoetherified to yield a phenoxy radical precursor as follows:

Tetraphenoxyhydroquinone is monomesylated in pyridine with one equivalent of MeSO₂Cl for 2-3 days at ambient temperature. The product is isolated in low to moderate yield by extractions and chromatography. (See Annalen 551:235 (1942)).

Example 36

2,6-Diphenylsulfonyl hydroquinone is monomesylated in pyridine with one equivalent of MeSO₂Cl for 2-3 days at ambient to high temperature. The product is isolated and purified by extractions and chromatography.

Example 37

Tetraethylthiohydroquinone is monomesylated with MeSO₂Cl in pyridine at room temperature for 2-4 days. The product is isolated by extractions and chromatography.

Radicals may be generated from the compounds of Examples 34-37 by conventional techniques.

Example 38

Tetraethylthiohydroquinone monomesylate is stirred with lead dioxide (excess) in the dark under an atomosphere of N₂. Small samples are taken, centrifuged or filtered through oxygen-free silica and analysed by ESR, or by OMRI signal enhancement measurements. The product is purified by centrifugation, filtration and

recrystallization or chromatography. Example 39

2,6-Dichlorohydroquinone monomethyl ether is stirred with an excess of K₃Fe(CN)₆ in benzene until samples taken show high conversion to the radical. The product is purified as described in Example 38.

Example 40

Six phenoxy radical precursors are prepared according to the following reaction schemes (See also Muller, E. et al. Chem. Ber. 93, 2649 (1960) and Müller, E. and Ley, K. Chem. Ber. 87, 922 (1954)):

The corresponding phenoxy radicals are generated by conventional techniques.

Example 41

A phenoxy radical precursor is prepared by a

trimerization-condensation reaction as set forth below (see Martinson, P. et al., Acta Chem. Scand. 23:751-64

(1969) ) :

In the first stage of the reaction scheme, to a hot solution of 1,3,5-tripivaloyl benzene and ethanedithiol in acetic acid is added dropwise BF₃·OEt₂ (48% in BF₃) and the reaction mixture is left overnight for

crystallization. After cooling, crystals separate.

These crystals are filtered off and recrystallized for use in the later reaction steps.

The phenol end product can be transformed into a radical directly or after oxidation of the sulphurs in the steric hindrance groups according to the reaction scheme below:

In step (b) 2-hydroxy-1,3,5-tripivaloyl benzene

trisethylenethioketal is dissolved in CH₂Cl₂ at ambient temperature. Magnesium monoperphtalic acid (MMPA) and tetra-n-butylammonium hydrogensulphate (Q+HSO_4- dissolved in water are added dropwise.

The reaction is complete after several hours. The phases are separated and the organic phase is washed with a saturated solution of NaHCO₃. The ether phase is dried (Na₂SO₄) and the solvent evaporated leaving the product, which can be purified via distillation,

crystallization or chromatography, or combinations thereof.

Example 42

(See Becker et al . New J . Chem 12 : 875-880 ( 1988 ) )

p-Benzoquinone is dissolved in acetic acid (60%). The thiophenol is slowly added at ambient temperature with efficient stirring. After stirring (3-4 days) a

voluminous red precipitate is formed and filtered off. The product can be crystallized and the two isomeric products (the 2,6- and 2,4-isomers, respectively) can be separated by chromatography. The reduction of the quinone product is performed in absolute EtOH with NaBH₄. After stirring, 2 M HCl is added until pH = 2-3. The ethanol is evaporated, and the residue is partitioned between ether and water. The ether phase is dried

(Na₂SO₄) and the solvent is evaporated, leaving a

residue, which was used without further purification. The O-alkylation of the product (2,6-bisphenylthio hydroquinone) can be performed in dry dioxane with isobutylene, condensed into the solution, and a

catalytic amount of concentrated sulfuric acid. The reaction flask is sealed and the reaction mixture is stirred at room temperature for 10 h. The reaction mixture is then neutralized with solid NaHCO₃ (until CO₂ evolution ceases). After drying (Na₂SO₄), the solvent is evaporated to give the t-butoxylated product.

2,6-Diphenylthio-4-t-butoxyphenole is dissolved in CH₂Cl₂ and mixed with metachloroperbenzoic acid (MCPBA) and Q+HSO-₄, dissolved in water. Efficient stirring is maintained at reflux for 20 h. Sodium sulphite is added to reduce the excess MCPBA. After concentration in high vacuum, the reaction mixture is worked up to give the product, which is purified via distillation,

crystallization or chroiuatography, or combinations thereof.

The reaction is performed according to the method of Ullman et al. Chem. Ber. 42: 2539-2548 (1909). If another oxidant is selected the same reaction sequence can be used to give the corresponding 5-COOH derivative.

Example 44

3 , 4 , 5-trimethoxyphenol is dissolved in a 2 M solution of NaOH at room temperature. Formaldehyde solution (37%) is added and the mixture is stirred for two days at room temperature. The reaction mixture is then neutralized with diluted (50%) acetic acid (to pH = 6-7) and the product is isolated and used without further

purification in the next reaction step.

The product of the first reaction step is dissolved in dry acetone and active MnO₂ is added. The mixture is stirred for 24 h at room temperature. The mixture is filtered, and the filtrate treated with an acidic ion exchanger (e.g. Dowex 50 x B) and filtered again. After evaporation of the solvent the di-aldehyde product can be isolated. This compound is dissolved in glacial aceitc acid with warming. After cooling, ethanedithiole and a few drops of BF₃·OEt₂ are added. Stirring is maintained for 20 h. The acetic acid is evaporated at reduced pressure (1-2 torr) and the residue is the desired product, compound (A). The oxidation of the compound (A) takes place in glacial acetic acid with H₂O₂ (35%). Stirring is continued at room temperature for 48 h. The excess peroxide is destroyed by the careful addition of a saturated

solution of sodium sulphite. Compound (B) can then be purified via distillation, crystallization or

chromatography, or combinations thereof.

Example 45

p-Hydroxymethyl phenol is etherified by dissolving it in dioxane, condensing isobutylene into the solution and adding a catalytic amount of mineral acid. This product can be converted to the di-hydroxymethyl derivative by addition to a solution of NaOH (50%) adn then adding, at room temperature, a solution of formaldehyde (37%). The oxidation of this product takes place with active MnO₂ (20 equivalents) in acetone. In the next reaction step, the starting product is dissolved in glacial acetic acid, and ethanedithiol (2.5 equivalents), and a few drops or BF₃·OEt₂ are added. After stirring overnight, the reaction mixture is worked up by evaporation of the solvent. The residue is then purified by crystallization, distillation or

chromatography, or combinations thereof. There follows another oxidation with MnO₂, and the aldehyde is isolated and then condensed with an active methylene compound to give compund (A), according to the general procedures given by Mullet et al. (see Example 40).

The phenol function can, by use of diazomethane, be

protected to give compound (B), which can be oxidized with hydrogen peroxide (20 equivalents) in acetic acid. The excess peroxide is reduced by the addition of sodium sulphite. The product can then be purified by

crystallization, distillation or chromatography, or

combinations thereof. The methyl ether is cleaved to the phenol with hydrogen iodide in acetone. The mixture is evaporated to dryness at high vacuum, and the phenol can be converted to its radical by anion formation and oxidation. S-oxidation can take place without prior phenol protection.

Example 46

The starting compound is dissolved in dry Et₂O, and t-BuLi is added via a syringe. Stirring is continued for several hours at room temperature. After quenching with water, the phases are separated and the organic phase is worked up. The product is used directly in the next reaction step. In this it is dissolved in acetone and oxidized with active MnO₂. After stirring at room temperature for 24 h, the mixture is filtered and the solvent is evaporated under reduced pressure. The product is then purified by

crystallization, distillation or chromatography, or

combinations thereof. The standard procedure for

thioketalisation, as given above, is followed. The

thioketal product is then purified by crystallization, distillation or chromatography, or combinations thereof. The thioketal is dissolved in acetone and MnO₂ is added. After work up the aldehyde product is used directly in next step. The aldehyde compound is mixed with diethylmalonate and pyridine, according to the procedure given by Mullet et al. (see above). The product is then purified by

crystallization, distillation or chromatography, or

combinations thereof. It is then oxidized with hydrogen peroxide in acetic acid. After work up, including

reduction of the excess peroxide, the product can then be purified by crystallization, distillation or

chromatography, or combinations thereof. Example 47

Radical formation

The following schemes illustrate phenoxy radical formation techniques:

Potassiumferricyanide (0.29 g, 0.8 mmol) was dissolved in water, which had been made alkaline with

potassiumhydroxide. Diethylether (80 mL) was added, and into the mixture was bubbled argon for 30 minutes. 3,5-di- tert-butyl-4-hydroxybenzaldehyde (0.1 g, 0.4 mmol) was added. After 45 minutes, the organic phase became yellow and the presence of the radical was established with ESR measurements.

3,5-Di-tert-butyl-4-hydroxyanisole (0.1 g, 0.4 mmol) was dissolved in diethylether (80 mL), and into the mixture was bubbled argon for 30 minutes. Potassiumferricyanide (0.29 g) was dissolved in water (100 mL), which had been made alkaline with potassiumhydroxide and bubbled with argon for 30 minutes. The solutions were mixed and after 10 minutes the organic phase was red and the presence of the radical was established with ESR measurements.

The S-methylated di-ketal (500 mg, 1.87 mmol) was dissolved in THF (50 mL, distilled over Na) under argon. The mixture was cooled to -70ºC. n-Butyllithium (0.8 mL, 2.0 mmol) was added through a syringe. The mixture was stirred at -70ºC for 2 hours. The Dewar flask was removed, and O₂ was bubbled through the mixture for 3 h. Diethylether (50 mL) was added, and a solid precipitated. This was filtered off and dissolved in 1 N NaOH and washed with Et₂O. The organic phase was extracted twice with 1 N NaOH (10 mL). The alkaline water phase was acidified with concentrated HCl to pH 2 and then extracted with CH₂Cl₂ (2 x 50 mL). After drying, filtering and evaporation the product was isolated (130 mg, 0.46 mmol; 25%). Radical formation is performed with KOH and K₃Fe(CN)₆, as described above.

Example 49

N,N-bis-(2-hydroxyethyl)-3,5-bis-(1,1-dimethylethyl)-4- hydroxybenzenecarboxamide

3, 5-bis-(1,1-dimetyletyl)-4-hydroxybenzenecarboxylic acid (2.5 g, 0.010 mol) and diethanolamine (1.05 g, 0.010 mol) were dissolved in 30 ml of dry dimethylformamide and dicyclohexylcarbodiimide (2.13 g, 0.0105 mol) in 10 ml of dry dimethylformamide was added over 5 minutes. After stirring overnight, the resulting colorless suspension was filtered, and the filtrate was evaporated, 3 X 25 ml of benzene was added and reevaporated to yield a white solid which was recrystallised from toluene.

Yield: 2.06 g (61%)

¹H NMR (CDCl₃, 300 MHz) δ : 7.34 (s, 2H, ArH), 5.42 (s, 1H, ArOH), 4.2-2.6 (m, 10H, CH₂CH₂OH), 1.43 (s, 18H, C(CH₃)₃) Mass spectrum (APcI, 25 V): m/e (%rel.int.) 338 (100) (M+1), 321 (5), 174 (4), 115 (6), 106 (31). Example 50

N,N-bis-(2,3-dihydroxypropyl)-3,5-bis-(1,1-dimethylethyl)- 4-hydroxybenzenecarboxamide

3,5-bis-(1,1-dimethylethyl)-4-hydroxybenzenecarboxylic acid (5.0 g, 0.020 mol) and bis-(2,3-dihydroxypropyl)-amine (3.3 g, 0.020 mol) were dissolved in 60 ml of dry

dimethylformamide and dicyclohexylcarbodiimide (2.13 g, 0.0105 mol) in 20ml of dry dimethylformamide was added. After stirring overnight the resulting colorless suspension was filtered, and the filtrate was evaporated, added 3 X 25 ml of benzene and reevaporated to yield a white solid;

according to HPLC a mixture of starting material, title compound and at least two other products. The title compound was isolated by preparative HPLC.

Yield: 0.1 g (1%) (not optimized, crude HPLC yield: ca

30%).

¹H NMR ((CD₃)₂SO, 300 MHz) δ : 7.20 (s, 2H, ArH), 5.02 (s,

IH, ArOH, 3.8-3.2 (m, 14H, CH₂CH(OH) CH₂OH), 1.36 (s, 18H,

C(CH₃)₃).

Mass spectrum: (APcI, 25V): m/e (rel.int.) 398 (100) M+1),

304 (5), 250 (7), 201 (9), 178 (2), 160 (16), 142 (48), 101

(45). Example 51

N ,N-bis-(2-hydroxyethyl)-2,6-bis-(1,1- dimethylethyl)benzene-4-carboxamide-1-oxy radical

To a saturated solution of N,N-bis-(2-hydroxyethyl)-3,5- bis-(1,1-dimethylethyl)-4-hydroxybenzenecarboxamide in redistilled, Argon-flushed water (50 mg in 50 ml) 1.0 g of lead dioxide was added in one portion while flushing with argon. The flask was sealed with an ordinary stopper and teflon tape and thoroughly shaken. The dark green solution thus obtained was used directly for ESR-measurements.

ESR-data (H₂O, 0.75 mM) : triplet, linewidth = 900 mG; a_H = 1650 mG.

Example 52

N,N-bis-(2,3-dihydroxypropyl)-2,6-bis-(1,1-dimethylethyl)- benzene-4-carboxamide-1-oxy radical To a saturated solution of N,N-bis-(2,3-dihydroxypropyl)- 3,5-bis-(1,1-dimethylethyl)-4-hydroxybenzenecarboxamide in redistilled. Argon-flushed water (50 mg in 50 ml) was added in one portion while flushing with argon, 0.5 g of lead dioxide. The flask was sealed with an ordinary stopper and teflon tape and thoroughly shaken. The dark green solution thus obtained was used directly for ESR-measurements.

ESR-data (H₂O, 3.79 mM) : triplet, linewidth = 900 mG; a_H = 1850 mG.

Example 53

8-Methoxy-3,3,5,5-tetraoxo-2,2,6,6-tetramethylbenzo[1,2,- d;4,5-d']-bis(1,3)oxathiole-4-oxyl

The title compound was prepared according to the following scheme:

a) 2,2,6,6-tetramethylbenzo[1,2,-d;4,5-d']-bis(1,3)- oxathiole

2,6-Dioxo-benzo[1,2-d:5,4-d']bis (1, 3) oxathiole (1.0 g, 4.4 mmol), prepared according to the literature (H. Fiedler, Berichte 95, 1771 (1962)) was suspended in dry methanol (30 mL) and a solution of sodium methoxide in methanol

(prepared from 20 mL methanol and 2.2 mmol sodium) was then added over a period of 15 minutes. After stirring for 15 minutes, the mixture was poured onto diethyl ether (50 mL) and 1 M aqueous HCl (25 mL). The aqueous phase was

extracted twice with ether and the combined organic phases were dried (MgSO₄) and evaporated. The residue (0.60 g) was dissolved in dry acetonitril (40 mL) containing acetone (6 mL) and BF₃.Et₂O (4 mL) was then added. After stirring for 20 minutes, water (100 mL) and dichloromethane (50 mL) were added. The aqueous phase was extracted twice with

dichloromethane and the combined organic phases were dried (MgSO₄) and evaporated. The brownish residue was passed through a short silica column using ethyl

acetate :cyclohexane (1:5) as the eluent to give 0.30 g of a yellow solid which was further purified by preparative HPLC (RP-18, acetonitrile:water 80:20). Yield 0.25 g (23%). ¹H NMR (CDCl₃) δ : 1.80 (s, 12H, CH₃), 6.35 (s, 1H), 6.75 (s, 1H). b) 8-t-Butoxy-2,2,6,6-tetramethylbenzo[1,2-d;5,4- d'bis (1,3) oxathiole

2,2,6, 6-Tetramethylbenzo [1,2-d:5,4-d'] bis (1,3) oxathiole (300 mg, 1.18 mmol) was dissolved in dry diethyl ether (30 mL) and the solution was cooled to -78°C. A solution of n- BuLi in hexane (2.5 M, 0.52 mL) was added and the reaction was allowed to attain room temperature. After stirring for 1 hour, the mixture was cooled to

-78°C and transferred into a solution of MgBr₂ in dry ether (prepared from magnesium, 60 mg and 1,2-dibromoethane, 0.2 mL in 2 mL ether) kept at -78°C. The mixture was stirred for 30 minutes at 0°C and then, a solution of t- butylperbenzoate (0.24 mL, 0.12 mmol) in dry ether (2 mL) was added. After stirring for 1 hour at 0°C, the mixture was poured onto a mixture of ice and 0.1 M aqueous HCl.

The aqueous phase was extracted three times with ether and the combined organic phases were washed with aqueous NaHSO₃, 2 M NaOH, dried (MgSO₄) and evaporated. The product was purified by preparative HPLC (RP-18, CH₃CN: H₂O 80:20).

Yield 124 mg (32%).

1H NMR (CDCl₃) δ : 1.36 (s, 9H, t-Bu), 1.82 (s, 12H, CH₃), 6.54 (s, 1H). c) 8-Methoxy-2,2,6,6-tetramethylbenzo[1,2-d;5,-4-d']bis

(1,3)oxathiole

7-t-Butoxy-2,2,6,6-tetramethylbenzo[1,2-d;5,4-d']bis- (1,3) oxathiole (152 mg, 0.47 mmol) was dissolved in 1,1,1- trifluoroethanol (4 mL) and cooled to -10°C. A solution of CF₃SO₃H in 1,1,1-trifluoroethanol (0.11 M, 0.52 mL) was then added and the mixture was stirred for 40 minutes at -5ºC. A solution of triethyl amine in ether (0.14 M, 0.41 mL) was then added, the solution was evaporated and the product purified by preparative HPLC (RP-18, CH₃CN: H₂O 80:20).

Yield 113 mg (90%). ¹H NMR (CDCl₃) δ : 1.86 (s, 12H, CH₃), 4.74 (s, 1H, 0H), 6.40 (s, 1H). This phenol was then methylated using phase-transfer conditions. Thus, a solution of the phenol (0.48 mmol, 130 mg) was dissolved in CH₂Cl₂ (20 mL) together with

tetrabutylammonium hydrogensulfate (163 mg, 0.48 mmol), 1M aqueous NaOH (20 mL) and methyl iodide (2.4 mmol, 0.15 mL). The mixture was stirred vigorously for 15 hours, the organic phase was evaporated and triturated with ether.

The organic phase was washed with brine, water, dried

(Na₂SO₄) and evaporated. The product was purified by preparative HPLC (RP-18, CH₃CN: H₂O 80:20). Yield 133 mg (97%). ¹H NMR (CDCl₃) δ: 1.86 (s, 12H, CH₃), 3.92 (s, 3H, 0ºCH₃), 6.52 (s, 1H). d) 4-t-Butoxy-8-methoxy-2,2,6,6-tetramethylbenzo[ 1,2- d;5,4-d'bis(1,3)oxathiole

7-Methoxy 2,2,6, 6-tetramethylbenzo [1,2-d;5,4-d'bis(1,3)- oxathiole (142 mg, 0.50 mmol) was dissolved in dry diethyl ether (20 mL) and cooled to -78°C. A solution of n-BuLi in hexane (2.5 M, 0.52 mL) was added and the reaction mixture was stirred for 2 hours at room temperature. After cooling to -78°C, the solution was transferred to a solution of MgBr₂ in ether (prepared from magnesium, 24 mg, and 1,2- dibromoethane, 0.086 ml in 2 mL ether) kept at -78°C.

After stirring for 45 minutes at 0°C, t-butylperbenzoate (0.6 mmol, 0.11 mL) in dry ether (2.0 mL) was added. After stirring for another hour, the mixture was poured onto a mixture of ice and 0.1 M HCl. The aqueous phase was extracted three times with ether, the combined organic extracts were washed with aqueous NaHSO₃, 2 M NaOH, dried (Na₂SO₄) and evaporated. The product was purified by preparative HPLC (RP-18, CH₃CN: H₂O 80:20). Yield 60 mg (34%).

¹H NMR (CDCl₃) δ : 1.39 (s, 9H, t-Bu), 1.84 (s, 12H, CH₃), 3.88 (s, 3H, 0ºCH₃). e) 4-Hydroxy-3,3,5,5-tetroxo-8-methoxy-2,2,6,6- tetramethyl-benzo[1,2-d;5,4-d']bis(1,3)oxathiole

4-t-Butoxy-8-methoxy-2,2,6,6-tetramethylbenzo[1,2-d;5,4- d']bis (1,3) oxathiole (60 mg, 0.17 mmol) was dissolved in glacial acetic acid and aqueous hydrogen peroxide (3 mL, 36%) was added. The solution was heated to 100°C for 1 hour. After neutralization of the solvent with aqueous 2 M NaOH, the mixture was extracted three times with ethyl acetate. The combined organic phases were dried (MgSO₄) and evaporated. The product was purified by preparative HPLC (RP-18, CH₃CN: H₂O 80:20). Yield 25 mg (35%). ¹H NMR (DMSO- d₆) δ : 1.69 (s, 12H, CH₃), 3.68 (s, 3H, 0ºCH₃), 3.8 (br s, 1H, OH). f) 8-Methoxy-3.3,5,5-tetraoxo-2,2,6,6-tetramethyl- benzo[1,2-d;5,4-d']bis(1,3)oxathiole-4-oxy

The radical is prepared from 4-hydroxy-3,3,5,5-tetroxo-8- methoxy-2,2,6,6-tetramethylbenzo[1,2-d;5,4-d']bis- (1,3) oxathiole using either PbO₂ or K₃Fe(CN)₆ as the

oxidant.

Example 54

N,N'-bis(2,3-dihydroxypropyI)-2,4,6-triiodophenoxide-3,5- dicarboxylic acid diamide The title compound was prepared according to the following scheme :

L

Dimethyl 1-hydroxybenzene-3,5-dicarboxylic acid

5-Hydroxyisophthalic acid (54.6 g, 0.30 mol, Aldrich 31,

127-8) was dissolved in absolute methanol (300 mL).

Concentrated sulfuric acid (15 mL) was added and the reaction mixture was heated to reflux temperature for 19 hours and then cooled to -20°C. The precipitate was collected by filtration and the crude product was

recrystallized in methanol.

¹H NMR (DMSO-d₆) 5: 3.90 (s, 6H, CH₃), 7.57 (d, 2H, J=1.5

Hz), 7.92 (t, 1H, J=1.5 Hz), 10.29 (s, 1H, OH).

¹³C NMR (DMSO-d₆) δ: 52.36, 120.2, 120.4, 131.3, 157.9,

165.4

N,N'-bis(2,3-dihydroxypropyl)-1-hydroxybenzene-3,5- dicarboxylic acid diamide

Dimethyl 5-hydroxyisophthalate (12.6 g, 60 mmol) was dissolved in methanol (36 mL) containing 3-amino-1,2- dihydroxypropane (16.4 g, 180 mmol). The mixture was heated to reflux temperature for 10 days, and, after cooling to room temperature, was evaporated. Acetone (100 mL) was added to the residue and the crystalline solid was collected by filtration. The product was purified by recrystallization from acetone. Yield 9.0 g (46%).

1H NMR (DMSO-d₆) δ : 3.18-3.28 (m, 2H), 3.36-3..47 (m, 4H), 3.67 (p, 2H, J-8.4 Hz), 4.40 (br s, 4H), 7.39 (s, 2H), 7.78 (s, 1H), 8.36 (t, 2H, J=6.3 Hz).

¹³C NMR (DMSO-d₆) δ : 63.04, 70.31, 116.6, 116.7, 135.9, 157.2, 166.2.

N,N-bis (2,3-dihydroxypropyl)-1-hydroxy-2,4,6-triodobenzene- 3,5-dicarboxylic acid diamide

N,N-bis (2 ,3-dihydroxypropyl)-1-hydroxy-2,4,6-triodo- benzene-3,5-dicarboxylic acid diamide (13.1 g, 40 mmol) was dissolved in water (160 mL) and pH was adjusted to 3.9 using aqueous HCl. To this solution, NaICl₂ (42.6 g, 50.3%, 40 mmol) was added dropwise during a period of 30 minutes. After standing overnight, the reaction mixture was

evaporated. The product was purified by preparative HPLC (RP-18, CH₃CN: H₂O 15:85, 1% TFA). Yield 22.3 g (79%).

1H NMR (DMSO-d₆) δ : 3.08-3.21 (m, 2H), 3.22-3.55 (m, 4H), 3.62-3.75 (m, 2H), 5.4 (br s, 4H), 7.97-8.12 (m, 1H), 8.33- 8.44 (m, 1H).

N,N-bis(2,3-dihydroxypropyl)-2,4,6-triiodophenoxide-3,5- dicarboxylic acid diamide

N,N-bis(2,3-dihydroxypropyl)-1-hydroxy-2,4,6-triodo- benzene-3,5-dicarboxylic acid diamide (100 mg, 0.14 mmol) was dissolved in water (7 mL) under an atmosphere of argon.

PbO₂ (1 g) was then added and, after stirring for 10

minutes, the solid was allowed to settle and a sample was withdrawn for ESR analysis.

Overhauser measurement: Enhancement of 38 (20 W microwave power).

ESR: singlet, linewidth 1.08 G.

Claims

Claims:

1. The use of a persistent aryloxy or arylthio free radicals, other than perhalo radicals, for the

manufacture of a contrast medium for use in magnetic resonance imaging.

2. Use as claimed in claim 1 of a said radical having an inherent linewidth in its esr spectrum of less than 500 mG for the manufacture of a contrast medium for use in OMRI.

3. Use as claimed in either of claims 1 and 2 of a said radical of formula Ar-Oº or Ar-Sº (where Ar is a 5-7 membered carbo- or heterocyclic cromatic ring optionally carrying one or more fused carbocyclic or heterocyclic rings, each ring in Ar optionally being substituted by one or more steric hindrance, electron withdrawing, electron donor or water-solubilizing groups).

4. Use as claimed in claim 3 of a phenoxy, indolizinyl or semiquinone anion radical.

5. Use as claimed in claim 4 of a phenoxy radical of formula

where each R³² independently represents a hydrogen atom, group R³¹ or a solubilizing group;

R³⁸ represents a group M²⁰ or R³¹;

each R³¹ independently represents a steric hindrance group or two R³¹ groups on adjacent carbons together represent a steric hindrance bridging group; and

M²⁰ represents an electron donor group.

6. Use as claimed in claim 4 of a indolizinyl radical of formula

where R⁵² is an electron withdrawing group, a steric hindrance group or a solubilizing group; and

each of R⁴⁸, R⁴⁹, R⁵⁰, R⁵¹ and R⁵³ is hydrogen or a steric hindrance or solubilizing group.

7. Use as claimed in claim 4 of a semiquinone anion radical of formula

where R⁶⁹ to R⁷¹ independently represent steric hindrance and/or solubilizing groups or R⁶⁹ and R⁷⁰ and/or R⁷¹ and R⁷² together with the intervening carbons form fused aryl rings optionally carrying steric hindrance and/or solubilizing groups.

8. Use as claimed in any one of claims 5 to 7 of a said radical carrying as a steric hindrance group a t- butyl-thio, t-butoxy or t-butyl group or a bridging steric hindrance group of formula -X⁷-CR⁷ ₂-X⁷- where each X⁷ is independently O, S, CO or SO₂ and R⁷ is hydrogen or C_1-6 alkyl optionally substituted by hydroxyl, C_1-6-alkoxy or carboxyl or an amide, ester or salt thereof.

9. Use as claimed in any one of claims 5 to 8 of a said radical carrying a water solubilizing group.

10. A persistent, water-soluble aryloxy or arylthio free radical as defined in any one of claims 3 to 9.

11. A magnetic resonance imaging contrast medium comprising a physiologically tolerable persistent aryloxy or arylthio free radical as defined in any one of claims 1 to 10 together with at least one

pharmacologically acceptable carrier or excipient.

12. A non-radical precursor to a radical as defined in claim 10.

13. A process for preparing a radical as defined in claim 14 comprising subjecting a non-radical precursor therefor to a radical generation procedure.

14. A method of magnetic resonance investigation of a sample, said method comprising introducing into said sample a persistent aryloxy or arylthio radical as defined in any one of claims 1 to 9, exposing said sample to a first radiation of a frequency selected to excite electron spin transitions in said free radical, exposing said sample to a second radiation of a

frequency selected to excite nuclear spin transitions in selected nuclei in said sample, detecting free induction decay signals from said sample, and, optionally,

generating an image or dynamic flow data from said detected signals.