WO1993006119A1

WO1993006119A1 - Deoxynucleoside derivatives

Info

Publication number: WO1993006119A1
Application number: PCT/GB1992/001777
Authority: WO
Inventors: Pawel Jerzy Serafinowski
Original assignee: The Institute Of Cancer Research
Priority date: 1991-09-27
Filing date: 1992-09-28
Publication date: 1993-04-01

Abstract

A process for efficient preparation of 2',3'-didehydro-2',3'-dideoxynucleoside derivatives from ribonucleosides, particularly inosine and adenosine, is disclosed in which the initial ribonucleoside is reacted in nitromethane with 2-acetoxyisobutyryl bromide under mild conditions to form 5'-O-dioxolano derivatives that, by prolonging the reaction, become converted exclusively into the 5'-O-(2-acetoxyisobutyryl)-2'(3')-acetoxy-3'(2')-bromo derivatives which are recovered and treated to bring about selective deacetylation at the 3' and/or 2' positions and conversion into the corresponding 5'-O-(2-acetoxyisobutyryl)-2',3'-didehydro-2',3'-dideoxynucleoside, followed by deacetylating to remove the acetoxyisobutyryl protective group at the 5' position.

Description

DEOXYNUCLEOSIDE DERIVATIVES

TECHNICAL FIELD

This invention relates to the preparation or synthesis of deoxynucleoside derivatives, and is concerned more particularly with the synthesis of 2',3'-didehydro- 2',3'-dideoxynucleosides and 2',3'-dideoxynucleosides. The invention is especially, although not exclusively, concerned with the synthetic preparation of 2',3'- didehydro-2',3'-dideoxyinosine and 2',3'-dideoxyinosine from inosine (hypoxanthine ribofuranoside). It is also concerned with the preparation of intermediates which in some cases are new compounds and which are useful in the overall process of preparing such 2',3'-didehydro-2',3'- dideoxynucleosides and 2',3'-dideoxynucleosides.

BACKGROUND ART

At the present time there is considerable interest in

2',3'-didehydro-2',3'-dideoxynucleosides and 2',3'- dideoxynucleosides because of some apparent anti-HIV activity displayed by certain of these compounds, and 2''3'-dideoxyinosine in particular has been the subject of clinical trials which have shown promising preliminary results for its use as a potentially effective drug against AIDS. If any of these compounds are in fact adopted for general use as drugs against AIDS, it is to be expected that an urgent need will then arise for an availability of improved methods of preparation or synthesis suited to economic large-scale production.

Although there are a number of conventional methods available, for synthetic preparation of 2',3'-didehydro- 2',3'-dideoxynucleosides (and hence, by hydrogenation, of the corresponding 2',3'-dideoxynucleosides), many of these methods often give rather poor yields, problems in purification, and are economically relatively inefficient, with very variable results depending upon the particular nucleoside concerned and upon the reagents and particular reaction conditions used. Starting from the corresponding ribonucleoside, it is known for example to treat the compound with 2-acetoxy- isobutyryl halide so as to form 2',3'-trans halo acetoxy derivatives in which the 5'-hydroxyl also carries a substituent, and then to subject those derivatives to reductive elimination using chromous salts to provide a direct route to the 2' 3'-unsaturated nucleosides (see Jain, T.C. et al, J. Org. Chem. (1974),39,30). This particular reaction scheme is not, however, entirely satisfactory for economic and efficient large scale production for the reasons mentioned above.

More recently, the inventor hereof has developed a more satisfactory process specifically for the preparation of 2',3'-didehydro-2',3'-dideoxyformycin A from 2',3'- trans bromo hydroxy derivatives of the corresponding ribonucleoside in which the 5'-hydroxyl is protected by a 2-acetoxyisobutyryl group substituent, .this process involving acylation of the 2' or 3' hydroxyl by treatment with O-phenylchlorothionoformate followed by a free radical β-elimination of bromo and phenoxythiocarbonyl leaving groups (see Serafinowski, P. Synthesis (1990), 5,411). However, it had seemed that formycin represented a special case for which this process was especially applicable because, as previously reported by Jain T.C. et al (J. Org. Chem. (1973), 18,3179). unlike many other nucleosides treatment of formycin with 2-acetoxyisobutyryl bromide under normal reaction conditions gives directly 2',3'-trans bromo acetoxy derivatives substituted at the 5' position exclusively as an acetoxyisobutyryl ester.

In general, when ribonucleosides are reacted with 2- acyloxyisobutyryl halides, such as 2-acetoxyisobutyryl halides, in an organic solvent, either 5'-0-dioxolano- 2'(3')-acyloxy-3'(2')-halo or 5'-O-(2-acyloxyisobutyryl)- 2'(3')-acyloxy-3'(2')-halo derivatives may be produced. The 5'-0-dioxolano group of the former, however, is basically unstable in mild acidic and alkaline conditions so that it is unsuitable as an effective protecting group in subsequent transformations of the. sugar moiety such as a selective removal of 2'(3')-0-acyl groups needed to provide the starting material for a process similar to that used for preparing a 2',3'-didehydro-2',3'-dideoxy derivative of formycin as referred to above - for this particular process it is known that a more stable protective group such as an the acyloxyisobutyryl ester is required. On the other hand, the reaction of 2- acyloxyisobutyryl halides with most ribonucleosides, other than formycin, has usually been found to lead to the formation of the 5'-0-dioxolano derivative, either alone or mixed in a substantial proportion with the 5'-0- acyloxyisobutyryl ester derivative. In particular, it has been reported that the reaction of 2-acetoxyisobutyryl halides with inosine and with adenosine lead exclusively to 5'-0-dioxolano derivatives (see Jain, T.C. et al, J. Org. Chem. (1974), 39,30; Lichtenthaler, F.W. et al, Angew. Chem. Int. Ed. (1974), 12 , 861; Russell, A.I. et al, J. Am. Chem. Soc. (1973),12, 4025; Robins, M.J. et al, Tetrahedron Lett. (1984), 25, 367).

DISCLOSURE OF INVENTION

The present invention is based on the finding that, contrary to expectations, reacting inosine, or certain other nucleosides (including adenosine), with 2-acyloxy- isobutyryl halides such as acetoxyisobutyryl halides in an appropriate organic solvent (preferably 2-acetoxyiso- butyryl bromide in nitromethane) under mild conditions (room temperature) can surprisingly lead to production almost exclusively of the 5'-0-(2-acyloxyisobutyryl) ester derivatives in high yield when the reaction time is sufficiently prolonged. This aspect of the invention has thus provided an unexpected useful and convenient new practical route to the preparation of such 5'-0-(2- acyloxyisobutyryl) ester derivatives having a high degree of purity (other than possibly being a mixture of 2' and 3' isomers) well suited for use as intermediates for subsequent production of corresponding 2',3'-didehydro- 2',3'-dideoxynucleosides and 2',3'-dideoxynucleosides, especially 2',3'-didehydro-2',3'-dideoxyinosine and 2',3'- di-eoxyinosine. For this latter purpose, the 5'-0-(2-aeyloxyisobutyryl) ester derivatives so produced can then readily be selectively deacylated at the 3' or 2' positions (e.g. by the action of 8M methanolic ammonia) ready for phenoxy- thiocarbonylation (acylation) of the 2' or 3' hydroxyl groups by treatment with O-phenylchlorothionoformate in the presence of a base such as dimethylaminopyridine. Upon then deoxygenating the products and bringing about a free radical β-elimination of the halo and phenoxythiocarbonyl groups by treatment with tributyltin hydride in the presence of 2,2'-azobis-(2-methylproprionitrile), the corresponding 5'-0-(2-acyloxyisobutyryl)-2',3'-didehydro- 2',3'-dideoxynucleosides can be obtained in good yield. Finally, the 5'-0-(2-acyloxyisobutyryl) protective group can be removed (e.g. 8M methanolic ammonia) to yield the 2',3'-didehydro-2',3'-dideoxynucleosides. Thereafter, the olefinic unsaturated products may readily be hydrogenated by conventional means to provide the corresponding 2',3'- dideoxy nucleosides, if so required.

The above constitutes a preferred basic reaction scheme and has been found to be especially satisfactory for preparing the 2',3'-didehydro-2',3'-dideoxy inosine derivatives. It can also be satisfactory for preparing the corresponding derivatives of adenosine.

As an alternative to that part of the reaction scheme mentioned above in which the 5'-0-(2-acyloxyiso- butyryl) ester derivatives are converted into the corresponding 5'-0-(2-acyloxyisobutyryl)-2',3'-didehydro- 2',3'-dideoxynucleosides, it has also been found that in the case of adenosine this conversion can be carried out with reasonable yield in a single direct step by. reacting the 5'-0-(2-acyloxyisobutyryl) ester derivative(s) with a freshly prepared Zn/Cu couple in anhydrous tetrahydrofuran (THF) and heating under reflux. On the other hand, attempts to apply this same direct conversion step in the case of the corresponding inosine derivatives using a Zn/Cu couple in THF (or DMF) failed to produce any useful results, but nonetheless it has been found possible to apply this direct conversion step to the corresponding inosine derivatives and to obtain worthwhile, albeit rather low, yields of the corresponding 5'-0-(2-acyloxyisobutyryl)-2',3'-didehydro-2',3'-dideoxynucleosides using a Zn/Cu couple in pyridine as the organic medium. Whilst this does not represent the presently preferred reaction route, it may still provide a useful variation of the overall synthetic process for producing the desired inosine nucleoside derivatives and it is believed that it may well be capable of improvement to provide more satisfactory yields under optimum conditions.

In a broad aspect, the invention provides a process for preparing 2',3'-didehydro-2',3'-dideoxy or 2',3'- dideoxy nucleoside derivatives from ribonucleosides that, when reacted with 2-acyloxyisobutyryl halides, tend to form 5'-0-dioxolano derivatives, said process including the steps of reacting the ribonucleoside with a 2-acyloxy- isobutyryl halide in an organic solvent for an extended time sufficient to cause substantially all the dioxolano derivative initially formed to be converted to the corresponding acyloxyisobutyryl ester, recovering the acyloxyisobutyryl ester product, and subjecting said acyloxyisobutyryl product to further treatment effective to bring about selective deacylation at the 3' and/or 2' positions and to produce the corresponding 5'-0-(2- aσyloxyisobutyryl)-2',3'-didehydro-2',3'-dideoxy- nucleoside, followed by deacylating to remove the acyloxyisobutyryl protective group at the 5' position thereby to yield the 2',3'-didehydro-2',3'-dideoxy derivative.

The process according to the invention is particularly useful for preparing 2',3'-didehydro-2',3'- dideoxy and 2',3'-dideoxy derivatives of inosine.

Thus, the invention may also be defined as providing a process for preparing a 2',3'-dideoxy nucleoside derivative from a ribonucleoside selected from inosine and adenosine, said process being characterised in that it includes the steps of: (a) reacting the said ribonucleoside with a 2-acyloxy- isobutyryl halide in an organic solvent to form a 5'-0- dioxolano derivative or derivatives and continuing said reaction until substantially all said 5'-0-dioxolano derivative(s) becomes converted to the corresponding acyloxyisobutyryl ester 2'(3')-acyloxy-3'(2')-halo derivative(s),

(b) recovering said acyloxyisobutyryl ester product of step (a),

(c) subjecting said acyloxyisobutyryl ester recovered in step (b) to further treatment effective to bring about selective deacylation at the 3' or 2' positions and to produce the corresponding 5'-0-(2-acyloxyisobutyryl)- 2',3'-didehydro-2',3'-dideoxynucleoside, followed by

(d) deacylating to remove the acyloxyisobutyryl protective group at the 5' position.

In preferred embodiments, as already indicated, after said selective deacylation at the 3' and/or 2' positions the acyloxyisobutyryl ester product is treated with O-phenylchlorothionoformate in the presence of an organic solvent so as to phenoxythiocarbonylate the 2' or 3' hydroxyl groups, and the products formed are then subjected to a deoxygenation reaction promoting a β- elimination of the halo, and phenoxythiocarbonyl groups.

Preferably, the initial reaction of the ribonucleoside with the acyloxyisobutyryl halide is carried out without application of heat in nitromethane as the organic solvent, and the preferred halide used is 2'- acetoxyisobutyryl bromide. In general, the extended time for which the initial reaction is allowed to continue will be in excess of 12 hours, and preferably will be at least 20 hours, for example within the range of 24-86 hours. Also, at least in the case of inosine, the subsequent selective removal of the 2'(3')-0-acyl groups is carried out using 8M methanolic ammonia as the deacylating agent.

The preferred reaction scheme is depicted below, where B represents the nucleoside base (hypoxanthine in the case of inosine, adenine in the case of adenosine).

In carrying out the process as depicted in the above reaction scheme, if in the first stage the ribonucleoside starting material, e.g. adenosine (Compound la) or inosine

(Compound lb), is allowed to react with the 2-acyloxyiso- butyryl halide in the preferred solvent, nitromethane (or possibly acetonitrile), at room temperature for only a relatively short period (say only 2 to 3 hours), it is generally found that the only products that can be isolated are the 5'-0-dioxolano derivatives. This is certainly true with inosine and adenosine, but by extending the reaction time it has been found, again at least for inosine and adenosine using 2-acetoxyisobutyryl bromide in nitromethane, that such 5'-0-dioxolano derivatives may be converted substantially completely into the 5'-0-(2-acetoxyisobutyryl). derivatives. Thus, with a reaction time of 24 hours for inosine, or 86 hours for adenosine, the 5' -0-(2-acetoxyisobutyryl) derivatives were isolated in high yield (86% for adenine; 92-94% for inosine), providing inseparable mixtures of the 3'-bromo-

3'-deoxy and 2'-bromo-2-deoxy isomers (Compounds 2a, 3a or

2b, 3b) with the former isomer predominating. It is, however, unnecessary to separate these isomers, and upon subsequently treating the mixture with 8M methanolic ammonia at room temperature, the 2'-0-acetyl and 3'-0- acetyl groups are removed selectively giving generally a mixture of 2'-bromo-2'-deoxy and 3'- bromo-3'-deoxy isomers (Compounds 4a, 5a or 4b, 5b) in over 70% yield.

In the case of the adenosine derivatives (4a, 5a) it was also found that the deacetylation at this stage could alternatively be carried out using a Zn/Cu couple in methanol, giving an even higher yield, but this alternative method of deacetylation was found not to work satisfactorily for the inosine derivatives (4b, 5b) and the methanolic ammonia is the preferred deacetylating reagent in this case.

In the the main reaction scheme, as shown, the deacetylated compounds 4 and 5 are then reacted with 0- phenylchlorothionoformate in the presence of an organic solvent such as dimethylaminopyridine to give inseparable mixtures of the phenoxythiocarbonylated isomers (Compounds 6 and 7). For inosine and adenosine, a yield in the region of 78-85% may be expected for this stage. The mixture of the isomers 6 and 7 can then be deoxygenated with tributyltin hydride in the presence of 2,2'-azobis(2- methylpropionitrile), promoting a β-elimination of the halo and phenoxythiocarbonyl groups, to give the corresponding 5'-0-(2-acetoxyisobutyryl)-2',3'-didehydro- 2',3'-dideoxy nucleoside (Compound 8a or 8b) in. virtually quantitative yield. Finally, the 5'-0-(2-acetoxyisobutyryl) group can be removed by further treatment with 8M methanolic ammonia as a deacetylating agent to give the 2',3'-didehydro-2',3'-dideoxy derivative ( Compound 9a or 9b).

If desired, this unsaturated olefinic didehydro compound can then be converted into the corresponding 2',3'-dideoxy derivative by hydrogenation using any convenient conventional hydrogenation process (see, for example, Chu, C.K. et al, J. Org. Chem. (1989), 2217). This provides an especially efficient and economical route for preparing in quantity the particularly useful compound 2',3'-dideoxyinosine.

It will be appreciated that the finding that it is possible in the way described to use a 2-acyloxybutyryl halide such as 2-acetoxyisobutyryl bromide in the first- stage of the process to ensure exclusive formation of the 5'-0-(2-acyloxybutyryl) ester derivatives in high yield plays a crucial role in the introduction of the halo (bromo) group into the sugar moiety to provide a suitable and stable derivative for the subsequent processing, including in the preferred reaction scheme the subsequent introduction of the phenoxythiocarbonyl group. This, and the fact that the basic reactions are carried out under mild conditions and proceed consistently in nearly quantitative yields, including the reaction involving the subsequent free radical β-elimination of halo and phenoxythiocarbonyl leaving groups, is a major factor in determining the efficiency and convenience of the method of at least the preferred embodiment of the present invention for producing the particular nucleoside derivatives concerned.

BEST MODE FOR CARRYING OUT THE INVENTION

By way of further illustration, a specific example of a preferred manner of carrying out the invention will now be more particularly described with reference to the schematic diagram previously referred to where adenosine (la) or inosine (lb) is used as the starting ribonucleoside material for preparing the 2',3'-didehydro- 2',3'-dideoxy derivatives thereof. EXAMPLE 1

STAGE 1 - Reaction of Nucleosides 1a, 1b with 1- Bromocarbonyl-1-methylethyl acetate; Products 2,3 To a suspension of adenosine (1a) or inosine (1b) in dry nitromethane (45 ml) a solution of 1-bromocarbonyl-1- methylethyl acetate (4.98 g, 24 mmol ) in dry nitromethane (15 ml) was added. The resulting pale-yellow solution which became clear after 1-2 hours was stirred at room temperature for 24 hours in the case of inosine or 86 hours in the case of adenosine. The solvent was removed In vacua, the residue partitioned between EtOAc/5% aq. NaHCO₃ (1:1,250 ml) and the aqueous layer was extracted further with EtOAc (3 x 40 ml). The combined EtOAc extracts were dried (Na₂SO₄), concentrated, under reduced pressure, dissolved in a small amount of CHCI₃ and applied to a short column of silica gel. Elution of the column with CHCl₃/EtOH (97:3) afforded the mixtures of isomers 2a/3a, 2b/3b as a colourless froth. Analytical samples were obtained when the crude products (0.25 g,- 0.5 mmol) were dissolved in a small amount of CHCI₃ (1 ml) and added dropwise to a stirred petroleum ether (b.p. 30-40°C) (50 ml). The resulting colourless precipitate was collected by centrifugation and dried in a desiccator (see Table 1). It was noted that if the nucleosides 1a or 1b were reacted with the 1-bromocarbonyl-1-methylethyl acetate in nitromethane under conditions identical to those described above but for a substantially shorter period, e.g. 2½ hours for inosine (1b) or 6-48 hours for adenosine (1a), after the same work-up and chromatographic separation as before the products obtained ( as a colourless froth) had spectroscopic data which agreed well with values characteristic of alkoxydioxolanones, e.g.

STAGE 2 - Deacetylation of the Products 2a/3a and 2b/3b with 8M Methanolic Ammonia The mixtures 2a/3a and 2b/3b in the ratio of 78/22 and 73/27 respectively (2 mmol) were dissolved in 8M methanolic ammonia (17 ml ) and the colourless solution was stirred at room temperature for 90 min. The solvent was removed in vacuo and the residue was dissolved in a small amount of CHCI₃ and applied to a short column of silica gel. The product was eluted with CHCl₃/EtOH in the ratio 97:3 for mixture 4a/5a and 23:2 for the mixture 4b/5b. The fractions containing the product were then combined and concentrated under reduced pressure to give the product as a colourless froth; analytical samples were prepared as described for compounds 2 and 3 (Table 1).

As mentioned, it was found that the adenosine derivative isomers of mixture 2a/3a ( in the ratio of 78/22) could alternatively be efficiently selectively deacetylated using a zinc/copper couple in methanol. Thus, on dissolving this mixture 2a/3a in methanol (20 ml) and adding a freshly prepared Zn/Cu couple (0.96 g) to the solution, stirring the resulting suspension at room temperature for 16 hours, filtering off the catalyst, evaporating the solvent in vacuo, followed by applying the residue to a short column of silica gel and eluting the product with CHCl₃/EtOH at the same polarity as described above for the same products of deacetylation with 8M methanolic ammonia, products 4a and 5a were obtained in high yield as a colourless froth (Table 1). However, this alternative procedure for selective deacetylation was not found to be successful in the case of inosine derivatives 2b/3b.

STAGE 3 - Acylation of the Nucleosides 4a/5a and 4b/5b with O-Phenylchlorothionoformate

To a stirred suspension of the nucleoside derivatives 4a/5a (77/23) or 4b/5b (67/33) and dimethyl- aminopyridine (2 mmol) in anhydrous CH₃CN (11 ml) a solution of O-phenylchlorothionoformate ( 1 ; 5 mmol ) in anhydrous CH₃CN (5 ml) was added in one portion. The resulting pale yellow solution was stirred at room temperature for 5 hours. The solvent was then removed under reduced pressure and the residue partitioned between EtOAc/H₂O (4:1, 100ml). The organic phase was washed with cold M HCl(2x20 ml), water (20 ml), 5% aq. NaHCO₃ (20 ml), water (20 ml) and dried (Na₂SO₄). The solvent was removed under reduced pressure and the residue was applied to a short column of silica gel. The phenoxythiocarbonylated product was eluted with CH₂Cl₂/EtOH in the ratio 24:1 for the mixture of the adenosine isomer derivatives 6a/7a (74/26), and in the ratio 47:3 for the mixture of the inosine isomer derivatives 6b/7b (72/28). The fractions containing the product were combined and evaporated to give the product as a colourless froth ( Table 1); analytical samples were again prepared as described for compounds 2 and 3. STAGE 4 - Reaction of Phenoxy(thiocarbonyl) Nucleosidas 6a/7a and 6b/7b with Tributyltin Hydride to prepare products 8a, 8b To a solution of the nucleoside mixture 6a/7a (74/26) or 6b/7b (72/28) in benzene (40 ml) were added tributyltin hydride (1.1 ml; 4 mmol) and 2,2'-azobis(2- methylpropionitrile). (AIBN, 0.050 g, 0.025 mmol). The stirred reactants were heated under reflux for 20 min. The solvent was removed under reduced pressure and the residue was applied to a short column of silica gel. The product was eluted with CHCl₃/EtOH (ratio 95.5:4.5). The fractions containing the product were combined and concentrated under reduced pressure to give the unsaturated olefinic product (8a or 8b) as a colourless froth; analytical samples were prepared as described for compounds 2 and 3 (see Table 2).

STAGE 5 - Preparation of 2'3'-Didehydro-2',3'- dideoxyadenosine (9a) and 2',3'-Didehydro-2',3'- dideoxyinosine (9b)

Compound 8a or 8b ( 1 mmol ) was dissolved in 8 M methanolic ammonia (10 ml ) and the colourless solution was stirred at room temperature for 48 hours. The solvent was removed in vacuo, the residue was dissolved in MeOH (50 ml), silica gel (0.5g) was added to the solution, and the resulting suspension was evaporated to dryness. The residue was treated with a small amount of CHCl₃/MeOH (ratio 24:1 for 9a, 19:1 for 9b). The resulting slurry was applied to a short column of silica gel (5 g, 20 x 32 mm). Elution of the column with CHCl₃/MeOH (49:6, 17:3) afforded the products 9a, 9b respectively as colourless glasses. Each colourless glass was dissolved in a small amount of water and lyophilised to give the products as colourless powders ( Table 2 ) that were homogenous on HPLC [retention times (sec) as determined using a Trilab 3000 (Trade Mark) multichannel chromatography data system were as follows:

for 9a - 149 (D), 114 (E); for 9b - 123 (D), 110 (E)]. As previously indicated, in that part of the main reaction scheme described in which the 5'-0-(2-acetoxyiso- butyryl) ester derivatives are converted into the corresponding 5'-0-(2-acetoxyisobutyryl)-2',3'-didehydro- 2',3'-dideoxynucleosides, it is also possible for this conversion alternatively to be carried out in a single direct step by reacting the 5'-0-(2-acetoxyisobutyryl) ester derivative(s) with a freshly prepared Zn/Cu couple in an appropriate carefully selected solvent. In the general reaction scheme previously depicted herein, the use of such Zn/Cu couple represents a direct conversion of Compounds 2a, 3a or 2b, 3b to Compound 8a or 8b.

By way of further illustration, a specific example of the use of this Zn/Cu couple direct conversion step applied both to adenosine derivatives 2a, 3a and to inosine derivatives 2b, 3b is set out below.

EXAMPLE 2

Reaction of Nucleosides 2a/3a and 2b/3b with Zn/Cu couple in THF or Pyridine;

General Procedure: To a solution of the nucleoside 2a/3a (78/22) (1 mmol) in anhydrous THF (15 ml) or 2b/3b (73/27) in anhydrous pyridine (15 ml), a freshly prepared Zn/Cu couple (0.96g) was added and the stirred suspension was heated under reflux for 4 hours (2a/3a) or at 100°C for 50 minutes (2b/3b). The catalyst was filtered off, the solvent was removed in vacua and the residue was applied to a short column of silica gel. The product was eluted with CHCl₃/EtOH at the same polarity as described for the same products of deoxygenation with tributytin hydride to give 8a and 8b as a colourless froth. A yield was obtained of 58% for the adenosine derivative 8a and 28% for the inosine derivative 8b

Further detailed analytical data relating to the above Examples of the preparation of the 2',3'-didehydro- 2',3'-dideoxy derivative of adenosine (9a) and inosine (9b) are presented in Tables 1 and 2 appended hereto. In general, in the preparative work short column chromatography was carried out on silica gel 60H (Merck), and solvents were removed in vacuo at 30-40°C unless otherwise indicated.

At least some of the new compounds prepared in carrying out the process described in these Examples, and which constitute useful intermediates, are summarised in the following list.

List of New Compounds:

1. 9-[5-0-(2-acetoxyisobutyrl)-2-0-acetyl-3-bromo-3- deoxy-β-D-xylofuranosyl]-adenine (2a) and 9-[5-0-(2- acetoxyisobutyrl)-3-0-acetyl-2-bromo-2-deoxy-β-D- arabinofuranosyljadenine (3a) - obtained as an inseparable mixture of both isomers.

C₁₈H₂₂BrN₅O₇-0.2H₂O Calc. C 42.90 H 4.48 N 13.90 (503.91) Found C 43.21 H 4.56 N 13.39

2. 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3-deoxy-β-D- xylofuranosyl]adenine (4a). C₁₆H₂₀BrN₅O₆ Calc. C 41.93 H 4.40 N 15.28

(458.27) Found C 41.89 H 4.34 N 15.05

3. 9-[5-0-(2-acetoxyisobutyryl)-2-bromo-2-deoxy-β-D- arabinofuranosyl]adenine (5a).

C₁₆H₂₀BrN₅O₆ Calc. C 41.93 H 4.40 N 15.28

(458.27) Found C 41.69 H 4.39 N 14.95 4. 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3-deoxy-2-0- phenoxy(thiocarbonyl)-β-D-xylofuranosyl] adenine (6a) and 9-[5-0-(2-acetoxyisobutyryl)-2-bromo-2-deoxy-3- 0-phenoxy(thiocarbonyl)-β-D-arabinofuranosyl] adenine (7a) - obtained as an inseparable mixture of both isomers.

C₂₃H₂₄BrN₅O₇S Calc. C 46.47 H 4.07 N 11.78

(594.44) Found C 46.32 H 4.07 N 11.69

5. 9- [ 5-0- ( 2-acetoxyisobutyryl ) -2 , 3-dideoxy- β-D- glyceropent-2-enofuranosyl] adenine ( 8a) .

C₁₆H₁₉N₅O-0.45H₂O Calc. C 52.01 H 5.43 N 18.96

(369.46) Found C 52.39 H 5.27 N 18.54

6. 9-[5-0-(2-acetoxyisobutyryl)-2-0-acetyl-3-bromo-3- deoxy-β-D-xylofuranosyl]-hypoxanthine (2b) and 9-[5- 0-(2-acetoxyisobutyryl)-3-0-acetyl-2-bromo-2-deoxy- β-D-arabinofuranosyl]hypoxanthine (3b) - obtained as an inseparable mixture of both isomers.

C₁₈H₂₁BrN₄O₈-0.5H₂O Calc. C 42.37 H 4.35 N 10.98 (510.30) Found C 42.67 H 4.31 N 10.46

7. 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3-deoxy-β-D- xylofuranosyl]hypoxanthine (4b) and 9-[5-0-(2- acetoxyisobutyryl)-2-bromo-2-deoxy-β-D-arabino- furanosyl]hypoxanthine (5b) - obtained as an inseparable mixture of both isomers.

C₁₆H₁₉BrN₄O₇ Calc. C.41.85 H 4.17 N 12.20

(459.25) Found C 41.84 H 4.02 N 12.07 8. 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3-deoxy-β-D- xylofuranosyl]hypoxanthine (4b).

C₁₈H₁₉BrN₄O₇-0.4H₂O Calc. C 41.20 H 4.28 N 12.01 (466.46) Found C 40.91 H 4.08 N 12.63

9. 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3-deoxy-2-0- phenoxy(thiocarbonyl)-β-D-xylofuranosyl]hypoxanthine (6b) and 9-[5-0-(2-acetoxyisobutyryl)-2-bromo-2- deoxy-3-0-phenoxy(thiocarbonyl)-β-D-arabino- furanosyl]hypoxanthine (7b) - obtained as an inseparable mixture of both isomers. C₂₃H₂₃BrN₄O₂S Calc. C 46.40 H 3.89 N 9.41 (595.42) Found C 46.02 H 3.91 N 9.31

10. 9-[5-0-(2-acetoxyisobutyryl)-2,3-dideoxy-β-D- glyceropent-2-enofuranosyl]hypoxanthine (8b).

C₁₆H₁₈N₄O₆-0.4H₂O Calc. C 52.00 H 5.13 N 15.16 (369.55) Found C 52.33 H 5.06 N 14.81

aSatisfactory microanalyses obtained: C ± 0.4, H ± 0.2, N ± 0.5; exception 5b N ± 0.6, 6b C ± 0.6. bThe products were obtained as amorphous powders having indefinite melting points.

cDeprotection with 8M NH₃/MeOH.

dDeprotection with Zn/Cu couple in MeOH.

to o

aSatisfaciory microanalysis obtained: C ± 0.4, H ± 0.2, N ± 0.4 for 8a, 8b.

b8 a, 8b, were obtained as amorphous powders having indefinite melting points; 9a had m p. 190-191ºC (MeO H), 9b > 300ºC (MeO H)

cObtained from 2 and 3 via elimination with Zn/Cu couple in THF or pyridine.

Claims

1. A process for preparing a 2',3'-dideoxy nucleoside derivative from a ribonucleoside that reacts with 2- acyloxyisobutyryl halides to form at least initially an acylated 2' or 3' halo 5'-0-dioxolano derivative, said process being characterised in that it includes the steps of:

(a) reacting said ribonucleoside with a 2-acyloxyisobutyryl halide in an organic solvent for an extended time sufficient to cause substantially all the 5'-0-dioxolano derivative initially formed to be converted to the corresponding acyloxyisobutyryl ester 2'('3')-acyloxy-

3'(2')-halo derivative(s),

(b) recovering said acyloxyisobutyryl ester product of step (a),

(c) subjecting said acyloxyisobutyryl ester recovered in step (b) to selective deacylation at the 3' and/or 2' positions and converting into the corresponding 5'-0-(2- acyloxyisobutyryl)-2',3'-didehydro-2',3'-dideoxy- nucleoside, followed by then

2. A process as claimed in Claim 1 wherein the initial ribonucleoside is selected from inosine and adenosine.

3. A process for preparing a 2',3'-dideoxy nucleoside derivative from a ribonucleoside selected from inosine and adenosine, said process being characterised in that it includes the steps of:

(a) reacting the said ribonucleoside with a 2-acyloxyisobutyryl halide in an organic solvent to form a 5'-0- dioxolano derivative or derivatives and continuing said reaction until substantially all said 5'-0-dioxolano derivative( s) becomes converted to the corresponding acyloxyisobutyryl ester 2'(3')-acyloxy-3'(2')-halo derivative(s),

(b) recovering said acyloxyisobutyryl ester product of step (a),

4. A process as claimed in Claim 2 or 3 wherein the initial ribonucleoside is inosine and wherein the acyloxyisobutyryl ester product recovered in step (b) is exclusively a mixture of the 2'-acyl-3'-halo and 3'-acyl- 2'-halo isomers and is obtained in more than 90% yield.

5. A process as claimed in any one of Claims 2, 3 or 4 wherein the initial ribonucleoside is inosine and in step

(c) the selective removal of the 2'(3')-0-acyl groups is carried out using 8M methanolic ammonia as the deacylating agent.

6. A process as claimed in Claim 2 or 3 wherein the initial ribonucleoside is adenine and wherein the acyloxyisobutyryl ester product recovered in step (b ) is exclusively a mixture of the 2'-acyl-3'-halo and 3'-acyl- 2'-halo isomers and is obtained in more than 85% yield.

7. A process as claimed in any one of Claims 2, 3 or 6 wherein the initial ribonucleoside is adenine and in step (c) the selective removal of the 2'(3')-0-acyl groups is carried out using a Zn/Cu couple in methanol as the deacylating agent.

8. A process as claimed in any of the preceding claims wherein after said selective deacylation in step (c) the production of said 5'-0-(2-acyloxyisobutyryl)-2',3'- didehydro-2',3'-dideoxynucleoside is carried out in two stages comprising (i) treating the acyloxyisobutyryl ester with 0-phenylchlorothionoformate in the presence of an organic solvent so as to phenoxythiocarbonylate the 2' or 3' hydroxyl groups thereof, and then (ii) treating the products formed so as to deoxygenate and promote a β- elimination of the halo and phenoxythiocarbonyl groups.

9. A process as claimed in any one of Claims 2, 3 or 4 wherein the initial ribonucleoside is inosine and step (c) is carried out by reacting the acyloxyisobutyryl ester with a Zn/Cu couple in pyridine.

10. A process as claimed in any one of Claims 2, 3 or 6 wherein the initial ribonucleoside is adenine and step (c) is carried out by reacting the acyloxyisobutyryl ester with a Zn/Cu couple in anhydrous tetrahydrofuran.

11. A process as claimed in any of the preceding claims wherein the reaction of the ribonucleoside with the acyloxyisobutyryl halide in step (a) is carried out without application of heat in nitromethane as the organic solvent.

12. A process as claimed in any of the preceding claims wherein the acyloxyisobutyryl halide is 2'-acetoxyisobutyryl bromide.

13. A process as claimed in any of the preceding claims wherein the extended time for which the initial reaction is allowed to continue is in excess of 12 hours.

14. A process as claimed in Claim 13 wherein the extended time for which the initial reaction is allowed to continue is at least 20 hours.

15. A process as claimed in Claim 13 wherein the extended time for. which the initial reaction is allowed to continue is within the range of 24-86 hours.

16. A process as claimed in any of the preceding claims wherein the 2',3'-didehydro-2',3'-dideoxy nucleoside initially obtained in step (d) is hydrogenated to convert it into the corresponding 2',3'-dideoxy nucleoside.

17. A mixture of 9-[5-0-(2-acetoxyisobutyrl)-2-0-acetyl- 3-bromo-3-deoxy-β-D-xylofuranosyl]-adenine and 9-[5-0-(2- acetoxyisobutyrl)-3-0-acetyl-2-bromo-2-deoxy-β-D-arabino- furanosyl]adenine.

18. 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3-deoxy-β-D- xylofuranosyl]adenine.

19. 9-[5-0-(2-acetoxyisobutyryl)-2-bromo-2-deoxy-β-D- arabinofuranosyl]adenine.

20. A mixture of 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3- deoxy-2-0-phenoxy(thiocarbonyl)-β-D-xylofuranosyl]adenine and 9-[5-0-(2-acetoxyisobutyryl)-2-bromo-2-deoxy-3-0- phenoxy(thiocarbonyl)-β-D-arabinofuranosyl]adenine.

21. 9-[5-0-(2-acetoxyisobutyryl)-2,3-dideoxy-β-D- glyceropent-2-enofuranosyl]adenine.

22. A mixture of 9-[5-0-(2-acetoxyisobutyryl)-2-0- acetyl-3-bromo-3-deoxy-β-D-xylofuranosyl]-hypoxanthine and 9-[5-0-(2-acetoxyisobutyryl)-3-0-acetyl-2-bromo-2-deoxy-β- D-arabinofuranosyl]hypoxanthine.

23. A mixture of 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3- deoxy-β-D-xylofuranosyl]hypoxanthine and 9-[5-0-(2- acetoxyisobutyryl)-2-bromo-2-deoxy-β-D-arabinofuranosyl]- hypoxanthine.

24. 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3-deoxy-β-D- xylofuranosyl]hypoxanthine.

25. A mixture of 9-[5-0-(2-acetoxyisobutyryl)-3-bromo-3- deoxy-2-0-phenoxy(thiocarbonyl)-β-D-xylofuranosyl]- hypoxanthine and 9-[5-0-(2-acetpxyisobutyryl)-2-bromo-2- deoxy-3-0-phenoxy(thiocarbonyl)-β-D-arabinofuranosyl]hypoxanthine.

26. 9-[5-0-(2-acetoxyisobutyryl)-2,3-dideoxy-β-D- glyceropent-2-enofuranosyl]hypoxanthine.