WO1999052897A1

WO1999052897A1 - Novel reagents for use as coupling activators and coupling reagents in organic synthesis

Info

Publication number: WO1999052897A1
Application number: PCT/US1999/008022
Authority: WO
Inventors: Chandra Vargeese; Venkata Palle
Original assignee: Nexstar Pharmaceuticals, Inc.
Priority date: 1998-04-14
Filing date: 1999-04-13
Publication date: 1999-10-21
Also published as: AU3556299A

Abstract

Novel reagents useful as coupling activators and coupling agents are described. The novel reagents of the invention include 1,1'-carbonyldiimidazole derivatives comprising substituted imidazole moieties, wherein the substituted imidazole moieties have pKa's between 5.0 and 6.8, and wherein the substituted imidazole moieties are more facile leaving groups than unsubstituted imidazole. A preferred reagent, 1,1'-carbonylbis-(4,5-dicyanoimidazole) (CBDCI) was found to be an effecting coupling activator and coupling agent. Carboxylic acid couplings utilizing CBDCI were found to give esters and amides (including peptides) in good yield and short reaction times and without racemization.

Description

NOVEL REAGENTS FOR USE AS COUPLING ACTIVATORS AND COUPLING REAGENTS IN ORGANIC SYNTHESIS

FIELD OF THE INVENTION

This invention describes versatile, easily synthesized reagents that can be used as coupling activators and coupling reagents in a wide variety of organic synthetic reactions.

BACKGROUND

Among the numerous methods available in organic synthesis for the derivatization of carboxylic acids, the use of coupling activators is by far the most efficient and convenient method. In particular, the subject of carboxyl activation of N-blocked amino acids, though explored in great detail, continues to attract new interest. N, N'-

Carbonyldiimidazole (CDI) (Staab and Mannschreck (1962) Chem. Ber. 95:1248; Ohta et al. (1982) Synthesis 833), N, N'-dicyclohexylcarbodiimide (DCC) / 4- dimethylaminopyridine (DMAP) (Neises and Steglich (1978) Angew. Chem. Int. Ed. Engl. l_7"-522; Hassner and Alexanian (1978) Tetrahedron Lett. 4475) and 2- halopyridinium salts (Saigo et al. (1977) Bull. Chem. Soc. Japan 50:1863) have been widely used as coupling activators for a number of coupling reactions. These conventional coupling activators, however, are not free of limitations. The coupling activator that continues to attract the most interest is carbonyldiimidazole (CDI), due to the mild conditions under which it can be used. In the proposed mechanism for the derivatization of carboxylic acids using CDI as a coupling activator (Scheme 1), the first step is the formation of the mixed anhydride 1 and one equivalent of imidazole, followed by rapid evolution of carbon dioxide to give the stable acyl imidazolide 2. The acyl imidazolide 2 is then reacted with a nucleophile (Nu), such as an amine or alcohol, which displaces another equivalent of imidazole to yield the acylated product 3. SCHEME 1

O O

RCOOH ₊ N^NH fast ^r O (Step l) R

O

N^NH R Λ slow

Nu (Step CO, Nu

The use of CDI as a coupling activator has several drawbacks. One limitation is that imidazole is a relatively poor leaving group, and therefore the nucleophilic displacement of imidazole from the intermediate acyl imidazolide 2 is the rate-limiting step in this reaction. Esterification of carboxylic acids, in particular, suffer from poor yields and long reaction times, since the intermediate acyl imidazolides are sluggish toward nucleophilic attack by alcohols, which are relatively poor nucleophiles (Staab (1962) Angew. Chem., Int. Ed. Engl. 1:351). Several methods have been reported in an effort to improve the reaction of alcohols with the intermediate acyl imidazole 2 in the esterification of carboxylic acids. Kamijo et al. (Chem. Pharm. Bull. (1984) 32:5044) describe a one-pot synthesis of esters from carboxylic acids using CDI in the presence of 2 to 5 equivalents of a reactive halide. This method, however, suffers from potential side reactions of the nucleophile with the reactive halide and racemization of chiral compounds. Ohta et al. (Synthesis (1982) 833) have attempted to accelerate the esterification of carboxylic acids using CDI in the presence of DBU (1,8- diazabicyclo[5.4.0]-7-undecene). These reactions still proceed slowly (5-24 hours) to give products in moderate yields. In addition, the presence of the basic DBU increases the potential of racemization. 3

Another limitation in the use of CDI as a carboxylic acid activator is that substantial racemization in the acyl component is observed (Weygand et al. (1966) Chem. Ber. 99:1451; Paul and Anderson (1960) J. Am. Chem. Soc. 82:4596). This undesirable racemization is attributed to the basic nature of the imidazole by-product. The prevention of racemization during the coupling of carboxylic acids and amines is one of the most serious problems to be solved in peptide synthesis (Bodanszky (1984) in Principles of Peptide Synthesis, Springer- Verlag, Berlin, p. 158; Benoiton (1983) in The Peptides, eds. Gross and Meienhofer, Academic Press, New York, vol. 5, ch. 4). In an effort to minimize racemization, the introduction of racemization-suppressing additives, such as 1- hydroxybenzotriazole (HOBT) (Sieber et al. (1977) Helv. Chim. Acta 60:27) or the simultaneous use of HOBT and copper(II) chloride (Miyazawa et al. (1988) J. Chem. Soc, Chem. Commun. 419) in the CDI activation method have been suggested. Unfortunately, even with these additives, couplings are not always free from racemization. Furthermore, these reaction conditions involve the use of insoluble catalysts, and therefore are not conducive to solid-phase peptide synthesis.

The risk of racemization would be minimized if the coupling reaction could be conducted in the total absence of base. Saha et al. (J. Am. Chem. Soc. (1989) 111 :4856) have reported the preparation of l, -carbonylbis(3-methylimidazolium) triflate (CBMIT) and its use as an alternative to CDI in the preparation of esters and peptides. Unfortunately, it was reported that this salt is only soluble in nitromethane, a seldom used and undesirable solvent which can be explosive. Furthermore, the CBMIT reaction medium is slightly acidic due to the presence of trace amounts of triflic acid in stored batches of the reagent solution. Therefore, this reagent is incompatible with acid-labile protecting groups, such as the tβrt-butoxycarbonyl (t-BOC) group, which is commonly used in peptide syntheses.

It would be advantageous to develop a coupling reagent that offers improved coupling rates and yields and also eliminates undesired racemization. 4

SUMMARY OF THE INVENTION

The present invention is directed to novel reagents that are useful as coupling activators for the synthesis of carboxylic acid derivatives and as coupling reagents in organic synthesis. The novel reagents of the present invention are derivatives of 1,1'- carbonyldiimidazole (CDI) comprising substituted imidazole moieties. The substituted imidazole moieties of the novel reagents described herein provide characteristics which offer improvements over the use CDI in the syntheses of carboxylic acid derivatives and in organic coupling reactions. First, the substituents on the imidazoles make the substituted imidazole moieties of the novel reagents better leaving groups than the unsubstituted imidazole moiety of CDI. Nucleophilic displacement of the intermediate acyl imidazolide 2 in Scheme 1 proceeds much faster when the novel reagents of the present invention are used as coupling activators. Therefore, the novel reagents described herein increase the overall reaction rate of carboxylic acid derivatization by increasing the rate of nucleophilic displacement (Scheme 1, Step 2) in the coupling reaction. Second, the substituted imidazole by-products formed during coupling reactions using the improved reagents of the present invention are non-basic, having pKa's between 5.0 and 6.8. As such, the novel reagents will not cause racemization of the product of the coupling reaction, and further are compatible with acid-labile protecting groups. Therefore, using the novel reagents of the present invention in organic coupling reactions results in increased coupling rates, higher product yields and maintenance of the chiral integrity of the product.

The novel reagents of the invention include 1,1 '-carbonyldiimidazole derivatives comprising substituted imidazole moieties, wherein the substituted imidazole moieties have pKa's between 5.0 and 6.8, and wherein the substituted imidazole moieties are more facile leaving groups than unsubstituted imidazole. The most preferred coupling activator and coupling reagent is l,l'-carbonylbis-(4,5-dicyanoimidazole) (CBDCI) (4).

O

The present invention further includes an improved method of synthesizing carboxylic acid derivatives using the novel reagents of the invention as coupling activators.

The present invention further includes the use of the novel reagents of the invention as coupling reagents.

DETAILED DESCRIPTION OF THE INVENTION

The present invention encompasses novel reagents that are useful as coupling activators for the synthesis of carboxylic acid derivatives and as coupling reagents in organic synthesis. The novel reagents of the present invention are derivatives of 1,1'- carbonyldiimidazole (CDI) comprising substituted imidazole moieties. The novel reagents of the invention include 1,1 '-carbonyldiimidazole derivatives comprising substituted imidazole moieties, wherein the substituted imidazole moieties have pKa' s between 5.0 and 6.8, and wherein the substituted imidazole moieties are better leaving groups than unsubstituted imidazole. The most preferred coupling activator and coupling reagent is l,r-carbonylbis-(4,5-dicyanoimidazole) (CBDCI) (4).

The preparation of CBDCI (4) following the literature procedure for the preparation of CDI (Staab and Wendel Organic Synthesis 48:44) was unsuccessful. The treatment of phosgene with four equivalents of 4,5-dicyanoimidazole in tetrahydrofuran did not yield any of the desired product. This is likely due to the fact that DCI, having a lower pKa (5.2) than imidazole (pKa = 6.9), is not basic enough to react with phosgene. Hence, CBDCI was synthesized in the presence of pyridine. It was found that the novel compound CBDCI can be easily prepared by reacting two equivalents of DCI with one 6 equivalent of phosgene in the presence of two equivalents of pyridine in tetrahydrofuran, to afford the desired product in quantitative yield (Example 1).

CBDCI can be isolated as a solid, however, for synthetic purposes CBDCI is most conveniently generated in solution just prior to use. In addition, CBDCI is soluble in most organic solvents routinely used in organic syntheses, making it a very versatile reagent.

The present invention further includes an improved method of synthesizing carboxylic acid derivatives using the novel reagents of the invention as coupling activators. In a preferred embodiment, the synthesis of carboxylic acid derivatives is performed by activating a carboxylic acid with CBDCI, as illustrated in Scheme 2. In general, a carboxylic acid reacts with CBDCI (4), leading to rapid evolution of CO₂ to give the activated form of the carboxylic acid, the acyl 4,5-dicyanoimidazolide intermediate 7, and one equivalent of 4,5-dicyanoimidazole (DCI) (6). The acyl 4,5-dicyanoimidazide intermediate 7 is then reacted with a nucleophile resulting in formation of a carboxylic acid derivative 3, along with another equivalent of DCI.

SCHEME 2

RCOOH O^' N ^N (Step 1) 0=

NC NC CN

NC CN H CN

O

N ^\

R Nu CO,

CN 3

7

The term "coupling activator" as used herein means a reagent which will react with a carboxylic acid to make the carbonyl group more reactive towards nucleophilic attack. The most preferred coupling activator is l,l'-carbonylbis-(4,5-dicyanoimidazole) (CBDCI) (4).

O

The term "nucleophile" as used herein is defined as an electron-rich compound that reacts with the carbonyl carbon of an acyl imidazolide. Suitable nucleophiles for use in the present invention include, but are not limited to, alcohols, amines, thiols, carboxylic acids and other nucleophiles known by those of skill in the art to undergo nucleophilic attack at a carbonyl carbon.

The term "peptide" as used herein refers to a polymer of amino acids chemically bound by amide linkages (CONH). An "amino acid" is defined as an organic molecule containing both an amino group (NH₂) and a carboxylic acid (COOH). Specifically, an "amino acid" is any compound of the general formula RCH(NH₂)COOH (α-amino acid), wherein R is selected from the group consisting of H or any suitably protected known amino acid side chain. Suitable protection for amino acid side chains is know to those skilled in the art. As used herein the term "peptide" includes peptides, polypeptides and proteins. The peptides synthesized by the method of this invention are depicted generally as follows:

wherein n = 1 -500 and R is as defined above. 8

Suitable N-protecting groups may be selected from the many reported N-protecting groups which are known to those in the art, including but not limited to urethanes, such as FMOC and t-BOC, benzyl groups, acyl groups or triphenylmethyl groups. The N- protecting group may also be a solid support. Suitable COOH-protecting groups may be selected from the many reported

COOH-protecting groups which are know to those in the art, including but not limited to ester protecting groups. The COOH-protecting group may also be a solid support.

The term "solid support" as used herein is defined as a solid support which can be covalently coupled to an amino acid or peptide, in such a fashion that the amino acid or peptide can be further modified using the chemistry disclosed herein, and subsequently removed using conventional chemistry. Suitable solid supports for use in the present invention include, but are not limited to, controlled pore glass (CPG) of various pore size and loading, polystyrene beads and polystyrene-polyethylene glycol copolymer support (TentaGel). The term "coupling reagent" as used herein means a reagent that will couple two compounds having the structures R^H and R₂Y₂H to form a compound having the structure R,Y_rC(O)-Y₂R₂, wherein Y, and Y₂ are independently selected from the group consisting of O, NH, S or C(O)O-; and R, and R₂ are independently selected from the group consisting of alkyl, alkenyl, alkynyl or aryl. The term "coupling reagent" further includes a reagent that will form a cyclic compound having the structure 9

(CH₂)₀,ι

/ \

R^R^C CR R

O 9 from a compound having the structure HY,R₁R₁'C-(CH₂)₀ ,-CR₂R₂'Y₂H, wherein Y, and Y₂ are independently selected from the group consisting of O, NH, S or C(O)O-; and R„ R , 9

R₂. and R₂' are independently selected from the group consisting of H, alkyl. alkynyl or aryl, or together R, and R₂ are part of a carbocyclic or heterocyclic ring.

The novel reagents of the present invention offer several advantages over the use of CDI as a coupling activator. First, the by-products of coupling reactions using the novel reagents described herein are weakly acidic, substituted imidazoles, having pKa's between 5.0 and 6.8. For example, the by-product of a coupling reaction in which a carboxylic acid is activated with CBDCI is 4,5-dicyanoimidazole (DCI), which has a pKa of 5.2. This DCI by-product is an innocuous by-product in the synthesis of carboxylic acid derivatives via CBDCI activation for several reasons. First, since DCI is non-basic, no loss of optical activity is observed with chiral compounds. This is particularly relevant in the synthesis of peptides. Second, since DCI is only weakly acidic, the presence of this by-product in the reaction mixture will be compatible with most amine and alcohol protecting groups, including N-protecting groups such as t-BOC (t-butoxycarbonyl) and O-protecting groups such as DMT (4,4'-dimethoxytrityl). The DMT protecting group was observed to be stable to DCI in copending United States Patent Application Serial No. 08/937,867, filed September 25, 1997, entitled "Improved Coupling Activators for Oligonucleotide Synthesis," the contents of which are incorporated herein by reference.

Another advantage in using the novel reagents of the present invention rather than CDI as a coupling activator is that the substituted imidazole moieties of these novel reagents are more facile leaving groups than the unsubstituted imidazole moieties of CDI. Again using the novel reagent CBDCI as an example, the presence of the two cyano groups on the imidazole moieties make the 4,5-dicyanoimidazole (DCI) moiety of this reagent a better leaving group than imidazole. As a result, nucleophilic displacement of the intermediate 4,5-dicyanoimidazolide 7 (Scheme 2) proceeds much faster than nucleophilic displacement of the unsubstituted acyl imidazolide 2 (Scheme 1), resulting in shorter reaction times and higher product yields.

Esters and amides (including peptides) were synthesized via CBDCI activation to illustrate the usefulness of the novel reagents of the present invention. In each example, the yields and reaction rates were found to be superior to those obtained using CDI. The results of the coupling reactions are summarized in Table 1. 10

Entries 1 and 2 of Table 1 summarize the reaction times and yields of an amide synthesized from benzoic acid and n-butyl amine using CBDCI and CDI, respectively. The reaction activated by CBDCI (entry 1) is clearly superior to the reaction activated by CDI (entry 2). The reaction activated by CBDCI afforded the amide product in quantitative yield in only 2 hours, whereas the reaction activated by CDI was only 50% complete after 2 hours.

The results shown in entries 3 and 4 of Table 1 support the hypothesis that the derivatized imidazole moiety of CBDCI is a better leaving group and therefore aids in accelerating the coupling reaction between a carboxylic acid and a weak nucleophile, such as an alcohol. The coupling of benzoic acid with ethanol via CBDCI activation (entry 3) was complete in 3 hours, affording an 80% isolated yield of the ester product. This result is striking when compared to the coupling reaction activated by CDI (entry 4) in which the reaction was not complete even after 24 hours.

A coupling reaction to be particularly noted is entry 5 of Table 1, in which L-CBZ- phenylalanine (CBZ - carbobenzoxy) was esterified with the sterically hindered long chain alcohol octadecanol. This reaction is significant in that amino acid esters of long-chain C- 18 alcohols are of pharmacological value (Penney et al. (1985) J. Org. Chem. 50:1457V For example, N-stearoyl amino acids possess nutritional value and stearyl amino acids are effective immunoadjuvants for vaccine production and allergy desensitization therapy. To date, the synthesis of these compounds has been problematic. The present invention demonstrates that the octadecyl ester of CBZ-phenylalanine can be easily prepared in quantitative yield in two hours by activating CBZ-phenylalanine with CBDCI (entry 5). This coupling reaction is in sharp contrast to the analogous coupling reaction utilizing CDI as the coupling agent (entry 6) in which the product was obtained in only 30% yield after 2 hours.

The novel reagents of the present invention are also useful as coupling activators in peptide synthesis. In one embodiment, the novel reagents described herein are utilized in the solid phase synthesis of peptides. In general, solid phase peptide synthesis proceeds from the C-terminal to the N-terminal amino acid. The carboxyl-terminal amino acid of the peptide to be synthesized is protected and covalently attached to a solid support. The subsequent amino acid monomers (which have been N-protected and side-chain protected) are then sequentially added in the form of a activated acyl imidazolides, such as acyl 4,5- dicyanoimidazolides formed by activation of protected amino acids with the novel reagent CBDCI. In another embodiment, the solid phase peptide synthesis proceeds from the N- terminal to the C-terminal amino acid. In this embodiment, the amino-terminal amino acid of the peptide is covalently attached to a solid support. The free carboxyl group is activated with a novel reagent of the present invention, and the subsequent amino acid monomer (which has been COOH-protected and side-chain protected) is then added. The COOH-protecting group is then removed, the free COOH is activated with a novel reagent of the present invention, and the procedure is repeated until the desired peptide is obtained. In another embodiment, the novel reagents described herein are utilized in solution phase synthesis of peptides. One method of solution phase peptide synthesis is described in United States Patent Application Serial No. 08/780,517, filed January 8, 1997, entitled "Method for Solution Phase Synthesis of Oligonucleotides and Peptides," now issued as United States Patent No. 5,874,532, the contents of which are incorporated herein by reference. This method is characterized by the utilization of an anchor group attached to the N-terminal amino acid end of the growing peptide product. The method comprises the reaction of an amino acid, protected with a suitable N-protecting group, with the N- terminal end of a growing peptide chain (whose C-terminal end is suitably protected with a standard COOH protecting group) in solution in the presence of a carboxylic acid activator.

Entry 7 in Table 1 demonstrates the utility of CBDCI in solution phase peptide synthesis. In this example, the esterification of the racemization sensitive L-CBZ- phenylalanine with valine methyl ester using CBDCI as the activator gave the peptide product in 90% yield in only 30 minutes. Analysis by ¹³CNMR indicated that the product was a single diastereomer. The analogous reaction employing CDI as the carboxylic acid activator afforded only 70% of the peptide product. 12

Table 1. Results of coupling reactions using CBDCI and CDI as couplin g activators

Entry Acyl Nucleophile Coupling Reaction Isolated Donor Activator Time Yield (%)

1 Benzoic acid n-butylamine CBDCI 2 hr 100

2 Benzoic acid n-butylamine CDI 2 hr 50

3 Benzoic acid Ethanol CBDCI 3 hr 80

4 Benzoic acid Ethanol CDI 24 hr rxn not complete

5 L-CBZ-Phe Octadecyl alcohol CBDCI 2 hr 100

6 L-CBZ-Phe Octadecyl alcohol CDI 2 hr 30

7 L-CBZ-Phe L-ValOCH₃ CBDCI 30 min 90

8 L-CBZ-Phe L-ValOCH₃ CDI 30 min 70

The novel reagents of the present invention may also be utilized as coupling reagents. Thus, the novel reagents of the instant invention may be used for the synthesis of ureas, carbonates, anhydrides, carbamates, thiocarbamates and thicarbonates. The synthesis of these compound is achieved by reacting a compound having the structure

R,Y,H with, for example, a novel reagent of the invention such as CBDCI to form a acyl

4,5-dicyanoimidazolide intermediate having the structure 8

O

RiY N^

CN 8 followed by reacting the acyl 4,5-dicyanoimidazolide intermediate 8 with a compound having the structure R₂Y₂H to form the compound having the structure R,Y₁-C(O)-Y₂R₂ , wherein Y, and Y₂ are independently selected from the group consisting of O, NH, S or C(O)O-; and R, and R₂ are independently selected from the group consisting of alkyl, alkenyl, alkynyl or aryl. 13

The following Examples are provided to explain and illustrate the present invention and are not intended to be limiting of the invention.

EXAMPLES Example 1. Preparation of l,l'-carbonylbis-(4,5-dicyanoimidazole) (CBDCI)

To a three necked, 500 mL round-bottomed flask containing imidazole (11.8 g, 100 mmol) and anhydrous pyridine (7.9 mL, 100 mmol) in anhydrous THF (100 mL), was added a 20% solution of phosgene in toluene (24 mL, 50 mmol) over 1 hour. The reaction mixture was stirred for an additional 15 minutes and the precipitated pyridinium salts were allowed to settle for 1 hour. The salts were filtered off and the solution (0.4 molar stock solution) was used as such in further reactions.

Example 2. Synthesis of n-butyl benzamide via CBDCI activation (Table 1, entry 1) A solution of CBDCI (10 mL) was cannulated into a solution of benzoic acid (488 mg, 4 mmol) in anhydrous THF and the mixture was stirred for 30 minutes. To this solution was added n-butylamine (0.5 mL) and mixture was stirred for 2 hours. TLC (10% ethyl acetate in hexanes) showed that the reaction was complete. The reaction mixture was then concentrated and applied to a silica gel column. Elution with 10% ethyl acetate in hexanes gave pure product in quantitative yield. Η NMR (300 MHz, CD₃CN): δ 0.93 (t, J=7 Hz, 3H), 1.36-1.41 (m, 2H), 1.53-1.58 (m, 2H), 3.33 (dt, J,=6, J₂=l Hz, 2H), 7.01 (bs, 1H), 7.41-7.5 (m, 3H), 7.75-7.79 (m, 2H).

Example 3. Synthesis of n-butyl benzamide via CDI activation (Table 1 , entry 2)

The reaction as described in Example 2 was repeated with CDI (1 eq.) in THF (10 mL). After 2 hours only 50% of the product was isolated by column chromatography (10% ethyl acetate in hexanes).

Example 4. Preparation of ethyl benzoate via CBDCI activation (Table 1 , entry 3) A solution of CBDCI (17.5 mL) was cannulated into a solution of benzoic acid (854 mg, 7 mmol) in anhydrous THF and the mixture was stirred. After 30 minutes 14 ethanol (0.4 mL) was added and mixture was stirred for 3 hours. TLC showed the completion of reaction. (10% ethyl acetate in hexanes). The reaction mixture was then concentrated and applied to a silica gel column. Elution with 10% ethyl acetate in hexanes gave pure ethyl benzoate in 80% yield. Η NMR (300 MHz, CD₃CN): δ 1.36 (t, J=6 Hz, 3H), 4.3 (q, J=6 Hz, 2H), 7.46-7.51 (m, 2H), 7.59-7.63 (m, 1H), 7.99-8.01 (m, 2H).

Example 5. Preparation of ethyl benzoate via CDI activation (Table 1 , entry 4)

The reaction as described in Example 4 was repeated using CDI (1 eq.) as the coupling activator in THF (10 mL). TLC showed that the reaction was not complete even after 24 hours.

Example 6. Preparation of CBZ-phenylalanyl octadecanoate via CBDCI activation

(Table 1 , entry 5)

A solution of CBDCI (2.6 mL) was cannulated into a solution of L-CBZ- phenylalanine (598 mg, 2 mmol) in anhydrous THF and the mixture was stirred for 30 minutes. After 30 minutes, octadecyl alcohol (260 mg, lmmol) was added and the mixture was stirred for 2 hours. The reaction was determined to be complete by TLC (10% ethyl acetate in hexanes). The reaction mixture was concentrated and applied to a silica gel column. Elution with 10% ethyl acetate in hexanes gave pure CBZ-phenylalanyl octadecanoate (m.p. 71 °C) in quantitative yield. Η NMR (300 MHz, CDC1₃): δ 0.88 (t, J=7.0 Hz, 3H), 1.26 (m, 30H), 1.56 (m, 2H), 3.10-3.13 (m, 2H), 4.06-4.12 (m, 2H), 4.62- 4.67 (m, 1H), 5.1 (m, 2H), 5.26 (d, 1H, D₂O exchangeable), 7.09-7.12 (m, 2H), 7.25-7.28 (m, 3H), 7.35-7.36 (m, 5H).

Example 7. Preparation of CBZ-phenylalanyl octadecanoate via CDI activation

(Table 1 , entry 6)

The reaction as described in Example 6 was repeated using CDI (1 eq.) as the coupling activator in THF (10 mL). After 2 hours the reaction was quenched, worked up and purified by column chromatography (10% ethyl acetate in hexanes) to yield 30% of the product. Starting material (60%) was also recovered from the column. Example 8. Synthesis of CBZ-phenylalanyl valine methyl ester via CBDCI activation

(Table 1 , entry 7)

A solution of CBDCI (5 mL) was cannulated into a solution of CBZ-phenylalanine (600 mg, 2 mmol) in anhydrous THF and the mixture was stirred for 30 minutes. To this was added L-valine methyl ester (162 mg, liberated from its hydrochloride with N-methyl morpholine) and the mixture was stirred for 2 hours. The reaction was determined to be complete by TLC (20% ethyl acetate in hexanes). The reaction mixture was concentrated and applied to a silica gel column. Elution with 20% ethyl acetate in hexanes gave pure product (m.p 81°C) in 90% yield. *H NMR(300 MHz, CDC1₃): δ 0.81-0.87 (m, 6H), 2.05 (m,lH), 3.05 (m, 2H), 3.67 (s, 3H), 4.47-4.61 (m, 2H), 5.06 (s, 2H), 5.78 (bd, 1H, D₂O exchangeable), 6.6 (bd, 1H, D₂O exchangeable), 7.1-7.31 (m, 2H). ^,3C NMR (300 MHz, CDC1₃): δ 17.82, 18.89, 31.05, 38.26, 52.10, 56.11, 57.50, 66.68, 116.97, 126.88, 127.85, 128.10, 128.55, 128.56, 129.47, 136.66, 136.88, 156.12, 171.45, 171.93.

Example 9. Synthesis of CBZ-phenylalanyl valine methyl ester via CDI activation

(Table 1, entry 8)

The reaction described in Example 8 was repeated using CDI (1 eq.) as the coupling activator in THF (10 mL). After 2 hours only 70% of product could be isolated by column chromatography (20% ethyl acetate in hexanes).

Claims

16CLAIMS

1. A method of activating a carboxylic acid comprising reacting a carboxylic acid with a substituted 1,1 '-carbonyldiimidazole comprising substituted imidazoles, wherein said substituted imidazoles have pKa's between 5.0 and 6.8, and wherein said substituted imidazoles are more facile leaving groups than imidazole.

2. The method of claim 1 wherein said substituted 1,1- carbonyldiimidazole is 1 ,1 '-carbonylbis-(4,5-dicyanoimidazole) (CBDCI).

3. A method of synthesizing a carboxylic acid derivative from a carboxylic acid comprising the steps of a) reacting a carboxylic acid with a substituted 1,1'- carbonyldiimidazole comprising substituted imidazoles, wherein said substituted imidazoles have pKa's between 5.0 and 6.8 and wherein said substituted imidazoles are more facile leaving groups than imidazole, to produce an acyl imidazolide intermediate; and b) reacting the acyl imidazolide intermediate with a nucleophile to form a carboxylic acid derivative.

4. The method of claim 3 wherein said carboxylic acid derivative is a carboxylic acid ester and said nucleophile is an alcohol.

5. The method of claim 3 wherein said carboxylic acid derivative is an amide and said nucleophile is an amine.

6. The method of claim 3 wherein said coupling activator is l,l'-carbonylbis- (4,5-dicyanoimidazole) (CBDCI).

7. A method of forming a peptide linkage between a first and second amino acid comprising the steps of: a) reacting a first amino acid having an amine protecting group with a substituted 1 , 1 '-carbonyldiimidazole comprising substituted imidazoles, wherein said substituted imidazoles have pKa's between 5.0 and 6.8 and wherein said substituted imidazoles are more facile leaving groups that imidazole, to produce an acyl imidazolide intermediate; and b) reacting a second amino acid having a COOH protecting group with said acyl imidazolide intermediate to form said peptide linkage.

8. The method of claim 7 wherein said substituted 1 , 1 '-carbonyldiimidazole is CBDCI.

9. The method of claim 7 wherein said COOH-protecting group is a solid support.

10. The method of claim 7 wherein said amine protecting group is a solid support.

11. A method for coupling two compounds having the structures Rj Y,H and R₂Y₂H to form a compound having the structure R,Y,-C(O)-Y₂R₂, comprising: a) reacting a compound having the structure R, Y,H with a substituted

1,1 '-carbonyldiimidazole comprising substituted imidazoles, wherein said substituted imidazoles have pKa's between 5.0 and 6.8 and wherein said substituted imidazoles are more facile leaving groups than imidazole, to form an acyl imidazolide intermediate; and b) reacting the acyl imidazolide intermediate with a compound having the structure R₂Y₂H to form a compound having the structure R,Y,-C(O)-Y₂R₂, wherein Y, and Y₂ are independently selected from the group consisting of O, NH, S or C(O)O-, and R, and R₂ are independently selected from the group consisting of alkyl, alkenyl, alkynyl or aryl.

12. The method of claim 11 wherein said coupling reagent is CBDCI. 18

13. A method for forming a compound having the following structure:

(CH₂)₀,╬╣

/ \

R-|R-|C CR₂R

O from a compound having a structure HY₁R,R,'C-(CH₂)₀ ,-CR₂R₂'Y₂H, wherein Y, and Y₂ are independently selected from the group consisting of O, NH, S or C(O)O-, and RΓÇ₧ R,', R₂, and R₂' are independently selected from the group consisting of H, alkyl, alkynyl or aryl, or together R, and R₂ are part of a carbocyclic or heterocyclic ring comprising reacting the compound having a structure HY,R,R,'C-(CH₂)₀ ,-CR₂R₂'Y₂H with a substituted 1,1 '-carbonyldiimidazole comprising substituted imidazoles, wherein said substituted imidazoles have pKa's between 5.0 and 6.8.

14. The method of claim 12 wherein said coupling reagent is CBDCI.

15. A compound having the following structure: