Hydroxyproline racemization

H Kuroda, S Kubo, N Chino, T Kimura, S Sakakibara. Unexpected racemization of pro line and hydroxyproline phenacyl ester during coupling reactions with Boc-amino acids. Int J Pept Prot Res 40, 114, 1992. 

An early example of this type of transformation was described in 1964 by Drehfahl and Horhold (310). These authors prepared racemic 4-hydroxyproline, albeit with low diastereoselectivity for the isoxazoline reduction step [Scheme 6.75, (1)] (310). Much higher selectivity was achieved using 5-halomethylisoxazolines bearing a 3-(l-oxyalkyl) side chain, which was introduced from the nitrile oxide portion. The examples presented in Scheme 6.75 outline the synthesis of 4-hydroxy-pyrrolidines, which contain a dioxyethyl or trioxypropyl side chain. Both of these substrates were converted into the corresponding imino acids of 4-hydroxyproline (23,225,234) and 4, 1 -dihydroxyhomoproline, respectively (23,207,225) (Eq. 2, 3). The latter compound is part of an N-Val dipeptide structure, that was mistakenly proposed for the antibiotic Tii 1718B (311,312). 

In the presence of the immobilized (2S,4S)-4-hydroxyproline Mo catalyst (13b), nerol and geraniol react selectively with r-BuOOH to form the 2,3-epoxide with ee values of 64 and 47%, respectively. Surprisingly, when Mo is complexed by the diastereomeric (2S,4R) form (13a), racemic epoxidation is observed. The enantioselective catalysis appears to be promoted by immobilization in the zeolite USY pores. Indeed, in the epoxidation of nerol, an ee of 10% was found for the homogeneous asymmetric Mo complex, whereas the supported catalyst favors the selective production of the (2S,3R)-epoxide (64% ee). 

Good results are also obtained, when functionalized substrates with additional coordinating sites 11 75 are converted in the presence of supported rhodium and platinum catalysts62,153,155, especially when using new hydroxyproline derived chiral phosphane ligands124,125,1 54,1 57,159. Suppression of racemization is possible by orthoformate addition to the reaction mixture (cf. p 2522). 

For the purpose of preparing such analogues, racemic 3-fluoropyrrolidine (R,R)-2 was prepared by nucleophilic displacement of A -benzyl-3-tosyloxypyrrolidine with KF. (25,4/ )-3-Hydroxyproline was the starting point for both (3>R)- and (35)-3-fluoropyrrolidine. Decarboxylation and protection with the carbobenzloxy (Cbz) group was followed by tosylation and displacement with fluoride and deprotection. A Mitsunobo inversion of the intermediate 3-hydroxypyrrolidine provided access to the other enantiomer by the same route (Fig. 3.1). ... 

The racemic tryptophan derivative 207 was condensed with optically pure proline derivatives following the BOP-Cl protocol. For the synthesis of notoamides C (213) and D (215), proline ethyl ester was used as coupling partner (Scheme 41), whereas the synthesis of stephacidin A (225) and notoamide B (227) employed hydroxyproline ethyl ester 219 (Scheme 43). Towards notoamides C (213) and D (215), the diaste-reomeric diketopiperazines 210 and 211 had to be separated by chromatography. The Fmoc can be replaced by a Boc protecting group and BOP-Cl by HATU [172]. 

Fales and Pisano (1964) have discussed the gas chromatography of amines, alkaloids, and amino acids. Pollock and Kawauchi (1968) have resolved derivatives of serine, hydroxyproline, tyrosine, and cysteine, as well as racemic aspartic acid and tryptophan. VandenHeuvel and Horning (1964) have listed derivatives of steroids that can be separated. VandenHeuvel et al. (1960) first described the separation of bile acid methyl esters and Sjovall (1964) has extended the methods to bile acids. Gas liquid chromatography (GLC) is useful in the analysis of pesticides, herbicides, and pharmaceuticals (Burchfield and Storrs, 1962). Analysis of alkaloids, steroids, and mixtures of anesthetics and expired air are other examples of the application of this very useful technique. Beroza (1970) has discussed the use of gas chromatography for the determination of the chemical structure of organic compounds at the microgram level. 

The synthesis of various proline derivatives has received attention this year. A route to proline itself from pyrrolidine in 45% overall yield has been reported, which appears to be an improvement on previous multistage methods.A one-pot synthesis of racemic 4-hydroxyproline in 30—40% yield from glyoxal and oxaloacetic acid has been developed, and a total synthesis of all four dias-tereoisomers of 3-hydroxy-5-methylproline has been achieved. An asymmetric synthesis of prolines is by addition of malonate carbanions to chiral aziridines derived from amino-acids. ... 

Natural hydroxyproline is racemized by heating with Ba(OH)2 or glacial acetic acid and acetyl chloride only on the a-carbon atom according to Leuchs and coworkers (498-500, 502) and Werner (852). 

The derivatives are used for amino acid analysis via HPLC separation. Instead of mercapto-ethanol, a chiral thiol, e.g., N-isobutyryl-L-cysteine, is used for the detection of D-amino acids. The detection hmit lies at 1 pmol. The very fast racemizing aspartic acid is an especially suitable marker. One disadvantage of the method is that proline and hydroxyproline are not detected. This method is apphed, e.g., in the analysis of fruit juices, in which high concentrations of D-amino acids indicate bacterial contamination or the use of highly concentrated juices. Conversely, too low concentrations of D-amino acids in fermented foods (cheese, soy and fish sauces, wine vinegar) indicate unfermented imitations. Fluorescamine reacts with primary amines and amino acids - at room temperature under alkaline conditions - to form fluorescent pyrrolidones (Aex = 390 nm, Aem - 474 nm). The detection limit lies at 50-100 pmol ... 

---