776. Amino-acids and peptides. Part XV. Racemisation during peptide synthesis

Abstract

Although a-benzyloxycarbonylamino-acids normally do not racemise when they are condensed with amino-esters to form peptides, benzyloxycarbonyldipeptides may do so when they are coupled by some of the procedures in current use, which therefore have limited applicability in peptide synthesis. The racemisation occurring when acetyl-L-leucine is condensed with glycine ethyl ester by various methods has now been studied. Coupling through the acid azide gave a product of high optical activity, from which no racemate could be isolated ; some racemisation was observed when dicyclohexylcarbodi-imide was the condensing agent (with dichloromethane or tetrahydrofuran as solvent). The racemisation found with the use of diethyl phosphorochloridite or tetraethyl pyrophosphite varied considerably with the conditions. The carbonic mixed anhydride and the " phosphorazo " method caused extensive racemisation. Methods which have been postulated to involve activation of the amino-component may therefore result in racemisation in the carboxylic component. The use of amino-ester hydrochloride with an equivalent of tertiary amine, in place of the free aminoester, led to increased racemisation in several cases.

N-Methylamino Acids in Peptide Synthesis. IV. Racemization and Yields in Peptide-bond Formation

“…DCCI-HONSu and EEDQ caused much less racemization than in experiments with salt present, the former yielding a stereochemically pure product. The enhancement of racemization by salts (22,23) has been attributed t o the basicity of the anion (the chloride ion in particular) (13), the increased ionic strength of the medium (24) and more recently to both of these (25). We have shown that the addition of an equivalent amount of triethylamine to a DCCI coupling did not increase the percentage of L,D-isomer.…”

“…This dependence on solvent polarity could explain why high extents of racemization were obtained for the NEPIS coupling where the solvent was acetonitrile. It is interesting to note that with this reagent under similar conditions (absence of chloride), Smart et al were able to couple the racemization-prone Bz-Leu without diastereomer formation (22). The absence of base catalysis indicates that the mechanism involved is not a simple cr-hydrogen abstraction.…”

Synthesis of new modified truncated peptides and inhibition of glycogen phosphorylase

“…For compounds 21-24 (Table 2), the peptides were unable to be isolated after solid-phase synthesis due to their highly liphophilic structures and low molecular weight. Instead, standard peptide coupling (HOBt, EDC.HCl, DIPEA) [35] of Ac-Leu-OH 25 with Gly-OEt gave Ac-Leu-Gly-OEt [36] (23) (Scheme 1, Table 2), and subsequent hydrolysis of 23 with lithium hydroxide gave Ac-Leu-Gly-OH [36] (24) (Scheme 1, Table 2). Similarly, Ac-Lys(Z)-OH [37] 26 was coupled with Leu-OEt [38] to give Ac-Lys(Z)-Leu-OEt (27) (Scheme 2).…”

“…The combined ethyl acetate layers were washed with brine (3 × 30 ml), dried (MgSO 4 , anhydrous) and the solvent was removed in vacuo to give a white solid. The solid was recrystallized (methanol/diethyl ether) to give a pure 24 [36]…”

7.4 Racemization Assays

“…A modification by Kemp, [10] utilizing 14 C-labeled carboxy components, extends the detection limit by two to three orders of magnitude by an isotopic dilution procedure. The Young test [11] addresses the coupling of Bz-Leu-OH to H-Gly-OEt, and the extent of epimerization is determined by measurement of the specific rotation of the dipeptide product. A method by Berger [12] utilized the stereoselective hydrolysis of diastereomeric peptides by leucine aminopeptidase.…”