Investigations on the stereospecificity of peptide active phenyl ester formation and coupling

Abstract

Phenolic active esters were investigated for stereospecificity during their formation and subsequent coupling. The different esters of ZGlyPheOH, ZGlyGlyPheOH and BzLeuOH were prepared either by: (a) the backing-off procedure, (b) the usual DCC method, (c) the reverse DCC procedure, or (d) by use of a crystalline “complex” consisting of the isourea derivative “A” and 2 PCPOH. In the preparation of active esters by methods (b), (c), and (d), it was found that the more acidic the phenol component the greater was the optical purity of the ester formed. Method (d) generally afforded the highest optical purity. Under coupling conditions similar to those employed for the preparation of sequential polypeptides using pentachlorophenyl esters, no racemization was encounted as shown by the Anderson racemization test. In esterification of ZGlyPheOH with PCPOH using DCC (1:1:1), the oxazolone formed rapidly, and the rate of ring opening showed the following order: DNPOH > PCPOH > NPOH. In the presence of triethylamine, the opening by the phenols was reversed. However, the rate of oxazolone ring opening by DNPOH and PCPOH was faster in the absence of base. The faster rate of ring opening by the more acidic phenols suggests a pathway indicated by Scheme 1. The rate of active ester formation from ZGlyOH using DCC gave the foUowing order: DNPOH (p K 4·1) > PCPOH (p K 5·3) > NPOH (p K 7·2). The major path to active ester is through direct attack of the phenols on the acylisourea intermediate, while the minor path is through (ZGly) 2O.