Esters of acylamino-acids with l-hydroxypiperidine , dimethylhydroxylamine, diethylhydroxylamine, and dibenzylhydroxylamine, have been prepared in good yield (A) by direct condensation using dicyclohexylcarbodiimide, (B) through the acid chloride, or (C) through the carbonic mixed anhydride. Such esters (1) are themselves slow to react with a-amino-esters but, in the presence of acetic acid, condensation occurs readily a t room temperature, giving high yields of pure protected peptides. The aminoester hydrochlorides with sodium acetate , or with triethylamine and acetic acid, may conveniently be used instead of the amino-ester and acetic acid. The acylamino-esters will also couple, rather more slowly, with the sodium salts of amino-acids, again in high yield. They have remarkable optical stability under basic conditions in which analogous 9-nitrophenyl esters racemise partially or completely. I n stringent racemisation tests, the condensation of benzoyl-L-leucine l-piperidyl ester with glycine ethyl ester or with the sodium salt of glycine, no racemisation was detected. These esters are therefore stable intermediates which can be activated at will t o provide reagents which couple without racemisation, giving products of high purity in excellent yield.Although acid catalysis is desirable for the reaction of these esters with a-amino-esters, with stronger nucleophiles such as aliphatic aniines and ammonia the uncatalysed reaction is also fast. Highly selective acylation can be effected in this way; e.g. , l-benzoyloxypiperidine reacts completely with n-butylamine under conditions in which t-butylamine and aniline are unattacked, and cyclohexylamine and isopropylamine scarcely attacked. Other uses include the benzyloxycarbonylation and pnitrobenzyloxycarbonylation of a-amino-esters by means of benzyl or p-nitrobenzyl l-piperidyl carbonate.
T. F. N. JOHNSONINVESTIGATIONS of the racemisation which may occur during the condensation of acylamino-acids with amino-esters have shown 2-6 that the acid azide route is unique among those examined, in preserving full optical activity in a wide range of cases and conditions. Furthermore, this route alone allows activation of the carboxyl group of an acylpeptide (via the alkyl ester and hydrazide) without danger of racemisation. Although, for example, the widely-used P-nitrophenyl esters will normally couple without racemisation, an acylpeptide cannot yet be converted into its p-nitrophenyl ester without this risk.* * We have shown 7 that the procedure using diphenylketen, which was reported to give optically pure aryl esters,s does in fact give some racemate when used to convert benzyloxycarbonylglycyl-Lphenylalanine into its p-nitrophenyl ester.
