TRI-ANDTETRA-PEPTIDES INCORPORATING AN a-AMINOAcID AT THE

Abstract: The first examples of peptides containing an a-amino acid residue in which the a-carbon is also the anomeric position of a carbohydrate arc described. The bromine-induced oxidative ring contraction of the aminolactone 1 to give the epimeric amino esters 28 and 2~2 in 6096 and 17 % yields, respectively, was the key step in the synthesis of some mannofuranose spirohydantoinsl as analogues of the naturally occurring plant growth regulator. hydantocidin.2 Although 2a and 2s slowly equilibrate in solution, reaction of the p anomer with phenyl isocyanate [Scheme l] gave the urea 38 which cycliscd in methanol to give the protected hydantoin 48; the minor a epimer on similar treatment gave 3a which cyclised to the epimeric hydantoin 4a, again without any apparent epimerisation. The reaction of the amines with phenyl isocyanate was faster than equilibration of 2a and 2s; also, it was clear that equilibration of the epimeric ureas 3 and hydantoins 4 was negligible other than under highly acidic or basic conditions. Scheme 1: (i) Bq, NaOAc, MeoH; then Et3N (ii) PhNCO. THF (iii) MeOH The chemical stability of 3 and 4, together with the stereointegrity of the different anomers, indicated that it might bc possible to incorporate amino acids at the anomecic position of sugars, such as 2, as components of peptides. Such amino acids would be of interest, inter alia. as N-linked glycoprotein analogues, as a.a