RESUMO
1. simple method is described for the preparation of optically active N-tert-butyloxycarbonyl (Boc) and N-benzyloxycarbonyl (Z) amino acid methyl and ethyl esters by papin-catalyzed esterification. 2. N-protected amino acid (50mM) were incubated with 25 micronM papain in 1 mM sodium acetate buffer, pH 4.7, containing 50% (v/v) methanol or ethanol for 24 h at 3-C. 3. Methyl and ethyl esters of 20 Boc-and Z-amino acids were prepared in yields from 28 to 98%. Among these, the following compounds have not previously been described (yield): Boc-Cys-(Mob)-OMe (98%); Boc-Lys-(Dmc)-OMe (78%); Boc-Ser-(Bzl)-OMe (53%); Boc-Arg-(Tos)-OMe (43%); Boc-Cys-(Mob)-OEt (52%); Boc-Trp-OEt (63%); Boc-Glu-(OBzl)-OEt (39%); Boc-Phe-OEt (43%); Z-Lys-(Z)-OEt (62%); /z-Trp-OEt (46%). 4. This method is effective for the enzymatic synthesis of N-protected amino acid esters and is sespecially useful for the preparation of Boc-amino acid esters because of the extreme lability of the Boc protecting group in acid medium