Note on the action of acetylating agents on amino-acids.

If amino-acids are treated in glacial acetic acid solution with the theoretical amount of acetic anhydride, optically active acetamino-acids are obtained, while if a large excess of acetic anhydride is used, racemization and/or formation of azlactones takes place [Bergmann et al. 1926; Bergmann & Zervas, 1928]. Excess of acetic anhydride causes racemization even in aqueous solution in the presence of alkali [du Vigneaud & Meyer, 1932]. If the reaction with acetic anhydride, however, is carried out at 1000 in the presence of pyridine, CO2 is split off and acetaminoketones result [Dakin & West, 1928]. It has been found by the present author that this anomalous reaction is suppressed at low temperature. Thus two typical amino-acids, histidine and phenylalanine, which by treatment with pyridine and acetic anhydride at 100' are almost quantitatively converted into the corresponding acetaminoketones, give, when the reaction is carried out at 2°, very good yields of acetamino-acids; only in one case was there an indication that a minute trace of a ketone was formed. It seems, however, that at least partial racemization of some amino-acids is induced by this method of acetylation also.