The treatment of N-benzyloxycarbonyl-L-alanyl-L-serine methyl ester (I) with phosgene afforded the corresponding O-chlorocarbonyl derivative (II), which was then cyclized to methyl L-3-(N-benzyloxycarbonyl-L-alanyl)-2-oxo-oxazolidine-4-carboxylate (III) accompanied with N-benzyloxycarbonyl-L-alanyl-3-chloro-L-alanine methyl ester (IV) by refluxing in xylene. By the mild alkaline hydrolysis of III, N-benzyloxycarbonyl-L-alanine ...
