SE322207

High content (i.e. 90% or more) D,L-threo-b - [p - methyl - 1,3 - dioxolan - 2 - yl) phenyl] serine alkyl or aryl ester hydrochlorides are made by contacting D,L-threo and D,L-erythro - N - [p - (2 - methyl - 1,3 - dioxolan - 2 - yl) benzylidene] - 3 - [p - 2 - methyl - 1,3 - dioxolan - 2 - yl) phenyl] serine alkyl or aryl ester of the formula wherein R is an alkyl or aryl group, preferably an ethyl group, with one equivalent of water in the presence of hydrogen chloride and recovering the high D,L-threo isomer residue which separates and if desired, treating the mother liquor with either whereby the high D,L-erythro hydrochloride isomer separates and may be recovered.