Rhodium acetate-mediated insertion of methanol into diazotyrosine methyl ester (I) provided the alpha-methoxy ester (IIa-b). Subsequent alkylation of (IIa-b) with mesylate (III) using NaH in DMF gave the racemic adduct (IVa-b). Finally, kinetic resolution of (Va-b) employing the lipase from Rhizopus delemar at a pH of 7.0 produced the desired (S)-acid while leaving the unchanged (R)-ester (V).