Derivatives of (5-Oxooxazolidin-4-yl)acetic acids in aspartic acid α-carboxyl protection: a word of caution

Isomeric products could be isolated in a reaction of pentafluorophenyl ester of (S)-2-(3-((benzyloxy)-carbonyl)-5-oxooxazolidin-4-yl)acetic acid with aminoacid esters. One product is an expected dipeptide, the isomeric one is an N-hydroxymethylamino succinimide – a product of methylene bridge cleavavge. Steric bulkness of aminoacid esters favors the formation of a peptide, however the reaction selectivity is quite unpredicatable. Moreover, the precise structure assignement requires high-temperature NMR experiments.