Racemization of Amino Acid Residues Fused in Thiazoline, Oxazoline, and Imidazoline Rings

Abstract For the purpose of study on racemization of exo methine carbon atom of thiazoline, oxazoline or imidazoline ring, the following four compounds were synthesized from amino acid imino ether: 2-(1-benzyloxycarbonylaminoethyl)-2-thiazoline (I), methyl 2-(1-benzyloxycarbonylaminoethyl)-2-thiazoline-4-carboxylate (II), methyl 2-(1-benzyloxycarbonylaminoethyl)-2-oxazoline-4-carboxylate (III), and 2-(1-benzyloxycarbonylaminoethyl)-2-imidazoline (IV). A racemization rate of the alanine residue was measured from an optical rotation of 2,4-dinitrophenyl derivative obtained from the acid hydrolyzate and from hydrogen-deuterium exchange reaction at α-carbon atom. All these compounds were found to be racemized very easily particularly in the presence of proton source. From the experimental results, a plausible mechanism for racemization peculiar to such heterocyclic compounds through protonation-deprotonation process was presented.