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

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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.
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