Regioselective synthesis of methyl 5-( N -Boc-cycloaminyl)-1,2-oxazole-4-carboxylates as new amino acid-like building blocks.

A convenient and efficient synthesis of novel achiral and chiral heterocyclic amino acid-like building blocks was developed. Regioisomeric methyl 5-( N -Boc-cycloaminyl)-1,2-oxazole-4-carboxylates were prepared by the reaction of β-enamino ketoesters (including azetidine, pyrrolidine or piperidine enamines) with hydroxylamine hydrochloride. Unambiguous structural assignments were based on chiral HPLC analysis, 1 H, 13 C, and 15 N NMR spectroscopy, HRMS, and single-crystal X-ray diffraction data.