4-Halo-1,3-oxazoles: Unambiguous structural assignment of 2-halo-2-benzoyl-2H-azirine-3-carboxylates thermal ring expansion products

IR spectroscopy in cryogenic argon matrix of methyl 4-chloro-5-phenyl-1,3-oxazole-2-carboxylate and methyl 4-chloro-5-phenylisoxazole-3-carboxylate was applied for the structural assignment of these isomeric heterocycles. It was demonstrated that methyl 2-benzoyl-2-halo-2H-azirine-3-carboxylates undergo thermal ring expansion to give 4-halo-5-phenyl-1,3-oxazole-2-carboxylates and not the isomeric isoxazoles.