Reactivity of 2-Halo-2H-azirines. Part II. Thermal Ring Expansion Reactions: Synthesis of 4-Haloisoxazoles

We had reported that under thermolysis 2-benzoyl-2-halo-2H-azirine-3-carboxylates 1 undergo ring expansion giving products in high yield which were identified as being 4-haloisoxazoles 3 (Scheme 1). The same products can also be obtained in high yield from the thermolysis of haloazidoalkenes 2 via intermediate 2-benzoyl-2-halo-2H-azirines 1.

Scheme 1

Recently, new data have been disclosed allowing to demonstrate 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 4 and not the isomeric isoxazoles (Scheme 2).1 These 1,3-oxazoles can also be obtained in high yield from haloazidoalkenes 2.

Scheme 2

Reference

(1) Lopes, S.; Nunes, C. M.; Fausto, R.; Pinho e Melo, T. M. V. D. J. Mol. Struct. 2009, 919, 47.