Synthesis of the first examples of the imidazo[4,5-c]isoxazole ring system

The synthesis of the first examples of the 4H-imidazo[4,5-c]isoxazole ring system are reported. These compounds are obtained in good yield by thermoly sis of 2-(4-nitro-1H-imidazol-5-yl) acetate and malonate derivatives, in tu rn readily available from reaction of 5-chloro-4-nitro-1H-imidazoles with a ctive methylene compounds. The X-ray crystal structure of an imidazo[4,5-c] [1,2]oxazine derived from a proposed intermediate has been obtained and a m echanism for the reaction is discussed.