Thermolysis of 1-(methylideneamino)-1H-pyrrole-2,3-diones. Synthesis of pyrazolooxazines by [4 + 2]-cycloaddition of azomethine imines to alkenes

Thermolysis of methyl 3-acyl-1-[(diphenylmethylidene)amino]-4,5-dioxo-4,5-dihydro-1H-pyrrole-2-carboxylates generates 4-acyl-1-(diphenylmethylidene)-3-(methoxycarbonyl)-1H-pyrazol-1-ium-5-olates which react with alkenes to give methyl 3-acyl-7,7-diphenyl-6,7-dihydro-5H-pyrazolo[5,1-b][1,3]oxazine-2- carboxylates. The product structure was confirmed by X-ray analysis.