The interaction of imines with isocyanides has been studied. The main product results from a sequential process involving the attack of two units of isocyanide, under Lewis acid catalysis, upon the carbon–nitrogen double bond of the imine to form the 4-membered ring system. The scope of the reaction regarding the imine and isocyanide ranges has been determined, and also some mechanistic variations and structural features have been described.
Pyrazole-fused cyclic anhydrides have been employed for the first time as the Castagnoli–Cushman reaction partners to produce hitherto unknown 4-oxo-4,5,6,7-tetrahydropyrazolo[1,5-a]pyrazine-7-carboxylates in remarkably convenient and speedy fashion. Attempts to realize the same chemistry with indole and benzimidazole analogues failed.