Reaction of dimethyl methylenesuccinate with hydrazine

The addition of hydrazine to dimethyl methylenesuccinate gave a product whose structure could be either a 1-aminopyrrolidone 1 or a hexahydropyridazinone 2. Chemical methods to establish the structure were contradictory. It gave a benzylidene derivative suggesting the former structure, but the N-benzoyl derivative of 1, prepared by a different route, differed from that of the product suggesting the latter structure. X-Ray structural analysis established that the product is 2 while the benzylidene derivative was actually that of 1, showing that rearrangement occurred when the product was heated with benzaldehyde.