Azomethine imines were generated in a controlled manner through a thermally allowed 1, 3- dipolar cycloreversion of 1, 3, 4-oxadiazolidines and subsequently trapped with dipolarophiles. This method results in the construction of the pyrazolidine heterocycles. A new method for the selective formation of the key semicarbazide substrate, from benzylidene hydrazone, is also disclosed.