Enantiopure furo[3,4-c]pyrazole derivatives by intramolecular nitrilimine cycloaddition: a stereoselectivity rationale based upon MP2 calculations

Silver carbonate treatment of hydrazonoyl chloride 4 promoted the in situ generation of the corresponding nitrilimine bearing a stereocentre at the α-position of the ethylenic dipolarophile. Intramolecular cycloaddition of the latter intermediate involves the formation of 4-(S)-methyl-6-oxo-3,3a,4,5-tetrahydro-furo[3,4-c]pyrazole derivatives with very good yield and diastereoselectivity. Full rationalization of the experimentally observed stereoselectivity has been pursued by means of MP2 calculations.