REGIO-DIPOLAR AND STEREOSELECTIVITY IN THE 1,3-DIPOLAR CYCLOADDITION OF C,N-DIARYLNITRONES TO 3,3-METHYLENE-5,5-DIMETHYL-2-PYRROLIDINONE

Regio- and stereoselectivity of the nitrone cycloaddition with 3,3-methylene-5,5-dimethyl-2-pyrrolidinone ((1) is discussed. Nitrones react regioselectively with 1 to give a mixture of diastereoisomeric spiro-cycloadducts 3 and 4, in which 3 always dominates. Both 3 and 4 result from the approach which binds the carbon of the nitrone with the exocyclic carbon of 1 and the oxygen to the spiro carbon. The structure and steric configuration of the adducts have been assigned on the basis of H-1- and C-13-NMR spectroscopy, mainly by nuclear Overhauser effect difference spectroscopy. AM1 calculations of the reactants were performed, the regio- and stereochemistry of the cycloaddition seems to be controlled by steric effects.