Studies on cycloaddition reactions of 2,2-dimethyl-3,4-dihydro-2H-pyrrole N-oxide and N-(benzylidene)methylamine N-oxide with α,β-unsaturated lactones & esters

Intermolecular asymmetric 1,3-dipolar cycloaddition of 2,2-dimethyl-3,4-dihydro-2H- pyrrole N-oxide and N-(benzylidene)methylamine N-oxide with optically active alpha,beta-unsaturated esters 1,2,4 and 7-9 provided the corresponding diastereomers while chiral sugar lactones gave stereoselectively the cycloadducts 17a and 17b (in the case of dipolarophile 3) and 18 in (the case of the dipolarophile 6) with the acyclic nitrone B. The stereochemistry of the products has been established using high field nmr techniques. The regioselectivities of these reactions are inconsistent with FMO coefficients and are explained on the basis of charge distribution obtained through AM1 calculations. (C) 1997 Elsevier Science Ltd.