Regio- and stereoselective synthesis of pyrrolo or azepine-fused cyclopenta[d]isoxazolines from 2-p-tolylsulfinylcyclopent-2-en-1-one

Reactions of enantiopure 2-p-tolylsulfinylcyclopent-2-en-1-one with cyclic nitrones afforded pyrrolo or azepine-fused cyclopentad]isoxazolidines in high yields under mild conditions. Comparison of these results with those obtained with cyclopent-2-en-1-one as the dipolarophile shows that the sulfinyl group increases the reactivity of the enonic system and efficiently controls the π-facial and endo/exo selectivities of the cycloadditions, which are also dependent on the easy cycloreversion of the resulting compounds. Results obtained in reactions with other dipoles (benzonitrile oxides and those resulting from allenoate and PPh₃) are also reported.