4.pi. Participation of 1-aza-1,3-butadienes in [4 + 2] cycloaddition reactions: intramolecular Diels-Alder reactions of .alpha.,.beta.-unsaturated N-sulfonylimines

The 4pi participation of alpha,beta-unsaturated N-sulfonylimines in intramolecular Diels-Alder reactions with unactivated dienophiles is detailed in studies which further extend the scope of the [4 + 2] cycloaddition reactions of this new class of 1-aza-1,3-butadienes. Similar to observations made in the intermolecular [4 + 2] cycloaddition reactions, a strong endo diastereoselectivity (>20:1) was observed to dominate in the cycloaddition reaction of 5b versus 5a and 5c. This endo specific reaction may be attributed to the combination of a pronounced, stabilizing secondary orbital interaction and preferred cycloaddition through an anti-endo transition state in which the lone pair on nitrogen and the C-aryl bond of the dienophile lie trans periplanar to one another benefiting from stabilization analogous to the ground-state anomeric effect.