EXPERIMENTAL AND THEORETICAL-ANALYSIS OF THE EFFECTS OF STRAIN DIMINUTION ON THE STEREOSELECTIVITY OF DIENOPHILIC CAPTURE BY PI-FACIALLY NONEQUIVALENT HOMOLOGS OF ISODICYCLOPENTADIENE

Four new cyclic dienes, constructed such that their conjugated pi networks are fused to bicyclic frameworks, viz. 6-9, have been prepared. All four are less strained than isodicyclopentadiene (1) and therefore lack the high-lying sigma orbital energy levels that presumably give rise to pi orbital tilting found uniquely in 1. The addition of various dienophiles to these dienes results in predominant Diels-Alder reaction from the top face, in contradistinction to the behavior of 1, which reacts with high below-plane pi-facial diastereoselectivity. The structures of the adducts were determined almost entirely by NOE methods as applied directly to the products themselves, their dihydro derivatives, or quadricyclane photoisomers. The relative energies of the two principal transition states for 6-9 were calculated by means of an MM2 model. The features uncovered by this means showed larger torsional angles between H(1) and C(4) to be invariably associated with bottom-face dienophile capture. Since this ordering of torsional strain effects does not conform with the experimental facts, such contributions cannot be responsible for controlling the stereoselectivity of these cycloadditions. Rather, in the absence of sigma/pi interaction, steric approach control operates, and dienophiles attack preferentially from the sterically less crowded pi surface.