Stereoselectivity control by torsional steering in an intramolecular Diels-Alder reaction of vinyl oxocarbenium ions.

[reaction: see text] Density functional theory (B3LYP/6-31+G) has revealed the origin of stereoselectivity in intramolecular Diels-Alder reactions of vinyl oxocarbenium ions. The cycloaddition has endo preference and occurs with remote stereocontrol syn to the substituent at the stereogenic center. Torsional steering, the preference for the staggered conformation about forming sigma-bonds, dictates the preferred transition structure.