. 64,793 (1986).E, E-and E,Z-a-phenyl-a'-acetoxyorthoquinodimethanes have been prepared from the corresponding cis-and trans-lacetoxy-3-phenyl-1,3-dihydrobenzo[c]thiophene-2,2-dioxides. The regio-and diastereoselectivity of the addition reactions with dimethyl fumarate, dimethyl maleate, maleic anhydride, and methyl crotonate have been determined. Ab initio calculations have been carried out on orthoquinodimethane and its a-phenyl and a-oxy derivatives. A correlation has been made between the steric and electronic properties of the orthoquinodimethanes and the regio-and diastereoselectivity of their Diels-Alder reactions.