Diastereoselectivity and asymmetric induction in the Diels–Alder reaction of o-quinodimethanes

Abstract

The extent of asymmetric induction in the bimolecular Diels-Alder reactions of chiral o-quinodimethanes with dimethyl fumarate, methyl acrylate, and maleic anhydride has been studied. o-Quinodimethanes with chiral .alpha.-alkoxy groups were prepared from 1-alkoxy-1,3-dihydrobenzo[c]thiophene-2,2-dioxides or 1-alkoxy-3-phenyl-1,3-dihydrobenzo[c]thiophene-2,2-dioxides by thermolysis. These alkoxybenzosulfones were prepared from the corresponding hydroxybenzosulfones and 1-phenylethanol, 2-phenyl-1-propanol, 4-phenyl-2-butanol, 1-phenyl-2-propanol, 3,3-dimethyl-2-butanol, or 1-cyclohexylethanol. The 1-phenylethoxy substituent yielded the largest asymmetric induction. The absolute configurations of the major cycloadducts of methyl acylate with the o-quinodimethanes generated from 1-(R-1-phenylethoxy)- and 1-(S-1-phenylethoxy)-1,3-dihydrobenzo[c]thiophene-2,2-dioxides were determined to be 1S,2S-1-(R-1-phenylethoxy)- and 1R,2R-1-(S-1-phenylethoxy)-2-carbomethoxy-1,2,3,4-tetrahydronaphthalene. The proposition that a chiral alkoxy substituent can block one face of the o-quinodimethane towards addition of a dienophile is discussed.