Formula omitted]-quinone methides 2. Stereoselectivity in cycloaddition reactions of [formula omitted]-quinone methides with vinyl ethers

Abstract

Cycloaddition reactions between vinyl ethers 3 and o -quinone methides 2, thermally generated from 2-hydroxybenzyl alcohols 1, have been studied. The structure and conformational preferences of the 4-substituted 2-ethoxy-(2,3)-dihydro-2 H -benzopyrans 4–9 obtained, which show new interesting features, are discussed together with competitive kinetic data. The cycloaddition process is concerted and involves o -quinone methides in the E -configuration. The OEt- endo transition state seems to be preferred with ethyl vinyl ether and Z -1-propenyl ethyl ether, whereas with E -1-propenyl ethyl ether the stereoselectivity of the cycloaddition process depends on substituents on the methylene group of the starting alcohol 1. These results are discussed in terms of endo and exo preference of the propenyl ether methyl group.