o-Quinonoid compounds. Part X. endo-Selectivity in the Diels–Alder additions of non-conjugated olefins to o-quinodimethanes

Abstract

The addition of cyclopentene, cis-but-2-ene, norbornadiene, and 1,4-dihydro-1,4-methanonaphthalene to 2-benzopyran-3-one (4) gives mainly endo-adducts; the addition of cycloheptene and norbornadiene to 1-methyl-2-benzopyran-3-one (5) is also endo-selective. endo-Addition predominates in the additions of cyclopentene to the valence tautomers (23; R = CN) and (23; R = CO₂Me) of 1-cyano- and 1-methoxycarbonyl-1,2-dihydrobenzocyclobutene. Only endo-adducts were isolated by trapping benzo[c]furan with cyclopentene, cyclopentadiene, and cycloheptene, but with norbornadiene as trap the exo-adduct predominated. An attractive interaction between an endo-alkyl group and the diene system will explain these observations.