o-Quinonoid compounds. Part XI. exo-Selectivity in the Diels–Alder reactions of phenyl-substituted o-quinonoid dienes

Buta-1,3-diene, isoprene, cyclopenta-1,3-diene, dimethyl maleate, norbornadiene, norbornene, cis-but-2-ene, and cyclopentene add to 1,4-diphenyl-2-benzopyran-3-one (2; R1= R2= Ph) to give mostly exo-adducts; only for the addition of furan is the endo-adduct preferred. By comparison with the preferred endo-addition to 2-benzopyran-3-one, its 1-phenyl derivative (2; R1= Ph, R2= H), and 1,3-diphenylinden-2-one (15) the exo-selectivity in additions to (2; R1= R2= Ph) is attributed to inhibition of secondary interactions by non-coplanar phenyl substituents.