Diels–Alder additions with exo-selectivity
Abstract
Butadiene, isoprene, cyclopentadiene, dimethyl maleate, norbornadiene, and cyclopentene add to the pyrone (1; R1= R2= Ph) to give mostly exo-adducts; by comparison with the exo–endo-ratios observed for addition to related systems this exo-selectivity is attributed to inhibition of secondary interactions by non-planar phenyl substituents.