Stereochemical π-Facial Selectivity of the Diels−Alder Reaction of Benz[a]aceanthrylene and 1,4-Diphenylbenz[a]aceanthrylene

The Diels−Alder (D−A) reaction of the twisted hydrocarbon 1,4-diphenylbenz[a]aceanthrylene (4) with dienophiles maleic anhydride, bromomaleic anhydride, and N-phenylmaleimide and with benzyne is reported. The stereochemistry of the products derived from the D−A reaction of 4 is compared with the products derived from reaction of planar benz[a]aceanthrylene (5) with maleic anhydride as a model. The endo regiochemical π-facial selectivity of 4 appears to be the result of the steric effect of a phenyl substituent as the transition state is reached. Compound 5 produces both endo and exo D−A adducts when treated with maleic anhydride. X-ray crystallographic analysis verifies the topology of the bromomaleic anhydride adduct of 4 and the maleic anhydride adduct of 5.