Integrity of alkene stereochemistry in the Diels—Alder reaction

The Diels—Alder reaction of hexachlorocyclopentadiene and α-methylstyrene produces both exo-phenyl and endo-phenyl adducts, with the latter in preponderance. A deuterium label at the β position of the alkene provides a probe for determining whether the vicinal stereochemical relationship between groups on the alkene is retained in the adducts. From the alkene labeled 77.6±2.3% E and 22.4±2.3% Z was produced the major (endo-phenyl) isomer with 79.4±1.1% exo-d and 20.6±1.1% endo-d and the minor (exo-phenyl) isomer with 78.9±0.5% endo-d and 21.1±0.5% exo-d. Thus both adducts are produced with complete retention of alkene stereochemistry.