Stereoselective Construction of Halogenated Quaternary Stereogenic Centers via Catalytic Asymmetric Diels-Alder Reaction

Highly enantioselective Diels-Alder reactions of alpha-halo-alpha,beta-unsaturated ketones with Lewis acid-activated chiral oxazaborolidine 1 are described. The reaction with alpha-fluoroenones provided the corresponding cyclohexane derivatives having a fluorinated quaternary stereogenic center with up to 99% de and 94% ee. The reaction with alpha-bromo cyclic enones provided the corresponding bromo bicyclic adducts with up to 99% de and 95% ee. A brominated cis-fused bicyclic adduct derived from 2-bromocyclopenten-1-one and Dane's diene was converted to the trans-fused bicyclic system via reductive alkylation with the bulky aluminum reagent aluminum tris(2,6-diphenylphenoxide) (ATPH). With this process, formal syntheses of (+)-estrone and norgestrel have been demonstrated.