Enantioselective Conversion of Achiral Cyclohexadienones to Chiral Cyclohexenones by Desymmetrization

The enantioselective reduction of
prochiral 4,4-disubstituted 2,5-cyclohexa­dienones
to chiral 2-cyclohexenones has been accomplished by the use of a carefully
selected chiral bisphosphine–CuI complex and diisobutyl­aluminum
hydride–hexa­methyl­phosphoric triamide complex.
This reagent has provided access to a key bicyclic intermediate for
the total synthesis of the natural enantiomer of the pentacyclic sesterterpene
retigeranic acid that involves spatial discrimination between CH3 and CH2CH2R substituents, an operation
that has been elusive previously. In addition, a second method for
desymmetrization is described using catalytic enantioselective [4
+ 2]-cycloaddition of cyclopentadiene to prochiral 4,4-disubstituted
2,5-cyclohexa­dienones.