Enantioselective Synthesis of (-)-Methoxyestrone

Enantioselective synthesis of unnatural (-)-methoxyestrone in 12 steps from the commercially available tetralone based on the formation of a chiral bicyclic intermediate having the A-B steroid ring framework was accomplished. The crucial synthetic step comprised the enantioselective conjugate addition of vinylmagnesium bromide to a chiral imine formed from a trisubstituted cyclic alpha,beta-unsaturated aldehyde with >98% ee, giving rise to the stereoselectively substituted building block possessing the A-B steroid rings. Further steps included the construction of the side chain containing the triple bond, and the obtained enyne was subjected to a Pauson-Khand reaction that furnished stereoselectively an intermediate tetracyclic ketone. Further functional group transformations yielded the target compound.