ASYMMETRIC-SYNTHESIS OF ALLYLIC SULFONES - USEFUL ASYMMETRIC BUILDING-BLOCKS

Construction of sulfones in enantiomerically pure form provides a great opportunity to enhance their value as synthetic building blocks. Allylic sulfones, in particular, have great flexibility derived from sulfone-controlled additions to the double bond. Two strategies have been developed based upon the ability to effect asymmetric allylic alkylations with palladium employing ligands derived from c(2) symmetric diamines and 2-(diphenylphosphino)benzoic acid. Desymmetrization of meso-2-ene-1,4-diol diesters does not involve the nucleophile in the enantiodiscriminating step and thus should, a priori, not depend upon the nature of the nucleophile. Indeed, such desymmetrization of such a diester in the presence of a sulfinate anion gave excellent enantioselectivity. On the other hand, conversion of both enantiomeric allylic esters to enantiomerically pure allylic sulfones requires sodium benzenesulfinate to participate in the enantiodiscriminating step. Five-, six-, and seven-membered substrates all gave excellent enantioselectivities. A catalytic phase transfer system proved most efficacious on larger scales. propagating the asymmetry requires diastereoselective functionalization of the double bond. While epoxidation proved excellent for the five-membered ring case and satisfactory for the six-membered ring case, it was unsatisfactory in the seven-membered ring case. Osmium tetroxide-catalyzed cis-dihydroxylation gave excellent diastereoselectivities in the six- and seven-membered ring cases. Reductive cleavages produced enantiomerically pure allylic alcohols. Base-catalyzed elimination generated enantiomerically pure gamma-hydroxy-alpha,beta-unsaturated sulfones from which further stereogenic centers were produced by diastereoselective conjugate additions. Notably, an asymmetric cyclopentenone annulation using palladium-catalyzed cycloadditions now derives from racemic allyl alcohols.