Stereochemical aspects of the enantioselective hydrogenation of 2-pyrones

The highly enantioselective (86-98% ee) hydrogenation of 2-pyrones, using cationic ruthenium catalysts containing the (6,6'-dimethoxybiphenyl-2,2'-diyl)bis[3,5-di(tert-butyl)phenylphosphine] ligand, takes place either regioselectively at the 5,6 position or involves both double bonds in the ring, depending on the substitution pattern. Both steps show a syn stereochemistry. The diastereoselectivity of the double hydrogenation is high (cis : trans greater than or equal to 90: 10). The second hydrogenation step shows kinetic resolution; however, when either enantiomeric catalyst was used essentially no double asymmetric induction effect was observed. The diastereoselectivity can be substantially reversed (cis : trans approximate to 12 : 88) when the alternative enantiomer of the diphosphine ligand is used for the second step.