Potential and limitations of palladium-cinchona catalyst for the enantioselective hydrogenation of a hydroxymethylpyrone

The palladium-catalyzed enantioselective hydrogenation of 4-hydroxy-6-methyl-2-pyrone afforded up to 85% excess to the (S)-enantiomer of the corresponding 5,6-dihydropyrone, under very mild conditions (1 bar, room temperature). This is the highest enantioselectivity achieved so far with chirally modified Pd, demonstrating the potential of this catalyst in the enantioselective hydrogenation of unsaturated compounds. A complicating feature of the reaction is the limited stability of cinchonidine under reaction conditions, which results in a decline of the initial enantiomeric excess (ee) with reaction time. Continuous feeding of a minute amount of cinchonidine during reaction allows maintenance of the high initial ee with an overall substrate/modifier molar ratio of ca. 20. (C) 2000 Academic Press.