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

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