Enantioselective hydrogenation of a methoxypyrone with cinchona-modified palladium

Hydrogenation of 4-methoxy-6-methyl-2-pyrone 1 has been investigated over cinchona-modified Pd/TiO2. The appropriate start-up procedure including a catalyst reduction-oxidation-reduction cycle and a short pretreatment with the modifier in the absence of reactant can remarkably enhance the enantiomeric excess (ee) to the dihydropyrone 2. Another key parameter is the alkaloid/Pd, molar ratio; the alkaloid concentration in the slurry or the alkaloid/reactant ratio is not crucial. Under the best conditions 94% ee and 95% chemoselectivity to 2 were achieved at 80% conversion of 1, in only 30 min reaction time under ambient conditions. The ee can be further increased by kinetic resolution of 2. In the second reaction step the diasteroselectivity to the cis-tetrahydropyrone 3 is about 99%.