Asymmetric Hydrogenation of Substituted 2-Pyrones

Various substituted 2-pyrones have been hydrogenated with high enantioselectivity (up to 97% ee) to the corresponding 5,6-dihydropyrones using cationic ruthenium catalysts containing the (6,6‘-dimethoxybiphenyl-2,2‘diyl)bis[3,5-di( tert -butyl)phenylphosphine] ligand. When substituents at position 3 are absent, 5,6-dihydropyrones are further hydrogenated to the fully saturated δ-lactones. In the case of 4,6-dimethyl-2 H -pyran-2-one, the diastereoselectivity of the second hydrogenation step was determined by the chirality of the applied catalyst, while for the 4,5,6-trimethyl-2 H -pyran-2-one a double asymmetric induction effect was observed. Other cyclic substrates with endo- or exocyclic double bonds were hydrogenated, although with substantially lower enantioselectivity with respect to the 2-pyrones.