Enantioselective hydrogenation of seven different esters of phenylglyoxylic acid was investigated by variation of the bulkiness at ester side of the substrates [Ph-C(O)-C(O)OR, R = Me (1), cyclohexyl (2), adamantyl (3), cis-decahydro-1-naphthyl (4), phenyl (5), 1-naphthyl (6), 2-naphthyl (7)] on Pt-aluntina-cinchonidine (CD) and Pt-alumina-beta-isocinchonine (beta-ICN) chiral catalysts using mild experimental conditions (273 K and room temperature, hydrogen pressure of 1-25 bar) in solvents of AcOH and toluene. Hydrogenation on Pt-alumina-CD chiral catalyst produced high values of ee for all but the 5 and 7 compounds. The formation of (R)-mandelic acid esters was 86-91% in AcOH and 79-94% in toluene. The experiments verified the general applicability of the Orito reaction in the preparation of enantionters of alpha-hydroxyesters. Under the experimental conditions applied, the magnitude of ee is affected by the steric size of alpha-ketoesters and by solvents. In the enantioselective hydrogenation of substrates over a Pt-alumina-beta-ICN catalyst in toluene, inversion of enantioselectivity occurred; (R)-mandelic acid esters formed with a medium ee of 30-54%. The conformation of the adsorbed modifier plays a more determining role than the bulkiness of the substrate in the formation of the intermediate complex responsible for enantioselection. (c) 2006 Elsevier Inc. All rights reserved.