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The hydrogenations of methyl 2-oxoeyclopentanecarboxylate (1), ethyl 2-oxocyelohexanecarboxylate (3), and 2-methylcyclohexanone (5) on unmodified Raney-Ni catalyst lead predominately to the formation of the cis-hydroxy diastereoisomers of 2, 4, and 6, respectively (Scheme 2). In the asymmetric hydrogenations on catalysts modified with chiral tartaric acid ((R, R )-C4H6O6/Raney-Ni and (R, R)-C4H6O6/NaBr/Raney-Ni), the predominance of the cis-isomer increases significantly. The hydrogenations of β-keto esters 1 and 3 proceed with an enantioselectivity of 10–15% on the modified catalysts, while the similar hydrogenation of 5 yields optically inactive 6. The (1S,2R)-enantiomers of the cis-isomers of 2 and 4 are formed in larger quantity, whereas the (lR,2R)-enantiomers of the corresponding trans-isomers predominate (Scheme 1). The enantioselective formation of trans-2 and trans-4 can be interpreted mainly in terms of the asymmetric hydrogenation of cyclic β-keto esters through the keto form, while that of the corresponding cis-hydroxy esters proceeds through the enol form. |
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