Heterogeneous asymmetric reactions.: 23.: Enantioselective hydrogenation of ethyl pyruvate over cinchonine- and α-isocinchonine-modified platinum catalysts

The enantioselective hydrogenation of ethyl pyruvate to (S)-ethyl lactate over cinchonine- and alpha -isocinchonine-modified Pt/Al2O3 catalysts was studied as a function of modifier concentration and reaction temperature. The maximum enantioselectivities obtained under the applied mild conditions were 89% ee using cinchonine (0.014 mmol dm(-3), 1 bar H-2, 23 degreesC, 6% AcOH in toluene). and 76% ee in the case of alpha -isocinchonine (0.14 mmol dm(-3), 1 bar H-2, - 10 degreesC, 6% AcOH in toluene). Since alpha -isocinchonine of rigid structure exists only in "anti-open" conformation these data provide additional experimental evidence to support the former suggestion concerning the dominating role of "anti-open" conformation in these cinchona-modified enantioselective hydrogenations.