Asymmetric syntheses of&-hydroxy carboxylic acids have gathered much interest for a long time, and a large number of reports have been made on the Grignard reaction and the reduction of&-keto esters. 1 As for the asymmetric reduction of&-keto esters, catalytic hydrogenation and metal hydride reduction of chiral esters of a-keto acids have been extensively studied. 1 However, relatively little is known about the asymmetric reduction of the d-keto esters by chiral reducing agents , and only the reductions of arylglyoxylic acid and its esters by the use of chiral magnesium alkoxides 2 and lithium aluminum hydride -chiral alcohol complexes 3 were reported. The optical yield attained by chiral magnesium alkoxides was reported to be 15-33%,2 and that obtained by lithium aluminum hydridechiral alcohol complexes was reported to be 4-17%.3
No catalytic asymmetric reduction of d-keto esters has been performed, however.
We wish to describe in this communication the first and effective asymmetric reduction ofO(-keto esters as an application of our work on the reduction of carbony and imino 5 functionalities using hydrosilylation catalyzed by a rhodium(I) complex with phosphine ligands.
We chose propyl pyruvate and ethyl phenylglyoxylate as a substrate and (+)_ EMPP6 and (+)-DIOP6 as chiral phosphine ligands for the rhodium(I) complex.
The asynnnetric reduction of propyl pyruvate is typically described. To a solu-