Novel camphor-derived chiral auxiliaries: Significant solvent and additive effects on asymmetric reduction of chiral α-keto esters

Stereoselective reduction of various α-keto esters 5a-d derived from exo-10,10-diphenyl-2,10-camphanediol 4a and exo-10,10-diphenyl-10-methoxy-2- camphanol 4b is described. High to excellent diastereomeric excess (99% de) with good chemical yields is obtained. The sense of stereoselectivity as a function of C10 modification is remarkable. Further, in the case of 5d, both diastereomers of α-hydroxyl esters can be obtained with excellent optical purity by the appropriate choice of reaction conditions. The influence of the solvents and additives on the reaction course has been investigated.