Stereodivergent Synthesis of Optically Active α-Hydroxy Acids via Diastereoselective Reduction of α-Keto Esters Derived from l -Quebrachitol

Reduction of an α-keto ester derived from chiral cyclitol, readily available from l-quebrachitol, with K-Selectride proceeded via re-face attack of the ketone with high diastereoselectivity to obtain the α-hydroxy ester in 92% de. On the other hand, reduction of the α-keto ester with K-Selectride in the presence of 18-Crown-6 took place via si-face attack of the ketone to furnish the corresponding α-hydroxy ester in 92% de. Thus both enantiomers of mandelic acid were obtained in optically pure form.