ORIGIN OF EPSILON-HYDROXY KETONES IN THE OSMIUM TETROXIDE-CATALYZED ASYMMETRIC DIHYDROXYLATION OF ALKENES

The origin and the mechanism of formation of alpha-hydroxy ketones in the osmium tetroxide-catalyzed asymmetric cis-dihydroxylation (ADH) of alkenes in the presence of tert-butyl hydroperoxide is described. The formation of alpha-hydroxy ketones has been established to proceed through either the hydration of monooxobisglycolate ester 2 followed by oxidation with tert-butyl hydroperoxide (TBHP) or by acid-catalyzed addition of TBHP on the intermediate bisglycolate ester 2. On the basis of the mechanistic insight, it has been possible to shut down the formation of alpha-hydroxy ketones and other side products in the ADH reaction, even when TBHP is used as an oxygen source. It is possible to prepare alpha-hydroxy ketones in good yields but the optical purity of ketols has been found to be very low, which not only shed significant light on the mechanism of their formation, but also delineated the improbability of synthesizing them in optically active forms by ADH reaction of alkenes.