Stereoselective hydroxylation reactions dihydroxylation

Asymmetric epoxidation, dihydroxylation, aminohydroxylation, and aziridination reactions have been reviewed.62 The use of the Sharpless asymmetric epoxidation method for the desymmetrization of mesa compounds has been reviewed.63 The conformational flexibility of nine-membered ring allylic alcohols results in transepoxide stereochemistry from syn epoxidation using VO(acac)2-hydroperoxide systems in which the hydroxyl group still controls the facial stereoselectivity.64 The stereoselectivity of side-chain epoxidation of a series of 22-hydroxy-A23-sterols with C(19) side-chains incorporating allylic alcohols has been investigated, using m-CPBA or /-BuOOH in the presence of VO(acac)2 or Mo(CO)6-65 The erythro-threo distributions of the products were determined and the effect of substituents on the three positions of the double bond (gem to the OH or cis or trans at the remote carbon) partially rationalized by molecular modelling. 

Optically active 1,2-diol units are often observed in nature as carbohydrates, macrolides or polyethers, etc. Several excellent asymmetric dihydroxylation reactions of olefins using osmium tetroxide with chiral ligands have been developed to give the optically active 1,2-diol units with high enantioselectivities. However, there still remain some problems, for example, preparation of the optically active anti-1,2-diols and so on. The asymmetric aldol reaction of an enol silyl ether derived from a-benzyloxy thioester with aldehydes was developed in order to introduce two hydroxyl groups simultaneously with stereoselective carbon-carbon bond formation by using the chiral tin(II) Lewis acid. For example, various optically active anti-a,p-dihydroxy thioester derivatives are obtained in good yields with excellent diastereo-... 

Next, dihydroxylation of homoaUyl alcohol 2.201 was examined for the installation of the stereoselective epoxide (Scheme 2.42). The hydroxyl in 2.201 was protected as the methyl ether 2.202, which was subsequently subjected to Sharpless asymmetric dihydroxylation reaction [149] conditions to afford 2.203, however, no diastereoselectivity was observed and the reaction resulted in the oxidation of the dithiane group under such oxidation reaction conditions. 

One of the foremost methods for oxidation of olefins is stereospecific syn-di-hydroxylation by treatment with OSO4. Because of the toxicity and cost of osmium, however, the need for methods that would employ substoichiometric quantities of this reagent was identified early on [180], A number of stoichiometric oxidants for catalytic dihydroxylations were examined, including metal chlorates, HjOa, NMO [181], f-BuOOH [182], and KjFeiCN) [40, 183], The mechanism of 0s04-catalyzed dihydroxylation reactions has been the subject of much debate details of these mechanistic considerations are beyond the focus of this book and are amply discussed elsewhere [40-42,47,49,184-186]. A number of interesting stereoselectivity trends of general importance in substrate-controlled diastereoselective dihydroxylations [40-43] and catalytic asymmetric dihydroxylations are discussed below (Section 9.8) [42, 44-49]. 

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