Alkenes dihydroxylation, stereoselectivity

It was of obvious interest to prepare the inhibitors 60 as their pure dia-stereoisomers, 66 and 67. Following on from our successful treatment of alkenyl D-glucosides under Sharpless asymmetric dihydroxylation conditions [21], we treated the alkenes 64 with the a-AD - and AD -mLxes - the results are summarized in Table 2. In no case did we ever obtain a satisfactory diastereo-isomeric excess of the diol 68 over the diol 69, or vice versa. A similar lack of stereoselectivity was also obtained with the triol 70 and the amine 71 [48]. 

In addition to the 1,3-ally lie strain concept, Houk has employed a model for 7t-facial stereoselection of electrophilic additions to chiral alkenes, such as hydroboration, epoxidation, and dihydroxylation, with similar predictive success. ... 

In the analysis of multiple stereochemical influences, if is useful fo classify the stereoselectivity as substrate (reactant) controlled or reagent controlled. For example, in the dihydroxylation of the chiral alkene 5, the product is determined primarily by the choice of hydroxylation catalyst, although there is some improvement in the diastereoselectivity with one pair. This is a case of reagent-controlled sfereoselecfion. 

In their approach, two key issues were addressed (i) stereoselective P-glycosidation in d,d- and L,D-heptoses and (ii) efficient generation of the 6-deoxy-P-Hepp unit with full regiocontrol at the 6-position and stereocontrol at the anomeric position. For the methodology study, various d,d- and L,D-heptoses were constructed mainly through the dihydroxylation of C6-C7 alkenes derived from D-mannose. This series of molecules allowed the study of the influence of... 

Transformation of an alkenylsilane to a 1,2-dihydroxyalkylsilane, followed by the Peterson reaction after conversion of its a-hydroxy group to methoxy group, provides a useful method for the stereoselective synthesis of an alkene. 1-Methoxy-fraus-cyclooctene 201 is prepared by syn-elimination from )S-hydroxyalkylsilane 200, which is derived from syn-dihydroxylation of 1-trimethylsilyl-cis-cyclooctene, although the product 201 is thermally unstable and easily isomerizes to the cis-cyclooctene (Scheme 2.127, for NMO, see Scheme 2.27) [356]. 

Fluorinated alcohols catalyse the dihydroxylation of alkenes with cyclopropyl mal-onoyl peroxide. Addition of perfluoro t-butyl alcohol to a toluene solution of alkene and the peroxide increases the rate of product formation and the stereoselectivity. The reaction is postulated to proceed via a dioxonium species (Scheme 19). ... 

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