Woodward modification of the Prevost

The present procedure offers a convenient alternative to the Prevost reaction and the Woodward modification of the Prevost reaction in which silver carboxylates are used instead of thal-lium(I) carboxylates. Thallium(I) salts have the advantages of being generally stable crystalline solids that can be readily prepared in high yield by neutralization of the appropriate carboxylic acid with thallium(I) ethoxide. Silver salts, on the other hand, are frequently unstable and difficult to dry. Thallium and its compounds are, however, extremely toxic, and great care must therefore be taken in the use and disposal of thallium salts. ... 

The Prevost Reaction allows the synthesis of anti-diols from alkenes by the addition of iodine followed by nucleophilic displacement with benzoate in the absence of water. Hydrolysis of the intermediate diester gives the desired diol.The Woodward Modification of the Prevost Reaction gives syn-diols. 

Hydrolysis of the ester does not change the configuration. The Woodward modification of the Prevost reaction is similar, but results in overall syn hydroxylation. The alkene is treated with iodine and silver acetate in a 1 1 molar ratio in acetic acid containing water. Here again, the initial product is a (3-halo ester the addition is anti and a nucleophilic replacement of the iodine occurs. However, in the presence of water, neighboring-group participation is prevented or greatly decreased by solvation of the ester function, and the mechanism is the normal Sn2 process, ... 

The desired diol can be isolated after hydrolysis. Woodward noted, that his modification of the Prevost reaction offers the opposite facial selectivity as compared to oxidations with 0s04 in the hydroxylation of synthetic steroid intermediates. Here, the steric approach factors first direct the stereochemistry of the iodination, which is followed by hydroxylation from the opposite face, whereas 0s04 leads to the isomeric cw-diol by direct attack from the most accessible face. 

The Woodward-Brutcher modification of the Prevost reaction was used by P.T. Lansbury to install the cis vicinal diol moiety of (+)-2,3-dihydrofastigilin The cis vicinal diacetate was formed in high yield and with good diastereoselectivity (5 1) when the reaction was conducted in wet acetic acid. 

Despite the dominance of dihydroxylation reactions employing Os(VlIl) for the production of vicinal diols, alternatives have been utilized in asymmetric syntheses. The best known of these is perhaps Woodward s modification [205] of the Prevost reaction [206]. This classic oxidation is known to generally yield anti-1,2-diacetates from olefinic substrates [207]. Woodward s key modification involves the inclusion of water in the reaction mixture. The intermediate iodo-acetate 303 undergoes hydrolytic cleavage... 

The use of expensive silver salts, the requirement for a stoichiometric amount of molecular halogen, and the formation of a relatively large amount of organic and inorganic wastes are definite drawbacks to this reaction. Sudalai recently reported on catalytic versions of both the Prevost Reaction and the Woodward-Modification. 

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