Sharpless bishydroxylation
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Sharpless bishydroxylation or asymmetric dihydroxylation (AD) is a chemical reaction of an alkene with osmium tetroxide to form an diol (dialcohol). Chiral products can be formed by adding quinine derivatives. A more efficient way to perform this reaction is reoxidise the osmate, which is formed in the reaction, with potassium ferricyanide. This reduces the amount of the highly toxic and very expensive osmium tetroxide needed by up to 95%. It is possible to buy these mixtures as AD-mix α and AD-mix β.
The chiral diols are important for further synthesis. The introduction of chirality (chemistry) into nonchiral reactants through small amounts of a chiral catalyst is an important concept in organic synthesis.
K. Barry Sharpless won the 2001 Nobel Prize in Chemistry.
Examples
Azasugar.gif
The first step in the synthesis of azasugar 6 is a dihydroxylation reaction [1] of the prochiral diene 1. Azasugars are pharmacologically relevant. The entire reaction sequence takes place in water as a reaction medium and protective groups were not required. Notice the chemoselectivity in the nucleophilic substitution of bromine by the hydroxyl group in step 2.
References
- [1] Efficient asymmetric synthesis of an azasugar in water Ulf M. Lindström, Rui Ding, Olle Hidestål Chemical Communications, 2005 p. 1773