The Woodward Reaction allows the synthesis of syn-diols from alkenes by the addition of iodine followed by nucleophilic displacement with acetate in the presence of water. Hydrolysis of the intermediate ester gives the desired diol.
The Prévost Reaction gives anti-diols.
Mechanism of the Woodward Reaction
Similar to the Prévost Reaction, initial addition of iodine leads to a cyclic iodonium ion, that is opened through nucleophilic substitution by acetate anion:
A cyclic acetoxonium ion is then formed:
In contrast to the course of the Prévost Reaction, water appears to add readily as a nucleophile to the partially positive carbon atom of the intermediate. The cyclic orthoacetate is then cleaved to a monoacylated diol:
The desired diol can be isolated after hydrolysis.
Woodward noted (J. Am. Chem. Soc. 1958, 80, 209-211), that his modification of the Prévost reaction offers the opposite facial selectivity as compared to oxidations with OsO4 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 OsO4 leads to the isomeric cis-diol by direct attack from the most accessible face.
In a recent modification described by Sudalai (see also the Prévost Reaction), NaIO4 acts both as the stoichiometric oxidant and as a source of water: