Steroid molecules Oppenauer oxidation

The most extensive application of the Oppenauer oxidation has been in the oxidation of steroid molecules. The most common aluminum catalysts are aluminum /-butoxide, i-propoxide, and phenoxide. While only catalytic amounts of the aluminum alkoxide are theoretically required, in practice at least 0.25 mole of alkoxide per mole of alcohol is used. Acetone and methyl ethyl ketone have proved valuable hydride acceptors due to their accessibility and ease of separation from the product, whereas other ketones such as cyclohexanone and p-benzoquinone are useful alternatives, due to their increased oxidation potentials.4 Although the reaction can be performed neat, an inert solvent to dilute the reaction mixture can reduce the extent of condensation, and, as such, benzene, toluene, and dioxane are commonly utilized. Oxidation of the substrate takes place at temperatures ranging from room temperature to reflux, with reaction times varying from fifteen minutes to twenty-four hours and yields ranging from 37% to 95%. 

The Oppenauer oxidation has also proved particularly useful in the partial oxidation of polyhydroxyl compounds and the order of oxidation appears to almost be the reverse of chromic anhydride.3 In many cases the selectivity is achieved by the unique steric environment of hydroxyl groups within the steroid molecules. Activation of one of the hydroxyl groups by unsaturation makes the differentiation even more facile.42... 

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