Cholesta-5,8-dien-3-ol 1, which is synthesised as described in the literature [J. Lip. Res. 37, 1529, (1996)], can be oxidised in an Oppenauer reaction to give cholesta-4,8-dien-3-one 2 (scheme 1). In this reaction, the sterol is treated with a ketone like acetone, quinone or cyclohexanone in the presence of aluminum isopropoxide or aluminum tert-butoxide [e.g. J. Chem. Soc. Perkin 12667 (1994)]. The sterol can also be oxidised with pyridinium dichromate [vide Synth. Commun. 20 (1990), 1167]. The same oxidation reaction can be carried out with cholesta-5,8(14)-dien-3β-ol, which is also synthesised as described in the literature [J. Lip. Res. 37 (1996), 1529,] to give cholesta-5,8(14)-dien-3-one. For this ketone, a laborious synthesis is described in the literature [Bull. Soc. Chim. Fr. 2037, (1971)]. Cholesta-4,7-dien-3-one is synthesised according to literature procedures [Liebigs Ann. Chem. 542 (1939), 218,] (in scheme 1, the series with the Δ8-double bond are shown as an example, analogous reactions have to be performed in the Δ7- and Δ8(14)-series).
