A synthetic approach to forskolin

Harrowven, David Charles (1988) A synthetic approach to forskolin. University of Southampton, Doctoral Thesis.

Abstract

A synthetic approach to the diterpene forskolin is described. Control of the C1 and C10 stereocentres is achieved by either tin or cobalt mediated, intramolecular, radical cyclisations. The differential reactivity of these, with cobalt additionally controlling the C5 stereochemistry, is discussed. A mechanism for the cobalt mediated radical cyclisation is proposed, involving cyclisation, reductive elimination and alkene isomerisation. Ethanolic rhodium thrichloride effected the isomerisation more efficiently; a rationalisation of this is presented. Closure of the B-ring is achieved by an intramolecular McMurry reaction. The use of α, ω-tricarbonyls and the relationship between structure and reaction pathway is discussed (cyclisation versus 1,2-reduction). Of additional interest is: a new method for preparing α-phenyl-selenoketones from vinyl acetates; the conversion of allylic alcohols to allylic ethers, with inversion, by treatment with ethanolic rhodium trichloride; and the stereoselective trisubstituted alkene formation in a tin mediated, intramolecular radical cyclisation of a β-alkoxy radical to a dienone. A review on forskolin is also presented. (DX86226)

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