Asymmetric synthesis of a taxol C-ring by aldol condensation and radical cyclization

In the context of a convergent approach to taxol, the asymmetric synthesis of a fully functionalized C-ring is described. Two asymmetric aldol reactions (Evans) are used to create the C5 and C7 stereocenters and the diastereoselective alkylation of a stereochemically and conformationally biased dioxanone to introduce, with the correct relative and absolute stereochemistry, the quaternary center at C8. Vinyl radical cyclization is used to close the C2,3 bond in good yield. This diastereoselective radical cyclization likely proceeds through a chair-like transition state with the maximum number of substituents equatorially disposed. (C) 1997 Elsevier Science Ltd.