STEREOCONTROLLED OXYGENATION OF CAMPHOR DERIVATIVES AS A PRELUDE TO THE COMPLETE BETA-RING FUNCTIONALIZATION OF POTENTIAL PRECURSORS TO TAXOL AND STRUCTURAL ANALOGS THEREOF

The possibility of realizing the predominant exo a-hydroxylation of ca mphor-related ketones is described. Both the direct oxidation of enola te anions with 1-phenyl-N-(phenylsulfonyl)oxaziridine or exposure of t he derived silyl enol ethers to dimethyl dioxirane are conducive to ex o attack. Following conversion to MOM derivatives, the acidity of the alpha-carbonyl proton is reduced sufficiently to allow direct condensa tion of these hindered ketones with cyclohexenyllithium reagents. The subsequent anionic oxy-Cope rearrangement of the resulting exo carbino ls results in direct conversion to tricyclic products that feature the characteristic C-2,C-9,C-10 B-ring oxygenation pattern resident in ta xol. Further, the stereogenic centers are introduced in their proper a bsolute configuration. The steric demands of the [3,3] sigmatropic shi ft are discussed and the stereochemical features conducive to the mini mization of steric compression subjected to experimental verification.