Rearrangements and biogenetic relationships of silphinane sesquiterpenes

Acid-catalyzed rearrangement of presilphiperfolanol to silphiperfol-6-ene and $\alpha$-terrecyclene formed by way of a silphinyl ion intermediate, suggests that the presilphiperfolanyl carbocation could be an important intermediate and a branch point in the biogenesis of triquinane sesquiterpenes such as silphiperfol-6-ene, silphinene, and related compounds.The absolute configuration of the tricyclic sesquiterpene alcohol ($-$)-presilphiperfolanol, structural precursor for the angular and propellane triquinane sesquiterpenes, was established by correlation with ($-$)-silphiperfol-6-ene. The relative stereochemistry of this alcohol was previously determined in this laboratory by an X-ray crystallographic analysis of its p-nitrobenzoate derivative.Solvolytic rearrangement both silphin-1$\alpha$-yl and 1$\beta$-yl methanesulfonates gave not only the previously reported tricyclic bridgehead alcohol and $\alpha$-terrecyclene by Wagner-Meerwein rearrangement products, but also isocomene and modhephene sesquiterpene natural products as minor rearrangement products arising by an initial competing 1,3-hydride shift. Exclusive rearrangement of the interannular bond occurred, and no products arising from migration of the peripheral cyclopentyl bond were detected. The large (ca. 10$\sp3$) rate enhancement for solvolysis of silphin-1$\alpha$-yl methanesulfonate over its epimer is consistent with C-7,C-8 bond participation in the transition state for the 1$\alpha$-isomer. $\alpha$-Terrecyclene has the same (4.3.2.0$\sp{1,5}$) undecane carbon skeleton ring system as the antibiotic quadrone and terrecyclic acid natural products. Thus, it is possible that silphinyl carbocation is another branch point in the biogenesis of sesquiterpenes such as isocomene, modhephene, quadrone and related compounds.U of I OnlyETDs are only available to UIUC Users without author permissio