The Tandem Radical Cyclization Approach to Angular Triquinanes : Model Studies and the Total Synthesis of (±)-Silphiperfolene and (±)-9-Episilphiperfolene

Abstract

Models studies and the total synthesis ofsilphiperfol-6-ene ( 4, seven steps) and 9-episilphiperfol-6-ene ( 14, five steps) are reported. The use of a tandem radical cyclization permits rapid construction of the requisite tricyclic nucleus. In the most direct approach, the key tandem cyclization ( 18 → 19) produces the C-9 epimer as the major product. In a modified approach, cyclization of the dioxolane ketal derived from 18 ( 27 → 28) produces a useful excess of the natural isomer. Interesting observations on the rapid prototropic isomerization of a vinyl silane to an allyl silane ( 21→ 22+23) are also discussed.