A Novel Conversion of C₁₉-Diterpenoid Alkaloids into Aconane-Type Diterpenes with Eight-Membered Ring System via Skeletal Rearrangement of Corresponding Diazonium Derivatives

Abstract

A new and efficient approach toward the conversion of C₁₉-diterpenoid alkaloids into diterpenes with [6+8+5+6] ring system is reported. Treatment of imines 5, 14, and 24 derived from the C₁₉-diterpenoid alkaloids with NaNO₂–NaOAc–HOAc afforded a series of novel rearrangement diterpenes 6—8, 15—19, and 25—27, respectively. The lactone 11 was obtained in 41% yield by treating 5 with NaNO₂–HBr–Br₂. The formation of diazonium intermediate is postulated, which was subsequently subjected to skeletal rearrangement, leading to the enlargement of B ring. All the new compounds were isolated and fully characterized.