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

A new and efficient approach toward the conversion of C-19-diterpenoid alkaloids into diterpenes with [6+8+5+6] ring system is reported. Treatment of imines 5, 14, and 24 derived from the C-19-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-Br-2. 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.