Novel skeletal rearrangements of a diterpene derived from deltaline

Treatment of imines 5 and 7, both prepared from the C19-diterpenoid alkaloid deltaline (1), with NaNO2–HCl resulted in the formation of diterpene 6 with an eight-membered ring through a sequence of denitrogenation and enlargement of ring B. In addition, two unusual skeletal rearrangements of diterpene 6 were also observed, leading to two structurally novel products 8 and 10. The structures of compounds 8 and 10 were confirmed by their X-ray crystallographic analyses and 2D NMR data.