Palcernuine, the first [5/6/6/6]-cernuane-type alkaloid from Palhinhaea cernua f. sikkimensis

One novel (named palcernuine, 1) and five known cernuane-type (2–6) alkaloids were isolated from the whole plant of Palhinhaea cernua f. sikkimensis. The structure of 1, possessing an unprecedented [5/6/6/6]-tetracyclic ring system containing two nitrogen atoms, was established on the basis of spectroscopic methods, and its absolute configuration was determined by comparison of the experimental and calculated electronic circular dichroism (ECD) spectra. A plausible biosynthetic pathway to 1 is proposed.