Criofolinine and Vernavosine, New Pentacyclic Indole Alkaloids Incorporating Pyrroloazepine and Pyridopyrimidine Moieties Derived from a Common Yohimbine Precursor

Two new indole alkaloids characterized
by previously unencountered
natural product skeletons, viz., criofolinine (1), incorporating
a pyrroloazepine motif within a pentacyclic ring system, and vernavosine
(2, isolated as its ethyl ether derivative 3, which on hydrolysis regenerated the putative precursor alkaloid 2), incorporating a pyridopyrimidine moiety embedded within
a pentacyclic carbon framework, were isolated from a Malayan Tabernaemontana species. The structures and absolute configuration
of these alkaloids were determined on the basis of NMR and MS analysis
and confirmed by X-ray diffraction analysis.