Gentrymine B, an N-Quaternary Ancistrocladus Alkaloid: Stereoanalysis, Synthesis, and Biomimetic Formation from Gentrymine A

The total synthesis of the N-quaternary isoquinoline alkaloid gentrymine B (1) and of its unnatural enantiomer as well as its oxidative degradation is described. A further proof of stereostructure and hints at the biosynthetic origin of the unusual S,S-configuration in gentrymine B (1) were obtained by an additional biomimetic synthesis of 1 from the related—but 1R-configured—natural product gentrymine A (2), by N-methylation and subsequent spontaneous epimerization at C-1.