A Pyridine Dearomatization Approach to the Matrine-type Lupin Alkaloids

(+)-Matrine and (+)-isomatrine are tetracyclic alkaloids isolated from the plant Sophora flavescens, the roots of which are used in traditional Chinese medicine. Biosynthetically, these alkaloids are proposed to derive from three molecules of (–)-lysine via the intermediacy of the unstable cyclic imine Δ1-piperidine. Inspired by the biosynthesis, a new dearomative annulation reaction has been developed that leverages pyridine as a stable surrogate for Δ1-piperidine. In this key transformation, two molecules of pyridine are joined with a molecule of glutaryl chloride to give the complete tetracyclic framework of the matrine alkaloids in a single step. Using this dearomative annulation, (+)-isomatrine is synthesized in four steps from inexpensive commercially available chemicals. (+)-Isomatrine then serves as the precursor to additional lupin alkaloids, including (+)-matrine, (+)-allomatrine, (+)-isosophoridine, and (–)-sophoridine.