Investigations into the Manzamine Alkaloid Biosynthetic Hypothesis.

The biomimetic synthesis of a pentacyclic alkaloid (keramaphidin B, 1), an intermediate in the biogenetic pathway to the manzamine alkaloids, has been achieved. Compound 1 was formed by an intramolecular Diels-Alder reaction of macrocycle 2 in buffer followed by reduction with NaBH4 . This reaction provides the first direct expeimental evidence for the authors' biosynthetic hypothesis.