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A crucial step in the biosynthesis of the spermine alkaloid aphelandrine and its diastereoisomer orantine is an intramolecular cyclization of the intermediate (S)-dihydroxyverbacine. In order to elucidate this step of the biosynthetic pathway, microsomes from the roots of Aphelandra squarrosa Nees were incubated with unlabeled and (D8)-labeled (S)-dihydroxyverbacine. It was shown that the microsomal fraction catalyzes the intramolecular coupling of (S)-dihydroxyverbacine to aphelandrine. This was proven by microsomal transformation of (D8)-labeled (S)-dihydroxyverbacine to (D8)-labeled aphelandrine. The reaction absolutely requires NAPDH and O2. The underlying reaction mechanism is probably an oxidative phenol coupling catalyzed by an aphelandrine synthase. This enzyme is proposed to be a cytochrome P-450 oxidase. The intramolecular cyclization of (S)-dihydroxyverbacine represents an important point in the biogenesis of the aphelandrine-type alkaloids.

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The asymmetric synthesis of the unlabeled and [D8]‐labeled terminal precursors, 4 ((−)‐(S)‐dihydroxyverbacine) and 19, respectively, in the biogenesis of the spermine alkaloids aphelandrine (5) and orantine (6), respectively, is described. A partial synthesis of the alkaloid (−)‐(S)‐[(E)‐4‐methoxycinnamoyl]buchnerine (10) is also presented.