Leurosine biotransformations: an unusual ring-fission reaction catalyzed by peroxidase.

The dimeric Vinca alkaloid leurosine undergoes an unusual fission of the piperidine ring of the Iboga substructure when reacted with horseradish peroxidase and hydrogen peroxide. A preparative-scale oxidation of leurosine provided 15'-hydroxycatharinine, which was identified by infrared and proton and carbon-13 NMR spectroscopies and high-resolution mass spectrometry. A proposed pathway for the formation of 15'-hydroxycatharinine involves radical and iminium intermediates and cleavages of a putative diol. The enzymatic transformation product is 3 orders of magnitude less active than leurosine or vinblastine in vitro, in inhibiting the polymerization of tubulin.