Anodic oxidation of kopsamine on platinum in acetonitrile results in cyanation or cyanomethylation, following ring closure, while oxidation on vitreous carbon in methanol yields a dimerization product with a C2 element of symmetry. This behaviour is reproduced in the aromatic congeners of kopsamine, kopsiflorine and 11-methoxykopsilongine. Oxidation of kopsingine on platinum in CH2Cl2–MeCN leads to a stable conjugated iminium salt which can be trapped by alcohol or water nucleophiles to give kopsidines A, B, and C. Kopsidines A and B can also be obtained directly from electrooxidation in methanol or ethanol. Oxidation of dihydrokopsingine in acetonitrile gives the 17-to-5 and 17-to-3 oxo-bridged compounds directly, while oxidation in methanol yields an additional product which is a caged kopsinitarine derivative. Oxidation of an aspidofractinine derivative without unsaturation in the piperidine ring and with a C(17)-αOH function gives a C2 symmetric dimerization product as well as a caged kopsinitarine-type product. Possible pathways leading to the various products are presented.
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Kam et al. (2001) studied this question.