Formation of a reactive iminium derivative by enzymatic and chemical oxidations of 16-O-acetylvindoline

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Abstract
16-O-Acetylvindoline (1a) [a derivative of vindolene, a compound whose structure is found intact in vincristine and vinblastine (human antineoplastic agents)] was oxidatively transformed into an iminium derivative (2a) by copper oxidases (laccase and human ceruloplasmin), an unknown enzyme system(s) of Streptomyces griseus, and the chemical oxidizing agent 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). The iminium derivative (2a) was isolated from enzymatic and chemical oxidation mixtures and was identified by spectral and chemical techniques. Reduction of the iminium compound with sodium borodeuteride provided monodeuterated 16-O-acetylvindoline (1b) as the sole product. Mass spectral analysis indicated that the deuterium atom was introduced into position C-3 of the piperidine portion of the alkaloid structure. Location and stereochemistry of the deuterium atom were confirmed by high-field 1H and 2H NMR analyses of the deuterated product to be in the 2H.alpha. orientation. Hydrolysis of the 16-O-acetyl functional group from the iminium derivative (2a) resulted in the production of a previously identified dimer (5), which forms by intramolecular etherification through the reactive enamine (3). The iminium derivative (2a) reacts with cyanide to provide complex mixtures of products, one of which was identified by mass spectrometry as a cyanide addition product. Results confirm the existence of a reactive iminium intermediate formed by all of the biochemical and chemical systems examined.