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Anodic Fragmentation of Catharanthine and Coupling with Vindoline. Formation of Anhydrovinblastine

1997; American Chemical Society; Volume: 62; Issue: 4 Linguagem: Inglês

10.1021/jo9621128

1520-6904

Ibro Tabaković, Esmir Gunić, Ivan O. Juranić,

Oxidative Organic Chemistry Reactions

The anodic oxidation of catharanthine (Cath) in the presence of vindoline (Vind) performed in MeCN−Et4NClO4 at a controlled potential yields (16'S)- and (16'R)-anhydrovinblastine (AVBL) in s 4.3:1 ratio. The favorable stereoselectivity found at room temperature is rationalized by proposing the coupling of generated dication 1a++ and Vind at the electrode surface. The mechanism of the overall fragmentation/coupling reaction was studied using electrochemical techniques and the technique of homogeneous redox catalysis. It was concluded that the rate-determining step in the overall reaction is the first electron transfer leading to Cath•+. MO calculations support the proposed mechanism and provide the evidence that C16−C21 bond fragmentation is a homolytic process.