ANODIC FRAGMENTATION OF CATHARANTHINE AND COUPLING WITH VINDOLINE - FORMATION OF ANHYDROVINBLASTINE

The anodic oxidation of catharanthine (Cath) in the presence of vindol ine (Vind) performed in MeCN-Et(4)NClO(4) at a controlled potential yi elds (16'S)- and (16'R)-anhydrovinblastine (AVBL) in s 4.3:1 ratio. Th e favorable stereoselectivity found at room temperature is rationalize d by proposing the coupling of generated dication 1a(++) and Vind at t he electrode surface. The mechanism of the overall fragmentation/coupl ing reaction was studied using electrochemical techniques and the tech nique of homogeneous redox catalysis. It was concluded that the rate-d etermining step in the overall reaction is the first electron transfer leading to Cath(.+). MO calculations support the proposed mechanism a nd provide the evidence that C16-C21 bond fragmentation is a homolytic process.