Anodic fragmentation of catharanthine and coupling with vindoline. Formation of anhydrovinblastine

The anodic oxidation of catharanthine (Cath) in the presence of vindoline (Vind) performed in MeCN-Et(4)NClO(4) 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.