I. Tabakovic et al., ANODIC FRAGMENTATION OF CATHARANTHINE AND COUPLING WITH VINDOLINE - FORMATION OF ANHYDROVINBLASTINE, Journal of organic chemistry, 62(4), 1997, pp. 947-953
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.