Electrochemical oxidation of aromatic ethers. Part 8. Evidence of homogeneous electron transfer during the ‘low potential’ oxidation of laudanosine

Photolysis of N-amino- and N-nitroso-derivatives of 1,2,3,4-tetrahydropapaverine fails to produce arylaryl coupled products analogous to O-methylflavinantine. Instead both the 1- and 2-substituents of the heterocyclic ring are cleaved to give 6,7-dimethoxy-3,4-dihydroisoquinolines and veratraldehyde, the latter being an oxidation product of the 3,4-dimethoxybenzyl radical and/or cation. From this evidence, and the results of an electrolysis experiment carried out at high dilution, it is considered that the anodic cyclisation of laudanosine to O-methylflavinantine proceeds through homogeneous electron transfer rather than via homoconjugative activation within the intermediate radical cation as proposed by other workers.