Anodic Cyclization-Rearrangement of Methoxybibenzyls to Dihydrophenanthrones

Anodic oxidations were performed in acetonitrile at platinum using controlled potentials. 2-Methyl-3′,4,4′,5-tetramethoxybibenzyl was converted in 98 % yield to 9,10-dihydro-10a-methyl-2,6,7-trimethoxy-3(10aH)-phenanthrone. 2-Methyl-2′,4,5′,5-tetramethoxybibenzyl produced 9,10-dihydro-10a-methyl-2,5,8-trimethoxy-3(10aH)-phenanthrone in 88% yield. 9,10-Dihydro-10a-methyl-2-ethoxy-6,7-dimethoxy-3(10aH)-phenanthrone was correspondingly formed from 2-methyl-3′,4′, 5-trimethoxy-4-ethoxybibenzyl. 2-Methyl-4,5-dimethoxybibenzyl did not cyclize under these conditions. A mechanism rationalizing these products as well as cyclic voltammetric data is proposed.