Intramolecular Anodic Carbon−Carbon Bond Formation from Oxidized Phenol Intermediates. Effect of Oxygenated Substituents on the Yields of Spiro Dienones in Electrochemical and Iodobenzene Diacetate Oxidations

Abstract

The single-cell, constant-current anodic oxidation of a series of 4-(2-alkenylaryl)phenols was studied in which the aryl substituents were 2-vinyl-4,5-dimethoxy, 6a; 2-propenyl-4,5-dimethoxy, 9a; 2-vinyl-4,5-methylenedioxy, 6b; 2-propenyl-4,5-methylenedioxy, 9b; 2-vinyl-4-methoxy, 6c; 2-propenyl-4-methoxy, 9c; 2-vinyl-5-methoxy, 6d; and 2-propenyl-5-methoxy, 9d. Two compounds having methoxyl groups on the phenolic ring were also studied: 4-(2‘-propenylphenyl)-2-methoxyphenol, 9e, and 4-(2‘-propenylphenyl)-3-methoxyphenol, 9f. A novel synthetic route to the methoxy compounds 6c, 9c, 6d, and 9d was developed. This involved as a key step the MAD, bis(2,6-di-tert-butyl-4-methylphenoxide)methylaluminum, mediated addition of organolithium and Grignard reagents to functionalize quinol ether derivatives. Anodic oxidation of compounds 6a, 9a, 9b, 6c, 9c, and 9d gave good yields of spiro dienones arising from trapping of the phenoxonium ion by the alkenyl side chain followed by reaction of the resulting cation with methanol. The products from the electrochemical oxidation of these 4-(2‘-alkenylaryl)phenols were compared with those obtained from the iodobenzene diacetate oxidations. In general, the compounds that gave good yields of spiro dienones from electrochemical oxidation gave good yields from iodobenzene diacetate oxidations. However, there were two cases in which the results of the two oxidations did not parallel each other. Whereas anodic oxidation of 9f gave a low yield of spiro dienone, the iodobenzene diacetate route gave a 74% yield of this product. In the case of 9d, the opposite effect was noted: the electrochemical route gave a higher yield than did the iodobenzene diacetate oxidation.