An Efficient Route to Benzothiophene Quinones, Quinone Bisketals, and Quinone Monoketals via Anodic Oxidation of Methoxylated Benzothiophenes. Studies Directed at the Synthesis of Functionalized Benzothiophene-4,7-quinones

Article abstract of DOI:10.1021/jo00171a031

A convenient route to 4-and 7-methoxybenzothiophenes has been developed in the course of electrochemical studies of these molecules.The anodic oxidations of 4-methoxy-, 5-bromo-4-methoxy-, 5-methyl-4-methoxy-, and 6-bromo-7-methoxybenzothiophene, 17a-d, respectively, in methanolic potassium hydroxide at a platinum anode have been investigated.The products obtained from the oxidation of these compounds were temperature dependent.This temperature dependence was most extensively studied for 17a wherein oxidation at or below room temperature, followed by workup, gave primarily 4,7-dimethoxybenzothiophene.However, oxidation of 17a in methanol at ca. 65 deg C gave primarily the quinone bisketal.In a similar manner oxidation of 17b,c gave the respective bisketals of benzothiophene-4,7-quinones in good yields.Extended acidic hydrolyses of these quinone bisketals afforded the corresponding quinones in 78-95 percent yield.Milder conditions for the hydrolyses afforded monoketals of benzothiophene-4,7-quinones, the products from 17b,c being formed with high regioselectivity.Other chemistry of these quinone bis- and monoketals, initially directed at a synthesis of Caldariellaquinone, is presented and discussed.