Chemistry of Aminochromes. Part XVIII. The Effects of pH and Substituents on some Aminochrome–Thiol Reactions

Abstract

Aminochromes react with thiols in the pH range ca. 3–4 to give mainly 4-thiosubstituted-5,6-dihydroxyindoles (or the corresponding 5,6-dihydroxyindolines) along with reduction products (5,6-dihydroxyindoles or 5,6-dihydroxyindolines). There is also some evidence, in this pH range, for the formation of an addition product involving one of the aminochrome carbonyl groups directly. Under somewhat less acidic conditions (pH 5–6), however, the major products formed initially are, in general, 3a-thiosubstituted-3a, 4-dihydroaminochromes. These products, which are formed reversibly, are gradually replaced in the reaction mixture by the indolic products, which are formed irreversibly. The increased rate of formation of the 4-thiosubstituted-5,6-dihydroxyindoles at lower pH's is probably due to the increased protonation of the C-6 carbonyl oxygen. The 3a-thiosubstituted-3a, 4-dihydroaminochromes may be formed more readily at higher pH's due to the increased ionization of the thiol.