Reaction of 1,4-Naphthoquinones with Tertiary Amines

Abstract

Substituted naphthoquinones and their analogues, such as streptonigrin, are involved in various important biochemical processes. It has been suggested that they are beneficial in the prevention of tumors in several major organs. Thus, synthesis of complicated naphthoquinone structures by applying the substitution reactions starting with 1,4-naphthoquinones is one endeavor that could prove to have beneficial applications for the medical field in particular. The detailed mechanisms governing these substitution processes have not been well established. In order to disclose the mechanistic details, the reactions of 1,4-naphthoquinone, 2-methoxy-1,4-naphthoquinone, 2,3-dimethoxy-1,4-naphthoquinone, 2-halogeno-1,4-naphthoquinone, and 2,3-dihalogeno-1,4-naphthoquinone are studied here. These quinones are reacted with tertiary amines such as triethylamine and diethylmethylamine in solvents: methanol, ethanol, and trifluoroethanol. With methanol as the solvent, the quinone reacts with methanol to produce a common tri-substituted product, 4-hydroxy-2,2,3-trimethoxy-1,2-dihydronaphthalene-1-one, with a typical yield of 38 percent. In some systems, 2-methoxy-1,4-naphthoquinone was isolated. We believe 2-methoxy-1,4-naphthoquinone and 2,3-dimethoxy-1,4-naphthoquinone are the intermediates toward the formation of the tri-substituted product. By increasing the reaction time and decreasing the solute concentration, the tri-substituted product is made to be the major product. We also believe that reactions occurring in ethanol and trifluoroethanol afford mono- and tri-substituted products.