The Chemoselective Substitution Reaction of 2-Bromo-3-methoxy-1, 4-naphthoquinone
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The substitution of quinones has long been studied, partly because a quinone moiety is incorporated into various biologically important molecules of complex structure. As of yet, there has not been a procedure established for introducing a particular substituent at the 2- and/or the 3-positions of 1,4-naphthoquinones lacking symmetry. In order to develop a method for regioselective substitution at the 2- and/or the 3-positions of 1,4-naphthoquinones, the chemoselective substitution reactions of 2-bromo-3-methoxy-1, 4-naphthoquinones were further studied. In the reactions of 2-bromo-3-methoxy-1, 4-naphthoquinone with a variety of alkylamines, 2-alkylamino-3-bromo-1, 4-naphthoquinones resulted in high yields. The rate of substitution was dependent upon the nature and the size of the alkyl group of the amines. Primary amines reacted fast, whereas secondary amines afforded the substitution production rather slowly. Tertiary amines were found not to form any substitution product with amines, even at elevated temperatures. Alkylthiols, on the other hand, were found to form 2-alkylthio-3-methoxy-1, 4-naphthoquinones in their reactions with 2-bromo-3-methoxy-1, 4-naphthoquinone in the presence of an amine. Clearly, the mechanisms involved in each substitution reaction are different and thus the details governing the course of reactions remain to be studied.