Derivatives of 1,4-naphthoquinone play an important role in various biological processes through reactions with a nucleophile. For example, the anticancer activities in tyrosine phosphatase are due to the reactivity of 1,4-naphthoquinones with a nucleophilic cysteine residue in the active site. Here we study the reactivity of 2-alkylthio-3-methyl-1,4-naphthoquinones against amines, one of the important nucleophiles in the biological environment. We first synthesized 2-alkylthio-3-methyl-1,4-naphthoquinone from the reaction of 2-methyl-1,4-naphthoquinone epoxide with alkanethiol in the presence of N-methylimidazole, a weak base. Then we reacted 2-alkylthio-3-methyl-1,4-naphthoquinone with various alkylamines. The alkylthiols studied were methylthiol, ethylthiol, 1-propanethiol, and 2-propanethiol, and the alkylamines studied were methylamine, ethylamine, and isopropylamine. The product is 2-alkylamino-3-alkylthio-1,4-naphthoquinone through demethylation. In the presence of oxygen, 2-alkylthio-3-methyl-1,4-naphthoquinone epoxide also was efficiently formed. However, in the reactions under argon atmosphere, the formation of epoxide was completely excluded, and only 2-alkylamino-3-alkylthio-1,4-naphthoquinone is detected.