A Novel Ring-closure Reaction between 1,4-Dihydroxyanthraquinone and Diamines Promoted by Copper Ions

Abstract The reaction of 1,4-dihydroxyanthraquinone(DHAQ, 1) with various diamines were carried out in the presence of CuCl2. Primary 1,2-ethylenediamines or 1,2-cyclohexanediamine afforded the ring-closure products, 6-hydroxy-1,2,3,4-tetrahydronaphtho[2,3-f]quinoxaline-7,12-dione derivatives, in good yields of 70–98%. On the other hand, in the cases of N-alkylethylenediamines or 2-(alkylamino)ethanols, the major products were the 2-aminated 1,4-dihydroxyanthraquinones. The direct 2-amination of 1 was greatly inhibited when an alkyl group was introduced to the amino group because of its steric requirement. The role of copper ions, the effect of a steric requirement of the N-alkyl substituent of amines, and the effect of the chain length of alkanediamines in the reaction of 1 with amines were studied. It was proposed that the reaction was initiated by the direct 2-amination of 1 via the copper complex, followed by the intramolecular nucleophilic substitution of the 2-amino group on the 2-alkylamino substituent to give the ring-closure product.