Direct amination of naphthopurpurin and mompain, sea urchin pigments, and their O-methyl ethers by the reaction with ammonia

The regiochemistry of direct amination of 2-hydroxynaphthazarin (naphthopurpurin) and 2,7-dihydroxynaphthazarin (mompain), sea urchin pigments, and their O-methyl ethers by the action of ammonia solutions in H2O and MeOH under mild conditions was studied. The predominant direction of the reactions of all substrates with NH3 was the 1,2-addition of the nucleophile to the C-O group at position 1 of the quinoid ring with subsequent elimination of H2O from the alpha-hydroxyamine intermediate. The resulting quinonimines further underwent conversion to more stable tautomers with the NH2 group. All substrates, except naphthopurpurin, can add two NH3 molecules to form different products. The mechanisms of the reactions of all substrates with NH3 and the problems of prototropic tautomerism of the starting substrates and the reaction products were discussed.