Condensation of hydroxy naphthazarins with aromatic aldehydes

Hydroxy naphthazarins (2,5,8-trihydroxy-1,4-naphthoquinones) react with aromatic aldehydes containing no hydroxy groups at position 2 under mild acid catalysis to form 3,3′-(arylmethylene)bisnaphthazarins. On the example of o-vanillin, it was shown that the reaction of 2-hydroxybenzaldehydes with hydroxy naphthazarins under the same conditions leads to 7,10-dihydroxy-12H-benzo[b]xanthene-6,11-dione derivatives.