D. Liu, S. Zhou, J. Gao, and D. Xu, “Phosphomolybdic Acid Catalyzed Synthesis of 13-Aryl-5H-dibenzo[b,i]-xanthene-5,7,12,14(13H)-tetraones”, ajc, vol. 25, no. 14, pp. 7864–7866, Aug. 2013.