Chao, S. et al. 2011. Amberlyst-15 Catalyzed Synthesis of 12-Aryl-12H-benzo[i][1,3]-dioxolo[4,5-b]xanthene- 6,11-diones and 14-Aryl-14H-dibenzo[a,i]xanthene-8,13-diones under Solvent-Free Condition. Asian Journal of Chemistry. 23, 9 (Jun. 2011), 3865–3869.