Kawade, Deepak S., et al. “Tannic Acid an Efficient Catalyst for the Synthesis of 12-Aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-One Derivatives”. Orbital: The Electronic Journal of Chemistry, vol. 7, no. 2, June 2015, pp. 155-9, https://periodicos.ufms.br/index.php/orbital/article/view/17884.