Kunkuma, Vijayalakshmi, et al. “An Efficient Synthesis of Quinoxalines Catalyzed by Monoammonium Salt of 12-Tungstophosphoric Acid”. European Journal of Chemistry, vol. 2, no. 4, Dec. 2011, pp. 495-8, doi:10.5155/eurjchem.2.4.495-498.413.