S. S. Shafqat, A. A. Khan, M. A. Khan, S. F. Salleh, M. S. Jamaludin, and P. S. Cem, “Green Synthesis and Characterization of 3-Carboxycoumarin and Ethylcoumarin-3-carboxylate via Knoevenagel Condensation”, ajc, vol. 29, no. 2, pp. 261–266, Nov. 2016.