K.K. EL-LOUH, ALI, et al. “Synthesis of Dihydropyridine, Alkylthiopyridine, Thienopyridine and Pyridothienopyrimidine Derivatives”. Asian Journal of Chemistry, vol. 19, no. 2, Mar. 2010, pp. 1283-92, https://asianpubs.org/index.php/ajchem/article/view/20244.