HARISH KUMAR, D.R., et al. “Synthesis and in Vitro Calcium Channel Blocking Activity of Symmetrical and Unsymmetrical Substituted 1,4-Dihydropyridine Derivatives”. Asian Journal of Chemistry, vol. 21, no. 6, Mar. 2010, pp. 4357-65, https://asianpubs.org/index.php/ajchem/article/view/18174.