Gyun Lee, Yong, et al. “Synthesis and Acid Catalytic Activity of 1,5,3,7-Diazadiphosphocine-1,5-Dicarboxylic Acids”. Asian Journal of Chemistry, vol. 27, no. 4, Feb. 2015, pp. 1481-3, doi:10.14233/ajchem.2015.18518.