Thatipally, Suresh, et al. “Pyridinium P-Toluenesulfonate: A Mild and Efficient Catalyst for The Regioselective Tetrahydropyranylation of Indazole Derivatives Under Solvent-Free Conditions”. Asian Journal of Chemistry, vol. 23, no. 1, Sept. 2010, pp. 451-4, https://asianpubs.org/index.php/ajchem/article/view/10059.