Juan Yang, Li, et al. “Synthesis of N-Acyl Carbamates and Oxazolidinones Using HClO4-SiO2 As Catalyst Under Solvent-Free Conditions”. Asian Journal of Chemistry, vol. 23, no. 3, Nov. 2010, pp. 1085-7, https://asianpubs.org/index.php/ajchem/article/view/16089.