Covalently Anchored Sulfonic Acid on Silica Gel as an Efficient and Reusable Heterogeneous Catalyst for the One-Pot Synthesis of Hantzsch 1,4-Dihydropyridines under Solvent-Free Conditions - Gupta, Raman; Gupta, Rajive; Paul, Satya; Loupy, Andre (Synthesis, 2007)