Research Article

p-Sulfonic Acid Calix[4]arene as an Efficient Catalyst for One-Pot Synthesis of Pharmaceutically Significant Coumarin Derivatives under Solvent-Free Condition

Figure 1

1H NMR spectra of calix[4]arene sulfonic acid in (a) DMSO-d6 and (b) D2O.
(a)
(b)