350 mg (1.83 mmol, 3 equiv.) of toluenesulfonic acid monohydrate (recrystallised from CHCl3) are added to a solution of 467 mg of 21-deoxyconcanamycin A (0.55 mmol) in 55 ml of CH3CN/H2O (5:1) and the reaction mixture is stirred at 38° C. for 12 hours. For working-up, the reaction mixture is cooled to 0° C. and 50 ml of saturated NaHCO3 solution are added; the organic solvent is removed in vacuo and the aqueous phase is extracted with CHCl3. TLC monitoring on silica gel (eluant: EtOAc/hexane 1:1) revealed two new zones in addition to the starting material at the starting spot. Column chromatography on silica gel (gradient EtOAc/hexane, 1:1 EtOAc) yields 63 mg (22% of the conversion) of 16-demethyl-21-deoxyconcanolide A, 122 mg of 21-deoxyconcanolide A (42% of the conversion) and 105 mg of starting material.
