To a solution of 7-benzyloxy-3-(4-chlorophenyl)-2-isopropyl-4-oxo-4H-chromene-8-carbaldehyde (8.03 g, 18.6 mmol) in CH2Cl2 (200 ml) is added m-chloroperbenzoic acid (9.24 g, 53.5 mmol). The reaction mixture is stirred at 50° C. for 4 h, washed with saturated NaHCO3 solution, dried (MgSO4) and concentrated in vacuo to give a yellow oil. To a solution of the oil in methanol (350 ml) is added aqueous KOH solution (10%, 35 ml), and the mixture is stirred at room temperature overnight. The mixture is concentrated to a ca. 50 ml volume, ice/water is added, and the mixture is acidified with concentrated HCl. A solid is isolated by filtration, washed with water and taken up into CH2Cl2. The CH2Cl2 solution is dried (MgSO4), and the solvent is removed in vacuo to afford a dark brown solid. This is stirred in hot hexane/ethyl acetate and filtered to afford the title compound as a colourless solid. A further crop of the desired product can be isolated from the filtrate by concentrating the hot solution to a quarter of its original volume and allowing it to stand at room temperature.
