Phenyl 4-(((2,2-dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)methyl)amino)-2-hydroxybenzoate (11.5 g, 0.030 mmol), benzyl bromide (5.64 g, 0.033 mmol) and potassium carbonate (4.56 g, 0.033 mmol) were stirred in dimethylformamide (45 ml) at 80° C. for 3 hours. The reaction solution was distributed between a diethyl ether-tetrahydrofuran mixed solvent and water, the organic layer was washed with water and saturated brine and dried over anhydrous magnesium sulfate, and the drying agent was filtered out and the filtrate distilled off under reduced pressure. The obtained crude product was suspended in ethanol, the suspension was diluted with hexane, and the crystals were filtered out, washed with hexane and blow-dried to obtain white crystals. The crystals were then heated and stirred for 1 hour in Dowtherm A (50 ml) at 200° C. After allowing the reaction solution to cool to room temperature, diethyl ether (25 ml) was added and the mixture was further stirred overnight. The precipitated crystals were filtered out, washed with diethyl ether and blow-dried to obtain the title compound (1.20 g, 3.23 mmol, 11%) as white crystals.
