The compound of step 1 (1.555 g, 6.84 mmol) and the compound of example 1, step 2 (1.677 g, 6.84 mmol) were dissolved in 28.5 ml of acetone and the solution was heated to reflux overnight. The mixture was then evaporated to dryness under reduced pressure and the residue dissolved in 28.5 ml of isopropanol. Sodium carbonate (2.176 g, 20.53 mmol) was added and the mixture was heated at 105° C. with stirring for 2 h. The solids were filtered off and the filtrate was evaporated to dryness under reduced pressure. The residue was purified by chromatography on silica gel (cyclohexane/EA, 100:0, then 95:5) to give 0.497 g of the title compound as a yellow solid melting at 55° C.
