Compound 4(10 g, 36.2 mmol) was dissolved in dry benzene (150 ml) mixed with ethylene glycol (25 ml, 280 mmol) and pyridinium p-toluenesulfonate (1 g, 3.9 mmol) and refluxed using a Dean-stark separator overnight till no starting material was detected by TLC. The cold solution was diluted with EtOAc (100 ml), washed twice with brine, dried on Na2SO4 and the solvents removed under reduced pressure. Flash-chromatography on silica gel with EtOAc: Hexane 4:6 as eluant to give 11.0 g (94.5% yield) the title compound as a white solid.
