2-Mesitylenesulfonyl chloride (7.2 g) was added to a suspension of diisopropylethylamine (5.5 mL) and the product from step (iii) (6.1 g) in THF (200 mL) and the mixture was stirred under reflux for 12 h. The resulting mixture was diluted with water and extracted with EtOAc. The combined organic layer was washed with brine, dried and concentrated under reduced pressure. The residue was purified by flash column chromatography on amino silica gel to afford the sub-title compound (8.6 g) as a white solid; 1H NMR (300 MHz, CDCl3); 6.92 (2H, s), 6.86 (1H, s), 6.77 (2H, s), 4.67 (2H, br s), 4.64 (2H, d), 3.82 (3H, s), 3.81 (2H, s), 2.57 (6H, s), 2.29 (3H, s), 2.21 (3H, s), 1.81 (1H, t).
