A solution of the product of step (j) (2.0 g, 4.4 mmol), dibenzyl diisopropylphosphoramidite (2.28 g, 6.6 mmol), tetrazole (0.92 g, 13.2 mmol) and 4-dimethylaminopyridine (50 mg) in dichloromethane (30 ml) was heated under reflux for 13 hours. The reaction mixture was cooled (0° C.) and m-chloroperbenzoic acid (1.52 g, 8.8 mmol) added. The solution was stirred at 0° C. for a further hour then allowed to warm to room temperature. The reaction mixture was washed with aqueous sodium sulphite solution (10%, 30 ml), saturated sodium bicarbonate solution (30 ml) and brine (30 ml). The organic layer was dried (Na2SO4) and solvent was evaporated in vacuo to give an oil. Purification by column chromatography (silica gel, 45 g, eluting with toluene to 3.5% diethylamine in toluene gradient) gave the required product as a colourless oil (0.8 g, 27%), m/z 671 (M++1). 1H N.M.R. (CDCl3) δ=1.3 (d, 3H); 3.8 (s, 3H); 3.85 (q, 1H); 4.8 (m, 2H); 4.9 (m, 2H), 5.2 (s, 2H); 6.25 (s, 1H); 6.6 (m, 1H); 6.8 (m, 1H); 7.05 (m, 1H); 7.15 (m, 2H); 7.2-7.35 (m, 12H); 7.5 (s,1H); 7.8 (s,1H); 8.23 (s,1H).
