To a solution of compound 3 (1.00 g, 2.11 mmol) in dry acetonitrile (65 mL) were added 4-(dimethylamino)pyridine (0.77 g, 6.32 mmol) and phenoxythiocarbonyl chloride (0.44 mL, 3.16 mmol). The mixture was stirred at room temperature for 2 h. After concentration, the residue was dissolved in dichloromethane (50 mL) and washed successively with water (2×30 mL), aqueous solution of hydrochloric acid 0.5 N (30 mL) and water (3×30 mL). The organic layer was dried, filtered and concentrated to dryness. The crude thiocarbonylated intermediate was directly treated with tris-(trimethylsilyl)silane hydride (0.78 mL, 5.23 mmol) and α,α′-azoisobutyronitrile (AIBN, 0.112 g, 0.69 mmol) in dry dioxane (17 mL) at reflux for 2 h. The solvent was removed under vacuum and the residue was purified by silica gel column chromatography (0-5% MeOH in dichloromethane) to give pure 4 (0.93 g, 96%) as a foam: 1H NMR (DMSO-d6): δ 2.9-3.1 (m, 2H, H-2′ and H-2″), 4.6-4.7 (m, 3H, H-4′, H-5′ and H-5″), 5.8 (br s, 1H, H-3′), 6.43 (dd, 1H, H-1′, J1′,2′=3.1 Hz, J1′,2″=7.6 Hz), 7.3 (br s, 2H, NH2-6), 7.4-7.9 (m, 10H, 2 benzoyls), 8.05 and 8.33 (2s, 2H, H-2 and H-8); ms : matrix G/T, (FAB+) m/z 460 [M+H]+, 325 [S]+, 136 [BH2]+, (FAB−) m/z 458 [M−H]−, 134 [B]−; UV (95% ethanol): λmax 261 mn (ε 14400), 231 nm (ε 26300), λmin 249 nm (ε 12000); [α]D20=−38 (c 1.04, DMSO).
