2-(2`,3`,5`-Tri-O-acetyl-b-D-ribofuranosyl)-4-(p-tolylazo)-5-trifluoromethyl-2,4-dihydropyrazol-3-one

To a solution of 4-(4-methylphenylazo)-5-trifluoromethyl-2,4-dihydropyrazol-3-one (1) [1] (0.81g, 3 mmol) in hexamethyldisilazine (HMDS) (25 ml) was added few crystals of anhydrous ammonium sulfate [2]. The mixture was refluxed for three hours, then it was evaporated under vacuum to dryness. The residue was mixed with anhydrous xylene (30 ml) and the resulted solution was re-evaporated under vacuum to dryness to remove any remaining traces of HMDS. To a solution of the residue in anhydrous 1,2-dichloroethane (25 ml) was added 1,2,3,5-tetra-O-acetyl-b-D-ribofuranose (0.954g, 3 mmol). The mixture was treated with SnCl₄ (1.1mmol) [2,3] and was then stirred at room temperature for two hours and followed by tlc. The reaction mixture was diluted with dichloromethane (25 ml), washed with saturated aqueous solution of sodium bicarbonate (50 ml) and water (3x30 ml). The organic layer was dried over anhydrous sodium sulfate, filtered, evaporated to a small volume and chromatographed over silica gel column using methylene chloride / methanol (98:2 v/v) to give 1.3 g (82%) of 3 as orange powder.

R_f: 0.253 (methylene chloride / methanol, 98 : 2 v/v).

UV (lmax , 95% ethanol): 384 nm

IR (KBr, cm^-1): 1747 (CO ester), 1674 (CO Pyrazolone).

MS (m/z): 528.

¹H-NMR (250 MHz, CDCl₃): 2.10 (s, 9H, COCH₃); 2.31 (s, 3H, CH₃); 2.42 (s, 1H, CH); 4.14-4.19 (dd, 1H, H-5`); 4.32-4.37 (m, 1H, H-4`); 4.43-4.48 (dd, 1H, H-5``); 5.56 (t, 1H, H-3`, J3`,4`=5.49 Hz); 5.72 (t, 1H, H-2`, J2`,3`=5.31 Hz); 6.03 (d, 1H, H-1`, J1`,2`=4.2 Hz); 7.23 (dd, 4H, aromatic CH).

¹³C-NMR (75 MHz, CDCl₃): 21.9 (CH₃); 48.5 (CH); 63.0 (C-5`), 70.84 (C-3`); 72.7 (C-2`); 79.7 (C-4`); 84.4 (C-1`); 116.9 (2 aromatic carbons), 121.1, 122.0 (2 aromatic carbons), 130.5, 144.1 (2 quaternary aromatic carbons); 137.5 (q, CF₃); 149.2 (C=N); 169.5, 169.8, 170.7, 173.0 (4 CO).