To a suspension of compound 3 (19 g, 0.0738 mol) in anhydrous N,N-dimethylformamide (150 ml) was added N,N-dimethylformamide dimethyl acetal (98 ml, 0.7385 mol) and the mixture was stirred at 20-22° C. After one hour TLC (silica gel, 30% methanol in dichloromethane, Rf for compound 3: 0.21 and for product 4: 0.55) indicated that reaction was complete. Solvent and reagent were removed under reduced pressure (temperature was kept below 40° C.). Ethanol (50 ml) was added to the obtained oily residue and the solvent was removed under reduced pressure. This process was repeated twice and crude product solidified. The crude product was stirred with 190 ml of ethanol at 20° C. for one hour and kept at 5° C. for 12 hours. Solids were collected by filtration and filter cake washed with 30 ml of cold ethanol and 30 ml of cold tert-butyl-methyl ether. Drying the solid under vacuum gave 14.7 g (64%) of compound (4) as a first crop. TLC (silica gel, 30% methanol in dichloromethane, Rf for product (4): 0.55) and (silica gel, 10% methanol in dichloromethane, Rf for product (4): 0.1) showed only a single spot for compound (4) Mother liquor from the ethanol purification was evaporated to dryness and the residue was stirred with ethanol (80 ml) at 20° C. for one hour and kept at 5° C. for 12 hours. Solids were collected by filtration and filter cake washed with 15 ml of cold ethanol and 15 ml of cold tert-butyl-methyl ether. After drying the solid under vacuum, 3.5 g (15%) was obtained as a second crop. TLC (silica gel, 30% methanol in dichloromethane, Rf for product (4): 0.55) and (silica gel, 10% methanol in dichloromethane, Rf for product (4): 0.1) showed only a single spot for compound (4); m.p. 201-209° C.; 1H NMR (DMSO-d6) δ ppm 8.62 (s, 1H, N═CH), 8.17 (d, 1H, H-6, J5-6=7.3 Hz), 5.91 (m, 2H, H-1′, H-5), 5.16 (t, 1H, OH-5′, D2O exchangeable), 5.06 (s, 1H, OH-2′, D2O exchangeable), 3.8-3.5 (m, 4H, H-3′, H-4′, H-5′ and H-5″), 3.15 and 3.02 (2s, 6H, N(CH3)2), 0.92 (s, 3H, CH3); FAB>0 (GT) 625 (2M+H)+, 313 (M+H)+, 167 (B+2H)+; FAB<0, (GT) m/z 419 (M+T−H)−, 403 (M+G−H)−, 311 (M−H)−, 165 (B)−.
