[CH3:1][O:2][C@@H:3]1[C@H:7]([OH:8])[C@@H:6]([CH2:9][OH:10])[O:5][C@H:4]1[N:11]1[C:21]2[N:20]=[C:18]([NH2:19])[NH:17][C:15](=[O:16])[C:14]=2[N:13]=[CH:12]1.C[Si](Cl)(C)C.[C:27](Cl)(=[O:31])[CH:28]([CH3:30])[CH3:29].[NH4+].[OH-]>N1C=CC=CC=1.O>[C:27]([NH:19][C:18]1[NH:17][C:15](=[O:16])[C:14]2[N:13]=[CH:12][N:11]([C:21]=2[N:20]=1)[C@@H:4]1[O:5][C@H:6]([CH2:9][OH:10])[C@@H:7]([OH:8])[C@H:3]1[O:2][CH3:1])(=[O:31])[CH:28]([CH3:30])[CH3:29]
2'-O-methylguanosine (3.5 g) in pyridine (100 ml) was treated with trimethylsilyl chloride (9 ml, 6 eq) and isobutyryl chloride (6.2 ml) at RT for 4 hr. The reaction mixture was cooled in an ice bath, H2O (20 ml) was added and stirring continued for an additional 20 min. NH4OH (20 ml) was added and after stirring for 30 min the reaction mixture was evaporated. The residue was triturated with H2O, filtered and the filtrate evaporated and purified by silica gel chromatography as per the procedure of Example 4 to yield the product as an off white solid (1.5 g). 1H NMR (DMSO-d6) δ 1.1 [d, 6, CH(CH3)2 ], 2.77 [m, 1, CH(CH3)2 ], 3.33-3.6 (m, 5, OCH3, H-5'), 3.93 (m, 1, H-4'), 4.22 (m, 1), 4.3 (m, 1), 5.1 (t, 1, 5'-OH), 5.28 (d, 1, 3'-OH), 5.9 (d, 1, H-1'), 8.28 (s, 1, H-8) and 11.9 (br s, 1, NH).
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