Guanosine 5 (14.16 g, 50 mmoles) was dried three times by coevaporation with dried pyridine and suspended in 250 ml of dry pyridine. To this was added trimethylsilyl chloride (48 ml, 360 mmoles) at room temperature. After the solution was stirred for 1 hour at room temperature, 4-bromophenylacetyl chloride (35 g, 150 mmoles) was added dropwise and the solution stirred for 15 hours. The reaction mixture was then cooled in an ice bath and 75 ml of cold water was added. After 30 minutes 20 ml of 29% aqueous ammonia was added and the mixture stirred for 30 minutes. The solution was then evaporated to near dryness and the residue dissolved in 500 ml water and 500 ml methylene chloride. The product crystalized out of the solution upon extraction with CH2Cl2 was filtered off, washed with CH2Cl2 (3×70 ml) and ether (3×70 ml) and air dried to yield 14.4 g (60%) of Ribo GBPA 6c. The product was characterized as following: m.p. 160°-170° C. (dec.); UV (EtOH): λ max 280 nm and 258 nm; IR (KBr): ν 1688 (vs, br, C=O of amides), 3000-3500 (NH, OH) cm-1. 1H-NMR (DMSO-d6): δ 3.52 (m, 2H, C5, CH2), 3.67 (s, 2H, CH2C6H4), 3.91(m, 1H, C4, H), 4.14 (m, 1H, C3, H), 4.45(m, 1H, C2, H), 5.81 (t, 1H, J1',2' =5.64 Hz, C1, H), 7.15-7.56 (m, 4H, C6H4), 8.29 (d, 1H, C8H), 11.92 and 12.04 (2s, 2H, 2×NHCO).
