Guanosine 5 (10.19 g, 36 mmoles) was dried three times by coevaporation with dry pyridine and suspended in 200 ml of dry pyridine. To this was added trimethylsilyl chloride (24 ml, 180 mmoles) at room temperature. After the solution was stirred for 1 hour at room temperature, phenylacetyl chloride (24 ml, 180 mmoles) was added dropwise and the solution was maintained at room temperature with mechanical stirring for 15 hours. The reaction mixture was cooled in an ice bath, and 75 ml of cold water added. After 30 minutes, 75 ml of 29% aqueous ammonia was added and the reaction mixture was stirred for 30 minutes. The solution was then evaporated to near dryness, and the residue was dissolved in 300 ml water and 300 ml methylene chloride. The product crystallizing out of the aqueous solution upon extraction with CH2Cl2 was filtered off, washed with CH2Cl2 (3×70 ml) and ether (3×70 ml) and air dried to yield 8.5 g (59% yield) of Ribo GPA 6. The product was characterized as follows: m.p. 160°-170° C. (dec.); UV (EtOH): λ max 280 nm and 258 nm; IR (KBr): ν 1680 (vs, C=O of amide), 1722 (s, C=O of amide ring), and 3000-3500 (NHOH) cm-1 ; 1H-NMR (DMSO-d6): δ 3.53 and 3.57 (2m, 2H, C5, CH2), 3.81 (s, 2H, C6H5CH2), 3.91 (d, 1H, C4'H), 4.13 (m, 1H, C3' H), 4.42 (d, 1H, C2' H), 5.08 (br, s, 1H, C5' OH), 5.21 (br, s, 1H, C3' OH), 5.81 (d, 1H, J1',2' =5.64 Hz, C1' H), 7.26-7.41 (m, 5H, C6H5), 8.28 (s, 1H, C8H), 11.99 and 12.01 (2s, 2H, 2×NHCO). Calcd for C18H19N5O6 (401.37): C, 53.86; H, 4.77; N, 17.45. Found: C, 53.49; H, 5.11; N, 17.36
