[CH:1]1[CH:2]=[CH:3][C:4]([CH2:7][O:8][C:9]2[C:10]3[N:14]=[CH:13][NH:12][C:11]=3[N:15]=[C:16](N)[N:17]=2)=[CH:5][CH:6]=1.[H+].[B-](F)(F)(F)[F:21].N([O-])=O.[Na+].C(=O)([O-])[O-].[Na+].[Na+]>O>[CH:1]1[CH:2]=[CH:3][C:4]([CH2:7][O:8][C:9]2[C:10]3[NH:14][CH:13]=[N:12][C:11]=3[N:15]=[C:16]([F:21])[N:17]=2)=[CH:5][CH:6]=1O6 -Benzylguanine (1.21 g, 5 mmol) was added to 100 mL of 48% fluoboric acid at -20° C.. Sodium nitrite (1.23 g, 35 mmole) was dissolved in water (5 mL) and 2.5 mL of this sodium nitrite solution was added slowly to the cold fluoboric acid solution. The resulting mixture was stirred for 1 h at or below -15° C. Additional fluoboric acid (25 mL) was added followed by an additional 2.5 mL of the aqueous sodium nitrite solution. After stirring for an additional 1 h below -15° C., fluoboric acid (25 mL) was again added and stirring was continued for 1 h. The resulting solution was neutralized with saturated aqueous sodium carbonate solution at -20° C. and was allowed to warm to room temperature. A white precipitate that formed was collected by filtration and was washed with water and dried under vacuum to afford crude 4c: yield, 0.52 g, 43%. An analytical sample was prepared by chromatography on a Sephadex LH-20 column (3×80 cm) eluted with methanol/water (1:1) at 1 mL/min. The desired 4c eluted in fractions 66-77: mp 182°-183° C. (184°-185° C.; Robins and Robins, J. Org. Chem., 34, 2160-2163 (1969)); UV (pH 1) λmax 256 nm (ε=1.117×104) ; (pH 6.9) 257 (1.078×104) (pH 13) 264 (1.063×104); 1H NMR δ5.60 (s, 2H, ArCH2), 7.37-7.57 (m, 5H, ArH), 8.40 (s, 1H, H-8), 13.60 (s, 1H, NH, exchanges with D2O), 19F NMR δ23.54 downfield from trifluoroacetic acid standard; MS (EI) calcd. m/z for C12H9 FN4O 244.0760, found 244.0756; Anal. (C12H9FN4 O.2/3 H2O) C, H, N.
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