Reaction SMILES: [Br:1]Br.[C:3]1([P:9]([C:16]2[CH:21]=[CH:20][CH:19]=[CH:18][CH:17]=2)[C:10]2[CH:15]=[CH:14][CH:13]=[CH:12][CH:11]=2)[CH:8]=[CH:7][CH:6]=[CH:5][CH:4]=1>CC(N(C)C)=O>[Br-:1].[Br-:1].[C:16]1([P:9]([C:3]2[CH:4]=[CH:5][CH:6]=[CH:7][CH:8]=2)[C:10]2[CH:15]=[CH:14][CH:13]=[CH:12][CH:11]=2)[CH:17]=[CH:18][CH:19]=[CH:20][CH:21]=1
Reactant: BrBr
Agent: c1ccc(P(c2ccccc2)c2ccccc2)cc1
Solvent: CC(=O)N(C)C
Product: [Br-]

Procedure:
Bromine (59.6 g, 0.373 mole) was added dropwise over a 30-min period to a stirred solution of triphenylphosphine (97.7 g, 0.373 mole) in anhydrous DMAC (486 ml) kept at ~10° (ice bath) and protected from atmospheric moisture. (Bromine remaining in the funnel was rinsed with 10 ml of DMAC). A smooth suspension containing finely divided, crystalline triphenylphosphine dibromide resulted. The 2,4-diamino-6-pteridinemethanol.HRr (25.4 g, 0.093 mole) described above was added in one portion through a powder funnel (with the aid of 10 ml DMAC). The ice bath was removed, and the stirred mixture was allowed to warm to 20°-25°. After about 1 hr, complete solution had occurred. The solution, which gradually developed a dark-red color, was kept at 20°-25° for 1 hr longer and was then chilled (ice bath) before it was treated with EtOH (72 ml). After overnight refrigeration, the solvents were removed by evaporation in vacuo (Swissco evaporator, pressure <1 mm, bath <45°). The dark, semisolid residue was stirred with two 300-ml portions of C6H6 (to remove triphenylphosphine oxide), and each portion was removed from the C6H6 -insoluble product by decantation. The solid that remained was dissolved with stirring in glacial AcOH (660 ml) which had been preheated to 80°. The mixture was kept in a bath at 80° until solution was complete Tan crystalline solid separated as the dark solution was allowed to cool. Overnight refrigeration caused the AcOH to partially freeze. When it had thawed, the solid was collected, washed with chilled AcOH followed by Et2O, and dried in vacuo (over P2O5 and NaOH pellets) at successive temperatures of 25°, 56°, and 110°. (The higher temperature was necessary for complete removal of AcOH). The yield was 15.3 g (49%). (Some runs afforded 60% yield). This sample was further purified by reprecipitation from MeOH solution (Norit) by addition of Et2O followed by drying in vacuo (25°, P2O5), yield 13.0 g (42%) of pale-yellow solid. Spectral data: λmax, nm (ε x 10-3), 0.1 N HCl, 249 (17.3), 339 (10.5), 353 (sh); pH 7, 258 (21.2), 370 (6.87); 0.1 N NaOH, 258 (21.5), 370 (6.94); pmr (CF3CO2D), δ 4.70 (s, 2, CH2) and 9.08 (s, 1, C7 -H); estimated proportion relative to the methyl-substituted contaminant, 25:1. These spectral properties are in close agreement with those of a similarly obtained sample that gave the following elemental analysis results. Anal. Calcd for C7H7BrN6.HBr: C, 25.02; H, 2.40; Br, 47.56; N, 25.01. Found: C, 25.22; H, 2.44; Br, 47.30; N, 24.99. The 13.0-g sample described above gave the following results. Anal. Found: C, 25.59; H, 2.79; N, 24.62. The preparation of I described above is typical of several runs that gave similar yields of material whose pmr spectra differed only slightly in the estimated proportion of I with respect to the methyl-substituted contaminant. The proportions usually ranged from 16:1 to 25:1, which corresponds to a percentage of I of 94 to 96%. Samples of I of this degree of purity proved to be suitable for use in the preparation of 2-31.