Trimethylsilyl triflate (150 μL, 0.81 mmol) was added in a single portion to a mixture of 1-methyl-4-[(1-methyl-1H-imidazole-2-carbonyl)-amino]-1H-imidazole-2-carboxylic acid (200 mg, 0.80 mmol) in anhydrous acetonitrile (8.00 mL). The solution which formed within a few moments was transferred to solid N-(1-chloro-2-methylprop-1-enyl)-N-methyl aminomethylpolystyrene (1.55 mequiv/g, 1.04 g, 1.61 mequiv). After stirring at ambient temperature for 10 minutes, an equal volume of anhydrous CHCl3 (stabilized with amylenes) was added and the mixture was filtered in an inert atmosphere. The resin was washed with an additional 10 mL of anhydrous CHCl3 (stabilized with amylenes), and the combined filtrates containing the 1-methyl-4-[(1-methyl-1H-imidazole-2-carbonyl)-amino]-1H-imidazole-2-carbonyl chloride were added dropwise to a vigorously stirred 2-phase mixture of a solution of 4-[(4-amino-1-methyl-1H-imidazole-2-carbonyl)-amino]-1-methyl-1H-pyrrole-2-carboxylic acid methyl ester (171 mg, 0.62 mmol) in CHCl3 (10 mL), and a solution of Na2CO3 (170 mg, 1.60 mmol) in H2O (5 mL). The resulting reaction mixture was stirred for 5 minutes, and then was diluted with CHCl3 and H2O. The CHCl3 solution was isolated, dried (MgSO4), and concentrated to afford a 59% yield (240 mg) of 1-methyl-4-{[1-methyl-4-({1-methyl-4-[(1-methyl-1H-imidazole-2-carbonyl)-amino]-1H-imidazole-2-carbonyl}-amino)-1H-imidazole-2-carbonyl]-amino}-1H-pyrrole-2-carboxylic acid methyl ester as a very pure off-white solid. Reverse phase HPLC of this material was carried out on an Agilent 1100 system using a Vydac 4.6×250 mm Protein & Peptide C18 column eluted at 1.2 mL/min with a linear gradient of 20% MeCN:80% H2O to 40% MeCN:60% H2O over a 15 minute period. Both solvents contained 0.1% TFA. A single 304 nm HPLC peak eluted at 10.847 minutes. 1H NMR (CD3CN plus TFA): δ 10.90 (s, 1H), 10.71 (s, 1H), 9.49 (s, 1H), 7.85 (s, 1H), 7.76 (s, 1H), 7.65 (m, 2H), 7.46 (d, J=1.8 Hz, 1H), 6.92 (d, J=1.9 Hz, 1H), 4.15 (s, 3H), 4.12 (s, 3H), 4.11 (s, 3H), 3.92 (s, 3H), 3.80 (s, 3H).
