To a solution of di-tert-butyl [iminobis(2,1-ethanediyl)]biscarbamate (2.13 g) and 3-ethoxy-4-{[1-methyl-5-(tritylamino)-1H-pyrazol-4-yl]amino}-3-cyclobutene-1,2-dione (1.91 g) in chloroform (10 ml) was added triethylamine (0.558 ml), and the mixture was stirred under reflux. After 9 hours, additional di-tert-butyl [iminobis(2,1-ethanediyl)]biscarbamate (1.82 g) was added to the reaction mixture, and the mixture was stirred under reflux for 1 day. To the reaction mixture was added diethyl ether, and the solution was washed successively with 5% aqueous citric acid solution, water and brine. The organic layer was dried over anhydrous sodium sulfate, filtered and concentrated in vacuo. The residue was chromatographed on silica gel eluting with methylene chloride/methanol (20:1) to give di-tert-butyl {[(2-{[1-methyl-5-(tritylamino)-1H-pyrazol-4-yl]amino}-3,4-dioxocyclobut-1-en-1-yl)imino]bis(2,1-ethanediyl)}biscarbamate (1.84 g) as a solid.
