To pteroic acid dihydrochloride (424 mg, 1.10 mmol) and triethylamine (0.75 mL, 5.50 mmol) in N,N-dimethylformamide (10 mL), propanephosphonic anhydride 50% solution in N,N-dimethylformamide (1.30 mL, 2.23 mmol) was added. After the reaction stirred for 45 minutes at ambient temperature, (5)-tert-butyl 21-amino-2,2-dimethyl-4,18-dioxo-3,8,11,14-tetraoxa-5,17-diazadocosan-22-oate (802 mg, 1.68 mmol) in N,N-dimethylformamide (5 mL) was added, and the reaction was stirred for 16 h at ambient temperature. After this time, the reaction was diluted with diethyl ether (150 mL) and the filtered to give (S)-tert-butyl 21-(4-((2-amino-4-hydroxypteridin-6-yl)methylamino)benzamido)-2,2-dimethyl-4,18-dioxo-3,8,11,14-tetraoxa-5,17-diazadocosan-22-oate (849 mg, obtained as a 1:1 mixture with pteroic acid, 50%) as a brown solid that was used in the next step without further purification: MM-APCI MS m/z=794 [M+Na]+.
