A solution of thieno[2,3-b]pyrazine-6-carboxylic acid (1.01 g, 5.61 mmol), HATU (1.8 g, 5.61 mmol) and DIPEA (1.953 mL, 11.21 mmol) in DMF (8 mL) was stirred for 15 min. (3-Amino-4-methyl-phenyl)-carbamic acid tert-butyl 27 (1.246 g, 5.61 mmol), DIPEA (1 eq) in DMF (8 mL) were added and stirred overnight at rt. Citric acid solution (3%) was added and extracted with EtOAc. Organic layer was washed with NaHCO3 solution, brine, dried and evaporated. Purification by chromatography (0-20% EtOAc in CH2Cl2) gave tert-butyl 4-methyl-3-(thieno[2,3-b]pyrazine-6-carboxamido)phenylcarbamate 28 (1.43 g, 66.4%). (m/z)=385 (M+H)+.
