To a solution of 7-(tert-butoxycarbonyl)-3-phenyl-5,6,7,8-tetrahydroimidazo[1,5-a]pyrazine-1-carboxylic acid (Intermediate 3B) (2.0 g, 5.82 mmol) in DMF was added (S)-methyl 2-amino-3,3-dimethylbutanoate (1.18 g, 8.15 mmol), followed by DIEA (2.26 g, 17.47 mmol). The resulting mixture was stirred for 20 min and TBTU (2.43 g, 7.57 mmol) was added and the mixture was stirred overnight. The reaction mixture was diluted with water and extracted with EtOAc, washed with water, brine, dried over anhydrous Na2SO4, filtered and concentrated. The residue was purified by flash chromatography eluting with ethyl acetate/hexanes 10-50% gradient to give Intermediate 22A 2.73 g (98%). LCMS (+ESI) m/z 470.0 [M+H]+.
