A mixture of (S)-3-amino-8-methyl-2,3-dihydrobenzo[b][1,4]oxazepin-4(5H)-one trifluoroacetate (1.60 g, 5.23 mmol, Eq: 1.00), (S)-2-(tert-butoxycarbonyl(methyl)amino)propanoic acid (1.06 g, 5.23 mmol, Eq: 1.00), HOBT (801 mg, 5.23 mmol, Eq: 1.00), HBTU (1.98 g, 5.23 mmol, Eq: 1.00) at 0° C. and DIEA (2.03 g, 2.73 mL, 15.7 mmol, Eq: 3.00) in DMF (50 mL) was stirred at 0° C. for 1 h, diluted with EtOAc, washed with water, brine, dried over Na2SO4, filtered and concentrated. The residue was purified by silica gel chromatography to afford methyl-[(S)-1-((S)-3-methyl-8-oxo-6,7,8,9-tetrahydro-5-oxa-9-aza-benzocyclohepten-7-ylcarbamoyl)-ethyl]-carbamic acid tert-butyl ester (2.04 g) which was used without purification.
