Cl[C:2]([O:4][CH2:5][C:6]1[CH:11]=[CH:10][CH:9]=[CH:8][CH:7]=1)=[O:3].[Br:12][C:13]1[C:22]2[O:21][CH2:20][CH2:19][NH:18][C:17]=2[CH:16]=[C:15]([CH3:23])[CH:14]=1.N1C=CC=CC=1.Cl>ClCCl>[CH2:5]([O:4][C:2]([N:18]1[C:17]2[CH:16]=[C:15]([CH3:23])[CH:14]=[C:13]([Br:12])[C:22]=2[O:21][CH2:20][CH2:19]1)=[O:3])[C:6]1[CH:11]=[CH:10][CH:9]=[CH:8][CH:7]=1A solution of benzyl chloroformate (1.32 g, 0.008 mol) in dichloromethane (20 mL) was added dropwise under nitrogen to an ice-cold solution of 8-bromo-6-methyl-3,4-dihydro-2H-benzo[1,4]oxazine (1.78 g, 0.007 mol) and pyridine (1.06 g, 0.013 mol) in dichlormethane (40 mL). After 1 hour at ambient temperature, a 10% aqueous HCl solution was added. The layers were separated and the organic phase was washed with water (50 mL×2), dried (Na2SO4) and concentrated to give 8-bromo-6-methyl-2,3-dihydro-benzo[1,4]oxazine-4-carboxylic acid benzyl ester as a colorless oil (2.34 g, 96%) which was used in the step 2 without purification.
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