P(Br)(Br)([Br:3])=O.[CH:6]1([CH2:9][N:10]2[CH:15]=[CH:14][C:13](O)=[CH:12][C:11]2=[O:17])[CH2:8][CH2:7]1>CN(C=O)C>[Br:3][C:13]1[CH:14]=[CH:15][N:10]([CH2:9][CH:6]2[CH2:8][CH2:7]2)[C:11](=[O:17])[CH:12]=1Phosphorus oxybromide (5.4 g, 18.9 mmol) was added to a solution of intermediate D2 (1.42 g, 8.6 mmol) in DMF (140 ml) and the mixture was heated at 110° C. for 1 hour. After cooling in an ice bath the solution was partitioned between water and EtOAc. After three extractions with EtOAc, the combined organic fractions were dried (Na2SO4) and the solvent was evaporated in vacuo. The crude product was purified by column chromatography (silica gel; DCM as eluent). The desired fractions were collected and evaporated in vacuo to yield intermediate D3 (1.82 g, 93%).
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