C([O:3][C:4]([C:6]1[S:7][C:8]2[CH:9](CC)[CH2:10][O:11][C:12]3[CH:19]=[C:18]([F:20])[C:17]([Br:21])=[CH:16][C:13]=3[C:14]=2[N:15]=1)=[O:5])C.O1CCCC1.[Li+].[OH-].Cl>O>[Br:21][C:17]1[C:18]([F:20])=[CH:19][C:12]2[O:11][CH2:10][CH2:9][C:8]3[S:7][C:6]([C:4]([OH:5])=[O:3])=[N:15][C:14]=3[C:13]=2[CH:16]=1
Into a 10-L 4-necked round-bottom flask purged and maintained with an inert atmosphere of argon was placed ethyl 9-Bromo-8-fluoro-4,5-dihydro-6-oxa-3-thia-1-aza-benzo[e]azulene-2-carboxylic acid ethyl ester (350 g, 940.33 mmol, 1.00 equiv), tetrahydrofuran (1.3 L), water (2.6 L), and LiOH (62 g, 2.59 mol, 2.73 equiv). The resulting solution was stirred at room temperature for 3 h, cooled to <10° C. and adjusted to pH=2-3 with hydrochloric acid. The precipitates were collected by filtration, washed with 4×1 L of water and dried to afford 260 g (80%) of 9-Bromo-8-fluoro-4,5-dihydro-6-oxa-3-thia-1-aza-benzo[e]azulene-2-carboxylic acid as a brown solid.
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