[CH2:1]([O:8][C:9]1[C:10]([CH2:18][CH3:19])=[CH:11][C:12](Br)=[C:13]([O:15][CH3:16])[CH:14]=1)[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1.C([Li])CCC.[B:25](OCC)([O:29]CC)[O:26]CC>C1COCC1>[CH2:1]([O:8][C:9]1[C:10]([CH2:18][CH3:19])=[CH:11][C:12]([B:25]([OH:29])[OH:26])=[C:13]([O:15][CH3:16])[CH:14]=1)[C:2]1[CH:7]=[CH:6][CH:5]=[CH:4][CH:3]=1Under a nitrogen atmosphere, 16.70 g (52.00 mmol) of 5-benzyloxy-2-bromo-4-ethyl-1-methoxybenzene (4) was added to 110 mL of anhydrous THF. The solution was cooled to −78° C., and 22.88 mL (57.19 mmol) of a 2.5 M solution of butyl lithium was added dropwise while maintaining the temperature below −70° C. The reaction mixture was stirred at −78° C. for 45 minutes. 9.73 mL (57.19 mmol) of triethyl borate was then added, and the reaction was allowed to stir at −78° C. for an additional 2 hours. The reaction mixture was then allowed to warm to 23° C. over a 30 minute period and was quenched with 100 mL of saturated NH4Cl. The pH was adjusted to 5.0 with 1M HCl, and the resultant solution was extracted with ethyl acetate (2×200 mL). The combined extracts were washed with brine (1×100 mL), dried over sodium sulfate, filtered and concentrated in vacuo to yield crude product as a greenish-tan solid. The crude product was triturated with hexane and filtered to afford 10.65 g (64%) of 4-benzyloxy-5-ethyl-2-methoxy-phenylboronic acid (5) as an off-white solid.
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