C[Li].Br[C:4]1[CH:5]=[C:6]([N:10]([CH3:19])[C:11]([NH:13][CH2:14][CH2:15][CH2:16][CH2:17][CH3:18])=[O:12])[CH:7]=[CH:8][CH:9]=1.C([Li])(C)(C)C.[B:25](OC)([O:28]C)[O:26]C.Cl>C(OCC)C.CCCCC.O1CCCC1>[CH3:19][N:10]([C:6]1[CH:5]=[C:4]([B:25]([OH:28])[OH:26])[CH:9]=[CH:8][CH:7]=1)[C:11]([NH:13][CH2:14][CH2:15][CH2:16][CH2:17][CH3:18])=[O:12]150 mL (0.24 mol) of methyllithium at 1.6M in diethyl ether are added to a solution cooled to −70° C. of 123 g (0.41 mol) of 1-(3-bromophenyl)-1-methyl-3-pentylurea in 1.21 of tetrahydrofuran. The reaction medium is stirred at −70° C. for 1 hour and 530 mL (0.91 mol) of a 1.7M solution of tert-butyllithium in pentane are added. The reaction medium is stirred at −70° C. for 45 minutes and 115 mL (0.91 mol) of trimethyl borate are then added. The reaction medium is stirred at room temperature for 1 hour and then hydrolyzed by addition of ice, acidified with 1 l of 2 N hydrochloric acid and extracted with ethyl acetate. The organic phases are combined, washed with water and dried over magnesium sulfate. The solvent is evaporated off and the residue is purified by chromatography on a column of silica eluted with a 1/1 heptane/ethyl acetate mixture and crystallization from a heptane/ethyl acetate mixture. 42 g (39%) of 3-(1-methyl-3-pentylureido)phenylboronic acid are obtained in the form of a pink solid.
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