Br[C:2]1[CH:12]=[C:11]([C:13]([CH3:16])([CH3:15])[CH3:14])[C:5]2[O:6][CH2:7][C:8]([CH3:10])([CH3:9])[C:4]=2[CH:3]=1.C([Li])CCC.C(=O)=O>C1COCC1>[C:13]([C:11]1[C:5]2[O:6][CH2:7][C:8]([CH3:10])([CH3:9])[C:4]=2[CH:3]=[CH:2][CH:12]=1)([CH3:16])([CH3:14])[CH3:15]A solution of 5-bromo-7-tert-butyl-3,3-dimethyl-2,3-dihydrobenzo[b]furan (12.4 g, 44.0 mmol) in freshly distilled THF (44 mL) is cooled to -78° C. under Ar, and a solution of n-butyl lithium (2.5M in hexane; 15.0 mL, 37.5 mmol) is added dropwise via syringe. A slight exotherm is apparent, and a milky suspension results. The suspension is allowed to warm to -20° C., and then is cooled to -50° C. To the resulting suspension is added a large excess of freshly pulverized solid CO2. The resulting mixture is allowed to warm to 0° C. After 1 h, the mixture is partitioned between 0.1N NaOH (40 mL) and hexane (3×30 mL). The hexane layer is dried (MgSO4), filtered and evaporated to yield 7-tert-butyl-3,3-dimethyl-2,3-dihydrobenzo[b]furan. The aqueous layer is acidified to pH 2 with 1N HCl, and is extracted with Et2O (3×20 mL). The Et2O layers are combined, dried (MgSO4), filtered and evaporated to provide the desired carboxylic acid.
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