[CH3:1][Si:2]([CH3:12])([CH3:11])[C:3]1[O:7][C:6]([C:8]([OH:10])=[O:9])=[CH:5][CH:4]=1.C([N-]C(C)C)(C)C.[Li+].[CH:21]([S:24]C(C)C)([CH3:23])[CH3:22].O>O1CCCC1>[CH:21]([S:24][C:5]1[CH:4]=[C:3]([Si:2]([CH3:12])([CH3:11])[CH3:1])[O:7][C:6]=1[C:8]([OH:10])=[O:9])([CH3:23])[CH3:22]A solution of 25.01 g (136.0 mmol) of 5-trimethylsilyl-2-furancarboxylic acid (Carpenter, A. J. and Chadwick, D. J., Tetrahedron Lett., 26, 1777 (1985)) in 50 mL of dry tetrahydrofuran is added over 10 minutes to a -76° C. solution of freshly generated lithium diisopropylamide (328.0 mmol, 2.41 equiv) in dry tetrahydrofuran (175 mL) under nitrogen atmosphere and the resulting mixture is stirred at -76° to 65° C. for 1.5 hours. The reaction is recooled to -76° C. and treated with 23.58 g (151.0 mmol, 1.11 equiv) of diisopropyl sulfide in 25 mL of tetrahydrofuran and the resulting mixture is stirred for 4 hours while slowly warming to room temperature. The reaction is poured onto 1.5 L of water and the aqueous mixture is extracted with diethyl ether (200 mL, 3x), treated with 400 g of ice, and acidified with concentrated hydrochloric acid. The resulting precipitate is collected by vacuum filtration and recrystallized from diethyl ether/cyclohexane to give 18.22 g (35.14 g theo., 52%) of analytically pure 3-isopropylthio-5-trimethylsilyl-2-furancarboxylic acid as a tan solid; mp 181°-183° C.
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