Br[C:2]1[CH:11]=[CH:10][C:5]2[S:6][CH2:7][CH2:8][S:9][C:4]=2[CH:3]=1.C([Li])CCC.CCCCCC.[CH:23](OCC)=[O:24].Cl>C(OCC)(=O)C.O1CCCC1>[S:6]1[C:5]2[CH:10]=[CH:11][C:2]([CH:23]=[O:24])=[CH:3][C:4]=2[S:9][CH2:8][CH2:7]1To 5 mL of a tetrahydrofuran solution containing 500 mg of 6-bromo-2,3-dihydro-1,4-benzodithiin, 0.67 mL of a 2.67 mol/L butyllithium/hexane was added at −78° C., the mixture was stirred for 0.5 hour, then 300 mg of ethyl formate was added thereto, and the mixture was further stirred for 2 hours. Thereto was added 1 mol/L hydrochloric acid, and ethyl acetate was then added to the mixture. The organic layer was separated, washed with an aqueous saturated sodium hydrogen carbonate solution and dried over anhydrous magnesium sulfate, and the solvent was removed under reduced pressure. The residue thus obtained was purified by silica gel column chromatography [silica gel; Silica gel 60N manufactured by Kanto Chemical Co., Inc., eluent; ethyl acetate:hexane 1:4] to obtain 121 mg of a brown oily substance, 2,3-dihydro-1,4-benzodithiin-6-carbaldehyde.
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