[SH:1][C:2]1[CH:3]=[C:4]([CH:8]=[CH:9][C:10]=1[OH:11])[C:5]([OH:7])=[O:6].B(F)(F)F.CCOCC.[C:21]1(C)[CH:26]=CC=C[CH:22]=1>CC(C)=O>[C:5]([C:4]1[CH:8]=[CH:9][C:10]2[O:11][C:21]([CH3:26])([CH3:22])[S:1][C:2]=2[CH:3]=1)([OH:7])=[O:6]16 g of 3-mercapto-4-hydroxybenzoic acid (III) was dissolved in a mixture of 300 ml of toluene and 50 ml of acetone. 50 ml of boron trifluoride etherate was added and the mixture was stirred at room temperature for 4 hours. Ice (50 g) was added and the mixture was stirred for a further 1 hour. The organic layer was separated, washed with brine, dried with magnesium sulfate and evaporated to leave a solid residue. The residue was recrystallized from aqueous methanol to yield 15 g of 5-carboxy-2,2-dimethyl-1,3-benzoxathiole (IV), m.p. 124°-125° C.
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