(The following is performed in a draft chamber.) A mixture of concentrated sulfuric acid (1.5 ml), water (10 ml), and glacial acetic acid (50 ml) was dropped for about 10 minutes into a suspension of 7-butoxy-3-[2-(trans-4-propylcyclohexyl)ethyl]-1,2-difluoronaphthalene(21.4 g, 0.053 mol), periodic acid dihydrate (2.6 g, 0.011 mol), and iodine (5.4 g, 0.021 mol) in glacial acetic acid (100 ml) with vigorously stirring. After stirring at 60° C. for 4 hours, the reaction liquid was cooled, poured into water, and the reaction was terminated. The precipitated solid was dissolved with toluene, the organic layer was separated, and was extracted with toluene from the aqueous layer. After collecting organic layers, it was rinsed, in order, with 10% sodium thiosulfate aqueous solution, with saturated sodium hydrogen carbonate aqueous solution, and with saturated brine, and was dried with anhydrous sodium sulfate. A light yellow solid (21.4 g) was obtained by removing the solvent under reduced pressure and by recrystallizing the obtained orange solid (from acetone). (Yield 73.9%)
