US 10,781,276 B2  21  22  (Synthesis Example 6) Synthesis of 9-( 4-methacry› loxypheny  1  )dibenzothiophenium-benzoate ( Com› pound A3)  <Synthesis of Compound A4 constituting Unit A>  (Synthesis Example 7) Synthesis of 9-( 4-methacry› loxyphenyl)dibenzothiophenium-hexafluoroanti› monate (Compound A4)  [Chem.18]  OH  ¢>--o  s+  0-0  - [Chem. 19]  (Compound A4)  3.0 g of 9-( 4-methacryloxyphenyl)dibenzothiophenium› benzoate obtained in Synthesis Example  6  is dissolved in 24  g of methylene chloride, 1.5 g of potassium hexafluoroan- timonate and 24 g of pure water are added thereto, and the  mixture is stirred at room temperature for 15 hours. There- after, phase separation is performed and the combined  organic layer is washed with 24 g of pure water. The  combined organic layer is concentrated and purified by  colunm chromatography (methylene chloride/methanol=S0/  20 (volume ratio)) to obtain 2.1 g of 9-( 4-methacryloxyphe› nyl )dibenzothiophenium-hexafluoroantimonate ( Compound  A4).  5.9 g of 9-(4-methacryloxyphenyl)dibenzothiophenium› benzoate (CompoundA3) is obtained in the same procedure  as Synthesis Example 3 except that 9-(4-hydroxyphenyl)  dibenzothiophenium-benzoate is used instead of 9-(4-hy› droxyphenyl)dibenzothiophenium-trifluoromethanesul- 60  fonate,  methacrylic  anhydride  is  used  instead  of  methacryloyl chloride, and the concentrated organic layer is 65  purified by colunm chromatography (methylene chloride/  methanol=70/30 (volume ratio)).  (Compound A3) 