Example 20: Preparation of Compound 50; [235] [236] 20-A. Preparation of Compound 20a; To a solution of 1,6-dihydroxynaphthalene (1.2 g, 7.68 mmol) in acetonitrile (50 mL) was added phosphorus tribromide (PBr3, 2.9 g, 10.8 mmol), and the mixture was heated and refluxed with stirring for 48 hours. Upon completion of the reaction, the mixture was cooled to room temperature, followed by the addition of methanol (100 mL) to precipitate the solid product. The solid was collected by filtration, washed well with methanol and dried under reduced pressure to afford compound 20a 1,6-dibromonaphthalene (1.6 g, 74% yield). [M] = 286
