TFA catalyzed C-C bond cleavage followed by intermolecular [4+2]-Diels-Alder cycloadditionTFA catalyzed C-C bond cleavage followed by intermolecular [4+2]-Diels-Alder cycloaddition
SyntheticPage 772
Published Jan 08, 2015
Preparation of 2,4,6-trimethoxy-1-styrylbenzene. To the stirred solution of benzyl triphenylphosphine bromide salt (1.23 g, 2.8 mmol) in anhydrous THF (10 ml) under N2 atmosphere, was added potassium tert-butoxide (0.342 g, 3.0 mmol) in small portions at 0-5 ºC and the resulting mixture stirred at 80 ºC for 30 min. Formation of orange color was observed in the reaction. Reaction was cooled to the room temperature and 2,4,6-trimethoxy benzaldehyde (0.5 g, 2.5 mmol) was added. The resulting reaction mixture was again refluxed at 80 ºC for 30 min. Reaction mixture was quenched with ethyl acetate (10 ml), and solvent was evaporated under vaccum, followed by partitioning between water and ethyl acetate (50 ml x 2). The organic layer was collected, dried on anhydrous sodium sulphate and evaporated on rotary evaporator to get the crude product. The crude product was purified by silica gel (#100-200) column chromatography using 20% EtOAc: hexane to get (E/Z)-1-styryl 2,4,6-trimethoxy benzene (0.667g) in 92% yield.
Procedure for synthesis of 2-phenylnaphthalene. The solution of 2,4,6-trimethoxy-1-styrylbenzene (100 mg) in 50% trifluoroacetic acid in water (1 ml) or 100% TFA (0.5 ml) was stirred at 80 ºC for 30 min. Reaction was monitored with TLC using 100% n-hexane as an eluent. Reaction mixture was then cooled to room temperature and neutralised slowly with saturated NaHCO3 solution; and extracted with ethyl acetate (25 ml x 2). The combined organic layer was dried over anhydrous sodium sulphate and evaporated on vacuo rotavapor to get crude product. The crude product was purified by silica gel (#60-120) column chromatography using 100% n-hexane as an eluent to get pure 2-phenylnaphthalenes (35 mg) in 94% yield.