GENERAL METHODS: 8-Hydroxy-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinoline-8-ol and N,N-dimethylformamide were used as raw materials to synthesize 8-hydroxy-1,1,7,7-tetramethyl-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinoline-9-carbaldehyde. The procedure was as follows: to a magnetically stirred solution of N,N-dimethylformamide (2 mL, 23 mmol) of phosphorous trichloride (2.2 mL, 23 mmol) under argon protection, 1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H-pyrido[3,2,1-ij]quinolin-8-aldehyde (2 g, 11.6 mmol) was slowly added at 0°C and additional of N,N-dimethylformamide (2 mL). Subsequently, the reaction mixture was stirred at room temperature for 3 h and the progress of the reaction was monitored by thin layer chromatography (TLC). Upon completion of the reaction, the reaction mixture was neutralized with ammonium hydroxide solution and then the product was extracted with ethyl acetate. The organic phase was washed several times with water to remove impurities. Finally, the residue was purified by column chromatography (silica gel as stationary phase and 10% ethyl acetate/hexane as eluent) to afford the target product 8-hydroxy-1,1,7,7-tetramethyl-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinoline-9-carboxaldehyde in a yield of 1.8 g (80% yield).
