3-Methylisoquinoline

PreparationBenzylamine (9.18 mL, 84.0 mmol) and 1,1-dimethoxypropan-2-one (9.95 mL, 84.0 mmol) were added to dichloromethane (350 mL) at room temperature. Sodium triacetoxyborohydride (25 g, 118 mmol) was added to the reaction mixture in one portion. The reaction was stirred at room temperature overnight. The reaction mixture was then diluted with 2.5% sodium bicarbonate (250 mL) and mixed for 30 minutes, becoming biphasic. The organic layer was discarded. The aqueous layer was basified to pH 14 using concentrated sodium hydroxide. The basified aqueous solution was washed three times with ethyl acetate and the organic layers were kept and combined. Then, the resulting solution was washed with 5% sodium chloride solution three times. The organic layers were combined and dried over sodium sulfate. The resulting solution was evaporated to a yellow oil, which was found to be N-benzyl-1,1-dimethoxypropan-2-one. The oil (2.62 g, 12.5 mmol) was added dropwise to chlorosulfonic acid (8.35 mL, 125 mmol) over ice. A few milliliters of dichloromethane was used to rinse the oil from its reaction flask and the oil dissolved in dichloromethane was also added to the chlorosulfonic acid flask over ice. The reaction mixture was placed over boiling water for 5 minutes with a condenser to evaporate the solvent used to rinse the N-benzyl-1,1-dimethoxypropan-2-one out of its previous flask while not letting any water vapor enter the flask before the reaction mixture was fully heated. Then, the reaction mixture was placed in the boiling water for 10 minutes without a condenser to allow methanol, a side-product of the reaction, to evaporate out of the flask and drive the reaction forward. After cooling, the reaction mixture was quenched with ice and then basified to pH 14 with concentrated sodium hydroxide. The mixture was washed with dichloromethane three times. The organic layers were combined and dried using magnesium sulfate. The resulting solution was evaporated to solid 3-methylisoquinoline (average percent yield: 2-5%) and the structure was confirmed with GC-MS and NMR.