1. Hydrogenation reaction: 2.0 g of 10% palladium carbon (50% water wetted) was added to a 500 mL Parr flask and covered with 50 mL of ethanol. 2-Methyl-5-nitroanisole (10.0 g, 59.8 mmol) was dissolved in 100 mL of ethanol and added to the catalyst suspension. The reaction system was hydrogenated at 50 psi hydrogen pressure for 3 hours. Upon completion of the reaction, the catalyst was removed by filtration through a diatomaceous earth pad and the filter cake was washed with 150 mL of ethanol. The filtrates were combined and concentrated under reduced pressure to give 8.05 g (98% yield) of 5-amino-2-methylanisole as a clear oil.
2. Diazotization reaction: 5-amino-2-methylanisole (8.05g, 58.7mmol) was dissolved in 244mL of water and 8.1mL of concentrated sulfuric acid, and cooled to 0℃. A 61 mL aqueous solution of sodium nitrite (4.86 g, 70.4 mmol) was added slowly and dropwise with stirring. The reaction mixture was stirred at 0 °C for 30 min and then urea (0.70 g, 11.7 mmol) was added and stirring was continued for 30 min. The resulting light yellow diazonium salt solution was transferred to a dropping funnel.
3. Iodination reaction: the above diazonium salt solution was slowly added to 122 mL of a stirred aqueous solution of potassium iodide (19.48 g, 117.4 mmol). After addition, the reaction solution was stirred at room temperature for 1 hour. The reaction mixture was extracted with ether (3×300mL), and the organic phases were combined, washed sequentially with 1M sodium thiosulfate solution (2×200mL), dried over anhydrous sodium sulfate, filtered, and concentrated under reduced pressure to give 9.60g (66% yield) of 5-iodo-2-methylanisole as a brown oil.
4. Suzuki coupling reaction: In a 250 mL round-bottomed flask, 5-iodo-2-methylanisole (9.60 g, 38.70 mmol) and diethyl-(3-pyridyl)borane (5.70 g, 38.70 mmol) were dissolved in 60 mL of tetrahydrofuran. Sodium carbonate (8.20 g, 77.40 mmol) and 30 mL of water were added, followed by tetrakis(triphenylphosphine)palladium(0) (0.90 g, 0.77 mmol) and 15 mL of ethanol. The reaction mixture was refluxed under nitrogen protection for 24 h. After cooling to room temperature, the mixture was diluted with 200 mL of water and extracted with ether (2 x 200 mL). The organic phases were combined, washed with saturated sodium chloride solution (3 x 150 mL), dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure to give 7.71 g (99% yield) of 2-methyl-5-(3-pyridinyl)-anisole as a brown oil.
5. Hydrogenation reduction: 0.77 g of platinum(II) oxide was added to a 500 mL hydrogenation vessel, and after displacing the air with nitrogen, 150 mL of acetic acid solution of 2-methyl-5-(3-pyridyl)-anisole (7.71 g, 38.7 mmol) was added. The reaction system was hydrogenated at 45 psi hydrogen pressure for 18 hours. The catalyst was removed by diatomaceous earth filtration and the filter cake was washed with 200 mL acetic acid. The filtrates were combined and concentrated under reduced pressure. The residue was dissolved in 300 mL of water, adjusted to alkaline with 5N sodium hydroxide solution and extracted with ethyl acetate (2 x 300 mL). The organic phases were combined, dried over anhydrous sodium sulfate, filtered and concentrated under reduced pressure.
6. Tartrate split: dissolve the above oil in 300mL of hot ethanol, add 50mL of hot ethanol solution of L-(+)-tartaric acid (5.81g, 38.7mmol), stir at room temperature for 24 hours, precipitate white solid, filter and collect. The white solid was recrystallized from hot 5% water/ethanol (200 mL) to give 4.88 g (35% yield) of 5-(3-piperidinyl)-2-methylanisole-L-tartrate. The mother liquors were combined and concentrated under reduced pressure and the residue was dissolved in 500 mL of ether and washed with 300 mL of saturated sodium bicarbonate solution. The organic phase was dried with anhydrous sodium sulfate, filtered and concentrated under reduced pressure. The resulting oil was dissolved in 200 mL of hot ethanol, 50 mL of hot ethanol solution of D-(-)-tartaric acid (3.75 g, 25.0 mmol) was added and stirred at room temperature for 48 h. A white solid was precipitated and collected by filtration. The white solid was recrystallized from hot 5% water/ethanol (300 mL) to give 5.36 g (39% yield) of 5-(3-piperidinyl)-2-methylanisole-D-tartrate.
7. Deprotection reaction: 3-(3-methoxy-4-methylphenyl)-1H-piperidine-L-tartrate (4.88 g, 13.73 mmol) was slowly added to 50 mL of hydrobromic acid and the mixture was heated at 140 °C for 2 hours. After cooling to room temperature, hydrobromic acid and water were removed by distillation.
