[Synthesis]
1. 8-Azabicyclo[3,2,1]octan-3-one hydrochloride (10.0 g; 71.84 mmol) was dissolved in dichloroethane (DCE, 60 mL), and 1-chloroethyl chloroformate (ACE-C1, 14.5 mL; 19.11 g; 133.7 mmol) was added slowly and dropwise. The reaction mixture was stirred overnight at room temperature, then diluted with ether (Et2O, 400 mL) and filtered. The filtrate was concentrated under reduced pressure to give crude chloroethyl carbamate.
2. The above crude product was dissolved in methanol (MeOH, 200 mL) and stirred at room temperature for 1 h. Subsequently, it was concentrated under reduced pressure at 55 °C to obtain crude desmethyltropinone hydrochloride (brown solid, 11.4 g, 98% yield).
3. The crude product was purified by recrystallization from formonitrile to give a white crystalline solid (5 g, 43% yield).1H NMR (400 MHz, DMSO-d6) δ 1.79 (dd, J = 15.0,6.9 Hz, 2H), 2.09 (m, 2H), 2.40 (d, J = 16.7 Hz, 2H), 3.02 (dd, J = 17.1,4.3 Hz, 2H), 4.23 (s, 2H), 10.00 (br s, 2H).
4. desmethyltropinone (5.10 g; 31.55 mmol) was dissolved in dichloromethane (CH2Cl2, 50 mL) and benzyl chloroformate (4.29 mL; 5.11 g; 29.98 mmol) and diisopropylethylamine (DIPEA, 16.48 mL; 12.23 g; 94.66 mmol) were added slowly and dropwise (note exothermic reaction). The reaction mixture was stirred at room temperature for 30 min and then diluted with dichloromethane (100 mL).
5. The organic phase was washed with 1N hydrochloric acid (2 x 100 mL), dried over anhydrous sodium sulfate (Na2SO4) and concentrated under reduced pressure to give the crude product (7.2 g, 88% yield).1H NMR (400 MHz, CDCl3) δ 1.71 (dd, J = 15.0,7.2Hz, 2H), 2.12 (m, 2H), 2.38 (d, J = 15.9 Hz, 2H), 2.67 (m, 2H), 4.62 (s, 2H), 5.22 (s, 2H), 7.38 (m, 5H). |