General procedure: Potassium carbonate (K2CO3, 1.190 g, 8.66 mmol) was added to a solution of 4-(tert-butoxycarbonyl)morpholine-2-carboxylic acid (CAS No. 189321-66-2; 0.500 g, 2.164 mmol) in anhydrous N,N-dimethylformamide (DMF, 3 mL), followed by iodomethane (CH3I, 0.922 g, 6.49 mmol) at room temperature. 0.922 g, 6.49 mmol) at room temperature. The reaction mixture was stirred at room temperature for 36 hours. Upon completion of the reaction, the mixture was poured into 1 M hydrochloric acid (HCl, 20 mL) and extracted with ethyl acetate (EtOAc, 3 × 50 mL). The organic phases were combined, washed with water (50 mL), dried over anhydrous sodium sulfate (Na2SO4), filtered and concentrated under reduced pressure to afford methyl 4-(tert-butoxycarbonyl)-2-morpholinecarboxylate (0.624 g, quantitative yield). The product could be used in the next reaction without further purification. Liquid chromatography-mass spectrometry (LCMS) analysis: method C, retention time 1.756 min; mass spectrometry (MS): electrospray positive ionization mode (ES+) m/z 246.48. The product was analyzed by liquid chromatography-mass spectrometry (LC-MS).
