N,N'-Bis(tert-butoxycarbonyl)-S-methylisothiourea

Experimental procedure: To a stirring solution of 1 (272 mg, 0.88 mmol) in anhydrous DMF (10 mL) and N,N'-bis(tert-butoxycarbonyl)-S-methylisothiourea (262 mg, 0.90 mmol) at 0 ℃ is added Et₃N (0.37 mL, 2.66 mmol) in one portion, the reaction is stirred for 15 minutes. Then AgNO₃ (164 mg, 0.96 mmol) is added, the solution is stirred at 0 ℃ for 3 hours, then room temperature overnight. Filtered the solution through celite, the residue is extracted with water and EtOAc for 3 times. The organic layers are combined and washed with saturated brine, dried over Na₂SO₄. The crude product is purified by silica gel flash chromatography eluting with 7% EtOAc/hexane to give 2 (455 mg, 86%) as colorless oil.