1: 46.3 percent / N,N'-dicyclohexylcarbodiimide / CH2Cl2 / 1 h / 20 °C 2: 67.3 percent / 1.5 h / 20 °C 3: 72.6 percent / phosphorus trichloride / CH2Cl2; dimethylformamide / 1.5 h / 21 °C 4: 1.) Zn, ZnCl2; 2.) Deacidite FF ion-exchange resin / 1.) formic acid, 22 deg C, 2 h View Scheme