acetic acid; 1,8-diazabicyclo[5.4.0]undec-7-ene in dichloromethane T=20°C; 0.75 h; Inert atmosphere;

1: 5'-O-dimethoxytrityl-thymidine in toluene T=45°C; 1 h; 2: succinic acid With triethylamine in dichloromethane; toluene T=25°C; 4 h;