3.1.3. General Procedure for the Phosphorylation of Morpholino Nucleosides **2**

Morpholino nucleosides **2** (0.1 mmol) were dissolved in dry Py (1 mL). The solution was cooled in an ice bath (−15 ◦C). POCl<sup>3</sup> (0.4 mmol, 0.037 mL) was added to the solution under stirring. In 20 min, 1 M TEAB (2 mL) was added to the cold reaction mixture. The reaction mixture was stirred for 10 min at room temperature. A) Compounds **2A,G,C,IU**: the reaction mixture was distributed between CH2Cl<sup>2</sup> (10 mL) and water (10 mL). An aqueous layer was washed with CH2Cl<sup>2</sup> (10 mL). Combined organic layers were dried (Na2SO4), filtrated and evaporated. CH2Cl<sup>2</sup> (1 mL) was added to the residue, and the target products **3A,G,C,IU** were precipitated by petroleum ether (12 mL). The suspension was cooled (−20 ◦C, 2 h), the precipitate was separated by centrifugation, washed by petroleum ether and dried under vacuum. B) Compounds **2U,T,BrU,ClU**: the reaction mixture was distributed between CH2Cl<sup>2</sup> (10 mL) and water (10 mL). An aqueous layer was evaporated, and the target monophosphates **3U,T,BrU,ClU** were purified by RPC in a gradient of MeCN (0%–75 %) in water. Appropriate fractions were combined and evaporated. The residue was dried under vacuum. Morpholino nucleotides **3A-ClU** were obtained as TEA salts
