3.1.1. General Procedure for the Synthesis of Morpholino Nucleosides **2**

Ribonucleosides **1** (4 mmol) were suspended in EtOH (80 mL). A warm solution of NaIO<sup>4</sup> (4.2 mmol, 0.9 g) in water (4 mL) was added to the suspension under vigorous stirring. In 15 min, (NH4)2B4O7·4H2O (4.8 mmol, 1.26 g) was added. The pH of the reaction mixture was maintained between 8.5 and 9.0 by adding triethylamine (TEA, total amount 0.6–1.0 mL). After 1.5 h of stirring, the precipitate was separated by filtration and washed with EtOH (2 × 4 mL). NaCNBH<sup>3</sup> (5.2 mmol, 0.326 g) was added to the combined filtrates and the stirring was continued. In 1 h, trifluoroacetic acid (TFA) was added till pH 3.0–4.0. In 2 h, solvents were evaporated and the residue was dried by coevaporation with MeCN (3×10 mL) and toluene (3×10 mL), suspended in DMF (16 mL) and Et3N (12 mmol, 1.7 mL). Triphenylmethyl chloride (TrCl) (3.5 mmol, 0.975 g) was added to the suspension, and the stirring was performed overnight. The reaction was quenched by the addition of MeOH (5 mL); the solution was evaporated up to half of a volume and poured into the water (300 mL). The precipitate was separated by filtration and dried. After that, the precipitate was dissolved in CH2Cl<sup>2</sup> (10 mL), and the target morpholino nucleoside was precipitated by the petroleum ether (250 mL). The suspension was cooled (−20 ◦C, 2 h) and the precipitate was separated by filtration, washed by petroleum ether, and dried under vacuum.
