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Synthetic method A to phosphorodiamidates 14–21. Reagents and conditions: (a) anhydrous Et3N (1.0–1.2 mol/eq), anhydrous THF, room temperature, 30 min; then POCl3 (1.0–1.2 mol/eq), −78 °C, 30 min; (b) amino acid ester p-TSA salt (3.0–5.0 mol/eq) in anhydrous CH2Cl2, room temperature; then anhydrous Et3N (5.0–10.0 mol/eq), −78 °C, then room temperature, 16–20 h.
