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Synthetic method B to phosphorodiamidates 24–40. Reagents and conditions: (a) POCl3 (1.0 mol/eq), trimethylphosphate or triethylphosphate, −5 °C, 4–5 h, or 5 °C, 16 h; (b) amino acid ester p-TSA or HCl salt (5.0 mol/eq), anhydrous CH2Cl2, anhydrous Et3N (10.0 mol/eq) or anhydrous DIPEA (5.0–10.0 mol/eq), −78 °C, 30 min, then room temperature, 16–45 h.
