Scheme 5. Solid-Phase Synthesis of N_PST Dimers.

Reagents and conditions: (i) 0.1 M monomer, 0.25 M condensing reagent in the presence of 1.8 M base or in the absence of base, CH₃CN, rt, 3 min; (ii) 3% DCA, CH₂Cl₂–Et₃SiH (1:1, v/v), rt, 4 × 15 s; (iii) 0.1 M Et₃N in CCl₄–2,6-lutidine–H₂O (5:12.5:1, v/v/v), rt, 90 min; (iv) concentrated aqueous NH₃–EtOH (3:1, v/v), 30 °C, 12 h for entries 1–4, 50 °C, 12 h for entries 5–7.