Scheme 7.

Stereoselective synthesis of PD2_{n-3 DPA} (12) [55]. Reagents and conditions: (i) EtSH, HCl, rt., 3 h; (ii) TBSOTf, 2,6-lutidine, 0 °C, 16 h; (iii) NBS, 2,6-lutidine, acetone/H₂O, 0 °C, 1 h; (iv) NaHMDS, BrPh₃P(CH₂)₂CH₃, CH₂Cl₂, −78 °C to rt; (v) PTSA, EtOH/MeOH (1:1), −20 °C to rt; (vi) DMP, NaHCO₃, CH₂Cl₂, rt; (vii) (triphenylphosphoranylidene)acetaldehyde, toluene, 95 °C; (viii) NaHMDS, HPMA, THF, then 59, −78 °C; (ix) CSA, MeOH/CH₂Cl₂ (2:1); (x) PPh₃, I₂, imidazole, CH₂Cl₂, then Ph₃P, MeCN, Δ; (xi) NaHMDS, THF, HMPA, −78 °C; (xii) TBAF, THF, 0 °C to rt; (xiii) LiOH, THF/MeOH/H₂O (2:2:1), 0 °C.