Scheme 2.

Synthesis of pentasialic acid 23

Reagents and conditions: (a) TMSOTf, CH₂Cl₂/CH₃CN (2:1), −78 °C, 1 h, then AC₂O, TfOH, CH₂Cl₂, 20 min; (b) dibutyl phosphate, NIS, TfOH, CH₂Cl₂, 0 °C, 12 h; (c) LiOH, MeOH/H₂O, reflux, 24 h, and NaHCO₃, AC₂O, H₂O, rt, 3 h, then NaOMe, MeOH, rt, 24 h, finally Pd/c, H_2, H₂O, rt, 12 h.