Scheme 3.

Synthesis of ribose-ketal analogs: Reagents and conditions: (a) TBSCl, imid., DMF, rt, 2–24 h; (b) CH₂Br₂, NaOH, TBAB, CH₂Cl₂/H₂O, 40 °C, 72 h; (c) Boc₂O, DMAP, DMF, 18 h; (d) TBAF, THF, 1–18 h; (e) MsCl, Et₃N, CH₂Cl₂, 1 h; (f) MeNH₂/EtOH, THF, 50 °C, 18 h; (g) 13, Et₃N, NaI, MeCN, rt, 3–18 h; (h) 1M H₂SO₄, MeOH, rt, 18 h; (i) cyclohexanone, TFA, rt; (j) TFA, CH₂Cl₂, rt, 1 h.