Scheme 2.

Reagents and conditions: (a) BzCl, pyridine, 0 °C, 2 h, 94%; (b) DAST, CH₂Cl₂, −65 to 0 °C, 2 h, 43%; (c) 7 M NH₃ in MeOH, rt, overnight, 81%; (d) Dess-Martin periodinane, CH₂Cl₂, 0 °C to rt; 2 h; (e) NaBH₄, MeOH, rt, overnight, 81% (two steps); (f) BzCl, pyridine, 0 °C to rt, 3 h, 88%; (g) Pd/C, H₂, MeOH, rt, overnight, >99%; (h) BzCl, pyridine, 0 °C to rt, overnight, 93%; (i) H₂SO₄, AcOH, Ac₂O, 0 °C, 3 h, 87% for 20, 12% for 21; (j) N⁶-benzoyladenine, BSA, TMSOTf, CH₃CN, 65 °C, 3 h; (k) 7 M NH₃ in MeOH, rt, overnight, 47% (two steps).