Scheme 5.

Reagents and conditions: (i) NaBH₄, MeOH, rt, 79%; (ii) BnBr, KOH, 18-crown-6, THF, H₂O, rt, 86%; (iii) conc. HCl, THF, rt; (iv) (Boc)₂O, NaHCO₃, MeCN, H₂O, rt, 69% (2 steps); (v) MsCl, Et₃N, CH₂Cl₂, rt; (vi) TFA, CH₂Cl₂, rt; (vii) Et₃N, MeCN, rt, 75% (3 steps); (viii) OsO₄, NaIO₄, 2,6-lutidine, t-BuOH, H₂O, rt; (ix) NaBH₄, MeOH, rt, 45% (2 steps); (x) MsCl, NaHCO₃, CH₂Cl₂, H₂O, rt, 61%.