Scheme 1.

Synthesis of Phosphorothioates 1^a

^a Conditions: (a) TIPDSCl₂, imidazole, Py, 88%; (b) MOMCl, DIPEA, DMF, 65 °C, 63%; (c) TBAF, THF, 77%; (d) N,N -dimethylphosphor-midite, 1H -tetrazole,; m-CPBA, 81%; (e) TBAF·3H₂O, DMF, 91%; (f) TESCl, imidazole, CH₂Cl₂, 88%; (g) Dibal-H, CH₂Cl₂, −78 °C, 84%; (h) Bis(2-cyanoethoxy) (diisopropylamino)phosphine, 1H-tetrazole; phenylacetyl disulfide, 72%; (i) NH₄F, MeOH, 85%; (j) 1H -tetrazole, CH₂Cl₂, rt; then t-BuOOH; (k) TEA, BSTFA, CH₃CN; (l) TMSBr/CH₂Cl₂ (2:3), rt, 40 min; (m) MeOH.