Scheme 1.
(A) Reagents and conditions: (i) 1. 7-hydroxyheptyl 2-phenoxyacetate (1.2 eqv.), PPh3 (1.2 eqv.), diisopropyl azodicarboxylate (1.2 eqv.), dioxane, 21 °C, 16 h; 2. TBAF (1 M in THF), THF, 21 °C, 30 min; (ii) N,N-diisopropylamino cyanoethyl phosphonamidic chloride (1.2 eqv.), N,N-diisopropylethylamine (1.5 eqv.), THF, 21 °C, 30 min. (B) Reagents and conditions: (iii) N,N-diisopropylamino cyanoethyl phosphonamidic chloride (1.2 eqv.), N,N-diisopropylethylamine (1.5 eqv.), THF, 21 °C, 30 min.