Scheme 10.

Synthesis of Cyclopropyl Bisphosphate Derivative, Compound 20^a

^a Reaction conditions: (i) Et₂Zn, CH₂I₂, CH₂Cl₂, rt, overnight, 96%; (ii) MsCl, TEA, CH₂Cl₂, rt, 1 h, 93%; (iii) 2-chloro-N⁶-methylaminopurine, K₂CO₃, 18-crown-6, DMF, 60 °C, overnight, 62%; (iv) AcOH:THF:H₂O = 1:2:2, 40 °C, overnight, 55%; (v) Et₂NP(OtBu)₂, tetrazole, THF, rt, 30 min then MCPBA, 40%; (v) 5% TFA:CH₂Cl₂:THF = 10:20:1, rt, 30 min, 55%.