Synthesis of Isopropyl Bisphosphate, Compound 5a
a Reaction conditions: (i) NaBH4, EtOH, 0 °C, 30 min, 91%; (ii) 2,6-dichloropurine, DEAD, PPh3, THF, rt, overnight, 68%; (iii) CH3NH2, THF, rt, 4 h, 88%; (iv) Dowex 50x-8 200, MeOH, reflux, overnight, 75%; (v) Et2NP(OtBu)2, tetrazole, THF, rt, 20 min, then MCPBA, 73%; (vi) Dowex 50x-8 200, MeOH:H2O = 1:1, 80 °C, 16 h, 63%.