Scheme 2. Reagents and Conditions.

a) Triphenylphosphine, I₂, imidazole, anhydrous THF, 74%, b) 10% Aq. trifluoroacetic acid, THF, 65 °C, 60%; c) Trisodium thiophosphate, H₂O, 57%; d) O,O′-diethyl chlorothiophosphate, anhydrous pyridine, rt, 13%; e) Dowex-50 resin, MeOH:H₂O (1:1, v/v) 55 °C, 12% (12), 48% (11), 59% (13); f) O,O-Diethyl thiophosphate potassium salt, anhydrous CH₃CN, Et₃N, rt, 52%; g) O,O′-Diethyl dithiophosphate, anhydrous CH₃CN, Et₃N, rt, 49%;