Adefovir dipivoxil can be obtained by two similar ways: 1) The reaction of adenine (I) with 2-(diisopropoxyphosphorylmethoxy)ethyl methanesulfonate (II) by means of cesium carbonate in DMF gives the expected condensation product (III), which is converted into the phosphonic acid (IV) by treatment with trimethylsilyl bromide in acetonitrile (1). Finally, this compound is condensed with chloromethyl pivalate (V) by means of N,N'-dicyclohexylmorpholine-4-carboxamidine in DMF. 2) The final condensation of the phosphonic acid (IV) can also be performed with iodomethyl pivalate (VI) in pyridine.
An optimized process for large-scale production of adefovir dipivoxil has been reported: The reaction of adenine (I) with 1,3-dioxolan-2-one (II) by means of NaOH in hot DMF gives 9-(2-hydroxyethyl)adenine (III), which is condensed with diethyl p-toluenesulfonyloxymethylphosphonate (IV) by means of sodium tert-butoxide, (instead of NaH, originally used) in DMF yielding 9-[2-(diethylphosphonomethoxy)ethyl]adenine (V). The hydrolysis of intermediate (V) by means of TMS-Br in hot acetonitrile affords the phosphonic acid (VI), which is finally condensed with chloromethyl pivalate (VII) by means of triethylamine in hot N-methylpyrrolidine.
The 9-[2-(phosphonomethoxy)ethyl]adenine (VIII), intermediate in the synthesis of adefovir has been obtained as follows: 1) The partial hydrolysis of 2-acetoxyethoxymethylphosphonic acid diethyl ester (I) with Dowex 50XB in ethanol gives the corresponding 2-hydroxyethoxymethylphosphonate (II), which is treated with CCl4/PPh3, with CBr4/PPh3 or with TsCl yielding 2-choroethoxy (III), 2-bromoethoxy (IV) or 2-tosyloxyethoxy (V) derivatives, respectively (1). The condensation of (III), (IV) or (V) with adenine (VI) by means of NaH in hot DMF affords 9-[2-(diethoxyphosphorylmethoxy)ethyl]adenine (VII), which is finally treated with TMS-Br in acetonitrile to give the desired intermediate 9-[2-(phosphonomethoxy)ethyl]adenine (VIII). 2) The condensation of N6-benzoyl-9-(2-hydroxyethyl)adenine (IX) with tosyloxymethylphosphonic acid dimethyl ester (X) by means of NaH in warm DMF gives 9-[2-(dimethoxyphosphorylmethoxy)ethyl]adenine (X), which is treated first with NaOH in DMF/water and finally with TMS-I in DMF to afford the desired intermediate 9-[2-(phosphonomethoxy)ethyl]adenine (VIII).