| http://www.w3.org/ns/prov#value | - A compound of the formula: or a pharmaceutically acceptable salt thereof, wherein R2 is an amino acid residue; and R3 and R4 are independently H, straight chained, branched or cyclic alkyl, dialkylaminoalkylene, CO-alkyl, CO-aryl, CO-alkoxyalkyl, CO-aryloxyalkyl, CO-substituted aryl, alkylsulfonyl, arylsulfonyl, aralkylsulfonyl, amino acid residue, mono, di, or triphosphate, or a phosphate prodrug
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