Methods and compositions for treating flaviviruses and pestiviruses

A method for the treatment of a flavivirus or pestivirus infection in a host, comprising administering to said host an anti-virally effective amount of a compound of Formula XVII:

• or a pharmaceutically acceptable salt or ester thereof, wherein:
• Base is a purine;
• R1 and R2 are independently H;
• phosphate;
• a stabilized phosphate prodrug;
• acyl;
• alkyl;
• sulfonate ester;
• benzyl, wherein the phenyl group is optionally substituted with one or more substituents selected from hydroxyl, amino, alkylamino, arylamino, alkoxy, aryloxy, nitro, cyano, sulfonic acid, sulfate, phosphonic acid, phosphate, phosphonate, either unprotected or protected as necessary;
• a lipid;
• an amino acid;
• a carbohydrate;
• a peptide;
• a cholesterol;
• or other pharmaceutically acceptable leaving group which when administered in vivo provides a compound wherein R1 and R2 are independently H or phosphate;
• R6 is alkyl, azido, cyano, alkenyl, alkynyl, Br-vinyl, —C(O)O(alkyl), —O(acyl), —O(alkyl), —O(alkenyl), chloro, bromo, fluoro, iodo, NO2, NH2, —NH(lower alkyl), —NH(acyl), —N(lower alkyl)2, or —N(acyl)2;
• R7 and R9 are independently OR2, hydroxy, alkyl, azido, cyano, alkenyl, alkynyl, Br-vinyl, —C(O)O(alkyl), —O(acyl), —O(alkyl), —O(alkenyl), chlorine, bromine, iodine, NO2, NH2, —NH(lower alkyl), —NH(acyl), —N(lower alkyl)2, or —N(acyl)2;
• R10 is H, alkyl, chlorine, bromine or iodine;
• X is O, S, SO2 or CH2.