Modified 2′ and 3′-nucleoside prodrugs for treating Flavivridae infections

A method for the treatment of a Flaviviridae virus infection in a host infected with a Flaviviridae virus comprising administering to the host an effective treatment amount of a compound of formula

• or a pharmaceutically acceptable salt thereof
• wherein:
• R1 is hydrogen, mono, di or triphosphate;
• acyl;
• an amino acid ester;
• a carbohydrate;
• a peptide;
• or a pharmaceutically acceptable leaving group which when administered in vivo provides a compound wherein R1 is independently H or phosphate;
• Base is selected from the group consisting of:
• wherein W1 is N, CH, CCN, CX3, CBr, CCl, or CF;
• X is O;
• each X3 is independently straight chained, branched or cyclic optionally substituted alkyl, CH3, NH2 or fluoro;
• R8, R10 and R11 are H;
• R9 is OH or OR2
• R12 is C(Y3)3;
• R13 is OR2 or fluoro;
• each Y3 is independently H or F;
• each R2 is independently hydrogen, acyl, an amino acid ester, a carbohydrate, a peptide, or a pharmaceutically acceptable leaving group which when administered in vivo provides a compound wherein R2 is H;
• and
• each R4 and R5 is independently hydrogen, acyl, alkyl, lower alkyl, alkenyl, alkynyl or cycloalkyl.