Antiviral compounds

A compound of formula I

• wherein:
• R1 is H, —P(═X)(OH)—O—P(═O)(OH)—P(═O)(OH)2, or —P(═X)(Ra)(Rb);
• R2 is H, F, Cl, Br, OH, (C1-C3)alkyl or (C1-C3)alkoxy, wherein any (C1-C3)alkyl and (C1-C3)alkoxy is optionally substituted with one or more halo;
• R3 is H, F, Cl, Br, OH, (C1-C3)alkyl or (C1-C3)alkoxy, wherein any (C1-C3)alkyl and (C1-C3)alkoxy is optionally substituted with one or more halo;
• R4 is selected from the group consisting of:
• Ra is an N-linked amino acid or a (C1-C6)alkyl ester of an N-linked amino acid, wherein the (C1-C6)alkyl is optionally substituted with phenyl;
• Rb is (C1-C6)alkoxy, aryloxy, or 5-10 membered heteroaryloxy, wherein any (C1-C6)alkoxy, aryloxy, and 5-10 membered heteroaryloxy is optionally substituted with one or more groups independently selected from the group consisting of halo, hydroxy, (C1-C3)alkoxy and (C1-C3)haloalkoxy;
• Rc is (C1-C3)alkyl, hydroxymethyl, formyl or carboxy;
• and
• X is S or O;
• or a salt thereof.