Azido nucleosides and nucleotide analogs

A method of ameliorating or treating a viral infection selected from a human respiratory syncytial virus (RSV) viral infection and an influenza A virus viral infection comprising administering a therapeutically effective amount of a compound of Formula (II), or a pharmaceutically acceptable salt thereof, to a subject suffering from the viral infection, wherein the compound of Formula (II) has the structure:

• wherein:
• B1a is selected from the group consisting of an optionally substituted heterocyclic base and an optionally substituted heterocyclic base with a protected amino group;
• R1a is selected from the group consisting of hydrogen, an optionally substituted acyl,
• na is 0, 1, or 2;
• R2a and R3a are independently selected from the group consisting of hydrogen, an optionally substituted C1-6 alkyl and an optionally substituted C1-6 haloalkyl;
• R4a is selected from the group consisting of hydrogen, halogen, an optionally substituted C1-6 alkyl, —OR18a and —OC(═O)R19a;
• R5a is selected from the group consisting of hydrogen, halogen, an optionally substituted C1-6 alkyl, —OR20a and —OC(═O)R21a;
• R6a is selected from the group consisting of hydrogen, halogen, an optionally substituted C1-6 alkyl, —OR22a and —OC(═O)R23a;
• or R5a and R6a are both oxygen atoms and linked together by a carbonyl group;
• R7a is selected from the group consisting of hydrogen, halogen, optionally substituted C1-6 alkyl, —OR24a and —OC(═O)R25a;
• R8a is selected from the group consisting of hydrogen, an optionally substituted C1-6 alkyl and an optionally substituted C1-6 haloalkyl;
• R9a and R12a are independently absent or hydrogen;
• R10a is absent or hydrogen;
• each R11a is independently absent or hydrogen;
• R13a is absent or hydrogen;
• R14a is selected from the group consisting of an —O-optionally substituted aryl, an —O-optionally substituted heteroaryl and an —O-optionally substituted heterocyclyl, and R15a is an optionally substituted N-linked amino acid or an optionally substituted N-linked amino acid ester derivative;
• or
• R14a is an optionally substituted N-linked amino acid or an optionally substituted N-linked amino acid ester derivative, and R15a is an optionally substituted N-linked amino acid or an optionally substituted N-linked amino acid ester derivative;
• or
• R14a is O−, hydroxy or an —O-optionally substituted C1-6 alkyl, and R15a and R5a together are O;
• R16a is selected from the group consisting of an —O-optionally substituted aryl, an —O-optionally substituted heteroaryl and an —O-optionally substituted heterocyclyl, and R17a is an optionally substituted N-linked amino acid or an optionally substituted N-linked amino acid ester derivative;
• or
• R16a is an optionally substituted N-linked amino acid or an optionally substituted N-linked amino acid ester derivative, and R17a is an optionally substituted N-linked amino acid or an optionally substituted N-linked amino acid ester derivative;
• or
• R16a is O−, hydroxy or an —O-optionally substituted C1-6 alkyl, and R17a and R5a together are O;
• R18a, R20a, R22a and R24 are independently hydrogen or an optionally substituted C1-6 alkyl;
• and
• R19a, R21a, R23a and R25a are independently selected from the group consisting of an optionally substituted C1-6 alkyl and an optionally substituted C3-6 cycloalkyl.