The invention provides unsaturated phosphonates of Formula I or a tautomer or pharmaceutically accepatble salt thereof, as described herein, as well as pharmaceutical compositions comprising the compounds, and therapeutic methods comprising administering the compounds. The compounds have anti-viral properties and are useful for treating viral infections (e.g. HCV) in animals (e.g. humans).
Compounds having the formula I wherein R
1
, R
2
and R
3
are as defined herein are Hepatitis C virus NS5
b
polymerase inhibitors. Also disclosed are compositions and methods for treating an HCV infection and inhibiting HCV replication.
Compounds having the formula I or II, wherein R
1
, R
2
, B, and V are as defined herein, are Hepatitis C virus NS5b polymerase inhibitors. Also disclosed are compositions and methods for treating an HCV infection and inhibiting HCV replication.