Disclosed are 5,6-dihydro-1H-pyridin-2-one compounds of formula I and a process for their preparation which are useful in treating or preventing a hepatitis C virus infection. An example of a compound of formula I is (rac-di-exo)-N- { 3-[3-(4-fluoro-benzyl)-6-hydroxy-4-oxo-3-azatricyclo[6.2.1.02,7]undec-5-en-5-yl]-1,1-dioxo-1,4-dihydro-1[lambda]6-benzo[1,2,4]thiadiazin-7-yl} -methanesulfonamide.