The present invention relates to compounds of formula I: or a pharmaceutically acceptable salt or mixtures thereof that inhibit serine protease activity, particularly the activity of hepatitis C virus NS3-NS4A protease. As such, they act by interfering with the life cycle of the hepatitis C virus and are also useful as antiviral agents. The invention further relates to compositions comprising these compounds either for ex vivo use or for administration to a patient suffering from HCV infection and to processes for preparing the compounds. The invention also relates to methods of treating an HCV infection in a patient by administering a composition comprising a compound of this invention. The invention further relates to processes for preparing these compounds.