Provided are compounds of Formula I: and pharmaceutically acceptable salts and esters thereof. The compounds, compositions, and methods provided are useful for the treatment of virus infections, particularly hepatitis C infections.提供式I化合物:及其藥學上可接受之鹽類和酯類。所提供之化合物、組成物和方法係用於治療病毒感染,特別是C型肝炎感染。