Process for preparing inhibitors of Hepatitis C virus

A process for preparing a compound of formula (Z) ** ** Formula or a pharmaceutically acceptable salt thereof, wherein: R4 and R5 are independently selected from hydrogen, alkyl, aryl, arylalkyl, heterocyclyl and heterocyclylalkyl; the process comprising: treating a compound of formula (IV) ** ** Formula with a base in the presence of ammonium acetate.