Disclosed are spiro compounds of formula (I), salts thereof, and methods for their preparation. A specific example of compounds of formula (I) is: 7-((S)-3-(benzo[b]thiophen-3-yl)-2-(2-cyclohexylacetamido)propanoyl)-3-(3-chlorophenyl)-N-((3R)-1-(cyclopropylamino)-2-hydroxy-1-oxohexan-3-yl)-1-oxa-2,7-diazaspiro[4.4]non-2-ene-8-carboxamide, Compounds of formula (I) are useful for treating hepatitis C virus