(1R,2S)-1-Amino-N-(cyclopropylsulfonyl)-2-vinylcyclopropanecarboxamide 4-methylbenzenesulfonate acts as a reagent in the preparation, SAR of methoxyisoquinoline derivatives and discovery of asunaprevir (BMS-650032), an orally efficacious NS3 protease inhibitor for the treatment of hepatitis C virus infection.