Discovery of (R)-6-Cyclopentyl-6-(2-(2,6-diethylpyridin-4-yl)ethyl)-3-((5,7-dimethyl-1,2,4triazolo1,5-apyrimidin-2-yl)methyl)-4-hydroxy-5,6-dihydropyran-2-one (PF-00868554) as a Potent and Orally Available Hepatitis C Virus Polymerase Inhibitor