Provided herein are compounds, compositions and methods for the treatment of Flaviviridae infections, including HCV infections. In certain embodiments, compounds and compositions of nucleoside derivatives are disclosed, which can be administered either alone or in combination with other anti-viral agents. In certain embodiments, the compounds are 1′,4′-thio nucleoside compounds which display remarkable efficacy and bioavailability for the treatment of, for example, HCV infection in a human. In certain embodiments, the compounds are of Formula 3001: or a pharmaceutically acceptable salt, solvate, stereoisomeric form, tautomeric form, or polymorphic form thereof; wherein Base, RA, RB, W, X, Y, and Z are as described herein.