The invention provides novel nucleoside compounds of formula I wherein R.sup.1, R.sup.2a, R.sup.2b, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.8a, R.sup.9 and R.sup.10 are as defined herein which are useful for the treatment of Hepatitis C Virus (HCV) mediated diseases. The invention further provides methods for treatment or prophylaxis of HCV mediated diseases with compounds of formula I and pharmaceutical compositions comprising these compounds,