An enantiomer, diastereoisomer or tautomer of a compound, represented by formula I: ##STR00001## wherein A, B, R.sup.1, R.sup.2, R.sup.3, R.sup.4, R.sup.5, R.sup.6, R.sup.7, R.sup.8, R.sup.9, and R.sup.10 are as defined herein, or a salt or ester thereof, as an inhibitor of HCV NS5B polymerase.