An isomer, enantiomer, diastereoisomer or tautomer of a compound, represented by formula I: wherein: A is O, S, N R 1 , or C R 1 , wherein R 1 is defined herein； ----- represents either a single or a double bond； R 2 is selected from: H, halogen, R 21 , O R 21 , S R 21 , COO R 21 , SO 2 N( R 22 ) 2 , N( R 22 ) 2 , CON( R 22 ) 2 , N R 22 C(O) R 22 or N R 22 C(O)N R 22 wherein R 21 and each R 22 is defined herein； B is NR 3 or CR 3 , with the proviso that one of A or B is either C R 1 or C R 3 , wherein R 3 is defined herein； K is N or CR 4 , wherein R 4 is defined herein； L is N or CR 5 , wherein R 5 has the same definition as R 4 ； M is N or CR 7 , wherein R 7 has the same definition as R 4 ； Y 1 is O or S； Z is N( R 6a ) R 6 or O R 6 , wherein R 6a is H or alkyl or N R 61 R 62 wherein R 61 and R 62 are defined herein； and R 6 is H, alkyl, cycloalkyl, alkenyl, Het, alkyl-aryl, alkyl- Het ； or R 6 is wherein R 7 and R 8 and Q are as defined herein； Y 2 is O or S； R 9 is H, (C 1-6 alkyl), (C 3-7 )cycloalkyl or (C 1-6 )alkyl-(C 3-7 )cycloalkyl, aryl, Het , (C 1-6 )alkyl-aryl or (C 1-6 )alkyl- Het, all of which optionally substituted with R 90 ； or R 9 is covalently bonded to either of R 7 or R 8 to form a 5- or 6-membered heterocycle； a salt or a derivative thereof, as an inhibitor of HCV NS5B polymerase.
