An isomer, enantiomer, diastereoisomer or tautomer of a compound, represented by formula I:wherein:A is O, S, NR1, or CR1, wherein R1 is defined herein----- represents either a single or a double bondR2 is selected from: H, halogen, R21, OR21, SR21, COOR21, SO2N(R22)2, N(R22 )2, CON(R22)2, NR22C(O)R22 or NR22C(O)NR22 wherein R21 and each R22 is defined hereinB is NR3 or CR3, with the proviso that one of A or B is either CR1 or CR3,wherein R3 is defined hereinK is N or CR4, wherein R4 is defined hereinL is N or CR5, wherein R5 has the same definition as R4M is N or CR7, wherein R7 has the same definition as R4Y1 is O or SZ is N(R6a)R6 or OR6, wherein R6a is H or alkyl or NR61R62 wherein R61 and R62 are defined herein and R6 is H, alkyl, cycloalkyl, alkenyl, Het, alkyl-aryl, alkyl-Hetor R6 iswherein R7 and R8 and Q are as defined hereinY2 is O or SR9 is H, (C1-6 alkyl), (C3-7)cycloalkyl or (C1-6)alkyl-(C3-7)cycloalkyl, aryl, Het, (C1-6)alkyl-aryl or (C1-6)alkyl-Het, all of which optionally substituted with R90 or R9 is covalently bonded to either of R7 or R8 to form a 5- or 6-membered heterocyclea salt or a derivative thereof, as an inhibitor of HCV NS5B polymerase.
