A compound of formula I or a salt thereof: wherein, X is a bond, -O-, -C -, -N(R x)-, -C N(R x)-, -N(R x)-C -, -S n -N(R x)- or - N(R x)-S n -; L is a bond, alkylene wherein one or more non-adjacent methylene groups of said alkylene are replaced with -O-; divalent aryl or divalent heteroaryl; or L is alkylene-Y-alkylene wherein Y is O, NR x, S, SO, SO 2 or a divalent heterocycle; wherein said alkylene groups are optionally substituted with OH, -C O-R 1, alkyl or alkyl substituted with OH or -C O-R 1; and wherein a carbon of said alkylene groups and R x optionally together form a heterocycle; provided that when X is other than a bond, then L is other than a bond; W is O or S; Q is a bond or alkylene; R 1 is H, alkyl, a carbocycle or a heterocycle wherein said alkyl, carbocycle and heterocycle are each optionally substituted with halogen, OH, amino, oxo, carboxy, acyloxy, alkoxycarbonyl, alkoxyacyloxy, alkoxycarbonyloxy, aminocarbonyl, a carbocycle optionally substituted with alkyl, haloalkyl, oxo, amino and halogen and a heterocycle optionally substituted with alkyl, oxo, amino and halogen; and a carbocycle or heterocycle optionally substituted with alkyl, haloalkyl, oxo, amino and halogen; V is -C O-, -C O-(CHR 5)-O-C -, -C O-(CHR 5)-O-C -O-, -C O-(CH 2) n -Y-, -C N(R 5)-, -C N(R 5)-O-, -NH-C -N(R 5)- or NH-S n -; wherein Y is a divalent heterocycle optionally substituted with alkyl, halogen, OH, amino, carboxyl and oxo; R 2 is H or R 5; R 3 is absent or alkyl; R 4 is absent, H, OH, =O, -R 6, -O-R 6, -C O-R 6, -O-C -Re, -O-C -O-Re, -O-C -NR 5 R 6, -NR 5 R 6, -NR 5 -C -R 6, -NR 5 -C -O-R 6, -NR 5 -SO 2 -R 6, =N-O-Rs; R 5 is H or alkyl optionally substituted with a carbocycle or heterocycle wherein said carbocycle and heterocycle are optionally substituted with halogen, OH, oxo and alkyl; R 6 is H, alkyl, a carbocycle, a heterocycle wherein said alkyl, carbocycle and heterocycle are optionally substituted with halogen, OH, SH, alkylthio, -S -alkyl, -SO 2 -alkyl, amino, -NHC -alkyl, oxo, alkyl, carboxyl, acyl, acyloxy, alkoxy, alkoxycarbonyl, a carbocycle optionally substituted with halogen, OH, amino or alkyl, or a heterocycle optionally substituted with halogen, OH, amino or alkyl; and wherein one or more non-adjacent methylene groups in each of said alkyl groups of R 6 are optionally replaced with -O- or -S-; R x is H, -C O-R 1, or alkyl optionally substituted with -C O-R 1; and n is 1 or 2, wherein the dashed lines between the 2- and 3- positions of the fused ring system indicate alternatively a single or double bond; and wherein the dashed lines converging inside the fused ring system indicate that R4, when present, and the group comprising -X-L-C -Q- may be attached alternatively at the 3-position or 4-position of the fused ring system. X is a bond, -O-, -C -, -N(R x)-, -C N(R x)-, -N(R x)-C -, -S n -N(R x)- or - N(R x)-S n -; L is a bond, alkylene wherein one or more non-adjacent methylene groups of said alkylene are replaced with -O-; divalent aryl or divalent heteroaryl; or L is alkylene-Y-alkylene wherein Y is O, NR x, S, SO, SO 2 or a divalent heterocycle; wherein said alkylene groups are optionally substituted with OH, -C O-R 1, alkyl or alkyl substituted with OH or -C O-R 1; and wherein a carbon of said alkylene groups and R x optionally together form a heterocycle; provided that when X is other than a bond, then L is other than a bond; W is O or S; Q is a bond or alkylene; R 1 is H, alkyl, a carbocycle or a heterocycle wherein said alkyl, carbocycle and heterocycle are each optionally substituted with halogen, OH, amino, oxo, carboxy, acyloxy, alkoxycarbonyl, alkoxyacyloxy, alkoxycarbonyloxy, aminocarbonyl, a carbocycle optionally substituted with alkyl, haloalkyl, oxo, amino and halogen and a heterocycle optionally substituted with alkyl, oxo, amino and halogen; and a carbocycle or heterocycle optionally substituted with alkyl, haloalkyl, oxo, amino and halogen; V is -C O-, -C O-(CHR 5)-O-C -, -C O-(CHR 5)-O-C -O-, -C O-(CH 2) n -Y-, -C N(R 5)-, -C N(R 5)-O-, -NH-C -N(R 5)- or NH-S n -; wherein Y is a divalent heterocycle optionally substituted with alkyl, halogen, OH, amino, carboxyl and oxo; R 2 is H or R 5; R 3 is absent or alkyl; R 4 is absent, H, OH, =O, -R 6, -O-R 6, -C O-R 6, -O-C -Re, -O-C -O-Re, -O-C -NR 5 R 6, -NR 5 R 6, -NR 5 -C -R 6, -NR 5 -C -O-R 6, -NR 5 -SO 2 -R 6, =N-O-Rs; R 5 is H or alkyl optionally substituted with a carbocycle or heterocycle wherein said carbocycle and heterocycle are optionally substituted with halogen, OH, oxo and alkyl; R 6 is H, alkyl, a carbocycle, a heterocycle wherein said alkyl, carbocycle and heterocycle are optionally substituted with halogen, OH, SH, alkylthio, -S -alkyl, -SO 2 -alkyl, amino, -NHC -alkyl, oxo, alkyl, carboxyl, acyl, acyloxy, alkoxy, alkoxycarbonyl, a carbocycle optionally substituted with halogen, OH, amino or alkyl, or a heterocycle optionally substituted with halogen, OH, amino or alkyl; and wherein one or more non-adjacent methylene groups in each of said alkyl groups of R 6 are optionally replaced with -O- or -S-; R x is H, -C O-R 1, or alkyl optionally substituted with -C O-R 1; and n is 1 or 2,