A compound of formula I or a salt thereof: wherein, X is a bond V is -C(O)O-, -C(O)N(R 5 )-, -C(O)N(R 5 )O-, -NH-C(O)-N(R 5 )- or NH-S(O) n -; L is a bond, alkylene wherein one or more non-adjacent methylene groups of said alkylene are optionally 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)O-R 1 , alkyl or alkyl substituted with OH or - C(O)O-Ri; and wherein a carbon of said alkylene groups and R x optionally together form a heterocycle; R 1 is alkyl 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; wherein one or more non-adjacent methylene groups in any of the foregoing alkyl groups is optionally replaced with O; R 2 is H or Ri; R 3 is absent, H, Me; provided that when -X-L-C(O)O-Ri depends from the carbon to which R 3 depends then R 3 is absent; or R 3 is -Z-L-C(O)O-R 1 wherein Z is a bond, -O-, -N(R x )-, -C(O)N(R x )-, -N(R x )-C(O)-, -S(O) n -N(R x )- or -N(R x )-S(O) n -; and R 4 is absent, H or OH; provided that when -X-L-C(O)O-Ri depends from the carbon to which R 4 depends then R 4 is H or absent; R 5 is H or alkyl; R x is H, -C(O)O-R 1 , or alkyl optionally substituted with -C(O)O-R 1 ; and n is 1 or 2. X is a bond V is -C(O)O-, -C(O)N(R 5 )-, -C(O)N(R 5 )O-, -NH-C(O)-N(R 5 )- or NH-S(O) n -; L is a bond, alkylene wherein one or more non-adjacent methylene groups of said alkylene are optionally 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)O-R 1 , alkyl or alkyl substituted with OH or - C(O)O-Ri; and wherein a carbon of said alkylene groups and R x optionally together form a heterocycle; R 1 is alkyl 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; wherein one or more non-adjacent methylene groups in any of the foregoing alkyl groups is optionally replaced with O; R 2 is H or Ri; R 3 is absent, H, Me; provided that when -X-L-C(O)O-Ri depends from the carbon to which R 3 depends then R 3 is absent; or R 3 is -Z-L-C(O)O-R 1 wherein Z is a bond, -O-, -N(R x )-, -C(O)N(R x )-, -N(R x )-C(O)-, -S(O) n -N(R x )- or -N(R x )-S(O) n -; and R 4 is absent, H or OH; provided that when -X-L-C(O)O-Ri depends from the carbon to which R 4 depends then R 4 is H or absent; R 5 is H or alkyl; R x is H, -C(O)O-R 1 , or alkyl optionally substituted with -C(O)O-R 1 ; and n is 1 or 2. <EOS>