A compound of formula (III): or a salt, solvate and chemically protected form thereof, wherein: R
5
is an optionally substituted C
5-20
aryl or C
4-20
alkyl group; L′ is a single bond, —O— or —C(═O)—; A is selected from the group consisting of: formulae (i) (ii) (iii) wherein X and Y are selected from the group consisting of: O and CR
3
; S and CR
3
; NH and CR
3
; NH and N; O and N; S and N; N and S; and N and O, and where the dotted lines indicate a double bond in the appropriate location, and where Q is either N or CH; D is selected from: formulae (i) (ii) (iii) (iv) (v) (vii) (viii) (ix) B is selected from the group consisting of: formulae (A) (B) where R
P6
is selected from fluoro and chloro; and R
2
is either: (i) —CO
2
H; (ii) —CONH
2
; (iii) —CH
2
—OH; or (iv) tetrazol-5-yl.