A compound of formula (I′) wherein R6 is H, C1-6alkyl, —(CH2)pCOOH, —(CH2)pCOOC1-6alkyl, —(CH2)pCONH2, —(CH2)pCONHC1-6alkyl, —(CH2)pCON(C1-6alkyl)2, R7 is as defined for R6; or and R7 taken together with the atoms joining them can form a 6-membered aromatic or non aromatic, saturated or unsaturated, carbocyclic or heteroatom containing (e.g. O, N or S containing) ring optionally substituted by up to 4 groups R8; each R8 is defined as for R6 or is oxo; R10 is C1-6alkyl Ar is a C6-14 aryl group, wherein the aryl group may be optionally substituted (preferably in the meta or para position relative to O) with one or more R9 groups; each R9 is halo, OH, CN, nitro, NH2, NHC1-6alkyl, N(C1-6alkyl)2, haloC1-6alkyl, C6-10 aryl group, C7-12 arylalkyl, a C1-10alkyl group, C2-10-mono or multiply unsaturated alkenyl group, OC1-10alkyl group, SC1-10alkyl group —OC1-10alkyl-O—C10alkyl, —C1-10alkyl-O—C1-10alkyl, —OC2-10-mono or multiply unsaturated alkenyl group, OAr2, O(CH2)qAr2, SAr2 or S(CH2)qAr2; wherein Ar2 is phenyl, optionally substituted with one or more of halo, trihalomethyl, C1-10-alkoxy, or C1-10 alkyl; each q is 1 to 3, preferably 1; each p is 0 to 3; or a salt, ester, solvate, N-oxide, or prodrug thereof.