A compound selected from the group consisting of a 5,6-dihydro-&agr;-pyrone of formula (I):
wherein bond a is oriented either or ;
X is O or NH;
R1 is selected from the group consisting of:
a group R3(O)C— wherein R3 is selected from the group consisting of: (i) a group of formula R4—CH═CH— wherein R4 is C1-C20 alkyl, C2-C20 alkenyl, an aryl group,
or a 5- or 6-membered unsaturated heterocyclic ring containing one or two O, N or S atoms, (ii) C2-C20 alkyl;
and (iii) an aryl group or a substituent which is a fused ring system of formula (1) or (2):
wherein each of R′ and R″, which are the same or different and may occupy any position on ring a or ring b of said fused ring system, is H or C1-C6 alkyl;
a group ArCH2— wherein Ar is an aryl group; and
a group R5O—CH2— wherein R5 is C1-C6 alkyl optionally interrupted by one or two O atoms; and
R2 is CH3 or, when R1 is a group of formula (A) as defined below, R2 is R6OOC— or R6NOC— wherein R6 is C1-C6 alkyl;
and the pharmaceutically and veterinarily acceptable salts and esters thereof; with the exception of compounds wherein bond a is oriented ,
X is O and R1 is a group of formula (A):
wherein R11 and R21 are H when R2 in formula (I) is CO2H or CH3, or one of R11 and R21 is H and the other is OH when R2 in formula (I) is CO2H.