Syntheses of (2E,4E)-5-arylpenta-2,4-dienoic acid hydroxyamides are described, some of which are potent inhibitors of histone deacetylase, a double bond conferring more than a 10-fold increase in potency compared with the triple bond analogue oxamflatin. Variation of substituents on the aromatic ring has a marked effect on potency, in vitro IC(50) values down to 50 nM being obtained.