New inhibitors of histone deacetylases having antitumor activity, and the process of preparation thereof are herein described. These compounds belong to the structural formula (I) where R1 is a linear or branched chain containing at least two conjugated double bonds, A is an optionally substituted phenyl or pyridyl ring, Ar is an aryl or heteroaryl group, and R3 is hydrogen or alkoxyalkyl. The application also describes the use of said compounds in the treatment of diseases associated to the deregulation of histone deacetylases activity, such as tumors, as well as the relevant pharmaceutical compositions for administration to patients requiring said treatment.