ne. [0027] A first class of preferred compounds of the invention is represented by the compounds of formula (I) wherein one of Ra or Rb is a hydrogen atom or an optionally substituted straight or branched C1-C6 alkyl group and the other is a group ???COR??? wherein R??? is an optionally substituted group selected from alkyl, cycloalkyl, cycloalkylalkyl, aryl, arylalkyl, heterocyclyl or heterocycly