A compound of formula (I) according to claim 11 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 ???COOR??? wherein R??? is a hydrogen atom or an optionally substituted group selected from alkyl, cycloalkyl, cycloalkylalkyl, aryl, arylalkyl, heterocyclyl or heterocyclylalkyl, as defined in claim 11, and R1, R2,