om, a lower alkyl radical or a radical --OR6, wherein R6 is as defined below, with the proviso that R5 has the same definition as R4 ; R6 is a hydrogen atom, a linear or branched alkyl radical having from 1 to 20 carbon atoms or a radical --CO--R11, wherein R11 is as defined below; R7 is a hydrogen atom or a lower alkyl radical; R8 is (a) a hydrogen atom, (b) a lower alkyl radical, (c) a radical o