two Y1 are the same; R5 is hydrogen atom or a methyl, ethyl, propyl or phenyl radical; and R6 is a hydrogen atom or R5 and R6 form a condensed benzene ring that can bear alkyl substituents; R7 is a hydrogen atom and R8 is a hydrogen atom or a methyl, ethyl, propyl or phenyl radical or R7 and R8 form a condensed benzene ring that can bear alkyl substituents.