ast one of the radicals R5 and R7 being a hydroxyl, alkyl or alkoxy radical, and R6 and R8, which may be identical or different, are each a hydrogen atom, or a hydroxyl radical, at least one of the radicals R6 and R8 being a hydroxyl radical, with the proviso that, when R5 and R8 are each a hydrogen atom and R6 is a hydroxyl radical, then R7 is other than an alkoxyl radical or a hydrogen atom.