. . . "esents a C.sub.1-C.sub.4 alkyl radical, R.sub.11 and R.sub.12, which may be identical or different, represent a hydrogen atom or a C.sub.1-C.sub.4 alkyl radical;##STR00009## wherein, R.sub.13 represents a hydrogen atom, a C.sub.1-C.sub.4 alkoxy radical, a halogen atom such as bromine, chlorine, iodine or fluorine, or an amino radical, R.sub.14 represents a hydrogen atom, a C.sub.1-C.sub.4 alkyl ra" . .