in which R20 represents a hydrogen atom, a CH3O, CH3CH2O or phenyl group, R21 denotes a hydrogen atom or a lower alkyl group such as methyl or ethyl, R22 denotes a hydrogen atom or a C1-C6 lower alkyl group such as methyl or ethyl, R23 denotes a C3-C6 lower alkyl group such as methyl or ethyl or a group corresponding to the formula: ???R24???N(R22)2, R24 representing a group ???CH2???CH2???, ???CH