I. When B represents a --S-- or --SO2 -- group and A represents an alkylene radical, by condensing, in the presence of an acid acceptor and in a polar solvent such as dimethylsulphoxide or an alcohol, for example butanol, or a ketone such as methyl ethyl ketone, or a non-polar solvent such as an aromatic hydrocarbon, for example benzene, toluene or xylene, a 4-alkoxyphenyl derivative of general fo