The invention concerns compounds of formula (I) wherein: R1 and R2, identical or different, represent a saturated or unsaturated alkyl radical, branched or not and containing 1 to 7 carbon atoms; R3 to R8, identical or different, represent a hydrogen, an alkyl branched or not and containing 1 to 5 carbon atoms, an acyloxy, branched or not and containing 1 to 5 carbon atoms, a halogeno, nitro, hydroxy, acyl or alkoxy group containing 1 to 5 carbon atoms, a dialkylamino group containing 1 to 5 carbon atoms, a trifluoromethyl or trifluoro methoxyl group; Z represents an oxygen or sulphur atom or methylene; m represents an integer between 0 and 4 inclusively; n represents an integer between 2 and 7 inclusively; and their pure enantiomers and mixtures, the therapeutically acceptable mineral or organic salts of the compounds of formula (I) and their possible hydrates.