1. The compounds of formula (I):   in which R represents: - alkyl, alkenyl or alkynyl containing from 1 to 18 carbon atoms, optionally substituted by O-alkyl, S-alkyl or SiRcRdRf group, in which Rc, Rd and Rf, identical or different, represent a linear, branched or cyclic alkyl containing up to 4 carbon atoms, which can be substituted by one or more halogen atoms; - (CH2)nAr, in which n is an integer ranging from 0 to 6, Ar is an aryl (phenyl or naphthyl) or heteroaryl optionally substituted by one or more substituents indicated above, and chosen from the group of the following radicals:     Z represents a hydrogen atom or the remainder of an acyl radical, containing up to 12 carbon atoms, as well as the addition salts thereof with acids. 2. The compounds of formula (I) according to Claim 1, in which Z represents a hydrogen atom. 3. The compounds of formula (I) according to Claim 1 or 2, in which R represents a (CH2)nAr in which n represents an integer ranging from 1 to 4 and Ar has the meanings indicated in Claim 1. 4. The compounds of formula (I) according to Claim 3 in which Ar is an optionally substituted phenyl. 5. The compounds of formula (I) according to Claim 3 in which R represents an optionally substituted   radical. 6. The compounds of formula (I) the names of which are: -11,12-didesoxy-3-de[(2,6-didesoxy-3-C-methyl-3-0-methyl-α-L-ribo-hexopyranosyl)oxy]-6-0-methyl-3-oxo-12,11-[oxycarbonyl[[[2-[4-(3-pyridinyl)-1H-imidazol-1-yl]ethoxy]methyl]imino]-erythromycin, -11,12-didesoxy-3-de[(2,6-didesoxy-3-C-methyl-3-0-methyl-α-L-ribo-hexopyranosyl)oxy]-6-0-methyl-3-oxo-12,11-[oxycarbonyl[[(2-phenylethoxy)methyl]imino]]-erythromycin. 7. Use of the compounds of formula (I) according to any one of Claims from 1 to 6 as an antibiotilc agent. 8. Pharmaceutical composition containing at least one of the compounds according to one of Claims from 1 to 6 as an active ingredient. 9. Preparation process for the compounds of formula (I) according to Claim 1, characterised in that a compound of formula (II):   in which Z? represents the remainder of a carboxylic acid containing up to 12 carbon atoms, is subjected to the action of a blocking agent of the ketone function in position 3 in the form of an enol ether or ester in order to obtain the compound of formula(III):   wherein E represents the remainder of an enol ether or ester and Z' has the meaning indicated above, which is subjected to the action of a compound of formula (IV): Hal-CH2OR (IV) wherein Hal represents a halogen atom, in order to obtain the corresponding compound of formula (V):   which is subjected, if necessary, to the action of an agent releasing the ketone function in position 3 and/or to the action of an agent releasing the hydroxyl in position 2', in order to obtain the corresponding compound of formula (I):   in which R and Z have the meanings indicated in Claim 1. 10. The compounds of formula (III):   in which E represents the remainder of an enol ether or ester and Z? represents the remainder of a carboxylic acid containing up to 12 carbon atoms, as chemical products. 11. The compounds of formula (V):   in which Z? represents the remainder of a carboxylic acid containing up to 12 carbon atoms and R has the meanings indicated in Claim 1, as chemical products.