Of the compounds where X???R2 and/or Y???R3 are represented by Formula II, preferred are those compounds the substituents of which are selected from the following groups: R2.1 is ???C(O)???O???R??? where R??? is hydrogen or lower alkyl, especially ethyl;R2.2 is hydrogen;R2.3 is ???CH2???S???;R2.4 is hydrogen, optionally substituted lower alkyl, optionally substituted