. . "neral formula (III) ##STR7## in which q has the meaning given for formula (I), R is a lower alkyl or aprotected aminoalkyl, and X' represents SH or an NH.sub.2 group, each protected by the usual acyl radicals such as, for example, acetyl, benzyloxycarbonyl or tert.-butoxycarbonyl, or one of the functional derivatives thereof, to obtain an acid derivativeof the general formula (IV) ##STR8## in whi" . .