Compounds of the general formula (I) and the salts thereof, preferably the pharmaceutically acceptable salts thereof; in which R.sup.1 is straight-chain C.sub.1-8-alkanoyloxy, straight-chain C.sub.1-8-alkoxy, straight-chain C.sub.1-8-alkoxy-straight-chain-C.sub.1-8-alkoxy, straight-chain C.sub.1-8-alkoxycarbonylamino, straight-chain C.sub.0-8-alkylcarbonylamino, optionally N-mono- or N,N-di-C.sub.1-8-alkylated amino or hydroxy or straight-chain omega-hydroxy-C.sub.1-8-alkyl. ##STR00001##