Abstract: The subject of the present invention is compounds which correspond to the following general formula: ##STR1## in which R.sub.1 and R.sub.3 independently of one another represent H, an alkyl, alkenyl, cycloalkyl, aryl, arylalkyl or an alkylaryl group, a heterocycle, or groups which are unsubstituted or substituted, in particular by one or more hydroxyl or alkoxy groups or halogen;n represents an integer from 1 to 5;i in each case takes values from 1 to n for the n successive ##STR2## links; R.sub.2, R.sub.4.sup.i and R.sub.5.sup.i independently of one another represent H, an alkyl, alkenyl, aryl, alkoxy, hydroxyalkyl, alkoxyalkyl, amido, acyl or carboxyalkyl group, or a salt, or an alkyl ester of the latter; or R.sub.4.sup.i and R.sub.5.sup.i together form an oxo group ##STR3## R.sub.6 and R.sub.7 represent H, or R.sub.6 and R.sub.7 together form an oxo group ##STR4## x represents a 5- or 6-membered heterocycle which contains at least one nitrogen atom, or, in the event that ##STR5## where R.sub.8 and R.sub.