pho or carboxyl, or R3 and R4 together with the nitrogen atom to which they are bonded are a saturated 5-membered or 6-membered heterocyclic ring, which additionally can also contain a nitrogen atom or oxygen atom as a ring member, R5 and R6 independently of one another are substituted or unsubstituted alkyl or aralkyl radical, R7 is a substituted or unsubstituted alkyl group having 1 to 6 carbon