Abstract: Compounds of the formula in which Z1 is an oxygen atom; or a sulfur atom; Z2 is an oxygen atom; or a sulfur atom; R1 is an aryl or heteroaryl group, which is unsubstituted or substituted; R2 is hydrogen; or an organic substituent; R3 is hydrogen; or an organic substituent; R4 is hydrogen; or an organic substituent; or R3 and R4, taken together, form, together with the nitrogen atom, to which they are attached, a ring, which is unsubstituted or substituted; R5 is hydrogen; or an unsubstituted or substituted alkyl group; or forms, taken together with R8 or with a monovalent substituent attached to that atom of R6, via which atom R6 is directly connected with the carbon atom, shown in the formula I, which carries R5, one additional bond; R6 and R7, taken together, form, together with the two carbon atoms, shown in the formula I, to which atoms they are attached, a bicyclic ring system, which ring system is carbocyclic or heterocyclic, which ring system is substituted, in the manner shown in the fo