BZDs of the general formula 1: ##STR1## where: R represents H, alkyl, alkenyl, cycloalkyl or cycloalkenyl each with up to 8 carbon atoms, phenylalkyl with an alkylene chain of 1 to 3 carbon atoms and optionally substituted on the phenyl radical with one or two substituents selected from the group consisting of Cl, Br, F, CN, CF.sub.3, NO.sub.2, lower alkyl, and OCH.sub.3, or represents a phenyl radical which is optionally substituted with one or two substituents selected from the group consisting of Cl, F, Br, CN, I, CF.sub.3 NO.sub.2, lower alkyl and OCH.sub.3, or represents a 5-membered or 6-membered heterocyclic radical with 1 or 2 heteroatoms from the group consisting of O, N and S; R.sub.1 represents H or, together with R.sub.2, forms a bond; R.sub.2 and R.sub.4 independently of one another represent H, or an alkyl radical with 1 to 6 carbon atoms, or represent a phenylalkyl radical with an alkyl chain of 1 to 3 carbon atoms and optionally substituted on the phenyl radical with one or two substituents selected from the group consisting of Cl, Br, F, CN, CF.sub.3, NO.sub.2, CH.sub.3, C.sub.2 H.sub.5 and OCH.sub.3, or R.sub.2 together with R.sub.1, forms a bond; R.sub.3 is (CH.sub.2).sub.n CH or (CH.sub.2).sub.n --CR.sub.14 .dbd.CR.sub.14, where n is between 0 and 5 inclusive and R.sub.14 .dbd.HOL a bond; R.sub.5 and R.sub.6 independently of one another represent H, Cl, F, Br, NO.sub.2, CN, CF.sub.3, lower alkyl, or OCH.sub.3 ; R.sub.7 and R.sub.9 are independently CH.sub.2 or C.dbd.0; R.sub.8 is ##STR2## where p and q are independently between 0 and 4 and the sum of p and q is 4 or less; and R.sub.11 and R.sub.12 are independently H or a lower alkyl of between 1 and 5 carbon atoms.