Compounds of the formula ##STR1## wherein m represents is the number 0 or 1; A is a single covalent bond, --CH.sub.2 --CH.sub.2 --, --OCH.sub.2 --, --COO-- or --OOC--; rings B, C and D are 1,4-phenylene optionally substituted with cyano, halogen or lower alkyl; Y.sup.1 and Y.sup.2 are hydrogen or one of them also is cyano; and R.sup.1 and R.sup.2 each represents C.sub.1 -C.sub.18 -alkyl or C.sub.2 -C.sub.18 alkenyl, either of which is optionally halogen-substituted, in which one CH.sub.2 group or two non-adjacent CH.sub.2 groups is/are optionally replaced by oxygen; with the provisos that at least one of R.sup.1 and R.sup.2 has a chiral carbon atom or a C--C double bond or both when A is --COO-- and at least one of R.sup.1 and R.sup.2 has a C--C double bond when A is a single covalent bond, their preparation, liquid crystalline mixtures containing these compounds and their use for electro-optical purposes are described.