Compounds of the formula ##STR1## wherein R.sup.1 is hydrogen or straight-chain alkyl; R.sup.2 is --CN, --R, --COR, --COOR or when R.sup.2 is positioned on an aromatic ring R.sup.2 also can be --OR, --OOCR or --F; R is alkyl; A is a group with 1 to 4 six-membered rings, these rings being linked directly with one another and with ring B in each case via a single covalent bond or being linked at one or two positions also via --COO--, --OOC-- or --CH.sub.2 CH.sub.2 --; the six-membered rings in A and ring B each are 1,4-phenylene or trans-1,4-cyclohexylene or one of these rings also is trans-2,5-disubstituted m-dioxane, 2,5-disubstituted pyrimidine or 3,6-disubstituted pyridazine, with the proviso that a maximum of two adjacent trans-1,4-cyclohexylene rings are linked directly via a single covalent bond; and m is the integer 2, or when ring B is trans-1,4-cyclohexylene or trans-2,5-disubstituted m-dioxane, m also can be the integer 0,their manufacture, liquid crystalline mixtures which contain these compounds as well as the use of these compounds for electro-optical purposes are described.Additionally, compounds of the formula ##STR2## wherein R.sup.8 is straight-chain C.sub.1 -C.sub.12 alkyl, their manufacture, liquid crystalline mixtures which contain these compounds and their use for electro-optical purposes are described.