Optically active compounds of the formula ##STR1## wherein m stands for the number 1 and R.sup.2 is an unsubstituted or halogen-substituted alkyl or alkenyl group in which optionally one methylene group is replaced by oxygen and/or optionally one methylene group is replaced by an ester group --COO-- or --OOC--; or m stands for the number 0 or 1 and R.sup.2 denotes a group of the formula ##STR2## R.sup.1 and R.sup.3 each independently represent an alkyl or alkenyl group in which optionally one methylene group is replaced by oxygen; Z.sup.1, Z.sup.2 and Z.sup.3 each independently are a single covalent bond, --CH.sub.2 O--, --OCH.sub.2 --, --CH.sub.2 CH.sub.2 --, --COO-- or --OOC--; n stands for the number 0 or 1; rings A.sup.1 an A.sup.2 each independently are unsubstituted or halogen, cyano and/or methyl-substituted 1,4-phenylene in which optionally 1 CH group or 2 CH groups is/are replaced by nitrogen; and C* denotes a chiral carbon atom; with the proviso that simultaneously R.sup.1 does not are an alkyl group, m and n do not are the number 1, Z.sup.1 does not are the group --CH.sub.2 CH.sub.2 -- or --CH.sub.2 O-- and ring A.sup.1, Z.sup.2, ring A.sup.2 and R.sup.2 together do not are 4-(5-alkyl-2-pyrimidinyl)-phenyl, their preparation, liquid crystalline mixtures which contain these compounds and their use for electro-optical purposes.