The present invention provides compounds of the general formula: ##STR1## wherein R is an alkyl group with 1 to 12 carbon atoms or a group of the formula ##STR2## Ring A.sup.1 is 1,4-phenylene, which is unsubstituted or substituted with fluorine, pyridine-2,5-diyl or pyrimidine-2,5-diyl;Ring A.sup.2 is trans-1,4-cyclohexylene or trans-1,3-dioxane-2,5-diyl;R.sup.1 is an alkyl, alkenyl or alkoxyalkyl group with 1 to 12 and, respectively, 2 to 12 carbon atoms;X.sup.1 is fluorine, chlorine, bromine, trifluoromethyl, trifluoromethoxy or difluoromethoxy; andX.sup.2, X.sup.3 each independently are hydrogen or fluorine.The invention is also concerned with liquid crystal mixtures which contain these compounds and with the use of these compounds and mixtures for electro-optical purposes.