| abstract |
Compounds of the formula wherein R 'is hydrogen or straight chain alkyl; R 2 represents -CN, -R, -COR, -COOR or on an aromatic ring also -OR, -OOCR or fluorine and R denotes alkyl; A represents a group with 1 to 4 six-membered rings, these rings being directly linked to one another and to ring B via a simple covalent bond or, at one or, if appropriate, 2 positions, also via -COO-, -OOC- or -CH 2 CH 2 - are linked; the six-membered rings in A and ring B are 1,4-phenylene or trans-1,4-cyclohexylene or one of these rings is also a trans-2,5-disubstituted m-dioxane ring, a 2,5-disubstituted pyrimidine ring or a 3, 6-disubstituted pyridazine ring, and no more than two of adjacent trans-1,4-cyctohexylene rings are directly linked via a simple covalent bond; and m denotes the number 2 or, if ring B is trans-1, 4-cyclohexylene or a trans-2,5-disubstituted m-dioxane ring, also denotes the number 0, their preparation, liquid-crystalline mixtures which contain such compounds, and the Use for electro-optical purposes is described. |