GB2190675A - 5-cyano-1,3-dioxanes useful as liquid crystals - Google Patents

Description

SPECIFICATION Liquid crystal compounds for mixtures of negative dielectric anisotropy and methods of producing the compounds which are used The invention relates to liquid crystal compounds for mixtures of negative dielectric anisotropy for optical arrangements for modulating the transient or rejected light and for coloured or black-and-white display of figures, symbols and moving or stationary images and methods of producing liquid crystal compounds. It is well-known that liquid crystal compounds can be used for modulating light and for displaying measured values or for reproducing data. These methods are based on the fact that the preferred orientation of thin layers of the liquid crystal substance can be varied by the application of an electrical field. The variation in the preferred orientation of liquid crystal substances is linked with a variation in the optical behaviour (double refraction, rotary capacity, light absorption) (M. Tobias: International Handbook of Liquid Crystal Displays 1975-76, Vlum Ltd., London 1976). In the past, no pure substance has been known which even approximately offers the required technical properties in terms of melting and clarification points, electro-optical threshold voltage, electrical conductivity, optical anisotropy, viscosity or thermal and chemical stability. Therefore, without exception, mixtures are used the properties of which are adapted to the particular application by a controlled choice of components. In order to be able to modify the properties of the mixtures under control, new substances offering favourable properties are time and again required. One demand which cannot be easily satisfied by the hitherto available substances is the production of mixtures of negative dielectric anisotropy for wide temperature ranges. What is mostly important here is to have a sufficiently large number of mixture components of negative dielectric anisotropy (Ae<-2) and immiscibility in the solid state so that when mixing takes place, a condition diagram of the eutectic type results so that the range of existence of the crystalline-liquid phase is widened to low temperatures. Such mixtures are in particular suitable for use in guest-most displays (T. Uchide, H. Soki, C. Shishido, M. Wada: Molecular and Liquid Crystals 54,161 (1979). The known laterally substituted cyclohexane derivatives (DE-OS 3231707 and 3320024) fulfil some of the requirements. Their great disadvantage, however, lies on the one hand in too low a structural variation capacity and, linked therewith, the small number of industrially applicable substances and on the other hand the complication of their synthesis. Furthermore, the dielectric anisotropy of this class of substance is relatively small. In order to manufacture the compounds used according to the invention, no method has been so far disclosed. The substances mentioned in DE-OS 3325727 have solely odd-numbered middle groups and have not been characterised in greater detail. Furthermore, no methods of producing them are known. The object of the invention are the new liquid crystal compounds for application in mixtures which are characterised by a high negative dielectric anisotropy, ready miscibility with other liquid crystals in the liquid crystalline state and ready producibility in electro-optical arrangements. The object of the invention is to suggest suitable compounds, the dielectric anisotropy of which is less than -2 and which have ready miscibility with other liquid crystals, and also a method of producing such compounds.

Y = -O-; -S-

Z = -O-; -S- It has been found that additives of one or more representatives of the r-2-substituted-t-5-substituted-c-5cyano-1,3-dioxanes to the general formula

in which A=R7 and B=R or A=COOR4 and B=R7 in which the number of rings is 2 to 4 and

R =-CnH2+1

in which X = -COO-; -OOC; single bond

If R CN

R8 = C m H2m+1

and in which p=0.2; q=2 to 8 and n=1 to 16 and m=1 to 16, are suitable for nematic mixtures and for use in opto-electronic components for modulating transient or reflected light and for the reproduction of figures, symbols and images which have a low negative anisotropy for wide temperature ranges. Chloresterinic liquid crystal compounds or chiralic smetic liquid crystals may be included in the mixtures. Also non-liquid crystal substances, particularly dyestuffs, may be included in the mixtures. Also in accordance with the invention, the liquid crystal r-2-substituted-t-5-substition-c-5-cyano-1,3dioxanes to the general formula M are produced by reaction of 2-substituted-2-cyano propane-1,3-diols with aldehydes in the presence of acid catalysts such as, for example, HC1, H2S04, p-toluene sulphonic acids, BF3, A1C13, acid ion exchangers in organic solvents such as, for example, benzene, toluene, CC14, CHC13, CH2C12, 1,2-dichloroethane, benzene in the presence of water binding agents such as molecular sieve, Na2SO4, CUS04 or in the absence of these, preferably with azeotropic removal of the reaction water, the isomer being surprisingly preferably created with an axial location of the nitrile function, which alone is capable of construction liquid crystal phases.Cleansing of this isomer is possible by simple recrystallisation from a suitable solvent or mixture such as methanol, benzene or hexane. Equation 1:

The 2-substituted-2-cyano-propane-1,3-diols are produced according to the invention by reaction of 2-substituted-cyano acetic acid esters or thiol esters with formaldehyde as a pure compound, as an aqueous solution (formalin), paraformaldehyde or polyoxymethylene in the presence of basic catalysts such as, for example, alkali metal carbonates, -hydrogen carbonates, -hydroxides, -alcoholates or other salts of weak acids or amines in the presence or absence of solvents, at temperatures between -78[deg]C and +100[deg]Cand preferably between -20[deg]C and +35[deg]C. Equation 2:

X = 0-alkyl, -alkyl The w-substituted 2-hydroxy methoyl-cyano acetic acid derivatives thus obtained can by selective reduction of the ester of thiol ester function with complex hydrides be converted to the 2-substituted-2-cyano propane-1,3-diols. Equation 3:

X - O-alkyl. S-alkyl It is thus possible to obtain selective reduction of the thiol alkyl ester function with NaBH4 in alcoholic solvents. Equation 4:

The selective reduction of the carboxylic acid ester function in addition to the nitrile function can take place by two methods: A) By reduction of the carboxylic acid ester function with LiBH4, of Ca(BH4)2 in etheric solvents such as THF or diglyms (M. M. Steward, J. Org.Chem. 26, 3360 (1961 )). Instead of LiBH4 or Ca(BH4)2, it is also possible to use mixtures of NaBH4 or a Li+ - or Ca2+ - salt (e.g. LiBr, CaCI2) (H. C. Brown, S. Narasimhan, Y. M. Choi, J. Org.Chem. 47, 4702-08). B) By reduction of the carboxylic acid ester function with Na(OCH3)3BH, the reduction preferably being carried out with NaBH4 in methanol, preferably at 0[deg]C to 20[deg]C or with NaBH4 in t-butanol or etheric solvents such as THFo or diglyms at elevated temperature (preferably 50 to 100[deg]C) with a slow addition of methanol (Bull. Chem. Soc. Jpn. 57,1948 (1984)). The hydroxy methyl function can thereby, to increase the yield, be during the reduction stages protected as a silyl ether, acetate or acetal (e.g. THP ether). By acetylation of the resultant 2-substituted-2-cyano propane-1,3-diols with suitable aldehydes, the 2,5-disubstituted 5-cyano-1,3-dioxane derivatives as described above are obtained in the correct configuration.The crystalline-liquid r-2-substituted-c-5-cyano-1,3-dioxane-t5-carboxylic acid esters are produced by the reaction of r-2-substituted-c-5-cyano-1,3-dioxane-t-5-carboxylic acids to the general formula V with alcohols or substituted phenols in the presence of a water soluble carbodiamide, e.g. 1-(3-dimethyl aminopropyl)-3-ethyl carbodiimide methiodide and catalytic quantities of an aceylating catalysts such as, for example, 4-dimethyl aminopyridin, in organic solvents, preferably in CH2Cl2As this happens, the r-2-substituted-c-5-cyano-1,3-dioxane-t-5-carboxylic acid esters are already obtained by single or double recrystallisation, suitable solvent, preferably ethanol, acetic acid ethyl ester, acetone and hydrocarbons or mixtures thereof in melting and clarification point-pure form with surprisingly high yields (60-95% of the theory) (Equation 5).

Other esterification methods produce lower yields and products of substantially lower purity. The r-2-substituted-c-5-cyano-1,3-dioxane-t-5-carboxylic acids not known in the literature are accessible by the reaction of corresponding aldehydes to the general formula VI with bis-(hydroxy methyl)-cyano acetic acid ethyl esters in the presence of acid catalysts and subsequent selective hydrolysis of the carbonic acid ester function with alkali or earth alkali metal hydroxides in suitable solvents or solvent mixtures, preferably at room temperature. Equation 6

The r-2-substituted-t-5-substituted-c-5-cyano-1,3-dioxanes are colourless substances which have nematic (N), smectic (S) and crystalline (K) and isotropic (I) phases. According to the intended application, so cholesterinic crystalline liquid compounds or chiral smectic liquid crystals or even non-liquid crystalline substances, particularly dyestuffs, can be admixed with these mixtures.

TABLE 1: Conversion temperatures of the compounds according to the invention

TABLE 2:

TABLE 3:

Examples of embodiment for the manufacturing process.

Example 1 Synthesis of 2-alkyl cyano acetic acid ethyl esters (alternative A). 1 mol (113 g) cyano acetic acid ethyl ester is added to a sodium ethanolate solution which has been obtained by dissolving 1 gramatom (23 g) Na in 500 ml absolute ethanol. After the addition of 1 g KJ and 1 mol alkyl bromide, heating takes place over 2 hours under reflux until the mixture reaches boiling point. After the reaction mixture has cooled, vacuum extraction from the precipitated NaBr is performed, the solvent is distilled off in a vacuum at the rotation evaporator, the residue is taken up in ether and washed twice with H20. Afterthe solvent is distilled off, fractional distillation is carried out in a vacuum. Yield: 30 to 40% of the theory.

Example 2 Synthesis of 2-alkyl cyano acetic acid ethyl ester (alternative B). A mixture of 1 mol freshly distilled aldehyde, 1 mol (113 g) cyano acetic acid ethyl ester, 500 ml benzene or toluene and 0.1 mol (7.7 g) ammonium acetate as well as 0.2 mol (12 g) acetic acid is heated for 6 hours in a water separator. After the reaction mixture has cooled to room temperature, it is subjected to four washings with water, dried over Na2SO4 and the solvent is distilled off in a vacuum in a rotary evaporator. The residue is dissolved in 500 ml methanol or ethanol and hydrogenated at room temperature over 2 g Pd on carbon (5 or 10%). When hydrogen absorption is completed, filtration from the catalyst follows and the solvent is distilled off. The residue is fractionated in a vacuum. Yield: 70 to 95. of the theory.

Example 3 Synthesis of 2-(4-ethyl cyclo hexyl)-cyano acetic acid ethyl ester (alternative C). Within a period of 10 minutes, a solution of 0.092 mol (20 g) 4-ethyl cyclo hexylidene cyano acetic acid ethyl ester and 0.4 mol (34.4 ml) t-butanol in 50 ml absolute THF is added dropwise to a solution of 0.2 gramatoms (1.4 g) lithium in 1000 ml of liquid ammonia. After 30 minutes reaction time, the excess lithium is destroyed by the addition of NH4CI and the NH3 is distilled off. The residue is mixed with 100 ml of ether and acidulated with 5% hydrochloric acid. The organic phase is separated off, then washed with water, NaHC03 solution and once again with water. Once the solvent has been distilled off, fractionation ensues, in a vacuum.

Example 4 TABLE 3 2-substituted-cyano acetic acid ethyl ester R-CH(CN)-COOC2H5

Synthesis of 2-substituted 2-hydroxy methyl cyano acetic acid ethyl ester 1 mol 2-substituted-cyano acetic acid ethyl ester is reacted with 120 ml formalin, 2 g K2CO3 and 700 ml methanol and agitated for 2 hours at room temperature. The reaction mixture is diluted with 2 litres water and extracted with a total of 500 ml ether three times. After careful drying over Na2S04, the solvent is distilled off in a rotary evaporator. The yield is quantitative.

Example 5 Synthesis of 2-substituted-2-cyano propane-1,3-diols (alternative A) A solution of 0.1 mol 2-substituted-2-hydroxy methyl cyano acetic acid ethyl ester in 50 ml absolute THF is added dropwise, accompanied by agitation, to a solution of 2.5 g LiBH4 in 200 ml absolute THF. The resulting solution is heated to boiling point under reflux forthree hours. After cooling, 100 ml methanol are added dropwise and the result agitated for 30 minutes at room temperature. The solvents are distilled off in a vacuum in a rotary evaporator. The residue is mixed with 250 ml water, carefully acidulated with concentrated hydrochloric acid and extracted three times with ether. The combined organic phases are washed with 200 ml 10% KOH, once with water and once with saturated NaCl solution. After drying over Na2SO4, the solvent is distilled off in a rotary evaporator and the residue recrystallised from n-hexane, possibly with the addition of a little acetic ether.

Example 6 Synthesis of 2-substituted-2-cyano propane-2,3-diols (alternative B) A solution of 0.1 mol 2-substituted 2-hydroxy methyl cyano acetic acid methyl ester in 50 ml THF is added dropwise, with agitation, to a solution of 3.8 g NaBH4 and 9.0 g LiBr in 400 ml absolute THF and heated for 3 hours under reflux to boiling point. Processing is similar to alternative A.

Example 7 Synthesis of 2-substituted-2-cyano propane 1,3-diols (alternative C) To a solution of 0.1 mol 2-substituted 2-hydroxy methyl cyano acetic acid ethyl ester in 150 ml absolute THF, 0.11 mol (11.9 g) trimethyl chlorosilane and 0.11 mol (11.1 g) triethyl amine are added and stirred for 30 minutes at room temperature. The resulting precipitate of triethyl ammonium chloride is extracted by vacuum and washed twice, in each case with 30 ml absolute THF. The solution obtained is mixed with 11 g NaBH4 and heated to boiling point. During the course of 2 hours, 100 ml absolute methanol are added slowly dropwise to the boiling solution, to the accompaniment of agitation. Allow to cool to approx. 40[deg]C and add 150 ml 15% hydrochloric acid and agitate for approx. 30 minutes.For the most part, the THF and methanol are distilled off in a rotary evaporator, the residue being diluted with water and extracted four times with ether. Further processing is similar to alternative A.

Example 8 Synthesis of 2-substituted-2-cyano propane-1,3-diols (alternative D) 0.1 mol 2-substituted-2-hydroxy methyl cyano acetic acid ethyl ester is silylated at specified under alternative C. The solution of 2-substituted-2-trimethyl silyl oxymethyl cyano acetic acid ester is THF is added dropwise to a solution of 2.5 g LiBH4 in 100 ml absolute THF and heated to boiling point for 3 hours. After cooling, 100 ml of methanol are added and the resulting solution further processes ad indicated under C.

Example 9 TABLE 4 2-substituted-2-cyano-propane-1,3-diols R-C(CN)(CH20H)2 R Alternative Yield % Fp [deg]C

Synthesis of r-2-substituted-t-5-substituted-c-5-cyano-1,3-dioxanes A mixture of 0.01 mol 2-substituted-2-cyano propane-1,3-diol, 0.01 mol aldehyde and 100 mg p-toluene sulphonic acid and 150 ml benzene is heated for approx. 20 minutes in a water separator to boiling point. The cooled reaction mixture is washed once with NaHC03 solution and twice with water. It is then dried over Na2S04 and the solvent distilled off in a rotary evaporator. The residue is recrystallised from methanol to constancy of conversion temperatures. Yield: 50 to 65% of the theory. The compounds produced in this way are summarised in Tables 1 and 2.

Example 10

0.2 mol trans-4-(t-4-alkyl-c-4-cyano-1,3-dioxane-2-r-yl) cyclo hexane carboxylic acid methyl ester is dissolved in 2 litres methanol and after the addition of a solution of 100 g KOH in 100 ml water, is heated for 20 minutes under reflex. The solvent is distilled off in a rotary evaporator and the residue mixed with approx. 2 litres of water. The solution is acidulated against Congo red with 25% sulphuric acid and extracted three times with acetic ether. The organic phases are washed with saturated NaCI solution and dried over NazS04. After the solvent has been distilled off at a rotary evaporator, the result is mixed with n-hexane and the product extracted by a vacuum.

Example 11

Esterification of trans-4-(t-4-alkyl-c-4-cyano-1,3-dioxane-2-r-yl)-cyclohexane carboxylic acids (alternative A) 0.01 mol trans-4-(t-4-alkyl-c-4-cyano-1,3-dioxane-2-r-yl)-cyclohexane carboxylic acid and 0.01 mol 4substituted phenol or 0.015 mol 4-alkyl cyclo hexanol and 100 mg DMAP are suspended in 200 ml absolute methylene chloride and mixed at room temperature with a solution of 0.011 mol (2.27 g) dicylohexyl carbodiimide in 30 ml methylene chloride. Agitation is pursued for 24 hours at room temperature, the separated dicyclohexyl urea is filtered off, the solvent distilled off at a rotary evaporator and the residue recrystallised from methanol. Yield: 40 to 70% of the theory.

Example 12 Esterification of trans-4-(t-4-alkyl-c-4-cyano-1,3-dioxane-2-r-yl)-cyclohexane carboxylic acid (alternative B) 0.002 mol trans-4-(t-4-alkyl-c-4-cyano-1,3-dioxane-2-r-yl)-cyclohexane carboxylic acid 0.002 mol 4substituted phenol or 0.003 mol trans-4-alkyl cyclohexanol and 20 mg DMAP are suspended in 20 ml methylene chloride and, after the addition of 0.0025 mol 1-(3-dimethyl amino propyl)1-ethyl carbo diimidemethiodide, agitated for 24 hours at room temperature. The reaction mixture is diluted with methylene chloride and washed three times with water, once with NaHC03 solution and once again with water. The solvent is distilled off at the rotary evaporator and the residue recrystallised from methanol. Yield: 60 to 80% of the theory.

Example 13 Synthesis of r-2-substituted-c-5-cyano-1,3-dioxane-t-5-carboxylic acid ethyl ester A mixture of 0.2 mol (34.6 g) bihydroxymethyl cyano acetic acid ethyl ester, 0.2 mol freshly distilled aldehyde, 5g p-toluene sulphonic acid and 1 litre of benzene are heated in a water separator with agitation until separation of the water is completed. After the reaction mixture has cooled, it is washed with 250 ml saturated NaHC03 solution and then with 500 ml water, dried over Na2S04 and the solvent distilled off in a rotary evaporator. The residue is mixed with 200 ml petrol ether (clarification point 30-60[deg]C) and cooled to -20[deg]C, when the product crystallises out. It is recrystallised again from n-hexane/acetic acid ethyl ester.

Example 14 TABLE 5 r-2-n-alkyl-c-5-cyano-1,3-dioxane-t-5-carboxylic acid ethyl ester

Synthesis of r-2-substituted-c-5-cyano-1,3-dioxane-t-5-carboxylic acids 0.2 mol r-2-substituted-c-5-cyano-1,3-dioxane-t-5-carboxylic acid ethyl ester is dissolved in 400 ml methanol and, at 20[deg]C, mixed with a solution of 0.24 mol (16.4 g - 83% KOH) in 200 ml methanol and 4 ml water. The ester is completely saponified after about 10 mins. The solvent is distilled off in a vacuum at about 30[deg]C in a rotary evaporator and the residue dissolved in 400 ml acetic acid ethyl ester. At -15 to -20[deg]C, it is mixed with 0.28 ml (24 ml-32%) hydrochloric acid and vibrated briskly for approx. 1 min. The precipitate consisting of KCI is extracted by a vacuum and washed three times with 200 ml acetic ether.The solution is dried with Na2S04 and the solvent distilled off in a rotary evaporator, the residue being mixed with 150ml petrol ether. At -20[deg]C, the product is crystallised out and is recrystallised from CHCl3/n-hexane.

Example 15 TABLE 6 r-2-n-alkyl-c-5-cyano-1,3-dioxane-t-5-carboxylic acids

Synthesis of r-2-substituted-c-5-cyano-1,3-dioxane-t-5-carboxylic acid ester 0.021 mol r-2-substituted-c-5-cyano-1,3-dioxane-t-5-carbonic acid, 0.020 mol 4-substituted phenol or trans4-alkyl-cyclohexonal as well as 100 mg DMAP are dissolved in 50 ml absolute methylene chloride. With ice cooling and agitation, 0.025 mol (7.5 g) 1-(3-dimethyl amino propyl)-3-ethyl carbodiimide methiodide are added. Then agitation is pursued for 30 minutes in an ice bath and for 12 hours at 20[deg]C. For processing, the reaction mixture is diluted with 250 ml methylene chloride, washed four times with in each case 200 ml H20, once with 5% NaHC03 solution and once with 100 ml H20, dried over Na2S04 and the solvent distilled off in a rotary evaporator. The residue is recrystallised from ethanol. Yield: 60 to 95% of the theory. The compounds which are thus synthesised are summarised in the aforementioned Tables 1 to 3.

EMBODIMENTS OF THE APPLICATION

Example I To a basic mixture consisting of 27 mol % 4-n-pentyl oxybenzoic acid-(4-n-octyl oxyphenyl ester), 24 mol % 4-n-methoxy benzoic acid-(4-n-hexyl oxyphenyl ester), 12 mol % 4-n-hexyl oxybenzoic acid-(4-n-heptyl oxyphenyl ester) and 37 mol % 4-n-hexyl benzoic acid-(4-n-heptyl oxyphenyl ester) in each case, 3 mol % of the compounds indicated in the following Table were added. Both the clarification temperatures K ȧr and also the dielectric anisotropies at 20[deg]C underwent a variation.

The magnitude B was calculated on the assumption of a linear relationship =(fXB) to

, It expresses the efficacy of the relevant dosage in the basic mixture at 20[deg]C.

Example II To the basic mixture from Example I were added 5 mol % r-2-n-pentyl-c-5-cyano-t-5-carboxylic acid-(4-n-octyl oxyphenyl ester (No. 2.19), 17 mol %r-2-(trans-4-n-heptyl cyclohexyl)-c-5-cyano-t-5-n-hexyl-1,3-dioxane (No. 1.2), 10 mol %r-2-(4(4-n-octyl oxyphenyl carbonyl oxy)phenyl-c-5-cyano-t-5-n-hexyl-1,3-dioxane (No. 1.14) and 8 mol %r-2-(4-n-octyl oxyphenyl)-c-5-cyano-t-5-n-hexyl-1,3-dioxane (No. 1.9). The mixture clarifies at 62[deg]C. Its dielectric anisotropy at 20[deg]C amounts to Ae=-3.2. The threshold voltage for the DAP effect was measured at room temperature at 2.3 V. Example /// The mixture indicated in Example 11 had added to it 1 % by weight of the red dyestuff

With homeotropic initial orientation, a colour reversal from slightly pinkto red was observed at U=2.2 V in a 15 Microm thick sandwich cell (measuring frequency 200 Hz, 25[deg]C).

Example IV A mixture was produced consisting of 33.3 mol %:2.5 bis-(trans-4-n-pentyl cyclohexyl)-1,3,4-thiazol, 22.2 mol % 2-(trans-4-n-propyl cyclohexyl)-5-(4-n-hexyl benzoyl)-1,3,4-thiazol, 13.3 mol % r-2-(trans-4-n-heptyl cyclohexyl)-c-5-cyano-t-5-n-hexyl-1,3-dioxane (No. 1.2) 14.5 mol % r-2-4(4-n-octyl oxyphenyl carbonyloxy)phenyl-c-5-cyano-t-5-n-hexyl-1,3-dioxane (No. 1.14) 11.1 mol % r-2-(4-n-octyl oxyphenyl)-c-5-cyano-t-5-hexyl-1,3-dioxane (No. 1.9) and 5.6 mol % r-2-n-pentyl-c-5-cyano-t-5-carboxylic acid-(4-n-octyl oxyphenyl)-ester (subst. 2.19) The mixture melts between 44 and 46[deg]C and clarifies between 93 and 95[deg]C. The dielectric anisotropy amounts to Ae=-5.7 at 45[deg]C. The threshold voltage for the DAP effect was measured in a 15 Microm sandwich cell at 50[deg]C as being 2.1 V.

Claims (6)

1. Liquid crystal compounds for mixtures of negative dielectric anisotropy for the modulation of transient or rejected light and for the reproduction of figures, symbols and images, characterised in that the ingredients in the mixture are one or a plurality of the r-2-substituted t-5-substituted c-5-cyano-1,3-dioxanes to the general formula

in which A=R and B=R or A=COOR4 and B=R7, the number of rings being

2 to 4 and

R = -CnH2n+1

in which X = -COO-; -ODC-; single Y = -O-; -SZ = -0; -S-

If R CN '

R8 = Cm H2m+1

and in which p=0.2; q=2 to 8 and n=1 to 16 and m=1 to 16. 2. Liquid crystal compounds for mixtures according to Claim 1, characterised in that cholesterinic liquid crystal compounds or chiralic smectic liquid crystals may be added to the mixtures.

3. Liquid crystal compounds for mixtures according to Claim 1, characterised in that non-liquid crystal substances, particularly dyestuffs, are added to these mixtures.

4. A method of producing the liquid crystal compounds to the general formula M according to Claim 1, characterised in that 2-substituted-2-cyano propane-1,3-diols to the general formula V are obtained from 2-substituted-cyano acetic acid esters or thiol esters to the general formula VI and formaldehyde, either as a pure compound or as an aqueous solution, paraformaldehyde or polyoxymethylene in the presence of basic catalysts such as, for example, alkali metal carbonates, -hydrogen carbonates, -hydroxides, -alcoholates or other sats of weak acids or amines, at temperatures between -78[deg]C and +100[deg]C and preferably between -20[deg]C and +35[deg]C followed by reduction of the thio or carboxylic acid ester group in known manner, being then reacted with aldehydes of the general formula IV in the presence of acid catalysts such as, for example, HCI, H2S04, p-toluene sulphonic acid, BF3, AICI3 or acid ion exchangers in organic solvents, such as for example benzene, toluene, CCI4, CHCI3, CI-CH2-CH2-CI, in the present of water-binding agents or in the absence thereof preferably with aziotropic removal of the reaction water, or in that r-2-substituted-c-5-cyano1,3-dioxane-t-5-carboxylic acids to the general formula V, which are obtained by reacting the correspondingly substituted aldehydes to the general formula VI with bis-(hydroxymethyl)-cyano acetic acid ethyl esters in the presence of acid catalysts and subsequent hydrolysis of the carbonic acid ester function with alkali or earth alkali metal hydroxides in solvents, are esterified with alcohols or substituted phenols to the general formula IV in the presence of a water soluble carbodiimide and catalytic quantities of an acetylating catalyst in organic solvents, preferably in methylene chloride, the stereoisomer-pure forms being obtained by recrystallisation from organic solvents.

5. A liquid crystal compound as claimed in Claim 1 involving the process substantially as described in any of Examples 1 to 15.

6. A liquid crystal mixture substantially as described in any of the Examples I, II, III or IV.