DE19525314A1 - Stable liq. crystalline or mesogenic di:oxanyl di:oxane(s) and intermediates - are prepd. by reducing di:oxanyl acetal(s), di:ester(s) or di:nitrile(s) to di:ol(s) and condensing with aldehyde or acetal, useful in liq. crystalline medium e.g. for LCD - Google Patents

Abstract

Dioxanyl-dioxanes (I) contg. a unit of the formula: -Diox-(A)m-Diox- (II) are new; Diox = 1,3-dioxan-2,5-diyl; A = -CH2CH2-, -CH=CH-, -Ctriple boundC- or 1,4-cyclohexylene; m = 0 or 1. Also claimed are: (a) dioxanyl diols contg. a unit of the formula: -Diox-(A)m-C(CH2OH)2 (III); (b) dioxane derivs. contg. a unit of the formula: -Diox-(A)m-CHXY (IV); X, Y = COR or CN; R = alkyl, aryl, alkoxy or aryloxy; and (c) a liq. crystalline (LC) medium contg. at least 2 LC components, including cpd(s). (I).

Description

The invention relates to dioxanyl-dioxane containing a structural element of general formula I

wherein
A is -CH₂CH₂-, -CH = CH-, -C = C- or 1,4-cyclohexylene, and
m is 0 or 1
means.

The invention further relates to a process for the preparation of the dioxanyl Dioxanes, in this process undergoing intermediates and the Use of dioxanyl-dioxanes as components of liquid-crystalline Media and liquid crystal and electro-optical display elements, the contain the liquid-crystalline media according to the invention.

The compound of the formula I can be used as components of liquid-crystalline Media are used, especially for displays based on the principle the twisted cell, the guest-host effect, the effect of deformation upright phases or the effect of dynamic dispersion based.

The invention was based on the object new stable liquid crystalline or to find mesogenic compounds that are liquid as components crystalline media are suitable and in particular at the same time a have relatively low viscosity and a relatively high dielectric anisotropy.  

It has now been found that compounds of the formula I as components liquid-crystalline media are eminently suitable. In particular They have comparatively low viscosities. With their help can be stable liquid-crystalline media with a broad mesophase range and before partial values for the optical and dielectric anisotropy. These media also have a very good low temperature behavior.

Liquid crystalline dioxane derivatives have long been known.

From EP 0 447 565 z. B. 1,3-dioxanes of the formula

and their use in STN blends.

In U.S. Pat. 5,322,638 are z. B. 1,3-dioxanes of the formula

where R¹ is an alkenyl radical, A² is a 1,4-phenyl ring or trans-1,4-cyclo hexene ring, X¹ is F or Cl and X² is fluorine.

From EP 0 400 861 z. B. phenyldioxanes of the formula

known.  

With regard to the various applications of 1,3-dioxanes with high Δε, however, it was desirable to have other high-level compounds Nematogeneity available to the respective appl have tailor-made properties.

With the provision of compounds of dioxanyl-dioxane is except in general, the range of liquid-crystalline substances that themselves from different application-technical points of view Production of liquid-crystalline mixtures are suitable, considerably widened.

The compounds of the formula I have a broad application Area. Depending on the choice of substituents, these may Compounds serve as base materials from which liquid crystalline Media are composed for the most part; but it can also compounds of formula I liquid-crystalline base materials be added to other classes of compounds, for example, the dielectric and / or optical anisotropy of such a dielectric to influence and / or the threshold voltage and / or to optimize its viscosity. The compounds of the invention are especially characterized by their broad mesophase range good viscosity properties and Δε values.

The compounds of the formula I are colorless in the pure state and form liquid-crystalline mesophases in one for electro-optical use conveniently located temperature range. Chemical, thermal and against Light are they stable.

The invention thus relates to the dioxanyl-dioxanes of the formula I, a process for the preparation of these compounds and the in this Process undergone, new intermediates and the use these compounds as components of liquid-crystalline media. against state of the invention are also liquid-crystalline media with a content on at least dioxanyl-dioxanes of the formula I and liquid crystal display elements, in particular electro-optical display elements, the contain such media.  

Preferred dioxanyl-dioxanes are those of the formula II:

wherein
A and m have the meaning given, and
R¹ and R² are each independently an unsubstituted or substituted by CN or at least one halogenated alkyl or alkenyl radical having up to 18 carbon atoms, wherein one or more non-adjacent CH₂ groups by a radical selected from the group -O- , -S-, -CO-, -O-CO-, -CO-O- and -C≡C- may be replaced,
R² is also halogen, CN, NCS,
Each of A¹ and A² is independently

• a) a 1,4-phenylene radical, wherein one or two CH groups can be replaced by N,
• b) a 1,4-cyclohexenylene or 1,4-cyclohexylene in which one or two non-adjacent CH₂- Groups can be replaced by -O- or -S-,
• c) a piperidine-1,4-diyl, a 1,4-bicyclo [2,2,2] - oxtylene or a naphthalene-2,6-diyl radical

where the radicals a) and b) may be substituted one or more times by halogen atoms, cyano and / or methyl groups,
Each of Z¹ and Z² independently represents -CO-O-, -O-CO-, -CH₂O-, -OCH₂-, -CH₂CH₂-, -CH = CH-, -C≡C- or a single bond
n 0, 1 or 2, and
0, 1 or 2
mean, in particular
Dioxanyl dioxanes which have a structural element of the formula Ia:

in which
L1 and L2 independently represent H or F, and
X is halogen, CN, NCS or a mono- or polysubstituted by halogen alkyl group having 1 to 10 carbon atoms, wherein one or two CH₂ groups may be replaced by -O- or -CH = CH-.

Another object of the invention is a process for the preparation the dioxanyl-dioxane according to claims 1 to 3, wherein a Dioxanyl-diol containing a structural element of the formula III

wherein
A and m have the meaning given, condensed with an aldehyde or acetal.

The invention further relates to novel dioxanyl-containing diols a structural element of the formula III

wherein
A and m have the meaning given.

Another object of the invention is a process for the preparation the dioxanyl-diols of formula III, wherein a dioxane derivative enthal tend a group of formula IV

in which
A and m have the meaning given and
X and Y are each independently of one another COR or CN,
wherein
R is alkyl, aryl, alkyloxy or aryloxy,
treated with a reducing agent.

The invention further relates to dioxane derivatives containing a Structural element of the formula IV

wherein
A, m, X and Y have the meaning given.

In the compounds of the formula I, R is preferably straight-chain Alkyl or alkenyl with 1-5 C atoms. The alkenyl radical is preferably 1E- Alkenyl, 3E-alkenyl or 4-alkenyl.

L 1 is preferably F.

R² or X in formula Ia is preferably F, Cl, OCF₃, OCHF₂, CF₃, CHF₂, OCHFCF₃, OCHFCHF₂, OCH₂CF₃, OC₂F₅, OC₃F₇, OCH = CHF, OCF = CHF, OCH = CF₂, OCF₂CHFCF₃, in particular F, OCF₃, OCHF₂, OCHFCF₃, OCHFCHF₂ and OCH = CF₂.

Particular preference is given to compounds of the formula Ia in which X = L 1 = F is.

If R 1 and R 2 in the compounds of the formula II are an alkyl radical and / or an alkoxy radical, this can be straight-chain or ver be branched. Preferably, it is straight-chain, has 2, 3, 4, 5, 6 or 7 C Atoms and therefore preferably means ethyl, propyl, butyl, pentyl, Hexyl, heptyl, ethoxy, propoxy, butoxy, pentoxy, hexoxy or heptoxy, furthermore methyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, Pentadecyl, methoxy, octoxy, nonoxy, decoxy, undecoxy, dodecoxy, Tridecoxy or tetradecoxy.  

Oxaalkyl is preferably straight-chain 2-oxapropyl (= methoxy methyl), 2- (= ethoxymethyl) or 3-oxabutyl (= 2-methoxyethyl), 2-, 3- or 4-oxapentyl, 2-, 3-, 4- or 5-oxahexyl, 2-, 3-, 4-, 5- or 6-oxaheptyl, 2-, 3-, 4-, 5-, 6- or 7-oxaoctyl, 2-, 3-, 4-, 5-, 6-, 7- or 8-oxanonyl, 2-, 3-, 4-, 5-, 6-, 7-, 8- or 9-oxadecyl.

If R¹ and R² represent an alkyl radical in which a CH₂ group by -CH = CH- is replaced, it may be straight-chain or branched. It is preferably straight-chain and has 2 to 10 C atoms. He means Accordingly, especially vinyl, prop-1, or prop-2-enyl, but-1, 2 or But-3-enyl, pent-1, 2-, 3- or pent-4-enyl, hex-1, 2-, 3-, 4- or Hex-5-enyl, hept-1, 2-, 3-, 4-, 5- or hept-6-enyl, oct-1, 2-, 3-, 4-, 5-, 6- or oct-7-enyl, non-1-, 2-, 3-, 4-, 5-, 6-, 7- or non-8-enyl, Dec-1, 2-, 3-, 4-, 5-, 6-, 7-, 8- or dec-9-enyl.

If R¹ and R² represent an alkyl radical in which a CH₂ group by -O- and one replaced by -CO-, these are preferably adjacent. Thus, these include an acyloxy group -CO-O- or an oxycar bonyl group -O-CO-. Preferably, these are straight-chain and have 2 up to 6 C atoms.

They therefore mean especially acetyloxy, propionyloxy, butyryloxy, Pentanoyloxy, hexanoyloxy, acetyloxymethyl, propionyloxymethyl, Butyryloxymethyl, pentanoyloxymethyl, 2-acetyloxyethyl, 2-propionyl oxyethyl, 2-butyryloxyethyl, 3-acetyloxypropyl, 3-propionyloxypropyl, 4-acetyloxybutyl, methoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, Butoxycarbonyl, pentoxycarbonyl, methoxycarbonylmethyl, ethoxy carbonylmethyl, propoxycarbonylmethyl, butoxycarbonylmethyl, 2- (methoxycarbonyl) ethyl, 2- (ethoxycarbonyl) ethyl, 2- (propoxycarbonyl) - ethyl, 3- (methoxycarbonyl) propyl, 3- (ethoxycarbonyl) propyl, 4- (methoxy carbonyl) -butyl.

If R¹ and R² are an alkyl or C₁ oder CF alkyl substituted by C₁ to C CF alkyl Alkenyl means, this radical is preferably straight-chain and the Substitution by CN or CF₃ in the ω position.  

If R¹ and R² are at least monosubstituted by halogen Alkyl or alkenyl radical, then this radical is preferred straight-chain and halogen is preferably F or Cl. At multiple Substitution is halogen, preferably F. The resulting radicals also include perfluorinated radicals. For simple substitution, the Fluorine or chlorine substituent in any position, preferably however in the ω position.

Compounds of the formula I with branched wing groups R¹ and R² can occasionally because of a better solubility in the usual liquid-crystalline base materials, in particular but as chiral dopants when they are optically active. smectic Compounds of this type are suitable as components for ferroelectric Materials.

Compounds of the formula I with S _A phases are suitable, for example, for thermally addressed displays.

Branched groups of this type usually contain no more than one Chain branching. Preferred branched radicals R are isopropyl, 2-butyl (= 1-methylpropyl), isobutyl (= 2-methylpropyl), 2-methylbutyl, Isopentyl (= 3-methylbutyl), 2-methylpentyl, 3-methylpentyl, 2-ethylhexyl, 2-propylpentyl, isopropoxy, 2-methylpropoxy, 2-methylbutoxy, 3-methyl butoxy, 2-methylpentoxy, 3-methylpentoxy, 2-ethylhexoxy, 1-methyl hexoxy, 1-methylheptoxy.

Formula II includes both the racemates of these compounds and the optical antipodes and their mixtures.

Among these compounds of formula II and the sub-formulas are those in which at least one of them contains Rests has one of the preferred meanings given.  

The compounds of the formula I are prepared by methods known per se as described in the literature (eg in the standard works such as Houben-Weyl, Methods of Organic Chemistry, Georg-Thieme-Verlag, Stuttgart), under reaction conditions suitable for the said reactions are known and suitable.

One can also of known per se, not mentioned here Make use of variants.

The compounds are easily accessible by the 1,3-dioxane ring By dehydration in the reaction of the corresponding diol with the aldehyde is formed or that by literature methods the substituent X directly via the corresponding hydrogen, OH, Tosylate, triflate, bromine, iodine, metal or aldehyde derivative.

The compounds of the invention can be z. B. as follows produce:

Scheme 1

Scheme 2

Scheme 3

Scheme 4

Scheme 5

The liquid-crystalline media according to the invention preferably contain in addition to one or more compounds of the invention as further constituents 2 to 40, in particular 4 to 30 components. Most preferably, these media contain one or more several compounds of the invention 7 to 25 components. These other ingredients are preferably selected from nematic or nematogenic (monotropic or isotropic) subunit especially substances from the classes of azoxybenzenes, Benzylideneanilines, biphenyls, terphenyls, phenyl or cyclohexyl benzoate, cyclohexane-carboxylic acid phenyl or cyclohexyl ester, Phenyl or cyclohexyl esters of cyclohexylbenzoic acid, phenyl or Cyclohexyl ester of cyclohexylcyclohexanecarboxylic acid, cyclohexyl phenylester of benzoic acid, the cyclohexanecarboxylic acid, or the Cyclohexylcyclohexanecarboxylic acid, phenylcyclohexanes, cyclohexyl biphenyls, phenylcyclohexylcyclohexanes, cyclohexylcyclohexanes, Cyclohexylcyclohexylcyclohexanes, 1,4-bis-cyclohexylbenzenes, 4,4'-bis cyclohexylbiphenyls, phenyl or cyclohexylpyrimidines, phenyl or Cyclohexylpyridines, phenyl or cyclohexyldioxanes, phenyl or Cyclohexyl-1,3-dithiane, 1,2-diphenylethane, 1,2-dicyclohexylethane, 1-phenyl-2-cyclohexylethane, 1-cyclohexyl-2- (4-phenylcyclohexyl) - ethane, 1-cyclohexyl-2-biphenylylethane, 1-phenyl-2-cyclohexyl phenylethanes, optionally halogenated stilbenes, benzylphenyl ethers, tolans and substituted cinnamic acids. The 1,4-phenylene groups in these compounds may also be fluorinated.

The most important as further constituents of inventive media in Questioning compounds can be represented by the formulas 1, 2, 3, 4 and 5 characterize:

R'-AER '' 1
R'-A-COO-ER '' 2
R'-A-OOC-ER "3
R'-A-CH₂CH₂-ER "4
R'-AC≡CER '' 5.

In the formulas 1, 2, 3, 4 and 5, A and E denote the same or ver can be separate, each independently a bivalent The residue of -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio, -G-Phe and -G-Cyc and their mirror images formed group, where Phe is unsubstituted or fluorine-substituted 1,4-phenylene, Cyc trans-1,4-cyclohexylene or 1,4-cyclohexylene, Pyr pyrimidine-2,5-diyl or pyridine-2,5-diyl, bio-1,3-dioxane-2,5-diyl and G2- (trans-1,4-cyclo hexyl) -ethyl, pyrimidine-2,5-diyl, pyridine-2,5-diyl or 1,3-dioxane-2,5-diyl mean.

Preferably, one of A and E is Cyc, Phe or Pyr. E is present preferably Cyc, Phe or Phe-Cyc. Preferably, the inventions contain According to the media one or more components selected from the compounds of formulas 1, 2, 3, 4 and 5, wherein A and E are selected are from the group Cyc, Phe and Pyr and at the same time one or more Components selected from the compounds of the formulas 1, 2, 3, 4 and 5, in which one of the radicals A and E is selected from the group Cyc, Phe and Pyr and the other rest is selected from the group -Phe-Phe, -Phe-Cyc, -Cyc-Cyc, -G-Phe and -G-Cyc-, and optionally one or more components selected from the Compounds of the formulas 1, 2, 3, 4 and 5, wherein the radicals A and E are selected from the group -Phe-Cyc, -Cyc-Cyc, -G-Phe- and G-Cyc.

R 'and R "in a smaller subgroup of the compounds mean Formulas 1, 2, 3, 4 and 5 are each independently alkyl, alkenyl, Alkoxy, alkoxyalkyl, alkenyloxy or alkanoyloxy of up to 8 carbon atoms. In the following, this smaller subgroup is called group A. and the compounds are described with sub-formulas 1a, 2a, 3a, 4a and 5a designated. For most of these compounds R 'and R "are of different from each other, wherein one of these radicals is usually alkyl, alkenyl, Alkoxy or alkoxyalkyl is.  

In a smaller subgroup of the compounds of the formulas 1, 2, 3, 4 and 5 designated as group B, R'-F, -Cl, -NCS or - (O) _i CH _{3 (k + 1)} denotes F _k Cl 1, where i is 0 or 1 and k + 1 is 1, 2 or 3; the compounds in which R "has this meaning are designated by the sub-formulas 1 b, 2 b, 3 b, 4 b and 5 b. Particularly preferred are those connec tions of the sub-formulas 1b, 2b, 3b, 4b and 5b, in which R '' is -F, -Cl, -NCS, -CF₃, -OCHF₂ or -OCF₃.

In the compounds of sub-formulas 1 b, 2b, 3b, 4b and 5b R 'has the at the meaning of the compounds of sub-formulas 1a-5a and is preferably alkyl, alkenyl, alkoxy or alkoxyalkyl.

In a further smaller subgroup of the compounds of the formulas 1, 2, 3, 4 and 5 are R "-CN; this subgroup is hereafter referred to as Group C and become the compounds of this subgroup according to sub-formulas 1c, 2c, 3c, 4c and 5c described. In the Compounds of sub-formulas 1c, 2c, 3c, 4c and 5c have R 'in the Compounds of the sub-formulas 1a-5a given meaning and is preferably alkyl, alkoxy or alkenyl.

In addition to the preferred compounds of groups A, B and C are also other compounds of formulas 1, 2, 3, 4 and 5 with other variants the proposed substituents in use. All these substances are by literature methods or in analogy available.

In addition to compounds of the formula I according to the invention, the media according to the invention preferably contain one or more compounds which are selected from group A and / or group B and / or group C. The mass fractions of the compounds from these groups on the media according to the invention are preferably
Group A: 0 to 90%, preferably 10 to 90%, in particular 30 to 90%
Group B: 0 to 80%, preferably 10 to 80%, in particular 10 to 65%
Group C: 0 to 80%, preferably 5 to 80%, in particular 5 to 50%
wherein the sum of the mass fractions of the compounds of the groups A and / or B and / or C contained in the respective inventive media is preferably 5 to 90% and in particular 10 to 90%.

The media according to the invention preferably contain 1 to 40%, in particular preferably 5 to 30% of inventive Ver bonds. Further preferred are media containing more than 40%, in particular 45 to 90% of compounds according to the invention. The Media preferably contain three, four or five inventive Links.

The preparation of the media according to the invention takes place in per se Wise. In general, the components are dissolved in each other, purpose moderately at elevated temperature. By suitable additives, the liquid crystalline phases according to the invention are modified so that they are in all hitherto known types of liquid crystal display elements can be used. Such additives are the subject known and described in detail in the literature (H. Kelker / R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, 1980). For example, pleochroic dyes for producing colored Guest host systems or substances for changing the dielectric Anisotropy, viscosity and / or orientation of the nematic Phases are added.

In the present application and in the following examples, the structures of the liquid crystal compounds are indicated by acronyms, wherein the transformation into chemical formulas according to the following Tabel len A and B takes place. All radicals C _n H _{2n + 1} and C _m H _{2m + 1} are straight-chain alkyl radicals with n or m C atoms. The coding according to Table B is self-evident. In Table A, only the acronym for the basic body is given. In the individual case, a code for the substituents R 1, R 2, L 1 and L 2 follows separately from the acronym for the main body with a dash:

Table A

Table B

The following examples are intended to illustrate the invention without being limits. Above and below mean percentages by weight center. All temperatures are in degrees Celsius. Mp means Melting point, bp. = Clearing point. Furthermore, K = crystalline state, N = nematic phase, S = smectic phase and I = isotropic phase. The  Indications between these symbols represent the transition temperatures Δn means optical anisotropy (589 nm, 20 ° C) and viscosity (mm² / sec) was determined at 20 ° C.

"Usual workup" means: if necessary, add water, extracted with dichloromethane, diethyl ether or toluene, separated, dried the organic phase is evaporated and the product is purified by distillation under reduced pressure or crystallization and / or chromato graphy. The following abbreviations are used:

DAST = diethylaminosulfur trifluoride
DMEU 1,3-dimethyl-2-imidazolidinone
KOT = potassium tertiary butoxide
THF = tetrahydrofuran
pTsOH = p-toluenesulfonic acid

example 1

A mixture of 40.0 ml (0.2 mol) ethoxymethylenediethyl malonate, 33.6 g (0.2 mol) of 2-hydroxymethylheptanol, 100 ml of toluene and 0.5 g of p-toluene sulfonic acid is heated at 130 ° C for 1.5 hours on a water separator.

After filtration, the solvent is distilled off under reduced pressure profiled.

This gives 70.1 g of diethyl 2- (5-pentyl-1,3-dioxan-2-yl) malonate 1A as cis / trans mixture (cis: trans = 30: 70).  

1 H-NMR: cis 5.1 ppm (d, J = 7 Hz)
80 MHz: trans 4.95 ppm (d, J = 7 Hz).

The chemical shifts refer to the proton in 2-position of the dioxane ring.

A mixture of 70.1 g of IA and 70 ml of THF slowly becomes an under Refluxed suspension of 13.8 g (0.366 mol) of lithium alumi niumhydride, 80 ml of toluene and 300 ml of THF.

After stirring for 1 hour, the mixture is cooled to 10 ° C and hydrolyzed with 40 ml of water. After usual work-up, 47.2 g of a cis / trans mixture of 5-pentyl-2- (1,3-di-hydroxyprop-2-yl) -1,3-dioxane are obtained.
1 H-NMR: cis 4.79 ppm (d, J = 4 Hz)
80 MHz: trans 6.48 ppm (d, J = 4 Hz).

Example 2

0.05 mol of IB, 0.055 mol of 3,4,5-trifluorobenzaldehyde, in 55 ml of toluene are mixed with 0.25 g of p-toluenesulfonic acid and ge ge for 1 h on a water. The mixture is allowed to cool to room temperature, mixed with water and worked up as usual, K 94 S _A (85) I.

Analogously, the following compounds of the formula

manufactured:

Example 2

0.05 mol of IB, 0.055 mol of 3,5-difluoro-4-hydroxybenzaldehyde in 55 ml of toluene are added with 0.25 g of p-toluenesulfonic acid and 1 h of water Cooked separator. Allow to cool to room temperature, mixed with Water and works up as usual.

0.033 mol of the 4- [5- (5-pentyl-1,3-dioxan-2-yl) -1,3-dioxane thus obtained 2-yl] -2,6-difluorophenol are dissolved in 100 ml of THF and heated to 50 ° C warms. After addition of 8.2 ml of 32% NaOH solution and 1.4 g of tetra Butylammonium hydrogen sulfate will chlorodifluoromethane in the reaction Mix until this starts to boil. The mixture is stirred for 1 h at 50 ° C. after, allowed to cool to room temperature and worked up as usual.

Analogously, the following compounds of the formula

manufactured:

R L¹ CH₃ H CH₃ F C₂H₅ H n-C₃H₇ H n-C₃H₇ F n-C₄H₉ H n-C₄H₉ F n-C₅H₁₁ H n-C₅H₁₁ F n-C₆H₁₃ H n-C₆H₁₃ F

Example 4

0.05 mol of IB, 0.05 mol, 100 ml of toluene and 0.6 g of p-toluenesulfonic acid are boiled for 0.5 h on a water. It is allowed to cool to room temperature, mixed with water and worked up as usual. K 105 S _B 217 I, Δn = 0.101, Δ∈ = 24.

Analogously, the following compounds of the formula

manufactured:

Example A

A nematic mixture is prepared consisting of:

Example B

A nematic mixture is prepared consisting of:

Claims (11)

1. dioxanyl-dioxane containing a structural element of the general formula I. wherein
A is -CH₂CH₂-, -CH = CH-, -C = C- or 1,4-cyclohexylene, and
m is 0 or 1
means. 2. dioxanyl-dioxanes according to claim 1 of the formula II wherein
A and m have the meaning given, and
R¹ and R² are each independently an unsubstituted or substituted by CN or at least one halogenated alkyl or alkenyl radical having up to 18 carbon atoms, wherein one or more non-adjacent CH₂ groups by a radical selected from the group -O- , -S-, -CO-, -O-CO-, -CO-O- and -C≡C- may be replaced,
R² is also halogen, CN, NCS,
Each of A¹ and A² is independently

• a) a 1,4-phenylene radical in which one or two CH groups may be replaced by N,
• b) a 1,4-cyclohexenylene or 1,4-cyclohexylene radical in which one or two non-adjacent CH₂ groups can be replaced by -O- or -S-,
• c) a piperidine-1,4-diyl, a 1,4-bicyclo [2,2,2] -oxylene or a naphthalene-2,6-diyl radical

where the radicals a) and b) may be substituted one or more times by halogen atoms, cyano and / or methyl groups,
Each of Z¹ and Z² independently represents COO, COO, CH₂O, COCH₂, CH₂CH₂, CH =CH, C≡C or a single bond
n 0, 1 or 2, and
0, 1 or 2
mean. 3. dioxanyl-dioxanes according to claim 1 or 2 containing a structural element of the formula Ia in which
L1 and L2 independently represent H or F, and
X is halogen, CN, NCS or a mono- or polysubstituted by halogen alkyl group having 1 to 10 carbon atoms, wherein one or two CH₂ groups may be replaced by -O- or -CH = CH-. 4. A process for preparing the dioxanyl-dioxanes according to claims 1 to 3, characterized in that a dioxanyl-diol containing a structural element of the formula III wherein
A and m have the meaning given, condensed with an aldehyde or acetal. 5. dioxanyl-diols containing a structural element of the formula III wherein
A and m have the meaning given. 6. A process for preparing the dioxanyl-diols of the formula III, characterized in that a dioxane derivative containing a group of the formula IV in which
A and m have the meaning given and
X and Y are each independently of one another COR or CN,
wherein
R is alkyl, aryl, alkyloxy or aryloxy,
treated with a reducing agent. 7. Dioxane derivatives containing a structural element of the formula IV wherein
A, m, X and Y have the meaning given. 8. Liquid-crystalline medium with at least two liquid-crystalline Components, characterized in that there is at least one Dioxanyl-dioxane according to claim 1 contains. 9. Liquid crystal display element, characterized in that it is a Liquid-crystalline medium according to claim 8 contains. 10. Electro-optical display element, characterized in that it as dielectric a liquid-crystalline medium according to claim 8 contains.