Optically active adsorbents

Absorbents based on optically active poly(meth)acrylamides, e.g. bonded to silica gel, can be used for separating racemic mixtures into their optical antipodes.

BACKGROUND OF THE INVENTION 
The separation of racemic mixtures into optical antipodes (enantiomers) 
represents a very important problem of preparative chemistry. In addition 
to the conventional methods of converting the racemic mixture into a 
mixture of diastereoisomers and separating them on the basis of different 
physical properties, interest has centered in recent years in the main on 
chromatographic methods for separating racemic mixtures, using in 
particular optically active adsorbents as the stationary phase. 
The problem in particular is finding optically active adsorbents which 
permit the separation of a large number of chemically different racemates 
and hence are universally applicable. 
It is known for example to polymerize optically active acrylamides and 
methacrylamides in the presence of a cross-linking agent in suspension, to 
let these cross linked polyamides swell in appropriate eluents and to use 
them for separating enantiomers (described in DEP 2,500,523). However, it 
is highly disadvantageous that these materials are only usable for low 
pressure liquid chromatography. Under pressures of greater than 5 bar, 
these crosslinked polyamides are compressed and become impermeable to the 
eluent, so that the rate of migration decreases significantly. 
Furthermore, their degree of swelling is dependent on the composition of 
the eluent. High pressure liquid chromatography of gradient solution is 
not possible on these gels. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to prepare and make 
available adsorbents which permit the separation of racemic mixtures of 
various structures with a high separation efficiency and which are 
universally also usable for high pressure liquid chromatography with 
gradient elution. 
Upon further study of the specification and appended claims, further 
objects and advantages of this invention will become apparent to those 
skilled in the art. 
It has now been found, surprisingly, that binding the optically active 
polyamides to a support such as silica gel produces particularly 
advantageous, optically active adsorbents for racemate splitting which are 
highly suitable for high pressure liquid chromatography and/or gradient 
elution. 
The objects therefore have been satisfied by the provision of adsorbents 
based on optically active poly(meth)acrylamides which can be used for 
separating racemic mixtures into their optical antipodes, wherein these 
amides are present bonded to, e.g., silica gel. 
Such amides are obtainable for example from polymerizable compounds of the 
formula I 
##STR1## 
in which 
##STR2## 
These polymers, bonded to silica gel, can be used as adsorbents for 
separating racemic mixtures in particular in chromatography, preferably in 
high pressure chromatography. 
The invention is not limited to these polyamides, however, and it is 
contemplated that any optically active polyamide may be used in accordance 
with the invention for the chromatographic separation of optical 
antipodes. The invention further provides adsorbents where the optically 
active acrylamides or methacrylamides are obtainable from polymerizable 
compounds of the formula I and wherein the optically active polyamides are 
attached to the silica gel directly or via (meth)acryloyl groups. 
The invention further provides a process for preparing optically active 
polymers, characterized in that the hydrophilic silica gel, which may be 
at least partially modified with (meth)acrylic acid, is reacted with 
optically active monomers of the formula I under conditions of 
polymerization reaction. 
Finally, the invention also provides a process for the chromatographic 
separation of racemic mixtures into the optical antipodes with the use of 
the polymer bonded to silica gel according to the invention. In general it 
is possible to use any desired optically active acrylamide or 
methacrylamide as monomers. Preference, however, is given to the monomer 
of the formula I. 
DETAILED DISCUSSION 
The preferred monomers of the invention are those in which R.sup.1 is H or 
methyl. 
The group 
##STR3## 
is preferably an optically active secondary or tertiary amine. Such amines 
include for example, (S)-phenylethylamine, 
(S)-cyclohexylphenylmethylamine, (S)-naphthylethylamine, 
(S)-cyclohexylethylamine or (1S, 2R)-phenylcyclopropylamine. X then has 
preferably the structure of 
##STR4## 
wherein A and B each are a ring component, e.g. phenyl, cyclopropyl, 
cyclohexyl, naphthyl, biphenyl, bicyclohexyl or cyclopentyl or an alkyl 
group having preferably 1-7 C-atoms with the proviso that A and B are not 
identical radicals. 
The group 
##STR5## 
is further preferably an optically active amino acid ester with up to 15 C 
atoms in the amino acid group. Preferably the ester is an alkyl ester, 
more preferably a C.sub.1-7 alkyl ester, in particular ethyl ester, and 
also an aryl ester, preferably a C.sub.6-10 aryl ester. Particular 
preference is given for example to alkyl esters of (S)-phenylalanine, in 
particular the ethyl ester, and also for example to the esters of leucine 
or valine. The amino acid group preferably has 3-9 C-atoms 
R.sup.2 is preferably H, not only in the amine but also in the amino acid 
derivatives. 
It is possible to use both antipodes of the optically active compounds. It 
is also possible to use different monomers to prepare copolymers. 
The monomers of the formula I are obtainable by conventional methods, for 
example by reacting ethyl (S)-phenylalaninate with acryloyl chloride or in 
general by reacting the optically active amines or amino acid ester 
##STR6## 
with methacylic acid or acrylic acid derivatives, preferably in the 
presence of a polymerization inhibitor such as 4-tertbutylpyrocatechol, at 
temperatures between about -5.degree. and 60.degree. C. Advantageously the 
reaction is carried out at room temperature in the presence of an inert 
organic solvent, in particular a halohydrocarbon such as methylene 
chloride or chloroform. The reaction times are between about 30 minutes 
and 4 hours and essentially depend on the reaction temperature. The 
reaction conditions have been repeatedly described in the literature, e.g. 
in Houben-Weyl, Methoden der Organischem Chemie, Georg-Thieme-Verlag, 
Stuttgart. 
Preferably the optically active polyamides are bonded to hydrophilic silica 
gels. Of these, preferred gel are those silica gels modified with diol, 
NH.sub.2 or cyano groups. Particularly preferred are silica gels 
containing diol groups. Silica gels modified in this way are known, e.g. 
as in K. K. Unger, in Porous Silica, J. of Chrom. Library, Vol. 16, 
Elsevier, Amsterdam, 1979, p. 8, D. P. Herman et al. in J. of Chrom. 
Science, Vol. 19, 1981 or in DE-PS 23 13 073. These materials are 
commercially available. However, it is also possible to use other support 
materials, for example aluminum oxides or titanium oxides. such material 
are disclosed in, e.g. J. H. Knox et al., J. of Chrom., 112 (1975) 
171-188. 
To prepare these polyamides bonded to silica gel there are two preferred 
variants. First, a silica gel modified for example with diol groups can be 
esterified in a conventional manner with (meth)acrylic acid. The 
esterification is preferably carried out in an inert solvent, preferably 
for example dioxane or tetrahydrofuran, at temperatures between 
-20.degree. and 80.degree. C., preferably at room temperature, and in 
general takes between 15 minutes and 48 hours. Preference is given to 
using a reactive derivative of (methy)acrylic acid, for example 
(meth)acrylic anhydride. This modification should be at least partial. 
Advantageously, the target degree of esterification should be over 50%, in 
particular over 70%. Loading of the silica gel with (meth) acrylate groups 
therefore yields a (meth)acrylate density of 0.5-4.0 .mu.mol/m.sup.2, in 
particular 2.0-3.0 .mu.mol/m.sup.2. 
Subsequently the (meth)acrylate bonded to silica gel is copolymerized in 
suspension with a monomer of the formula I. The polymerization is carried 
out in a conventional manner in the presence of a free radical former, for 
example a peroxide such dibenzoyl peroxide, dilauroyl peroxide, di-o-tolyl 
peroxide, or an azo compound such as azobisisobutylronitrile. The 
reactants are suspended in an inert organic solvent, preferably an 
aromatic hydrocarbon such as benzene or toluene, or in a halohydrocarbon 
such methylene chloride, chloroform or 1,2-dichloroethane. The stirred 
mixture is heated under an inert gas atmosphere, preferably under 
nitrogen, to temperature between +40.degree. and +100.degree. C., 
preferably between 70.degree. and 90.degree.. The polymerization time is 
between about 10 to 60 minutes, preferably between 15 to 30 minutes. The 
polymerization is discontinued by adding an inhibitor, for example 
tertbutylpryocatechol. The polyamide obtained in this way, which is bonded 
covalently to the silica gel, is separated off by filtration, thoroughly 
washed and dried in vacuum. 
In the same way it is possible to react in place of a diol-modified silicon 
gel an amino-modified silica gel with a (meth)acrylic acid derivative and 
to subject the product to copolymerization with monomeric 
(meth)acrylamides. The materials obtained are likewise suitable for 
enantiomer separation. 
In another process variant, hydrophilic silica gel is reacted directly with 
monomers of the formula I under conventional polymerization conditions. 
The result is likewise an attachment of the polyamides to the silica gel, 
and the materials obtained are likewise suitable for direct enantiomer 
separation by means of high pressure liquid chromatography. The 
polymerization reaction in this variation of the process may be carried 
out substantially as described above. 
The ratio of silica gel:monomer (in parts by weight) can be varied in the 
reactions described between about 1:0.05 and 1:10, preferably within the 
range from 1:0.1 and 1:0.5, being in particular 1:0.25. 
The loading of the silica gel with poly(meth)acrylamides therefore yields a 
poly(methyl)acrylamide density within the range of 0.5-8.0 
.mu.mol/m.sup.2, preferably within 2.0-4.0 .mu.mol/m.sup.2. 
The materials according to the invention permit the separation of racemic 
mixtures in high optical yields by means of high pressure liquid 
chromatography, which is impossible with polymers which are not attached 
to the silica gel. The method of high pressure liquid chromatography for 
the preparative separation of racemic mixtures into their optical 
antipodes is familiar to those skilled in the art. Customarily, the 
adsorbent is suspended in the eluent and packed under pressure into steel 
columns by means of a filling apparatus. The racemate to be separated, in 
the form of a solution in an appropriate eluent, in introduced into the 
column. Finally, this is followed by elution with eluents, optionally with 
gradient elution, at a certain flow rate and by detection. The eluents 
used are advantageously hydrocarbons, in particular aromatic hydrocarbons, 
and ethers of mixtures thereof. Mixtures of dioxane and hexane are 
particularly suitable. 
The advantages of enantiomer separations which are carried out by means of 
high pressure liquid chromatography using the adsorbent according to the 
invention lie, compared with separation over nonattached polymers by means 
of low pressure liquid chromatography, in a significantly shorter 
separating time and higher separating efficiency. By means of the polymers 
according to the invention it is possible to carry out separation of 
racemates of a great many classes of compounds.

In what follows, preparation and separation examples will illustrate the 
polymers according to the invention in more detail. 
EXAMPLES 
Without further elaboration, it is believed that one skilled in the art 
can, using the preceding description, utilize the present invention to its 
fullest extent. The following preferred specific embodiments are, 
therefore, to be construed as merely illustrative, and not limitative of 
the remainder of the disclosure in any way whatsoever. 
In the foregoing and in the following examples, all temperatures are set 
forth uncorrected in degrees Celsius and unless otherwise indicated, all 
parts and percentages are by weight. 
The entire text of all application, patents and publications, if any, cited 
above and below are hereby incorporated by reference. 
EXAMPLE 1 
a. A solution of 1.5 g of methacrylic anhydride in 10 ml of dioxane and 0.8 
g of diisopropylethylamine in 30 ml of dioxane is added to a suspension of 
2.5 g of diol phase (silica gel LiChrosorb.RTM. Diol 5 .mu.m, E. Merck), 
in 20 ml of dioxane in a nitrogen stream, the mixture is stirred at room 
temperature for 30 minutes, is allowed to stand for 24 hours, is filtered 
off with suction, washed with dioxane and dried. This gives a white 
product (C=8.27%, H=1.76%). 
b. A solution of 7.5 g of (S)-N-acryloylphenylalanine ethyl ester in 20 ml 
of toluene and a solution of 22 mg of azobisisobutylronitrile in 10 ml of 
toluene are added under nitrogen to a suspension of 2.5 g of the silica 
gel methacrylate prepared as in Example 1a in 10 ml of toluene, and the 
mixture is maintained at 80.degree. C. with stirring for 15 minutes. The 
polymerization is discontinued by adding a solution of 200 mg of 
tert-butylpyrocatechol in 10 ml of toluene, which is followed by 
filtering, washing and drying. This gives 3.0 g of a white adsorbent 
(C=21.35%, H=3.1%, N=1.16%) which has a loading of about 20% by weight of 
polyacrylamide. 
EXAMPLE 2 
By the method of Example 1b, silica gel methacrylate (prepared in 
accordance with Example 1a) and (S)-phenylethylmethylacrylamide are 
reacted to prepare the corresponding silica gel polymethacrylamide, except 
that the polymerization time is 30 minutes. 
In the same way (S)-cyclohexylethylmethacrylamide and silica gel 
methacrylate are used to prepare a silica gel 
(S)-cyclohexylethylpolymethacrylamide. 
EXAMPLE 3 
A suspension of 2.5 g of silica gel (LiChrosorb.RTM. Diol 5 .mu.m, E. 
Merck) is heated with stirring in a nitrogen atmosphere in a solution of 
7.5 g of (S)-N-acryloylphenylalanine ethyl ester and 22 mg of 
azobisisobutyronitrile in 40 ml of toluene at 80.degree. C. for 15 
minutes, the reaction is discontinued, and the product is washed and 
dried. The result obtained is 2.8 g of a white adsorbent (C=12.48%, 
H=2.06%, N=0.65%). 
EXAMPLE 4 
A suspension of 5.0 g of silica gel (LiChrosorb.RTM. Diol 5 .mu.m, E. 
Merck) is heated with stirring in a nitrogen atmosphere in a solution of 
1.25 g of (S)-acryloylphenylalanine ethyl ester and 44 mg of 
azobisisobutylronitrile in 100 ml of cyclohexane at 80.degree. C. for 15 
minutes, the reaction is discontinued, and the product is washed and 
dried. 
EXAMPLE A 
The adsorbents are introduced in the form of a suspension in 30 ml of 
isopropanol into steel columns (250.times.4 mm) under a pressure of 
450-500 bar. 
In each case .mu.g solutions of 3-15 ug of a racemate in dioxane are 
chromatographed at a rate of migration of 0.5-2.0 ml, a pressure of 20-100 
bar becoming established depending on eluent and rate of migration. 
Detection is effected with a flow cell at 254 nm. 
The table below shows the results of the chromatographic separation tests 
on various racemates. 
______________________________________ 
Eluent Elution volume 
Adsor- n--hexane/ 
Flow of enantiomers 
bent Racement dioxane ml/min. 
(ml) 
______________________________________ 
A Oxazepam 75:25 1.0 51.8 55.8 
A Oxazolam 80:20 0.5 12.3/ 15.9/ 
13.8 18.3 
(diast.) 
A Penflutizide 53:47 1.5 41.4 46.8 
B N--desmethyl- 
65:35 1.0 35.4 43.8 
chlormezanone 
B Cyclopenthiazide 
40:60 2.0 54.9 86.7 
C Thalidomide 75:25 1.0 17.1 23.4 
C Acenocomarol 75:25 1.0 34.2 39.0 
D Lopirazepam 50:50 1.0 40.8 49.2 
D Chlormezanone 
75:25 1.0 19.8 23.4 
D Chlorthalidone 
55:45 0.7 30.2 36.1 
______________________________________ 
From these results it is clear that the materials according to the 
invention are highly suitable for enantiomer separation by means of high 
pressure liquid chromatography. The elution volumes of the enantiomers are 
sufficiently wide apart, and generally very good base line separation is 
obtained. 
The preceding examples can be repeated with similar success by substituting 
the generically or specifically described reactants and/or operating 
conditions of this invention for those used in the preceding examples. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from the spirit and scope thereof, can make various changes and 
modifications of the invention to adapt it to various usages and 
conditions.