Synthesis and Utilization of Trialkylammonium-Substituted Cyclodextrins as Water-Soluble Chiral NMR Solvating Agents for Anionic Compounds

Article abstract of DOI:10.1002/chir.22582

Cationic trialkylammonium-substituted α-, β-, and γ-cyclodextrins containing trimethyl-, triethyl-, and tri-n-propylammonium substituent groups were synthesized and analyzed for utility as water-soluble chiral nuclear magnetic resonance (NMR) solvating agents. Racemic and enantiomerically pure (3-chloro-2-hydroxypropyl)trimethyl-, triethyl-, and tri-n-propyl ammonium chloride were synthesized from the corresponding trialkyl amine hydrochloride and either racemic or enantiomerically pure epichlorohydrin. The ammonium salts were then reacted with α-, β-, and γ-cyclodextrins at basic pH to provide the corresponding randomly substituted cationic cyclodextrins. The ¹H NMR spectra of a range of anionic, aromatic compounds was recorded with the cationic cyclodextrins. Cyclodextrins with a single stereochemistry at the hydroxy group on the (2-hydroxypropyl)trialkylammonium chloride substituent were often but not always more effective than the corresponding cyclodextrin in which the C-2 position was racemic. In several cases, the larger triethyl or tri-n-propyl derivatives were more effective than the corresponding trimethyl derivative at causing enantiomeric differentiation. None of the cyclodextrin derivatives were consistently the most effective for all of the anionic compounds studied.

Full text of DOI:10.1002/chir.22582

CHIRALITY 28:299–305 (2016)
Special Issue Article
Synthesis and Utilization of Trialkylammonium-Substituted
Cyclodextrins as Water-Soluble Chiral NMR Solvating Agents for
Anionic Compounds
ALISON E. DOWEY, CIRA MOLLINGS PUENTES, MIRA CAREY-HATCH, KEYANA L. SANDRIDGE, NIKHIL B. KRISHNA, AND
*
THOMAS J. WENZEL
Department of Chemistry, Bates College, Lewiston, Maine, USA
Abstract
Cationic trialkylammonium-substituted α-, β-, and γ-cyclodextrins containing trimethyl-,
triethyl-, and tri-n-propylammonium substituent groups were synthesized and analyzed for utility as
water-soluble chiral nuclear magnetic resonance (NMR) solvating agents. Racemic and
enantiomerically pure (3-chloro-2-hydroxypropyl)trimethyl-, triethyl-, and tri-n-propyl ammonium
chloride were synthesized from the corresponding trialkyl amine hydrochloride and either racemic
or enantiomerically pure epichlorohydrin. The ammonium salts were then reacted with α-, β-, and
γ-cyclodextrins at basic pH to provide the corresponding randomly substituted cationic cyclodex-
1
trins. The H NMR spectra of a range of anionic, aromatic compounds was recorded with the
cationic cyclodextrins. Cyclodextrins with a single stereochemistry at the hydroxy group on the
(2-hydroxypropyl)trialkylammonium chloride substituent were often but not always more effective
than the corresponding cyclodextrin in which the C-2 position was racemic. In several cases, the
larger triethyl or tri-n-propyl derivatives were more effective than the corresponding trimethyl deriv-
ative at causing enantiomeric differentiation. None of the cyclodextrin derivatives were consistently
the most effective for all of the anionic compounds studied. Chirality 28:299–305, 2016. © 2016 Wiley
Periodicals, Inc.
KEY WORDS: cyclodextrins; chiral solvating agents; NMR; enantiomeric differentiation; chiral
analysis
Nuclear magnetic resonance (NMR) spectroscopy is
frequently used for the analysis of enantiomeric purity and
the assignment of absolute configuration.1–6 NMR methods
for chiral analysis rely on the utilization of chiral derivatizing
agents, solvating agents, metal complexes, or liquid crystals
to achieve nonequivalent environments for enantiomeric com-
pounds leading to differences in chemical shifts in the NMR
spectrum. Among the different NMR systems, chiral solvat-
ing agents are noteworthy because of their ease of use and
less stringent need for complete enantiomeric purity of the
chiral reagent. Also, whereas there is often a concern that
reaction with a chiral derivatizing agent could result in some
loss of configuration of the analyte, this rarely if ever occurs
with chiral solvating agents that associate through noncovalent
interactions. Enantiomeric differentiation in NMR spectra of
substrates with chiral solvating agents occurs because the
complexes are diastereomeric and/or the association constants
of the two enantiomers with the chiral solvating agent are
different. It is important that the exchange of bound and
unbound substrate with the chiral solvating agent be fast on
the NMR time scale.
allowing the cyclodextrin to be tailored for certain families
of analytes. Native, underivatized cyclodextrins are soluble
in water and have been used in NMR spectroscopy to
enantiomerically differentiate a range of compounds. Many
of the water-soluble organic analytes previously examined
with cyclodextrins are charged species.
Prior studies have shown that anionic cyclodextrins contain-
ing sulfate17,18 or carboxymethylate18–21 groups are often much
more effective chiral NMR solvating agents for cationic sub-
strates than the corresponding native cyclodextrin. Similarly,
cationic cyclodextrins containing protonated amino,22–26 bis(6-
trimethylammonium),27 or mono(6-xylylethyenediamine)28
groups were found to be more effective chiral NMR solvating
agents for anionic substrates than the corresponding native
cyclodextrin. A more recent report examined cyclodextrins
with higher degrees of substitution of trimethylammonium
groups and found that these were much more effective chiral
[This article is part of the Special Issue: “27th International Symposium on
Chiral Discrimination” (ISCD27). See the first articles for this special issue
previously published in Volume 28:3. More special articles will be found in
this issue as well as in those to come.]
Contract grant sponsor: Major Instrumentation Program; Contract grant
number: CHE-0115579.
Cyclodextrins are an especially versatile family of chiral
NMR solvating agents.2,3,5–16 Cyclic oligosaccharides
comprised of D-glucose rings, the different sizes of the cavity
in α-, β-, and γ-cyclodextrin accommodate different size
analytes. Derivatization of the hydroxyl groups expands the
versatility of cyclodextrins by providing chiral reagents with
different solubility properties and different functionalities,
Contract grant sponsor: Research at Undergraduate Institutions Program;
Contract grant number: CHE-1145061.
*Correspondence to: T. J. Wenzel, Department of Chemistry, Bates College,
Lewiston, ME 04240. E-mail: twenzel@bates.edu
Received for publication 8 October 2015; Accepted 8 January 2016
DOI: 10.1002/chir.22582
Published online 16 February 2016 in Wiley Online Library
(wileyonlinelibrary.com).
© 2016 Wiley Periodicals, Inc.

300
DOWEY ET AL.
CDCl3. The product was extracted into 30 mL distilled water and used
directly in a reaction with the appropriate quantity or α-, β-, or γ-cyclodextrin.
NMR solvating agents for anionic substrates than native cyclo-
dextrins and derivatives with only two trimethylammonium
substituent groups.29 Of the cationic cyclodextrins, an advan-
tage of the trimethylammonium derivative is that it contains
quaternary amines that are cationic at all pH values. Cyclodex-
trin derivatives with primary amine groups require careful
adjustment of pH to maintain the cyclodextrin as a cationic spe-
cies and analyte as an anionic species.
Synthesis of (R)-(3-chloro-2-hydroxypropyl)triethylammonium chlo-
ride. The procedure was identical to that used to prepare racemic
(3-chloro-2-hydroxypropyl)triethylammonium chloride except that (R)-epi-
chlorohydrin was used in the reaction. The reaction was monitored for com-
pletion by 1H NMR spectroscopy in CDCl3.
Synthesis of racemic (3-chloro-2-hydroxypropyl)tri-n-propylammonium
chloride. Tri-n-propylamine (18.08 g, 126 mmol) was added dropwise to
60 mL of 2 M HCl in a 250-mL round-bottom flask at 0°C. Rotary evaporation
provided tri-n-propylammonium chloride as a white solid. The product was ver-
ified by 1H NMR spectroscopy in CDCl3. Racemic (3-chloro-2-hydroxypropyl)tri-
n-propylammonium chloride was prepared by a procedure identical to that used
to prepare racemic (3-chloro-2-hydroxypropyl)triethylammonium chloride
except that tri-n-propylammonium chloride was used in the reaction. The reac-
The procedure used to synthesize cyclodextrins that
contain a trimethylammonium group is shown in Figure 1.
Commercially available glycidyltrimethylammonium chloride
was reacted with the cyclodextrin for several days at basic
pH. Prior reports of these cationic cyclodextrins as chiral
NMR solvating agents involved the use of derivatives pre-
pared using racemic glycidyltrimethylammonium chloride.27,29
Opening of the epoxide ring in the reaction produces a new
stereocenter at the carbinol group on the substituent. When
racemic glycidyltrimethylammonium chloride is used in the
reaction, the stereochemistry of the carbinol group is racemic
as well. It is possible that having only a single stereochemistry
of the carbinol group may enhance the enantiomeric recogni-
tion properties of the trimethylammoniated cyclodextrins by
providing another site for enantiomeric differentiation. Also,
since the degree of enantiomeric differentiation is often depen-
dent on the quality of the fit of the substrate into the cyclodex-
trin cavity, derivatives with ammonium groups containing
larger ethyl or n-propyl groups may have varying degrees of
effectiveness as chiral NMR solvating agents.
1
tion, which typically required 4–7 days, was monitored for completion by H
NMR spectroscopy in CDCl3. The product was separated using 30 mL distilled
water. The top, clear, aqueous layer contained the product and was subse-
quently used to react with cyclodextrins.
Synthesis of (R)-(3-chloro-2-hydroxypropyl)tri-n-propylammonium
chloride. The procedure was identical to that used to prepare racemic
(3-chloro-2-hydroxypropyl)tri-n-propylammonium chloride except that
(R)-epichlorohydrin was used in the reaction. The reaction was moni-
tored for completion by 1H NMR spectroscopy in CDCl3.
Synthesis of (rac-2-hydroxypropyl)triethylammonium-α-cyclodextrin.
Racemic (2-hydroxypropyl)triethylammonium-α-cyclodextrin was synthe-
sized by a modification of a published procedure.31 To a 250-mL round bottom
flask, 4.67 g (4.8 mmol) of α-cyclodextrin and a catalytic amount of
tetrabutylammonium iodide was added to the 30 ml aqueous solution of
racemic (3-chloro-2-hydroxypropyl)triethylammonium chloride (126 mmol).
The pH of the solution was raised to 12 with 4 M sodium hydroxide. The
reaction mixture was stirred at 50°C. The pH was monitored during the first
10 hours of the reaction and often more sodium hydroxide was needed to
maintain the pH at 12. The reaction was monitored by 1H NMR spectroscopy
until the desired degree of substitution was achieved (7–10 days). The reac-
tion mixture was placed on ice and the pH adjusted to 6 by the addition of
2 M HCl and the total volume was brought up to 75 mL with distilled water.
A 30–40 cm long piece of Spectra/Por CE tubing was allowed to soak in dis-
tilled H2O for 0.5 h. The reaction mixture was added to the dialysis tubing
and dialyzed against distilled H2O for 10 h. Fresh distilled water was added
and dialysis was continued for an additional 6 h. The volume of the liquid in
the dialysis tube was reduced by rotary evaporation and then in a vacuum
desiccator to afford either a clear, colorless syrup or white crystalline material.
Integration of the appropriate signals in the 1H NMR spectrum was used to
determine the degree of substitution.
We report herein the preparation of cyclodextrin derivatives
where the carbinol in the substituent group has only a single
stereochemistry and where the substituent has trimethyl-,
triethyl-, or tri-n-propylammonium groups. The utility of these
cationic cyclodextrins as chiral NMR solvating agents for a
range of anionic analytes is reported.
MATERIALS AND METHODS
Reagents
Substrate compounds for NMR analysis were obtained from commer-
cial sources either as a sodium salt or neutral compound. Epichlorohy-
drin, (R)-epichlorohydrin, triethylammonium chloride, tri-n-propylamine,
(S)-(3-chloro-2-hydroxypropyl)trimethylammonium chloride, α-, β-, and
γ-cyclodextrin and tetrabutylammonium iodide were obtained from
Sigma-Aldrich (St. Louis, MO). Spectra/Por Cellulose Ester dialysis tub-
ing, 100–500 MWCO was purchased from Spectrum Laboratories (Ran-
cho Dominguez, CA).
Synthesis of (rac-2-hydroxypropyl)triethylammonium-β-cyclodex-
Synthetic Procedures
trin,
(rac-2-hydroxypropyl)triethylammonium-γ-cyclodextrin,
Synthesis of racemic (3-chloro-2-hydroxypropyl)triethylammonium
chloride. Racemic (3-chloro-2-hydroxypropyl)triethylammonium chloride
was synthesized by a modification of a published procedure.30 In a 250-mL
round bottom flask, 11.60 g (126 mmol) of racemic epichlorohydrin,
17.34 g (126 mmol) of triethylammonium chloride, and 50 mL of chloroform
were added. The solution was stirred at 65°C under nitrogen for 2–5 days.
The reaction was monitored for completion by 1H NMR spectroscopy in
(rac-2-hydroxypropyl)tri-n-propylammonium-α-cyclodextrin, (rac-
2-hydroxypropyl)tri-n-propylammonium-β-cyclodextrin and (rac-2-
hydroxypropyl)tri-n-propylammonium-γ-cyclodextrin. The proce-
dure was identical to that used to prepare (rac-2-hydroxypropyl)trie-
thylammonium-α-cyclodextrin with similar equivalents of reactants except
the appropriate ammonium salt and cyclodextrin were used in the reaction.
Synthesis of ((S)-2-hydroxypropyl)trimethylammonium-β-cyclodex-
trin. The procedure was identical to that used to prepare (rac-2-hy-
droxypropyl)triethylammonium-α-cyclodextrin except that (S)-(3-chloro-
2-hydroxypropyl)trimethylammonium chloride was used in the reaction.
Synthesis of ((R)-2-hydroxypropyl)triethylammonium-β-cyclodextrin
and ((R)-2-hydroxypropyl)tri-n-propylammonium-β-cyclodextrin.
The procedure was identical to that used to prepare (rac-2-hydroxypropyl)
triethylammonium-α-cyclodextrin except that (R)-(3-chloro-2-hydroxypropyl)
triethyl- or tri-n-propylammonium chloride and the appropriate cyclodextrin
were used in the reaction.
Fig. 1. Reaction of glycidyltrimethylammonium chloride with α-, β-, or
γ-cyclodextrin.
Chirality DOI 10.1002/chir

CATIONIC CYCLODEXTRINS AS CHIRAL NMR SOLVATING AGENTS
301
Procedures for Chiral Differentiation Studies
Concentrated solutions (0.10 M) of substrates enriched in one enantio-
mer (when available) were prepared in deuterium oxide. When the
substrate was available as a neutral compound instead of its sodium salt,
a stoichiometric equivalent of sodium deuteroxide in deuterium oxide
was added to the solution. Each 600 μL NMR sample contained the
necessary weight of each cyclodextrin to provide a 10 mM, 20 mM, or
30 mM concentration as needed. Additionally, 540 μL of deuterium oxide
and 60 μL of the 0.1 M stock solution of a substrate were added to provide
a 10 mM concentration of substrate. 1H NMR spectra were obtained
using a Bruker Avance (Billerica, MA) 400 MHz NMR spectrometer with
eight scans at ambient temperature.
Fig. 3. Reaction of (S)-(3-chloro-2-hydroxypropyl)trimethylammonium
chloride with α-, β-, or γ-cyclodextrin.
RESULTS AND DISCUSSION
Synthesis of Cationic Cyclodextrins
Racemic (2-hydroxypropyl)trimethylammonium-substituted
α-, β-, and γ-cyclodextrin (rac-TMA) is commonly prepared by
the reaction of commercially available glycidyltrimethy-
lammonium chloride with the cyclodextrin (Fig. 1).29,31 The
starting ammonium salts needed for a similar preparation of
the racemic (2-hydroxypropyl)triethyl- and tri-n-propylam-
monium and enantiomerically pure (2-hydroxypropyl)-
trimethyl-, triethyl-, and tri-n-propyl ammonium cyclodextrins
are not commercially available. Initial attempts at preparing
these other ammonium salts involved the reaction of either
racemic- or (R)-epichlorohydrin with triethylamine or
tri-n-propylamine (Fig. 2). Two different previously published
procedures were used in these attempts.32,33 A solution of
trimethylamine in ethanol, which is commercially available,
was used in the reaction with (R)-epichlorohydrin. The reaction
in Figure 2 was ineffective for our purposes, as we were unable
to avoid the production of substantial quantities of an alkene
byproduct (N-(3-hydroxy-1-propenyl)trialkylammonium chlo-
ride – (CH3)3N+CH = CHCH2OH Cl–) with all three amines.
The precedence for forming the alkene has been previously
observed.34 The yield of the desired trialkylammonium epoxide
using the reaction shown in Figure 2 was too low to merit a
subsequent reaction with cyclodextrin.
Fig. 4. Reaction used to prepare (3-chloro À2-hydroxypropyl)trie-
thylammonium chloride and (3-chloro-2-hydroxypropyl)tri-n-propylammonium
chloride.
epichlorohydrin by extraction into water. The resulting aque-
ous solution was reacted with the cyclodextrin as in Figure 3.
A
large excess of the (3-chloro-2-hydroxypropyl)trial-
kylammonium salt is used relative to the cyclodextrin to raise
the degree of substitution. A peak at about 4.4 ppm in the 1
H
NMR spectrum of the product corresponds to the methine res-
onance of the substituent group. Representative 1H NMR spec-
tra obtained for rac-TEA-α and rac-TEA-γ are shown in
Figure 5a,b, respectively. The area of the peak at 4.4 ppm rela-
tive to either the area of the H1 or H2-H6 resonances of the
cyclodextrin can be used to determine the degree of substitu-
tion (DS). Typical DS ranged from 7 to 12 for the
triethylammonium (rac-TEA and (R)-TEA) and tri-n-
propylammonium-substituted cyclodextrins (rac-TPA and
(R)-TPA). Only derivatives with a DS in the range of 10–12
were used in studies aimed at producing enantiodifferentiation
in the NMR spectra of substrates. Purification of the cationic
cyclodextrins was achieved by dialysis using membranes with
a 500 MW cutoff.
An alternative procedure to prepare (2-hydroxypropyl)-
trimethylammonium-substituted cyclodextrins with a single
stereochemistry at the carbinol of the substituent group
involves the reaction of commercially available (S)-(3-chloro-
2-hydroxypropyl)trimethylammonium chloride with α-, β-, or
γ-cyclodextrin at basic pH, as shown in Figure 3.31 This proce-
dure was effective at producing the desired (S)-2-hydroxypro-
NMR Studies of Anionic Substrates
The effectiveness of rac-TMA-β, rac-TEA-β, and rac-TPA-β
as chiral NMR solvating agents is compared to (S)-TMA-β
(R)-TEA-β and (R)-TPA-β. Thirteen different substrates (1–
13, Fig. 6) that span a range of compound classes and are an-
ionic when deprotonated as herein were examined. Enantio-
pyl)trimethylammonium-substituted
cyclodextrins
((S)-
1
TMA). Typical degrees of substitution for the (S)-(2-hydroxy-
propyl)trimethylammonium derivatives of β-cyclo-dextrin
ranged from 5 to 12.
meric differentiation in the H NMR spectrum of 1–13 with
Racemic- and (R)-(3-chloro-2-hydroxypropyl)triethyl- and
tri-n-propylammonium chloride salts were prepared as shown
in Figure 4.30 The progress of the reaction can be monitored
by the diminishment of the trialkylamine methyl resonance
and growth of the trialkylammonium methyl resonance in the
1H NMR spectrum. The (2-hydroxypropyl)trialkylammonium
salt was separated from unreacted trialkylamine and
Fig. 5. 1H NMR spectrum (400 MHz, D2O, 25°C) of (a) rac-TEA-α and (b)
rac-TEA-γ. The peak at about 4.4 ppm corresponds to the methine hydrogen of
the substituent group.
Fig. 2. Reaction used in an attempt to prepare glycidyltrialkylammonium
chloride salts.
Chirality DOI 10.1002/chir

302
DOWEY ET AL.
TABLE 2. Enantiomeric differentiation (ΔΔδ) in ppm in the 1
H
NMR spectrum of substrates (10 mM) in the presence of cat-
ionic β-TEA-cyclodextrin species
rac-TEA-βa
(R)-TEA-β
3
4
CH
0.010 (20 mM)
0.008 (20 mM)
0.030 (20 mM)
0.002 (10 mM)
0.005 (10 mM)
0.018 (30 mM)
0
0.007 (20 mM)
0.010 (20 mM)
0.010 (10 mM)
0.053 (20 mM)
0.010 (20 mM)
0.011 (30 mM)
0.013 (20 mM)
0
0.011 (30 mM)
0
0.008 (20 mM)
0.008 (20 mM)
0.014 (30 mM)
0.010 (30 mM)
0.063 (30 mM)
Hm
CH2’
Ha
CH
CH3
H5
7
9
10
11
H7
12
13
CH3
CH2
CH2’
aThe concentration of the cyclodextrin is indicated in parentheses.
TABLE 3. Enantiomeric differentiation (ΔΔδ) in ppm in the 1
H
NMR spectrum of substrates (10 mM) in the presence of cat-
ionic β-TPA-cyclodextrin species
rac-TPA-βa
(R)-TPA-β
Fig. 6. Structure of substrates: indoline-2-carboxylic acid (1), tryptophan
(2), 3-indolelactic acid (3), tyrosine (4), phenylalanine (5), phenylserine (6),
histidine (7), mandelic acid (8), 3-phenyllactic acid (9), 2-phenylpropionic acid
(10), ibuprofen (11), 2-phenylbutyric acid (12), 3-phenylbutyric acid (13). All
substrates were examined as their sodium salts.
4
Hm
CHOH
CH
0
0
0.014 (20 mM)
0.006 (30 mM)
0.014 (30 mM)
0.006 (30 mM)
0.013 (30 mM)
0.016 (30 mM)
0.065 (30 mM)
0.009 (30 mM)
6
b
9
-
11
12
13
H7
0.005 (20 mM)
CH3
CH2
CH2’
CH3
0
rac-TMA-β or (S)-TMA-β) (Table 1), rac-TEA-β or (R)-TEA-β
(Table 2), and rac-TPA-β or (R)-TPA-β (Table 3) are provided.
In each case, the substrate concentration was 10 mM and
0.011 (30 mM)
0.042 (20 mM)
0
aThe concentration of the cyclodextrin is indicated in parentheses.
bResonance obscured by another resonance in the NMR spectrum.
TABLE 1. Enantiomeric differentiation (ΔΔδ) in ppm in the 1
H
NMR spectrum of substrates (10 mM) in the presence of cat-
spectra with cyclodextrin were recorded at concentrations
typically ranging from 10 to 30 mM. The values of enantio-
meric differentiation reported in Tables 1–3 are the maximum
observed among the different cyclodextrin concentrations
where no overlap with other substrate or cyclodextrin reso-
nances occurred.
ionic β-TMA-cyclodextrin species
rac-TMA-βa b
(S)-TMA-β
,
1
3
CH2
H4
H6
H2
H4
H5
H6
H7
CH
CH2
Ho
Hm
CH
CH2
CHOH
Ha
CH2
CH2’
CH
CH
CH3
H5
0.032 (20 mM)
0.019 (20 mM)
0.020 (10 mM)
0.015 (30 mM)
0.017 (30 mM)
0.006 (20 mM)
0.006 (20 mM)
0.005 (20 mM)
0.005 (20 mM)
0.006 (20 mM)
0.011 (30 mM)
0
0
0
0
0
0
0
The results for TMA-β and TEA-β show that neither the
racemic nor enantiomerically pure derivative is consistently
more effective at causing enantiomeric differentiation in the
NMR spectrum (Tables 1, 2). The inconsistency in effective-
ness between cyclodextrin derivatives with a racemic or
enantiomerically pure carbinol in the substituent group even
occurs for different resonances of the same substrate. For
example, (S)-TMA-β causes enantiomeric differentiation for
the H6 resonance of 1 that is not observed with rac-TMA-β;
however, the enantiomeric differentiation of one of the meth-
ylene hydrogen atoms and H4 of 1 with rac-TMA-β is larger
than that with (S)-TMA-β. (S)-TMA-β causes enantiomeric
differentiation of several hydrogen resonances in 3 and 4 that
is not observed with rac-TMA-β. However, rac-TMA-β pro-
duces larger enantiomeric differentiation of one or two reso-
nances of 3 and 4 than (S)-TMA-β. More resonances show
larger enantiomeric differentiation with (R)-TEA-β (six over-
all) than with rac-TEA-β (three overall) (Table 2). However,
with 7 and 10, enantiomeric differentiation is only observed
with the rac-TEA-β. Enantiomeric differentiation was always
larger with the (R)-TPA-β than with rac-TPA-β, although fewer
0.021 (20 mM)
0.037 (20 mM)
0.021 (20 mM)
0
4
0
0.010 (20 mM)
0.006 (10 mM)
0.019 (10 mM)
0.003 (30 mM)
0.002 (10 mM)
0
0
0
0
0
6
7
0.004 (20 mM)
0.005 (20 mM)
0.006 (20 mM)
0.020 (20 mM)
0.005 (20 mM)
0.009 (20 mM)
0.006 (20 mM)
0
8
9
10
11
0.010 (30 mM)
0.013 (30 mM)
0.008 (30 mM)
0
H7
0.006 (20 mM)
aThe concentration of the cyclodextrin is indicated in parentheses.
bData from reference 29.
Chirality DOI 10.1002/chir

CATIONIC CYCLODEXTRINS AS CHIRAL NMR SOLVATING AGENTS
303
TABLE 5. Enantiomeric differentiation (ΔΔδ) in ppm in the 1
H
resonances show any differentiation with the TPA derivatives
than with the TMA or TEA derivatives (Table 3).
NMR spectrum of substrates (10 mM) in the presence of
cationic β-cyclodextrin species
These data indicate that in some cases the fixed stereo-
chemistry at the carbinol of the substituent group is impor-
tant in causing enantiomeric differentiation and is influential
in the magnitude of the chemical shift difference between
the two enantiomers. The inconsistency in cases where the
rac-TMA-β caused larger enantiomeric differentiation for
some resonances, whereas the (S)-TMA-β caused larger
enantiomeric differentiation for others, suggests that the
diastereomeric nature of the complexes species is more im-
portant than differences in the association constants in caus-
ing the enantiomeric differentiation.
A comparison of the effectiveness of the TMA-, TEA-, and
TPA-substituted cyclodextrins is provided in Tables 4–6 for
the rac-α, rac-β, and rac-γ derivatives, respectively. One of
the advantages of cyclodextrins is that the different cavity
sizes of α-, β-, and γ-cyclodextrin provide complementary fits
for different sized substrates, thereby expanding the number
of compounds that can be analyzed.
rac-TMA-βa b
rac-TEA-β
rac-TPA-β
,
1
3
4
CH2
H4
CH
CH2
Ho
Hm
CH2’
Ha
CH2
CH2’
CH
CH
CH3
H5
0.032 (20 mM)
0.019 (20 mM)
0.021 (20 mM)
0.037 (20 mM)
0.021 (20 mM)
0
0
0
0
0
c
0.010 (20 mM)
-
-
c
0
0
0
0
0
0
0
0
-
0.008 (20 mM)
0.030 (20 mM)
0.002 (10 mM)
0
0
7
0.004 (20 mM)
0.005 (20 mM)
0.006 (20 mM)
0.020 (20 mM)
0.005 (20 mM)
0.009 (20 mM)
0.006 (20 mM)
0
0
d
8
9
10
11
-
d
0.005 (10 mM)
0.018 (30 mM)
0
0.007 (20 mM)
0.010 (20 mM)
0.010 (10 mM)
0.053 (20 mM)
-
0
0
H7
0.005 (20 mM)
0
0.011 (30 mM)
0.042 (20 mM)
c
12
13
CH3
CH2
CH2’
-
c
-
Among the three α-cyclodextrin derivatives, rac-TMA-α is
often far more effective at causing enantiomeric differentia-
c
-
1
aThe concentration of the cyclodextrin is indicated in parentheses.
bData from reference 29.
tion in the H NMR spectrum than rac-TEA-α and rac-TPA-α
(Table 4). For those substrates studied with all three of the ra-
cemic cyclodextrins, enantiomeric differentiation of only one
of the methylene hydrogen atoms of 9 is largest with
rac-TEA-α and the H3 resonance of 11 is largest with
rac-TPA-α. Given that the size of the cavity of α-cyclodextrin
is the smallest of the three cyclodextrins, this suggests that
the larger ethyl and n-propyl substituent groups likely block
access of some substrates to the cavity.
cSample not recorded.
dResonance obscured by another resonance in the NMR spectrum.
TABLE 6. Enantiomeric differentiation (ΔΔδ) in ppm in the 1
H
NMR spectrum of substrates (10 mM) in the presence of cat-
ionic γ-cyclodextrin species
For the β-cyclodextrin derivatives, rac-TMA-β and rac-TEA-
β each exhibit complementary degrees of effectiveness and
rac-TMA-γa b
rac-TEA-γ
rac-TPA-γ
,
1
2
CH2
H4
H5
CH2
CH2
Ho
CH2
CH2
CH
0
0.005 (20 mM)
0
0
0
0
0.010 (20 mM)
0.010 (20 mM)
0
0
0
TABLE 4. Enantiomeric differentiation (ΔΔδ) in ppm in the 1
NMR spectrum of substrates (10 mM) in the presence of
cationic α-cyclodextrin species
H
0.040 (20 mM)
0.010 (20 mM)
3
4
0
0
0.010 (20 mM)
0.007 (10 mM)
0
0
0
rac-TMA-αa b
rac-TEA-α
rac-TPA-α
,
0.011 (20 mM)
0.008 (20 mM)
0.007 (20 mM)
0.004(20 mM)
0.007 (20 mM)
0
0
5
8
9
2
3
H4
H5
H6
CH2
H4
H5
H7
0.007 (20 mM)
0.031 (20 mM)
0.004 (20 mM)
0.037 (20 mM)
0.031 (20 mM) 0.018 (30 mM) 0.017 (30 mM)
0.007 (20 mM)
0.030 (20 mM) 0.021 (20 mM) 0.019 (30 mM)
0
0
0
0
0
c
d
-
-
0.013 (30 mM)
CH
CH2
CH2’
0.004 (20 mM)
0.007 (20 mM)
0.017 (20 mM)
0.006 (20 mM)
0.008 (20 mM)
0.017
0
0
d
13
-
0
0
aThe concentration of the cyclodextrin is indicated in parentheses.
bData from reference 29.
cResonance obscured by another resonance in the NMR spectrum.
dSample not recorded.
CH
CH2
Hm
CH2
0.010 (20 mM)
0.010 (20 mM)
0.028 (20 mM)
0.037 (20 mM)
0
0
0
0
0
0
0
0
0
0
0
0
4
each is best for several resonances (Table 5). The general
lack of enantiomeric differentiation in the H NMR spectra
1
6
7
9
CHNH2 0.025 (20 mM)
Hb
CH
CH2
CH2’
0.016 (20 mM)
0.040 (20 mM) 0.030 (30 mM) 0.010 (10 mM)
of many substrates with rac-TPA-β suggests that the bulkier
n-propyl groups likely restrict access of some substrates to
the cavity. For the γ-cyclodextrin derivatives, the rac-TMA-γ,
rac-TEA-γ, and rac-TPA-γ cause the largest enantiomeric dif-
ferentiation of five, three, and four resonances, respectively
(Table 6). These data show that the larger ethyl and n-propyl
groups can be advantageous in enhancing the extent of enan-
tiomeric differentiation, especially with β- and γ-cyclodextrin.
Enantiomeric differentiation of the diastereotopic H3 and
H3’ methylene resonances of 9 using rac-TEA-α, rac-TEA-β,
and rac-TEA-γ is shown in Figure 7. This set of spectra shows
the degree to which changes in the size of the cyclodextrin
0.015 (20 mM) 0.009 (30 mM)
0.015 (20 mM)
0.008 (20 mM) 0.001 (10 mM)
0
0
0
0
11 H1
H3
H7
0
0
0
0.014 (20 mM)
0
0.008 (20 mM)
c
13 CH2
-
0.025 (30 mM) 0.033 (20 mM)
0.035 (30 mM) 0.016 (20 mM)
c
CH2’
CH3
-
c
-
0
0.007 (20 mM)
aThe concentration of the cyclodextrin is indicated in parentheses.
bData from reference 29.
cSample not recorded.
Chirality DOI 10.1002/chir

304
DOWEY ET AL.
Fig. 9. 1H NMR spectrum (400 MHz, D2O, 25°C) of the diastereotopic H2
and H2’ methylene resonances of (a) 13 (10 mM, (S)-enriched) with 20 mM
(b) rac-TEA-α, (c) rac-TEA-β, and (d) rac-TEA-γ.
Fig. 7. 1H NMR spectrum (400 MHz, D2O, 25°C) of the diastereotopic H3
and H3’ methylene resonances of (a) 9 (10 mM, (D)-enriched) with 30 mM (b)
rac-TEA-α, (c) rac-TEA-β, and (d) rac-TEA-γ. *Resonances from an impurity.
enantiomeric differentiation of H2’ and no enantiomeric differ-
entiation of H2.
The cationic cyclodextrin responsible for the largest enan-
tiomeric differentiation of each resonance of each substrate
is reported in Table 7. A total of 38 resonances within the 13
cavity can influence the degree of enantiomeric differentia-
tion. The rac-TEA-α and rac-TEA-γ cause a small degree of
enantiomeric differentiation for H3, whereas the rac-TEA-β
does not. For H3’, rac-TEA-α causes significant enantiomeric
differentiation, whereas neither rac-TEA-β nor rac-TEA-γ
causes any enantiomeric differentiation.
TABLE 7. Enantiomeric differentiation (ΔΔδ) in ppm in the 1
H
NMR spectrum of substrates (10 mM) in the presence of cat-
ionic cyclodextrin species
The effects of (S)-TMA-β, (R)-TEA-β, and (R)-TPA-β on the
diastereotopic H2 and H2’ methylene resonances of 13 are
shown in Figure 8. All three cyclodextrins produce consider-
able enantiomeric differentiation of H2’, the largest of which
occurs with (S)-TMA-β. A lesser degree of enantiomeric
differentiation is observed in the H2 resonance with all three
of the cyclodextrins. The spectrum with (R)-TPA-β shows
slightly more broadening than those with (S)-TMA-β and
(R)-TEA-β. The broadening is likely caused by a slower
exchange of substrate between its unbound and complexed
form when mixed with (R)-TPA-β. This broadening, especially
with the larger more sterically hindered TPA derivatives, is
observed with some other substrates as well. One other fea-
ture worth noting is that the 3-bond coupling constants
between H2 and H3 changes when comparing the spectrum
of the substrate (Fig. 8a) with those with the cyclodextrins
(Fig. 8b,c). Presumably, complexation of 13 with the cationic
cyclodextrins influences the rotational motion within the
substrate thereby changing the time-averaged dihedral angle
between H2 and H3.
ΔΔδ
CDa
1
2
CH2
H4
H6
H4
H5
H6
CH2
H2
H4
H5
H6
H7
CH
CH2
Ho
Hm
CH
CH2
CH2’
CH2
CHNH2
Ha
0.032
0.019
0.017
0.010
0.031
0.005
0.040
0.006
0.031
0.007
0.005
0.030
0.021
0.037
0.021
0.028
0.006
0.055
0.030
0.008
0.025
0.002
0.016
0.004
0.004
0.010
0.040
0.015
0.015
0.018
0.008
0.014
0.009
0.008
0.018
0.033
0.075
0.008
rac-TMA-β (20 mM)
rac-TMA-β (20 mM)
(S)-TMA-β (30 mM)
rac-TMA-γ (20 mM)
rac-TMA-α (20 mM)
rac-TMA-α (20 mM)
rac-TEA-γ (20 mM)
(S)-TMA-β (20 mM)
rac-TMA-α (20 mM)
rac-TMA-α (20 mM)
(S)-TMA-β (20 mM)
rac-TMA-α (20 mM)
rac-TMA-β (20 mM)
rac-TMA-β (20 mM)
rac-TMA-β (20 mM)
rac-TMA-α (20 mM)
(S)-TMA-β (10 mM)
rac-TMA-α (20 mM)
rac-TEA-β (20 mM)
rac-TMA-γ (20 mM)
rac-TMA-α (20 mM)
rac-TEA-β (10 mM)
rac-TMA-α (20 mM)
rac-TMA-β (20 mM)
rac-TMA-β (20 mM)
(S)-TMA-β (30 mM)
rac-TMA-α (20 mM)
rac-TMA-α (20 mM)
rac-TEA-α (20 mM)
rac-TEA-β (30 mM)
rac-TMA-α (20 mM)
rac-TPA-α (20 mM)
rac-TMA-β (20 mM)
(R)-TEA-β (20 mM)
(S)-TMA-β (20 mM)
rac-TPA-α (20 mM)
(S)-TMA-β (30 mM)
(S)-TMA-β (10 mM)
3
4
A comparison of the enantiomeric differentiation of the
resonances of diastereotopic H2 and H2’ of 13 with
rac-TEA-α, rac-TEA-β, and rac-TEA-γ is shown in Figure 9.
The largest enantiomeric differentiation of H2’ is caused with
rac-TEA-β, whereas the enantiomeric differentiation of H2 is
slightly larger with rac-TEA-α. rac-TEA-γ causes only a slight
5
6
7
Hb
CH2
CH2’
CH
8
9
CH
CH2
CH2’
CH3
H1
H3
H5
10
11
H7
12
13
CH3
CH2
CH2’
CH3
aValues reported are the best result obtained among all of the cationic cyclo-
dextrins that were studied. The concentration of the cyclodextrin is indicated
in parentheses.
Fig. 8. 1H NMR spectrum (400 MHz, D2O, 25°C) of the diastereotopic H2
and H2’ methylene resonances of (a) 13 (10 mM, (S)-enriched) with 30 mM
(b) (S)-TMA-β, (c) (R)-TEA-β, and (d) (R)-TPA-β.
Chirality DOI 10.1002/chir

CATIONIC CYCLODEXTRINS AS CHIRAL NMR SOLVATING AGENTS
305
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number of resonances. However, there are some examples
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