Enantioselective sensing of chiral amino alcohols with a stereodynamic arylacetylene-based probe

Article abstract of DOI:10.1002/chir.22066

Enantioselective induced circular dichroism analysis of amino alcohols has been accomplished using a conformationally flexible arylacetylene-based probe exhibiting two terminal aldehyde groups. The chirality of the amino alcohol substrates is imprinted on

Full text of DOI:10.1002/chir.22066

CHIRALITY 24:584–589 (2012)
Enantioselective Sensing of Chiral Amino Alcohols with a
Stereodynamic Arylacetylene-based Probe
*
DANIEL P. IWANIUK, KEITH W. BENTLEY, AND CHRISTIAN WOLF
Department of Chemistry, Georgetown University, Washington, District of Columbia
ABSTRACT
Enantioselective induced circular dichroism analysis of amino alcohols has
been accomplished using a conformationally flexible arylacetylene-based probe exhibiting
two terminal aldehyde groups. The chirality of the amino alcohol substrates is imprinted on
the stereodynamic receptor upon [1 + 2] condensation, which ultimately generates a strong
chiroptical response. The distinct induced circular dichroism effects of the diimines obtained
can be used for enantioselective sensing and enantiomeric excess determination of a wide
range of substrates. Chirality 24:584–589, 2012. © 2012 Wiley Periodicals, Inc.
KEY WORDS: enantiodifferentiation; stereoselective analysis; induced circular dichroism;
molecular recognition
INTRODUCTION
quantitative ee analysis of the substrates used. In addition, 1
was found to give strong Cotton effects suitable for CD analysis
when monoamines were employed. We therefore decided to
investigate if the substrate scope of 1 can be extended to
aromatic and aliphatic amino alcohols.
Chiroptical spectroscopy, in particular circular dichroism
(CD), has received significant attention in recent years, and
an increasing number of efficient and broadly useful assays
for the stereochemical analysis of chiral compounds have
appeared in the literature.1 Induced circular dichroism (ICD)
is observed when covalent or noncovalent association between
a chiral substrate and a UV active, stereodynamic probe favors
population of a chiral conformation or configuration of the
latter, thus causing a distinct CD output.2–4 The development
of conformationally flexible receptors capable of reporting a
molecular recognition event upon binding to a substrate
provides new avenues for chiroptical analysis.5–10 For example,
Rosini and Toniolo demonstrated that the absolute configura-
tion of chiral amino acids, carboxylic acids, and alcohols can
be correlated to the ICD output of a covalently linked stereody-
namic biphenyl reporter unit.11–16 The same principles have
been exploited with molecular bevel gears,17 propellers,18 and
other well-defined arrangements.9,19,20 Berova, Nakanishi,
Anslyn, Canary, and others have introduced elegant probes
showing distinct chiral amplification and ICD signals that can
be used for both reliable structural analysis and enantiomeric
excess (ee) determination of chiral substrates.7,19,21–37
Because of the ever-increasing demand for enantiopure phar-
maceuticals, the development of fast, accurate, and practical
methods for the stereochemical analysis of amino alcohols,
which serve as chiral building blocks of many bioactive com-
pounds, has become very important.38,39 To date, few probes
that exploit the practicality of CD spectroscopy for enantioselec-
tive recognition of amino alcohols have been identified, and
examples of quantitative analysis of the enantiomeric excess
of scalemic mixtures are rare.40–44 Our group has previously
developed 1,8-diheteroarylnaphthalene-derived sensors45–50
and stereodynamic triaryl-derived probes43,44 for enantioselec-
tive recognition and UV, fluorescence, or CD analysis of
amines, carboxylic acids, amino acids, and amino alcohols.
Recently, we introduced 1,4-bis(2(2-formylphenylethynyl)
phenylethynyl)benzene (1, Fig. 1). This dialdehyde was
designed to undergo central-to-axial chirality induction upon
cyclocondensation with diamines.51,52 The macrocyclic
diimines obtained proved to be highly CD active, which allowed
© 2012 Wiley Periodicals, Inc.
MATERIALS AND METHODS
General
All reagents and solvents were commercially available and used
without further purification. Reactions were carried out under inert
atmosphere and anhydrous conditions. Electrospray ionization mass
spectra (ESI-MS) were collected with samples dissolved in methanolic
chloroform (0.1:10, 0.5 mg/ml).
Enantioselective Sensing Experiments
A stock solution of 1 (0.00375 M) in anhydrous CDCl3 was prepared,
and 350-ml aliquots of this solution were placed in 4-ml vials. Then,
solutions of the substrates (0.2828 M) in CDCl3 were prepared. For each
diimine formation, 10 ml of a substrate stock solution was placed in a vial
containing the sensor solution (350 ml) over molecular sieves (4 Å, 8–12
mesh). The reactions were stirred at room temperature for 90 min. Prior
to each use, the CD instrument was purged with nitrogen for 20 min at
room temperature. CD spectra were collected with a standard sensitivity
of 100 mdeg, a data pitch of 0.5 nm, a band width of 1 nm, a scanning
speed of 500 nm sÀ1, and a response of 0.5 s using a quartz cuvette
(1 cm path length). The data were baseline corrected and smoothed using
a binomial equation. The CD analysis was conducted with sample concen-
trations of 9.38 Â 10À5 M, generally showing acceptable optical density.
Control experiments with 5–14 at the same concentration range showed
that the free substrates are CD silent in the region of interest.
Calibration Curve and ee Determination
A calibration curve was constructed using samples of 5 with varying ee.
A stock solution of 1 (0.00375 M) in anhydrous CDCl3 was prepared, and
350-ml aliquots of this solution were placed in 4-ml vials. Stock solutions of
Additional Supporting Information may be found in the online version of this
article.
Contract grant sponsor: NSF; Contract grant number: CHE-0910604.
*Correspondence to: Christian Wolf, Georgetown University, Chemistry
Department, 37th and
cw27@georgetown.edu
O Streets NW, Washington DC, 20057. Email:
Received for publication 17 February 2012; Accepted 5 April 2012
DOI: 10.1002/chir.22066
Published online 24 May 2012 in Wiley Online Library
(wileyonlinelibrary.com).

ENANTIOSELECTIVE SENSING OF AMINO ALCOHOLS
585
O
H
NH
NH
2
N
2
N
H
H
H N
2
H
H
N
N
1
H
O
distinct ICD signal
CD silent
distinct ICD signal
Fig. 1. Schematic illustration of the reaction between 1 and diamines or monoamines towards [1 + 1] or [1 + 2] diimine condensation products, respectively.
5 (0.1313 M) with varying ee composition (+100.0, +80.0, +60.0, +20.0, 0.0,
À20.0, À40.0, À60.0, À80.0, À100.0) were prepared in anhydrous CDCl3.
For each diimine formation, 10 ml of a substrate stock solution was placed
in a vial containing the sensor solution, and molecular sieves (4 Å, 8–12
mesh) were added. The reactions were stirred at room temperature for
90 min. Upon completion, the reaction solution was diluted to
9.38 Â 10À5 M for CD analysis. The data were baseline corrected and
smoothed using a binomial equation. The CD amplitudes (mdeg) at
290 nm were plotted versus % ee. The calibration curve shows a linear
relationship (mdeg = 0.0813 [% ee] + 0.5762) with R2 = 0.992.
Treatment of 1 with all the aromatic and aliphatic amino
alcohols 5–14 shown in Figure 2 gave the expected [1 + 2]
condensation products. The reaction was monitored by ESI-
MS, IR, and NMR spectroscopic analysis, which showed the
disappearance of the formyl protons of 1 (Fig. 3 and
Supplementary Information). Comparison between the IR
spectra of the diimine product and 1 clearly shows that the
carbonyl stretching of the aldehyde moiety in 1 (1695 cmÀ1
)
disappeared, whereas the relatively weak imine stretching
absorption (1637 cmÀ1) emerged (Fig. 3).
The chiroptical properties of the diimines were then
determined by CD spectroscopy without further purification.
We were pleased to find that the diimines of aromatic
substrates 5–11 provided strong Cotton effects even at
RESULTS AND DISCUSSION
Stereodynamic probe 1 was prepared in four steps and
62% overall yield as described previously (Scheme 1).51,52
Br
TMS
I
Pd(PPh3)4
Si
Br
(10 mol%)
3, 93%
+
CuI
(10 mol%)
Pd(PPh3)4 (10 mol%)
CuI (10 mol%)
TMS
Br
2, 99%
Br
O
TBAF
O
H
H
Pd(PPh3)4 (10 mol%)
CuI (10 mol%)
H
4
, 76%
O
1, 88%
Scheme 1. Synthesis of sensor 1.
H2N
OH
H2N
OH
H2N
OH
OH
NH2
OH
NH2
5
6
7
8
9
NH2
H3C
OH
OH
OH
HO
NH2
CH3 NH2
NH2
NH2
OH
10
11
12
13
14
Fig. 2. Structures of amino alcohols tested (only one enantiomer is shown for simplicity).
Chirality DOI 10.1002/chir

586
IWANIUK ET AL.
Fig. 3. Top: Stacked IR spectra of 1 (solid line) and the diimine (dashed line) obtained with (1R,2S)-5. Bottom: ESI-MS spectrum of the diimine obtained from 1
and (1R,2S)-5 (m/z = 924).
micromolar concentrations albeit with less intensity than previ-
ously reported for the condensation products obtained with
amines and diamines.51,52 The CD spectra of the diimines ob-
served for the enantiomers of 2-amino-1,2-diphenylethanol, 5,
show ICD signals that originate from substrate-controlled in-
duction of axial chirality across the arylacetylene framework
of the probe (Fig. 4). Similarly, the condensation products
obtained with aliphatic substrates 12–14 exhibit distinct
Cotton effects suitable for CD analysis. A representative
example of the ICD effects obtained by condensation of 1 with
the enantiomers of 2-amino-4-methyl-1-pentanol, 12, is shown
in Figure 4.
We believe that the diimines formed exist as a complex
mixture of rapidly interconverting isomers, each having dif-
ferent thermodynamic stability and chiroptical properties.
Therefore, both solvent and temperature effects on the CD
amplitude of the 1-derived condensation product obtained
with (1R,2S)-5 were investigated. First, the Cotton effects
in several solvents (methanol, acetonitrile, diethyl ether,
ethyl acetate, hexanes, tetrahydrofuran, and chloroform)
were studied. In all cases, a negative Cotton effect was ob-
served. However, the ICD amplitude of the diimine in-
creased about twofold when methanol was used as solvent
Chirality DOI 10.1002/chir
(Fig. 5). In addition, we found that the ICD signal can be fur-
ther increased by ~15% when the temperature is reduced
from 25 to 5 ꢀC. Both effects are attributed to selective stabi-
lization of a highly CD active conformation of the 1-derived
diimine.
To demonstrate the practical use of sensor 1 in quantita-
tive enantioselective sensing applications, a calibration
curve was constructed using amino alcohol 5 in varying
ee. The corresponding 1-derived diimines were prepared
in chloroform at 3.75 mM, and the samples were diluted
to 9.38 Â 10À5 M for CD analysis. Plotting of the CD ampli-
tudes at 290 nm versus % ee showed a linear relationship
(R2 = 0.992) (Fig. 6). Four scalemic samples of 5 were then
prepared and treated with sensor 1 as described previ-
ously. Using the linear regression equation calculated
from the calibration curve and the measured CD ampli-
tudes at 290 nm, the enantiomeric excess of these samples
was determined. Experimentally obtained data were within
4.4% of the actual values, which is perfectly acceptable for
high throughput screening purposes (Table 1). We like to
point out that the diimine preparation under the unopti-
mized conditions used requires 90 min for completion.
However, the time for the condensation reaction can

ENANTIOSELECTIVE SENSING OF AMINO ALCOHOLS
587
Fig. 6. Calibration curve of the diimines generated from amino alcohol 5
(in varying % ee) and 1.
TABLE 1. Experimentally determined ee’s of four scalemic
samples of 5
Actual % ee (1R,2S)-5
Calculated % ee (1R,2S)-5
70.0
26.0
74.4
24.9
À36.0
À31.6
À69.0
À68.2
Fig. 4. Top: ICD spectra of the diimines formed from 1 (9.38 Â 10À5 M in
chloroform) and (1R,2S)-5 (solid line) and (1S,2R)-5 (dashed line). Bottom:
ICD spectra of the diimines formed from 1 and (R)-12 (solid line) and (S)-12
(dashed line) at the same concentration.
CONCLUSIONS
In summary, the stereodynamic sensor 1 was prepared with
high overall yield in four steps and used for enantioselective
sensing of aromatic and aliphatic amino alcohols. Upon diimine
formation, this CD silent probe undergoes substrate-controlled
chiral induction resulting in bisignate ICD signals that can be
used for quantitative ee determination. The Cotton effects
obtained with the diimines derived from 1 are both tempera-
ture and solvent dependent and generally occur at high wave-
lengths, which eliminates potential interference from chiral
impurities. This probe can be used for enantioselective metal-
free in situ CD analysis of chiral amino alcohols and avoids
cumbersome isolation and purification of reaction products.
ACKNOWLEDGMENT
This material is based upon work supported by the NSF
under CHE-0910604.
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