Isomeric chiral pyrrole diamides and their efficacy in enantioselective sensing of tartrate in sol–gel medium

Article abstract of DOI:10.1016/j.tetlet.2016.06.128

Pyridinium motif-based chiral pyrrole diamide clefts 1 and 2 have been designed and synthesized for chiral recognition of hydroxycarboxylates in sol–gel medium. Of the two isomeric chiral receptors, receptor 1 shows selective sensing of D-tetrabutylammonium tartrate over its mirror image isomer in CH₃CN. The isomeric receptor 2 did not show any enantioselectivity in the recognition. Moreover, the receptor 1 validates prompt visual sensing of D-tartrate through gelation. The recognition properties of the receptors have been studied by fluorescence, UV–vis,¹H NMR and CD spectroscopic methods.

Full text of DOI:10.1016/j.tetlet.2016.06.128

Tetrahedron Letters 57 (2016) 3629–3634
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Isomeric chiral pyrrole diamides and their efficacy in
enantioselective sensing of tartrate in sol–gel medium
⇑
Kumaresh Ghosh , Anupam Majumdar
Department of Chemistry, University of Kalyani, Kalyani 741235, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Pyridinium motif-based chiral pyrrole diamide clefts 1 and 2 have been designed and synthesized for chi-
Received 20 April 2016
Revised 19 June 2016
Accepted 29 June 2016
Available online 30 June 2016
ral recognition of hydroxycarboxylates in sol–gel medium. Of the two isomeric chiral receptors, receptor
1
shows selective sensing of
isomeric receptor 2 did not show any enantioselectivity in the recognition. Moreover, the receptor 1 val-
idates prompt visual sensing of -tartrate through gelation. The recognition properties of the receptors
have been studied by fluorescence, UV–vis, H NMR and CD spectroscopic methods.
Ó 2016 Elsevier Ltd. All rights reserved.
3
D-tetrabutylammonium tartrate over its mirror image isomer in CH CN. The
D
1
Keywords:
Pyrrole diamides
Enantioselective sensing
Tartrate sensing
Fluorometric discrimination
Gelation property
Design and synthesis of chiral receptors for molecular recogni-
tion studies of chiral guests is an important topic in chiral recogni-
tion. Chiral recognition is inevitably significant as chiral
an effective receptor for chiral discrimination of
monium tartrate from its mirror image isomer
receptor 2, in contrast, under identical conditions, did not exhibit
good chiral discrimination of tartrate.
D
-tetrabutylam-
L-tartrate. The
1
biomolecules such as proteins, nucleic acids, and carbohydrates
play a vital role in life. Thus the development of synthetic chiral
receptors which find applications to separation processes, cataly-
sis, sensing, and biochemical studies draws attention.2 For chiral
discrimination, molecular receptor requires effective chiral envi-
ronment. Because of high sensitivity of fluorescence, the presence
of a fluorescent probe near the binding center of the chiral receptor
to transfer the recognition event into a fluorescence signal is
appealing for rapid discrimination of enantiomers of a chiral
Tartaric acid is a biologically relevant compound. It is a mus-
cle toxin and works by inhibiting the production of malic acid. In
high doses it causes paralysis and death. Therefore, recognition
of this small molecule especially its anionic form draws atten-
tion. Many hydrogen bonding receptors for recognition of neu-
8
9
tral and anionic forms of tartaric acid or its derivatives are
known. But chiral recognition of the anionic form of tartaric
acid is less explored. We recently reported the chiral recognition
3
guest. In last several years, enantioselective recognition of car-
of
bis-urea receptor.
Compound 1 was synthesized according to Scheme 1A. Boc pro-
tected -valine was reacted with 1-naphthylamine in the presence
L-tartrate by L-valine-derived neutral benzimidazole-based
1
0
boxylate-based substrates by different architectural-based fluores-
cent receptors is reported.4
Of the different chiral carboxylates,
a-hydroxymono- and dicar-
L
boxylates (e.g., lactate, mandelate, tartrate etc.) draw attention
because of their biological relevance. Recently, we have reported
of dicyclohexylcarbodiimide (DCC) to get the coupled product 3.
Removal of the Boc-group in 3 using trifluoroacetic acid (TFA)
afforded the amine analogue of 3 which on reaction with chloroa-
cetyl chloride gave the compound 4. On the other hand, pyrrole-
2,5-dicarboxylic acid 5 was converted to the diacid chloride 6.
Reaction of 3-aminopyridine with the pyrrole-2,5-diacid chloride
6 gave the diamide 7 which on reaction with 4 under refluxing con-
chiral receptors for lactate,5 -tartrate6 utilizing pyridinium motif
L
as the principle binding site in the designs. Pyridinium motif with
good electrophilic character binds anions involving H-bonding and
charge-charge interactions. Use of this motif in building up chiral
1
1
7
receptors is less explored. In this manuscript, we report the design
and synthesis of two new 3-aminopyridinium-based chiral
receptors 1 and 2 (Fig. 1) of which the isomer 1 is established as
dition in DMF-CH
of 1. Anion exchange of this dichloride salt using NH
the desired compound 1. The isomeric compound 2 was synthe-
sized according to Scheme 1B. In the reaction sequence, -valine
derived pyridine amide 8 through deprotection of Boc group
3
CN mixture solvent afforded the dichloride salt
4
6
PF led to
⇑
Corresponding author. Fax: +91 33 2582 8282.
E-mail address: ghosh_k2003@yahoo.co.in (K. Ghosh).
L
5
http://dx.doi.org/10.1016/j.tetlet.2016.06.128
040-4039/Ó 2016 Elsevier Ltd. All rights reserved.
0

3
630
K. Ghosh, A. Majumdar / Tetrahedron Letters 57 (2016) 3629–3634
O
O
O
O
N
H
N
H
NH
HN
NH
HN
N
HN
N
NH
N
N
O
O
O
PF6
PF6
O
NH
NH
HN
HN
PF6
PF6
O
O
O
O
NH
HN
1
2
CO NBu
CO NBu
2 4
2
4
HO
H
H
OH
H
HO
OH
H
2
OH
CO NBu4
2
H
CO NBu4
H
HO
Ph
Ph
CO NBu
CO NBu
2 4
2
4
D-Tartrate
L-Tartrate
D-Mandelate
L-Mandelate
Figure 1. Structures of 1 and 2 (upper) for fluorescence sensing of chiral guests (lower).
O
O
When solution of 1 in CH
two emission maxima at 348 nm and 518 nm. The peak at
48 nm is attributed to the emission of naphthalene while a
weak emission at ꢀ518 nm is presumably ascribed for aggrega-
tion-induced formation of either excimer between naphthalenes
or exciplex between naphthalene and pyridinium moieties. Upon
3
CN was excited at 315 nm, it exhibited
NH2
HN
HN
3
NHBoc
HN
i
ii, iii
Cl
(
a)
O
4
3
addition of small amounts of tetrabutylammonium salt of
D
-tar-
O
O
vi, vii
N
O
O
iv
O
O
v
À5
N
1
taric acid to the solution of 1 (c = 2.5 Â 10 M) in CH
3
CN, the
(
b)
N
N
H
H
OH
Cl
H
Cl
NH
HN
peak at 518 nm was abolished and a new emission at 507 nm
was developed.
HO
5
6
N
7
However, on progression of titration, while the emission at
Scheme 1A. Synthesis of 1: a. i. Boc protected
L
-valine, DCC, CH
Cl , 6 h; b. iv. SOCl
, 12 h; vi. 4, DMF, CH CN, reflux, 72 h;
2
Cl
2
, 18 h; ii. TFA,
5
3
07 nm was considerably intensified, the monomer emission at
48 nm was changed to the small extent (Fig. 2a). In comparison,
CH
reflux, 12 h; v. 3-amino pyridine, Et
vii. NH PF , CH OH/H O.
2
Cl
2
, 3.5 h; iii. Chloroacetyl chloride, Et
3
N, CH
Cl
2
2
2
, DMF, CH Cl
2
2
,
3
N, CH
2
2
3
there was little fluorescence enhancement at 507 nm when 1
was titrated with -tartrate under the same conditions (Fig. 2b).
Titration of 1 with -/ -mandelates brought change in emission
4
6
3
2
L
D
L
followed by reaction with the diacid chloride 6 gave the bis amide
9
was obtained from the reaction of 1-naphthylamine with chloroa-
at ꢀ400 nm without showing any peak at 507 nm (Fig. S2).
As can be seen from the fluorescence ratio in Figure 3, the
receptor 1 shows sharp fluorometric discrimination between
. Reaction of this bis amide 9 with the chloro amide 101 which
2
cetyl chloride, yielded the dichloride salt of 2. Chloride exchange in
D- and L-tartrates and it has been observed to be significant upon
2
using NH
4
PF
6
gave compound 2 in appreciable yield. All the com-
addition of 4 equiv amounts of the guests. In presence of
20 equiv amounts of guest the discrimination between the
tartrate isomers is considerably less and this occurs due to
pounds were characterized by usual spectroscopic techniques
(Supplementary data).
Prior to study the chiral recognition behaviors of 1 and 2
L-tartrate-induced greater change in emission at 507 nm in
toward the tetrabutylammonium salts of
mandelic acids, the fluorescence and absorbance of the com-
pounds were investigated in some solvents (DMSO, DMF and
CH CN) considering their solubility. While the emission of 2
3
was less sensitive to solvent polarity, compound 1 showed a
D
-/
L
-tartaric and
D
-/
L
-
higher equivalent. Addition of more than 20 equiv amounts of
tartrates showed mere change in emission at 507 nm and
thereby indicated the saturation of complexation. Under identi-
cal conditions, enantiomers of mandelate are scarcely discrimi-
nated. It is worth mentioning that the receptor 1 exhibits
fluorescence difference ratio (ef) 18.38 at the point of addition
of 4 equiv amounts of tartrates. This is significantly greater than
the ef (3.77) noted upon addition of 20 equiv amount of tar-
trates. These findings demonstrate that the fluorescence
response of 1 toward the tartrates is highly enatioselective.
change in emission spectra with variation of solvent (Fig. S1).
O
O
H
N
N
H
NH2
NH
NH
HN
i
NHBoc ii, iii
In UV–vis titration, progressive addition of
D-tartrate to the
O
solution of 1 in CH CN decreased the absorption at 319 nm grad-
3
N
N
O
O
HN
9
ually and a new absorption at 345 nm was developed with sig-
nificant intensity. This resulted in a ratiometric change in the
absorption spectra (Fig. 4a). This was also true for the combina-
8
O
Cl
HN
iv, v
N
N
2
10
tion of 1 with
points were observed. To our opinion, the isosbestic point at
27 nm, in the beginning, represents 1:1 stoichiometric
interaction of the receptor with tartrates. The appearance of
L-tartrate (Fig. 4b). In both cases, two isosbestic
Scheme 1B. Synthesis of 2: i. Boc protected
L
-valine, DCC, CH
2
2
Cl , 20 h; ii. TFA,
6
PF ,
3
a
CH
CH
2
Cl
2
, 3 h; iii. 6, Et
O.
3
N, CH
2
Cl
2
, 10 h; iv. 10, DMF, CH CN, reflux, 72 h; v. NH
3
4
3
OH/H
2

K. Ghosh, A. Majumdar / Tetrahedron Letters 57 (2016) 3629–3634
3631
5
6
x10
5
5
5
5
5
(a)
20.0
17.0
5x10
4x10
3x10
2x10
5
5
5
5
5
5
4
3
2
1
.50x10
.40x10
.30x10
.20x10
.10x10
1
9
7
5
4
3.0
.0
.0
.0
.0
3.0
2
1
.0
.5
1
x10
1.0
0
0
0
.5
-
5
-4
-4
-4
0
.0
9.0x10 1.8x10 2.7x10 3.6x10
[
G]
D-tartrate
0.00
350
400
450
500
550
600
Wavelength (nm)
5
2
.25x10
2
3.0
0.0
7.0
3.0
(
b)
5
5
4
4
2
1
1
1.80x10
5
5
5
5
5
5
4
3
2
1
.50x10
.40x10
.30x10
.20x10
.10x10
9.0
1
9
4
.35x10
.00x10
.50x10
7
5
.0
.0
4.0
3
2
.0
.0
1.5
1
0
0
.0
.5
-
5
-4
-4
-4
0.0
9.0x10 1.8x10 2.7x10 3.6x10
[G]
L-tartrate
0.00
350
400
450
500
550
600
Wavelength (nm)
À5
-tartaric acids (in all cases [anion] = 1 Â 10À3 M;
Figure 2. Fluorescence titration spectra for 1 (c = 2.5 Â 10 M) in CH
3
CN with tetrabutylammonium salts of (a) D- and (b) L
k
exc = 315 nm). Plots in the insets corroborate the change in emission intensity at 507 nm with guest concentration.
sion at ꢀ420 nm was observed. However, upon progressive addi-
À5
tion of
CH
the small extent with a red shift of ꢀ10 nm (Fig. S4). Almost a
similar change in emission was observed with -tartrate. The
D
-tartrate to the solution of 2 (c = 2.5 Â 10 M) in
3
CN, the intensity of the peak at ꢀ420 nm was increased to
L
enantiomeric fluorescence difference ratios, ef at the points of
addition of 4 and 30 equiv amounts of tartrates are determined
to be 0.64 and 2.38, respectively (Fig. 5). Negligible change in
emission was noticed with the isomers of mandelate (Fig. S4).
Thus the compound 2 was neither good to discriminate fluori-
metrically the
D-/
L-isomers of tartrate nor the stereoisomers of
3
mandelate. UV–vis titrations of 2 in CH CN introduced similar
changes in absorption spectra for all the guests (Fig. S5) and
indicated the inefficacy of 2 in chiral recognition in the ground
state.
However, a closer look into the titration profiles (insets of Figs. 2
and 4, Figs. S2 and S3) for receptor 1 with the guests reveals that
binding follows two-step process and the usual Job plots for deter-
mining stoichiometry of the complexes were difficult to obtain.
The binding constant values for the tartrates were also not
determined.
À5
Figure 3. Change in fluorescence ratios of 1 (c = 2.5 Â 10 M) upon addition of
À3
different equivalent amounts of guests (c = 1 Â 10 M) in CH
3
CN at 507 nm;
(kexc = 315 nm).
the second isosbestic point at 333 nm followed by the destruc-
tion of the initial one at the later stage suggests the formation
of a new equilibrium complex in the medium. On the other
hand, the ratiometric response in absorption with a fixed isos-
bestic point at 327 nm was observed with the stereoisomers of
mandelate. The change in absorption intensity at ꢀ345 nm was
The enantioselective response of
stereoisomer of tartrate was understood from the change in emis-
sion of 1 in presence of its mirror image isomer. While -tartrate-
induced change in emission was further perturbed to a consider-
1 toward a particular
L
much smaller than the case with D-/L-tartrates (Fig. S3).
To understand the positional role of the chiral centers around
the 3-aminopyridinium binding site in enantioselection of tar-
trates, we considered the isomeric compound 2 in the study.
able extent upon addition of
insignificant (Fig. 6b).
D-tartrate (Fig. 6a), the reverse was
The enantioselective sensing of 1 toward
D-tartrate was further
On excitation of 2 in CH
3
CN at 290 nm, a non structured emis-
realized from CD spectra in CH CN. In the presence of 12 equiv
3

3
632
K. Ghosh, A. Majumdar / Tetrahedron Letters 57 (2016) 3629–3634
(
a) 1
0.6
λmax = 319 nm
λmax = 345 nm
.2
.0
.8
.6
.4
.2
.0
0
0
.3
.0
1
0
0
0
0
0
-
-
0.3
0.6
0
-5
-5
-5
-4
.00 2.50x10 5.00x10 7.50x10 1.00x10
[
G]D-tartrate
3
00
350
400
450
Wavelength (nm)
0
.6
.3
.0
λmax = 319 nm
λmax = 345 nm
(
b) 1.2
0
1.0
0.8
0.6
0.4
0.2
0.0
0
-
-
0.3
0.6
0
-5
-5
-5
-4
.00 2.50x10 5.00x10 7.50x10 1.00x10
[G]L-tartrate
300
350
400
450
Wavelength (nm)
À5
-tartaric acids (in all cases [anion] = 1 Â 10À3 M).
Figure 4. Absorption titration spectra for 1 (c = 2.5 Â 10 M) in CH
3
CN with tetrabutylammonium salts of (a) D- and (b) L
Plots in insets represent the change in absorbance at two different wavelengths.
spongy network with cavities (Fig. 8). Compound 1 also formed
gel in DMSO-CHCl (1:3, v/v) mixture solvent with almost similar
3
morphology (Fig. S7); but the gel was not so hard like hydrogel
of 1. Table S1 indicates the gelation nature of 1 and 2 in different
solvent systems. Interestingly, compound 1 formed thermo irre-
versible gels from CH
physical attributes in presence of 0.5 equiv amounts of
-tartrates. While in the presence of 0.5 equiv amounts of
-tartrate, gel (T = 78 °C) was formed instantly at room tempera-
ture, -tartrate produced gel (T = 108 °C) on cooling at 0 °C for at
least 20 min.
3
CN containing 5% DMSO with different
D- and
L
D
g
L
g
Photographs in Fig. 9 indicate the gelation properties of 1 and 2
in the presence of chiral anionic guests. These findings in gel chem-
istry enable compound 1 also to discriminate the D-/L-isomers of
tartrate effectively.
À5
Figure 5. Change in fluorescence ratios of 2 (c = 2.5 Â 10 M) upon addition of
À3
different equivalent amounts of guests (c = 1 Â 10 M) in CH
3
CN at 420 nm
(kexc = 290 nm).
To identify the interacting protons in binding, we tried to fol-
1
low the H NMR titration of 1 with both D- and L-tartrates. We
amounts of
304 nm of 1 was changed into bisignate Cotton effect which corre-
sponds to guest-induced helical bias of the receptor. This effect is
more pronounced in the presence of -tartrate and distinguishable
from its -isomer (Fig. 7a). However, under identical conditions, in
the presence of -/ -mandelates the change in CD spectra was negli-
gible (Fig. 7b). It is mentionable that a similar observation in CD was
noted in presence of 1 equiv amount of - and -tartrates (Fig. S6).
D
- and
L
-tartrates, the weak negative cotton effect at
became unsuccessful due to appearance of gel in the NMR tube
upon addition of less than 1 equiv amount of tartrates. Figure S8
ꢀ
shows that in the presence of small amounts of
D-tartrate, the
L
amide protons of types ‘a’, ‘b’, and ‘c’ underwent positional shift
with broadening. Pyrrole N–H at 11.0 ppm became invisible. Pro-
ton ‘d’ showed small downfield chemical shift suggesting the
involvement of pyrrole in complexation. The pyridinium ring
proton ‘e’ underwent downfield shift and other signals in the
range 7.0 ppm to 8.0 ppm became broad showing small down-
D
D
L
D
L
In addition to the different binding characteristics in solution,
compound 1 exhibited gelation behavior in the absence and pres-
ence of guests. While compound 1 gave thermo reversible hydro
field chemical shift. Addition of 0.5 equiv amounts of
brought gelation and no signal was observed. A similar observa-
tion was noticed with -tartrate at the point of addition of
0.5 equiv amounts of guest (Fig. S8). Based on these observa-
tions, we believe that the best fit of -tartrate into the binding
D-tartrate
2
gel (Tgel = 110 °C) in DMSO–H O (1:3, v/v), under identical condi-
L
tions the isomeric compound 2 did not form gel. Analysis of the
morphology of the xero gel of 1 using SEM image represents
D

K. Ghosh, A. Majumdar / Tetrahedron Letters 57 (2016) 3629–3634
3633
(
b)
(
a)
5
5
5
5
5
5
5
x10
5x10
4x10
3x10
2x10
1x10
1
+ 15 eqv. D-tartrate = B
B + 15 eqv. L-tartrate
5
5
5
5
4
x10
A + 15 eqv. D-tartrate
+ 15 eqv. L-tartrate = A
3x10
2x10
1x10
1
1
1
0
0
350
400
450
500
550
600
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
À5
-tartrate (c = 1 Â 10À3 M) in the presence of
-tartrate (c = 1 Â 10À3 M) and (b) L -tartrate (c = 1 Â 10À3 M) in
Figure 6. Fluorescent response of 1 (c = 2.5 Â 10 M) to (a)
D
L
À3
presence of
D
-tartrate (c = 1 Â 10 M) in CH
3
CN (kexc = 315 nm).
(
a)
(
b)
3
0
5
0
3
0
1
+ 12 equiv. L-tartrate
1
+ 12 equiv. of D-mandelate
1
+ 12 equiv. D-tartrate
1
1
15
0
1
+ 12 equiv. of L-mandelate
1
-
15
30
-15
-30
-
250
300
350
400
250
300
350
400
Wavelength (nm)
Wavelength (nm)
Figure 7. Change in CD spectra of 1 (c = 2.5 Â 10À5 M) upon addition of 12 equiv amounts of guests in CH
CN.
3
2
Figure 8. (a) Photograph of gel of compound 1 and (b) SEM image of the xerogel of 1 in DMSO–H O (1:3, v/v) (scale bar = 1 lm).
cavity of 1 induces marked change in fluorescence for which it is
discriminated from its -isomer. Considering the 1:1 stoichiomet-
ric interaction, DFT calculation13 on the complexes of 1 with
- and -tartrates reveals that 1. -tartrate is lower in energy by
.91 kcal/mole than 1. -tartrate complex and is thus relatively
stable (Fig. S9).
In conclusion, we have thus reported pyridinium motif-based
pyrrole diamide chiral cleft 1 that fluorometrically discriminates
-tartrate from its mirror image isomer in CH CN with a maximum
3
ef of 18.38. The gelation behavior of 1 itself and in the presence of
tartrate isomers also draws attention in their visual distinction.
-valine was used as the chiral source in the receptor. The isomeric
L
D
D
L
D
2
L
L

3
634
K. Ghosh, A. Majumdar / Tetrahedron Letters 57 (2016) 3629–3634
À2
Figure 9. Photographs of gels and solutions in CH
and (d)
3
CN containing 5% DMSO of (A) compound 1 (c = 1.71 Â 10 M) in presence of (a)
D-tartrate, (b) L-tartrate, (c) D-mandelate
À2
L
-mandelate; (B) compound 2 (c = 1.71 Â 10 M) in presence of (a)
D-tartrate, (b) L-tartrate, (c) D-mandelate and (d) L-mandelate.
receptor 2 with chiral centers in the vicinity of the pyrrole moiety
was unable to discriminate the stereoisomers of tartrate under
identical conditions.
Wang, L.; Jiao, L. J.; Cheng, Y.; Zhu, C. J. Org. Chem. 2012, 77, 4759–4764. and
references cited therein.
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K.; Majumdar, A. RSC Adv. 2015, 5, 24499–24506.
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.
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Ghosh, K.; Sarkar, A. R.; Sarkar, T.; Panja, S.; Kar, D. RSC Adv. 2014, 4, 20114–
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We thank DST, New Delhi, India for financial support [project
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(
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