Enantioselective sensing of lactate by pyridinium-based chiral receptor

Article abstract of DOI:10.1016/j.tetlet.2013.08.003

A simple pyridinium-based chiral receptor 1 containing l-valine as the chiral source has been designed and synthesized. The receptor 1 fluorometrically recognizes d-lactate over l-lactate in CH₃CN with an enantiomeric fluorescence ratio (ef) of 5.32.

Full text of DOI:10.1016/j.tetlet.2013.08.003

Accepted Manuscript
Enantioselective sensing of lactate by pyridinium-based chiral receptor
Kumaresh Ghosh, Anupam Majumdar
PII:
S0040-4039(13)01340-3
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.08.003
TETL 43360
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
25 June 2013
29 July 2013
1 August 2013
Please cite this article as: Ghosh, K., Majumdar, A., Enantioselective sensing of lactate by pyridinium-based chiral
receptor, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/j.tetlet.2013.08.003
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers
we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and
review of the resulting proof before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Enantioselective sensing of lactate by pyridinium-based chiral receptor
*
Kumaresh Ghosh , Anupam Majumdar
Department of Chemistry, University of Kalyani, Kalyani, Nadia-741235, India,
Email: ghosh_k2003@yahoo.co.in, Fax: +91 33258282
_________________________________________________________________________________________
_
Abstract- A simple pyridinium-based chiral receptor 1 containing L-valine as the chiral source has been designed and
synthesized. The receptor 1 fluorometrically recognizes D-lactate over L-lactate in CH CN with enantiomeric fluorescence
3
ratio (ef) of 5.32.
_
_________________________________________________________________________________________
Keywords: Enantioselective sensing, Chiral receptor, Pyridinium – based receptor, L-valine derived receptor, D-lactate sensing.
Application of pyridinium-based chiral receptor to the enantiomeric discrimination of a particular carboxylate-
1
based substrate is little explored although achiral pyridinium receptors are known for molecular recognition of
2
3
4
carboxylates, phosphate/dihydrogenphosphates, halides etc. Chiral fluororeceptors are worthy in chiral
recognition as they not only greatly facilitate rapid discrimination of enantiomeric composition of chiral
5
compounds, but also allow a rapid screening of high throughput catalysts for asymmetric synthesis. In the last
several years, there has been a growing interest in the enantioselective recognition of -hydroxycarboxylic
6
7
acids as well as of -amino acids and their derivatives due to their biological significance. Of the different -
hydroxycarboxylates, lactate is a simple example that is involved in biological metabolism. Compared to the
exploration of anion sensing by achiral receptors, enantioselective recognition using a chiral host is relatively
8
1,9
less explored, especially with regard to the enantioselective binding of lactate or its derivatives. In relation to
this, we wish to report here a simple L-valine derived pyridinium-based chiral receptor 1 that shows significant
perturbation in emission in the presence of D-lactate over L-lactate in CH CN.
3
Structure 1
The receptor 1 was synthesized according to Scheme 1. L-valine ester was reacted with chloroacetyl chloride in
the presence of Et N to give the chloride 3 (Scheme 1a) which was used for alkylation on the pyridine ring
3
nitrogen in Scheme 1b. In Scheme 1b, N-Boc protected L-valine ester 4 was hydrolyzed to give the acid 5.
Coupling of 3-aminopyridine with this acid 5 using DCC afforded the compound 6. Removal of the Boc group
in 6 furnished the amine 7 which was next reacted with 1-naphthyl isocyanate, obtained from the reaction of 1-
naphthyl amine with triphosgene to give pyridyl urea 8. Reaction of 8 with the chloroamide 3 in dry CH CN
3
under refluxing conditions yielded the chloride salt 9, which on anion exchange gave the desired compound 1 in
9
0% yield.
Scheme 1

The chiral recognition properties of 1 were investigated for tetrabutylammonium salts of D- and L- hydroxy
-5
acids such as lactic and mandelic acids. The fluorescence spectra were recorded for a solution of 1 (c = 3 x 10
M) in CH CN in the presence and absence of anions. Enantioselective fluorescence enhancement was observed.
3
While upon gradual addition of D-lactate to the solution of 1 in CH CN the emission at ~ 400 nm was increased
3
significantly, a small increase in emission with the addition of L-lactate was observed. Figure 1 represents the
titration spectra for 1 with both L- and D-lactates. Under identical conditions, titration experiments for 1 with D-
and L-mandelates were performed and negligible change in emission was found (supporting information).
Figure 2 shows the change in emission of 1 in the presence of 11 equiv. amounts D/L- lactate and mandelate in
CH CN. As can be seen from Fig. 2, while the receptor 1 shows sharp fluorometric discrimination between D-
3
and L-lactates, enantiomers of mandelate are scarcely discriminated.
Figs. 1, 2 and 3
The selective recognition effect on the guest of the D- and L-isomers of lactate can be understood from the
enantiomeric fluorescence difference ratio, ef [ef = (I – I )/(I – I )] where I represents the fluorescence
D
0
L
0
0
emission intensity in the absence of the chiral substrate. I and I are the fluorescence intensities in the presence
L
D
of L- and D-lactates, respectively. The value of ef is determined to be 5.32 for the sensor 1, which indicates that
this sensor can exhibit highly enantioselective response toward D-lactate. In the interaction process, the
stoichiometry of the complexes of 1 with both D- and L-lactates was determined to be 1:1 as confirmed by Job’s
1
0
plot (supporting information). Figure 3 shows the UV Job plot for 1 with D-lactate. The binding constant
1
1
3
value was determined from non linear fit of the emission titration data and it was found to be (4.17 ± 0.7) x 10
-1
3
-1
M for D-lactate. L-lactate gave a binding constant value of (1.92 ± 0.33) x 10 M which is ~2.17 times less
compared to D-lactate. Thus, like the difference in ef, the considerable difference in binding constant values also
in particular describes the enantioselectivity. Due to minor change in emission we were unable to determine
binding constant values for the mandelates.
Fig. 4
However, the enantioselectivity of 1 towards a particular stereoisomer of lactate was further realized from the
change in emission of 1 in the presence of its mirror image isomer. Figure 4, in this regard, demonstrates these
features. While L-lactate induced change in emission of 1 was further perturbed to a significant extent upon
addition of D-lactate (Fig. 4a) , the reverse one was observed to be insignificant (Fig. 4b). Thus the preferential
binding of D-lactate over L-lactate may be attributed to the steric fit of the D-lactate into the open chiral cavity
12
of 1 due to which strong complexation occurs. DFT calculation in CH CN was carried out with both D- and L-
3
lactates (Fig. 5). As can be seen from Fig. 5, sensor 1 binds D-lactate more strongly than L-lactate (E = - 42.45
kcal/mole). The naphthyl urea and pyridinium moieties are involved in complexation of carboxylates for the two
isomers in different ways involving a number of hydrogen bonds as shown in Fig. 5.
Fig. 5

Like fluorescence, UV-vis titrations of 1 with all the guests were conducted in CH CN. The change in
3
absorption spectra for 1 with all the anions was observed to be negligible (supporting information). This
interpreted that the receptor 1 is not efficient in discriminating the enantiomers of lactate in the ground state.
Fig. 6
1
The interaction of 1 with D/L - lactate was further realized from H NMR, recorded in CD CN (Fig. 6). On
3
gradual addition of D-lactate to 1 in CH CN, the pyridinium amide proton ‘a’ appeared at 9.63 ppm shifted to
3
1
0.5 ppm with broadening when [G]/[H] was 0.25 and then was not visible for the next addition. The pyridinium
ortho proton ‘b’ progressively moved in the downfield direction by 0.16 ppm indicating the formation of weak
hydrogen bond with the guest. The urea protons ‘g’ and ‘d’ suffered considerable downfield shifts of 1.64 ppm
and 0.45 ppm, respectively upon complexation. On the other hand, the aliphatic amide proton ‘e’ became broad
and was not visible during complexation. In the binding event the naphthyl ring proton ‘c’ underwent downfield
chemical shift by 0.20 ppm and thereby suggested its participation in hydrogen bonding with the guest. During
interaction, the bifurcation of the signals for the –CH - protons (labeled as type f) indicates an asymmetric
2
nature of the complex formed in solution. However, under identical condition, at [G]/[H] = 1 with L-tartarate, all
the labelled protons as highlighted in Fig. 6 underwent downfield chemical shift but relatively to a lesser extent
(
supporting information).
In conclusion, L-valine derived pyridinium-based chemosensor 1 has been established as good candidate to
distinguish the D-lactate from its mirror image isomer in CH CN fluorometrically. The sensor 1 is capable of
3
sensing D-lactate with ef 5.32. A combination of both the urea and pyridinium motifs to the guests involving
hydrogen bonding and charge – charge interactions and the steric fit apparently play the key roles in chiral
discrimination. To the best of our knowledge, the present example is a new addendum on the chiral recognition
1,9
of lactate to the existing very few reports in the literature.
Acknowledgements
We thank DST New Delhi, India for financial support [project SR/S1/OC-76/2010(G); Date: 23.08.2011]. We
also thank DST, New Delhi for the facility under PURSE program in the university. A.M. thanks CSIR, New
Delhi, India for a fellowship.
Supplementary data
Figures showing the change in fluorescence and UV-vis titrations of receptor 1 with the chiral anions, binding
1
1
13
constant curves, H NMR spectral changes of 1 with the addition of L-lactate, experimental procedure, H, C
NMR spectra and mass spectra are available.
References
1
. Barnard, A.; Dickson, S. J.; Peterson, M. J.; Todd, A. M.; Steed, J. W. Org. Biomol. Chem. 2009, 7, 1554
1561.
–
2
. (a) Ghosh, K.; Sarkar, A. R. Org. Biomol. Chem., 2011, 9, 6551- 6558; (b) Ghosh, K.; Masanta, G.;
Chattopadhyay, A. P. Eur. J. Org. Chem. 2009, 4515- 4524; (c) Ghosh, K.; Sarkar, A. R. Tetrahedron
Lett., 2009, 50, 85- 88.

3
. (a) Ghosh, K.; Sarkar, A. R.; Sommader, A.; Khuda-Bukhsh, A. –R. Org. Lett. 2012, 14, 4314- 4317; (b)
Ghosh, K.; Sarkar, A. R.; Chattopadhyay, A. P. Eur. J. Org. Chem. 2012, 1311- 1317; (d) Ghosh, K.;
Saha, I. New J. Chem. 2011, 35, 1397- 1402
4
5
. Turner, D. R.; Peterson, M. J.; Steed, J. W. J. Org. Chem. 2006, 71, 1598 – 1608.
. (a) Pu, L. Chem. Rev. 1998, 98, 2405- 2494; (b) Pu, L. Chem. Rev, 2004, 104, 1687- 1716; (c) Duke, R.
M.; Gunnlaugsson, T. Tetrahedron Lett. 2007, 48, 8043- 8047; (d) Li, Z. B.; Lin, J.; Qin, Y. C.; Pu, L.
Org. Lett. 2005, 7, 3441- 3444.
6
. (a) Jadhav, V. D.; Schmidtchen, F. P. Org. Lett. 2006, 8, 2329- 2332; (b) Lavigne, J. J.; Anslyn, E. V.
Angew. Chem. Ind. Ed. 1999, 38, 3666 – 3669; (c) Bencini, A.; Coluccini, C.; Garau, A.; Giorgi, C.;
Lippolis, V.; Messori, L.; Pasini, D.; Puccioni, S. Chem. Commun. 2012, 48, 10428- 10430; (d) Han, F.;
Chi, L.; Liang, X.; Ji, S.; Liu, S.; Zhou, F.; Wu, Y.; Han, K.; Zhao, J.; James, T. D. J. Org. Chem. 2009,
7
4, 1333- 1336; (e) Xu, K. –X.; Wu, X. – J.; He, Y. – B.; Liu, S. – Y.; Qing, G. – Y.; Meng, L. – Z.
Tetrahedron Asymmetry 2005, 16, 833 – 839; (f) Lin, J.; Zhang, H. –C.; Pu, L. Org. Lett. 2002, 4, 3297 –
300; (g) Chi, L.; Zhao, J.; James, T. D. J. Org. Chem. 2008, 73, 4684 - 4687; (h) Song, F.; Wei, G.;
3
Wang, L.; Jiao, J.; Cheng, Y.; Zhu, C. J. Org. Chem. 2012, 77, 4759 – 4764 and references cited therein.
. (a) Qin, H.; He, Y.; Hu, C.; Chen, Z.; Hu, L. Tetrahedron Asymmetry 2007, 18, 1769 - 1774; (b)
Rodriguez, M. H.; Juaristi, E. Tetrahedron 2007, 63, 7673 – 7678; (c) Li, Z. –B.; Lin, J.; Zhang, H. – C.;
Sabat, M.; Hyacinth, M.; Pu, L. J. Org. Chem. 2004, 69, 6284 – 6293.
7
8
9
. (a) Uppadine, L. H.; Keene, F. R.; Beer, P. D. J. Chem. Soc., Dalton Trans. 2001, 2188- 2198; (b) Best,
M. D.; Tobey, S. L.; Anslyn, E. V. Coord. Chem. Rev. 2003, 240, 3- 15; (c) Bondy, C. R.; Loeb, S. J.
Coord. Chem. Rev. 2003, 240, 77- 99.
. (a) Schmuck, C.; Schwegmann, J. Am. Chem. Soc. 2005, 127, 3373- 3379; (b) Budnikov, G. K.; G. A.
Evtyugin, G. A.; Budnikova, Y. G.; Al’fonsov, V. A. J. Anal. Chem. 2008, 63, 2.
1
1
1
0. Job, P. Ann. Chim., 1928, 9, 113- 203.
1. Valeur, B.; Pouget, J.; Bourson, J.; Kaschke, M.; Eensting, N. P. J. Phys. Chem.1992, 96, 6545- 6549.
2. DFT calculation was done by using Gaussian 09 (Revision A.02): Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, Jr. T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi,
M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda,
R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J.
E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.;
Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin,
R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P.
M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian, Inc., Wallingford CT,
2004.

Figures and captions
O
N
O
O
N
H
H
NH
N+
PF6-
H
N
CO Me
2
1
H
a.
i
H
HN COOMe
H N COOMe 81%
2
O
3
2
Cl
O
H
N
H
H
ii
iii
iv
H
HN COOMe
HN COOH
N
H
O
b.
85%
H N COOMe 90%
74%
O
2
N
6
O
O
O
O
5
4
2
O
N
O
O
N
H
O
H
H
N
H
O
H
N
NH
v
vi
69%
HN
N
H
vii
76%
NH2
viii
1
8
8%
O
90%
CO Me
N+
N
N
Cl-
H
N
2
7
8
9
Scheme 1. Reagents and conditions: a) i. chloroacetyl chloride, Et N, CH Cl , 2 h, yield 81.5%; b) ii. di-t-butyl
3
2
2
pyrocarbonate, Et N, THF, 8 h; iii. LiOH, CH OH, 8 h; iv. 3-aminopyridine, DCC, CH Cl , 20 h; v. TFA, CH Cl , 3 h; vi.
3
3
2
2
2
2
1
-naphthyl amine, triphosgene, Et N, CH Cl , 16 h; vii. 3, CH CN, reflux, 4 days; viii. NH PF , CH OH – H O.
3 2 2 3 4 6 3 2

5
4
3
2
1
00000
00000
00000
00000
00000
0
150000
(
a)
(b)
1
00000
5
0000
0
3
00
350
400
450
500
550
600
3
00
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
-
5
Figure 1. Fluorescence titration spectra for 1 (c = 3 x 10 M) in CH CN with tetrabutylammonium
3
-
4
salts of (a) D - and (b) L -lactic acids (in all cases [anion] = 6 x 10 M ;  = 300 nm).
exc
(
a) 1.0x10-5
-6
-6
-6
-6
8
6
4
2
.0x10
.0x10
.0x10
.0x10
0
.0
0
.0
0.2
0.4
XH
0.6
0.8
1.0
Figure 3. UV-vis Job plot for 1 ([H] = [G] =
Figure 2. Change in fluorescence ratio of 1 (c = 3 x
-5
-
5
3 x 10 M) with D-lactate in CH CN.
3
1
0
M)) upon addition of 11 equiv. amounts of
guests in CH CN at 405 nm.
3
3
2
1
00000 (a)
(b)
A + 10 eqv. D-lactate
3
2
1
00000
00000
00000
0
1 + 10 eqv. D-lactate = B
B + 10 eqv. L-lactate
00000
00000
0
1
+ 10 eqv. L-lactate = A
1
1
3
00
350
400
450
500
550
600
3
00
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
-
5
-4
Figure 4. Fluorescent response of 1 (c = 3 x 10 M) to (a) D- lactate (c = 6 x 10 M) in the presence of L-
-
4
-4
-4
lactate (c = 6 x 10 M) and (b) L-lactate (c = 6 x 10 M) in presence of D-lactate (c = 6 x 10 M) in CH CN.
3

(
A)
(B)
Figure 5. DFT optimized geometries of 1 with (A) D-lactate (a = 2.33 Å, b = 1.88 Å, c = 1.85 Å, d = 2.34 Å, e = 1.60 Å)
and (B) L-lactate (a = 2.17Å, b = 1.80 Å, c = 2.65 Å, d = 1.59 Å, e = 2.22 Å, f = 1.82 Å).
c
O
N
Hg
O
N
H
d
k
NH
a
h
b
N+
f
He
PF6-
N
CO Me
2
O
1
1
-3
Figure 6. Partial H NMR (400 MHz, CD CN) of 1 (c = 9.45 x 10 M) in the absence and presence of D-lactate.
3

GRAPHICAL ABSTRACT
___________________________________________________________________________________
_
Enantioselective sensing of lactate by pyridinium-based chiral receptor
*
Kumaresh Ghosh , Anupam Majumdar
O
A simple pyridinium-based chiral receptor 1
containing L-valine as the chiral source has
been designed and synthesized. The receptor
N
O
N
H
H
NH
1
fluorometrically recognizes D-lactate over
N+
-
PF
6
H
N
L-lactate in CH CN with enantiomeric
2
CO Me
3
fluorescence difference ratio (ef) of 5.32.
1
O