Recognition of a flavin analogue by novel bile acid-based receptors: Effects of hydrogen bonding and aromatic π-stacking interactions

Article abstract of DOI:10.1071/CH17220

Molecular recognition properties are reported for novel bile acid-based receptors that incorporate 2,6-diaminopyridine as a recognition unit. Apart from hydrogen-bonding interactions, the bile acid receptors exhibit significant aromatic π-stacking interactions with the aromatic fused ring of the flavin derivative. These studies provide rationalisation for the differences in binding behaviour of bile acid receptors having differing aromatic arm lengths towards a flavin analogue.

Full text of DOI:10.1071/CH17220

CSIRO PUBLISHING
Aust. J. Chem. 2017, 70, 1263–1268
Full Paper
https://doi.org/10.1071/CH17220
Recognition of a Flavin Analogue by Novel Bile Acid-Based
Receptors: Effects of Hydrogen Bonding and Aromatic
p-Stacking Interactions
A
A
A
Pradeep Kumar Muwal, Rajesh Kumar Chhatra, Shubhajit Das,
and Pramod S. Pandey
A B
,
A
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas,
New Delhi 110016, India.
B
Corresponding author. Email: pramod@chemistry.iitd.ac.in
Molecular recognition properties are reported for novel bile acid-based receptors that incorporate 2,6-diaminopyridine as a
recognition unit. Apart from hydrogen-bonding interactions, the bile acid receptors exhibit significant aromatic p-stacking
interactions with the aromatic fused ring of the flavin derivative. These studies provide rationalisation for the differences in
binding behaviour of bile acid receptors having differing aromatic arm lengths towards a flavin analogue.
Manuscript received: 22 April 2017.
Manuscript accepted: 29 June 2017.
Published online: 28 July 2017.
Introduction
The effect of different aromatic moieties on flavin redox
properties have also been studied. The authors also reported
intramolecular interactions of the flavins in solution. In this
model, a flavin-containing compound featuring the hydrogen-
bonding 2,6-diaminopyridine (DAP) moiety and an electron-
rich naphthalene unit was designed to undergo intramolecular
self-assembly through hydrogen-bonding and p-stacking inter-
actions.[8,9] Rotello and co-workers reported flavin-incorporating
rotaxanes and catenanes to create flavin-based molecular
machines, where aromatic stacking could potentially be one of
the interactions used to drive their mechanical behaviour.[10,11]
Rotello and co-workers also reported the formation of com-
plexes between flavin and diamidopyridine-functionalised por-
phyrin systems via hydrogen-bonding and p-stacking
interactions.[12] The binding behaviour was studied by 1H
NMR spectroscopy which suggested simultaneous hydrogen-
bonding and p-stacking interactions of porphyrin receptors with
flavin derivatives.
Flavin adenine dinucleotide (FAD) and flavin mononucleotide
(FMN) catalyse a variety of biological transformations, such as,
redox reactions, activation of dioxygen, electron transfer,
emission of light, and rearrangements.[1] The flavin cofactor is
strongly bound to the apoenzyme through non-covalent inter-
actions which not only provide recognition but also serve to
modulate the behaviour of the flavin cofactor.[2] Flavoenzymes
exhibit significant variation in their catalytic properties
depending on the active site micro-environment and the chem-
ical variations of the isoalloxazine nucleus. The oxidised flavin
(Flox) can be reduced in a two-electron process straight for-
wardly to the flavohydroquinone anion (FlredHꢀ) or can also
occur via two one-electron processes involving the flavosemi-
quinone radical, in either the anionic (Flꢀrad) or neutral (FlradH)
form.[3] Several flavin models have been developed to mimic the
ubiquitous flavoenzyme’s hydrogen bonds between the apoen-
zyme and the O(2), N(3), and O(4) positions of the flavin
nucleus and understand the mechanism of the catalysed reac-
tions. The significance of hydrogen bonds on flavin reactivity
has also been pointed out from theoretical studies. Molecular
orbital calculations indicate that the hydrogen bonding involving
N(1), C(2)=O, N(3)–H, C(4)=O, and N(5) is the most effective in
lowering unoccupied molecular orbital energy levels.[4] Nishi-
moto et al. have demonstrated, based on ab initio calculations,
that hydrogen bonds at C(2)=O are central for lowering the
activation energy for amino acid oxidation.[5]
Previously we reported bile acid-based receptors containing
2,6-diaminopyridine (DAP) moieties.[13,14] The binding behav-
iour of N(10)-hexyl and N(10)-octyl flavin derivatives were
1
studied with DAP-based receptors, using H NMR titrations.
These bis-cholyl-2,6-diaminopyridine derivatives showed a low
to moderate binding affinity (Ka 50–400 Mꢀ1) with flavin
analogues, mainly due to steric congestion.
In this communication we report the synthesis of bile acid-
based DAP receptors 4a–d containing different aromatic moie-
ties to examine the role of aromatic p-stacking interactions
and steric effects on the binding with a N(10)-hexylflavin
derivative. N(10)-Hexylflavin was synthesised from ortho-
phenylenediamine.[15] Due to their lipophilic nature, bile acid-
based receptors have been successfully used to transport anions
through biological membranes,[16–18] as such these flavin recep-
tors may be considered as potential flavin transporters.
Aromatic stacking interactions also play an important role
in flavoenzyme function.[6] It provides a recognition platform
for the flavin cofactor and efficiently modulates its reactivity.
Rotello and co-workers developed a model system of the
aromatic stacking in flavoenzymes. Xanthene-based receptors
with a diaminotriazine (DAT) moiety orients the flavin over an
aromatic surface through hydrogen-bonding interactions.[7]
Journal compilation Ó CSIRO 2017
www.publish.csiro.au/journals/ajc

1264
P. K. Muwal et al.
Results and Discussion
of the bile acid 1 were first protected by the reaction with formic
acid at 608C for 4 h, which resulted in the formation of formyl bile
acid 2. The formylated cholic acid 2 was treated with ethyl
chloroformate in the presence of triethylamine to give the anhy-
dride that was subsequently condensed with DAP to give
Synthesis
The bile acid-basedreceptors 4a–d were synthesisedby following
the reaction sequence as shown in Scheme 1. The hydroxy groups
O
HCOOH/60ЊC
4 h
OH
OH
H
O
O
OHCO
O
OCHO
H
O
HO
OH
1
2
i Ethyl chloroformate
Et3N/THF/rt/4 h
ii DAP/reflux/24 h
O
i
O
O
Cl
OH
R
OH
H
N
Et3N/THF/60ЊC/12 h
O
H
HO
O
OHCO
OCHO
ii LiOH/THF–H2O
N
O
H
N
N
3
H
N
R
NH2
4a; R ϭ
4c; R ϭ
4b; R ϭ
4d; R ϭ
Scheme 1. Synthesis of bile acid-based flavin receptors.
O
OH
OH
H
O
N
HO
R
N
N
N
NH
O
N
H
N
O
R ϭ Hexyl
11
10
9
8
[ppm]
Fig. 1. Partial 1H NMR titration stacking plot of receptor 4a (10 mM, CDCl3, 298 K) with increasing equivalent of
N(10)-hexylflavin (0–4.6 equiv.) from bottom to top.

Bile Acid-Based Receptors for Flavin Recognition
1265
compound 3. Treatment of 3 with phenylacetyl chloride, benzoyl
chloride, 2-naphthoyl chloride, or anthracene-9-carbonyl chloride
in the presence of triethylamine in dry THF at 608C for 12h fol-
lowed by basic hydrolysis using LiOH in THF–H2O led to the
formation of 4a, 4b, 4c, or 4d, respectively.
with amidic protons by a N–H?N hydrogen bond interaction. In
addition, significant downfield shifts were also observed for the
pyridine ring protons of the receptor. Also, an upfield shift and
splitting for the phenyl protons was observed which suggested an
aromatic p-stacking interaction between the flavin ring and the
phenyl unit of the receptor along with hydrogen bonding with
amide protons (Fig. 1).
Flavin Binding Studies
The change in the chemical shift of the amidic protons was
followed as a function of increasing guest concentration until
saturation of the chemical shift values was reached. The com-
plex stoichiometry was determined using Job’s method of
continuous variations, which indicated a 1 : 1 stoichiometry of
the complex formation (Fig. 2). The titration was carried out at
least twice. The association constants were determined from
their titration curves by monitoring chemical shift changes of the
amide proton using the WinEQNMR computer program (Fig. 3).
The binding constant of the complex of the receptor 4a with
The binding behaviour of the N(10)-hexylflavin towards the bile
acid-based receptors containing a DAP unit 4a–d, was deter-
mined using 1H NMR titration in CDCl3. An ,10 mM solution
of the receptor in CDCl3 was titrated with aliquots from a stock
solution of N(10)-hexylflavin (60 mM) in the same solvent.
Uponadditionof N(10)-hexylflavintothe receptor, significant
downfield shifts were observed for the amidic protons of the
receptor 4a suggesting the complexation ofthe N(10)-hexylflavin
N(10)-hexylflavin is 3500 Mꢀ1
.
0.0040
0.0035
0.0030
0.0025
0.0020
0.0015
0.0010
A similar trend in the NMR titrations was observed for 4b
and 4c (Figs 4–7). Although, in these cases, a significant upfield
shift of the aromatic ring protons was observed indicating a
better p-stacking interaction, they gave lower binding constants,
2000 and 1000 Mꢀ1, respectively. This may be due to some
steric effects which decrease the hydrogen bonding interaction
(N–H?N) between the receptor and the flavin derivative. In the
case of 4d, very little change in the chemical shift of the amide
and other protons was observed (Fig. 8). The observed binding
constants for the complexation of receptors 4a–d with
N(10)-hexylflavin are listed in Table 1.
The unexpected decrease in the binding of 4d, presumably,
is due to the large steric factor arising from the anthracene ring
which prevents the N–H?X hydrogen bond interaction.
Receptor 4a showed the highest affinity for flavin with a
binding constant of 3500 Mꢀ1, which may be due to the proper
orientation of the phenyl ring towards the flavin ring suitable
for the hydrogen bonding as well as aromatic p-stacking
interactions.
0
0.2
0.4
0.6
0.8
1.0
Mole fraction of host
Fig. 2. Job’s plot showing 1 : 1 complex formation between 4a and
N(10)-hexylflavin.
9.6
9.4
9.2
9.0
8.8
8.6
8.4
8.2
Conclusions
In conclusion, we synthesised novel bile acid-based DAP
receptors complementary to the specific hydrogen bond patterns
present in the isoalloxazine core of flavin. Binding studies of
flavin with bile acid-based DAP systems showed a prominent
influence of the steric factor and aromatic stacking interactions.
The receptor 4a with a benzyl group showed better binding
behaviour than the other receptors. The receptor 4a seems to
0.005
0.010
0.015
0.020
0.025
Concentration [M]
Fig. 3. 1H NMR titration binding isotherm for receptor 4a and
N(10)-hexylflavin in CDCl3 (Ka 3500 Mꢀ1).
[ppm]
9.5
9.0
8.5
8.0
7.5
Fig. 4. Partial 1H NMR titration stacking plot of receptor 4b (10 mM, CDCl3, 298 K) with increasing equivalent of
N(10)-hexylflavin (0–4.6 equiv.) from bottom to top.

1266
P. K. Muwal et al.
have the least sterically hindered binding site and the phenyl ring
is closer to the flavin ring suitable for aromatic p-stacking
interactions.
12 h. THF was evaporated and the residue was extracted with
chloroform. The solution was washed with 1 N HCl, saturated
NaHCO3, and brine, dried over Na2SO4, and evaporated. The
residue was purified by column chromatography on a silica-gel
column (60–65 % (v/v) ethyl acetate in hexane) to give 3. Yield:
85 %; mp 80–828C. vmax (KBr)/cmꢀ1 3383, 2942, 2871, 1716,
1457. dH (300 MHz, CDCl3, TMS) 0.76 (s, 3H, 18-Me), 0.88
(d, 3H, J 6.0, 21-Me), 0.95 (s, 3H, 19-Me), 1.08–2.45 (24H,
steroidal H), 4.41 (br s, 2H, NH2), 4.72 (m, 1H, 3b-H), 5.08
(s, 1H, 7b-H), 5.28 (s, 1H, 12b-H), 6.23 (d, 1H, J 7.8, Py-5-H),
7.46 (t, 1H, J 7.8, Py-4-H), 7.54 (d, 1H, J 7.8, Py-3-H), 8.03
(s, 1H, ꢀOCHO), 8.12 (s, 1H, ꢀOCHO), 8.17 (s, 1H, ꢀOCHO),
8.44 (br s, 1H, ꢀNHCO). dC (75 MHz, CDCl3, TMS) 11.94,
17.33, 22.11, 22.56, 25.33, 26.36, 26.96, 28.33, 30.89, 31.11,
31.22, 34.05, 34.22, 34.28, 34.67, 37.46, 40.55, 42.73, 44.80,
47.15, 70.52, 73.57, 75.11, 77.20 (steroid carbons); 102.99,
104.03, 140.50, 149.47, 156.65 (pyridine carbons); 160.43,
172.07, 178.94 (carbonyl carbons). HRMS (ES) m/z 606.3147
[M þ Na]þ; calcd for C32H45N3O7Na 606.3150.
Experimental
Synthesis of Formylated Cholic Acid 2[14]
Cholic acid 1 (4.5 g, 11.01 mmol) was dissolved in 20 mL of
100 % formic acid. The solution was stirred at 608C for 4 h. The
reaction mixture was cooled to room temperature and was added
drop wise to water (100 mL) with stirring to obtain a white
precipitate. The precipitate was filtered and vacuum dried to
give 2. Yield: 97 %; mp 192–1938C. vmax (KBr)/cmꢀ1 3420,
1954, 1721. dH (300 MHz, CDCl3, TMS) 0.76 (s, 3H, 18-Me),
0.84 (d, 3H, J 6.0, 21-Me), 0.95 (s, 3H, 19-Me), 1.08–2.40 (24H,
steroidal H), 4.72 (m, 1H, 3b-H), 5.07 (s, 1H, 7b-H), 5.27 (s, 1H,
12b-H), 8.02 (s, 1H, ꢀOCHO), 8.10 (s, 1H, ꢀOCHO), 8.16
(s, 1H, ꢀOCHO). dC (75 MHz, CDCl3, TMS) 12.05, 17.32,
22.21, 22.64, 25.42, 26.44, 27.02, 28.42, 30.29, 30.76, 31.20,
34.15, 34.31, 34.38, 34.58, 37.58, 40.66, 42.84, 44.89, 47.07,
70.59, 73.65, 75.15 (steroid carbons); 160.47, 160.57, 179.81
(carbonyl carbons). HRMS (ES) m/z 515.2625 [M þ Na]þ; calcd
for C27H40O8Na 515.2621.
General Procedure for Synthesis of Bile Acid-Based
Receptors 4a–d
To a solution of 3 (1g, 1.71mmol) and triethylamine (0.290mL)
in dry THF (15 mL), was added phenylacetyl chloride (0.264 g,
1.71mmol), benzoyl chloride (0.240g, 1.71mmol), 2-naphthoyl
chloride (0.325g, 1.71 mmol), or anthracene-9-carbonyl chloride
(0.411 g, 1.71mmol). The reaction mixture was refluxed for 15 h.
After completion of the reaction, THF was evaporated and the
residue was dissolved in water and extracted with chloroform.
The solution was washed with saturated NaHCO3, and the crude
Synthesis of Bile Acid-Based DAP Derivative 3
To a solution of 2 (4 g, 8.12 mmol) and triethylamine (1.4 mL) in
dry THF in a nitrogen atmosphere, was added ethyl chlor-
oformate (0.88 g, 8.12 mmol) drop wise over a period of 10 min.
The reaction mixture was stirred at room temperature for 4 h. A
solution of 2,6-diaminopyridine (1.06 g, 9.71 mmol) in dry THF
(15 mL) was added and the reaction mixture was refluxed for
9.35
9.30
9.25
9.20
9.15
9.10
9.05
9.00
8.95
8.90
8.85
9.05
9.00
8.95
8.90
8.85
8.80
8.75
8.70
8.65
8.60
8.55
8.50
0.005
0.010
0.015
0.020
0.025
0.030
0.005
0.010
0.015
0.020
0.025
0.030
Concentration [M]
Concentration [M]
Fig. 7. 1H NMR titration binding isotherm for receptor 4c and
N(10)-hexylflavin in CDCl3 (Ka 1000 Mꢀ1
Fig. 5. 1H NMR titration binding isotherm for receptor 4b and
N(10)-hexylflavin in CDCl3 (Ka 2000 Mꢀ1).
)
[ppm]
9.5
9.0
8.5
8.0
7.5
Fig. 6. Partial 1H NMR titration stacking plot of receptor 4c (10 mM, CDCl3, 298 K) with increasing equivalent of
N(10)-hexylflavin (0–4.8 equiv.) from bottom to top.

Bile Acid-Based Receptors for Flavin Recognition
1267
product obtained was hydrolysed using LiOH in THF–H2O. After
the completion of the hydrolysis, the solution was evaporated
under vacuum. The residue was worked up and purified by col-
umn chromatography on silica gel (40 % (v/v) ethyl acetate in
hexane) to give 4a–d.
12b-H), 7.55–7.64 (m, 2H, Ar-H), 7.76 (t, 1H, J 8.1, Py-H),
7.89–7.99 (m, 3H, Ar-H, Py-H), 8.02 (br s, 1H, ꢀNHCO), 8.11
(d, 1H, J 8.1, Py-H), 8.48 (s, 2H, Ar-H), 8.95 (br s, 1H,
ꢀNHCO). dC (75 MHz, CDCl3, TMS) 12.25, 17.20, 22.21,
23.09, 26.21, 27.27, 27.92, 29.23, 29.56, 30.05, 30.66, 32.51,
34.52, 35.19, 39.17, 41.27, 41.51, 45.59, 46.06, 68.30, 71.63,
73.20, 77.20 (steroid carbons); 109.83, 123.64, 126.68, 127.61,
127.91, 128.15, 128.39, 129.00, 131.18, 132.39, 134.85, 140.50,
149.99 (naphthalene carbons and pyridine carbons); 166.03,
173.25 (carbonyl carbons). HRMS (ES) m/z 676.3713 [M þ
Na]þ; calcd for C40H51N3O5Na 676.3721.
Compound 4a
Yield: 82 %; mp 80–828C. vmax (KBr)/cmꢀ1 3499, 2932,
1678, 1585. dH (300 MHz, CDCl3, TMS) 0.68 (s, 3H, 18-Me),
0.89 (s, 3H, 19-Me), 0.97 (d, 3H, J 3.6, 21-Me), 1.04–2.49 (24H,
steroidal H), 3.45 (m, 1H, 3b-H), 3.73 (s, 2H, ꢀCOCH2), 3.85
(s, 1H, 7b-H), 3.97 (s, 1H, 12b-H), 7.28–7.39 (m, 5H, Ar-H),
7.66 (t, 1H, J 8.1, Py-H), 7.85 (d, 2H, J 8.3, Py-H), 7.91 (br s, 1H,
–NHCO), 8.02 (br s, 1H, ꢀNHCO). dC (75 MHz, CDCl3, TMS)
12.28, 17.25, 22.26, 22.53, 23.15, 26.12, 27.31, 27.89, 29.19,
29.52, 30.06, 30.66, 32.79, 34.59, 35.21, 39.24, 41.39, 44.28,
45.75, 46.09, 68.35, 71.64, 73.16, 77.20 (steroid carbons);
109.31, 109.67, 127.18, 128.72, 129.16, 134.26, 140.48,
149.60, 149.77 (benzene and pyridine carbons); 169.89,
173.04 (carbonyl carbons). HRMS (ES) m/z 640.3721 [M þ
Na]þ; calcd for C37H51N3O5Na 640.3721.
Compound 4d
Yield: 70 %; mp 130–1328C. vmax (KBr)/cmꢀ1 3408, 2929,
1678, 1584. dH (300 MHz, CDCl3, TMS) 0.61 (s, 3H, 18-Me),
0.68 (s, 3H, 19-Me), 0.80 (d, 3H, J 5.1, 21-Me), 1.04–1.92 (24H,
steroidal H), 2.51 (br, 1H, 3a-OH), 2.68 (br, 1H, 7a-OH), 2.98
(m, 1H, 3b-H), 3.46 (s, 1H, 7b-H), 3.88 (s, 1H, 12b-H), 5.05 (br,
1H, 12a-OH) 7.07 (d, 1H, Ar-H), 7.17 (t, 1H, J 7.9, Py-H), 7.33–
7.46 (m, 4H, Ar-H, 1H, Py-H), 7.88–8.03 (m, 5H, Ar-H, 1H, Py-
H), 8.33 (br s, 1H, ꢀNHCO), 8.44 (s, 1H, Ar-H), 10.20 (br s, 1H,
ꢀNHCO). dC (75 MHz, CDCl3, TMS) 12.62, 17.76, 21.95,
23.12, 24.46, 26.47, 27.26, 27.79, 27.99, 29.07, 29.60, 31.82,
34.68, 35.14, 39.65, 39.97, 40.25, 43.81, 45.03, 68.43, 71.79,
73.44, 77.21 (steroid carbons); 108.61, 108.79, 123.55, 124.07,
124.86, 125.41, 126.36, 127.16, 127.23, 127.86, 128.18, 128.67,
129.34, 129.47, 130.48, 130.74, 130.88, 140.18, 149.28, 149.44
(anthracene and pyridine carbons); 168.56, 171.06 (carbonyl
carbons). HRMS (ES) m/z 726.3857 [M þ Na]þ; calcd for
C44H53N3O5Na 726.3877.
Compound 4b
Yield: 80 %; mp 92–948C. vmax (KBr)/cmꢀ1 3416, 2926,
1675, 1585. dH (300 MHz, CDCl3, TMS) 0.67 (s, 3H, 18-Me),
0.87 (s, 3H, 19-Me), 0.95 (d, 3H, J 3.7, 21-Me), 1.04–2.49 (24H,
steroidal H), 3.47 (m, 1H, 3b-H), 3.84 (s, 1H, 7b-H), 3.96 (s, 1H,
12b-H), 7.48–7.56 (m, 3H, Ar-H), 7.73 (t, 1H, J 8.1, Py-H),
7.90–7.95 (m, 2H, Ar-H, Py-H), 8.05 (d, 1H, J 7.8, Py-H), 8.59
(br s, 1H, ꢀNHCO), 8.85 (br s, 1H, ꢀNHCO). dC (75 MHz,
CDCl3, TMS) 12.33, 17.27, 22.31, 23.20, 26.26, 27.31, 27.99,
28.85, 29.58, 30.21, 30.68, 31.81, 32.56, 33.70, 34.62, 35.28,
39.26, 41.37, 41.56, 45.67, 46.14, 68.36, 71.70, 73.24, 77.21
(steroid carbons); 109.75, 109.89, 113.98, 127.33, 128.57,
132.09, 134.02, 139.15, 140.56, 149.96, 150.02 (benzene and
pyridine carbons); 165.91, 173.18 (carbonyl carbons). HRMS
(ES) m/z 626.3566 [M þ Na]þ; calcd for C36H49N3O5Na
626.3564.
Table 1. Association constants Ka (M21) for 1 : 1
complexes of DAP-based receptors 4a–d with
N(10)-hexylflavin in CDCl3 at 298 K
Receptor
KAa [Mꢀ1
]
4a
4b
4c
4d
3500
2000
1000
Compound 4c
B
—
Yield: 75 %; mp 111–1138C. vmax (KBr)/cmꢀ1 3418, 2931,
1670, 1584. dH (300 MHz, CDCl3, TMS) 0.68 (s, 3H, 18-Me),
0.87 (s, 3H, 19-Me), 0.96 (d, 3H, J 4.8, 21-Me), 1.04–2.49 (24H,
steroidal H), 3.50 (m, 1H, 3b-H), 3.84 (s, 1H, 7b-H), 3.98 (s, 1H,
AEstimated error , 10 %.
BBinding constant cannot be calculated as chemical
shifts were too small to allow determination.
[ppm]
10.0
9.5
9.0
8.5
8.0
7.5
Fig. 8. Partial 1H NMR titration stacking plot of receptor 4d (10 mM, CDCl3, 298 K) with increasing equivalent of N(10)-
hexylflavin (0–5.0 equiv.) from bottom to top.

1268
P. K. Muwal et al.
Supplementary Material
[7] (a) E. C. Breinlinger, V. M. Rotello, J. Am. Chem. Soc. 1997, 119,
1165. doi:10.1021/JA9612110
NMR and mass spectra associated with this article are available
on the Journal’s website.
(b) A. J. Goodman, E. C. Breinlinger, C. M. Mclntosh, L. N. Grimaldi,
V. M. Rotello, Org. Lett. 2001, 3, 1531. doi:10.1021/OL015838L
(c) M. Gray, A. J. Goodman, J. B. Carrol, K. Bardon, M. Markey,
G. Cooke, V. M. Rotello, Org. Lett. 2004, 6, 385. doi:10.1021/
OL036279G
Conflicts of Interest
The authors declare no conflicts of interest.
[8] A. S. F. Boyd, J. B. Carroll, G. Cooke, J. F. Garety, B. J. Jordan,
S. Mabruk, G. Rosair, V. M. Rotello, Chem. Commun. 2005, 2468.
doi:10.1039/B501887K
Acknowledgements
The authors thank the Council of Scientific and Industrial Research,
New Delhi, for financial support.
[9] S. T. Caldwell, G. Cooke, S. G. Hewage, S. Mabruk, G. Rabani, V. M.
Rotello, B. O. Smith, C. Subramani, P. Woisel, Chem. Commun. 2008,
4126. doi:10.1039/B809762C
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