An activated coumarin-enamine Michael acceptor for CN-

Article abstract of DOI:10.1039/c4nj00862f

Two coumarin-enamine chemodosimeters have been synthesized in three steps. They have been shown to selectively detect the cyanide ion with a fluorescent response (t_1/2 = 20 s) and a limit of detection approximately 4.2 ppb in DMSO. The X-ray crystal structure of the thermodynamically stable E-(keto) enamine isomer of one compound was obtained and exhibited offset π-stacking. This journal is

Full text of DOI:10.1039/c4nj00862f

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An activated coumarin-enamine Michael
acceptor for CNˠ
Cite this:DOI: 10.1039/c4nj00862f
Aaron B. Davis,a Rachel E. Lambert,a Frank R. Fronczek,b Peter J. Craggc and
Karl J. Wallace*a
Received (in Montpellier, France)
25th May 2014,
Accepted 7th July 2014
Two coumarin-enamine chemodosimeters have been synthesized in three steps. They have been shown
to selectively detect the cyanide ion with a fluorescent response (t1/2 = 20 s) and a limit of detection
approximately 4.2 ppb in DMSO. The X-ray crystal structure of the thermodynamically stable E-(keto)
enamine isomer of one compound was obtained and exhibited offset p-stacking.
DOI: 10.1039/c4nj00862f
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e.g., aldehydes,14 iminium groups15 and carbon–carbon double
Introduction
bonds,16–18 as well as Lewis acid centers.19 Needless to say these
Anion recognition is now a well-established area of research, so functional groups have been incorporated into colorimetric,
much so that Gale has written regular updates on the area over a fluorescent and electrochemical molecular probes that detect
period of 10 years.1–6 One aspect of anion recognition of practical cyanide. A recent review by Yoon et al. highlights some elegant
importance is the design and synthesis of molecular probes to examples of such sensors.20
detect and monitor anions by colorimetric,7 fluorescent8 or electro-
Derivatives of 1,2-benzopyrone (commonly known as coumarin)
chemical9 means, however, the main challenges to the design of have been used extensively as molecular probes due to their unusual
molecular probes are sensitivity and selectivity. One of the most photophysical properties in different media.21 As a consequence,
interesting but ‘‘simple’’ anions to monitor is the cyanide ion due numerous coumarin molecular probes have been utilized as
to its environmental,10 biological11 and industrial importance.12 colorimetric and fluorescent sensors.8,15,17,21–23 Although a
The specific challenge of cyanide detection is a consequence of recent coumarin-enamine derivative was used in the histo-
the different species that exist in solution. At a pH of 9.3–9.5, CNÀ chemical identification of b-amloid plaque,24 the use of coumarin
and HCN are in equilibrium, with equal amounts of each present, enamine molecular probes is not particularly prevalent in the
referred to as free cyanide. At a pH of 11, over 99% of the cyanide realms of anion recognition.
remains in solution as CNÀ known as the cyanide ion, while at
As part of our ongoing interest in the design of new
pH 7, over 99% of the cyanide will exist as molecular cyanide, molecular probes, we have synthesized coumarin enamines 3
HCN. Other species that exist that are prevalent in the mining and 4 (Fig. 1) that can undergo Michael addition of cyanide at
industry are simple cyanides (i.e., KCN), complex cyanides the a,b-unsaturated carbonyl. It is well known that electron
(i.e., Ag(CN)2), weak acid dissociable cyanides, e.g., (Cd(CN)2) withdrawing functional groups attached to a carbonyl moiety will
and strong acid dissociable cyanides, e.g., [Co(CN)6]4À. The pull electron density away from the carbon atom. Consequently
summation of these cyanide species is known as the total making this region more electrophilic will render it more susceptible
cyanide concentration.13
towards rapid nucleophilic attack. Our intention was also to synthe-
The cyanide ion displays an intrinsic nucleophilic behavior size a planar molecular with a high degree of conjugation which
towards carbon centers, particularly carbonyl groups,
a Department of Chemistry and Biochemistry University of Southern Mississippi,
Hattiesburg, Mississippi 39406, USA. E-mail: karl.wallace@usm.edu
b Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana,
70803, USA
c School of Pharmacy and Biomolecular Sciences, University of Brighton,
Brighton BN2 4GJ, UK
† Electronic supplementary information (ESI) available: Full characterization,
experimental details, molecular modelling calculation and X-ray crystallographic
data. CCDC 1004495. For ESI and crystallographic data in CIF or other electronic
Fig. 1 Coumarin-enamine target chemodosimeters.
format see DOI: 10.1039/c4nj00862f
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can be easily perturbed to produce a spectroscopic response. Thus
we took advantage of intramolecular hydrogen bonding, available
to 3 and 4, that forms a resonance assisted hydrogen bonding
interaction (RAHB).25 Upon addition of the cyanide it was antici-
pated that a significant spectroscopic response in the UV-vis and
fluorescence spectra would occur through perturbation of chemo-
dosimeter conjugation. To the best of our knowledge this is the first
time that a coumarin-enamine scaffold has been used to detect
cyanide ions in this manner.
Scheme 1 The proposed Michael addition of the CNÀ breaking up the
pseudo conjugated molecular probe 3 and perturbing the delocalisation
due to the hybridization change.
depending on the polarity of the solvent. This has led to some
very unusual photophysical behavior.21,22 The spectroscopic
responses of many molecular probes are often only investigate
in one particular solvent system, normally a water–organic
solvent mixture, presumably to show that a particular system
works in water-based environments; more often than not other
solvent systems are ignored. Therefore, before we investigated
the response between 3 and 4 towards various anions, we
examined the basic photophysical properties in a range of
nonpolar to polar solvents. A plot of solvent polarity factor, DF,
against absorption and fluorescence lmax values for 3 is shown in
the ESI,† Fig. S3(A). The hypsochromic shift observed in the UV-vis
spectra as solvent polarity increases (negative solvatochromism) is
a consequence of the ground state of 3 being stabilized by the
solvent than the excited state. There was no effect on 4 as the ring
system contains a carbon atom instead of the nitrogen atom.
However, the lfl (Fig. S3(B), ESI† red line) exhibits a greater solvent
effect and a bathochromic shift as the polarity increases, this is
seen for both 3 and 4 (Fig. S3, ESI†). This is in excellent agreement
with coumarin dyes with a dialkylamino group tethered at the
7-position on the coumarin scaffold and has been attributed to
the stabilization of the intramolecular charge transfer (ICT), in
turn leading to stabilization of the excited singlet state.22 The
more non polar the solvent the more a blue/green the fluores-
cence emission appears under a UV lamp (Fig. S2, ESI†).
After investigating the photophysical properties in the different
solvent systems, we decided to use one with an uncomplicated
spectroscopic signal. The spectroscopic behavior of 3 and 4
towards a number of monoanionic analytes was determined
in DMSO. Potassium cyanide was chosen to ensure complete
dissociation in the solvent. Upon the addition of CNÀ a distinct
change can be observed from a yellow to colorless solution
(Fig. 3B), whereas the color intensity for the other anions remained
unchanged, with the exception of the fluoride (Fig. 3C) and acetate
ions (Fig. 3N). These two exceptions may result from the two
anions’ hydrogen bonding and inhibition of the excited state
intramolecular proton transfer (ESIPT) mechanism as proposed
in the literature.27–29 However, a more likely explanation is that
partial deprotonation is responsible for the spectroscopic change as
both the fluoride anion and acetate anion are basic in DMSO
solution and the abstraction of a proton by both of these anions is a
well-known phenomenon in supramolecular chemistry.30
Results and discussion
Synthesis and crystallography studies
The two coumarin-enamine compounds were prepared in three
straightforward steps (Scheme S1, ESI†). Solution studies on 3
showed that the product existed as a mixture of two E and Z isomers
in a solvent independent 1 : 3 ratio. Tautomeric transformations can
also occur, (Scheme S2, ESI†), however the E-(keto) enamine and
Z-(keto) emamine, were the only species observed in solution.
Fortunately, X-ray quality crystals of E-3 were obtained by
heating a concentrated solution of 3 in DMSO and allowing it to
stand for several days. Even though DMSO is known as a competitive
solvent, and often hinders hydrogen bonding interactions, the
crystal packing of 3 is dominated by two sets of bifurcated hydrogen
C–HÁÁÁO interactions between C(10)–H and C(12)–H of E-(keto)-
enamine 3 and the O(2) of an inversion-related molecule that is in
the anti-orientation (Fig. 2). The crystal packing shows that each
molecule is offset such that the electron rich aniline B ring system
(Scheme 1) is stacked via a p–p arrangement with the electron
deficient enamine-derived A ring system with a separation of 3.64 Å
(Fig. S1(b), ESI†), which is in excellent agreement with other
aromatic stacking arrangements.26 The planarity of the molecular
probe is also stabilized by a very strong intramolecular hydrogen
bonding interaction between N(1)–HÁÁÁO(3) at 1.95(3) Å, in essence
forming a pseudo ring system.
UV-Vis and fluorescence spectroscopic studies
There have been many studies on the photophysics of coumarin
molecules and it is well known that changes in the dipole moment
of these molecules significantly influences the Stokes shift
The UV-vis absorption spectra of all the anions studied are
shown in Fig. S4 and S5, ESI.† Compound 3 shows a broad
band at 405 nm that is shifted on the addition of CNÀ, with
the appearance of a hyperchromic band at 335 nm through a
Fig. 2 Crystal structure of 3, illustrating the two set of bifurcated hydrogen
bonding interactions (dashed lines). Donor–acceptor distances (Å): intra-
molecular hydrogen bond N(1)–H(1)NÁÁÁO(3) = 2.642(2); intermolecular hydro-
gen bond C(10)–H(10)ÁÁÁO(2) = 3.342(3) and C(12)–H(12)ÁÁÁO(2) = 3.191(3).
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at 365 nm. It is reasonable to believe that this new species is a
consequence of an acid base reaction whereby the primary amine
has deprotonated the molecule in the E-s-cis-hydroxyimine form,
thus significantly changing the electronics of the system. A plot of
the absorbance ratio at 335/405 nm upon the addition of two
equivalents of KCN shows that the ratiometric response towards
CNÀ is approximately six times greater than the fluoride ion
(Fig. 4, top). Even though there was a response on the addition
of cyanide we wanted to see if the two anions can be further
distinguished and turned to the more sensitive technique of
fluorescence spectroscopy (Fig. S6 and S7, ESI† for all anions).
Upon the addition of cyanide the florescence spectrum exhibited
an emission shift of 219 nm with an isoemissive point at 475 nm.
A plot of the nominalized fluorescence ratio at 356/575 nm
upon the addition of two equivalents of KCN shows a clear
response towards cyanide, over the other anions studied, by a
factor of 350 (Fig. 4, bottom).
Fig. 3 (top) Colorimetric response via naked eye in DMSO (bottom) under
the UV-lamp: probe 3 (0.2 mol dmÀ3) and anions (0.3 mol dmÀ3);
(A) compound 3 (B) KCN (C) TBAF (D) TBAH2PO4 (E) TBABr (F) TBACl (G)
TBAI (H) TBANO3 (I) NaBF4 (J) NaN3 (K) TBAHSO4 (L) NH4SCN (M) NH4ClO4
(N) TBACH3CO2.
As we could clearly distinguish CNÀ in the fluorescence
spectrum we used fluorescence spectroscopy to determine the
limit of detection (LOD) of cyanide upon reaction with 3, a key
requirement for any molecular probe for cyanide. The LOD was
determined by least squares linear regression. The confidence
limit of the slope is defined as b Æ tsb, where t is the t-value
taken from the desired confidence level and n À 2 degrees of
freedom; here a 98% confidence level (t-value 2.72, df = 11) was
adopted (Fig. S33, ESI†). It is generally accepted that the LOD is
the analyte concentration giving a signal equal to that of the
blank signal plus three standard deviations from the blank,
i.e. y = yB + 3SB. The LOD values for chemodosimeter 3 were
calculated to be 4.2 ppb, in the range of other examples31,32 and
well below the threshold set by the European Union (50 ppb)
and the United States (200 ppb). Interestingly, the guidelines
set by WHO in 2011 stated that it is not necessary to establish
short-term detection limits of cyanide ions in drinking water
due to the rapid detoxification by the liver.33 However, the
guidelines do point out that high doses can give rise to thyroid
toxicity.33,34 which is of particular importance in developing
countries due to the lucrative business of gold mining that
requires cyanide as part of the gold extraction process. To confirm
that the cyanide ion added to the chemodosimeter in a 1 : 1
manner, we ran Job plot analysis and ESI-MS (Fig. S32 and S33,
ESI† respectively); the Job plot showed that the mole fraction was
at 0.5 and a m/z = 363 assigned to the [M À CN]À signal. We then
undertook an extensive NMR investigation on 3.
well-defined isosbestic point at 340 nm (Fig. 4 top insert). The
absorption band at the longer wavelength can be attributed to
resonance assisted hydrogen bonding, which in essence means
that there are four ring systems that span the molecule. On the
addition of the nucleophile to the enamine ring C is broken, thus
the conjugation is hampered due to the hybridization change
from sp2 to sp3 (Scheme 1).
The UV-vis spectrum of the fluoride does show some similar
spectroscopic changes, whereas octylamine spectrum shows a
very different response (Fig. S4, ESI†) with a new band appearing
NMR studies
The enamine can exist as two geometrical isomers (E or Z) or
one of a plausible number of tautomers (Fig. S35, ESI†). No
tautomers were detected in the NMR spectra; only the E-(keto)
enamine and the Z-(keto) enamine isomers of 3 were evident.
Integration of the 1H NMR showed a 3 : 1 ratio for E-(keto)
enamine and the Z-(keto) enamine respectively, in a mixture of
CHCl3-d : DMSO-d6 (6 : 1), consistent with other coumarin
enamines.35 The E isomer was found to be lower in energy than
the Z and the only product that formed X-ray quality crystals.
As the UV-Vis and fluorescence data showed an optical
Fig. 4 UV-Vis titration of 3 (16 mmol dmÀ3; DMSO, lEx 339 nm) upon the
addition of two equivalents of KCN (top). The bar plot illustrates the
ratiometric response of probe 3 in the presence of the anions studied
(two equivalents); normalized fluorescence titration data of probe 3
(0.16 mol dmÀ3), upon the addition of two equivalent KCN (bottom).
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Fig. 5 1H NMR spectral change of the dosimeter 3 (DMSO, 9.5 mM) on
the addition of 1.2 equivalents of KCN (DMSO, 7.6 mmol dmÀ3).
response upon the addition of KCN, we investigated the change
in the NMR spectra upon the addition of 1.2 equivalents of
KCN to 3.
Due to the poor solubility of KCN in CHCl3 100% DMSO-d6 was
used for the NMR studies. As a consequence of the 1,4 addition of
cyanide the E and Z isomers no longer exist in solution so both
1H and 13C spectra became significantly easier to assign (Fig. 5).
Interestingly, there is only one addition product observed in the
NMR. The two downfield doublets at d 11.52 and 13.50 ppm Z-3
and E-3 respectively are assigned to the NH proton, confirmed by
HSCQ (Fig. S19, ESI†). As these signals were not correlated to any
corresponding carbon atoms, and they also disappeared upon a
D2O shake. Both of these signals subsequently disappeared upon
the addition of cyanide with a new sharp doublet appearing at
d 6.95 ppm. The 13C NMR was significantly more informative,
in particular the 13C APT spectrum which distinguishes between
CH/CH3 (positive signals) and CH2/C (negative signals). Upon
the addition of the cyanide C(3) and C(9) are observed at d 99.2
(E-3); 99.7 (Z-3) and 151.8 ppm which shifted up-field to d 88.0
and 125.0 ppm C(3) and C(9) respectively and the quaternary
carbon atom of the cyanide is seen as a negative signal at
119.5 ppm in the 13C ATP experiment (Fig. S27, ESI†).
Fig. 6 Reaction between cyanide and 3 as calculated by DFT (B3LYP/
6-311G*): NCÁ Á ÁC distance (from top) 5.00 Å, 3.15 Å, 2.80 Å, 2.48 Å, 2.15 Å,
1.80 Å and 1.48 Å. Structures (left) are shown superimposed with HOMOs
on the right (see ESI† for details).
Computational studies and kinetic analysis
Computational methods were used to calculate eight different
conformers of 3 in the gas phase (Fig. S35, ESI† structures A to H). had been identified as the optimum C–CN bond distance from
From the NMR of 3 and the product formed from the reaction a DFT/B3LYP/6-311G* calculation. The effect that this has on
with cyanide the only structures that could be involved were that the HOMO of the system is illustrated in Fig. 6. As expected,
shown in Fig. 1, with the N–H Z to the ketone, and its E isomer. Of electron density is initially associated with the incoming cyanide
these, the former was preferred on energetic grounds.
but, as a C–C bond is formed, this migrates to the coumarin
A sequence of structures was generated in which CNÀ region of probe 3.
approached the target carbon down a vector. A reaction trajec-
The cyanide ion is known to stop cellular respiration by
tory was attempted using DFT/B3LYP/6-311G* but simulations inhibiting the cytochrome C oxidase enzyme at high concentra-
resulted in proton abstraction rather than cyanide addition so a tions.36 Therefore it is imperative that the detection of the
simpler method was employed. A PM6 semi-empirical simula- cyanide ion is achieved quickly. As pyridine groups are more
tion was more successful, indicating that there were no orbital electron withdrawing than the analogous benzene group we
interactions between CNÀ and the target at distances exceeding anticipated that the kinetics between compound 3 would occur
3.14 Å. This was taken as a starting point and the C–CN faster than probe 4, as the electron density is being pulled away
distance was reduced in steps of 0.33 Å down to 1.48 Å which from the enamine carbon making it more electrophilic for the
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nucleophile to attack. Our initial studies indicate that this is true. carried out on a QuantaMastert 40 intensity based spectro-
A 2.5 Â 10À5 mol dmÀ3 solution of 3 and a 1.25 Â 10À4 mol dmÀ3 fluorometer from PTI technologies in the steady-state. High
solution of KCN were mixed in equal amounts and the reaction resolution mass spectra (HRMS) were recorded at the Depart-
was monitored for 3 min at 335 nm. Pseudo first-order kinetics ment of Chemistry at University of Alabama, Tuscaloosa. X-Ray
were assumed and, by monitoring the increase at 335 nm over crystallography was carried in the department of chemistry at
time, the rate of the reaction was calculated from ln(Ao/(Ao À AI)) Louisiana State University, see pages 42–47 in ESI† for experi-
versus time (Fig. S34, ESI†), where Ao is the final absorbance mental details, crystal data and tables.
intensity and AI is the absorbance intensity recorded at each time
Synthesis and characterization
interval measured. The rate constant, k, was calculated to be
0.03 sÀ1 giving a half-life (t1/2 = ln2/k) of approximately 23 seconds.
Synthesis of bis-(2,4,6-trichlorophenyl)malonate (1)23,24
.
We are currently investigating the kinetics of a number of different 2,4,6-Trichlorophenol (15.8 g, 79.8 mmol), malonic acid (4.16 g,
substituted isomers containing electron withdrawing and donating 39.9 mmol) and phosphorus (V) oxychloride (10 mL, 107 mmol)
groups to increase the rate of reaction.
were refluxed for three hours. The reaction was allowed to cool
to room temperature and quenched with cold deionized water
(50 mL). The precipitate was collected and dissolved in deionized
water (10 mL). The solution was then brought to pH = 7 by the
addition of saturated sodium bicarbonate solution. The resulting
solid was collected by vacuum filtration and recrystallized from
ethylacetate (16.6 g, 90% yield). 1H NMR (300 K, DMSO-d6,
400 MHz): d 7.53 (s, 4H, ArH), 3.85 (s, 2H, CH2); 13C NMR (300 K,
DMSO-d6, 100 MHz): 168.8, 149.0, 128.6, 123.8, 123.5.
Conclusions
In summary, two coumarin-enamine dyes have been synthesized
and their spectroscopic behavior, in particular fluorescence, has
demonstrated their selectivity for CNÀ over 12 common anions
with a LOD in the region of 4 ppb. Michael reactions are reversible
under certain conditions, yet we were unable to observe any
reversibility in our probes, despite numerous attempts to achieve
this. Therefore, the current probes act as true, stable chemodosi-
meters. We have also investigated the spectroscopic properties in
various organic solvents, something that is often omitted in the
published literature. Despite a spectroscopic response from FÀ
and CH3CO2À, which is most likely due to partial deprotonation,
we now have a good understanding that these molecules can be
used to detect cyanide ions, we are currently incorporating them
into ‘‘test strips’’ and polymeric matrices for onsite detection of
hydrogen cyanide in air.
Synthesis of 7-(diethylamino)-4-hydroxycoumarin (2)23
Bis-(2,4,6-trichlorophenyl)malonate (9.26 g, 20.0 mmol) and
3-diethylaminophenol (3.30 g, 20.0 mmol), and anhydrous
toluene (50 mL) were refluxed for three hours. The reaction was
allowed to cool to room temperature. The precipitate was collected
by vacuum filtration and washed with toluene (3.50 g, 75% yield).
1H NMR (300 K, DMSO-d6, 400 MHz): 11.91 (s, 1H, OH), 7.55
(d, 1H, J = 9.0 Hz, ArH), 6.65 (dd, 1H, J = 9.0 and 2.2 Hz, ArH),
6.45 (d, 1H, J = 2.2 Hz, ArH), 5.25 (s, 1H, CH), 3.42 (dq, 4H,
J = 14.0 and 7.0 Hz, CH2), 1.11 ppm (t, 6H, J = 7.0, CH3);
13C NMR (300 K, DMSO-d6, 100 MHz): 166.9, 163.2, 156.6, 151.3,
124.6, 108.6, 103.9, 98.9, 86.6, 44.4, 12.8.
Experimental
General techniques
General procedure for molecular probes (3) and (4).
7-(Diethylamino)-4-hydroxycoumarin (2) (1.0 mmol), and the
One-dimensional (1H, 13C and 13C APT) and two-dimensional appropriate primary amine (1.0 mmol), and triethylortho-
(HMBC, HSQC and ROESY) NMR spectra were recorded Bruker formate (1.5 mmol) were refluxed in propan-2-ol (5 mL) for two
Avance 600 spectrometer operating at a proton frequency of hours. The reaction was allowed to cool to room temperature. The
600.13 MHz and equipped with a standard BFO 5 mm two resulting solid was collected by vacuum filtration and washed
channel probe in the appropriate deuterated solvents or were with cold propan-2-ol (typical yields 80 to 85%).
recorded on a Bruker Ultrashield plus 400 MHz spectrometer.
Characterization of probe (3). Yield 282 mg, 0.84 mmol, 84%
Chemical shifts are reported in parts per million (ppm) down- yield; 1H NMR (300 K, DMSO-d6, 600 MHz) and 13C NMR (300 K,
field from tetramethylsilane (0 ppm) as the internal standard DMSO-d6, 150 MHz): see pages 15–27 for 1D and 2D spectrum.
and coupling constants ( J) are recorded in hertz (Hz). The ESI-MS; m/z for [M + H]+ = 338.2; IR (ATR solid); 3059 (w)
multiplicities in the 1H NMR spectra are reported as (br) broad,
(s) singlet, (d) doublet, (dd) doublet of doublets, (ddd) doublet
n
n
CQC(enamine), 2970 (w) nCH, 1716 (s) nCO (delta lactone), 1571
CO (ketone) cmÀ1; HRMS observed for C19H19N3O3 = 337.1430;
of doublet of doublets, (t) triplet, (sp) septet and (m) multiplet. Calculated for C19H19N3O3 = 337.1426.
All spectra are recorded at ambient temperature, unless other-
Characterization of probe (4). Yield 268 mg, 0.98 mmol, 80%
wise stated. UV-Vis experiments were performed on a Beckman yield; 1H NMR (300 K, CHCl3-d, 400 MHz): d 13.65 (d, 1H,
DU-70 UV-Vis spectrometer. Low resolution mass spectra were J = 12.5 Hz, NH), 8.82 (d, 1H, J = 13.3 Hz, CHenamine), 7.87 (d, 1H,
measured with Finnigan TSQ70 IR spectra were recorded on a J = 9.0 Hz, CHcoumarin), 7.45 (t, 2H, J = 7.9 Hz, CHaromatic), 7.30
Nicolet Nexus 470 FT-IR paired with a Smart Orbit ATR attach- (dt, 3H, J = 15.9 and 7.5 Hz, CHaromatic), 6.59 (dd, 1H, J = 9.0 and
ment. The characteristic functional groups are reported in 2.4 Hz, CHcoumarin), 6.38 (d, 1H, J = 2.3 Hz, CHcoumarin), 3.45
wavenumbers (cmÀ1), and are described as weak (w), medium (m), (q, 4H, J = 7.1 Hz, CH2), 1.15 ppm (t, 6H, J = 7.1 Hz, CH3);
strong (s) and very strong (vs). Fluorescence experiments were 13C NMR (300 K, CHCl3, 100 MHz): 181.0, 164.5, 157.2, 153.5,
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ed. D. A. Dzomback, R. S. Ghosh and G. M. Wong-Chong,
CRC press, Baco Raton, 2006, p. 57.
152.9, 138.2, 130.8, 127.3, 126.6, 118.0, 108.8, 108.4, 98.1, 97.2,
44.9, 12.9; ESI-MS m/z [M + H]+ = 337.0; IR (ATR solid) IR
14 K.-S. Lee, H. J. Kim, G.-H. Kim, I. Shin and J.-I. Hong, Org.
Lett., 2008, 10, 49.
(ATR solid); 3239 (w) nNH 3059 (w) nCQC(enamine), 2970 (w) nCH
,
;
1716 (s) nCO (delta lactone), 1571 nCO (ketone) cmÀ1
15 H. J. Kim, K. C. Ko, J. H. Lee, J. Y. Lee and J. S. Kim, Chem.
Commun., 2011, 47, 2886.
HRMS observed for C20H20N2O3 = 336.1484; calculated for
19H19N3O3 = 336.1474.
C
16 W.-C. Lin, S. C. Fang, J.-W. Hu, H.-Y. Tsai and K.-Y. Chen,
Anal. Chem., 2014, 86, 4648.
Acknowledgements
17 G.-J. Kim and H.-J. Kim, Tetrahedron Lett., 2010, 51, 2914.
18 S. H. Mashraqui, R. Betkar, M. Chandiramani, C. Estarellas
and A. Frontera, New J. Chem., 2011, 35, 57.
19 R. Tirfoin and S. Aldridge, Dalton Trans., 2013, 42, 12836.
20 Z. Xu, X. Chen, H. N. Kim and J. Yoon, Chem. Soc. Rev., 2010,
39, 127.
The KJW group is grateful for the financial support from NSF
Grant OCE-0963064.
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