Photocontrolled Single-/Dual-Site Alternative Fluorescence Probes Distinguishing Detection of H2S/SO2 in Vivo

Article abstract of DOI:10.1021/acs.orglett.9b01879

Herein, a novel fluorescent probe by integrating 4-azide-1,8-naphthalic anhydride and spiropyran was obtained. NT-N₃-SP was capable of specifically monitoring H₂S by azide reduced. Upon irradiation by alternate ultraviolet and visible light, both the structure and emission of spiropyran moiety in NT-N₃-SP can be reversibly tuned. Importantly, the alternation will be interrupted in the presence of SO₂. Additionally, NT-N₃-SP was successfully used for detection of H₂S/SO₂ in cells and mice.

Full text of DOI:10.1021/acs.orglett.9b01879

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Photocontrolled Single-/Dual-Site Alternative Fluorescence Probes
Distinguishing Detection of H S/SO in Vivo
2
2
†
‡
,†
Weijie Zhang, Fangjun Huo, and Caixia Yin*
†Key Laboratory of Chemical Biology and Molecular Engineering of Ministry of Education, Key Laboratory of Materials for Energy
Conversion and Storage of Shanxi Province, Institute of Molecular Science, Shanxi University, Taiyuan 030006, China
‡
Research Institute of Applied Chemistry, Shanxi University, Taiyuan 030006, China
*
S Supporting Information
ABSTRACT: Herein, a novel fluorescent probe by integrat-
ing 4-azide-1,8-naphthalic anhydride and spiropyran was
obtained. NT-N -SP was capable of specifically monitoring
3
H S by azide reduced. Upon irradiation by alternate
2
ultraviolet and visible light, both the structure and emission
of spiropyran moiety in NT-N -SP can be reversibly tuned.
3
Importantly, the alternation will be interrupted in the
presence of SO . Additionally, NT-N -SP was successfully
2
3
used for detection of H S/SO in cells and mice.
2
2
eactive sulfur species (RSS) are a class of sulfur-containing
which displays a highly selective and distinctive response for H2S
1
−3
R
compounds that play vital roles in human physiology.
Among these RSS, hydrogen sulfide (H S) and sulfur dioxide
and SO simultaneously is highly desirable but even more
2
2
challenging. Studies suggest that the aryl azide group shows high
26,27
(
SO ) seem to be the most attractive sulfur compounds due to
selectivity with hydrogen sulfide.
Therefore, we speculate
2
their physiological role as the endogenous gasotransmitters
that azide derivative seems to be an ideal probe for H2S
detection. Spiropyran, as a typical photoswitchable mole-
4
−9
following nitric oxide (NO) and carbonic oxide (CO). From
a chemistry perspective, H S and SO are redox partners;
28−30
2
2
cule,
was a latent fluorophore that can be reversibly
therefore, they could coexist in biological systems. In fact, it is
evident that SO and H S are generated from the same
tuned to merocyanine, which shows high selectivity for SO
2
31
2
2
detection. To this end, we coupled the spiropyran derivative to
the weakly fluorescent molecule 4-azide-1,8-naphthalic anhy-
dride with ethylenediamine as a tether. The full working strategy
is described in Scheme 1.
metabolism of sulfur-containing amino acids, and sometimes the
two gasotransmitters share the same signal pathway, even the
1
0−13
same target residue.
In addition, increasing studies
suggested that H S and SO exhibit similar anti-inflammatory
2
2
In order to confirm our hypothesis for SO and H S detection,
14
2
2
activities against ROS, especially in cardiovascular systems.
first we tested the absorbance and fluorescence responses of NT-
Obviously, there is a significant correlation between H S and
2
N -SP with UV irradiation. As expected, upon irradiation of UV
3
SO in the living systems. Because of these similar properties,
2
light (365 nm, 12W), a gradual fluorescence emission at 630 nm
researchers cannot distinguish one specific from another in the
complicated physiological environment of the cells. In fact, some
biological mechanisms that were originally attributed to H2S
Scheme 1. Molecular Structure and Proposed Sensing
Mechanism of NT-N -SP for H S and SO
3
2
2
may actually be mediated by SO and vice versa.
2
In order to better understand the physiological roles of SO2
and H S, several methods, such as electrochemistry, chromatog-
2
raphy, and capillary electrophoresis, have been reported for
1
5−18
19,20
SO2
and H2S
detection. However, these methods
cannot be directly applied to living cells due to their destructive
2
1
nature. Fluorescence probes, which can display rapid,
noninvasive, and sensitive detection of target analytes, have
22−25
become powerful tools for in vivo imaging.
Although a few
fluorescent probes for the detection of SO or H S have been
2
2
reported in recent years, most still suffer from drawbacks in
terms of the selectivity. The design of a single fluorescent probe
Received: May 31, 2019
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01879
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
was observed (Figure 1a), which fully proved the formation of
MR isomer. In addition, with the photoconversion of the SP
Then the UV light-controlled recognition of NT-NH -SP for
2
SO was performed in solution. The decrease of the maximum
2
absorption band around 535 nm confirmed the disruption of the
double bond in the merocyanine moiety (Figure 2c).
Simultaneously, the fluorescent emission at 630 nm decreased
gradually with the addition of Na SO (Figure 2d), suggesting
2
3
formation of the Michael adduct between Na SO e and MR-stat.
2
3
The detection limit of NT-NH -MR toward the SO derivative
2
2
was calculated to be 0.121 μM (Figure S3).
We then tested the selectivity of NT-N -SP for H S and SO .
3
2
2
It was found that only the addition of Na S induced significant
2
fluorescence enhancement at 540 nm (Figure S4a). Next, we
recorded the spectra change of NT-N -MR with other analytes.
3
As shown in Figure S4b, other interfering species hardly caused
any fluorescence change at 630 nm except Na SO . Then the
2
3
real-time fluorescence responses of NT-N -SP toward SO and
3
2
H S were examined by recording the fluorescence intensity of
2
Figure 1. (a) Fluorescence spectra and (b) absorption spectra of probe
the two emission wavelengths at 540 or 630 nm, respectively. As
illustrated in Figure S5a, the fluorescence intensity at 540 nm
notably enhanced and finally leveled off at around 25 min after
NT-N -SP (10 μM) upon irradiation with a UV lamp. (c) Fluorescence
3
emission and (d) UV−vis absorbance photoswitching of NT-N -SP
3
with UV/vis cycle. Test medium: PBS/C H OH solution (v/v = 1/1,
2
5
pH 7.4).
treatment with 200 μM Na S. The fluorescence signals of NT-
2
NH -MR at 630 nm leveled off within 5 min upon addition of
2
moiety, the azide group was also reduced, and the absorption
peaks at 440 and 535 nm are enhanced during the UV irradiation
Na SO (200 μM), suggesting that the MR state is highly
2
3
reactive with SO . However, one may wonder if hydrogen sulfide
2
(
Figure 1b). The light-controlled reversible SP/MR switch
may also show high reactivity toward such a conjugated system.
To address this issue, we further investigated the time-
could be realized for several cycles without obvious degradation
in intensity (Figure 1c,d).
dependent fluorescence response of NT-NH -MR at 630 nm
2
Then UV−vis and fluorescence measurements were carried
with hydrogen sulfide. Upon addition of Na S, the fluorescence
2
out. As described in Figure 2a, upon addition of Na S (0−200
signals at 630 nm decreased more slowly than those of SO , and
2
2
the detection process was still ongoing until 60 min (Figure
S5c). As a result, the azido group seems to be more sensitive
toward H S compared with the MR state in NT-NH -MR. The
2
2
pH effect of NT-N -SP toward SO and H S was subsequently
3
2
2
investigated. Probe NT-N -SP is stable in a broad pH range
3
(
3.0−8.0), and an obvious enhancement of intensity at 540 nm
was found after it was treated with Na S within the normal
2
physiological ranges from 5.0 to 7.4 (Figure S6a). NT-NH -MR
2
was also steady in a broad pH range (3.0−8.0) after UV
irradiation, and the addition of Na SO displayed the best
2
3
fluorescence response within the pH range from 5.0 to 8.0
(
Figure S6b).
For the mechanistic study of probe NT-N -SP for H S and
3
2
SO , the mass spectrometry analysis was carried out here. ESI-
2
Figure 2. (a) Absorption spectra and fluorescence spectra changes of
MS in Figure S7a shows a main peak at m/z 644.22525 [M −
probe NT-N -SP (10 μM) with increasing Na S (0−200 μM), λ = 440
+
3
2
ex
H] , which corresponds to NT-N -SP. The mass peak appearing
3
nm. (c) Fluorescence emission and (d) UV−vis absorbance of activated
at m/z 618.23474 corresponded to the product of NT-N -SP
3
NT-NH -MR (10 μM) in the presence of Na SO (60 μM), λ = 535
2
2
3
ex
with Na S (Figure S7b). Then the mixture of NT-N -SP with
2
3
nm.
Na S was exposed to UV light. As expected, MS analysis gave the
2
expected mass shift at m/z 618.23444 of deserved product NT-
μM), the maximal absorption at 370 nm decreased and a new
red-shifted absorbance peak at 440 nm was centered. Mean-
while, the fluorescence intensity was significantly enhanced at
NH
2
-MR, and the dominant peaks at 640.21674 were attributed
-MR + Na (Figure S7c). Addition-
to the complex of NT-NH
ally, it is worth pointing out no signal appeared at 651.2152 in
the mass spectrometry, which should belong to NT-NH -MR +
2
5
40 nm (Figure 2b), and the detection limit of NT-N -SP for
3
2
Na S was calculated to be 0.101 μM (Figure S1a) based on
Na S (Figure S7d). However, a noticeable signal peak was
2
2
32,33
IUPAC (CDL = 3S /m).
observed at m/z 698.1931 with the mixture of NT-NH -SP +
b
2
Probe NT-N -SP is essentially nonfluorescent at around 630
Na SO (Figure S7e). All these results were consistent with our
3
2
3
nm due to the ring-closed spiropyran moiety. In addition, the
presence of Na SO caused almost no changes in the absorption
hypothesis.
Next, fluorescence imaging in living cells was carried out.
2
3
and fluorescence spectra, suggesting that the caged compound is
stable and it will not transform into the fluorescence form
without UV irradiation. In contrast, upon 150 s UV irradiation, a
First, the cytotoxicity of NT-N -SP was evaluated by MTT assay
3
(Figure S8), and the result showed that NT-N -SP was well
3
suited for bioimaging applications. We then carried out
huge enhancement of NT-NH -MR in both the absorption and
fluorescence spectra was observed as shown in Figure S2a,b.
fluorescence imaging of exogenous H S and endogenous H S
(induced by SNP: sodium nitroprusside). As shown in Figure 3
2
2
2
B
DOI: 10.1021/acs.orglett.9b01879
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
light irradiation for 3 min (a1). Subsequently, we tested the
ability of NT-NH -MR for imaging endogenous SO induced by
2
2
lipopolysaccharide (LPS: 1 μg/mL). As shown in Figure 5 (b1),
Figure 3. Confocal microscopy images of living HepG 2 cells. (a1−a3)
Fluorescence imaging probe NT-N -SP (10 μM). (b1−b3) Fluo-
3
rescence imaging probe NT-N -SP (10 μM) with endogenous H S
3
2
(
(
induced by SNP). (c1−c3) Fluorescence imaging probe NT-N -SP
3
10 μM) with exogenous H S (Na S: 50 μM). Green channel λ = 530
Figure 5. Confocal microscopy images of Na SO in living HepG2 cells.
2 3
2
2
em
±
20 nm (λ = 458 nm).
(a1−a3) Cells were pretreated with probe NT-N -SP (10 μM) and
ex
3
then treated with UV irradiation. (b1−b3) Fluorescence imaging of
(b1), after pretreatment with SNP (500 μM) for 60 min, the
UV-activated probe NT-N -SP (10 μM) with endogenous SO
3
2
(
induced by LPS). (c1−c3) Fluorescence imaging of UV-activated
cells incubated with NT-N -SP showed an enhancement in
3
probe NT-N -SP (10 μM) with exogenous SO (Na SO : 50 μM). Red
fluorescence in green channel. Similarly, a remarkable enhance-
ment of the green fluorescence signal was observed when NT-
N -SP pretreated cells were further incubated with Na S for 20
3
2
2
3
channel λem = 630 ± 20 nm (λ = 561 nm).
ex
3
2
min, which indicates that NT-N -SP was capable of monitoring
upon pretreatment with LPS for 60 min, the fluorescence signal
3
both exogenous and endogenous H S in living cells.
of NT-NH -MR-loaded HepG 2 cells decreased in the red
2
2
Then NT-N -SP-loaded HepG 2 cells were treated with UV
channel. Also, upon incubation with Na SO for 20 min, the
3
2
3
light for conversion to the MR state. It is also known that UV
causes photobleaching of azide-containing dyes. As a result of
fluorescence intensity in the red channel decreased Figure 5
(c1). Hence, we speculate that NT-NH -MR was potentially
2
the Fo
̈
rster resonance energy transfer (FRET) from naph-
suitable for response of exogenous and endogenous SO in living
2
thalimide donor to the MR acceptor, it was found that HepG 2
cells showed negligible fluorescence in the green channel but a
strong fluorescence in the red channel after 3 min of UV
irradiation (Figure 4b). To our delight, after the visible-light
cells.
In order to further expand the biological applications of the
NT-N -SP, we next examined its capability for visualization of
3
H S and SO in a mouse model. As shown in Figure 6a1, no
2
2
fluorescence signals were observed in mice before injection of
the NT-N -SP and NT-NH -MR. Then the solutions of NT-N -
3
2
3
SP (100 μM) with or without UV irradiation were separately
injected into two groups of tested mice. As shown in Figure 6a2,
distinctive fluorescence signal appeared in the mice after
injection of NT-NH -MR in group 1. However, another group
2
Figure 4. Photochromism of probe NT-N -SP in living cells. (a1, a2)
3
HepG 2 cells were first incubated with probe NT-N -SP (10 μM). (b1,
3
b2): Cells were further treated with Na S (100 μM). (c1−f1, c2−f2):
2
UV/vis cycling of the two imaging channels of NT-NH -MR and NT-
2
NH -SP. Red channel λ = 630 ± 20 nm (λ = 561 nm), green channel
2
em
ex
λem = 535 ± 20 nm (λ = 458 nm).
ex
irradiation (10 min), the fluorescence was recovered due to the
reversible isomerization from the MR state back to the initial SP
state (Figure 4c). Furthermore, we obtained satisfactory
intracellular photochromic actions with alternate UV/vis
irradiation (Figure 4d,e). As a whole, these results clearly
Figure 6. Visual imaging of Na SO and Na S in mice model. (a1, b1)
2
3
2
Fluorescence imaging of the control group. Fluorescent signals of NT-
NH -MR 100 μM (a2) and NT-N -SP 100 μM (b2) in living mice.
validate the smart light-control of NT-N -SP by alternate UV/
3
2
3
vis irradiation in living cells.
Fluorescent signals of NT-NH -MR 100 μM (a2) and NT-N -SP 100
2
3
Eventually, the UV light-controlled conversion of NT-N -SP
3
μM (b2) upon injection of Na SO 200 μM (a3, a4) and Na S 200 μM
2
3
2
from the SP state to the activated MR state for SO detection was
2
(b3, b4) at 5 and 20 min, respectively. Group 1: red channel λem = 600 ±
20 nm (λ = 530 nm). Group 2: green channel λ = 540 ± 20 nm (λ =
carried out in HepG 2 cells. Probe NT-N -SP was first
3
ex
em
ex
internalized by HepG 2 cells and then treatment with UV
450 nm).
C
DOI: 10.1021/acs.orglett.9b01879
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
of mice per treatment with NT-N -SP exhibited a weak
(3) Fukuto, J. M.; Ignarro, L. J.; Nagy, P.; Wink, D. A.; Kevil, C. G.;
Feelisch, M.; Cortese-Krott, M. M.; Bianco, C. L.; Kumagai, Y.; Hobbs,
A. J.; Lin, J.; Ida, T.; Akaike, T. FEBS Lett. 2018, 592, 2140−2152.
3
fluorescent signal. After 15 min, we injected Na SO and Na S
2
3
2
to the two groups of mice and then anesthetized them for
fluorescence imaging. As time went by, the fluorescence
intensity in the mice gradually decreased with the injection of
Na SO , and remarkable enhancement of fluorescence signals
(4) Zhao, Y.; Henthorn, H. A.; Pluth, M. D. J. Am. Chem. Soc. 2017,
1
(
1
39, 16365−16376.
5) Cerda, M. M.; Zhao, Y.; Pluth, M. D. J. Am. Chem. Soc. 2018, 140,
2574−12579.
2
3
appeared in the other group which was treated with Na S.
2
(6) Du, Z. B.; Song, B.; Zhang, W. Z.; Duan, C. C.; Wang, Y. L.; Liu, C.
L.; Zhang, R.; Yuan, J. L. Angew. Chem., Int. Ed. 2018, 57, 3999−4004.
(7) Pardeshi, K. A.; Ravikumar, G.; Chakrapani, H. Org. Lett. 2018, 20,
4−7.
Therefore, we envision that NT-N -SP was suitable for
3
screening the increased concentration of H S and SO in living
2
2
mice models.
To conclusion, inspired by the light-controlled recognition
strategy, for the first time we have successfully engineered a
(8) Li, J. L.; Meng, Z. Q. Nitric Oxide 2009, 20, 166−174.
(9) Xu, W.; Teoh, C. L.; Peng, J. J.; Su, D. D.; Yuan, L.; Chang, Y. T.
Biomaterials 2015, 56, 1−9.
10) Chen, S. Y.; Huang, Y. Q.; Liu, Z. W.; Yu, W.; Zhang, H.; Li, K.;
Yu, X. Q.; Tang, C. S. Clin Sci. (Lond). 2017, 131, 2655−2670.
11) Xiao, J.; Zhu, X. Y.; Kang, B.; Xu, J. B.; Wu, L. H.; Hong, J.;
Zhang, Y. F.; Wang, Z. N. Cell Physiol Biochem 2015, 37, 2444−2453.
12) Chen, Q. H.; Zhang, L. L.; Chen, S. Y.; Huang, Y. Q.; Li, K.; Yu,
novel fluorescent probe NT-N -SP based on 4-azide-1,8-
3
(
naphthalic anhydride and spiropyran derivative. The azide
group in NT-N -SP could be reduced in the presence of H S,
3
2
(
and the activated spiropyran moiety could serve as a new SO2
recognition site upon UV radiation. Importantly, the “smart”
(
sensor NT-N -SP exhibits high selectivity and sensitivity for SO
3
2
X. Q.; Zhang, C. Y.; Tang, C. S.; Du, J. B.; Jin, H. F. Int. J. Cardiol 2016,
and H S, and it was also successfully applied to in vivo imaging in
225, 392−401.
2
living cells and mice. Relying on these merits, we believe that this
work provides a powerful tool for better understanding the
contributions of SO and H S in physiological and pathological
(13) Ji, K. X.; Xue, L.; Cheng, J. W.; Bai, Y. Brain Res. Bull. 2016, 121,
68−74.
(14) Zhang, D.; Wang, X. L.; Tian, X. Y.; Zhang, L. L.; Yang, G. S.;
2
2
Tao, Y. H.; Liang, C.; Li, K.; Yu, X. Q.; Tang, X. J.; Tang, C. S.; Zhou, J.
Front Immunol 2018, 9, 882−899.
processes.
(
15) Jimenez, D.; Martínez-Manez, R.; Sancenon, F.; Ros-Lis, J. V.;
̃
́ ́ ́
ASSOCIATED CONTENT
Supporting Information
■
Benito, A.; Soto, J. J. Am. Chem. Soc. 2003, 125, 9000−9001.
*
S
(16) Searcy, D. G.; Peterson, M. A. Anal. Biochem. 2004, 324, 269−
275.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b01879.
(
17) Radford-Knoery, J.; Cutter, G. A. Anal. Chem. 1993, 65, 976−
82.
18) de Macedo, A. N.; Jiwa, M. I. Y.; Macri, J.; Belostotsky, V.; Hill,
S.; Britz-McKibbin, P. Anal. Chem. 2013, 85, 11112−11120.
19) Jiang, G. W.; Li, M.; Wen, Y. Y.; Zeng, W. L.; Zhao, Q.; Chen, C.
L.; Yuan, H.; Liu, C. R.; Liu, C. L. ACS Sens 2019, 4, 434−440.
20) Filipovic, M. R.; Zivanovic, J.; Alvarez, B.; Banerjee, R. Chem. Rev.
018, 118, 1253−1337.
21) Ong, J. X.; Lim, C. S. Q.; Le, H. V.; Ang, W. H. Angew. Chem., Int.
Ed. 2019, 58, 164−167.
22) Zhang, W. J.; Huo, F. J.; Yin, C. X. J. Mater. Chem. B 2018, 6,
919−6929.
23) Jiang, M.; Gu, X.; Lam, J. W. Y.; Zhang, Y.; Kwok, R. T. K.; Wong,
K. S.; Tang, B. Z. Chem. Sci. 2017, 8 (8), 5440−5446.
24) Cheng, P. H.; Zhang, J. J.; Huang, J. G.; Miao, Q. Q.; Xu, C. J.; Pu,
9
(
Experimental procedures, additional UV−vis and fluo-
rescence spectra, and analytical and spectral data for all
compounds (PDF)
(
(
2
(
AUTHOR INFORMATION
■
Corresponding Author
(
6
(
*E-mail: yincx@sxu.edu.cn.
ORCID
Caixia Yin: 0000-0001-5548-6333
Notes
(
K. Y. Chem. Sci. 2018, 9, 6340−6347.
The authors declare no competing financial interest.
(25) Gao, M.; Yu, F. B.; Lv, C. J.; Choo, J. B.; Chen, L. X. Chem. Soc.
Rev. 2017, 46, 2237−2271.
(26) Thorson, M. K.; Majtan, T.; Kraus, J. P.; Barrios, A. M. Angew.
ACKNOWLEDGMENTS
■
(
Chem., Int. Ed. 2013, 52, 4641−4644.
We thank the National Natural Science Foundation of China
Nos. 21775096, 21672131, and 21705102), One hundred
(27) Brito da Silva, C.; Gil, E. S.; da Silveira Santos, F.; Moras, A. M.;
Steffens, L.; Bruno Goncalves, P. F.; Moura, D. J.; Ludtke, D. S.;
Rodembusch, F. S. J. Org. Chem. 2018, 83, 15210−15224.
(28) Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Chem. Rev.
people plan of Shanxi Province, Shanxi Province “1331 project”
key innovation team construction plan cultivation team (2018-
CT-1), Shanxi Province Foundation for Returness (2017-026),
the Shanxi Province Science Foundation for Youths (No.
2
(
014, 114, 12174−12277.
29) Dong, M.; Babalhavaeji, A.; Samanta, S.; Beharry, A. A.; Woolley,
G. A. Acc. Chem. Res. 2015, 48, 2662−2670.
2
01701D221061), Shanxi Collaborative Innovation Center of
(
(
30) Tian, Z.; Li, A. D. Q. Acc. Chem. Res. 2013, 46, 269−279.
High Value-added Utilization of Coal-related Wastes, China
Institute for Radiation Production and Scientific Instrument
Center of Shanxi University (201512). We also thank Dr. J. J.
Wang of Shanxi University for her assistance with confocal laser
scanning microscopy imaging.
31) Zhang, J. J.; Fu, Y. X.; Han, H. H.; Zang, Y.; Li, J.; He, X. P.;
Feringa, B. L.; Tian, H. Nat. Commun. 2017, 8 (1), 987−996.
(32) Zhang, W. J.; Liu, T.; Huo, F. J.; Ning, P.; Meng, X. M.; Yin, C. X.
Anal. Chem. 2017, 89, 8079−8083.
(33) Zhang, W. J.; Huo, F. J.; Liu, T.; Yin, C. X. J. Mater. Chem. B 2018,
6
, 8085−8089.
REFERENCES
■
(
1) Jiao, X. Y.; Li, Y.; Niu, J. Y.; Xie, X. L.; Wang, X.; Tang, B. Anal.
Chem. 2018, 90, 533−555.
2) Liu, C. R.; Chen, N. W.; Shi, W.; Peng, B.; Zhao, Y.; Ma, H. M.;
(
Xian, M. J. Am. Chem. Soc. 2014, 136, 7257−7260.
D
DOI: 10.1021/acs.orglett.9b01879
Org. Lett. XXXX, XXX, XXX−XXX