Carbene-Catalyzed Alkylation of Carboxylic Esters via Direct Photoexcitation of Acyl Azolium Intermediates

Article abstract of DOI:10.1021/acscatal.1c00165

A carbene-catalyzed reductive coupling reaction of carboxylic esters and substituted Hantzsch esters is disclosed. Key steps of this reaction include one-electron reduction of a carbene catalyst-bound acyl azolium intermediate to generate the corresponding radical intermediate for subsequent alkylation reactions. The reaction is promoted by irradiation with visible light without the involvement of transition-metal photocatalysts. Mechanistic studies suggest that direct photoexcitation of the in situ formed acyl azolium intermediate is likely responsible for this light-induced one-electron-reduction process. Photoexcitation converts the acyl azolium intermediate to a single-electron oxidant, enabling single-electron oxidation of Hantzsch esters to generate radical intermediates. Our reactions work well for a broad range of aryl carboxylic ester and Hantzsch ester substrates. Sophisticated structures, including those present in medicines, can be incorporated into ketone molecules using our approach via very mild conditions that tolerate various functional groups.

Full text of DOI:10.1021/acscatal.1c00165

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Research Article
Carbene-Catalyzed Alkylation of Carboxylic Esters via Direct
Photoexcitation of Acyl Azolium Intermediates
Shi-Chao Ren, Wen-Xin Lv, Xing Yang, Jia-Lei Yan, Jun Xu, Fang-Xin Wang, Lin Hao, Huifang Chai,*
Zhichao Jin, and Yonggui Robin Chi*
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ABSTRACT: A carbene-catalyzed reductive coupling reaction of carboxylic esters and substituted Hantzsch esters is disclosed. Key
steps of this reaction include one-electron reduction of a carbene catalyst-bound acyl azolium intermediate to generate the
corresponding radical intermediate for subsequent alkylation reactions. The reaction is promoted by irradiation with visible light
without the involvement of transition-metal photocatalysts. Mechanistic studies suggest that direct photoexcitation of the in situ
formed acyl azolium intermediate is likely responsible for this light-induced one-electron-reduction process. Photoexcitation converts
the acyl azolium intermediate to a single-electron oxidant, enabling single-electron oxidation of Hantzsch esters to generate radical
intermediates. Our reactions work well for a broad range of aryl carboxylic ester and Hantzsch ester substrates. Sophisticated
structures, including those present in medicines, can be incorporated into ketone molecules using our approach via very mild
conditions that tolerate various functional groups.
KEYWORDS: N-heterocyclic carbene, reductive-radical-coupling reaction, acyl azolium, photocatalyst-free, ketone synthesis
Hantzsch esters to form ketones in the presence of an Ir(III)
catalyst (Figure 1b).11 Key steps in Scheidt’s approach
involved an Ir(III) complex photocatalyst and light-promoted
generation of alkyl radical intermediates from the Hantzsch
ester substrates and single-electron reduction of the acyl
azolium intermediate to form acyl azolium radical intermedi-
ates enabled by the in situ-generated Ir(II) complex (Figure
1b). Around the same period, Studer reported NHC-mediated
three-component coupling of acyl fluorides, alkenes, and
Langlois reagent, in which the iridium photocatalyst-mediated
one-electron reduction of the NHC-bound acyl azolium
intermediate was postulated as a key step (Figure 1b).11b
arboxylic esters and related carbonyl compounds are
Cbasic building blocks and ubiquitous functional groups in
natural and non-natural molecules. The use of N-heterocyclic
carbenes (NHCs) as organic catalysts has been proved
effective in activating this class of molecules for diverse
transformations.1 Traditionally, NHC-catalyzed reactions are
designed based (or assumed to be based) on electron-pair
transfers as the key reaction steps.1 In recent years, single-
electron-transfer radical reactions mediated by NHCs have
received increasing attention, in part, due to their potential to
cover a broader range of substrates including otherwise inert
molecules.2 Till this point, the reported NHC-mediated radical
reactions are mainly based on single-electron oxidation of
aldehyde-derived Breslow acyl anion intermediates for further
reactions, as developed by Scheidt,3 Studer,4 our own
laboratory,5 Rovis,6 Sun,7 Ye,8 Ohmiya,9 and a few others10
(Figure 1a). In contrast, single-electron reduction of NHC-
bound azolium ester intermediates for radical reactions
remains less explored. (Figure 1a). Recently, Scheidt reported
NHC-mediated photoredox coupling of acyl imidazoles and
Received: January 13, 2021
Revised: February 10, 2021
Published: February 18, 2021
https://dx.doi.org/10.1021/acscatal.1c00165
© 2021 American Chemical Society
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Figure 1. Approaches of NHC-mediated radical reactions.
Our entry to this objective of one-electron reduction of an
NHC-bound acyl azolium intermediate (Figure 1a, right part)
was, in part, inspired by the tremendous success in the area of
light-induced photocatalyst-free reactions that proceed via
electron donor−acceptor (EDA) complex pathways12,13 or
direct photoexcitation of organic molecules/intermediates.14 In
particular, direct photoexcitation can convert organic mole-
cules/intermediates to the corresponding single-electron-trans-
fer reductants or oxidants. For example, Melchiorre and co-
workers reported direct photoexcitation of an electron-
deficient iminium ion for enantioselective β-alkylation of
enals, in which the excited state of the iminium ion acts as a
strong oxidant that removes an electron from Hantzsch
ester.14d The excited states of electron-rich imines, Hantzsch
esters, and alkylborates have also been utilized as single-
electron reductants by the Melchiorre14 and Ohmiya group.14i
As important intermediates in NHC catalysis, acyl azolium
intermediates have found wide application in catalytic organic
reactions.1 Here, we disclose that under the influence of visible
light and the NHC catalyst, aryl carboxylic esters can couple
with Hantzsch esters to form ketone products (Figure 1c).
Unlike the studies from Scheidt11a and Studer,11b our reaction
proceeds without the involvement of iridium or other metal
complexes as the photoredox catalysts. Mechanistic studies
suggest that direct photoexcitation of an electron-deficient acyl
azolium intermediate is likely responsible for its one-electron
reduction to generate the corresponding radical intermediate.
Specifically, photoexcitation converts the acyl azolium
intermediate to its excited state and thus acts as a single-
electron oxidant to trigger the single-electron-transfer process
with the electron-rich Hantzsch ester substrate.
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a
Table 1. Condition Optimization
b
entry
variation from standard conditions
yield [%]
c
1
none
70 (68)
2
0.2 equiv Cs2CO3
0
3
0.5 equiv Cs2CO3
22
4
5
6
7
8
9
K2CO3, Li2CO3, and Na2CO3 instead of Cs2CO3
K2CO3, Li2CO3, Na2CO3, and CH3CN as a solvent
K2CO3 and acetone as a solvent
DBU, DMAP, DIPEA, and DABCO instead of Cs2CO3
t-BuONa instead of Cs2CO3
B instead of A
0−trace
2−30
16
0
14
32
10
11
12
13
14
15
16
C instead of A
D instead of A
E instead of A
440, 456, and 467 nm instead of 427 nm
400 nm instead of 427 nm
without the NHC catalyst
without light irradiation (in dark), rt
without light irradiation (in dark), 80 °C
40
24
trace
66−69
0
0
0
0
17
a
Standard conditions: 1a (0.2 mmol), 2a (0.1 mmol), A (20 mol %), and Cs2CO3 (1.5 equiv) in DCE (1.5 mL), blue LED (Kessil PR160 series,
b
λmax = 427 nm), Ar, 30−40 °C, and 12 h. Nuclear magnetic resonance (NMR) yield using 1,1,2,2-tetrachloroethane as an internal standard.
c
Isolated yield is shown in parentheses.
We started to search for suitable radical coupling conditions
using 4-nitrophenyl carboxylic ester (1a),15 a readily available
and stable acyl azolium precursor, and Hantzsch ester (2a)16 as
the model substrate to form ketone product 3a (Table 1). One
acceptable condition that led to the formation of 3a in 68%
yield involved the use of azolium A9,16 as the NHC precatalyst
(20 mol %), Cs2CO3 as a base (150 mol %), and blue light-
emitting diode (LED) (λmax = 427 nm) as the visible-light
source (entry 1). The amount of Cs2CO3 was found to be
important, as decreasing its loading from 150 to 20 or 50 mol
% led to dramatic losses on reaction yields (entries 2 and 3).
Under the condition with 1,2-dichloroethane (DCE) as the
solvent, the use of K2CO3, Li2CO3, or Na2CO3 as the
carbonate sources led to little formation of the ketone product,
presumably due to the low solubilities of these bases in DCE
(entry 4). The desired radical coupling reaction under these
carbonates could be partially restored when DCE was replaced
by CH3CN or acetone as the solvent (entries 5 and 6).
Replacing the carbonates with organic bases [such as 1,8-
Diazabicyclo[5.4.0]undec-7-ene (DBU), 4-Dimethylaminopyr-
idine (DMAP), N, N-Diisopropylethylamine (DIPEA), and
Triethylenediamine (DABCO)] led to no formation of 3a
(entry 7). The use of t-BuONa as the base gave 3a in 14%
yield (entry 8). The steric and electronic natures of the NHC
catalysts have a clear influence on the reaction outcomes
(entries 9−12).9 Additionally, the influence of the illumination
wavelength was explored. The reactions, under irradiation of
different sources of visible light (λmax = 440, 456, and 467 nm)
gave similar yields with that at 427 nm (entry 13). In contrast,
LEDs (λmax = 400 nm) resulted in no product (entry 14). No
coupling reactions were observed in the absence of light or the
NHC precatalyst (entries 15−17).
With optimized conditions in hand, we set out to investigate
the generality of this NHC-catalyzed light-induced alkylation
reaction (Tables 2 and 3).
We first evaluated the scope of 4-nitrophenyl carboxylic
esters (Table 2). Various substituents on the aryl ring of the
ester substrates, such as halogen atoms (3b−3e), trifluor-
omethyl (3h), and cyano (3f) units, were all tolerated to give
the corresponding ketone products with moderate to good
yields. It is worth noting that due to the mild coupling
conditions, functional groups (such as esters and ketone
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a
Table 2. Scope of Carboxylic Esters
a
Reaction conditions: 2 (0.1−0.2 mmol), 1 (2.0 equiv), A (20 mol %), and Cs2CO3 (1.5 equiv) in 1,2-dichloroethane (1.5 mL), blue LED (Kessil
b
PR160 series, λmax = 427 nm), Ar atmosphere, 30−40 °C, and 12 h. 4.0 equiv of Cs2CO3 was used.
moieties) typically incompatible with a traditional ketone
synthesis method such as Grignard reactions were well
tolerated in our approach (3a and 3g). Heteroaryl carboxylic
esters were effective substrates as well (3i−3k). The electronic
property and the substitution patterns of the substituents on a
benzene ring have a considerable influence on the reaction
outcomes. The use of benzoic ester (with an unsubstituted
benzene ring) could give the corresponding ketone product 3l
in an encouraging yield. Placing a methyl substituent on the
meta-carbon of the benzene ring led to 3m with 41% yield.
However, when the electron-releasing methyl substituent was
placed on the para-carbon of the benzene ring of the ester
(3n), a sharp drop in the reaction yield (<5%) was observed
with the use of the standard condition (1.5 equiv of Cs2CO3).
To our delight, the yield could be improved to 31% yield when
a large excess of Cs2CO3 (6.0 equiv) was used. The reason
regarding the beneficial effects from excess Cs2CO3 remains
unclear at this point. Replacing the methyl substituent with a
stronger electron-releasing methoxyl (CH3O−) unit led to
nearly a complete loss of the radical coupling reactions even in
the presence of 6 equiv of Cs2CO3 (3o).
substituents on a benzene ring were tolerated to give the
corresponding ketone products with moderate to good yields,
regardless of their electronic nature (4a−4f). The substituent
on the meta position of the benzene ring was also tolerated,
giving the ketone product with moderate yield (4g). The
methyl group of 2a could be replaced with other alkyl
substituents such as an ethyl (4h) or n-butyl (4i) unit without
affecting the reaction yield. Replacing the methyl group of 2a
with a phenyl unit led to a dropped yield (4j). The phenyl
group of 2a could be switched into a 2-naphthalene group to
give 4k with 63% yield. Additionally, cyclic alkyl units such as
cyclohexyl and cyclopentyl could also be installed to the
Hantzsch esters to give the corresponding ketone products
with moderate to high yields (4l and 4m). It should be noted
that alkenes and primary alkyl halides were well tolerated
under our conditions, affording the ketone products (4a and
4n) bearing readily transferrable functional groups. Substrates
bearing alkyl substituents other than a benzyl moiety are
incompatible with this method, presumably due to the stability
of the corresponding radicals.
Our protocol could also be used to install functional groups
to complex molecules (Table 4). For example, probenecid is a
medicine for the treatment of psoriasis, acne, and photo-
We then examined the scope of the 4-substituted Hantzsch
esters using 1a as a model ester substrate (Table 3). Various
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a
Table 3. Scope of 4-Substituted Hantzsch Esters
a
Reaction conditions: 2 (0.1−0.2 mmol), 1 (2.0 equiv), A (20 mol %), and Cs2CO3 (1.5 equiv) in 1,2-dichloroethane (1.5 mL), blue LED (Kessil
b
PR160 series, λmax = 427 nm), Ar atmosphere, 30−40 °C, and 12 h. 6.0 equiv of Cs2CO3 was used.
a
Table 4. Coupling of Medicinal Fragments
a
Reaction conditions: 2 (0.05−0.2 mmol), 1 (2.0 equiv), A (20 mol %), and Cs2CO3 (1.5 equiv) in 1,2-dichloroethane (1.5 mL), blue LED (Kessil
b
PR160 series, λmax = 427 nm), Ar atmosphere, 30−40 °C, and 12 h. 6.0 equiv of Cs2CO3 was used.
damage. The ester of probenecid could be readily converted to
the corresponding ketone product (5) using our method.
Similar transformations could be performed for many other
drugs (such as tazarotene) containing carboxylic acids or their
derivatives to give various ketone adducts (6). The drug
molecules (such as flurbiprofen) may also be incorporated into
the Hantzsch ester substrate and thus be transferred to the
corresponding ketone adduct (7). Our method also allows for
direct coupling of two medicinal fragments to form a new
ketone entity that may show alternative activities. Here, we
showed that the carboxylic ester from tazarotene could couple
with Hantzsch ester bearing the key fragment of flurbiprofen
(8). This study indicates that our method can likely be used to
readily assemble complex molecules.
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Scheme 1. Synthetic Transformation of Our Ketone Product
Figure 2. (a) LEDs of 467 nm with a band pass at 450 nm were used; (b) acyl azolium intermediate (I) was prepared using the corresponding acyl
chloride and NaH, see the Supporting Information for details; (c) LEDs of 400 and 467 nm (with a band pass at 450 nm) were used, respectively;
(d) UV−vis absorption spectra of 2a (10−4 M in DCE, black line) and emission spectra of LEDs (red, blue and green lines); (e) UV−vis
absorption spectra of acyl azolium I (10−3 M in DCE, red line) and 2a (10−3 M in DCE, black line). Fluorescence spectrum of acyl azolium I (10−3
M in DCE, blue line); (f) Lambert−Beer linear correlation experiments, see the Supporting Information for details; and (g) cyclic voltammograms
of the preformed acyl azolium intermediate I (0.001 M) in [0.1 M] TBAPF6 in CH3CN. Sweep rate: 100 mV/s. A Pt electrode was used as a
working electrode, a calomel electrode as a reference electrode, and a Pt wire as an auxiliary electrode.
The ketone product from our catalytic reaction can undergo
further transformations to prepare bioactive molecules. For
example, product 4n was transformed to an analogue of a
serotonin 5HT1A receptor antagonist17 (9) in one step with
67% yield (Scheme 1).
Multiple experiments were conducted to gain insight into
the reaction mechanism. When carboxylic ester 1b was
employed to react with Hantzsch ester 2b, the desired ketone
(10) was generated in 36% yield. A small amount of
dimerization adduct (11, Figure 2a) was formed from self-
coupling of the Hantzsch ester-derived radical intermediate,
suggesting that our reaction proceeds through a radical
pathway. Considering the ability of carbonate18 and Hantzsch
ester13i,l to form an electron donor−acceptor (EDA) complex
with electron-deficient aromatic rings, we first proposed that an
EDA complex between acyl azolium (I) and carbonate anion
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Scheme 2. Plausible Reaction Pathway
or Hantzsch ester is responsible for this radical reaction.
However, direct irradiation of a solution of preformed acyl
azolium (I) and Hantzsch ester (2a) in DCE under the light of
λ > 450 nm without the presence of Cs2CO3 (Figure 2b)
resulted in 76% yield of the desired product (3a) with 90% of
the NHC precatalyst recovered. Further UV−vis absorption
experiments showed that no EDA complex was formed
between acyl azolium (I) and Hantzsch ester (see Figure
S10 for details). These results exclude the possible pathway via
an EDA complex involving Cs2CO3 or Hantzsch ester (2a).
Instead, the results point toward a reaction pathway with direct
photoexcitation of acyl azolium (I) or Hantzsch ester (2a).
We next performed experiments to exclude photoexcitation
of Hantzsch ester (2a) as the driving force for our reaction.
Hantzsch ester (2a) can be excited (to behave as reductant)
under the light of a shorter wavelength (around 400 nm).14h
However, it has little absorption at the visible-light region (λ >
420 nm).14g This was further confirmed by the emission
spectra of LEDs and the absorption spectrum of 2a (Figure
2d). However, our reaction works well (Figure 2c) under the
irradiation of long-wavelength visible lights (λ > 450 nm),
where Hantzsch ester has no absorption (Figure 2d, green
line).19 In contrast, using LEDs of λmax = 400 nm, with the
emission region overlapping lightly with the strong absorption
region of 2a (Figure 2d, black and red lines), did not result in
We then turn our attention to investigate the photophysical
behaviors of preformed acyl azolium intermediate (I) to
evaluate the feasibility of its direct photoexcitation. The UV−
vis absorption spectrum of acyl azolium (I) revealed a
significant absorption of visible light, and the tail wavelength
reached over 520 nm (Figure 2e, red line). The absorption
spectra of preformed I were measured at different concen-
trations in DCE. The absorbances showed a typical Lambert−
Beer linear correlation with the concentrations (Figure 2f).
The corresponding emission spectrum of I upon excitation at
400 nm was also recorded (Figure 2e, a blue dotted line). A
cyclic voltammetry experiment was used to measure the redox
potential of ground state of I (Figure 2g). The cyclic
voltammogram of preformed intermediate I (as a solution in
MeCN) features a reversible peak at E1/2 = −0.48 V vs a
saturated calomel electrode (SCE), which could be attributed
to the redox couple of acyl azolium (I) and its reduced radical
intermediate.20 With the UV−vis, fluorescence, and cyclic
voltammetry data in hand, the excited state potential of I was
estimated to be +1.9 V vs SCE (see the Supporting
Information for details).21 This redox potential (+1.9 V) is
higher than that of the Hantzsch ester (2a, Eox = +1.1 V vs
SCE, see Figure S8 for details), indicating that thermodynami-
cally single-electron-transfer (SET) oxidation of 2a by I (at its
excited state) is feasible.
To provide further evidence in supporting the photo-
excitation of acyl azolium (I), we conducted the Stern−Volmer
quenching experiments. N-Methyl-N-((trimethylsilyl)methyl)-
aniline (Eox = +0.62 V vs SCE, see Figure S9 for details), which
radical coupling product (3a). Under this condition (λmax
=
400 nm), 2a decomposed completely to form the correspond-
ing pyridine and alkane (Figure 2c). These results (Figure 2c)
suggest that direct excitation of the Hantzsch ester was not
responsible for the radical coupling reactions.
cannot react with the ground state of acyl azolium I (E1/2
=
−0.48 V vs SCE), was chosen as the quenching agent.22 It was
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found that N-methyl-N-((trimethylsilyl)methyl)aniline could
effectively quench the emission of I (see Figure S9 for details),
supporting that the acyl azolium I at its excited state can
behave as an effective oxidant.
Nanyang Technological University, Singapore 637371,
Singapore
Wen-Xin Lv − Division of Chemistry & Biological Chemistry,
School of Physical & Mathematical Sciences, Nanyang
Technological University, Singapore 637371, Singapore
Xing Yang − Division of Chemistry & Biological Chemistry,
School of Physical & Mathematical Sciences, Nanyang
Technological University, Singapore 637371, Singapore;
orcid.org/0000-0003-4156-2061
Jia-Lei Yan − Division of Chemistry & Biological Chemistry,
School of Physical & Mathematical Sciences, Nanyang
Technological University, Singapore 637371, Singapore
Jun Xu − Division of Chemistry & Biological Chemistry, School
of Physical & Mathematical Sciences, Nanyang Technological
University, Singapore 637371, Singapore; School of
Pharmacy, Guizhou University of Traditional Chinese
Medicine, Guiyang 550025, China
Fang-Xin Wang − Division of Chemistry & Biological
Chemistry, School of Physical & Mathematical Sciences,
Nanyang Technological University, Singapore 637371,
Singapore
Lin Hao − Division of Chemistry & Biological Chemistry,
School of Physical & Mathematical Sciences, Nanyang
Technological University, Singapore 637371, Singapore;
orcid.org/0000-0001-9221-5751
Based on the results from the mechanistic studies above, a
plausible reaction pathway is proposed (Scheme 2). The
reaction starts with addition of an NHC catalyst to the
carboxylic ester (1a) to generate an electron-deficient acyl
azolium intermediate I.15 Photoexcitation converts intermedi-
ate I to its electronically excited state (I*) that can act as a
single-electron oxidant (E1/2 = +1.9 V vs SCE). A subsequent
single-electron transfer between electron-rich Hantzsch ester
(2a, Ered = +1.1 V vs SCE) and the excited acyl azolium (I*)
leads to a Hantzsch ester-derived radical cation II and an
NHC-bound radical intermediate III. This radical cation (II)
undergoes a homolytic C−C bond cleavage to generate an
alkyl radical intermediate IV. Subsequent radical coupling
between the alkyl radical (IV) and the NHC-bound radical
(III) intermediate eventually affords the desired ketone
product (3a) and regenerates the carbene catalyst.
In conclusion, we have developed NHC-catalyzed light-
induced alkylation of aryl carboxylic esters with 4-substituted
Hantzsch esters. A transition-metal photocatalyst is not
involved in the photopromoted process. Instead, the direct
excitation of an acyl azolium intermediate contributes to a
visible-light-induced one-electron-transfer process that reduces
an acyl azolium intermediate to the corresponding radical
species for subsequent coupling reactions. The reaction
conditions are very mild and various functional groups are
well tolerated. Sophisticated ketone products, including those
bearing one or two medicinal fragments, can be readily
prepared. Our study provides a new approach in NHC-
catalyzed reductive-radical-coupling reactions. Additional
mechanistic studies, including density functional theory
(DFT) calculation, are in progress in our laboratories.
Zhichao Jin − Laboratory Breeding Base of Green Pesticide
and Agricultural Bioengineering, Key Laboratory of Green
Pesticide and Agricultural Bioengineering, Ministry of
Education, Guizhou University, Guiyang 550025, China;
orcid.org/0000-0003-3003-6437
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c00165
Notes
The authors declare no competing financial interest.
ASSOCIATED CONTENT
■
ACKNOWLEDGMENTS
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c00165.
The authors thank Dr. Wangsheng Liu (NTU) for assistance
with cyclic voltammetry experiments. The authors acknowl-
edge financial support from the Singapore National Research
Foundation under its NRF Investigatorship (NRF-NRFI2016-
06), the Ministry of Education, Singapore, under its MOE
AcRF Tier 1 Award (RG108/16, RG5/19, and RG1/18),
MOE AcRF Tier 3 Award (MOE2018-T3-1-003), the Agency
for Science, Technology and Research (A*STAR) under its
A*STAR AME IRG Award (A1783c0008 and A1783c0010),
the GSK-EDB Trust Fund, Nanyang Research Award Grant,
Nanyang Technological University, the National Natural
Science Foundation of China (21772029, 21801051
21961006, 82360589, and 81360589), The 10 Talent Plan
(Shicengci) of Guizhou Province ([2016]5649), the Guizhou
Province Returned Oversea Student Science and Technology
Activity Program [(2014)-2], the Science and Technology
Department of Guizhou Province ([2018]2802 and [2019]
1020), the Program of Introducing Talents of Discipline to
Universities of China (111 Program, D20023) at Guizhou
University, the Guizhou Province First-Class Disciplines
Project [(Yiliu Xueke Jianshe Xiangmu)-GNYL(2017)008],
the Guizhou University of Traditional Chinese Medicine
(China), and Guizhou University.
Experimental procedures, analytical and spectroscopic
data for new compounds, and copies of NMR (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
Huifang Chai − School of Pharmacy, Guizhou University of
Traditional Chinese Medicine, Guiyang 550025, China;
Email: hfchai@126.com
Yonggui Robin Chi − Division of Chemistry & Biological
Chemistry, School of Physical & Mathematical Sciences,
Nanyang Technological University, Singapore 637371,
Singapore; Laboratory Breeding Base of Green Pesticide and
Agricultural Bioengineering, Key Laboratory of Green
Pesticide and Agricultural Bioengineering, Ministry of
Education, Guizhou University, Guiyang 550025, China;
orcid.org/0000-0003-0573-257X; Email: robinchi@
ntu.edu.sg
Authors
Shi-Chao Ren − Division of Chemistry & Biological
Chemistry, School of Physical & Mathematical Sciences,
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