One-Pot, Metal- And Azide-Free Synthesis of 1,2,3-Triazoles from α-Ketoacetals and Amines

Article abstract of DOI:10.1055/s-0039-1691526

An efficient one-pot two-step synthesis of 1,4-disubstituted 1,2,3-triazoles from α-ketoacetals and amines is presented. The method does not use metals, azides, or oxidants, and is compatible with a variety of functional groups, including heterocycles, esters, nitriles, and carbamates.

Full text of DOI:10.1055/s-0039-1691526

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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–D
letter
A
en
L. R. Zehnder et al.
Letter
Synlett
One-Pot, Metal- and Azide-Free Synthesis of 1,2,3-Triazoles from
-Ketoacetals and Amines
Luke R. Zehndera
Joel M. Hawkinsb
Scott C. Sutton*a
O
1,2,3-triazole
R3
1. DMSO, r.t.
N
OMe
N
N
R1
+
TsNHNH2
2. R2NH2
80 °C
R1
R2
OMe
R3
a Oncology Chemistry, Pfizer Medicine Design, 10770 Science
Center Drive, San Diego, CA 92121, USA
scott.sutton@pfizer.com
b Pharmaceutical Sciences, Pfizer Inc., Eastern Point Road, Gro-
ton, CT 06340, USA
a-ketoacetal
alkyl
alkyl, aryl, hetaryl
20 examples
yields: 45–91%
Me, cyclic
Received: 16.10.2019
Accepted after revision: 19.11.2019
Published online: 03.12.2019
zones with primary amines at room temperature could lead
to regioselective formation of 1,4-disubstituted 1,2,3-tri-
azoles (Scheme 1a).7 Sakai’s reaction was found to be highly
chemoselective, but was not studied in detail until 2012,
when van Berkel et al. examined the mechanism, scope, and
limitations of this transformation.8 One reason for the lim-
ited adoption of Sakai’s work involves the fragile nature of
,-dichloro ketones and the limited number of robust
methods for preparing these base-sensitive intermediates.9
Recently, Anthore-Dalion and Zard developed a xanthate
functionalization/reductive dexanthylation route to ,-di-
chloro ketones.10 Their work is a rare example of a new
method for the synthesis of ,-dichloro ketones from read-
ily available building blocks.11 The sensitivity of ,-di-
chloro ketones originates from a base-catalyzed Favorskii
rearrangement leading to undesired byproducts.
DOI: 10.1055/s-0039-1691526; Art ID: ss-2019-v0564-l
Abstract An efficient one-pot two-step synthesis of 1,4-disubstituted
1,2,3-triazoles from -ketoacetals and amines is presented. The meth-
od does not use metals, azides, or oxidants, and is compatible with a
variety of functional groups, including heterocycles, esters, nitriles, and
carbamates.
Key words triazoles, ketoacetals, metal-free, azide-free, one-pot syn-
thesis, Sakai reaction
1,2,3-Triazoles are important heterocycles used in phar-
maceutical agents, in materials research, in ionic liquids,
and as synthetic intermediates.1 The most popular method
for synthesizing 1,2,3-triazoles involves the dipolar cy-
cloaddition between azides and alkynes, first described in
the 1960s by Huisgen et al., who used heat to drive the re-
action.2 Since that time, metal-catalyzed cycloadditions be-
tween azides and alkynes have been developed to an ad-
vanced state now known as click chemistry.3 The Cu-cata-
lyzed alkyne–azide cycloaddition reaction to form 1,4-
disubstituted 1,2,3-triazoles is the most common reaction,4
whereas Ru- and Ir-catalyzed alkyne–azide cycloaddition
reactions are employed for the formation of 1,5-disubstitut-
ed 1,2,3-triazoles.5 Metal-free synthetic methods are desir-
able to reduce waste-disposal costs and to eliminate the po-
tential for trace transition-metal impurities in commercial
products. Moreover, the toxicity and potential thermal haz-
ards of azides make them undesirable intermediates for
batch syntheses of 1,2,3-triazoles. There are several meth-
ods for constructing 1,2,3-triazoles without the use of
azides or transition metals. A review of these methods ap-
peared in 2017, and we refer the reader to this for a compi-
lation of metal- and azide-free triazole work before 2017.6
The seminal work by Sakai and co-workers in 1986 demon-
strated that condensation of ,-dichloro-N-tosylhydra-
(a) Synthesis of 1,2,3-triazoles from α,α-dichloro-N-tosylhydrazones
and amines (Sakai, Westermann, Wang)
NNHTs
N
base
N
N
Cl, F
R2NH2
R1
R1
+
R2
Cl, F
(b) Synthesis of 1,2,3-triazoles from acetophenones, TsNHNH2,
and amines (Zhang, et al.)
O
I2, DMSO
100 °C
N
N
N
R2NH2
+
TsNHNH2
+
Me
Ar1
Ar1
R2
mostly Ar
(c) this work (one-pot)
O
R3
1. r.t.
N
N
OMe
+ TsNHNH2
R1
R1
2. R2NH2
80 °C
N
R2
OMe
R3
alkyl
alkyl, aryl, hetaryl
Scheme 1 Synthesis of 1,2,3-triazoles under metal and azide-free con-
ditions
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–D
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
L. R. Zehnder et al.
Letter
Synlett
Much of the recent work on azide-free 1,2,3-triazole
synthesis has moved away from ,-dichlorohydrazones.
For example, Zhang and co-workers reported a 1,2,3-tri-
azole synthesis from N-tosylhydrazones and amines by us-
ing stoichiometric Cu(OAc)2 and, in work closer to ours, the
same group used I2 and TsNHNH2 in DMSO to oxidize meth-
yl ketones flanked by aryl groups to -oxidized tosylhydra-
zones that underwent a similar cyclization to that discussed
here (Scheme 1b).12 The use of alkyl amines in Zhang’s
work12b led to low yields of product and required an extra
oxidant (TBHP or Oxone) in addition to I2. Ji and co-workers
reported an I2/TBHP-mediated synthesis of 1,2,3-triazole by
using acetophenone-derived N-tosylhydrazones and ani-
lines, as well as -chlorocarbonyl-derived N-tosylhydra-
zones and anilines under aerobic conditions.13 Wang and
co-workers used ,-difluoro-N-tosylhydrazones and
amines to prepare 1,2,3-triazoles through treatment with t-
BuOLi in toluene at 100 °C.14 The strong base and high-tem-
perature conditions for the ,-difluoro reaction appeared
to limit the substrate scope to nonfunctionalized alkyl, aryl,
or pyridyl groups. Other metal-free triazole methods simi-
lar to the current work published since 2017 include (a) a
metal-free cascade [4+1] cyclization to give 4-aryl-NH-1,2,
3-triazoles from N-tosylhydrazones and sodium azide,15 (b)
a synthesis of 5-thiolated 1,2,3-triazoles from -thioenami-
nones through a diazo-transfer reaction under aqueous
conditions,16 (c) reactions of -haloacroleins with azides to
give formyl triazoles,17 (d) a catalyst-free route to 4-acyl-
NH-1,2,3-triazoles by water-mediated cycloaddition reac-
tions of enaminones and tosyl azide,18 (e) [3+2] annulation
of alkyl propiolates for the synthesis of 1,5- and 1,4-disub-
stituted 1,2,3-triazoles containing an ester group by using
tosyl azide or tosylhydrazine as the nitrogen source,19 and
(f) TBAI- or KI-promoted oxidative coupling of enamines
containing electron-withdrawing groups with N-tosylhy-
drazine to prepare triazoles.20 Of the six metal-free triazole
methods published since 2017, five used some form of
azide. We sought to expand upon Sakai’s original work, be-
cause it had the benefits of being mild, regiospecific, and
metal-, azide-, and oxidant-free, and it gave generally high
yields. We also wanted a reaction that would tolerate both
alkyl and aryl groups at the 1- and 5-positions of the tri-
azole to broaden the scope of existing methods. To this end,
our strategy was to employ a less-sensitive N-tosylhydra-
zone intermediate at the same oxidation state as the ,-di-
chloro-N-tosylhydrazone. Our approach involved replacing
the ,-dichloro functional group with an ,-dimethoxy or
,-diethoxy acetal (Scheme 1c). The expected stability of
the -ketoacetals to synthetic manipulation and the signifi-
cant number of known methods for preparing them21 in-
spired the research presented here.
after precipitation of the white solid from water. Treatment
of 2 with benzylamine in DMSO at 80 °C for four hours af-
forded the desired triazole 3 in 59% yield for an overall two-
step yield of 53%.
Having established that the reaction sequence shown in
Scheme 2 works, we sought to simplify the procedure by
preparing the N-tosylhydrazone and the 1,2,3-triazole in
the same reaction vessel. Telescoping the reaction sequence
of Scheme 2 afforded an 87% yield of 3a, supporting the ex-
pectation that, in most cases, yields from the two-step one-
pot method are at least equivalent to those from the step-
wise approach, if not better.
O
NNHTs
i
ii
N
MeO
MeO
N
TsNHNH2
+
CH3
N
Bn
CH3
OMe
OMe
H3C
1
2
3a
Scheme 2 Two-step process using -keto acetal 1. Reagents and condi-
tions: (i) DMSO, r.t., 89%; (ii) BnNH2, DMSO, 80 °C, 4 h (59% two steps;
87%, one-pot).
To establish the scope and limitations of the reaction,
we coupled a variety of primary amines and -ketoacetals.
Alkyl amines, anilines, and heterocyclic amines worked
well giving yields of 55–91% from the telescoped procedure
(Table 1).22 We observed that both dimethoxy and diethoxy
-ketoacetals work in this transformation, with the dime-
thoxy -ketoacetal 1 giving a slightly better yield of 3a (87%
as against 77% the diethoxy analogue). We also noted that
methyl substitution on the quaternary acetal carbon (R3 in
Table 1) works well, leading to trisubstituted 1,2,3-triazoles
with a methyl group at the 5-position (Table 1, entries 5
and 15). Weakly nucleophilic anilines and hetaryl amines
gave the corresponding products in yields of 55–70% (en-
tries 7–11, 13, 14). Functional groups, including esters, car-
bamates, carboxylic acids, and nitriles, were tolerated un-
der the mild conditions employed. A comparison of the -
ketoacetal approach with the ,-dichloro-N-tosylhydra-
zone method is shown in entry 9; we obtained a similar
yield (64%) to that achieved by van Berkel et al. (59%).8 Chi-
rality was maintained in the -amino ester example under
the mild reaction conditions (entry 16).
Table 1 Reaction Scope Using Various Amines and -Ketoacetalsa
O
R3
1. DMSO, r.t.
N
N
N
OMe
R1
+
TsNHNH2
R1
2. R2NH2
80 °C
R2
OMe
R3
Entry Product R1
R2NH2
BnNH2
R3
Yieldb
(%)
In our first attempt to test the feasibility of this ap-
proach, we used the commercial -ketoacetal 1. Treatment
of 1 with one equivalent of 4-methylbenzenesulfonohydra-
zide (TsNHNH2) in DMSO afforded hydrazone 2 in 89% yield
1
2
3
3a
3b
3c
Me
Me
Me
H
87
Ph(CH2)2NH2
Ph2CHNH2
H
H
56
80
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–D

C
L. R. Zehnder et al.
Letter
Synlett
Entry Product R1
R2NH2
R3
Yieldb
(%)
O
N
MeO
MeO
1. DMSO, r.t.
N
TsNHNH2
+
N
O
2. BnNH2
80 °C
Bn
4
3d
Me
H
61
N
NH2
3p (53%)
EtO
Scheme 3 Reagents and conditions: (i) DMSO, r.t., 30 min (hydrazone
formation not observed); (ii) BnNH2, DMSO, 80 °C, 4 h (53%).
5
6
7
8
3e
3f
Me
Me
Me
Me
Ph2CHNH2
t-BuO2CCH2NH2
PhNH2
Me
H
65
75
70
55
3g
3h
H
Finally, a tangible application of this reaction in the
preparation of an intermediate leading to the IRAK4 inhibi-
tor BMS-98623623 was examined to highlight the advantag-
es of this azide-free method (Scheme 4). The first-genera-
tion synthesis of BMS-986236 used an energetic azide in-
termediate, necessitating a second-generation synthesis for
scale-up that used a less-energetic hetaryl azide. Heating a
mixture of -ketoacetal 524 with TsNHNH2 and aniline 6 at
80 °C for six hours afforded a 51% yield of triazole 7.25 In
this reaction, mixing all three components together at the
beginning of the reaction resulted in slightly higher yields
compared with the usual stepwise formation of the N-to-
sylhydrazone at room temperature followed by addition of
the amine and then heating to 80 °C. It is conceivable that
the N-tosylhydrazone of 5 has a short half-life, even at room
temperature, and immediate consumption leading to tri-
azole 7 is beneficial to the yield. Conversion of 7 into 8 was
accomplished by inverse addition of the ester to MeMgBr in
THF at 0 °C, affording the penultimate intermediate leading
to BMS-986236. The 1H NMR of 8 generated in this way was
identical to the reported spectrum.
In conclusion, this 1,2,3-triazole synthesis represents a
convenient modification of the Sakai method and is expect-
ed to expand the scope of the reaction by avoiding the use
of base-sensitive ,-dichloro ketones.24 The scope of the
present reaction extends to alkyl, aryl, or hetaryl groups at
the 1-position and to alkyl groups at the 4-position. 2,2-
Diethoxyacetophenone was investigated (not shown), but
afforded an average of 30% yield after three replicates. Be-
cause the acetophenone scope is covered broadly by the
other methods cited in the introduction, we did not focus
our attention on the optimization of the acetophenone-de-
rived -ketoacetals. The orthogonal scope of the current
method complements previous work in the acetophenone
space. Finally, a scaled-up method not involving an azide
(or a metal) might offer advantages for the synthesis of
1,2,3-triazoles in terms of mitigation of thermal hazards
4-MeO2CC6H4NH2
H
9c
3i
3j
Me
Me
H
H
64
62
NH2
HO2C
NH2
10
N
N
11
12
3k
3l
Me
H
H
H
60
45
NH2
N
BnNH2
NH2
13
3m
Me
H
63
N
N
Bn
S
H3C
NH2
14
15
3n
3o
Me
Me
H
56
91
N
BnNH2
Me
O
16d
17
3q
3r
3s
Me
Me
Me
H
H
H
60e
75
EtO
Ph
NH2
NH2
NH2
18
72
NC
a Reaction conditions: (i) DMSO, TsNHNH2 (1.0 equiv), r.t., 1 h; (ii) amine,
80 °C, 4 h.
b Isolated yield.
c See ref. 8 for the Sakai reaction on this substrate.
d The HCl salt of the amine was used with one equivalent of pyridine.
e 95% ee.
A cyclic example, 2,2-dimethoxycyclohexan-1-one, was
surveyed to determine if the scope could be extended to the
formation of bicyclic products. Indeed, a 53% yield of the
5,6-fused heterocycle 3p was obtained in one step from
commercial starting materials (Scheme 3).
CO2Me
HO
O
HN
HN
MeMgBr
1. TsNHNH2, DMSO, r.t.
2.
MeO
OMe
THF, 0 °C
Cl
N
Cl
N
OMe
O
N
N
N
N
5
HN
Cl
N
N
8, 84%
7, 55%
N
H2N
6
Scheme 4 Azide-free synthesis of 8, leading to BMS-986236
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–D

D
L. R. Zehnder et al.
Letter
Synlett
and the elimination of trace metals. However, it is import-
ant to note that hydrazides, hydrazones, 1,2,3-triazoles, and
DMSO are known to have either low decomposition onsets,
high decomposition energies, or both. The safety of reac-
tions involving low-onset or high-energy compounds needs
to be evaluated on a case-by-case basis.
(17) Zhang, D.; Fan, Y.; Yan, Z.; Nie, Y.; Xiong, X.; Gao, L. Green Chem.
2019, 21, 4211.
(18) Yang, L.; Wu, Y.; Yang, Y.; Wen, C.; Wan, J.-P. Beilstein J. Org.
Chem. 2018, 14, 2348.
(19) Cao, S.; Liu, Y.; Hu, C.; Wen, C.; Wan, J.-P. ChemCatChem 2018,
10, 5021.
(20) Huang, W.; Zhu, C.; Li, M.; Yu, Y.; Wu, W.; Tu, Z.; Jiang, H. Adv.
Synth. Catal. 2018, 360, 3117.
(21) (a) Ayala-Mata, F.; Barrera-Mendoza, C.; Jiménez-Vázquez, H. A.;
Vargas-Díaz, E.; Zepeda, L. G. Molecules 2012, 17, 13864.
(b) Adamczyk, M.; Johnson, D. D.; Mattingly, P. G.; Pan, Y.;
Reddy, R. E. Synth. Commun. 2002, 32, 3199. (c) Smith, D. S.;
Winnick, J.; Ding, Y.; Bottomley, L. A. Electrochim. Acta 1997, 43,
335. (d) El-Wareth, A.; Sarhan, A. O.; Hoffmann, H. M. R. J. Prakt.
Chem./Chem.-Ztg. 1997, 339, 390. (e) Tamura, Y.; Annoura, H.;
Fuji, M.; Yoshida, T.; Takeuchi, R.; Fujioka, H. Chem. Pharm. Bull.
1987, 35, 4736.
Acknowledgment
We gratefully acknowledge the support of this research by Pfizer
management.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0039-1691526.
(22) Methyl 4-(4-Methyl-1H-1,2,3-triazol-1-yl)benzoate (3h);
Typical Procedure
S
u
p
p
orti
n
gInformati
o
n
S
u
p
p
orit
n
gInformati
o
n
TsNHNH2 (308 mg, 1.65 mmol) was added to a solution of 1,1-
dimethoxypropan-2-one (195 mg, 1.65 mmol) in DMSO (2 mL),
and the mixture was stirred for 1 h at r.t. Methyl 4-aminobenzo-
ate (262 mg, 1.74 mmol) was then added and the mixture was
heated at 80 °C for 4 h, then cooled. The solution was filtered
and purified by supercritical fluid chromatography (Princeton
HA-Morpholine column) to give a white solid; yield: 198 mg
(55%).
References and Notes
(1) Dheer, D.; Singh, V.; Shankar, R. Bioorg. Chem. 2017, 71, 30.
(2) (a) Huisgen, R.; Knorr, R.; Moebius, L.; Szeimies, G. Chem. Ber.
1965, 98, 4014. (b) Huisgen, R.; Szeimies, G.; Moebius, L. Chem.
Ber. 1967, 100, 2494.
(3) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem. Int. Ed.
2001, 40, 2004.
(4) (a) Tornøe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002,
67, 3057. (b) Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.;
Sharpless, K. B. Angew. Chem. Int. Ed. 2002, 41, 2596.
(5) (a) Zhang, L.; Chen, X.; Xue, P.; Sun, H. H. Y.; Williams, I. D.;
Sharpless, K. B.; Fokin, V. V.; Jia, G. J. Am. Chem. Soc. 2005, 127,
15998. (b) Ding, S.; Jia, G.; Sun, J. Angew. Chem. Int. Ed. 2014, 53,
1877. (c) Song, W.; Zheng, N. Org. Lett. 2017, 19, 6200.
(6) Ahmed, M.; Razaq, H.; Faisal, M.; Siyal, A. N.; Haider, A. Synth.
Commun. 2017, 47, 1193.
1H NMR (400 MHz, DMSO-d6):  = 8.66 (s, 1 H), 8.19–8.10 (m, 2
H), 8.08–7.98 (m, 2 H), 3.89 (s, 3 H), 2.34 (s, 3 H). 13C NMR (101
MHz, DMSO-d6):  = 165.3, 143.6, 139.9, 130.9, 128.9, 120.5,
119.4, 52.3, 10.4. HRMS (ESI): m/z [M
11H12N3O2: 218.0924; found: 218.0930.
+
H]+ Calcd for
C
(23) Arunachalam, P. N.; Kuppusamy, P.; Ganesan, S.;
Krishnamoorthy, S.; Nimje, R. Y.; Jarugu, L. B.; Kanikahalli Chik-
kananjaiah, N.; Anki Reddy, C.; Anjanappa, P.; Botlagunta, M.;
Vanteru, S.; Maddala, N.; Shankar, M.; Nair, S.; Hynes, J. Jr;
Santella, J. B. III; Carter, P. H.; Rampulla, R.; Vetrichelvan, M.;
Gupta, A.; Gupta, A. K.; Mathur, A. Org. Process Res. Dev. 2019,
23, 912.
(24) Tiecco, M.; Testaferri, L.; Tingoli, M.; Bartoli, D. J. Org. Chem.
1990, 55, 4523.
(25) Methyl 3-{1-[6-Chloro-4-(isopropylamino)pyridin-3-yl]-1H-
1,2,3-triazol-4-yl}propanoate (7)
(7) (a) Sakai, K.; Hida, N.; Kondo, K. Bull. Chem. Soc. Jpn. 1986, 59,
179. (b) Harada, K.; Oda, M.; Matsushita, A.; Shirai, M. Heterocy-
cles 1998, 48, 695.
(8) van Berkel, S. S.; Brauch, S.; Gabriel, L.; Henze, M.; Stark, S.;
Vasilev, D.; Wessjohann, L. A.; Abbas, M.; Westermann, B.
Angew. Chem. Int. Ed. 2012, 51, 5343.
To a solution of TsNHNH2 (98 mg, 0.53 mmol) were added ester
5 (100 mg, 0.55 mmol), prepared according to ref. 24, and
diamine 6 (100 mg, 0.53 mmol) in DMSO (1 mL), and the
mixture was heated at 80 °C for 6 h, then cooled. The resulting
solution was purified by reverse-phase chromatography [Tele-
dyne Isco ACCQPrep; Luna Omega 5m Polar C18 column (250 ×
30 mm), gradient elution: H2O–HOAc/MeCN]. The pure frac-
tions were combined, and MeCN was removed by evaporation
under reduced pressure. The resulting aqueous mixture was
frozen and lyophilized to give a white solid; yield: 91 mg (55%).
1H NMR (400 MHz, DMSO-d6):  = 8.30 (s, 1 H), 8.05 (s, 1 H),
6.91 (s, 1 H), 6.41 (br d, J = 7.9 Hz, 1 H), 3.86–3.77 (m, 1 H), 3.62
(s, 3 H), 3.03–2.96 (m, 2 H), 2.80–2.73 (m, 2 H), 1.14 (d, J = 6.4
Hz, 6 H). 13C NMR (101 MHz, DMSO-d6):  = 172.4, 151.3, 148.2,
145.9, 144.5, 123.5, 119.3, 105.2, 51.4, 43.4, 32.4, 21.6, 20.5.
HRMS (ESI): m/z [M + H]+ Calcd for C14H19ClN5O2: 324.1222;
found: 324.1223.
(9) Barluenga, J.; Llavona, L.; Concellón, J. M.; Yus, M. J. Chem. Soc.,
Perkin Trans. 1 1991, 297.
(10) Anthore-Dalion, L.; Zard, S. Z. Org. Lett. 2017, 19, 5545.
(11) Cho, E.; Kim, M.; Jayaraman, A.; Kim, J.; Lee, S. Eur. J. Org. Chem.
2018, 781.
(12) (a) Chen, Z.; Yan, Q.; Yi, H.; Liu, Z.; Lei, A.; Zhang, Y. Chem. Eur. J.
2014, 20, 13692. (b) Chen, Z.; Yan, Q.; Yi, H.; Liu, Z.; Lei, A.;
Zhang, Y. Chem. Eur. J. 2014, 20, 17635.
(13) (a) Cai, Z.-J.; Lu, X.-M.; Zi, Y.; Yang, C.; Shen, L.-J.; Li, J.; Wang, S.-
Y.; Ji, S.-J. Org. Lett. 2014, 16, 5108. (b) Bai, H.-W.; Cai, Z.-J.;
Wang, S.-Y.; Ji, S.-J. Org. Lett. 2015, 17, 2898.
(14) Zhou, Q.; Fu, Z.; Yu, L.; Wang, J. Asian J. Org. Chem. 2019, 8, 646.
(15) Shu, W.-M.; Zhang, X.-F.; Zhang, X.-X.; Li, M.; Wang, A.-J.; Wu,
A.-X. J. Org. Chem. 2019, 84, 14919.
(16) Deng, L.; Cao, X.; Liu, Y.; Wan, J.-P. J. Org. Chem. 2019, 84, 14179.
© 2019. Thieme. All rights reserved. Synlett 2019, 30, A–D