Potassium tert-Butoxide-Mediated Condensation Cascade Reaction: Transition Metal-Free Synthesis of Multisubstituted Aryl Indoles and Benzofurans

Article abstract of DOI:10.1021/acs.orglett.9b01093

An efficient and facile method to synthesize valuable disubstituted 2-aryl indoles and benzofurans in good yields has been demonstrated, based on a tert-butoxide-mediated condensation reaction involving a vinyl sulfoxide intermediate. Products are obtained from N- or O-benzyl benzaldehydes using dimethyl sulfoxide as a carbon source. The methodology features a wide functional group tolerance and transition metal-free environment. Preliminary mechanistic studies suggest that the reaction involves a tandem aldol reaction/Michael addition/dehydrosulfenylation/isomerization sequence through an ionic protocol.

Full text of DOI:10.1021/acs.orglett.9b01093

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Potassium tert-Butoxide-Mediated Condensation Cascade Reaction:
Transition Metal-Free Synthesis of Multisubstituted Aryl Indoles and
Benzofurans
Pengfei Yang, Weiyan Xu, Rongchao Wang, Min Zhang, Chunsong Xie, Xiaofei Zeng,
and Min Wang*
College of Materials, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou 311121, P. R. China
S
* Supporting Information
ABSTRACT: An efficient and facile method to synthesize
valuable disubstituted 2-aryl indoles and benzofurans in good
yields has been demonstrated, based on a tert-butoxide-
mediated condensation reaction involving a vinyl sulfoxide
intermediate. Products are obtained from N- or O-benzyl
benzaldehydes using dimethyl sulfoxide as a carbon source.
The methodology features a wide functional group tolerance
and transition metal-free environment. Preliminary mecha-
nistic studies suggest that the reaction involves a tandem aldol
reaction/Michael addition/dehydrosulfenylation/isomerization sequence through an ionic protocol.
ubstituted indoles1 and benzofurans2 are frequently
encountered in various biologically active natural products,
as well as in pharmaceuticals and other organic materials. Among
these, 2-aryl multisubstituted derivatives,3,4 especially indoles,
have been the subject of much cooperative research as a result of
the synthetic challenges that remain and the diverse biological
properties that can be obtained from these compounds (for
examples, seeScheme1). Traditionalsyntheticapproachestothe
reactions.8 However, in most cases, a stoichiometric amount of
an expensive metal additive or oxidant is required.
S
Recently, few examples of the synthesis of 2-arylindole
derivatives under transition metal-free conditions have been
reported. Onesuchmethodusesarynesasthearylsource, reacted
with amines, 2H-azirines, or hydrazines under Lewis acid or
Lewis acid-free conditions (Scheme 2a).9 However, epoxide has
been developed as C2 fragment to react with anilines forming 2-
arylindoles in the presence of Lewis acid (Scheme 2b).10 In
addition, the intramolecular amination reaction of anilines with
different oxidants was a direct approach to aryl indole synthesis
(Scheme2c).11 Veryrecently, potassiumtert-butoxide(t-BuOK)
was employed as an initiator in various types of coupling
reactions, which provided an effective method for the direct
silylation12 and arylation13 of indoles. Yan developed an
intramolecular coupling reaction of tertiary amines and ketones
witht-BuOKtoform2-arylindolesviaaradicalpathway(Scheme
2d).14 Even so, there remains a need to develop new, generic, and
convenient transition metal-free strategies for synthesizing aryl
indoles using other pathways.
Scheme 1. Selected Examples of Biologically Active
Compounds, Natural Products, and Pharmaceuticals
Meanwhile, sulfoxides are employed as versatile regents in
various organic transformations,15 such as Swern oxidation,
Pummererreaction, andCorey−Chaykovskyreaction. However,
a sulfoxide undergoes pyrolysis upon heating to form alkene and
sulfenic acid has been known for over a century,16,17 which needs
two steps for preparation (substitution and oxidation) from the
initial carbonyl compounds.18 Hence, more efficient methods
basedonsulfurstrategiesfortherapidsynthesisofalkenesarestill
required. Recently, some examples to formation of both alkenes
synthesis of 2-arylindole include the Fischer, Larock, and Bartoli
methods.5 Recently, the exploration of newer flexible method-
ologies for the highly efficient syntheses of these biologically
active structures has gained significant attention.6 Two main
strategies through direct C−H functionalization with transition
metals have been developed: direct coupling annulation of
anilines with C2 fragments (intermolecular and intramolecular)7
and direct arylation for indole modification via direct coupling
Received: March 28, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01093
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Synthesis of 2-Arylindoles under Transition Metal-
Free Conditions
Table 1. Optimization of the Reaction Conditions for the
a
Synthesis of 2a
b
entry
base (equiv)
temp (°C)
time (h)
yield (%)
1
2
3
4
5
6
7
8
t-BuOK (2.0)
NaH (2.0)
90
90
90
90
90
90
90
90
90
90
90
90
60
40
60
60
60
60
0.5
4
24
4
24
4
0.5
0.5
0.5
0.5
0.5
0.5
0.5
2
62
32
15
52
trace
42
80
81
80
KHMDS (2.0)
t-BuONa (2.0)
P4-t-Bu (2.0)
t-BuOK (1.0)
t-BuOK (3.0)
t-BuOK (4.0)
t-BuOK(2.5)
t-BuOK(2.5)
t-BuOK(2.5)
t-BuOK(2.5)
t-BuOK(2.5)
t-BuOK(2.5)
t-BuOK(2.5)
t-BuOK(2.5)
t-BuOK(2.5)
t-BuOK(2.5)
9
c
10
11
12
13
14
15
16
17
18
10
51
60
d
e
82
70
f
0.5
0.5
4
84
58
83
g
h
i
and aromatic compounds have been reported by using DMSO as
a carbon source.19,20 However, there are no examples of
dehydrosulfenylation approaches that proceed through the in
situ preparation of sulfoxides to generate heteroaromatic rings
starting from carbonyl compounds. As part of our continued
interest in benzylic C−H bond functionalization,21 we herein
report the transition metal-free condensation of N- or O-benzylic
C−H bonds with aldehydes using DMSO as the carbon source
through a cascade protocol including dehydrosulfenylation to
furnish 2-aryl-3-methylindoles and benzofurans in good yields.
Initially, 2-(benzyl(methyl)amino)benzaldehyde 1a was
selected as the model substrate to investigate the optimal
conditions to form disubstituted 2-aryl indoles via this process,
using DMSO as the solvent. Although this transformation may
appear simple, there are aspects that require careful consid-
eration. First, direct intramolecular coupling reactions between
the carbonyl group and the benzyl group to form the C3−H
indole under basic conditions could complete with the formation
of the desired C3−C indole product.14,22 Second, intermolecular
coupling reactions between the intermediate vinyl sulfoxide and
thenucleophile couldoccur.23 Finally, the radicalpathwaywith t-
BuOK may lead to the synthesis of other arylheterocycles,
especially in the presence of a tertiary amine.24 To our pleasure,
when 1a was reacted with DMSO using t-BuOK as the base, the
desiredannulation product2a wasobtained in62%yieldat 90 °C
for 0.5 h (Table 1, entry 1). After screening a series of inorganic
bases, t-BuOK proved to be the optimum candidate, as it resulted
in higher yields and shorter reaction times (entries 2−4). When
substituting the organic super base P4-t-Bu for t-BuOK, only a
small yield of the desired product 2a was observed (entry 5).25
Thereafter, the effect of the amount of base was examined, and it
was determined that 2.5 equiv of the base produced similar
resultstothoseobtainedfrom3.0or4.0equiv(entries7−9). The
results of solvent screening indicated that adopting a mixed
solvent approach also did not enhance the product yield (entries
10−12). The effect of reaction temperature was assessed, and the
data showed that the yield of 2a was increased to 82% when the
reaction was performed at 60 °C (entry 13).The reaction
0.5
83
a
Reaction conditions: 1a (0.2 mmol) and base in 2 mL of DMSO
were stirred at the desired temperature in a glass vial with cap.
b
c
d
Isolated yields. DMSO (1 mL), t-BuOH (2 mL). DMSO (1 mL),
e
f
toluene (2 mL). DMSO (1 mL), THF (2 mL). DMSO (3 mL).
g
h
i
DMSO (1 mL). DMSO (1 mL). With the presence of trace metals
from the t-BuOK and DMSO.
concentration also had an effect on the product yield, such that
employing 3 mL of DMSO gave the highest yield of 84%. Thus,
the reaction conditions listed in entry 15 of Table 1 were optimal.
In addition, applying these optimal conditions and in the
presence of trace metals from the t-BuOK and DMSO, the
desired product 2a was obtained in a similar yield (83%), which
ruled out a transition metal effect.
Subsequently, using the optimized conditions, the scope of
ortho-N-substitutedaryl aldehydestowhich thisprocesscould be
applied was investigated. As shown in Scheme 3, various aryl
aldehydes possessing both electron-withdrawing and electron-
donating substituents on the aromatic ring could participate in
this transformation to afford the desired products in moderate-
to-good yields. The reaction was found to proceed when using
benzaldehydesubstratessubstitutedindifferentpositions, togive
the corresponding C4, C5, or C6 substituted 3-methyl-2-phenyl
indoles (2b−2j). Notably, the effect of the electron densityof the
phenyl ring was evident because products 2c, 2e, and 2h
possessing electron-withdrawing groups, were isolated in lower
yields compared with those obtained using reactants possessing
electron-donating groups. The substrate scope was further
expanded to includediverse substituted tertiaryanilines. Tertiary
amines bearing o-, m-, or p-substituted phenyl rings in the benzyl
group were screened, and the results showed that products 2i−
2n could be obtained with 55−89% yields. Interestingly, the
reaction of 1m under optimized conditions yielded 2m at 89%. It
shouldbenotedthatthereactioninthepresenceoftertiaryamine
substrates possessing N-Et, N-Pr, N-Bu, or N-Bn substituents
proceeded smoothly to givethe desired N-alkyl indoles (2o−2r),
although a slight decrease in yield was observed. The
B
DOI: 10.1021/acs.orglett.9b01093
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Aryl Aldehyde Substrates Screening To Form Aryl
Indoles
Scheme 5. Derivatization of 2a and Gram-scale Reaction of 2r
a
Scheme 6. Experiments Used To Elucidate the Reaction
Mechanism
a
Reaction performed on a 0.2 mmol scale in a glass vial with cap;
yields of isolated products.
tetrahydroisoquinoline benzaldehyde 1s was also examined and
was found toproduce the corresponding product 2s in 78% yield.
Encouraged by these results, we further explored the scope of
this transformation while synthesizing benzofuran products by
slightly modifying the reaction conditions. As shown in Scheme
4, the desired product 4a could be obtained with 84% yield in the
Scheme 4. Aryl Aldehyde Substrates Screening To Yield Aryl
a
Benzofurans
diphenylethene to trap this transformation, the desired product
was observed with slightly decreased yields of 78% and 79%,
respectively. This result suggests that the reaction is based on a
base anion process, which is not in agreement with previous
studies of t-BuOK and tertiary amine systems.24 The reaction of
1a with DMSO-d6 was subsequently investigated to gain insight
into the carbon pathway. The desired products 2a-d2 and 2a-d3
were obtained in 60% and 19% yields, respectively. In addition,
the desired deuterated product was not detected following the
reaction of 2a with DMSO-d6 under standard conditions, even
prolonged reaction times. These results indicate that the methyl
group was obtained from DMSO but not from t-BuOK. When
the reaction was allowed to proceed for 5 min at room
temperature under standard conditions, the desired product 2a
wasobtainedin10%yieldalongwiththevinylsulfoxide6ain50%
yield. Furthermore, 2a was obtained when the key intermediate
6a was subjected to the same reaction conditions under an argon
atmosphere. Finally, when CH3SO2CH3 was substituted for
DMSO under standard conditions, a trace amount of 2a was
formed. It is therefore evident that an intermediate sulfoxide, but
not a sulfone, is the key factor in realizing the completion of the
overall reaction.23
a
Reaction performed on a 0.2 mmol scale in a glass vial with cap;
yields of isolated products.
presence of 2-(benzyloxy)benzaldehyde 3a using this t-BuOK/
DMSO system in conjunction with an increased temperature of
100 °C and 4.0 equiv of t-BuOK. Diverse 2-aryl benzofuran
scaffolds were also successfully constructed under these reaction
conditions, although slightly lower yields were observed when
the electron-donating group was changed to an electron-
withdrawing group.
To demonstrate the synthetic utility of this process,
derivatization reactions of 2,3-disubstituted indoles were
performed (Scheme 5). When the N-methyl-2,3-disubstituted
indole 2a was reacted with2-hydroxy-5-nitrobenzyl bromide, the
tetrahydrochromeno[2,3-b]indole 5 was obtained in 74%
yield.26 This t-BuOK-mediated condensation reaction could
also be performed on a gram scale with 2r in 60% yield.
To better understand the mechanism of this reaction, a series
of control experiments were designed and conducted (Scheme
6). Using the stoichiometric radical scavengers TEMPO or
Based on the experimental results and in combination with
previous literature, a plausible reaction mechanism is proposed,
starting from the methylsulfinyl carbanion species,27 as shown in
Scheme7. Thisnucleophilicreagentaddstothealdehydetoform
the β-hydroxyl sulfoxide. Thereafter, one molecule of water is
C
DOI: 10.1021/acs.orglett.9b01093
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(c) Liu, Y.; Ai, K.; Lu, L. Chem. Rev. 2014, 114, 5057−5115. (d) Taylor,
R. D.; MacCoss, M.; Lawson, A. D. G. J. Med. Chem. 2014, 57, 5845−
5859.
Scheme 7. Plausible Reaction Mechanism
(2) (a) Nevagi, R. J.; Dighe, S. N.; Dighe, S. N. Eur. J. Med. Chem. 2015,
97, 561−581. (b)Khanam, H.;Shamsuzzaman. Eur. J. Med. Chem. 2015,
97, 483−504. (c) Hiremathad, A.; Patil, M. R.; Chethana, K. R.; Chand,
K.; Santos, M. A.; Keri, R. S. RSC Adv. 2015, 5, 96809−96828.
(3) Yet, L. Privileged Structures in Drug Discovery: Medicinal Chemistry
and Synthesis, 1st ed.; John Wiley & Sons, Inc., 2018; pp 321−355.
(4) (a) Chand, K.; Hiremathad, A.; Singh, M.; Santos, M. A.; Keri, R. S.
́
Pharmacol. Rep. 2017, 69, 281−295. (b) Salome, C.; Narbonne, V.;
Ribeiro, N.; Thuaud, F.; Serova, M.; Gramont, A.; Faivre, S.; Raymond,
E.; Desaubry, L. Eur. J. Med. Chem. 2014, 74, 41−49.
(5) (a) Bartoli, G.; Dalpozzo, R.; Nardi, M. Chem. Soc. Rev. 2014, 43,
4728−4750. (b) Larock, R. C.; Yum, E. K.; Refvik, M. D. J. Org. Chem.
1998, 63, 7652−7662. (c) Larock, R. C.; Yum, E. K. J. Am. Chem. Soc.
1991, 113, 6689−6690. (d) Bartoli, G.; Palmieri, G.; Bosco, M.;
Dalpozzo, R. Tetrahedron Lett. 1989, 30, 2129−2132. (e) Robinson, B.
Chem. Rev. 1969, 69, 227−250. (f) Robinson, B. Chem. Rev. 1963, 63,
373−401.
released to yield the vinyl sulfoxide 6a through an aldol
condensation reaction. Subsequently, 6a undergoes a Michael
additionreaction toobtain the intermediate 6b, which eliminates
sulfenic acid (CH3SOH) at high temperatures by thermolysis to
form alkene 6c. Under basic conditions, this alkene rapidly
isomerizes to form the desired indole product 2a.
In summary, we have developed an efficient transition metal-
free method for the synthesis of important disubstituted 2-aryl-
indolesandbenzofurans, usingDMSOasthecarbonsource. This
t-BuOK-mediated process provides a straightforward protocol
that uses widely available starting materials. Various 2-aryl-3-
methylindoles and benzofurans were readily obtained in
moderate-to-good yields under mild conditions. It is important
to note that, in this reaction, DMSO functions not only as a
solvent but also as a reactant for the in situ generation of a
sulfoxide in the basic system. Mechanistic studies show that the
reaction proceeds via a four-step sequence through an anion
pathway that includes aldol condensation, Michael addition,
dehydrosulfenylation, and isomerization reactions.
(6) (a) Mancuso, R.; Dalpozzo, R. Catalysts 2018, 8, 458−458.
(b) Kumar, I.; Kumar, R.; Sharma, U. Synthesis 2018, 50, 2655−2677.
(c)Liang, T.;Li, J.;Liu,Y.;Wu,C.;Wang, J.;Lan,J.;Liu,K. YoujiHuaxue
2016, 36, 2619−2633. (d) Inman, M.; Moody, C. J. Chem. Sci. 2013, 4,
29−41. (e) Vicente, R. Org. Biomol. Chem. 2011, 9, 6469−6480.
(7) For selected intramolecular examples, see: (a) Andries-Ulmer, A.;
Brunner, C.;Rehbein, J.;Gulder, T. J. Am. Chem. Soc. 2018, 140, 13034−
13041. (b) Heckman, L. M.; He, Z.; Jamison, T. F. Org. Lett. 2018, 20,
3263−3267. (c) Yu, R.; Zeng, F. J. Org. Chem. 2018, 83, 323−329.
(d) Takeda, Y.; Kajihara, R.; Kobayashi, N.; Noguchi, K.; Saito, A. Org.
Lett. 2017, 19, 6744−6747. (e) Liu, W.-Q.; Lei, T.; Song, Z.-Q.; Yang,
X.-L.; Wu, C.-J.; Jiang, X.; Chen, B.; Tung, C.-H.; Wu, L.-Z. Org. Lett.
2017, 19, 3251−3254. (f) Alt, I. T.; Plietker, B. Angew. Chem., Int. Ed.
2016, 55, 1519−1522. (g) Wu, C.-J.; Meng, Q.-Y.; Lei, T.; Zhong, J.-J.;
Liu, W.-Q.; Zhao, L.-M.; Li, Z.-J.; Chen, B.; Tung, C.-H.; Wu, L.-Z. ACS
Catal. 2016, 6, 4635−4639. For selected intermolecular examples, see:
(h) Liu, Y.; Yang, Y.; Wang, C.; Wang, Z.; You, J. Chem. Commun. 2019,
55,1068−1071.(i)Zhou,J.;Li,J.;Li,Y.;Wu,C.;He,G.;Yang,Q.;Zhou,
Y.; Liu, H. Org. Lett. 2018, 20, 7645−7649. (j) Sengupta, D.;
Radhakrishna, L.; Balakrishna, M. S. ACS Omega 2018, 3, 15018−
15023. (k) Luo, Y.-G.; Basha, R. S.; Reddy, D. M.; Xue, Y.-J.; Chen, T.-
H.; Lee, C.-F. Org. Lett. 2018, 20, 6872−6876. (l) Ren, L.; Nan, G.;
Wang, Y.; Xiao, Z. J. Org. Chem. 2018, 83, 14472−14488. (m) Jie, L.;
Wang, L.; Xiong, D.; Yang, Z.; Zhao, D.; Cui, X. J. Org. Chem. 2018, 83,
10974−10984. (n) Vaitla, J.; Bayer, A.; Hopmann, K. Angew. Chem., Int.
Ed. 2017, 56, 4277−4281.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.9b01093.
Experimental procedures, characterization data, and
copies of 1H and 13C NMR spectra (PDF)
AUTHOR INFORMATION
(8) (a) Moncea, O.; Poinsot, D.; Fokin, A. A.; Schreiner, P. R.; Hierso,
J.-C. ChemCatChem 2018, 10, 2915−2922. (b) Nareddy, P.; Jordan, F.;
Szostak, M. Org. Lett. 2018, 20, 341−344. (c) Gemoets, H. P. L.; Kalvet,
I.; Nyuchev, A. V.; Erdmann, N.; Hessel, V.; Schoenebeck, F.; Noel, T.
Chem. Sci. 2017, 8, 1046−1055. (d) Duan, L.; Fu, R.; Zhang, B.; Shi, W.;
Chen, S.; Wan, Y. ACS Catal. 2016, 6, 1062−1074. (e) Sandtorv, A. H.
Adv. Synth. Catal. 2015, 357, 2403−2435. (f) Modha, S. G.; Greaney, M.
F. J. Am. Chem. Soc. 2015, 137, 1416−1419.
■
Corresponding Author
*E-mail: mwang@hznu.edu.cn.
ORCID
Xiaofei Zeng: 0000-0003-4222-1365
Min Wang: 0000-0002-8776-1560
Notes
(9) (a) Thangaraj, M.;Bhojgude, S. S.; Jain, S.; Gonnade, R. G.; Biju, A.
T. J. Org. Chem. 2016, 81, 8604−8611. (b) Hovey, M. T.; Check, C. T.;
Sipher, A. F.; Scheidt, K. A. Angew. Chem., Int. Ed. 2014, 53, 9603−9607.
The authors declare no competing financial interest.
́
(c)Fra,L.;Millan, A.;Souto,J.A.;Muniz, K.Angew.Chem.,Int.Ed. 2014,
̃
ACKNOWLEDGMENTS
■
53, 7349−7353. (d) Jang, Y. H.; Youn, S. W. Org. Lett. 2014, 16, 3720−
3723. (e) Bunescu, A.; Piemontesi, C.; Wang, Q.; Zhu, J. Chem.
Commun. 2013, 49, 10284−10286. (f) Yang, Q.-Q.; Xiao, C.; Lu, L.-Q.;
An, J.; Tan, F.; Li, B.-J.; Xiao, W.-J. Angew. Chem., Int. Ed. 2012, 51,
9137−9140. (g) McAusland, D.; Seo, S.; Pintori, G. D.; Finlayson, J.;
Greaney, M. F. Org. Lett. 2011, 13, 3667−3669. (h) Hong, D.; Chen, Z.;
Lin, X.; Wang, Y. Org. Lett. 2010, 12, 4608−4611.
We gratefully acknowledge the Natural Science Foundation of
China (No. 21372056), the PCSIRT (IRT 1231), and The
Pandeng Plan Foundation for Youth Scholars of College of
Material, Chemistry and Chemical Engineering, Hangzhou
Normal University.
REFERENCES
(10) (a) Xu, C.; Xu, J. J. Org. Chem. 2018, 83, 14733−14742.
(b) Rodrigues, I.; Bonnet-Delpon, D.; Begue, J. J. Org. Chem. 2001, 66,
2098−2103.
■
(1) (a) Konopelski, P.; Ufnal, M. Curr. Drug Metab. 2018, 19, 883−
890. (b) Sravanthi, T. V.; Manju, S. L. Eur. J. Pharm. Sci. 2016, 91, 1−10.
D
DOI: 10.1021/acs.orglett.9b01093
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(11) (a) Ma, A.-L.; Li, Y.-L.; Li, J.; Deng, J. RSC Adv. 2016, 6, 35764−
35770. (b)Li, Y.-L.; Li, J.;Ma, A.-L.; Huang, Y.-N.; Deng, J. J. Org. Chem.
2015, 80, 3841−3851. (c) Jang, Y. H.; Youn, S. W. Org. Lett. 2014, 16,
3720−3723.
(12) (a) Banerjee, S.; Yang, Y.-F.; Jenkins, I. D.; Liang, Y.; Toutov, A.
A.; Liu, W.-B.; Schuman, D. P.; Grubbs, R. H.; Stoltz, B. M.; Krenske, E.
H.; Houk, K. N.; Zare, R. N. J. Am. Chem. Soc. 2017, 139, 6880−6887.
(b) Liu, W.-B.; Schuman, D. P.; Yang, Y.-F.; Toutov, A. A.; Liang, Y.;
Klare, H. F. T.; Nesnas, N.; Oestreich, M.; Blackmond, D. G.; Virgil, S.
C.; Banerjee, S.; Zare, R. N.; Grubbs, R. H.; Houk, K. N.; Stoltz, B. M. J.
Am. Chem. Soc. 2017, 139, 6867−6879. (c) Toutov, A. A.; Liu, W.-B.;
Betz, K. N.; Fedorov, A.; Stoltz, B. M.; Grubbs, R. H. Nature 2015, 518,
80−84.
(13) Chen, J.; Wu, J. Angew. Chem., Int. Ed. 2017, 56, 3951−3955.
(14) Wei, W.-T.; Dong, X.-J.; Nie, S.-Z.; Chen, Y.-Y.; Zhang, X.-J.; Yan,
M. Org. Lett. 2013, 15, 6018−6021.
(15) (a) Wu, X.-F.; Natte, K. Adv. Synth. Catal. 2016, 358, 336−352.
(b) Jones-Mensah, E.; Karki, M.; Magolan, J. Synthesis 2016, 48, 1421−
1436.
(26) Douglas, N.; Neef, C. J.; Rogers, R. A.; Stanley, J. A.; Armitage, J.;
Martin, B.; Hudnall, T. W.; Brittain, W. J. J. Phys. Org. Chem. 2013, 26,
688−695.
(27) Corey, E. J.; Chaykovsky, M. J. Am. Chem. Soc. 1965, 87, 1345−
1353.
(16) (a) Field, L. Synthesis 1978, 10, 713−740. (b) Field, L. Synthesis
1972, 3, 101−133.
(17) (a) Moghaddam, F. M.; Baradjee, G. R. J. Sulfur Chem. 2005, 26,
325−329. (b) Blades, K.; Percy, J. M. Tetrahedron Lett. 1998, 39, 9085−
9088. (c) Moghaddam, F. M.; Ghaffarzadeh, M. Tetrahedron Lett. 1996,
37, 1855−1858. (d) Trost, B. M. Acc. Chem. Res. 1978, 11, 453−461.
(18)(a)Trost,B.M.;Salzmann,T.N.;Hiroi,K. J.Am. Chem. Soc.1976,
98,4887−4902.(b)Kingsbury,C.A.;Cram,D.J.J. Am.Chem.Soc.1960,
82, 1810−1819.
(19) (a) Liu, Y.-F.; Ji, P.-Y.; Xu, J.-W.; Hu, Y.-Q.; Liu, Q.; Luo, W.-P.;
Guo, C.-C. J. Org. Chem. 2017, 82, 7159−7164. (b) Zhao, X.-J.; Zhao, J.;
Sun, X.; Liu, J.-K.; Wu, B. Tetrahedron 2017, 73, 3463−3477. (c) Liu, Y.;
Zhan, X.; Ji, P.; Xu, J. W.; Liu, Q.; Luo, W.; Chen, T.; Guo, C. Chem.
Commun. 2017, 53, 5346−5349. (d) Wen, Z. K.; Liu, X. H.; Liu, Y. F.;
Chao, J. B. Org. Lett. 2017, 19, 5798−5801. (e) Pothikumar, R.; Sujatha,
C.; Namitharan, K. ACS Catal. 2017, 7, 7783−7787.
(20) (a) Wang, M.; Tang, B.-C.; Ma, J.-T.; Wang, Z.-X.; Xiang, J.-C.;
Wu, Y.-D.; Wang, J.-G.; Wu, A.-X. Org. Biomol. Chem. 2019, 17, 1535−
1541. (b) Liu, Y.; Hu, Y.; Cao, Z.; Zhan, X.; Luo, W.; Liu, Q.; Guo, C.
Adv. Synth. Catal. 2018, 360, 2691−2695. (c) Xu, C.; Jiang, S.-F.; Wen,
X.-H.; Zhang, Q.; Zhou, Z.-W.; Wu, Y.-D.; Jia, F.-C.; Wu, A.-X. Adv.
Synth. Catal. 2018, 360, 2267−2271. (d) Phanindrudu, M.; Wakade, S.
B.; Tiwari, D. K.; Likhar, P. R.; Tiwari, D. K. J. Org. Chem. 2018, 83,
9137−9143. (e) Jiang, T.-S.; Wang, X.; Zhang, X. Tetrahedron Lett.
2018, 59, 2979−2982. (f) Zheng, K.; Zhuang, S.; Shu, W.; Wu, Y.; Yang,
C.; Wu, A. Chem. Commun. 2018, 54, 11897−11900. (g) Jadhav, S. D.;
Singh, A. Org. Lett. 2017, 19, 5673−5676. (h) Wakade, S. B.; Tiwari, D.
K.;Ganesh, P. S. K. P.; Phanindrudu, M.; Likhar, P. R.;Tiwari, D. K. Org.
Lett. 2017, 19, 4948−4951.
(21) (a) Kong, S.; Zhang, L.; Dai, X.; Tao, L.; Xie, C.; Shi, L.; Wang, M.
Adv. Synth. Catal. 2015, 357, 2453−2456. (b) Jiao, Z.; Tu, Y.-Q.; Zhang,
Q.; Liu, W.-X.; Wang, S.-H.; Wang, M. Org. Chem. Front. 2015, 2, 913−
916. (c) Wang, M. ChemCatChem 2013, 5, 1291−1293.
(22) (a) Li, W.; Duan, Y.; Zhang, M.; Cheng, J.; Zhu, C. Chem.
Commun. 2016, 52, 7596−7599. (b) Jones, C. D. J. Org. Chem. 1972, 37,
3624−3625.
(23) (a) Chang, M. Y.; Chen, H.-Y.; Tsai, Y.-L. Org. Lett. 2019, 21,
1832−1836. (b) Li, J.; Liu, G.; Shi, L.; Xing, Q.; Li, F. Green Chem. 2017,
19, 5782−5788. (c) Yang, K.; Song, Q. Org. Biomol. Chem. 2015, 13,
2267−2272. (d) Zhang, Q.; Cai, S.; Li, L.; Chen, Y.; Rong, H.; Niu, Z.;
Liu, J.; He, W.; Li, Y. ACS Catal. 2013, 3, 1681−1684.
(24) (a) Wei, W.-T.; Liu, Y.; Ye, L.-M.; Zhang, X.-J.; Yan, M. Org.
Biomol. Chem. 2015, 13, 817−824. (b) Hu, W.; Lin, J.-P.; Song, L.-R.;
Long, Y.-Q. Org. Lett. 2015, 17, 1268−1271. (c) Chen, Y.; Chen, J.-H.;
Zhang, N.-N.; Ye, L.-M.; Zhang, X.-J.; Yan, M. Tetrahedron Lett. 2015,
56,478−481. (d)Chen, Y.Y.;Zhang,X. J.;Yuan, H.M.;Wei, W. T.;Yan,
M. Chem. Commun. 2013, 93, 10974−10976.
(25) (a) Kanazawa, C.; Goto, K.; Terada, M. Chem. Commun. 2009,
5248−5250. (b) Kraus, G. A.; Gupta, V. Tetrahedron Lett. 2009, 50,
7180−7183.
E
DOI: 10.1021/acs.orglett.9b01093
Org. Lett. XXXX, XXX, XXX−XXX