2 H-Chromene-3-carboxylic Acid Synthesis via Solvent-Controlled and Rhodium(III)-Catalyzed Redox-Neutral C-H Activation/[3 + 3] Annulation Cascade

Article abstract of DOI:10.1021/acs.orglett.8b01477

An efficient and redox-neutral synthesis of 2H-chromene-3-carboxylic acids from N-phenoxyacetamides and methyleneoxetanones has been realized via a solvent-controlled and rhodium(III)-catalyzed C-H activation/unusual [3 + 3] annulation sequence. This tran

Full text of DOI:10.1021/acs.orglett.8b01477

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
2H‑Chromene-3-carboxylic Acid Synthesis via Solvent-Controlled
and Rhodium(III)-Catalyzed Redox-Neutral C−H Activation/[3 + 3]
Annulation Cascade
Zhi Zhou,†,§ Mengyao Bian,†,§ Lixin Zhao,†,§ Hui Gao,† Junjun Huang,† Xiawen Liu,† Xiyong Yu,*,†
Xingwei Li,‡ and Wei Yi*,†
†Key Laboratory of Molecular Target & Clinical Pharmacology, School of Pharmaceutical Sciences & the Fifth Affiliated Hospital,
Guangzhou Medical University, Guangzhou, Guangdong 511436, China
‡Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
S
* Supporting Information
ABSTRACT: An efficient and redox-neutral synthesis of 2H-chromene-3-
carboxylic acids from N-phenoxyacetamides and methyleneoxetanones has
been realized via a solvent-controlled and rhodium(III)-catalyzed C−H
activation/unusual [3 + 3] annulation sequence. This transformation represents
the first example of using an α-methylene-β-lactone unit as the three-carbon
source in transition-metal-catalyzed C−H activations through selective alkyl C−
O bond cleavage. Synthetic applications and mechanistic details, including
further derivatization of 2H-chromene-3-carboxylic acids, the isolation and
identification of a five-membered rhodacycle, as well as the theoretical studies for
reasoning a plausible Rh(III)−Rh(V)−Rh(III) process, have also been discussed.
product class was obtained through the O−N bond cleavage.
ransition-metal-catalyzed cascade C−H activation/annu-
Tlations have emerged as a powerful and straightforward Clearly, more efforts are still needed to explore new, redox-
neutral and O-NHAc-directed transition-metal-catalyzed cas-
cade C−H activation/annulations for direct synthesis of other
key structural motifs such as 2H-chromene, for which, to the
best of our knowledge, there has been only one elegant Rh(III)-
catalyzed example documented in the literature using highly
strained unsaturated cyclopropenes as the three-carbon
components.6b
On the other hand, more recently, Howell’s group disclosed a
highly selective Pd-catalyzed ring opening of methyleneoxeta-
nones with amines to give β-hydroxy amides through an acyl C−
O bond cleavage pathway (Scheme 1, eq 1).14 Almost
simultaneously, Li’s group also revealed a Rh(III)-catalyzed
C−H allylation of (hetero)arenes with methyleneoxetanones to
deliver acrylic acid derivatives via ortho-C−H activation,
followed by selective olefin insertion and β-oxygen elimination
(unusual alkyl C−O bond cleavage pathway) (Scheme 1, eq
2).15
Inspired by these developments, and in consideration of the
reactivity of N-phenoxyacetamide substrates in C−H function-
alization reactions, we reasoned that a promising transition-
metal-catalyzed cascade C−H activation/annulation between
N-phenoxyacetamides and methyleneoxetanones can be
reached under proper reaction conditions as the highly reactive
and negatively charged phenol species is easy to produce via
ortho-C−H alkenylation of the α-methylene part/the cleavage of
synthetic strategy for the one-pot assembly of many important
structural motifs, such as ubiquitous nitrogen-containing
heterocycle scaffolds, in an atom-economic fashion.1 In general,
the compulsive use of a combination of traditional directing
groups (DGs) and stoichiometric amounts of external oxidants
is required to achieve the above regioselective C−H activation/
annulation reactions, which to some extent hindered their
synthetic applications.
To conquer the aforementioned limitation, recently, the
development of an autocleavable oxidizing directing group
(ODG) that acts simultaneously as both the DG and internal
oxidant and then to be incorporated into the structural backbone
of the product or transformed into a highly valuable functional
group has received intensive research focus in this hot area.2 As a
result, several versatile ODGs have stood out2−13 thus far, of
which O-NHAc played a particularly prominent role. Followed
by the pioneering work from the research group of Lu,4 to date,
Glorius,5 Wang,6 You,7 Zhao,8 Liu,9 Rovis,10 Zhou,11 us,12 and
others13 have made significant progress in O-NHAc-directed
C−H functionalization reactions. However, by reviewing these
notable achievements, some important issues remain to be
addressed in terms of substrate scope and reaction mode.
Indeed, previous O-NHAc-directed C−H functionalization
reactions are mostly limited to the employment of diazo
compounds,6a,9a,11,12a alkenes,4b,5a−c,6b,9b,10,13a and alkyne-
s4a,c,d,8a,13d as the coupling partners, and the simple alkylation,
direct alkenylation, or classical annulation mode were frequently
observed. Accordingly, a fairly limited and oxygen-involved
Received: May 10, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01477
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Letter
in Scheme 2a. We found that the developed Rh(III)-catalyzed
system proved to be broadly applicable, thus furnishing the
Scheme 1. Transition-Metal-Catalyzed C−H
Functionalization with Methyleneoxetanones
a,b
Scheme 2. Scope of Substrates
the O−N bond, which undergoes a cyclization with a β-lactone
moiety via acyl and/or alkyl C−O cleavage and subsequent
intramolecular nucleophilic attack. On the basis of our efforts,
we now report the first solvent-controlled and Rh(III)-catalyzed
redox-neutral C−H activation, followed by an unusual [3 + 3]
annulation of N-phenoxyacetamides with methyleneoxetanones
via selective alkyl C−O bond cleavage, leading to direct and
efficient access to a 2H-chromene-3-carboxylic acid framework
with excellent regioselectivity (Scheme 1, eq 3).
We commenced our exploration with optimization studies on
the coupling of N-phenoxyacetamide 1a with methyleneox-
etanone 2a (eq 1; see the Supporting Information (SI) for
a
Reaction conditions: 1 (0.2 mmol, 1 equiv), 2 (0.3 mmol, 1.5 equiv),
[Cp*RhCl2]2 (2.5 mol %), and CsOAc (1 equiv) in CH3CN (0.1 M)
at 60 °C for 24 h without exclusion of air or moisture; isolated yields
b
were reported. Contained an inseparable mixture of isomers; the
1
ratio was determined by H NMR.
details). The desired 2H-chromene-3-carboxylic acid 3a was
delivered with 42% isolated yield under the initial conditions, in
which 1,4-dioxane was employed as the solvent. Control
experiments confirmed that both [Cp*RhCl2]2 and CsOAc
were indispensable for the reaction outcome. Very interestingly,
changing the solvent 1,4-dioxane to MeOH gave an improved
yield of 3a (60%) along with a lactonization byproduct 3a′ (the
ratio of 3a/3a′ = 6/1), indicating that two different ring-opening
pathways of β-lactone were simultaneously involved in MeOH,
probably due to competing alkyl C−O and acyl C−O bond
cleavages in β-lactone.16 Moreover, these results revealed that
the solvent played a key role in controlling the reaction process
and the selectivity. Considering the unusual [3 + 3] annulation
mode for direct construction of the valuable 2H-chromene core,
in which very few transition-metal-catalyzed protocols6b,17 have
been reported for its synthesis, we were thus interested to further
optimize the related reaction parameters for this transformation,
with the ultimate aim of improving the yield and the selectivity
toward the formation of 3a. After a number of trials, we found
that the desired product 3a was solely obtained in 84% isolated
yield via selective alkyl C−O bond cleavage. Encouraged by
these results, we thus identified the following conditions for
subsequent studies: 1a (0.2 mmol), 2a (1.5 equiv),
[Cp*RhCl2]2 (2.5 mol %), and CsOAc (1.0 equiv) in MeCN
at 60 °C for 24 h under an atmosphere of air.
corresponding 2H-chromene-3-carboxylic acids in moderate to
good yields. We found that the electrical property of the
substituent played a crucial role in the reaction because N-
methoxybenzamides bearing the electron-withdrawing group
(the range of yields from 23 to 76%) gave yields relatively lower
than those bearing the electron-donating group (the range of
yields from 67 to 85%). Importantly, various substitutions,
including many useful functional groups such as halogens (3e−
h, 3k, and 3p), trifluoromethyl (3i), and ester (3j) either at the
para- (3b−g), meta- (3k−n), or ortho- (3o,p) position, were
fully tolerated. Of note, substrate 1k bearing the chloro
functional group at the meta-position provided the desired
product 3k with exclusive regioselectivity toward the less-
hindered site, whereas the meta-methyl- and methoxy-
substituted derivatives all gave a mixture of regioisomers with
different ratios (3m/3m′ = 1:0.1, 3n/3n′ = 1:0.3), revealing that
the nature of the substituent at the meta-position had a clear
effect on determining the reaction outcome and the
regioselectivity. This protocol could be extended to polyar-
omatic diphenyl and naphthalene substrates, giving the
predicted products 3d and 3q in decent isolated yields.
To better probe the scope of the reaction, several
methyleneoxetanones bearing substituents of different types
and natures such as alkyl, alkenyl, cycloalkyl, and aryl were
further investigated (Scheme 2b). Gratifyingly, the reaction
showed good compatibility with these substrates to deliver the
With the reaction conditions established, we first explored the
scope of N-phenoxyacetamides, and the results are summarized
B
DOI: 10.1021/acs.orglett.8b01477
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corresponding [3 + 3] annulation products in synthetically
useful yields. In particular, the two methyleneoxetanone
substrates, derived from the well-known and FDA-approved
food flavors citronellal and cyclamen aldehyde, also worked well,
affording the desired products 4h and 4i in good yields, which
further demonstrated the versatility of the developed Rh(III)-
catalyzed [3 + 3] annulation.
Scheme 4. Experimental Mechanistic Studies
To demonstrate the synthetic application potentiality, gram-
scale synthesis of product 3a was performed under the
optimized conditions, and a decent yield of 81% was isolated
smoothly (Scheme 3a). Additionally, two representative
Scheme 3. Gram-Scale Synthesis and Derivatization of 3a
derivatization reactions have been carried out to explore the
synthetic transformation of the obtained 2H-chromene-3-
carboxylic acid products. As illustrated in Scheme 3b, further
C−H functionalization of 3a with methyl acrylate afforded an
intriguing scaffold, 1H-furo[3,4-c]chromen-3(4H)-one deriva-
tive 5 in 65% yield. In addition, treatment of 3a with HOTf at
100 °C resulted in intramolecular cyclization to deliver the
interesting tetracyclic product 6 in 52% yield (Scheme 3c).
To further cast light on the reaction mechanism, a series of
experimental investigations were conducted (Scheme 4). First,
the reversibility of the C−H activation was defined by removing
the methyleneoxetanone substrate and running the reaction in
deuterated methanol. As shown in Scheme 4a, 1H NMR analysis
showed approximately 29 and 55% deuteration at the ortho-
positions of the recovered N-phenoxyacetamide 1a after 2 or 24
h, respectively, revealing that the C−H activation process was
reversible. Second, carrying out the same reaction in the
presence of 2a led to no deuterium incorporation in product 3a
and unreacted 1a, suggesting that the reaction rate of the
migratory insertion of 2a was far faster than that of the C−H
bond activation step. Then, the isotope-labeling experiment was
performed to further probe the C−H activation process
(Scheme 4b). Both results from a competitive experiment and
two parallel, side-by-side reactions showed that significant
kinetic isotope effect was involved based on two rate constants,
thus suggesting that the C−H cleavage might be related to the
rate-determining step. Moreover, to capture the potential
intermediates, we first treated [Cp*RhCl2]2 with 1a, and the
five-membered rhodacyclic complex A was smoothly obtained.
As predicted, complex A displayed good activity in the next stage
of the stoichiometric reaction (Scheme 4c), which provided
clear evidence that C−H activation was involved in the catalytic
cycle, and also revealed that the rhodacyclic complex A species
should act as one of the active intermediates for this
transformation. Finally, two control experiments were carried
out to define the role of the β-lactone moiety and the reaction
sequence with regard to the olefination and the ring opening. As
demonstrated in Scheme 4d, no reaction was monitored when 2-
carboxyl allylic alcohol 7 and its ester form 8 were treated with
1a under standard conditions, showing that a four-membered β-
lactone moiety was crucial for the reaction outcome and also
implying that its ring-opening step should occur after the C−H
activation/olefination reaction.
On the basis of these experimental results and literature
precedents, we proposed the plausible mechanism of this
reaction in Scheme 5. First, the reactive acetate Rh catalyst was
generated through a ligand exchange, followed by the
coordination with the N atom of 1a and ortho-C−H activation
Scheme 5. Proposed Mechanism
C
DOI: 10.1021/acs.orglett.8b01477
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Letter
of the directing group to yield the five-membered rhodacycle
intermediate A. Subsequent regioselective migratory insertion of
the α-methylene moiety of 2a into the Rh−C bond of A forms
the seven-membered intermediate B. Then, both the Rh(III)−
Rh(I)−Rh(III) and the Rh(III)−Rh(V)−Rh(III) pathways are
possible, mainly depending upon the oxidation of Rh by the O−
N bond.5a Thus, in pathway a, an oxidative addition of Rh(III)
into the O−N bond produces the six-membered Rh(V) species
C, followed by HOAc-assisted addition and subsequent anti-β-
H elimination to give the intermediate F. Alternatively, β-H
elimination of B provides the ring-opening Rh(I) species E.
Then, intramolecular oxidative addition of Rh(I) into the O−N
bond affords F′ (pathway b). Finally, protonation of F′
regenerates the active catalyst and delivers the intermediate G,
which undergoes a classical intramolecular SN2-type nucleo-
philic substitution reaction along with the alkyl C−O bond
cleavage to release the product 3a.
To further confirm which pathway is more feasible for this
transformation, DFT calculations were performed with
Gaussian 09 by selecting the isolated rhodacycle A as the
starting point (see the SI for details). The results demonstrate
that rhodacycle A proceeds via a Rh(III)−Rh(V)−Rh(III)
pathway to give the INT-5 (intermediate F) with an overall 41.9
kcal/mol exothermicity (pathway a), in which TS-2 and TS-3
are used as the transition states with a free energy of 21.8 and
27.6 kcal/mol, respectively. However, in pathway b, a higher free
energy barrier is required to overcome a Rh(III)−Rh(I)−
Rh(III) pathway (TS-2′ = 26.4 kcal/mol; TS-3′ = 30.1 kcal/
mol). Moreover, a low overall exothermicity of 28.3 kcal/mol is
involved from A to INT-5′ (intermediate F′). Taken together,
the results reveal that pathway b is clearly disfavored in
comparison to the pathway a.
In summary, we have developed, for the first time, a mild
solvent-controlled and rhodium(III)-catalyzed redox-neutral
cascade C−H activation/unusual [3 + 3] annulation of diverse
N-phenoxyacetamides for the one-pot synthesis of a 2H-
chromene-3-carboxylic acid framework with broad substrate
tolerance and good functional group compatibility, in which
methyleneoxetanones were employed as an innovative three-
carbon source through selective alkyl C−O bond cleavage of the
β-lactone moiety. The synthetic utilities of the obtained
products have also been successfully exemplified by the gram-
scale synthesis of the products and subsequent derivatization
reactions for the one-pot construction of interesting 1H-
furo[3,4-c]chromen-3(4H)-one derivatives and tetracyclic
compounds. Through a set of experimental investigations
together with the theoretical calculations, an active five-
membered rhodacycle intermediate has been established, and
a tandem C−H activation/olefination/intramolecular SN2-type
nucleophilic substitution process employing the Rh(III)−
Rh(V)−Rh(III) pathway to turn over the catalytic cycle has
also been deduced rationally. Further studies on understanding
the reaction mechanism and exploring the new type of reaction
modes of methyleneoxetanones are in progress.
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: yiwei@gzhmu.edu.cn
*E-mail: yuxycn@aliyun.com
ORCID
Hui Gao: 0000-0002-8736-4485
Xingwei Li: 0000-0002-1153-1558
Wei Yi: 0000-0001-7936-9326
Author Contributions
§Z.Z., M.B., and L.Z. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSFC (81502909, 81330007 and U1601227), the
Guangdong Natural Science Funds for Distinguished Young
Scholars (2017A030306031), the Science and Technology
Programs of Guangdong Province (2015B020225006), and the
Natural Science Foundation of Guangdong Province
(2017A030313058) for financial support of this study.
REFERENCES
■
(1) For recently selected reviews, see: (a) Mishra, N. K.; Sharma, S.;
Park, J.; Han, S.; Kim, I. S. ACS Catal. 2017, 7, 2821. (b) Wei, Y.; Hu, P.;
Zhang, M.; Su, W. Chem. Rev. 2017, 117, 8864. (c) Park, Y.; Kim, Y.;
Chang, S. Chem. Rev. 2017, 117, 9247. (d) Xia, Y.; Qiu, D.; Wang, J.
Chem. Rev. 2017, 117, 13810. (e) Ping, L.; Chung, D. S.; Bouffard, J.;
Lee, S.-G. Chem. Soc. Rev. 2017, 46, 4299. (f) Kim, D.-S.; Park, W.-J.;
Jun, C.-H. Chem. Rev. 2017, 117, 8977.
(2) (a) Patureau, F. W.; Glorius, F. Angew. Chem., Int. Ed. 2011, 50,
1977. (b) Huang, H.; Ji, X.; Wu, W.; Jiang, H. Chem. Soc. Rev. 2015, 44,
1155−1171. (c) Huang, H.; Cai, J.; Deng, G. J. Org. Biomol. Chem.
2016, 14, 1519.
(3) For selected examples, see: (a) Chan, C.-M.; Zhou, Z.; Yu, W.-Y.
Adv. Synth. Catal. 2016, 358, 4067. (b) Yu, C.; Zhang, J.; Zhong, G.
Chem. Commun. 2017, 53, 9902. (c) Dateer, R. B.; Chang, S. J. Am.
Chem. Soc. 2015, 137, 4908. (d) Hong, S. Y.; Jeong, J.; Chang, S. Angew.
Chem., Int. Ed. 2017, 56, 2408. (e) Mo, J.; Wang, L.; Cui, X. Org. Lett.
2015, 17, 4960. (f) Huang, X.; Huang, J.; Du, C.; Zhang, X.; Song, F.;
You, J. Angew. Chem., Int. Ed. 2013, 52, 12970. (g) Yu, S.; Liu, S.; Lan,
Y.; Wan, B.; Li, X. J. Am. Chem. Soc. 2015, 137, 1623. (h) Chuang, S.-C.;
Gandeepan, P.; Cheng, C.-H. Org. Lett. 2013, 15, 5750. (i) Li, B.; Ma, J.;
Wang, N.; Feng, H.; Xu, S.; Wang, B. Org. Lett. 2012, 14, 736.
(j) Rakshit, S.; Grohmann, C.; Besset, T.; Glorius, F. J. Am. Chem. Soc.
2011, 133, 2350. (k) Yu, X.; Chen, K.; Wang, Q.; Zhang, W.; Zhu, J.
Chem. Commun. 2018, 54, 1197. (l) Hyster, T. K.; Ruhl, K. E.; Rovis, T.
J. Am. Chem. Soc. 2013, 135, 5364. (m) Liu, B.; Song, C.; Sun, C.; Zhou,
S.; Zhu, J. J. Am. Chem. Soc. 2013, 135, 16625. (n) Wang, C.; Sun, H.;
Fang, Y.; Huang, Y. Angew. Chem., Int. Ed. 2013, 52, 5795. (o) Wu, X.;
Wang, B.; Zhou, S.; Zhou, Y.; Liu, H. ACS Catal. 2017, 7, 2494. (p) Wu,
J.-Q.; Zhang, S.-S.; Gao, H.; Qi, Z.; Zhou, C.-J.; Ji, W.-W.; Liu, Y.; Chen,
Y.; Li, Q.; Li, X.; Wang, H. J. Am. Chem. Soc. 2017, 139, 3537.
(4) (a) Liu, G.; Shen, Y.; Zhou, Z.; Lu, X. Angew. Chem., Int. Ed. 2013,
52, 6033. (b) Shen, Y.; Liu, G.; Zhou, Z.; Lu, X. Org. Lett. 2013, 15,
3366. (c) Zhou, Z.; Liu, G.; Chen, Y.; Lu, X. Org. Lett. 2015, 17, 5874.
(d) Zhou, Z.; Liu, G.; Shen, Y.; Lu, X. Org. Chem. Front. 2014, 1, 1161.
(5) (a) Wang, X.; Lerchen, A.; Daniliuc, C. G.; Glorius, F. Angew.
Chem., Int. Ed. 2018, 57, 1712. (b) Wang, X.; Lerchen, A.; Gensch, T.;
Knecht, T.; Daniliuc, C. G.; Glorius, F. Angew. Chem., Int. Ed. 2017, 56,
1381. (c) Lerchen, A.; Knecht, T.; Daniliuc, C. G.; Glorius, F. Angew.
Chem., Int. Ed. 2016, 55, 15166. (d) Wang, X.; Gensch, T.; Lerchen, A.;
Daniliuc, C. G.; Glorius, F. J. Am. Chem. Soc. 2017, 139, 6506.
(6) (a) Hu, F.; Xia, Y.; Ye, F.; Liu, Z.; Ma, C.; Zhang, Y.; Wang, J.
Angew. Chem., Int. Ed. 2014, 53, 1364. (b) Zhang, H.; Wang, K.; Wang,
ASSOCIATED CONTENT
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S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b01477.
Experimental procedures, characterization of products,
and copies of 1H and 13C NMR spectra (PDF)
D
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B.; Yi, H.; Hu, F.; Li, C.; Zhang, Y.; Wang, J. Angew. Chem., Int. Ed.
2014, 53, 13234.
(7) (a) Li, B.; Zhou, L.; Cheng, H.; Huang, Q.; Lan, J.; Zhou, L.; You,
J. Chem. Sci. 2018, 9, 1213. (b) Li, B.; Tang, G.; Zhou, L.; Wu, D.; Lan,
J.; Zhou, L.; Lu, Z.; You, J. Adv. Funct. Mater. 2017, 27, 1605245. (c) Li,
B.; Lan, J.; Wu, D.; You, J. Angew. Chem., Int. Ed. 2015, 54, 14008.
(8) (a) Chen, Y.; Wang, D.; Duan, P.; Ben, R.; Dai, L.; Shao, X.; Hong,
M.; Zhao, J.; Huang, Y. Nat. Commun. 2014, 5, 4610. (b) Hu, S.; Lu, L.;
Zhu, T.; Wu, Q.; Chen, Y.; Li, J. J.; Zhao, J. Org. Biomol. Chem. 2018, 16,
43. (c) Wu, Q.; Chen, Y.; Yan, D.; Zhang, M.; Lu, Y.; Sun, W.-Y.; Zhao,
J. Chem. Sci. 2017, 8, 169. (d) Wu, Q.; Yan, D.; Chen, Y.; Wang, T.;
Xiong, F.; Wei, W.; Lu, Y.; Sun, W.-Y.; Li, J. J.; Zhao, J. Nat. Commun.
2017, 8, 14227. (e) Duan, P.; Lan, X.; Chen, Y.; Qian, S.-S.; Li, J. J.; Lu,
L.; Lu, Y.; Chen, B.; Hong, M.; Zhao, J. Chem. Commun. 2014, 50,
12135. (f) Duan, P.; Yang, Y.; Ben, R.; Yan, Y.; Dai, L.; Hong, M.; Wu,
Y.-D.; Wang, D.; Zhang, X.; Zhao, J. Chem. Sci. 2014, 5, 1574.
(9) (a) Hu, Z.; Liu, G. Adv. Synth. Catal. 2017, 359, 1643. (b) Hu, Z.;
Tong, X.; Liu, G. Org. Lett. 2016, 18, 1702.
(10) Piou, T.; Rovis, T. Nature 2015, 527, 86.
(11) Wu, Y.; Chen, Z.; Yang, Y.; Zhu, W.; Zhou, B. J. Am. Chem. Soc.
2018, 140, 42.
(12) (a) Zhou, J.; Shi, J.; Liu, X.; Jia, J.; Song, H.; Xu, H. E.; Yi, W.
Chem. Commun. 2015, 51, 5868. (b) Zhou, J.; Shi, J.; Qi, Z.; Li, X.; Xu,
H. E.; Yi, W. ACS Catal. 2015, 5, 6999. (c) Chen, W.; Liu, F.-X.; Gong,
W.; Zhou, Z.; Gao, H.; Shi, J.; Wu, B.; Yi, W. Adv. Synth. Catal. 2018,
DOI: 10.1002/adsc.201800322.
(13) For representative examples, see: (a) Prakash, S.; Muralirajan, K.;
Cheng, C.-H. Chem. Commun. 2015, 51, 13362. (b) Wang, H.; Wang,
B.; Li, B. J. Org. Chem. 2017, 82, 9560. (c) Zhang, Y.; He, Y.; Li, L.; Ji,
M.; Li, X.-Z.; Zhu, G. J. J. Org. Chem. 2018, 83, 2898. (d) Xie, Y. Chem.
Commun. 2016, 52, 12372. (e) Li, Y.; Tang, Y.; He, X.; Shi, D.; Wu, J.;
Xu, S. Chem. - Eur. J. 2017, 23, 7453.
(14) Malapit, C. A.; Caldwell, D. R.; Sassu, N.; Milbin, S.; Howell, A.
R. Org. Lett. 2017, 19, 1966.
(15) Xia, J.; Kong, L.; Zhou, X.; Zheng, G.; Li, X. Org. Lett. 2017, 19,
5972.
(16) Nelson, S. G.; Dura, R. D.; Peelen, T. J. Org. React. 2012, 82, 471.
(17) For recent transition-metal-catalyzed examples for the synthesis
of the 2H-chromene core, see: (a) Majumdar, N.; Paul, N. D.; Mandal,
S.; de Bruin, B.; Wulff, W. D. ACS Catal. 2015, 5, 2329. (b) Kuppusamy,
R.; Muralirajan, K.; Cheng, C.-H. ACS Catal. 2016, 6, 3909.
(c) Romanov-Michailidis, F.; Ravetz, B. D.; Paley, D. W.; Rovis, T. J.
Am. Chem. Soc. 2018, 140, 5370.
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