Synthesis of 6 H-Benzo[ c]chromene Scaffolds from O-Benzylated Phenols through a C-H Sulfenylation/Radical Cyclization Sequence

Article abstract of DOI:10.1021/acs.orglett.1c00087

S-Aryl dibenzothiophenium salts, obtained through a highly regioselective C-H sulfenylation of o-benzyl-protected phenols, are used as precursors of 6H-benzo[c]chromenes. The reaction starts with a photocatalytically triggered single-electron transfer to the sulfonium salt, which promotes the formation of an aryl radical via selective mesolitic cleavage of the S-Arexo bond. Mechanistic studies reveal that this initial radical species cyclizes following a kinetically favored 5-exo-trig pathway. Subsequent ring expansion, favored by rearomatization, delivers the desired tricyclic systems.

Full text of DOI:10.1021/acs.orglett.1c00087

pubs.acs.org/OrgLett
Letter
Synthesis of 6H-Benzo[c]chromene Scaffolds from O‑Benzylated
Phenols through a C−H Sulfenylation/Radical Cyclization Sequence
Steve Karreman, Simon B. H. Karnbrock, Sebastian Kolle, Christopher Golz, and Manuel Alcarazo*
Cite This: Org. Lett. 2021, 23, 1991−1995
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ABSTRACT: S-Aryl dibenzothiophenium salts, obtained through a highly regioselective C−H sulfenylation of o-benzyl-protected
phenols, are used as precursors of 6H-benzo[c]chromenes. The reaction starts with a photocatalytically triggered single-electron
transfer to the sulfonium salt, which promotes the formation of an aryl radical via selective mesolitic cleavage of the S−Arexo bond.
Mechanistic studies reveal that this initial radical species cyclizes following a kinetically favored 5-exo-trig pathway. Subsequent ring
expansion, favored by rearomatization, delivers the desired tricyclic systems.
he ability of sulfonium salts to generate organic radicals
Tafter accepting one electron via mesolitic S−C bond
cleavage was reported by Kellogg as early as 1978. Although
the specific transformation chosen in his study, namely, the
desulfuration of phenacyl sulfonium salts 1 to acetophenones,
has limited synthetic applications, the seminal work already
contained all of the ingredients that later contributed to the
flourishing of photoredox catalysis into a versatile synthetic
tool (Scheme 1a).1 After that original finding, the further
employment of sulfonium salts in the photocatalysis arena
remained dormant for decades until Fensterbank, Goddard,
and Ollivier, making use of the conditions developed by
Kellogg, reported the formation of aryl radicals from triaryl
sulfonium salts 2.2 Interestingly, the thus-prepared radicals
were found to participate in carbon−carbon bond-forming
reactions, including allylation and addition to olefins; however,
the scope and practical utility of their protocol was severely
limited to structurally simple and easily available aryl groups.
Note that whereas the three aryl substituents at the central
sulfur of the sulfonium salt need to be identical to avoid
chemoselectivity issues, only one of them can finally be
transferred (Scheme 1b).
That contribution attracted the attention of many
researchers toward the chemistry of sulfonium salts, and a
number of investigations followed, in particular, those focused
on the generation of benzyl radicals.3 However, it has not been
until very recently that the full synthetic potential of sulfonium
salts could be exploited by the identification of suitable
platforms, such as the dibenzothiophene and thianthrene
skeletons, which chemoselectively promote the cleavage of the
S−Ar(exo) bond after a single-electron transfer process.4 This
has significantly increased the range of applications of
sulfonium salts.5 Among the recently developed trans-
formations, the metal-free C−H cross-coupling between
unfunctionalized (hetero)arenes arguably stands out for its
synthetic potential and simplicity (Scheme 1c).4a It also worth
noting that all of these newly developed transformations
enormously benefit from the recently developed routes for the
synthesis of sulfonium salts from unfunctionalized arenes via
highly regioselective C−H sulfenylation.6,7
Being aware of the practical utility of the two-step C−H
sulfenylation/Ar-radical generation strategy just mentioned, we
envisaged the possibility of transforming that methodology
into an efficient cyclization tool when applied to appropriately
designed polyaromatic substrates. Crucial for the success of
this concept is the initial sulfenylation step, which must be
highly regioselective. In fact, the substrate needs to be
engineered in a way that the sulfenylation occurs only at the
position that geometrically allows the subsequent radical
cyclization, and this must occur in the presence of at least
two different aromatic rings. Not making things easier, the
conditions need to be settled in a way that the transient radical
intermediate cyclizes in the predicted manner and does not
evolve via any other plausible competing reaction pathway.8
Considering all of these requirements, p-substituted aryl benzyl
ethers 4 were selected as appropriate model substrates, which
should reliably undergo sulfenylation at the o-position of the
phenol moiety to deliver sulfonium salts 5. The subsequent
Received: January 10, 2021
Published: March 1, 2021
© 2021 The Authors. Published by
American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.1c00087
Org. Lett. 2021, 23, 1991−1995
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a
Scheme 1. Selected Precedents in Photocatalytic Radical
Generation Using Sulfonium Salts
Scheme 2. Scope of the Sulfenylation Step
photo-redox-promoted radical generation is expected to
initiate a Pschorr-type cyclization to afford the desired 6H-
benzo[c]chromenes (Scheme 1d).9,10 Herein we report the
practical realization of that initial hypothesis as well as a series
of control experiments focused on determining the actual
reaction path operative under the applied conditions.
To our delight, the initial sulfenylation step proceeds as
programmed, and a range of O-benzylated phenols are
selectively functionalized at the o-position of the oxygenated
substituent 5a−j,p (Scheme 2). Note that alkyl groups (Me- or
tBu-) are needed to block the otherwise more sterically
accessible p-position of the most electron-rich ring. Halogens
and phenyl and methoxy groups are not adequate for this task;
the former dramatically reduces the yield of the reaction (5p),
whereas the latter suffer sulfenylation themselves; see
compounds 5p−r. Complex reaction mixtures are obtained
when additional methoxy groups are installed in the substrates.
Yields of 5a−j are moderate to good, and all compounds
survive column chromatography purification on silica gel
despite their saline nature. Bis-sulfonium products 5k−o are
also obtained in acceptable yields provided that a slight excess
of the sulfenylation reagent is employed. The connectivity of
the newly prepared products has been additionally confirmed
by X-ray analyses of compounds 5e,f,h,i,r. (See Scheme 1 for
5h and 5r and the Supporting Information for the others.)
Short contacts between the sulfur atom and one oxygen from
the triflate counteranion are detected along the complete
series, revealing the remarkable electrophilic character of sulfur
in these compounds.6b
a
Reaction conditions: (a) arene (1.1 equiv), pyridine (1.1 equiv) in
CH2Cl2, −60° → r.t., 16 h; b) arene (0.5 equiv), pyridine (1.05
equiv) in CH2Cl2, −60° → r.t., 16 h. For the X-ray structures of 5h
and 5r, ellipsoids are represented at 50% probability, and triflate
anions and hydrogen atoms are omitted for clarity.
In this stage, we continued the characterization of the
obtained salts by determining the reduction potential of model
substrate 5a through cyclic voltammetry (CV). This experi-
ment showed an irreversible reduction with Ered = −1.50 V (vs
Fc+/0 in CH3CN) (Scheme 3a).4a This value is considerably
less negative than that determined for the excited state of
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a
DIPEA (5.0 equiv) mainly delivers the dimeric structure 7
(Scheme 3c).
Scheme 3. Mechanistic Studies
Control experiments and cumulative evidence from the
literature indicate that the operating mechanism is, with high
probability, the one shown in Scheme 3d.4a,8,10 Preliminary
experiments indicate that the reaction does not proceed in the
dark, and the quantum yield of the formation of 6a is 0.47,
suggesting that a radical chain process cannot predominate.12
Hence, we do believe that upon the initial generation of radical
A, a kinetically favored 5-exo-trig ipso-attack of the aryl radical
to the pending arene results in the formation of spiro
cylohexadienyl radical B. The installation of a pyridine-N-
oxide substituent as a radical trapping agent in the substrate
allows the detection of species of this structure.13 Thus, the
reaction of sulfonium salt 9 with 1 equiv of Cp2Co (Ered
=
−1.33 V vs Fc+/0) affords radical 10, which we have
characterized by standard electron paramagnetic resonance
(EPR) techniques. Its spectrum shows resolved hyperfine
splitting as result of hyperfine coupling to the N atom (aN =
8.39 G) and two pairs of equivalent H atoms (aH = 5.83 and
1.99 G, respectively). This pattern fits with that expected for
the Cs symmetric structure of 10.
Another hint indicating the formation of B comes from the
isolation of substantial amounts of its dimer 7 when B is
formed under the reducing environment generated by the
[Ru(bpy)3]Cl2/DIPEA catalytic system. Under the applied
conditions, this mixture is not capable of effectively promoting
the further reduction of B (E1/2 = −1.74 V vs Fe+/0) into the
corresponding cyclohexadienyl anion8a or its oxidation to C.
For that reason, B accumulates and finally dimerizes. The
isolation of 7, and not a dimer of D, also indicates that the
[1,2]-aryl migration must be a slow process under our working
conditions, in the case that it actually takes place. Moreover,
the evolution of B into D is reported to be nonselective,
delivering regioisomeric product mixtures.14 We have observed
the formation of regioisomeric mixtures only in the case of 6f
and 6f′ (Scheme 4). Hence, we proposed that this reaction
preferentially proceeds via the oxidation of B into carbocation
C, followed by [1,2]-aryl rearrangement and deprotonation to
deliver 6.4a On the contrary, the sulfone substituent in the
precursor of 6f and 6f′ is expected to hinder the oxidation of
this species from B to C. In that case, the ring expansion step
probably takes place at the radical intermediate, resulting in a
low-selectivity process.
Finally, we have also submitted to standard reaction
conditions substrate 5i, which has been conveniently designed
with two methyl substituents at the o-positions of the tether.
Because 1,2-migration is hindered here, a scission of the CH2−
a
For the X-ray structure of 7, ellipsoids are represented at 50%
probability.
C
spiro bond takes place, and the thus-generated intermediate is
trapped with MeOH, delivering the MOM-protected biaryl 8
(Scheme 3c).
Ir(ppy)3 (Ered* = −2.13 V vs Fc+/0 in CH3CN) or that of
[Ru(bpy)3]Cl2 after photoexcitation and reductive quenching
(Ered = −1.73 V vs Fc+/0), indicating the feasibility of the
necessary single-electron transfer event from the catalyst to the
substrate in both cases.11 In line with these findings, Stern−
Volmer experiments confirm that 5a effectively quenches the
excited state of Ir(ppy)3 (Scheme 3b). Actually, both catalytic
systems promote the formation of transient aryl radicals via
C−S bond cleavage; however, while Ir(ppy)3 efficiently
transforms the model substrate 5a into the desired 6H-
benzo[c]chromene 6a, the combination of [Ru(bpy)3]Cl2 and
This light-driven reaction effectively engages a range of
substrates 5a−o in the desired cyclization toward 6H-
benzo[c]chromenes in good to excellent yields (Scheme 4).
Fluoro, chloro, sulfone, and trifluoromethyl substituents are
tolerated, which allows the further functionalization of the
products obtained, for example, by traditional cross-coupling
chemistry. The scalability of the protocol has been
demonstrated by the preparation of 6a on a scale ten times
higher than the initial scale (350 mg) with no loss of yield;
moreover, dibenzothiophene (96%) is recovered from that
experiment and can be recycled.
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a
polyaromatic structures, phosphorines, and pyridinium and
thiinium salts.16
Scheme 4. Scope of the Photocatalyzed Cyclization
a
Scheme 5. Synthesis of Pyrylium Salts From 6a
a
For the X-ray structure of 11a ellipsoids are represented at 50%
probability. Triflate anions and hydrogen atoms are omitted for
clarity.
In summary, a mild and efficient protocol for the rapid
synthesis of 6H-benzo[c]chromenes from easily available
benzyl ethers is described, which operates via the selective
C−H sulfenylation of electron-rich aromatic moieties followed
by photocatalyzed radical cyclization. The utility of the
method, which also allows multicyclizations, is exemplified
by the synthesis of 6H-benzo[c]chromenes of different
substitution patterns and their one-step transformation into
synthetically versatile pyrylium salts.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00087.
General experimental procedures, characterization data
including 1H, 13C, and 19F NMR spectra of new
compounds, and dynamic NMR studies (PDF)
Accession Codes
CCDC 2051252−2051263 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
a
■
For the X-ray structures of 6j, 6m, and 6m′, ellipsoids are
represented at 50% probability. Triflate anions and hydrogen atoms
are omitted for clarity.
Manuel Alcarazo − Institut für Organische und Biomolekulare
Chemie, Georg-August-Universität Göttingen, 37077
Göttingen, Germany; orcid.org/0000-0002-5491-5682;
Email: manuel.alcarazo@chemie.uni-goettingen.de
Double cyclizations also proceed satisfactorily; however, the
second [1,2]-rearrangement from intermediate C to 6 is not
regioselective when the adjacent positions to the spiranic
carbon are not chemically equivalent (6m, 6m′). In these
cases, mixtures of the helicoidal and linearly cyclized
regioisomers are observed. The connectivity of both types of
products is confirmed by X-ray diffraction analyses (Scheme
4). The Supporting Information contains the X-ray structure of
6b. The free energy of activation (ΔG#) for the inversion of
the helicoidal structure 6j is estimated by temperature-
dependent nuclear magnetic resonance (NMR) to be 81.2
kJ/mol. From this number, it can be deduced that the half life
at 20 °C of the enantiomers is approximately half a minute,
making their separation impossible at this temperature.
Authors
Steve Karreman − Institut für Organische und Biomolekulare
Chemie, Georg-August-Universität Göttingen, 37077
Göttingen, Germany
Simon B. H. Karnbrock − Institut für Organische und
Biomolekulare Chemie, Georg-August-Universität Göttingen,
37077 Göttingen, Germany
Sebastian Kolle − Institut für Organische und Biomolekulare
Chemie, Georg-August-Universität Göttingen, 37077
Göttingen, Germany
Christopher Golz − Institut für Organische und
Biomolekulare Chemie, Georg-August-Universität Göttingen,
37077 Göttingen, Germany
Finally, to further explore the synthetic utility of the method
reported, 6a is further transformed into the corresponding
pyrylium salt 11a by the reaction with SOCl2/PCl5 (Scheme
5).15 These salts are well known precursors of condensed
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00087
1994
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Org. Lett. 2021, 23, 1991−1995

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́
2019, 58, 8779−8783. (g) Fernandez-Salas, J. A.; Pulis, A. P.; Procter,
D. J. Chem. Commun. 2016, 52, 12364−12367. (h) Shevchenko, N.
E.; Karpov, A. S.; Zakurdaev, E. P.; Nenajdenko, V. G.; Balenkova, E.
S. Synthesis of methylthiosubstituted heterocycles using the complex
of trifluoromethanesulfonic anhydride with dimethyl sulphide. Chem.
Heterocycl. Compd. 2000, 36, 137−143.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support from the DFG (INST 186/1237-1 and INST 186/
1324-1) and the European Commission (ERC CoG 771295)
is gratefully acknowledged. We also thank Mr. M. Simon
(7) For the synthesis of alkynyl sulfonium salts from sulfoxides, see:
(a) Waldecker, B.; Kafuta, K.; Alcarazo, M. Preparation of 5-
(Triisopropylalkynyl) dibenzo[b,d]thiophenium triflate. Org. Synth.
2019, 96, 258−276. (b) Waldecker, B.; Kraft, F.; Golz, C.; Alcarazo,
M. 5-(Alkynyl) dibenzothiophenium Triflates: Sulfur-Based Reagents
for Electrophilic Alkynylation. Angew. Chem., Int. Ed. 2018, 57,
12538−12542. For the synthesis of alkenyl sulfonium salts from
sulfoxides, see: (c) Chen, J.; Li, M.; Plutschack, M. B.; Berger, F.;
Ritter, T. Regio-and Stereoselective Thianthrenation of Olefins To
Access Versatile Alkenyl Electrophiles. Angew. Chem., Int. Ed. 2020,
̈
(University of Gottingen) for HPLC analyses and our NMR
and MS departments for support. S. Karreman thanks the
Studienstiftung des deutschen Volkes for his doctoral fellow-
ship.
REFERENCES
■
(1) (a) Van Bergen, T. J.; Hedstrand, D. M.; Kruizinga, W. H.;
Kellogg, R. M. Chemistry of dihydropyridines. 9. Hydride transfer
from 1,4-dihydropyridines to sp3-hybridized carbon in sulfonium salts
and activated halides. Studies with NAD(P)H models. J. Org. Chem.
1979, 44, 4953−4962. (b) Hedstrand, D. M.; Kruizinga, W. H.;
Kellogg, R. M. Light induced and dye accelerated reductions of
phenacyl onium salts by 1,4-dihydropyridines. Tetrahedron Lett. 1978,
19, 1255−1258.
́
59, 5616−5620. (d) Aukland, M. H.; Talbot, F. J. T.; Fernandez-Salas,
J. A.; Ball, M.; Pulis, A. P.; Procter, D. J. An Interrupted Pummerer/
Nickel-Catalysed Cross-Coupling Sequence. Angew. Chem., Int. Ed.
2018, 57, 9785−9789.
(8) (a) Flynn, A. R.; McDaniel, K. A.; Hughes, M. E.; Vogt, D. B.;
Jui, N. T. Hydroarylation of Arenes via Reductive Radical-Polar
Crossover. J. Am. Chem. Soc. 2020, 142, 9163−9168. (b) Harrowven,
D. C.; Nunn, M. I. T.; Newman, N. A.; Fenwick, D. R. New Cascade
Radical Reaction for the Synthesis of Biaryls and Triaryls from Benzyl
Iodoaryl Ethers. Tetrahedron Lett. 2001, 42, 961−964. (c) Studer, A.;
Bossart, M. Radical Aryl Migration reactions. Tetrahedron 2001, 57,
9649−9667. (d) Bowman, W. R.; Mann, E.; Parr, J. Bu3SnH mediated
oxidative radical cyclisations: synthesis of 6H-benzo[c]chromen-6-
ones. J. Chem. Soc., Perkin Trans. 1 2000, 2991−2999.
(2) Donck, S.; Baroudi, A.; Fensterbank, L.; Goddard, J.-P.; Ollivier,
C. Visible-Light Photocatalytic Reduction of Sulfonium Salts as a
Source of Aryl Radicals. Adv. Synth. Catal. 2013, 355, 1477−1482.
(3) Selected examples: (a) Varga, B.; Gonda, Z.; Tóth, B. L.;
́
Kotschy, A.; Novak, Z. A Ni−Ir Dual Photocatalytic Liebeskind
Coupling of Sulfonium Salts for the Synthesis of 2-Benzylpyrrolidines.
Eur. J. Org. Chem. 2020, 2020, 1466−1471. (b) Otsuka, S.; Nogi, K.;
Rovis, T.; Yorimitsu, H. Photoredox-Catalyzed Alkenylation of
Benzylsulfonium Salts. Chem. - Asian J. 2019, 14, 532−536.
(9) (a) He, J.-Y.; Bai, Q.-F.; Jin, C.; Feng, G. Organic Photoredox
Catalysis for Pschorr Reaction: A Metal-Free and Mild Approach to
6H-Benzo[c]chromenes. Synlett 2018, 29, 2311−2315. (b) Deng, Q.;
Tan, L.; Xu, Y.; Liu, P.; Sun, P. Synthesis of 6-Fluoroalkyl 6H-
Benzo[c]chromenes via Visible-Light-Promoted Radical Addition/
Cyclization of Biaryl Vinyl Ethers. J. Org. Chem. 2018, 83, 6151−
6161.
(10) For similar cylizations with different tethers, see: (a) Ohno, H.;
Iwasaki, H.; Eguchi, T.; Tanaka, T. The first samarium(II)-mediated
aryl radical cyclisation onto an aromatic ring. Chem. Commun. 2004,
2228−2229. (b) Hey, D. H.; Jones, G. H.; Perkins, M. J. lnternuclear
Cyclisation. Part XXX. The Photolysis of 2-lodo-2’-, −3′-, and −4’-
methoxy-N-alkylbenzanilides in Benzene. J. Chem. Soc., Perkin Trans. 1
1972, 1150−1155. (c) Hey, D. H.; Rees, C. W.; Todd, A. R.
Internuclear Cyclisation. Part XXII. Catalysed Decomposition of
Diazonium Fluoroborates from Alkoxy-N-alkyl-2-aminobenzanilides.
J. Chem. Soc. C 1967, 1518−1525.
(11) Shon, J.-H.; Teets, T. S. Photocatalysis with Transition Metal
Based Photosensitizers. Comments Inorg. Chem. 2020, 40, 53−85.
(12) For the intermolecular version of this reaction, Φ = 0.18 has
been determined. See ref 4a.
(13) For a comparison, see: Kubota, T.; Nishikida, K.; Miyazaki, H.;
Iwatani, K.; Oishi, Y. Electron spin resonance and polarographic
studies of the anion radicals of heterocyclic amine N-oxides. J. Am.
Chem. Soc. 1968, 90, 5080−5090.
(14) Roman, D. S.; Takahashi, Y.; Charette, A. B. Potassium tert-
Butoxide Promoted Intramolecular Arylation via Radical Pathway.
Org. Lett. 2011, 13, 3242−3245.
́
́
́
(c) Simkó, D. C.; Elekes, P.; Pazmandi, V.; Novak, Z. Sulfonium
Salts as Alkylating Agents for Palladium-Catalyzed Direct Ortho
Alkylation of Anilides and Aromatic Ureas. Org. Lett. 2018, 20, 676−
679.
̌
̌
(4) (a) Aukland, M. H.; Siauciulis, M.; West, A.; Perry, G. J. P.;
Procter, D. J. Metal-free photoredox-catalysed formal C−H/C−H
coupling of arenes enabled by interrupted Pummerer activation. Nat.
Catal. 2020, 3, 163−169. (b) Berger, F.; Plutschack, M. B.; Riegger, J.;
Yu, W.; Speicher, S.; Ho, M.; Frank, N.; Ritter, T. Site-selective and
versatile aromatic C−H functionalization by thianthrenation. Nature
2019, 567, 223−228.
́
(5) For recent reviews on the topic, see: (a) Péter, A.; Perry, G. J. P.;
Procter, D. J. Radical C−C Bond Formation using Sulfonium Salts
and Light. Adv. Synth. Catal. 2020, 362, 2135−2142. (b) Kozhushkov,
S.; Alcarazo, M. Synthetic Applications of Sulfonium Salts. Eur. J.
Inorg. Chem. 2020, 2020, 2486−2500. (c) Kaiser, D.; Klose, I.; Oost,
R.; Neuhaus, J.; Maulide, N. Bond-Forming and -Breaking Reactions
at Sulfur(IV): Sulfoxides, Sulfonium Salts, Sulfur Ylides, and Sulfinate
Salts. Chem. Rev. 2019, 119, 8701−8780.
(6) See ref 4 and: (a) Xu, P.; Zhao, D.; Berger, F.; Hamad, A.;
Rickmeier, J.; Petzold, R.; Kondratiuk, M.; Bohdan, K.; Ritter, T. Site-
Selective Late-Stage Aromatic [18F]Fluorination via Aryl Sulfonium
Salts. Angew. Chem., Int. Ed. 2020, 59, 1956−1960. (b) Kafuta, K.;
̈
Korzun, A.; Bohm, M.; Golz, C.; Alcarazo, M. Synthesis, Structure,
and Reactivity of 5-(Aryl)dibenzothiophenium Triflates. Angew.
Chem., Int. Ed. 2020, 59, 1950−1955. (c) Zhang, Z.; He, P.; Du,
H.; Xu, J.; Li, P. Sulfur-Mediated Electrophilic Cyclization of Aryl-
Substituted Internal Alkynes. J. Org. Chem. 2019, 84, 4517−4524.
(d) Ming, X.-X.; Tian, Z.-Y.; Zhang, C.-P. Base-Mediated O-Arylation
of Alcohols and Phenols by Triarylsulfonium Triflates. Chem. - Asian J.
2019, 14, 3370−3379. (e) Yan, J.; Pulis, A. P.; Perry, G. J. P.; Procter,
D. J. Metal-Free Synthesis of Benzothiophenes by Twofold C− H
Functionalization: Direct Access to Materials-Oriented Heteroar-
omatics. Angew. Chem., Int. Ed. 2019, 58, 15675−15679. (f) Siauciulis,
M.; Ahlsten, N.; Pulis, A. P.; Procter, D. J. Transition-Metal-Free
Cross-Coupling of Benzothiophenes and Styrenes in a Stereoselective
Synthesis of Substituted (E,Z)-1,3-Dienes. Angew. Chem., Int. Ed.
(15) Bringmann, G.; Schoner, B.; Peters, K.; Peters, E.-M.; von
Schnering, H. G. Synthesis and Structure of a Protected Lactolate-
Bridged Biaryl with Relevance to the Atropisomer-Selective Ring
Opening of Biaryl Lactones. Liebigs Ann. Chem. 1994, 1994, 439−444.
(16) Eicher, T.; Hauptmann, S.; Speicher, A. The Chemistry of
Heterocycles, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2012.
̌
̌
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