Thiolate-Initiated Synthesis of Dibenzothiophenes from 2,2′-Bis(methylthio)-1,1′-Biaryl Derivatives through Cleavage of Two Carbon-Sulfur Bonds

Article abstract of DOI:10.1055/s-0037-1611974

A catalytic reaction involving the cleavage of two carbon-sulfur bonds in 2,2′-bis(methylthio)-1,1′-biaryl derivatives is reported. This reaction does not require a transition-metal catalyst and is promoted by a thiolate anion. Notably, based on DFT calculations, the product-forming cyclization step is shown to proceed through a concerted nucleophilic aromatic substitution (CS _N Ar) mechanism.

Full text of DOI:10.1055/s-0037-1611974

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Thiolate-Initiated Synthesis of Dibenzothiophenes from 2,2′-
Bis(methylthio)-1,1′-Biaryl Derivatives through Cleavage of Two
Carbon–Sulfur Bonds
Yoshihiro Masuya
Yuki Kawashima
Takuya Kodama
Naoto Chatani*
Mamoru Tobisu*
MeS
cat. MeS–
Two C-S Bonds Cleavage
S
S
Me
0
0
0
0-0
0
0
2-8
4
1
5-
2
2
2
5
Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Suita, Osaka 565-0871, Japan
chatani@chem.eng.osaka-u.ac.jp
tobisu@chem.eng.osaka-u.ac.jp
Received: 12.11.2018
Accepted after revision: 11.12.2018
Published online: 14.01.2019
the reaction of 1a-Me in the presence of a catalytic amount
of KOtBu (20 mol%) in N,N-dimethylformamide (DMF) at
160 °C for 4 h afforded 2a in 56% yield (entry 3).
DOI: 10.1055/s-0037-1611974; Art ID: st-2018-b0740-c
Abstract A catalytic reaction involving the cleavage of two carbon–
sulfur bonds in 2,2′-bis(methylthio)-1,1′-biaryl derivatives is reported.
This reaction does not require a transition-metal catalyst and is pro-
moted by a thiolate anion. Notably, based on DFT calculations, the
product-forming cyclization step is shown to proceed through a con-
certed nucleophilic aromatic substitution (CSNAr) mechanism.
cat. Pd
PPh2
(a)
PPh3
+
PPh
P
Ph
Ph
Morandi (2017)
Our group (2017)
Key words C–S cleavage, C–O cleavage, metathesis, thiophene
Given the fact that heteroatom analogues of cyclopenta-
diene, such as siloles,1 phospholes,2 and thiophenes,3 which
are referred as heteroles,4 have emerged as privileged scaf-
folds in the field of organic materials and pharmaceuticals,
the development of synthetic methods for preparing these
compounds has attracted considerable interest. In this con-
text, we have been investigating the synthesis of heteroles
that proceed through cleavage of a carbon–heteroatom
bond.5 Morandi and our group both independently report-
ed on a palladium-catalyzed method for the synthesis of
phosphole derivatives from bisphosphines through the for-
mal metathesis of carbon–phosphorus bonds (Scheme 1a).6
These reactions allow for the rapid, efficient construction of
a range of elaborate phosphole derivatives by using com-
mercially available bisphosphines as substrates. We report
herein on a sulfur variant of this reaction and its use in the
synthesis of thiophenes (Scheme 1b).
We hypothesized that 2,2′-bis(methylthio)-1,1′-biphe-
nyl (1a-Me) could be cyclized by palladium catalysis to give
the dibenzothiophene (2a) through a mechanism similar to
that reported for the corresponding bisphosphines.6a Con-
sistent with this expectation, the desired product was ob-
tained in 97% yield.7 To our surprise, 2a was formed, even in
the absence of a palladium catalyst (Table 1).8 For example,
cat. MeS–
SMe
(b)
MeSMe
+
S
S
Me
Scheme 1 Synthesis of heteroles through the formal metathesis of
carbon–heteroatom bonds
Screening a series of bases led to an improvement in the
yield of 2a, with NaSMe being the most effective, giving 2a
in 87% isolated yield9 (Table 1, entry 5). The use of a polar
aprotic solvent, such as DMF, was essential for the success
of this reaction (entries 6–8), suggesting that this cycliza-
tion reaction proceeded through a nucleophilic substitution
process. Having established that NaSMe is the optimal initi-
ator for this reaction, we proceeded to explore the effect of
the leaving groups on the sulfur atoms (Table 2). Increasing
the steric bulk on the sulfur substituent led to a dramatic
decrease in yield, which would be predicted based on the
assumption that an SN2 mechanism is involved in the C(al-
kyl)–S bond cleavage process. Although the desired cycliza-
tion of substrates having ethyl and phenethyl groups took
place with low efficiency, it was possible to improve the
yields by changing the reaction conditions (entries 1 and 2).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

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In the case of an isopropyl group, no desired product was
formed (entry 3). When a benzyl-substituted substrate was
used, dibenzothiophene was successfully formed in 78%
yield, along with dibenzyl sulfide (71%) (entry 4). These re-
sults suggest that the benzyl thiolate, which is generated af-
ter the cyclization reaction via a C(aryl)–S bond cleavage,
also functions as a nucleophile in the cleavage of the C(al-
kyl)–S bond.
amide (2e and 2f) groups. Interestingly, the introduction of
an electron-donating group, such as a methyl group at the
para-position to the SMe group was tolerated, with the cy-
clized product 2g being formed in 94% yield. Our formal
metathesis method also allowed us to incorporate alkenes
(2h and 2i), naphthalenes (2j) and a pyridine ring (2k) into
the molecule, resulting in the synthesis of a variety of π-ex-
tended thiophenes. Pleasingly, it was also possible to pre-
pare six-membered rings (2l) by using this method.
Table 1 Optimization of Reaction Conditionsa
MeS
NaSMe (20 mol%)
MeS
Ar
Ar
Ar
Ar
Me
nucleophile (20 mol%)
DMF, 160 °C, 4 h
S
solvent, 160 °C, 4 h
S
S
1b–l
2b–l
S
Me
1a-Me
2a
O
CN
Ph
Entry
Nucleophile
Solvent
NMR yield of 2a (%)
1
2
3
4
5
6
7
8
LiOtBu
NaOtBu
KOtBu
DMF
0
S
S
DMF
3
2b 96%
2c 96%
O
DMF
56
F3C
CF3
NHPh
NaOMe
NaSMe
NaSMe
NaSMe
NaSMe
DMF
16
DMF
99 (87b)
S
S
2e 98%a,b
2d 93%
toluene
1,4-dioxane
tAmyl-OH
0
0
0
O
Me
Me
NEt2
a Reaction conditions: 1a-Me (0.20 mmol) and the nucleophile (0.04 mmol)
in DMF (1.0 mL) at 160 °C for 4 h.
S
S
2f 93%
2g 77%b (94%a,b
)
b Isolated yield.
S
Table 2 Effect of the Leaving Groupsa
S
S
RS
2h 99%b
2i 95%b
NaSMe (20 mol%)
DMF, 160 °C, 18 h
S
O
N
S
R
1a-R
2a
S
S
S
2j 98%
2k 95%
2l 85%b
Entry
R
Isolated yield of 2a (%)
16 (79b)
39 (87c)
0 (15c)
78d
Scheme 2 Substrate Scope. Reaction conditions: 1b-1l (0.20 mmol)
and NaSMe (0.04 mmol) in DMF (1.0 mL) at 160 °C for 4 h. Isolated
yields are shown. a 180 °C. b 18 h.
1
2
3
4
Et
Phenylethyl
iPr
Benzyl
To get insights into the mechanism, we used DFT calcu-
lations to further investigate the cyclization of 1a-Me with
an SMe anion (Figure 1).10 The energy changes at the
B3LYP/6-311+G* level of theory [SCRF (pcm, solvent=N,N-
dimethylformamide)] are shown in kcal/mol (Figure 1). An
exothermic reaction pathway with two transition states
(TS1 and TS2) was obtained. The first step is the cleavage of
a C(alkyl)–S bond via an SN2 mechanism and the calculated
Gibbs energy of activation (∆G‡) was estimated to be 33.4
kcal/mol. The calculations also indicate that the second step
a Reaction conditions: 1a-R (0.20 mmol) and NaSMe (0.04 mmol) in DMF
(1.0 mL) at 160 °C for 18 h.
b NaSMe (0.08 mmol) was used.
c NaOtBu (0.40 mmol) was used instead of NaSMe.
d Dibenzyl sulfide was also obtained (71% isolated yield).
We next investigated the scope of the reaction with re-
spect to SMe-substituted biaryl substrates (Scheme 2).
Gratifyingly, this method allowed us to synthesize various
biaryl substrates containing a range of functional groups,
including ketone (2b), cyano (2c), trifluoromethyl (2d), and
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proceeds through a concerted nucleophilic aromatic substi-
tution reaction (CSNAr)11 pathway with a ∆G‡ of 35.0
kcal/mol,12 which is the rate-determining step. Notably, a
Meisenheimer type intermediate could not be obtained by
intrinsic reaction coordinate (IRC) calculations at TS2. Since
the negative charge in the TS2 is also dispersed at the sulfur
atoms in addition to the arene ring, the reaction would be
expected to be less sensitive to the electronic effect of the
arene ring, compared with a pathway that proceeds
through a classical SNAr mechanism involving a Meisen-
heimer intermediate, in which the negative charge is ac-
commodated over the aromatic ring. The involvement of a
CSNAr mechanism is consistent with the successful cycliza-
tion of the electron-rich substrate 1g (Scheme 2).
clization of the more challenging biphenyl-based substrate
1n. DFT calculations revealed that the cyclization of 1m and
1n proceeds via a Meisenheimer intermediate,16 probably
because the OMe is a poorer leaving group than an SMe
group.
MeO
KOtBu (8 equiv)
DMF, 190 °C, 18 h
O
O
Me
1m–n
2m–n
O
O
ΔGB3LYP/6-311+g* (kcal/mol)
2m 70%
2n 49%
MeS
Scheme 3 Synthesis of dibenzofurans via metathesis of carbon-oxygen
bonds. Reaction conditions: 1m–n (0.10 mmol) and KOtBu (0.80 mmol)
in DMF (1.0 mL) at 190 °C for 18 h. Isolated yields are shown.
δ−
S
Me
SMe
δ−
δ−
S
S
δ−
33.4
Me
We also investigated substrates bearing both carbon–
sulfur and carbon–oxygen bonds (Scheme 4). Treatment of
the biaryl substrate 1o, which is readily accessible from
simple naphthalene derivatives,17 with a stoichiometric
amount of KOtBu gave the dibenzofuran derivative 2m se-
lectively in a yield of 52%. The selective O-cyclization of 1o
is not surprising given that the OH group is a far poorer
leaving group than the SMe group. In contrast, selective S-
cyclization is possible by the reaction of the ethylated biaryl
1p under our conditions to form the dibenzothiophene de-
rivative 2j in 71% yield. The selectivity for the cyclization of
TS1
26.9
TS2
MeSMe
SMe
0.0
SM
–8.1
S –
SMe–
+
Int2
1a-Me
2a
–24.4
Product
SMe
demethylation
CSNAr
Figure 1 Reaction pathways for carbon–sulfur bond metathesis
O
S
OH
Me
+
We subsequently investigated the possibility of extend-
ing the C–S bond metathesis reaction to the corresponding
C–O bonds. A C(sp2)–O bond is typically an inert bond and
transition metals are normally required to activate them.13
It should be noted, however, that nucleophilic aromatic
substitution reactions in which an OMe group serves as a
leaving group have recently been reported. However, the
use of substrates bearing strong electron-withdrawing
groups, such as cyano groups at ortho- or para-positions are
required for such reactions to proceed.14,15 We initially ex-
amined the reaction of 2,2′-dimethoxy-1,1′-binaphthalene
(1m) in the presence of NaSMe (400 mol%) at 160 °C for 18
h, but the expected dibenzofuran derivative 2m was not
formed. The lower reactivity of 1m compared with 1j can
be attributed to the lower nucleophilicity of the phenoxide
anion and poorer leaving ability11c of an OMe group com-
pared to an SMe group. Optimization of the reaction of 1m
led us to discover that when 8 equivalents of KOtBu were
used as a base at 190 °C, 2m was produced in 70% yield
(Scheme 3). These conditions can also be used for the cy-
Tf2O
KOtBu (2 equiv)
SMe
OH
O
DMF, 190 °C, 18 h
1o
2m 52%
NaH
then THF
EtI
NaSMe (1 equiv)
SMe
OEt
S
DMF, 180 °C, 18 h
1p
2j 71%
Scheme 4 Chemoselective ring closure via formal C-O/C-S metathesis
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1p is determined by the initial de-alkylation step, in which
a less hindered methyl group reacts more rapidly than an
ethyl group.
Carbon–silicon bond cleavage reaction for the synthesis of silole
derivatives: (e) Tobisu, M.; Onoe, M.; Kita, Y.; Chatani, N. J. Am.
Chem. Soc. 2009, 131, 7506. (f) Onoe, M.; Baba, K.; Kim, Y.; Kita,
Y.; Tobisu, M.; Chatani, N. J. Am. Chem. Soc. 2012, 134, 19477.
(g) Onoe, M.; Morioka, T.; Tobisu, M.; Chatani, N. Chem. Lett.
2013, 42, 238. Carbon-phosphorus bond cleavage reaction for
the synthesis of phosphole derivatives: (h) Baba, K.; Tobisu, M.;
Chatani, N. Angew. Chem. Int. Ed. 2013, 52, 11892. (i) Baba, K.;
Tobisu, M.; Chatani, N. Org. Lett. 2015, 17, 70. Carbon–sulfur
bond cleavage reaction for the synthesis of thiophene deriva-
tives: (j) Tobisu, M.; Masuya, Y.; Baba, K.; Chatani, N. Chem. Sci.
2016, 7, 2587. (k) Masuya, Y.; Tobisu, M.; Chatani, N. Org. Lett.
2016, 18, 4312. (l) Carbon–germyl bond cleavage reaction for
the synthesis of germole derivatives: Tobisu, M.; Baba, K.;
Chatani, N. Org. Lett. 2011, 13, 3282.
In summary, we report herein on the thiolate-initiated
formal double carbon–sulfur bonds metathesis for use in
the synthesis of dibenzothiophene derivatives. The C(aryl)–
S bond cleavage process was found to proceed through a
concerted nucleophilic aromatic substitution (CSNAr) path-
way. Furthermore, this metathesis protocol enables the
method to be expanded to double carbon–oxygen bonds
and carbon–oxygen/carbon–sulfur metathesis.
Funding Information
(6) (a) Baba, K.; Masuya, Y.; Chatani, N.; Tobisu, M. Chem. Lett. 2017,
46, 1296. (b) Lian, Z.; Bhawal, B. N.; Yu, P.; Morandi, B. Science
2017, 356, 1059.
(7) Treatment of 1a-Me (0.20 mmol) with [(allyl)PdCl]2 (5.0 mol%)
and NaOtBu (200 mol%) in DMF (1 mL) under 160 °C for 18 h
gave 2a in 97% NMR yield.
This work was supported by Grant-in-Aid for Scientific Research
(18H01978) and Scientific Research on Innovative Area "Precisely De-
signed Catalysts with Customized Scaffolding" (18H04259) from
MEXT, Japan.()
(8) Treatment of 1j with a stoichiometric amount of NaSMe was
reported to give a demethylated compound, along with a small
amount of 2j (11%). However, this was not investigated in detail.
See: Furia, F. D.; Licini, G.; Modena, G.; Valle, G. Bull. Soc. Chim.
Fr. 1990, 134.
Acknowledgment
We thank the Instrumental Analysis Center, Faculty of Engineering,
Osaka University, for their assistance with HRMS.
(9) Dibenzothiophene (2a) [CAS: 132-65-0]; Typical Procedure
An oven-dried 5 mL screw-capped vial was charged with 1a-Me
(49.2 mg, 0.20 mmol), NaSMe (2.8 mg, 0.04 mmol), and DMF (1
mL) under a gentle stream of nitrogen. The vessel was then
sealed and heated at 160 °C for 4 h. The mixture was cooled to
r.t. and filtered through a short pad of silica gel, eluting with
EtOAc. The eluent was evaporated to give a residue, which was
purified by flash chromatography (Rf 0.43, hexane) to give 2a as
a white solid (49 mg, 87%). 1H NMR (CDCl3, 399.78 MHz): δ =
7.44-7.47 (m, 4 H), 7.84–7.87 (m, 2 H), 8.15–8.17 (m, 2 H). 13C
NMR (CDCl3, 100.53 MHz): δ = 121.7, 122.9, 124.5, 126.8, 135.7,
139.6. HRMS (EI): m/z calcd for C12H8S: 184.0347; found:
184.0349.
(10) Calculations were performed with the Gaussian 09, Rev. D01
program (see the Supporting Information): Frisch, M. J.; Trucks,
G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J.
R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.;
Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A.
F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.;
Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.;
Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A. Jr.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.;
Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.;
Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi,
J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross,
J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.;
Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.;
Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.;
Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels,
A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox,
D. J. Gaussian 09, Revision D.01; Gaussian, Inc: Wallingford CT,
2013.
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611974.
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References and Notes
(1) Selected reviews: (a) Yamaguchi, S.; Tamao, K. J. Synth. Org.
Chem., Jpn. 1998, 56, 500. (b) Yamaguchi, S.; Tamao, K. Chem.
Lett. 2005, 34, 2. (c) Shimizu, M.; Hiyama, T. Synlett 2012, 973.
(d) Hill, A. F.; Fink, M. J. Advances in Organometallic Chemistry;
Academic Press: London, 2011.
V
o
l.
5
9
(2) Selected reviews: (a) Baumgartner, T.; Réau, R. Chem. Rev. 2006,
106, 4681. (b) Crassous, J.; Réau, R. Dalton Trans. 2008, 6865.
(c) Matano, Y.; Imahori, H. Org. Biomol. Chem. 2009, 7, 1258.
(d) Baumgartner, T. Acc. Chem. Res. 2014, 47, 1613. (e) Stolar,
M.; Baumgartner, T. Chem. Asian J. 2014, 9, 1212. (f) Matano, Y.
Chem. Res. 2015, 15, 636. (g) Duffy, M. P.; Delaunay, W.; Bouit,
P.-A.; Hissler, M. Chem. Soc. Rev. 2016, 45, 5296. (h) Joly, D.;
Bouit, P.-A.; Hissler, M. J. Mater. Chem. C 2016, 4, 3686.
(i) Hibner-Kulicka, P.; Joule, J. A.; Skalik, J.; Bałczewski, P. RSC
Adv. 2017, 7, 9194.
(3) Selected reviews: (a) Takimiya, K.; Shinamura, S.; Osaka, I.;
Miyazaki, E. Adv. Mater. 2011, 23, 4347. (b) Takimiya, K.;
Nakano, M. Bull. Chem. Soc. Jpn. 2018, 91, 121.
(4) Selected reviews: (a) Wu, B.; Yoshikai, N. Org. Biomol. Chem.
2016, 14, 5402. (b) Kodama, T.; Chatani, N.; Tobisu, M. J. Synth.
Org. Chem., Jpn. 2018, 76, 1185.
(5) Selected reviews: (a) Carbon–silicon bond cleavage: Komiyama,
T.; Minami, Y.; Hiyama, T. ACS Catal. 2017, 7, 631. (b) Carbon–
phosphorus bond cleavage: Wang, L.; Chen, H.; Duan, Z. Chem.
Asian J. 2018, 13, 2164. (c) Carbon–sulfur bond cleavage:
Modha, S. G.; Mehta, V. P.; Van der Eycken, E. V. Chem. Soc. Rev.
2013, 42, 5042. (d) Pan, F.; Shi, Z.-J. ACS Catal. 2014, 4, 280.
(11) (a) The first report: Neumann, C. N.; Hooker, J. M.; Ritter, T.
Nature 2016, 534, 369. (b) Review on carbon–fluorine bond
forming reactions via CSNAr mechanism: Neumann, C. N.;
Ritter, T. Acc. Chem. Res. 2017, 50, 2822. (c) Mechanistic study:
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E

E
Y. Masuya et al.
Cluster
Synlett
Kwan, E. E.; Zeng, Y.; Besser, H. A.; Jacobsen, E. N. Nat. Chem.
2018, 10, 917. (d) Lennox, A. J. J. Angew. Chem. Int. Ed. 2018, 57,
14686.
(b) Intramolecular amination of methoxy arenes: Pang, J. H.;
Kaga, A.; Chiba, S. Chem. Commun. 2018, 10324. (c) Acid-medi-
ated reactions: Mishra, A. K.; Verma, A.; Biswas, S. J. Org. Chem.
2017, 82, 3403. (d) Murai, M.; Origuchi, K.; Takai, K. Chem. Lett.
2018, 47, 927. (e) Light-mediated reaction: Tay, N. E. S.;
Nicewicz, D. A. J. Am. Chem. Soc. 2017, 139, 16100.
(12) A pathway initiated by a single electron transfer from NaSMe to
1a is unlikely, because this process was found to be endother-
mic by 66.3 kcal/mol. See Supporting Information for details.
(13) Selected reviews: (a) Tobisu, M.; Chatani, N. Acc. Chem. Res.
2015, 48, 1717. (b) Tobisu, M.; Chatani, N. Top. Curr. Chem. 2016,
374, 1. (c) Zeng, H.; Qiu, Z.; Domínguez-Huerta, A.; Hearne, Z.;
Chen, Z.; Li, C.-J. ACS Catal. 2017, 7, 510.
(14) (a) Wang, X.; Li, C.; Wang, X.; Wang, Q.; Dong, X.-Q.; Duan, A.;
Zhao, W. Org. Lett. 2018, 20, 4267. (b) Nucleophilic aromatic
substitution reaction of aryl alkyl thioethers: Wang, Xi.; Tang,
Y.; Long, C.-Y.; Dong, W.-K.; Li, C.; Xu, X.; Zhao, W.; Wang, X.-Q.
Org. Lett. 2018, 20, 4749.
(16) (a) See the Supporting Information for further information
regarding the reaction pathway of the C-O/C-O bond meta-
thesis. (b) C-O/C-S metathesis was also investigated by DFT cal-
culations, which indicated that the nature of the leaving group
determines the mechanism, rather than the nature of the
nucleophile. A better SMe leaving group favors CSNAr, whereas
an OMe leaving group favors SNAr.
(17) Yanagi, T.; Otsuka, S.; Kasuga, Y.; Fujimoto, K.; Murakami, K.;
Nogi, K.; Yorimitsu, H.; Osuka, S. J. Am. Chem. Soc. 2016, 138,
14582.
(15) (a) Intermolecular amination of methoxy pyridines: Kaga, A.;
Hayashi, H.; Hakamata, H.; Oi, M.; Uchiyama, M.; Takita, R.;
Chiba, S. Angew. Chem. Int. Ed. 2017, 56, 11807.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–E