Carbon-Oxygen Homocoupling of 2-Naphthols through Electrochemical Oxidative Dearomatization

Article abstract of DOI:10.1055/s-0037-1611777

A homocoupling reaction of 2-naphthols with formation of a C-O bond through electrochemical oxidative dearomatization in the presence of catalytic amounts of ferrocene and a ruthenium complex was developed. Mechanistic studies revealed that the reaction might proceed through coupling between two identical radical species. Moreover, a gram-scale experiment was performed to illustrate the potential practicability of this methodology in organic synthesis.

Full text of DOI:10.1055/s-0037-1611777

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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–G
letter
A
en
T. Chen et al.
Letter
Synlett
Carbon–Oxygen Homocoupling of 2-Naphthols through Electro-
chemical Oxidative Dearomatization
Ting Chena
Song Chena
Shaomin Fu*a
RVC(+) - Pt(–), 10 mA
[RuCl2(p-cymene)]2 (2.5 mol%)
R1
R1
Ar
Song Qina
Bo Liu*a,b
OH
Ar
Ferrocene (0.2 equiv), KPF6 (2.0 equiv)
H2O/1,4-dioxane (4:1), 90 °C, 4 h
16 examples, up to 92% yield
0
0
0
0
–
0
0
0
2
–
6
2
3
5
–
1
5
7
3
O
O
R1
Ar
a Key Laboratory of Green Chemistry and Technology of Ministry
of Education, College of Chemistry, Sichuan University, Chengdu
610064, P. R. of China
b State Key Laboratory of Natural Medicines, China Pharmaceutical
University, Nanjing 210009, P. R. of China
chembliu@scu.edu.cn
RVC = reticulated vitreous carbon
fsm09@aliyun.com
Received: 14.02.2019
Accepted after revision: 11.03.2019
Published online: 11.04.2019
Sudan I [1-(phenyldiazenyl)-2-naphthol] is a diazo dye
once used as a food coloring in some countries,6 but is now
recommended as unsafe because it causes cancers in some
animals such as mice and rabbits,7 and is also considered to
be a potential human carcinogen. Stiborová and co-workers
reported peroxidase-mediated oxidation of Sudan I, and
characterized the Sudan I dimer (Figure 1, right) as one a
major peroxidase-mediated metabolites. The isolation and
characterization of this dimer will help understand the
mechanism by which metabolites are formed during the
oxidation of Sudan I by peroxidase.8
DOI: 10.1055/s–0037–1611777; Art ID: st–2019–k0091–l
Abstract A homocoupling reaction of 2-naphthols with formation of a
C–O bond through electrochemical oxidative dearomatization in the
presence of catalytic amounts of ferrocene and a ruthenium complex
was developed. Mechanistic studies revealed that the reaction might
proceed through coupling between two identical radical species. More-
over, a gram-scale experiment was performed to illustrate the potential
practicability of this methodology in organic synthesis.
Key words electrochemistry, homocoupling, C–O bond formation,
Because these C–O bonded dimeric molecules have po-
tential bioactivities and uses, the discovery of efficient
methodologies for the construction of such compounds
could play a vital role in further investigations. Intermolec-
ular C–O homocoupling reactions through oxidative dearo-
matization form an efficient method for synthesizing the
core skeletons of these C–O bonded dimeric compounds.
Kovtonyuk and co-workers have disclosed such reactions of
tetra- or pentafluoro phenols in the presence of various ox-
idants, such as Pb(OAc)4, XeF2, VOF3, or VF5 [Scheme 1(a)].9
C–O homocouplings of 2-naphthols have been conducted
through oxidative dearomatization with PhNMe3·Br3 (PT-
naphthols, dearomatization, naphthyloxynaphthalenones
Deconjugated C–O bonded dimeric molecules have
found uses in organic syntheses and bioresearch. Lapachol,
a naphthoquinone isolated from the heartwood of the lapa-
cho tree (Handroanthus impetiginosus, formerly Tabebuia
avellanedae) ,1 has diverse bioactivities, such as anticancer,2
analgesic,3 and trypanocidal activities.4 The C–O bonded di-
mer of laplachol can be prepared by Hooker’s oxidation
method (Figure 1, left).1c–e Biological probing has shown
that this dimer exhibits activity against HCT-8 (colon) can-
cer cells (IC50, 9.76 M).5
10
AB)/H2O2 or TBAI/t-BuOOH.11 The substrates in these
works were limited to 1-alkyl-2-naphthols [Scheme 1(b)]. It
is apparent that to complement current progress in relation
to this reaction, more attempts need to be made to discover
novel C–O bond homocoupling approaches through oxida-
tive dearomatization.
In the past decade, electroorganic chemistry has been
shown to provide a green alternative to common chemical
oxidizing or reducing agents12 and, in some cases, to induce
extraordinary reactivity, superior to that attainable by con-
ventional methods.13 Notably, Quideau et al. reported an
electrochemical C–O cross-coupling reaction of 2-methoxy-
phenols through oxidative dearomatization in MeOH, al-
O
O
O
O
O
O
O
N
N
N
N
O
metabolite from peroxidase-
catalyzed oxidation of Sudan I
Hooker' s lapachol dimer
Figure 1 Deconjugated C–O bond naphthol dimers
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

B
T. Chen et al.
Letter
Synlett
a) previous report: stochiometric C–O homo-coupling via oxidative dearomatization
OH
F
F
R
F
F
F
F
F
F
F
R
O
F
F
F
F
F
Pb(OAc)4
O
F
R
O
R
XeF2, VF5
VOF3, etc.
F
F
F
O
R
F
F
R = F or Me
Alk
Alk
PhNMe3 Br3 , H2O2
OH
O
nBu4NI, t-BuOOH
Alk
O
b) previous report: electrochemical C-O cross-coupling via oxidative dearomatization
O
OH
OMe
OMe
OMe
only one example
limited scope
electricity
64% yield
MeOH
Br
Br
c) this report: electrochemical C–O homo-coupling via oxidative dearomatization
R1
2.5 mol%
[Ru(p-cymene)Cl2]2
Ar
R1
OH
Ar
20 mol% Cp2Fe
O
O
R1
electricity
Ar
16 examples, up to 92% yield
Scheme 1 C–O bond coupling reactions through oxidative dearomatization
though only one example was showcased [Scheme 1(b)].14
We were therefore motivated by the prospect of exploiting
electrochemical synthesis to induce a dearomatization re-
action leading to C–O homocoupling although, to the best
of our knowledge, there was no precedent for this. Here, we
described our findings on an electrochemical oxidative
dearomatization approach to the C–O homocoupling of 2-
naphthols [Scheme 1(c)]. The reaction is compatible with
various 1-aryl-2-naphthols in the presence of inexpensive
ferrocene (Cp2Fe) as a redox catalyst.
We began this project by using 1-phenyl-2-naphthol
(1a) as a substrate to probe the optimal reaction condition
to give the homocoupling product 2a (Table 1). Besides ex-
tensive NMR characterization, the molecular structure of 2a
was confirmed by single-crystal X-ray diffraction analysis
(see Scheme 2, below).15 After extensive examination [See
Supplementary Information (SI) for details], a maximum
yield of 92% was obtained when the electrochemical syn-
thesis was conducted at a constant current of 10 mA in an
undivided cell with a reticulated vitreous carbon anode
[RVC (+)] and a platinum plate cathode [Pt (–)] with 20
mol% Cp2Fe as a redox catalyst and 2.5 mol% [RuCl2(p-cy-
mene)]2 as an additive in an electrolyte solution of KPF6 in
H2O–1,4-dioxane (4:1) at 90 °C.
electrode materials other than an RVC anode and a plati-
num plate cathode (Table 1, entries 1–4). Notably, when the
platinum cathode was replaced with an inexpensive copper
plate, the product yield decreased markedly (entry 3). Not
surprisingly, performing the electrolysis without an electric
current or in the absence of Cp2Fe suppressed the reaction
completely (entries 5 and 6). It has been reported that cy-
clizations of phenols or benzoic acid can be catalyzed by a
Ru complex through weak O-coordination under electro-
chemical conditions.16 Because our reaction was also per-
formed without a base, we reasoned that the introduction
of a Ru complex to coordinate with naphthols 1a and to in-
crease the acidity of the proton would accelerate the pro-
duction of H2 at the cathode and would promote the reac-
tion. Notably, the product was obtained in 65% yield under
electrolysis in the absence of [RuCl2(p-cymene)]2 (entry 7),
demonstrating that this ruthenium salt was not essential
for the reaction and might act as an additive, albeit in a cat-
alytic amount. Changing the amount of Cp2Fe from 20 mol%
in the absence of [RuCl2(p-cymene)]2 gave inferior results
(entries 8 and 9) (See SI for details). A slightly lower yield
was observed when the amount of ruthenium salt was re-
duced from 2.5 mol% to 1 mol% (entry 10). Not surprisingly,
the supporting electrolyte also affected the efficacy of this
reaction (entries 11 and 12). In addition, altering the cur-
rent from 10 mA to 20 mA or 5 mA proved detrimental to
this electrochemical reaction (entries 13 and 14). Control
It was found that the electrode materials had a marked
effect on the reaction, since a lower yield was obtained with
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

C
T. Chen et al.
Letter
Synlett
Table 1 Screening of reaction conditions
the corresponding homocoupling products in satisfactory
yields of 69–86%. The presence of other electron-deficient
groups on the aryl ring, such as ester (2j) or cyano (2k)
groups, was found to benefit the electrochemical homocou-
pling under the standard conditions. Notably, it had previ-
ously been found that a styryl group could be electrochemi-
cally oxidized to a radical-cation intermediate that was cap-
tured by alcohols, H2O, or other nucleophiles.17
Interestingly, a styryl group was left intact under our reac-
tion conditions, and compound 2l was obtained in a satis-
factory 80% yield; this might be due to the higher electrical
potential (Ep/2 > 1.5 V vs. SCE)17a,18 preventing oxidation of
the styryl group to alkene radical-cation intermediates.
Additionally, a variety of functional groups at the C-6
position of the naphthalene ring, including methyl (2m),
chloro (2n), bromo (2o), or ester groups (2p), were found to
be compatible with the reaction. Unfortunately, substrates
with heteroaryl substituents at the C-1 position of 2-naph-
thol did not give the desired product (see SI for details).
Specifically, a methoxy-substituted substrate did not
give the corresponding homocoupling product, but instead
gave the hydroxylated compound 2q (Scheme 3). We rea-
soned that the presence of a methoxy group would make
the electrochemically generated intermediate with a radical
at C-1 prone to release an electron to generate a cationic
carbon atom at the C-1 position. This cationic species would
be stabilized by two phenyl groups, one of which is elec-
tron-rich, and then captured by water to give compound 2q.
To probe the reaction mechanism, we performed cyclic
voltammetry (CV) on the RVC electrode in a solution of 0.15
g/L Cp2Fe in 2:1 H2O–1,4-dioxane with 0.8 M KPF6 as the
supporting electrolyte at a scan rate of 10 mV∙s–1 at 50 °C
(Figure 2). A pair of quasi-reversible peaks were observed
centered at about 0.1 V versus Ag/AgCl; these were super-
imposed on a large capacitive background in the absence of
the substrate 1a [Figure 2(a)]. The waves showed a typical
CV pattern of a Cp2Fe(II) ↔ Cp2Fe(III)+ pair. When substrate
1a was gradually added to the above system, the reduction
peak for Cp2Fe(III)+ fell markedly when the concentration of
1a reached 3.5 g/L, showing that most of the Cp2Fe(III)+ spe-
cies was probably reduced in solution rather than on the
electrode. On the other hand, two oxidation peaks appeared
in the CV curves [Figure 2(b)]. The first peak, centered at 0.1
V, arose from oxidation of Cp2Fe(II), and its peak current (ip)
showed a linear relationship to the amount of Cp2Fe(II). The
second peak was attributed to oxidation of substrate 1a,
and its ip also showed a good linear correlation with the
concentration of Cp2Fe(II) [Figure 2(c)]. These results indi-
cate that Cp2Fe(III)+, generated by electronic oxidation on
the anode, mediates the oxidation of 1a, which is followed
by reduction to Cp2Fe(II) in the solution.19
RVC(+), Pt(–), 10 mA
Ph
1
[RuCl2(p-cymene)]2 (2.5 mol%)
1
OH
O
O
Ferrocene (0.2 equiv), KPF6 (2.0 equiv)
H2O/1,4-dioxane (4:1), 90 °C, 4 h
Ph
1a
2a 92%
undivided cell
Entry
Deviations from standard conditions
Yielda (%)
1
2
graphite (–)
79
RVC (–)
72
3
Cu (–)
trace
4
Pt (+)
76
5
no current
0
6
no ferrocene
trace
7
no [RuCl2(p-cymene)]2
ferrocene (0.1 equiv), no [RuCl2(p-cymene)]2
ferrocene (0.3 equiv), no [RuCl2(p-cymene)]2
[RuCl2(p-cymene)]2 (1.0 mol%)
Bu4NOAc as electrolyte
Bu4NClO4 as electrolyte
current: 5 mA
65
8
45
9
62
10
11
12
13
14
15
16
17
18
19
87
83
54
69
current: 20 mA
76
H2O–acetone (1:4), 70 °C
H2O–MeOH, 60 °C
70 °C
50
14
32
r.t.
trace
gram scale
76 (0.765g)
a Isolated yield.
experiments verified that the best solvent system is a 4:1
mixture of H2O and 1,4-dioxane; other solvent systems
such as H2O–acetone or H2O–MeOH gave inferior yields (en-
tries 15 and 16), unlike the results obtained by Quideau et
al. [Scheme 1(b)].14 Notably, when the reaction was per-
formed at a lower temperature, the electrolysis gave disap-
pointing results (entries 17 and 18). The reaction could be
scaled up to a gram scale, with 76% yield (entry 19).
With these optimal reaction conditions, we set about
exploring the scope of this method (Scheme 2). A broad
range of functional groups on the phenyl ring (Ar) were tol-
erated. Substrates with 4-methyl or 4-tert-butyl substitu-
ents on the phenyl ring (1b and 1c, respectively) gave the
corresponding homocoupling products 2b and 2c in yields
of 71 and 49%, respectively. Fluoro- (2d and 2e), chloro-
(2f), bromo- (2g), and trifluoromethyl-substituted sub-
strates (2h and 2i) exhibited good reactivity and afforded
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

D
T. Chen et al.
Letter
Synlett
R1
Ar
1
RVC(+), Pt(–), 10 mA, 90 °C
6
R1
OH
[RuCl2(p-cymene)]2 (2.5 mol%)
Ar
1
6
Ferrocene (0.2 equiv), KPF6 (2.0 equiv)
H2O–1,4-dioxane (4:1), undivided cell
O
O
2
R1
Ar
1
tBu
Me
H
O
O
O
O
O
O
tBu
Me
H
2a, 92%
2b, 71%
2c, 49%
F
F
Cl
O
O
O
O
O
O
F
F
Cl
2d, 83%
2e, 86%
2f, 73%
CF3
F3C
Br
O
F3C
O
O
O
O
O
CF3
CF3
F3C
Br
2g, 70%c
2h, 71%
2i, 69%
NC
MeOOC
O
O
O
O
O
O
CN
COOMe
Me
2l, 80%
2j, 72%
2k, 73%
Cl
Br
O
O
O
O
O
O
Cl
Br
Me
2m, 58%
2n, 64%
2o, 71%
COOEt
O
O
EtOOC
2p, 81%
ORTEP of single crystal of 2a
Scheme 2 Substrate scope. Reaction conditions: RVC anode [100 pores per linear inch, 10 × 10 × 12 mm], Pt cathode (10 × 12 × 0.1 mm), substrate
(0.15 mmol), H2O (12 mL), 1,4-dioxane (3.0 mL), 4 h. Isolated yields are reported.a 0.1 mmol scale.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

E
T. Chen et al.
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(Scheme 4). The electrolytic process commences with an-
odic oxidation of Cp2Fe to generate Cp2Fe+. Meanwhile, sub-
strate 1a coordinates with the [Ru] complex to give inter-
mediate I, which is reduced at the cathode to produce II and
H2 efficiently. Then, single-electron transfer (SET) between
II and Cp2Fe+ gives the oxygen-centered radical III, with re-
generation of Cp2Fe. A carbon-centered radical IV might
then be formed, which has a resonance structure associated
with intermediate III.20 Theoretically, three coupling path-
ways should then be possible. For Path a, O–O bond forma-
tion through homocoupling of III might generate peroxide
V, which is likely to be unstable and would be reduced at
the cathode to regenerate II under the current electrochem-
ical condition. For Path b, C–O bond formation through cou-
pling of III and IV produces the stable product. Hence, this
pathway is predominant among the three possible path-
ways. For Path c, C–C bond formation through homocou-
pling of radical IV would suffer from considerable steric
hindrance, disfavoring this pathway; this is supported by
the fact that we did not observe any compound VI in the re-
action.
OMe
MeO
RVC(+), Pt(–), 10 mA, 90 °C
[RuCl2(p-cymene)]2 (2.5 mol%)
OH
1
O
1
Ferrocene (0.2 equiv), KPF6 (2.0 equiv)
OH
2
2
H2O–1,4-dioxane (4:1), undivided cell
2q, 67% yield
Scheme 3 Electrolysis of a methoxy-substituted substrate
(a)
(b)
peak 2
peak 1
E vs. Ag/AgCl (V)
(c)
E vs. Ag/AgCl (V)
Concentration of Cp2Fe (g/L)
a) CV of 0.15g/L Cp2Fe; b) CV of 1a of 3.5 g/L with the different amount of
Cp2Fe. CV was conducted on the RVC electrode in the H2O/1,4-dioxane (2:1)
solution with 0.8 M KPF6 over 10 mV/s scan rate at 50 °C.
In summary, we have reported an electrochemical oxi-
dative dearomatization method for C–O homocoupling of 2-
naphthols.21 A gram-scale experiment illustrated the po-
tential practicability of our method. Further studies on ap-
plications of this electrochemical system in syntheses of re-
lated C–O homocoupling dimers that are bioactive and use-
ful are underway in our laboratory.
Figure 2 Cyclic voltammetry of the tested reaction system
Based on the above study, a possible mechanism for
electrochemical synthesis of the homocoupling product
was proposed with 2-naphthol (1a) as a model substrate
Ph
O
Ph
Ph
II
+ 2 e–
Ph
O
O
O
path a
unstable
O–O
peroxide V
coupling
IV
Ph
O
path b
target product 2a
O
C–O
coupling
Pt
RVC
favored pathway
Ph
Ph
O
O
sterically hindered VI
unfavored pathway
Ph
path c
Cp2Fe
Ph
C–C
coupling
O
III
more acidic
H
Ph
–
– e
Ph
SET
Ph
OH
[Ru]
O
O
[Ru]
I
1a
Cp2Fe
Ph
– 1/2 H2
–
II
O
[Ru]
II
Scheme 4 Plausible mechanism
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

F
T. Chen et al.
Letter
Synlett
Funding Information
(12) For selected reviews, see: (a) Horn, E. J.; Rosen, B. R.; Baran, P. S.
ACS Cent. Sci. 2016, 2, 302. (b) Yan, M.; Kawamata, Y.; Baran, P. S.
Chem. Rev. 2017, 117, 13230. (c) Tang, S.; Liu, Y.; Lei, A. Chem.
2018, 4, 27. (d) Yang, Q.-L.; Fang, P.; Mei, T.-S. Chin. J. Chem.
2018, 36, 338. (e) Jiang, Y.; Xu, K.; Zeng, C. Chem. Rev. 2018, 118,
4485. (f) Cao, Y.; He, X.; Wang, N.; Li, H.-R.; He, L.-N. Chin. J.
Chem. 2018, 36, 644. (g) Sauermann, N.; Meyer, T. H.; Qiu, Y.;
Ackermann, L. ACS Catal. 2018, 8, 7086. For selected recent
examples, see. (h) Hou, Z.-W.; Mao, Z.-Y.; Zhao, H.-B.; Melcamu,
Y. Y.; Lu, X.; Song, J.; Xu, H.-C. Angew. Chem. Int. Ed. 2016, 55,
9168. (i) Gieshoff, T.; Schollmeyer, D.; Waldvogel, S. R. Angew.
Chem. Int. Ed. 2016, 55, 9437. (j) Tang, S.; Gao, X.; Lei, A. Chem.
Commun. 2017, 53, 3354. (k) Zhao, H.-B.; Hou, Z.-W.; Liu, Z.-J.;
Zhou, Z.-F.; Song, J.; Xu, H.-C. Angew. Chem. Int. Ed. 2017, 56,
587. (l) Tang, S.; Wang, S.; Liu, Y.; Cong, H.; Lei, A. Angew. Chem.
Int. Ed. 2018, 57, 4737. (m) Zhang, S.; Li, L.; Xue, M.; Zhang, R.;
Xu, K.; Zeng, C. Org. Lett. 2018, 20, 3443. (n) Tian, C.; Massignan,
L.; Meyer, T. H.; Ackermann, L. Angew. Chem. Int. Ed. 2018, 57,
2383. (o) Shao, A.; Li, N.; Gao, Y.; Zhan, J.; Chiang, C.-W.; Lei, A.
Chin. J. Chem. 2018, 36, 619. (p) Hou, Z.-W.; Yan, H.; Song, J.-S.;
Xu, H.-C. Chin. J. Chem. 2018, 36, 909. (q) Xu, F.; Li, Y. J.; Huang,
C.; Xu, H. C. ACS Catal. 2018, 8, 3820. (r) Sauer, G. S.; Lin, S. ACS
Catal. 2018, 8, 5175. (s) Yuan, Y.; Cao, Y.; Qiao, J.; Lin, Y.; Jiang,
X.; Weng, Y.; Tang, S.; Lei, A. Chin. J. Chem. 2019, 37, 49.
(13) For selected examples, see: (a) Mihelcic, J.; Moeller, K. D. J. Am.
Chem. Soc. 2004, 126, 9106. (b) Rosen, B. R.; Werner, E. W.;
O’Brien, A. G.; Baran, P. S. J. Am. Chem. Soc. 2014, 136, 5571.
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575.
This project was supported by the Open Fund of State Key Laboratory
of Natural Medicines in China Pharmaceutical University,
(Grant/Award Number: SKLNMKF201810) and the National Natural
Science Foundation of China (Grant/Award Number: 21672153).
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Acknowledgment
We would like to thank the Analytical and Testing Center of Sichuan
University for x-ray diffraction work and we are grateful to Dr.
Daibing Luo for his help in the single-crystal analysis
Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611777.
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References and Notes
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A 25 mL four-necked flask equipped with stirrer bar, a reflux
condenser, an RVC anode (100 pores/inch, 10 × 10 × 12 mm;
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equiv), H2O (12.0 mL), and 1,4-dioxane (3.0 mL). Constant-
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G

G
T. Chen et al.
Letter
Synlett
r.t., the layers was separated, and the aqueous phase was
extracted with EtOAc (3 × 20 mL). The organic phases were
combined, dried (Na2SO4), filtered, and concentrated under
reduced pressure. The crude product was purified by column
chromatography.
(m, 2 H), 7.62 (d, J = 10.0 Hz, 1 H), 7.57–7.47 (m, 6 H), 7.39 (td, J
= 7.2, 1.8 Hz, 1 H), 7.35–7.27 (m, 4 H), 7.16–7.11 (m, 1 H), 7.06
(dd, J = 10.1, 4.9 Hz, 2 H), 7.01–6.97 (m, 2 H), 6.42 (d, J = 9.1 Hz,
1 H), 6.25 (d, J = 10.0 Hz, 1 H).13C NMR (101 MHz, CDCl3):
 = 196.78, 150.02, 145.09, 143.89, 139.57, 137.11, 133.97,
131.81, 131.37, 130.79, 130.56, 129.71, 128.91, 128.87, 128.44,
128.21, 128.16, 127.82, 127.19, 126.94, 126.37, 125.93, 125.75,
125.48, 123.78, 116.41, 85.58, 29.84. HRMS (ESI): m/z [M + Na]+
calcd for C32H22NaO2: 461.1512; found: 461.1508.
1-Phenyl-1-[(1-phenyl-2-naphthyl)oxy]naphthalen-2(1H)-
one (2a)
Yellow foam solid; yield: 40.1 mg (92%); TLC: Rf = 0.45 (EtOAc–
PE, 1:5). IR (neat): 2973, 1923, 1734, 1684, 1507, 1053, 697 cm–1
.
1H NMR (400 MHz, CDCl3):  = 7.85 (d, J = 7.4 Hz, 1 H), 7.71–7.64
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–G