Platinum-on-Carbon-Catalyzed Aqueous Oxidative Lactonization of Diols by Using Molecular Oxygen

Article abstract of DOI:10.1055/s-0037-1611917

A lactonization of various diols catalyzed by platinum on carbon (Pt/C) in water under an atmosphere of molecular oxygen was developed. Derivatives of 1,4- 1,5- and 1,6-diols were transformed into the corresponding five-, six-, and seven-membered lactones by the present oxidative lactonization method.

Full text of DOI:10.1055/s-0037-1611917

SYNLETT
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
©
Georg Thieme Verlag Stuttgart · New York
2019, 30, A–E
letter
A
en
Synlett
R. Takakura et al.
Letter
Platinum-on-Carbon-Catalyzed Aqueous Oxidative Lactonization
of Diols by Using Molecular Oxygen
Ryoya Takakura
Kazuho Ban
HO
OH
O
O
1
0% Pt/C, O2 (1 atm)
H2O, 80 °C, 12–24 h
Hironao Sajiki*
Yoshinari Sawama*
•
•
•
mild & neutral conditions
environmentally friendly solvent
13 examples, up to 88% yield
0
0
0
0-0
0
0
2-9
9
2
3-
2
4
1
2
Laboratory of Organic Chemistry, Gifu Pharmaceutical University,
-25-4 Daigaku-nishi, Gifu 501-1196, Japan
1
sajiki@gifu-pu.ac.jp
sawama@gifu-pu.ac.jp
Received: 12.07.2019
Accepted after revision: 08.08.2019
Published online: 27.08.2019
total synthesis.17 We now report the Pt/C-catalyzed clean
oxidative lactonization of diols in water under an atmo-
sphere of molecular oxygen.
DOI: 10.1055/s-0037-1611917; Art ID: st-2019-u0364-l
We have previously reported a dehydrogenative oxida-
tion of primary alcohols to carboxylic acids catalyzed by
palladium on carbon (Pd/C) in water, which proceeds via
the corresponding aldehyde and a subsequent hydrate in-
termediate.18 When 1,2-phenylenedimethanol (1a), bearing
two benzylic alcohol functions, was used as a substrate un-
der Pd/C-catalyzed dehydrogenation conditions in water
under an Ar atmosphere at 80 °C for six hours, the desired
lactone product, the phthalide 2a, was obtained in a low
(10%) yield (Table 1, entry 1). This lactonization of 1a pro-
ceeds through oxidation of the primary benzylic alcohol
moiety to the corresponding aromatic aldehyde as an inter-
mediate during the first oxidative step (see Scheme 2, be-
low). We therefore performed the reaction of 1a under an
oxygen atmosphere with the aim of quenching the generat-
ed hydrogen, which is responsible for the reverse reaction
Abstract A lactonization of various diols catalyzed by platinum on car-
bon (Pt/C) in water under an atmosphere of molecular oxygen was de-
veloped. Derivatives of 1,4- 1,5- and 1,6-diols were transformed into
the corresponding five-, six-, and seven-membered lactones by the
present oxidative lactonization method.
Key words lactones, diols, lactonization, platinum catalysis, water,
green chemistry
Lactones are important frameworks in the organic,
pharmaceutical, and industrial fields. Various synthetic
methods have been developed for the construction of lac-
tone skeletons, including intramolecular hydroacyloxyl-
1
ation of olefinic acids, intramolecular esterification of hy-
2
droxy acids, and Baeyer–Villiger reactions of cyclic ke-
3
tones. The oxidative lactonization of easily available diol
substrates is also useful and has been extensively investi-
(catalytic hydrogenation of the aromatic aldehyde), with
4–15
4
19
gated.
Stoichiometric metal reagents such as silver or
O . As a result, 1a was transformed into 2a in moderate
2
5
chromium have been traditionally used in the oxidative
lactonization of diols. Meanwhile, several catalytic methods
that use a transition metal with an oxidant such as oxy-
yield (entry 2). Encouraged by this result, we examined the
effects of various metal-on-carbon catalysts (entries 2–8).
Pt/C and Ir/C showed higher catalytic activities, giving 2a in
yields of 74 and 75%, respectively (entries 3 and 4), whereas
the use of Au/C produced incomplete lactonization to give
2a in 57% yield, together with recovered 1a in 26% yield (en-
6–13
14
15
gen,
hydrogen peroxide, or a carbonyl compound
have been developed, and transition-metal-catalyzed dehy-
drogenations of diols to give lactones have also been report-
12,16
20
ed.
Among these, the lactonization of diols under an at-
try 5). Rh/C, Ru/C, and Cu/C were ineffective catalysts (en-
mosphere of O as an oxidant is valuable from the view-
tries 6–8), although Ru/C has been reported to be an effi-
cient catalyst for the oxidation of alcohols by oxygen in tol-
uene to give carbonyl compounds. On decreasing the
catalyst loading of Ir/C to 1 mol%, the reaction efficiency
drastically decreased (entry 9). In contrast, 1 mol% of Pt/C
retained sufficient catalytic activity (entry 10), and 2a was
obtained in 88% isolated yield on extending the reaction
time to 12 hours, even in the presence of only 0.5 mol% of
2
point of atom economy and green sustainability, because
6
21
the only byproduct is water. Although homogeneous Ru,
7
8
9
10
11
12
Pd, or Ir species or heterogeneous Mn, Ru, Pd, Co, or
Au species have been reported to catalyze the lactoniza-
13
tion of diols under O , the use of an organic solvent was es-
2
sential in all cases. A transformation using preactivated
PtO in acetone–water has also been reported as part of a
2
©
2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E
Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
Synlett
R. Takakura et al.
Letter
Pt/C (entry 11). A further decrease in the loading of Pt/C
0.1 mol%) or a lower reaction temperature (60 °C) led to a
lower yield of 2a (entries 12 and 13). The starting material
a was not completely consumed under an air atmosphere
entry 14), and the lactonization did not proceed at all un-
The scope of the substrates was next examined in the
presence of Pt/C (0.5 mol%) in water under an O2 atmo-
sphere at 80 °C for 12 hours (Table 2). A 5 mmol (0.69 g)-
scale reaction with 1a as a substrate was successfully per-
formed to give 2a in 82% yield (Table 2, entry 1). 1,2-Phenyl-
enedimethanol derivatives 1b–d bearing various substitu-
ents at the 4-position of their aromatic nuclei were
smoothly transformed into the corresponding phthalide de-
rivatives 2b–d as isomeric mixtures (entries 2–4). In these
reactions, the 5-substituted phthalides 2ba–da were ob-
(
1
(
der an argon atmosphere (entry 15).
Table 1 Catalyst Effects
O
OH
OH
catalyst (X mol%), O2 (balloon)
H2O (1 mL), 80 °C
22
tained as the major products. The reactions of 4,5-dime-
O
thoxy-1,2-phenylenedimethanol (1e) and of 4,5-dichloro-
1,2-phenylenedimethanol (1f) proceeded similarly to give
the corresponding lactones 2e and 2f, together with residu-
al starting material and/or a dialdehyde as a byproduct, as
1a (0.25 mmol)
2a
a
Entry
Catalyst
Mol%
Time (h)
Yield (%)
1
1
a
2a
confirmed by H NMR analysis (entries 5 and 6). Substrate
1
g, bearing both primary and secondary alcohol moieties
1b
2
3
4
5
6
7
8
9
10% Pd/C
10% Pd/C
10% Pt/C
10% Ir/C
10% Au/C
10% Rh/C
10% Ru/C
10% Cu/C
10% Ir/C
10% Pt/C
10% Pt/C
10% Pt/C
10% Pt/C
10% Pt/C
10
10
10
10
10
10
10
10
1
6
6
34
0
10
within the molecule, was also converted into the desired
lactone product 2g in moderate yield (entry 7). The reaction
of the unsymmetrical diol 1h gave a mixture of 1-isochro-
manone (2ha) and 3-isochromanone (2hb) (entry 8); pref-
erential oxidation of the benzylic alcohol moiety provided
55
74
75
57
15
4
6
0
6
0
6
26
34
20
78
33
9
2
ha as the major isomer. 1,8-Bis(hydroxymethyl)naphtha-
lene (1i) was smoothly converted into the 1,8-naphthalide
i (entry 9). The present method was also applicable to the
6
6
2
6
0
synthesis of the seven-membered-ring lactone 2j from 2,2′-
bis(hydroxymethyl)-1,1′-biphenyl (1j), albeit in low yield
(entry 10). The yield of 2j was not improved by raising the
6
10
81
1
1
1
1
1
0
1
6
1
0.5
12
12
12
12
12
2
90 (88)c
temperature to 120 °C or increasing the O pressure to 3
2
atm. The lactonization of 2-phenylbutane-1,4-diol (1k) af-
forded an 88:12 mixture of isomers 2ka and 2kb in good
yield (entry 11). Butane-1,4-diol (1l), as a simple linear ali-
phatic diol, was transformed into -butyrolactone (2l) in
only 19% yield, with the formation of the acyclic dicarboxyl-
ic acid 3l as an undesired byproduct (entry 12), whereas the
reaction of cis-di(hydroxymethyl)cyclohexane (1m), as an
alicyclic diol, provided the corresponding lactone 2m in
moderate yield (entry 13).
2
0.1
0.5
0.5
0.5
36
26
56
82
45
66
30
0
3d
4e
5b
1
10% Pt/C
a
1
Determined by H NMR spectroscopy with 1,3-benzodioxole (30.5 mg,
0
.25 mmol) as an internal standard.
Under argon.
b
c
Isolated yield.
The reaction was carried out at 60 °C.
Under air.
d
e
Table 2 Pt/C-catalyzed oxidative lactonization of diols.
HO
OH
O
O
1
0% Pt/C (0.5 mol%), O2 (balloon)
H2O (1 mL), 80 °C, 12 h
a
Entry
1
Substrate
product
Yield (%)
R
O
R
OH
OH
R
O
88 (82)b
O
O
R = H (1a)
2
a
–
2
3
4
R = Me (1b)
R = Br (1c)
R = F (1d)
2ba
2ca
2da
2bb
2cb
2db
82 (32:68)c
26 (12:88)c
89 (29:71)c
©
2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

C
Synlett
R. Takakura et al.
Letter
Table 2 (continued)
Entry
a
Substrate
product
Yield (%)
O
MeO
OH
OH
MeO
MeO
O
5
6
43
MeO
1
e
2
e
O
Cl
Cl
OH
OH
Cl
Cl
O
40 (21)d
1f
2f
O
OH
OH
O
7e
8e
9e
56 (20)d
1
g
2
g
O
OH
O
O
c
d
OH
50 (72:28) (10)
O
2hb
1
h
2ha
HO HO
O
O
70 (24)d
1i
2i
HO
OH
O
O
2
1
1
0e
f
g
(
15) (29)
1j
2j
OH
O
O
O
O
OH
c
d
1
1
1
1e
78 (88:12) (5)
2
ka
1
k
2kb
O
CO2H
HO
l: 25i
2
2h
OH
O
CO2H
3l: 19
1
l
2l
3l
H
H
O
OH
OH
O
3
47
H
H
1m
2m
a
b
c
d
e
f
Isolated yield.
5
mmol scale.
1
The ratio of the two isomers was determined by H NMR spectroscopy of the crude product.
Recovered starting material.
The reaction was stirred for 24 h.
At 120 °C.
2
Under 3 atm O pressure.
Succinic acid was obtained in 19% yield.
The yield was determined by H NMR using 1,3-benzodioxole as an internal standard.
g
h
i
1
©
2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

D
Synlett
R. Takakura et al.
Letter
When the benzyl alcohol derivative 4 was used as a sub-
strate instead of the diol 1, a mixture of 4-methylbenzalde-
hyde (5) and 4-methylbenzoic acid (6) was obtained, even
after 12 hours (Scheme 1, Equation 1). In addition, 2-(hy-
droxymethyl)benzoic acid (7) was converted into the
phthalide 2a in both the presence and absence of Pt/C
hydration of the aldehyde moiety in A gives intermediate C,
which undergoes oxidation to produce the hydroxy acid in-
termediate D. Subsequent intramolecular condensation of
D produces 2a (path B). Whereas the lactonization in organ-
ic solvents (Scheme 1, Equation 3) proceeds by path A with-
out the formation of hydrate C, the lactonization in water
can proceed by both pathways A and B, as demonstrated in
Scheme 1, Equations 1 and 2.
We have accomplished an oxidative lactonization of a
variety of diols by using Pt/C24 as a catalyst in aqueous me-
dium under an oxygen atmosphere. The present catalytic
method in environmentally benign water, with molecular
oxygen as a clean oxidant (hydrogen acceptor), is valuable
from the viewpoint of green sustainability and atom effi-
ciency. Furthermore, the use of water is advantageous as it
suppresses the potential ignition properties of Pt/C.
(Scheme 1, Equation 2). The present reaction could also be
carried out in various organic solvents [DMF, cyclopentyl
methyl ether (CPME), or toluene] instead of water, giving 2a
in good yields (Scheme 1, Equation 3).
CHO
+
CO2H
OH
Pt/C (0.5 mol%), O2 (balloon)
(1)
H2O (1 mL), 80 °C, 12 h
5
6
4
(0.25 mmol)
39%
40%
O
O
CO2H
OH
Pt/C (0.5 mol%), O2 (balloon)
2
(2)
H O (1 mL), 80 °C, 12 h
Funding Information
7
(0.25 mmol)
2a
with Pt/C
84 (%)
This study was supported by a Grant-in-Aid for JSPS Research Fellows
from the Japan Society for the Promotion of Science (JSPS, Number:
without Pt/C 90 (%)
18J23126) for R.T.
J
a
p
a
n
S
o
c
i
ety forth
e
Pro
m
oti
o
n
of
S
c
i
e
n
c
e
(1
8
J
2
3
1
2
6)
O
Pt/C (0.5 mol%), O2 (balloon)
solvent (1 mL), 80 °C, 12 h
OH
OH
O
(3)
Acknowledgment
1
a (0.25 mmol)
2a
DMF : 73 (%)
CPME : 77 (%)
toluene : 81 (%)
We thank the N.E. Chemcat Corporation for their kind gift of the vari-
ous metal-on-carbon catalysts.
Scheme 1
Supporting Information
Two plausible reaction pathways for lactonization of the
diols are conceivable, as shown in Scheme 2. Dehydrogena-
tion of 1a first gives the hydroxy aldehyde A. O2 can
Supporting information for this article is available online at
https://doi.org/10.1055/s-0037-1611917.
S
u
p
p
orit
n
g Inform ati
o
n
S
u
p
p
orit
n
g Inform ati
o
n
18
quench the generated H in the presence of Pt/C to generate
2
References and Notes
H O , which is nearly instantaneously degraded to H O and
2
2
2
19
23
O . The direct oxygen oxidation of 1a to A is also possi-
(1) March, J. Advanced Organic Chemistry: Reactions, Mechanisms,
and Structure, 3rd ed; Wiley: New York, 1985.
2
ble. The desired lactone 2a is constructed through hemiace-
tal formation between the benzyl alcohol and aromatic al-
dehyde functionalities to give B, and a subsequent dehydro-
genative or oxygen oxidation process (path A). Alternatively,
(2) (a) March, J. Advanced Organic Chemistry: Reactions, Mecha-
nisms, and Structure, 3rd ed; Wiley: New York, 1985, 348.
(b) Mukaiyama, T. Angew. Chem. 1979, 91, 798; Angew. Chem.
Int. Ed. 1979, 18, 707.
O
OH
OH
10% Pt/C (0.5 mol%), O2 (balloon)
H2O (1 mL), 80 °C, 12 h
O
1a
2a
Pt/C + O2
ref. 19
H2
OH
O
H2
Path A
O
H2O2
H
B
OH
OH
CO2H
OH
OH
OH
H2
A
H2O
Path B
H2O
H2O
C
D
Scheme 2 Proposed mechanism
©
2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E

E
Synlett
R. Takakura et al.
Letter
(
(
(
(
(
3) (a) March, J. Advanced Organic Chemistry: Reactions, Mecha-
(16) (a) Touchy, A. S.; Shimizu, K.-i. RSC Adv. 2015, 5, 29072.
(b) Dutta, I.; Sarbajna, A.; Pandey, P.; Rahaman, S. M. W.; Singh,
K.; Bera, J. K. Organometallics 2016, 35, 1505. (c) Gülcemal, S.;
Gülcemal, D.; Whitehead, G. F. S.; Xiao, J. Chem. Eur. J. 2016, 22,
10513. (d) Wada, E.; Tyagi, A.; Yamamotoa, A.; Yoshida, H. Pho-
tochem. Photobiol. Sci. 2017, 16, 1744.
(17) (a) Fried, J.; Sih, J. C. Tetrahedron Lett. 1973, 14, 3899.
(b) Kretchmer, R. A.; Thompson, W. J. J. Am. Chem. Soc. 1976, 98,
3379. (c) Lansbury, P. T.; Hangauer, D. G. Jr. Vacca J. P. 1980, 102,
3964.
(18) (a) Sawama, Y.; Morita, K.; Yamada, T.; Nagata, S.; Yabe, Y.;
Monguchi, Y.; Sajiki, H. Green Chem. 2014, 16, 3439.
(b) Sawama, Y.; Morita, K.; Asai, S.; Kozawa, M.; Tadokoro, S.;
Nakajima, J.; Monguchi, Y.; Sajiki, H. Adv. Synth. Catal. 2015, 357,
1205.
(19) Monguchi, Y.; Ida, T.; Maejima, T.; Yanase, T.; Sawama, Y.; Sasai,
Y.; Kondo, S.; Sajiki, H. Adv. Synth. Catal. 2014, 356, 313.
(20) 2-Benzofuran-1(3H)-one (2a); Typical Procedure
A test tube was charged with 1,2-phenylenedimethanol (1a;
34.5 mg, 0.25 mmol), 10% Pt/C (2.5 mg, 0.00125 mmol, 0.5
nisms, and Structure, 3rd ed; Wiley: New York, 1985. (b) Leisch,
H.; Morley, K.; Lau, P. C. K. Chem. Rev. 2011, 111, 4165.
4) (a) Fétizon, M.; Golfier, M.; Louis, J.-M. J. Chem. Soc. D 1969,
1118. (b) Fétizon, M.; Golfier, M.; Louis, J.-M. Tetrahedron 1975,
31, 171.
5) (a) Stenberg, V. I.; Perkins, R. J. J. Org. Chem. 1963, 28, 323.
b) Johnston, P.; Sheppard, R. C.; Stehr, C. E.; Turner, S. J. Chem.
(
Soc. C 1966, 1847.
6) (a) Lee, M.; Chang, S. Tetrahedron Lett. 2000, 41, 7507. (b) Arita,
S.; Koike, T.; Kayaki, Y.; Ikariya, T. Chem. Asian J. 2008, 3, 1479.
(
c) Endo, Y.; Bäckvall, J.-E. Chem. Eur. J. 2011, 17, 12596.
7) (a) Aït-Mohand, S.; Muzart, J. J. Mol. Catal. A: Chem. 1998, 129,
35. (b) Nishimura, T.; Onoue, T.; Ohe, K.; Uemura, S. J. Org.
1
Chem. 1999, 64, 6750. (c) Bai, X.-F.; Ye, F.; Zheng, L.-S.; Lai, G.-
Q.; Xia, C. G.; Xu, L.-W. Chem. Commun. 2012, 48, 8592.
(8) Ikariya, T.; Kuwata, S.; Kayaki, Y. Pure Appl. Chem. 2010, 82,
1471.
(9) Biswas, S.; Mullick, K.; Chen, S.-Y.; Gudz, A.; Carr, D. M.;
Mendoza, C.; Angeles-Boza, A. M.; Suib, S. L. Appl. Catal., B 2017,
203, 607.
mol%), and H O (1 mL), then sealed with a septum. The atmo-
2
(
(
10) (a) Ji, H.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Tetrahedron Lett.
sphere in the test tube was replaced with O (balloon), and the
2
2
9
1
002, 43, 7179. (b) Yamaguchi, K.; Mizuno, N. Chem. Eur. J. 2003,
, 4353. (c) Mizuno, N.; Yamaguchi, K. Catal. Today 2008, 132,
8.
mixture was stirred with a ChemiStation (EYELA Tokyo Rikaki-
kai Co., Ltd., Tokyo, Japan) and heated at 80 °C for 12 h. The
mixture was then cooled to r.t. and passed through a membrane
filter (Millex-LH 0.20 m; Millipore) to remove the insoluble
catalyst. The filtrate was extracted with EtOAc (3 × 5.0 mL), and
11) (a) Kakiuchi, N.; Nishimura, T.; Inoue, M.; Umemura, S. Bull.
Chem. Soc. Jpn. 2001, 74, 165. (b) Kwon, M. S.; Kim, N.; Park, C.
M.; Lee, J. S.; Kang, K. Y.; Park, J. Org. Lett. 2005, 7, 1077.
the combined organic layers were dried (MgSO ) and concen-
4
(
(
12) Zhong, W.; Liu, H.; Bai, C.; Liao, S.; Li, Y. ACS Catal. 2015, 5, 1850.
13) (a) Abad, A.; Corma, A.; García, H. Chem. Eur. J. 2008, 14, 212.
trated in vacuo. The residue was purified by column chromatog-
raphy (silica gel, hexane–EtOAc) to give a white solid; yield:
(
1
b) Huang, J.; Dai, W.-L.; Fan, K. J. Phys. Chem. C 2008, 112,
6110. (c) Mitsudome, T.; Noujima, A.; Mizugaki, T.; Jitsukawa,
29.5 mg (0.22 mmol, 88%).
1
H NMR (CDCl ):  = 7.93 (d, J = 7.5 Hz, 1 H), 7.70 (t, J = 7.5 Hz,
3
K.; Kaneda, K. Green Chem. 2009, 11, 793. (d) Zheng, J.; Huang, J.;
Li, X.; Dai, W.-L.; Fan, K. RSC Adv. 2012, 2, 3801. (e) Oberhauser,
W.; Lavacchi, A.; Vizza, F.; Capozzoli, L. Appl. Catal., A 2013, 451, 58.
14) (a) Ishii, Y.; Osakada, K.; Ikariya, T.; Saburi, M.; Yoshikawa, S.
Tetrahedron Lett. 1983, 24, 2677. (b) Suzuki, T.; Morita, K.;
Tsuchida, M.; Hiroi, K. Org. Lett. 2002, 4, 2361. (c) Ito, M.; Osaku,
A.; Shiibashi, A.; Ikariya, T. Org. Lett. 2007, 9, 1821. (d) Buntara,
T.; Noel, S.; Phua, P. H.; Melián-Cabrera, I.; de Vries, J. G.; Heeres,
H. J. Angew. Chem. Int. Ed. 2011, 50, 7083. (e) Nicklaus, C. M.;
Phua, P. H.; Buntara, T.; Noel, S.; Heeres, H. J.; de Vries, J. G. Adv.
Synth. Catal. 2013, 355, 2839. (f) Aellig, C.; Jenny, F.; Scholz, D.;
Wolf, P.; Giovinazzo, I.; Kollhoff, F.; Hermans, I. Catal. Sci. Tech-
nol. 2014, 4, 2326.
1 H), 7.55 (t, J = 7.5 Hz, 1 H), 7.52 (d, J = 7.5 Hz, 1 H), 5.34 (s, 2 H).
(21) Mori, S.; Takubo, M.; Makida, K.; Yanase, T.; Aoyagi, S.;
Maegawa, T.; Monguchi, Y.; Sajiki, H. Chem. Commun. 2009,
5159.
(22) Fujita, K.; Ito, W.; Yamaguchi, R. ChemCatChem 2014, 6, 109.
(23) (a) Ng, Y. H.; Ikeda, S.; Harada, T.; Morita, Y.; Matsumura, M.
Chem. Commun. 2008, 3181. (b) Ng, Y. H.; Ikeda, S.; Morita, Y.;
Harada, T.; Ikeue, K.; Matsumura, M. J. Phys. Chem. C 2009, 113,
12799. (c) Karimi, B.; Behzadnia, H.; Bostina, M.; Vali, H. Chem.
Eur. J. 2012, 18, 8634.
(24) Analysis by atomic absorption spectrometry indicated that no
Pt metal (<1 ppm) leached into the reaction mixture during the
first run (Table 1, entry 11). When recovered Pt/C was used in a
second run, 2a was obtained in 77% yield together with the
starting material 1a (11% yield).
(
(15) Zhu, Q.-J.; Dai, W.-L.; Fan, K.-N. Green Chem. 2010, 12, 205.
©
2019. Thieme. All rights reserved. — Synlett 2019, 30, A–E