Selective Synthesis of 5-Substituted N-Aryloxazolidinones by Cycloaddition Reaction of Epoxides with Arylcarbamates Catalyzed by the Ionic Liquid BmimOAc

Article abstract of DOI:10.1002/ejoc.201600474

A selective procedure for the synthesis of 5-substituted N-aryloxazolidinones by the coupling of epoxides with arylcarbamates catalyzed by ionic liquids has been developed. The effects of reaction time, reactant molar ratio, amount of catalyst, and temper

Full text of DOI:10.1002/ejoc.201600474

DOI: 10.1002/ejoc.201600474
Full Paper
Oxazolidinones Synthesis
Selective Synthesis of 5-Substituted N-Aryloxazolidinones by
Cycloaddition Reaction of Epoxides with Arylcarbamates
Catalyzed by the Ionic Liquid BmimOAc
Elnazeer H. M. Elageed,[a][‡] Bihua Chen,[a][‡] Binshen Wang,[a] Yongya Zhang,[a] Shi Wu,[a]
Xiuli Liu,[a] and Guohua Gao*[a]
Abstract: A selective procedure for the synthesis of 5-substi- dition, chiral 5-substituted oxazolidinones were synthesized by
tuted N-aryloxazolidinones by the coupling of epoxides with this procedure in good-to-excellent yields with enantiomeric
arylcarbamates catalyzed by ionic liquids has been developed. excesses in excess of 99.9 % starting from chiral terminal epox-
The effects of reaction time, reactant molar ratio, amount of ides. A possible reaction mechanism is discussed in accord with
1
catalyst, and temperature were investigated. Under the optimal the results obtained by H NMR spectroscopy and DFT calcula-
reaction conditions, BmimOAc exhibited efficient catalytic activ- tions, which indicate the cooperative activation by BmimOAc
ity compared with other ionic liquids leading to the formation through the formation of hydrogen bonds with the substrates.
of 5-substituted N-aryloxazolidinones in excellent yields. In ad-
Introduction
ates. As an alternative process for the synthesis of oxazol-
idinones, the cycloaddition reaction between epoxides and
carbamates has attracted significant attention due to the use
of nontoxic materials. Many catalyst systems have been em-
ployed for this reaction, including binary metal oxides, organo-
tin complexes, Al(aminotriphenolate) complexes, Co(III) salen
complexes, and other alkaline agents.[9] Very recently, Deng and
co-workers reported that amine-functionalized ionic liquids effi-
ciently catalyzed the cycloaddition of epoxides with carb-
amates.[10] Good-to-excellent yields of 2-oxazolidinones were
achieved, however, this process is limited to aliphatic carb-
amates.
During our studies on the utilization of ionic liquids as cata-
lysts we have reported approaches to the synthesis of oxazol-
idinones by the straightforward conversion of CO2 using epox-
ides and aromatic amines in the presence of ionic liquids, syner-
getic 1,8-diazobicyclo[5.4.0]undec-1-ene (DBU), and DBU-de-
rived ionic liquids as catalysts, the direct condensation of 2-(aryl-
amino) alcohols with diethyl carbonate, and the reaction of cy-
clic carbonates with aromatic amines in the presence of a cata-
lytic amount of an ionic liquid.[11] Herein we wish to report our
recent success in the synthesis of oxazolidinones in high yields
and high regioselectivities by the coupling of epoxides with
arylcarbamates in the presence of a catalytic amount of the
ionic liquid BmimOAc (Scheme 1).
Oxazolidinones are important heterocyclic compounds in the
field of fine chemicals and synthetic organic chemistry, and in
recent decades they have received significant attention due to
their outstanding biological activity. They have been used as a
new class of antibiotics, HIV-1 protease inhibitors, fungicides,
antibacterials, antimicrobial agents, and key structural units in
pharmaceuticals as well as agrochemicals.[1]
Therefore, many strategies have been developed for the syn-
thesis of oxazolidinones, including the reactions of amino alco-
hols with phosgene or its derivatives,[2] carbon dioxide with ꢀ-
amino alcohols[3] or aziridines,[4] oxidative carbonylation of ꢀ-
amino alcohols with CO/O2,[5] and the carbonylation of ꢀ-amino
alcohols with dialkyl carbonate.[6] However, these approaches
required relatively harsh reaction conditions (5.0 MPa of CO2
pressure, 110–180 °C, and a prolonged reaction time of 9–20 h)
and the catalysts are highly toxic or expensive. In addition, syn-
thetic approaches through cycloaddition reactions between iso-
cyanates and epoxides, which are usually catalyzed by quater-
nary ammonium salts, lithium halides,[7] chromium(III) and va-
nadium(V) salen complexes, bimetallic aluminium (salen) com-
plexes, or rare-earth-metal complexes,[8] have also been re-
ported. However, these approaches suffer from high reaction
temperature, long reaction time, and the use of toxic isocyan-
[a] Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University,
North Zhongshan Road 3663, Shanghai 200062, P. R. China
E-mail: ghgao@chem.ecnu.edu.cn
http://faculty.ecnu.edu.cn/s/376/t/3755/main.jspy
[‡] Authors contributed equally to this work.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/ejoc.201600474.
Scheme 1. Reaction of arylcarbamates with epoxides catalyzed by BmimOAc.
3650 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Results and Discussion
Next, using BmimOAc as catalyst, the reaction time was re-
duced to 2 and 1 h; the yield of the product fell to 77 and
53 %, respectively (entries 7 and 8). On lowering the reaction
temperature from 100 to 90 and 80 °C, the yield decreased to
84 and 75 %, respectively (entries 9 and 10). Reducing the mo-
lar ratio from 1:2 to 1:1 led to a lower yield of the product
(40 %; entry 11), whereas a ratio of 1:1.5 gave a similar yield of
the product (entry 12). When the catalyst amount was de-
creased from 10 to 5 and 1 %, the yield dramatically dropped to
80 and 47 %, respectively (entries 13 and 14). Thus, the optimal
reaction conditions for the coupling reaction between aryl-
carbamates and epoxides were found to be a molar ratio of 1:2,
10 % of catalyst, a reaction temperature of 100 °C, and a reac-
tion time of 3 h.
Optimization of the Reaction Parameters and Catalyst
System
Imidazolium-based ionic liquids are commonly known to act as
reaction media and catalysts.[12] In our previous work on the
synthesis of 3-aryloxazolidin-2-ones,[11b,11d] various ionic liquids
exhibited different catalytic activities depending on their struc-
tures. On this basis we investigated the reactions of arylcarb-
amates with epoxides in the presence of a series of ionic liquids.
To optimize the reaction parameters and catalyst system, the
reaction of ethyl phenylcarbamate with propylene oxide was
performed in the presence of different catalysts and at different
reaction times and reaction temperatures as well as with differ-
ent amounts of catalyst and substrate molar ratios, as shown in
Table 1. Without any catalyst, no conversion was observed (en-
try 1), which indicates that the catalyst is very important to
activate the reaction. Upon addition of a catalytic amount of
tetrabutylammonium acetate (TBAA), a yield of only 12 % was
obtained (entry 2). When BmimOAc was used, the conversion
reached 100 % with an isolated yield of 98 % of 5-methyl-3-
phenyloxazolidin-2-one as the sole product (entry 3). Mean-
while, by using 1-butyl-2,3-dimethylimidazolium acetate
(BmmimOAc), which bears a methyl group at C-2 of the imid-
azolium ring, a yield of 33 % (entry 4) was obtained, which is
significantly lower than that obtained with the corresponding
Bmim-based ionic liquid (entry 3). This implies that the cation
of the ionic liquid plays a significant role in activating the reac-
tion, which has previously been well established by us and
Synthesis of 5-Substituted 3-Aryloxazolidin-2-ones from
the Reaction of Arylcarbamates with Epoxides in the
Presence of BmimOAc
The scope of the reaction was explored by studying the reac-
tions of several arylcarbamates with various epoxides. The re-
sults are shown in Table 2.
Ethyl phenylcarbamate was easily converted into the corre-
sponding oxazolidinones upon treatment with epoxides in the
presence of a catalytic amount of BmimOAc. Ethylene oxide
gave a high yield of 99 % of 3-phenyl-2-oxazolidinone (1a;
Table 2, entry 1). Terminal epoxides bearing methyl, phenyl, and
n-butyl groups gave excellent yields of 87–98 % of the corre-
sponding 5-substituted oxazolidinones 1b–1d (entries 2–4) as
the sole products. Glycidyl phenyl ether gave a 95 % yield of
1e (entry 5) and cyclohexene oxide gave a 53 % yield of 1f
(entry 6). The lower yield of 1f might be attributed to the high
steric hindrance of cyclohexene oxide. Ethyl (4-methoxy-
phenyl)carbamate and ethyl (4-ethoxyphenyl)carbamate, which
have electron-donating groups, could also be used for this reac-
tion and were smoothly converted into the corresponding 5-
substituted 3-aryloxazolidin-2-ones 1g–1p (entries 7–16) in
yields ranging from 81 to 94 %. Ethyl (4-chlorophenyl)carb-
amate gave high yields of 85–94 % of 1q–1u (entries 17–21),
whereas ethyl (4-nitrophenyl)carbamate gave good yields of
68–80 % of 1v–1z (entries 22–26); the lower yields were attrib-
uted to deactivation by the nitro group. In all cases the 5-substi-
tuted oxazolidinones were observed as the sole products and
no byproducts were formed, which reveals that BmimOAc se-
lectively catalyzes the ring-opening of epoxides to form the
desired products.
others.[11b,11d,13]
1-Butyl-3-methylimidazolium
chloride
(BmimCl) and 1-butyl-3-methylimidazolium bromide (BmimBr)
also gave lower yields of 70 and 57 %, respectively (entries 5
and 6). These results show that the yield of the product is also
affected by the anion of the ionic liquid, with stronger
hydrogen bond basicity,[14] which decreases in the order
OAc– > Cl– > Br–, giving higher yields.
Table 1. Optimization of the reaction conditions and catalyst system.[a]
Entry
Catalyst
Time
[h]
T [°C]
EPC/PO
molar ratio
Conv.[b] [%] Yield[c]
[%]
1
2
3
4
5
6
7
8
none
TBAA
BmimOAc
BmmimOAc
BmimCl
3
3
3
3
3
3
2
1
3
3
3
3
3
3
100
100
100
100
100
100
100
100
90
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:1
1:1.5
1:2
1:2
0
15
100
35
70
60
80
55
85
77
40
100
85
50
0
12
98
33
70
57
77
53
84
75
40
98
82
47
BmimBr
BmimOAc
BmimOAc
BmimOAc
BmimOAc
BmimOAc
BmimOAc
BmimOAc[d]
BmimOAc[e]
9
10
11
12
13
14
80
100
100
100
100
Synthesis of Chiral 5-Substituted Oxazolidinones from the
Reaction of Arylcarbamates with Chiral Epoxides in the
Presence of BmimOAc
[a] Reagents and conditions: Ethyl phenylcarbamate (EPC; 5 mmol), propylene
oxide (PO), EPC/PO molar ratio 1:1–1:3, 10 mol-% catalyst (based on EPC), 80–
100 °C, 1–3 h. [b] Conversion of ethyl phenylcarbamate, determined by GC.
[c] Isolated yield. [d] Reaction carried out with 5 % catalyst. [e] Reaction car-
ried out with 1 % catalyst.
Chiral oxazolidinones have attracted significant attention in
drug synthesis and have been used as synthetic and essential
subunits in pharmaceuticals. To date, several synthetic ap-
proaches have been developed for the preparation of chiral 5-
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Table 2. Reaction of arylcarbamates with various epoxides.[a]
[a] Reagents and conditions: Arylcarbamates (5 mmol), epoxides (10 mmol), BmimOAc (0.5 mmol), 100 °C, 3 h. [b] Isolated yield. [c] Reaction was carried out
at 120 °C for 6 h.
substituted oxazolidinones.[15] Herein we wish to report the Studies of the Reaction Mechanism
synthetic application of the ionic liquid BmimOAc for the pro-
duction of chiral oxazolidinones by the reaction of arylcarb- The interactions between the substrates and ionic liquids were
amates with chiral terminal epoxides (Table 3). Arylcarbamates studied by 1H NMR spectroscopy and DFT calculations with
bearing electron-donating or -withdrawing groups were exam- propylene oxide and ethyl phenylcarbamate used as models. A
ined and in all cases regioselective ring-opening of the epox- 1H NMR titration based on the addition of aliquots of propylene
ides occurred and chiral 5-substituted oxazolidinones were ob- oxide and ethyl phenylcarbamate to BmimOAc was carried out.
tained as the sole products. (R)-Propylene oxide was easily con- Figure 1 shows the shift of the resonance of the 2-H of imid-
verted into the corresponding (R)-5-methyl-3-oxazolidin-2-ones azolium upon interaction with propylene oxide. When 1 equiv.
2a–2e in yields ranging from 56 to 95 % with ee > 99.9 % (en- of propylene oxide was added to BmimOAc, the 2-H of the
tries 1–5). On the other hand, (S)-propylene oxide gave (S)-5- imidazolium cation shifted upfield from 10.51 to 10.15 ppm.
methyl-3-oxazolidin-2-ones 2f–2j in yields of 73–95 % with ee This might be a result of a change in the hydrogen-bond do-
> 99.9 % (entries 6–10).
nor–acceptor pair. In BmimOAc, the 2-H of the imidazolium cat-
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Table 3. Reaction of arylcarbamates with chiral terminal epoxides.[a]
Figure 1. 1H NMR spectra showing the shift of the 2-H resonance of BmimOAc
in CDCl3. a) BmimOAc, b) BmimOAc/propylene oxide = 1:1, c) 1:2, d) 1:5,
e) 1:10 (mol ratio), respectively, f) BmimOAc/propylene oxide/ethyl phenyl-
carbamate = 1:10:10 (mol ratio).
nance of 2-H from δ = 9.60 to 9.07 ppm was observed. In a
competitive environment, both the imidazolium and ethyl
phenylcarbamate act as hydrogen-bond donors to interact with
the acetate anion. Therefore, upon addition of ethyl phenyl-
carbamate, the acetate anion interacts with N–H, leaving the 2-
H of the imidazolium cation free to interact with the oxygen
atom of propylene oxide, resulting in the extreme upfield shift
to δ = 9.07 ppm. This demonstrates that ethyl phenylcarbamate
may be activated by the acetate anion through hydrogen bond-
ing. These NMR studies have illustrated that BmimOAc could
cooperatively activate ethyl phenylcarbamate and propylene
oxide by hydrogen bonding.
To investigate the interaction between ethyl phenylcarb-
amate and BmimOAc alone, another 1H NMR titration based on
the addition of aliquots of the ionic liquid to ethyl phenyl-
carbamate was carried out and the results are shown in Fig-
ure 2.
When 1 equiv. of BmimOAc was added to ethyl phenyl-
carbamate, the peak of N–H underwent a downfield shift (from
δ = 7.64 to 8.03 ppm). This demonstrates that ethyl phenyl-
carbamate could be activated by the interaction between N–H
and the acetate counter ion of the ionic liquid. After the step-
wise addition of further BmimOAc to ethyl phenylcarbamate,
the N–H underwent a step-by-step downfield shift with peak
broadening until it nearly disappeared after the addition of
10 equiv. of BmimOAc. This result can be explained by the acet-
ate counter ion activating ethyl phenylcarbamate through
hydrogen bonding.
[a] Reagents and conditions: Arylcarbamates (5 mmol), epoxides (10 mmol),
BmimOAc (0.75 mmol), 40 °C, 48 h. [b] Isolated yield. [c] Measured by HPLC.
ion acts as a hydrogen-bond donor to interact with the acetate
counter ion, which, in turn, acts as a hydrogen-bond acceptor.
After the stepwise addition of propylene oxide to BmimOAc,
the intramolecular hydrogen bond between the acetate and 2-
H is weakened and an intermolecular hydrogen bond between
the imidazolium and propylene oxide emerges. These results
imply that the propylene oxide could be activated by the 2-H
of imidazolium.
It is clear that the 2-H of the imidazolium ring progressively
shifts upon the addition of increasing amounts of propylene
oxide due to its interaction with the oxygen atom of propylene
oxide. Similar results have previously been reported by us and
others.[11c,11d,16] Upon addition of 10 equiv. of propylene oxide,
the 2-H resonance underwent a significant shift from δ = 10.51
to 9.60 ppm. This indicates that a stronger interaction between
the 2-H of imidazolium and propylene oxide occurs when using
an excess amount of propylene oxide. Meanwhile, when
10 equiv. of ethyl phenylcarbamate was added to the mixture
The interactions between BmimOAc and the model sub-
of propylene oxide and BmimOAc, a dramatic shift in the reso- strates, propylene oxide and ethyl phenylcarbamate, were also
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Figure 3. DFT-optimized geometries of a) propylene oxide, b) ethyl phenylcar-
bamate, and c) the complex of ethyl phenylcarbamate and propylene oxide
with BmimOAc. Bond lengths: a) C1–O1 1.492 Å; b) N1–H1 1.009 Å; c) C1–O1
1.511 Å, N1–H1 1.039 Å, O1···H2 1. 966 Å, O2···H1 1.713 Å. Some hydrogen
atoms have been omitted for clarity.
azolidin-2-ones was obtained as the final product with reten-
tion of configuration. A similar result was obtained with (S)-
propylene oxide. These results indicate that the nitrogen atom
of ethyl phenylcarbamate regioselectively attacks the propylene
oxide at the carbon atom that has no chiral center to give inter-
mediate iii, which in turn regioselectively undergoes ring clo-
sure to give the final product with retention of configuration.
Figure 2. 1H NMR spectra showing the shift of the N–H resonance of ethyl
phenylcarbamate in CDCl3. a) Ethyl phenylcarbamate, b) ethyl phenyl-
carbamate/BmimOAc = 1:1, c) 1:2, d) 1:3, e) 1:5, f) 1:7, g) 1:10 (mol ratio),
respectively.
investigated by DFT calculations. The optimized geometrical
structures of propylene oxide, ethyl phenylcarbamate, and the
complex of propylene oxide and ethyl phenylcarbamate with
BmimOAc were simulated at the B3LYP/6-31G level of theory
(Figure 3). The length of the Cl–O1 bond of propylene oxide is
elongated from 1.492 to 1.511 Å after complexation with
BmimOAc and the hydrogen bond O1···H2 is formed with a
bond length of 1.966 Å. These results imply that propylene ox-
ide is activated by a hydrogen-bonding interaction between the
oxygen atom and the 2-H of the imidazolium ring of the ionic
liquid. Meanwhile, the length of the N1–H1 bond of ethyl
phenylcarbamate is elongated from 1.009 to 1.039 Å after com-
plexation with BmimOAc and the hydrogen bond O2···H1 is
formed with a bond length of 1.713 Å. This demonstrates that
ethyl phenylcarbamate is activated by hydrogen bonding with
the acetate anion. The results of these DFT calculations indicate
that both the cation and anion of BmimOAc cooperatively acti-
vate the substrates through the formation of hydrogen bonds.
This phenomenon has been well established by us in our previ-
ous work.[11c,11d]
Scheme 2. Proposed mechanism for the reaction of ethyl phenyl-
carbamate with propylene oxide catalyzed by BmimOAc.
Conclusions
Based on these results, a reaction mechanism is proposed in
Scheme 2. First, the 2-H of the imidazolium cation activates the We have described a simple procedure for the synthesis of 5-
ring-opening of propylene oxide through the formation of a substituted N-aryloxazolidinones by the coupling of epoxides
hydrogen bond with the oxygen atom. The activation of ring- with arylcarbamates catalyzed by ionic liquids. Under mild reac-
opening of epoxides by this kind of interaction has also been tion conditions, BmimOAc can selectively catalyze the forma-
reported previously.[17] At the same time, the acetate anion acti- tion of various 5-substituted N-aryloxazolidinones in excellent
vates the ethyl phenylcarbamate through the formation of a yields. Moreover, this procedure is applicable to the synthesis
hydrogen bond with the N–H atom. Secondly, the nitrogen of chiral 5-substituted oxazolidinones starting from chiral epox-
atom of ethyl phenylcarbamate nucleophilically attacks the acti- ides. The high catalytic activity of BmimOAc is attributed to the
vated propylene oxide to form the intermediate species iii. strong hydrogen-bond basicity of its anion and the hydrogen-
Lastly, the species iii undergoes intramolecular cyclization to bond acidity of the 2-H of its imidazolium ring. DFT calculations
form the final product 5-methyl-3-phenyloxazolidin-2-one and and 1H NMR spectroscopy indicate that both the anion and
BmimOAc is recycled to complete the reaction circle. When (R)- cation of the ionic liquid cooperatively activate the substrates
propylene oxide was used as reagent, (R)-5-methyl-3-phenylox- through the formation of hydrogen bonds.
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Experimental Section
General: Ethyl phenylcarbamate, propylene oxide, and other epox-
ides were supplied by TCI. (S/R)-Propylene oxides were purchased
from Adamas-beta. All ionic liquids were supplied by Center of
Green Chemistry and Catalysis, LICP, and CAS. The other reagents
were commercial reagents of AR grade and were used without fur-
ther purification. The arylcarbamates were synthesized by the reac-
tion of arylamines with diethyl carbonate in the presence of the
ionic liquid BmimOAc as catalyst (see the Supporting Information).
GC analysis was performed by using a Shimadzu GC-14B instrument
equipped with a DM-1701capillary column (30 m × 0.32 mm × 0.25 μm)
and a flame ionization detector. The NMR spectra were recorded
with Bruker Ascend 400 and DRX500 spectrometers with tetrameth-
ylsilane as the internal standard. HRMS analyses were performed
with a Bruker Microtof II instrument. The enantiomeric excesses
(ees) were determined by HPLC analysis, which was performed with
an Agilent series instrument and a Daicel Chiralcel OD-H column.
The DFT calculations were carried out by using the B3LYP functional
with the 6-311g basis set as implemented in the Gaussian 09[18]
program package. Vibrational frequency calculations, from which
the zero-point energies were derived, were performed for each opti-
mized structure at the same level of theory to identify the natures
of all the stationary points.
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[9]
General Procedure for Synthesis of 5-Substituted Oxazolidin-
ones from the Reaction of Arylcarbamates with Epoxides: In a
typical procedure, the reactions of arylcarbamates with epoxides
were carried out in a 15 mL thick-walled pressure bottle. Aryl-
carbamates (5.0 mmol), epoxides (10.0 mmol), and BmimOAc (10 %,
0.5 mmol, 0.10 g) were mixed together and stirred at 100 °C for 3 h.
The reaction mixtures were analyzed by GC with n-dodecane as the
internal standard. The pure products were obtained by chromatog-
raphy on silica gel and structurally characterized by NMR spectro-
scopy.
[10]
[11]
General Procedure for Synthesis of Chiral 5-Substituted Ox-
azolidinones from the Reaction of Arylcarbamates with Chiral
Terminal Epoxides: In a typical procedure, the reactions of aryl-
carbamates with epoxides were carried out in a 15 mL thick-walled
pressure bottle. Arylcarbamates (5.0 mmol), chiral epoxides
(10.0 mmol), and BmimOAc (15 %, 0.75 mmol, 0.15 g) were mixed
together and stirred at 40 °C for 48 h. The reaction mixtures were
analyzed by GC with n-dodecane as the internal standard. The pure
products were obtained by chromatography on silica gel and struc-
turally characterized by NMR spectroscopy. The enantiomeric ex-
cesses (ee values) were determined by HPLC analysis.
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Acknowledgments
The authors thank the National Natural Science Foundation of
China (NSFC) (grant numbers 21273078 and 21573072) and the
Shanghai Leading Academic Discipline Project (project number
B409) for financial support.
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Keywords: Homogeneous catalysis · Reaction mechanisms ·
Cycloaddition · Ionic liquids · Nitrogen heterocycles
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Full Paper
J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J.
Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski,
G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O.
Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian 09,
revision A.02, Gaussian, Inc., Wallingford, CT, 2009.
Received: April 14, 2016
Published Online: June 27, 2016
Eur. J. Org. Chem. 2016, 3650–3656
www.eurjoc.org
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© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim