Metal-free stereoselective annulation of quinolines with trifluoroacetylacetylenes and water: An access to fluorinated oxazinoquinolines

Article abstract of DOI:10.1039/c7cc09725e

Metal-free reaction between quinolines, aryltrifluoroacetylacetylenes and water at -18 °C-rt in MeCN resulted in stereoselective assembly of trifluoromethylated oxazinoquinolines with up to 99% yield that was essentially in contrast to a similar reaction with pyridines. The annulation proceeded via the 1,3-dipolar adducts of quinolines with trifluoroacetylacetylenes followed by intramolecular cyclization involving the trifluoroacetyl group and a molecule of water.

Full text of DOI:10.1039/c7cc09725e

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: B. Trofimov, K.
Belyaeva, L. Nikitina, A. Afonin, A. V. Vashchenko, V. Muzalevskiy and V. Nenajdenko, Chem. Commun.,
2018, DOI: 10.1039/C7CC09725E.
This is an Accepted Manuscript, which has been through the
Volume 52 Number
1 4 January 2016 Pages 1–216
Royal Society of Chemistry peer review process and has been
accepted for publication.
ChemComm
Accepted Manuscripts are published online shortly after
Chemical Communications
www.rsc.org/chemcomm
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1359-7345
COMMUNICATION
Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
first methanofullerene derivatives of Sc N@D5h-C80
=
3
rsc.li/chemcomm

Please do not adjust margins
ChemComm
Page 1 of 4
View Article Online
DOI: 10.1039/C7CC09725E
Journal Name
COMMUNICATION
Metal‐free stereoselective annulation of quinolines with
trifluoroacetylacetylenes and water: an access to fluorinated
oxazinoquinolines
Received 00th October 2017,
Accepted 00th January 20xx
B. A. Trofimov,a K. V. Belyaeva,a L. P. Nikitina,a A. V. Afonin,a A. V. Vashchenko,a V. M. Muzalevskiyb
DOI: 10.1039/x0xx00000x
and V. G. Nenajdenkob
www.rsc.org/
Metal‐free
reaction
between
quinolines, by the reversible formation of the 1,3‐dipole
A, which after
aryltrifluoroacetylacetylenes and water at ‐18 ‐ rt in MeCN 1,4‐addition of water, gives intermediate
resulted in stereoselective assembly of trifluoromethylated with the ring opening.
B further rearranging
oxazinoquinolines in up to 99% yield that essentially contrasted to
Surprisingly, quinoline under the same conditions
similar reaction with pyridines. The annulation proceeds via the appeared to be completely unreactive towards non‐
1,3‐dipolar adducts of quinolines with trifluoroacetylacetylenes fluorinated acylacetylenes.1 This is likely a result of a low
followed by intramolecular cyclization involving the trifluoroacetyl basicity (nucleophilicity) of the quinoline compared to the
group and a molecule of water.
pyridine (pKaBH+ 4.85 and 5.14, respectively)3 and hence a
much low equilibrium concentration of intermediate 1,3‐
Recently1 it was revealed that the transition‐metal free
reaction of pyridines with electron‐deficient acetylenes
including phenyltrifluroacetylacetylene2 in the presence of
water proceeds at rt via ring‐opening to deliver 5‐(Z‐alkenyl)‐
amino‐2,4‐pentadienals in up to 92% yields (Scheme 1).
dipole
A.
Meanwhile,
trifluoroacetyl‐containing
quinoline
derivatives, either with pyridine‐ring‐opened or those with
quinoline core retained represent rewarding objects for
pharmaceutical research. Just to mention, functionalized
quinolones, in particular, containing fluorine atoms which
exhibit strong antibacterial activity (well‐known antibiotics
Ciprofloxacin, Moxifloxacin, Gatifloxacin, Levofloxacin).4,5
A
number of antimalarial drugs have fluorine‐containing
quinolines as a key fragment, for example Mefloquine.6
Here it is relevant to mention that 1,3‐oxazine moiety, like
the quinoline one, is a remarkable object for drug discovery.
The compounds containing this framework have demonstrated
antidepressant properties,7 tuberculostatic,8 antibacterial and
fungicidal activities.9 They are recommended for the treatment
of Alzheimer's disease and type 2 diabetes.10 1,3‐Oxazines are
also useful intermediates in the stereoselective synthesis of
1,3‐amino alcohol derivatives.11 The construction of fluorine‐
containing oxazine particularly in combination with quinoline
scaffold is among important challenges of organic synthesis.
For instance, the fused 1,3‐oxazinoquinoline systems were
claimed to possess an antimalarial activity.12
Scheme 1 Reaction of pyridine with electron‐deficient acetylenes
The N‐C(2) bond cleavage in the pyridine ring is triggered
This is why certain efforts to design the 1,3‐
oxazinoquinoline scaffold are now being undertaken.13
Therefore we have tried to implement the reaction
between quinolines and aryltrifluoroacetylacetylenes in the
presence of water in hope that due to the strong electron‐
withdrawing effect of trifluoroacetyl moiety the pyridine ring
of the quinoline core will become able to form the 1,3‐dipole
a. A. E. Favorsky Irkutsk Institute of Chemistry, Siberian Branch, Russian Academy
of Sciences, 1 Favorsky Str., Irkutsk, 664033 Russian Federation; Fax: (395‐2)41‐
93‐46.
intermediate of the type
A
in the concentration enough to
b. M. V. Lomonosov Moscow State University, Department of Chemistry, Leninskie
Gory 1, Moscow, 119991 Russian Federation.
boris_trofimov@irioch.irk.ru; nenajdenko@org.chem.msu.ru.
†Electronic Supplementary Information (ESI) available: Experimental details, X‐ray
crystallographic data for 3a (1545030) and 3k’ (1545031), NMR spectra for all
compounds (PDF). For ESI see DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 1
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 2 of 4
View Article Online
DOI: 10.1039/C7CC09725E
COMMUNICATION
render the further transformation. Eventually, in view of the following conditions, 1a
previous results for this reaction with pyridines, we might temperature range ‐18 ‐ 25 C, time 20 h (entry 6). A larger
expect to have also the quinoline ring cleavage affording amount of water (5 molar excess relative to other reactants)
trifluoroacetyl substituted quinoline related analogues of 5‐ and solvent‐free synthesis accelerated the consumption of
Journal Name
:
2a:H2O molar ratio was 0.95:1:1, the
o
amino‐2,4‐pentadienals.
acetylene 2a but the yields dropped considerably (8% and
However, to our surprise the catalyst(metal)‐free reaction 43%, entries 3 and 4, respectively).
of quinoline 1a with phenyltrifluoroacetylacetylene 2a in the
Having the above optimal conditions in hand, the
presence of water in MeCN takes quite another direction: generality of this annulation was evaluated. First, the reaction
instead of the anticipated ring opening, the three‐component of
parent
quinoline
1a
with
a
series
of
annulation involving trifluoroacetyl group of acetylene 2a and aryltrifluoroacetylacetylenes
2 was checked (Scheme 3). To our
1
a molecule of water has been observed. According to H, 13C, delight, the annulation was found to be general for all
15N and 19F NMR and IR spectroscopy and X‐Ray analysis the trifluoroacetylacetylenes examined. The formation of 1,3‐
product proved to be oxazinoquinoline 3a (a single 3R*,4aR* oxazinoquinolines
3
proceeded smoothly in all cases to give
diastereomer), the isolated yield being 65% (Scheme 2).
the target molecules in up to 99% isolated yield.
Scheme 2 Reaction of quinoline 1a with phenyltrifluoroacetylacetylene 2a and
water and X‐ray structure of 3a
In this communication, we report a further development of
this unexpected reaction in terms of its optimal conditions and
substrate scope. For the optimization we have chosen the
same reactant pair, i.e. 1a and 2a. The some selected
experiments illustrating the influence of the reaction
conditions (the reactant ratio, temperature, time) on the yield
of adduct 3a are collected in Table 1. Acetonitrile was proved a
solvent of choice, while other solvents (DMF, DMSO, THF, 1,2‐
DCE, Et2O, PhMe, CH2Cl2, Et3N, EtOAc) were found to be
inferior: a longer reaction times (84 h) instead of 20 h with
MeCN, lower yields (see Table S1 in SI).
a
Scheme 3 Scope of trifluoroacetyleacetylenes 2 a
.
General reaction conditions:
1a (0.95 equiv., 0.475 mmol),
2 (1.00 equiv., 0.500 mmol), H2O (1.00 equiv.,
Table 1 Optimization of Reaction Conditionsa
0.500 mmol), MeCN (3.0 mL), ‐18 – 25 oC
However, the reaction with less electrophilic acetylenes 2e
and 2i having electron‐donating methoxy‐group requited a
longer time to complete (up to 184 h for 2i). This is in
agreement with the initial step of the pyridine‐ring cleavage
Isolated
1a/2a/H2O
ratio
Time
(h)
Entry
Temp. (oC)
yield of
3a (%)
65
88
8
43
80
90
1
2
3b
4b
5
1:1:1
1:1:1
1:1:5
1:1:5
1:1:1
20‐25
‐5~0 ‐ +5
20‐25
‐5 ‐ +5
‐5~0 ‐ 25
‐18 – 25
24
96
1
1
20
20
R
H
O
O
N
H2O
Ph
R
+
+
OH
MeCN
CF3
N
N
Ph
CF3
2a
3
1
Br
Me
O
6
0.95:1:1
а
O
Conditions:
quinoline
1a
(0.5
or
0.475
mmol)
and
N
O
N
OH
OH
CF3
OH
CF3
b
phenyltrifluoroacetylacetylene 2a (0.5 mmol), MeCN (3 mL).
Solvent‐free
Ph
CF3
Ph
Ph
synthesis.
3j, 75% (24 h)b
3k, 68% (144 h)b
3a, 90% (20 h)
The completion of the reaction was monitored by IR
spectra by disappearance of the band at 2174‐2202 cm‐1
Cl
(νC
≡
C bond of
2
) or using 19F NMR.
N
O
OH
CF3
As the optimization performed was shown (Table 1), the
Ph
yield of adduct 3a reached its maximum (90%) under the
3l, 68% (24 h)b
3k' (complex of 3k with 1c), 13% (144 h)b
CN
This jourBnral is © The Royal Society of Chemistry 20xx
Cl
2 | J. Name., 2012, 00, 1‐3
N
O
N
O
Cl
N
O
OH
OH
OH
Ph
CF3
Ph
CF3
Ph
CF3
Please do not adjust margins
3m, 48% (80 h)
3n, 87% (40 h)
3o, 85% (45 h)

Please do not adjust margins
ChemComm
Page 3 of 4
View Article Online
DOI: 10.1039/C7CC09725E
Journal Name
COMMUNICATION
under the action of electron‐deficient acetylenes as previously 1,3‐oxazine ring via the nucleophilic addition of the hydroxyl
reported.1,2
group to the carbonyl function (Scheme 5).
Next, tolerance of the annulation toward substituents in
Scheme 5 A possible mechanisms for formation of
3
quinolines was assessed using phenyltrifluoroacetylacetylene
The experiments with D2O for the reference reactant pair
1a and 2a resulted in isolation of the expected
2a as a reference compound (Scheme 4).
(
)
oxazinoquiniline having deuterium in the position 2 of the
oxazine ring and in hydroxyl group that is in agreement with
Scheme 5.
a
Scheme 4 Scope of quinolines
conditions: 1a (0.95 equiv., 0.475 mmol),
equiv., 0.500 mmol), MeCN (3.0 mL), ‐18 – 25 oC. b Reaction conditions: 1a (1.00
1
and X‐ray structure of 3k’ a
.
General reaction
2
(1.00 equiv., 0.500 mmol), H2O (1.00
equiv., 0.500 mmol),
2
(1.00 equiv., 0.500 mmol), H2O (1.00 equiv., 0.500 mmol),
To gain a better understanding an annulation mechanism,
the reaction progress was monitored (on the example
quinoline 1a/acetylene 2a interaction) by the 19F NMR spectra
where the following signals were observed: −78.3, −83.0 and ‐
85.9 ppm, assignable to the starting acetylene 2a, final product
3a and an intermediate, correspondingly (Fig. 1, SI). In time,
the first signal was disappearing while the intensity of the
second one was increasing. As to the third signal, it was
growing up to a maximum then gradually dropping until
disappearing that corresponds to its intermediate nature. The
position of the signal and its evolution character allows it to be
MeCN (3.0 mL), ‐5~0 – 25 oC.
Notably, in this reaction crystalline H‐bonding complex
between 1,3‐oxazinoquinoline 3k and starting quinoline 1c was
isolated in 13% yield. As follows from Schemes 3 and 4 the
annulation is general enough tolerating electronically and
sterically diverse substituents both in quinolines and
acetylenes. Expectedly, the scope of the reaction appeared to
be not extendable over 2‐substituted quinolines. For instance,
the reaction with 2‐methylquinoline gave only a mixture of
oligomeric products. The latter may be formed via the
nucleophilic addition of methylene carbanion to other
molecules of electron‐deficient acetylene, like it was observed
in.14 Also, understandable was an adverse effect of
substituents at position 8 of the quinoline ring as imposing a
steric screening on the nitrogen atom. In fact, the reaction was
blocked completely when at this position were such
substituents as Me‐, Br‐, OH.
assignment to the intermediate D. As seen from Fig. 1 (SI) the
life‐time of the intermediate is short (its concentration begins
dropping after c.a. 4 h) and consequently it is not isolable.
The stereoselectivity of the annulation can be understood
if we consider the ionic forms of intermediate
particularly paying attention at the cyclic intermediate
Since the oxazine‐cationic moiety (in structure ) is almost
orthogonal to the quinoline plane, the attack of hydroxide
anion at the carbocationic centre in structure should
D
(Scheme 6,
E
).
E
The structure of all 1,3‐oxazinoquinolines
3 was confirmed
1
by H, 13C, 15N and 19F NMR and IR spectroscopy. The X‐ray
data show that 1,3‐oxazinoquinolines 3a and 3k’ in the
crystalline state consist of the (3R*,4aR*) diastereomers. As
mentioned above, 1,3‐oxazinoquinoline 3k’ forms bimolecular
crystal with the 3‐bromoquinoline due to the intermolecular
D
preferably be directed from outside, i.e. from a sterically less
demanded position, thereby giving the observed (3R*,4aR*)
diastereomers. Additionally, these diastereomers should be
stabilized by the anomeric effect, which requires the each
oxygen atom at the position 3 to be situated on the opposite
sites of the plane, that also corresponded to the experimental
stereochemistry.
OH
N hydrogen bond (the ON distance is 2.74 Å).
Diastereomer ratio of 1,3‐oxazinoquinolines appeared to
3
be changeable during the passing through chromatographic
column (SiO2) thus implying the kinetic nature of
stereoselectivity. However, isolation of pure diastereomers
(3R*,4aR*) can be attained using NEt3‐treated silica gel. Their
1
configuration can be determined by NMR. For example, in H
NMR spectra of 3a, the H‐4a signal is slightly downfield shifted,
while the H‐5 signal is slightly upfield shifted for the
(3R*,4aR*) diastereomer as compared with the corresponding
signals for the (3S*,4aR*) diastereomer. The most reliable
criterion for these diastereomers recognition is the position of
CF3‐group signal in 19F NMR spectra. The (3R*,4aR*)
diastereomers have singlet in the range from ‐82.8 to ‐82.3
ppm whereas the signal of the (3S*,4aR*) diastereomers are in
a range ‐79.8 to ‐79.0 ppm.
Scheme 6 A possible ionic forms of intermediate
D
The first step of the suggested mechanism, i.e. the
formation of intermediate , is supported by slowing down of
the reactions with methoxy‐substituted acetylenes 2e and 2i
C
:
for the first one π‐donating effect of the methoxy group
decreases electrophilicity of the triple bond, while the naphtyl
substituent renders the sterical hindrance, in both the cases
inhibiting nucleophilic addition of the nitrogen site to the
acetylene moiety (Scheme 2).
Apparently, in view of Scheme 1 a plausible mechanism of
the annulation studied involves the reversible formation of
1,3‐dipole intermediate
pyridine case), which undergoes 1,4‐concerted addition of
water molecule to afford intermediate further closing the
C (analogue of intermediate A for the
The observed rough correlation between the reaction time
and nucleophilicity (basicity) of quinolines
D
1 is also in keeping
with Scheme 5. In most the cases, 20‐45 h was enough to have
the reactions completed (Scheme 4), whereas a significant
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1‐3 | 3
Please do not adjust margins

Please do not adjust margins
ChemComm
Page 4 of 4
View Article Online
DOI: 10.1039/C7CC09725E
COMMUNICATION
Journal Name
‡ Solution of quinoline
solution of acetylene
(1.5 mL) were cooled (‐18
screw cap. The reaction mixture was left at room temperature
for appropriate time (TLC or 19F NMR control). The solvent was
removed under reduced pressure, the residue was purified by
column chromatography on silica gel which was treated with
NEt3 (about 3‐5 weight %). CH2Cl2, and mixtures of CH2Cl2 and
MeOH (300:1, 150:1, 100:1, 40:1) were consistently used as
eluents.
1
(0.475 mmol) in MeCN (1.5 mL) and
increase of the reaction time up to 144 h was found for 3‐
bromoquinoline (1c). In the light of the above mechanism, this
result is well understood in terms of a weaker basicity of 3‐
bromoquinoline compared to unsubstituted quinoline (pKaBH+
in water 2.69 vs 4.85, respectively) and, consequently, its
lower nucleophilicity.
2
(0.5 mmol) and H2O (0.5 mmol) in MeCN

C) and then mixed in 4 mL vial with a
The reaction is not restricted by quinolines, but other
azines can be successfully also involved into similar
transformations. It was found that isoquinoline
naphthyridine reacted with 2a under the same conditions
like quinolines to afford the corresponding 1,3‐oxazinoazines
and in 63 and 91% yield correspondingly (Scheme 7). The
products and were isolated as a single diastereomers:
4 and 1,8‐
6
1
2
3
B. A. Trofimov, L. V. Andriyankova, K. V. Belyaeva, L. P.
Nikitina, A. V. Afonin and A. G. Mal’kina, Eur. J. Org. Chem.,
2015, 7876.
L. V. Andriyankova, L. P. Nikitina, K. V. Belyaeva, A. G.
Mal’kina, A. V. Afonin, V. M. Muzalevskii, V. G. Nenaidenko
and B. A. Trofimov, Rus. J. Org. Chem., 2016, 52, 1857.
H .C. Brown, D. H. McDaniel and O. Hafliger, Dissociation
Constants in Determination of Organic Structures by Physical
Methods, ed. E. A. Braude and F. C. Nachod, Academic Press,
1955, vol. 1, pp. 567‐662.
5
7
5
7
(2R*,11bR*) and (6aR,8R*) correspondingly.
H
N
N
N
O
N
N
O
N
Ph
6
4
OH
Ph
H
MeCN/H2O
CF3
Ph
CF3
O
MeCN/H2O
2a
4
(a) P. M. S. Chauhan, N. Sunduru and M. Sharma, Future
Med. Chem., 2010, 2, 1469; (b) Fluorine in Heterocyclic
7, 91% (36 h)
HO
CF3
5, 63% (4h)
Chemistry, ed. V. G. Nenajdenko, Springer International
Publishing, Switzerland, 2014, vols. 1‐2. (c) Fluorinated
Heterocyclic Compounds: Synthesis, Chemistry, and
Applications, ed. V. A. Petrov, John Wiley & Sons, Inc.:
Hoboken, New Jersey, 2009. (d) Fluorinated Heterocycles, ed.
A. A. Gakh and K. L. Kirk, ACS Symposium Series 1003,
American Chemical Society, Washington, DC, 2009.
(a) A. Kleemann, J. Engel, B. Kutscher and D. Reicher in
Pharmaceutical substances (syntheses, patents,
Scheme 7 The reaction with isoquinoline and 1,8‐naphthyridine
As to the question why the reactions with quinolines
proceed through annulation rather than ring opening as in the
pyridine case, it can be explained by a higher basicity (as
mentioned above) of the pyridines relative to quinolines. This
should be resulted in a higher polarization of the N‐C(2) bond
5
applications), Thieme, Stuttgart – New York, 2001, vol. 1, p.
1380; (b) M. Baumann and I. R. Baxendale, Beilstein J. Org.
Chem., 2013,
P. Schlagenhauf, M. Adamcova, L. Regep, M.T. Schaerer and
H. G. Rhein, Malar. J., 2010, , 357.
9, 2265.
in the starting intermediate 1,3‐dipole
positive charge on the nitrogen atom (Scheme 1).
Consequently the covalent intermediate is more prone (then
the corresponding intermediate in the quinolines, Scheme 5)
A due to a stronger
6
7
8
9
C. Altomare, A. Carotti, G. Casini and M. Ferappi, J.
Heterocyclic Chem., 1984, 21, 777.
B
D
(a) T. Sauvaître, M. Barlier, D. Herlem, N. Gresh, A. Chiaroni,
D. Guenard and C. Guillou, J. Med. Chem. 2007, 50, 5311; (b)
B. D. Palmer, A. M. Thompson, H. S. Sutherland, A. Blaser, I.
Kmentova, S. G. Franzblau, B. Wan, Y. Wang, Z. Ma and W. A.
Denny, J. Med. Chem. 2010, 53, 282.
T. J. Sindhu, S. D. Arikkatt, V. Girly, M. Chandran, A. R. Bhat
and K. Krishnakumar, Int. J. Pharm. Sci. Res., 2013, 4, 134.
to the rearmament with the cleavage of N‐C(2) bond to form
pentadienal species.
In summary, metal‐free annulation of quinolines with
trifluoroacetylacetylenes in the presence of water proceeds
under exceptionally mild conditions (rt and lower) to give CF3‐
substituted 1,3‐oxazinoquinolines in up to quantitative yields.
The reaction has broad scope in terms of substituted
quinolines and acetylenes. The possible reaction mechanism
includes the formation of 1,3‐dipolar adducts as it is supported
by the experiment using D2O. The reaction is c.a. 100%
diastereoselective to form cyclo‐products in (3R*,4aR*)‐
configuration. The results highlight new facets of fundamental
and applied chemistry of pharmaceutically‐targeted
fluorinated quinolines and oxazines.
9
10 H. Hilpert, R. Narquizian, E. Pinard, A. Polara, M. Rogers‐
Evans, T. Woltering and W. Wostl, Preparation of 1,3‐
oxazines as BACE1 and BACE2 inhibitors. WO 2012/156284
A1, 2012. Chem. Abstr. 2012, 158, 11628.
11 J. P. Cherkauskas, A. M. Klos, R. M. Borzilleri, J. Sisko and S.
M. Weinreb, Tetrahedron, 1996, 52, 3135.
12 A. T. Biju, A. Bhunia and D. Shanmugam, Multicomponent
process for diastereoselective preparation of
benzoxazino(iso)quinoline compounds and use as
antimalarial agents. US 9206196 B1 20151208, 2015; Chem.
Abstr. 2015, 164, 69156.
The main results were obtained with Baikal analytical centre of
collective using SB RAS.
13 (a) F. Rostami‐Charati, Z. Hossaini, E. Gharaee and M. A.
Khalilzadeh, J. Heterocycl. Chem., 2013, 50(S1), E174; (b) A.
Bhunia, T. Roy, P. Pachfule, P. R. Rajamohanan and A. T. Biju,
Angew. Chem., Int. Ed., 2013, 52, 10040; (c) A. Bhunia, D.
Conflicts of interest
“There are no conflicts to declare”.
Porwal, R. G. Gonnade and A. T. Biju, Org. Lett., 2013, 15
4620; (d) P. Liu, M. Lei and L. Hu, Tetrahedron, 2013, 69
,
,
10405; (e) T. Sun, Q. Cai, M. Li, Z. Wang, J. Chen, H. Deng, M.
Shao, H. Zhang and W. Cao, Tetrahedron, 2015, 71, 622.
14 K. V. Belyaeva, L. P. Nikitina, A. V. Afonin, A. V. Vashchenko
and B. A. Trofimov, Izv. Akad. Nauk., Ser. Khim., 2017, 2258.
Notes and references
4 | J. Name., 2012, 00, 1‐3
This journal is © The Royal Society of Chemistry 20xx
Please do not adjust margins