Synthesis of 5-aryl-2-hydroxy-2-(trifluoromethyl)furan-3(2H)-ones and their reactions with aromatic 1,2-diamines, hydrazine and hydroxylamine

Article abstract of DOI:10.1016/j.tet.2015.09.035

2-(Trifluoromethyl)furanones were obtained in good yields via the Claisen condensation of acetophenones with methyl 2-methoxytetrafluoropropionate, followed by sulfuric acid-mediated deprotection of the reaction products. These compounds react with 2,3-di

Full text of DOI:10.1016/j.tet.2015.09.035

Tetrahedron 71 (2015) 8535e8543
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Synthesis of 5-aryl-2-hydroxy-2-(trifluoromethyl)furan-3(2H)-ones
and their reactions with aromatic 1,2-diamines, hydrazine and
hydroxylamine
,
,b
Alexander V. Safrygin a, Roman A. Irgashev a b, Pavel A. Slepukhin a
,
c
a,
*
€
Gerd-Volker Roschenthaler , Vyacheslav Y. Sosnovskikh
a Department of Chemistry, Institute of Natural Sciences, Ural Federal University, 620000 Ekaterinburg, Russian Federation
b Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences, 620137 Ekaterinburg, Russian Federation
c School of Engineering and Science, Jacobs University Bremen, 28759 Bremen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2015
Received in revised form 1 September 2015
Accepted 14 September 2015
Available online 15 September 2015
2-(Trifluoromethyl)furanones were obtained in good yields via the Claisen condensation of acetophe-
nones with methyl 2-methoxytetrafluoropropionate, followed by sulfuric acid-mediated deprotection of
the reaction products. These compounds react with 2,3-diaminopyridine, o-phenylenediamine and 2,3-
diaminonaphthalene in refluxing acetic acid to give the corresponding quinoxaline derivatives, the re-
gioisomeric and tautomeric composition of which was investigated. Reactions of 2-(trifluoromethyl)
furanones with hydrazines and hydroxylamine proceeded regioselectively and gave pyridazine, pyrazole
and isoxazole derivatives in high yields. Their regiochemistry was established by X-ray diffraction
analysis.
Keywords:
2-(Trifluoromethyl)furanones
Nitrogen nucleophiles
Quinoxalines
Ó 2015 Elsevier Ltd. All rights reserved.
Pyridazines
Pyrazoles
Isoxazoles
1. Introduction
carbon atom connected to the CF3 group to give regioisomeric
products.4 Particularly interesting synthons, prepared in this series
The synthesis of specifically trifluoromethylated molecules has
remarkable interest due to the unique physical and biological
properties imparted by the CF3 group.1 Therefore, the development
of a simple method for the preparation of trifluoromethylated
building blocks and their further utilization for the synthesis of
desired CF3-containing heterocycles are essential to organic
chemistry. In recent years, considerable attention has been given to
the development of new procedures for the synthesis of CF3-con-
of trifluoromethylated building blocks, are 2-(trifluoroacetyl)chro-
mones 3, which can serve as versatile 1,2-dielectrophiles for the
double nucleophilic addition of aliphatic and aromatic 1,2-
diamines to produce pyrazine and quinoxaline derivatives. In this
case, the first step of the reaction involves an attack of the more
nucleophilic amino group at C-2 with concomitant opening of the
pyrone ring; subsequent intramolecular attack of the second amino
group at the trifluoroacetyl group leads to the heterocyclic
products.5
Although the reactivity of a wide range of CF3-containing
diketones has been studied intensively over the last years,6 to our
knowledge, there are only two communications about the prepa-
ration of the CF3-containing triketones 47a and 5,8 existing in a cy-
clic furanone form. Due to the concentration of electron-deficient
groups in a small carbon skeleton, these compounds are potent
trielectrophiles (Fig. 1). Very recently,7b it has been found that the
methoxy derivative of furanone 4 is a valuable building block for
the preparation of a wide range of trifluoromethylated N-hetero-
cycles. In connection with our interest in the synthesis of simple
and accessible fluoroorganic synthons,9 we now report on the
taining synthons such as
b-chloro-, b-alkoxy and b-aminovinyl
trifluoromethyl ketones 1.2 These compounds readily react with
a wide range of nucleophiles and they are extensively used for the
synthesis of various trifluoromethylated heterocycles, which may
be of interest in both medicine and agricultural chemistry.2,3
Generally, nucleophiles initially attack their b-carbon atom with
elimination of the leaving group (chloro, alkoxy or amino). On the
other hand, the reactions of 1-trifluoromethyl-1,3-diketones 2 with
the same nucleophiles usually start with an attack at the carbonyl
* Corresponding author. Fax: þ7 343 261 59 78; e-mail address: vy.sosnovskikh@
urfu.ru (V.Y. Sosnovskikh).
http://dx.doi.org/10.1016/j.tet.2015.09.035
0040-4020/Ó 2015 Elsevier Ltd. All rights reserved.

8536
A.V. Safrygin et al. / Tetrahedron 71 (2015) 8535e8543
diketones 6 and furanones 5 were confirmed by elemental analy-
O
sis, 1H, 19F, 13C NMR, and IR spectroscopies. Recently, this very
practical and convenient approach has been applied by us to the
synthesis of 2-(trifluoroacetyl)chromones 3.5
R
R
CF3
R
CF3
CF3
X
O
O
O
O
1
2
3
O
X = Cl, R'O, R'2N
H
O
O
O
O
O
O
CF3
O
MeO
CF3
F OMe
F3C
Me
OH
Me
Me
1) NaOEt, EtOH
2) HCl
F
OMe
F3C
O
O
O
O
R
R
6a,b
4
SiO2 H2SO4
O
Ar
OH
CF3
O
OH
O
Ar
CF3
O
O
CF3
OH
5
CF3
O
O
R
R
Fig. 1. Trifluoromethylated building blocks 1e5 (red arrows indicate the site of the
initial attack).
5a,b
Ar
Ph
4-ClC6H4
6
a
b
Yield (%)
79
73
5
a
b
Yield (%)
64
61
synthesis of furanones 5, which contain two masked carbonyl
groups, and their facile conversion into various five- and six-
membered nitrogenated heterocycles bearing a CF3 group, in-
cluding quinoxaline derivatives that will be of interest to synthetic
and medicinal chemists.
Scheme 1. Synthesis of compounds 5a,b and 6a,b.
To demonstrate the ability of furanones 5 to undergo regiose-
lective heterocyclization reactions and to determine the site of the
initial nucleophilic attack, they were treated with 2,3-
diaminopyridine, having two amino groups with different nucleo-
philicity. In connection with this, it should be noted that when 2-
2. Results and discussion
Despite the unique biological and physical properties imparted
by the CF3 group, and the obvious versatility of tricarbonyls as
valuable scaffolds for heterocyclic chemistry, 1-trifluoromethyl-
1,2,4-triketones have not received much attention, probably owing
to the lack of general methods for their synthesis. Our approach to
these compounds involves a Claisen condensation of methyl ke-
tones with methyl 2-methoxy-2,3,3,3-tetrafluoropropionate, pre-
pared from hexafluoropropene epoxide. It is known that methanol
readily attacks the central carbon atom of hexafluoropropene ep-
oxide, resulting in the formation of 2-methoxytetrafluoropropionyl
fluoride, which further reacts with an additional molecule of
methanol, forming methyl 2-methoxytetrafluoropropionate.10 We
reasoned that the use of this ester in the Claisen condensation,
which represents a readily available starting fluorinated material,
would be extremely advantageous, particularly on a preparative
multigram scale. To the best of our knowledge, little is known about
the Claisen-type preparation of 1,3-diketones from methyl 2-
methoxytetrafluoropropionate and their deprotection to tri-
fluoromethylated 1,2,4-triketones, which we deemed suitable for
the synthesis of more complex CF3-containing molecules. Indeed,
apart from two our communications,5,8 there is only one report, in
which this ester was used as a carbonyl component in the con-
densation with cyclohexanone without deprotection.11
(trifluoroacetyl)chromones
3
were
reacted
with
2,3-
diaminopyridine in acetic acid at room temperature within one
week, only 3-CF3-isomers as a mixture of tautomers A-7 and B-7
precipitated from the reaction mixture5 (Scheme 2).
initial attack
O
H2N
R
+
CF3
H2N
N
O
3
O
ref. 5
AcOH
HO
HO
O
O
R
R
H
N
N
2
3
N
N
CF3
N
N
CF3
A-7
B-7
Scheme 2. Synthesis of 3-CF3-pyrido[2,3-b]pyrazines 7 (red arrow indicates the site of
the initial attack).
In contrast to these results, furanones 5a,b have now been
shown to react under the same conditions with 2,3-
diaminopyridine to form 2-CF3-isomers 8a,b, existing principally
as the enamine tautomeric form A-8 in 88e94% yield. This reaction
proceeds very cleanly without any undesirable side-products.
Pyrido[2,3-b]pyrazines 8a,b (structure assignment see below)
precipitated from the reaction mixture and a simple filtration
provides analytically pure products in both cases. However, the
reaction conditions were important for the regioselectivity and
tautomeric composition. Thus, when furanones 5a,b were reacted
with 2,3-diaminopyridine in refluxing AcOH for 4 h, apart from 2-
CF3-isomers A-8 3-CF3-isomers 9 as a tautomic mixture and im-
ine tautomers B-8 were observed in a small amount (4e13% for
tautomers 9 and 4e6% for B-8, see experimental part). The
We found that methyl 2-methoxytetrafluoropropionate reacted
with acetophenone and p-chloroacetophenone under Claisen re-
action conditions (refluxing ethanol and NaOEt as a base) affording,
after hydrochloric acid hydrolysis, 1,3-diketones 6a,b in high yield
(73e79%). Note that in refluxing THF in the presence of LiH the yield
of compounds 6a,b was only 33e35%. Deprotection of these dike-
tones, which were fully enolic in CDCl3 (dOH¼15.5e15.6 ppm), was
10b
carried out using 96% H2SO4 and SiO2
to afford the correspond-
ing 1,2,4-triketones. The latter underwent spontaneous intra-
molecular cyclization at the COCF3 group to give 5-aryl-2-hydroxy-
2-(trifluoromethyl)furan-3(2H)-ones 5a,b in 61e64% yield. These
compounds contain three carbonyl groups, with two of them
masked in enol ether and ketal forms, i.e., with the two carbonyl
groups differently masked (Scheme 1). The structures of 1,3-

A.V. Safrygin et al. / Tetrahedron 71 (2015) 8535e8543
8537
proportions of these tautomers were determined by integration of
the CF3 signals in the 19F NMR spectra. Note that in the case of
regioisomers 9a,b, regardless of the reaction conditions, tautomers
B-9a,b were found in the crude reaction mixture in only very small
amounts (ca. 1%). The first step of the reaction leading to major
products 8a,b apparently involves an attack of the more nucleo-
philic 3-NH2 group at C-2 of compounds 5 with concomitant
opening of the furanone ring. Subsequent intramolecular attack of
the less nucleophilic 2-NH2 group at the carbonyl group leads to
pyrido[2,3-b]pyrazines 8 (Scheme 3).
The structure of the major products A-8a,b was established by
the usual spectroscopic analyses. A characteristic feature of their 1H
NMR spectra in a CDCl3 solution is the appearance of a quartet at
d
6.46e6.52 (5JH,F¼1.7 Hz) and a singlet at
d 15.1 ppm for the ]CH
and NH protons, respectively. The CF3 group in the 19F NMR spectra
manifests itself as a doublet at d 93.4 ppm due to coupling with the
]CH proton, in good accordance with the given structure. The
assignment of structure 8 to the 2-CF3-regioisomer is based on the
1H NMR chemical shifts of the pyridine protons H-6, H-7 and H-8 in
its enamine tautomeric form A-8. A comparison of these chemical
shifts in 8a with the values reported5 for 2-CF3-regioisomer A-10
confirms the validity of the structure and shows that 2-CF3-isomer
8a exists as a single tautomer A with the aminoenone fragment. In
the case of 3-CF3-pyrido[2,3-b]pyrazine 9a, the observed chemical
shift of the same pyridine protons is a weighted average of the
contributions from A-7 and B-75 indicating an approximately 1:1
mixture of the tautomers A-9a and C-9a (Scheme 3). This suggested
that in a CDCl3 solution of compound 9a the proton exchange be-
tween the ketoenamine (A-9) and iminoenol (C-9) tautomeric
forms is too fast and only average signals were observed. The signal
of the CF3 group in 8a and 9a as well as the 1H and 19F NMR spectra
of 8b and 9b are in accordance with this conclusion. At the same
time, the enamine form A-8a is stable and interconversion of type
A$C does not occur, probably because the prototropic shift pro-
ceeds very slowly as a result of interaction between the N-5 lone
pair and NH proton (Scheme 3, Fig. 2).
Obviously, the observed differences in regioselectivity of chro-
mones 3 and furanones 5 is a result of different directions of the
initial nucleophilic attack (addition of the more nucleophilic 3-NH2
group to C-2 of 3, the former carbonyl adjacent to the CF3CO group,
and to C-2 of 4, the former carbonyl connected to the CF3 group)
with the formation of 3-CF3-quinoxalines 7 from 3 and 2-CF3-qui-
noxalines 8 from 5. The regiochemical outcome of the reaction
reveals that 2,3-diaminopyridine is a useful dinucleophile for
evaluation of the site of the initial nucleophilic attack.
O
H2N
H2N
N
OH
+
Ar
CF3
O
5a,b
initial attack
AcOH
O
Ar
O
Ar
H
N
N
N
N
N
6
7
slow
3
2
8a,b
CF3
N
CF3
8
A-8
B-8
+
O
Ar
H
N
8
7
6
2
3
O
Ar
N
N
CF3
A-9
N
slow
fast
9a,b
N
N
CF3
HO
Ar
B-9
N
We also found that furanones 5a,b reacted with o-phenyl-
enediamine in refluxing acetic acid for 4 h to give products 11a,b as
a mixture of enamine and imine tautomers A and B in 68e87% yield
(Scheme 4). In the 19F NMR spectra of 11a,b, the CF3 group of tau-
tomer A appeared as a doublet (5JF,H¼1.7e1.9 Hz) at 96.8e96.9 ppm
and the CF3 group of B appeared as a triplet (5JF,H¼1.6 Hz) or
a broadened singlet at 98.9e99.0 ppm (DMSO-d6, C6F6). Thus, this
reaction afforded a mixture of two tautomeric forms (ca.1:1) as was
observed previously for their analogues prepared from chromones
3.5
N
N
CF3
C-9
Ar
Ph
4-ClC6H4
8+9a
a
b
A-8/B-8
90:6
83:4
A,C-9/B-9 Yield (%)
3:1
88
94
12.5:0.5
a At reflux in AcOH for 4 h.
Scheme 3. Synthesis and composition of compounds 8 and 9 (red arrow indicates the
site of the initial attack).
HO
HO
O
H
O
O
Ph
H
N
8.50
7.74
8.18
N
N
7.59
8.70
7.87
9.31
7.73
9.00
3
3
3
5
5
5
N
N
CF3
92.8
N
N
CF3
96.5
N
N
CF3
94.9
A-7
B-7
9a
HO
HO
O
Ph
O
O
H
N
H
N
N
N
N
N
8.69
7.34
8.60
7.31
9.32
7.86
5
5
5
2
2
2
N
CF3
93.4
N
CF3
97.0
N
CF3
92.9
8.13
8.10
8.63
A-8a
A-10
B-10
Fig. 2. Diagnostic 1H and 19F NMR signals (
d, ppm, CDCl3) of compounds 7e10.

8538
A.V. Safrygin et al. / Tetrahedron 71 (2015) 8535e8543
O
O
O
Ar
H2N
H2N
CF3
H
N
N2H4
OH
HCl
Ar
AcOH
CF3
O
N
5a,b
Ar
N
H
AcOH
5a,b
N
CF3
A
NH2
NH2
12a,b
MeNHNH2
H2SO4
AcOH
AcOH
13a,b
O
O
Ar
O
Ar
OH
CF3
OH
H
CF3
Ar
N
N
N
O
O
N
CF3
N
Ar
N
N
CF3
CF3
N
B
Ar
B-11a,b
A-11a,b
Me 14a,b
Ar
Ph
4-ClC6H4
11 A:B
Yield (%)
68
87
12 Yield (%)
Ar
Ph
4-ClC6H4
13 A:B
Yield (%)
81
88
14 Yield (%)
a
b
45:55
51:49
a
b
84
97
a
95:5
a
b
39
45
b
80:20
Scheme 4. Synthesis of compounds 11a,b and 12a,b.
Scheme 5. Synthesis of compounds 13a,b and 14a,b.
Under similar conditions, 2,3-diaminonaphthalene gave benzo[g]
quinoxalines 12a,b in excellent yields (84e97%). According to the 1H,
19F and 13C NMR spectra, these compounds are predominantly in the
form of enamine tautomer, probably because of the additional ben-
zene ring (Scheme 4). In the 19F NMR spectra of 12a,b, the CF3 group
appeared as a broadened singlet at
d 93.8e93.9 ppm (CDCl3). Since
only one structural isomer can be produced in reactions involving
the symmetrical diamines such as o-phenylenediamine and 2,3-
diaminonaphthalene, the structures of the products 11 and 12 re-
quire no special discussion. Thus, our results showed that tri-
fluoromethylated 1,2,4-triketones, existing principally as the
furanones 5, behave as a latent 1-trifluoromethyl-1,2-diketone and
react with aromatic 1,2-diamines to give various quinoxaline de-
rivatives in high yields. Attempts to prepare the corresponding pyr-
azine derivative from furanone 5a and ethylenediamine under
various conditions were fruitless due to the more nucleophilic
character of this diamine, leading to cleavage of the starting material.
Our next concern was focused on the reactions of furanones 5
with other dinucleophiles, such as hydrazines and hydroxylamine.
We found that treatment of compounds 5a in refluxing AcOH for
8 h with N2H4$2HCl (2 equiv) resulted in the formation of 6-aryl-3-
(trifluoromethyl)pyridazin-4(1H)-ones 13a,b (yield 81e88%),
which exist as a mixture of two tautomeric forms (A/B¼95:5 and
80:20, respectively) in DMSO-d6 solution (Scheme 5). It should be
noted that isomeric 6-phenyl-4-(trifluoromethyl)pyridazin-3(2H)-
one can be prepared from acetophenone, methyl trifluoropyruvate,
and hydrazine hydrate.12 Interestingly, the reaction of methoxy
derivative of furanone 4 with hydrazine hydrate in methanol gave
pyrazole derivative as a sole product;7b a similar pyrazole forma-
tion was observed in the reaction of nonfluorinated furanones.13
Several competing pathways in both the initial step of nucleo-
philic attack and the subsequent heterocyclization would have been
expected for the reaction of furanones 5 with methylhydrazine.
However, we found that the principal direction of the reaction of
MeNHNH2$H2SO4 with 5a,b in refluxing AcOH for 12 h is the for-
mation of pyridazin-4-ones 14a,b, which were isolated in 39e45%
yield as a 1:1 complex with a furanone molecule (Scheme 5). The
crystal structure of complex with 14a was confirmed by XRD anal-
ysis. In according to XRD data, molecules 14a and 5a have standard
bonds lengths and angles. The furanone ring is planar in limits of
Fig. 3. ORTEP view of compound 14a according to XRD data (thermal ellipsoids at 50%
probability).
Another reaction route was observed for the reaction of fur-
anones 5a,b with phenylhydrazine hydrochloride (3.5 equiv) under
the same conditions (AcOH, reflux, 8 h). In this case, the only iso-
lated products were pyrazoles 15a,b (25e33% yields), which
resulted from the double nucleophilic addition of two phenyl-
hydrazine molecules at the C-1 and C-2 atoms of the 1,2,4-
tricarbonyl system. The alternative addition of the second phenyl-
hydrazine molecule, involving the C-4 atom, does not occur, and no
formation of the regioisomeric pyrazole was detected (Scheme 6).
The regiochemistry and E-configuration at the C]N double bond of
pyrazole 15a was confirmed by X-ray single crystal analysis (Fig. 4).
When compounds 5a,b were treated with NH2OH$HCl (2 equiv,
AcOH, reflux, 8 h), isoxazoles 16a,b were isolated in 71e74% yield as
beige crystals. The regiochemistry of 16b was unambiguously
confirmed by X-ray single crystal analysis (Fig. 5). The difference in
behaviour between the hydrochlorides of hydroxylamine and
phenylhydrazine in the reaction with furanones 5 is remarkable. In
this case, the double nucleophilic addition of two hydroxylamine
molecules occurs at the C-1 and C-4 atoms of the 1,2,4-tricarbonyl
system. Obviously, the change in the reaction course is a result of
the replacement of the phenylamino group by hydroxy group in the
binucleophilic agent (Scheme 6).
ꢀ
0.015 A and due to the OH group forms strong intermolecular hy-
drogen bond OH/O with the carbonyl oxygen atom of 14a with
ꢀ
parameters d(O(2)/O(4) [1ꢀx, 2ꢀy, 1ꢀz])¼2.634(2) A, d(O(2)e
ꢀ
ꢀ
H(2))¼0.87(3) A, d(H(2)/O(4) [1ꢀx, 2ꢀy, 1ꢀz])¼1.77(3) A, angle
O(2)H(2)O(4) [1ꢀx, 2ꢀy, 1ꢀz] 172(3)ꢁ. No any significantly short-
ened contacts in the crystal packing are observed (Fig. 3).

A.V. Safrygin et al. / Tetrahedron 71 (2015) 8535e8543
8539
X
N
O
Ar
NH2X HCl
OH
CF3
Ar
Ar
CF3
O
O
O
A
5a,b
X = NHPh
X = OH
NOH
NNHPh
CF3
Ar
Ar
CF3
O
NNHPh
NOH O
B
C
Ar
OH
N
N
CF3
N
Ph
N
O
N
CF3
16a,b
16 Yield (%)
HN
Ph
15a,b
Fig. 5. ORTEP view of compound 16b according to XRD data (thermal ellipsoids at 50%
Ar
Ph
4-ClC6H4
15 Yield (%)
probability).
a
b
25
33
a
b
74
71
O
O
O
Scheme 6. Synthesis of compounds 15a,b and 16a,b.
R
R
NH2OH HCl
CF3
CF3
O
O
N
3
17a,b
HO
Ar
17
Yield (%)
Ph
4-ClC6H4
a
b
83
90
Scheme 7. Synthesis of compounds 17a,b.
furanone system offers the potential as an entry to the synthesis of
trifluoromethylated analogues of these compounds.
3. Conclusion
In summary, we have developed a simple and convenient two-
step synthesis of 5-aryl-2-hydroxy-2-(trifluoromethyl)furan-
3(2H)-ones starting from commercially available acetophenones
and hexafluoropropene epoxide via introduction of a CF3COCO
moiety into a ketone methyl group. The compounds obtained,
which contain two masked carbonyl groups due to the cyclic fur-
anone form, are of interest as precursors for the synthesis of various
trifluoromethylated nitrogenated heterocycles. The latter result
from the initial addition of the more nucleophilic centre to the C-2
atom of 2-(trifluoromethyl)furanones.
Fig. 4. ORTEP view of compound 15a according to XRD data (thermal ellipsoids at 50%
probability).
4. Experimental
4.1. General
Finally, we found that 2-(trifluoroacetyl)chromones 35 react
with NH2OH$HCl (2 equiv) under the same conditions for 4 h to
give E-oximes 17a,b in excellent yield (Scheme 7). Their configu-
ration was deduced from the chemical shift of the CF3 group
1H, 19F and 13C NMR spectra were recorded on Bruker DRX-400
(400, 376, 100 MHz) and AVANCE-500 (500, 470.5, 126 MHz) spec-
trometers in CDCl3 or DMSO-d6 with TMS and C6F6 as internal stan-
dards. IR spectra were recorded on a PerkineElmer Spectrum BX-II as
KBr discs and Nicolet 6700 instruments (FTIR mode). Elemental
analyses were performed at the Microanalysis Services of theInstitute
of Organic Synthesis, Ural Branch, Russian Academy of Sciences. All
solvents used were dried and distilled per standard procedures.
(
dCF3¼97.9 ppm for 16a,b and dCF3¼98.4e98.5 ppm for 17a,b).
It is evident from the reactions of furanones 5 with a number of
dinucleophiles that the C-2 atom, due to the electron-withdrawing
effect of the CF3 group, is very susceptible to nucleophilic attack,
which makes them useful for constructing biologically and medic-
inally important products. Since many natural products are based on
the quinoxaline, pyridazine, pyrazole and isoxazole structures, this

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A.V. Safrygin et al. / Tetrahedron 71 (2015) 8535e8543
4.2. Synthesis of diketones 6a,b
for 1 h at this temperature. The cooled mixture was poured on
crushed ice (125 g) and extracted with ethyl acetate (3ꢂ40 mL). The
combined extracts were washed with brine (3ꢂ50 mL), dried over
anhydrous Na2SO4 and evaporated under reduced pressure. The
solid that formed was recrystallized from cyclohexane/methyl tert-
butyl ether (10:1) to give 5a in 64% yield, beige crystalꢀs,1 mp
4.2.1. (Z)-4,5,5,5-Tetrafluoro-3-hydroxy-4-methoxy-1-phenylpent-2-
en-1-one (6a)
4.2.1.1. Method A. A mixture of methyl 2-methoxy-2,3,3,3-
tetrafluoropropionate (9.5 g, 50.0 mmol) and acetophenone
(5.0 g, 42.0 mmol) was added dropwise to an alcoholic solution of
NaOEt obtained by dissolution of sodium (2.9 g, 126.1 mmol) in
anhydrous EtOH (50 mL). The resulting reaction mixture was
heated at reflux with stirring for 5 h and allowed to stand over-
night. Concentrated HCl (20 mL) and water (175 mL) were added to
the disodium salt, and the oily product was extracted with ether
(3ꢂ65 mL). The combined extracts were washed with water
(50 mL), dried over anhydrous Na2SO4 and evaporated to afford
a colourless solid. The solid was recrystallized from petroleum
ether (40e70 ꢁC) to give 6a in 79% yiꢀel1d, beige crystals, mp
143e144 ꢁC. IR (KBr): 3066, 1696, 1600, 1588, 1567, 1489 cm
;
1H
NMR (400 MHz, CDCl3)
d 4.0e5.8 (br s, 1H, OH), 6.12 (s, 1H, H-4),
7.52e7.57 (m, 2H, H-30, H-50), 7.66 (tt, J¼7.6, 1.2 Hz, 1H, H-40),
7.88e7.91 (m, 2H, H-20, H-60); 1H NMR (500 MHz, DMSO-d6)
d 6.70
(s, 1H, H-4), 7.63 (t, J¼7.6 Hz, 2H, H-30, H-50), 7.73 (tt, J¼7.4, 1.1 Hz,
1H, H-40), 7.99e8.03 (m, 2H, H-20, H-60), 9.79 (s, 1H, OH); 19F NMR
(376 MHz, CDCl3)
d
79.4 (s, CF3); 19F NMR (376 MHz, DMSO-d6)
2
d
81.2 (s, CF3); 13C NMR (126 MHz, CDCl3)
d
97.3 (q, JC,F¼34.8 Hz),
99.3, 120.3 (q, 1JC,F¼285.9 Hz), 127.1, 127.9, 129.2, 134.4, 186.8, 194.7;
13C NMR (126 MHz, DMSO-d6)
d
97.7 (q, JC,F¼32.8 Hz), 99.9, 120.7
2
(q, 1JC,F¼285.8 Hz), 127.2, 127.4, 129.4,134.1, 184.2,194.0; MS (EI): m/
z (%) 244 [M]þ (48), 216 [MꢀCO]þ (20), 147 [MꢀCOCF3]þ (16), 105
[C6H5CO]þ (11), 102 (100), 77 [C6H5]þ (11), 69 [CF3]þ (16), 51 (16).
Anal. Calcd for C11H7F3P3: C, 54.11; H, 2.89. Found: C, 53.99; H, 2.50.
56e57 ꢁC. IR (KBr): 1620, 1600, 1573 cm
;
1H NMR (400 MHz,
CDCl3)
d
3.61 (d, 4JH,F¼0.9 Hz, 3H, MeO), 6.70 (d, 4JH,F¼2.1 Hz, 1H, ]
CH), 7.48e7.53 (m, 2H, H-30, H-50), 7.62 (tt, J¼7.4, 1.3 Hz, 1H, H-40),
7.95e7.99 (m, 2H, H-20, H-60), 15.62 (br s, 1H, OH); 19F NMR
3
3
(376 MHz, CDCl3)
d
22.8 (q, JF,F¼3.5 Hz, F), 80.6 (d, JF,F¼3.5 Hz,
3
CF3); 13C NMR (126 MHz, CDCl3)
d
53.3 (d, JC,F¼3.1 Hz), 95.4 (d,
2
4.3.2. 5-(4-Chlorophenyl)-2-hydroxy-2-(trifluoromethyl)furan-
3(2H)-one (5b). This compound was prepared by the procedure
described for 5a. Yield 61%, beige crystals, mp 146e147 ꢁC. IR (ATR):
3285, 3101, 1700, 1598, 1584, 1558, 1487 cmꢀ1; 1H NMR (400 MHz,
3JC,F¼3.6 Hz), 105.7 (dq, JC,F¼241.2 Hz, JC,F¼34.9 Hz), 119.5 (qd,
1
1JC,F¼287.0 Hz, JC,F¼35.1 Hz), 127.6, 129.0, 133.4, 133.8, 182.1 (d,
2
2JC,F¼35.0 Hz), 185.7 (d, JC,F¼0.8 Hz); MS (EI, 29 ꢁC): m/z (%) 278
4
[M]þ (5), 147 [MꢀC(F) (OMe)CF3]þ (100), 105 [C6H5CO]þ (19), 77
[C6H5]þ (14), 69 [CF3]þ (60); MS (EI, 239 ꢁC): m/z (%) 228
[MꢀFꢀOMe]þ (32), 147 [MꢀC(F) (OMe)CF3]þ (58), 105 [C6H5CO]þ
(49), 102 (100), 77 [C6H5]þ (30), 69 [CF3]þ (44), 44 (50). Anal. Calcd
for C12H10F4O3: C, 51.81; H, 3.62. Found: C, 52.16; H, 3.45.
CDCl3)
d
6.10 (s, 1H, H-4), 6.3e6.7 (br s, 1H, OH), 7.53 (d, J¼8.6 Hz,
2H, H-30, H-50), 7.84 (d, J¼8.6 Hz, 2H, H-20, H-60); 1H NMR (500 MHz,
DMSO-d6)
d
6.74 (s, 1H, H-4), 7.71 (m, 2H, H-30, H-50), 8.02 (m, 2H,
H-20, H-60), 9.82 (s, 1H, OH); 19F NMR (376 MHz, CDCl3)
d
79.4 (s,
CF3); 19F NMR (376 MHz, DMSO-d6)
d
81.3 (s, CF3); 13C NMR
2
(126 MHz, DMSO-d6)
d
97.8 (q, JC,F¼32.6 Hz), 100.4, 120.6 (q,
1JC,F¼286.0 Hz), 126.0, 129.1, 129.6, 138.9, 183.0, 194.0. Anal. Calcd
4.2.1.2. Method B. A mixture of methyl 2-methoxy-2,3,3,3-
tetrafluoropropionate (9.5 g, 50.0 mmol) and acetophenone
(5.0 g, 42.0 mmol) was added dropwise to a suspension of LiH
(0.67 g, 83.8 mmol) in anhydrous THF (40 mL). The resulting re-
action mixture was heated at reflux with stirring for 6 h. After that,
the mixture was concentrated under reduced pressure, the residue
was washed with ether (50 mL), filtered and quenched by addition
of 20% H2SO4 (100 mL). The oily product obtained was extracted
with ether (3ꢂ65 mL) and dried over anhydrous Na2SO4. After
evaporation of the solvent the solid was recrystallized from pe-
troleum ether (40e70 ꢁC) to give 6a in 35% yield.
for C11H6ClF3O3: C, 47.42; H, 2.17. Found: C, 47.40; H, 2.12.
4.4. Synthesis of pyrido[2,3-b]pyrazines 8
4.4.1. (Z)-1-Phenyl-2-(2-(trifluoromethyl)pyrido[2,3-b]pyrazin-
3(4H)-ylidene)ethanone (8a). A solution of furanone 5a (244 mg,
1.0 mmol) and 2,3-diaminopyridine (131 mg, 1.2 mmol) in AcOH
(5 mL) was heated at reflux for 4 h and allowed to stand at room
temperature overnight. The solid that formed was filtered and
washed with cooled ethanol. Yield 88%, red crystals, mp
195e196 ꢁC. IR (ATR): 1620, 1575, 1539 cmꢀ1
;
1H NMR (400 MHz,
4.2.2. (Z)-1-(4-Chlorophenyl)-4,5,5,5-tetrafluoro-3-hydroxy-4-
methoxypent-2-en-1-one (6b). This compound was prepared by the
procedure described for 6a. Yield 73% (method A), yield 33%
(method B), beige crystals, mp 65e66 ꢁC. IR (ATR): 3119, 3064, 3001,
CDCl3)
d
(A-8a, 90%) 6.53 (q, J¼1.7 Hz, 1H, ]CH), 7.34 (dd, J¼8.0,
4.6 Hz, 1H, H-7), 7.45e7.55 (m, 3H, Ph), 7.92e7.97 (m, 2H, Ph), 8.13
(dd, J¼8.0, 1.7 Hz, 1H, H-8), 8.70 (dd, J¼4.6, 1.7 Hz, 1H, H-6), 15.09 (s,
1H, NH); (B-8a, 6%) 5.04 (q, J¼1.3 Hz, 2H, CH2), 7.45e7.55 (m, 2H,
Ph), 7.64 (m, 1H, Ph), 7.83 (dd, J¼8.4, 4.2 Hz, 1H, H-7), 8.02e8.06 (m,
2H, Ph), 8.60 (dd, J¼8.4, 1.8 Hz, 1H, H-8), 9.30 (dd, J¼4.1, 1.8 Hz,
1H, H-6); (Z)-1-phenyl-2-(3-(trifluoromethyl)pyrido[2,3-b]pyrazin-
2(1H)-ylidene)ethanone (9a) (A-9a, 3%) 6.63 (q, J¼1.7 Hz, 1H, ]CH),
7.45e7.55 (m, 3H, Ph), 7.73 (dd, J¼8.4, 4.1 Hz, 1H, H-7), 7.92e7.97
(m, 2H, Ph), 8.18 (dd, J¼8.4, 1.7 Hz, 1H, H-8), 9.00 (dd, J¼4.1, 1.7 Hz,
1H, H-6), 15.31 (s, 1H, NH); (B-9a, 1%) 4.99 (br q, J¼1.3 Hz, 2H, CH2),
7.45e7.55 (m, 2H, Ph), 7.64 (m, 1H, Ph), 7.83 (dd, J¼8.4, 4.1 Hz, 1H,
H-7), 8.02e8.06 (m, 2H, Ph), 8.48 (dd, J¼8.5, 1.9 Hz, 1H, H-8), 9.30
2958, 2858,1594,1557,1486 cmꢀ1; 1H NMR (400 MHz, CDCl3)
d 3.61
(d, 4JH,F¼0.8 Hz, 3H, MeO), 6.65 (d, 4JH,F¼2.1 Hz, 1H, ]CH), 7.48 (dt,
J¼8.7, 2.2 Hz, 2H, H-30, H-50), 7.90 (dt, J¼8.7, 2.2 Hz, 1H, H-20, H-60),
15.54 (br s, 1H, OH); 19F NMR (376 MHz, CDCl3)
d 22.9 (q,
3
3JF,F¼3.4 Hz, F), 80.7 (d, JF,F¼3.4 Hz, CF3); 13C NMR (126 MHz,
3
3
CDCl3)
d
53.3 (d, JC,F¼3.1 Hz), 95.3 (d, JC,F¼4.1 Hz), 105.7 (dq,
2 1
1JC,F¼241.2 Hz, JC,F¼35.0 Hz), 119.4 (qd, JC,F¼287.1 Hz,
2JC,F¼35.1 Hz), 128.9, 129.3, 131.8, 140.3, 182.2 (d, JC,F¼35.1 Hz),
2
184.4 (d, 4JC,F¼0.9 Hz). Anal. Calcd for C12H9ClF4O3: C, 46.10; H, 2.90.
Found: C, 46.24; H, 2.75.
(m, 1H, H-6); 19F NMR (376 MHz, CDCl3)
d (A-8a, 90%) 93.4 (d,
5
5JF,H¼1.7 Hz, CF3), (B-8a, 6%) 97.0 (t, JF,H¼1.2 Hz, CF3), (A-9a, 3%)
5
5
4.3. Synthesis of furanones 5a,b
94.9 (d, JF,H¼1.7 Hz, CF3), (B-9a, 1%) 96.5 (t, JF,H¼1.2 Hz, CF3); 13C
4
NMR (126 MHz, CDCl3)
d
(A-8a) 90.2 (q, JC,F¼3.2 Hz), 120.0 (q,
4.3.1. 2-Hydroxy-5-phenyl-2-(trifluoromethyl)furan-3(2H)-one
(5a). To a suspension of SiO2 (480 mg, 8.0 mmol) in concentrated
sulfuric acid (15 mL), diketone 6a (6.7 g, 24 mmol) was added in
small portions with stirring. The resulting solution was slowly
heated with stirring to 90e95 ꢁC for 45 min and allowed to stand
1JC,F¼276.6 Hz), 121.4, 127.2, 128.7, 129.1, 132.0, 137.9, 138.0, 143.9,
2
144.0, 144.6 (q, JC,F¼35.6 Hz), 154.5, 186.3. Anal. Calcd for
C16H10F3N3O: C, 60.57; H, 3.18; N, 13.24. Found: C, 60.31; H, 3.17; N,
13.10. Signals for A-9a and B-9a were abstracted from the spectra of
products, isolated from the mother liquor. When the reaction was

A.V. Safrygin et al. / Tetrahedron 71 (2015) 8535e8543
8541
conducted in AcOH at room temperature within one week, only 2-
CF3-isomer A-8a was obtained in 90% yield.
CH2), 7.68 (d, J¼8.6 Hz, 2H, Ar), 8.01e8.11 (m, 2H, H-6, H-7), 8.13 (d,
J¼8.6 Hz, 2H, Ar), 8.19 (dd, J¼8.1, 1.4 Hz, 1H, H-8), 8.30 (dd, J¼8.1,
1.4 Hz, 1H, H-5); 19F NMR (376 MHz, DMSO-d6)
d (A, 51%) 96.9 (d,
4.4.2. (Z)-1-(4-Chlorophenyl)-2-(2-(trifluoromethyl)pyrido[2,3-b]
pyrazin-3(4H)-ylidene)ethanone (8b). This compound was pre-
pared by the procedure described for 8a. Yield 94%, oraꢀn1ge crystals,
5JF,H¼1.7 Hz, CF3), (B, 49%) 99.0 (br s, CF3); 13C NMR (126 MHz,
DMSO-d6)
d
(AþB) 45.8 (q, 4JC,F¼1.8 Hz), 88.6 (q, 4JC,F¼3.4 Hz), 120.7
1
1
(q, JC,F¼277.2 Hz), 121.3 (q, JC,F¼276.2 Hz), 122.9, 127.9, 128.5,
mp 223e224 ꢁC. IR (ATR): 1615, 1574, 1536, 1489 cm
;
1H NMR
128.98, 129.03, 129.3, 129.5, 130.2, 131.7, 133.3, 134.0, 134.4, 134.5,
2
(400 MHz, CDCl3)
d
(A-8b, 83%) 6.47 (q, J¼1.7 Hz, 1H, ]CH), 7.37
135.5, 135.6, 136.1, 138.5, 138.8, 140.1 (q, JC,F¼33.8 Hz), 141.0 (q,
(dd, J¼8.0, 4.6 Hz, 1H, H-7), 7.45 (d, J¼8.6 Hz, 2H, Ar), 7.89 (d,
J¼8.6 Hz, 2H, Ar), 8.15 (dd, J¼8.0, 1.7 Hz, 1H, H-8), 8.71 (dd, J¼4.6,
1.7 Hz, 1H, H-6), 15.09 (s, 1H, NH); (B-8b, 4%) 4.99 (q, J¼1.2 Hz, 2H,
CH2), 7.51 (d, J¼8.5 Hz, 2H, Ar), 7.83e7.92 (m, 1H, H-7), 7.99 (d,
J¼8.6 Hz, 2H, Ar), 8.61 (dd, J¼8.4, 1.8 Hz, 1H, H-8), 9.31 (dd, J¼4.0,
1.8 Hz,1H, H-6); (Z)-1-(4-chlorophenyl)-2-(3-(trifluoromethyl)pyrido
[2,3-b]pyrazin-2(1H)-ylidene)ethanone (9b) (A-9b, 12.5%) 6.58 (q,
J¼1.7 Hz,1H, ]CH), 7.46 (d, J¼8.7 Hz, 2H, Ar), 7.75 (dd, J¼8.4, 4.2 Hz,
1H, H-7), 7.86 (d, J¼8.7 Hz, 2H, Ar), 8.20 (dd, J¼8.4, 1.7 Hz, 1H, H-8),
9.02 (dd, J¼4.2, 1.7 Hz, 1H, H-6), 15.29 (s, 1H, NH); (B-9b, 0.5%) 4.95
(br q, J¼1.2 Hz, 2H, CH2), 7.51 (d, J¼8.5 Hz, 2H, Ar), 7.83e7.92 (m,1H,
H-7), 7.99 (d, J¼8.6 Hz, 2H, Ar), 8.48 (dd, J¼8.4, 1.7 Hz, 1H, H-8), 9.30
2JC,F¼33.8 Hz), 142.2, 146.4, 148.5, 173.4, 195.2. Anal. Calcd for
C17H10F3ClN2O: C, 58.22; H, 2.87; N, 7.99. Found: C, 57.93; H, 2.82;
N, 7.96.
4.5.3. (Z)-1-Phenyl-2-(3-(trifluoromethyl)benzo[g]quinoxalin-2(1H)-
ylidene)ethanone (12a). Yield 84%, vinous crystals, mp 196e197 ꢁC.
IR (ATR): 3049, 1587, 1543, 1460 cmꢀ1 1H NMR (400 MHz, CDCl3)
;
d
6.52 (q, J¼1.8 Hz, 1H, ]CH), 7.45 (ddd, J¼8.2, 6.9, 1.2 Hz, 1H, H-8),
7.46e7.55 (m, 3H, Ph), 7.56 (ddd, J¼8.2, 6.9, 1.2 Hz, 1H, H-7), 7.69 (s,
1H, H-10), 7.84 (d, J¼8.3 Hz, 1H, H-9), 7.92 (d, J¼8.3 Hz, 1H, H-6),
7.95 (m, 2H, Ph), 8.36 (s, 1H, H-5), 15.19 (s, 1H, NH); 19F NMR
(376 MHz, CDCl3)
d d 90.1
93.8 (s, CF3); 13C NMR (126 MHz, CDCl3)
(m, 1H, H-6); 19F NMR (376 MHz, CDCl3)
d
(A-8b, 83%) 93.4 (d,
(q, JC,F¼3.3 Hz), 113.3, 120.2 (q, 1JC,F¼276.5 Hz), 125.8, 127.1, 127.2,
128.7 (2C), 129.1 (2C), 130.2, 131.0, 131.9, 132.4, 135.9, 138.6, 141.5,
144.7 (q, 2JC,F¼35.1 Hz),188.0. Anal. Calcd for C21H13F3N2O: C, 68.85;
H, 3.58; N, 7.65. Found: C, 68.70; H, 3.56; N, 7.61.
3
5JF,H¼1.7 Hz, CF3), (B-8b, 4%) 97.0 (t, 5JF,H¼1.2 Hz, CF3), (A-9b, 12.5%)
95.0 (d, 5JF,H¼1.7 Hz, CF3), (B-9b, 0.5%) 96.5 (t, 5JF,H¼1.2 Hz, CF3); 13
C
4
NMR (126 MHz, CDCl3)
d
(A-8b) 89.8 (q, JC,F¼3.4 Hz), 120.0 (q,
1JC,F¼276.5 Hz), 121.6, 128.6, 128.9, 129.2, 136.4, 137.9, 138.3, 143.8,
2
144.0, 144.5 (q, JC,F¼35.6 Hz), 154.7, 185.0. Anal. Calcd for
4.5.4. (Z)-1-(4-Chlorophenyl)-2-(3-(trifluoromethyl)benzo[g]quinox-
alin-2(1H)-ylidene)ethanone (12b). Yield 97%, vinous crystals, mp
244e245 ꢁC. IR (ATR): 3098, 3044, 1589, 1539, 1487 cmꢀ1; 1H NMR
C
16H9ClF3N3O: C, 54.64; H, 2.58; N, 11.95. Found: C, 54.37; H, 2.46;
N,11.86. Signals for A-9b and B-9b were abstracted from the spectra
of products, isolated from the mother liquor. When the reaction
was conducted in AcOH at room temperature within one week,
only 2-CF3-isomer A-8b was obtained in 90% yield.
(400 MHz, CDCl3)
d
6.48 (q, J¼1.8 Hz, 1H, ]CH), 7.49 (d, J¼8.7 Hz,
2H, Ar), 7.49 (ddd, J¼8.2, 6.8, 1.1 Hz, 1H, H-8), 7.60 (ddd, J¼8.2, 6.8,
1.1 Hz, 1H, H-7), 7.76 (s, 1H, H-10), 7.88 (d, J¼8.2 Hz, 1H, H-9), 7.90
(d, J¼8.7 Hz, 2H, Ar), 7.97 (d, J¼8.1 Hz, 1H, H-6), 8.42 (s, 1H, H-5),
4.5. General procedure for the synthesis of quinoxalines 11
and 12
15.26 (s,1H, NH); 19F NMR (376 MHz, CDCl3)
d
93.9 (s, CF3); 13C NMR
3
(126 MHz, CDCl3)
d
89.8 (q, JC,F¼3.3 Hz), 113.5, 120.2 (q,
1JC,F¼276.4 Hz), 125.9, 127.1, 128.4, 128.5, 128.9, 129.1, 129.2, 130.3,
131.1, 132.4, 136.0, 136.9, 138.1, 141.6, 144.5 (q, 2JC,F¼35.1 Hz), 186.6.
Anal. Calcd for C21H12ClF3N2O: C, 62.93; H, 3.02; N, 6.99. Found: C,
63.06; H, 3.02; N, 6.92.
A solution of the corresponding furanone 5 (1.0 mmol) and o-
phenylenediamine or 2,3-diaminonaphthalene (1.2 mmol) in AcOH
(5 mL) was heated at reflux for 4 h and allowed to stand at room
temperature overnight. The solid that formed was filtered and
washed with cooled ethanol to give compounds 11 or 12 as red or
orange crystals.
4.6. Reactions with hydrazines
4.6.1. 6-Phenyl-3-(trifluoromethyl)pyridazin-4(1H)-one (13a). A so-
lution of furanone 5a (244 mg, 1.0 mmol) and N2H4$2HCl (210 mg,
2.0 mmol) in AcOH (5 mL) was heated at reflux for 8 h. The cooled
mixture was quenched by addition of water (15 mL) and the solid
that formed was filtered, washed with water and recrystallized
from ethanol. Yield 81%, beige crystals, mp 291e292 ꢁC. IR (ATR):
3154, 3117, 3078, 3014, 2755, 2361, 1697, 1673, 1627, 1603, 1574,
4.5.1. (Z)-1-Phenyl-2-(3-(trifluoromethyl)quinoxalin-2(1H)-ylidene)
ethanone (11a). Yield 68%, red crystals, mp 150e151 ꢁC. IR (ATR):
3064, 1617, 1593, 1572, 1532, 1493 cmꢀ1; 1H NMR (400 MHz, DMSO-
d6)
d
(A, 45%) 6.45 (q, J¼1.9 Hz, 1H, ]CH), 7.54e7.64 (m, 2H, Ar),
7.71e7.76 (m, 2H, Ar), 7.90e7.95 (m, 3H, Ar), 8.02e8.14 (m, 2H, Ar),
15.51 (br s, 1H, NH); (B, 55%) 5.10 (q, J¼1.6 Hz, 2H, CH2), 7.54e7.64
(m, 3H, Ar), 8.02e8.14 (m, 4H, Ar), 8.20 (dd, J¼8.3, 1.3 Hz, 1H, H-8),
1552, 1538, 1498, 1471 cmꢀ1 1H NMR (400 MHz, DMSO-d6)
; d (A,
8.30 (dd, J¼8.3, 1.3 Hz, 1H, H-5); 19F NMR (376 MHz, DMSO-d6)
d
(A,
95%) 6.96 (s, 1H, H-5), 7.57e7.66 (m, 3H, Ph), 7.78e7.84 (m, 2H, Ph),
14.07 (br s, 1H, NH); (B, 5%) 7.08 (s, 1H, H-5), 7.43e7.55 (m, 3H, Ph),
7.87e7.90 (m, 2H, Ph), 14.48 (br s, 1H, OH); 19F NMR (376 MHz,
5
5
45%) 96.8 (d, JF,H¼1.9 Hz, CF3), (B, 55%) 98.9 (t, JF,H¼1.6 Hz, CF3);
13C NMR (100 MHz, DMSO-d6)
d
(AþB) 45.9 (q, JC,F¼2.3 Hz), 88.5
4
(q, JC,F¼3.1 Hz), 120.8 (q, 1JC,F¼276.2 Hz), 121.3 (q, 1JC,F¼276.0 Hz),
123.1, 126.0, 128.2, 128.4, 128.5, 128.94, 129.0, 129.4, 129.6, 131.4,
131.7, 133.3, 133.9, 134.0, 135.5, 135.6, 135.7, 135.9, 138.5, 140.1 (q,
DMSO-d6)
(100 MHz, DMSO-d6)
d
(A, 95%) 95.2 (s, CF3); (B, 5%) 97.3 (s, CF3); 13C NMR
4
d
2
117.5, 121.1 (q, 1JC,F¼274.8 Hz), 127.5, 129.2,
130.2, 131.4, 142.7 (q, JC,F¼30.6 Hz), 152.4, 167.0; MS (EI): m/z (%)
240 [M]þ (100), 212 [MꢀN2]þ (23), 164 [MþHꢀPh]þ (22), 145
[MꢀCF3C]N]þ (38), 117 [PhC(NH)]CH]þ (25), 104 [PhC(NH)]þ
(85), 89 [PhC]þ (11), 77 [Ph]þ (28), 68 (13), 51 [CHF2]þ (17). Anal.
Calcd for C11H7F3N2O: C, 55.01; H, 2.94; N, 11.66. Found: C, 54.77; H,
2.94; N, 12.06.
2JC,F¼34.2 Hz), 141.1 (q, JC,F¼33.9 Hz), 142.3, 146.7, 148.8, 174.3,
2
196.2. Anal. Calcd for C17H11F3N2O: C, 64.56; H, 3.51; N, 8.86. Found:
C, 64.29; H, 3.51; N, 8.74.
4.5.2. (Z)-1-(4-Chlorophenyl)-2-(3-(trifluoromethyl)quinoxalin-
2(1H)-ylidene)ethanone (11b). Yield 87%, orange crystals, mp
182e183 ꢁC. IR (ATR): 3062, 1618, 1591, 1531, 1487, 1470 cmꢀ1
;
1H
4.6.2. 6-(4-Chlorophenyl)-3-(trifluoromethyl)pyridazin-4(1H)-one
(13b). This compound was prepared by the procedure described
for 13a. Yield 88%, beige crystals, mp>300 ꢁC. IR (ATR): 3224, 3106,
NMR (400 MHz, DMSO-d6)
d
(A, 51%) 6.44 (br q, J¼1.7 Hz, 1H, ]CH),
7.62 (d, J¼8.6 Hz, 2H, Ar), 7.74 (ddd, J¼8.1, 7.0, 1.1 Hz, 1H, H-6), 7.93
(ddd, J¼8.1, 7.0, 1.1 Hz, 1H, H-7), 7.95 (d, J¼8.6 Hz, 2H, Ar), 8.05e8.11
(m, 2H, H-5, H-8),15.56 (s,1H, NH); (B, 49%) 5.09 (br q, J¼1.3 Hz, 2H,
2680, 1673, 1634, 1601, 1573, 1531, 1496, 1466 cmꢀ1 1H NMR
;
(500 MHz, DMSO-d6)
d
(A, 80%) 6.98 (s, 1H, H-5), 7.68 (d, J¼8.5 Hz,

8542
A.V. Safrygin et al. / Tetrahedron 71 (2015) 8535e8543
2H, Ar), 7.84 (d, J¼8.5 Hz, 2H, Ar), 14.09 (br s, 1H, NH); (B, 20%) 7.14
(s, 1H, H-5), 7.60 (d, J¼8.5 Hz, 2H, Ar), 7.92 (d, J¼8.5 Hz, 2H, Ar),
yellow crystals, mp 174e175 ꢁC. IR (ATR): 3211, 3154, 3102, 3069,
3033, 1714, 1581, 1528, 1498, 1487, 1453 cmꢀ1 1H NMR (500 MHz,
;
14.53 (br s, 1H, OH); 19F NMR (376 MHz, DMSO-d6)
d
(A, 80%) 95.4
117.6,
DMSO-d6)
d
6.95 (br q, J¼1.8 Hz, 1H, H-4), 7.01 (br t, J¼7.2 Hz, 1H, H-
(s, CF3); (B, 20%) 97.5 (s, CF3); 13C NMR (126 MHz, DMSO-d6)
d
40), 7.23 (d, J¼7.6 Hz, 2H, Ar), 7.33e7.39 (m, 4H, Ar), 7.46e7.56 (m,
119.9, 122.1, 125.4 (q, 1JC,F¼273.7 Hz), 127.6, 129.2, 129.4,136.3,142.7
7H, Ar), 12.18 (br s, 1H, NH); 19F NMR (376 MHz, DMSO-d6)
d 99.0 (d,
2
(q, JC,F¼30.9 Hz), 151.4, 166.8. Anal. Calcd for C11H6ClF3N2O: C,
J¼1.8 Hz, CF3); 13C NMR (126 MHz, DMSO-d6)
d
106.7 (q, J¼2.4 Hz),
2
48.11; H, 2.20; N, 10.20. Found: C, 48.27; H, 2.41; N, 10.56.
113.8 (2C), 119.5 (q, JC,F¼33.9 Hz), 122.2 (q, 1JC,F¼272.2 Hz), 122.3,
125.2 (2C), 127.4, 128.7, 129.4, 129.5, 130.6, 134.0, 138.6, 142.6, 142.7.
Anal. Calcd for C23H16ClF3N4: C, 62.66; H, 3.66; N, 12.71. Found: C,
62.27; H, 3.54; N, 12.68.
4.6.3. 1-Methyl-6-phenyl-3-(trifluoromethyl)pyridazin-4(1H)-one as
complex with 2-hydroxy-5-phenyl-2-(trifluoromethyl)furan-3(2H)-
one (14a). A solution of furanone 5a (244 mg, 1.0 mmol) and
MeNHNH2$H2SO4 (290 mg, 2.0 mmol) in AcOH (5 mL) was heated
at reflux for 12 h. The cooled mixture was quenched by addition of
water (15 mL), kept at room temperature for 24 h and the solid that
formed was filtered and washed with water. Yield 39%, colourless
crystals, mp 126e127 ꢁC. IR (ATR): 3125, 3073, 2967, 2823, 2688,
4.7. Reactions with hydroxylamine
4.7.1. (E)-2,2,2-Trifluoro-1-(3-phenylisoxazol-5-yl)ethanone oxime
(16a). A solution of furanone 5a (244 mg, 1.0 mmol) and
NH2OH$HCl (140 mg, 2.0 mmol) in AcOH (5 mL) was heated at
reflux for 8 h. The cooled mixture was quenched by addition of
water (15 mL), the solid that formed was filtered and washed with
water. Yield 74%, beige crystals, mp 209e210 ꢁC. IR (ATR): 3146,
3032, 2861, 2807, 1609, 1594, 1577, 1486, 1468, 1446 cmꢀ1; 1H NMR
2616, 2458, 1719, 1604, 1572, 1540, 1492, 1463 cmꢀ1
;
1H NMR
(500 MHz, DMSO-d6) (pyridazine part) 3.71 (s, 3H, Me), 6.60 (s,
d
1H, H-5), 7.53e7.66 (m, 5H, Ph); (furan part) 6.70 (s, 1H, H-4), 7.63
(t, J¼7.6 Hz, 2H, H-30, H-50), 7.73 (tt, J¼7.4, 1.1 Hz, 1H, H-40),
7.99e8.03 (m, 2H, H-20, H-60), 9.78 (s, 1H, OH); 19F NMR (376 MHz,
(500 MHz, DMSO-d6)
d 7.53e7.57 (m, 3H, Ph), 7.98e8.02 (m, 2H,
DMSO-d6)
d
(pyridazine part) 95.3 (s, CF3); (furan part) 81.2 (s, CF3);
(pyridazine part) 46.0, 120.8 (q,
Ph), 7.99 (s, 1H, H-5), 14.41 (s, 1H, OH); 19F NMR (470.5 MHz, DMSO-
13C NMR (126 MHz, DMSO-d6)
d
d6) d d 109.0, 120.3 (q,
97.9 (s, CF3); 13C NMR (126 MHz, DMSO-d6)
1JC,F¼275.2 Hz), 122.0, 128.4, 128.8, 130.4, 132.0, 142.2 (q,
2JC,F¼30.6 Hz), 155.2, 166.0; (furan part) 97.7 (q, 2JC,F¼32.8 Hz), 99.9,
120.7 (q, 1JC,F¼285.8 Hz), 127.1, 127.3, 129.3, 134.0, 184.2, 194.0. Anal.
Calcd for C12H9F3N2O$C11H7F3O3: C, 55.43; H, 3.24; N, 5.62. Found:
C, 55.25; H, 2.91; N, 5.66.
1JC,F¼273.6 Hz), 126.9, 127.4, 129.2, 130.7, 133.5 (q, JC,F¼33.1 Hz),
155.5, 162.4; MS (EI): m/z (%) 256 [M]þ (75), 144 [MꢀCF3C]NOH]þ
(91), 116 [PhC(]N)CH]þ (39), 104 [PhC]NH]þ (47), 89 [PhC]þ (17),
77 [Ph]þ (100), 69 [CF3]þ (11), 51 [CHF2]þ (35). Anal. Calcd for
2
C11H7F3N2O2: C, 51.57; H, 2.75; N, 10.94. Found: C, 51.17; H, 2.83; N,
10.74.
4.6.4. 1-Methyl-6-(4-chlorophenyl)-3-(trifluoromethyl)pyridazin-
4(1H)-one as complex with 2-hydroxy-5-(4-chlorophenyl)-2-(tri-
fluoromethyl)furan-3(2H)-one (14b). This complex was prepared by
the procedure described for 14a. Yield 45%, colourless crystals, mp
130e131 ꢁC. IR (ATR): 3130, 3059, 2997, 2853, 2689, 2624, 2469,
1717, 1603, 1563, 1547, 1486, 1460 cmꢀ1; 1H NMR (500 MHz, DMSO-
4. 7. 2. (E)-1-(3-(4-Chlorophenyl)isoxazol-5-yl)-2, 2, 2-
trifluoroethanone oxime (16b). This compound was prepared by the
procedure described for 16a. Yield 71%, beige crystals, mp
220e221 ꢁC. IR (ATR): 3147, 3038, 2865, 2809, 1604, 1588, 1567,
1489 cmꢀ1
;
1H NMR (500 MHz, DMSO-d6)
d
7.62 (d, J¼8.5 Hz, 2H,
d6)
d
(pyridazine part) 3.70 (s, 3H, Me), 6.63 (s, 1H, H-5), 7.60e7.68
Ar), 8.03 (s, 1H, H-5), 8.04 (d, J¼8.5 Hz, 2H, Ar), 14.43 (s, 1H, OH); 19
F
(m, 5H, Ph); (furan part) 6.74 (s, 1H, H-4), 7.71 (m, 2H, H-30, H-50),
NMR (376 MHz, DMSO-d6) d
97.9 (s, CF3); 13C NMR (126 MHz,
8.02 (m, 2H, H-20, H-60), 9.84 (s, 1H, OH); 19F NMR (376 MHz, DMSO-
d6)
DMSO-d6)
d
109.1, 120.3 (q, 1JC,F¼273.3 Hz), 126.3,128.8,129.3, 133.4
d
(pyridazine part) 95.3 (s, CF3); (furan part) 81.3 (s, CF3); 13C
(q, 2JC,F¼33.1 Hz), 135.5, 155.6, 161.5. Anal. Calcd for C11H6ClF3N2O2:
NMR (126 MHz, DMSO-d6)
d
(pyridazine part) 46.1, 120.7 (q,
C, 45.46; H, 2.08; N, 9.64. Found: C, 45.43; H, 2.12; N, 9.61.
1JC,F¼275.2 Hz), 122.2, 128.9, 130.5, 130.8, 135.4, 142.2 (q,
2JC,F¼30.7 Hz), 154.2, 166.0; (furan part) 97.8 (q, JC,F¼32.6 Hz),
4.7.3. (E)-2-(2,2,2-Trifluoro-1-(hydroxyimino)ethyl)-4H-chromen-4-
one (17a). A solution of chromone 3 (R]H, 300 mg, 1.15 mmol) and
NH2OH$HCl (160 mg, 2.3 mmol) in AcOH (5 mL) was heated at
reflux for 4 h. The resulting mixture was allowed to stand at room
temperature overnight and the solid that formed was filtered and
washed with cooled ethanol. Yield 83%, colourless crystals, mp
218e219 ꢁC. IR (ATR): 3112, 2992, 2724, 1633, 1606, 1590, 1567,
2
1
100.4, 120.6 (q, JC,F¼286.0 Hz), 126.0, 129.1, 129.6, 138.9, 183.0,
194.0. Anal. Calcd for C12H8ClF3N2O$C11H6ClF3O3: C, 48.70; H, 2.49;
N, 4.94. Found: C, 48.76; H, 2.37; N, 4.94.
4.6.5. (E)-1,5-Diphenyl-3-(2,2,2-trifluoro-1-(2-phenylhydrazono)
ethyl)-1H-pyrazole (15a). A solution of furanone 5a (244 mg,
1.0 mmol) and PhNHNH2$HCl (505 mg, 3.5 mmol) in AcOH (5 mL)
was heated at reflux for 8 h. The resulting mixture was evaporated
and the solid that formed was washed with ethanol (2 mL) and
recrystallized from ethanol. Yield 25%, light yellow crystals, mp
161e162 ꢁC. IR (ATR): 3203, 3134, 3060, 3027, 1583, 1540, 1525,
1486 cmꢀ1 1H NMR (400 MHz, DMSO-d6)
; d 7.02 (s, 1H, H-3), 7.56
(ddd, J¼7.9, 7.2, 0.7 Hz, 1H, H-6), 7.67 (d, J¼8.5 Hz, 1H, H-8), 7.88
(ddd, J¼8.5, 7.2, 1.7 Hz, 1H, H-7), 8.07 (dd, J¼8.0, 1.7 Hz, 1H, H-5),
14.28 (s,1H, OH); 19F NMR (470.5 MHz, DMSO-d6) 98.4 (s, CF3); 13
d C
NMR (126 MHz, DMSO-d6)
d
115.2, 118.5, 120.1 (q, 1JC,F¼273.9 Hz),
1496, 1452 cmꢀ1
;
1H NMR (500 MHz, DMSO-d6)
d
6.90 (br q,
123.3, 125.0, 126.2, 135.2, 137.5 (q, 2JC,F¼32.9 Hz), 149.8, 155.3, 176.3.
Anal. Calcd for C11H6F3NO3: C, 51.37; H, 2.35; N, 5.45. Found: C,
51.23; H, 2.33; N, 5.27.
J¼1.5 Hz, 1H, H-4), 7.01 (br t, J¼7.3 Hz, 1H, H-40), 7.23 (d, J¼7.9 Hz,
2H, Ph), 7.31e7.38 (m, 4H, Ph), 7.38e7.43 (m, 3H, Ph), 7.45e7.54 (m,
5H, Ph), 12.22 (br s, 1H, NH); 19F NMR (470.5 MHz, DMSO-d6)
d
99.0
5
(d, JF,H¼1.3 Hz, CF3); 13C NMR (126 MHz, DMSO-d6)
d
106.6 (q,
4.7.4. (E)-6-Chloro-2-(2,2,2-trifluoro-1-(hydroxyimino)ethyl)-4H-
chromen-4-one (17b). This compound was prepared by the pro-
cedure described for 17a. Yield 90%, colourless crystals, mp
231e232 ꢁC. IR (ATR): 3143, 3094, 3011, 2732, 1612, 1591, 1564,
J¼2.3 Hz), 113.9, 119.6 (q, JC,F¼33.7 Hz), 122.2 (q, 1JC,F¼272.2 Hz),
122.4, 125.2, 128.5, 128.6, 128.7, 128.8, 129.2, 129.4, 129.5, 138.8,
142.6, 142.7, 143.8. Anal. Calcd for C23H17F3N4: C, 67.97; H, 4.22; N,
13.79. Found: C, 67.75; H, 4.16; N, 13.69.
2
1490, 1466 cmꢀ1; 1H NMR (500 MHz, DMSO-d6)
d 7.06 (s, 1H, H-3),
7.74 (d, J¼9.0 Hz, 1H, H-8), 7.91 (dd, J¼9.0, 2.7 Hz, 1H, H-7), 7.99 (d,
4.6.6. (E)-5-(4-Chlorophenyl)-1-phenyl-3-(2,2,2-trifluoro-1-(2-
phenylhydrazono)ethyl)-1H-pyrazole (15b). This compound was
prepared by the procedure described for 15a. Yield 33%, light
J¼2.6 Hz, 1H, H-5), 14.34 (s, 1H, OH); 19F NMR (470.5 MHz, DMSO-
d6)
d d 115.1, 120.0 (q,
98.5 (s, CF3); 13C NMR (126 MHz, DMSO-d6)
1JC,F¼274.0 Hz), 121.0, 123.9, 124.4, 130.7, 135.0, 137.2 (q, J¼33.0 Hz),

A.V. Safrygin et al. / Tetrahedron 71 (2015) 8535e8543
8543
150.0, 153.9, 175.3. Anal. Calcd for C11H5ClF3NO3: C, 45.31; H, 1.73;
N, 4.80. Found: C, 45.14; H, 1.83; N, 5.00.
State Task from the Ministry of Education and Science of Russian
Federation.
4.8. Crystallographic data for compounds 14a, 15a, and 16b
Supplementary data
XRD experiments were accomplished on ‘Xcalibur E’ diffrac-
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.tet.2015.09.035.
tometer with CCD detector on standard procedure (Mo K
a irradi-
ation, graphite monochromator,
u
-scans with step 1ꢁ). The crystals
of compounds 14a and 16b were analysed at T¼295(2) K, com-
pound 15a was analysed at T¼150.0(1) K; empirical absorption
correction was applied. Using Olex2,14 the structures 14a, 15a, and
16b were solved with the ShelXS structure solution program using
direct methods and refined in anisotropic approximation for non-H
atoms with the ShelXL refinement package using least-squares
minimisation.15
References and notes
1. (a) Hiyama, T. Organofluorine Compounds. Chemistry and Application; Springer:
Berlin, Germany, 2000; (b) Organofluorine Compounds in Medicinal Chemistry
and Biomedical Applications; Filler, R., Kobayashi, Y., Yagupolskii, L. M., Eds.;
Elsevier: Amsterdam, The Netherlands, 1993.
2. (a) Druzhinin, S. V.; Balenkova, E. S.; Nenajdenko, V. G. Tetrahedron 2007, 63,
7753; (b) Nenajdenko, V. G.; Balenkova, E. S. Arkivoc 2011, i, 246.
3. (a) Kirk, K. L. J. Fluorine Chem. 2006, 127, 1013; (b) Begue, J.-P.; Bonnet-Delpon,
D. J. Fluorine Chem. 2006, 127, 992; (c) Dolbier, W. R., Jr. J. Fluorine Chem. 2005,
126, 157.
4. (a) Takahashi, M.; Nagaoka, H.; Inoue, K. J. Heterocycl. Chem. 2004, 41, 525; (b)
Lang, R. W.; Wenk, P. F. Helv. Chim. Acta 1988, 71, 596; (c) Peng, W.; Zhu, S.-Z. J.
Fluorine Chem. 2002, 116, 81; (d) Rozin, Y. A.; Leban, J.; Dehaen, W.; Nenajdenko,
V. G.; Muzalevskiy, V. M.; Eltsov, O. S.; Bakulev, V. A. Tetrahedron 2012, 68, 614;
ꢁ
ꢁ
4.8.1. Crystal data for 14a. C23H16F6N2O4, M¼498.38, triclinic,
ꢀ
ꢀ
ꢀ
a¼9.9966(7) A, b¼10.8579(4) A, c¼11.6639(10) A,
a
¼102.076(5)ꢁ,
3
ꢀ
ꢁ
b
¼94.618(6)ꢁ,
g
m
¼111.803(5) , V¼1131.85(13) A , space group P-1
(no. 2), Z¼2,
(Mo K
a
)¼0.133 mmꢀ1, 9943 reflections measured,
ꢁ
(e) Diab, J.; Laurent, A.; Le Drean, I. J. Fluorine Chem. 1997, 84, 145; (f) Kislyi, V. P.;
5617 unique (Rint¼0.0300), which were used in all calculations. The
Nikishin, K. G.; Kruglova, E. Y.; Shestopalov, A. M.; Semenov, V. V.; Gakh, A. A.;
Buchanan, A. C. Tetrahedron 1996, 52, 10841; (g) Konakahara, T.; Hojahmat, M.;
Tamura, S. J. Chem. Soc., Perkin Trans. 1 1999, 2803.
final wR2 was 0.1850 (all data) and R1 was 0.0580 (I>2s(I)). De-
position number CCDC 1412562.
€
5. Irgashev, R. A.; Safrygin, A. V.; Ezhikova, M. A.; Kodess, M. I.; Roschenthaler, G.-
V.; Sosnovskikh, V. Y. Tetrahedron 2015, 71, 1822.
4.8.2. Crystal data for 15a. C23H17F3N4, M¼406.41, monoclinic,
6. Isakova, V. G.; Khlebnikova, T. S.; Lakhvich, F. A. Russ. Chem. Rev. 2010, 79, 849.
¼92.110(2)ꢁ,
ꢀ
ꢀ
ꢀ
a¼12.9413(3) A, b¼11.2241(3) A, c¼13.3660(3) A,
b
€
7. (a) Bazhin, D. N.; Chizhov, D. L.; Roschenthaler, G.-V.; Kudyakova, Y. S.; Burgart,
3
ꢀ
V¼1940.16(8) A , space group P21/n (no. 14), Z¼4,
m
(Mo K
a
)¼
Y. V.; Slepukhin, P. A.; Saloutin, V. I.; Charushin, V. N. Tetrahedron Lett. 2014, 55,
0.105 mmꢀ1
,
10,684 reflections measured, 5304 unique
€
5714; (b) Bazhin, D. N.; Kudyakova, Y. S.; Roschenthaler, G.-V.; Burgart, Y. V.;
Slepukhin, P. A.; Isenov, M. L.; Saloutin, V. I.; Charushin, V. N. Eur. J. Org. Chem.
http://dx.doi.org/10.1002/ejoc.201500737.
8. Irgashev, R. A.; Sosnovskikh, V. Y.; Kalinovich, N.; Kazakova, O.; Roschenthaler,
G.-V. Tetrahedron Lett. 2009, 50, 4903.
9. (a) Sosnovskikh, V. Y. Fluorinated pyrones, chromones and coumarins In-
Nenajdenko, V., Ed.. Fluorine in Heterocyclic Chemistry; Springer: Cham, Heidel-
berg, New York, Dordrecht, London, 2014; Vol. 2, p 211; (b) Korotaev, V. Y.;
Sosnovskikh, V. Y.; Barkov, A. Y. Russ. Chem. Rev. 2013, 82, 1081; (c) Sosnovskikh,
V. Y. Russ. Chem. Rev. 2003, 72, 489.
(Rint¼0.0200), which were used in all calculations. The final wR2
€
was 0.1437 (all data) and R1 was 0.0437 (I>2s(I)). Deposition
number CCDC 1412561.
4.8.3. Crystal data for 16b. C11H6ClF3N2O2, M¼290.63, monoclinic,
ꢀ
3
ꢀ
ꢀ
ꢀ
b
a¼4.8234(4) A, b¼27.103(5) A, c¼9.576(2) A,
¼99.320(13)ꢁ,
V¼1235.3(4) A , space group P21/c, Z¼4,
m
(Mo K
a
)¼0.345 mmꢀ1
,
ꢁ
10. (a) Sianesi, D.; Pasetti, A.; Tarli, F. J. Org. Chem. 1966, 31, 2312; (b) Dolensky, B.;
7309 reflections measured, 3061 unique (Rint¼0.0316), which were
ꢂ
ꢂ
Kvícala, J.; Palecek, J.; Paleta, O. J. Fluorine Chem. 2002, 115, 67.
11. Sevenard, D. V.; Khomutov, O. G.; Boltachova, N. S.; Filyakova, V. I.; Vogel, V.;
used in all calculations. The final wR2 was 0.1201 (all data) and R1
€
Lork, E.; Sosnovskikh, V. Y.; Iaroshenko, V. O.; Roschenthaler, G.-V. Z. Natur-
forsch. 2009, 64b, 541.
was 0.0458 (I>2s(I)). Deposition number CCDC 1412560.
12. Sibgatulin, D. A.; Volochnyuk, D. M.; Kostyuk, A. N. Synlett 2005, 1907.
13. Shurov, S. N.; Pavlova, E. Y.; Livantsova, L. I.; Zaitseva, G. S.; Andreichikov, Y. S.
Russ. J. Org. Chem. 1993, 29, 2275.
Acknowledgements
14. Dolomanov, O. V.; Bourhis, L. J.; Gildea, R. J.; Howard, J. A. K.; Puschmann, H. J.
This work was financially supported by the Russian Foundation
Appl. Crystallogr. 2009, 42, 339.
for Basic Research (Grant 14-03-00179) and performed within the
15. Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.