Synthesis of nonracemic polysubstituted cyclohexanes through cascade reactions of (2R,3S)-2-Acetyl-3-aryl-4-nitrobutanoates, Adducts of Ni(II)-catalyzed michael addition

Article abstract of DOI:10.1134/S1070428013110067

Stereoselective synthesis of ethyl-4,6-diaryl-2-hydroxy-2-methyl-5-nitro-3- formylcyclohexanecarboxylates was described. The formation of the asymmetric site of the required configuration is achieved by an enantioselective Michael addition of ethyl acetoa

Full text of DOI:10.1134/S1070428013110067

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2013, Vol. 49, No. 11, pp. 1600−1604. © Pleiades Publishing, Ltd., 2013.
Original English Text © A.N. Reznikov, E.A. Sidnin, Yu.N. Klimochkin, 2013, published in Zhurnal Organicheskoi Khimii, 2013, Vol. 49, No. 11, pp. 1623−1626.
Synthesis of Nonracemic Polysubstituted Cyclohexanes
through Cascade Reactions
of (2R,3S)-2-Acetyl-3-aryl-4-nitrobutanoates,
Adducts of Ni(II)-Catalyzed Michael Addition
A. N. Reznikov, E. A. Sidnin, and Yu. N. Klimochkin
Samara State Technical University, Samara, Russia
e-mail: orgchem@samgtu.ru
Received April 17, 2013
Abstract—Stereoselective synthesis of ethyl-4,6-diaryl-2-hydroxy-2-methyl-5-nitro-3-formyl-
cyclohexanecarboxylates was described. The formation of the asymmetric site of the required configuration is
achieved by an enantioselective Michael addition of ethyl acetoacetate to nitroalkenes in the presence of a chiral
Ni(II) complex with (R,R)-N,N'-dibenzylcyclohexane-1,2-diamine. Further reaction of the products obtained with
cinnamic aldehyde led to the formation of polysubstituted cyclohexanes.
DOI: 10.1134/S1070428013110067
The utilization of cascade reactions in the asym-
metric synthesis of polysubstituted alicyclic compounds
with several stereocenters is lately a vigorously growing
synthetic strategy for the preparation of biologically ac-
tive compounds and chiral building blocks [1]. Several
examples were described of the synthesis of functionally
substituted chiral cyclohexane derivatives via cascade
reactions of adducts of alkyl acetoacetate and nitroalkenes
with the use of iminium-enamine activation [2]. The key
target in this case is the building of a chiral center of the
desired configuration governing the stereodirection of the
subsequent processes. In this event the organic catalysis
is applied with chiral thiourea derivatives [2].
dibenzylcyclohexane-1,2-diamine.
The approach we developed to the stereoselective
synthesis of polyfunctional derivatives of cyclohexane
VIа–VId is based on the use of successive reactions of
Ni-catalyzed Michael addition of ethyl acetoacetate (I) to
nitroalkenes IIa–IId with the subsequent cascade trans-
formation (Michael addition–aldol condensation) of the
obtained chiral adducts IVa–IVd with cinnamic aldehyde
(V) under the conditions of pyrrolidine catalysis.
The addition of ethyl acetoacetate (I) to nitroalkenes
IIа–IId in the presence of complex III affords adducts
IVа–IVd with the prevailing formation of (3S)-enantio-
mers [3] and dr ~1 : 1. By the subsequent recrystallization
from the mixture toluene–petroleum ether we isolated in-
dividual diastereomers IVа–IVd (de 86–96%). The ratio
of diastereomers was determined by the analysis of the
1H NMR spectra of adducts IVа–IVd, where the chemical
shifts of the protons of methyl groups of two diastereo-
mers differed by 0.23–0.27 ppm. The spectra contain two
triplets of the methyl protons of the ester groups with the
triplet of the main diastereomer shifted upfield, and two
singlets of the methyl protons of the acetyl groups with the
singlet of the main diastereomer shifted downfield. The
assignment of the signals of diastereomers of compounds
In [3] the enantioselective Michael addition of
β-ketoesters to nitroalkenes catalyzed by chiral Ni(II)
complexes was described for the first time. Ni(II) com-
plexes with diamine ligands were successfully used in
reactions of 1,3-dicarbonyl compounds with various un-
saturated nitro derivatives [4]. We formerly studied the
catalytic activity of complexes of various metals in the
asymmetric Michael reaction [5]. Synthetic approaches
were developed to the preparation of (R)-rolipram, (R)-
phenotropil, and 4-amino-(3R)-methyl-butanoic acid
[6], underlain by the addition of diethyl malonate to
nitroalkenes catalyzed by the Ni(II) complex with N,N'-
1600

SYNTHESIS OF NONRACEMIC POLYSUBSTITUTED CYCLOHEXANES
1601
Ph
Ph
Br
Ni
NH
NH
NH
NH
O
O
Br
O
R
O
Ph
Ph
H3C
OEt
III
(1 mol%)
I
+
H3C
OEt
NO2
PhMe, 25°C
NO2
IIа_IId
R
IVа_IVd
O
H3C
R
O
H3C OH
O
R
O
N
OEt
NO2
O
OHC
Ph
COOEt
H2O
H
H3C
OEt
+
PhMe, 25°C,
_H2O
Ph
H
NO2
_
R
IVа_г
N
H
Ph
N
NO2
VIа_VId
V
R = Ph (а), 4-ClC6H4 (b), 4-MeOC6H4 (c), 4-NO2C6H4 (d).
IVа–IVd and establishing their absolute configuration
were performed based on the published data [3, 7]. The
subsequent cascade reaction catalyzed by pyrrolidine
consists in the Michael addition of the obtained adducts
IVа–IVd to cinnamic aldehyde (V) followed by the in-
tramolecular aldol condenation. The reaction results in
the corresponding cyclohexanes VIа–VId as confirmed
by the NMR spectra 1Н, 13C, DEPT, and HMQC. Thus
in the spectrum of the 2D experiment HMQC 1H–13C of
compound VIа the cross-peak 4.30/41.6 ppm corresponds
to the linked C–H atoms at the aryl group R, the cross-
peak 4.00/44.3 ppm belongs to the linked C–H atoms at
the phenyl group, the cross-peak 3.94/61.3 ppm is due to
the correlation of the bound C–H atoms of the methylene
group of the ester fragment, the cross-peaks 3.83/49.6 and
3.50/54.7 ppm correspond to the methane groups at the
ester and formyl substituents, respectively. The spectrum
contains also the cross-peak 5.02/93.1 ppm corresponding
to the linked C–H atoms at the nitro group.
VId was established from the measured angles of the
optical rotation and the published values for the methyl
(1S,2R,3R,4S,5R,6R)-2-hydroxy-2-methyl-5-nitro-4,6-
diphenyl-3-formylcyclohexanecarboxylate prepared
from the methyl (3R)-2-acetyl-4-nitro-3-phenylbutanoate
under analogous conditions [2].
EXPERIMENTAL
Mass spectra were measured on an instrument Thermo
Finnigan Trace DSQ, capillary column BPX-5 (30 × 0.32
mm), ionizing electrons energy 70 eV. The optical rota-
The relative configuration of compounds VIа–VId
corresponds to the data on the stereochemistry of the
reaction of methyl 2-acetyl-3-aryl-4-nitrobutanoates with
the cinnamic aldehyde under similar conditions [2]. It is
confirmed by the values of the vicinal spin-spin coupling
constants in the 1Н NMR spectrum of compound VId.
R = 4-NO2-C6H4.
The absolute configuration of cyclohexanes VIа–
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 11 2013

REZNIKOV et al.
1602
(78), 193 (46), 179 (42), 165 (12), 138 (24), 115 (25),
103 (12), 75 (7), 43 (100). Found, %: C 53.98; H 5.45;
N 4.19. C14H16ClNO5. Calculated, %: C 53.60; H 5.14;
N 4.46. M 313.74.
tion was measured on an automatic polarimeter Rudolph
Research Analytical Autopol V Plus. NMR spectra H,
1
13C, DEPT, and HMQC were registered on a spectrom-
eter JEOL JNM-ECX400 [399.78 (1H) and 100.53 MHz
(13C)] in CDCl3. The signals of deuterated solvent served
as internal reference. Elemental analysis was carried out
on a CHNS-analyzer Euro Vector EA-3000.
Ethyl (2R,3S)-2-acetyl-3-(4-methoxyphenyl)-
4-nitrobutanoate (IVc). Yield 4.1 g (41%), de 86%
(1H NMR data), mp 82–85°С, [α]D +91.9° (c 0.01 g
20
mL−1, CHCl3). 1Н NMR spectrum, δ, ppm: 1.03 t (3H,
CH2CH3, 3JHH 7.3 Hz), 2.29 s (3H, COCH3), 3.76 s (3H,
OCH3), 3.97 q (2H, CH2CH3, 3JHH 7.1 Hz), 4.06 d (1H,
Chiral catalyst III was prepared by procedure [3].
Ethyl (2R,3S)-2-acetyl-3-aryl-4-nitrobutanoates
IVа–IVd. To a solution of 1 equiv. of nitroalkene IIa–IId
and 2 equiv. of ethyl acetoacetate (I) in 30 mL of toluene
was added 1 mol% of nickel complex III, and the mixture
was maintained for 24 h at room temperature. Toluene and
excess etyl acetoacetate were distilled off in a vacuum,
the residue was dissolved in 20 mL of chloroform and
was filtered through a silica gel bed. Chloroform was
distilled off, the residue was recrystallized from a mixture
of toluene–petroleum ether, 2 : 1, at –20°C.
3
CHCOOEt, JHH 10.1 Hz), 4.11–4.18 m (1H, CHAr),
4.70 d (1H, CH2NO2, 2JHH 2.1 Hz), 4.71 s (1H, CH2NO2),
6.82 d (2H, Hm, 3JHH 8.7 Hz), 7.11 d (2H, Ho, 3JHH 8.7 Hz).
13C NMR spectrum, δ, ppm: 13.85 (CH2CH3), 30.17
(COCH3), 41.74 (CHAr), 55.30 (OCH3), 62.02 (CH2Me),
62.26 (CHCOMe), 78.20 (CH2NO2), 114.39 (Cm), 128.30
(Ci), 129.17 (Co), 159.47 (Ci), 166.01 (COOEt), 201.36
(COMe). Found, %: C 58.65; H 6.32; N 4.24. C15H19NO6.
Calculated, %: C 58.25; H 6.19; N 4.53.
Ethyl (2R,3S)-2-acetyl-4-nitro-3-phenylbutanoate
(IVа). Yield 3.5 g (35%), de 98% (1H NMR data), mp 72–
75°С, [α]D20 +146.6° (c 0.01 g mL−1, CHCl3). 1Н NMR
Ethyl (2R,3S)-2-acetyl-4-nitro-3-(4-nitrophenyl)
butanoate (IVd). Yield 0.58 g (35%), de 96% (1H NMR
3
data), mp 102–106°С, [α]D +102.1° (c 0.01 g mL−1,
20
spectrum, δ, ppm: 0.99 t (3H, CH2CH3, JHH 7.1 Hz),
2.29 s (3H, COCH3), 3.95 q (2H, CH2CH3, 3JHH 7.1 Hz),
4.11 d (1H, CHCOOEt, 3JHH 10.1 Hz), 4.16–4.22 m (1H,
CHPh), 4.74 d (2H, CH2NO2, 3JHH 6.1 Hz), 7.18–7.20 m
(2Harom), 7.24–7.32 m (3Harom). 13C NMR spectrum, δ,
ppm: 13.77 (CH2CH3), 30.22 (COCH3), 42.39 (CHPh),
62.06 (CHCOMe), 62.09 (CH2Me), 77.98 (CH2NO2),
128.05 (Co), 128.39 (Cp), 129.05 (Cm), 136.50 (Ci),
166.95 (COOEt), 201.28 (COMe). Mass spectrum, m/z
(Irel, %): 233 (4) [M – NO2]+, 189 (80), 159 (48), 145 (65),
115 (48), 91 (33), 77 (11), 43 (100). Found, %: C 60.89;
H 6.41; N 4.78. C14H17NO5. Calculated, %: C 60.21;
H 6.14; N 5.01. m 279.29.
1
CHCl3). Н NMR spectrum, δ, ppm: 1.05 t (3H,
CH2CH3, 3JHH 7.1 Hz), 2.28 s (3H, COCH3), 3.99 q (1H,
CH2CH3,3JHH 7.1Hz), 4.004q(1H,CH2CH3,3JHH 7.1Hz),
4.13 d (1H, CHCOOEt, 3JHH 10.1 Hz), 4.29–4.35 m (1H,
CHAr), 4.76 s (1H, CH2NO2), 4.78 d (1H, CH2NO2,
2JHH 2.1 Hz), 7.41 d (2H, Ho, 3JHH 8.7 Hz), 8.18 d (2H,
3
Hm, JHH 8.7 Hz). 13C NMR spectrum, δ, ppm: 13.88
(CH2CH3), 30.18 (COCH3), 41.94 (CHAr), 61.39
(CHCOMe), 62.50 (CH2Me), 77.29 (CH2NO2), 124.22
(Cm), 129.06 (Co), 144.06 (Ci), 147.85 (Ci), 166.38
(COOEt), 200.32 (COMe). Found, %: C 54.38; H 5.30;
N 8.21. C14H16N2O7. Calculated, %: C 51.85; H 4.97;
N 8.64.
Ethyl (2R,3S)-2-acetyl-4-nitro-3-(4-chlorophe-
nyl)butanoate (IVb). Yield 2.9 g (29%), de 92%
Ethyl (1R,2S,3S,4R,5S,6S)-4,6-diaryl-2-hydroxy-
2-methyl-5-nitro-3-formylcyclohexanecarboxylates
VIа–VId. To a solution of 1 equiv. of compound IVa–IVd
in 10 mL of toluene was added 1.1 equiv. of cinnamic
aldehyde (V) and 1 equiv. of pyrrolidine, the mixture was
stirred at room temperature for 24 h. The separated pre-
cipitate was filtered off and recrystallized from methanol.
(1H NMR data), mp 79–82°С, [α]D +115.7° (c 0.01
20
g mL−1, CHCl3). 1Н NMR spectrum, δ, ppm: 1.04 t
3
(3H, CH2CH3, JHH 7.3 Hz), 2.29 s (3H, COCH3),
3
3.98 q (2H, CH2CH3, JHH 7.3 Hz), 4.07 d (1H,
3
CHCOOEt, JHH 10.1 Hz), 4.14–4.22 m (1H, CHAr),
4.70 d (1H, CH2NO2, 2JHH 1.4 Hz), 4.72 s (1H, CH2NO2),
7.14 d (2H, Ho, 3JHH 8.5 Hz), 7.28 d (2H, Hm, 3JHH 8.5 Hz).
13C NMR spectrum, δ, ppm: 13.83 (CH2CH3), 30.15
(COCH3), 41.75 (CHAr), 61.84 (CHCOMe), 62.24
(CH2Me), 77.76 (CH2NO2), 129.25 (Co), 129.47 (Cm),
134.34 (Ci), 135.08 (Ci), 166.72 (COOEt), 200.88
(COMe). Mass spectrum, m/z (Irel, %): 313 (1) [M]+, 225
Ethyl (1R,2S,3S,4R,5S,6S)-2-hydroxy-2-meth-
yl-5-nitro-4,6-diphenyl-3-formylcyclohexane-
carboxylate (VIа). Yield 0.95 g (33%), mp 220–
222°С (decomp.), [α]D –13.8° (c 0.011 g mL−1,
20
1
CHCl3). Н NMR spectrum, δ, ppm: 0.89 t (3H,
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 11 2013

SYNTHESIS OF NONRACEMIC POLYSUBSTITUTED CYCLOHEXANES
1603
COOCH2CH3, 3JHH 7.1 Hz), 1.47 s (3H, CH3), 3.52 d.d.d
(1H,CHCHO,3JHH 12.6,3JHH 3.0,4JHH 1.6Hz),3.81–3.84m
(2H, CHCOOEt, OH), 3.88–4.02 m (3H, COOCH2CH3,
(OCH3), 61.40 (COOCH2CH3), 72.20 [C(CH3)OH],
93.51 (CHNO2), 114.31 (Cm), 127.52 (Ci), 128.21 (Cm),
128.62 (Cp), 129.31 (Cm), 129.41 (CHo), 135.41 (Ci),
159.61 (Ci), 174.42 (COOEt), 203.10 (CHO). Found,
%: C 65.58; H 6.02; N 3.58. C24H27NO7. Calculated, %:
C 65.29; H 6.16; N 3.17.
3
3
CHPh), 4.30 d.d (1H, CHPh, JHH 12.6, JHH 4.6 Hz),
5.03 t (1H, CHNO2, 3JHH 4.6 Hz), 7.14–7.30 m (10Harom),
9.51 d (1H, CHO, 3JHH 4.6 Hz). 13C NMR spectrum, δ,
ppm: 13.71 (COOCH2CH3), 27.32 (CH3), 41.80 (CHPh),
44.41 (CHPh), 49.82 (CHCOOEt), 54.80 (CHCHO),
61.42 (COOCH2CH3), 72.11 [C(CH3)OH], 93.33
(CHNO2), 128.22 (Co), 128.24 (Co), 128.65 (Cp), 128.68
(Cp), 129.01 (Cm), 129.42 (Cm), 135.41 (Ci), 135.50 (Ci),
174.30 (COOEt), 202.9 (CHO). Found, %: C 66.58;
H 6.04; N 3.84. C23H25NO6. Calculated, %: C 67.14;
H 6.12; N 3.40.
Ethyl (1R,2S,3S,4R,5S,6S)-2-hydroxy-2-meth-
yl-5-nitro-6-(4-nitrophenyl)-4-phenyl-3-formyl-
cyclohexanecarboxylate (VId). Yield 0.08 g (20%),
mp 242–245°С, [α]D –11.1° (c 0.011 g mL−1,
20
1
CHCl3). Н NMR spectrum, δ, ppm: 0.97 t (3H,
COOCH2CH3, 3JHH 7.3 Hz), 1.50 s (3H, CH3), 3.52 d.d.d
(1H, CHCHO, 3JHH 12.8, 3JHH 4.5, 4JHH 1.8Hz), 3.59d(1H,
OH, 3JHH 1.8 Hz), 3.85 d (1H, CHCOOEt, 3JHH 12.4 Hz),
3
3.98 q (1H, COOCH2CH3, JHH 7.3 Hz), 3.98 q
Ethyl (1R,2S,3S,4R,5S,6S)-2-hydroxy-2-meth-
yl-5-nitro-4-phenyl-3-formyl-6-(4-chlorophe-
nyl)cyclohexanecarboxylate (VIb). Yield 0.9 g
3
(1H, COOCH2CH3, JHH 7.3 Hz), 4.16 d.d (1H,
3
3
CHPh, JHH 12.8, JHH 4.1 Hz), 4.30 d.d
3
3
(1H, CHAr, JHH 12.4, JHH 4.5 Hz), 5.02 t (1H,
20
(32%), mp 228–230°С (decomp.), [α]D –10.18°
3
CHNO2, JHH 4.5 Hz), 7.14–7.16 m (2H, CHo), 7.27–
(c 0.011 g mL−1, CHCl3). 1Н NMR spectrum, δ, ppm:
7.32 m (3Harom), 7.41 d (2H, Ho, JHH 8.7 Hz), 8.16 d
3
3
0.96 t (3H, COOCH2CH3, JHH 7.1 Hz), 1.47 s (3H,
(2H, Hm, 3JHH 8.7 Hz), 9.50 d (1H, CHO, 3JHH 4.6 Hz).
13C NMR spectrum, δ, ppm: 13.93 (COOCH2CH3), 27.17
(CH3), 41.72 (CHPh), 44.09 (CHAr), 49.58 (CHCOOEt),
54.60 (CHCHO), 61.80 (COOCH2CH3), 72.10 [C(CH3)
OH], 92.62 (CHNO2), 124.22 (CHm), 128.20 (Co), 128.93
(Cp), 129.29 (Co), 129.56 (Cm), 134.77 (Ci), 142.89 (Ci),
148.06 (Ci), 173.51 (COOEt), 202.40 (CHO). Found, %:
C 60.96; H 5.16; N 6.53. C23H24N2O8. Calculated, %:
C 60.52; H 5.30; N 6.14.
CH3), 3.47–3.51 d.d.d (1H, CHCHO, 3JHH 12.6, 3JHH 4.4,
4JHH 1.6 Hz), 3.81–3.84 m (2H, CHCOOEt, OH),
3.88–4.02 m (3H, COOCH2CH3, CHPh), 4.28 d.d
3
3
(1H, CHAr, JHH 12.6, JHH 6.5 Hz), 5.00 t (1H,
CHNO2, 3JHH 4.3 Hz), 7.13–7.15 m (4Harom), 7.24–7.30 m
(5Harom), 9.50 d (1H, CHO, JHH 4.4 Hz). 13C NMR
3
spectrum, δ, ppm: 13.80 (COOCH2CH3), 27.20 (CH3),
41.70 (CHAr), 43.71 (CHPh), 49.70 (CHCOOEt), 54.73
(CHCHO), 61.62 (COOCH2CH3), 72.10 [C(CH3)OH],
93.10 (CHNO2), 128.20 (Cm), 128.70 (Cp), 129.20 (Co),
129.40 (Cm) 129.50 (Co), 134.10 (Ci), 134.62 (Ci), 135.21
(Ci), 173.91 (COOEt), 202.80 (CHO). Found, %: C 62.28;
H 5.31; N 3.44. C23H24ClNO6. Calculated, %: C 61.95;
H 5.43; N 3.14.
ACKNOWLEDGMENTS
The study was carried out under the financial support
of the Ministry of Education and Science of the Russian
Federation (agreement 14.В37.21.1917, State contract
no. 16.552.11.7076) with the application of the Center of
the GeneralAccess “Examination of the physicochemical
properties of substances and materials”.
Ethyl (1R,2S,3S,4R,5S,6S)-2-hydroxy-2-meth-
yl-6-(4-methoxyphenyl)-5-nitro-4-phenyl-3-for-
mylcyclohexanecarboxylate (VIc). Yield 0.93 g
(33%), mp 165–166°С, [α]D20 –7.45° (c 0.011 g mL−1,
1
CHCl3). Н NMR spectrum, δ, ppm: 0.93 t (3H,
REFERENCES
3
COOCH2CH3, JHH 7.1 Hz), 1.46 s (3H, CH3), 3.46–
1. Toure, B.B. and Hall, D.G., Chem. Rev., 2009, vol. 109,
p. 4439; Pellissier, H., Adv. Synth. Catal., 2012, vol. 354,
p. 237.
2. Enders, D., Urbanietz, G., Cassens-Sasse, E., Keeß, S.,
Raabea, G., Adv. Synth. Catal., 2012, vol. 354, p. 1481.
3. Evans, D.A., Seidel, D. J. Am. Chem. Soc., 2005, vol. 127,
p. 9958; Evans, D.A., Mito, S., and Seidel, D., J. Am. Chem.
Soc., 2007, vol. 129, p. 11583.
3.51 m (1H, CHCHO), 3.74 с (3H, OCH3), 3.76–3.81 m
(2H, CHCOOEt, OH), 3.91–3.98 m (3H, COOCH2CH3,
CHPh), 4.27 d.d (1H, CHAr, 3JHH 12.3, 3JHH 4.6 Hz), 5.00 t
(1H, CHNO2, 3JHH 4.3 Hz), 6.79 d (2H, CHm, 3JHH 8.7 Hz),
7.09–7.15 m (4Harom), 7.22–7.30 m (3Harom), 9.50 d (1H,
CHO, 3JHH 4.6 Hz). 13C NMR spectrum, δ, ppm: 13.82
(COOCH2CH3), 27.32 (CH3), 41.71 (CHAr), 43.62
(CHPh), 50.00 (CHCOOEt), 54.71 (CHCHO), 55.30
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 11 2013

REZNIKOV et al.
1604
Klimochkin, Yu.N., Russ. J. Org. Chem., 2012, vol. 48, p.
1526.
6. Reznikov,A.N., Osyanin, V.A., and Klimochkin, Yu.N., Izv.
Vuzov, Ser. Khim. i Khim. Tekhnol., 2012, vol. 55, p. 86;
Reznikov, A.N., Golovin, E.V., and Klimochkin, Yu.N.,
Russ. J. Org. Chem., 2013, vol. 49, p. 663.
7. Manzano, R., Andrés, J.M., and Pedrosa, R., Synlett., 2011,
no. 15, p. 2203; Ballini, R., Maggi, R., Palmieri, A., Sarto-
rib, G., Synthesis, 2007, vol. 19, p. 3017.
4. Zhu, Q., Huang, H., Shi, D., Shen, Z., and Xia, C., Org. Lett.,
2009, vol. 11, p. 4536; Chen, W.-Y., Ouyang, L., Chen, R.-
Y., and Li, X.-S., Tetrahedron Lett., 2010, vol. 51, p. 3972;
Wilckens, K., Duhs, M.-A., Lentz, D., and Czekelius, C.
Eur. J. Org. Chem., 2011, p. 5441; Jin, R., Liu, K., Xia,D.,
Qian, Q., Liu, G., and Li, H., Adv. Synth. Catal., 2012,
vol. 354, p. 3265.
5. Reznikov,A.N., Golovin, E.V., and Klimochkin, Yu.N., Russ.
J. Gen. Chem., 2012, vol. 82, p. 1827; Reznikov, A.N. and
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 49 No. 11 2013