Novel L-threonine-based ionic liquid supported organocatalyst for asymmetric syn-aldol reactions: activity and recyclability design

Article abstract of DOI:10.24820/ark.5550190.p010.149

A novel recyclable threonine-derived ionic-liquid-supported organocatalyst of asymmetric cross-aldol reactions has been developed. In its presence, aromatic aldehydes react with hydroxyacetone, methoxyacetone and 2-butanone to afford the corresponding syn

Full text of DOI:10.24820/ark.5550190.p010.149

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Arkivoc 2017, part iii, 241-249
Novel L-threonine-based ionic liquid supported organocatalyst for asymmetric
syn-aldol reactions: activity and recyclability design
Vasiliy V. Gerasimchuk,a Roman R. Romanov,b Gladys H.-T. Woo,c Igor A. Dmitriev,a
Alexander S. Kucherenko,a and Sergei G. Zlotin*a
a Zelinsky Institute of Organic Chemistry, Leninsky prospect 47, 119 991, Moscow, Russia
b Moscow Chemical Lyceum, Tamogeny proezd 4, 111 033, Moscow, Russia
c National Junior College, 37 Hillcrest Road, 288 913, Singapore
Email: zlotin@ioc.ac.ru
Dedicated to Professor Oleg A. Rakitin on the occasion of his 65th anniversary
Received 04-25-2017
Accepted 07-04-2017
Published on line 07-29-2017
Abstract
A novel recyclable threonine-derived ionic-liquid-supported organocatalyst of asymmetric cross-aldol
reactions has been developed. In its presence, aromatic aldehydes react with hydroxyacetone, methoxy-
acetone and 2-butanone to afford the corresponding syn-aldol products in moderate to high yields with
excellent diastereo- (syn/anti up to 96:4) and enantio-selectivity (up to 95 % ee), which was retained over five
recycling experiments.
Keywords: Aldol reaction; asymmetric catalysis; organocatalysis; recyclable catalysts
DOI: https://doi.org/10.24820/ark.5550190.p010.149
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Introduction
Asymmetric organocatalysis is an intensively developing area of modern organic chemistry.1-3 One of the most
important organocatalytic reactions is the asymmetric aldol reaction, which occur in Nature and is widely used
in chemical research for enantioselective formation of the carbon-carbon bonds in organic compounds.4 As a
rule, major products of aldol reactions catalyzed by secondary amines have anti-configuration5-7 whereas syn-
aldols, which are key structural fragments of carbohydrates, are formed in aldolase-catalyzed enzymatic aldol
reactions.8-9 Some of these native catalysts (aldolases of type II) have a peptide structure with primary amino
acid fragments as active sites.10 Over the past decade, a number of similar syn-aldol reactions have been
realized in laboratory (though, with a somewhat lower stereoselectivity) in the presence of properly designed
primary aminocatalysts. Among them, O-protected serine or threonine amino acids,11-16 their amides,17
valine,18 leucine,19 iso-leucine20 or tert-leucine derivatives21 and some primary–tertiary 1,2-diamine organo-
catalysts22-27 exhibited promising catalytic performance. However, unlike enzymes, these valuable catalysts
could be used just once and until recently no information on their recovery and reuse in the catalytic process
has been available.
A few years ago we presented the first “conditionally” recyclable catalyst 1a of syn-aldol reactions, an
ionic-liquid-supported (S)-threonine amide bearing an α,α-diphenylvalinol structural unit (Scheme 1).28
Unfortunately, the catalytic activity of compound 1a became lower after the first recovery and after the third
one it became nearly inactive. Very recently, we discovered that main reason for this deactivation is the
undesirable intramolecular O-N migration of the acyl fragment attached to ionic group which resulted in the
amidation of the primary amino group which is key for the enamine catalysis.29 To make the migration
thermodynamically unfavorable, we designed catalyst 1b, in which the acyl linker is located distantly from the
amino group. Indeed, catalyst 1b appeared much more sustainable and could be recycled 7 times with
complete retention of stereoselectivity and only a slight conversion decrease.
Scheme 1. Research strategy.
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We hypothesized that the parasitic rearrangement may also be suppressed by a Brønsted-acidic group,
which being incorporated into the catalyst would reduce nucleophilicity of the threonine amino group via the
protonation. Furthermore, we expected that a remote carboxyl group in catalyst 1c would simultaneously act
as an acidic co-catalyst and reduce catalyst leaching during workup. A number of catalytic aldol reactions are
known to proceed with a higher rate and better enantioselectivity in the presence of acidic additives.5 A few
examples of favorable impact of the incorporated carboxy group on the catalytic performance and
recyclability of ionic-liquid-supported primary-amine-based chiral organocatalysts in asymmetric Michael30
and anti-aldol reactions31 have also been reported. However, to the best of our knowledge, this approach has
never been used to improve the catalytic performance of primary amino acid-derived supported organo-
catalysts in asymmetric syn-aldol reactions.
Results and Discussion
To verify this hypothesis, we synthesized the carboxylated analog 1c, in which the imidazolium cation is
attached to a carboxylic group. The synthetic scheme included alkylation of O-protected 1-(4-
benzyloxycarboxybutyl)-imidazole 3 with bromoester 2 followed by the conversion of the imidazolium
bromide 4 into the carboxylated IL-supported catalyst 1c via a sequence of anion exchange and catalytic
hydrogenation (5% Pd/C) reactions (Scheme 2).
Scheme 2. Synthesis of carboxylated catalyst 1c.
Having catalyst 1c in hand, we at first optimized reaction conditions using hydroxyacetone 5a and 4-nitro-
benzaldehyde 6a as model substrates (Table 1). It was found that in nonpolar aprotic solvent (e.g. toluene)
product 7a was generated with higher selectivity and conversion of 6a than in other solvents.
Under optimal conditions, hydroxyacetone (5a) reacted with benzaldehyde derivatives (6a-i) bearing
acceptor or donor substituents in the aromatic ring to afford corresponding syn-aldols 7a-i with high
conversion and with good to excellent diastereo- and enantio-selectivity (for compounds 7a-h, the dr and ee
values were similar or even higher that reported with catalyst 1a28) (Scheme 3). The methoxyacetone (5b) also
appeared a suitable ketone-donor for the catalytic syn-aldol reactions with aldehydes 6a and 6d to give
corresponding aldols 7j and 7k with reasonably high diastereo- and enantio-selectively. In case of 2-butanone
(5c) the conversion and dr and ee values of generated aldol 7l were significantly lower.
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Table 1. Optimization of reaction conditions a
Solvent
MeOH
NMP
CH2Cl2
Toluene
o-Xylene
Conv., % b,d
dr (syn/anti) b,d
70:30 (70:30)
90:10 (70:30)
94:6 (80:20)
93:7 (92/8)
90/10
ee (syn), % c,d
88 (83)
14 (39)
53 (91)
99 (99)
90
79 (68)
90 (90)
92 (88)
95 (94)
88
a Unless otherwise specified, all reactions were carried out with 5a (15 mg, 14 µL, 0.2 mmol),
6a (0.066 mmol), 1c (6.5 mg, 0.01 mmol), and solvent (90 µL). b 1H NMR spectroscopic data
c
(J3-4 = 1.8–4.2 Hz, J3-4
= 6.1–8.8 Hz); HPLC data (Daicel Chiralpak AD-H) for crude
syn
anti
compound 7a. d Corresponding data for catalyst 1a are given in parentheses.
Scheme 3. The reaction scope.
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It is worthy of note that the previously unknown compound 7i is a close structural analog of flavanonol –
an inhibitor of nitric oxide (NO) production in inflammatory cells (Figure 1).32
Figure 1. Biologically active flavanonol – a structural analog of 7i.
Finally, we examined the recyclability of catalyst 1c in the asymmetric syn-aldol reaction between
compounds 5a and 6d (Table 2). After completion of the reaction, the solvent was evaporated under reduced
pressure, aldol product 7d was extracted with Et2O, and a fresh solution of the starting compounds in toluene
was added to the remaining catalyst. In this manner, catalyst 1c was successfully recycled five times without
any reduction of the dr and ee values, though, with a slight conversion decrease. These data are in agreement
with a favorable impact of the carboxy group in catalyst 1c on its sustainability and recyclability under
proposed conditions as compared with catalyst 1a.
Table 2. Recyclability of catalyst 1c in the model reaction between 5a and 6d
Cycle
Conv., %
95
dr, syn/anti, %
95/5
ee, %
95
95
95
95
1
2
3
4
5
94
89
83
79
95/5
95/5
95/5
95/5
95
Conclusions
The obtained results show that simple modification with the carboxylic group may be considered as a
promising approach to improve sustainability of IL-supported primary amino acid derived organocatalysts in
asymmetric syn-aldol reactions. Based on this approach, a novel carboxylated threonine amide derived IL-
tagged catalyst of asymmetric aldol reactions between aromatic aldehydes and linear ketones has been
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developed which exhibited improved catalytic activity and good diastereo- (syn/anti up to 96/4) and enantio-
selectivity (up to 95% ee) over five recycling experiments.
Experimental Section
1
General. H and 13C NMR spectra were recorded with a Bruker AM 300 spectrometer in CDCl3 and DMSO-d6.
1
The chemical shifts of H and 13C signals were measured relative to Me4Si or CDCl3, respectively. The high-
resolution mass spectra (HRMS) were measured with a Bruker microTOF II spectrometer using electrospray
ionization (ESI). The measurements were taken either in the positive ion mode (interface capillary voltage
4500 V) or in the negative ion mode (3200 V) in a mass range m/z = 50–3000 Da; external or internal
calibration was done with electrospray calibrant solution (Fluka). Syringe injection was used for solution in
MeCN/H2O (1:1, v/v) (flow rate 3 μL/min). Nitrogen was applied as a dry gas, and the interface temperature
was set at 180 °C. Silica gel 0.060–0.200 µm (Acros) was used for column chromatography. Threonineamide (2)
and benzyl 5-(1H-imidazol-1-yl)pentanoate (3) were synthesized according to known methods. Compounds 5
and 6 were purchased from Aldrich and used without purification. The solvents were purified by standard
procedures. For experimental details and spectral or HPLC data see Supporting Information.
3-[5-(Benzyloxy)-5-oxopentyl]-1-(5-[((2R,3S)-3-{[(benzyloxycarbonyl)amino]-4-[((S)-1-hydroxy-3-methyl-1,1-
diphenylbutan-2-yl)amino]-4-oxobutan-2-yl}oxy)-5-oxopentyl]-1H-imidazol-3-ium hexafluorophosphate (4).
Benzyl 5-(1H-imidazol-1-yl)pentanoate (3) (0.22 g, 0.83 mmol) was gradually added to a solution of (2R,3S)-
3-[(benzyloxycarbonyl)amino]-4-[((S)-1-hydroxy-3-methyl-1,1-diphenylbutan-2-yl)amino]-4-oxobutan-2-yl
5-bromopentanoate (2) (0.45 g, 0.69 mmol) in CH3OH (2 mL). The reaction mixture was kept at ambient
o
temperature for 10 min and evaporated under reduced pressure (20 Torr) at 40 C. The residue was heated at
o
the same pressure (rotary evaporator, 80 C) for 5 min, cooled to ambient temperature and diluted with
distilled water (3.0 mL). A solution of KPF6 (128 mg, 0.69 mmol) in distilled water (1.5 mL) was added to the
resulting aqueous solution and the reaction mixture was stirred for 1 h at ambient temperature. The
precipitate was filtered, washed successively with distilled water (3 x 3 mL) and Et2O (2 x 1 mL), and dried in air
o
1
to afford 4 (0.612 g, 90%). White powder, mp 97-100 C. H NMR (600 MHz, DMSO-d6): 0.65 (d, J 6.5 Hz, 3H,
CH3); 0.70 (d, J 6.5 Hz, 3H, CH3); 0.87 (d, J 6.5 Hz, 3H, CH3); 1.38-1.45 (m, 2H, CH2); 1.48-1.55 (m, 2H, CH2); 1.69-
1.78 (m, 3H, CH2 + CH(CH3)2); 1.78-1.85 (m, 2H, CH2); 2.13-2.24 (m, 2H, CH2); 2.40 (t, J 7.3 Hz, 2H, CH2); 3.99 (t,
J 8.2 Hz, 1H, CH); 4.11 (t, J 6.9 Hz, 2H, CH2); 4.18 (t, J 6.9 Hz, 2H, CH2); 4.84 (m, 1H, CH); 4.89 (d, J 9.5 Hz, 1H,
CH); 5.04 (2H, CH2 AB system, JHH 12.66 Hz); 5.10 (s, 2H, CH2); 5.64 (s, 1H, OH); 7.08 (t, J 7.2 Hz, 1H, CH); 7.13-
7.21 (m, 3H, CH); 7.26-7.41 (m, 12H, CH); 7.46-7.55 (m, 4H, CH); 7.60 (d, J 10.0 Hz, 1H, NH); 7.71 (d, J 8.9 Hz,
1H, NH); 7.79 (d, J 5.0 Hz, 2H, NCHCHN); 9.15-9.24 (m, 1H, NCHN); 13C NMR (125.76 MHz, DMSO-d6): 16.6,
18.2, 21.3, 21.4, 23.2, 29.05, 29.15, 33.1, 33.2, 48.9, 58.2, 59.2, 65.9, 69.8, 81.3, 122.9, 125.7, 125.8, 126.6,
128.0, 128.1, 128.3, 128.4, 128.5, 128.8, 128.9, 136.4, 136.6, 137.5, 146.5, 147.7, 156.5, 169.3, 172.1, 172.9.
Elemental analysis: calcd for C49H59F6N4O8P: C, 60.24; H, 6.09; N, 5.73; found: C, 60.06; H, 6.14, N, 5.79%.
1-(5-{(2R,3S)-3-Amino-4-[((S)-1-hydroxy-3-methyl-1,1-diphenylbutan-2-yl)amino]-4-oxobutan-2-yl}oxy)-5-
oxopentyl}-3-(4-carboxybutyl)-1H-imidazol-3-ium hexafluorophosphate (1c). 5% Pd/C (50 mg) was added to
a solution of 4 (120 mg, 0.12 mmol) in freshly distilled methanol (3 mL) and the reaction mixture was
vigorously stirred under H2 atmosphere ( 1 bar) for 5 h at ambient temperature. The reaction mixture was
~
filtered and evaporated under reduced pressure (20 Torr). The residue was dried in vacuo (2 Torr) at 40 oC for
o
1 h to afford 1c (89 mg, 96%). A yellow powder, mp 89-91 C. 1H NMR (600 MHz, DMSO-d6): 0.58 (d, J 3.2 Hz,
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3H, CH3); 0.68-0.73 (m, 3H, CH3); 0.80-0.90 (m, 3H, CH3); 1.40-1.53 (m, 4H, 2xCH2); 1.62-1.74 (m, 1H, CH i-Pr);
1.71-1.87 (m, 4H, 2xCH2); 2.18 (t, J 7.1 Hz, 2H, CH2); 2.28 (t, J 7.2 Hz, 2H, CH2); 3.64-3.72 (m, 1H, CH3CHOH);
3.98 (t, J 7.4 Hz, 1H, CH(NH)CONH); 4.13-4.24 (m, 4H, 2xCH2); 4.50-4.62 (m, 1H, CH(i-Pr)NH); 4.87 (d, J 9.5 Hz,
1H, OH); 5.67 (s, 1H, OH); 7.06-7.23 (m, 4H, CH); 7.29 (t, J 7.7 Hz, 2H, CH); 7.42 (d, J 10.1 Hz, 1H, NH); 7.49 (t, J
6.7 Hz, 4H, CH); 7.81 (d, J 11.7 Hz, 2H, NCHCHN); 7.94 (d, J 8.4 Hz, 1H, NH); 9.24 (s, 1H, NCHN); 12.08 (s, 1H,
COOH); 13C NMR (125.76 MHz, DMSO-d6): 18.2, 19.9, 21.5, 22.2, 23.3, 28.9, 29.1, 29.3, 29.6, 33.3, 34.6, 49.0,
49.1, 57.9, 59.4, 66.1, 81.3, 122.9, 125.6, 126.0, 126.6, 128.1, 128.5, 136.4, 146.7, 170.8, 172.1, 174.5; HRMS
+
(ESI): m/z calcd. for C34H47N4O6 : 607.3490, found: 607.3493.
General procedure for syn-aldol reactions. Aldehyde 6a-i (0.066 mmol) and catalyst 1c (7.5 mg, 0.01 mmol)
were dissolved in dry toluene (90 µL). Then, ketone 5a-c (0.2 mmol) was added to the resulting solution. The
reaction mixture was stirred at ambient temperature for 24-48 h (TLC-monitoring), filtered through a silica gel
o
1
pad and evaporated (40 C, 8 mbar). Conversions and dr values of aldol products 7a-l were measured by H
NMR spectroscopy. The ee values of aldol products 7a-l were determined by chiral HPLC column (Daicel
Chiralpak AD-H).
1
(3R,4S)-4-(2,4-Dimethoxyphenyl)-3,4-dihydroxybutan-2-one (7i). Pale yellow oil. H NMR (500 MHz, CDCl3):
2.27 (s, 3H, CH3), 3.82 (d, 6H, (OCH3)2), 4.41 (s, 1H), 5.31 (s, 1H), 6.42-6.60 (m, 2H, Ar), 7.31 (m, 1H, Ar); 13C
NMR (125 MHz, CDCl3): 26.4, 55.8, 56.0, 69.5, 71.5, 80.2, 80.4, 99.0, 104.8, 104.9, 121.5, 128.1, 129.0, 157.3,
161.13, 208.86; HRMS (ESI) m/z calcd. for [C12H16O5+Na]: 263.0890; found: 263.0890.
General procedure for catalyst 1c recycling. After 24 h, the mixture of hydroxyacetone (5a) (74 mg, 70µl, 1
mmol), 2-chlorobenzaldehyde (6d) (46.8 mg, 0.33 mmol), catalyst 1c (37.5 mg, 0.05 mmol) and toluene (0.45
o
mL) was gently evaporated (40 C, 8 mbar). Product 7d and unchanged starting compounds were carefully
extracted from the residue by Et2O (3 x 0.7mL). Fresh portions of reagents and toluene were added to the
remaining catalyst 1c and the catalytic procedure was performed again as described.
Acknowledgements
This research was supported by the President of the Russian Federation (Grant for young PhDs No.
7441.2016.3), by the Russian Foundation of Basic Research (project 16-03-00767), and by the Scientific
Research Program No. III.5.1 of the Department of Chemistry and Material Sciences of the Russian Academy of
Sciences.
Supplementary Material
1H, 13C, 1H-13C HSQC, 1H-13C HMBC and ESI-MS spectra for compounds 4 and 1c. HPLC traces for compounds 7.
References
1. Science of Synthesis: Asymmetric Organocatalysis, Eds.: List, B.; Maruoka, K. Thieme: Stuttgart, 2012.
2. Comprehensive Enantioselective Organocatalysis: Catalysts, Reactions, and Applications, Eds.: Dalco, P. I.
Wiley-VCH: Weinheim, 2013.
Page 247
©ARKAT USA, Inc

Arkivoc 2017, iii, 241-249
Gerasimchuk, V. V. et al.
3. Shaikh, I. R. Journal of Catalysts, 2014, Article ID 402860, p. 1-35.
http://doi.org/10.1155/2014/402860
4. Guillena, G. in Modern Methods in Stereoselective Aldol Reactions, Eds.: R. Mahrwald, Wiley-VCH:
Weinheim, 2013; pp155–268.
5. Mukherjee, S.; Yang, J.; Hoffman, W. S.; List. B. Chem. Rev. 2007, 107, 5471-5569.
http://doi.org/10.1021/cr0684016
6. Zlotin, S. G.; Kucherenko, A. S.; Beletskaya, I. P. Russ. Chem. Rev. 2009, 78, 737-784.
http://doi.org/10.1070/RC2009v078n08ABEH004040
7. Trost, B. M.; Brindle, C. S. Chem. Soc. Rev. 2010, 39, 1600–1632.
http://doi.org/10.1039/B923537J
8. Koeller, K. M.; Wong, C. H. Nature 2001, 409, 232-240.
http://doi.org/10.1038/35051706
9. Wong, C. H.; Machajewski, T. D. Angew. Chem., Int. Ed. 2000, 39, 1352-1375.
http://doi.org/10.1002/(SICI)1521-3773(20000417)39:8<1352::AID-ANIE1352>3.0.CO;2-J
10. Dean, S. M.; Greenberg, W. A.; Wong, C. H. Adv. Synth. and Catal. 2007, 349, 1308-1320.
http://doi.org/10.1002/adsc.200700115
11. Ramasastry, S. S. V.; Zhang, H.; Tanaka, F.; Barbas, C. F., III. J. Am. Chem. Soc. 2007, 129, 288-289.
http://doi.org/10.1021/ja0677012
12. Utsumi, N.; Imai, M.; Tanaka, F.; Ramasastry, S. S. V.; Barbas, C. F., III. Org. Lett. 2007, 9, 3445-3448.
http://doi.org/10.1021/ol701467s
13. Teo, Y. C.; Chua, G. L.; Ong, C. Y.; Poh, C. Y. Tetrahedron Lett. 2009, 50, 4854-4856.
http://doi.org/10.1016/j.tetlet.2009.06.037
14. Markert M.; Scheffler U.;Mahrwald, R. J. Am. Chem. Soc. 2009, 131, 16642-16643.
http://doi.org/10.1021/ja907054y
15. Yong, F.-F.; Poh, C.-Y.; Chua,G.-L.; Teo,Y.-C. Chem. Lett. 2010, 39, 490-492.
http://doi.org/10.1246/cl.2010.490
16. A. H. Henseler, C. Ayats, M. A. Pericàs, Adv. Synth. Catal. 2014, 356, 1795–1802.
http://doi.org/10.1002/adsc.201400033
17. Paradowska, J.; Pasternak, M.; Gut, B.; Gryzło, B.; Mlynarski, J. J. Org. Chem. 2012, 77, 173−187.
http://doi.org/10.1021/jo201584w
18. Sarkar, D.; Harman, K.; Ghosh, S.; Headley. A. D. Tetrahedron: Asymmetry 2011, 22, 1051–1054.
http://doi.org/10.1016/j.tetasy.2011.05.021
19. C. Nicolas, R. Pluta, M. Pasternak-Suder, O.R. Martin, J. Mlynarski. Eur. J. Org. Chem. 2013, 1296–1305.
http://doi.org/10.1002/ejoc.201201413
20. Kumar, A.; Singh, S.; Kumar, V.; Chimni, S. S. Org. Biomol. Chem. 2011, 9, 2731–2742.
http://doi.org/ 10.1039/C0OB00898B
21. Umehara, A.; Kanemitsu, T.; Nagata, K.; Itoh. T. Synlett 2012, 453–457.
http://doi.org/10.1055/s-0031-1290316
22. Raj, M.; Parashari, G. S.; Singh, V. K. Adv. Synth. Catal. 2009, 351,1284-1288.
http://doi.org/10.1002/adsc.200900122
23. Luo, S.; Xu, H.; Zhang, L.; Li, J.; Cheng, J. P. Org. Lett. 2008, 10, 653-656.
http://doi.org/10.1021/ol703023t
24. Luo, S.; Xu, H.; Chen, L.; Cheng, J. P. Org. Lett. 2008, 10, 1775-1778.
http://doi.org/10.1021/ol800471b
Page 248
©ARKAT USA, Inc

Arkivoc 2017, iii, 241-249
Gerasimchuk, V. V. et al.
25. Li, J.; Luo, S.; Cheng, J. P. J. Org. Chem. 2009, 74, 1747-1750.
http://doi.org/10.1021/jo802557p
26. Luo, S.; Xu, H.; Li, J.; Zhang, L.; Cheng, J. P. J. Am. Chem. Soc. 2007, 129, 3074-3075.
http://doi.org/10.1021/ja069372j
27. Demuynck, A. L. W.; Vanderleyden, J.; Sels, B. F. Adv. Synth. Catal. 2010, 352, 2421–2426.
http://doi.org/10.1002/adsc.201000419
28. Larionova, N. A.; Kucherenko, A. S.; Siyutkun, D. E.; Zlotin, S. G. Tetrahedron 2011, 67, 1948-1954.
http://doi.org/10.1016/j.tet.2011.01.017
29. Gerasimchuk, V. V.; Kucherenko, A. S.; Fakhrutdinov, A. N.; Medvedev, M. G.; Nelyubina, Y. V.; Zlotin, S. G.
Eur. J. Org. Chem. 2017, 2540-2544.
http://doi.org/10.1002/ejoc.201700166
30. Kucherenko, A. S.; Lisnyak,; V. G. Chizhov,; A. O. Zlotin, S. G. Eur. J. Org. Chem. 2014, 3808–3813.
http://doi.org/10.1002/ejoc.201400045
31. Kucherenko, A. S.; Gerasimchuk, V. V.; Lisnyak, V. G.; Nelyubina, Y. V.; Zlotin, S. G. Eur. J. Org. Chem. 2015,
25, 5649-5654.
http://doi.org/10.1002/ejoc.201500775
32. Jiang, W.-J.; Ishiuchi, K.; Furukawa, M.; Takamiya, T.; Kitanaka, S.; Iijima, H. Bioorg. Med. Chem. 2015, 23,
6922-6929.
http://doi.org/10.1016/j.bmc.2015.09.042
Page 249
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