Trifluoroethanol as an efficient reaction media for the synthesis of pyran skeleton through domino Knoevenagel-hetero-Diels-Alder reaction with non-activated alkynes

Article abstract of DOI:10.1007/s13738-014-0521-8

Trifluoroethanol as an efficient media for the domino Knoevenagel-hetero-Diels-Alder reaction of O-propargyloxy benzaldehydes as non-activated terminal alkynes with some active methylene compounds has been described. Short reaction time, easy work-up, good to high yields, and mild reaction conditions are advantages of this new media. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s13738-014-0521-8

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0521-8
ORIgINAl PAPER
Trifluoroethanol as an efficient reaction media for the synthesis
of pyran skeleton through domino Knoevenagel–hetero‑Diels–
Alder reaction with non‑activated alkynes
Saeed Balalaie · Javad Azizian · Abolghasem Shameli ·
Hamid Reza Bijanzadeh
Received: 3 April 2014 / Accepted: 30 July 2014
© Iranian Chemical Society 2014
Abstract Trifluoroethanol as an efficient media for the
domino Knoevenagel–hetero-Diels–Alder reaction of
O-propargyloxy benzaldehydes as non-activated terminal
alkynes with some active methylene compounds has been
described. Short reaction time, easy work-up, good to high
yields, and mild reaction conditions are advantages of this
new media.
reaction steps, time and saving energy and raw materials [2,
3].
The hetero-Diels–Alder reaction using 1-oxo-1,3-butadi-
ene is one of the most efficient and powerful synthetic tools
for the synthesis of chroman and isochroman derivatives,
and also some natural products with a broad range of bio-
logical and pharmaceutical activities [4]. Moreover, Tietze
et al. extensively described the domino Knoevenagel–het-
ero-Diels–Alder reaction (DKHDA) of unsaturated aro-
matic- and aliphatic aldehydes with several 1,3-dicarbonyl
compounds for the synthesis of heterocycles containing
pyran rings [5–9]. However, the use of alkynes as dieno-
phile in domino Knoevenagel–hetero-Diels–Alder reac-
tions was limited due to their low reactivity compared to
alkenes [10, 11]. Consequently, the activation of alkynes
toward a variety of organic transformations is an interest-
ing field in organic synthesis [12–17]. In this regard, previ-
ously, we reported CuI as an efficient lewis acid for acti-
vation and using various non-activated alkynes in domino
Knoevenagel–hetero-Diels–Alder reactions using different
organic solvents [18–26]. However, many of the previously
reported domino Knoevenagel–hetero-Diels–Alder reac-
tions with inactivated alkynes suffer from high reaction
temperatures, low to moderate yields, long reaction times,
and in some cases limitation of the substituents in the struc-
ture of starting materials. Due to the importance of domino
Knoevenagel–hetero-Diels–Alder reactions and the biolog-
ical activities of the compounds containing pyran skeletons,
optimization of these reaction conditions are necessary in
organic synthesis.
Keywords Trifluoroethanol · Domino Knoevenagel–
hetero-Diels–Alder reaction · Pyran skeleton · Non-
activated alkynes · CuI
Introduction
Domino reactions have become important tools in the prep-
aration of structurally diverse chemical libraries of drug-
like poly functional compounds [1]. However, to ensure
sufficient molecular diversity and complexity of new chem-
ical entities, there is a continuous need for novel reactions
carrying out in mild and efficient conditions with high effi-
ciency and selectivity. Designing domino reactions is an
efficient approach through reducing the chemical waste,
S. Balalaie (*) · H. R. Bijanzadeh
Peptide Chemistry Research Center, K. N. Toosi University
of Technology, P. O. Box 15875-4416, Tehran, Iran
e-mail: balalaie@yahoo.com; balalaie@kntu.ac.ir
S. Balalaie
One of the main principles of “green” chemistry is to
develop cost-effective and environmentally benign sys-
tems which have become one of the main themes of cur-
rent synthetic chemistry. For example, in this way, highly
fluorinated alcohols exhibit high hydrogen bonding donor
Medical Biology Research Center, Kermanshah University
of Medical Sciences, Kermanshah, Iran
J. Azizian · A. Shameli
Department of Chemistry, Science and Research Branch,
Islamic Azad University, P.O. Box 19395-1775, Tehran, Iran
1 3

J IRAN CHEM SOC
Scheme 1 Domino Knoev-
enagel hetero- Diels–Alder
reaction for the synthesis of
polycyclic compounds that
contained pyran skeleton in TFE
at room temperature
X
O
CHO
O
CuI, NEt3 (20%)
TFE, RT
O
O
+
O
X
O
X: H, 5-Br, 5-NO2
1
2
3
Table 1 Effect of different
catalysts on the domino Kno-
evenagel–hetero-Diels–Alder
reaction of 1a with dimethyl
barbituric acid
O
N
O
O
N
CHO
O
Cat. (30% mol), Et3N
TFE
N
O
O
O
N
O
1a
2a
3a
Catalists (30 %)
Time
Yield (%)
Ag (Nanopowder)
10 h
90
75
70
90
95
Sc(OTf)3
TiO2
16 h
Reaction conditions: 1.1 mmol
dimethy barbituric acid, 1 mmol
O-propargylated salicylalde-
hydes, CuI (30 %mol) and Et3N
(20 %mol) in 2 ml of TFE
24 h
ZrO2
15 min
10 min
CuI
ability, low nucleophilicity, high ionizing power, and ability
for the hydrogen bonding with water. Reactions in fluori-
nated alcohols are generally selective, with no by-products,
and they can be easily recovered by distillation [27–31].
Among several of fluorinated alcohols, the most com-
monly used and the cheapest are trifluoroethanol (TFE)
and hexafluoroisopropanol (HFIP), which are commer-
cially available on a commercial scale [32–34]. Due to
their activity and capacity, these solvents have been used
as a medium for different reactions, such as activation of
orthoesters [35], synthesis of heterocycles [36, 37] and also
condensation reactions [38, 39].
In continuation of our interest to the domino Knoev-
enagel–hetero-Diels–Alder reactions [18–26], we would
like to report carrying out the DKHDA of alkynes as dieno-
phile in trifluoroethanol as reaction media for the synthesis
of polycyclic compounds which have pyran moiety in the
presence of cuprous iodide at room temperature Scheme 1.
construction of different heterocyclic compounds contain-
ing the pyran skeleton [18–26]. Initially, O-propargylated
salicylaldehydes (1) as the starting material were prepared
in excellent yields using reaction of propargyl bromide
and salicylaldehyde derivatives in dimethylformamide
and in the presence of K2CO3 ([11] a). Reaction of O-pro-
pargylated salicylaldehydes 1a with dimethy barbituric
acid was selected as the model reaction. To optimize the
reaction conditions, we have used various metal catalysts
such as silver nanopowder, titanium dioxide nanopowder,
and scandium triflate, zirconium dioxide, and cuprous
iodide. Among these catalysts, only zirconium oxide, and
cuprous iodide led to good yields in short reaction time.
(Table 1) Considering the above results, cuprous iodide
was found to be the most suitable lewis acid in this
reaction.
Meanwhile, reaction of compound 1b with 1-phenyl-
3-methyl pyrrazolone 2c was checked, and the reaction
was studied using different lewis acids. The best result was
obtained as another model reaction using cuprous iodide in
TFE. The optimization of reaction was done using differ-
ent ratios of cuprous iodide in the presence of two bases
triethylamine, and potassium carbonate in TFE as reaction
media. The results are summarized in Table 2. The best
results was obtained with CuI (30 %) and Et3N (20 %).
Results and discussion
In our recent research work, we surveyed different dom-
ino Knoevenagel–hetero-Diels–Alder (DKHDA) reac-
tion with inactivated alkynes using cuprous iodide for
1 3

J IRAN CHEM SOC
Table 2 Effect of different
reaction condition for DKHDA
reaction of 1b and 1-phenyl-3-
methyl pyrrazolone 2c in TFE
as reaction media
Br
O
CHO
O
Br
Metal Catalyst
TFE, Base
N
O
N
+
N
N
O
Ph
1b
3g
2c
Entry
lewis acid (mol %)
Base
Yield %
Temperature
Time
1
2
3
4
5
6
7
8
Sc(OTf)4 (40)
Ag (50)
–
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
K2CO3
78
65
–
RT
RT
RT
RT
RT
RT
RT
RT
16 h
10 h
72 h
CuI (10)
CuI (20)
CuI (30)
CuI (40)
CuI (30)
40
70
95
85
60
2 h
20 min
10 min
10 min
10 min
The reactions were checked with different active meth-
ylene compounds; such as barbituric acid, dimethyl bar-
bituric acid, indandione, pyrazolone, and Meldrum’s acid.
The experimental results of the current optimized reac-
tion in TFE and the previously reported reactions in other
solvents are summarized in Table 3. In entry 6, when the
substrate 1b was treated with pyrazolone 2b in MeOH in
the presence of CuI at reflux condition for 8 h, the desired
product 3f was obtained in 78 % overall yield, but this reac-
tion was done in TFE with 84 % yield in about 10 min with
the same percent of CuI (20 %) and in the presence of 30 %
CuI, the yield was 90 %. We also repeated the reaction with
indandione and benzoylacetonitrile in TFE at room tem-
perature. The reactions were completed after 5–15 min and
the desired products were obtained in 90–95 % yields. The
same previously reported reaction with CuI need more than
8 h in refluxing MeOH or EtOH. The scope and limitations
of the reaction with respect to the electron-withdrawing
and halogen on the aldehyde component was examined.
The optimized used cuprous acid was 30 % in the TFE as
reaction media.
reaction. Consequently, the reaction of compound (4)
with aryl pyrazolone (2b) in the presence of CuI at
the same condition as previously mentioned in TFE
(Scheme 2) does not give the desired product, and it
could confirm the formation of copper acetylide for pro-
gressing the reaction.
The energy difference between the HOMO and lUMO
plays a major role for the rate of a Diels–Alder reac-
tion [40]. Due to the polarity of TFE, it seems that it can
affect the HOMO–lUMO gaps of the heterodiene and the
copper acetylide and these gaps are smaller than those
in MeOH or EtOH. Therefore, the formation of the cop-
per acetylide allows a more favorable pathway, and the
reaction can proceed better in TFE as reaction media.
According to the energy data, when the HOMO dieno-
phile-lUMO diene energy gap is smaller, leads to a bet-
ter interaction between the lUMO of the diene and the
HOMO of the dienophile. Figure 1 shows the HOMO–
lUMO interaction in TFE.
In conclusion, we have developed a novel and effi-
cient protocol for the synthesis of polycyclic compounds
containing the pyran skeleton. Combination of CuI and
TFE has an important role in domino Knoevenagel–het-
ero-Diels–Alder reaction. The combined use of TFE and
coprous iodide for the synthesis of heterocyclic compounds
containing pyran skeleton offers great potential for rapid
and easily accessible developments in this area, due to
the efficient, economical and easily performed operations.
High yields, low reaction times, carrying out the reaction at
room temperature, and easy set-up and work-up are advan-
tages of this method compared to the reported methods.
Meanwhile, the high polarities, low boiling point of TFE
are the advantages of carrying out the reaction compared to
other organic solvents.
In all cases, the structures of the products were con-
1
firmed by their spectroscopic data. In H-NMR spectra
of the products, the −OCH2 group resonates in region
δ = 4.50–5.00 ppm as two distinguished doublets with
J = 11.5–11.8 Hz. The −CH proton appears as a singlet at
δ 4.70–4.95 ppm. The alkene peak resonates at δ = 7.10–
7.30 ppm as a singlet. The corresponding signal of the
−OCH2 and the shielded alkene carbons in 13C-NMR
appear at δ 64–68 ppm and 83.0–90.0 ppm.
The formation of an intermediate copper acetylide
is consistent with the observation that only inactivated
terminal acetylenes participate in the reaction, and end-
capped substrates do not undergo the cycloaddition
1 3

J IRAN CHEM SOC
CuI(%)a,b
CuI(%)c
Trifluoro ethanol
Yield
†%
Table 3 Synthesis of polycyclic
compounds contained pyran
skeleton in TFE as reaction
media at room temperature
Substrates
Products
Yield†
Time
Time
(min)
Solvent
%
(hour)*
O
N
CHO
N
O
O
O
O
O
N
1
75a
CH3CN
6 11b
95
40
O
N
O
3a
1a
2a
O
Br
Br
CHO
O
N
O
O
O
N
O
N
2
3
73b
CH3CN
25 11b
90
60
O
N
O
3b
3c
3d
1b
2a
O
O N
2
O2N
CHO
O
N
N
O
N
O
O
O
75a
CH3CN
21 11b
92
70
O
N
O
1c
2a
N
O
O
CHO
O
N
N
N
4
80b
CH3OH
811d
90
12
O
Cl
ClC H
6
4
1a
2b
O2N
N
O
O2N
CHO
N
O
O
5
6
89b
CH3OH
CH3OH
8 11d
95
90
10
10
N
N
ClC6H4
Cl
O
3e
3f
1c
2b
Br
N
O
O
Br
CHO
O
N
78b
811d
N
N
ClC6H4
O
Cl
1b
2b
Br
N
O
Br
CHO
O
N
O
7
8
82b
CH3OH
8 11d
95
95
10
N
N
Ph
O
3g
1b
2c
O2N
N
O
O2N
CHO
O
N
O
78b
CH3OH
8 11d
4
N
N
Ph
O
3h
1c
2c
1 3

J IRAN CHEM SOC
CuI(%)a,b
CuI(%)c
Table 3 continued
Trifluoro ethanol
Substrates
Products
Yield†
%
Time
Yield
†%
Time
(min)
Solvent
(hour)*
N
O
CHO
O
N
9
75b
CH3OH
8 11d
48 11e
20 11e
95
90
92
5
O
N
N
O
Ph
3i
1a
2c
O
CHO
CN
O
O
56b
CH3OH
CH3OH
55
40
NC
10
11
Ph
Br
O
1a
2d
3j
O
Br
CHO
O
CN
O
70b
NC
Ph
O
1b
2d
3k
O
O
O
O
†
Isolated Yields
References for our previously
O
12
13
85b
EtOH
EtOH
10 11c
93
95
5
5
*
CHO
O
experiments
a
All reactions were carried out
3l
1a
2e
using 20 %mol of CuI
Br
O
O
O
b
Br
All reactions were carried out
O
using 40 %mol of CuI
O
c
80b
24 11c
Reactions with trifluroetha-
CHO
nol were carried out at room
temperature and using 30 %mol
of Cu
O
1b
2e
3m
Experimental section
5–70 min at room temperature (Table 3). The progress of
reaction was monitored by TlC (Eluent Petroleum ether:
EtOAc 3:2). After completion of reaction, the mixture was
extracted with EtOAc. The solvent was evaporated under
vacuum and the obtained precipitate was separated. Further
purification was done using crystallization in dichlorometh-
ane. The desired products were obtained in 90–95 % yields.
The details about full spectroscopic data for the synthe-
sized compounds are in the references ([11] a–h).
general
Commercially available materials were used without further
purification. Compounds 3a–m was prepared in excellent
purity. Melting points were determined on an Electrother-
mal 9100 apparatus and were uncorrected. IR spectra were
1
obtained on an ABB FTIR (FTlA 2000) spectrometer. H
NMR and 13C NMR spectra were run on Bruker DRX-300
and DRX-500 AVANCE spectrometers at 300 and 500 MHz
for 1H NMR, and 75 and 125 MHz for 13C NMR. CDCl3 and
DMSO-d6 were used as solvents. High resolution mass spec-
tra were recorded on JEOl JMS-700 (HR-EI) spectrometer.
Selected spectroscopic data
Compound 3a
m.p. 254–255 °C; IR(KBr, cm−1): ν˜ = 1,704, 1,638, 1,521,
1
General procedure for the synthesis of compounds 3a–m
1,340. H NMR (300 MHz, DMSO-d6): δ = 3.27 (s, 3H,
NMe), 3.30 (s, 3H, NMe), 4.77 (s, 1H, CH), 4.86 (d,
J = 11.6 Hz, 1H, OCH), 4.98 (d, J = 11.6 Hz, 1H, OCH),
6.95 (d, J = 9.0 Hz, 1H, HAr), 7.18 (s, 1H, = CH), 7.97 (s,
1H, HAr), 8.02 (d, J = 8.0 Hz, 1H, HAr); 13C-NMR (75 MHz,
A solution of O-propagylated salicyclaldehyde (1 mmol),
active methylene compound (1.2 mmol), CuI (30 % mol)
and Et3N (20 % mol) in 2 ml of TFE was stirred for
1 3

J IRAN CHEM SOC
Scheme 2 The reaction of
compound (4) with aryl pyra-
zolone (2b) in the presence of
CuI (30 %) in TFE
O
Br
CHO
O
CuI (30%)
N
N
No reaction
Et3N (20%), TFE
r.t
Cl
2b
4
Fig. 1 Energy-level diagram
for molecular orbitals of
heterodiene and dienophile and
comparison of their energy gap
LUMO
HOMO
LUMO
gap
gap
LUMO
HOMO
E
HOMO
TS
Α (4 π)
Β (2π)
Α (4 π)
HOMO Control (normal)
TS
LUMO Control (Inverse Electron Demand)
DMSO-d6): δ = 28.6, 29.5, 30.0, 67.6, 85.2, 111.0, 118.0,
124.0, 124.5, 128.0, 136.4, 140.8, 150.5, 154.4, 159.8,
163.6; HR-MS (EI): C16H13N3O6 [M]+ found 343.0805, cal.
343.0804. Elemental analysis (C16H13N3O6): cal. C 55.98, H
3.82, N 12.24, found C 55.75, H 3.69, N 12.08 %.
Compound 3 m
m.p 195–196 °C. IR (KBr, cm−1): 1,698, 1,636, 1,589,
1,480; 1H-NMR (300 MHz, DMSO-d6): 4.64 (d, 1H,
J = 11.5 Hz, −OCH), 4.73 (d, 1H, J = 11.5 Hz, −OCH),
4.74 (s, 1H, CH), 6.72 (d, 1H, J = 8.7 Hz, H–Ar), 7.26–7.30
(m, 2H, H–Ar), 7.32 (brs, 1H, = CH), 7.44–7.53 (m, 3H,
H–Ar), 7.75 (brs, 1H, H–Ar); 13CNMR (75 MHz, DMSO-
d6): 28.8, 65.3, 106.7, 111.4, 111.6, 118.5, 118.8, 121.4,
127.3, 130.4, 130.5, 130.6, 130.7, 132.8, 135.2, 137.1,
152.6, 169.5, HRMS (EI): Calcd C19H11O739Br 365.9892.
Found: 365.9920. Calcd C19H11O3 81Br 367.9871. Found:
367.9863.
Compound 3e
m.p. 215–216 °C; IR (KBr, cm−1): ν˜ = 1,686, 1,599, 1,580,
1
1,510; H-NMR (500 MHz, DMSO-d6): δ = 2.48 (s, 3H,
Me), 4.97 (s, 2H, OCH and -CH), 5.01(s, 1H, OCH), 7.00
(d, J = 9.0 Hz, 1H, HAr), 7.24 (s, 1H, = CH), 7.37 (dd,
J = 6.5, 1 Hz, 1H, HAr), 7.51 (t, J = 8.0 Hz, 1H, HAr), 7.67
(dd, J = 8.1 Hz, 1H, HAr), 7.74 (t, J = 2.0 Hz, 1H, HAr),
7.87 (d, J = 1.8, 1H, HAr), 8.05 (dd, J = 9.0, 2.8 Hz, 1H,
HAr); 13C-NMR (125 MHz, DMSO-d6): δ = 13.9, 30.6,
66.3, 67.6, 95.5, 108.3, 117.8, 118.7, 119.9, 121.9, 124.1,
126.2, 127.7, 131.0, 133.6, 136.9, 138.5, 140.3, 145.9,
146.6, 159.2; HR-MS (EI): C20H14N3O345Cl [M]+· found
395.0663, cal. 395.0673, C20H14N3O347Cl [M + 2]+· found
397.0622, cal. 397.0643.
Acknowledgments Saeed Balalaie thanks Alexander von Humboldt
foundation for the research fellowship.
References
1. l.F. Tietze, g. Brasche, K.M. gericke, Domino reactions in
organic synthesis (Wiley, Weinheim, 2006)
1 3

J IRAN CHEM SOC
2. l. F.Tietze, F. Hauner, in Stimulating Concepts in Chemistry,
ed by M. Shibasaki, J. F. Stoddart (Wiely, Weinheim, 2000), pp.
39–64
3. P. Tundo, A. Perosa, F. Zecchini, Methods and reagents for green
chemistry (Wiley, Hoboken, 2007)
4. K.C. Majumdar, A. Taher, R.K. Nandi, Tetrahedron 68, 5693–
5718 (2012)
5. l. F. Tietze, N. Rackelmann, in Multicomponent Reactions ed by
J. Zhu, H. Bienayme (Wiley, Weinheim, 2005), pp. 121–167
6. l.F. Tietze, N. Rackelmann, Pure Appl. Chem. 76, 1967–1983
(2004)
22. M. Mollazadeh, M.J. Khoshkholgh, S. Balalaie, F. Rominger,
H.R. Bijanzadeh, J. Heterocycl. Chem. 47, 1200–1208 (2010)
23. M.J. Khoshkholgh, M. lotfi, S. Balalaie, F. Rominger, Tetrahe-
dron 65, 4228–4234 (2009)
24. S. Balalaie, A. Poursaeed, M.J. Khoshkholgh, H.R. Bijanzadeh,
E. Wolf Comptes Rendus Chemie 15, 283–289 (2012)
25. M.J. Khoshkholgh, M.R. Hosseindokht, S. Balalaie, M.R. Bozorg-
mehr, H.R. Bijanzadeh, H.R. Helv, Chim. Acta 95, 52–60 (2012)
26. S. Balalaie, J. Azizian, A. Shameli, H.R. Bijanzadeh, Synth.
Commun. 43, 1787–1795 (2013)
27. R. Yanada, S. Obika, Y. Kobayashi, T. Inokuma, M. Oyama, K.
Yanada, Y. Takemoto, Adv. Synth. Catal. 347, 1632–1642 (2005)
28. M. Arthuis, R. Pontikis, J.-C. Florent, Tetrahedron lett. 48,
6397–6400 (2007)
7. l.F. Tietze, Chem. Rev. 96, 115–136 (1996)
8. l.F. Tietze, N. Rackelmann, I. Müller, Chem. Eur. J. 10, 2722–
2731 (2004)
9. l. T. Tietze, N. Rackelman, in Multicomponent Reactions, ed by
J. Zhu, H. Bienayme (Wiley, Weinheim, 2005), pp. 121–167
10. Y. Yammamoto, J. Org. Chem. 72, 7817–7831 (2007). and refer-
ences cited therein
11. V. gouverneur, M. Reiter, Chem. Eur. J. 11, 5806–5815 (2005)
12. A. Fürstner, C.C. Stimson, Angew. Chem. Int. Ed. 46, 8845–8849
(2007)
29. l.A. Mag, T.M. Koenig, F.O. ginah, J.D. Copp, D. Mitchel, J.
Org. Chem. 63, 5050–5058 (1998)
30. S. gorsuch, V. Bavetsias, M.g. Rowlands, g.W. Aherne, P. Work-
man, M. Jarman, E. McDonald, Bioorg. Med. Chem. 17, 467–474
(2009)
31. B. gabriele, P. Plastina, g. Salerno, R. Mancuso, Synthesis
4247–4251 (2006)
13. K. C. Majumdar, A. Taher, S. Ponra, Synthesis 4043–4050 (2010)
14. K.C. Majumdar, A. Taher, S. Ponra, Tetrahderon lett. 51, 147–
150 (2010)
32. M.J. Kornet, J. Pharm. Sci. 68, 350–353 (1979)
33. R. Patra, S.B. Maiti, A. Chatterjee, A.K. Chakravarty, Tetrahe-
dron lett. 32, 1363–1366 (1991)
15. M. Kiamehr, F.M. Moghaddam, Tetrahedron lett. 50, 6723–6727
(2009)
16. F.M. Moghaddam, M. Kiamehr, S. Taheri, Z. Mirjafary, Helv.
34. J.-P. Begue, D. Bonnet-Delpon, B. Crousse, Synlett 18–29 (2004)
35. M.E. Kraft, J.A.l. Wright, V.R. Bonaga, Can. J. Chem. 83, 1006–
1016 (2005)
Chim. Acta 93, 964–973 (2010)
36. A. Heydari, S. Khakhsar, M. Tajbakhsh, H.R. Bijanzadeh, J.
17. A.O. Bryhas, Y.I. Horak, Y.V. Ostapiuk, M.D. Obushak, V.S. Mat-
iychuk, Tetrahedron lett. 52, 2324–2326 (2011)
18. M.J. Khoshkholgh, S. Balalaie, R. gleiter, F. Rominger, Tetrahe-
dron 64, 10924–10929 (2008)
19. M.J. Khoshkholgh, S. Balalaie, H.R. Bijanzadeh, F. Rominger,
J.H. gross, Tetrahedron lett. 49, 6968–6971 (2008)
20. M.J. Khoshkholgh, S. Balalaie, H.R. Bijanzadeh, J.H. gross,
Arkivoc ix, 114 (2009)
Fluor. Chem. 130, 609–611 (2009)
37. S. Khakhsar, A. Heydari, M. Tajbakhsh, S.M. Vahdat, J. Flour.
Chem. 131, 1377–1381 (2010)
38. S. Balalaie, H. Motaghedi, D. Tahmassebi, M. Bararjanian, H.R.
Bijanzadeh, E. Wolf Tetrahedron lett. 53, 6177–6181 (2012)
39. S. Khakhsar, A. Heydari, M. Tajbakhsh, H.R. Bijanzadeh, J.
Flour. Chem. 131, 106–110 (2010)
40. J.W. Wijnen, S. Zavarise, J.B.F.N. Engberts, J. Org. Chem. 61,
2001–2005 (1996)
21. M. J. Khoshkholgh, S. Balalaie, H. R. Bijanzadeh, J. H. gross,
Synlett 55–58 (2009)
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