Guanidine Hydrochloride: A Highly Efficient Organocatalyst for the Synthesis of 12-Aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-ones under Solvent-Free Conditions

Article abstract of DOI:10.1002/jhet.1879

A new green protocol has been developed for the synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones using guanidine hydrochloride as an organocatalyst under solvent-free conditions. Operational simplicity, mild reaction conditions, enhanced r

Full text of DOI:10.1002/jhet.1879

May 2016
Guanidine Hydrochloride: A Highly Efficient Organocatalyst for the Synthesis
of 12-Aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-ones Under
Solvent-Free Conditions
981
a
b
b
*
Mahdieh Sadeghpour, Abolfazl Olyaei, and Mortaza Rezaei
aDepartment of Chemistry, Takestan Branch, Islamic Azad University, Takestan, Iran
bDepartment of Chemistry, Payame Noor University, PO BOX 19395-3697, Tehran, Iran
*
E-mail: m.sadeghpour@tiau.ac.ir
Received August 1, 2012
DOI 10.1002/jhet.1879
Published online 12 October 2015 in Wiley Online Library (wileyonlinelibrary.com).
A new green protocol has been developed for the synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]
xanthen-11-ones using guanidine hydrochloride as an organocatalyst under solvent-free conditions.
Operational simplicity, mild reaction conditions, enhanced rates, high isolated yields of the pure
products, and purification of products by nonchromatographic methods are significant advantages of the
protocol presented here.
J. Heterocyclic Chem., 53, 981 (2016).
INTRODUCTION
Organocatalysts have been used widely in many reactions
as mono and bifunctional catalysts because of economic and
environmental considerations. Among many organocatalysts,
hydrogen-bonding compounds such as guanidine derivatives
are becoming powerful tools for activation of the carbonyl
functionality in organic transformations. Recently, guanidi-
nium salts have been successfully employed as novel chiral
phase-transfer catalyst in the conjugate addition of nitroalk-
anes with enones [16]. Moreover, these organocatalysts
provide an environment to the process activating the nucleo-
phile, the electrophile, or both reagents through weak
interactions, such as hydrogen bonding or ion pairing or much
stronger interactions such as covalent bonding.
Because of our interest in developing solvent-free multi-
component reactions [17–19], we report here a simple and
facile protocol for the synthesis of a series of 12-aryl-
8,9,10,12-tetrahydrobenzo[a]xanthen-11-one derivatives from
aldehydes, naphthols, and dimedone using guanidine hydro-
chloride as a catalyst under solvent-free conditions. To the best
of our knowledge in the open literature, one-pot synthesis of
xanthenone derivatives catalyzed by guanidine hydrochloride
has not previously been reported.
Xanthenes and benzoxanthenes are important hetero-
cycles that are known to possess multiple biological activi-
ties. Although not widely found in nature, xanthenes and
compounds based on these core templates exhibit a broad
spectrum of pharmaceutical activities such as antibacterial
[1], anti-inflammatory [2], and antiviral [3]. These structural
motifs have also found a niche as antagonists for paralyzing
the action of zoxazolamine [4] and demonstrate efficacy in
photodynamic therapy [5]. In addition, these compounds
have been employed as dyes [6] and pH-sensitive fluorescent
materials for visualization of biomolecular assemblies [7]
and utilized in laser technologies [8]. Thus, a broad utility
range has made xanthenes prime synthetic candidates
thereby accentuating the need to develop newer synthetic
routes for scaffold manipulation of xanthene derivatives.
Previously, tetrahydrobenzo[a]xanthenes have been synthe-
sized under reflux for 4–5 h for long hours in dichloromethane
or 1,2-dichloroethane in the presence of acid catalysts such as
Sr(OTf)2 [9], InCl3 or P2O5 [10], pTSA in ionic liquid [11],
2,4,6-trichloro-1,3,5-triazine [12], BF3.Et2O [13], tetrabuty-
lammonium fluoride (TBAF) [14], and NaHSO4—SiO2 [15].
These synthetic methods afforded good yields but have
limitations of long reaction time, harsh reaction conditions,
and often expensive catalysts. Moreover, the synthesis has
been usually carried out in the solvent leading to complex
isolation and recovery procedures. Consequently, the devel-
opment of new and simple synthetic methods such as further
improvement towards lower reaction time and improved
yields for the preparation of heterocyclic compounds con-
taining xanthene fragment remains an interesting challenge.
RESULTS AND DISCUSSION
On the basis of the green chemistry approach, we report an
efficient and environmentally benign protocol for the synthe-
sis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-one
derivatives by multicomponent condensation of naphthols,
aromatic aldehydes, and dimedone catalyzed by guanidine
hydrochloride under solvent-free neat conditions. The
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M. Sadeghpour, A. Olyaei, and M. Rezaei
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products were obtained in high to excellent yields by a simple
work-up. The condensation of salicylaldehyde (1.0 mmol),
2-naphthol (1.0 mmol), and dimedone (1.0 mmol) using
10 mol% of catalyst was attempted at various temperatures.
Our investigation demonstrated that 80ꢀC is an effective
temperature in terms of reaction time and yield obtained.
The reaction was then attempted under similar conditions in
the presence of different catalyst concentrations. The reactions
were carried out in the presence of 2, 5, 8, 10, and 15mol%
of catalyst. The yield of the product improved remarkably to
90% after heating the components at 80ꢀC in the presence of
catalytic amount of guanidine hydrochloride. After optimiz-
ing the conditions, several syntheses of 12-aryl-8,9,10,
12-tetrahydrobenzo[a]xanthen-11-one derivatives from the
condensation of 2-naphthol (2) and 2,7-dihydroxynaphtha-
lene (3) with a wide range of aromatic aldehydes (1) and
dimedone utilizing guanidine hydrochloride as organocata-
lyst under solvent-free conditions at 80ꢀC were examined
(Scheme 1). All reactions were complete within 15–60min,
as indicated in Table 1; in all cases, the reactions afforded
the desired products in high to excellent yields.
of the expected xanthenes-11-one derivatives, but the
desirable products were obtained in moderate yields, and the
reaction times are too long. It should be noted that, in these
reactions, steric factors play a major role in the product forma-
tion, and electronic factors play a limited role.
All products were well characterized by 1H-NMR, 13C-NMR,
FTIR, mass spectra, elemental analyses, and melting point.
For example, the 1H-NMR spectrum of synthesized
compounds contained two singlet signals at about d 0.90
and 1.06ppm corresponding to two chemically different
methyl protons. Moreover, methylene protons appeared as
two AB quartet patterns at about d 2.10 and 2.30 ppm
(J = 16.0 Hz) and at d 2.50 and 2.70ppm (J = 17.2 Hz)
corresponding to two chemically different moieties. Methine
and aromatic protons showed as a sharp singlet at about
d 5.50 and multiplet at about d 6.50–7.70 ppm, respectively.
Hydroxyl protons appeared as a sharp singlet at d 9.90ppm.
To the best of our knowledge, synthesis of compounds 5d–g
has not previously been reported in literature.
CONCLUSION
Concerning the reaction mechanism, we proposed that
guanidine hydrochloride is a polyfunctional catalyst. The
catalyst initially acts as a hydrogen-bond donor to activate
aldehyde by formation of six-membered ring with aldehyde
oxygen. Subsequently, the reaction proceeds through the
nucleophilic addition of naphthol to aldehyde that is further
attacked by dimedone. Then, deprotonation of the hydroxyl
group on naphthol occurred by the amine of guanidine
hydrochloride (as the base), and naphthoxy ion is formed.
Finally, the reaction proceeds through the nucleophilic addi-
tion of naphthoxy ion to carbonyl of dimedone, and dehydra-
tion occurred in the presence of hydrochloride of guanidine
hydrochloride (as the acid) to give the final product. Various
aromatic aldehydes containing electron-withdrawing and elec-
tron-donating substituent at ortho, meta, or para-positions
show equalease towards the product formation in good to high
yields. Also, aliphatic aldehydes were used for the production
In summary, a novel and highly efficient method for the syn-
thesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones
by condensation reaction of naphthols with aromatic aldehydes
and dimedone catalyzed by guanidine hydrochloride has been
described. The attractive features of this protocol are simple
experimental procedure, solvent-free reaction conditions, utiliza-
tion of an inexpensive and readily available organocatalyst, short
reaction time, and its adaptability for synthesis of a diverse set of
benzoxanthenone derivatives. To the best of our knowledge, this
is the first report on synthesis of 12-aryl-8,9,10,12-tetrahydro-
benzo[a]xanthen-11-ones with guanidine hydrochloride.
EXPERIMENTAL
All commercially available chemicals and reagents were used
without further purification. Melting points were determined with
Scheme 1. Guanidinium chloride promoted one-pot synthesis of xanthenones (4, 5). [Color figure can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
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Synthesis of Xanthenones
983
Table 1
Guanidine hydrochloride catalyzed one-pot synthesis of xanthenones.
Entry
1
Aldehyde
Product
Time (min)
60
Yield %
90
4a
2
4b
60
88
3
4c
35
92
(Continued)
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Table 1
(Continued)
Entry
4
Aldehyde
Product
Time (min)
30
Yield %
89
4d
4e
4f
5
25
91
6
30
93
(Continued)
Journal of Heterocyclic Chemistry
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Synthesis of Xanthenones
985
Table 1
(Continued)
Entry
7
Aldehyde
Product
Time (min)
15
Yield %
89
5a
5b
5c
8
20
90
9
20
93
(Continued)
Journal of Heterocyclic Chemistry
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M. Sadeghpour, A. Olyaei, and M. Rezaei
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Table 1
(Continued)
Entry
Aldehyde
Product
Time (min)
20
Yield %
10
5d
5e
5f
95
91
88
89
11
12
13
30
25
15
5g
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Synthesis of Xanthenones
987
an Electrothermal model 9100 apparatus (Bibby Scientific
Limited, UK) and are uncorrected. IR spectra were recorded on a
Shimadzu 4300 spectrophotometer (Shimadzu Scientific Instru-
ments, Columbia, MD). The 1H-NMR and 13C-NMR spectra were
recorded in DMSO-d6 on Bruker DRX-400 AVANCE spectro-
meters (Bruker Optics Inc., Billerica, MA). Chemical shifts (d) are
reported in parts per million and are referenced to the NMR solvent.
Mass spectra of the products were obtained with an high performing
Agilent 5973 Mass Selective Detector (Palo Alto, CA). Elemental
analyses were carried out by a Carbon-Hydrogen-Nitrogen
(CHN)-Rapid Heraeus elemental analyzer (Wellesley, MA).
(d, 1H, J= 17.6 Hz, CH2), 3.72 (s, 3H, OCH3), 5.80 (s, 1H,
methine-H), 6.56–8.32 (m, 9H, Ar—H), 8.95 (s, 1H, OH).
12-(4-Chlorophenyl)-9,9-dimethyl-2-hydroxy-8,9,10,12-
tetrahydrobenzo-[a]xanthen-11-one (5a). White solid;
mp= 308–310ꢀC; IR (KBr): 3187 (OH), 1626 (C═O), 1231
1
(C—O—C) cmÀ1; H-NMR (400 MHz, DMSO-d6): 0.87 (s, 3H,
CH3), 1.06 (s, 3H, CH3), 2.12 (d, 1H, J = 16.4Hz, CH2), 2.34
(d, 1H, J = 16.4 Hz, CH2), 2.56 (d, 1H, J = 17.6 Hz, CH2), 2.67
(d, 1H, J = 17.6Hz, CH2), 5.35 (s, 1H, methine-H), 6.97–7.79
(m, 9H, Ar—H), 9.95 (s, 1H, OH) ppm.
12-(4-Methylphenyl)-9,9-dimethyl-2-hydroxy-8,9,10,12-
tetrahydrobenzo-[a]xanthen-11-one (5b). White solid;
mp= 298–300ꢀC; IR (KBr): 3173 (OH), 1625 (C═O), 1228
General procedure for the synthesis of xanthenones (4, 5). To
a mixture of aldehyde (1 mmol), 5,5-dimethyl-1,3-cyclohexanedione
(1 mmol), and naphthol (1 mmol), guanidinium chloride (10 mol%)
was added and heated at 80ꢀC for the appropriate amount of time
as indicated in Table 1. The progress of the reaction was monitored
by TLC. After completion, the reaction mixture was cooled to RT,
and EtOH (5 mL) was added until white solid products precipitated.
The precipitate was filtered, washed with cold ethanol, and dried.
The obtained products 3 were found to be pure upon TLC examination.
9,9-Dimethyl-12-(2-hydroxyphenyl)-8,9,10,12-tetrahydrobenzo-[a]
xanthen-11-one (4a). White solid; mp = 221–227ꢀC; IR (KBr): 3199
1
(C—O—C) cmÀ1; H-NMR (400 MHz, DMSO-d6): 0.88 (s, 3H,
CH3), 1.06 (s, 3H, CH3), 2.11 (d, 1H, J = 16.0Hz, CH2), 2.33
(d, 1H, J = 16.0 Hz, CH2), 2.55 (d, 1H, J = 17.6 Hz, CH2), 2.67
(d, 1H, J = 17.6Hz, CH2), 5.30 (s, 1H, methine-H), 6.90–7.76
(m, 9H, Ar—H), 9.91 (s, 1H, OH) ppm.
12-(4-Methoxyphenyl)-9,9-dimethyl-2-hydroxy-8,9,10,12-
tetrahydrobenzo-[a]xanthen-11-one (5c). White solid;
mp= 296–297ꢀC; IR (KBr): 3190 (OH), 1625 (C═O), 1232
1
(C—O—C) cmÀ1; H-NMR (400 MHz, DMSO-d6): 0.88 (s, 3H,
CH3), 1.06 (s, 3H, CH3), 2.11 (d, 1H, J = 16.4Hz, CH2), 2.33
(d, 1H, J = 16.4 Hz, CH2), 2.55 (d, 1H, J = 17.2 Hz, CH2), 2.67
(d, 1H, J = 17.2 Hz, CH2), 3.64 (s, 3H, OCH3), 5.29 (s, 1H,
methine-H), 6.75–7.76 (m, 9H, Ar—H), 9.20 (s, 1H, OH) ppm.
12-(2-Chlorophenyl)-9,9-dimethyl-2-hydroxy-8,9,10,12-
tetrahydrobenzo-[a]xanthen-11-one (5d). White solid;
mp= 280–282ꢀC; IR (KBr): 3396 (OH), 1628 (C═O), 1228
1
(OH), 1625 (C═O), 1228 (C—O—C) cmÀ1; H-NMR (400 MHz,
DMSO-d6): 0.94 (s, 3H, CH3), 1.08 (s, 3H, CH3), 2.11 (d, 1H,
J= 16.4 Hz, CH2), 2.35 (d, 1H, J= 16.4 Hz, CH2), 2.58 (d, 1H,
J= 17.2 Hz, CH2), 2.72 (d, 1H, J= 17.2 Hz, CH2), 5.75 (s, 1H,
methine-H), 6.60–8.33 (m, 10H, Ar—H), 9.69 (s, 1H, OH) ppm.
9,9-Dimethyl-12-(4-bromophenyl)-8,9,10,12-tetrahydrobenzo-[a]
xanthen-11-one (4b). White solid; mp= 182–184ꢀC; IR (KBr):
1
(C—O—C) cmÀ1; H-NMR (400 MHz, DMSO-d6): 0.90 (s, 3H,
1640 (C═O), 1221 (C—O—C) cmÀ1 1H-NMR (400 MHz,
;
CH3), 1.07 (s, 3H, CH3), 2.09 (d, 1H, J = 16.0Hz, CH2), 2.34
(d, 1H, J = 16.0 Hz, CH2), 2.57 (d, 1H, J = 17.2 Hz, CH2), 2.71
(d, 1H, J = 17.2Hz, CH2), 5.62 (s, 1H, methine-H), 7.01–7.77
(m, 9H, Ar—H), 9.92 (s, 1H, OH) ppm; 13C-NMR (100MHz,
DMSO-d6): 26.54, 29.42, 32.18, 33.29, 50.61, 106.02, 112.61,
114.03, 115.18, 117.47, 125.87, 127.56, 128.51, 129.60, 130.16,
130.64, 132.56, 133.34, 142.02, 148.31, 156.85, 157.02, 158.52,
164.75, 196.20 ppm; MS (EI): m/z 406 (M + 2)+, 404 (M)+, 369,
293, 237, 209, 181, 152; Anal. Calcd for C25H21ClO3: C, 74.16;
H, 5.19. Found: C, 74.20; H, 5.23.
DMSO-d6): 0.88 (s, 3H, CH3), 1.07 (s, 3H, CH3), 2.14 (d, 1H,
J = 16.0Hz, CH2), 2.35 (d, 1H, J = 16.0Hz, CH2), 2.59 (d, 1H,
J = 17.2Hz, CH2), 2.70 (d, 1H, J = 17.2 Hz, CH2), 5.58 (s, 1H,
methine-H), 7.24–8.02 (m, 10H, Ar—H).
9,9-Dimethyl-12-(4-chlorophenyl)-8,9,10,12-tetrahydrobenzo-[a]
xanthen-11-one (4c). White solid; mp= 185–187ꢀC; IR (KBr):
1640 (C═O), 1221 (C—O—C) cmÀ1 1H-NMR (400 MHz,
;
DMSO-d6): 0.88 (s, 3H, CH3), 1.07 (s, 3H, CH3), 2.14 (d, 1H,
J = 16.0Hz, CH2), 2.35 (d, 1H, J = 16.0Hz, CH2), 2.59 (d, 1H,
J = 17.6Hz, CH2), 2.70 (d, 1H, J = 17.6 Hz, CH2), 5.59 (s, 1H,
methine-H), 7.24–8.02 (m, 10H, Ar—H).
9,9-Dimethyl-12-(4-methoxyphenyl)-8,9,10,12-tetrahydrobenzo-[a]
xanthen-11-one (4d). White solid; mp= 200–201ꢀC; IR (KBr): 1645
(C═O), 1227 (C—O—C) cmÀ1; 1H-NMR (400 MHz, DMSO-d6):
0.90 (s, 3H, CH3), 1.07 (s, 3H, CH3), 2.10 (d, 1H, J = 16.0Hz,
CH2), 2.34 (d, 1H, J = 16.0 Hz, CH2), 2.58 (d, 1H, J = 17.2Hz,
CH2), 2.69 (d, 1H, J = 17.2 Hz, CH2), 3.63 (s, 3H, OCH3), 5.52 (s,
1H, methine-H), 6.73–8.05 (m, 10H, Ar—H).
12-(2-Hydroxy-3-methoxyphenyl)-9,9-dimethyl-2-hydroxy-
8,9,10,12-tetrahydrobenzo-[a]xanthen-11-one (5e). White
solid; mp= 269–270ꢀC; IR (KBr): 3406 (OH), 1633 (C═O), 1228
1
(C—O—C) cmÀ1; H-NMR (400 MHz, DMSO-d6): 0.91 (s, 3H,
CH3), 1.07 (s, 3H, CH3), 2.10 (d, 1H, J = 16.0Hz, CH2), 2.34
(d, 1H, J = 16.0 Hz, CH2), 2.53 (d, 1H, J = 17.2 Hz, CH2), 2.68
(d, 1H, J = 17.2Hz, CH2), 5.57 (s, 1H, methine-H), 6.57–7.72
(m, 8H, Ar—H), 8.72 (s, 1H, OH), 9.79 (s, 1H, OH) ppm;P
13C-NMR (100MHz, DMSO-d6): 26.64, 28.88, 29.45, 32.10,
32.32, 50.60, 55.92, 106.21, 109.74, 113.32, 114.06, 116.67,
117.33, 119.37, 121.96, 125.84, 128.78, 130.38, 132.02, 133.52,
143.16, 148.12, 156.55, 158.42, 165.14, 197.11ppm; MS (EI):
m/z 416 (M)+, 312, 293, 257, 237, 241, 209, 181, 160, 152, 131;
Anal. Calcd for C26H24O5: C, 75.00; H, 5.77. Found: C, 75.08;
H, 5.71.
9,9-Dimethyl-12-(4-methylphenyl)-8,9,10,12-tetrahydrobenzo-[a]
xanthen-11-one (4e). White solid; mp= 175–177ꢀC; IR (KBr):
1645 (C═O), 1231 (C—O—C) cmÀ1 1H-NMR (400 MHz,
;
DMSO-d6): 0.89 (s, 3H, CH3), 1.07 (s, 3H, CH3), 2.11 (d, 1H,
J = 16.0Hz, CH2), 2.14 (s, 3H, CH3), 2.34 (d, 1H, J = 16.0Hz,
CH2), 2.58 (d, 1H, J = 17.6 Hz, CH2), 2.70 (d, 1H, J = 17.6Hz,
CH2), 5.53 (s, 1H, methine-H), 6.97–8.05 (m, 10H, Ar—H).
9,9-Dimethyl-12-(2-hydroxy-3-methoxyphenyl)-8,9,10,12-
12-(5-Bromo-2-hydroxyphenyl)-9,9-dimethyl-2-hydroxy-
8,9,10,12-tetrahydrobenzo-[a]xanthen-11-one (5f). White
solid; mp= 308–310ꢀC; IR (KBr): 3381 (OH), 1625 (C═O), 1222
tetrahydrobenzo-[a]xanthen-11-one
(4f).
White solid;
1
(C—O—C) cmÀ1; H-NMR (400 MHz, DMSO-d6): 0.94 (s, 3H,
mp = 213–215ꢀC; IR (KBr): 3276 (OH), 1622 (C═O), 1238
1
(C—O—C) cmÀ1; H-NMR (400 MHz, DMSO-d6): 0.94 (s, 3H,
CH3), 1.07 (s, 3H, CH3), 2.11 (d, 1H, J = 16.0Hz, CH2), 2.34
(d, 1H, J = 16.0 Hz, CH2), 2.56 (d, 1H, J = 17.2 Hz, CH2), 2.69
(d, 1H, J = 17.2Hz, CH2), 5.47 (s, 1H, methine-H), 6.66–7.73
CH3), 1.08 (s, 3H, CH3), 2.11 (d, 1H, J= 16.0 Hz, CH2), 2.35
(d, 1H, J= 16.0 Hz, CH2), 2.58 (d, 1H, J= 17.6 Hz, CH2), 2.72
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(m, 8H, Ar—H), 9.88 (s, 1H, OH), 9.94 (s, 1H, OH) ppm;
13C-NMR (100MHz, DMSO-d6): 26.48, 29.47, 29.88, 32.10,
50.60, 106.03, 110.39, 112.42, 114.03, 115.61, 117.33, 117.44,
118.32, 125.83, 129.07, 130.46, 132.99, 133.41, 148.28, 154.01,
156.68, 156.84, 165.22, 196.69 ppm; MS (EI): m/z 466 (M+ 2)+,
464 (M)+, 307, 293, 237, 209, 181, 152; Anal. Calcd for
C25H21BrO4: C, 64.51; H, 4.51. Found: C, 64.53; H, 4.54.
12-(2-Hydroxyphenyl)-9,9-dimethyl-2-hydroxy-8,9,10,12-
tetrahydrobenzo-[a]xanthen-11-one (5g). White solid;
mp= 276–278ꢀC; IR (KBr): 3316 (OH), 1626 (C═O), 1224
1
(C—O—C) cmÀ1; H-NMR (400 MHz, DMSO-d6): 0.92 (s, 3H,
CH3), 1.07 (s, 3H, CH3), 2.10 (d, 1H, J = 16.4 Hz, CH2), 2.34
(d, 1H, J = 16.4 Hz, CH2), 2.55 (d, 1H, J = 17.2Hz, CH2), 2.69
(d, 1H, J = 17.2 Hz, CH2), 5.52 (s, 1H, methine-H), 6.61–7.72
(m, 9H, Ar—H), 9.55 (s, 1H, OH), 9.80 (s, 1H, OH) ppm;
13C-NMR (100MHz, DMSO-d6): 26.61, 29.47, 32.32, 40.86,
50.61, 106.27, 113.17, 114.03, 116.34, 116.43, 117.27, 119.59,
125.86, 127.73, 128.73, 130.38, 130.63, 131.43, 135.52, 148.28,
154.28, 156.55, 165.13, 197.03 ppm; MS (EI): m/z 386 (M)+, 293,
237, 227, 209, 181, 171, 152; Anal. Calcd for C25H22O4: C,
77.72; H, 5.70. Found: C, 77.75; H, 5.78.
[14] Gao, S.; Tsai, C. H.; Yao, C. F. Synlett 2009, 949.
[15] Das, B.; Laxminarayana, K.; Krishnaiah, M.; Srinivas, Y. Synlett
2007, 3107.
[16] (a) Davis, A. P.; Dempsey, K. J. Tetrahedron: Asymmetry
1995, 6, 2829; (b) Allingham, M. T.; Howard-Jones, A.; Murphy, P. J.;
Thomas, D. A.; Caulkett, P. W. R. Tetrahedron Lett 2003, 44, 8677.
[17] Shockravi, A.; Sadeghpour, M.; Olyaei, A. Synth Commun
2009, 39, 2347.
Acknowledgments. The authors thank the Research Council of
Payame Noor University for financial support.
REFERENCES AND NOTES
[18] Olyaei, A.; Chehrehgosha Parashkuhi, E.; Raoufmoghaddam, S.;
Sadeghpour, M. Synth Commun 2010, 40, 3609.
[19] Olyaei, A.; Shams, B.; Sadeghpour, M.; Gesmati, F.;
Razaziane, Z. Tetrahedron Lett 2010, 51, 6086.
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet