Novel one-pot access to 2-formyl/acetyl-1-substituted pyrrolo[2,3-b] quinoxalines under Sonogashira reaction conditions

Article abstract of DOI:10.1007/s00706-012-0887-1

A one-pot reaction of N-alkyl-3-chloroquinoxaline-2-amines with acetylenic alcohols in the presence of a Pd-Cu catalyst leads to the formation of 2-formyl/acetyl-N-substituted pyrrolo[2,3-b]quinoxalines in good to high yields. A possible mechanism for the conversion has also been proposed.

Full text of DOI:10.1007/s00706-012-0887-1

Monatsh Chem (2013) 144:859–863
DOI 10.1007/s00706-012-0887-1
ORIGINAL PAPER
Novel one-pot access to 2-formyl/acetyl-1-substituted
pyrrolo[2,3-b]quinoxalines under Sonogashira reaction conditions
•
Ali Keivanloo Mohammad Bakherad
•
•
Mahrokh Rahmani Amin Rahimi
Received: 18 February 2012 / Accepted: 22 November 2012 / Published online: 15 January 2013
Ó Springer-Verlag Wien 2013
Abstract
A
one-pot reaction of N-alkyl-3-chloro-
We were intrigued by the prospect of applying this meth-
odology to the synthesis of new heterocyclic compounds.
Although a few methods have been reported [21, 22] for
the synthesis of pyrrolo[2,3-b]quinoxalines, a survey of the
literature showed that there are no references concerning
the use of acetylenic alcohols as a starting material. In
continuation of our recent studies [23–26] on the Pd-cata-
lyzed reaction of acetylenes leading to new heterocyclic
compounds, and in particular pyrrolo[2,3-b]quinoxalines
[27], we decided to investigate the reaction of N-alkyl-3-
chloroquinoxaline-2-amines 1a–1h with acetylenic alco-
hols 2a, 2b in organic solvents.
quinoxaline-2-amines with acetylenic alcohols in the
presence of a Pd–Cu catalyst leads to the formation of
2-formyl/acetyl-N-substituted pyrrolo[2,3-b]quinoxalines
in good to high yields. A possible mechanism for the
conversion has also been proposed.
Keywords Acetylenic alcohol Á Pyrroloquinoxaline Á
Cross-coupling Á Pd–Cu catalyst
Introduction
Quinoxalines and their derivatives are a very important
class of nitrogen-containing heterocycles showing various
biological activities, and can act as antiviral, antibacterial,
antibiotic, anti-inflammatory, and kinase inhibitors [1–7].
They are potential building blocks for the synthesis of
organic semiconductors [8], electroluminescent materials
[9], cavitands [10, 11], and dehydroannulenes [12]. Quin-
oxalines also serve as useful rigid subunits in macrocyclic
receptors [13], for molecular recognition, and chemically
controllable switches [14].
Results and discussion
When compounds 1a–1h were reacted with acetylenic
alcohols 2a, 2b in dry morpholine in the presence of
Pd(PPh3)2Cl2 and CuI at 70 °C, 2-formyl/acetyl-N-substi-
tuted pyrrolo[2,3-b]quinoxalines 3a–3l were obtained in
moderate to good yields (Scheme 1). The reactions were
carried out under an argon atmosphere, and solvents were
degassed prior to use.
The Pd-catalyzed cross-coupling reactions of aryl
halides or triflates with terminal alkynes, commonly
referred to as Sonogashira reactions, are powerful, versa-
tile, and popular tools for selective construction of new
C–C bonds [15, 16]. This strategy with subsequent cycli-
zation has been successfully utilized for the synthesis of
carbocylic [17] and heterocyclic compounds alike [18–20].
We recently reported the synthesis of pyrrolo[2,3-b]qui-
noxaline-2-carbaldehydes from the reaction of N-alkyl-3-
chloroquinoxaline-2-amines with propargyl bromide [28].
In this article we report the use of acetylenic alcohols
instead; this has many advantages over propargyl bromide,
which is not only an expensive chemical but also a very
toxic, odorous, and unstable starting material with high
vapor pressure.
For optimization of the reaction conditions, the reaction
of 1b with propargyl alcohol 2a was chosen as a model
reaction. The effect of solvents and bases was also exam-
ined. Several solvents and bases which were screened for
A. Keivanloo (&) Á M. Bakherad Á M. Rahmani Á A. Rahimi
School of Chemistry, Shahrood University of Technology,
Shahrood, Iran
e-mail: akeivanloo@yahoo.com; keivanloo@shahroodut.ac.ir
123

860
A. Keivanloo et al.
Scheme 1
1
N
Cl
R
N
O
Pd(Ph3P)2Cl2, CuI
HC
+
1
N
dry morpholine
70 °C
N
N
NH
R
R
OH
R
1a-1h
2a-2b
3a-3l
R= Bn, Me, Et, n-Pr, i-Pr, n-Bu, i-Bu, MeOCH CH
2
R1= H, Me
2
the reaction in the presence of a catalytic amount of
Pd(PPh3)2Cl2 and CuI are shown in Table 1.
Table 2 Synthesis of 2-formyl/acetyl-N-substituted pyrrolo[2,3-b]quino-
xalines
Amongst the various solvents and bases which were
tested, cyclic secondary amines such as morpholine and
pyrrolidine were the most suitable solvents and bases
(Table 1, entries 15 and 16).
Entry
R
R1
Product
Yield/%
1
Bn
H
3a
3b
3c
3d
3e
3f
93
88
80
85
68
76
70
65
75
80
74
83
2
Me
H
3
Me
Me
H
In order to explore the scope and generality of this
protocol, several N-alkyl-3-chloroquinoxalin-2-amines
1a–1h were prepared [28], and then reacted with acetylenic
alcohols 2a, 2b to afford 2-formyl/acetyl-N-substituted
pyrrolo[2,3-b]quinoxalines 3a–3l in moderate to good
yields (Table 2).
4
Et
5
n-Pr
Me
H
6
n-Pr
7
i-Pr
H
3g
3h
3i
8
i-Pr
Me
H
9
n-Bu
i-Bu
The structural assignments of compounds 3a–3l were
based on the elemental analyses and the following spectral
1
data. In H NMR spectra of compounds 3a–3l, a singlet
10
11
12
H
3j
Et
Me
H
3k
3l
aromatic proton appeared at 7.55–7.92 ppm, which was
characteristic of a fused pyrrole ring. For 2-formyl com-
pounds an unexchangeable peak at 10.21–10.25 ppm was
assigned to the aldehyde proton, and for 2-acetyl com-
pounds a singlet for methyl protons was observed at
2.76–2.78 ppm. Quinoxaline ring protons in all compounds
MeOCH2CH2
appeared at 7.55–8.23 ppm. The IR spectra of all com-
pounds 3a–3l showed a strong absorption band at
1,668–1,685 cm-1 which was assigned to conjugated car-
bonyl groups. Therefore spectral and elemental analyses
data strongly supported the synthesis of 2-formyl/acetyl-N-
substituted pyrrolo[2,3-b]quinoxalines 3a–3l.
Table 1 Effect of solvent and base on the heterocyclization via
cross-coupling of compound 1b with propargyl alcohol (2a)
Entry
Solvent
Base
Yield/%
The following steps could be postulated for the mech-
anism of the formation of 2-formyl/acetyl-N-substituted
pyrrolo[2,3-b]quinoxalines 3a–3l (Scheme 2) [29]: (1)
standard Sonogashira cross-coupling reaction between N-
alkyl-3-chloroquinoxalin-2-amine 1 and acetylenic alco-
hols 2 gives rise to [A], (2) ring closure by hydroamination/
cyclization leads to compound [B], (3) aerial oxidation
gives the aromatic products 3a–3l. It seems that aerial
oxidation took place during aqueous work-up and column
chromatography.
1
DMF
Et3N
22
33
45
25
28
50
72
68
42
65
58
70
63
67
88
78
2
DMF
DIEA
3
DMF
Morpholine
Et2NH
Pyrrolidine
Et3N
4
DMF
5
DMF
6
CH3CN
CH3CN
CH3CN
CH3CN
Dioxane
Dioxane
Dioxane
Et3N
7
Morpholine
Pyrrolidine
Et2NH
Et3N
8
9
Control of the reaction temperature was critical.
Because when the temperature rose to 100 °C, nucleophilic
displacement of the chlorine atom on the aromatic ring was
observed, and N-benzyl-3-(morpholin-4-yl)quinoxalin-2-
amine (4) was formed as the major product (Scheme 3).
10
11
12
13
14
15
16
Morpholine
Pyrrolidine
–
Et2NH
Morpholine
Pyrrolidine
–
1
But on the basis of H NMR data, 15 % of 1-benzyl-1H-
pyrrolo[2,3-b]quinoxaline-2-carbaldehyde (3a) was still
produced under the reaction conditions used.
–
–
123

Novel one-pot access to 2-formyl/acetyl-1-substituted pyrrolo[2,3-b]quinoxalines
861
1
1
1
Scheme 2
R
R
R
CuI
0
Pd(PPh )Cl
(
)
2
3
2
+
Pd
C
+
OH
Base
HO
OH
2
1
R
R1
C
OH
CCu
N
N
N
N
Cl
N
Pd-Cl
HO
0
Pd
+
NH
R
(i)
NH
R
N
NH
R
[A]
1
cyclization
(ii)
N
N
N
N
O
R
OH
Oxidation
1
1
N
R
N
R
R
(iii)
[B]
3a-3l
Scheme 3
O
O
N
N
N
N
Cl
O
H
N
N
N
100 °C
Pd - Cu
C
CH
+
+
+
N
NH
N
H
NH
HO
4
Conclusion
heated at 70 °C with stirring for 20 h. After completion of
the reaction, the resulting solution was concentrated in
vacuum, then 5 cm3 hydrochloric acid (10 %) was added,
and the mixture was extracted with CHCl3. The chloroform
layer was separated, washed with water, and dried over
anhydrous magnesium sulfate. Concentration of the solu-
tion gave the crude product which was subjected to column
chromatography using CHCl3/CH3OH (98:2) as eluent to
afford an analytically pure product.
We have developed a one-pot reaction for the synthesis of
2-formyl/acetyl-N-substituted pyrrolo[2,3-b]quinoxalines
via Pd-catalyzed reaction of readily available N-alkyl-3-
chloroquinoxaline-2-amines with acetylenic alcohols in dry
morpholine.
Experimental
1-Benzyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde
(3a, C18H13N3O)
All the reagents used were of general reagent grade. IR
spectra were obtained from potassium bromide pellets in
the range of 400–4,000 cm-1 on a Shimadzu model 460
M.p.: 180–182 °C; Rf = 0.55; 1H NMR (500 MHz,
DMSO-d6): d = 5.91 (s, 2H, CH2), 7.16 (d, J = 7.3 Hz,
2H, 2CH), 7.23 (t, J = 6.9 Hz, 1H, CH), 7.29 (t,
J = 7.2 Hz, 2H, 2CH), 7.81 (t, J = 7.7 Hz, 1H, CH),
7.87 (t, J = 7.7 Hz, 1H, CH), 7.92 (s, 1H, CH pyrrole),
8.16 (d, J = 8.5 Hz, 1H, CH), 8.25 (d, J = 8.4 Hz, 1H,
CH), 10.21 (s, 1H, CHO) ppm; 13C NMR (125 MHz,
DMSO-d6): d = 185.4, 143.3, 142.0, 141.5, 140.7, 140.1,
137.8, 130.0, 129.5, 128.5, 128.2, 127.7, 127.3, 126.6,
1
spectrometer. H NMR spectra were recorded on a Bruker
BRX 500 AVANCE spectrometer. Elemental analyses
were performed on a Thermo Finnigan Flash EA micro-
analyzer.
General procedure for the preparation of 2-formyl/
acetyl-N-substituted pyrrolo[2,3-b]quinoxalines 3a–3l
ꢀ
113.6, 45.8 ppm; IR (KBr): m = 1,675 (C=O), 1,525,
1,433, 1,090, 750 cm-1
.
A mixture of N-alkyl-3-chloroquinoxaline-2-amines 1
(0.56 mmol), 19.6 mg (PPh3)2PdCl2 (0.028 mmol), and
10.7 mg CuI (0.056 mmol) was stirred in 2 cm3 dry mor-
pholine under an argon atmosphere. Acetylenic alcohol 2
(1.12 mmol) was then added slowly, and the mixture was
1-Methyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde
(3b, C12H9N3O)
M.p.: 176–177 °C; Rf = 0.52; 1H NMR (500 MHz,
DMSO-d6): d = 4.15 (s, 3H, CH3), 7.76 (s, 1H, CH
123

862
A. Keivanloo et al.
pyrrole), 7.77 (t, J = 7.6 Hz, 1H, CH), 7.87 (t, J = 7.7 Hz,
1H, CH), 8.16 (d, J = 8.5 Hz, 1H, CH), 8.22 (d,
J = 8.5 Hz, 1H, CH), 10.22 (s, 1H, CHO) ppm; 13C
NMR (125 MHz, DMSO-d6): d = 185.1, 148.6, 147.6,
143.7, 142.6, 141.4, 130.3, 128.9, 125.8, 125.1, 112.7,
1-Isopropyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde
(3g, C14H13N3O)
M.p.: 134–135 °C; Rf = 0.54; 1H NMR (500 MHz,
DMSO-d6): d = 1.73 (d, J = 6.9 Hz, 6H, 2CH3), 5.73
(m, J = 7.4 Hz, 1H, CH), 7.81 (t, J = 7.6 Hz, 1H, CH),
7.85 (s, 1H, CH pyrrole), 7.87 (t, J = 7.7 Hz, 1H, CH),
8.15 (d, J = 8.5 Hz, 1H, CH), 8.22 (d, J = 8.5 Hz, 1H,
CH), 10.23 (s, 1H, CHO) ppm; 13C NMR (125 MHz,
DMSO-d6): d = 185.0, 148.2, 145.4, 143.3, 141.3, 140.1,
ꢀ
30.6 ppm; IR (KBr): m = 1,680 (C=O), 1,454, 1,095,
777 cm-1
.
2-Acetyl-1-methyl-1H-pyrrolo[2,3-b]quinoxaline
(3c, C13H11N3O)
130.8, 129.1, 126.0, 125.2, 116.2, 48.8, 20.6 ppm; IR
-1
M.p.: 169–171 °C; Rf = 0.53; 1H NMR (500 MHz,
CDCl3): d = 2.77 (s, 3H, CH3), 4.23 (s, 3H, CH3), 7.71
(s, 1H, CH pyrrole), 7.68–7.80 (m, 2H, 2CH), 8.16 (d,
J = 8.0 Hz, 1H, CH), 8.22 (d, J = 8.1 Hz, 1H, CH) ppm;
13C NMR (125 MHz, DMSO-d6): d = 188.7, 146.8, 144.5,
143.7, 143.2, 141.5, 130.3, 128.9, 125.7, 125.1, 112.2,
ꢀ
(KBr): m = 1,670 (C=O), 1,450, 1,010, 770 cm
.
2-Acetyl-1-isopropyl-1H-pyrrolo[2,3-b]quinoxaline
(3h, C15H15N3O)
M.p.: 124–125 °C; Rf = 0.51; 1H NMR (500 MHz,
CDCl3): d = 1.59 (d, J = 7.3 Hz, 6H, 2CH3), 4.23 (m,
J = 7.1 Hz, 1H, CH), 7.30 (s, 1H, CH pyrrole), 7.68–7.79
(m, 2H, CH), 8.16 (d, J = 7.8 Hz, 1H, CH), 8.28 (d,
J = 7.9 Hz, 1H, CH) ppm; 13C NMR (125 MHz, DMSO-
d6): d = 189.1, 149.5, 148.2, 145.1, 143.6, 140.5, 131.1,
128.8, 125.8, 125.2, 115.3, 50.5, 27.3, 20.6 ppm; IR (KBr):
ꢀ
30.4, 27.3 ppm; IR (KBr): m = 1,668 (C=O), 1,442, 1,210,
749 cm-1
.
1-Ethyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde
(3d, C13H11N3O)
M.p.: 130–132 °C; Rf = 0.54; 1H NMR (500 MHz,
CDCl3): d = 1.48 (t, 3H, J = 7.2 Hz, CH3), 4.86 (q,
J = 7.2 Hz, 2H, CH2), 7.55 (s, 1H, CH pyrrole), 7.73 (t,
J = 7.7 Hz, 1H, CH), 7.81 (t, J = 7.6 Hz, 1H, CH), 8.17
(d, J = 8.3 Hz, 1H, CH), 8.25 (d, J = 8.4 Hz, 1H, CH)
ppm; 13C NMR (125 MHz, DMSO-d6): d = 185.2, 149.2,
144.7, 144.2, 142.5, 138.8, 130.3, 128.6, 125.7, 125.2,
ꢀ
m = 2,900, 1,677 (C=O), 1,400, 1,420, 1,350, 1,210, 1,190,
1,110, 760 cm-1
.
1-Butyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde
(3i, C15H15N3O)
M.p.: 146–147 °C; Rf = 0.52; 1H NMR (500 MHz,
DMSO-d6): d = 0.90 (t, J = 7.5 Hz, 3H, CH3), 1.27 (m,
J = 7.6 Hz, 2H, CH2), 1.79 (m, J = 7.5 Hz, 2H, CH2),
4.71 (t, J = 7.4 Hz, 2H, CH2), 7.82 (t, J = 8.3 Hz, 1H,
CH), 7.86 (s, 1H, CH pyrrole), 7.89 (t, J = 8.9 Hz, 1H,
CH), 8.16 (d, J = 8.6 Hz, 1H, CH), 8.23 (d, J = 8.1 Hz,
1H, CH), 10.23 (s, 1H, CHO) ppm; 13C NMR (125 MHz,
DMSO-d6): d = 184.8, 150.0, 145.8, 143.4, 141.2, 129.9,
ꢀ
115.8, 39.6, 13.6 ppm; IR (KBr): m = 1,680 (C=O), 1,460,
1,095, 780 cm-1
.
2-Acetyl-1-propyl-1H-pyrrolo[2,3-b]quinoxaline
(3e, C15H15N3O)
M.p.: 129–130 °C; Rf = 0.55; 1H NMR (500 MHz, DMSO-
d6): d = 0.85 (t, J = 7.6 Hz, 3H, CH3), 1.77 (m,
J = 7.5 Hz, 2H, CH2), 2.76 (s, 3H, CH3), 4.63 (t,
J = 7.4 Hz, 2H, CH2), 7.73 (m, 3H, CH and CH pyrrole),
8.14 (d, J = 8.4 Hz, 1H, CH), 8.20 (d, J = 8.3 Hz, 1H, CH)
ppm; 13C NMR (125 MHz, DMSO-d6): d = 189.5, 147.3,
146.6, 146.1, 145.7, 139.3, 130.5, 128.9, 125.7, 125.2, 113.2,
128.9, 124.9, 124.2, 114.9, 44.0, 30.3, 20.7, 14.0 ppm; IR
-1
ꢀ
(KBr): m = 1,675 (C=O), 1,450, 1,100, 770 cm
.
1-(2-Methylpropyl)-1H-pyrrolo[2,3-b]quinoxaline-2-carb-
aldehyde (3j, C15H15N3O)
M.p.: 142–143 °C; Rf = 0.54; 1H NMR (500 MHz,
DMSO-d6): d = 0.83 (d, J = 6.6 Hz, 6H, 2CH3), 2.30
(m, J = 6.8 Hz, 1H, CH), 4.51 (d, J = 7.5 Hz, 2H, CH2),
7.77 (t, J = 7.3 Hz, 1H, CH), 7.81 (s, 1H, CH pyrrole),
7.86 (t, J = 7.4 Hz, 1H, CH), 8.12 (d, J = 8.5 Hz, 1H,
CH), 8.21 (d, J = 8.4 Hz, 1H, CH), 10.24 (s, 1H, CHO)
ppm; 13C NMR (125 MHz, DMSO-d6): d = 185.1, 148.9,
146.8, 146.0, 143.5, 140.6, 130.5, 128.7, 125.8, 125.3,
ꢀ
49.2, 27.3, 21.5, 12.2 ppm; IR (KBr): m = 1,675 (C=O),
1,440, 1,418, 1,350, 1,290, 1,150, 745 cm-1
.
1-Propyl-1H-pyrrolo[2,3-b]quinoxaline-2-carbaldehyde
(3f, C14H13N3O)
M.p.: 137–138 °C; Rf = 0.53; 1H NMR (500 MHz,
DMSO-d6): d = 0.88 (t, J = 7.4 Hz, 3H, CH3), 1.81 (m,
J = 7.3 Hz, 2H, CH2), 4.67 (t, J = 7.2 Hz, 2H, CH2), 7.80
(t, J = 7.6 Hz, 1H, CH), 7.84 (s, 1H, CH pyrrole), 7.87 (m,
1H, CH), 8.14 (d, J = 8.5 Hz, 1H, CH), 8.21 (d,
J = 8.5 Hz, 1H, CH), 10.23 (s, 1H, CHO) ppm; 13C
NMR (125 MHz, DMSO-d6): d = 184.9, 147.0, 145.3,
143.2, 140.4, 131.3, 128.9, 125.7, 125.1, 115.8, 47.2, 21.3,
ꢀ
114.9, 52.5, 26.1, 20.2 ppm; IR (KBr): m = 1,680 (C=O),
1,445, 1,095, 775 cm-1
.
2-Acetyl-1-ethyl-1H-pyrrolo[2,3-b]quinoxaline
(3k, C14H13N3O)
M.p.: 150–151 °C; Rf = 0.50; 1H NMR (500 MHz,
CDCl3): d = 1.44 (t, 3H, J = 7.5 Hz, CH3), 4.82 (q,
J = 7.3 Hz, 2H, CH2), 7.35 (s, 1H, CH pyrrole), 7.55–7.79
ꢀ
11.8 ppm; IR (KBr): m = 1,678 (C=O), 1,447, 1,110,
770 cm-1
.
123

Novel one-pot access to 2-formyl/acetyl-1-substituted pyrrolo[2,3-b]quinoxalines
863
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(m, 2H, 2CH), 8.18 (d, J = 7.9 Hz, 1H, CH), 8.21 (d,
J = 8.0 Hz, 1H, CH) ppm; 13C NMR (125 MHz,
DMSO-d6): d = 190.1, 147.6, 146.7, 146.5, 145.8, 139.5,
130.6, 129.3, 125.7, 125.1, 113.6, 41.1, 27.3, 13.6 ppm; IR
ꢀ
(KBr): m = 2,900, 1,676 (C=O), 1,448, 1,260, 1,100,
750 cm-1
.
1-(2-Methoxyethyl)-1H-pyrrolo[2,3-b]quinoxaline-2-carb-
aldehyde (3l, C14H13N3O2)
M.p.: 126–127 °C; Rf = 0.53; 1H NMR (500 MHz,
CDCl3): d = 3.32 (s, 3H, CH3), 3.82 (t, J = 5.6 Hz, 2H,
CH2), 5.08 (t, J = 5.6 Hz, 2H, CH2), 7.56 (s, 1H, CH
pyrrole), 7.72 (t, J = 6.4 Hz, 1H, CH), 7.80 (t, J = 6.8 Hz,
1H, CH), 8.18 (d, J = 8.8 Hz, 1H, CH), 8.22 (d,
J = 8.0 Hz, 1H, CH), 10.21 (s, 1H, CHO) ppm; 13C
NMR (125 MHz, DMSO-d6): d = 184.9, 150.0, 145.9,
143.4, 140.1, 131.5, 129.1, 125.9, 125.3, 114.9, 72.9, 57.8,
ꢀ
41.8 ppm; IR (KBr): m = 1,680 (C=O), 1,465, 1,098,
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(2008) Tetrahedron Lett 49:3819
N-Benzyl-3-(morpholin-4-yl)quinoxalin-2-amine
(4, C19H20N4O)
1H NMR (500 MHz, DMSO-d6): d = 3.23 (m, 4H, 2CH2),
3.85 (m, 4H, 2CH2), 4.70 (d, J = 6.0 Hz, 2H, CH2), 7.18–
7.39 (m, 8H, ArH and NH), 7.47 (d, J = 8.5 Hz, 1H, CH),
7.60 (d, J = 8.0 Hz, 1H, CH) ppm; 13C NMR (125 MHz,
DMSO-d6): d = 151.5, 141.5, 139.9, 137.1, 136.8, 129.8,
128.3, 127.8, 127.3, 123.7, 123.6, 65.8, 45.8, 43.6 ppm; IR
-1
ꢀ
(KBr): m = 3,400 (NH), 1,560, 1,500, 1,085 cm
.
Acknowledgments We are grateful to the Research Council of
Shahrood University of Technology for the financial support of this
work.
26. Bakherad M, Isfahani HN, Keivanloo A, Sang G (2008) Tetra-
hedron Lett 49:6188
27. Keivanloo A, Bakherad M, Rahimi A, Taheri SAN (2010) Tet-
rahedron Lett 51:2409
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