Ligand-free, PdCl2(PPh3)2 catalyzed, microwaveassisted Suzuki coupling of 1-chloro-3-phenylisoquinoline in the synthesis of diversified 1,3-disubstituted isoquinolines

Article abstract of DOI:10.1007/s11164-011-0350-7

An efficient ligand-free, microwave-assisted, Pd-catalyzed, Suzuki coupling of 1-chloro-3-phenylisoquinoline and boronic acids, in the presence of sodium carbonate base and 1,4-dioxane solvent, is reported for the synthesis of diversified 1,3-disubstitute

Full text of DOI:10.1007/s11164-011-0350-7

Res Chem Intermed (2012) 38:337–346
DOI 10.1007/s11164-011-0350-7
Ligand-free, PdCl2(PPh3)2 catalyzed, microwave-
assisted Suzuki coupling of 1-chloro-3-
phenylisoquinoline in the synthesis of diversified
1,3-disubstituted isoquinolines
•
K. Prabakaran F. Nawaz Khan Jong Sung Jin
•
Received: 22 March 2011 / Accepted: 16 July 2011 / Published online: 4 August 2011
Ó Springer Science+Business Media B.V. 2011
Abstract An efficient ligand-free, microwave-assisted, Pd-catalyzed, Suzuki coupling
of 1-chloro-3-phenylisoquinoline and boronic acids, in the presence of sodium carbonate
base and 1,4-dioxane solvent, is reported for the synthesis of diversified 1,3-disubstituted
isoquinolines.
Keywords Suzuki coupling Á Ligand-free Á 1,3-disubstituted isoquinolines Á
PdCl2(PPh3)2 Á Boronic acids
Introduction
The 2010 Nobel prize in chemistry was awarded to Prof. Suzuki for his discovery
and development of the Suzuki coupling reaction of an aryl- or vinyl-boronic acid
with an aryl-, vinyl-, or alkyl-halide catalyzed by palladium. It is widely used to
synthesize poly-olefins, styrenes, and substituted biphenyls. A variety of catalysts
and development of the method in this reaction have broadened the applications and
the scope of the reaction. The Suzuki cross-coupling reaction is an extremely
versatile methodology for the generation of carbon–carbon bonds [1–3]. This
reaction has paved a way for the synthetic chemist to develop an efficient
K. Prabakaran Á F. Nawaz Khan (&)
Organic & Medicinal Chemistry Research Laboratory, Organic Chemistry Division,
School of Advanced Sciences, Vellore Institute of Technology (VIT) University,
Vellore 632 014, Tamil Nadu, India
e-mail: nawaz_f@yahoo.co.in
F. Nawaz Khan Á J. S. Jin (&)
Division of High Technology Materials Research, Busan Center, Korea Basic Science Institute
(KBSI), Busan 618 230, Korea
e-mail: jsjin@kbsi.re.kr
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K. Prabakaran et al.
methodology in organic synthesis and also in the total synthesis of targeted
compounds.
The reaction was first reported by Akira Suzuki and his group in 1979 [4, 5]. The
Suzuki reaction has found a wide application in modern synthetic organic chemistry
for the preparation of biaryl compounds [6–9]. Over the years, the Suzuki cross-
coupling has been developed further to include the reaction between boronic acids,
boronate esters, or organoboranes and organic halides or pseudohalides [10]. There
are many advantages of the reaction, e.g., mild reaction conditions, commercial
availability of the environmentally safe boronic acids, easy removal of inorganic
byproducts, stability of boronic acids towards heat, air, and moisture, and tolerance
to a wide variety of functional groups [11]. Byproducts which can be formed are
coupling products of phosphine-bound aryls and self-coupled products. It is
necessary to have a base present during the reaction, since it will be able to form
borates which are more electron-rich than the boronic acid, thereby, facilitating the
transmetallation step [12]. The most commonly used base is Na2CO3, but it is often
ineffective with sterically hindered substrates. Instead, Ba(OH)2 and K3PO4 can be
used in such reactions in order to obtain good yields [13, 14]. The addition of ligands,
which stabilize the metal catalyst, has made it possible to couple aryl chlorides [15]
and hindered substrates, which has presented problems in the past [16].
1,3-disubstituted isoquinoline skeleton is an integral part of many naturally
occurring substances and pharmaceutically important compounds [17–19]. 1,3-
disubstituted isoquinolines have resulted in them being used as building blocks [20]
in pharmaceutical compounds [21], as chiral ligands for the transition metal
catalysts [22], and their iridium complexes in organic light-emitting diodes
(OLEDs) [23]. A number of methods have been developed for the synthesis of 1,3-
disubstituted isoquinolines, including palladium-catalyzed cross-coupling of iodine-
mediated electrophilic cyclization of 2-alkynylbenzaldoximes [24], copper cata-
lyzed tandem synthesis of [1, 2, 3] triazolo [5,1-a]isoquinolines [25], irradiation of
1-methoxy-2-azabuta-1,3-dienes in neutral medium [26], isobenzofuran-nitrile
Diels–Alder reaction [27], light-induced iminyl radical cyclization of acyloximes
[28], and silver [29] and gold [30] catalyzed cyclization of 2-alkynyl benzyl azides.
Disubstituted isoquinolines are endowed with an extensive range of biological
activities [31]. Moreover, they display potent thrombin inhibitory activity and
antibacterial activity against Gram-positive bacteria [32].
In our continued interest on isoquinolines and heterocyclics [33–48], the present
study involved the Suzuki coupling, in order to explore an efficient catalytic system
in the synthesis of diversified 1,3-disubstituted isoquinolines. The optimization
envisaged that the use of PdCl2(PPh3)2 catalyst, Na2CO3 base, and 1,4-dioxane
solvent under aqueous-free microwave irradiation conditions offered greater
efficiency in the synthesis of 1,3-disubstituted isoquinolines.
Results and discussion
In our continued interest of isoquinolines and heterocyclics [33–48], in the present
letter, the coupling of a series of boronic acid and 1-chloroisoquinoline in the
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Ligand-free, PdCl2(PPh3)2 catalyzed, microwave-assisted
339
synthesis of 1,3-disubstituted isoquinoline through ligand-free Suzuki coupling is
reported. The reaction of 1-chloro-3-phenylisoquinoline 1 and boronic acids 2 in
1,4-dioxane in the presence of PdCl2(PPh3)2 catalyst and sodium carbonate base
under microwave irradiation at 150 W at 110 °C for 30 min afforded 1,3-
disubstituted isoquinolines 3 in excellent yields and high purity (Scheme 1;
Table 1).
The optimization of the reaction conditions was carried out by choosing the
Suzuki coupling of 1-chloro-3-phenylisoquinoline 1 and 4-pyridylboronic acid 2a as
the model reaction. The development of optimized conditions included the
screening of various catalysts in the presence of sodium carbonate base, the effect
of ligands, and the effect of bases (Tables 2–4). As shown in Table 2, the reaction
proceeds well in the presence of PdCl2(PPh3)2 catalyst (entry 4), with an excellent
product yield. However, Pd(PPh3)4 and palladium acetate/X-Phos catalysts
produced lower yields (Table 2, entries 5 and 6); the rest of the catalysts were
found to be less effective.
The influence of phosphine ligands in the reaction was then explored by choosing
the Suzuki coupling of 1-chloro-3-phenylisoquinoline 1 with phenylboronic acid 2c
as a model reaction (Scheme 2). The reaction results indicated that a major quantity
of byproduct 3-phenyl isoquinoline-1-ol 4c was formed in the presence of phosphine
ligand, with a lesser amount of required product 3c.
Further, to optimize this reaction condition, variation of the amount of ligand
load and aqueous-free conditions were investigated, which suggested the result as
indicated in Table 3, entries 1–6. From the results, it is clear that the phosphine
ligand and palladium acetate catalytic system is not a suitable catalyst system for the
desired product 3c; however, they are fruitful in achieving the undesired product,
3-phenyl isoquinoline-1-ol 4c.
Further investigations were made to explore the influence of bases on the Suzuki
reaction of 1 and 2a (Table 4). Among the bases tested, sodium carbonate (Table 4,
entry 6) proved to be an excellent base at 110 °C in 1,4-dioxane solvent under
microwave irradiation conditions, while sodium bicarbonate, potassium carbonate,
and cesium carbonate are found to be moderately effective (entries 1, 3, and 5,
R1
Cl
R1B(OH)2 (1.1eq)
2
PdCl2(PPh3)2 (5 mol%)
Na2CO3 (2.0 eq)
N
N
1,4-dioxane
Microwave
110°C/30min
3a-g
1
R1= 4-pyridyl, 4-formylphenyl, phenyl, p-tolyl, 3-methoxyphenyl,
4-biphenyl, thiophene-2-carbonyl
Scheme 1 Synthesis of 1,3-disubstituted isoquinolines
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K. Prabakaran et al.
Yieldb (%)
Table 1 Suzuki coupling of 1-chloro-3-phenylisoquinolines 1 and boronic acids 2a
Entry
Boronic acids 2
Product 3
N
B(OH)2
N
2a
N
1
2
90
73
3a
OHC
B(OH)2
CHO
2b
N
3b
B(OH)2
N
3
4
5
82
87
72
3c
3d
3e
2c
CH3
H3C
B(OH)2
B(OH)2
N
2d
H3CO
OCH3
N
2e
B(OH)2
N
6
7
65
78
3f
2f
CHO
S
B(OH)2
S
2g
OHC
N
3g
a
Reaction conditions: 1 (0.1 mmol), 2 (0.11 mmol), PdCl2(PPh3)2 (5 mol%), Na2CO3 (2.0 eq), 1,4-
dioxane (4 mL) microwave irradiated at 150 W at 110 °C for 30 min
b
Isolated yield
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Ligand-free, PdCl2(PPh3)2 catalyzed, microwave-assisted
341
Table 2 Effect of catalysts in the Suzuki coupling of 1 with boronic acid 2a
Entry
Catalyst
Base
Water
Yielda (%)
1
2
3
4
5
6
Pd(PPh3)4
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
–
55
30
25
90
20
22
PdCl2(dppf)2
–
Pd2(dba)3
–
PdCl2(PPh3)2
–
Pd(OAc)2/(X-Phos, 0.1 eq)
Pd(OAc)2/(ruphos, 0.1 eq)
0.1 mL
0.1 mL
K2CO3
Reaction conditions: 1 (0.1 mmol), 2 (0.11 mmol), catalyst (5 mol %), base (2.0 eq), 1,4-dioxane (4 mL),
microwave irradiated at 150 W at 110 °C for 30 min
a
Isolated yield
2c
B(OH)2
OH
Cl
Pd (OAc) 2/ X-Phos
N
K2CO3
N
N
1,4-dioxane/Water
Microwave
110°C/30min
3c (20%)
4c (60%)
1
Scheme 2 Suzuki coupling of 1-chloro-3-phenylisoquinoline 1 with phenylboronic acid 2c
Table 3 Effect of phosphine ligands in the Suzuki reaction of 1 with boronic acid 2c
Entry
Ligand
Water (mL)
K2CO3 (eq)
Product 3c (%)
Byproduct 4c (%)
1
2
3
4
5
6
X-phos (0.5 eq)
Ruphos (0.5 eq)
X-phos (0.1 eq)
X-phos (0.1 eq)
X-phos (0.1 eq)
X-phos (0.1 eq)
0.1
0.1
0.1
–
2.0
2.0
2.0
2.0
–
20
22
18
–
60
65
60
62
20
15
0.1
–
–
–
–
Reaction conditions: 1 (0.1 mmol), 2c (0.11 mmol), PdCl2(PPh3)2 (5 mol%), base, 1,4-dioxane (4 mL)
microwave irradiated at 150 W at 110 °C for 30 min
respectively). The rest of the bases were found to be less effective (entries 2 and 4).
The influence of the amount of base was also investigated, and the result indicated
that lower the amount of base could yield loss of product 3a (Table 4, entries 7 and
8) and that 2.0 eq of base is necessary for effective conversion (Table 4, entries 9
and 10). With the optimized result in hand, various 1,3-disubstituted isoquinolines
were synthesized and the results are reported (as shown in Scheme 1; Table 1). The
desired products 3a–g, were obtained in high yield (65–90%) and purity. The
1
purified products were characterized by different spectral techniques, including H,
13C, LC-MS, IR, and CHNS analysis. The proposed mechanism of the reaction is
depicted in Scheme 3. The mechanism of the Suzuki reaction involved the oxidative
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K. Prabakaran et al.
addition of aryl halide to palladium in the formation of organo-palladium species;
meanwhile, the boron atom of boronic acid gets activated by the base, which
enhances the polarization of the organic ligand, and facilitates transmetallation and,
finally, reductive elimination to give rise to the final isoquinolines.
In conclusion, an efficient, experimentally simple, ligand-free, Pd-catalyzed,
microwave-assisted, Suzuki coupling in the presence of sodium carbonate and 1,4-
dioxane system for diversified 1,3-disubstituted isoquinolines has been established.
The highlights of this catalytic system include high catalytic activity, ready
availability of catalyst, and aqueous-free reaction conditions.
Experimental section
All reagents were purchased from Sigma-Aldrich, India, and used without further
purification. All boronic acids were purchased from Sigma-Aldrich, India, and used as
received. IR spectra were recorded using Avatar 330 equipped with a DTGS detector.
The NMR was recorded on a Bruker Spectrospin Avance DPX400 Ultrashield
(400 MHz) spectrometer instrument. Mass spectra were obtained using Agilent mass
spectrometry 1200 series. Microwave irradiation was performed in 8-mL vials. The
Biotage microwave equipment operating with a frequency of 20 kHz and output
power 0–150 W was utilized for the study. The CHNS analyses were carried out on a
Vario Micro elemental analyzer. Melting points were checked on an open capillary
tube in Elchem microprocessor-based DT apparatus and the values are uncorrected.
General procedure for the synthesis of 1,3-disubstituted isoquinolines (3a–g)
A mixture of 1-chloro-3-phenylisoquinoline 1, (0.1 mmol) boronic acids 2,
(0.11 mmol) sodium carbonate (2.0 eq), and 1,4-dioxane (4 mL) was degassed
Table 4 Effect of bases in the Suzuki reaction of 1 with boronic acid 2a
Entry
Base
Amount of base (eq)
Yielda (%)
1
2
3
4
5
6
7
8
9
10
NaHCO3
NaOtBut
K2CO3
2.0
1.1
1.5
1.0
2.0
2.0
1.0
1.5
2.5
3.0
65
20
60
40
65
90
55
75
90
90
NaOH
Cs2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Reaction conditions: 1 (0.1 mmol), 2c (0.11 mmol), catalyst, base, 1,4-dioxane (4 mL) microwave
irradiated at 150 W at 110 °C for 30 min
a
Isolated yield
123

Ligand-free, PdCl2(PPh3)2 catalyzed, microwave-assisted
343
R1
N
Cl
N
Pd(0)L2
1
4
II
L
II
Pd
L
L
Pd
N R1
L
N
Cl
B
(OH)2 Nu
3
2
Na2CO3
II
L
Pd
L
OH
OH
NNu
R1
B
NaCl
L= PPh3
R1= 4-pyridyl, 4-formylphenyl, phenyl, p-tolyl, 3-methoxyphenyl,
4-biphenyl, thiophene-2-carbonyl
Scheme 3 Mechanism of reaction
twice using nitrogen gas. Then, PdCl2(PPh3)2 (5.0 mol%) was added and again
degassed twice and microwave irradiated at 150 W at 110 °C for 30 min. The
reaction was monitored by TLC. After completion of the reaction, the resulting
solution was filtered off using celite pad (to remove catalyst) and the filtrate was
concentrated in vacuo. The crude product was subjected to silica-gel (230–400
mesh) flash column chromatography using hexane-ethyl acetate (90:10) eluent to
1
afford the pure products (Table 1). The compounds were confirmed by FTIR, H
NMR, 13C NMR, CHNS, and MS techniques. The analysis data of 3a–g are given
below.
3a: Yellow solid: mp 87–88 °C; 1H NMR (400 MHz, CDCl3): d = 8.83–8.82 (d,
2H, J 5.6), 8.21–8.18 (d, 2H, J 8.4), 8.14 (s, 1H), 8.07–8.05 (d, 1H, J 8.4), 7.98–7.96
(d, 1H, J 8.4), 7.74–7.71 (m, 3H), 7.58–7.49 (m, 3H), 7.44–7.41 (t, 1H, J 7.2); 13C
NMR (100 MHz, CDCl3): d 157.58, 150.45, 149.91, 147.34, 139.11, 137.84,
130.44, 128.82, 128.75, 127.74, 127.51, 127.01, 126.53, 125.36, 124.80, 116.67; IR
(m cm-1) 3029.3, 2918.5, 2850.3, 2283.4, 2167.1, 2132.1, 2027.9, 1988.0, 1951.9,
1716.3, 1615.9, 1588.7, 1562.9, 1539.0, 1492.6, 1447.5, 1408.5, 1379.0, 1333.5,
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K. Prabakaran et al.
1211.5, 1176.5, 1137.8, 1067.8, 1026.4, 977.8, 881.0, 856.3, 828.4, 761.6, 680.6,
643.2, 621.0, 594.3, 564.1, 516.8; LC-MS: m/e 283.2, C20H14N2 requires Mol. Wt.:
282.12. Elemental analysis, calculated: C, 85.08; H, 5.00; N, 9.92%. Found: C,
85.09; H, 5.03; N, 9.95%.
1
3b: Yellow solid: mp 134–135 °C; H NMR (400 MHz, CDCl3): d = 10.18 (s,
1H), 8.54 (s, 1H), 8.29–8.27 (d, 2H, J 8.4), 8.18–8.14 (m, 3H), 8.03–8.01 (d, 3H,
J 8.0), 7.87–7.83 (t, 1H, J 7.6), 7.69–7.65 (t, 1H, J 8.0), 7.57–7.53 (t, 2H, J 7.2),
7.48–7.44 (t, 1H, J 7.2); 13C NMR (100 MHz, CDCl3): d 192.00, 150.16, 137.96,
136.33, 130.99, 130.62, 129.68, 128.81, 127.69, 127.52, 127.16, 126.99, 125.61,
116.73; IR (m cm-1) 3368.4, 3034.4, 2919.2, 2850.4, 2724.4, 2185.3, 2131.6,
2029.1, 1934.3, 1693.1, 1598.2, 1550.4, 1490.6, 1439.9, 1376.6, 1329.7, 1300.1,
1258.8, 1202.5, 1168.8, 1070.8, 1019.8, 970.0, 880.1, 823.3, 761.1, 679.7, 646.5,
589.6, 512.6; LC-MS: m/e 310.2, C22H15NO requires Mol. Wt.: 309.12. Elemental
analysis, calculated: C, 85.41; H, 4.89; N, 4.53; O, 5.17%. Found: C, 85.46; H, 4.93;
N, 4.50%.
3c: White solid: mp 165–165.8 °C; 1H NMR (400 MHz, CDCl3): d = 7.85-7.82
(d, 2H, J 8.31), 7.40-7.11 (m, 11H), 6.68-6.66 (d, 1H, J 7.59), 6.51 (s, 1H); 13C
NMR (100 MHz, CDCl3): d 151.18, 143.88, 134.43, 133.25, 131.81, 128.68,
128.54, 128.37, 128.28, 128.22, 127.87, 127.75,127.61, 127.26, 126.05, 124.95,
124.07; IR (m cm-1) 3056.5, 2920.3, 2850.5, 1722.2, 1620.3, 1566.4, 1494.7,
1443.9, 1384.7, 1332.7, 1266.9, 1138.4, 1070.5, 1025.1, 963.7, 878.3, 848.4, 742.0,
687.7, 581.8, 557.5, 511.8; LC-MS: m/e 282.2, C21H15N requires Mol. Wt.: 281.12.
Elemental analysis, calculated: C, 89.65; H, 5.37; N, 4.98%. Found: C, 89.62; H,
5.34; N, 4.94%.
3d: White solid: mp 112–112.6 °C; 1H NMR (400 MHz, CDCl3): d 8.14–8.11(d,
3H, J 8.16), 8.06 (s, 1H), 7.94–7.91 (d, 1H, J 8.10), 7.83–7.81 (d, 2H, J 7.95), 7.68
(m, 1H), 7.57–7.50 (m, 4H), 7.32–7.30 (d, 2H, J 7.89), 2.43 (s, 3H); 13C NMR
(100 MHz, CDCl3): d 160.24, 150.18, 139.85, 138.38, 137.88, 136.74, 130.23,
130.0, 129.42, 128.56, 128.25, 125.55, 127.38, 126,95, 126.70, 125.66, 115.23,
21.28; IR (m cm-1) 3051, 2916, 2854, 2729, 2360, 1904, 1872, 1616, 1588, 1561,
1515, 1488, 1441, 1385, 1335, 1308, 1185, 1022, 975, 829, 823, 813, 766, 744, 698,
678, 643, 616; LC-MS: m/e 296.2, C22H17N requires Mol. Wt.: 294.14. Elemental
analysis, calculated: C, 89.46; H, 5.80; N, 4.74%. Found: C, 89.43; H, 5.77; N,
4.70%.
1
3e: White solid: Mp 109–110 °C; H NMR (400 MHz, CDCl3): d = 8.21–8.14
(m, 3H), 8.09 (s, 1H), 7.96–7.94 (d, 1H, J 8.13), 7.74–7.69 (t, 1H, J 8.1), 7.53–7.36
(m, 7H), 7.12–7.10 (d, 1H, J 8.10), 3.91 (s, 3H); 13C NMR (100 MHz, CDCl3): d
160.18, 159.59, 149.9, 139.11, 137.89, 130.42, 129.26, 128.7, 128.61, 127.71,
127.43, 127.22, 127.11, 125.81, 122.72, 116.26, 115.66, 114.62, 55.44; IR (m cm-1),
3063, 3032, 2992, 2960, 2918, 2851, 2829, 2361, 1951, 1923, 1662, 1617,1606,
1578, 1559, 1484, 1446, 1431, 1378, 1363, 1337, 1284, 1178, 1244, 1170, 1141,
1086, 1057, 1044, 983, 921, 894, 865, 858, 806, 799, 780, 765, 758, 690, 564; LC-
MS: m/e 312.2, C22H17NO requires Mol. Wt.: 311.13. Elemental analysis,
calculated: C, 84.86; H, 5.50; N, 4.50; O, 5.14%. Found: C, 84.81; H, 5.47; N,
4.50%.
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Ligand-free, PdCl2(PPh3)2 catalyzed, microwave-assisted
1
345
3f: White solid: mp 188.5–189.5 °C; H NMR (400 MHz, DMSO d6): d = 8.47
(s, 1H), 8.30–8.28 (d, 2H, J 7.2), 8.15–8.12 (d, 2H, J 8.0), 7.90–7.85 (m, 4H),
7.82–7.80 (d, 3H, J 7.2), 7.68–7.64 (t, 1H, J 7.6), 7.56–7.52 (t, 4H, J 7.6), 7.47–7.42
(m, 2 H); 13C NMR (100 MHz, DMSO d6): d 159.57, 149.34, 140.95, 140.14,
139.23, 138.81, 138.03, 131.02, 129.54, 129.25, 129.15, 128.21, 127.30, 127.16,
127.07, 125.07, 125.52, 116.07; IR (m cm-1), 3056, 3302, 2921, 2851., 1635, 1597,
1567, 1481, 1451, 1444, 1342, 1303, 1204, 1180, 1157, 1077, 1051, 1025, 964, 924,
815, 776, 752, 709, 693, 631; LC-MS: m/e 358.2, C27H19N requires Mol. Wt.:
357.15. Elemental analysis, calculated: C, 90.72; H, 5.36; N, 3.92%. Found: C,
90.72; H, 5.36; N, 3.88%.
1
3g: Yellow solid: mp 144–145 °C; H NMR (400 MHz, CDCl3): d = 10.06 (s,
1H), 8.56–8.54 (d, 2H, J 9.6), 8.31–8.29 (d, 2H, J 7.2), 8.19–8.16 (m, 2H),
8.08–8.07 (d, 1H, J 2.0), 7.90–7.87 (t, 1H, J 7.6), 7.80–7.76 (t, 1H, J 7.6), 7.60–7.56
(t, 2H, J 7.6), 7.50–7.46 (t, 1H, J 7.2); 13C NMR (100 MHz, CDCl3): d 185.33,
152.23, 151.09, 149.09, 144.71, 138.71, 138.43, 138.31, 131.48, 130.50, 129.54,
129.42, 129.32, 128.72, 126.93, 126.17, 124.69, 117.48; IR (m cm-1) 3057.2,
2924.3, 2852.0, 2030.7, 1664.7, 1619.4, 1555.0, 1495.5, 1459.6, 1369.2, 1335.9,
1283.6, 1221.9, 1056.5, 948.0, 884.7, 760.5, 690.3, 524.8; LC-MS: m/e 316.2,
C20H13NOS requires Mol. Wt.: 315.07. Elemental analysis, calculated: C, 76.16; H,
4.15; N, 4.44; O, 5.07; S, 10.17%. Found: C, 76.13; H, 4.13; N, 4.41; S, 10.15%.
Acknowledgment The authors wish to express their gratitude to the Vellore Institute of Technology
(VIT) University Management for their support and facilities.
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