Cycloisomerization of acetylenic oximes and hydrazones under gold catalysis: Synthesis and cytotoxic evaluation of isoxazoles and pyrazoles

Article abstract of DOI:10.1007/s12039-015-0993-9

The synthesis of substituted isoxazoles and pyrazoles through a general cycloisomerization methodology has been reported. The capability of gold(III) chloride to promote cycloisomerization of both α, β-acetylenic oximes and α, β-acetylenic hydrazones is t

Full text of DOI:10.1007/s12039-015-0993-9

c
J. Chem. Sci. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0993-9
Cycloisomerization of acetylenic oximes and hydrazones under gold
catalysis: Synthesis and cytotoxic evaluation of isoxazoles and pyrazoles
J C JEYAVEERANa, CHANDRASEKAR PRAVEENb,∗, Y ARUNc,
A A M PRINCEa and P T PERUMALc
aDepartment of Chemistry, Ramakrishna Mission Vivekananda College, Mylapore, Chennai 600 004, India
bFunctional Materials Division, Central Electrochemical Research Institute (CSIR laboratory),
Karaikudi 630 006, India
cOrganic Chemistry Division, Central Leather Research Institute (CSIR laboratory), Adyar,
Chennai 600 020, India
e-mail: chandrasekar.praveen@gmail.com
MS received 16 September 2015; revised 12 October 2015; accepted 20 October 2015
Abstract. The synthesis of substituted isoxazoles and pyrazoles through a general cycloisomerization metho-
dology has been reported. The capability of gold(III) chloride to promote cycloisomerization of both α, β-
acetylenic oximes and α, β-acetylenic hydrazones is the centrepiece of the strategy. A range of acetylenic pre-
cursors were investigated to afford 28 examples of the products with good to excellent chemical yields. Selected
compounds were screened for their cytotoxic potential towards COLO320 cancer cell lines. The IC50 values of
the tested compounds were in the micromolar range, with the best compound, 5-(6-Methoxy-naphthalen-2-yl)-
3-phenyl-isoxazole (3h) displaying an IC50 of 38.9 μM. For this compound, the crystal structure in complex
with Aurora-A kinase was obtained which revealed details of its binding mode within the active site with a free
energy of binding -9.54 kcal/mol.
Keywords. Gold catalysis; cycloisomerization; isoxazoles; pyrazoles; cytotoxicity; molecular docking.
1. Introduction
and their 1,3-dielectrophilic equivalents including α, β-
unsaturated aldehydes and ketones or on the intermole-
cular 1,3-dipolar cycloaddition of diazoalkanes and ni-
trilimines with alkenes/alkynes.5 The synthetic scope of
the aforementioned protocols is limited by strong reac-
tion conditions, high temperature, poor reactivity and
lack of regioselectivity. In the context of heterocycle
synthesis by π-Lewis acids, we have already addressed
this problem by employing AuCl3 as an effective cata-
lyst for the cyclization of α, β-acetylenic oximes leading
to isoxazoles.2e As an extension of this methodology,
we investigated the scope of related α, β-acetylenic hy-
drazones for the synthesis of pyrazoles. Following our
previous communication concerning the synthesis of
isoxazoles, we report herein a comprehensive study
which includes the cyclization of α, β-acetylenic hydra-
zones leading to pyrazoles, their cytotoxic evaluation
and molecular docking studies.
Gold catalyzed cyclization of alkynes possessing proxi-
mate nucleophiles has emerged as one of the imperative
topics in the field of contemporary metal catalysis, since
they offer a wide range of biologically significant carbo-
and heterocyclic derivatives.1 Our research group for
the past few years has been actively engaged in the de-
velopment and application of gold catalyzed cyclization
methodologies for the synthesis of basic heterocycles
like indoles,2a,b furans,2c quinolines,2d isoxazoles,2e car-
bazoles2f and pyranoindolones.2g On the other hand,
the heteroaromatic pharmacophore of isoxazoles3a and
pyrazoles3b as depicted in figure 1 exhibits a wide range
of biological properties and has led to continued interest
in chemical biology and medicinal chemistry.3c−j The
most useful synthetic access of isoxazoles includes the
[3+2] cycloaddition of alkenes/alkynes with nitrile oxi-
des or the reaction of hydroxyl amine with a three-carbon
component.4 Typical methods for the synthesis of pyra-
zoles involve the approaches based either on the conden-
sation of hydrazines with 1,3-dicarbonyl compounds
2. Experimental
2.1 Materials and methods
All commercially available solvents and reagents were
used without further purification unless otherwise stated.
∗For correspondence

J C Jeyaveeran et al.
O
N
Cl
Cl
Cl
Me
NH
Me
Ph
O
N
NH2
R
O
N
O
F
SO2NH2
H
Cl
O
H
F2HC
O
N
HO
O
O
N
N
N
N
H
Valdecoxib
(NSAID)
R = COOH, Ibotenic acid
(ionotropic on G-protein)
R = H, Muscimol
Me
O
HOOC
Dicloxacillin
-lactam antibiotic)
OMe
Bradykinin B1
(antiinflammatory)
(
(GABAA receptor agonist)
H2NO2S
NH2
H
O
N
HO
HO
NH
N
N
N
COOH
N
N
N
CF3
O
N
H
Ph
N
OHHO
H2N
p-Tolyl
3-n-Nonylpyrazole Celecoxib
L- -Amino- -(pyrazolyl-N)-
propanoic acid
(anti-diabetic)
Pyrazofurin
(C-nucleoside antibiotic) (CNS depressant)
Withasomnine
(antibiotic)
(COX-2 inhibitor)
Figure 1. Natural products and clinical drugs containing isoxazole and pyrazole nucleus.
All solvents used in the reactions were distilled for COLO320 adenocarcinoma colorectal cancer cell line
purity. Solutions in organic solvents were dried with was obtained from National Institute of Cell Sciences,
anhydrous sodium sulphate. Solvents were evaporated Pune. Absorption (λmax) was red at 570 nm in an ELISA
under reduced pressure. The inert atmosphere was cre- reader. Cytotoxicity of the compounds were statistically
ated was created by a slight positive pressure (ca. 0.1 analyzed by Duncan multiple range test at P = 0.05
psi) of nitrogen. All glassware were dried in an oven with the help of SPSS 11.5 version software package.
prior to use. Melting points were obtained using open All computations for molecular docking studies were
capillaries and are uncorrected. Infrared (IR) spectra carried out on a Dell Desktop D510 personal computer
were recorded on a Perkin-Elmer FTIR spectropho- (2.4 GHz Pentium 4 processor, Intel, Santa Clara, CA)
tometer as KBr pellets for solid compounds and neat running Red Hat Enterprise Linux Client release 5.5.
1
sample for liquid compounds. H and 13C NMR spec- The time required for each simulation run was on the
tra were obtained in DMSO-d6 and CDCl3 on a JEOL order of Real = 1h 13m 54.67s, CPU = 1h 08m 54.57s,
spectrometer at 500 and 125 MHz, respectively. Pro- System = 11.75s on the Pentium machine.
ton chemical shifts (δ) are relative to tetramethylsilane
(TMS, δ = 0.00) as internal standard and expressed in
2.2 General Procedure for the synthesis of isoxazoles
parts per million. The number of protons (n) for a given
(3a−3r)
resonance was indicated as nH. Spin multiplicities are
To a solution of oxime 2 (1.0 mmol) in dry dichloro-
methane was added AuCl3 (0.01 mmol) under N2 atmo-
sphere and stirred at the specified time and temperature
(table 1). After completion of the reaction as indicated
by TLC the reaction mixture was concentrated under
reduced pressure and purified by column chromatogra-
phy over silica gel (100-200 mesh) to afford the pure
product of isoxazole 3.
given as s (singlet), d (doublet), t (triplet), dd (doublet of
doublet), dt (doublet of triplet), ddd (doublet of doublet
of doublet) and m (multiplet). Coupling constants (J)
are given in hertz. GC-MS spectra were recorded on a
Perkin Elmer. Elemental analyses were recorded using
a Thermo Finnigan FLASH EA 1112CHN analyzer. All
the compounds gave C, H and N analysis within 0.5%
of the theoretical values. Column chromatography was
performed using thick-walled glass columns along with
a mixture of petroleum ether and ethyl acetate on silica
gel (100-200 mesh, SRL, India). The relative propor-
tions of solvents in chromatography solvent mixtures
refer to the volume to volume ratio. Analytical TLC
was performed on precoated plastic sheets of silica gel
G/UV-254 of 0.2 mm thickness (Macherey-Nagel, Ger-
many) using analytical grade solvents and visualized
with iodine spray (10% w/w I2 in silica gel), UV light
(λ = 254 and 365 nm) and alkaline KMnO4 solution.
2.2a 3-Methyl-5-phenyl-isoxazole (3a): Colourless
solid; M.p. 76-78◦C; IR (KBr): 3100, 1566, 1157, 1046,
896, 763 cm-1. 1H NMR (500 MHz, CDCl3): δH2.34 (s,
3H, -CH3); 6.35 (s, 1H, isoxazolinyl-H); 7.40-7.44 (m,
3H, Ar-H); 7.74 (d, 2H, J = 6.1 Hz, Ar-H). 13C NMR
(125 MHz, CDCl3): δC 11.6, 100.2, 125.8, 127.6, 129.0,
130.0, 160.4, 169.7. MS (ESI): m/z = 159 [M+].
Anal. Calcd. for C10H9NO: C, 75.45; H, 5.70; N, 8.80%.
Found: C, 75.65; H, 5.67; N, 8.65%.

Isoxazoles and Pyrazoles
Table 1. Cyclization of oximes 2 to isoxazoles 3
1479, 1446, 1300, 1075, 1000, 820 cm−1. 1H NMR
(500 MHz, CDCl3): δH 2.39 (s, 3H, -CH3); 6.76 (s, 1H,
isoxazolinyl-H); 7.26 (d, 2H, J = 7.6 Hz, Ar-H); 7.46
(d, 3H, J = 7.6 Hz, Ar-H); 7.71 (d, 1H, J = 8.4 Hz,
Ar-H); 7.75 (d, 1H, J = 8.4 Hz, Ar-H); 7.86 (d, 2H,
J = 7.6 Hz, Ar-H). 13C NMR (125 MHz, CDCl3):
δC 21.5, 96.9, 125.0, 125.8, 125.9, 126.8, 129.0, 129.7,
130.0, 140.6, 163.0, 170.6. MS (ESI): m/z = 235
[M+]. Anal. Calcd. for C16H13NO: C, 81.68; H, 5.57;
N, 5.95%. Found: C, 81.75; H, 5.52; N, 6.01%.
Product Time Yield
Entry
R
R1
3a
(min) (%)b
1
2
3
4
5
6
7
8
Me
Ph
Ph
Ph
Ph
Ph
Ph
Ph
p-tolyl
Ph
Ph
Ph
Ph
Me
H
Ph
p-anisyl
m-anisyl
p-tolyl
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
10
10
10
10
10
20
20
15
10
25
30
25
10
30
10
10
10
10
92
94
91
95
93
82
83
81
90
78
75c
80
93
79c
92
95
89
88
m-tolyl
o-cyanophenyl
p-fluorophenyl
6-MeO-2-naphthyl
Me
2-hydroxybutyl
SiPhMe2
H
9
2.2e 3-Phenyl-5-m-tolyl-isoxazole (3e): Colourless
10
11
12
13
14
15
16
17
18
solid; M.p. 106-108◦C; IR (KBr): 3416, 3043, 1600,
1
1476, 1442, 1295, 1085, 1004, 823 cm−1. H NMR
Ph
SiMe3
(500 MHz, CDCl3): δH 2.43 (s, 3H, -CH3); 6.81 (s,
1H, isoxazolinyl-H); 7.26 (s, 1H, Ar-H); 7.36 (t, 1H,
J = 7.6 Hz, Ar-H); 7.47 (d, 3H, J = 6.8 Hz, Ar-H);
7.63-7.66 (m, 2H, Ar-H); 7.84 (d, 1H, J = 6.1 Hz,
Ar-H); 7.88 (d, 1H, J = 8.4 Hz, Ar-H). 13C NMR
(125 MHz, CDCl3): δC 21.5, 97.4, 123.1, 125.9, 126.5,
126.9, 129.0, 129.1, 130.0, 130.9, 131.1, 138.8, 163.0,
170.6. MS (ESI): m/z = 235 [M+]. Anal. Calcd. for
C16H13NO: C, 81.68; H, 5.57; N, 5.95%. Found: C,
81.55; H, 5.52; N, 6.10%.
m-anisyl
p-tolyl
H
H
H
npropyl
npentyl
aAll products were characterized by IR, NMR and mass.
bIsolated yield after column chromatography.
cReaction was performed under reflux.
2.2b 5-(4-Methoxy-phenyl)-3-phenyl-isoxazole (3b):
Colourless solid; M.p. 118-120◦C; IR (KBr): 3113,
2939, 1615, 1501, 1461, 1397, 1252, 1025, 919 cm−1.
1H NMR (500 MHz, CDCl3): δH 3.84 (s, 3H, -OCH3);
6.69 (s, 1H, isoxazolinyl-H); 6.98 (d, 2H, J = 9.2 Hz,
Ar-H); 7.46 (d, 3H, J = 6.9 Hz, Ar-H); 7.76 (d, 2H,
J = 9.1 Hz, Ar-H); 7.85 (d, 2H, J = 7.6 Hz, Ar-H).
13C NMR (125 MHz, CDCl3): δC 55.4, 96.2, 114.4,
125.9, 126.8, 127.2, 128.9, 129.0, 130.0, 161.2, 163.0,
170.4. MS (ESI): m/z = 251 [M+]. Anal. Calcd. for
C16H13NO2: C, 76.48; H, 5.21; N, 5.57%. Found: C,
76.65; H, 5.17; N, 5.45%.
2.2f 2-(3-Phenyl-isoxazol-5-yl)-benzonitrile
(3f):
Colourless solid; M.p. 116-118◦C; IR (KBr): 3411,
1
3009, 2222, 1501, 1499, 783 cm−1. H NMR (500
MHz, CDCl3): δH 7.15 (s, 1H, isoxazolinyl-H);
7.34-7.35 (m, 4H, Ar-H); 7.55-7.56 (m, 2H, Ar-H);
7.82-7.84 (m, 2H, Ar-H); 8.00 (d, 1H, J = 8.4 Hz,
Ar-H). 13C NMR (125 MHz, CDCl3): δC 101.8, 109.1,
118.0, 126.0, 127.0, 127.8, 129.1, 129.7, 130.1, 130.4,
133.5, 134.3, 163.4, 165.8. MS (ESI): m/z = 246
[M+]. Anal. Calcd.. for C16H10N2O: C, 78.04; H, 4.09;
N, 11.38%. Found: C, 77.89; H, 4.15; N, 11.50%.
2.2c 5-(3-Methoxy-phenyl)-3-phenyl-isoxazole (3c):
Colourless solid; M.p. 76-78◦C; IR (KBr): 3302, 1604,
1461, 1338, 1223, 1085, 757 cm−1. 1H NMR (500 MHz,
CDCl3): δH 3.86 (s, 3H, -OCH3); 6.81 (s, 1H, isoxa-
zolinyl-H); 6.98 (d, 1H, J = 7.6 Hz, Ar-H); 7.37
(d, 1H, J = 7.6 Hz, Ar-H); 7.39-7.47 (m, 5H, Ar-H);
7.86 (dd, 2H, J1 = 7.7 Hz, J2 = 2.7 Hz, Ar-H). 13C
NMR (125 MHz, CDCl3): δC 55.5, 97.8, 111.0, 116.2,
118.4, 125.9, 126.9, 128.6, 129.0, 130.1, 130.2, 160.0,
163.0, 170.3. MS (ESI): m/z = 251 [M+]. Anal. Calcd.
for C16H13NO2: C, 76.48; H, 5.21; N, 5.57%. Found: C,
76.75; H, 5.17; N, 5.49%.
2.2g 5-(4-Fluoro-Phenyl)-phenyl-isoxazole (3g): Pale
yellow solid; M.p. 74-76◦C; IR (KBr): 3399, 3014,
1
1507, 1487, 927, 755 cm−1. H NMR (500 MHz,
CDCl3): δH 6.76 (s, 1H, isoxazolinyl-H); 7.11-7.22
(m, 2H, Ar-H); 7.41-7.54 (m, 3H, Ar-H); 7.79-7.92
(m, Ar-H). 13C NMR (125 MHz, CDCl3): δC 97.3,
116.2, 116.4, 125.9, 126.8, 127.9, 128.0, 129.0, 130.1,
163.1, 169.5. MS (ESI): m/z = 239 [M+]. Anal. Calcd.
for C15H10FNO: C, 75.30; H, 4.21; N, 5.85%. Found: C,
75.01; H, 4.25; N, 5.93%.
2.2h 5-(6-Methoxy-naphthalen-2-yl)-3-phenyl-isoxa-
2.2d 3-Phenyl-5-p-tolyl-isoxazole (3d): Pink colour zole (3h): Brown solid; M.p. 181-183◦C; IR (KBr):
solid; M.p. 134-136◦C; IR (KBr): 3420, 3032, 1601, 3238, 2961, 1638, 1572, 1406, 1362, 1177, 1099, 847

J C Jeyaveeran et al.
1
cm−1. H NMR (500 MHz, CDCl3): δH 3.91 (s, 3H,
-OCH3); 6.87 (s, 1H, isoxazolinyl-H); 7.46-7.53 (m,
4H, Ar-H); 7.68-7.71 (m, 4H, Ar-H); 7.81-7.83 (m,
3H, Ar-H). 13C NMR (125 MHz, CDCl3): δC 55.5,
97.2, 119.8, 120.0, 123.6, 125.7, 126.9, 127.6, 128.6,
129.0, 130.0, 130.1, 130.3, 130.4, 135.5, 158.9, 163.1,
170.7. MS (ESI): m/z = 301 [M+]. Anal. Calcd. for
C20H15NO2: C, 79.72; H, 5.02; N, 4.65%. Found: C,
79.89; H, 4.95; N, 4.50%.
m/z = 145 [M+]. Anal. Calcd. for C9H7NO: C, 74.47;
H, 4.86; N, 9.65%. Found: C, 74.89; H, 4.81; N, 9.50%.
2.2m 3,5-Diphenyl-isoxazole
(3m): Colourless
solid; M.p. 140-142◦C; IR (KBr): 3113, 3046, 1613,
1571, 1449, 1256, 1073, 821, 762 cm−1. M.p. 142-
1
144◦C; H NMR (500 MHz, CDCl3): δH 6.83 (s, 1H,
isoxazolinyl-H); 7.44-7.49 (m, 6H, Ar-H); 7.84 (d,
2H, J = 6.9 Hz, Ar-H); 7.88 (d, 2H, J = 7.6 Hz,
Ar-H). 13C NMR (125 MHz, CDCl3): δC 97.6, 125.9,
126.9, 127.5, 129.0, 129.1, 129.2, 130.1, 130.5, 163.0,
170.5. MS (ESI): m/z = 221 [M+]. Anal. Calcd. for
C15H11NO: C, 81.43; H, 5.01; N, 6.33%. Found: C,
81.65; H, 4.97; N, 6.25%.
2.2i 5-Methyl-3-p-tolyl-isoxazole (3i): Colourless
solid; M.p. 59-61◦C; IR (KBr): 3098, 1577, 1152,
1035, 899, 752 cm−1. 1H NMR (500 MHz, CDCl3): δH
2.39 (s, 3H, Ar-CH3); 2.46 (s, 3H, isoxazolinyl-CH3);
6.26 (s, 1H, isoxazolinyl-H); 7.24 (d, 2H, J = 9.2 Hz,
Ar-H); 7.67 (2H, d, J = 8.0 Hz, Ar-H). 13C NMR (125
MHz, CDCl3): δC 12.3, 21.4, 99.6, 126.5, 126.7, 128.8,
129.5, 139.9, 162.5, 169.6. MS (ESI): m/z = 174
[M+H]+. Anal. Calcd. for C11H11NO: C, 76.28; H,
6.40; N, 8.09%. Found: C, 75.99; H, 6.45; N, 8.20%.
2.2n 3-Methyl-5-trimethylsilyl-isoxazole (3n): Yel-
low liquid; IR (neat): 3302, 2912, 1604, 1419, 1338,
1085, 885, 757 cm−1. 1H NMR (500 MHz, CDCl3): δH
0.29 (s, 9H, -Si(CH3)3); 2.29 (s, 3H, -CH3); 6.24 (s,
1H, isoxazolinyl-H). 13C NMR (125 MHz, CDCl3): δC
-1.8, 10.8, 113.5, 157.7, 177.8. MS (ESI): m/z = 155
[M+]. Anal. Calcd. for C7H13NOSi: C, 54.15; H, 8.44;
N, 9.02%. Found: C, 53.95; H, 8.47; N, 9.15%.
2.2j 2-(3-Phenyl-isoxazol-5-yl)-butan-2-ol (3j): Brown
paste; IR (neat): 3479, 3218, 2832, 1573, 1466, 1493,
1254, 689 cm-1. 1H NMR (500 MHz, CDCl3): δH 0.93
(t, 3H, J = 6.9 Hz, -CH2CH3); 1.55 (s, 3H, -CH3);
1.91 (m, 2H, -CH2CH3); 2.10 (brs, -OH); 6.46 (s, 1H,
isoxazolinyl-H); 7.39-7.44 (m, 3H, Ar-H); 7.75-7.78
(m, 2H, Ar-H); 13C NMR (125 MHz, CDCl3): δC 7.7,
26.8, 33.2, 89.0, 91.6, 125.9, 126.6, 128.7, 130.9, 166.4,
168.5. MS (ESI): m/z = 217 [M+]. Anal. Calcd. for
C13H15NO2: C, 71.87; H, 6.96; N, 6.45%. Found: C,
72.01; H, 6.90; N, 6.38%.
2.2o 5-(3-Methoxy-phenyl)-isoxazole (3o): Yellow
oil; IR (KBr): 3302, 2219, 1604, 1419, 1338, 1085, 885,
1
757 cm−1. H NMR (500 MHz, CDCl3): δH 3.83 (s,
3H, -OCH3); 6.48 (d, 1H, J = 2.2 Hz, isoxazolinyl-
H); 6.94-6.96 (m, 1H, Ar-H); 7.31 (s, 1H, Ar-H); 7.34
(d, 2H, J = 5.3 Hz, Ar-H); 8.26 (d, 1H, J = 1.5
Hz, isoxazolinyl-H). 13C NMR (125 MHz, CDCl3): δC
55.4, 99.0, 111.0, 116.2, 118.4, 128.4, 130.2, 150.9,
160.0, 169.2. MS (ESI): m/z = 175 [M+]. Anal. Calcd.
for C10H9NO2: C, 68.56; H, 5.18; N, 8.00%. Found: C,
68.75; H, 5.10; N, 7.95%.
2.2k 3-Methyl-5-dimethylphenylsilylisoxazole (3k):
Yellow paste; IR (neat): 1590, 1411, 1250, 1100, 1000,
701 cm−1. 1H NMR (500 MHz, CDCl3): δH 0.64 (s, 6H,
-SiPh(CH3)2); 2.33 (s, 3H, -CH3); 6.29 (s, 1H, -isoxa-
zolinyl-H); 7.32-7.36 (m, 3H, Ar-H); 7.58-7.62 (m, 2H,
Ar-H). 13C NMR (125 MHz, CDCl3): δC -0.05, 10.7,
114.6, 127.7, 129.2, 139.9, 157.6, 176.0. MS (ESI):
m/z = 217 [M+]. Anal. Calcd. for C12H15NOSi: C,
66.31; H, 6.96; N, 6.44%. Found, C, 66.73; H, 7.19; N,
6.31%.
2.2p 3-p-Tolyl-isoxazole (3p): Colourless solid;
M.p. 56-58◦C; IR (KBr): 3301, 1609, 1420, 889, 729
1
cm−1. H NMR (500 MHz, CDCl3): δH 2.39 (s, 3H,
-CH3); 6.63 (d, 1H, J = 1.8 Hz, 3rd-Isoxazolinyl-H);
7.27 (d, 2H, J = 7.8 Hz, Ar-H); 7.70 (d, 2H, J = 8.1
Hz, Ar-H); 8.42 (d, 1H, J = 1.8 Hz, 2nd-Isoxazolinyl-
H). 13C NMR (125 MHz, CDCl3): δC 21.5, 102.5,
126.0, 126.6, 129.7, 140.2, 158.9, 161.6. MS (ESI):
m/z = 159 [M+]. Anal. Calcd. for C10H9NO: C, 75.45;
H, 5.70; N, 8.80%. Found: C, 75.75; H, 5.62; N, 8.69%.
2.2l 3-Phenyl-isoxazole (3l): Colourless oil; IR (neat):
3102, 1567, 1147, 1041, 901, 763 cm-1. 1H NMR
(500 MHz, CDCl3): δH 6.64 (d, 1H, J = 1.8 Hz, 3-
Isoxazolinyl-H); 7.42-7.47 (m, 3H, Ar-H); 7.80-7.84
(m, 2H, Ar-H); 8.44 (d, 1H, J = 1.8 Hz, 2-Isoxazo- 2.2q 5-Propyl-isoxazole (3q): Yellow oil; IR (neat):
1
linyl-H). 13C NMR (125 MHz, CDCl3): δC 102.2, 2979, 1730, 1469, 1354, 1328, 1118, 738 cm−1. H
126.7, 128.6, 128.7, 130.0, 159.0, 161.6. MS (ESI): NMR (500 MHz, CDCl3): δH 0.97 (t, 3H, J = 6.9 Hz,

Isoxazoles and Pyrazoles
-CH3); 1.54-1.58 (m, 2H, -CH2CH3); 2.30 (t, 2H, J = 128.1, 128.3, 128.7, 129.0, 130.5, 140.1, 142.9. MS
7.6 Hz, -CH2CH2CH3); 6.73 (s, 1H, isoxazolinyl-H); (ESI): m/z = 220 [M+]. Anal. Calcd. for C15H12N2: C,
7.34 (d, 1H, J = 1.5 Hz, isoxazolinyl-H). 13C NMR 81.79; H, 5.49; N, 12.72%. Found: C, 82.02; H, 5.45;
(125 MHz, CDCl3): δC 13.5, 21.4, 21.6, 96.8, 130.9, N, 12.53%.
135.0. MS (ESI): m/z = 111 [M+]. Anal. Calcd. for
C6H9NO: C, 64.84; H, 8.16; N, 12.60%. Found: C,
2.3b 1-Phenyl-5-(p-tolyl)-1H-pyrazole
(5b): IR
65.00; H, 8.12; N, 12.55%.
(neat) 3452, 2922, 1611, 1451, 1309, 1159, 1072,
761 cm−1. 1H NMR (500 MHz, CDCl3): δH 2.38 (s,
3H); 6.50 (d, 1H, J = 1.6 Hz); 7.12-7.18 (m, 4H);
7.31-7.38 (m, 5H); 7.75 (d, 1H, J = 1.5 Hz), 7.35 (m,
5H), 7.15 (m, 4H), 6.51 (d, J = 1.5 Hz, 1H), 2.37 (s,
3H). MS (ESI): m/z = 235 [M+H+]. Anal. Calcd. for
C16H14N2: C, 82.02; H, 6.02; N, 11.96%. Found: C,
81.82; H, 6.07; N, 12.11%.
2.2r 5-Pentyl-isoxazole (3r): Yellow oil; IR (neat):
2980, 1727, 1475, 1360, 1720, 740 cm−1. 1H NMR (500
MHz, CDCl3): δH 0.87 (t, 3H, J = 6.9 Hz, -CH3);
1.45-1.59 (m, 4H, -(CH2)2CH3); 1.68 (q, 2H, J =
6.8 Hz, -CH2(CH2)2CH3); 2.74 (t, 2H, J = 7.6 Hz, -
CH2(CH2)3CH3); 5.94 (s, 1H, isoxazolinyl-H); 8.11 (d,
1H, J = 1.5 Hz, isoxazolinyl-H). 13C NMR (125 MHz,
CDCl3): δC 13.9, 22.3, 26.5, 27.2, 31.2, 99.9, 150.2,
173.1. MS (ESI): m/z = 139 [M+]. Anal. Calcd. for
C8H13NO: C, 69.03; H, 9.41; N, 10.06%. Found: C,
68.92; H, 9.45; N, 10.10%.
2.3c 5-(4-Methoxyphenyl)-1-phenyl-1H-pyrazole (5c):
IR (neat): 3133, 2930, 1600, 1499, 1441, 1385, 1245,
1
1175, 960, 759 cm−1. H NMR (500 MHz, CDCl3):
δH 3.75 (s, 3H); 6.41 (d, 1H, J = 1.2 Hz); 6.78 (d,
1H, J = 8.5 Hz); 7.15 (d, J = 8.6 Hz, 2H); 7.27-
7.32 (m, 5H); 7.67 (d, 1H, J = 1.2 Hz). 13C NMR
(125 MHz, CDCl3): δC 55.1, 107.2, 114.0, 122.9, 125.3,
127.2, 129.0, 130.1, 140.1, 140.2, 142.9, 159.5. MS
(ESI): m/z = 250 [M+]. Anal. Calcd. for C16H14N2O:
C, 76.78; H, 5.64; N, 11.19%. Found: C, 77.00; H, 5.60;
N, 11.11%.
2.3 General Procedure for the synthesis of pyrazoles
(5a–5j)
To a solution of hydrazone 4 (1.0 mmol) in dry
dichloromethane was added AuCl3 (0.01 mmol) under
N2 atmosphere and stirred at the specified time and
temperature (table 2). After completion of the reac-
tion as indicated by TLC the reaction mixture was con-
centrated under reduced pressure and purified by col-
umn chromatography over silica gel (100-200 mesh) to
afford the pure product of pyrazole 5.
2.3d 5-Pentyl-1-phenyl-1H-pyrazole (5d): IR (neat)
2954, 2929, 286, 1598, 1537, 1500, 1454, 1394, 1012,
1
923, 761 cm−1. H NMR (500 MHz, CDCl3): δH 0.82
(t, 3H, J = 7.6 Hz); 1.22-1.27 (m, 4H); 1.55 (p, 2H,
J = 7.6 Hz); 2.61 (t, 2H, J = 7.6 Hz); 6.20 (s, 1H);
7.37-7.41 (m, 5H); 7.65 (d, 1H, J = 1.6 Hz). 13C NMR
(125 MHz, CDCl3): δC 13.2, 22.2, 26.1, 28.5, 31.3,
105.2, 125.3, 127.9, 129.0, 139.8, 140.0, 143.7. MS
(ESI): m/z = 215 [M+]. Anal. Calcd. for C14H18N2: C,
78.46; H, 8.47; N, 13.07%. Found: C, 78.66; H, 8.42;
N, 12.92%.
2.3a 1,5-Diphenyl-1H-pyrazole (5a): 1H NMR (500
MHz, CDCl3): δH 6.55 (d, 1H, J = 1.6 Hz); 7.28-7.30
(m, 2H); 7.32-7.37 (m, 8H); 7.77 (s, 1H, J = 1.6 Hz) .
13C NMR (125 MHz, CDCl3): δC 107.7, 125.1, 127.3,
Table 2. Cyclization of hydrazones 4 to 5.
Product Time Yield
Entry
R
R1
R2
5a
(h)
(%)b
2.3e 3-Methyl-1,5-diphenyl-1H-pyrazole (5e): Yel-
low paste, IR (CH2Cl2): 3060, 2958, 2865, 1596, 1500,
1451, 1369 cm−1. 1H NMR (500 MHz, CDCl3): δH 2.40
(s, 3H); 6.35 (s, 1H); 7.22-7.25 (m, 2H); 7.27-7.35 (m,
8H). 13C NMR (125 MHz, CDCl3): δC 13.7, 107.5,
125.0, 127.8, 128.8, 128.9, 129.7, 129.9, 130.9, 140.5,
143.9. MS (ESI): m/z = 234 [M+]. Anal. Calcd. for
C14H18N2: C, 82.02; H, 6.02; N, 11.96%. Found: C,
82.22; H, 6.07; N, 12.11%.
1
2
3
4
5
6
7
8
9
10
H
H
H
Ph
p-tolyl
p-anisyl
Pentyl
Ph
Ph
Ph
Ph
Ph
Ph
5a
5b
5c
5d
5e
5f
5g
5h
5i
1.5
1.5
1.5
2.0
1.0
1.0
4.5
4.0
4.5
4.0
75
80
79
67
85
88
55
62
59
64
H
Me
Ph
Me
H
Ph
H
Ph
Ph
Ph
p-CF3Ph
p-tolyl p-CF3Ph
Ph p-CF3Ph
p-anisyl p-CF3Ph
5j
aAll products were characterized by IR, NMR and mass.
bIsolated yield after column chromatography.
2.3f 1,3,5-Triphenyl-1H-pyrazole (5f): IR (neat)
3064, 1612, 1550, 1523, 1483, 1458, 1323, 1166,

J C Jeyaveeran et al.
1
1056, 968, 763 cm−1. H NMR (500 MHz, CDCl3):
δH 6.85 (s, 1H); 7.33-7.37 (m, 8H); 7.41-7.45 (m, 3H);
7.50 (t, 2H, J = 7.6 Hz); 8.0 (d, 2H, J = 7.6 Hz).
13C NMR (125 MHz, CDCl3): δC 105.2, 125.3, 126.0,
127.3, 128.0, 128.2, 128.6, 128.8, 128.9, 129.0, 128.2,
128.5, 128.6, 128.9, 129.0, 130.7, 133.1, 140.2, 144.5,
152.1. MS (ESI): m/z = 296 [M+]. Anal. Calcd. for
C21H16N2: C, 85.11; H, 5.44; N, 9.45%. Found: C,
84.90; H, 5.50; N, 9.60%.
2.3j 5-(4-Methoxyphenyl)-1-(4-(trifluoromethyl)phe-
nyl)-1H-pyrazole (5j): IR (neat) 2846, 1614, 1521,
1498, 1251, 1161, 1029, 960, 837, 792 cm−1. 1H NMR
(500 MHz, CDCl3): δH 3.79 (s, 3H); 6.45 (d, 1H, J =
1.2 Hz); 6.85 (d, 2H, J = 8.6 Hz); 7.15 (d, 2H, J =
8.6 Hz); 7.45 (d, 2H, J = 8.6 Hz); 7.57 (d, 2H, J =
8.6 Hz); 7.71 (d, 1H, J = 1.2 Hz). 13C NMR (125
MHz, CDCl3): δC 55.2, 108.3, 114.1, 122.5, 124.0 (d,
J = 276 Hz), 124.8, 125.9, 129.0 (d, J = 35 Hz),
130.0, 141.1, 143.0, 143.1, 160.0. MS (ESI): m/z =
318 [M+]. Anal. Calcd. for C17H13F3N2O: C, 64.15; H,
4.12; N, 8.80%. Found: C, 63.96; H, 4.18; N, 8.91%.
2.3g 3-Methyl-5-phenyl-1-(4-(trifluoromethyl)phenyl)-
1H-pyrazole (5g): IR (neat) 3060, 2923, 2851, 1615,
1520, 1499, 1361, 1162, 760 cm−1. H NMR (500
1
2.4 Experimental procedure for the evaluation of
cytotoxicity
MHz, CDCl3): δH 2.30 (s, 3H); 6.25 (s, 1H); 7.11-
7.15 (m, 2H); 7.19-7.23 (m, 3H); 7.29 (d, 2H, J =
8.1 Hz); 7.45 (d, 2H, J = 8.4 Hz). 13C NMR (125
MHz, CDCl3): δC 13.4, 109.0, 124.5 (d, J = 269 Hz);
124.6, 123.0, 128.5, 128.7, 128.8, 129.0 (q, J = 9.1
Hz); 130.5, 143.0, 144.0, 150.3. MS (ESI): m/z = 302
[M+]. Anal. Calcd. for C17H13F3N2: C, 67.54; H, 4.33;
N, 9.27%. Found: C, 67.00; H, 4.38; N, 9.35%.
The MTT [3-(4,5-dimethylthiazo-2-yl)-2,5-diphenyl-
tetrazolium bromide] cell proliferation assay was used
to evaluate the cytotoxic activity of the synthesized
compounds against COLO320 adenocarcinoma col-
orectal cancer cell lines.6 COLO320 cancer cell line
was maintained in complete tissue culture medium
RPMI with 10% Fetal Bovine Serum and 2 mM L-
Glutamine, along with antibiotics (about 100 IU/mL of
2.3h 5-(p-Tolyl)-1-(4-(trifluoromethyl)phenyl)-1H-py-
razole (5h): IR (neat) 2921, 1616, 1521, 1419, 1380, penicillin, 100 μg/mL of streptomycin) with the pH
1059, 920, 817, 777 cm−1. 1H NMR (500 MHz, adjusted to 7.2. Cells (5× 105) were seeded in 96 well
CDCl3): δH 2.36 (s, 3H); 6.47 (d, 1H, J = 1.6 Hz); 7.12 plates containing medium with different concentrations
(d, 2H, J = 8.4 Hz); 7.15 (d, 2H, J = 8.4 Hz); 7.41 (d, such as 500 μg/mL, 250 μg/mL and 100 μg/mL. The
2H, J = 8.4 Hz); 7.55 (d, 2H, J = 8.4 Hz); 7.70 (d, cells were cultivated at 37◦C with 5% CO2 and 95% air
1H, J = 1.6 Hz, 1H), 7.57 (d, J = 8.5 Hz, 2H), 7.42 in 100% relative humidity. After various durations of
(d, J = 8.4 Hz, 2H), 7.14 (d, J = 8.3 Hz, 2H), 7.11 cultivation, the solution in the medium was removed.
(d, J = 8.3 Hz, 2H), 6.48 (d, J = 1.4 Hz, 1H), 2.35 An aliquot of 100 μL of medium containing 1 mg/mL
(s, 3H). 13C NMR (125 MHz, CDCl3): δC 21.3, 108.5, of MTT was loaded to the plate. Incubation at 37◦C for
124.0 (d, J = 276 Hz), 124.7, 126.0, 127.4, 128.6, 4 h allowed reduction of MTT by mitochondrial dehy-
129.0 (d, J = 35 Hz), 129.3, 138.6, 141.0, 143.0, drogenase to an insoluble formazan product. Well con-
143.5. MS (ESI): m/z = 288 [M+]. Anal. Calcd. for tents were removed and the formazan product was sol-
C16H11F3N2: C, 66.66; H, 3.85; N, 9.72%. Found: C, ubilized by the addition of 100 μL of DMSO, which
66.83; H, 3.80; N, 9.60%.
led to the formation of purple colour. The amount of
formazan product is directly proportional to the num-
ber of living cells. Absorbance of the converted dye
2.3i 3,5-Diphenyl-1-(4-(trifluoromethyl)phenyl)-1H-py- in each well was read on ELISA reader at 570 nm.
razole (5i): IR (neat) 3065, 1614, 1550, 1523, 1455, From the absorbance, % of inhibition was calculated by
1
1362, 1105, 1056, 916, 810, 760 cm−1. H NMR (500 using the formula, % of inhibition = (Ac-At)/Ac X 100,
MHz, CDCl3): δH 6.75 (s, 1H); 7.17-7.21 (m, 4H); 7.35 where Ac is the mean absorbance of control and At is
(t, 2H, J = 7.6 Hz); 7.39 (d, 2H, J = 8.4 Hz); 7.50 (d, that of test. From the results, non-linear regression graph
2H, J = 8.4 Hz); 7.82 (d, 2H, J = 7.6 Hz). 13C NMR was plotted between % cell growth inhibition and log10
(125 MHz, CDCl3): δC 106.2, 124.0 (d, J = 276 Hz), concentration (μM). The half maximal inhibitory con-
124.8, 126.0, 126.1, 127.9, 128.2, 128.6, 128.8, 128.9, centration (IC50 value) was determined and averaged
129.0 (d, J = 35 Hz), 130.2, 132.7, 143.0, 144.6, from three replicate experiments. Cytotoxicity was sta-
152.6. MS (ESI): m/z = 364 [M+]. Anal. Calcd. for tistically analyzed by Duncan multiple range test at P
C22H15F3N2: C, 72.52; H, 4.15; N, 7.69%. Found: C, = 0.05 with the help of SPSS 11.5 version software
72.01; H, 4.23; N, 7.80%.
package.

Isoxazoles and Pyrazoles
after reflux for 30 min. This can be attributed to the
3. Results and Discussion
presence of electron withdrawing silyl group which
renders the triple bond electron deficient, thus mak-
ing the nucleophile (=N-OH), difficult to attack the
triple bond. The cycloisomerization of silyl oximes (2k
and 2n) by this methodology seems to be beneficial as
the silyl group was left intact, whereas in the related
K2CO3 method, desilylated product was obtained.8 One
of the noteworthy advantages of this methodology is
the cycloisomerization of oximes possessing termi-
nal alkyne (2l) leading to 3-substituted isoxazole (3l),
whilst similar type of cyclization uses only internal
alkynes leading to the formation of 3,5-disubstituted
isoxazoles.7
3.1 Chemistry
As part of our research endeavour of gold catalysis
for carbo- and heterocycle construction, we have pre-
viously reported the AuCl3 catalyzed cyclization of
α, β-acetylenic oximes leading to substituted isoxa-
zoles (scheme 1).2e The chemical synthesis of requi-
site oximes 2 commenced from the condensation of
acetylenic aldehydes/ketones 1 with hydroxylamine.7
The cyclization of acetylenic oximes 2 was effected by
the catalytic use of AuCl3 (1 mol %) in dichloromethane
at room temperature which results in the formation of
isoxazoles 3. Under this reaction condition, all sub-
strates underwent the reaction completely and resulted
in good yields of the product as summarized in table 1.
However, the rate of the reaction seems to be depen-
dent on the substituent on the substrates. Oximes having
electron releasing group on the alkyne residue (entries
1-5, 9, 13 and 15-18) more readily underwent the
cyclization than those possessing electron withdraw-
ing groups (entries 6 and 7). The reaction condition
was also amenable to substituent possessing hydroxyl
and polyaromatic functionalities albeit in moderate
yields (entries 8 and 10). The substrates (2k and 2n)
possessing silyl group underwent the cyclization only
The isoxazole products were characterized by IR, 1H
NMR, 13C NMR and mass. All compounds exhibited a
1
sharp singlet in their H NMR spectrum at δH = 6.2-
6.8 ppm, characteristic of isoxazolinyl proton. In 13C
NMR, the characteristic peak of C5 carbon of the isox-
azole ring was exhibited at δC = 170.0-170.9 ppm. As
a final proof, single crystal of compounds 3c and 3e
was obtained by recrystallization and the structure was
unambiguously confirmed by X-ray diffraction studies
(figures 2 and 3).9
With an efficient protocol for the synthesis of isoxa-
zoles in hand, we next set out to investigate the scope
and limitations of our gold catalytic system towards
.
l
.
l
l
l
:
l
Scheme 1. Synthesis of isoxazoles.
Figure 2. ORTEP diagram of compound 3c.

J C Jeyaveeran et al.
Figure 3. ORTEP diagram of compound 3e.
attack of the tethered nucleophile (OH, NHR2) leads
to the cyclized intermediate III. Proto-deauration of
intermediate III results in the formation of the product
IV.
α, β-acetylenic hydrazones for the synthesis of pyra-
zoles (scheme 2). The requisite α, β-acetylenic hydra-
zones 4 has been prepared in satisfactory yield through
the condensation of acetylenic aldehydes/ketones 1
with arylhydrazines.10 By subjecting the precursors 4
under our previously demonstrated conditions (1 mol%
AuCl3, CH2Cl2), we found that the reaction proceeded
3.2 Cytotoxicity
smoothly and afforded the pyrazole products, albeit not Ten compounds of isoxazoles (3a, 3f-3j, 3l, 3m, 3o and
with the exceptional efficiency of the isoxazoles as evi- 3q) and pyrazoles (5a-5j) were selected to evaluate the
denced by the reduced yield (table 2). The effect of cytotoxic potential against COLO320 cancer cell lines.
the substituent on the hydrazone nitrogen was explored The cytotoxic results were compared with reference
using phenyl and p-CF3Ph groups. We found that drug cyclophosphamide which showed 90% inhibition
hydrazones having a highly electron withdrawing sub- at a concentration of 156 μM. The tested compounds
stituent such asp-CF3Ph (5g-5j) led to reasonable con- exhibited maximum cytotoxicity against COLO320
version with only an increase of the reaction time as cells at a concentration of 200 to 50 μM. All con-
compared to that of phenyl groups (5a-5f). The pres- centrations used in the experiment could decrease the
ence of other substituent viz. R and R1 had shown little cell viability significantly (P<0.05) in a concentration-
effect on the conversion. Our gold catalyzed method- dependent manner. Cytotoxicity of each sample was
ology for the cyclization of α, β-acetylenic hydrazones expressed as IC50 value. The IC50 value is the con-
seems to be advantageous compared to some of the centration of test sample that causes 50% inhibition
relevant protocols developed in recent times. Most of of cell growth averaged from three replicate experi-
these protocols suffer the stoichiometric use of reagents ments (table 3). Analysis of the screening data revealed
as in the case of electrophilic cyclization11 and tan- that isoxazole series (3a-3q, IC50 = 38.9 to 49.6 μM)
dem aminofluorination reactions.12 However, our proto- exhibited better activity compared to the pyrazole series
col circumvents this limitation and entails only a very (5a-5j, IC50 = 50.8 to 55.9 μM). Among the tested
low catalytic loading of AuCl3. The structure of pyra- series, isoxazole 3h possessing 6-MeO-2-naphthyl
zole products was established by routine spectroscopy group emerged as the best active with an IC50 of 38.9
techniques and compared with literature reports.11
Based on our previous observations,2e a tentative and lipophilicity offered by the β-naphthyl residue. To
mechanism is proposed for the formation of isoxazoles our surprise, compound with a propyl chain (3q, IC50
μM and this could be ascribed to the increased size
=
and pyrazoles (scheme 3). According to which, the 40.1 μM) emerged as the second most active among
gold metal centre activates the alkyne residue I to form the screened compounds. The compound devoid of any
the gold-alkyne π-complex II. Subsequent nucleophilic substitution at the C5 position (3l, IC50 = 43.9 μM)
.
l
l
l
.
Scheme 2. Synthesis of pyrazoles.

Isoxazoles and Pyrazoles
l
-
-
l
Scheme 3. Plausible mechanism for the formation of isoxazoles and pyrazoles.
Table 3. IC50 and FEB values of isoxazoles 3 and pyrazoles
in many tumour cells. Moreover, inhibition studies of
Aurora-A kinase expression in the cells induce the
apoptosis and subsequently results in the cell death.13
Twenty compounds (3a, 3f-3j, 3l, 3m, 3o, 3q and 5a-5j)
were subjected to molecular docking studies using the
AutoDock Tools (ADT) version 1.5.6 and AutoDock
version 4.2.5.1 docking program14 to investigate the
potential binding mode of inhibitors with Aurora-A
kinase receptor (PDB ID: 3P9J). To start with, the
reproducibility of docking calculations was verified by
extracting the bound ligand from the complexes and
submitted for one-ligand run calculation. This repro-
duced top scoring conformation falling within root-
mean-square deviation (RMSD) value of 0.50 to 0.61
Å with bound X-ray conformation for 3P9J, suggest-
ing this method is valid enough to be used for dock-
ing studies of other compounds (figure S1). Docking
of different ligands to Aurora-A kinase was performed
using AutoDock, following the same protocol used in
the validation study (figure S2). All dockings were
taken into 2.5 million energy evaluations and were per-
formed for each of the test molecules. Docked ligand
conformations were analyzed in terms of energy, hydro-
gen bonding and hydrophobic interaction between lig-
and and receptor protein Aurora-A. Detailed analyses
of the ligand-receptor interactions were carried out and
final coordinates of the ligand and receptor were saved.
PyMOL software was used for display of the receptor
with the ligand binding site. From the docking scores,
the free energy of binding (FEB) of all compounds were
calculated (table 3). The results revealed that the com-
pounds are efficiently bound with the 3P9J receptor and
exhibit free energy of binding (FEB) from -4.53 to -9.54
kcal/mol. All the docked compounds fit in the active
site of 3P9J receptor and interact with the nine amino
acids namely LEU-139, VAL-147, LYS-162, LEU-194,
LEU-210, GLU-211, TYR-212, ALA-213 and LEU-
263. Among all the compounds docked, compound 3h,
although not significant, but exhibits highest binding
energy of -9.54 kcal/mol (figure S3). In compound 3h,
the isoxazole ring oxygen interacts with N-H of the
ALA-213 and forms a hydrogen bond with the bond
length of 2.1 Å. Also, methoxy oxygen interacts with N-
H of the LYS-162 and forms a hydrogen bond with the
bond length of 2.0 Å. Furthermore, the polar interaction
5.
Entry
Compound
IC50 (μM)
FEB (kcal/moL)a
1
2
3
4
5
6
7
8
3a
3f
3g
3h
3i
3j
3l
3m
3o
3q
5a
5b
5c
5d
5e
5f
45.4
49.6
46.9
38.9
46.2
46.4
43.9
47.3
45.0
40.1
52.5
53.5
52.8
50.8
51.9
54.8
52.3
53.9
55.9
53.4
−6.43
−8.03
−7.74
−9.54
−6.58
−7.14
−5.72
−7.85
−6.21
−9.02
−6.72
−7.05
−6.97
−6.05
−6.62
−8.10
−6.73
−7.22
−8.26
−7.03
9
10
11
12
13
14
15
16
17
18
19
20
5g
5h
5i
5j
aFree energy of binding calculated using Autodock 4.
bCyclophosphamide was used as the standard for cytotoxic
studies which showed 90% inhibition (156 μM).
also demonstrated significant cytotoxicity. Other com-
pounds of the isoxazole series exhibited comparable
inhibitory potencies. However, compound 3f having o-
cyanophenyl showed poor activity (IC50 = 49.6 μM)
among the isoxazole series suggesting that the nitrile
function at those sites is not tolerated. Assessment
of the pyrazole series revealed that compound with a
pentyl chain (5d, IC50 = 50.8 μM) exhibited good
activity compared to other compounds in the same
series.
3.3 Molecular docking
Aurora-A is a family of serine/threonine kinases that
are involved in the correct centrosome maturation and
separation. Aurora-A kinases have attracted significant
attention for small molecule inhibition because its ele-
vation may play a vital role in the support of tumour
progression which is evident by its high expression

J C Jeyaveeran et al.
(e) Hashmi A S K and Bührle 2010 Aldrichim. Acta. 43
between isoxazole nitrogen and C=O of the ALA-213
occurs in the distance of 2.8 Å. Importantly, π-cationic
interaction results between naphthyl ring and (NH3)+ of
LYS-162. It is noteworthy to mention that the naphthyl
ring fits in the lipophilic pocket of the active site con-
sist of five amino acids namely, LEU-139, VAL-147,
LUE-194, LEU-210 and LEU-263.
27; (f) Wegner H A and Auzias M 2011 Angew. Chem.
Int. Ed. 50 8236; (g) Patil N 2012 Chem. Asian. J. 7
2186; (h) Inamdar S M, Konala A and Patil N T 2014
Chem. Commun. 50 15124; (i) Bandini M 2011 Chem.
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4. Conclusions
In summary, a general strategy that supports the
synthesis of both isoxazoles and pyrazole involving
gold-catalyzed cycloisomerization of α, β-acetylenic
oximes/hydrazones has been developed. From a syn-
thetic chemist perspective, the key features of our proto-
col include low catalyst loading, short reaction time and
an overall satisfactory chemical yields. Cytotoxic eval-
uation of these compounds against COLO320 cells had
shown activity with the IC50 values ranging between
38.9 and 55.9 μM. Compound 3h emerged as the most
active (IC50 = 38.9 μM) amongst the tested series.
Molecular docking of compound 3h into Aurora-A
kinase receptor exhibited the largest binding energy (-
9.54 kcal/mol) compared to other compounds and thus
supports the in vitro results. Further investigations on
application of this methodology towards natural prod-
uct synthesis are in progress.
Supplementary Information
Docking poses of all the screened compounds and copy
of IR, 1H NMR, 13C NMR and mass of compounds 3m
and 5b are available at www.ias.ac.in/chemsci.
Acknowledgements
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34 75
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P T 2013 Acta Cryst. E69 o597; (b) Balakrishnan B,
Praveen C, Seshadri P R and Perumal P T 2011 Acta
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García-Estrada J G and Penieres-carrillo J G 2008 Tetra-
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C.P. gratefully acknowledges the financial support of
the Department of Science & Technology (DST), India
for providing INSPIRE faculty award. C.P. also thanks
Dr. Vijayamohanan K. Pillai and Dr. D. Jeyakumar,
CSIR-CECRI for infrastructure facilities.
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