Synthesis, spectroscopic and crystal structure analysis of two dihydropyrimidines

Article abstract of DOI:10.3184/030823409X425064

The preparation of two reduced pyrimidine derivatives, ethyl 3-acetyl-4-(4-methoxyphenyl)-6-methyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5- carboxylate and ethyl 4-(3,4-dimethoxyphenyl)-6-methyl-2-thioxo-1,2,3,4- tetrahydropyrimidine-5-carboxylate, is des

Full text of DOI:10.3184/030823409X425064

JOURNAL OF CHEMICAL RESEARCH 2009
APRILꢊꢀꢉꢇꢆ±204
RESEARCH PAPER 201
Synthesis, spectroscopic and crystal structure analysis of two
dihydropyrimidines
Noor Shahina Begum* and D.E. Vasundhara
Department of Studies in Chemistry, Bangalore University, Central College Campus, Bangalore 560 001, India
The preparation of two reduced pyrimidine derivatives, ethyl 3-acetyl-4-(4-methoxyphenyl)-6-methyl-2-thioxo-
1,2,3,4-tetrahydropyrimidine-5-carboxylate and ethyl 4-(3,4-dimethoxyphenyl)-6-methyl-2-thioxo-1,2,3,4-tetrahydro-
pyrimidine-5-carboxylate, is described, along with details of their crystal structure analysis.
Keywords: dihydropyrimidines, calcium channel blockers, crystal structure
Dihydropyridines (DHPs) are the most studied class of
organic calcium channel modulators and have become almost
indispensable for the treatment of cardiovascular diseases
such as hypertension, cardiac arrhythmias, and angina.1
In recent years, interest has also been focused on their aza-
analogs such as dihydropyrimidines (DHPMs), which exhibit
DꢀSKDUPDFRORJLFDOꢀSUR¿OHꢀVLPLODUꢀWRꢀFODVVLFDOꢀGLK\GURS\ULGLQHꢀ
calcium channel modulators. These inherently asymmetric
dihydropyrimidine derivatives are better calcium channel
blockers, and they have been extensively studied to expand
the existing structure-activity relationships and to gain further
insight into molecular interactions at the receptor level.2-10
The DHPMs can be easily synthesised by the so-called
Biginelli dihydropyrimidine synthesis,11,12 which is a simple,
one-pot, acid-catalysed condensation reaction.
The present work is part of our research involving a series
of novel dihydropyrimidine derivatives which can be potent
mimics of the dihydropyridines.13,14 A knowledge of the
molecular geometry and the probable bioactive structure of
a compound is a prerequisite for any understanding of its
pharmacological properties. We report here the structures of
two dihydropyrimidine derivatives, 1 and 2, both of them
potential calcium channel blockers.
Table 1 Selected bond lengths [Å] and angles [°] for
compound 1
Bond
Length/Å
Bonds
Angle/°
C3–O3
C7–C10
C8–N1
C8–N2
C8–S1
C9–C10
C9–C14
C9–N2
C10–C11
C11–O1
C11–O2
C12–C13
C15–C16
C15–N1
C15–O4
C17–O3
1.369(4)
1.513(3)
1.373(3)
1.363(3)
1.644(3)
1.343(4)
1.498(4)
1.401(3)
1.470(4)
1.199(3)
1.334(4)
1.427(7)
1.493(4)
1.431(3)
1.193(4)
1.419(4)
C2–C3–O3
C4–C3–O3
124.4(3)
116.4(3)
113.7(2)
125.07(19)
121.08(19)
129.3(2)
116.6(2)
114.2(3)
121.7(2)
122.2(2)
125.5(3)
111.7(2)
122.7(3)
119.6(3)
122.2(3)
118.1(3)
126.9(2)
125.1(2)
117.8(3)
N1–C8–N2
N1–C8–S1
N2–C8–S1
C10–C9–C14
C10–C9–N2
C14–C9–N2
C7–C10–C11
C9–C10–C11
C10–C11–O1
C10–C11–O2
O1–C11–O2
C16–C15–N1
C16–C15–O4
N1–C15–O4
C8–N1–C15
C8–N2–C9
C3–O3–C17
Ethyl 3-acetyl-4-(4-methoxyphenyl)-6-methyl-2-thioxo-1,2,3,4-
tetrahydropyrimidine-5-carboxylate (1)
In this molecule, the aryl ring is essentially planar,
positioned axially, perpendicular to and bisecting the boat-
like dihydropyrimidine ring (Fig. 1). The dihedral angle
between the planes of the aryl and dihydropyrimidine rings
is 86.462(5)°, indicating the extent of orthogonality between
WKHꢀ WZRꢀ ULQJꢀ V\VWHPVꢁꢀ 7KLVꢀ µDU\OꢂJURXSꢀ XS¶ꢀ FRQ¿JXUDWLRQꢀ LVꢀ
considered crucial for enhanced calcium antagonist activity
in these compounds. The methoxy substituent on the phenyl
ULQJꢀDGRSWVꢀDꢀFRQ¿JXUDWLRQꢀZKLFKꢀLVꢀDOPRVWꢀQRUPDOꢀWRꢀWKHꢀ&ꢃ±
H7 bond. The carbonyl group of the ester at C10 is cis to the
GLK\GURS\ULPLGLQHꢀGRXEOHꢀERQGꢀꢄ&ꢅ±&ꢆꢇꢈꢁ
Results and discussion
The DHPM compounds 2 and 3 were prepared by the standard
three-component condensation reaction.15 Compound 3 was
acetylated at N(3) using acetic anhydride, forming 1 (Scheme 1).
Figures 1 and 4 show the ORTEP diagrams of the molecules
1 and 2. Figures 2, 3 and 5 show the crystal packing of the
two compounds. Selected bond distances and angles are given
in Table 1 and Table 2 for compounds 1 and 2, and Tables 3
and 4 show the hydrogen-bond interactions for compounds 1
and 2 respectively. In the following two subsections of this
discussion, except for the compound names, the atoms are
numbered using the crystallographic numbering scheme as
shown in Figs 1 and 4.
All bond lengths and angles are within the normal ranges.
The molecular packing shows the presence of the less frequent
…
…
1±+ 6ꢀ K\GURJHQꢀ ERQGVꢁꢀ 7KHVHꢀ 1ꢉ±+ꢉ1 S1 interactions
OMe
OMe
R
O
O
OMe
CHO
R
H C
3
OEt
EtO C
2
EtO C
2
COCH
S
3
+
LiBr
NH
N
Ac O
2
S
+
MeCN
[R = H]
H C
3
H C
3
N
H
S
N
H
NH
2
H N
2
2; R = OMe
3; R = H
1
Scheme 1
* Correspondent. E-mail: noorsb@rediffmail.com, noorsb@yahoo.com

202 JOURNAL OF CHEMICAL RESEARCH 2009
Table
2
Selected bond lengths [Å] and angles [°] for
Table 3 Non-bonded interactions and possible hydrogen
bonds (Å,°) for compound
H – hydrogen)
compound 2
1
(D
–
donor;
A
–
acceptor;
Bond
Length/Å
Bonds
Angle/°
…
…
…
…
D–H
A
D–H
H
A
D
A
angle D–H
A
S1–C5
1.685(3)
1.328(3)
1.326(3)
1.210(3)
1.468(4)
1.517(3)
1.353(3)
1.364(3)
1.394(3)
1.528(4)
1.380(3)
1.425(4)
1.498(4)
1.369(4)
1.408(4)
1.380(4)
C2–C3–O3
C6–C2–C8
O3–C3–O4
O3–C3–C2
O4–C3–C2
C5–N1–C8
S1–C5–N2
S1–C5–N1
N2–C5–N1
C10–O2–C17
C2–C8–N1
C2–C8–C7
N1–C8–C7
O2–C10–C9
O2–C10–C14
C9–C10–C14
119.6(2)
122.7(2)
114.9(2)
122.4(2)
124.2(2)
123.90(19)
120.67(18)
115.4(2)
114.5(3)
118.4(2)
128.7(2)
112.9(2)
119.0(2)
120.9(3)
120.1(3)
118.1(3)
…
N2–C5
O3–C3
O4–C3
C2–C3
C16–H16B O1i 0.96(3) 2.429(3) 3.284(4)
148(2)
157(2)
N2–H2N· · ·S1iI
0.86(2) 2.546(1) 3.356(3)
Symmetry codes: (i) –x + 1, –y + 1, –z + 1; (ii) –x, –y + 1, –z + 1.
C2–C6
C2–C8
Table Non-bonded interactions and possible hydrogen
4
bonds (Å,°) for compound
H – hydrogen)
2
(D
A
–
donor;
…
A
–
acceptor;
…
N1–C5
N1–C8
C4–C6
O2–C10
O2–C17
C11–C15
O1–C14
O1–C16
C12–C14
…
…
D–H
A
D–H
H
D
A
angle D–H
A
N2–H2N· · ·S1i
N1–HN1· · ·O4iI
0.85(7) 2.511(5) 3.340(3)
0.80(2) 2.175(2) 2.937(3)
165(6)
158(2)
Symmetry codes: (i) –x,–y + 1,–z; (ii) –x + 1, + y, + z.
…
Fig. 3 View of the molecular packing in 1, showing CH
O
…
and NH S interactions, down the a axis.
Fig. 1 ORTEP view of compound 1, showing 50% probability
boat-like dihydropyrimidine ring. The dihedral angle between
the planes of aryl and dihydropyrimidine rings in this case is
88.767(2)°, which predisposes the molecule towards excellent
receptor-binding properties. Additionally, it can be seen that
both the substituents are added to the biologically less important
‘right hand side’ of the molecule, thereby not interfering with
the receptor-sensitive groups of the ‘left hand side’.9
The ester group is, in contrast to compound 1, considered
to be in the trans conformation because the carbonyl group
LVꢀ IDFLQJꢀ DZD\ꢀ IURPꢀ WKHꢀ &ꢉ±&ꢌꢀ GRXEOHꢀ ERQGꢀ DQGꢀ GRHVꢀ QRWꢀ
eclipse it. The 1,4-dihydropyrimidine ring, as before, adopts
a ‘boat’ conformation. The deviations from the least-squares
ellipsoids and the atom numbering scheme.
2
give rise to centrosymmetric dimers of graph set R2 (8) (Fig. 2).
This observation is in accord with the general tendency of the
dihydropyrimidines to form dimers through intermolecular
hydrogen bonds. The crystal cohesion is further enhanced by
…
…
C-H 2ꢀVKRUWꢀFRQWDFWVꢀ>&ꢆꢋ±+ꢆꢋ% O1].
Ethyl 4-(3,4-dimethoxyphenyl)-6-methyl-2-thioxo-1,2,3,4-tetra-
hydropyrimidine-5-carboxylate (2)
The classical dihydropyrimidine structural features are
emphasised further in this molecule, due to the effect of the
second methoxy substituent on the aryl ring (Fig.4).
Disubstitution by bulky methoxy groups bring about enhanced
conformational restriction resulting in the aryl group having
an enhanced axial position, perpendicular to and bisecting the
Fig. 4 ORTEP diagram of compound 2, showing 50% probability
displacement ellipsoids and the atom numbering scheme.
…
Fig. 2 The NH S interactions in 1 viewed along the c axis.

JOURNAL OF CHEMICAL RESEARCH 2009 203
Experimental
plane through the dihydropyrimidine ring atoms suggest that
the greatest displacement from zero occurs about the bonds
from N1 and C6 (carbon with aryl substituent), indicating that
the greatest degree of ring puckering occurs at these positions,
the distortion being greatest at the C6 position. The magnitudes
of ring torsion angles indicate that both C6 and N1 are
displaced from the ring in the same direction, which imparts the
boat-type conformation to the dihydropyrimidine ring
Synthesis and characterisation
Melting points were determined in open capillaries. The IR spectra
of samples as KBr pellets were recorded on a Shimadzu FTIR 8400
instrument. NMR spectra of the samples in CDCl3 were recorded on
an AMX 400 NMR spectrometer.
Ethyl 3-acetyl-4-(4-methoxyphenyl)-6-methyl-2-thioxo-1,2,3,4-tetra-
hydropyrimidine-5-carboxylate (1): Ethyl acetoacetate (3.12 g, 24 mol),
anisaldehyde (2.72 g, 20 mmol), thiourea (1.83 g, 24 mmol), and
/L%UꢀꢄꢇꢁꢆꢃꢍꢀJꢊꢀꢉꢀPPROꢈꢀZHUHꢀUHÀX[HGꢀWRJHWKHUꢀIRUꢀꢍꢀKꢀLQꢀDFHWRQLWULOHꢀ
(25 mL). The reaction mixture was cooled, poured onto crushed ice,
and stirred for several minutes. The ethyl 4-(4-methoxyphenyl)-6-
methyl-2-thioxo-1,2,3,4-tetrahydropyrimidine-5-carboxylate (3) was
¿OWHUHGꢀ RIIꢊꢀ ZDVKHGꢀ ZLWKꢀ FROGꢀ ZDWHUꢊꢀ GULHGꢀ DQGꢀ UHFU\VWDOOLVHGꢀ IURPꢀ
ethanol, from which it separated as a white solid (5.02 g, 82%), m.p.
172°C (lit.16ꢀPꢁSꢁꢀꢆꢍꢇ±ꢆꢍꢉꢀƒ&ꢈꢁ
Due to the aryl group axially bisecting this boat like
dihydropyrimidine ring, the second methoxy group, being an
additional aryl substituent, is forced into the synperiplanar
RULHQWDWLRQꢀUHODWLYHꢀWRꢀ&ꢋ±+ꢁ
…
The molecular packing of compound 2ꢀ UHYHDOVꢀ 1±+
S
…
LQWHUDFWLRQVꢀZLWKꢀ1ꢉ±+1ꢉ S1 hydrogen bonds generating a
centrosymmetric dimer of graph set R22 (8) (Fig. 5). In addition
Compound 3 (2.0 g) was mixed with acetic anhydride (10 mL) and
UHÀX[HGꢀIRUꢀꢎꢀKRXUVꢁꢀ7KHꢀUHDFWLRQꢀPL[WXUHꢀZDVꢀFRROHGꢀDQGꢀGLOXWHGꢀ
by addition of water (20 mL). The acetyl derivative separated as an
RLOꢊꢀ ZKLFKꢀ VROLGL¿HGꢀ RQꢀ SURORQJHGꢀ VWLUULQJꢁꢀ 7KHꢀ VROLGꢀ ZDVꢀ ZDVKHGꢀ
with more water. White crystals of this solid (1, 1.95 g, 86%) were
obtained by slow evaporation from a solution in chloroform.
IR: Qmax 3186, 3132, 2993, 1705, 1643, 1227, 1026 cm-1. NMR: GH
ꢌꢁꢍꢀꢄVꢊꢀꢆ+ꢈꢊꢀꢋꢁꢌꢉ±ꢃꢁꢉꢉꢀꢄPꢊꢀꢎ+ꢈꢊꢀꢋꢁꢋꢇꢀꢄVꢊꢀꢆ+ꢈꢊꢀꢎꢁꢉꢆꢀꢄTꢊꢀJ = 7 Hz, 2H), 3.77
(s, 3H), 2.77 (s, 3H), 2.41 (s, 3H), 1.27 (t, J = 7 Hz, 3H).
…
WRꢀ WKHVHꢀ LQWHUDFWLRQVꢀ WKHUHꢀ DUHꢀ DOVRꢀ 1±+ O interactions
linking the molecules into a chain along the crystallographic
…
aꢀ D[LVꢀ 7KHꢀ 1±+ S hydrogen bonds link the molecules in
dimers about centres of symmetry, and the dimers are linked
in chains, in a ladder formation, parallel to the a axis.
Conclusion
Ethyl 4-(3,4-dimethoxyphenyl)-6-methyl-2-thioxo-1,2,3,4-tetrahydro-
pyrimidine-5-carboxylate (2):Asolution of ethyl acetoacetate (3.12 g,
24 mM), veratraldehyde (3.32 g, 20 mM), thiourea (1.83 g, 24 mM)
DQGꢀ/L%UꢀꢄꢇꢁꢆꢃꢍꢀJꢊꢀꢉꢀP0ꢈꢀLQꢀDFHWRQLWULOHꢀꢄꢉꢍꢀP/ꢈꢀZDVꢀUHÀX[HGꢀIRUꢀꢍꢀK.
After cooling, the reaction mixture was poured onto crushed ice and
VWLUUHGꢀIRUꢀDꢀIHZꢀPLQXWHVꢁꢀ7KHꢀVROLGꢀSURGXFWꢀZDVꢀ¿OWHUHGꢀRIIꢊꢀZDVKHGꢀ
with cold water, dried and recrystallised from ethanol. Yield: 5.24
g (78%), m.p. 152°C (lit.17ꢀPꢁSꢁꢀꢆꢎꢅ±ꢆꢍꢇꢀƒ&ꢈꢁꢀ3DOHꢀEURZQꢀFU\VWDOVꢀ
suitable for crystallography were obtained by slow evaporation from
a mixture of ethanol and ethyl acetate.
In conclusion, the authors consider that, given the intense
interest generated by the therapeutic activities of the
dihydropyrimidines and subsequent studies conducted on
their structural aspects, the two compounds discussed here
are analogues possessing the special structural requirements
for calcium channel modulation. They are therefore highly
relevant to the design of new cardiovascular drugs and are,
therefore, suitable for further pharmacological testing.
The crystal structure analyses of the two compounds
revealed several structural aspects which are prerequisites
for calcium antagonist activity as suggested by the well-
documented structure-activity studies carried out on both
DHPs and DHPMs. These include the latent structural
asymmetry, the substituted phenyl ring at C4, the resultant
perpendicular conformation of the aromatic ring at C4 with
respect to the dihydropyrimidine ring, the non-identical
substitution at positions 3 and 5, methyl substitution at C6
and the sulfur group at C2. Further, it is interesting to note
WKHꢀLQÀXHQFHꢀRIꢀDQꢀDGGLWLRQDOꢀVXEVWLWXHQWꢀRQꢀWKHꢀSKHQ\OꢀULQJꢀLQꢀ
compound 2, which causes an enhancement of these desirable
features, suggesting optimum calcium antagonist activity.
Inspection of the extended crystal structures also showed the
IR: Qmax 3209, 3155, 2962, 1705, 1643, 1227, 1026, 1589 cm-1.
NMR: GHꢀꢌꢁꢌꢏꢀꢄVꢊꢀꢆ+ꢈꢊꢀꢋꢁꢃꢏ±ꢋꢁꢅꢇꢀꢄꢏ+ꢈꢊꢀꢋꢁꢍꢌꢀꢄVꢊꢀꢆ+ꢈꢊꢀꢎꢁꢉꢆꢀꢄTꢊꢀJ = 7 Hz,
2H), 3.81 (s, 6H), 2.39 (s, 3H), 1.26 (t, J = 7 Hz, 3H).
Crystal structure determination
The X-ray diffraction data for compounds 1 and 2 were collected
on
a Bruker Smart CCD Area Detector System using MoKa
(0.71073Å) radiation. The data were processed using SAINTPLUS.18
The structures were solved by direct methods and difference
Fourier synthesis using SHELXS97.19 The positions and anisotropic
displacement parameters of all non-hydrogen atoms were included in
WKHꢀIXOOꢀPDWUL[ꢀOHDVWꢂVTXDUHꢀUH¿QHPHQWꢀXVLQJꢀ6+(/;/ꢅꢃꢁ20 Molecular
diagrams were generated using ORTEP.21
Ethyl 3-acetyl-4-(4-methoxyphenyl)-6-methyl-2-thioxo-1,2,3,4-tetra-
hydropyrimidine-5-carboxylate (1)
…
…
OHVVꢀFRPPRQꢀ1±+ 6ꢀLQWHUDFWLRQVꢀDORQJꢀZLWKꢀ&±+ O and
Intensity data were collected up to a maximum of 28.13° for the
compound in the Z±ɮꢀVFDQꢀPRGHꢁꢀ$ꢀWRWDOꢀRIꢀꢆꢍꢊꢇꢋꢇꢀUHÀHFWLRQVꢀZHUHꢀ
collected, resulting in 4130 (Rintꢀ ꢀ ꢇꢁꢇꢉꢎꢈꢀ LQGHSHQGHQWꢀ UHÀHFWLRQVꢀ
RIꢀZKLFKꢀWKHꢀQXPEHUꢀRIꢀUHÀHFWLRQVꢀVDWLVI\LQJꢀI !ꢉꢀıꢄI) criteria were
3175. These were treated as observed. The hydrogen atoms were
¿[HGꢀ JHRPHWULFDOO\ꢀ DQGꢀ ZHUHꢀ UH¿QHGꢀ LVRWURSLFDOO\ꢁꢀ 7KHꢀ 5ꢀ LQGLFHVꢀ
for all data were R1 = 0.0903 and wR2 = 0.2026. The R factor for
µREVHUYHG¶ꢀ GDWDꢀ ¿QDOO\ꢀ FRQYHUJHGꢀ WRꢀ ꢇꢁꢇꢃꢇꢃꢀ ZLWKꢀ Z52 = 0.1880.
The maximum and minimum values of residual electron density were
ꢇꢁꢋꢏꢇꢀDQGꢀ±ꢇꢁꢎꢆꢋꢀHc-3.
…
1±+ O hydrogen bonds in the packing of both compounds.
Crystal data for 1: C17H20N2O4S, formula weight = 348.41,
monoclinic, P21/c, a = 9.092(4)Å, b = 20.483(10)Å, c = 9.867(5)Å,
ß = 106.573(7)0, V = 1761.3(14)Å3, Z = 4, μ = 0.206 mm-1, Dx = 1.314
Mg m-3, T = 293(2)K.
Ethyl 4-(3,4-dimethoxyphenyl)-6-methyl-2-thioxo-1,2,3,4-tetrahydro-
pyrimidine-5-carboxylate (2)
Intensity data were collected up to a maximum of 28.33° for the
compound in the Z±ɮꢀVFDQꢀPRGHꢁꢀ$ꢀWRWDOꢀRIꢀꢆꢎꢊꢏꢆꢎꢀUHÀHFWLRQVꢀZHUHꢀ
collected, resulting in 3967 (Rintꢀ ꢀꢇꢁꢇꢉꢎꢈꢀLQGHSHQGHQWꢀUHÀHFWLRQVꢀRIꢀ
ZKLFKꢀWKHꢀQXPEHUꢀRIꢀUHÀHFWLRQVꢀVDWLVI\LQJꢀI !ꢀꢉꢀıꢄI) criteria were
2983. These were treated as observed. The hydrogen atoms (a few
located in ')ꢀ PDSVꢀ DQGꢀ Dꢀ IHZꢀ JHRPHWULFDOO\ꢀ ¿[HGꢈꢀ ZHUHꢀ UH¿QHGꢀ
isotropically. The R indices for all data were R1 = 0.1043 and wR2 =
ꢇꢁꢀꢆꢎꢍꢅꢁꢀ7KHꢀ5ꢀIDFWRUꢀIRUꢀµREVHUYHG¶ꢀGDWDꢀ¿QDOO\ꢀFRQYHUJHGꢀWRꢀꢇꢁꢇꢃꢎꢇꢀ
with wR2 = 0.1344. The maximum and minimum values of residual
HOHFWURQꢀGHQVLW\ꢀZHUHꢀꢇꢁꢏꢏꢀDQGꢀ±ꢇꢁꢉꢉHc-3.
…
Fig. 5 Packing in 2 due to N-H S dimers viewed along the
a axis.

204 JOURNAL OF CHEMICAL RESEARCH 2009
4
K. Atwal, G.C. Rovnyak, S.D. Kimball, D.M. Floyd, S. Moreland,
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J.Z. Gougoutas, A. Hedberg, M. Malley, J.P. McCarthy, R. Zhang and
S. Moreland, J. Med. Chem., 1995, 38, 119.
Crystal data for 2: C16H20N2O4S, formula weight = 336.40,
monoclinic, P21/c, a = 7.2181(9)Å, b = 24.592(3)Å, c = 10.1024(13)Å,
ß = 107.845(2)0, V = 1707.0(4)Å3, Z = 4, μ = 0.210 mm-1, Dx = 1.309
Mg m-3, T = 293(2)K.
5
6
Supplementary material
Crystallographic data for the structures reported in this paper
have been deposited with the Cambridge Crystallographic
Data Centre. The deposition numbers are CCDC 686220
(compound 1) and CCDC 686219 (compound 2).
7
8
9
DEV thanks UGC-FIP for the Teacher Fellowship.
The authors thank the University Grants Commission for the
DRS grant. They also thank the Department of Science and
Technology, for data collection on the CCD facility set up
under the IRHPA-DST program.
10 D.J. Triggle and S. Padmanabhan, Chemtracts: Org. Chem., 1995, 8, 191.
11 P. Biginelli, Gazz. Chim. Ital., 1893, 23, 360.
12 C.O. Kappe, Tetrahedron, 1993, 49, 6937.
13 N.S. Begum and D.E. Vasundhara, Acta. Crystalogr., 2006, E62, o5796.
14 N.S. Begum and D.E. Vasundhara, Acta. Crystalogr., 2007, E63, o3741.
15 M. Gourhari, K. Pradip and G. Chandrani, Tetrahedron Lett., 2003, 44,
2757.
Received 30 April 2008; accepted 13 January 2009
Paper 08/5252 doi:10.3184/030823409X425064
Published online: 29 April 2009
16 N.Y. Fu, Y.F. Yuan, Z. Cao, S.W. Wang, J.T. Wang and C. Peppe,
Tetrahedron, 2002, 58, 4801.
17 M.S. Akhtar, M. Seth and A.P. Bhaduri, Indian J. Chem. Sect. B, 1987, 26,
556.
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UH¿QHPHQWꢊꢀ8QLYHUVLW\ꢀRIꢀ*|WWLQJHQꢊꢀ*HUPDQ\ꢁ
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