Knoevenagel condensation catalyzed by K2NiP2O 7. Synthesis of (E)-methyl-α-cyanocinnamates in high yields

Article abstract of DOI:10.1023/A:1026155323394

The Knoevenagel reaction of benzaldehyde and several chloroderivatives with methyl cyanoacetate catalyzed by K₂NiP₂O₇ leads to methyl (E)-α-cyanocinnamate derivatives in 40 min with yields of 71.65-83.45%. Pure products ar

Full text of DOI:10.1023/A:1026155323394

C
Journal of Chemical Crystallography, Vol. 33, No. 10, October 2003 ( 2003)
Knoevenagel condensation catalyzed by K2NiP2O7.
Synthesis of (E)-methyl- -cyanocinnamates in high yields
Aicha El Maadi,(1) Chance L. Matthiesen,(2) Philip Ershadi,(2) Jennifer Baker,(2)
Derrick M. Herron,(2) and Elizabeth M. Holt(2)
Received October 28, 2002
The Knoevenagel reaction of benzaldehyde and several chloroderivatives with methyl
cyanoacetate catalyzed by K2NiP2O7 leads to methyl (E)- -cyanocinnamate derivatives in
40 min with yields of 71.65–83.45% . Pure products are easily obtained in crystalline form,
uncontaminated by side products or by stereoisomers. Methyl 2-chlorocyanocinnamate,
methyl 2,4-dichlorocyanocinnamate, and a new polymorph of methyl cyanocinnamate have
been characterized by single crystal X-ray diffraction.
KEY WORDS: heterogeneous catalysis; diphosphate catalysis; Knoevenagel condensation; methyl (E)- -
cyanocinnamate derivatives.
Introduction
The Knoevenagel condensation of an alde-
hyde and an active methylene compound is nor-
mally carried out in the presence of a base to re-
move the somewhat acidic or “active” hydrogen
atom (Scheme 1).
Often secondary amines or piperidine are
used as the base. Of late there has been interest
in carrying out synthetic organic reactions under
conditions which do not require separation of the
product from other organic materials (such as the
base), which are fast, effective, and which do not
require heating or the use of large quantities of
solvent.
In an attempt to meet these conditions for the
Knoevenagel condensation, a number of efforts
have been directed at the use of heterogeneous
catalysts which can be removed from the reaction
mixture by filtration. Weak basic ion exchange
resins,1 KF, CsF,2,3 TiCl44, CdI2,5 and ZnCl62 have
found use in this domain.
Al2O3, which has both basic and acidic
sites, has been used to catalyze the condensa-
tion of active methylene compounds and aldehy-
des on the surface of solid Al2O3. The reaction
is reported to be fast (2–3 min). The product is
extracted from the surface of the solid Al2O3 with
dichloromethane.7
4-Chlorobenzaldehyde plus ethyl cyanoac-
etate gives high yields of the E product (84%)
using Xonotlite doped with Me3COK at ambient
temperatures.8
Alkali and alkaline earth containing phos-
phate materials (Na2CaP2O7, Na4P2O7, and
Ca2P2O7 have shown effectiveness in Knoeve-
nagel catalysis.9 Natural phosphate (similar to
(1) Departement de Chimie, Laboratoire de Physico-Chimie du
Solide, Faculte´ des Sciences, Universite´ ibn Tofail, Ke´nitra,
Morocco.
(2) Department of Chemistry, Oklahoma State University, Stillwater,
Oklahoma, 74078.
To whom correspondence should be addressed. E-mail: betsy@
biochem.okstate.edu
757
C
1074-1542/03/1000-0757/0 2003 Plenum Publishing Corporation

758
El Maadi, Matthiesen, Ershadi, Baker, Herron, and Holt
Table 1. Crystallographic Data for Methyl (E)- -Cyanocinnamate
(I), Methyl (E)-2-Chloro- -Cyanocinnamate (II), and Methyl (E)-
2,4-Dichloro- -Cyanocinnamate (III)
Compound
I
II
III
CODC deposit no. 192568
Molecular formula C11H9NO2
192628
C11H8NO2Cl C11H7NO2Cl2
192629
Scheme 1.
Mass
187.19
5.531(1)
10.053(1)
17.374(4)
90.0
90.0
90.0
966.0(3)
4
1.287
392
221.63
18.887(9)
7.505(4)
7.308(3)
90.0
101.07(1)
90.0
1016.6(8)
4
256.08
3.919(2)
8.377(4)
17.539(8)
87.75(3)
88.98(4)
80.25(4)
567.0(5)
2
˚
a, A
˚
b, A
Ca10(PO4)6F2) doped with KF and/or sodium
nitrate has been reported to catalyze Kno-
evenagel reactions.10 Magnesium phosphates,
NaMg4(PO4)3, Mg3(PO4)2, -Mg2P2O7, and
NaMgPO4 have been studied as catalysts.11
However phosphates containing transition
metals such as K2NiP2O7 etc. have also shown
effectiveness.12,13 In this effort, we have investi-
gated the use of K2NiP2O7 as a heterogeneous
catalyst in the reaction of methyl cyanoacetate
with benzaldehyde, 2-chlorobenzaldehyde, and
2,4-dichlorobenzaldehyde, stirred in methanol for
40 min.
Besides the product yield and the identity
of the product, of specific interest was the stere-
ochemistry of the methyl- -cyanocinnamates
produced. Both (E)- and (Z)-methyl- -cyano-
cinnamate are known.7,14 Only the (E) form of
methyl 2-chloro- -cyanocinnamate is known15
and there have been reports of methyl 2,4-
dichloro- -cyanocinnamate of undetermined
stereochemistry.16 Thus we have identified the
stereochemistry of the products of the phosphate-
catalyzed Knoevenagel reaction by single crystal
X-ray diffraction (Table 1).
˚
c, A
, deg
, deg
, deg
3
˚
Volume, A
Z
d, Mg/m3
F(000)
1.448
456
0.352
1.500
260
0.554
1
, mm
0.090
Crystal system
Space group
Diffractometer
Radiation
Orthorhombic Monoclinic
Triclinic
P212121
Siemens P4
Mo K
P21/c
Siemens P4
Mo K
P
1
Siemens P4
Mo K
Monochromator
Highly
oriented
graphite
crystal
0.1 0.1
0.1
Highly
oriented
graphite
crystal
0.05 0.1
0.1
Highly
oriented
graphite
crystal
0.1 0.1
0.1
Crystal size
Crystal form
Temperature, K
Data collecting
mode
range, deg
Reciprocal lattice
segment
needle
301
prism
301
needle
301
2
2
2
2.34 to 28.70 2.20 to 29.99 2.32 to 25.02
1 < h <
7; 1 < k <
13;
26 < h < 1;
10 < k <
1; 10 <
1 < 10
4 < h < 1;
9 < k < 9;
20 < 1 < 20
1 < 1 < 13
Reflections
measured
Number of
symmetry
independent
reflections
Method of
solution
2026
2119
2911
1852
1843
1997
Direct
Direct
Direct
Method of
refinement
Full-matrix
least-
Full-matrix
least-
Full-matrix
least-
Experimental
squares
128/1852
0.0233
squares
137/1843
0.0878
squares
145/2911
0.0733
Parameter/Fo
Requiv
Synthesis
R[I > 2 (I)]
Rw
0.0521
0.0822
0.0597
0.0865
0.0697
0.0954
Benzaldehyde(I), 2-chlorobenzaldehyde(II)
or 2,4-dichlorobenzaldehyde(III) (3 mmol), and
methyl cyanoacetate (3 mmol) and four drops
of water in methanol (1.6 mL (I), 7 mL (II),
and 10 mL (III)) were mixed to give a solution.
K2NiP2O7(0.200 g) was added and the mixture
stirred for 40 min at ambient temperature. The
catalyst was removed by filtration and washed
with methanol. The filtrate and the MeOH used
for washing were combined and used for GC-MS
analysis. After standing for 3–5 h, crystallization

Methyl (E)- -cyanocinnamate derivatives
759
took place. Percent yields: 71.65% (I). 77.10%
(II), 83.45% (III). Melting points, (I) 87; (II) 104;
(III) 104 C.
using K2NiP2O7 led to a yield of 71.65% of
the condensation product. Higher yields (77.10%
(II) and 83.45% (III)) were observed when the
benzaldehyde was substituted with electron with-
drawing groups. The purity of the product was es-
tablishedbyGC-MS. Asingleproductwaspresent
in each case. Because of the low solubility of
the products in methanol, crystalline condensation
products, (I), (II), and (III), were easily isolated.
Single crystal X-ray diffraction allowed identifi-
cation of the stereochemistry of the product.
The single product produced in each
case was an (E)-methyl- -cyanocinnamate;
methyl cyanocinnamate (I), methyl 2-chloro-
cyanocinnamate (II), and methyl 2,4-dichloro-
cyanocinnamate (III). Figures 1(a–c) are based
on the positional parameters of Tables 2, 3, and
4. One might predict the stability of the E isomer
on the basis of relief of steric crowding about the
double bond, i.e. the bulky phenyl and methylac-
etate groups would prefer to be trans to each other
about the double bond; however both (E) and (Z)
methyl cyanocinnamate isomers are known. Only
the (E) isomer of methyl 2-chlorocyanocinnamate
is known and the conformation of the methyl
2,4-dichlorocinnamate previously isolated has
not been reported.
Gas chromatography/mass spectral analysis
Methanol filtrates were examined by GC/MS
analysis using a Hewlett Packard G1800A GCD
System [column: crosslinked 5% Ph Me Sili-
cone coating in 30 m
0.25 mm
0.25
m
film thickness; helium flow 1.0 mL/min, inlet
150 C, detector 280 C, oven 70–290 C, ramp
rate 4 /min] retention times 35.89 (I), 39.62 (II),
44.4 min (III).
Single crystal X-ray diffraction
Crystals of (I), (II), and (III) were mounted
on glass fibers for single crystal X-ray diffraction
analysis. Diffraction data were collected using an
automated four-circle Siemens P4 diffractometer
equipped with graphite monochromated Mo K
˚
radiation ( = 0.71073 A). Data was collected
at ambient temperature with a -2 data collec-
tion mode (XSCANS).17 Three standard reflec-
tions were remeasured after each 97 data points
and these showed no decline in intensity during
data collection. Data were corrected for Lorentz,
polarization, and background effects. Atomic po-
sitional parameters were determined using di-
rect methods (SHELXS)18 and refinement was
carried out using full-matrix least-squares tech-
niques (SHELXL).19 Semiempirical absorption
corrections were applied using XEMP.20 Hydro-
gen atoms were placed in theoretical positions
Table 2. Positional Parameters for (I)
U(eq)
Atom
x( (x))
y( (y))
z( (z))
103a
N1
O1
O2
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
0.5756(5) 0.5005(3)
0.7753(3) 0.25627(15) 0.34343(8)
0.23095(11) 0.0672(7)
0.0484(4)
0.0580(5)
0.5271(3) 0.2147(2)
0.5831(4) 0.2794(2)
0.4453(4) 0.3977(2)
0.5179(4) 0.4571(2)
0.2622(4) 0.4421(2)
0.1017(4) 0.5557(2)
0.1261(5) 0.6572(2)
0.0404(5) 0.7592(2)
0.2289(5) 0.7636(2)
0.2528(4) 0.6660(2)
0.0883(4) 0.5638(2)
0.9249(5) 0.1450(2)
0.44228(8)
0.38666(11) 0.0386(4)
0.36021(10) 0.0371(4)
0.28850(11) 0.0455(5)
0.40401(10) 0.0388(4)
0.39700(10) 0.0385(4)
0.34227(13) 0.0504(5)
0.33975(14) 0.0575(6)
0.39101(15) 0.0551(6)
0.44633(14) 0.0515(6)
0.44946(11) 0.0447(5)
0.36645(15) 0.0570(6)
˚
0.97 A from the bonded carbon atom. Details of
data collection and crystal structure are presented
in Table 1.
Discussion
Catalysis of the Knoevenagel reaction of
benzaldehyde, 2-chlorobenzaldehyde and 2,4-
dichlorobenzaldehyde with methyl cyanoacetate
aU(eq) is defined as one third of the trace of the orthogonalized U
ij
tensor.

760
El Maadi, Matthiesen, Ershadi, Baker, Herron, and Holt
Fig. 1. (a) Projection view of methyl (E)- -cyanocinnamate (I); (b) Projection view of methyl (E)-2-chloro- -cyanocinnamate
(II); (c) Projection view of methyl (E)-2,4-dichloro- -cyanocinnamate(III). Thermal ellipsoids for (I), (II), and (III) are shown
at the 50% probability level.

Methyl (E)- -cyanocinnamate derivatives
761
Table 3. Positional Parameters for Methyl (E)-2-Chloro- -
Cyanocinnamate (II)
U(eq)
Atom
x( (x))
y( (y))
z( (z))
103a
Cl1
O1
O2
N1
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
0.41072(5)
0.08479(12) 0.8280(3)
0.18291(15) 0.7079(4)
0.92590(11) 0.3462(2) 0.0615(5)
0.2373(5) 0.0467(9)
0.4115(6) 0.0626(11)
0.0791(7) 0.0561(13)
0.3226(6) 0.0418(11)
0.2921(6) 0.0371(10)
0.1750(6) 0.0405(10)
0.3677(6) 0.0391(10)
0.3552(6) 0.0366(10)
0.3555(6) 0.0419(11)
0.3480(7) 0.0506(14)
0.3404(7) 0.0540(14)
0.3391(7) 0.0503(13)
0.3481(6) 0.0419(11)
0.2446(7) 0.0622(15)
0.1172(2)
0.1530(2)
0.1892(2)
0.1488(2)
0.2587(2)
0.3056(2)
0.2822(2)
0.3286(2)
0.3999(2)
0.4242(2)
0.3779(2)
0.0463(2)
1.2333(5)
0.8282(4)
0.9997(4)
1.1294(5)
1.0159(4)
1.1671(4)
1.3429(4)
1.4817(4)
1.4485(5)
1.2770(5)
1.1399(4)
0.6627(5)
aU(eq) is defined as one third of the trace of the orthogonalized U
ij
tensor.
The structure of the (E)-isomer, (E)-methyl-
-cyanocinnamate(I) has already been reported
in monoclinic space group C2/c with param-
˚
˚
eters a = 12.386(2) A, b = 8.237(1) A, c =
˚
19.320(2) A, = 98.49(5) , and Z = 8. Those
Fig. 2. (a) Packing of (E)-methyl- -cyanocinnamate (mono-
clinic cell based on parameters of Ref. 21). Projection down
the b-axis. (b) Packing of (I) (orthorhombic cell). Projection
down the a axis. Thermal ellipsoids are shown at the 50%
probability level.
crystals were reported to be prisms.17
Table 4. Positional Parameters for Methyl 2,4-Dichloro- -
Cyanocinnamate (III)
U(eq)
Atom
x( (x))
y( (y))
z( (z))
103a
The crystals isolated in this work were
needlelike. The single crystal X-ray diffrac-
tion data showed the material isolated to be
a polymorph of the reported (E)-methyl- -
cyanocinnamate and to crystallize in orthorhom-
bic space group P212121 with cell parameters a =
Cl1
Cl2
O1
O2
N1
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C11
0.1631(4)
0.4525(4)
0.23745(15) 0.91978(9) 0.0694(5)
0.40406(14) 0.90434(8) 0.0647(5)
1.2315(10) 0.5485(5)
0.9702(11) 0.6567(5)
1.1635(15) 0.1468(7)
1.0515(13) 0.5410(7)
0.9623(11) 0.3708(6)
1.0715(13) 0.2467(8)
0.7897(11) 0.3493(6)
0.6556(11) 0.2041(5)
0.6667(12) 0.0451(6)
0.5252(12) 0.0899(6)
0.3581(11) 0.0695(5)
0.3461(11) 0.0820(5)
0.4932(10) 0.2158(5)
0.5928(2)
0.6913(2)
0.5991(3)
0.6579(3)
0.6843(3)
0.6372(3)
0.7508(3)
0.7915(3)
0.7635(3)
0.8021(3)
0.8715(3)
0.9024(3)
0.8627(3)
0.5622(4)
0.0789(12)
0.0821(12)
0.0918(16)
0.0617(14)
0.0526(12)
0.0645(14)
0.0504(11)
0.0457(11)
0.0554(12)
0.0552(12)
0.0497(11)
0.0501(11)
0.0430(10)
0.097(2)
˚
˚
˚
5.531(1) A, b = 10.053(1) A, c = 17.374(4) A.
While both structures have E conformation, the
packing of molecules in the unit cell differs in
the two polymorphs. Both show stacking of aro-
matic rings. The monoclinic crystal shows stacks
of molecules with their long axes parallel and
with alternating placement of the aromatic ring
(Fig. 2a). The orthorhombic crystal isolated in
this work shows that the regular overall parallel
packing has been disrupted to yield two different
1.335(2)
0.7074(8)
aU(eq) is defined as one third of the trace of the orthogonalized U
tensor.
ij

762
El Maadi, Matthiesen, Ershadi, Baker, Herron, and Holt
Table 5. Bond Angles and Distances for (I), (II), and (III)
(II)
(I)
(III)
---
N1 C3
---
N1 C3
---
1.137(5) N1 C3
1.137(3)
1.322(3)
1.448(3)
1.205(2)
1.485(3)
1.343(3)
1.439(3)
1.451(3)
1.393(3)
1.402(3)
1.379(3)
1.372(4)
1.380(3)
1.374(3)
1.152(7)
1.329(6)
1.445(7)
1.200(6)
1.498(7)
1.342(6)
1.420(8)
1.450(7)
1.399(6)
1.409(6)
1.371(7)
1.390(7)
1.369(6)
1.379(6)
1.728(5)
1.742(4)
---
---
---
O1 C1
O1 C1
1.319(5) O1 C1
---
O1 C11
---
O1 C11
---
1.444(4) O1 C11
---
---
---
O2 C1
O2 C1
1.190(5) O2 C1
---
C1 C2
---
C1 C2
---
1.494(4) C1 C2
---
---
---
C2 C4
C2 C4
1.329(6) C2 C4
---
C2 C3
---
C2 C3
---
1.419(6) C2 C3
---
---
---
C4 C5
C4 C5
1.453(4) C4 C5
---
C5 C10
---
C5 C10
---
1.392(4) C5 C10
---
---
---
C5 C6
C5 C6
1.392(4) C5 C6
---
C6 C7
---
C6 C7
---
1.369(5) C6 C7
---
---
---
C7 C8
C7 C8
1.381(5) C7 C8
---
C8 C9
---
C8 C9
---
1.367(5) C8 C9
---
---
---
C9 C10
C9 C10
1.360(5) C9 C10
---
Cl1 C10
---
1.723(3) Cl1 C8
---
Cl2 C10
--- ---
--- ---
--- ---
--- ---
--- ---
--- ---
--- ---
--- ---
--- ---
--- ---
115.8(3) C1 O1 C11
C1 O1 C11 116.00(17) C1 O1 C11
116.7(5)
123.9(5)
124.2(5)
111.9(5)
125.7(5)
117.1(5)
117.3(4)
179.3(6)
131.7(5)
115.3(4)
120.2(4)
124.5(4)
123.0(4)
118.7(4)
120.9(4)
119.3(4)
119.8(4)
119.2(4)
122.9(4)
116.3(3)
120.8(3)
--- ---
--- ---
O2 C1 O1 124.57(19) O2 C1 O1
125.2(3) O2 C1 O1
--- ---
--- ---
123.5(4) O2 C1 C2
O2 C1 C2 123.24(19) O2 C1 C2
--- ---
--- ---
O1 C1 C2 112.17(16) O1 C1 C2
111.3(3) O1 C1 C2
--- ---
--- ---
124.2(3) C4 C2 C3
C4 C2 C3 124.23(19) C4 C2 C3
--- ---
--- ---
C4 C2 C1 118.55(17) C4 C2 C1
117.5(3) C4 C2 C1
--- ---
--- ---
118.2(3) C3 C2 C1
C3 C2 C1 117.21(19) C3 C2 C1
--- ---
--- ---
N1 C3 C2 N1 C3 C2
178.0(2)
178.8(3) N1 C3 C2
--- ---
--- ---
--- ---
--- ---
--- ---
127.7(4) C2 C4 C5
C2 C4 C5 132.48(18) C2 C4 C5
--- ---
--- ---
C10 C5 C6 118.30(19) C10 C5 C6
116.9(3) C10 C5 C6
--- ---
--- ---
120.2(3) C10 C5 C4
C10 C5 C4 116.88(17) C10 C5 C4
--- ---
--- ---
--- ---
C6 C5 C4
124.82(19) C6 C5 C4
122.9(3) C6 C5 C4
--- ---
C7 C6 C5
--- ---
C7 C6 C5
--- ---
121.0(3) C7 C6 C5
119.9(2)
120.7(2)
120.1(2)
119.8(2)
--- ---
--- ---
--- ---
C8 C7 C6
C6 C7 C8
120.0(3) C6 C7 C8
--- ---
C7 C8 C9
--- ---
C9 C8 C7
--- ---
120.1(3) C9 C8 C7
--- ---
--- ---
--- ---
C10 C9 C8
C10 C9 C8
119.5(3) C9 C8 Cl1
--- ---
C9 C10 C5
---
---
--- ---
122.4(3) C7 C8 Cl1
121.14(19) C9 C10 C5
--- ---
--- ---
C9 C10 Cl1
--- ---
118.0(2) C8 C9 C10
---
---
C5 C10 Cl1
119.6(2) C9 C10 C5
--- ---
C9 C10 Cl2
--- ---
C5 C10 Cl2
directions of placement of the long molecular axis
(Fig. 2b).
(I); 37.6, (II); 6.8 (III)). Since both (II) and (III)
involve ortho substitution of chloride groups on
the phenyl ring, the larger deviation from copla-
narity observed for (II) must be due to packing
influences. Bond angles and distances are similar
for the three compounds (Table 5).
The catalyst has been consistent in producing
only the E isomer. Further experimental work will
be needed to establish if the stereochemical pref-
erence is a due to thermodynamic considerations
or to the noncentric space group22 of the solid
K2NiP2O7 used as catalyst.
Supplementry material
Products (I), (II), and (III) show relative
coplanarity of the cyanoacetate moiety of the
molecule. However, the angles subtended by the
phenyl ring and this moiety differ greatly: (5.1,
Crystallographic Data for (I), (II), and
(III) have been deposited with the Cambridge
Crystallographic Data Center as supplementary

Methyl (E)- -cyanocinnamate derivatives
763
10. S. Sebti, S.; Smahi, A.; Solhy, A. Tetrahedron Lett. 2002, 43,
1813–1815.
11. Aramendia, M.A.; Boreau, V.; Jimenez, C.; Marinas. J.M.;
Romero, F.J. Kinet. Cobal. Lett. 2000, 69(2), 311–314.
12. Bennazha, J. These d’Etat, Universite Mohammed V. Rabat
Maroc, 2002.
publication numbers CCDC 192568, 192628,
192629. Copies of the data can be obtained, free of
charge, on application to CCDC, 12 Union Road,
Cambridge, CB2, 1EZ, UK [FAX: +44(0)-1223-
336033 or e-mail: deposit@ccdc.cam.ac.uk].
13. El Maadi, A. These d’Etat, Universite Mohammed V. Rabat
Maroc, 2002.
14. Ishikawa, K.; Griffin, G.W.; Lev, I.J. J. Org. Chem. 1976, 41(23),
3747–3752.
References
15. Pita, I.; da R.; Boucherie, A.; Pillon, D. Eur. J. Med. Chem.
1974, 9(3), 347–349.
16. Kim, K.J.; Morales, M.; Scully, M.J.; Seitz, C.D.; Sikora, A.M.;
Spaulding, A.M.; Sudman, R.; Sullivan, A.C.; Dean, B.C.;
Kharas, G.B.; Watson, K. Des. Monom. Polym. 1999, 2(4), 333–
341.
17. XSCANS Users Manual; Siemens Analytical X-Ray Instru-
ments, Inc., Madison, WI, 1991.
18. Sheldrick, G.M. Acta Cryst. A46 1990, 467.
19. Sheldrick, G.M. SHELXL-98, A Program for the Refinement of
Crystal Structures From Diffraction Data; University of Gottin-
gen, Germany, 1998.
1. Jones, G. Organic Reactions, Vol 15; Wiley; New York, 1967,
pp. 15, 204–207.
2. Rand, L.; Swisher, J.V.; Cronin, C.J. J. Org. Chem. 1962, 27,
3505–3509.
3. Rand, L.; Haidukewych, D.; Dolinski, R.H. J. Org. Chem. 1966,
31, 1272–1274.
4. Lehnert, W. Tetrahedron 1974, 30, 301–307.
5. Prajapati, D.; Sandhu, J. S. 1993, J. Chem. Soc., Perkin Trans I,
739–740.
6. Rao, P.S.; Venkataratnam, R.V. Tetrahedron Lett. 1991, 32,
5821–5822.
7. Texier-Boullet, F.; Foucaud, A. Tetrahedron Lett. 1982, 23(47)
4927–4928.
20. Siemens XEMP; Siemens Analytical X-Ray, Instruments, Inc.,
Madison, WI, 1991.
8. Chalais, S.; Laszlo, P.; Mathy, A. Tetrahedron Lett. 1985, 26(37)
4453–4454.
9. Bennazha, J.; Zahouily, M.; Sebti, S.; Boukhari, A.; Holt, E.M.
Catal. Commun. 2001, 2, 101–104.
21. Busetti, V.; Mayoral, J.A.; Cativiela, C.; Ajo, D.; DeZuane, F.
J. Cryst. Spect. Res. 1990, 20(6), 545–550.
22. El Maadi, A.; Boukhari, A.; Holt, E.M. J. Chem. Cryst. 1995,
25, 531–536.