Catalytic C-C bond formation promoted by Mg-Al-O-t-Bu hydrotalcite

Article abstract of DOI:10.1016/S0040-4020(00)00906-6

Mg-Al-O-t-Bu-hydrotalcite catalyst was found to be an efficient, environmentally attractive and selective solid base catalyst for 1,4 Michael addition. Mg-Al-O-t-Bu hydrotalcite is also effective for simple synthesis of α,β-unsaturated esters and nitriles by condensation of the corresponding activated carboxylic esters or nitriles with various aldehydes by Knoevenagel condensation. These reactions proceeded at room temperature at a greater rate in the presence of Mg-Al-O-t-Bu hydrotalcite than in the presence of any of the other catalysts examined. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4020(00)00906-6

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 9357±9364
Catalytic C±C Bond Formation Promoted
by Mg±Al±O±t-Bu Hydrotalcite
a,*
a
B. M. Choudary, M. Lakshmi Kantam, B. Kavita, Ch. Venkat Reddy and F. Figueras
a
a
b
a
Indian Institute of Chemical Technology, Hyderabad 500007, India
Institut de Recherches sur la Catalyse, 2, Avenue Albert Einstein 69626 Villeurbanne Cedex, France
b
Received 26 June 2000; revised 5 September 2000; accepted 21 September 2000
AbstractÐMg±Al±O±t-Bu-hydrotalcite catalyst was found to be an ef®cient, environmentally attractive and selective solid base catalyst
for 1,4 Michael addition. Mg±Al±O±t-Bu hydrotalcite is also effective for simple synthesis of a,b-unsaturated esters and nitriles by
condensation of the corresponding activated carboxylic esters or nitriles with various aldehydes by Knoevenagel condensation. These
reactions proceeded at room temperature at a greater rate in the presence of Mg±Al±O±t-Bu hydrotalcite than in the presence of any of
the other catalysts examined. q 2000 Elsevier Science Ltd. All rights reserved.
1
2
Introduction
sodium azide afford strongly basic sites, which catalyse
side chain alkylations also. In view of these limitations, the
development of an ef®cient and selective solid acid-base
catalyst for the construction of a C±C bond continues to
be a challenging exploration in organic synthesis.
There has been increasing attention on the design and use
of environmentally compatible solid acid/base catalysts
targeted to minimise the emission of ef¯uents in the chemi-
cal industry due to the increasing demands of environmental
legislation. The challenge is to perform heterogeneous cata-
lytic reactions for C±C bond formation in the laboratory,
bulk and ®ne chemical industries. The versatile Michael and
Knoevenagel reactions have numerous applications in the
Hydrotalcites
Layered double hydroxides (LDHs) or hydrotalcite-like
1
3
1
,2
compounds (HTLCs) are of current interest and have
several applications in organic chemistry. The structure of
hydrotalcite consists of brucite [Mg (OH) ] type octahedral
elegant synthesis of ®ne chemicals and are classically
3
,4
catalysed by bases or suitable combinations of amines
and carboxylic or Lewis acids under homogeneous con-
ditions. The employment of these bases/acids in the
reactions encounters two main environmental problems
affecting environment, i.e. the necessity to dispose huge
amounts of organic waste due to formation of undesirable
side products resulting from polymerisation, bis-addition
and self condensation and total dissolved salts formed
requiring the neutralisation of soluble bases with acids or
acids with bases. There are few reports available under the
category of heterogeneous catalysis for Michael additions
and Knoevenagel condensations mediated particularly
2
layers in which a part of the M(II) cations are isomorphously
substituted by M(III) cations. The excess positive charge of
the octahedral layers resulting from this substitution is
compensated by interstitial layers built of anions such as
carbonates, nitrates, chlorides, cyanides and crystal water.
These materials are represented by the general formula
x2
n2
x2
[
is a divalent cation such as Mg, Cu, Ni, Co, Mn, Zn; M(III)
M(II)(12x) M(III) .(OH) ] [(A ) Y´H O] where M(II)
x 2 x/n 2
n2
is a trivalent cation such as Al, Fe, Cr, Ga; A is the inter-
±
±
layer anion such as OH , Cl , CO , NO , SO and the
22
3
2
3
22
4
5
6,7
value of x is in the range of 0.1±0.33. As part of our research
programme aimed at unravelling and developing the
synthetic utility of solid bases in ®ne chemicals synthesis,
we envisaged the use of layered double hydroxides (LDHs)
or hydrotalcite-like compounds (HTLCs) in view of their
potential usefulness as adsorbents, anion exchangers and
by aluminum oxide, xonotlite/potassium tert-butoxide,
8
cation-exchanged zeolites, alkali containing MCM-41,9
AlPO ±Al O catalysts. The incorporation of alkali
metal cations
1
0
4
2
3
9
,11
1
1
such as Cs or Na in zeolites and meso-
porous molecular sieves by cationic exchange provides low
basicity useful for a small range of organic reactions only.
Na clusters introduced in the zeolites by impregnation with
1
4
1
most importantly as basic catalysts. LDHs upon thermal
decomposition at about 4508C provide a highly active
homogeneous mixed oxide which is a potential basic cata-
lyst for a variety of organic transformations such as aldol
Keywords: Mg±Al±O±t-Bu-HT hydrotalcite; solid base catalyst; Michael
additions; Knoevenagel condensation.
* Corresponding author. Tel.: 191-40-7170921; fax: 191-40-7170921;
e-mail: choudary@iict.ap.nic.in
1
5
16
condensation, ole®n isomerisation, epoxidations of
1
7
activated ole®ns with hydrogen peroxide, alkylations of
0040±4020/00/$ - see front matter q 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(00)00906-6

9
358
B. M. Choudary et al. / Tetrahedron 56 (2000) 9357±9364
Table 1. Michael addition catalysed by Mg±Al±O±t-Bu hydrotalcite catalyst
a
b
Entry
a
Acceptor (1)
Donor (2)
Time [h]
Product (3)
Isolated yield (%)
93
95
STY
NM
0.16
30.00
6.38
c
c
20
b
c
NE
0.5
98
6.70
0.04
0.14
d
d
2
0
2
93
100
e
e
NM
0.75
96
77
4.12
0.11
f
2
f
d
e
DMM
DEM
0.83
94
98
4.82
0.5
9.01
0.06
0.55
0.05
g
g
3
2
78
70
90
90
h
i
h
7
4
i
8
4
j
j
f
DEM
2
95
3.49
21.85
0.11
h
h
4
8
75
96
k
k
6
g
h
DMM
DMM
1
1
90
96
4.10
6.79
i
j
DMM
DMM
1.3
1
95
92
86
5.27
6.25
k
l
EAA
EAA
0.16
33.02
17.00
c
c
8
85
0.33
90
11.71
0.58
0.09
0.04
g
g
3
81
88
80
l
j
l
4
4
j
a
f
DMM: Dimethyl malonate; NM: Nitromethane; NE: Nitroethane;
DEM: Diethyl malonate; EAA: Ethyl acetoacetate.
Space time yield (STY)ˆgram of product obtained per gram of
catalyst per hour.
Using KF/Alumina (basic) as catalyst, 5 g of catalyst per 9.2 mmol of
substrate, Ref. 24.
b
g
h
i
Using zeolite as catalyst, 1 g of catalyst per 1 mmol of substrate, Ref. 28.
Using potassium tert-butoxide on Xonotlite as catalyst, Ref. 7a.
Using Ni(acac) as catalyst, Ref. 27.
c
Using Ba(OH) as a catalyst Ref. 7b.
Using Amberlyst A-27 as catalyst, 200 mg of catalyst per 1 mmol
of substrate, Ref. 25.
2
2
d
j
using Al O as catalyst, 1 g of catalyst per 1 mmol of substrate, Ref. 26b.
2 3
Using rehydrated Mg±Al hydrotalcite as catalyst, 1 g of catalyst per 2 mmol
of substrate, Ref. 21.
k
e
Using KF/Alumina (basic) as catalyst, 3 g of catalyst per 5 mmol of
substrate, Ref. 24.
l
2 3
Using Al O as catalyst, 500 mg of catalyst per 1 mmol of substrate Ref. 26a.

B. M. Choudary et al. / Tetrahedron 56 (2000) 9357±9364
9359
Scheme 2.
Scheme 1.
action between a,b-unsaturated ketones and malonate
derivatives catalysed by 1 provides a manifold increase in
STY of Michael addition products. For instance, 1 shows an
increased activity of 130 times over zeolite, 8 times over
xonotlite, 18 times over Al O in entry e, 31 times over
rehydrated hydrotalcite in entry f. Similarly, in the reaction
of a,b-unsaturated ketones with ethyl acetoacetate the cata-
lyst 1 showed enhanced activity, say, 2 times over Ba(OH)2
1
diketones, nucleophilic halide exchange and Claisen±
8
1
Schmidt condensation, etc. We have recently reported
9
2
0
aldol, Knoevenagel condensations
and Michael
2
1
additions catalysed by rehydrated Mg±Al hydrotalcite
catalyst.
2
3
Later, we designed and developed Mg±Al±O±t-Bu hydro-
2
2
talcite for the ®rst time. It was prepared by incorporating
tert-butoxide anion into the interlayer of hydrotalcite to
enhance the basicity of the hydrotalcite and we found it to
in entry k, 234 times over zeolite and 292 times over Al O3
2
in entry l.
2
2
be excellent for aldol condensation and epoxidation of
The Michael addition of cyclic enones (entries b, d, g, Table
1) with malonic esters and nitroalkanes was performed to
demonstrate the versatility of the method with acceptors
other than the usual Michael acceptors such as methyl
vinyl ketone or acrylic esters. Excellent yields of adducts
were obtained.
2
3
ole®ns. Herein, we wish to report highly selective Michael
and Knoevenagel reactions promoted by a Mg±Al±O±t-Bu
hydrotalcite catalyst.
Results and Discussion
Michael addition
Knoevenagel condensation
Knoevenagel condensation involving various aromatic
carbonyl compounds with (a) malononitrile and (b) ethyl
cyanoacetate (Scheme 2) as the active methylene compound
was carried out with Mg±Al±O±t-Bu hydrotalcite at room
temperature (Table 2). The aromatic aldehydes readily
condensed with malononitrile, while with ethyl cyano-
acetate, the reaction is slightly slow. This may be attributed
to the fact that abstraction of a proton from the active
methylene group of ethyl cyanoacetate is dif®cult due to
lower acidity.
Results of Michael addition reactions are compiled in Table
1. The Mg±Al±O±t-Bu-hydrotalcite catalyst, 1, was found
to be an ef®cient and selective catalyst for 1,4 addition.
Several structurally varying donors such as diethyl
malonate, nitromethane, nitroethane, dimethyl malonate
and ethyl acetoacetate underwent clean and remarkably
fast Michael addition with a variety of acceptors including
methyl vinyl ketone, methyl acrylate, ethyl acrylate, chal-
cones and substituted chalcones by this procedure (Scheme
1). The activity of catalyst 1 was compared with that of a
variety of bases and other catalysts such as potassium tert-
As can be seen from Table 2, all reactions proceeded selec-
tively to the dehydrated products without any side reaction.
No self-condensation, Cannizaro products or hydrated
products of Knoevenagel adducts were obtained. The reac-
tion between benzaldehyde and ethyl cyanoacetate in the
presence of Mg±Al±O±t-Bu hydrotalcite (entry b, Table
2) gave selectively the Knoevenagel adduct while the
same reaction promoted by an alkali metal containing
7
butoxide on xonotlite, barium hydroxide, alumina-
a
7b
2
supported potassium ¯uoride, Amberlyst A-27, alumi-
4
25
2
6
27
28
num oxide, Ni(acac)2 and NaY±Zeolite (Table 1).
The Michael reaction with earlier reported catalysts
required longer reaction times and high catalyst loading
and afforded low yields of the adducts. In the case of potas-
sium tert-butoxide on xonotlite and alumina supported
potassium ¯uoride, excess potassium tert-butoxide/¯uoride
was used for the preparation of the catalyst.
9
MCM-41 yielded a mixture of hydrated and dehydrated
products. The rate of reaction is quite impressive and
comparable with the recently reported Knoevenagel
2
9
The Michael addition of nitroalkanes is a convenient
method for the preparation of a number of useful synthetic
intermediates, since the nitro group can be transformed
into various functionalities. Several catalysts have been
examined for this purpose but none has displayed optimum
effectiveness. These reactions have various limitations. In
particular, a large excess of nitroalkane is required. The
catalyst 1 displayed increased activity by several-fold, in
terms of space-time yield (STY) in the activation of
nitroalkanes, over the other catalysts described in Table 1,
condensation under microwave irradiation employing
phosphorous pentoxide as dehydrating agent and chloro-
benzene as an energy transfer medium for the removal of
water.
6
The xonotlite-tert-butoxide catalyst fails to initiate the
condensation of furfuraldehyde with malononitrile (entry i,
Table 2). However, the use of untreated xonotlite led to
moderate yields of the adduct upon a longer reaction time.
The Knoevenagel condensation for furfuraldehyde in the
presence of AlPO ±Al O catalyst, required a large amount
say, for example, 4.7 times greater activity as Ba(OH) in
2
4
2
3
entry a, 223 times as active as Amberlyst A-27 and 48 times
as active as KF-Al O in entry b, 37 times as active as
KF-Al O in entry 3c. It is signi®cant to note that the inter-
of catalyst and afforded poor yields of adducts.10 Though
this reaction was relatively facile with Al O , it required a
2
2
3
3
5
large amount of catalyst. In contrast to all of the above
2
3

9
360
B. M. Choudary et al. / Tetrahedron 56 (2000) 9357±9364
Table 2. Knoevenagel condensation catalysed by Mg±Al±O-t-Bu hydrotalcite
0
a
b
Entry
a
R (4)
R (5)
Time (h)
Isolated Yields (6) (%)
M.P.
STY
C
6
6
H
5
5
CN
0.16
99
82
33.72
6.10
0.48
3.02
9.09
22.20
15.49
0.46
c
c
1
4
43
81
d
d
2
e
f
e
0
.25
80
96
f
0.05
0.33
b
c
C
H
CO
2
Et
98
52
98
c
c
1
82
74
81
e
1
e
g
g
7
0.07
4-OMeC
6
H
H
4
4
CN
0.33
96
21.19
18.40
0.66
c
c
1
4
77
86
d
d
2
e
e
79
0.25
0.33
3.87
d
e
4-OMeC
6
CO
2
Et
98
86
27.44
19.86
0.40
42.86
6.39
0.47
c
c
1
86
52
e
e
1
4-Cl C
6
6
H
4
CN
0.16
95
164
c
c
1
4
34
65
d
d
2
2
4
.06
f
4-Cl C
H
4
CO
2
Et
0.33
98
87
27.63
19.27
0.83
0.73
38.86
27.23
c
c
1
82
85
d
d
4
e
1
e
47
g
h
6
3-OMeC H
4
CN
CN
0.16
0.33
88
93
93
±
3,4,5(OMe)
3
6
C H
2
i
CN
CO
CN
0.08
4
96
63
84
82
65.89
0.50
2.135
29.79
0.67
d
d
2
84
56
e
e
0.25
0.25
j
2
Et
98
d
d
2
4
85
89
e
1
e
0.56
k
4-NO
2
2
C
6
H
H
4
0.16
96
45.84
7.96
0.66
c
c
1
4
40
80
d
d
2
e
e
82
0.25
0.33
4.35
l
4-NO
C
6
4
CO
2
Et
95
170
26.44
19.25
0.91
c
c
1
4
83
94
d
e
1
d
2
e
85
1.31
m
n
o
2-OMeC
2-OMeC
6
H
H
4
4
CN
CO
CN
CO
0.16
96
97
96
95
78
69
±
170
42.39
53.77
39.39
32.98
6
2
Et
Et
0.16
0.16
0.25
4-OHC
4-OHC
6
H
H
4
4
p
6
2
a
Melting points reported are uncorrected.
Space time yield (STY)ˆgram of product obtained per gram of catalyst per hour.
b
c
d
e
f
Reaction with xonotlite-tert-butoxide as catalyst, Ref. 6.
Reaction with untreated xonotlite as catalyst, Ref. 6.
Reaction with AlPO
4
±Al
as catalyst, 3 g for 10 mmol, Ref. 5.
Alkali metal containing MCM-41 as catalyst, Ref. 9.
2 3
O as catalyst, 1.5±3 g for 10 mmol, Ref. 10.
Reaction with Al O
2
3
g
mentioned methodologies, the Mg±Al±O±t-Bu-hydro-
talcite is very effective for the said reaction, requiring
very small amounts of the catalyst and proceeding at a
much faster rate, viz. 5 min.
on xonotlite. The superactivity of catalyst 1 is ascribed to
the presence of the mobile interstitial anion, ±OtBu paired
with Mg and Al main frame hydrotalcite which more effec-
tively abstracts a proton from activated methylene groups to
promote the higher rate of reaction.
The superactivity of the catalyst, 1, in comparison with
catalysts previously described for both Michael and Knoe-
venagel reactions is well established as is evident from
Tables 1 and 2. The activation of methylenes via abstraction
of a proton to generate an anion is a prerequisite for the
initiation of the reaction both in Michael and Knoevenagel
reactions. The much greater activity of 1 over potassium
tert-butoxide impregnated on the basic clay, xonotlite,
assumes importance, as both the catalysts have the same
species, viz. tert-butoxide but with a much higher loading
The absence of any reaction, when Mg±Al hydrotalcite was
employed as such in Knoevenagel and Michael reactions
with any of the substrates described in Tables 1 and 2 is
2
1
noteworthy. The recently reported rehydrated hydrotalcite
shows several disadvantages in the Michael additions such
as high catalyst loading, longer reaction times and limited
scope of utility (activating only the methylene group ¯anked
by at least one electron-withdrawing nitrile group), when
compared with the present catalyst 1. The base strength

B. M. Choudary et al. / Tetrahedron 56 (2000) 9357±9364
9361
Scheme 3.
derived from Br oÈ nsted hydroxyl anions intercalated in rehy-
drated hydrotalcite of the rehydrated catalyst is not adequate
to abstract a proton from the active methylene group of
compounds such as dimethyl malonate, diethyl malonate
and ethyl acetoacetate and consequently, there is no reaction
with these substrates. In contrast, in the case of Mg±Al±O±
t-Bu-hydrotalcite, the tert-butoxide anions are strongly
basic to deprotonate even weakly activated methylenes to
facilitate Michael reactions.
higher order of basicity to enable the formation of C±C
bonds by Knoevenagel condensations as well as Michael
reactions with a high degree of selectivity at faster rates
than any of the earlier catalysts. This new solid base catalyst
is a practical alternative for application for both Knoevenagel
and Michael reactions in view of the following advantages:
(a) high catalytic activity under very mild liquid phase
conditions (b) easy separation of the catalyst by simple
®ltration (c) waste minimization without any side reactions
such as self condensation, bis-addition, dimerization or rear-
rangements (d) use of non-toxic and inexpensive catalysts
(e) involvement of environmentally benign processes and
(f) reusability of catalyst.
4
c
The plausible mechanism of the Michael or Knoevenagel
reactions mediated by catalyst 1 is probably identical to that
of the reactions promoted by homogeneous bases. In
Michael reaction, the abstraction of a proton from the
donor generates a carbanion [A], which can be stabilized
by the cationic charge of hydrotalcite, as suggested in
Scheme 3. It could be proposed that this carbanion further
adds to the a,b-unsaturated ketone to form an intermediate
enolate [C], which would take a proton from in situ formed
tert-butanol to afford the ®nal product and regenrated hydro-
talcite alkoxide. Similarly, in the case of Knoevenagel
condensation, the carbanion [B] formed by the abstraction
of a proton from active methylene compound gets stabilized
by the cationic charge of hydrotalcite. This stabilized carb-
anion attacks the electrophilic carbonyl carbon to form an
intermediate [D] which in turn, abstracts a proton from in
situ formed tert-butanol. Dehydration then takes place to
form the Knoevenagel product.
Experimental
Tetrahydrofuran (THF) was distilled from sodium/benzo-
phenone prior to use. All the reactions were conducted
under an atmosphere of nitrogen. Melting points reported
1
here are uncorrected. H NMR spectra were recorded at
200 MHz using CDCl as internal reference. Mass spectra
3
were obtained at an ionization potential of 70 eV [scanned
on VG 70-70H (micro mass)]; only selected ions are
reported here. Infrared spectra were recorded either as
neat liquids or KBr pellets. Thin layer chromatography
was performed on silica gel 60F254 plates procured from
E. Merck. Elemental analysis was performed using a C,
H, N analyser. Starting materials purchased from Aldrich
or Fluka were used as such.
An attempt was made to check the reusability of the catalyst
in the following manner. The substrates, catalyst and solvent
(
Table 1, entry l) were taken, and the reaction was
Preparation of catalyst
performed as usual for a speci®ed time. Then the stirring
was stopped and the reaction mixture withdrawn by a glass
syringe through a septum. Fresh substrates and solvent were
added to the residual catalyst and the reaction performed
under identical conditions. The activity and selectivity of
the catalyst was found to be the same.
(a) Preparation of Mg±Al±NO hydrotalcite: Mg±Al±NO3
3
hydrotalcite catalyst was prepared in nitrogen atmosphere to
avoid carbonation in air. Magnesium nitrate hexahydrate
(30.8 g, 0.12 mol) and aluminum nitrate nonahydrate
(15.0 g, 0.04 mol) were dissolved in 100 mL deionised
and decarbonated water. The pH of the solution was
adjusted to 10 by the addition of NaOH (2 M). The slurry
was stirred for 2 h at room temperature, ®ltered and then
dried under vacuum at 808C. The reaction and ®ltration
processes were executed under nitrogen atmosphere.
Conclusion
In summary, a new solid base catalyst Mg±Al±O±t-Bu-
hydrotalcite has been developed by incorporating tert-
butoxide into the brucite layer of hydrotalcite to achieve a
(b) Preparation of Mg±Al±O±t-Bu hydrotalcite: Mg±Al±

9
362
B. M. Choudary et al. / Tetrahedron 56 (2000) 9357±9364
O±t-Bu hydrotalcite was prepared from Mg±Al±NO3
hydrotalcite. A 0.1 M solution of potassium tert-butoxide
was prepared by dissolving 1.12 g of potassium tert-
butoxide in 100 mL of freshly dried THF. 1.214 g of Mg±
4.5 Hz), 1.30(t, 3H, CH , Jˆ4.5 Hz), 2.02±2.55 (m, 4H,
3
1
CH , CH ), MS (EI) m/z (M 2NO) 141 (10), 123 (7), 55
(48), 41 (68).
2
2
Al±NO hydrotalcite was added and the solution was stirred
3
for 24 h in nitrogen atmosphere. It was then ®ltered under
nitrogen. A white solid, Mg±Al±O±t-Bu hydrotalcite
3-[Bis(methoxycarbonyl)methyl]cyclopentanone (3d).
Brown coloured liquide; IR (neat) 1710, 1160 cm ; H
NMR (200 MHz, CDCl ) d 3.27 (d, 1H, COCH,
3
2
1
1
(
1.382 g) was obtained. t-Butoxide was exchanged onto
nitrate-saturated (MgAl) LDH to an extent of 12%
0.99mmol/ g t-butoxide) as indicated by chemical analysis.
Jˆ8.1 Hz), 2.62±2.82 (m, 1H, cyclic CH), 3.72 (s, 6H,
1
OCH ), 1.48±2.44 (m, 6H, (CH ) ); MS (EI) m/z (M 2
3
2 2
(
OCH ) 183 (7), 132 (42), 83 (33), 69 (45), 55 (90), 39
3
(100). Anal. calcd for C H O : C, 56.07; H, 6.54. Found:
10 14 5
C, 56.54, H, 6.96.
General procedure for Michael reactions
2
7
4-[Bis(ethoxycarbonyl)methyl]2-butanone (3e). brown
In a typical procedure, the acceptor (2 mmol) and Mg±Al±
O±t-Bu hydrotalcite (0.1 g) were stirred in 10 ml of metha-
nol for 5 min. Then the donor (2 mmol) was added and the
stirring continued till the completion of the reaction (as
monitored by thin layer chromatography (TLC)). The cata-
lyst was removed by ®ltration and the ®ltrate was concen-
trated under reduced pressure. The crude product was
puri®ed by column chromatography (Acme Synthetic
Chemicals, 60±120 mesh silica gel using ethyl acetate/
hexane).
2
1
1
coloured liquid; IR (neat) 1740, 1360 cm ; H NMR
(200 MHz, CDCl ) d 4.16 (q, 4H, OCH
, Jˆ8.1 Hz), 3.32
(t, 1H, COCH, Jˆ7.7 Hz), 2.52 (t, 2H, COCH , Jˆ7.2 Hz),
2.04±2.18 (s, m, 5H, COCH , CH CH ), 1.25 (t, 6H, CH
3
2
2
,
3
3
1
2
2
Jˆ8.1 Hz); MS (EI) m/z (M peak) 230 (13), 184 (9), 173
(18), 55 (65), 43 (100).
7
a
Diethyl-2-(3-oxo-1,3-diphenylpropyl)malonate
(3f).
2
1
1
White solid, mp 658C; IR (neat) 1720, 1680 cm ; H
NMR (200 MHz, CDCl ) d 1.00 (t, 3H, CH , Jˆ5.4 Hz);
3
3
General procedure for Knoevenagel condensation
1.28 (t, 3H, CH
3
, Jˆ5.4 Hz); 3.32±3.62 (m, 2H, COCH
);
2
3
4.10±4.25 (m, 4H, CH ); 7.10±7.52 (m, 8H, aromatic), 7.85
.78 (d, 1H, COCH, Jˆ8.6 Hz); 3.88±3.98 (m, 1H, PhCH);
In a typical procedure, aldehyde (2 mmol) and Mg±Al±O±
t-Bu hydrotalcite (0.05 g) were stirred in 10 ml of dimethyl
formamide for 5 min. To it, active methylene compound
2
1
(d, 2H, aromatic, Jˆ5.9 Hz); MS (EI) m/z (M peak) 368
(1), 249 (13), 209 (16), 105 (100), 77 (86), 51 (22).
(
2 mmol) was added and stirring continued till the com-
pletion of the reaction (as monitored by TLC). The catalyst
was removed by ®ltration and the product was extracted
with ethyl acetate. The ethyl acetate solution was then
dried on anhydrous sodium sulfate and concentrated under
reduced pressure. The crude product was puri®ed by column
chromatography (ethyl acetate/hexane, 95/5, v/v).
3-[Bis(methoxycarbonyl)methyl]cyclohexanone (3g).
Brown coloured liquid; IR (neat) 1740, 1780, 1140 cm ;
2
1
1
H NMR (200 MHz, CDCl
3.42 (m, 1H, CH), 1.45±2.58 (m, 9H, (CH )
2 5
) d 3.78 (s, 6H, OCH
); MS (EI) m/z
(M peak) 228 (1), 197 (7), 132 (33), 97 (96), 69 (69), 59
), 3.30±
3
3
1
(72), 41 (100). Anal. calcd for C11
Found: C, 57.64; H, 6.70.
H O : C, 57.89; H, 7.01.
16 5
1
The products were fully characterised by their mp, H NMR,
Mass and IR spectroscopy and elemental analysis. These
data for the Michael and Knoevenagel reactions are
presented below in order of the products in Tables 1 and
Dimethyl 2-[1-(4-methylphenyl)-3-oxo-3-phenylpropyl]-
malonate (3h). White solid, mp 1468C; IR (neat) 1740,
2
1 1
1760, 1630 cm ; H NMR (200 MHz, CDCl
3H, CH ), 3.35±3.55 (m, 2H, COCH ), 3.78 (2s, 6H, OCH
3
) d 2.30 (s,
3
2, respectively.
3
2
),
3
.86 (d, 1H, COCH, Jˆ9.5 Hz), 4.05±4.22 (m, 1H, PhCH),
Michael reaction
7.32±7.75 (m, 7H, aromatic), 7.90 (d, 2H, aromatic, Jˆ
1
.1 Hz); MS (EI) m/z (M peak) 354 (3), 223 (50), 105
7
7
b
-Nitro-1,3-diphenyl-1-butanone (3a). white solid, mp
1
4
(100), 77 (60). Anal. calcd for C21
6.26 Found: C, 70.66; H, 6.11.
H O : C, 71.15; H,
22 5
2
018C; IR (neat) 1690, 1580, 1380 cm ; H NMR
1
1
(
200 MHz, CDCl ) d 3.45 (d, 2H, COCH , Jˆ6.8 Hz)
3
2
4
7
6
.12±4.28 (m, 1H, PhCH), 4.61±4.88 (m, 2H, CH NO ),
2
Dimethyl 2-[1-(4-methoxylphenyl)-3-oxo-3-phenylpropyl]-
malonate (3i): White solid, mp 1418C; IR (neat) 1720,
2
.12±7.64 (m, 9H, aromatic), 7.95 (d, 2H, aromatic, Jˆ
1
.3 Hz); MS (EI) m/z (M 2NO) 222 (7), 117 (6), 105
21
1680, 1510 cm ; H NMR (200 MHz, CDCl
1
) d 3.35±
3
(
100), 77 (42), 51 (12).
3.50 (m, 2H, COCH ), 3.74 (s, 9H, OCH ), 3.84 (d, 1H,
2
3
COCH, Jˆ5.2 Hz), 4.05±4.18 (m, 1H, PhCH), 7.32±7.75
(m, 7H, aromatic), 7.90 (d, 2H, aromatic, Jˆ4.5 Hz). MS
2
-(1-Nitroethyl)cyclohexanone (3b). brown coloured
5
3
liquid, IR (neat) 1720, 1560 cm ; H NMR (200 MHz,
2
1
1
1
(EI) m/z (M 1 peak) 371 (6), 239 (21), 105 (100), 77 (31).
CDCl ) d 4.39 (q, 1H, CHNO , Jˆ5.4 Hz), 1.82±2.52 (m,
Anal. calcd for C21
68.10; H, 5.88.
H O : C, 68.08; H, 5.99. Found: C,
22 6
3
2
9
H, (CH ) ); 1.45±1.62 (m, 3H, CH ); MS (EI) m/z (MS (EI)
2 5 3
1
m/z (M 2NO) 125 (40), 124 (57), 97 (48), 81 (48), 69 (63),
5 (100), 39 (98).
7
b
5
Dimethyl-2-(3-oxo-1,3-diphenylpropyl)malonate (3j).
2
1
1
White solid, mp 1458C; IR (neat) 1660, 1710 cm ; H
NMR (200 MHz, CDCl ) d 3.42±3.50 (m, 2H, COCH ),
3.72±3.80 (m, 6H, OCH ), 3.86 (d, 1H, COCH,
Jˆ9.5 Hz), 4.05±4.20 (m, 1H, PhCH), 7.32±7.75 (m, 8H,
2
Ethyl 4-nitrobutanoate (3c). brown coloured liquid; IR
4
3
2
21 1
neat) 1730, 1560, 1320 cm ; H NMR (200 MHz, CDCl )
(
3
3
d 4.48 (t, 2H, CH NO , Jˆ5.4 Hz), 4.21 (q, 2H, OCH , Jˆ
2
2
2

B. M. Choudary et al. / Tetrahedron 56 (2000) 9357±9364
9363
1
1
aromatic), 8.03 (d, 2H, Jˆ6.8 Hz); MS (EI) m/z (M peak)
2H, aromatic, Jˆ9.3 Hz); MS (EI) m/z (M peak) 235 (100),
3
40 (3), 249 (13), 209 (48), 105 (100),77 (33).
207 (56), 190 (80), 162 (50), 127 (48), 76 (50), 51 (27).
7
b
Ethyl 2-acetyl-5-oxo-3,5-diphenyl pentanoate (3k).
m-Methoxybenzylidenemalonitrile (6g). Pale yellow
solid, mp 938C; IR (KBr pellets) 2240, 1610, 1190 cm ;
2
1
1
21
White solid, mp 1208C; IR (neat) 1720, 1680 cm ; H
NMR (200 MHz, CDCl ) d 1.28 (t, 3H, CH , Jˆ6.3 Hz),
1
H NMR (200 MHz, CDCl ) d 3.90 (s, 3H, OCH ), 7.74 (s,
3
3
3
3
2
4
.32 (s, 3H, COCH ), 3.28±3.46 (m, 2H, COCH ), 3.86±
3
1H, alkenic), 7.12±7.20 (m, 1H, aromatic), 7.38±7.48 (m,
3H, aromatic); MS (EI) m/z (M peak) 184 (90), 141 (33),
2
1
.05 (m, 2H, CH,CH); 4.12±4.25 (m, 2H, CH ); 7.12±7.55
2
(
m, 8H, aromatic), 7.82±7.95 (m, 2H, aromatic); MS (EI)
114 (100), 88 (39), 64 (50); Anal. calcd for C H N O: C,
1
71.73, H, 4.34; N, 15.21. Found: C, 71.82; H, 4.31; N, 15.01.
1
8
2
1
m/z (M peak) 320 (3), 247 (33), 144 (62), 105 (96), 77
100), 51 (45), 43 (54).
(
0
liquid; IR (KBr pellets) 2260, 1580, 1120 cm ; H NMR
m,m ,p-Trimethoxybenzylidenemalonitrile (6h). Viscous
2
-Acetyl-5-methyl 2-acetyl pentanedioate (3l). Pale
8
21 1
1
yellow coloured liquid; IR (neat) 1760, 1700 cm ; H
2
1
1
(200 MHz, CDCl ) d 3.98 (s, 3H, OCH ), 3.90 (s, 6H,
3
3
NMR (200 MHz, CDCl ) d 1.32 (t, 3H, CH , Jˆ6.6 Hz)
OCH ), 7.62 (s, 1H, alkenic), 7.18 (s, 2H, aromatic); MS
3
3
3
1
(EI) m/z (M peak) 244 (3), 229 (7), 201 (13), 115 (30), 77
2
.08±2.42 (m, 7H, CH CH , COCH ), 3.52 (t, 1H, COCH,
2
2
3
Jˆ4.0 Hz), 3.70 (s, 3H, OCH ), 4.12 (q, 2H, CH , Jˆ
(100); Anal. calcd for C H N O : C, 63.93; H, 4.93; N,
13 12
11.47. Found: C, 63.02; H, 4.91; N, 10.82.
3
2
2
3
1
.6 Hz); MS (EI) m/z (M 2OCH ) 174 (9), 100 (60), 85
18), 43 (100).
6
3
(
1
Furfurylidenemalonitrile (6i). Pale yellow solid, mp
0
2
1
1
Knoevenagel condensation
638C; IR (KBr pellets) 2226, 1607, 1020 cm ; H NMR
200 MHz, CDCl ) d 7.82 (s, 1H, aromatic), 7.45 (d, 1H,
(
3
1
Benzylidenemalonitrile (6a). Pale yellow solid, mp 828C;
0
aromatic, Jˆ5.2 Hz), 7.54 (s, 1H, alkenic), 6.70±6.78 (m,
1
1H, aromatic); MS (EI) m/z (M peak) 144 (46), 115 (19),
88 (46), 51 (66), 38 (100).
2
1
1
IR (KBr pellets) 2280, 1560, 1090 cm ; H NMR
(
200 MHz, CDCl ) d 7.90 (d, 2H, aromatic, Jˆ5.7 Hz),
3
7
(
.76 (s, 1H, alkenic), 7.48±7.68 (m, 3H, aromatic); MS
1
EI) m/z (M peak) 154 (100), 127 (53), 103 (48), 76 (10).
10
Ethyl 3-furfurylidene-2-cyanopropionate (6j).
yellow solid, mp 848C; IR (KBr pellets) 2221, 1717, 1620,
Pale
1
Ethyl 2-cyanocinnamate (6b). Pale yellow solid, mp
0
21 1
1210 cm ; H NMR (200 MHz, CDCl ) d 1.45 (t, 3H, CH ,
3 3
2
1
28C; IR (KBr pellets) 2240, 1610, 1200 cm ; H NMR
1
5
Jˆ5.7 Hz), 4.38 (q, 2H, CH , Jˆ5.7 Hz), 8.04 (s, 1H,
2
(
2
2
200 MHz, CDCl ) d 1.45 (t, 3H, CH , Jˆ7.1 Hz), 4.42 (q,
alkenic), 7.45 (d, 1H, aromatic, Jˆ5.4 Hz), 6.64±6.72 (m,
1H, aromatic), 7.76 (d, 1H, aromatic, Jˆ5.4 Hz); MS (EI)
3
3
H, CH , Jˆ7.1 Hz), 7.48±7.64 (m, 3H, aromatic), 8.02 (m,
2
1
1
m/z (M peak) 191 (100), 163 (22), 146 (50), 119 (7), 63
(37).
H, aromatic), 7.98±8.08 (s, 1H, alkenic); MS (EI) m/z (M
peak) 201 (100), 172 (56), 156 (90), 128 (81), 77 (67), 51
53), 43 (45).
(
6
p-Nitrobenzylidenemalonitrile (6k). Pale yellow solid,
6
p-Methoxybenzylidenemalonitrile (6c). Pale yellow
21
1
mp 828C; IR (KBr pellets) 2180, 1560, 1180 cm ; H
NMR (200 MHz, CDCl ) d 8.62 (s, 1H, alkenic), 8.18 (d,
2
solid, mp 988C; IR (KBr pellets) 2280, 1560, 1210 cm ;
1
3
1
H NMR (200 MHz, CDCl ) d 7.62 (s, 1H, alkenic), 3.90 (s,
3
2H, aromatic, Jˆ9.2 Hz), 8.38 (d, 2H, aromatic, Jˆ9.2 Hz);
1
MS (EI) m/z (M peak) 199 (100), 153 (78), 126 (84), 141
(56), 114 (31).
3
H, OCH ), 7.92 (d, 2H, aromatic, Jˆ9.0 Hz), 7.01 (d, 2H,
3
1
aromatic, Jˆ9.0 Hz), MS (EI) m/z (M peak) 184 (100), 141
(
19), 114 (36), 88 (12), 57 (22), 43 (34).
-Propionic acid, 2-cyano-3-(4-methoxyphenyl) ethyl
ester (6d). Pale yellow solid, mp 868C; IR (KBr pellets)
2-Propionic acid, 2-cyano-3-(4 nitrophenyl) ethyl ester
(6l). Pale yellow solid, mp 1708C; IR (KBr pellets) 2260,
6
2
6
21
1
1740, 1580, 1120 cm ; H NMR (200 MHz, CDCl
(t, 3H, CH
) d 1.46
3
, Jˆ7.1 Hz), 8.14
2
1 1
2
180, 1710, 1560 cm ; H NMR (200 MHz, CDCl ) d 1.4
3
3
, Jˆ7.1 Hz), 4.45 (q, 2H, CH
2
(
(
t, 3H, CH , Jˆ8.5 Hz), 4.38 (q, 2H, CH , Jˆ8.5 Hz), 3.90
(d, 2H, aromatic, Jˆ9.5 Hz), 8.25 (s, 2H, alkenic), 8.35 (d,
3
2
1
s, 3H, OCH ), 8.13 (s, 1H, alkenic), 8.02 (d, 2H, aromatic,
3
2H, aromatic, Jˆ9.5 Hz); MS (EI) m/z (M peak) 246 (27),
Jˆ9.5 Hz), 6.98 (d, 2H, aromatic, Jˆ9.5 Hz); MS (EI) m/z
218 (18), 202 (21), 127 (30), 57 (100).
1
M peak) 231 (100), 186 (24), 158 (7).
(
o-Methoxybenzylidenemalonitrile (6m). Pale yellow
solid, mp 788C; IR (KBr pellets) 2210, 1580, 1160 cm ;
H NMR (200 MHz, CDCl ) d 3.94 (s, 3H, OCH ), 8.28 (s,
3 3
6
p-Chlorobenzylidenemalonitrile (6e). Pale yellow solid,
21
2
mp 1648C; IR (KBr pellets) 2205, 1560, 1110 cm ; H
1
1
1
NMR (200 MHz, CDCl ) d 7.75 (s, 1H, alkenic), 7.56 (d,
3
1H, alkenic), 8.20 (d, 1H, aromatic, Jˆ9.5 Hz), 7.10 (t, 1H,
2
H, aromatic, Jˆ9.0 Hz), 7.88 (d, 2H, aromatic, Jˆ9.0 Hz);
aromatic, Jˆ9.0 Hz), 7.58 (t, 1H, aromatic, Jˆ7.1 Hz)),
1
MS (EI) m/z (M peak) 188 (100), 153 (51), 126 (16), 87
1
6.98 (d, 1H, aromatic, Jˆ8.5 Hz); MS (EI) m/z (M peak)
(
7), 51 (30), 50 (36).
-Propionic acid, 2-cyano-3-(4-chlorophenyl) ethyl ester
6f). Pale yellow solid, mp 878C; IR (KBr pellets) 2240,
184 (24), 141 (15), 114 (69), 88 (69), 39 (100); Anal. calcd
for C H N O: C, 71.73; H, 4.34; N, 15.21. Found: C, 71.80;
H, 4.24; N, 14.75.
1
1
8
2
2
(
1
1
0
2
1 1
760, 1580, 1160 cm ; H NMR (200 MHz, CDCl ) d 1.46
3
2-Propionic acid, 2-cyano-3-(2-methoxyphenyl) ethyl
ester (6n). Pale yellow solid, mp 698C; IR (KBr pellets)
(
(
t, 3H, CH , Jˆ6.9 Hz), 4.39 (q, 2H, CH , Jˆ8.8 Hz), 8.18
3
2
2
2364, 1707, 1598, 1258 cm ; H NMR (200 MHz,
1
1
s, 1H, alkenic), 7.95 (d, 2H, aromatic, Jˆ9.3 Hz), 7.42 (d,

9
364
B. M. Choudary et al. / Tetrahedron 56 (2000) 9357±9364
CDCl ) d 1.45 (t, 3H, CH , Jˆ7.6 Hz), 4.40 (q, 2H, CH ,
2648. (b) Garcia-Raso, A.; Garcia-Raso, J.; Campaner, B.;
Mestres, R.; Sinisterra, J. V. Synthesis 1982, 1037±1041. (c)
Iglesias, M.; Marinas, J. M.; Sinisterra. J. V. Tetrahedron 1987,
43, 2335±2342.
3
3
2
Jˆ7.6 Hz), 3.95 (s, 3H, OCH ), 8.32 (d, 1H, aromatic, Jˆ
3
8
8
.7 Hz), 8.78 (s, 1H, alkenic), 7.10 (t, 1H, aromatic, Jˆ
.2 Hz), 7.48±7.56 (m, 1H, aromatic), 6.98 (d, 1H,
1
aromatic, Jˆ10.2 Hz); MS (EI) m/z (M peak) 231 (100),
8. Corma, A.; Fornes, V.; Martin-Aranda, R. M.; Garcia, H.;
Primo, J. Appl. Catal. 1990, 59, 237±248.
186 (33), 172 (69), 89 (19), 77 (30); Anal. calcd for
C H NO : C, 67.53; H, 5.62; N, 6.06. Found: C, 67.36;
H, 5.52; N, 5.88.
9. Kloetstra, K. R.; Van Bekkum, H. J. Chem. Soc., Chem.
Commun. 1995, 1005±1006.
1
3
13
3
1
J. M. J. Org. Chem. 1984, 49, 5195±5197.
0. Cabello, J. A.; Campelo, J. M.; Garcia, A.; Luna, D.; Marinas,
p-Hydroxybenzylidenemalonitrile (6o). Viscous liquid;
2
1
1
IR (KBr pellets) 2280, 1580, 1220 cm ; H NMR
200 MHz, CDCl ) d 7.72 (s, 1H, alkenic), 6.92 (d, 2H,
11. Corma, A.; Martin-Aranda, R. M. J. Catal. 1991, 130, 130±
137.
(
3
aromatic, Jˆ8.0 Hz), 7.89 (d, 2H, aromatic, Jˆ8.0 Hz);
12. Martens, L.; Grobet, P.; Jacobs, P. A. Nature 1985, 315, 568±
570.
1
MS (EI) m/z (M peak) 170 (100), 142 (7), 119 (22), 115
(
1
9), 39 (7); Anal. calcd for C H N O: C, 70.58; H, 3.52; N,
2
6.47. Found: C, 70.70; H, 3.63; N, 15.48.
13. Cavani, F.; Tri®ro, F.; Vaccari, A. Catal. Today 1991, 11,
173±301.
1
0
8
1
4. (a) Reichle, W. T. J. Catal. 1985, 94, 547±557. (b) Numan,
2-Propionic acid, 2-cyano-3-(4-hydroxyphenyl) ethyl
ester (6p). Pale yellow solid, mp 1708C; IR (KBr pellets)
J. G.; Himelfarb, P. B.; Herman, R. G.; Klier, K.; Bogdan, C. E.;
Simmons, G. W. Inorg. Chem. 1989, 28, 3868±3874. (c) Busetto,
C.; Delpiero, G.; Manara, G.; Tri®ro, F.; Vaccari, A. J. Catal.
1984, 85, 260±266.
21 1
230, 1720, 1620, 1220 cm ; H NMR (200 MHz, CDCl )
2
3
d 1.42 (t, 3H, CH , Jˆ9.1 Hz), 4.38 (q, 2H, CH , Jˆ9.1 Hz),
3
2
8
.12 (s, 1H, alkenic), 7.94 (d, 2H, aromatic, Jˆ5.7 Hz), 6.96
15. Suzuki, E.; Ono, Y. Bull. Chem. Soc. Jpn. 1988, 61, 1008±
1010.
1
(
(
d, 2H, aromatic, Jˆ5.7 Hz); MS (EI) m/z (M peak) 217
92), 189 (43), 172 (100), 144 (33), 89 (45); Anal. calcd for
16. Suzuki, E.; Okamoto, M.; Ono, Y. J. Mol. Catal. 1990, 61,
283±294.
C H NO : C, 66.35; H, 6.45; N, 5.06. Found: C, 66.16; H,
3
1
2
11
6.43; N, 5.11.
17. Cativiela, C.; Figueras, F.; Garcia, J. I.; Mayoral, J. A.;
Zurbano, M. Synth. Commun. 1995, 25, 1745±1750.
18. Cativiela, C.; Figueras, F.; Fraile, J. M.; Garcia, J. I.; Mayoral,
J. A. Tetrahedron Lett. 1995, 36, 4125±4128.
Acknowledgements
1
2
9. Reichle, W. T. US Patent 4, 458, 026, 1984 to Union Carbide.
0. Lakshmi Kantam, M.; Choudary, B. M.; Venkat Reddy, Ch.;
B. K. and Ch. VR gratefully acknowledge the University
Grants Commission (UGC) and Council of Scienti®c and
Industrial Research (CSIR), New Delhi, India, for a
Research Fellowship.
Koteswara Rao, K.; Figueras, F. J. Chem. Soc., Chem. Commun.
1
998, 1033±1034.
1. Choudary, B. M.; Lakshmi Kantam, M.; Venkat Reddy, Ch.;
2
Koteswara Rao, K.; Figueras, F. J. Mol. Catal. 1999, 146, 279±
References
284.
2
2. Choudary, B. M.; Lakshmi Kantam, M.; Kavita, B.; Venkat
Reddy, Ch.; Koteswara Rao, K.; Figueras, F. Tetrahedron Lett.
1998 , 39, 3555±3558.
1. (a) Trost, B. M. Comprehensive Organic Synthesis, Pergamon:
Oxford, 1991; Vol. 2, pp 133±340. (b) Clark, J. H. Chem. Rev.
1
2
1
980, 80, 429±452.
. (a) Ganesh, S.; Sarkar, A. Tetrahedron Lett. 1991, 32, 1085±
088. (b) Machida, S.; Hashimoto, Y.; Saigo, K.; Inoue, J.;
23. Choudary, B. M.; Lakshmi Kantam, M.; Bharathi, B.; Venkat
Reddy, Ch. Synlett 1998, 1203 ±1204.
24. Bergbreiter, D. E.; Lalonde, J. J. J. Org. Chem. 1987, 52,
1601±1603.
Hasegawa, M. Tetrahedron 1991, 47, 3737±3752.
. (a) Knoevenagel, E. Chem. Ber. 1894, 27, 2345. (b) Jones, G.
Org. React. 1967, 15, 204±599.
3
25. Ballini, R.; Marziali, P.; Mozzicafreddo, A. J. Org. Chem.
1996, 61, 3209±3211.
4
. (a) Bergmann, E. D.; Ginsburg, D.; Pappo. R. Org. React. 1959,
0, 179±555. (b) Oare, D. A.; Heathcock, C. H. In Topics in
Stereochemistry; Eliel, Willen, S. H., Eds.; Wiley: New York,
26. (a) Ranu, B. C.; Bhar, S. Tetrahedron 1992, 48, 1327±1332.
(b) Ranu, B. C.; Bhar, S.; Sarkar. D. C. Tetrahedron Lett. 1991, 32,
2811±2814.
1
1
5
4
6
4
7
989; Vol. 19, pp 277.
. Texier-Boullet, F.; Foucaud, A. Tetrahedron Lett. 1982, 23,
927±4928.
27. Nelson, J. H.; Howells, P. N.; DeLullo, G. C.; Landen, G. L.;
Henry, R. A. J. Org. Chem. 1980, 45, 1246±1249.
28. Sreekumar, R.; Rugmini, P.; Padmakumar, R. Tetrahedron
Lett. 1997, 37, 6557±6560.
. Chalais, S.; Laszlo, P.; Mathy, A. Tetrahedron Lett. 1985, 26,
453±4454.
. (a) Laszlo P.; Pennetreau, P. Tetrahedron Lett. 1985, 26, 2645±
29. Kim, S-Y.; Kwon, P-S.; Kwon, T-W.; Chung, S-K.; Chang,
Y-T. Synth. Commun. 1997, 27, 533±541.