Scandium(III) zeolites as new heterogeneous catalysts: [4+2] cyclocondensation of in situ generated aryl imines with alkenes

Article abstract of DOI:10.1002/chem.201002649

Scandium(III)-exchanged zeolite was used as heterogeneous ligand-free catalysts for the [4+2]cyclocondensation of amines and aldehydes with alkenes through the in situ formation of imines. Such catalysts offered a versatile, efficient, and highly regio-and stereoselective synthesis of tetrahydroquinoline derivatives. These catalysts exhibited a wide scope and compatibility with functional groups. They are very simple to use, easy to remove (by simple filtration), and they are recyclable (up to three times without loss of activity).

Full text of DOI:10.1002/chem.201002649

FULL PAPER
DOI: 10.1002/chem.201002649
Scandium
ACHTUNGTRENNGNU( III) Zeolites as New Heterogeneous Catalysts:
[
4+2]Cyclocondensation of in situ Generated Aryl Imines with Alkenes
[
a]
[b]
[a]
Andrea Olmos, Jean Sommer, and Patrick Pale*
Abstract:
Scandium
A
H
U
G
R
N
N
(III)-exchanged
regio- and stereoselective synthesis of
tetrahydroquinoline derivatives. These
catalysts exhibited a wide scope and
compatibility with functional groups.
They are very simple to use, easy to
remove (by simple filtration), and they
are recyclable (up to three times with-
out loss of activity).
zeolite was used as heterogeneous
ligand-free catalysts for the [4+2]cy-
clocondensation of amines and alde-
hydes with alkenes through the in situ
formation of imines. Such catalysts of-
fered a versatile, efficient, and highly
Keywords: catalysis · cycloaddition ·
imines · scandium · zeolites
Introduction
Quinoline and its hydrogenated forms represent a very im-
[1]
portant subclass of alkaloid natural products. Such motifs
can be found in organisms as different as plants with i.a. an-
gustura, a Venezuelan tree used in folk medicine principally
[2]
to combat fevers (Scheme 1, top left) and frogs with the
[3]
neurotoxins excreted on their skins (Scheme 1, top right).
Among them, tetrahydroquinolines exhibit the largest range
[4]
of activities. They are not only used or developed as anti-
[
5]
[6]
bacterial agents, as antitumor agents, as HIV protease in-
[7]
hibitors, as therapeutic agents for i.a. allergic diseases trig-
gered by prostaglandin (CRTH2 inhibitors; Scheme 1), or
for antiemetic and analgesic effects through activation of
[8]
[9]
cannabinoid receptors (Levonantradol, Scheme 1),
but
also as compounds for altering the lifespan of eukaryotic or-
Scheme 1. Examples of biologically active natural tetrahydroquinolines
and related drugs and therapeutic agents.
[10]
ganisms
or imaging G protein-coupled estrogen recep-
[
11]
tors.
Therefore, a considerable number of synthetic approaches
towards (hydro)quinoline derivatives have been devel-
[
12]
oped. Among them, the formal [4+2]cycloaddition reac-
tions between N-arylimines acting as heterodienes towards
dienophiles have long been recognized as one of the most
convenient and rapid methods for the synthesis of (hydro)-
quinolines. Usually performed under acidic conditions, such
cycloadditions have been reported with various Lewis acid
[
a] Dr. A. Olmos, Prof. Dr. P. Pale
Laboratoire de Synthꢀse et Rꢁactivitꢁ Organiques
Institut de Chimie de Strasbourg, associꢁ au CNRS
4
rue Blaise Pascal, 67000 Strasbourg (France)
[
13]
Fax : (+33)368851517
E-mail: ppale@chimie.u-strasbg.fr
catalysts, such as bismuth AHCTUNGTRENNUN(G III) bromide, niobium(V) chlo-
[14]
[15]
ride,
trate,
samarium diiodide,
cerium(IV) ammonium ni-
[
b] Prof. Dr. J. Sommer
[16]
[17]
[18]
magnesium perchlorate,
copper(II) bromide,
Laboratoire de Physicochimie des Hydrocarbures
Institut de Chimie de Strasbourg, associꢁ au CNRS
[19]
[20]
indium trichloride, and lanthanide salts. However, sup-
ported and reusable catalysts remained scarcely used despite
obvious advantages, and only indium trichloride supported
4
rue Blaise Pascal, 67000 Strasbourg (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002649.
[21]
[22]
on polyaniline,
antimony chloride on hydroxyapatite,
Chem. Eur. J. 2011, 17, 1907 – 1914
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1907

[23]
[24]
lanthanide polyoxometalate, and montmorillonites have
so far been mentioned. This relative lack led us to consider
applying metal-doped zeolites as catalysts for such cycload-
ditions (Scheme 2), in connection with our recent zeo-click
Results and Discussion
Set up of reaction conditions: In a first series of experi-
ments, we examined the behavior of the stable (E)-imine
3a, readily accessible from aniline (1a) and benzaldehyde
[25–28]
syntheses.
(
2a), with cyclopentadiene (4a) in the presence of USY zeo-
lites modified with different metals under various conditions
Table 1). This zeolite was selected first, since upon metal
doping, it proved efficient for other organic transforma-
(
[24–26]
tions
and because it holds large pores, able to accom-
modate several molecules together.
Table 1. Condition screening for the m-doped zeolite-catalyzed cycload-
[
a]
dition of N-phenylbenzimine or its precursors with cyclopentadiene.
Scheme 2. Possible metal-doped zeolite-catalyzed cycloaddition of imines
with alkenes.
I
We indeed recently demonstrated that Cu zeolites are
very effective and reusable catalysts for the cycloaddition of
alkynes with azides or azomethine imines (Scheme 3a and
Entry
Catalyst
Solvent
T
t
Recovered
3a [%]
Yield of
5a [%]
[
b]
[b]
[
8C]
[h]
[
25,26]
b).
Therefore, other cycloadditions, such as the reaction
[
[
[
c]
c]
c]
1
2
3
4
5
6
7
8
9
none
CuI
CH
CH
CH
CH
CH
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
20
20
20
20
20
60
40
60
60
20
20
72
72
24
72
72
72
12
24
18
24
48
98
80–97
99–58
99
0
of N-arylimines with alkenes should be amenable under sim-
traces
0–41
0
traces
16
12
4
96
23
CuBr
2
I
[c]
[c]
Cu -USY
II
Cu -USY
97
79
80
81
2
58
95
III
Sc -USY
PhMe
CH Cl
THF
III
Sc -USY
2
2
III
Sc -USY
III
Sc -USY
MeCN
MeOH
MeCN
III
1
1
0
1
Sc -USY
H-USY
traces
[
0
a] Reaction performed on 0.3 mmol of 1a, 0.3 mmol of 2a, and
III
.45 mmol of 4a in 1 mL of solvent, with 7–10 mg of Sc zeolite (i.e., ap-
III[30]
proximately 5 mol% of Sc
). [b] Yields of isolated pure product, ob-
tained as a single diastereoisomer. [c] Toluene, THF, acetonitrile, and
methanol were also examined without success.
No reaction took place without catalyst whatever the sol-
I
vent (Table 1, entry 1). Cu alone was not a true catalyst,
Scheme 3. Selected reactions catalyzed by m-zeolites.
only giving a very small amount of the expected adduct 5a
[18]
II
in very low yield (entry 2). As reported, Cu salts alone
did catalyze the cycloaddition and led to the expected
adduct 5a although in moderate yield in a slow reaction
ilar conditions. Moreover, we also showed that one-pot mul-
I
ticomponent reactions could also be catalyzed by Cu zeo-
[27]
I
II
lites (Scheme 3c). Since the latter proceeds by the in situ
formation of imines, it was thus tempting to examine a
direct one-pot reaction from aldehydes and amines in the
presence of alkenes. On the other hand, we also recently
(entry 3). Under similar conditions, Cu - as well as Cu -USY
were not effective as catalysts and only traces of adduct
could be detected after a prolonged reaction time while the
starting materials were mostly recovered (entries 4–5). Re-
III
III
showed that Sc zeolites are useful and reusable catalysts
wardingly, Sc -USY gave the expected adduct 5a in low to
[28]
for Mukaiyama-type aldolizations (Scheme 3d).
Lantha-
high yields depending on the solvent used (entries 6–10). A
slow but clean transformation was observed in apolar or
slightly polar solvents and despite heating, the adduct 5a
was isolated with modest yields (entries 6–7). Surprisingly, in
more polar solvents, such as THF, the reaction was even
lower although the conversion was similar (entry 8 vs. 6–7).
In sharp contrast, acetonitrile allowed the adduct 5a to be
obtained in excellent and almost quantitative yield (entry 9).
III
nide and Sc salts are known to promote hetero-Diels–
[29]
III
Alder reactions. Thus, Sc zeolites seem good candidates
for catalyzing cycloadditions leading to hydroquinolines.
We report here our results in this area, revealing the first
zeolite-catalyzed cycloaddition of N-arylimines with alkenes
for the synthesis of hydroquinolines (Scheme 2).
1908
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Chem. Eur. J. 2011, 17, 1907 – 1914

Scandium ACHTUNGTRENNUNG( III) Zeolites as New Heterogeneous Catalysts
FULL PAPER
Protic solvents, such as methanol, slowed the reaction again
and only modest yields could be achieved (entry 10).
Control experiments with the native H-USY showed its
total inactivity, confirming the key role of scandium ions
loaded into the zeolite during this reaction (entry 11). As
expected, the same results were obtained when starting
from aniline (1a) and benzaldehyde (2a), in situ leading to
imine 3a.
Interestingly, a single regio- and diastereoisomer was iso-
lated in these experiments with cyclopentadiene. Compari-
[19,31]
son of NMR spectroscopic data with known data
and
further NOESY experiments revealed the all-cis stereo-
chemistry of the 5a adduct. This result suggested a [4+2]cy-
cloaddition with a concerted endo mechanism (Scheme 4;
see the mechanism section below).
III
Figure 1. Sc -USY recycling and reuse for the one-pot imine forma-
tion—cycloaddition reaction between benzaldehyde (1a), aniline (2a),
and cyclopentadiene (4a).
III
Table 2. Sc -USY-catalyzed cycloaddition of various anilines 1 with ben-
[
a]
zaldehyde.
Scheme 4. Observed NOESY proximities and the resulting hypothetic
endo [4+2]pathway for the reaction catalyzed by Sc zeolite.
Entry Amine
T
Yield Ratio
Adduct
[b]
[
8C] [%]
cis/
[
c]
Catalyst recycling: An important and useful feature of any
heterogeneous catalyst is its recovery and recyclability. The
Sc-doped zeolites could easily be recovered through a
simple filtration over Nylon membrane. A simple wash al-
lowed us to recover the organic materials on one hand and
trans
1
2
20
60
40
96
100:0
100:0
III
on the other, the Sc zeolite.
The recyclability was examined by performing the one-
pot imine formation–cycloaddition reaction between aniline
3
4
5
60
60
60
35
29
47
79:21
100:0
100:0
(
1a), benzaldehyde (2a), and cyclopentadiene (4a) with the
III
same Sc -USY catalyst several times. At each run, the cata-
lyst was recovered by filtration and regenerated or not by
heating before being reused in the next run. As shown in
Figure 1, the catalyst could be recycled at least up to five
times, but the catalyst efficiency dramatically changed upon
regeneration or not between each run (Figure 1, back and
front row, respectively). A slight decrease in yields occurred
every two runs with regeneration through heating at 5508C
(
Figure 1, back row), whereas a sharp drop was observed
6
60
51
100:0
after the first run without regeneration (Figure 1, front row).
Nevertheless, the catalyst was still active after the 5th reuse,
still giving the product with a reasonable 47% isolated yield
with regeneration.
[
a] Reactions performed with 1 equiv of 1, 1 equiv of 2a, 1.5 equiv of 4a,
III [30]
and 5 mol% Sc -USY in 2 mL of acetonitrile over 18 h. [b] Yields of
isolated pure product. The starting amine and aldehyde usually account-
ed for the mass balance. [c] Determined by NMR spectroscopic analysis
of the crude mixtures.
Scope and limitations: With these conditions in hand, we
then explored the scope of this heterogeneously catalyzed
cyclocondensation. In one series of experiments, various ani-
lines 1a–e were submitted to benzaldehyde (2a) in the pres-
adduct but with a modest yield, whereas at 608C the reac-
tion was quantitative (Table 2, entry 1 vs. 2). Under both
conditions, the reaction was completely regio- and stereose-
lective, yielding a single regio- and diastereoisomer. With
methoxy groups on the aniline, the reaction became less ef-
III
ence of cyclopentadiene (4a) and a catalytic amount of Sc -
USY (5 mol%) as the catalyst in acetonitrile (Table 2). At
room temperature, the simplest aniline gave the expected
Chem. Eur. J. 2011, 17, 1907 – 1914
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1909

P. Pale et al.
fective, even upon heating (entries 3–4). Surprisingly, the
presence of one para-methoxy group induced a drop in ste-
reoselectivity, whereas with one para- and one meta-me-
thoxy group, the very high cis stereoselectivity was main-
tained (entry 3 vs. 4). Electron-withdrawing groups instead
of -donating groups did not reverse the yield lowering and
whatever the electron-withdrawing character of the group,
moderate yields were achieved (entries 5–6). Nevertheless, a
single cis adduct was again produced in each case.
vinyl ether, which again provided a single adduct (entry 7).
However, its stereoselectivity was established as trans by
[32]
comparison with known compounds and by NOESY ex-
periments. This stereochemical reversion suggests another
but not concerted mechanism with enol ethers (see the
mechanism section below).
In a third series of experiments, various aldehydes 2a–h
were examined (Table 4). As for anilines (Table 2), benzal-
dehydes carrying electron-donating or -withdrawing groups
In a second series of experiments, various alkenes 4a–d
were examined, and thus submitted to aniline (1a) and ben-
zaldehyde (2a) under the same conditions (Table 3). Enol
III
Table 4. Sc -USY-catalyzed cyclocondensation of various aldehydes 2
[a]
and aniline with cyclopentadiene or dihydrofurane.
III
Table 3. Sc -USY-catalyzed cyclocondensation of various alkenes 4 with
[
a]
aniline and benzaldehyde.
[
c]
Entry
Aldehyde
Alkene Yield Ratio
Adduct
[
b]
[
%]
cis/trans
1
2
PhCHO
2a
40
96
100:0
100:0
Entry
Alkene
T
Yield
[%]
Ratio
cis/trans
Adduct
[
b]
[c]
[
8C]
1
2
20
60
40
96
100:0
100:0
pMePhCHO
2
b
3
4
5
32
13
63
100:0
100:0
100:0
3
4
20
0
76
70
63:37
62:38
pMeOPhCHO
2
c
5
6
20
60
34
23
48:52
69:31
pFPhCHO
2
d
7
60
60
0:100
2
pO NPhCHO
2
e
[
a] Reactions performed with 1 equiv of 1, 1 equiv of 2a, 1 equiv of 4a,
III [30]
6
50
100:0
and 5 mol% Sc -USY, in 2 mL of acetonitrile over 18 h. [b] Yields of
isolated pure product. The starting amine and aldehyde usually account-
ed for the mass balance. [c] Determined by NMR spectroscopic analysis
of the crude mixture.
7
8
40
10
100:0
n.d.
ethers were clearly more reactive than cyclopentadiene at
room temperature, as revealed by reactions with dihydrofur-
an and ethyl vinyl ether (Table 3, entries 3–4, 7 vs. 1–2).
With the former, the reaction could even be performed at
0
8C without dramatic changes in yield and stereoselectivity
(
entry 4 vs. 3 vs. 1). In contrast, dihydropyrane did not react
as well, even at 608C. At room temperature, the correspond-
ing adduct was formed with low yield and almost no stereo-
selectivity, but at 608C the stereoselectivity was improved
but to the detriment of yield (entry 6 vs. 5 vs. 1).
With such enol ethers, the regioselectivity was still very
high and a single regioisomer was observed (Table 3, en-
tries 3–4, 5–6, 7). However, the stereoselectivity was dramat-
ically lowered (entries 3–4, 5–6 vs. 1–2), except with ethyl
9
58
62
100:0
88:12
10
1910
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Chem. Eur. J. 2011, 17, 1907 – 1914

Scandium ACHTUNGTRENNUNG( III) Zeolites as New Heterogeneous Catalysts
FULL PAPER
Table 4. (Continued)
Entry Aldehyde
Organic reactions performed in zeolites could occur with
mechanisms similar or different from the same reaction in
[
c]
Alkene Yield Ratio
Adduct
[
b]
I
[%]
cis/trans
homogeneous conditions, as observed for Cu -zeolite-cata-
[25–28]
III
EtOOC
ÀCHO
lyzed reactions.
The present Sc -zeolite-catalyzed cy-
2
h
clocondensations could thus follow either normal or reverse
electron demand Diels–Alder pathways or a stepwise cation-
ic route, or both of them.
1
1
2
96
95
100:0
The differences in stereoselectivity observed here with cy-
clopentadiene (see Table 1) relative to other alkenes (see
Table 2) suggested that different mechanisms operated in
these cyclocondensations. To look for such mechanistic dual-
ity, correlations between Hammet coefficients and various
EtOOCÀCHO
2
h
1
72:28
[36]
reaction parameters are often used.
We thus evaluated
[
a] Reactions performed with 1 equiv of 1, 1 equiv of 2a, 1 equiv of 4a,
III
the Sc -zeolite-catalyzed reaction efficiency upon substitu-
ent effects (Figure 2). Both para-substituted anilines and
benzaldehydes showed similar trends with maximum yields
for nonsubstituted substrates and decreasing yields for sub-
strates with either electron-donating or -withdrawing sub-
stituents.
III
[30]
and 5 mol% Sc -USY, in 2 mL of acetonitrile over 18 h. [b] Yields of
isolated pure product. The starting amine and aldehyde usually account-
ed for the mass balance. [c] Determined by NMR spectroscopic analysis
of the crude mixture.
proved to be less reactive and they required heating
(
Table 4, entries 3–6 vs. 1–2). The presence of an electron-
donating group was particularly deleterious, inducing low
yields of adducts, and the more electron-donating the group,
the lower the yield (entry 4 vs. 3). Although the effect of the
withdrawing groups was less dramatic with modest to good
yields of adducts, the same tendency was observed (entry 5
vs. 6). In contrast, all these reaction gave a single adduct of
cis stereochemistry.
Heteroaromatic aldehydes were also less reactive, leading
to modest yields of the corresponding adducts (Table 4, en-
tries 7–8), even with more reactive dienes, such as dihydro-
furan (entry 8 vs. 7).
Aliphatic aldehydes gave better results and the expected
adducts between isobutyraldehyde (2g) and cyclopentadiene
or dihydrofuran were isolated in good yields (Table 4,
entry 9–10). As for the preceding examples, a single adduct
was formed with cyclopentadiene, whereas a mixture of ste-
reoisomers was obtained with dihydrofuran.
Ethyl glyoxylate (2h) proved to be very reactive, giving
the expected adducts 5q–r with cyclopentadiene or dihydro-
furane in almost quantitative yields at room temperature
Figure 2. Correlation between yields and the substitution of either aniline
or benzaldehyde (^: aldehyde;
: aniline).
&
The presence of two different slopes in the correlation
graphic confirms the implication of different mechanisms in
III
this Sc -zeolite-catalyzed cyclocondensation (Figure 2). For
electron-withdrawing groups, the dependence of the reaction
efficiency on Hammet parameters and the formation of a
single diastereoisomer point to a normal electronic demand
concerted reaction (Scheme 4, right). On the other hand,
the isolation of two different diastereoisomers from para-
methoxyaniline suggests a dominant stepwise mechanism for
electron-donating-substituted substrates. The latter mecha-
nism is also supported by the reactions of enol ethers, which
gave a mixture of diastereoisomers, and by the rise in diaste-
reoselectivity upon a rise in temperature to above room
temperature for dihydropyrane and the reversal of selectivi-
ty observed with ethyl vinyl ether (Table 3).
(
Table 4, entries 11 and 12). Again, a single adduct was
formed with cyclopentadiene, and a mixture of diastereoiso-
mers but with the same very high regioselectivity was ob-
tained with dihydrofuran (see NOESY in the Supporting In-
formation).
Mechanism: The mechanism of aza-Diels–Alder reactions is
still a matter of debate. Under classical conditions, that is, in
the presence of a Lewis acid, a concerted Diels–Alder type
[33]
mechanism is often proposed, although a delicate balance
between normal and reverse electron demand is invoked to
It is worth noting that literature precedents for reactions
with enol ethers often mentioned mixtures of diastereoiso-
[19d,31,34]
explain differences in reactivity and stereoselectivity.
However, evidence for a stepwise cationic mechanism has
III
IV
mers, with the cis isomer as a major product for Sm , Ce ,
[35]
IV
[15,16,32]
been reported.
or Ti as the catalyst
but with the trans as the major
III
product in the presence of Yb , I , TMSCl, or even 4-nitro-
2
Chem. Eur. J. 2011, 17, 1907 – 1914
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1911

P. Pale et al.
[
37]
13
phtalic or phosphomolybdic acids. The overall cis prefer-
77.23 ppm for C). Carbon multiplicities were determined by DEPT135
experiments. The cis/trans conformation of the products was determined
by NOESY experiments. Electrospray (ESIMS) low/high-resolution mass
spectra were obtained from the mass spectrometry department of the
Service Commun d’Analyses, Institut de Chimie, Strasbourg.
III
ence observed with Sc zeolite whatever the “dienophile”
could also be due to the spherical shape of the zeolite used
USY), favoring more or less spherical rather than extended
transition states, especially in the stepwise mechanism
Scheme 5).
(
III
Preparation of Sc zeolites
(
III
Sc -USY: Commercial NH
4
USY was loaded in an oven and heated at
(OTf) (214 mg,
.1 equiv) were mixed by using a mortar and charged in a closed reactor.
5
0
508C during 4 h giving H-USY. H-USY (1 g) and Sc
A
H
U
G
R
N
U
G
3
The mixture of powders was heated at 3508C during over three days
III
under a nitrogen flow, quantitatively yielding Sc -USY. These modified
zeolites have been fully characterized and this will be described in a
more specific article.
III
General procedure for the Sc -zeolite-catalyzed [4+2]cyclocondensation
of anilines, aldehydes, and alkenes: Aniline 1 (0.5 mmol, 1.0 equiv), alde-
hyde 1 (0.5 mmol, 1.0 equiv), and then alkene 4 (0.75 mmol, 1.5 equiv)
III
were successively added to
0
a suspension of Sc -USY (45 mg,
[
30]
.05 equiv) in acetonitrile (2 mL). After 18 h stirring at 20 or 608C (see
Tables 1–4), the mixture was cooled to room temperature and the catalyst
was removed by filtration over Nylon membrane (0.20 mm). Solvent
evaporation provided the resulting crude product. Column chromatogra-
phy was performed when necessary.
Some of the so-formed adducts are known compounds, and hydroquino-
[
38]
[39]
[40]
[41]
[42]
[15]
[32]
[39]
[40]
[37b]
lines 5a
have been reported.
-Fluoro-4-phenyl-3a,4,5,9b-tetrahydro(3H)cyclopenta[c]quinoline (5e):
5b,
5c,
5d,
5 f,
5g
5h,
5I,
5l,
and 5n
8
Only the cis adduct was obtained. Yield: 51%; pale-green solid; m.
1
p. 1138C; H NMR (300 MHz, CDCl3): d=7.43–7.36 (m, 2H), 7.08–7.00
(
m, 3H), 6.97 (d, J=7.6 Hz, 1H), 6.75 (dd, J=7.9, 7.1 Hz, 1H), 6.61 (d,
J=7.9 Hz, 1H), 5.84 (dm, J=5.4, 1.5 Hz, 1H), 5.64 (ddd, J=5.5, 2.3,
1.8 Hz, 1H), 4.61 (d, J=3.3 Hz, 1H), 4.09 (d, J=8.9 Hz, 1H), 3.69 (brs,
1H), 2.95 (qd, J=9.0, 3.4 Hz, 1H), 2.60 (ddddd, J=16.3, 9.4, 2.7, 2.4,
Scheme 5. Proposed stepwise mechanism taking into account the spheri-
cal shape of the Sc-USY used as the catalyst (the bold curve indicates
steric repulsion between the “dienophilic” part and zeolite wall).
1
3
2
.1 Hz, 1H), 1.79 ppm (dddd, J=16.0, 8.7, 2.5, 1.7 Hz, 1H); C NMR
75 MHz, CDCl ): d=156.7 (d, JCÀF =236 Hz), 142.8, 141.9, 133.6, 131.1,
28.7, 127.7 (d, JCÀF =6 Hz), 127.5, 126.6, 116.6 (d, JCÀF =7 Hz), 115.0 (d,
(
3
1
J
CÀF =22 Hz), 113.0 (d, JCÀF =23 Hz), 58.6, 46.9, 45.8, 31.7 ppm; HRMS
Conclusion
(
ESI, positive mode): m/z: calcd for C18 17FN: 266.134; found: 266.134
H
+
[
M+H] .
We have shown for the first time that scandium ACHUTNGRENNUG( III)-modi-
4
-(4-Methoxyphenyl)-3a,4,5,9b-tetrahydro(3H)cyclopenta[c]quinoline
fied zeolites can be used as catalysts for the aza-Diels–Alder
reaction of imines with alkenes. This heterogeneous method
offers a mild and efficient access to tetrahydroquinoline de-
rivatives with a reasonably wide scope, tolerating various
functional groups, and with high regioselectivity. Moreover,
this heterogeneous scandium (III)-modified zeolite catalyst
can be reused at least three times without significant loss of
activity.
(
5j): Only the cis adduct was obtained. Yield: 13%; pale-yellow oil;
1
H NMR (300 MHz, CDCl ): d=7.34 (d, J=8.5 Hz, 2H), 7.04 (d, J=
7.0 Hz, 1H), 6.97 (td, J=7.7, 1.4 Hz, 1H), 6.89 (d, J=8.5 Hz, 2H), 6.73
3
(
5
(
td, J=7.4, 1.2 Hz, 1H), 6.60 (dd, J=7.9, 1.0 Hz, 1H), 5.85–5.80 (m, 1H),
.66–5.61 (m, 1H), 4.57 (d, J=3.3 Hz, 1H), 4.08 (d, J=9.1 Hz, 1H), 3.81
s, 3H), 3.70 (brs, 1H), 2.95 (qd, J=9.0, 3.3 Hz, 1H), 2.62 (dddd, J=16.5,
9
.4, 4.4, 2.2 Hz, 1H), 1.82 ppm (dddd, J=16.0, 8.8, 2.5, 1.7 Hz, 1H);
1
3
C NMR (75 MHz, CDCl ): d=158.9, 145.9, 135.1, 134.2, 130.6, 129.2,
3
127.7, 126.5, 126.3, 119.3, 116.0, 114.0, 57.7, 55.5, 46.6, 46.3, 31.7 ppm;
HRMS (ESI, positive mode): m/z: calcd for C19 20NO: 278.154; found:
2
H
+
78.152 [M+H] .
4
-(4-Fluorophenyl)-3a,4,5,9b-tetrahydro(3H)cyclopenta[c]quinoline
Experimental Section
(5k): Only the cis adduct was obtained. Yield: 19%; pale-brown solid;
1
m. p. 1468C; H NMR (300 MHz, CDCl
.00 (m, 3H), 6.97 (d, J=7.6 Hz, 1H), 6.75 (t, J=7.5 Hz, 1H), 6.61 (d,
J=6.9 Hz, 1H), 5.86–5.82 (m, 1H), 5.65–5.63 (m, 1H), 4.61 (d, J=3.3 Hz,
3
): d=7.43–7.36 (m, 2H), 7.09–
7
General: All starting materials were commercial (used as received) or
prepared according to known literature procedures (see the Supporting
Information for details). The reactions were monitored by TLC carried
out on silica plates (silica gel 60 F254, Merck) by using UV-light and
phosphomolybdic acid with cerium(IV) for visualization. Column chro-
matography was performed on silica gel 60 (0.040–0.063 mm, Merck) by
using mixtures of ethyl acetate or toluene and cyclohexane as the eluents.
Evaporation of the solvents was conducted under reduced pressure at
temperatures less than 308C. Melting points (M.p.) were measured in
1
1
2
2
1
H), 4.09 (d, J=8.4 Hz, 1H), 3.69 (brs, 1H), 2.95 (qd, J=9.0, 3.4 Hz,
H), 2.60 (ddq, J=16.3, 9.4, 2.4 Hz, 1H), 1.79 ppm (dddd, J=16.1, 8.3,
.3, 1.7 Hz, 1H); C NMR (75 MHz, CDCl ): d=162.2 (d, J =
3 CÀF
44 Hz), 145.6, 138.8, 134.2, 130.5, 129.2, 128.2 (d, JCÀF =8.6 Hz), 126.6,
26.2, 119.5, 116.2, 115.5 (d, JCÀF =19 Hz), 57.7, 46.5, 46.3, 31.6 ppm;
1
3
HRMS (ESI, positive mode): m/z: calcd for C18H17FN: 266.134; found:
+
2
66.134 [M+H] .
4-(Furan-2-yl)-3a,4,5,9b-tetrahydro(3H)cyclopenta[c]quinoline
Only the cis adduct was obtained. Yield: 39%; white solid; m. p. 1048C;
1
13
open capillary tubes and are uncorrected. H and C NMR spectra were
recorded on a Bruker Avance 300 spectrometer at 300 and 75 MHz, re-
spectively. Chemical shifts d and coupling constants J are given in ppm
(5m):
1
3
H NMR (500 MHz, CDCl ): d=7.38 (d, J=1.7 Hz, 1H), 7.05 (d, J=
and Hz, respectively. Chemical shifts d are reported relative to residual
7.8 Hz, 1H), 6.98 (td, J=7.7, 1.8 Hz, 1H), 6.76 (td, J=7.4, 1.3 Hz, 1H),
6.63 (dd, J=7.9, 1.1 Hz, 1H), 6.36 (dd, J=3.1, 1.8 Hz, 1H), 6.25 (dt, J=
1
solvent as an internal standard ([D
1
]chloroform: d=7.24 for H and
1912
www.chemeurj.org
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 1907 – 1914

Scandium ACHTUNGTRENNUNG( III) Zeolites as New Heterogeneous Catalysts
FULL PAPER
3
3
3
8
1
3
2
.2, 0.8 Hz, 1H), 5.82–5.78 (m, 1H), 5.68–5.66 (m, 1H), 4.63 (d, J=
Acknowledgements
.3 Hz, 1H), 4.09 (d, J=8.9 Hz, 1H), 3.85 (brs, 1H), 3.19 (qd, J=8.9,
.3 Hz, 1H), 2.65 (ddq, J=16.5, 8.8, 2.4 Hz, 1H), 2.17 ppm (ddt, J=16.5,
AO thanks the CNRS and the “Ministerio EspaÇol de Educaciꢃn” for
post-doctoral fellowships. The authors gratefully acknowledge financial
support from the Loker Institute (Los Angeles), the CNRS, and the
French Ministry of Research.
1
3
.9, 2.1 Hz, 1H); C NMR (75 MHz, CDCl
3
): d=155.9, 145.1, 141.7,
34.2, 130.5, 129.2, 126.5 (2C), 119.7, 116.1, 110.4, 105.5, 56.8, 46.3, 42.7,
2.7 ppm; HRMS (ESI, positive mode): m/z: calcd for
38.123; found: 238.124 [M+H] .
16
C H16NO:
+
4
-Isopropyl-3a,4,5,9b-tetrahydro(3H)cyclopenta[c]quinoline (5o): Only
1
the cis adduct was obtained. Yield: 58%; colorless oil; H NMR
300 MHz, CDCl ): d=6.98 (d, J=7.6 Hz, 1H), 6.93 (ddd, J=7.8, 7.5,
.5 Hz, 1H), 6.68 (td, J=7.4, 1.3 Hz, 1H), 6.55 (d, J=7.8 Hz, 1H), 5.81
dtd, J=5.7, 2.8, 1.5 Hz, 1H), 5.69 (ddd, J=5.8, 2.5, 1.6 Hz, 1H), 3.94 (d,
J=3.9 Hz, 1H), 3.63 (brs, 1H), 2.94 (dddd, J=9.9, 3.5, 2.8, 2.4 Hz, 1H),
(
1
(
3
[
1] a) T. H. Jones, M. S. Blum, H. G. Robertson, J. Nat. Prod. 1990, 53,
29–435; b) J. W. Daly, H. M. Garraffo, T. F. Spande in Alkaloids:
Chemical and Biological Perspectives, Vol. 13 (Ed.: S. W. Pelletier),
Pergamon, New York, 1999, p. 1–161; c) M. F. Grundon, Nat. Prod.
Rep. 1987, 4, 225–236; d) M. F. Grundon in The Alkaloids, Vol. 32
4
2
1
1
1
2
2
.51 (ddq, J=16.0, 9.0, 2.4 Hz, 1H), 2.23 (dddd, J=16.1, 8.4, 2.4, 1.7 Hz,
H), 1.66 (ddm, J=13.3, 9.4, 6.5 Hz, 1H), 1.05 (d, J=6.5 Hz, 3H),
(
Ed.: A. Brossi), Academic Press, San Diego, 1988, p. 341–362.
1
3
.00 ppm (d, J=6.9 Hz, 3H); C NMR (75 MHz, CDCl
3
): d=145.8,
[
2] a) I. Jacquemond-Collet, F. Benoit-Vical, M. A. Valentin, E. Stani-
slas, M. Mallie, I. Fouraste, Planta Med. 2002, 68, 68–69; b) I. Jac-
quemond-Collet, S. Hannedouche, N. Fabre, I. Fouraste, C. Moulis,
Phytochemistry 1999, 51, 1167–1169.
34.6, 130.4, 129.5, 129.0, 126.3, 118.9, 115.7, 60.4, 46.6, 41.2, 31.1, 30.8,
0.3, 19.5 ppm; HRMS (ESI, positive mode): m/z: calcd for C15
14.160; found: 214.159 [M+H] .
H
20N:
+
4
-Isopropyl-2,3,3a,4,5,9b-hexahydrofuro
A
H
U
G
R
N
N
[3,2-c]quinoline (5p): Yield:
[3] For a seminal review, see: J. W. Daly, T. F. Spande, H. M. Garraffo,
J. Nat. Prod. 2005, 68, 1556–1575.
[4] A SciFinder survey revealed 1932 tetrahydroquinolines with biologi-
cal activities, one third being patented, revealing the importance of
this family of compounds.
[5] R. Frechette, PCT Int. Appl., WO 2009073550A2 20090611; CAN
151:33434, 2009.
[6] K. Schiemann, D. Finsinger, C. Amendt, F. Zenke, Ger. Offen., DE
102007013855 A1 20080925, 2008 and PCT Int. Appl., WO
2008113451A1 20080925; CAN 149: 378717, 2008.
6
2%; cis/trans 88:12, diastereoisomers could not be separated; white
1
solid; m. p. 1108C; cis isomer: H NMR (300 MHz, CDCl
J=7.8, 1.2 Hz, 1H), 7.03 (td, J=7.6, 1.6 Hz, 1H), 6.73 (td, J=7.4, 1.2 Hz,
3
): d=7.28 (dd,
1
2
2
4
1
1
1
2
H), 6.51 (dd, J=8.0, 0.7 Hz, 1H), 5.11 (d, J=8.0 Hz, 1H), 3.83–3.72 (m,
H), 3.68 (brs, 1H), 3.03 (dd, J=9.1, 2.7 Hz, 1H), 2.73 (dtd, J=10.4, 8.1,
.6 Hz, 1H), 2.01 (ddt, J=12.1, 10.3, 8.5 Hz, 1H), 1.85 (ddd, J=8.7, 7.1,
.2 Hz, 1H), 1.71 (dm, J=9.1, 6.5 Hz, 1H), 1.05 (d, J=6.6 Hz, 3H),
1
3
.00 ppm (d, J=6.7 Hz, 3H); C NMR (75 MHz, CDCl
3
): d=145.1,
30.0, 128.3, 122.8, 118.8, 114.6, 76.1, 66.7, 59.0, 40.8, 31.3, 24.0, 20.1,
9.2 ppm; HRMS (ESI, positive mode): m/z: calcd for
C
14
H
20NO:
[7] H. A. Carlson, K. L.; Damm, K. L. Meagher, PCT Int. Appl., WO
2009036341A2 20090319; CAN 150:345456, 2009.
+
18.154; found: 218.154 [M+H] .
Ethyl
3a,4,5,9b-tetrahydro(3H)cyclopenta[c]quinoline-4-carboxylate
[8] J. Liu, Y. Wang, Y. Sun, D. Marshall, S. Miao, G. Tonn, P. Anders, J.
Tocker, H. L. Tang, J. Medina, Bioorg. Med. Chem. Lett. 2009, 19,
6840–6844.
[9] For a review, see: E. Russo, J. Cannabis Ther. 2003, 163–174.
[10] D. S. Goldfarb, U. S. Pat. Appl. Publ. , US 2009163545A1 20090625;
CAN151: 115085, 2009.
[11] C. Ramesh, T. K. Nayak, R. Burai, M. K. Dennis, H. J. Hathaway,
L. A. Sklar, E. R. Prossnitz, J. B. Arterburn, J. Med. Chem. 2010, 53,
1004–1014.
[12] For reviews, see: a) G. Jones in Comprehensive Heterocyclic Chemis-
try II, Vol. 5 (Eds.: A. Katritzky, C. W. Rees, E. F. V. Scriven), Per-
gamon, Oxford, 1996, p. 67; b) A. Katritzky, S. Rachwal, B. Rachwal,
Tetrahedron 1996, 52, 15031–15070; c) V. V. Kouznetsov, L. Y.
Vargas Mꢁndez, C. M. Melꢁndez Gꢃmez, Curr. Org. Chem. 2005, 9,
(
5q): Only the cis adduct was obtained. Yield: 96%; white solid; m.
1
p. 66 8C; H NMR (300 MHz, CDCl
.4 Hz, 1H), 6.63 (dd, J=7.9, 1.2 Hz, 1H), 5.74–5.70 (m, 1H), 5.65–5.63
m, 1H), 4.69 (brs, 1H), 4.28 (dq, J=27.1, 7.1 Hz, 1H), 4.24 (dq, J=27.1,
J=7.1 Hz, 1H), 4.09 (d, J=3.3 Hz, 1H), 4.06 (d, J=2.0 Hz, 1H), 3.33
qd, J=8.9, 3.5 Hz, 1H), 2.47 (ddq, J=16.0, 8.7, 2.4 Hz, 1H), 2.31 (dddd,
3
): d=6.98 (m, 2H), 6.71 (td, J=7.5,
1
(
(
1
3
J=16.2, 9.0, 2.3, 1.9 Hz, 1H), 1.31 ppm (t, J=7.1 Hz, 3H); C NMR
75 MHz, CDCl ): d=172.1, 144.1, 134.4, 130.0, 128.8, 126.7, 126.2, 119.5,
16.0, 61.4, 56.7, 46.6, 40.9, 32.9, 14.5 ppm; HRMS (ESI, positive mode):
(
1
3
+
m/z: calcd for C15
Ethyl 2,3,3a,4,5,9b-hexahydrofuro
Both cis and trans diastereoisomers were obtained; yield: 97%; cis/trans
H18NO
2
: 244.134; found: 244.132 [M+H] .
AHCTUNGTR[ENNNUG 3,2-c]quinoline-4-carboxylate (5r):
1
7
2:28; both products are white solids; cis diastereoisomer: H NMR
1
41–161.
13] a) J. L. Rogers, J. J. Ernat, H. Yung, R. S. Mohan, Catal. Commun.
009, 10, 625–626; b) B. V. S. Reddy, R. Srinivas, J. S. Yadav, T.
(
300 MHz, CDCl ): d=7.27 (dd, J=7.7, 1.6 Hz, 1H), 7.06 (td, J=7.6,
3
[
1
5
.5 Hz, 1H), 6.76 (td, J=7.4, 1.1 Hz, 1H), 6.59 (dd, J=8.1, 1.3 Hz, 1H),
.19 (d, J=8.1 Hz, 1H), 4.99 (s, 1H), 4.28 (dq, J=20.0, 7.1 Hz, 1H), 4.25
2
Ramalingam, Synth. Commun. 2001, 31, 1075–1080.
14] L. C. Da Silva-Filho, V. Lacerda Jr, M. G. Constantino, G. V. Jose da
Silva, Synthesis 2008, 2527–2536.
15] Z. Zhou, F. Xu, X. Han, J. Zhou, S. Qi, Eur. J. Org. Chem. 2007,
5265–5269.
(
dq, J=20.0, 7.1 Hz, 1H), 4.18 (d, J=3.3 Hz, 1H), 3.82–3.70 (m, 2H),
[
[
[
[
3
3
1
.08 (qd, J=8.8, 3.1 Hz, 1H), 2.04–1.82 (m, 2H), 1.31 ppm (t, J=7.1 Hz,
H); C NMR (75 MHz, CDCl
19.4, 115.0, 75.7, 66.7, 61.7, 55.5, 40.5, 25.4, 14.4 ppm; trans diastereoiso-
1
3
3
): d =171.5, 143.7, 130.1, 128.8, 122.3,
1
mer: H NMR (300 MHz, CDCl
3
): d=7.30 (dd, J=7.5, 1.8 Hz, 1H), 7.08
16] V. Sridharan, C. Avendano, J. C. Menendez, Synthesis 2008, 1039–
(
td, J=7.7, 1.6 Hz, 1H), 6.76 (td, J=7.5, 1.1 Hz, 1H), 6.65 (dd, J=8.1,
1
044.
17] V. T. Kamble, V. R. Ekhe, N. S. Joshi, A. V. Biradar, Synlett 2007,
379–1382.
[18] A. Semwal, S. Nayak, Synth. Commun. 2006, 36, 227–236.
0
7
3
2
.9 Hz, 1H), 4.60 (d, J=5.9 Hz, 1H), 4.24 (q, J=7.1 Hz, 1H), 4.23 (q, J=
.1 Hz, 1H), 3.95 (td, J=8.4, 5.6 Hz, 1H), 3.80 (td, J=8.5, 7.1 Hz, 1H),
.58 (d, J=9.5 Hz, 1H), 2.59 (dddd, J=9.5, 7.8, 5.8, 4.3 Hz, 1H), 2.32–
1
1
3
3
.13 (m, 2H), 1.29 ppm (t, J=7.1 Hz, 3H); C NMR (75 MHz, CDCl ):
[
19] a) J. S. Yadav, B. V. S. Reddy, R. S. Rao, S. K. Kumar, A. C. Kunwar,
Tetrahedron 2002, 58, 7891–7896; b) G. Babu, R. Nagarajan, R. Na-
tarajan, P. T. Perumal, Synthesis 2000, 661–666; c) G. Babu, P. T. Pe-
rumal, Tetrahedron Lett. 1998, 39, 3225–3228; d) D. Duvelleroy, C.
Perrio, O. Parisel, M-C. Lasne, Org. Biomol. Chem. 2005, 3, 3794–
d=172.5, 143.3, 130.7, 129.1, 120.3, 119.0, 115.2, 75.2, 65.7, 61.7, 55.7,
9.2, 29.7, 14.4 ppm.
3
Detailed experimental procedures for 5a–r and for 4a and NMR spectra
of all new compounds are given in the Supporting Information.
3
804.
[
20] a) R. A. Batey, D. A. Powell, A. Acton, A. J. Lough, Tetrahedron
Lett. 2001, 42, 7935–7939; Powell, A. Acton, A. J. Lough, Tetrahe-
dron Lett. 2001, 42, 7935–7939; b) Y. Ma, C. Qian, M. Xie, J. Sun, J.
Org. Chem. 1999, 64, 6462–6467; c) Y. Makioka, T. Shindo, Y. Tani-
guchi, K. Takai, Y. Fujiwara, Synthesis 1995, 801–804.
Chem. Eur. J. 2011, 17, 1907 – 1914
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1913

P. Pale et al.
[
[
[
21] M. L. Kantam, M. Roy, S. Roy, S. M. Subhas, B. Sreedhar, B. M.
Choudary, R. L. De, J. Mol. Catal. A 2007, 265, 244–249.
22] D. Mahajan, B. A. Ganai, R. L. Sharma, K. K. Kapoor, Tetrahedron
Lett. 2006, 47, 7919–7921.
23] C. Boglio, G. Lemiere, B. Hasenknopf, S. Thorimbert, E. Lacote, M.
Malacria, Angew. Chem. 2006, 118, 3402–3405; Angew. Chem. Int.
Ed. 2006, 45, 3324–3327.
lites, see: B. Louis, S. Walspurger, J. Sommer, Catal. Lett. 2004, 93,
81–84.
[31] V. Lucchini, M. Prato, G. Scorrano, P. Tecilla, J. Org. Chem. 1988,
53, 2251–2258.
[32] G. Sundararajan, N. Prabagaran, B. Varghese, Org. Lett. 2001, 3,
1973–1976.
[33] Y-S. Cheng, E. Ho, P. S. Mariano, H. L. Ammon, J. Org. Chem.
1985, 50, 5678–5686.
[
24] a) T. Kawabata, M. Kato, T. Mizugaki, K. Ebitani, K. Kaneda,
Chem. Eur. J. 2005, 11, 288–297; b) K. V. N. S. Srinivas, B. Das, Syn-
lett 2004, 1715–1718; c) J. S. Yadav, B. V. S. Reddy, K. Sadasiv,
P. S. R. Reddy, Tetrahedron Lett. 2002, 43, 3853–3856.
[34] E. Eibler, P. Hçcht, B. Prantl, H. Rossmaier, H. M. Scuhbauer, H.
Wiest, J. Sauer, Liebigs Ann. 1997, 2471–2484.
[35] a) S. Hermitage, D. A. Jay, A. Whiting, Tetrahedron Lett. 2002, 43,
9633–9636; b) S. Hermitage, A. K. Howard, D. Jay, R. G. Pritchard,
M. R. Probert, A. Whiting, Org. Biomol. Chem. 2004, 2, 2451–2460;
c) M. J. Alves, N. G. Azoia, A. G. Fortes, Tetrahedron 2007, 63, 727–
734.
[36] T. H. Lowry, K. Schueller-Richardson, Mechanism and Theory in
Organic Chemistry, Harper & Row, New York, 1987.
[37] a) X. Xing, J. Wu, W-M. Dai, Tetrahedron 2006, 62, 11200–11206;
b) S. V. More, M. N. V. Sastry, C.-F. Yao, Synlett 2006, 1399–1403;
c) K. Nagaiah, D. Sreenu, R. S. Rao, G. Vashishta, J. S. Yadav, Tetra-
hedron Lett. 2006, 47, 4409–4413; d) X.-F. Lin, S.-L. Cui, Y.-G.
Wang, Tetrahedron Lett. 2006, 47, 4509–4512; e) A. Srinivasa, K. M.
Mahadevan, K. M. Hosamani, V. Hulikal, Monatsh. Chem. 2008,
139, 141–145.
[38] S. Kobayashi, H. Ishitani, S. Nagayama, Synthesis 1995, 1195–1202.
[39] G. Babu, P. T. Perumal, Tetrahedron Lett. 1998, 39, 1627–1638.
[40] G. Sartori, F. Bigi, R. Maggi, A. Mazzacani, G. Oppici, Eur. J. Org.
Chem. 2001, 2513–2518.
[41] R. Nagarajan, S. Chitra, P. T. Perumal, Tetrahedron 2001, 57, 3419–
3423.
[
25] a) S. Chassaing, M. Kumarraja, K. Sani Souna Sido, P. Pale, J.
Sommer, Org. Lett. 2007, 9, 883–886; b) S. Chassaing, A. Sani
Souna Sido, A. Alix, M. Kumarraja, P. Pale, J. Sommer, Chem. Eur.
J. 2008, 14, 6713–6721; c) A. Alix, S. Chassaing, P. Pale, J. Sommer,
Tetrahedron 2008, 64, 8922–8929; d) P. Kuhn, A. Alix, M. Kumarra-
ja, B. Louis, P. Pale, J. Sommer, Eur. J. Org. Chem. 2009, 423–429;
e) P. Kuhn, P. Pale, J. Sommer, B. Louis, J. Phys. Chem. C 2009, 113,
2
903–2910; f) S. Chassaing, A. Alix, T. Boningari, K. Sani Souna Si-
do, M. Keller, B. Louis, P. Kuhn, P. Pale, J. Sommer, Synthesis 2010,
557–1567.
26] M. Keller, A. Sani Souna Sido, P. Pale, J. Sommer, Chem. Eur. J.
009, 15, 2810–2817.
1
[
[
2
27] a) M. K. Patil, M. Keller, M. B. Reddy, P. Pale, J. Sommer, Eur. J.
Org. Chem. 2008, 4440–4445; b) V. Bꢁnꢁteau, A. Olmos, T. Bonin-
gari, J. Sommer, P. Pale, Tetrahedron Lett. 2010, 51, 3673–3677.
28] a) A. Olmos, A. Alix, J. Sommer, P. Pale, Chem. Eur. J. 2009, 15,
[
[
[
1
1229–11234; b) S. Chassaing, A. Alix, A. Olmos, M. Keller, J.
Sommer, P. Pale, Z. Naturforsch. 2010, 65, 783–790.
29] a) S. Kobayashi, Eur. J. Org. Chem. 1999, 15–27; b) S. Kobayashi M.
Sugiura, H. Kitagawa, W. W.-L. Lam, Chem. Rev. 2002, 102, 2227–
[42] S. Kobayashi, S. Komiyama, H. Ishitani, Biotechnol. Bioeng. 1998,
61, 23–31.
2
302.
III
30] 5 mol% catalyst correspond to around 5 mol% Sc species based
on the number of native acidic sites of the corresponding H-zeolite.
For a recent method of determination of Brçnsted acid sites on zeo-
Received: September 14, 2010
Published online: January 12, 2011
1914
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Chem. Eur. J. 2011, 17, 1907 – 1914