The CeCl3·7H2O/NaI/SiO2 system as an efficient promoter for the friedel-crafts reaction of indoles to nitroalkenes under solvent-free conditions

Article abstract of DOI:10.1055/s-2007-990815

The cheap, nontoxic and easy-to-handle CeCl₃·7H ₂O/NaI Lewis acid promoter is optimal with regard to economic and ecological consideration and allows for useful applications in the synthesis of heterocyclic polyfunctionalized molecul

Full text of DOI:10.1055/s-2007-990815

320
PRACTICAL SYNTHETIC PROCEDURES
The CeCl3·7H2O/NaI/SiO2 System as an Efficient Promoter for the Friedel–
Crafts Reaction of Indoles to Nitroalkenes under Solvent-Free Conditions
F
riedel–Crafts Rea
i
ction o
u
f
I
ndoles seppe Bartoli,a Giustino Di Antonio,b Sandra Giuli,b Enrico Marcantoni,*b Mauro Marcolini,b Melissa Paolettib
a
Dipartimento di Chimica Organica ‘A. Mangini’, Università di Bologna, v. le Risorgimento 4, 40136 Bologna, Italy
b
Dipartimento di Scienze Chimiche, Università di Camerino, v. S. Agostino 1,
62032 Camerino (MC), Italy
PSP
Fax +39(0737)402297; E-mail: enrico.marcantoni@unicam.it.it
Received 13 February 2007
No109
Abstract: The cheap, nontoxic and easy-to-handle CeCl3⋅7H2O/NaI Lewis acid promoter is optimal with regard to economic and ecological
consideration and allows for useful applications in the synthesis of heterocyclic polyfunctionalized molecules. The procedure becomes ef-
ficient if the reaction is carried out under solvent-free conditions and on the surface of CeCl3⋅7H2O/NaI supported on silica gel. The sim-
plicity of our method, especially that no precautions need to be taken to exclude moisture or oxygen from the reaction system, permit us to
perform the Friedel–Crafts-type conjugate addition of indoles to nitroalkenes. The success of the reaction is independent of the type of in-
dole or nitroalkene used, and provides 3-(2-nitroethyl)indolyl derivatives which are useful building blocks for the synthesis of various types
of 3-(2-aminoethyl)indolyl derivatives. These can be subsequently transformed to the b-carbolines with different substituents.
Key words: addition reactions, alkaloids, heterocycles, indoles, lanthanides, Lewis acids, natural products
R1
H
CeCl3⋅7H2O
NO2
R2
NO2
+
X
R1
NaI, SiO2
N
R2
X
H
N
1
2
H
3
reduction
R1
R1
X
R2
NH
R3CHO
R2
NH2
X
cyclization
N
H
N
R3
H
5
4
aromatization
X
R1
R2
N
N
H
R3
6
Scheme 1
Introduction
cycles that contain this moiety, a structure which is broad-
ly found in a large number of natural products2 and
The Friedel–Crafts reaction is one of the most important biologically active substances.3 The plethora of ingenious
reactions in organic synthesis, in that it provides an useful methods developed for Lewis acid promoted conjugate
method for the direct introduction of a functional group addition of indoles to electron-poor alkenes is a clear tes-
onto heterocycles or aromatic compounds.1 Among these, timony to their paramount importance. However, most of
the formation of 3-substituted indoles in good yield from these procedures are associated with serious disadvantag-
indole is quite a powerful tool for the synthesis of hetero- es involving strictly anhydrous conditions. To solve these
problems, several lanthanoid triflates have been applied in
the Friedel–Crafts-type conjugate addition of indoles in
SYNTHESIS 2008, No. 2, pp 0320–0324
Advanced online publication: 25.09.2007
DOI: 10.1055/s-2007-990815; Art ID: Z03407SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
0
7
water.4 Unfortunately, these Lewis acid catalyzed
reactions5 in water with triflates generally require a cer-
tain amount of organic solvents such as THF and ethanol

PRACTICAL SYNTHETIC PROCEDURES
Friedel–Crafts Reaction of Indoles
321
mixable with water to dissolve organic substrates. In this ous conditions, we observed that an equimolar ratio of the
way the reaction is promoted efficiently, but it is not opti- reagents was necessary, and in optimizing the reaction
mal with regard to economic and ecological consider- conditions we found that 0.3 equivalent of CeCl3·7H2O
ations. Their use should be minimized as far as possible or and 0.3 equivalent of NaI supported on SiO2 (0.5 g/mmol
even avoided altogether. The best solvent from this point of nitrolakene) were crucial in these reactions. When the
of view therefore is, without a doubt, no solvent at all.6
same reactions were performed in their absence, no alky-
lation took place. The best results were obtained under
solvent-free conditions, and taken together, these data al-
low important advances in chemical processes to mini-
mize waste, a demanding challenge for synthetic chemists
when atom-economy and green chemistry are considered.
Recently, there has been a great development in reactions
that can be carried out in the absence of solvent,7 and the
area of growth includes reactions on supported inorganic
reagents.8 Thus, due to the current challenge for develop-
ing solvent-free and environmentally benign synthetic
system, and extending our interest in the applications of
cerium trichloride for various organic transformations,9
we have focused our attention on the use of a CeCl3⋅7H2O/
NaI system supported on SiO2 in the Friedel–Crafts reac-
tions.10 Although SiO2 was originally introduced only as a
support that not only facilitated the work-up of the reac-
tion mixture, it was found to also enhance the rate con-
stant of the reaction. This fact, together the acceleration
effect caused by addition of NaI to CeCl3⋅7H2O, make our
system optimal with regard to suppress the tendency of
electron-rich heteroaromatic rings such as indoles to poly-
merize under acid-catalyzed conditions. Thus, useful ap-
plications in bond forming reactions in solvent-free
conditions for the preparation of heterocyclic polyfunc-
tionlized molecules have been realized.11 The treatment of
indoles with a,b-disubstituted nitroalkenes in the presence
of our CeCl3⋅7H2O/NaI system supported on SiO2 gives
the corresponding b-indolyl nitroalkanes.12
Table 1 Friedel–Crafts-Type Conjugate Addition of Indoles 1 with
trans-b-Nitrostyrene (2a)a
Ph
X
X
CeCl3⋅7H2O
NO2
+
Ph
N
NaI, SiO2
NO2
H
N
1
2a
H
3
Entry
Indole
1a
X
Time (h) Producta Yield (%)b
1
2
H
8
4
3aa
3ba
3ca
3da
96
92
85
74
1b
OMe
CN
OH
3
1c
18
24
12
1d
a All products were identified by their IR, NMR and GC/MS spectra.
b Yields of products isolated by column chromatography.
It is known that nitroalkenes are one of the strongest
Michael acceptors13 providing as common pathway to ni-
troalkanes,14 which could serve as stock compounds for
the corresponding amino compounds. Thus, our system is
a useful addition to the synthetic toolbox of indole chem-
istry,15 and this Friedel–Crafts-type conjugate addition
can be exploited in the synthesis of different pyrido[3,4-
b]indoles and b-carbolines (Scheme 1). The product 3 ob-
tained by reaction promoted by our CeCl3⋅7H2O/NaI/SiO2
combination is converted to tryptamine derivative 4 by re-
duction. Conversion of this compound into the tetrahydro-
b-carboline 5 proceeds under Pictet–Spengler conditions.
Finally, the aromatization by palladium-on-carbon treat-
ment gives the fully aromatic b-carbolines 6 with different
substituents. These heterocycles are of interest to the
pharmaceutical industry due to their numerous reported
biological activities.16 Herein, we describe a reliable, scal-
able synthesis of methyl 9H-b-carbolines-4-carboxylate
without requiring protection of the nitrogen.
The reaction proceeded in good yields even in the case of
poorly reactive indoles (Table 1, entry 3). The rate was ac-
celerated when an electron-donating group was present on
the indole nucleus (Table 1, entry 2) and, interestingly, in
the case of an indole derivative containing an hydroxyl
group (1d). It is known that a direct Lewis acid promoted
reaction of hydroxy indole substrates is generally prob-
lematic and normally results in low yield due to the inter-
action of the indolyl hydroxyl group with the Lewis acid
catalyst.17 Analogously, the utilization of commercially
available basic alumina as solid catalyst18 gave low yield
of the corresponding adduct 3da, because the free hy-
droxy group present in the benzene ring of the indole is
preferentially deprotonated by the basic catalyst, leading
to a consistent acidity and hence reduction of the pyrrole
nucleus. With our system, this problem with hydroxy
indoles is much reduced. The presence of electron-with-
drawing groups in the pyrrole ring reduces the overall nu-
cleophilicity,
and
indole
substrates
with
a
methoxycarbonyl or a benzenesulfonyl group on the pyr-
role moiety do not give the corresponding Friedel–Crafts
adducts. Thus, our Friedel–Crafts-type conjugate addition
of indoles under solvent-free conditions was also efficient
with a- or b-substituted enones. The reaction did not work
for a,b-unsaturated sulfones and nitriles. The correspond-
ing esters are less reactive, and in the case of a,b-unsatur-
ated aldehydes, the reaction suffered from regiochemical
restriction caused by competing 1,2- and 1,4-addition.
Scope and Limitations
The Friedel–Crafts-type conjugate addition was opti-
mized using indole (1a) and trans-b-nitrostyrene (2a) as
the test substrates (Table 1, entry 1). The substitution on
the indole nucleus occured exclusively at the 3-position
giving b-indolyl nitroalkanes in good yields, and N-alky-
lation products were not observed. By screening the vari-
Synthesis 2008, No. 2, 320–324 © Thieme Stuttgart · New York

322
G. Bartoli et al.
PRACTICAL SYNTHETIC PROCEDURES
With nitroalkenes substituted in the a- or b-position, the
reaction proceeded with good yields, and the synthetic po-
tential of this system was applied to promote the synthesis
of 3-(2-nitroethyl)indolyl derivative 3ab (Scheme 2).19
This can be converted, in several steps,20 into the b-carbo-
line ring of (–)-(S)-brevicolline, the major alkaloid of the
plant Carex brevicollis, which is biologically important
for its phototoxic effect against bacteria and fungi. The
chiral nitroalkene synthon 2b was obtained starting from
the natural amino acid (S)-proline.21 These findings sug-
gest a suitable route to the synthesis of carbolines starting
from an easy and convenient method of making
tryptamines by Friedel–Crafts-type conjugate addition of
indoles to nitroalkenes.
NO2
CeCl3⋅7H2O
+
MeO2C
2c
N
NaI, SiO2
H
1a
r.t., 2 h, 89%
1. H2 (1 atm)
Raney nickel
r.t., 16 h, EtOH
CO2Me
2. 4 N HCl
dioxane, 78%
NO2
N
H
3ac
CO2Me
1. 37% aq HCHO
MeOH, r.t., 18 h
NH2⋅HCl
2. 6% aq K2CO3
N
H
EtOAc, r.t., 1 h
90%
8
Boc
NO2
CeCl3⋅7H2O
N
CO2Me
+
N
NaI, SiO2
Pd/C, xylene
H
1a
r.t., 2 h, 89%
2b
NH
reflux, 4 h, 85%
N
H
9
N
N
CO2Me
N
Me
Boc
H
N
N
NO2
N
H
N
H
H
Me
10
7
3ab
Scheme 2
Scheme 3
We have been interested in evaluating the general utility
of our silica gel supported CeCl3·7H2O/NaI combination
by synthesizing methyl 9H-b-carboline-4-carboxylate
(10). The method reported in literature requires N-protec-
tion,22 whereas our linear approach involves introduction
of the 4-substituent in the first synthetic step (Scheme 3).
The product 3ac obtained by the reaction of indole (1a) to
the readily available trans-b-nitroacrylate (2c) promoted
by CeCl3⋅7H2O/NaI/SiO2 under solvent-free conditions
was converted into the tryptamine derivative 8 by hydro-
genation in the presence of Raney nickel in ethanol. The
corresponding 3-(2-aminoethyl)-1H-indole is part of an
important class of compounds frequently used as building
blocks in the construction of numerous indole alkaloids.
Unfortunately, these are not stable; thus, compound 8 was
isolated as its stable hydrochloride by treating with aque-
ous 4 N HCl in dioxane. This was directly used without
purification in the next step. However, its treatment with
formaldehyde under protic conditions followed by Pictet–
Spengler cyclization of the imine23 afforded a modest
yield of the 4-substituted 1,2,3,4-tetrahydro-b-carboline
9. On the other hand, preparation of the compound 9 pro-
ceeded in good yield when a 37% formalin solution was
added to the hydrochloride of 8 in methanol, followed by
the conversion into the free base. Finally, aromatization
with palladium-on-carbon afforded the fully aromatic b-
carboline 10 with a substituent at the 4-position.
In conclusion, we have achieved a practical and highly
efficient method for the preparation of 2-indolyl-1-nitro-
alkane derivatives in good yields by using CeCl3⋅7H2O/
NaI/SiO2 as a ‘friendly’ Lewis acid promoter. The sim-
plicity of this approach provides facile access to the syn-
thesis of 4-substituted b-carbolines, and the use of
solvent-free conditions reduces the harmful effects of or-
ganic solvents on the environment.
Most solvents and reagents were used without purification unless
otherwise mentioned. Solvents (EtOAc and hexane) for chromatog-
raphy were distilled. Anhyd xylene was prepared by distillation
from Na under N2 and used immediately. All air- or moisture-sensi-
tive reactions were carried out in flame-dried glassware under N2.
All the products obtained were characterized by IR, GC/MS, 1H and
13C NMR spectroscopy. The compounds 312 are all known and their
1
structures were consistent with their published physical data. H
NMR (200 MHz) spectra were recorded in CDCl3 with residual sig-
nals of solvent as an internal standard. 13C NMR (50 MHz) spectra
were recorded in CDCl3 and chemical shifts are reported relative to
the center line of the triplet at 77.00 ppm for CDCl3. Mass spectra
were recorded on a Hewlett-Packard 5988 gas chromatography with
a mass-selective detector MSD HP 5790 MS, utilizing electron ion-
ization (EI) at an ionizing energy of 70 eV. Microanalyses were per-
formed with a EA1108 CHNS-D Fisons Instruments. IR spectra
were recorded on PerkinElmer FTIR Paragon 500 spectrometer on
NaCl plates. Only the characteristic peaks are quoted. Analytical
GC was performed with a capillary fused silica column (0.32
mm × 25 cm), stationary phase OV1 (film thickness 0.40–0.45 mm).
Solutions were evaporated under reduced pressure with a rotary
evaporator and the residue was chromatographed on a Baker silica
gel (230–400 mesh) column using an EtOAc in hexanes as eluent.
Synthesis 2008, No. 2, 320–324 © Thieme Stuttgart · New York

PRACTICAL SYNTHETIC PROCEDURES
Friedel–Crafts Reaction of Indoles
Methyl 9H-b-Carboline-4-carboxylate (10)
323
Analytical TLC was performed using precoated glass-backed plates
(Merck Kieselgel 60 F254) and visualized by ultraviolet, Von’s re-
agent, KMnO4, or I2 stain.
The pure tetrahydro-b-carboline 9 (0.90 g, 3.9 mmol) was dissolved
in anhyd xylene (120 mL), and 10% Pd/C catalyst (0.60 g) was add-
ed. The mixture was heated under reflux with vigorous stirring for
4 h until no starting material remained, as indicated by TLC using
toluene–EtOH (9:2) as eluent. The catalyst was removed by filtra-
tion over Celite and washed with EtOAc (5 × 50 mL). Evaporation
of the filtrate gave a residue which was purified by chromatography
on silica gel (eluent, hexanes–EtOAc–EtOH, 6:3:1) to afford the
crystalline b-carboline 10; yield: 0.762 g (85%); mp 192–195 °C
(EtOH).
IR (Nujol): 3380, 1730 cm–1.
1H NMR (DMSO-d6): d = 4.10 (s, 3 H), 7.23–7.36 (m, 1 H), 7.60–
7.71 (m, 2 H), 8.75–8.82 (m, 2 H), 9.12 (s, 1 H), 12.90 (s, 1 H).
13C NMR (DMSO-d6): d = 50.9, 112.8, 120.7, 121.0, 125.8, 127.3,
130.3, 136.2, 137.8.
MS (EI): m/z = 226 ([M+], 100%), 195, 167, 140, 113.
3-(1-Methoxycarbonyl-2-nitroethyl)-1H-indole (3ac)
Silica gel (5 g) was added to a mixture of CeCl3⋅7H2O (11.2 g, 3
mmol) and NaI (0.134 g, 3 mmol) in MeCN (100 mL), and the mix-
ture was stirred overnight at r.t. The MeCN was removed by rotary
evaporation and the resulting mixture was stored in a bottle at r.t. To
the CeCl3⋅7H2O/NaI combination supported on silica gel (4.5 g)
prepared as above was added indole (1a; 0.9 g, 7.63 mmol) and
trans-b-nitroacrylate (2c;24 1.0 g, 7.63 mmol). The mixture was
stirred at r.t. for 4 h by using a mechanical stirrer. After addition of
Et2O (250 mL), the mixture was passed through a short pad of Celite
and the filtrate was concentrate under reduced pressure. The crude
was purified by chromatography on a silica gel column (eluent:
EtOAc in hexanes, 30:70) to give the adduct 3ac (1.62 g, 86%) as
an oil.
IR (neat): 3406, 3030, 1745, 1552, 1376 cm–1.
Anal. Calcd for C13H10N2O2: C, 69.02; H, 4.46; N, 12.38. Found: C,
68.98; H, 4.38; N, 12.36.
1H NMR (CDCl3): d = 3.70 (s, 3 H), 4.65–4.87 (m, 2 H), 5.10–5.20
(m, 1 H), 7.02–7.19 (m, 3 H), 7.38–7.42 (m, 1 H), 7.86 (d, J = 8.5
Hz, 1 H), 8.30 (br s, 1 H, NH).
Acknowledgment
13C NMR (CDCl3): d = 40.1, 52.9, 75.4, 112.0, 120.5, 123.9, 124.4,
125.8, 136.7, 172.7.
We thank the Italian MIUR (National Project ‘Studio degli Aspetti
Teorici ed Applicativi degli Aggregati di Molecole Target su Siti
catalitici Stereoselettivi’) and the University of Camerino for finan-
cial support. We thank Pfizer Ascoli Piceno Plant and Teuco Guz-
zini Montelupone for funding post-graduate fellowships to M. P.
and M. M., respectively.
MS (EI): m/z = 248 [M+], 201, 160, 143 (100), 142, 115, 73, 62.
Anal. Calcd for C12H12N2O4: C, 58.06; H, 4.87; N, 11.29. Found: C,
58.01; H, 4.80; N, 11.23.
Methyl 3-Amino-2-(1H-3-indolyl)propionate Hydrochloride
(8·HCl)
A vigorously stirred mixture of 3-(2-nitroethyl)indolyl derivative
3ac (1.57 g, 6.33 mmol), EtOH (325 mL), and Raney Ni (9.15 g)
was hydrogenated at atmospheric pressure for 16 h. The Ni was then
removed by filtration over Celite and washed with hot EtOH (850
mL). Evaporation of the filtrate under vacuum afforded a syrup that
was treated with 1 equiv of 4 N HCl in dioxane. The product (1.25
g, 78%) was filtered, after which it was sufficiently pure for further
use.
1H NMR (D2O): d = 3.35–3.67 (m, 5 H), 4.35 (t, J = 7.50 Hz, 1 H),
4.75 (s, 3 H), 6.75–7.16 (m, 2 H), 7.34 (s, 1 H), 7.38–7.50 (m, 1 H),
8.00 (br s, 1 H, NH).
References
(1) Olah, G. A.; Krishnamurti, A. R.; Prakash, G. K. S. In
Comprehensive Organic Synthesis, Vol. 3; Trost, B. M.;
Fleming, I., Eds.; Pergamon Press: Oxford, 1999, 293–299.
(2) Ninomiya, I. J. Nat. Prod. 1992, 55, 541.
(3) (a) Zhang, H.; Larock, R. C. Org. Lett. 2001, 3, 3083.
(b) Jiang, B.; Yang, C.-G.; Wang, J. J. Org. Chem. 2001, 66,
4865. (c) Sakagami, M.; Muratake, H.; Natsume, M. Chem.
Pharm. Bull. 1994, 42, 1393. (d) Vaillancouirt, V.; Albizati,
K. F. J. Am. Chem. Soc. 1993, 115, 3499.
(4) (a) Manabe, K.; Aoyama, N.; Kobayashi, S. Adv. Synth.
Catal. 2001, 343, 174. (b) Mori, Y.; Kakumoto, K.;
Manabe, K.; Kobayashi, S. Tetrahedron Lett. 2000, 41,
3107. (c) Manabe, K.; Mori, Y.; Wekabayashi, T.;
Nagayama, S.; Kobayashi, S. J. Am. Chem. Soc. 2000, 122,
7202.
(5) Generally, triflates are rather expensive and their use in
large-scale synthetic methodology is very limited.
(6) Tundo, P.; Anastas, P.; Black, D. C.; Breen, J.; Collins, T.;
Memoli, S.; Miyamoto, J.; Polyakoff, M.; Tumas, W. Pure
Appl. Chem. 2000, 72, 1207.
(7) (a) Metzger, J. O. Angew. Chem. Int. Ed. 1998, 37, 2975.
(b) Toda, F.; Tanaka, K. Chem. Rev. 2000, 100, 1025.
(8) (a) Cornelis, A.; Laszlo, P. Synthesis 1985, 909.
(b) McKillop, A.; Young, D. W. Synthesis 1979, 401.
(9) (a) Bartoli, G.; Bosco, M.; Carlone, A.; Locatelli, M.;
Marcantoni, E.; Melchiorre, P.; Sambri, L. Adv. Synth.
Catal. 2006, 348, 905. (b) Bartoli, G.; Bartolacci, M.;
Giuliani, A.; Marcantoni, E.; Massaccesi, M.; Torregiani, E.
J. Org. Chem. 2005, 70, 169. (c) Bartoli, G.; Bosco, M.;
Giuliani, A.; Marcantoni, E.; Palmieri, A.; Petrini, M.;
Sambri, L. J. Org. Chem. 2004, 69, 1290. (d) Bartoli, G.;
Marcantoni, E.; Sambri, L. Synlett 2003, 2101; and
references cited therein.
Methyl 1,2,3,4-Tetrahydro-1H-b-carboline-4-carboxylate (9)
A mixture of 8 (1.10 g, 4.35 mmol) and 37% formalin (0.45 g, 5.25
mmol) in MeOH (100 mL) was stirred at r.t. for 18 h. The mixture
was diluted with Et2O (350 mL), and the resulting crystals were col-
lected by filtration and dried to give 9⋅HCl as colorless needles. The
HCl salt was converted into the free base by treatment with aq 6%
K2CO3 (38 mL) in EtOAc (125 mL). The resulting mixture was
stirred at r.t. for 1 h; then the organic layer was dried (MgSO4), and
the solvent was evaporated under vacuum. The residue was chro-
matographed on silica gel by eluting with hexanes–EtOAc–EtOH
(6:3:1) to give 9; yield: 1.0 g (90%); mp 162–164 °C (EtOH).
IR (Nujol): 3402, 1730, 1463, 1424 cm–1.
1H NMR (CDCl3): d = 2.26 (br s, 1 H, NH), 3.05 (dd, J = 13.73,
4.56 Hz, 1 H, ), 3.60 (dd, J = 13.75, 2.16 Hz, 1 H), 3.72 (s, 3 H),
3.78–3.87 (m, 1 H), 4.01–4.12 (m, 2 H), 7.10–1.20 (m, 2 H), 7.34–
7.42 (m, 1 H), 7.65–7.71 (m, 1 H), 8.10 (br s, 1 H, NH).
13C NMR (CDCl3): d = 39.3, 46.6, 52.7, 53.4, 104.8, 112.2, 121.3,
122.9, 123.5, 124.9, 140.6, 142.3, 175.2.
Anal. Calcd for C13H14N2O2: C, 67.81; H, 6.13; N, 12.17. Found: C,
67.81; H, 6.09; N, 12.23.
Synthesis 2008, No. 2, 320–324 © Thieme Stuttgart · New York

324
G. Bartoli et al.
PRACTICAL SYNTHETIC PROCEDURES
(10) (a) Bartoli, G.; Bosco, M.; Foglia, G.; Giuliani, A.;
Marcantoni, E.; Sambri, L. Synlett 2004, 895. (b) Bartoli,
G.; Bartolacci, M.; Bosco, M.; Foglia, G.; Giuliani, A.;
Marcantoni, E.; Sambri, L.; Torregiani, E. J. Org. Chem.
2003, 68, 4594.
(11) (a) Bartoli, G.; Giovannini, R.; Giuliani, A.; Marcantoni, E.;
Massaccesi, M.; Melchiorre, P.; Paoletti, M.; Sambri, L.
Eur. J. Org. Chem. 2006, 1472. (b) Attanasi, O. A.; Favi,
G.; Filippone, P.; Forzato, C.; Giorgi, G.; Morganti, S.; Nitti,
P.; Pitacco, G.; Rizzato, E.; Spinelli, D.; Valentini, E.
Tetrahedron 2006, 62, 6420. (c) Sabitha, G.;
(16) (a) Cacchi, S.; Fabrizi, G.; Parisi, L. M. Org. Lett. 2003, 5,
3843. (b) Wang, H.; Usui, T.; Osada, H.; Ganesan, A. J.
Med. Chem. 2000, 43, 1577. (c) Saxton, J. E. J. Nat. Prod.
Rep. 1997, 559.
(17) Yamamoto, H. In Lewis Acid Reagents; Oxford University
Press: New York, 1999.
(18) Ballini, R.; Clemente, R. R.; Palmieri, A.; Petrini, M. Adv.
Synth. Catal. 2006, 348, 191.
(19) Mahboobi, S.; Wiegrebe, W.; Popp, A. J. Nat. Prod. 1999,
62, 577.
(20) Towers, G. H. N.; Abramovski, Z. J. Nat. Prod. 1983, 46,
572.
(21) Mahboobi, S.; Popp, A.; Burgemeister, T.; Schollmeyer, D.
Tetrahedron: Asymmetry 1998, 9, 2369.
(22) Busacca, C. A.; Eriksson, M. C.; Dong, Y.; Prokopowicz, A.
S.; Salvagno, A. M.; Tschantz, M. A. J. Org. Chem. 1999,
64, 4564.
Kiran Kuma Reddy, G. S.; Bhaska Reddy, K.;
Mallikarjun Reddy, N.; Yadav, J. S. Adv. Synth. Catal. 2004,
346, 921. (d) Yadav, J. S.; Reddy, B. V. S.; Srinivas, M.;
Padmavani, B. Tetrahedron 2004, 60, 3261.
(12) Bandini, M.; Melchiorre, P.; Melloni, A.; Umani-Ronchi, A.
Synthesis 2002, 1110.
(13) Berner, O. M.; Tedeschi, L.; Enders, D. Eur. J. Org. Chem.
2002, 1877.
(14) Rosini, G.; Ballini, R. Synthesis 1988, 833.
(15) Bartoli, G.; Bosco, M.; Giuli, S.; Giuliani, A.; Lucarelli, L.;
Marcantoni, E.; Sambri, L.; Torregiani, E. J. Org. Chem.
2005, 70, 1941.
(23) Zhou, H.; Liao, X.; Cook, J. M. Org. Lett. 2004, 6, 249.
(24) Chandler, M.; Contoy, R.; Cooper, A. W. J.; Lamont, R. B.;
Scicinski, J. J.; Smart, J. E.; Storer, R.; Weir, N. G.; Wilson,
R. D.; Wyatt, P. G. J. Chem. Soc., Perkin Trans. 1 1995,
1189.
Synthesis 2008, No. 2, 320–324 © Thieme Stuttgart · New York