Fast, solvent-free, microwave-promoted friedlaender annulation with a reusable solid catalyst

Article abstract of DOI:10.1080/00397910902957407

A fast, solvent-free method is described for the synthesis of substituted quinoline derivatives via Friedlander cyclization, employing a reusable solid catalyst (silica-propylsulfonic acid). Although it worked best under microwave irradiation (with generally more than 90% isolated yields in 30min), the reaction was also feasible under conventional heating (with fair to good yields in about 5h).

Full text of DOI:10.1080/00397910902957407

This article was downloaded by: [Stony Brook University]
On: 29 October 2014, At: 14:42
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Synthetic Communications: An
International Journal for Rapid
Communication of Synthetic Organic
Chemistry
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/lsyc20
Fast, Solvent-Free, Microwave-Promoted
Friedländer Annulation with a Reusable
Solid Catalyst
a
a
a
Davide Garella , Alessandro Barge , Dharita Upadhyaya , Zalua
a b
c
a
Rodríguez
a
, Giovanni Palmisano & Giancarlo Cravotto
Dipartimento di Scienza e Tecnologia del Farmaco , Università di
Torino , Torino , Italy
b
Centro de Química Farmacéutica , Ciudad de La Habana , Cuba
c
Dipartimento di Scienze Chimiche e Ambientali , Università
dell'Insubria , Como , Italy
Published online: 09 Dec 2009.
To cite this article: Davide Garella , Alessandro Barge , Dharita Upadhyaya , Zalua Rodríguez ,
Giovanni Palmisano & Giancarlo Cravotto (2009) Fast, Solvent-Free, Microwave-Promoted
Friedländer Annulation with a Reusable Solid Catalyst, Synthetic Communications: An International
Journal for Rapid Communication of Synthetic Organic Chemistry, 40:1, 120-128, DOI:
10.1080/00397910902957407
To link to this article: http://dx.doi.org/10.1080/00397910902957407
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“
Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.

This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
Conditions of access and use can be found at http://www.tandfonline.com/page/terms-
and-conditions

1
Synthetic Communications , 40: 120–128, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910902957407
FAST, SOLVENT-FREE, MICROWAVE-PROMOTED
¨
FRIEDLANDER ANNULATION WITH A REUSABLE
SOLID CATALYST
1
Davide Garella, Alessandro Barge, Dharita Upadhyaya,
1
1
1,2
3
Zalua Rodr ´ı guez, Giovanni Palmisano,
and Giancarlo Cravotto
1
1
Dipartimento di Scienza e Tecnologia del Farmaco, Universit a` di Torino,
Torino, Italy
2
Centro de Qu ı´ mica Farmac e´ utica, Ciudad de La Habana, Cuba
3
Dipartimento di Scienze Chimiche e Ambientali, Universit a` dell’Insubria,
Como, Italy
A fast, solvent-free method is described for the synthesis of substituted quinoline derivatives
via Friedl a¨ nder cyclization, employing a reusable solid catalyst (silica-propylsulfonic acid).
Although it worked best under microwave irradiation (with generally more than 90%
isolated yields in 30 min), the reaction was also feasible under conventional heating (with
fair to good yields in about 5 h).
Keywords: Friedl a¨ nder synthesis; microwaves; quinoline; solid catalyst; solvent-free
INTRODUCTION
In the Friedl a¨ nder synthesis, a classic example of heterocyclic chemistry, o-
aminoaryl aldehydes or ketones typically react with enolizable carbonyl compounds
in the presence of a Brønsted or Lewis acid catalyst. After an initial amino-ketone
condensation, the intermediate product undergoes a base- or acid-catalyzed cyclo-
condensation to afford a quinoline derivative. Unfortunately, the yield can be
lowered by self-condensation of o-aminoaryl carbonyl compounds.
[
1,2]
Quinoline derivatives are widespread in natural products
their wide range of biological activities, play pivotal roles in medicinal chemis-
and, owing to
[
3–6]
[7]
try.
and electronics.
In the past decade, several improved versions of the Friedl a¨ nder synthesis have
They have also found very interesting applications in polymer chemistry
[8–10]
[11–15]
appeared in the literature,
[
proof that interest in this old cyclization, dating from
is far from waning. Just in the past two years, more than one hundred
16]
1882,
peer-reviewed papers covered some useful application of it. The best protocols,
Received December 20, 2008.
Address correspondence to Giancarlo Cravotto, Dipartimento di Scienza e Tecnologia del
Farmaco, Universit a` di Torino, Via Giuria 9, 10125 Torino, Italy. E-mail: giancarlo.cravotto@unito.it
1
20

¨
SOLVENT-FREE FRIEDLANDER ANNULATION
121
benefiting from microwave (MW) irradiation, proved more efficient than those
[
17–19]
carried out under conventional heating.
also been explored, including p-toluenesulfonic acid, molecular iodine, neody-
A wide variety of newer catalysts have
[21]
[
20]
[22]
[23]
mium(III) nitrate hexahydrate,
and a group of amino compounds.
cular interest are recyclable catalysts such as silver phosphotungstate,
Of parti-
a Lewis
and an HCl-catalyzed Friedl a¨ nder reaction
[
24]
[25]
acid–surfactant combined catalyst,
carried out in plain water without any added metal catalyst and phase transfer cata-
[
26]
lyst (PCT).
Recently Das et al. described the use of solid acid catalysts such as
NaHSO ꢀ SiO , H SO ꢀ SiO , Amberlyst-15, and HClO ꢀ SiO working under reflux
4
2
2
4
2
4
2
[
27]
in ethanol.
viously introduced by Narasimhulu et al. working under reflux in acetonitrile.
The HClO ꢀ SiO -catalyzed Friedl a¨ nder annulation had been pre-
4
2
[
28]
In the present communication, we report a new solvent-free protocol by which
the Friedl a¨ nder synthesis is promoted, either under conventional heating or under
MW irradiation, by a solid catalyst, namely a derivatized silica bearing alkylsulfonic
acid groups.
According to the guidelines of green chemistry, we aimed to achieve a fast,
[
29]
efficient, solventless procedure using a recyclable catalyst. It is well documented that
the use of solid acids as catalysts, besides simplifying the isolation of products, often
allows reactions to be run under milder conditions and improves their selectivity.
RESULTS AND DISCUSSION
The present preparation of quinoline derivatives by Friedl a¨ nder annulation
draws on our previous experience in developing more efficient and greener synthetic
[
30,31]
[32]
and MW irradiation. We started
protocols exploiting solid reusable catalysts
by preparing the solid catalyst, propylsulfonic silica (PSS) (Fig. 1), by the published
procedure.
[
33,34]
Preliminary experiments showed that cyclization occurred under conventional
heating conditions, but reactions were quite sluggish with prolonged reaction time.
Although we monitored the temperature in the MW oven with an infrared (IR)
pyrometer, under solvent-free conditions it is very likely that localized hot spots
did reach somewhat greater temperatures, which can account for such a difference
in the results. Whereas the (scientific) debate on the existence of nonthermal MW
[35]
effects seems to be closed,
there are no doubts that dielectric heating does
optimize heat transfer. Table 1 reports reaction times and yields under MW as well
as conductive heating.
With the exception of entry 7 (A and B), all the products listed in Table 1 are
[
36–42]
known, and spectroscopic data are available in the literature. With solid car-
bonyl compounds such as dimedone and 4-hydroxycoumarin (entries 3/10 and 7,
respectively), longer times were required even under MW. With 4-hydroxycoumarin,
Figure 1. Structure of propylsulfonic silica (PSS).

122
D. GARELLA ET AL.
Table 1. MW vs. conventional heating: reaction times and yields
MW
irradiation
Conventional
heating
Time
(min)
Yield
(%)
Time
(h)
Yield
(%)
Entry
Reagent
Product
1
45
30
88
61
5
5
80
24
2
3
4
5
210
30
92
90
91
5
5
5
80
83
84
30
6
7
90
60
92
5
12
13
22
36
5
5
6
0
20
81
8
9
45
45
89
70
5
5
22
(
Continued )

¨
SOLVENT-FREE FRIEDLANDER ANNULATION
123
Table 1. Continued
MW
irradiation
Conventional
heating
Time
(min)
Yield
(%)
Time
(h)
Yield
(%)
Entry
Reagent
Product
10
210
89
5
75
1
1
2
30
30
86
93
5
5
81
85
1
13
90
90
5
13
a masked b-ketoester (Scheme 2), the reaction afforded, alongside the expected tetra-
cyclic derivative (A), the product of lactone hydrolysis–decarboxylation (B). The
˚
addition of activated 4-A-molecular-sieved powder to the reacting mixture increased
the yield of A (40%) while dramatically reducing the yield of B (11%). o-Hydroxya-
cetophenone (C) was the main product (about 50%) generated by the partial degra-
dation of excess 4-hydroxycoumarin.
Results reported in Table 2 indicate that the reaction with cyclopentanone,
cyclohexanone and methyl acetoacetate occurred even in the absence of catalyst.
A study carried out with the last-mentioned substrate demonstrated that the catalyst
could be filtered off and reused to afford comparable product yields (90% !87%
!
85%).
Scheme 1. General synthetic scheme.

124
D. GARELLA ET AL.
Scheme 2. Reaction of o-aminobenzophenone with 4-hydroxycoumarin.
EXPERIMENTAL
Preparation of 3-Mercaptopropyl Silica (MPS)
˚
Silica gel (average pore diameter 60 A) was activated by refluxing in
concentrated hydrochloric acid (6 M) for 24 h, then thoroughly washed with distilled
water and dried under vacuum before undergoing chemical surface modification.
This was carried out by refluxing the activated silica gel (10 g) with 3-mercaptopro-
pyltrimethoxysilane (MPTMS, 5 mmol) in dry toluene for 18 h. The solid product
was isolated by centrifugation (2000 rpm), washed with hot toluene (three times),
ꢁ
and oven-dried at 110 C overnight.
Preparation of Propylsulfonic Silica (PSS)
The thiol groups of the modified silica (MPS, 5 g) were oxidized with 30% H O
2
2
(50 ml), to which two drops of concentrated H SO in 15 ml methanol were added.
2 4
After the mixture had stood 12 h at room temperature, the solid was isolated by
centrifugation (2000 rpm) and washed three times with 50-ml portions of distilled
water. To ensure that the sulfonic acid groups were fully protonated, the solid was
suspended for 4 h in 10 wt% aqueous H SO (30 ml), centrifuged off, thoroughly
2
4
Table 2. Reaction times and yields in the absence of catalyst or using recycled catalyst
Time
(h)
Yield
(%)
Time
(h)
Yield
(%)
Time
(h)
Yield
(%)
Conditions
Catalyst
None
None
MW
2
5
69
48
2
5
26
7
0.5
5
63
17
Conventional
heating
MW
Sandard
1st recycle
2nd recycle
—
—
—
—
—
—
—
—
—
—
—
—
0.5
0.5
0.5
90
87
85
MW
MW

¨
SOLVENT-FREE FRIEDLANDER ANNULATION
125
Scheme 3. Operative conditions.
washed with distilled water (until the pH of washing was close to neutrality), and
ꢁ
dried at 60–70 C overnight. About 5 g of a buff-colored solid was recovered, to be
directly used as catalyst in the reactions described next. Its full characterization
[
33,34]
followed methods reported in the literature.
General Reaction Conditions
Under Conventional Heating. A well-blended mixture of o-aminobenzophe-
none (0.5 mmol), the ketone (1.5 mmol), and PSS catalyst (100 mg) was poured into a
ꢁ
Pyrex tube that was stoppered and heated at 80 C in an electric oven. After o-ami-
nobenzophenone was shown by thin-layer chromatography (TLC) to have
disappeared, the reacted mixture was cooled and purified by column chromato-
graphy on silica gel (Merck, 100–200 mesh, petroleum ether–EtOAc, 9:1) to afford
the pure quinoline derivative.
Under MW. A well-blended mixture of o-aminobenzophenone (0.5 mmol), the
ketone (1.5 mmol), and PSS catalyst (100 mg) was poured into a stoppered Pyrex
ꢁ
tube and irradiated with MW (80 C, 200 W) for the appropriate time (see Table 1).
After o-aminobenzophenone was shown by TLC to have disappeared, the reacted
mixture was cooled and purified by column chromatography on silica gel (Merck,
1
00–200 mesh, petroleum ether–EtOAc, 9:1) to afford the pure quinoline derivative.
In either version, when catalyst recovery was desired, the reacted mixture was
treated with EtOAc (15 ml), run through a 10-mm sintered glass filter, and washed
with EtOAc (5 ml ꢂ 3 times) to recover the clean catalyst. This was activated at
ꢁ
6
0 C prior to reuse.
Reaction times and results are listed in Tables 1 and 2.
Data
7
-Phenyl-6H-chromen[4,3-b]quinolin-6-one (7a). Yellow powder; R ¼ 0.6
f
1
(
7
PE=EtOAc 6:4). H NMR (300 MHz, CDCl ): d ¼ 8.15–8.10 (overlap, 2H, H-4,11),
3
.86 (t, J ¼ 6.9 Hz, 1H, H-10), 7.75 (t, J ¼ 6.9 Hz, 1H, H-3), 7.61–7.55 (overlap, 5H,
1
3
H-1,8, Ph), 7.46 (t, J ¼ 6.6, 1H, H-9), 7.36–7.29 (overlap, 3H, H-2, Ph). C NMR
(
75 MHz, CDCl ): d ¼ 178.2, 157.97, 155.75, 155.48, 148.75, 137.16, 135.85,
3
1
1
1
33.31, 128.42, 128.27, 128.14, 128.03, 127.28, 127.16, 126.27, 124.36, 121.99,
18.03, 113.52. FT-IR (KBr): t ¼ 1670, 1612, 1576, 1556, 1466, 1375, 1358, 1330,
325, 1246, 1115, 837, 752, 719, 108. MS (ESI) m=z (%) ¼ 324 (M þ 1).
2
-(4-Phenylquinolin-2-yl)phenol (7b). Pale yellow powder. R ¼ 0.4
f
1
(
8
PE=EtOAc 9:1). H NMR (300 MHz, CDCl ): d ¼ 8.11 (d, J ¼ 8.4 Hz, 1 H, H-8),
3
0
.0–7.95 (overlap, 2H, 3,6 ), 7. 90 (d, J ¼ 8.4 Hz, 1H, H-5), 7.75 (t, J ¼ 8.1 Hz, 1H,

126
D. GARELLA ET AL.
H-7), 7.60–7.55 (overlap, 5H, Ph), 7.52 (t, J ¼ 7.2 Hz, 1H, H-6), 7.37 (t, J ¼ 7.2 Hz,
0
0
0
13
H, H-5 ), 7.10 (d, J ¼ 8.1 Hz, 1H, H-4 ), 6.94 (t, J ¼ 7.2 Hz, 1H, H-3 ). C NMR
3
1
(
1
1
1
75 MHz, CDCl ): d ¼ 161.24, 157.57, 150.34, 145.4, 138.06, 132.19, 130.47,
29.60, 128.89, 128.85, 128.11, 127.10, 126.82, 125.99, 125.48, 119.11, 118.87,
18.78, 117.71. FT-IR (KBr): t ¼ 3485.2, 1605.0, 1589.5, 1549.0, 1508.5, 1493.1,
ꢃ1
466.1, 1213.4, 1122.7, 1074.5, 873.9, 765.8, 704.1 cm . MS (CI) m=z (%) ¼ 298
(
M þ 1).
CONCLUSION
Our solvent-free protocol for Friedl a¨ nder’s annulation is simple, fast, and
efficient. It employs a catalyst that is easily recovered by filtration and can be reused
without appreciable loss of activity. When carried out under MW, the reaction was
faster and gave somewhat better yields.
ACKNOWLEDGMENTS
Support of this research by the University of Turin is gratefully acknowledged.
Z. R. thanks the World Wide Style (WWS) Project. Part of this work comes from the
degree thesis of Christian Speranza.
REFERENCES
1
2
3
4
. Dewick, P. M. Medicinal Natural Products: A Biosynthetic Approach; Wiley, Chichester,
UK 2002.
. Michael, J. P. Quinoline, quinazoline, and acridone alkaloids. Nat. Prod. Rep. 1997, 14,
605–618.
. Markees, D. G.; Dewey, V. C.; Kidder, G. W. Antiprotozoal 4-aryloxy-2-aminoquinolines
and related compounds. J. Med. Chem. 1970, 13, 324–326.
. Campbell, S. F.; Hardstone, J. D.; Palmer, M. J. 2,4-Diamino-6,7-dimethoxyquinoline
derivatives as a 1-adrenoceptor antagonists and antihypertensive agents. J. Med. Chem.
1988, 31, 1031–1035.
5
. Giardina, G. A. M.; Raveglia, L. F.; Grugny, M.; Sarau, H. M.; Farina, C.; Medhurst, A. D.;
Graziani, D.; Schmidt, D. B.; Rigolio, R.; Luttmann, M.; Cavagnera, S.; Foley, J. J.;
Vecchietti, V.; Hay, D. W. P. Discovery of a novel class of selective non-peptide antagonists
for the human neurokinin-3 receptor, 2: Identification of (S)-N-(1-phenylpropyl)-3-hydroxy-
2-phenylquinoline-4-carboxamide (SB 223412). J. Med. Chem. 1999, 42, 1053–1065.
6
. Ko, T. C.; Hour, M. J.; Lien, J. C.; Teng, C. M.; Lee, K. H.; Kuoa, S. C.; Huang, L. J.
Synthesis of 4-alkoxy-2-phenylquinoline derivatives as potent antiplatelet agents. Bioorg.
Med. Chem. Lett. 2001, 11, 279–282.
7. Zhang, X.; Shetty, A. S.; Jenekhe, S. A. Electroluminescence and photophysical properties
of polyquinolines. Macromolecules 1999, 32, 7422–7429.
8. Jenekhe, S. A.; Lu, L.; Alam, M. M. New conjugated polymers with donor–acceptor
architectures:
Synthesis
and
photophysics
of
carbazole-quinoline
and
phenothiazine-quinoline copolymers and oligomers exhibiting large intramolecular charge
transfer. Macromolecules 2001, 34, 7315–7324.
9
. Agrawal, A. K.; Jenekhe, S. A. Synthesis and processing of heterocyclic polymers as elec-
tronic, optoelectronic, and nonlinear-optical materials, 3: New conjugated polyquinolines
with electron-donor or electron-acceptor side groups. Chem. Mater. 1993, 5, 633–640.

¨
SOLVENT-FREE FRIEDLANDER ANNULATION
127
1
1
0. Jegou, G.; Jenekhe, S. A. Highly fluorescent poly(arylene ethynylene)s containing quino-
line and 3-alkylthiophene. Macromolecules 2001, 34, 7926–7928.
1. Cho, C. S.; Kim, B. T.; Kim, T. J.; Shim, S. C. Ruthenium-catalysed oxidative cyclisation
of 2-aminobenzyl alcohol with ketones: Modified Friedlaender quinoline synthesis. Chem.
Commun. 2001, 24, 2576–2577.
1
1
1
2. McNaughton, B. R.; Miller, B. L. A mild and efficient one-step synthesis of quinolines.
Org. Lett. 2003, 5, 4257–4259.
3. Ranu, B. C.; Hajra, A.; Dey, S. S.; Jana, U. Efficient microwave-assisted synthesis of quino-
lines and dihydroquinolines under solvent-free conditions. Tetrahedron 2003, 59, 813–819.
4. Cho, C. S.; Kim, B. T.; Choi, H. J.; Kim, T. J.; Shim, S. C. Ruthenium-catalyzed oxidative
coupling and cyclization between 2-aminobenzyl alcohol and secondary alcohols leading
to quinolines. Tetrahedron 2003, 59, 7997–8002.
1
5. Motokura, K.; Mizugaki, T.; Ebitaniu, K.; Kaneda, K. Multifunctional catalysis of a
ruthenium-grafted hydrotalcite: One-pot synthesis of quinolines from 2-aminobenzyl alco-
hol and various carbonyl compounds via aerobic oxidation and aldol reaction. Tetra-
hedron Lett. 2004, 45, 6029–6032.
¨
1
6. Friedl a¨ nder, P. Uber o-Amidobenzaldehyd. Chem. Ber. 1882, 15, 2572–2575.
17. Song, S. J.; Cho, S. J.; Park, D. K.; Kwon, T. W.; Jenekhe, S. A. Microwave enhanced
solvent-free synthesis of a library of quinoline derivatives. Tetrahedron Lett. 2003, 44,
255–257.
1
1
8. Haudhuri, M. K.; Sahid, H. An efficient synthesis of quinolines under solvent-free con-
ditions. J. Chem. Sci. 2006, 118, 199–202.
9. Jia, C. S.; Zhang, Z.; Tu, S. J.; Wang, G. W. Rapid and efficient synthesis of
poly-substituted quinolines assisted by p-toluene sulphonic acid under solvent-free con-
ditions: Comparative study of microwave irradiation versus conventional heating. Org.
Biomol. Chem. 2006, 4, 104–110.
2
2
2
0. Mogilaiah, K.; Rama Sudhakar, G. PTSA-catalyzed Friedlander condensation in the
solid state. Indian J. Chem. Sect. B 2003, 42, 1170–1171.
1. Wu, J.; Xia, H. G.; Gao, K. Molecular iodine: A highly efficient catalyst in the synthesis
of quinolines via Friedlander annulation. Org. Biomol. Chem. 2006, 4, 126–129.
2. Varala, R.; Enugala, R.; Adapa, S. R. Efficient and rapid Friedlander synthesis of func-
tionalized quinolines catalyzed by neodymium(III) nitrate hexahydrate. Synthesis 2006,
22, 3825–3830.
2
2
3. Dormer, P. G.; Eng, K. K.; Farr, R. N.; Humphrey, G. H.; McWilliams, J. C.; Reider, P.
J.; Sager, J. W.; Volante, R. P. Highly regioselective Friedlander annulations with unmo-
dified ketones employing novel amine catalysts: Syntheses of 2-substituted quinolines,
1,8-naphthyridines, and related heterocycles. J. Org. Chem. 2003, 68, 467–477.
4. Yadav, J. S.; Reddy, B. V. S.; Sreedhar, P.; Rao, R. S.; Nagaiah, K. Silver phosphotung-
state: A novel and recyclable heteropoly acid for Friedlander quinoline synthesis.
Synthesis 2004, 14, 2381–2385.
25. Zhang, L.; Wua, J. Friedlaender synthesis of quinolines using a lewis acid–surfactant com-
bined catalyst in water. Adv. Synth. Catal. 2007, 349, 1047–1051.
2
6. Wang, G. W.; Jia, C. S.; Dong, Y. W. Benign and highly efficient synthesis of quinolines
from 2-aminoarylketone or 2-aminoarylaldehyde and carbonyl compounds mediated by
hydrochloric acid in water. Tetrahedron Lett. 2006, 47, 1059–1063.
27. Das, B.; Damodar, K.; Chowdhury, N.; Kumar, R. A. Application of heterogeneous solid
acid catalysts for Friedlander synthesis of quinolines. J. Mol. Catal. A. Chem. 2007, 274,
148–152.
28. Narasimhulu, M.; Reddy, T. S.; Mahesh, K. C.; Prabhakar, P.; Rao, C. B.;
Venkateswarlu, Y. Silica supported perchloric acid: A mild and highly efficient

128
D. GARELLA ET AL.
heterogeneous catalyst for the synthesis of poly-substituted quinolines via Friedlander
hetero-annulation. J. Mol. Catal. A: Chem. 2007, 266, 114–117.
9. Clark, J. H. Solid acids for green chemistry. Acc. Chem. Res. 2002, 35, 791–797.
0. Curini, M.; Rosati, O.; Campagna, V.; Montanari, F.; Cravotto, G.; Boccalini, M.
Layered zirconium sulfophenyl phosphonate as heterogeneous catalyst in the synthesis
of pyrazoles and 4,5,6,7-tetrahydro-1(2)H-indazoles. Synlett 2005, 19, 2927–2930.
1. Upadhyaya, D. J.; Barge, A.; Stefania, R.; Cravotto, G. Efficient, solventless N-Boc
protection of amines carried out at room temperature using sulfamic acid as recyclable
catalyst. Tetrahedron Lett. 2007, 48, 8318–8322.
2. Cravotto, G.; Mendicuti, F.; Martina, K.; Tagliapietra, S.; Robaldo, B.; Barge, A. A new
access to homo- and heterodimers of a-, b-, and c-cyclodextrin by a microwave-promoted
huisgen cycloaddition. Synlett 2008, 18, 2642–2646.
3. Karimi, B.; Khalkhali, M. Solid silica-based sulfonic acid as an efficient and recoverable
interphase catalyst for selective tetrahydropyranylation of alcohols and phenols. J. Mol.
Catal. A: Chem. 2005, 232, 113–117.
4. Karimi, B.; Khalkhali, M. Silica functionalized sulfonic acid as a recyclable interphase
catalyst for chemoselective thioacetalization of carbonyl compounds in water. J. Mol.
Catal. A: Chem. 2007, 271, 75–79.
5. Herrero, M. A.; Kremsner, J. M.; Kappe, C. O. Nonthermal microwave effects revisited:
On the importance of internal temperature monitoring and agitation in microwave
chemistry. J. Org. Chem. 2008, 73, 36–47.
6. Palimkar, S. S.; Siddiqui, S. A.; Rajgopal, T. D.; Lahoti, J.; Srinivasan, K. V. Ionic liquid–
promoted regiospecific Friedlander annulation: Novel synthesis of quinolines and fused
polycyclic quinolines. J. Org. Chem. 2003, 68, 9371–9378.
7. Yadav, J. S.; Reddy, B. V. S.; Premalatha, K. Bi(OTf)(3)-catalyzed Friedlander
hetero-annulation: A rapid synthesis of 2,3,4-trisubstituted quinolines. Synlett 2004, 6,
2
3
3
3
3
3
3
3
3
963–966.
3
8. Shaabani, A.; Soleimani, E.; Badri, Z. Silica sulfuric acid as an inexpensive and recyclable
solid acid catalyzed efficient synthesis of quinolines. Monatsh. Chem. 2006, 137, 181–184.
9. Varala, R.; Enugala, R.; Adapa, S. R. Efficient and rapid Friedlander synthesis of func-
tionalized quinolines catalyzed by neodymium(III) nitrate hexahydrate. Synthesis 2006,
3
22, 3825–3830.
4
4
0. Yadav, J. S.; Purushothama Rao, P.; Sreenu, D.; Srinivasa Rao, R.; Naveen Kumar, K.;
Prasad, A. R. Sulfamic acid: An efficient, cost-effective, and recyclable solid acid catalyst
for the Friedlander quinoline synthesis. Tetrahedron Lett. 2005, 46, 7249–7253.
1. Bose, D. S.; Kumar, R. K. High-yielding microwave assisted synthesis of quinoline and
dihydroquinoline derivatives under solvent-free conditions. Heterocycles 2006, 68,
549–559.