Room-temperature synthesis of pyrazoles, diazepines, β-enaminones, and β-enamino esters using silica-supported sulfuric acid as a reusable catalyst under solvent-free conditions

Article abstract of DOI:10.1080/00397910802441551

Silica-supported sulfuric acid (H2SO4·SiO2) has been utilized as a heterogeneous recyclable catalyst for a highly efficient regio- and chemoselective condensation of hydrazines/hydrazides, diamines, and primary amines with various β-dicarbonyl compounds at room temperature to afford pyrazoles, diazepines, and β-enaminones/β-enamino esters under solvent-free conditions within 5-15 min. Copyright Taylor & Francis Group, LLC.

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Room-Temperature Synthesis
of Pyrazoles, Diazepines, β-
Enaminones, and β-Enamino
Esters Using Silica-Supported
Sulfuric Acid as a Reusable
Catalyst Under Solvent-Free
Conditions
Xiang Chen a , Jin She a , Zhi-Cai Shang a , Jun Wu a
& Peizhi Zhang b
a Department of Chemistry, Zhejiang University,
Hangzhou, China
b School of Biological and Chemical Engineering,
Zhejiang University of Science and Technology,
Hangzhou, China
Version of record first published: 25 Feb 2009.
To cite this article: Xiang Chen , Jin She , Zhi-Cai Shang , Jun Wu & Peizhi Zhang
(2009): Room-Temperature Synthesis of Pyrazoles, Diazepines, β-Enaminones, and
β-Enamino Esters Using Silica-Supported Sulfuric Acid as a Reusable Catalyst Under
Solvent-Free Conditions, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 39:6, 947-957
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Synthetic Communications1, 39: 947–957, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802441551
Room-Temperature Synthesis of Pyrazoles,
Diazepines, b-Enaminones, and b-Enamino Esters
Using Silica-Supported Sulfuric Acid as a
Reusable Catalyst Under Solvent-Free Conditions
Xiang Chen,1 Jin She,1 Zhi-Cai Shang,1 Jun Wu,1 and Peizhi Zhang2
1Department of Chemistry, Zhejiang University, Hangzhou, China
2School of Biological and Chemical Engineering, Zhejiang University of
Science and Technology, Hangzhou, China
Abstract: Silica-supported sulfuric acid (H2SO4ꢀSiO2) has been utilized as a
heterogeneous recyclable catalyst for a highly efficient regio- and chemoselective
condensation of hydrazines=hydrazides, diamines, and primary amines with
various b-dicarbonyl compounds at room temperature to afford pyrazoles, diaze-
pines, and b-enaminones=b-enamino esters under solvent-free conditions within
5–15 min.
Keywords: Diazepines, b-enaminones=b-enamino esters, H2SO4ꢀSiO2, hetero-
geneous recyclable catalyst, pyrazoles, solvent-free conditions
INTRODUCTION
Pyrazoles and diazepines are valuable bio-active heterocycles, which are
shown to possess important biological and pharmaceutical activities[1]
such as antimicrobial, antiviral, antitumor, anti-inflammatory, antifungal,
antidepressant, and anticonvulsant activities. Meanwhile, b-enaminones=
b-enamino esters are useful synthones for the synthesis of various pharma-
ceuticals[2] and bioactive heterocycles.[3]
Received July 6, 2008.
Address correspondence to Zhi-Cai Shang, Department of Chemistry,
Zhejiang University, Hangzhou 310027, China. E-mail: shangzc@zju.edu.cn
947

948
X. Chen et al.
Because of these prominent roles, the synthesis of these three kinds of
compounds (pyrazoles, diazepines, and b-enaminones=b-enamino esters)
has attracted considerable attention recently. By now, various synthetic
approaches have been reported. For preparing pyrazoles, among the
reported methods,[4] the most preferred are the reactions of 1,3-dicarbonyl
compounds with hydrazines in the presence of acid, such as polystyrene-
supported sulfonic acid (PSSA),[4a] sulfuric acid,[4b] and so on. In regard to
synthesis of diazepines, several routes have been reported, such as micro-
wave heating of diazepine-diones in boiling POCl3,[5a] hydrogen transfer
N-heterocyclization using catalyst [CpꢁIrCl2]2=K2CO3,[5b] condensation
of 1,3-diketones with diamines using catalyst PSSA,[4a] and others.[5c–5e]
Moreover, preparation of b-enaminones=b-enamino esters has been
reported to proceed by condensation of b-dicarbonyl compounds with
amines utilizing SiO2=microwaves,[6a] montmorillonite K-10,[6b] NaAu-
ClO4,[6c] Bi(TFA)3,[6d] Zn(ClO4)2ꢀ6H2O,[6e] CeCl3ꢀ7H2O,[6f] SiO2=
HClO4,[6g] and so on. However, most of the methods to prepare these
three kinds of compounds suffer from certain drawbacks including long
reaction times, unsatisfactory yields, higher temperatures, employment
of organic solvents, and use of expensive nonreusable catalyst. Thus, there
is still a need to develop greener and more efficient pathways for such
synthesis.
Recently, carrying out organic reactions under solvent-free conditions
has become highly desirable.[7] Solventless organic reactions usually are
rapid, are regio- or chemoselective, occur in high yields, and have environ-
mental and economic advantages.[7a] These advantages become even more
attractive if such reactions can be performed using reusable catalysts.
Solid acid catalysts prepared by employing acid supported on oxides have
been known and used for a long time; for instance, H2SO4ꢀSiO2,[8]
HClO4ꢀSiO2,[6g] and NaHSO4ꢀSiO2[9] have been reported to efficiently cat-
alyze some reactions. As we know, H2SO4ꢀSiO2 can easily be prepared
from readily available silica gel and H2SO4, which is much cheaper and
more stable than HClO4, and H2SO4ꢀSiO2 was also found to be a good
protic acid source under milder and safer conditions than HClO4ꢀSiO2
in some reaction systems.[8] For these reasons, we studied synthesis of pyr-
azoles, diazepines, b-enaminones, and b-enamino esters by the solventless
condensation of b-dicarbonyl compounds with hydrazines=hydrazides,
diamines, and primary amines at room temperature using silica-supported
sulfuric acid (H2SO4ꢀSiO2) as a heterogeneous recyclable catalyst.
Preparation of H2SO4ꢀSiO2: To a slurry of silica gel (10 g, 200–400
mesh) in dry diethyl ether (50 mL) was added commercially available
concd. H2SO4 (3 mL) with shaking for 5 min. The solvent was evaporated
under reduced pressure resulting in free-flowing H2SO4ꢀSiO2, which was
then dried at 110 ꢂC for 3 h.

Synthesis Catalyzed by Reusable H2SO4ꢀSiO2
949
RESULTS AND DISCUSSION
Initially, condensation of hydrazines=hydrazides (1.1 mmol) and
1,3-dicarbonyl compounds (1 mmol) with H2SO4ꢀSiO2 (20 mg) under
solvent-free conditions was examined. Hydrazines=hydrazides reacted
efficiently with various b-diketones to afford the desired pyrazoles in good
to excellent yields (Table 1). All of the reactions between hydrazines=
hydrazides and symmetrical b-diketones gave single components
1a–10a (entries 1–10 in Table 1). When an unsymmetrical b-diketone,
1-phenylbutane-1,3-dione, was employed to react with PhNHNH2, two
regioisomers, 14a and 14b, were obtained in the ratio of 19:1 (entry 14
in Table 1). Moreover, the b-ketoesters can also be used as a substitute
a
Table 1. Room temperature solventless synthesis of pyrazoles using H2SO4ꢀSiO2
Entry
R1
R2
R3
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
OEt
OEt
OEt
Ph
H
Me
H
Me
H
Me
H
Me
H
Me
H
H
H
Ph
Ph
6
7
5
5
7
8
5
5
6
7
8
8
8
8
93
85
95
90
89
85
96
93
92
89
87c
90c
82c
94
4-MePh
4-MePh
4-ClPh
4-ClPh
H
H
CH3CO
CH3CO
Ph
4-MePh
4-ClPh
Ph
H
aReactions performed with 1.0 mmol of b-dicarbonyl compound, 1.1 mmol of
hydrazine=hydrazide, and 20 mg of H2SO4ꢀSiO2 mixed for 5–8 min under
solvent-free condition, at room temperature.
bIsolated yields after column chromatography.
cYields of reactions performed in 50ꢂC.

950
X. Chen et al.
for diketones in this synthesis (entries 11–13 in Table 1). In such case,
only single components were afforded, in low yields at room temperature
and in high yields at 50 ꢂC.
Next, reactions of diamines with b-dicarbonyl compounds were
investigated (Table 2). 1,2-Diaminobenzene reacted efficiently with
pentane-2,4-diketone and 3-methylpentane-2,4-diketone to yield diaze-
pines in a single step (entries 1 and 2 in Table 2). The reaction proceeded
at room temperature, delivering excellent yields. The reaction of 1,2-
diaminobenzene with b-ketoesters, however, failed to give the desired
products (entries 3 and 4 in Table 2).
Finally, this protocol was also extended to the synthesis of various
b-enaminones and b-enamino esters by the condensation of primary
amines with various b-diketones and b-ketoesters at room temperature
under solvent-free conditions, and the results are summarized in
Table 3. All of the reactions between primary amines and symmetrical
b-diketones and b-ketoesters gave single components in good to high
yields (entries 1–13 in Table 3). In the case of the reaction of unsymme-
trical diketone 1-phenylbutane-1,3-dione with PhNH2, two regioisomers
(14a and 14b) were obtained in the ratio of 27:1 (entry 14 in Table 3).
The method was found to be highly chemo- and regioselective.
Groups –NH2 in RNH2 and RNHNH2 attack only at the ketone carbo-
nyl for b-ketoesters (entries 11–13 in Table 1, entries 12 and 13 in Table 3)
and in priority at the certain ketone carbonyl connecting with a relatively
a
Table 2. Room-temperature solventless synthesis of diazepines using H2SO4ꢀSiO2
Entry
R1
R2
Yield (%)b
1
2
3
4
Me
Me
OMe
OEt
H
Me
H
93
90
NR
NR
H
aReactions performed with 1.0 mmol of b-dicarbonyl compound, 1.1 mmol of
diamine, and 20 mg of H2SO4ꢀSiO2 mixed for 15 min under solvent-free condi-
tions at room temperature.
bIsolated yields after column chromatography.

Synthesis Catalyzed by Reusable H2SO4ꢀSiO2
951
Table 3. Room-temperature solventless synthesis of b-enaminone=b-enamino
a
ester using H2SO4ꢀSiO2
Entry
R1
R2
R3
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
10
11c
12
13
14
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
OEt
OEt
Ph
H
Me
H
Me
H
Me
H
Me
H
H
H
H
4-OMePh
4-OMePh
4-MePh
4-MePh
Ph
6
6
6
6
7
8
9
9
8
6
6
10
12
10
97
94
95
90
94
88
87
82
90
94
95
95
95
91
Ph
4-ClPh
4-ClPh
2-MePh
Benzyl
NH2CH2CH2
4-OMePh
Benzyl
Ph
H
H
aReactions performed with 1.0 mmol of b-dicarbonyl compound, 1.1 mmol of
primary amines, and 20 mg of H2SO4ꢀSiO2 mixed for 6–12 min under solvent-free
conditions at room temperature.
bIsolated yields after column chromatography.
cReactions performed with 2.0 mmol of b-dicarbonyl compound, 1.1 mmol of
primary amines, and 20 mg of H2SO4ꢀSiO2 mixed for 6–12 min under solvent-free
conditions at room temperature.
weaker electron-donating group when using unsymmetrical diketone
(entry 14 in Table 1, entry 14 in Table 3). Moreover, the (Z)-selectivity
in the products b-enaminones=b-enamino esters (entries 1–14 in
Table 3) derived from b-dicarbonyl compounds with primary amines
1
was secured by intramolecular hydrogen bonding. In the H NMR spec-
tra the proton of the –NH– group appeared in the region of d 8.5–13.5.
The possibility of recycling the catalyst was examined. For this
reason, the reaction of pentane-2,4-diketone and aniline at room
temperature in the presence of H2SO4ꢀSiO2 was studied (Table 4). After

952
X. Chen et al.
a
Table 4. Recycle experiment of H2SO4ꢀSiO2
Run
Yield (%)b
1
2
3
4
5
6
94
95
94
92
94
94
aReactions performed with 1.0 mmol of pentane-2,4-diketone, 1.1 mmol of ani-
line, and 20 mg of H2SO4ꢀSiO2 mixed for 7 min under solvent-free condition at
room temperature.
bIsolated yields after column chromatography.
completion of the reaction, the catalyst was recovered by simple filtration
and reused in subsequent reactions with consistent activity.
CONCLUSION
In conclusion, we have developed a green and efficient approach
for room-temperature solventless synthesis of pyrazoles, diazepines,
b-enaminones, and b-enamino esters by condensation of hydrazines=
hydrazides, diamines, and primary amines with various b-dicarbonyl
compounds in the presence of H2SO4ꢀSiO2 as a heterogeneous catalyst,
which may provide a useful route for drug discovery. The solvent-free
conditions, simple experimental procedure, mildness of the conversion,
clear reaction profiles, high yields and chemo- and regioselectivities, short
reaction times, and low cost, stability, and reusability of the catalyst are
the noteworthy advantages of the protocol.
EXPERIMENTAL
General Considerations
Silica-supported sulfuric acid was prepared by a reported method.[8] All the
1
reagents were obtained from commercial sources. H NMR spectra were
recorded on a Bruker Avance DMX 500-MHz spectrometer in CDCl3

Synthesis Catalyzed by Reusable H2SO4ꢀSiO2
953
solution. Low-resolution MS analyses were measured on a Bruke Esquire
3000 spectrometer using the ESI (electrospray ionization) technique.
General Procedure for the Synthesis of Pyrazoles, Diazepines, and
b-Enaminones/b-Enamino Esters
To a mixture of a dicarbonyl compound (1 mmol) and a hydrazine=
hydrazide (a diamine, a primary amine) (1.2 mmol), H2SO4ꢀSiO2 (20 mg)
was added. The mixture was stirred for 5–8 min (15 min, 6–12min) at room
temperature. After completion of the reaction, the reaction mixture was
diluted with ethyl acetate (2ml) and filtered. The catalyst was recovered
from the residue. The filtrate was concentrated. The residue, on purifica-
tion by column chromatography (silica gel, petroleum ether-ethyl acetate)
afforded pure pyrazole (diazepine, b-enaminone=b-enamino ester).
Condensation of b-enamino esters (1 mmol) with hydrazine
(1.2 mmol) was conducted at both room temperature and 50 ꢂC.
When ethanediamine (1.2 mmol) was used as an amine, a dicarbonyl
compound (2 mmol) was required.
Selected Data
5-Ethoxy-3-methyl-1-phenyl-1H-pyrazole (Table 1, Entry 12)
1H NMR (CDCl3): d 1.42–1.45 (t, J ¼ 7.0 Hz, 3H, CH3), 2.28 (s, 3H,
CH3), 4.11–4.16 (q, J ¼ 7.1 Hz, 2H, CH2), 5.48 (s, 1H, CH), 7.21–7.72
(m, 5H, Ph). 13C NMR (CDCl3): d 14.77, 14.86, 68.01, 86.49, 121.97,
125.94, 128.96, 139.03, 148.94, 155.07. MS (ESI) m=z 203 ([M þ H]þ).
3-Methyl-1,5-diphenyl-1H-pyrazole (Table 1, Entry 14a)
1H NMR (CDCl3): d 2.39 (s, 3H, CH3), 6.55 (s, 1H, CH), 7.32–7.91 (m,
10H, Ph). 13C NMR (CDCl3): d 13.76, 107.97, 125.41, 127.40, 128.34,
128.63, 128.87, 129.09, 130.89, 140.27, 143.37, 149.70. MS (ESI) m=z
235 ([M þ H]þ).
5-Methyl-1,3-diphenyl-1H-pyrazole (Table 1, Entry 14b)
1H NMR (CDCl3): d 2.40 (s, 3H, CH3), 6.33 (s, 1H, CH), 7.22–7.32 (m, 10H,
Ph). 13C NMR (CDCl3): d 12.75, 104.57, 125.16, 125.90, 127.79, 127.94,
128.75, 129.27, 133.52, 140.10, 140.36, 151.67. MS (ESI) m=z 235
([M þ H]þ).

954
X. Chen et al.
(1Z,4Z)-2,4-Dimethyl-3H-benzo[b][1,4]diazepine (Table 2, Entry 1)
1H NMR (CDCl3): d 2.34 (s, 6H, CH3), 2.81 (s, 2H, CH2), 7.20–7.22
(m, 2H, Ph), 7.35–7.38 (m, 2H, Ph). 13C NMR (CDCl3): d 27.87, 43.38,
125.09, 127.68, 140.43, 157.99. MS (ESI) m=z 173 ([M þ H]þ).
(Z)-Ethyl-3-(benzylamino)but-2-enoate (Table 3, Entry 13)
1H NMR (CDCl3): d 1.25–1.28 (t, J ¼ 7.0 Hz, 3H, CH3), 1.92 (s, 3H,
CH3), 4.09–4.13 (q, J ¼ 7.1 Hz, 2H, CH2), 4.43–4.44 (d, J ¼ 6.4 Hz, 2H,
CH2), 4.55 (s, 1H, CH), 7.26–7.36 (m, 5H, Ph), 8.96 (s, 1H, NH).
13C NMR (CDCl3): d 14.83, 19.54, 47.00, 58.57, 83.45, 126.92, 127.54,
128.98, 138.98, 161.97, 170.79. MS (ESI) m=z 220 ([M þ H]þ).
(Z)-1-Phenyl-3-(phenylamino)but-2-en-1-one (Table 3, Entry 14a)
1H NMR (CDCl3): d 2.16 (s, 3H, CH3), 5.92 (s, 1H, CH), 7.19–7.95
(m, 10H, Ph), 13.13 (s, 1H, NH). 13C NMR (CDCl3): d 20.66, 94.48,
124.98, 125.99, 127.28, 128.50, 129.38, 131.12, 138.85, 140.23, 162.44,
188.88. MS (ESI) m=z 238 ([M þ H]þ).
(Z)-4-Phenyl-4-(phenylamino)but-3-en-2-one (Table 3, Entry 14b)
1H NMR (CDCl3): d 2.24 (s, 3H, CH3), 5.44 (s, 1H, CH), 6.77–7.35
(m, 10H, Ph), 9.71 (s, 1H, NH). 13C NMR (CDCl3): d 29.62, 100.33,
122.98, 123.79, 127.64, 128.20, 128.59, 129.18, 135.40, 139.60, 159.50,
197.36. MS (ESI) m=z 238 ([M þ H]þ).
REFERENCES
1. (a) Hatheway, G. J.; Hansch, C.; Kim, K. H.; Milstein, S. R.; Schimidt, C. L.;
Smith, R. N.; Quin, F. R. Antitumor 1-(X-aryl)-3,3-dialkyltriazenes, 1: Quantita-
tive structure–activity relationships vs L1210 leukemia in mice. J. Med. Chem.
1978, 21, 563–574; (b) Katayama, H.; Oshiyama, T. Preparation and bioactivity
of pyrazole derivativies as potential cross-linking agent. Can. J. Chem. 1997, 75,
913–919; (c) Badawey, E. A. M.; El-Ashmawey, I. M. Nonsteroidal antiinflamma-
tory agents, part 1: Antiinflammatory, analgesic, and antipyretic activity of some
new 1-(pyrimidin-2-yl)-3-pyrazolin-5-ones and 2-(pyrimidin-2-yl)-1,2,4,5,6,7-hex-
ahydro-3H-indazol-3-ones. Eur. J. Med. Chem. 1998, 33, 349–361; (d) Bailey, D.
M.; Hansen, P. E.; Hlavac, A. G.; Baizman, E. R.; Pearl, J.; Defelice, A. F.;
Feigenson, M. E. 3,4-Diphenyl-1H-pyrazole-1-propanamine antidepressants.
J. Med. Chem. 1985, 28, 256–260.

Synthesis Catalyzed by Reusable H2SO4ꢀSiO2
955
2. (a) Edafiogho, I. O.; Moore, J. A.; Alexander, M. S.; Scott, K. R. Nuclear
magnetic resonance studies of anticonvulsant enaminones. J. Pharm. Sci.
1994, 83, 1155–1170; (b) Foster, J. E.; Nicholson, J. M.; Butcher, R.; Stables,
J. P.; Edafiogho, I. O.; Goodwin, A. M.; Henson, M. C.; Smith, C. A.; Scott,
K. R. Synthesis, characterization, and anticonvulsant activity of enaminones,
part 6: Synthesis of substituted vinylic benzamides as potential anticonvul-
sants. Bioorg. Med. Chem. 1999, 7, 2415–2425; (c) Wang, Y. F.; Izawa, T.;
Kobayashi, S.; Ohno, M. Stereocontrolled synthesis of (þ)-negamycin from
an acyclic homoallylamine by 1,3-asymmetric induction. J. Am. Chme. Soc.
1982, 104, 6465–6466; (d) Michael, J. P.; De Koning, C. B.; Hosken, G. D.;
Stanbury, T. V. Reformatsky reactions with N-arylpyrrolidine-2-thiones:
Synthesis of tricyclic analogues of quinolone antibacterial agents. Tetahedron
2001, 57, 9635–9648; (e) Boger, D. L.; Ishizaki, T.; Wysocki, R. J.; Munk, J.
S. A.; Kitos, P. A.; Suntornwat, O. Total synthesis and evaluation of (ꢃ)-N-
(tert-butoxycarbonyl)-CBI, (ꢃ)-CBI-CDPI1, and (ꢃ)-CBI-CDPI2: CC-1065
functional agents incorporating the equivalent 1,2,9,9a-tetrahydrocyclo-
propa[1,2-c]benz[1,2-e]indol-4-one (CBI) left-hand subunit. J. Am. Chem.
Soc. 1989, 111, 6461–6463.
3. Spivey, A. C.; Srikaran, R.; Diaper, C. M.; Turner, D. J. Traceless solid phase
synthesis of 2-substituted pyrimidines using an ‘‘off-the-shelf’’ chlorogermane-
functionalised resin. Org. Biomol. Chem. 2003, 1, 1638–1640.
4. (a) Polshettiwar, V.; Varma, R. S. Greener and rapid access to bio-active
heterocycles: Room temperature synthesis of pyrazoles and diazepines in
aqueous medium. Tetrahedron Lett. 2008, 49, 397–400; (b) Wang, Z.; Qin,
H. Solventless synthesis of pyrazole derivatives. Green Chem. 2004, 6, 90–
92; (c) Mohamed Ahmed, M. S.; Kobayashi, K.; Mori, A. One-pot
construction of pyrazoles and isoxazoles with palladium-catalyzed four-com-
ponent coupling. Org. Lett. 2005, 7, 4487–4489; (d) Heller, S. T.; Natarajan,
S. R. 1,3-Diketones from acid chlorides and ketones: A rapid and general
one-pot synthesis of pyrazoles. Org. Lett. 2006, 8, 2675–2678; (e) Deng,
X.; Mani, N. S. Reaction of N-monosubstituted hydrazones with nitroole-
fins: A novel regioselective pyrazole synthesis. Org. Lett. 2006, 8, 3505–
3508; (f) Armstrong, A.; Jones, L. H.; Knight, J. D.; Kelsey, R. D. Oxazir-
idine-mediated amination of primary amines: Scope and application to a
one-pot pyrazole synthesis. Org. Lett. 2005, 7, 713–716; (g) Greenberg, S.;
Marcuccio, S. M.; Leznoff, C. C.; Tomer, K. B. Selective synthesis of
binuclear and trinuclear phthalocyanines covalently linked by a one atom
oxygen bridge. Synthesis 1986, 409; (h) Siva, A.; Murugan, E. A new tri-
meric cinchona alkaloid as a chiral phase-transfer catalyst for the synthesis
of asymmetric a-amino acids. Synlett 2005, 2927.
5. (a) Rault, S.; Gillard, A. C.; Foloppe, M. P.; Robba, M. A new rearrange-
ment under microwave heating conditions: Synthesis of cyclopenta[b][1,4]-
benzodiazepines and tetrahydrodibenzo[b,f][1,4]diazepines. Tetrahedron
Lett. 1995, 36, 6673–6674; (b) Eary, C. T.; Clausen, D. Preparation of sub-
stituted 1,2,3,4-tetrahydroquinoxalines and 2,3,4,5-tetrahydro-1H-benzo[b][
1,4]diazepines from catalytic CpꢁIr hydrogen transfer N-heterocyclization

956
X. Chen et al.
of anilino alcohols. Tetrahedron Lett. 2006, 47, 6899–6902; (c) Wang, Q. F.;
Hu, B.; Luo, B. H.; Hu, C. M. Synthesis of 5-per(poly)fluoroalkyl-2,3-
dihydro-1,4-diazepines. Tetrahedron Lett. 39, 2377–2380; (d) Bonacorso,
H. G.; Lourega, R. V.; Deon, E. D.; Zanatta, N.; Martins, M. A. P. The first
synthesis of dihydro-3H-pyrido[2,3-b][1,4]diazepinols and a new alternative
approach for diazepinone analogues. Tetrahedron Lett. 2007, 48, 4835-
4838; (e) Cairns, J.; Clarkson, T. R.; Hamersma, J. A. M.; Rae, D. R. 11-
(Tetrahydro-3 and 4-pyridinyl)dibenzo[b,e][1,4]diazepines undergo novel
rearrangements on treatment with concentrated HBr. Tetrahedron Lett.
2002, 43, 1583–1585.
6. (a) Rechsteiner, B.; Texier-Boullet, F.; Hamalin, J. Synthesis in dry media
coupled with microwave irradiation: Application to the preparation of
enaminoketones. Synthesis in dry media coupled with microwave irradia-
tion: Application to the preparation of enaminoketones. Tetrahedron Lett.
1993, 34, 5071–5074; (b) Braibante, M. E. F.; Braibante, S.;
Missio, L.; Andricopulo, A. Synthesis and reactivity of b-amino a,b-unsatu-
rated ketones and esters using K-10 montmorillonite. Synthesis 1994, 898;
(c) Arcadi, A.; Bianchi, G.; Di Giuseppe, S.; Marinelli, F. Gold catalysis
in the reactions of 1,3-dicarbonyls with nucleophiles. Green Chem. 2003,
5, 64–67; (d) Khosropour, A. R.; Khodaei, M. M.; Kookhazadeh, M. A
mild, efficient and environmentally friendly method for the regio- and che-
moselective synthesis of enaminones using Bi(TFA)3 as a reusable catalyst
in aqueous media. Tetrahedron Lett. 2004, 45, 1725-1728; (e) Bartoli, G.;
Bosco, M.; Locatelli, M.; Marcantoni, E.; Melchiorre, P.; Sambri, L.
Zn(ClO4)2ꢀ6H2O as a powerful catalyst for the conversion of b-Ketoesters
into b-enamino esters. Synlett 2004, 239; (f) Khodaei, M. M.; Khosropour,
A. R.; Kookhazadeh, M. Enamination of b-dicarbonyl compounds cata-
lyzed by CeCl3ꢀ7H2O at ambient conditions: Ionic liquid and solvent-free
media. Synlett 2004, 1980; (g) Das, B.; Venkateswarlu, K.; Majhi, A.;
Reddy, M. R.; Reddy, K. N.; Rao, Y. K.; Ravikumar, K.; Sridhar, B.
Highly efficient, mild and chemo- and stereoselective synthesis of enami-
nones and enamino esters using silica supported perchloric acid under
solvent-free conditions. J. Mol. Catal. A: Chem. 2006, 246, 276–281.
7. (a) Cave, G. W. V.; Raston, C. L.; Scott, J. L. Recent advances in solventless
organic reactions: Towards benign synthesis with remarkable versatility. Tet-
rahedron Lett. 2001, 1, 2159–2169; (b) Raston, C. L.; Scott, J. L. Chemoselec-
tive, solvent-free aldol condensation reaction. Green Chem. 2000, 2, 49–52; (c)
Yoshizawa, K.; Toyota, S.; Toda, F. Solvent-free Claisen and Cannizzaro
reactions. Tetrahedron Lett. 2001, 42, 7983–7985; (d) Tanaka, K.; Sugino, T.;
Toda, F. Selective Stobbe condensation under solvent-free conditions. Green
Chem. 2000, 2, 303–304; (e) Yoshizawa, K.; Toyota, S.; Toda, F. Efficient
solvent-free Thorpe reactions. Green Chem. 2002, 4, 68–70.
8. (a) Rajput, V. K.; Roy, B.; Mukhopadhyay, B. Sulfuric acid immobilized on
silica: An efficient reusable catalyst for selective hydrolysis of the terminal
O-isopropylidene group of sugar derivatives. Tetrahedron Lett. 2006, 47,
6987–6991; (b) Roy, B.; Mukhopadhyay, B. Sulfuric acid immobilized on

Synthesis Catalyzed by Reusable H2SO4ꢀSiO2
957
silica: An excellent catalyst for Fischer type glycosylation. Tetrahedron Lett.
2007, 48, 3783–3787.
9. Das, B.; Veeranjaneyulu, B.; Krishnaiah, M.; Balasubramanyam, P. Synthesis
of gem-dihydroperoxides from ketones using silica-supported sodium
hydrogen sulfate as a heterogeneous catalyst. J. Mol. Catal. A: Chem. 2008,
284, 116–119.