Silica sulfuric acid: A reusable solid catalyst for one pot synthesis of densely substituted pyrrole-fused isocoumarins under solvent-free conditions

Article abstract of DOI:10.3762/bjoc.9.269

A convenient and efficient methodology for the synthesis of densely substituted pyrrole-fused isocoumarins, which employs solidsupported silica sulfuric acid (SSA) as catalyst, has been developed. When the mixture of ninhydrin adducts of acetylacetone/ethyl acetoacetate and primary amines was heated on the solid surface of SSA under solvent-free conditions, the pyrrole-fused isocoumarins were formed in good yields. This synthetic method has several advantages such as the employment of solvent-free reaction conditions without the use of any toxic reagents and metal catalysts, the ease of product isolation, the use of a recyclable catalyst, the low cost, the easy availability of the starting materials, and the excellent yields of products.

Full text of DOI:10.3762/bjoc.9.269

Silica sulfuric acid: a reusable solid catalyst for one
pot synthesis of densely substituted pyrrole-fused
isocoumarins under solvent-free conditions
Sudipta Pathak, Kamalesh Debnath and Animesh Pramanik*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 2344–2353.
Department of Chemistry, University of Calcutta, 92, A. P. C. Road,
Kolkata-700 009, India; Fax: +91-33-2351-9755; Tel:
+91-33-2484-1647
doi:10.3762/bjoc.9.269
Received: 01 August 2013
Accepted: 07 October 2013
Published: 04 November 2013
Email:
Animesh Pramanik* - animesh_in2001@yahoo.co.in
Associate Editor: J. P. Wolfe
* Corresponding author
© 2013 Pathak et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
green chemistry; pyrrole-fused isocoumarin; reusable solid support;
silica sulfuric acid; solvent-free condition
Abstract
A convenient and efficient methodology for the synthesis of densely substituted pyrrole-fused isocoumarins, which employs solid-
supported silica sulfuric acid (SSA) as catalyst, has been developed. When the mixture of ninhydrin adducts of acetylacetone/ethyl
acetoacetate and primary amines was heated on the solid surface of SSA under solvent-free conditions, the pyrrole-fused
isocoumarins were formed in good yields. This synthetic method has several advantages such as the employment of solvent-free
reaction conditions without the use of any toxic reagents and metal catalysts, the ease of product isolation, the use of a recyclable
catalyst, the low cost, the easy availability of the starting materials, and the excellent yields of products.
Introduction
Isocoumarins are an important class of naturally occurring process [10,11], an iridium-catalyzed oxidative lactonization or
lactones [1-3], which has attracted the attention of chemists an intramolecular cyclization reaction of δ-ketoaldehydes [12],
because of their various biological activities such as antioxida- a ruthenium-catalyzed aerobic oxidative cyclization of aromatic
tive [4], anticancer [5] and antifungal activities [6]. The devel- acids with alkynes [13], an FeCl3-promoted regioselective
opment of a new and efficient methodology for the synthesis of annulation of o-(1-alkynyl)benzoates with disulfides [14], a
biologically potent isocoumarins and their carbo/hetero annu- Heck–Matsuda cyclization reaction [15], a 6-endo-dig cycliza-
lated analogues has drawn great attention of synthetic as well as tion of heteroaryl esters to alkynes [16], or a Pd(II)-mediated
medicinal chemists [7-9]. Various methodologies for the syn- cyclization of o-allylbenzaldehydes [17]. Salvinorin A, a natural
thesis of isocoumarins have been reported such as the reaction product isolated from the hallucinogenic sage Salvia divinorum,
of o-halobenzoic acids and 1,3-diketones through a copper- which also contains a saturated isocoumarin ring, has been
catalyzed tandem sequential cyclization/addition/deacylation synthesized [18]. Although these methods are useful for the
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Beilstein J. Org. Chem. 2013, 9, 2344–2353.
synthesis of isocoumarin derivatives, the reactions involved in in protection–deprotection reactions of multistep syntheses [31-
the synthesis still suffer from some serious limitations such as 33], in esterifications [34] and in syntheses of heterocycles [35].
the use of expensive and hazardous reagents [12] and toxic Since we are actively involved in the synthesis of biologically
metal catalysts [10,11,15,17]. Some of the reactions need labo- important heterocycles [36-42], we wish to report herein a
rious and time consuming procedures [12,13,17], or drastic green methodology for the construction of pyrrole-fused
reaction conditions and with only low to moderate yields [16]. isocoumarins, which uses SSA as a solid-supported acid cata-
On the other hand, although a number of synthetic methods lyst under solvent-free conditions (Scheme 1, present work).
have been developed for the construction of densely substituted
pyrrole rings [19-21], not a single report has been given on the Results and Discussion
synthesis of pyrrole-fused isocoumarins with the help of green Recently, we have reported that the enamines 3 generated from
methodology, so far. Therefore, the development of an environ- acetylacetone (1) and amines 2 react with ninhydrin to form the
mentally friendly and safer reaction methodology following the cyclic hemiaminal dihydroxyindenopyrroles 4. Subsequently
green chemistry principles is essential for the synthesis of intermediates 4 produce the pyrrole-fused isocoumarins 5 upon
pyrrole-fused isocoumarins.
heating in glacial acetic acid with a catalytic amount of conc.
H2SO4 (Scheme 1, previous work) [38]. It was observed that in
The employment of a reusable solid supported/heterogeneous the above synthesis the intermediate dihydroxyindenopyrroles 4
catalyst for the efficient synthesis of heterocyclic compounds were needed to be isolated for further reaction to get the final
remains a challenge to chemists in laboratories and in the products 5 in pure form. Otherwise some acetylated amines
industry [22,23]. Reactions with reagents that are immobilized were always produced as byproducts. Besides, the formation of
on inorganic solid supports show several advantages over the 4 from 3 did not proceed significantly when the enamines of
conventional reactions in solution because of simple work-up ethyl acetoacetate were employed, because under acidic condi-
procedures, improved product yields, greater ease of purifica- tions, the enamines of ethyl acetoacetate readily hydrolyze and
tion, shorter reaction times, milder reaction conditions, and re- the free amines react with ninhydrin to form Schiff bases. To
cyclability of the catalyst [24]. In the recent years, silica sulfuric overcome the above problems we have designed an opera-
acid (SSA) has shown immense potentiality as an efficient and tionally simple one-pot reaction for the synthesis of pyrrole-
easily retrievable solid catalyst in various important organic fused isocoumarins (5 or 8) from the ninhydrin adducts of
syntheses under solvent-free conditions [25]. The high catalytic acetylacetone/ethyl acetoacetate (6 or 7) [43] and primary
activity, the operational simplicity and the recyclability of SSA amines 2 under solvent-free conditions (Scheme 1, present
can be exploited in the industry for the synthesis of various work).
drugs and pharmaceuticals. SSA, a product that is easily synthe-
sized from silica gel and chlorosulfonic acid [26], was observed In order to explore the role of the different catalysts and
to improve the reactivity and selectivity in carbon–carbon bond- solvents in the preparation of pyrrole-fused isocoumarins, an
formation reactions [27,28], in cycloaddition reactions [29,30], optimisation study was carried out with the model reaction
Scheme 1: Synthesis of pyrrole-fused isocoumarins.
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Beilstein J. Org. Chem. 2013, 9, 2344–2353.
between dihydroxyindenofuran ethyl ester 7 [43] and aniline in entry 6). The structure of the product 8a was confirmed by IR,
a molar ration of 1.00:1.50 (Scheme 2). When the reaction was 1H NMR and 13C NMR spectroscopy and elemental analysis.
carried out in aqueous solution under reflux the reaction did not Surprisingly, when the above-mentioned reaction was carried
proceed at all (Table 1, entry 1). Previous results showed that an out with aliphatic amines, only the acetylated amines were
activation by a Brønsted acid was necessary to carry out the obtained instead of the desired products 8. These results influ-
reaction successfully [38]. Therefore, we screened various enced us to carry out the reaction under greener and milder
Brønsted acid catalysts, e.g., lactic acid, formic acid, citric acid reaction conditions, but with satisfying yield of the desired
and acetic acid in aqueous solution under reflux. But the yields product, both for aromatic and aliphatic amines. We restrained
were very low even after prolonged reaction time (Table 1, the reaction to using PEG–OSO3H as a Brønsted acid–surfac-
entries 2–5). On the basis of the assumption that more acidic tant combined catalyst in aqeous solution under refluxing condi-
conditions might be necessary to furnish the desired products in tions as well as under solvent-free conditions (Table 1, entries 7
high yields, we carried out the reaction in acetic acid with and 8). Although under solvent-free conditions the required
adding a catalytic amount of H2SO4. Intriguingly, the yield of temperature was lower and the yields of the products were
the product increased from less than 10% to 64% (Table 1, higher, the yields were still only moderate. This encouraged us
Scheme 2: Reaction scheme for the synthesis of pyrrole-fused isocoumarins.
Table 1: Optimization of reaction conditions for the synthesis of 8a.
entry
catalyst
solvent
catalyst load
temperature (°C)
time (h)
yield (%)a
1
2
—
lactic acid
H2O
H2O
H2O
H2O
H2O
acetic acid
H2O
—
—
100
100
100
100
100
85
24
24
24
24
24
1
—
5
20 mol %
20 mol %
20 mol %
20 mol %
20 mol %
500 mg
500 mg
500 mg
500 mg
500 mg
500 mg
500 mg
400 mg
300 mg
3
formic acid
8
4
citric acid
5
5
acetic acid
6
6
H2SO4
64
45
66
—
—
45
58
90
90
83
7
PEG–OSO3H
PEG–OSO3H
silica gel
100
80
2
8
1.5
24
24
0.5
0.5
1
9
—
100
100
100
85
10
11
12
13
14
15
melamine sulphonic acid
silica sulfuric acid
silica sulfuric acid
silica sulfuric acid
silica sulfuric acid
silica sulfuric acid
—
—
—
—
65
—
65
1
—
65
1.5
aOptimization studies were carried out with 1.0 mmol 7 and 1.5 mmol of aniline.
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Beilstein J. Org. Chem. 2013, 9, 2344–2353.
to execute the optimization study in presence of a solid acid evident from Table 2, all the primary amines reacted well with
catalyst under solvent-free conditions. This is one important adducts 6 and 7 affording the desired products 5 and 8 in good
facet of green chemistry: the eradication of solvents in chem- yields. The results show that solvent-free conditions and the
ical processes. Hence, we have carried out the synthesis by SSA catalyst are crucial carrying out the reaction succesfully
dissolving the substrate 7 and aniline in a minimum volume of even with aliphatic amines. The structures of the new products
chloroform, soaked them on the solid surface of solid Brønsted 8a–o were determined by using spectroscopic data and
acid catalysts, such as silica gel and melamine sulfonic acid elemental analysis. X-ray crystal data analysis of compound 8c
(MSA), dried the mixture under vacuum, and heated the reac- further confirmed the product formation (Figure 1). The forma-
tion mixture to 100 °C (Table 1, entries 9 and 10). Unfortu- tion of products 5a–l was confirmed by comparing the reported
nately, the reactions on silica gel and MSA failed to give the spectral data and melting points (Table 2) [38].
desired product 8a. In the search of a suitable solid acid cata-
lyst we employed silica sulfuric acid (SSA) at 100 °C.
However, the reaction mixture got charred after 0.5 h and a
considerable amount of impurities along with the desired prod-
uct 8a was formed (Table 1, entry 11). When lowering the reac-
tion temperature (65–100 ºC) and varying the amount
(300–500 mg) of solid catalyst (Table 1, entries 12–15), the
maximum yield (90%) of 8a was obtained at 65 °C using
400 mg of SSA (Table 1, entry 14).
Having successfully prepared 8a, we decided to explore the
scope and generality of this reaction in the synthesis of other
analogues. Accordingly, the ninhydrin adducts of acetylacetone/
ethyl acetoacetate (6 and 7) [43] were reacted with a variety of
commercially available aliphatic and aromatic primary amines
under the optimized conditions (Table 1, entry 14). As becomes
Figure 1: ORTEP diagram of 8c with atom numbering scheme.
Thermal ellipsoids are shown at 50% probability with CCDC number
949317.
Table 2: Formation of isocoumarins 5 and 8 from adducts 6 and 7 respectively on an SSA surface.
entry
1
R1
R2
adduct
product
yield (%)a
91
mp observed/lit. [38] (°C)
248–250/248
Me
6
5a
2
Me
6
5b
82
205–207/205
3
4
Me
Me
6
6
5c
5d
88
89
262–264/262
258–260/258
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Table 2: Formation of isocoumarins 5 and 8 from adducts 6 and 7 respectively on an SSA surface. (continued)
5
6
Me
Me
6
6
5e
5f
84
80
220–222/220
172–174/172
7
Me
6
5g
79
236–238/236
8
9
Me
Me
6
6
5h
5i
82
86
260–262/260
>320/>320
10
Me
6
5j
84
>320/>320
11
12
Me
Me
6
6
5k
5l
90
88
150–152/150
182–184/182
13
14
OEt
OEt
7
7
8a
8b
90
89
208–210
252–254
15
16
OEt
OEt
7
7
8c
8d
79
85
233–235
230–232
17
18
OEt
OEt
7
7
8e
8f
87
80
218–220
194–196
19
20
OEt
OEt
7
7
8g
8h
83
81
198–200
254–256
21
22
OEt
OEt
7
7
8i
8j
86
81
202–204
190–192
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Table 2: Formation of isocoumarins 5 and 8 from adducts 6 and 7 respectively on an SSA surface. (continued)
23
24
OEt
OEt
7
7
8k
8l
83
86
212–214
260–262
25
26
OEt
7
8m
91
180–182
OEt
OEt
7
7
8n
8o
89
87
132–134
125–127
27
aIsolated yield.
The comparative studies in terms of overall reaction times and tives, since the starting materials dihydroxy indenofurans 6 and
product yields show that although the present method takes 7 need only one step for preparation, (b) the employment of
more time than the previous method [38], the overall yields of milder acidic conditions, (c) a lower reaction temperature
the products are larger in the present method (Table 3). More (65 °C), (d) solvent-free conditions, and (e) more cost-effective
importantly the present method enables us to access a new because of the reusability of the solid-supported SSA. More-
series of pyrrole-fused isocoumarins with ester functionality over the starting materials 6 and 7 can also be prepared through
(Table 3, compounds 8a–o), which were not possible to synthe- a green methodology [43].
size with the previous method. Apart from that the present
method is more advantageous in terms of product formation and Based on the results of Table 2 and the fact that SSA plays the
greener characteristics than the previous one in many respects role of transferring protons from its solid surface, a probable
such as (a) a less laborious and more step-economical reaction mechanism for the formation of isocoumarins 5 or 8 is expli-
for the library synthesis of pyrrole-fused isocoumarin deriva- cated in Scheme 3. The protonation and activation of the
Table 3: Comparison between the present and the previous method for the synthesis of pyrrole-fused isocoumarins from ninhydrin.
entry
product
overall reaction time (min)
present method previous method
overall yield (%)
present method
previous method
1
2
5a
5b
5c
5d
5e
5f
61
37
87
79
84
85
81
77
76
79
83
81
86
81
64
74
80
80
66
58
77
56
68
78
61
61
61
61
61
61
61
61
61
61
46
45
37
39
51
37
36
37
46
21
3
4
5
6
7
5g
5h
5i
8
9
10
11
5j
5k
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Table 3: Comparison between the present and the previous method for the synthesis of pyrrole-fused isocoumarins from ninhydrin. (continued)
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
5l
8a
8b
8c
8d
8e
8f
61
61
61
61
61
61
61
61
61
61
61
61
61
61
61
61
25
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
84
84
83
73
79
81
74
77
75
80
75
77
80
85
83
81
84
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
8g
8h
8i
8j
8k
8l
8m
8n
8o
Scheme 3: Mechanism of formation of isocoumarins 5 or 8 on the surface of SSA.
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Beilstein J. Org. Chem. 2013, 9, 2344–2353.
hydroxy group of dihydroxy indenofuran (6 or 7) by the pyrrole-fused isocoumarins in presence of the solid-supported
sulfonic group of SSA generates dehydrated cationic intermedi- Brønsted acid catalyst silica sulfuric acid (SSA). The method-
ate 9. This reactive intermediate 9 provokes a nucleophilic ology has a series of intrinsic advantages such as easy prepar-
attack of primary amines to form bicyclo[3.3.0]octanamino ation of the solid supported SSA from chlorosulfonic acid and
compound 10. Then the α-hydroxy group of 10 attacks the adja- silica gel, less energy and manpower usage, easy product isola-
cent carbonyl carbon to generate epoxy intermediate 11. This tion/purification and operational simplicity, which lead to the
unstable epoxy intermediate 11 produces a six-membered synthetic route ‘‘benign by design’’. This is the first report, in
lactone intermediate 12 through the breaking of a C–C bond. which a rearrangement reaction has been carried out on the
Subsequently, intermediate 12 tautomerizes to 13 under forma- solid surface of SSA. Overall this greener and environmentally
tion of the isocoumarin skeleton. The dihydropyrrole-fused friendly method may attract the fellow chemists in chemical and
isocoumarin intermediate 14 is formed through the intramolec- pharmaceutical industries for the synthesis of biologically
ular nucleophilic attack of the secondary amine group to the important pyrrole-fused isocoumarins.
carbonyl carbon of 13. Finally, intermediate 14 loses water to
furnish pyrrole-fused isocoumarins 5 or 8. It is worth Experimental
mentioning that in the previous method instead of the forma- General information: Starting materials and solvents were
tion of epoxy intermediate like 11 a transannular rearrangement purchased from commercial suppliers and used without further
was proposed for the product formation [38], because the for- purification. Melting points were determined in open capillary
mation of epoxy intermediate is less probable in the presence of tubes and were uncorrected. IR spectra were recorded on a
a strong acid and a nucleophilic solvent as well as at higher Perkin-Elmer 782 spectrophotometer. 1H (300 MHz) and
temperatures.
13C NMR (75 MHz) spectra were recorded on a Bruker
300 MHz instrument in CDCl3 and d6-DMSO. Elemental
Furthermore, a test with respect to recovery and reusability of analyses (C, H and N) were performed by using a Perkin-Elmer
SSA for the formation of 8a was carried out. After heating the 240C elemental analyzer. The X-ray diffraction data for crystal-
mixture of aniline and adduct 7 on the solid surface of SSA for lized compounds were collected with Mo Kα radiation at 296 K
1 h, the product 8a was isolated easily with ethylacetate by using the Bruker APEX-II CCD System. The crystals were
sonication of the reaction mixture. The recovered solid- positioned at 50 mm from the CCD. Frames were measured
supported SSA was reused five times, and the yield of the prod- with a counting time of 5 s. Data analyses were carried out with
uct 8a varied from 90–83%, which indicates a substantial reten- the Bruker APEX2 and Bruker SAINT program. The structures
tion of catalytic activity and efficiency of SSA even after were solved using direct methods with the SHELXS97
repeated application (Figure 2).
program.
General experimental procedure for synthesis of pyrrole-
fused isocoumarins 5 and 8: A mixture of primary amines 2
(1.5 mmol) and dihydroxy indenofurans 6 or 7 (1.0 mmol) in
chloroform (5 mL) was soaked in SSA (400 mg) by stirring for
10 min and then the solvent was removed under reduced pres-
sure to get a solid mass. The solid mass was heated at 65 °C for
1 h under continuous stirring, until the complete disappearance
of dihydroxyindenofuran was observed (as monitored by TLC).
After cooling the solid mass to room temperature, ethylacetate
(15 mL) was added to it, shaken thoroughly, ultra-sonicated and
filtered to remove the SSA catalyst. The separated organic
phase was evaporated under reduced pressure to get the crude
product 5 or 8 which was purified by column chromatography
(hexane/EtOAc).
Figure 2: Reusability of SSA for the synthesis of pyrrole-fused
isocoumarins.
Reusability of the SSA: After completion of the reaction, the
product was isolated by ultra-sonication with ethylacetate and
decanted. Then the separated solid catalyst SSA was dried
Conclusion
In conclusion, a facile and convenient methodology has been under vacuum and reused directly for a new reaction set. The
developed for the synthesis of a diverse range of N-substituted yield of the product 8a varied from 90–83% after five consecu-
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Acknowledgements
S.P. and K.D. thank CSIR and UGC, New Delhi, India, for
offering a Senior Research Fellowship (SRF) and a Junior
Research Fellowship (JRF) respectively. The financial assis-
tance of CSIR, New Delhi is gratefully acknowledged [Major
Research Project, No. 02(0007)/11/EMR-II]. Crystallography
was performed at the DST-FIST, India-funded Single Crystal
Diffractometer Facility at the Department of Chemistry, Univer-
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