On water one-pot synthesis of quaternary centered 3-hydroxy-3-(1H-tetrazol-5-yl)indolin-2-ones

Article abstract of DOI:10.1039/c3ra00021d

An efficient green protocol has been developed for the synthesis of 3-hydroxy-3-(1H-tetrazol-5-yl)indolin-2-ones and 3-(phenylamino)-3-(1H-tetrazol- 5-yl)indolin-2-ones as a novel class of oxindole derivatives by metal-free azide-nitrile [2 + 3] cycloaddition with excellent yields.

Full text of DOI:10.1039/c3ra00021d

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Graphical Abstract
“On water” one-pot synthesis of quaternary centered 3-hydroxy-3-
(1H-tetrazol-5-yl)indolin-2-ones
An efficient one-pot green protocol has been developed for the synthesis of 3-hydroxy-3-(1H-
tetrazol-5-yl)indolin-2-ones and 3-(phenylamino)-3-(1H-tetrazol-5-yl)indolin-2-ones as a novel
class of oxindole derivatives by metal-free azide-nitrile [2+3] cycloaddition. The process is
simple, eco-friendly and applicable for diverse functionalities of isatin with excellent yields at
room temperature.




Atom economic one-pot reaction
Non-toxic catalyst
Excellent yields with broad substrate scope
Water as solvent at RT

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ARTICLE TYPE
“On water” one-pot synthesis of quaternary centered 3-hydroxy-3-(1H-
tetrazol-5-yl)indolin-2-ones
Parvathaneni Saiprathima,a Keesara Srinivas, aBalasubramanian Sridharb and Mandapati Mohan Rao,*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
45 Apart from their wide spectrum of biological activities, recently
they have been reported for their antiꢀHIV activity.5
An efficient one-pot green protocol has been developed for
the synthesis of 3-hydroxy-3-(1H-tetrazol-5-yl)indolin-2-ones
and 3-(phenylamino)-3-(1H-tetrazol-5-yl)indolin-2-ones as a
novel class of oxindole derivatives by metal-free azide-nitrile
10 [2+3] cycloaddition. The process is simple, eco-friendly and
applicable for diverse functionalities of isatin with excellent
yields at room temperature.
In pursuit of our continuous research in Cꢀ3 functionalization
of isatins,6 we envisioned the combination of both tetrazole and
oxindole framework linked at appropriate position to generate
50 quaternary centred αꢀhydroxytetrazole and αꢀaminotetrazole. As
majority of approved drugs contain tetrazole and indoline
moieties, this approach may provide the most potent scaffold,
which represents an attractive endeavor in drug discovery
(Scheme 1).
Tetrazole derivatives are prized as energetic, nitrogenꢀrich,
metabolically stable surrogates for terminal carboxylic acid
15 groups as well as for cisꢀamide bonds, with high thermal stability
(due to aromaticity).1 Much attention has been focused on this
functionality, as it constitutes a common structural skeleton for
family of heterocycles, which has significant applications in
organocatalysis,coordination chemistry and medicinal chemistry.
20 Valsartan, Irbesartan, Losartan, Candesartan are few examples of
55
antihypertensive
drugs
with
tetrazole
scaffold.2
Scheme 1. Direct oneꢀpot synthetic route to tetrazole substituted oxindole
derivatives.
C3 functionalized tetra substituted, quaternary centred,
oxindole framework form the core unit of many natural
molecules, bioactive compounds and FDA approved drugs.3
25 Notably, 3ꢀ substitutedꢀ3ꢀhydroxyꢀ2ꢀoxindole derivatives serves
as potential synthons for pharmaceutical candidates.3 Oxindole
motif with a hydroxyꢀbearing C3 substitution, encountered in a
large variety of pharmacologically active alkaloids, such as
maremycins A & B, celogentin K, CPCꢀ1, convolutamydines,
30 TMCꢀ95A, etc.4 Representative molecules of oxindoles and
A common route for synthesis of 5ꢀsubstituted 1Hꢀtetrazole
60 derivatives involves [2+3] cycloaddition of azide salts to organic
nitriles (RN3 + RC≡N) at 100ꢀ150 oC.7 This cycloaddition is very
slow, require harsh reaction conditions such as high temperatures
and long reaction times with high boiling solvents such as
dimethylformamide (DMF). Apart from sodium azide, other
65 azides like trimethylsilyl, trialkyl tin, organoaluminium azides8
have been reported. Lewis acids, heterogeneous catalysts, Iron
salts, copper salts, Zinc salts, amine salts also have been used to
activate the azideꢀnitrile cycloaddition.9 The main bottlenecks
involved in the reported synthetic methods are use of expensive
70 reagents, toxic metals, long reaction hours and high temperatures.
To date, Nꢀalkylated nitriles (nitrilium salts), trichloroacetonitrile,
perflouronitriles, phenyl cyanate, pꢀtoluenesulfonyl cyanide, Nꢀ
cyano sulfoximines, malononitrile etc have been employed as
dienophiles in tetrazole formation.10 The insitu generated 3ꢀ
75 Cyano 3ꢀhydroxy oxindoles as dipolarophiles for the tetrazole
formation has not been explored. Herein, we report the synthesis
of various interesting tetrazole derivatives by facile inter
molecular [2+3] cycloaddition between an azide and a nitrile
group bound to quaternary centre of oxindole at C3 position.
tetrazoles
are
shown
in
(Fig.
1).
35 Fig. 1 Representative C3 functionalized oxindoles (A, B) and tetrazole
based drugs (C, D).
80
In recent years “On water” catalysis is considered to be a
challenging research area for synthetic chemists due to stringent
environmental restrictions.11 The significant rate enhancements
are observed in water compared to organic solvents has been
attributed to factors, such as hydrogen bonding or interactions
aInorganic and Physical Chemistry Division, Indian Institute of Chemical
Technology (IICT), Tarnaka, Hyderabad-500 607 (India)
40 Fax: (+)91 40 27160921, Tel: (+) 91 40 27193181
E-mail: mandapati@iict.res.in
bX-ray Crystallography Division,(IICT)
85 related to polarity, acidity and hydrophobicity etc make water
interesting from an industrial as well as laboratory perspective.
Recently R. A. Marcus reported a detailed study describing the
†Electronic Supplementary Information (ESI) available: CCDC 857953,
857954 See DOI: 10.1039/b000000x/
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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organic catalysis “On Water”.11c This has provided a strong
impetus to develop efficient green protocol in aqueous media for 55 in yield due to dispersion of the reagents. The best result was
Further increase in water content, there is considerable reduction
the synthesis of new Oxindoleꢀtetrazole motif 4a. The threeꢀ
component reaction of isatin 1a, TMSCN 2, NaN3 3, is similar to
the Passerini type reaction (Pꢀ3CR) of aldehydes, isocyanides and
hydrazoic acid affording tetrazoles (Scheme 2). The fourꢀ
component reaction which involves 1a, 2, 3 and an amine is
comparable to the Ugi fourꢀcomponent reaction (Uꢀ4CR)
(Scheme 5).12 To the best of our knowledge, this is the first report
obtained in the presence of 2.0 mL of water (98%, Fig. 2).
N
N
N
O
HO
5
N
H2O
H
O + TMSCN
NaN3
+
O
N
N
H
2
3
4a
1a
H
Scheme 2. Synthesis of 3ꢀhydroxyꢀ3ꢀ(1Hꢀtetrazolꢀ5ꢀyl)indolin 2one 4a
10 for oneꢀpot synthesis of 3ꢀhydroxyꢀ3ꢀ(1Hꢀtetrazolꢀ5ꢀyl)indolinꢀ2ꢀ
ones 4a (Scheme 2) on water directly from isatins.
by oneꢀpot three component reaction.
The straightforward preparation of compound 4a is C3
functionalisation by nucleophilic addition of isatins. We
envisaged that the use of TMSCN as a nucleophile would allow
15 the synthesis of 3ꢀCyano 3ꢀhydroxy oxindoles 5 and their direct
conversion into corresponding tetrazole derivatives 4 by facile
[2+3] cycloaddition of azides. By employing the most popular
“clickꢀchemistry” approach, we first examined the reaction of
isatin 1a as a model substrate for tetrazole formation on water. To
20 our surprise, the reaction proceeded very well at room
temperature, affording tetrazole 4a in 60 % and 85% yields with
TMSCN 2, sodium azide 3 at 2.0 and 3.0 equiv relative to isatin
1a, respectively (Table 1, entry 1 and 2). With nearꢀequimolar
nitrileꢀTMSCNꢀazide ratio the reaction is sluggish with low yield
25 (entry 3). To improve the yields and enhance the rate of reaction
with near stoichimetric molar ratios of TMSCN and azide (1.0
equiv relative to isatin 1a), several amine catalysts were
experimented (Table 1). Pyridine.HCl was found to be most
efficient catalyst, giving the tetrazole 4a in 98% yield with 100%
30 conversion within 11 h using water as solvent. (Table 1, entry
15). All the other catalysts led to less satisfactory results (Table 1,
entry 4ꢀ11) and few gave good yields. (Table 1, entry 12ꢀ14). We
were surprised to find, however, that water gave far best results
than the other solvents (Table 1, entry 15ꢀ17) due to the
35 formation of hydrogen bonds on the interface which are
responsible for accelaration of reaction “on water“.11c The low
yield of the desired tetrazole 4a under solventꢀfree condition
indicates (entry 18) the essential role of water for the tetrazole
formation. Also the amount of water is found to be crucial in
40 determining the rates of reaction and yields of the product. When
using 0.5, 1.0, 1.5 and 2.0 mL of water, 4a was obtained in 40%,
60%, 80% and 98% yields successively within 11 h of reaction.
The reaction mixture is a slurry in the presence of small quantity
of water, where as with the increase in volume the increase in
45 yields is more likely due to a better dissolution of the reactants.
This might be in favor of a strong Hꢀbond of water with the
carbonyl group which causes “Electrophilic activation” making it
60 Table1. Conditions optimization for tetrazole formationa
Time [h]
72
Yield [%]b
60c
85d
Entry
Solvent
H2O
Catalyst
No catalyst
No catalyst
1
2
48
3
4
No catalyst
Et3N
20
10
96
48
5
6
Diethanolamine
Piperidine
Pyrrolidine
48
20
15
30
10
10
48
24
7
8
oꢀPhenylene diamine
N,N diethyl amine
48
48
9
10
Pyridine
48
48
30
50
83
11
12
Dimethyl amine hydrochloride
Methyl ammonium chloride
48
13
14
NH4Cl
48
11
11
24
24
80
95
98
Pyridine HBr
Pyridine HCl
Pyridine HCl
Pyridine HCl
15
16
MeOH
DMSO
85
80
10
17
18
No solvent
Pyridine HCl
24
aUnless otherwise indicated, all reactions were carried out on water with isatin
(1 mmol), TMSCN (1 mmol) , NaN3 (1.2 mmol), and the catalyst (0.02 mmol)
in the solvent (2.0 mL) at room temperature. bYield of isolated product after
purification. cusing 2 equivalents of TMSCN, NaN3. dusing 3 equivalents of
TMSCN, NaN3.
As we anticipated the reaction proceeding a mechanism via
65 nucleophilic addition of cyanide 2 to 1a affords the intermediate
(B), which on concurrent nucleophilc attack of azide ion on the
nitrile group (Py.HCl activates azide nitrile addition) provides C
followed by ring closure of the iminoazide D to form tetrazole 4a
(Scheme 3).15i
H
more susceptible to nucleophilic attack
the yield slightly decreased to 95%.
.
11e With 2.5 mL of water
O+
Me
Me
O
S
Me
i
O
Me
Si
H
ꢀ
N
H
CN
50
O
Me
O
ꢀ
A
H
N
O
Me
1a
H
C
N
Me
Me
i
H
S
OH
O
Me
HCl
N
N
O
Py
O
B
H
H 2
N
NaN3+ Py.HCl
N
N
N
N
N
H
O
N
N
E
N
H
4a
O
Me
Me
PyH+Nꢀ3+ NaCl
i
S
i
Me
Me
Me
S
Me
N
O
C
O
Py
N
Py
D
N
N
N
H
O
H
O
N
N N
N+
C
Nꢀ
70
Scheme 3. Proposed mechanism for the formation of tetrazole 4a
Fig. 2 Effect of quantity of water on the yield of 3ꢀhydroxyꢀ3ꢀ(1Hꢀ
tetrazolꢀ5ꢀyl)indolinꢀ2ꢀones 4a
We next examined the generality of the reaction using different
isatin derivatives bearing both electronꢀdonating (methyl) and
2
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Scheme 4. Direct one-pot Synthesis of 3-hydroxy-3-(1H-
tetrazol-5-yl)indolin-2-ones 4 (a-r).
Table 2. Direct one-pot Synthesis of 3-hydroxy-3-(1H-
tetrazol-5-yl)indolin-2-one derivatives on water.a
5
10 electronꢀwithdrawing groups (chloro, bromo, iodo, fluoro, nitro
and trifluoromethoxy) at Cꢀ5, chloro at Cꢀ7 and substituents
(methyl, benzyl, allyl, halo substituted benzyl) at Nꢀ1 position.
The cycloaddition reaction proceeded smoothly and produced the
desired tetrazoles (4a-r) in excellent yields (83–98%) as
15 illustrated in (Scheme 4). The electronic nature of the substituent
and its position on the isatin ring did not show considerable
influence on the yields. The rate of reaction is rapid with 100%
conversion, with isatin bearing substituents chloro at Cꢀ7, OCF3
at Cꢀ5 and benzyl at Nꢀ1 position (Scheme 4, 4g, 4h, 4m).
20
Having the nitro substituent at Cꢀ5 position gave the
corresponding tetrazole 4k with 96% yield. Several Nꢀalkylated
isatins with substituents (methyl, benzyl, allyl, halo substituted
benzyl) at Nꢀ1 position are synthesized by adapting the reported
procedure and subjected to functionalisation at Cꢀ3 position.13
25 When Nꢀallyl isatin is employed the reaction is comparatively
slow with 83 % yield of the product (Table 2, 4j). In case of Nꢀ
methyl isatin also the reaction observed to be slow with 89%
yield (Table 2, 4i). All examples of Nꢀalkylated isatins with
electron withdrawing substituents at Cꢀ5 position, (Table 2, entry
30 12) aromatic nucleus (Table 2, entry 14, 16 & 17) and on both
(Table 2, entry 15 & 18) of the oxindole core lead to equally
satisfying results. During the reaction no by products were
observed, with a simple workup procedure the products can be
easily isolated in excellent purity. The structures of tetrazoles
35 were determined by spectroscopic analysis. Furthermore, the
structure of the Tetrazoles 4a and 5a were unambiguously
confirmed by Xꢀray crystallographic analysis (Figure 3).14
The nucleophilic addition of TMSCN 2 to isatin derived
ketimines for synthesis of 3ꢀphenyl amino 3ꢀcyano oxindoles 6 is
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very less explored.15 Next we extended the scope of the reaction
for synthesis of novel 3ꢀ(phenylamino)ꢀ3ꢀ(1Hꢀtetrazolꢀ5ꢀ
yl)indolinꢀ2ꢀone 5a from insitu generated 3ꢀphenyl amino3ꢀcyano
oxindoles
6
as dipolarophiles for the tetrazole formation
(Scheme 5). We were pleased to observe that certain substituted
amines as well as isatins were viable for direct Oneꢀpot four
component reaction for synthesis of their corresponding 3ꢀ
15 (phenylamino)ꢀ3ꢀ(1Hꢀtetrazolꢀ5ꢀyl)indolinꢀ2ꢀones 5(a-f) in good
yields.
In conclusion, we have developed a simple green method for
the synthesis of novel 3ꢀhydroxyꢀ3ꢀ(1Hꢀtetrazolꢀ5ꢀyl)indolinꢀ2ꢀ
ones 4 and 3ꢀ(phenylamino)ꢀ3ꢀ(1Hꢀtetrazolꢀ5ꢀyl)indolinꢀ2ꢀone 5
20 from readily available isatins as reaction partners. The method
has excellent generality and applicable for various substituted
isatins under “on water” conditions. Further investigation for
asymmetric version of this structural frame works is under
progress in our laboratory. The new compounds reported herein
25 could find potential application in medicinal chemistry.
5
Fig. 3 ORTEP diagram of 3ꢀhydroxyꢀ3ꢀ(1Hꢀtetrazolꢀ5ꢀyl) indolinꢀ2ꢀone
4a & 3ꢀ(phenylamino)ꢀ3ꢀ(1Hꢀtetrazolꢀ5ꢀyl)indolinꢀ2ꢀone 5a with thermal
ellipsoids set at 30% probability
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