Cu-mediated 1,3-dipolar cycloaddition of azomethine ylides with dipolarophiles: A faster access to spirooxindoles of potential pharmacological interest

Article abstract of DOI:10.1016/j.tetlet.2013.07.126

CuI facilitated three-component reaction of isatin derivatives, l-proline and terminal alkynes containing an amide or ester functional group. The multi-component reaction (MCR) afforded a faster and practical synthesis of spirooxindole derivatives. A range of novel spirooxindoles were synthesized by using this straightforward and one-pot efficient methodology. A representative compound showed significant inhibition of PDE4B enzyme in vitro and good interactions with this protein in silico.

Full text of DOI:10.1016/j.tetlet.2013.07.126

Tetrahedron Letters xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Cu-mediated 1,3-dipolar cycloaddition of azomethine ylides with
dipolarophiles: a faster access to spirooxindoles of potential
pharmacological interest
Shambhu Nath Singh a,b, Sridhar Regati a, Abir Kumar Paul a, Mohosin Layek a, Sarva Jayaprakash a,
K. Venkateshwara Reddy b,c, Girdhar Singh Deora d, Soumita Mukherjee e, Manojit Pal e,
⇑
a Custom Pharmaceuticals Services and Dr. Reddy’s Laboratories Ltd, Bollaram Road, Miyapur, Hyderabad 500049, India
b Department of Chemistry, JNTUH College of Engineering, JNT University, Kukatpally, Hyderabad 500085, India
c CMR Engineering College Affiliated to JNTU, Medchal Road, Kandlaykoya, Hyderabad 501401, India
d Biomolecular Pharmaceutical Chemistry Laboratory, School of Pharmacy, The University of Queensland, 20 Cornwall Street, Woolloongabba, Qld 4102, Australia
e Dr. Reddy’s Institute of Life Sciences, University of Hyderabad Campus, Gachibowli, Hyderabad 500046, India
a r t i c l e i n f o
a b s t r a c t
Article history:
CuI facilitated three-component reaction of isatin derivatives, L-proline and terminal alkynes containing
Received 21 June 2013
Revised 22 July 2013
Accepted 24 July 2013
Available online xxxx
an amide or ester functional group. The multi-component reaction (MCR) afforded a faster and practical
synthesis of spirooxindole derivatives. A range of novel spirooxindoles were synthesized by using this
straightforward and one-pot efficient methodology. A representative compound showed significant inhi-
bition of PDE4B enzyme in vitro and good interactions with this protein in silico.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
MCR
CuI
Spirooxindole
PDE4B
The unique structural features of spiro heterocycles and the
presence of spiro linkage as a basic skeleton in several natural
products have increased their importance to a great extent. The
naturally occurring spiro heterocycles for example spirooxindoles
(with five membered nitrogen containing ring) present in horsfi-
line1 (analgesic activity), spirotryprostatins A2 (inhibitors of mam-
malian cell cycle at G2/M phase), and elacomine3 showed
interesting biological properties (Fig. 1). Mitraphylline (Fig. 1), a
natural compound, containing the spirooxindole framework pos-
sesses anti-tumor activity against human brain cancer cell lines
and malignant glioma GAMG.4 Rhynchophylline, another natural
product is used as antipyretic, anti-hypertensive, and anti convul-
sant medications for the treatment of headache, vertigo, and epi-
lepsy.5 The oxindole class of compounds attracted our particular
attention due to their impressive PDE4 inhibitory properties re-
ported earlier. For example, the PDE4 inhibitor A (Fig. 2) signifi-
cantly reduced antigen-induced bronchoconstriction in animal
models and in asthmatic patients.6 This prompted us to synthesize
and assess the PDE4 inhibitory properties of novel spiro derivatives
of oxindoles B (Fig. 2) possessing an amide/ester at C-20. We antic-
ipated that this group may facilitate the interactions of B with
PDE4B. To the best of our knowledge PDE4 inhibitory properties
of B have not been reported in the literature earlier.
The most common synthetic routes reported so far for the syn-
thesis of spiro cyclic compounds include alkylation methods, rear-
rangement based approaches, ring closure of geminally substituted
compounds, radical cyclizations, organometallic processes (metals
include Rh, Pd, Cu, Ir etc.,), organocatalytic approaches, cleavage of
bridged ring systems, or cycloaddition reactions.7 Recently, the
H
H
N
HO
N
O
O
MeO
Horsfiline
N
NH
Elacomine
Me
O
H
H
O
O
H
HN
N
O
H
N
HN
N
CO2Me
H
O
HO
Spirotryprostatin A
Mitraphylline
⇑
Corresponding author. Tel.: +91 40 6657 1500; fax: +91 40 6657 1581.
E-mail address: manojitpal@rediffmail.com (M. Pal).
Figure 1. Structure of natural products containing spirooxindole framework.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.126
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2
S. N. Singh et al. / Tetrahedron Letters xxx (2013) xxx–xxx
Table 1
OMe
O
Effect of reaction conditions on three-component reaction of 1a, 2, and 3aa
N
R3OC
R1
N
H2NOC
O
NH2
Solvent
O
O
O
OH
NH
O
N
N
N
O
R2
+
+
O
80oC
O
N
H
N
O
B
H
A
2
3a
4a
1a
Figure 2. Known PDE4 inhibitor A and our target spirooxindoles B.
Entry Solvent
Catalyst
Time (h) Temp (°C) Yieldb (%)
1
2
3
4
5
6
7
DMF
EtOH
CH3CN
CH3CN
CH3CN
CH3CN
No catalyst 24
No catalyst 24
No catalyst 24
80
80
80
80
80
80
80
60
55
70
92
78
75
32
multi component reaction or MCR strategy has been successfully
adopted by many researchers for the synthesis of spirooxindoles.
This reaction involved 1,3-dipolar cycloaddition of azomethine
ylide generated in situ with different dipolarophiles in a single
pot. The azomethine ylides were efficiently generated by the reac-
CuI
CuBr
CuCl
2
8
12
6
i-PrOH:H2O (1:1) CuI
tion of isatin with
of the spirooxindole pyrrolidine ring by three-component coupling
of isatin, -proline, and 1,4-naphthoquinone in methanol under
L-proline. Chen et al. have reported the synthesis
a
Reactions were carried out using 1a (1.0 mmol), 2 (1.0 mmol), and propiola-
mide (3a) (1.0 mmol) in a solvent (15 mL) under nitrogen.
L
b
Isolated yield.
ultrasound.8 Kang and co-workers reported a similar type of cou-
pling in 1,4-dioxane by varying the dipolarophile for the synthesis
of spiropyrrolidine oxindoles.9 They used dimethyl maleate instead
of 1,4-naphthoquinone. The synthesis of spiropyrrolidine was also
reported by using terminal and internal alkynes as dipolarophiles
in acetonitrile by Pardasani et al.10 that required a relatively longer
reaction time for example 20 h. Moreover, the use of only one ter-
minal alkyne that is phenyl acetylene was examined in their study.
Herein, we report a faster and efficient method leading to our tar-
Table 2
Synthesis of spirooxindoles 4 via Cu-mediated MCR of 1, 2, and 3a
R3
O
R3OC
R1
O
OH
NH
O
N
CuI
O
+
+
R1
O
N
MeCN
N
R2
80 o
C
R2
get spiropyrrolidine oxindoles B or 4 (Scheme 1) using isatin (1),
L
-
2
3
4
1
proline (2), and terminal alkynes (3) containing ester or amide sub-
stituents in the presence of catalytic CuI in acetonitrile. Notably the
use of alkyne 3 in a similar MCR is not common in the literature.
Initially, the coupling reaction was performed by heating a mix-
Entry
Isatin derivative
Alkyne
Time (h)
Product
4
Yieldb (%)
(R1, R2=) 1
(R3=) 3
1
2
3
4
H, H
1a
1a
NH2
3a
NHMe
3b
NHEt
3c
3a
2.0
2.0
2.0
2.5
4a
4b
4c
4d
92
90
91
85
ture of isatin (1a), L-proline (2), and propiolamide (3a) in DMF at
80 °C. After 24 h, the spiro compound 4a was obtained in 60% yield
(Table 1, entry 1). To improve the product yield the reaction was
carried out in different solvents like ethanol (Table 1, entry 2)
and acetonitrile (Table 1, entry 3). In case of ethanol, the yield de-
creased to 55%, but improved in acetonitrile (70%). However, the
duration of the reaction was not satisfactory for a quick access to
the compound library related to 4a. After screening a range of cat-
alysts a major improvement in yield (92%) as well as reaction time
(2 h) was observed when catalytic amount of CuI was added to the
reaction mixture in acetonitrile (Table 1, entry 4). While the reac-
tion proceeded in the presence of other copper salts for example
CuBr and CuCl, the yield of product 4a was poor (Table 1, entries
5 and 6) in these cases. The MCR was found to be less effective
in an aqueous media for example in 1:1 i-PrOH/H2O when 4a
was obtained only in 32% yield (Table 1, entry 7). Thus based on
the observation that CuI in acetonitrile decreased the reaction time
from 20 to 2 h, the combination of CuI and acetonitrile was used for
our further study.
1a
5-Br, H
1b
1b
1b
5,7-Di-NO2, H
1c
5-F, H
1d
1a
5
6
7
3b
3c
3a
2.5
2.0
2.0
4e
4f
4g
82
85
90
8
9
3a
2.5
3.0
4h
4i
80
81
OEt
3d
3d
3d
3a
10
11
12
1b
1c
2.5
2.5
3.0
4j
4k
4l
80
85
75
H, –CH2C6H4Cl-m
1e
a
Reactions were carried out using isatin (1) (1.0 mmol), proline (2) (1.0 mmol),
alkyne (3) (1.0 mmol), and CuI (0.01 mmol) in acetonitrile (15 mL) at 80 °C under
nitrogen.
b
Isolated yield.
The scope and generality of the reaction were further tested by
performing the reactions using a range of isatin derivatives (1) and
terminal alkynes (3) containing various carbonyl functionalities
(Table 2).11 Substituents such as Br, F, NO2, and aryl group on the
isatin ring were well tolerated. The terminal alkynes employed
contained an N-unsubstituted or substituted amide (e.g. NH2,
NHMe, or NHEt) or an ester moiety. The reaction proceeded well
in all these cases affording desired spirooxindoles 4 in good to
excellent yield.
All the spirooxindole derivatives (4a–l) synthesized were char-
acterized by their 1H and 13C NMR, IR, and mass spectral data. The
sharp peak at ꢀ3300 cmꢁ1 in IR, a singlet at 10.0 ppm in 1H NMR
correspond to the spirooxindole NH group. Moreover, a sharp IR
absorption at ꢀ1640 cmꢁ1 and appearance of a quaternary carbon
signal at ꢀ178.0 ppm in the 13C NMR indicated the presence of the
C@O group. While a sharp IR absorption at ꢀ1735 cmꢁ1 and a
signal at ꢀ161.0 ppm in the 13C NMR indicated the presence of
the ester carbonyl group (e.g. 4i–k), the amide derivatives (4a–h)
R3
O
R3OC
R1
O
OH
NH
O
N
CuI
O
+
+
R1
O
N
MeCN
80 oC
N
R2
R2
2
3
4
1
Scheme 1. Cu-mediated faster synthesis of spirooxindoles 4.
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S. N. Singh et al. / Tetrahedron Letters xxx (2013) xxx–xxx
3
H2'
1'
EtO2C
H3'
H3'
regioselectivity of the reaction was addressed by NOE experiment
2'
2'
4'
4'
0
using 4i (Fig. 4). The ring juncture proton of 4i that is H3 that ap-
3'
3'
EtO2C
H1'
peared as a triplet of doublet at 4.43 ppm in the 1H NMR spectra
was irradiated to examine its relationship with other nearby pro-
tons. It was found that the intensity of the proton at 7.21 ppm
and the multiplet at ꢀ2.0 ppm for the C-40 aliphatic protons was
increased (Fig. 4). This observation suggested that (i) the proton
at 7.21 ppm is the olefinic one and (ii) the regioisomer isolated
was 4i–I and not 4i–II (because in case of 4i–II the intensity of
3
3
N
2
N
1'
2
O
O
N
H
N
H
1
1
4i-I
4i-II
Figure 3. The possible regioisomers of compound 4i.
0
the olefinic proton (H1 ) was not expected to be affected during
showed IR absorption at ꢀ1640 cmꢁ1 and a carbonyl signal at
ꢀ163.0 ppm in the 13C NMR. Based on the fact that the methodol-
ogy may lead to the formation of regioisomeric products for exam-
ple 4i–I and 4i–II of a representative compound 4i (Fig. 3), the
0
the NOE experiment as it was not adjacent to H3 ).
A plausible mechanism that reconciles the structure of the syn-
thesized spirooxindole derivatives 4a–1 has been outlined in
Scheme 2. The reaction proceeds via the formation of an imine
Figure 4. 1H NMR spectra and NOE experiment of compound 4i.
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4
S. N. Singh et al. / Tetrahedron Letters xxx (2013) xxx–xxx
intermediate E-1 as a result of the reaction between zwitterionic L-
I
OH
H2O
proline 5 and isatin 1. This step was assisted by CuI which was
regenerated after the conversion of E-1 to a spirocyclic intermedi-
ate E-2. Elimination of CO2 from E-2 generated the dipolar azome-
thine ylide E-3 which on cycloaddition with dipolarophile that is
the terminal alkyne 3 in the presence of CuI afforded the desired
spirooxindole 4. A theoretical study10 on azomethine ylide E-3
(R1 = R2 = H) indicated that it has a planar structure where the pro-
line ring is planar instead of having an envelope shape, and co-pla-
nar with the isatin moiety. Thus, the cycloaddition of alkyne 3 on
the planar azomethine ylide can occur from either side leading to
the formation of product 4 having three chiral centers that is the
possibility of generation of 23 = 8 isomers. Once again based on
earlier theoretical studies10 the generation of four isomers as a re-
sult of front side attack was ruled out. Similarly, two isomers aris-
ing from the back side attack can also be ruled out as their
formation does not follow a concerted mechanism. Moreover, fur-
ther theoretical calculations favored an endo approach leading to
the (3S)-4 isomer instead of (3R)-4.
All the spirooxindoles (4) synthesized were tested for their
PDE4 inhibitory properties in vitro using PDE4B enzyme assay.12
Notably, recent studies have indicated that among the four sub-
types of PDE4 for example A, B, C, and D, the PDE4B subtype is
linked to inflammatory cell regulation.13 It was therefore hypothe-
sized that inhibition of the PDE4B may provide a means to achieve
efficacy while potentially mitigating the adverse effects.14 We have
used a known inhibitor rolipram15 as a reference compound in our
assay. Among all the compounds tested, 4k showed >40%
1
R1
CO2
O
CO2 Cu
+
N
N
H
7
N
CuI
H
H
R2
5
6
O
O
N
I
N
CuI
CO2
O
OCu
R1
O
R1
O
N
N
R2
R2
E-2
E-1
R3
O
O
C
N
R2
N
N
N
R3OC
H 3
CuI
R1
R1
O
O
N
N
R1
4
R2
R2
E-3
Scheme 2. Proposed mechanism for the Cu-mediated MCR leading to 4.
inhibition of PDE4B when tested at 30 lM. To understand its inter-
action with the PDE4B protein, docking studies were performed
using co-crystal structural coordinates of PDE4B from the protein
data bank (PDB) and the (3S)-isomer of 4k. The in silico study
showed that both C@O groups of 4k were involved in two strong
hydrogen bonds with His 278 and Met 347 residues of the active
site. Additionally, a few hydrophobic interactions with hydropho-
bic clamp residue of Q-pocket were also observed. Both nitro
groups of the molecule facilitate the deep insertion in the enzyme
pocket (Fig 5). The glide score of ꢁ5.6 obtained for 4k (see Supple-
mentary data, Table S-1) indicates its good interaction with the
PDE4B protein. Notably, 4k showed selectivity toward PDE4B over
D in silico as indicated by its docking score ꢁ3.3 obtained during
its interaction with PDE4D (see Supplementary data, Fig. S4 and
Table S-2).
In conclusion, we have reported the first Cu-mediated one-pot
three-component reaction of isatin derivatives,
L-proline, and
terminal alkynes leading to spiropyrrolidine oxindoles in good
yields. This remarkably faster and operationally simple one-pot
methodology seemed to have advantages over the previously
reported methods. This also demonstrates the first use of terminal
alkynes containing an ester/amide group in the synthesis of a
range of spirooxindoles. A representative compound 4k showed
significant inhibition of PDE4B when tested in vitro. Docking
studies indicated that both the carbonyl and nitro groups of this
molecule played key roles in the interactions with the PDE4B
protein. Overall, the spirooxindole framework described here
represents a new template for the identification of novel inhibi-
tors of PDE4 and the synthetic methodology could be useful in
constructing a library of molecules related to spirooxindoles of
potential pharmacological interest.
Acknowledgments
The authors thank Dr. Kishore and his team for PDE4 assay.
S.N.S. thanks Dr. Vilas Dahanukar, Dr. Upadhya Timmanna and
the analytical group of DRL. S.M. thanks CSIR for a Research
Associate fellowship.
Figure 5. Binding modes and interactions of molecule 4k with the inhibitor binding
site of PDE4B.
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S. N. Singh et al. / Tetrahedron Letters xxx (2013) xxx–xxx
5
9. Xie, Y.-M.; Yao, Y.-Q.; Sun, H.-B.; Yan, T.-T.; Liu, J.; Kang, T.-R. Molecules 2011,
16, 8745.
Supplementary data
10. Pardasani, R. T.; Pardasani, P.; Chaturvedi, V.; Yadav, S. K.; Saxena, A.; Sharma, I.
Heteroat. Chem. 2003, 14, 36.
11. General procedure for the preparation of 4: CuI (0.01 mmol) was added to a
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013.
07.126.
mixture of isatin (1.0 mmol), L-proline (1.0 mmol) and alkyne (1.0 mmol) in
acetonitrile (15 mL) and the reaction mixture was stirred at 80 °C for 2–3 h
under nitrogen. After completion of the reaction (monitored by TLC) the
solvent was then removed under reduced pressure and the residue was
purified by column chromatography on silica gel using 50% hexane-ethyl
acetate to afford the desired compound. Spectral data of selected compounds:
compound 4a; off white solid; mp: 260–263 °C; lH NMR (400 MHz, DMSO-d6):
d 10.0 (s, 1H), 7.50 (s, 1H), 7.20–7.15 (m, 1H), 7.00–6.70 (m, 5H), 4.45–4.35 (m,
1H), 2.50–2.40 (m, 2H), 2.05–1.95 (m, 1H), 1.80–1.45 (m, 3H); 13C NMR
(100 MHz, DMSO-d6): d 178.5, 163.8, 143.2, 139.1, 137.7, 128.8, 127.3, 125.4,
120.4, 109.4, 76.9, 71.6, 47.8, 30.6, 26.8; IR (KBr): 3275, 2873, 1663, 1598,
1472, 1384, 1205, 745 cmꢁ1; m/z (ES): 270.12 (M+1, 100%); HRMS: m/z [M+H]
calcd for C15H16N3O2: 270.1243; found: 270.1240. Compound4b; orange solid;
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d 10.0 (s, 1H), 8.00 (d,
J = 4.9 Hz, 1H), 7.20ꢁ7.15 (m, 1H), 6.95–6.85 (m, 3H), 6.75 (d, J = 7.3 Hz, 1H),
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