Assembly of spirooxindole derivatives via organocatalytic iminium-enamine cascade reactions

Article abstract of DOI:10.1021/ol302876c

The assembly of complex spirocyclopentaneoxindoles via a novel organocatalytic iminium-enamine cascade process is reported. Reactions between 3-substituted oxindoles and α,β-unsaturated aldehydes catalyzed by second generation prolinol ethers provided the desired products in high yield with excellent levels of enantioselectivity in a single step.

Full text of DOI:10.1021/ol302876c

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Assembly of Spirooxindole Derivatives via
Organocatalytic Iminium-Enamine
Cascade Reactions
Klaus Albertshofer, Kimberly E. Anderson, and Carlos F. Barbas III*
The Skaggs Institute for Chemical Biology and the Departments of Chemistry and
Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road,
La Jolla, California 92037, United States
carlos@scripps.edu
Received October 18, 2012
ABSTRACT
The assembly of complex spirocyclopentaneoxindoles via a novel organocatalytic iminium-enamine cascade process is reported. Reactions
between 3-substituted oxindoles and R,β-unsaturated aldehydes catalyzed by second generation prolinol ethers provided the desired products in
high yield with excellent levels of enantioselectivity in a single step.
Highly substituted carbocyclic spirooxindole units are
often found in natural products or synthetic molecules of
pharmaceutical interest (Figure 1).1 For example, spiroin-
dolones have potential for treatment of malaria.2 The high
density of substituents on the spirocyclic ring system, often
with multiple quaternary stereocenters, makes the assembly
of these complex building blocks challenging. The enantio-
pure construction of spirocyclopentaneoxindoles has been
carried out using transition metal catalysts,3 nucleophilic
phosphine catalysts,4 and cinchona alkaloid catalysts5 and
through cycloaddition processes.6 Due to the promising
medicinal properties of these compounds, novel methods
for the diversity-oriented assembly of spirocyclopentaneox-
indoles in enantiopure forms are needed.
(1) (a) Fensome, A.; Adams, W. R.; Adams, A. L.; Berrodin, T. J.;
Cohen, J.; Huselton, C.; Illenberger, A.; Kern, J. C.; Hudak, V. A.;
Marella, M. A.; Melenski, E. G.; McComas, C. C.; Mugford, C. A.;
Slayden, O. D.; Yudt, M.; Zhang, Z.; Zhang, P.; Zhu, Y.; Winneker,
R. C.; Wrobel, J. E. J. Med. Chem. 2008, 51, 1861. (b) Shangary, S.; Qin,
D.; McEachern, D.; Liu, M.; Miller, R. S.; Qiu, S.; Nikolovska-Coleska,
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R. F.; Boeyens, J. C. A.; Holzapfel, C. W.; Steyn, P. S. J. Chem. Soc.,
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Spencer, K. R.; Gonzalez-Paez, G. E.; Lakshminarayana, S.; Goh, A.;
Suwanarusk, R.; Jegla, T.; Schmitt, E. K.; Beck, H.-P.; Brun, R.; Nosten,
F.; Renia, L.; Dartois, V.; Keller, T. H.; Fidock, D. A.; Winzeler, E. A.;
Diagana, T. T. Science (Washington, DC, U.S.) 2010, 329, 1175.
(3) (a) Trost, B. M.; Cramer, N.; Silverman, S. M. J. Am. Chem. Soc.
2007, 129, 12396. (b) Corkey, B. K.; Toste, F. D. J. Am. Chem. Soc. 2007,
129, 2764. (c) Sun, W.; Zhu, G.; Hong, L.; Wang, R. Chem.;Eur. J.
2012, 18, 13959.
Since the development of modern organocatalysis at the
turn of this century, iminiumꢀenamine reaction cascades
(4) (a) Voituriez, A.; Pinto, N.; Neel, M.; Retailleau, P.; Marinetti, A.
Chem.;Eur. J. 2010, 16, 12541. (b) Tan, B.; Candeias, N. R.; Barbas,
C. F., III J. Am. Chem. Soc. 2011, 133, 4672. (c) Zhong, F.; Han, X.;
Wang, Y.; Lu, Y. Angew. Chem., Int. Ed. 2011, 50, 7837.
(5) (a) Tan, B.; Candeias, N. R.; Barbas, C. F., III. Nat. Chem. 2011,
3, 473. (b) Li, X.; Li, Y.-M.; Peng, F.-Z.; Wu, S.-T.; Li, Z.-Q.; Sun,
Z.-W.; Zhang, H.-B.; Shao, Z.-H. Org. Lett. 2011, 13, 6160. (c) Li,
Y.-M.; Li, X.; Peng, F.-Z.; Li, Z.-Q.; Wu, S.-T.; Sun, Z.-W.; Zhang,
H.-B.; Shao, Z.-H. Org. Lett. 2011, 13, 6200. (d) Albertshofer, K.; Tan,
B.; Barbas, C. F., III. Org. Lett. 2012, 14, 1834. (e) Sun, W.; Zhu, G.; Wu,
C.; Hong, L.; Wang, R. Chem.;Eur. J. 2012, 18, 6737.
(6) (a) Tan, B.; Hernandez-Torres, G.; Barbas, C. F., III. J. Am. Chem.
Soc. 2011, 133, 12354. (b) Peng, J.; Huang, X.; Jiang, L.; Cui, H.-L.; Chen,
Y.-C. Org. Lett. 2011, 13, 4584. (c) Li, X.; Li, Y.-M.; Peng, F.-Z.; Wu, S.-T.;
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r
10.1021/ol302876c
XXXX American Chemical Society

Initially, a series of primary and secondary amine-
containing catalysts IꢀVI were tested for their ability to
promote a model reaction between 3-substituted oxindole
1a and trans-2-butenal 2a in dichloromethane (DCM) at
37 °C (Figure 3). Secondary amine-containing catalysts
provided reaction products in higher selectivity and yield
than their counterparts with primary amines. Catalysts
I10a and II12,13 did not promote the transformation in
satisfactory yields under the evaluated conditions (Table 1,
entries 1, 2). When the Takemoto-type catalyst III14 was
employed, spirocyclic oxindole 3a was obtained in 15%
yield and 92% enantiomeric excess (ee) (entry 3). Methox-
ymethylpyrrolidine IV, the first prolinol ether catalyst
developed,15 provided 3a in 18% yield and 15% ee (entry 4).
Use of the second generation diarylprolinol derivative V16
resulted in an ee of 95%, although the yield was a moderate
20% (entry 5).
Figure 1. Naturally occurring and bioactive natural products
containing the spirooxindole core structure.
have been at the center of the explosive growth in what are
now more generally termed as organocascade reactions.7
There are now numerous examples of organocascade reac-
tions that produce stereochemically complex compounds in
a single step.8 This approach has been used successfully for
the synthesis of spirocyclic oxindole derivatives.4ꢀ6,9ꢀ11
Recently, our own laboratory disclosed the assembly of spiro-
cyclic oxindoles via MichaelꢀAldol,5a MichaelꢀHenry,5d
and other organocascade strategies.4b,6a,6f Inspired by these
studies, a novel iminiumꢀenamine cascade reaction between
3-substituted oxindole derivatives and R,β-unsaturated alde-
hydes was designed (Figure 2). This strategy allowed for the
construction of molecules containing two chiral centers, one
of them a quaternary carbon center, in a single step.
Figure 2. Retrosynthetic analysis for the construction of spir-
ocyclo-3,3-oxindoles via an iminiumꢀenamine cascade reaction.
(7) Bui, T.; Barbas, C. F., III. Tetrahedron Lett. 2000, 41, 6951.
(8) For reviews, see: (a) Enders, D.; Grondal, C.; Huttl, M. R. M.
Angew. Chem., Int. Ed. 2007, 46, 1570. (b) Barbas, C. F., III. Angew.
Chem., Int. Ed. 2008, 47, 42. (c) Westemann, B.; Muhammad, A.; van
Berkel, S. S. Angew. Chem., Int. Ed. 2010, 49, 846.
Figure 3. Organocatalysts employed in the asymmetric iminiumꢀ
enamine cascade reaction.
(9) (a) Bencivenni, G.; Wu, L.-Y.; Mazzanti, A.; Giannichi, B.;
Pesciaioli, F.; Song, M.-P.; Bartoli, G.; Melchiorre, P. Angew. Chem.,
Int. Ed. 2009, 48, 7200. (b) Jiang, K.; Jia, Z.-J.; Yin, X.; Chen, Y.-C. Org.
Lett. 2010, 12, 2766. (c) Jiang, K.; Jia, Z.-J.; Chen, S.; Wu, L.; Chen,
Y.-C. Chem.;Eur. J 2010, 16, 2852. (d) Jia, Z.-J.; Jiang, H.; Li, J.-L.;
Gschwend, B.; Li, Q.-Z.; Yin, X.; Grouleff, J.; Chen, Y.-C.; Jørgensen,
K. A. J. Am. Chem. Soc. 2011, 133, 5053. (e) Liu, Y.; Nappi, M.; Arceo,
E.; Vera, S.; Melchiorre, P. J. Am. Chem. Soc. 2011, 133, 15212. (f)
Zhang, S.-L.; Xie, H.-X.; Zhu, J.; Li, H.; Zhang, X.-S.; Li, J.; Wang, W.
Nat. Commun. 2011, 2, 1214.
(10) (a) Galzernao, P.; Bencivenni, G; Pesciaioli, F.; Mazzanti, A.;
Giannichi, B.; Sambri, L.; Bartoli, G.; Melchiorre, P. Chem.;Eur. J.
2009, 15, 7846. (b) Bergonzini, G.; Melchiorre, P. Angew. Chem., Int. Ed.
2012, 51, 971.
(12) II was first reported by: Brunner, H; Bugler, J.; Nuber, B.
Tetrahedron: Asymmetry 1995, 6, 1699.
(13) For reviews on primary amine-containing cinchona alkaloids as
organocatalysts, see: (a) Chen, Y.-C. Synlett 2008, 13, 1919. (b) Xu,
L.-W.; Luo, J.; Lu, Y. Chem. Commun. 2009, 14, 1807.
(14) Mei, K.; Jin, M.; Zhang, S.; Li, P.; Liu, W.; Chen, X.; Xue, F.;
Duan, W.; Wang, W. Org. Lett. 2009, 11, 2864.
(15) Cordova, A.; Barbas, C. F., III. Tetrahedron Lett. 2002, 43, 7749.
(16) (a) Marigo, M.; Wabnitz, T. C.; Fielenbach, D.; Jørgensen,
K.-A. Angew. Chem., Int. Ed. 2005, 44, 794. (b) Hayashi, Y.; Gotho,
H.; Hayashi, T.; Shoji, M. Angew. Chem., Int. Ed. 2005, 44, 4212. For
recent reviews, see: (c) Palomo, C.; Mielgo, A. Angew. Chem., Int. Ed.
2006, 45, 7876. (d) Mielgo, A.; Palomo, C. Chem.;Asian J. 2008, 3, 922.
(e) Jensen, K. L.; Dickmeiss, G.; Jiang, H.; Albrecht, L.; Jørgensen,
K. A. Acc. Chem. Res. 2012, 45, 248.
(11) Sun, W.; Hong, L.; Wang, R. Tetrahedron: Asymmetry 2010, 50,
2493.
B
Org. Lett., Vol. XX, No. XX, XXXX

TMS-protected diphenylprolinol VI16 afforded the
desired product 3a in 96% ee and 25% yield (entry 6).
Unprotected diphenylprolinol VII7,17 did not catalyze our
model reaction, indicating the importance of a sterically
bulky hydroxyl protecting group. Inanattempt toimprove
the yield, the concentration of aldehyde 2a was decreased
incrementally from 4 to 1 equiv (entries 8ꢀ10). With
decreasing amounts of enal, the formation of 3a was
favored in the presence of catalyst VI, leading to an improved
yield of 60% with a diastereomeric ratio (dr) of 6:1 and ee of
92% (entry 10).
As catalyst VI was the most efficient of those tested, the
effect of solvent on the model reaction in the presence of
this catalyst was evaluated (see Supporting Information).
Aprotic, nonpolar solvents such as toluene were optimal
(Table 1, entry 11). Lowering the reaction temperature
reduced the yield compared to the reaction at 37 °C (entry
12). In a final attempt to increase yield and stereoselectiv-
ity, additiveswereevaluated(see Supporting Information).
Addition of benzoic acid affored 3a with 17:1 dr, 94% ee,
and 90% yield (entry 13).
with products obtained in excellent selectivity and yield in
24 h (Table 2). Enals with alkyl substituents such as pentyl,
ethyl, and methyl reacted to afford products in diast-
eremoeric ratios between 9:1 and 17:1 and enantiomeric
ratios of 94ꢀ97% (Table 2, entries 1ꢀ3). With the least
sterically hindered trans-2,3-butenal 2a, the reaction yield
was 91% (entry 3). R,β-Unsaturated aldehydes bearing no
substituent on C-3 provided reaction product 3d in 96%
yield; however, a drop of ee to 23% was observed (entry 4).
Neutral, electron-donating, and electron-withdrawing
substituents on the aryl moiety of the enal had little to no
effect on selectivity and yield. Products were obtained in
95% ee (99% ee after single recrystallization) and 9:1 dr
and yields of 93ꢀ95% (entries 5ꢀ7).
Table 2. Scope of the Reactiona
Table 1. Catalyst Screen and Optimization Studiesa
yieldb drc
product (%) (%)
eed
(%)
entry R0
R00
Pent
X
1
Me
Me
Me
Me
Me
Me
Me
Me
Me
Me
Et
H
H
H
H
H
H
H
3b
3c
3a
3d
3e
3f
81
82
91
96
95
93
95
89
92
91
94
94
92
>9:1 96
9:1 97
17:1 94
2
Et
3
Me
4
H
ꢀ
23
5
Ph
9:1 95
9:1 95 (99)f
9:1 95
10:1 95
10:1 97
14:1 94
9:1 94
6:1 80
7:1 90
6
2-NO2-Ph
2-MeO-Ph
Me
equiv of
temp
yieldb
(%)
drc
eed
(%)
7
3g
3h
3i
entry
catalyst
aldehyde
(°C)
(%)
9
5-Me
5-OMe
6-Cl
H
1
I
4
37
37
37
37
37
37
37
37
37
37
37
rt
<5
<5
15
18
20
25
<5
35
55
60
90
49
90
ꢀ
ꢀ
10
11
12
13e Ph
Me
2
II
4
ꢀ
ꢀ
Me
3j
3
III
IV
V
4
n.d.
n.d.
n.d.
n.d.
n.d.
7:1
92
15
95
96
n.d.
94
93
92
92
89
94
Me
3k
3l
4
4
Me
H
14e furyl Me
H
3m
5
4
6
VI
VII
VI
VI
VI
VI
VI
VI
4
a General reaction conditions: 3-substituted oxindole (0.1 mmol, 1 equiv),
aldehyde (0.1 mmol, 1 equiv), catalyst (10 mol %), benzoic acid (10 mol %),
toluene 0.4 mL (0.25 M); reaction time 24 h; work up employed ice cold
1 M HCl. b Isolated yield. c Determined by 1H NMR of crude product.
d Determined by chiral-phase HPLC analysis of major diastereomer.
e Reaction time 48 h, 20 mol % of catalyst, 20 mol % of benzoic acid, and
0.12 mmol, 1.2 equiv of aldehyde. f After single recrystallization.
7
4
8
2
9
1.5
1
8:1
10
11e
12e
13e,f
6:1
1
13:1
14:1
17:1
1
1
37
a Reaction conditions unless otherwise indicated: 3-substituted oxindole
(0.035 mmol, 1 equiv), aldehyde (0.144 mmol, 4 equiv), catalyst (10 mol %),
DCM (140 μL, 0.25 M); no workup. b Isolated yield. c Determined by
1H NMR of crude product. d Determined by chiral-phase HPLC analysis.
e Toluene (140 μL, 0.25 M) used as solvent. f 10 mol % benzoic acid.
To further investigate the scope of the reaction, we
focused on 3-substituted oxindoles 1bꢀg. Substituents on
the aromatic ring system of the molecule were varied.
5-Methyl, 5-O-methyl, and 6-chloro provided products
3hꢀj in high yields (89ꢀ91%) and selectivity (dr’s 10:1 to
14:1, ee 94ꢀ97%) (Table 2, entries 9ꢀ11). We further
extended the scope of the reaction from methyl-substituted
alkylketone-derived oxindoles to sterically more hindered
ethyl and aryl ketones 1eꢀg. With ethyl-substituted oxi-
ndole 1e, product 3k was obtained in 9:1 dr and 94% ee
(entry 12). Arylketone-derived oxindoles 1f and 1g af-
forded products 3lꢀm in 80ꢀ90% ee, 6:1ꢀ7:1 dr, and
The generality of the methodology developed was eval-
uated by screening a wide range of R,β-unsaturated alde-
hydes (2aꢀg) in the presence of 3-substituted oxindole 3a,
diarylprolinol silyl ether VI, benzoic acid at 37 °C. Alkyl as
well as aryl substituents were tolerated on enal reactants
(17) For a recent review on diarylprolinol catalysts, see: Lettanzi, A.
Chem. Commun. 2009, 12, 1452.
Org. Lett., Vol. XX, No. XX, XXXX
C

92ꢀ94% yield, although an increased catalyst loading of
20 mol %, 1.2 equiv of aldehyde, and an extended reaction
time of 48 h were necessary.
stepreaction to provide products with diastereoselectivities
up to 17:1 and enantioselectivities up to 99%.
In summary, a highly efficient tandem process was
developed for the enantioselective construction of complex
spirocyclopentaneoxindoles from simple starting materials
in a single step. A wide variety of spirocyclopentaneox-
indoles was obtained in high chemical and optical yield
from 3-substituted oxindoles and various R,β-unsaturated
aldehydes, demonstrating the broad scope of this trans-
formation. The absolute configuration of the products
obtained through the iminiumꢀenamine cascade process
was determined by X-ray analysis of compound 3f (see
Supporting Information). Commercially available second-
generation prolinol ethers promoted the high-yielding one
Acknowledgment. Support from the Skaggs Institute
for Chemical Biology is gratefully acknowledged. We also
thank Drs. A. L. Rheingold and C. E. Moore for X-ray
crystallographic analysis (Department of Chemistry and
Biochemistry, University of San Diego).
Supporting Information Available. Experimental pro-
cedures and compound characterization (1H NMR, 13C
NMR, HPLC) including X-ray data. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX