Access to spirocyclic oxindoles via N-heterocyclic carbene-catalyzed reactions of enals and oxindole-derived α,β-unsaturated imines

Article abstract of DOI:10.1021/ol3008028

A diastereoselective access to β-lactam fused spirocyclic oxindoles and related compounds bearing all carbon spiro centers is described. This N-heterocyclic carbene-catalyzed process employed challenging β,β-disubstituted α,β-unsaturated imines to react with enals.

Full text of DOI:10.1021/ol3008028

ORGANIC
LETTERS
2012
Vol. 14, No. 9
2382–2385
Access to Spirocyclic Oxindoles via
N-Heterocyclic Carbene-Catalyzed
Reactions of Enals and Oxindole-Derived
r,β-Unsaturated Imines
Kun Jiang, Bhoopendra Tiwari, and Yonggui Robin Chi*
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical
Sciences, Nanyang Technological University, Singapore 637371, Singapore
robinchi@ntu.edu.sg
Received March 29, 2012
ABSTRACT
A diastereoselective access to β-lactam fused spirocyclic oxindoles and related compounds bearing all carbon spiro centers is described. This
N-heterocyclic carbene-catalyzed process employed challenging β,β-disubstituted r,β-unsaturated imines to react with enals.
Spirocyclic oxindole is a class of unique scaffolds widely
present in naturally occurring alkaloid-type products1 and
synthetic molecules2 of interesting bioactivities (Figure 1).
The synthesis of spirocyclic oxindoles has received con-
siderable attention employing various methods such as
Heck reactions,3 cyclopropane expansions,4 DielsꢀAlder
reactions,5 and [3 þ 2] cycloadditions.6 Recentlywithsmall
organic molecule catalysts, a set of spirocyclic oxindole
core structures has been constructed by the groups of
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r
10.1021/ol3008028
Published on Web 04/23/2012
2012 American Chemical Society

Barbas,7 Melchiorre,8 Chen,9 Gong,10 Williams,11 Scheidt,12g
Lu,12h Wang,12d,i and others12 using secondary amine, cinch-
ona alkaloid, phosphine, or phosphoric acid catalysts. With
N-heterocyclic carbene (NHC) catalysis,13 the reactions of
aldehydes, enals, or ketenes with isatins as activated ketone
electrophiles can afford hetero-spirocyclic oxindoles
(oxindole β- or γ-lactones), as disclosed by the groups of
Nair14 and Ye.15 Built on the development of NHC-cata-
lyzed enal activations by Bode, Glorius, Scheidt, Nair,
Rovis, You, and others,16 we are interested in the catalyti-
cally generated enal-derived homoenolate intermediates
pathway is similar to that reported by Bode et al. in related
reactions between enals and β-monosubstituted unsatu-
rated imines.16i
Table 1. Optimization of a Model Reactiona,b
entry
solvent
t (h)
yieldc (%)
drd
1e
2
THF
THF
24
1
0h
93
78
0h
97
97
82:18
86:14
3
CH2Cl2
2
4
CH3CN
24
24
4
5f
6g
THF/CH3CN (1:1)
THF/CH3CN (3:1)
92:8
95:5
a Reaction conditions: 1a (0.12 mmol), 2a (0.10 mmol), B (0.02 mmol),
solvent (1.0 mL) at rt. b For the results of the reactions on 0.5 and
1.0 mmol scales of 2a, see ref 19. c Isolated yield of combined diaster-
eomers (3a and 3a0) based on 2a. d dr (3a:3a0) determined via 1H NMR of
unpurified reaction mixture; relative stereochemistry of both diastereo-
mers was determined via X-ray structures of 3a and 3a0.18 e Used
NHC precatalyst A. f THF/CH3CN (1.0 mL þ 1.0 mL). g THF/CH3CN
(1.5 mL þ 0.5 mL). h No detectable formation of product via TLC and 1H
NMR analysis of crude reaction mixture.
Figure 1. Examples of natural products and synthetic inhibitors
containing spiro oxindole scaffolds.
containing three reactive carbons to build relatively sophis-
ticated molecules.17 Here we report an NHC-catalyzed
synthesis of spiroindoles containing two quaternary carbons
with one all carbon spiro center (Table 1). The catalytic
reaction involves a cascade process of the three consecutive
reactive carbons of enals and oxindole-derived β,β-disub-
stituted R,β-unsaturated imines. The postulated reaction
Our initial studies using cinnamaldehyde (1a) and oxi-
ndole-derived β,β-disubstituted unsaturated imine 2a
as model substrates are briefed in Table 1. The imidazo-
lium-based precatalyst A, previously used in enal
activations,13,14,16aꢀ16c,16j,16k,16m,16p was not effective in
this reaction (Table 1, entry 1). When the triazolium-based
catalyst B was used, the desired spiroindole product 3a
could be obtained in excellent isolated yield with 82:18 dr
(Table 1, entry 2). Given the fact that simple β,β-disub-
stituted R,β-unsaturated imines or ketones were unreactive
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2383

solvents could significantly affect the reaction yields
and diastereoselectivities (see the Supporting Information).
Finally, by using a mixture of THF and CH3CN as the
solvents and DBU as the base, the reaction could afford 3a
with 97% yield and 95:5 dr (Table 1, entry 6).18,19
Table 2. Synthesis of Spirocyclic Oxindole 3a
We next evaluated the scope of the reactions with respect
to both the enal and the unsaturated imine substrates for
the formation of the spiro bicyclic lactam products 3
(Table 2). Enals with β-(hetero)aryl substituents all gave
the products in excellent yields and dr (Table 2, 3bꢀh). One
notable deviation was that when β-aryl enal with a strong
electron-donating substituent (MeO) on the phenyl group
was used, a drop in yield was observed (Table 2, 3f).
β-Alkyl enals reacted as well, albeit with relatively lower
diastereoselectivity (3i and 3j). The imines (2) bearing both
electron-withdrawing and -donating functionalities (R2)
were found to be excellent substrates and gave the desired
products in high yields and diastereoselectivities (3kꢀo).
Reactions were slower with imines having R3 as electron-
withdrawing substituents (3pand 3q) and required a higher
catalyst loading compared to that using an imine with an
electron-donating methyl substituent (Table 2, 3r).
Our attempt to realize asymmetric reactions led to mod-
erate enantioselectivites (Scheme 1). The sterically hindered
β,β-disubstituted R,β-unsaturated imines (as well as the
analogous enones) pose relatively low reactivities and have
remained as challenging substrates in NHC catalysis. The
currently explored chiral NHC catalysts (designed mainly
for substrates such as β-monosubstituted enones and un-
saturated imines)13,16dꢀ16g,16i,16l,16o showed moderate enan-
tioselectivities in our reactions after extensive optimizations.
The product 3a was obtained in 89% yield and 51% ee using
the aminoindanol-derived catalyst D (Scheme 1).
3
1, R1
2, R2, R3
time (h) yield (%)b drc
3b 4-BrC6H4
3c 3-FC6H4
2a
2a
3
3
93
89
85
93
69
94
76
90
93
96
94
90
86
77
72
70
91
9/1
9/1
9/1
8/1
7/1
9/1
6/1
2/1
2/1
12/1
9/1
12/1
8/1
6/1
9/1
4/1
9/1
3d 3-OMe, 4-AcOC6H3 2a
4
3e 4-MeC6H4
3fd 4-OMeC6H4
3g 2-naphthyl
3h 2-furyl
3i Me
2a
4
2a
4
2a
3
2a
6
2a
2
3j n-Pr
3k 1a
2a
1
4-BrC6H4, H
4-ClC6H4, H
4-FC6H4, H
4-PhC6H4, H
4-MeC6H4, H
Ph, Br
Ph, Cl
3
3l 1a
4
3m 1a
3
3n 1a
3
3o 1a
2
3pe 1a
24
24
2
3qe 1a
3r 1a
Ph, Me
a Unless otherwise noted, reactions were performed with 1 (0.12 mmol),
2 (0.1 mmol), B (0.02 mmol), DBU (0.02 mmol), and THF/CH3CN
(1.5 mL þ 0.5 mL). b Isolated yield (with a trace of minor diastereomers
in some cases). c Determined via 1H NMR of unpurified reaction
mixture. d B (0.03 mmol), DBU (0.03 mmol). e B (0.04 mmol), DBU
(0.04 mmol).
toward enals in NHC catalysis (see the Supporting In-
formation), we were very delighted to see this challenging
homoenolate addition to the sterically hindered β-carbon
of 2a to generate an all carbon quaternary center. Further
condition optimizations indicated both the bases and
Scheme 1. Enantioselective Synthesis of 3a
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The β-lactam adduct 3a, stable at room temperature and
with typical handlings (such as SiO2 chromatography),
(17) Fang, X.; Jiang, K.; Xing, C.; Hao, L.; Chi, Y. R. Angew. Chem.,
Int. Ed. 2011, 50, 1910–1913.
(18) The relative configurations of 3a, 3a0, 6, and 8 were determined
via X-ray analysis (see the Supporting Information). CCDC 865692 (3a),
CCDC 865695 (3a0), CCDC 865694 (6), and CCDC 865693 (8) contain
the supplementary crystallographic data.
(19) The reactions of 2a on 0.5 mmol (208 mg) and 1.0 mmol (416 mg)
scale under the conditions as in Table 1 (entry 6) gave the desired product
3a in 84% and 81% yields, respectively, and 9:1 dr in both cases.
(20) For elegent enantioselecitive approaches to this type of oxindole
spirocyclopentenes, see: (a) Voituriez, A.; Pinto, N.; Neel, M.; Retailleau,
P.; Marinetti, A. Chem.;Eur. J. 2010, 16, 12541–12544. (b) Tan, B.;
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50, 7837–7841.
2384
Org. Lett., Vol. 14, No. 9, 2012

was found to undergo a facile conversion tocyclopentene20
4a (Table 3) at elevated temperatures. We later found that
isolation of the lactam 3a was not necessary; the NHC
catalytic reactions carried out at 50 °C efficiently afforded
4a (Table 3). This cyclopentene-forming reaction likely
proceeded through a process similar to the decarboxyla-
tions observed in the related β-lactones derived from
β-monosubstituted enones and enals.16d,f,h,l Both elec-
tron-rich and -deficient (hetero)aryl and alkylenals reacted
smoothly with various unsaturated imines to produce
spirocyclopenteneꢀoxindoles in good yields and diaster-
eoselectivities (Table 3).
Scheme 2. Synthetic Transformations of 3a
Table 3. Synthesis of Spirocyclopentene Oxindole 4a
an unexpected rearrangement of 3a was observed to give
pyrrolidinone 8 as a single diastereomer containing one
quaternary carbon center. Such pyrrolidine derivatives are
expected to have some applicatons in catalysis or as syn-
thetic building blocks. The exact pathway of this transfor-
mation remains unclear at this point.
In summary, we have developed a diastereoselective
method for a facile access to spirocyclic oxindoles contain-
ing two quaternary carbons including an all carbon spiro
center. The reactions proceeded through enal-derived NHC-
bounded homoenolate intermediates bearing three consecu-
tive reactive carbons. Oxindole-derived R,β-unsaturated imi-
nes with β,β-disubstituents were used as the substrates.
Ongoing work in our laboratory includes the evaluation of
other challenging (e.g., sterically demanding) substrates un-
der NHC catalysis, and the development of new catalytic
strategies to control the enantioselectivities for this class of
substrates that lead to products with chiral quaternary
(spiro)carbon centers.
4
1, R1
2, R2, R3
2a
time (h)
yieldb (%)
drc
4a
4b
4c
4d
4e
4f
1a
9
9
93
92
91
52
62
66
61
63
91
9/1
4-BrC6H4
4-MeC6H4
2-furyl
Me
2a
13/1
12/1
6/1
2a
9
2a
9
2a
9
6/1
1a
4-BrC6H4,H
4-MeC6H4,H
Ph, Cl
Ph, Me
7
17/1
9/1
4g
4hd
4i
1a
9
1a
24
2
13/1
9/1
1a
a Reaction conditions: 1 (0.12 mmol), 2 (0.10 mmol), B (0.02 mmol),
DBU (0.02 mmol), THF/CH3CN (1.0 mL þ 1.0 mL). b Isolated yield (with
trace of minor diastereomers in some cases). c Determined via 1H NMR of
unpurified reaction mixture. d B (0.04 mmol), DBU (0.04 mmol).
Acknowledgment. We acknowledge the generous finan-
cial support from the Singapore National Research Foun-
dation (NRF), Singapore Economic Development Board
(EDB), GlaxoSmithKline (GSK), and Nanyang Techno-
logical University (NTU).
A few potentially useful synthetic transformations of the
β-lactam products (3) were also examined (Scheme 2). For
example, hydrolysis of the lactams (e.g., 3a) under basic
conditions could give protected cyclic β-amino acids (e.g., 5)
that are privileged building blocks for non-natural
peptidic mimics (foldamers) of interesting properties.21
Under acidic conditions, a retro-alkene amination adduct
6, a substituted spiro cyclopentene, was obtained in good
yield. On reduction with NaBH4, 3a produced cyclic
γ-amino alcohol 7. Under the conditions of Na/naphthalene,
Supporting Information Available. Experimental pro-
cedures and spectral data for all new compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(21) For a review, see: Cheng, R. P.; Gellman, S. H.; DeGrado, W. F.
Chem. Rev. 2001, 101, 3219–3232.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 9, 2012
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