Phosphine-catalyzed asymmetric formal [4+2] tandem cyclization of activated dienes with isatylidenemalononitriles: Enantioselective synthesis of multistereogenic spirocyclic oxindoles

Article abstract of DOI:10.1002/adsc.201301070

The first highly enantioselective formal [4+2] tandem cyclizations between isatylidenemalononitriles and activated dienes catalyzed by bifunctional chiral phosphine catalysts have been developed, yielding multistereogenic spirocyclic oxindoles in high yie

Full text of DOI:10.1002/adsc.201301070

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DOI: 10.1002/adsc.201301070
Phosphine-Catalyzed Asymmetric Formal [4+2]Tandem
Cyclization of Activated Dienes with Isatylidenemalononitriles:
Enantioselective Synthesis of Multistereogenic Spirocyclic
Oxindoles
Fang-Le Hu,a Yin Wei,b,* and Min Shia,b,*
a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, 130 Mei Long Road, Shanghai 200237, Peopleꢀs Repuiblic of China
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
b
Sciences, 354 Fenglin Lu, Shanghai 200032, Peopleꢀs Republic of China
Fax : (+86)-21-6416-6128; e-mail: weiyin@mail.sioc.ac.cn or mshi@mail.sioc.ac.cn
Received: November 29, 2013; Published online: February 20, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201301070.
Abstract: The first highly enantioselective formal
[4+2]tandem cyclizations between isatylidenemalo-
nonitriles and activated dienes catalyzed by bifunc-
tional chiral phosphine catalysts have been devel-
oped, yielding multistereogenic spirocyclic oxin-
doles in high yields and excellent optical purity.
tivity and high to excellent levels of enantioselectivi-
ty.[7] Shortly thereafter, Barbas and Lu have inde-
pendently disclosed the enantioselective synthesis of
spirooxindoles via the [3+2]cyclization between
isatin-derived alkenes and MBH adducts.[8] Recently,
Luꢀs group developed the first highly enantioselective
[4+2]annulations between activated alkenes and a-
substituted allenoates catalyzed by amino acid-based
phosphine catalysts to construct chiral functionalized
cyclohexenes.[9] With the expanded ring size, this
strategy can lead to more stereocenters and increased
functionality. Very recently, Marinetti and co-workers
reported the [4+2]reaction between 3-arylideneoxin-
doles and d-aryl-substituted penta-2,3-dienoates in
This reaction is accomplished via
a tandem
Rauhut–Currier/Michael/Rauhut–Currier reaction
sequence.
Keywords: activated dienes; asymmetric [4+2]cycli-
zation; chiral phosphines; multistereogenic spirocy-
clic oxindoles; organocascade reactions
The spirooxindole scaffolds are commonly presented
in a variety of natural products as well as biologically
active compounds,[1] such as the spirooxindoles con-
taining cyclohexane backbones marcfortine A,[2a]
MDM2-p53 inhibitor B[2b] and gelsemine C2c]
(Figure 1). Thus, the efficient synthetic protocols
based on organocatalytic[3] and organometallic[4] cycli-
zation of isatin derivatives to construct multistereo-
genic spirocyclic oxindoles have been developed over
the past years.[5] Among these protocols, phosphine-
catalyzed cyclization of electron-poor olefins derived
from isantin with allenic esters or Morita–Baylis–Hill-
man (MBH) adducts leading to spirooxindoles have
been reported.[6] In 2010, Marinetti and co-workers
reported the first example of chiral phosphine-cata-
lyzed [3+2]annulations of activated allenes with elec-
tron-poor olefins derived from isatin, affording spiro-
Figure 1. Examples of natural and bioactive compounds con-
cyclic oxindolic cyclopentanes with good regioselec- taining spirocyclohexane oxindole scaffolds.
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Phosphine-Catalyzed Asymmetric Formal [4+2]Tandem Cyclization
Table 1. Screening of conditions for the asymmetric [4+2]cyclization.
[a]
1a (0.1 mmol), 2a (0.25 mmol) and catalyst (0.01 mmol) were stirred in 2 mL of solvent
for 12 h at room temperature.
Isolated yield of the major isomer.
[b]
[c]
Determined by chiral HPLC.
[d]
1
Determined by H NMR analysis of the crude reaction mixture.
the presence of triphenylphosphine, producing spiro- a variety of catalysts derived from an axially chiral bi-
cyclic oxindoles with six-membered rings.[10] However, naphthyl scaffold to exploit this interesting reaction
to the best of our knowledge, there is no report on and the results are summarized in Table SI-1 in the
the asymmetric organocascade [4+2] reaction of acti- Supporting Information. The bifunctional chiral phos-
vated dienes with isatylidenemalononitriles in the phine catalysts (LB1, LB2) and monofunctional chiral
presence of chiral phosphine catalysts.[11] 1,4-Dien-3- phosphines (LB3, LB4) could not catalyze this reac-
ones, which are useful multifunctional building blocks tion efficiently, only affording trace products. To our
in the synthesis of carbocyclic compounds,[12] possess delight, the bifunctional chiral phosphine catalyst
a potentially nucleophilic C=C double bond and an LB5, which was demonstrated as an efficient catalyst
electronically poor C=C double bond, allowing for in MBH or aza-MBH reaction,[14] was the best catalyst
the design of a cascade procedure to construct an all- for this [4+2] reaction, giving the corresponding prod-
carbon ring system in the presence of organocatalysts. uct 3a in 61% yield along with moderate optical
As part of our continuing efforts on seeking efficient purity (9:1 dr, 66% ee) within 12 h. Encouraged by
approaches toward the construction of the spirooxin- these promising results, a series of bifunctional chiral
dole motif,6a,6b,13] herein we wish to report an asym- phosphine catalysts LB6–LB10 with various substitu-
metric formal [4+2] reaction of activated dienes with ents on the phosphorus center were synthesized based
isatylidenemalononitriles catalyzed by chiral phos- on our previous work[6] and then tested in this
phines derived from an axially chiral binaphthyl scaf- [4+2]cycloaddition reaction, and the results are sum-
fold.
marized in Table 1. The bifunctional chiral phosphine
We initially examined the reaction of isatylidene- catalyst LB9, having methoxy groups on the 3,5-posi-
malononitrile 1a (0.1 mmol, 1.0 equiv.) with (E)-1- tions of the benzene ring, gave the best performance,
phenylpenta-1,4-dien-3-one 2a (0.2 mmol, 2.0 equiv.) affording the major product 3a in 85% yield with 18:1
catalyzed by PPh3 in toluene (2 mL) at room tempera- dr and 94% ee. However, using bifunctional chiral
ture during 24 h. It was found that the major product phosphine catalyst LB10, having the more sterically
3a, the three-component cascade product, was ob- hindered tert-butyl groups on the 3,5-positions of ben-
tained in 70% yield. Based on the preliminary investi- zene, decreased the dr value from 18:1 to 7:1 with
gation of this formal [4+2]cyclization, we screened almost the same yield and ee value. Finally, the sol-
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Fang-Le Hu et al.
Table 2. Substrate scope of the asymmetric [4+2] cyclization of 1a with 2.
[a]
1a (0.1 mmol), 2 (0.25 mmol) and LB9 (0.01 mmol) were stirred in 2 mL of toluene for
12 h at room temperature.
Isolated yield of the major isomer.
[b]
[c]
[d]
Determined by chiral HPLC.
1
Determined by H NMR analysis of the crude reaction mixture.
vent effects have been also examined using LB9 as duced at the other C=C double bond, no desired
the catalyst. In the presence of chloroform, dichloro- product was formed (Table 2, entry 12), which is pre-
methane (DCM), tetrahydrofuran (THF) or acetoni- sumably due to the fact that the activity of diene 2m
trile (MeCN), the reaction could take place, however decreases and the nucleophilicity of catalyst LB9 is
the dr value of 3a decreased remarkably (Table 1, en- no longer enough to promote this reaction. It should
tries 7–10). Therefore, the best reaction conditions be pointed out that the dr values were determined
1
have been determined in that the reaction should be from H NMR spectra of the crude reaction mixtures
catalyzed by LB9 (10 mol%) in toluene at room tem- and that the main diastereomer can be isolated by
perature within 12 h.
column chromatography.
With the optimized conditions in hand, we next fo-
Having examined the substrate generality of 2 in
cused on the examination of scope and limitations of this asymmetric [4+2] reaction, the scope of the sub-
this reaction by using isatylidenemalononitrile 1a with stituted isatylidenemalononitriles 1 with (E)-1-phenyl-
a variety of activated dienes 2. It was found that the penta-1,4-dien-3-one 2a was further examined. A vari-
reaction could proceed smoothly to give product 3 in ety of isatylidenemalononitriles 1 having either elec-
good to high yields (up to 92%) with excellent optical tron-donating or electron-withdrawing groups as sub-
purity under the standard conditions (up to >20:1 dr, stituents on the 5-, 6- or 7-position of the benzene
up to 95% ee) (Table 2, entries 1–7), irrespective of rings underwent the reactions smoothly, giving the
the variations of the electronic properties and the po- corresponding product 4 in good to high yields (up to
sitions of the substituents attached to the phenyl rings 98%) along with excellent enantioselectivities (90%–
of dienes 2. When two substituents were attached on 94% ee) and diastereoselectivities (up to >20:1)
the benzene ring of the activated diene, the corre- (Table 3, entries 1–11).
1
sponding cascade product 3m was obtained in 82%
Their structures have been determined by H and
yield with more than 20:1 dr and 94% ee (Table 2, 13C NMR spectroscopic data as well as MS and HR-
entry 8). Moreover, the dienes 2j, 2k and 2i with het- MS analyses. The absolute configuration of 4h has
eroaromatic or aliphatic rings were also suitable for been unambiguously determined by X-ray diffraction
this reaction, producing the desired products 3j, 3k to be R,R,R. The ORTEP drawing of 4h and its CIF
and 3i in high yields and excellent optical purities (up data are presented in the Supporting Information.
to >20:1 dr, 94% ee, respectively) (Table 2, entries 9–
Instead of using PPh3 as a catalyst, we also exploit-
11). Unfortunately, when the methyl group was intro- ed this cascade reaction by using 1,4-diazabicyclo-
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Phosphine-Catalyzed Asymmetric Formal [4+2]Tandem Cyclization
Table 3. Substrate scope of the asymmetric [4+2]cyclization of 1 with 2a.
[a]
1 (0.1 mmol), 2a (0.25 mmol) and LB9 (0.01 mmol) were stirred in 2 mL of toluene for
12 h at room temperature.
Isolated yield of the major isomer.
[b]
[c]
Determined by chiral HPLC.
[d]
1
Determined by H NMR analysis of the crude reaction mixture
ACHTUNGTRENNUNG[2.2.2]octane (DABCO) as a catalyst under the stan- cyclic product 6a in 95% yield with 94% ee as a single
dard conditions, giving the desired product 3aa in stereoisomer upon treatment with Et2O·BF3 in DCM
42% yield along with two-component product 3aa’ in (Scheme 2). The absolute configuration of 6a has
41% yield, although using PPh3 as the catalyst, a trace been determined to be R,R,R,S by X-ray diffraction
of 3aa’ was detected and when using LB9 as the cata- and the ORTEP drawing has been indicated in Figure
lyst, no 3aa was formed due to the fact that PPh3 and SI-2 in the Supporting Information.
LB9 are more reactive than DABCO in this reaction
Mechanistically, we propose that this asymmetric
(Scheme 1). It should be noted that 3aa was obtained [4+2]tandem cyclization might proceed via a tandem
as the same diastereoisomer as that when using PPh3 Rauhut–Currier/Michael/Rauhut–Currier reaction se-
1
as a catalyst on the basis of H NMR analysis. More- quence (Scheme 3).[15] The reaction might be initiated
over, 3aa’ could be transformed to 3aa in 85% yield with the in situ formation of a zwitterionic intermedi-
with >20:1 dr value in the presence of PPh3, suggest- ate A by nucleophilic addition of chiral phosphine
ing that 3aa is indeed derived from 3aa’ via a cascade catalyst LB9 to the vinyl ketone 2a, which undergoes
process.
the nucleophilic attack with 1a to give the corre-
To demonstrate the synthetic utility of the obtained sponding intermediate B. Subsequent intramolecular
products 3, we transformed 3a to the 1,5-diol product Michael addition reaction produces intermediate C,
5a or 5a’ in 78% total yield along with 2:1 dr via the which undergoes the proton transfer and elimination
Luche reduction method, which could afford the poly- of LB9 to give intermediate 3a’. Finally, 3a’ containing
Scheme 1. DABCO-catalyzed formal [4+2]tandem reaction of 1a with 2a.
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Fang-Le Hu et al.
Scheme 2. Representative transformations of product 3a.
Scheme 3. A plausible mechanism for the asymmetric cascade [4+2]cyclization.
an activated C=C double bond can react with 2a alyst without phenol moiety (LB4) could not catalyze
through a Rauhut–Currier reaction in the presence of this reaction at all (see Table SI-1 in the Supporting
LB9, giving the cascade product 3a. We believe that Information). The possible key transition states illus-
the phenol in LB9 plays a crucial role in this reaction trated in Figure 2 may account for the enantioselectiv-
through hydrogen bonding because the phosphine cat- ity. Presumably, the zwitterionic intermediate A is
favoured to approach 1a via the re-face due to less
steric hindrance and the potential p–p interactions of
the electron-rich Ar group of LB9 with the electron-
deficient double bond of 1a.
In summary, we have disclosed the first highly
enantioselective formal [4+2]tandem cyclization be-
tween isatylidenemalononitriles and activated dienes
catalyzed by bifunctional chiral phosphine catalysts,
yielding the multi-stereogenic spirocyclic oxindoles in
high yields along with excellent enantioselectivities
and diastereoselectivities. This reaction is proposed to
proceed via
a
tandem Rauhut–Currier/Michael/
Rauhut–Currier reaction sequence. Further efforts
will be focused on applications of this reaction for or-
ganic and medicinal synthesis.
Figure 2. Proposed transition states.
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Phosphine-Catalyzed Asymmetric Formal [4+2]Tandem Cyclization
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Experimental Section
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General Procedure
Into a 25-mL oven-dried reaction flask under an argon at-
mosphere were added (E)-2 (0.25 mmol) and 1 (0.1 mmol).
The catalyst LB9 (0.01 mmol, 6 mg) in toluene (2 mL) was
added and the reaction mixture was stirred at room temper-
ature for 12 h. After removal of the solvent under reduced
pressure, product 3 or 4 from crude reaction mixture was
1
subjected to HNMR spectroscopic analysis for the determi-
nation of the diastereoselectivity. The reaction mixture was
purified by flash chromatography on silica gel (eluent:
EtOAc:petroleum ether 1:8 to 1:4) to give the pure product
3 or 4.
Supporting Information Available
Spectroscopic data and chiral HPLC traces of the com-
pounds shown in Table 1, Table 2, Table 3 and Scheme 1, de-
tailed descriptions of experimental procedures and the crys-
tal structures are presented in the Supporting Information.
CCDC 947613 (4h) and CCDC 970096 (6a) contain the sup-
plementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
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Acknowledgements
We thank the Shanghai Municipal Committee of Science and
Technology (11JC1402600), National Basic Research Pro-
gram of China ACHTUNGTRENNUNG(973)-2010CB833302, the Fundamental Re-
search Funds for the Central Universities, and the National
Natural Science Foundation of China (21072206, 20472096,
21102166, 21372241, 21302203, 20672127, 21121062 and
20732008) for financial support.
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