Core-structure-motivated design of a phosphine-catalyzed [3 + 2] cycloaddition reaction: Enantioselective syntheses of spirocyclopenteneoxindoles

Article abstract of DOI:10.1021/ja110147w

A novel organocatalytic asymmetric [3 + 2] cycloaddition reaction between methyleneindolinones and allylic compounds yielding complex spirocyclopentaneoxindoles has been developed. It provides extraordinary levels of enantioselective control involving a c

Full text of DOI:10.1021/ja110147w

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pubs.acs.org/JACS
Core-Structure-Motivated Design of a Phosphine-Catalyzed [3 þ 2]
Cycloaddition Reaction: Enantioselective Syntheses of
Spirocyclopenteneoxindoles
Bin Tan,† Nuno R. Candeias,†,‡ and Carlos F. Barbas, III*,†
†The Skaggs Institute for Chemical Biology and Departments of Chemistry and Molecular Biology, The Scripps Research Institute,
10550 North Torrey Pines Road, La Jolla, California 92037, United States
‡Faculdade de Farmꢀacia, Universidade de Lisboa, Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal
S Supporting Information
b
ABSTRACT: A novel organocatalytic asymmetric [3 þ 2]
cycloaddition reaction between methyleneindolinones and
allylic compounds yielding complex spirocyclopentaneox-
indoles has been developed. It provides extraordinary levels
Figure 1. Natural products containing the spirocyclopentaneoxindole
of enantioselective control involving a chiral phosphine as a
scaffold.
nucleophilic organocatalyst. Simple precursors were used
under mild conditions to construct oxindole derivatives with
high enantiopurity and structural diversity. This method
should be useful in medicinal chemistry and diversity-
oriented syntheses of these intriguing compounds.
Scheme 1. Core-Structure-Motivated Strategy for Spirocy-
clopentaneoxindole Scaffold Synthesis
nspired by Williams and co-worker’s recent pioneering synth-
eses of prenylated indole alkaloids and the asymmetric total
I
synthesis of (þ)- and (-)-versicolamide B,1 we have developed a
simple and efficient approach to the construction of the spir-
ocyclopentaneoxindole scaffold featured in a large number of
natural products2 and drug candidates3 (Figure 1). The potential
clinical significance of these enantiomerically pure backbones has
led to a demand for efficient methods for their synthesis. The
regioselective and stereoselective preparation of biologically
relevant spirooxindoles, which contain several chiral centers
including the stereogenic carbon at C3, is challenging and has
been addressed in part in elegant studies4 founded on a few
asymmetric transformations, such as cycloaddition processes5
and the intramolecular Heck reaction.6 Thus, developing new,
highly enantioselective methods for the direct construction of
this skeleton is very important.
we envisioned that spirooxindole skeletons with three stereo-
centers could be constructed through [3 þ 2] cycloaddition
reactions between modified allylic compounds and methylenein-
dolinones catalyzed by chiral phosphine catalysts (Scheme 1).
Herein we report the successful implementation of this
strategy to provide spirocyclopenteneoxindole derivatives with
high enantiopurity and significant opportunities for structural
diversification.
We initiated our studies by evaluating the reaction between
Boc-protected methyleneindolinone 1a and Morita-Baylis-
Hillman carbonate 2a using the triarylphosphine (R)-BINAP
as the catalyst in dichloromethane (DCM) at room temperature
(Table 1, entry 1). No product was obtained after 24 h, and 1a
was completely recovered. We assumed that the lack of reaction
was due to steric hindrance and the weak nucleophilic ability of
the phosphine. Therefore, we turned our attention to more active
diphenylphosphine catalysts (entries 2-4). With DIOP as the
catalyst, the desired cycloaddition product was obtained in
modest yields (entry 2). On the basis of previous reports by
Jacobsen14 and Zhao,15 we then evaluated some bifunctional
phosphine catalysts without success (entries 3 and 4).
Biphosphines16 such as DuPhos derivatives (entry 6) were
promising catalysts. (þ)-Et-BPE containing a trialkylphosphine
moiety proved to be the best candidate for further optimization
In recent years, organocatalytic7 enantioselective domino/
cascade reactions8,9 have been employed for the synthesis of
increasingly complex enantiomerically enriched molecules.
Although a wide variety of phosphine-catalyzed reactions have
been discovered,10 relatively few enantioselective variants have
been reported,11 and to date there has been only one report
concerning the enantioselective synthesis of spirocyclopente-
neoxindoles via phosphine catalysis.11l Recently, Lu and co-
workers reported annulation reactions of Morita-Baylis-Hill-
man carbonates using a modified allylic phosphonium ylide
strategy that provides a reactive functionalized three-carbon
synthon,12a-e and Zhou reported the asymmetric intramolecular
cyclization of these carbonates and olefins with phosphine
catalysts.12g Encouraged by these results and the fact that
methyleneindolinones are highly reactive Michael acceptors,5,13
Received: November 19, 2010
Published: March 11, 2011
r
2011 American Chemical Society
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Table 1. Screening of Reaction Conditionsa
Table 2. Generality of Organocatalytic [3 þ 2] Cycloaddition
Reactionsa
a All reactions were carried out using 1c (0.1 mmol, 1 equiv) and 2 (0.15
mmol, 1.5 equiv) with 10 mol % (þ)-Ph-BPE in DCM at room temperature
(22°C). b Isolated yields of the major products. Other diastereoisomers were
detected by NMR analysis of the crude products. Some of drs were estimated
as follows: entry 3, dr = 4.1:1:0.2:0.1; entry 7, dr = 3.7:1.0:0.7:0.04; entry 8,
dr = 2.3:1.0:0.6:0.1; entry 9, dr = 2.0:1.0:0.2:0.2. c Determined by chiral-phase
HPLC analysis. d For the procedure, see the SI.
Scheme 2. Effect of Methyleneindolinone Substituents
a Unless otherwise specified, all reactions were carried out using
methyleneindolinone 1a, 1b, or 1c (0.05 mmol, 1 equiv) and allylic
compound 2a (0.075 mmol, 1.5 equiv) with 20 mol % phosphine catalyst
at 22 °C. b Isolated yields of the major products. Other diastereoisomers
were detected by NMR analysis of the crude product. c Determined by
chiral-phase HPLC analysis. d The reaction was conducted at þ4 °C.
e The reaction was carried out using 1c (0.1 mmol, 1 equiv) and 2a (0.15
mmol, 1.5 equiv) with 10 mol % catalyst.
(91-99% ee). The position and electronic properties of the
substituent on the aromatic ring had little or no effect on the
stereoselectivity. Neutral (entry 1), electron-withdrawing (entries
2-6), and electron-donating (entry 7) groups as well as substrates
substituted at different positions on the aromatic ring were good
substrates. A substrate with a heteroaromatic group also reacted,
affording the corresponding spirocyclopenteneoxindole derivative
with excellent enantioselectivity (entry 8). The use of a methyl
Morita-Baylis-Hillman carbonate failed to yield the desired
product with high enantioselectivity (entry 9).
Further exploration of the substrate scope focused on the
methyleneindolinone. As shown in Scheme 2, different substit-
uents in the methyleneindolinone and different esters and
ketones yielded the desired products with good selectivity. The
absolute configuration of 3m was determined by X-ray analysis
and used together with NMR analysis to establish the relative and
absolute configurations of our compounds (see the SI).
To further demonstrate the substrate scope and improve the
diastereoselectivity, some Boc-protected methyleneindolino-
nes were tested. However, lower temperatures were needed
to achieve the excellent enantioselectivities observed for the
amide-protected methyleneindolinones. Boc-protected bispiro-
cyclopenteneoxindoles were obtained in good yields and excel-
lent enantioselectivities in reactions performed at -20 °C for
24 h (Scheme 3). The absolute configuration of 3s was
(entry 7), as with this catalyst the desired product was obtained
with good enantioselectivity (86% ee).
We envisioned that the aromatic π-π stacking interaction
between the catalyst and substrate might have some effect on the
stereoselectivity. In the presence of (þ)-Ph-PBE, the yield was
dramatically improved relative to that of the reaction catalyzed by
(þ)-Et-BPE, although the enantioselectivity dropped slightly
(entry 8). When the protecting group on substrate 1 was Bn
rather than Boc, the ee of the reaction catalyzed by (þ)-Ph-BPE
was improved significantly (entry 9). These results led to design
of a substrate containing an amide protecting group, which we
hypothesized would form a six-membered ring through hydro-
gen bonding. With this substrate (1c), the desired product was
obtained in good yield and excellent enantioselectivity (entry
10), demonstrating that interactions between the substrate and
catalyst are crucial. Evaluation of other solvents led to no
significant change in the results (entries 11 and 12). Similar
results were also obtained when the catalyst loading was
decreased to 10 mol % (entry 14).
The generality of the [3 þ 2] cycloaddition process was
investigated by using various Morita-Baylis-Hillman carbo-
nates (Table 2). Most of the reactions evaluated were completed
within 2 h with good yields (63-85%) and enantioselectivities
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Scheme 3. Boc-Protected Methyleneindolinones
Figure 2. 31P NMR experiments.
Scheme 6. Effect of the Methyleneindolinone Carbonyl
Group
Scheme 4. Deprotection of the Amide Moiety
Scheme 7. Proposed Mechanism
Scheme 5. Control Experiment for Mechanism Studies
determined by X-ray analysis (see the SI) and found to be in
accordance with that of the amide-protected products.
In order to demonstrate the synthetic utility of this method,
we developed conditions for deprotection of the amide-pro-
tected spirocyclopenteneoxindoles. We were delighted to ob-
serve that using KOH and silica gel allowed the amide moiety to
be deprotected without affecting the ester functionalities or
changing the enantioselectivity (Scheme 4).
While the mechanism of this reaction is not completely eluci-
dated, we believe the second phosphine has a major impact on the
stereoselectivity, as evidenced by the reaction with a monopho-
sphine analogue catalyst, DPDP (Scheme 5). We propose the
following mechanism based on previous reports of phosphine-
catalyzed annulation reactions (Scheme 7).12b,c The reaction is
initiated by displacement of the carbonate moiety by the phosphine
via an addition-elimination mechanism,12a which is followed by
deprotonation to generate ylide B. Regioselective nucleophilic
attack on methyleneindolinone 1 by B activated by the second
phosphine, with subsequent ring closing via intramolecular con-
jugate addition, affords intermediate D. The second phosphine
moiety in the catalyst seems to interact with the carbonyl of the
methyleneindolinone, since a strong interaction between the cata-
lyst and 1c (in comparison with 1d) was observed by 31P NMR
spectroscopy (Figure 2). This interaction may explain the lower
enantioselectivity obtained for 1d (Scheme 6). Alternatively, a
reversible addition to 1c might occur. Nucleophilic attack of the
carbon bonded to the phosphine would be disfavored because of
steric hindrance between the phosphine and the carboxylic ester
of 1. Finally, elimination of the phosphine completes the catalytic
cycle, yielding the product 3 and the free catalyst. While further
studies need to be performed in order to more fully understand all
aspects of the asymmetric induction in this reaction, the trans
diastereoselectivity between the carboxylic ester and the phenyl
moieties can be explained by the steric hindrance between the two
groups in the first nucleophilic attack in going from B to C. While
excellent enantioselectivities were observed for both Boc- and
amide-protected oxindoles, the presence of the amide protecting
group allowed the reaction to proceed at higher temperatures,
probably as a result of the formation of a six-membered ring through
intramolecular hydrogen bonding between the amide and the
carbonyl of the oxindole, locking the conformation of the substrate.
In summary, we have developed a highly efficient organoca-
talytic [3 þ 2] cycloaddition reaction for the direct construction
of spirocyclopenteneoxindole derivatives containing three chiral
centers, including one spiro quaternary chiral center. When phos-
phine was used as the nucleophilic organocatalyst, this straightfor-
ward process provided the products with excellent enantioselectivity
(up to 99% ee) from simple starting materials and under mild
conditions. This strategy will be useful in medicinal chemistry and
diversity-oriented synthesis. We believe it to be likely that novel
compounds based on spirocyclopenteneoxindole skeletons, such as
those prepared here, will provide novel therapeutic agents and useful
biological tools.
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b
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’ AUTHOR INFORMATION
Corresponding Author
carlos@scripps.edu
’ ACKNOWLEDGMENT
Research support from the Skaggs Institute for Chemical
Biology is gratefully acknowledged. N.R.C. thanks Fundac-~ao
para a Ci^encia e Tecnologia (SFRH/BPD/46589/2008) for
financial support. We also thank Dr. A. L. Rheingold for the
X-ray crystallographic analysis.
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