Enantioselective [3 + 2] cycloaddition of allenes to acrylates catalyzed by dipeptide-derived phosphines: Facile creation of functionalized cyclopentenes containing quaternary stereogenic centers

Article abstract of DOI:10.1021/ja1106282

A new family of dipeptide-based chiral phosphines was designed and prepared. d-Thr-l-tert-Leu-derived catalyst 4c promoted [3 + 2] cycloaddition of allenoates to α-substituted acrylates in a regiospecific and stereoselective manner, furnishing functionalized cyclopentenes with quaternary stereogenic centers in high yields and with excellent enantioselectivities.

Full text of DOI:10.1021/ja1106282

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pubs.acs.org/JACS
Enantioselective [3 þ 2] Cycloaddition of Allenes to Acrylates
Catalyzed by Dipeptide-Derived Phosphines: Facile Creation of
Functionalized Cyclopentenes Containing Quaternary Stereogenic
Centers
Xiaoyu Han,† Youqing Wang,‡ Fangrui Zhong,† and Yixin Lu*,†
†Department of Chemistry and Medicinal Chemistry Program, Life Sciences Institute, National University of Singapore,
3 Science Drive 3, Republic of Singapore 117543
‡Provincial Key Laboratory of Natural Medicine and Immuno-Engineering, Henan University, Jinming Campus, Kaifeng,
Henan 475004, P.R. China
S Supporting Information
b
Scheme 1. Cyclopentane Structures with a Quaternary
Carbon
ABSTRACT: A new family of dipeptide-based chiral phos-
phines was designed and prepared. D-Thr-L-tert-Leu-derived
catalyst 4c promoted [3 þ 2] cycloaddition of allenoates to
R-substituted acrylates in a regiospecific and stereoselective
manner, furnishing functionalized cyclopentenes with qua-
ternary stereogenic centers in high yields and with excellent
enantioselectivities.
unctionalized five-membered carbocycles are structural motifs
Creation of quaternary stereocenters is a challenging task in
organic synthesis,12 and we recently became interested in devising
organocatalytic methods to access molecules with chiral quaternary
centers.13 Five-membered carbocycles with a quaternary stereo-
genic center are interesting substructures often found in many
natural products and bioactive molecules (Scheme 1);14 we envi-
sioned that phosphine-catalyzed [3 þ 2] annulations between R-
substituted acrylates and allenes may be utilized to construct such
five-membered ring systems. Our group has been actively inves-
tigating asymmetric organic transformations that can be promoted
by organocatalysts derived from primary amino acids in the past few
years,13a-13d,15 and thus, we have keen interest in deriving versatile
amino acid-based novel phosphines. To ensure effective chiral
communications with the substrates and to make the catalysts
readily accessible, we chose dipeptide16 as the basic chiral backbone
for our catalyst development (Scheme 2). The carboxylic acid
group can be easily converted to a phosphine, which is expected to
be highly nucleophilic as the phosphorus atom is connected to a
primary carbon. The substrate-interacting chiral pocket derived
from the dipeptide is highly tunable by simply varying the amino
acid side chains. Herein, we describe the first enantioselective [3 þ
2] cycloaddition between R-substituted acrylates and allenoates
mediated by dipeptide-based novel phosphine catalysts, creating
chiral cyclopentenes containing a quaternary stereogenic center.
We began our investigation by selecting [3 þ 2] cycloaddition
between 2-phenyl-substituted acrylate 5a and benzyl allenoate 6a as
a model reaction (Table 1). It should be noted that employment of
F
often found in natural products and medicinally important
agents.1 Among the known synthetic methods, phosphine-cata-
lyzed [3 þ 2] cycloaddition, developed by Lu in 1995,2 is
considered to be one of the most efficient synthetic approaches.
By employing electron-deficient olefins and imines, cyclopentenes
and pyrrolidines can be prepared via phosphine-catalyzed cycload-
ditions, respectively.3 The first asymmetric [3 þ 2] cycloaddition
between allenoates and acrylates catalyzed by a bicyclic chiral
phosphine was reported by Zhang in 1997.4 Recently, enantiose-
lective cyclizations of allenoates and enones were achieved by Fu5
and Miller,6 utilizing a binaphthyl-based C2-symmetric chiral
phosphine and a multifunctional phosphine-containing R-amino
acid, respectively. Jacobsen designed a series of bifunctional
phosphine-thiourea catalysts and applied them to the enantiose-
lective imine-allene annulations.7 Planar chiral 2-phospha[3]-
ferrocenophanes, introduced by Marinetti, were shown to promote
enantioselective [3 þ 2] additions of allenic esters and phospho-
nates with enones.8 Very recently, Loh discovered that commer-
cially available chiral phosphines could promote the cycloaddition
of 3-butynoates to enones.9 Zhao reported bifunctional N-acyl
amino phosphines were effective catalysts for the asymmetric [3 þ
2] cycloadditions of allenoates and activated olefins.10 Despite the
above impressive achievements, comparing to the widespread
applications of phosphine-mediated processes, the design and
development of chiral phosphine catalysts are still under-explored.
When the phosphine-catalyzed [3 þ 2] cyclizations are concerned,
acrylates remain as elusive substrates;11 thus, an enantioselective [3
þ 2] cycloaddition applicable to substituted acrylates is highly
desirable.
Received: December 6, 2010
Published: January 12, 2011
r
2011 American Chemical Society
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|

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COMMUNICATION
Scheme 2. Phosphine Catalysts Based on Dipeptides
Table 2. Optimization of Reaction Conditionsa
entry
R1/R2
R:γb
yield (%)c
ee (%)d
1
2-naphthyl/Et
94:6
96:4
91
94
95
72
92
93
95
95
92
93
84
95
93
74
84
84
24
76
81
91
80
70
67
88
91
90
Table 1. [3 þ 2] Cycloaddition of Allenoates with Acrylates
Catalyzed by Different Amino Acid-Based Phosphinesa
2
2-naphthyl/t-Bu
2-naphthyl/Ph
i-Pr/t-Bu
3
89:11
97:3
4
5
Ph/t-Bu
97:3
6
1-napthyl/t-Bu
9-phenanthryl/t-Bu
9-anthryl/t-Bu
98:2
7
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
>99:1
8
9e
10f
11g
12h
13i
9-phenanthryl/t-Bu
9-phenanthryl/t-Bu
9-phenanthryl/t-Bu
9-phenanthryl/t-Bu
9-phenanthryl/t-Bu
a Unless otherwise specified, reactions were performed with 5 (0.05
mmol), 6 (0.075 mmol), and 4c (10 mol %) in toluene (0.5 mL) at room
temperature. b Determined by 1H NMR analysis of the crude reaction
mixture. c Isolated yield. d The ee value of the major stereoisomer,
determined by HPLC analysis on a chiral stationary phase. e CH2Cl2 was
used as the solvent. f THF was used as the solvent. g The reaction was
performed at -20 °C for 5 h. h The catalyst loading was 5 mol %. i The
catalyst loading was 2 mol %.
entry
cat.
7ar:7aγb
yield (%)c
ee (%)d
1
2
3
4
5
6
7
8
1
83:17
84:16
89:11
74:26
92:8
88
90
96
90
93
93
95
96
-36
-52
-60
-57
-61
63
2
3a
3b
3c
4a
4b
4c
Finally, O-TBDPS-D-Thr-L-tert-Leu-derived 4c was found to be
the best catalyst, affording the desired adduct 7a in 96% yield, with
an R to γ ratio of 95:5 and 78% ee (entry 8).
93:7
94:6
74
95:5
78
Having identified the best catalyst 4c, we then focused on tuning
the ester moieties in acrylates and allenoates (Table 2). The tert-
butyl ester proved to be superior to other esters in the allenoate
structures, the ratio of R- to γ-isomer could be improved to 96:4,
and ee of the major isomer reached 84% (entries 1-3). Among the
different acrylate esters, 9-phenanthryl acrylate was found to be the
best, and its cycloaddition with tert-butyl allenoate led to the
formation of only R-isomer in 95% yield and 91% ee (entry 7).
Performing reactions in different solvents19 and lowering the
reaction temperature did not result in further improvement
(entries 9-11). To make the methodology more practical, catalyst
loading was further decreased. With 5 mol % 4c, the [3 þ 2]
cycloaddition could be completed within half an hour, furnishing R-
isomer in 95% yield and with 91% ee (entry 12). It should be noted
that the catalyst loading could go as low as 2 mol %, with marginally
reduced yield and enantioselectivity (entry 13).
With the optimized reaction conditions in hand, the substrate
scope of 4c-catalyzed enantioselective [3 þ 2] cycloaddition
between allenes and acrylates was examined (Table 3). Different
R-aryl-substituted acrylates could be employed, R-isomers were
regiospecifically formed, and enantioselectivities were excellent in
all the examples examined. Reactions of acrylates bearing electron-
withdrawing aryl substituents proceeded very fast, typically com-
pleting in 10 min, while longer reaction time was required for the
cyclization of the acrylate with electron-rich phenyl group at its R-
position (entry 5). The acrylate with 1- or 2-naphthyl substitution,
or disubstituted phenyl was well-tolerated for the reaction (entries
a Reactions were performed with 5a (0.05 mmol), 6a (0.075 mmol) and
the catalyst (10 mol %) in toluene (0.5 mL) at room temperature.
b Determined by 1H NMR analysis of the crude reaction mixture.
c Isolated yield. d The ee value of the major stereroisomer, determined
by HPLC analysis on a chiral stationary phase.
R-substituted acrylates in asymmetric [3 þ 2] cycloadditions is
virtually unexplored.17 For the design of effective catalysts, given
our success in threonine-based catalytic systems,15b-15e we chose
threonine as the first amino acid residue, and a number of
dipeptide-derived phosphines 2-4 were prepared. We hypothesize
judicious selection of the side chains may facilitate the dipeptide
catalyst to adopt a relatively rigid conformation, favoring its
interactions with substrates. L-Threonine-derived phosphine 1 led
to the formation of R-selective product with low ee (entry 1). On
the other hand, dipeptide-based phosphines turned out to be more
effective. L-Thr-L-Val-derived 2 led to substantially improved
results; moderate ee was attainable (entry 2). Combining L-Thr
and D-Val yielded a better catalytic system, and the ee value was
further improved to 60% (entry 3). Employment of sulfonamide as
Brønsted acid moiety18 in the catalyst did not offer better results
(entry 4), and higher R-selectivity was achieved by utilizing an even
more sterically hindered carbamate (entry 5). The catalyst struc-
tures were further tuned by engaging tert-leucine as the second
amino acid residue and varying the siloxy groups on the OH of
threonine. To make the catalyst more economical, D-Thr-L-tert-Leu
dipeptidic backbone was selected for structural elaborations.
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Journal of the American Chemical Society
COMMUNICATION
Table 3. Enantioselective Allene-Acrylates [3 þ 2] Cy-
Scheme 4. Proposed Mechanism and Transition State Model
cloadditions Catalyzed by 4ca
entry
product (R1)
7b (Ph)
time
yield (%)b
ee (%)c
1
30 min
10 min
10 min
3 h
95
96
97
81
61
87
97
96
96
94
96
92
91
91
94
93
90
87
90
94
88
90
92
80
91
68
2
7c (4-ClC6H4)
7d (4-BrC6H4)
7e (4-MeC6H4)
7f (4-OMeC6H4)
7g (4-tBuC6H4)
7h (4-CNC6H4)
7i (3-MeC6H4)
7j (2-NO2C6H4)
7k (3,4-ClC6H4)
7l (1-naphthyl)
7m (2-naphthyl)
7n (CH2Ph)
3
4
5
24 h
6
3 h
7
10 min
3 h
hydrogen-bonding interactions with the acrylate substrate. The
phosphonium enolate intermediate, generated from the nucleo-
philic attack of the phosphine catalyst at the allene, approaches
the acrylate from its Re face to yield the major stereoisomer. The
formation of the γ-regioisomer is suppressed by the unfavorable
steric interactions of the bulky tert-butyl group with the acrylate sub-
strate and the sterically hindered carbamate moiety in the cata-
lyst, which is analogous to Lu’s utilization of tert-butyl allenoate
in an R-selective cycloaddition.2e
In summary, we have developed a new family of dipeptide-based
chiral phosphines; such phosphine catalysts are highly reactive, and
their structures are easily tunable. We have also employed R-
substituted acrylates in the enantioselective cycloaddition reactions
for the first time. D-Thr-L-tert-Leu-based phosphine 4c catalyzed the
allene-acrylate [3 þ 2] cyclizations efficiently, affording functiona-
lized cyclopentenes containing quaternary stereocenters in a regio-
specific and enantioselective manner. Detailed mechanistic investiga-
tions and applications of this class of novel catalysts to other organic
transformations are currently ongoing in our laboratory.
8
9
10 min
10 min
30 min
30 min
5 h
10
11
12
13
a Reactions were performed with 5 (0.05 mmol), 6b (0.075 mmol), and
4c (5 mol %) in toluene (0.5 mL) at room temperature. b Isolated yield.
c The ee value was determined by HPLC analysis on a chiral stationary
phase.
Scheme 3. Preparation of a Spiral Oxindole Derivative
’ ASSOCIATED CONTENT
10-12). However, the use of R-alkyl-substituted acrylate resulted
in the formation of the desired product in high yield, but only with
moderate enantioselectivity20 (entry 13). The absolute configura-
tions of the cycloaddition products were determined on the basis of
the X-ray crystal structure of 7k (see the Supporting Information
for details).
The optically enriched functionalized cyclopentenes 7 are
valuable molecules, due to the importance of five-membered ring
structures in natural products and medicinal chemistry.1,14 With
the established synthetic protocols,5,21 such structures are also
attractive synthetic intermediates. As oxindoles are important
structural scaffolds in pharmaceutical industry,22 synthetic value
of the cycloaddition products was further demonstrated by
converting 7j into a spiral oxindole. As illustrated in Scheme 3,
reduction of the nitro group resulted in a spontaneous lactam
formation and yielded spiral oxindole core 8, which was readily
transformed to 9, an agent displaying interesting cytotoxic
activities.23
S
Supporting Information. Optimization of reaction con-
b
ditions, preparation of catalysts and substrates, representative
experimental procedures, X-ray crystallographic data of 7k,
HPLC chromatogram, and NMR spectra of products. This
material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
chmlyx@nus.edu.sg
’ ACKNOWLEDGMENT
This work was financially supported by the National Uni-
versity of Singapore and the Ministry of Education (MOE) of
Singapore (R-143-000-362-112). We thank Prof. Dr. Kuo-Wei
Huang for helpful discussions.
Mechanism of this reaction has not been rigorously investigated
at this stage, based on Lu’s initial proposal2a and recent excellent
mechanistic studies;24 a plausible mechanism and transition state
model are presented in Scheme 4. We propose that the dipeptidic
backbone of the catalyst adopts a conformation favoring its
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