Hydrogen-Bonding Network Promoted [3+2] Cycloaddition: Asymmetric Catalytic Construction of Spiro-pseudoindoxyl Derivatives

Article abstract of DOI:10.1002/asia.201600013

The enantioselective construction of a spirocyclic quaternary stereogenic carbon center at the C2 position of indole has long been an elusive problem in organic synthesis. Herein, by employing a rationally designed hydrogen-bonding network activation strategy, for the first time, 2,2′-pyrrolidinyl-spirooxindole, which is a valuable and prevalent indole alkaloid scaffold, was directly obtained through a catalytic asymmetric [3+2] cycloaddition reaction with high yields and excellent stereoselectivities. Spiro of the moment: 2,2′-pyrrolidinyl-spirooxindole for the first time was directly obtained through catalytic asymmetric reaction with high yields and excellent stereoselectivities through a rationally designed hydrogen-bonding network activation strategy.

Full text of DOI:10.1002/asia.201600013

DOI: 10.1002/asia.201600013
Communication
Asymmetric Synthesis
Hydrogen-Bonding Network Promoted [3+2] Cycloaddition:
Asymmetric Catalytic Construction of Spiro-pseudoindoxyl
Derivatives
Liang-Jie Zhang,[a] Yao Wang,[b] Xiu-Qin Hu,*[a] and Peng-Fei Xu*[a]
as mitragynine pseudoindoxyl,[2] fluorocurine,[3] diketopipera-
zine,[4] and rauniticine,[5] display important and diverse biologi-
Abstract: The enantioselective construction of a spirocyclic
cal properties.[6] For instance, mitragynine pseudoindoxyl was
quaternary stereogenic carbon center at the C2 position
of indole has long been an elusive problem in organic
found to have potent opioid agonistic activity, anti-viral and
anti-cancer properties.[2,7] To date, the established approaches
synthesis. Herein, by employing a rationally designed hy-
drogen-bonding network activation strategy, for the first
to spiro-pseudoindoxyl skeleton are mainly racemic syntheses
time, 2,2’-pyrrolidinyl-spirooxindole, which is a valuable
and prevalent indole alkaloid scaffold, was directly ob-
and oxidative rearrangement of indole derivatives has been
employed as the primary strategy to construct this structural
unit.[2b,3b,8] Among these significant advances, Glorius and co-
tained through a catalytic asymmetric [3+2] cycloaddition
reaction with high yields and excellent stereoselectivities.
workers elegantly developed an enantioselective N-heterocy-
clic carbene (NHC)-catalyzed annulation of enals with azaaur-
ones to synthesize spiro-pseudoindoxyl (Figure 2).[9] Consider-
The spiro-pseudoindoxyl scaffold has been found in a wide
range of indole alkaloids.[1] In particular, as shown in Figure 1,
some alkaloids incorporating a spiro ring fusion at the 2-posi-
tion of the oxindole skeleton with a pyrrolidinyl moiety, such
ing the remarkable achievements in the construction of 3,3’-
pyrrolidinyl-spirooxindole alkaloids that also show promising
biological activities,[10] it will naturally be an ideal and direct
method for the asymmetric synthesis of spiro-[pseudoindoxyl-
2,3’-pyrrolidine] through 1,3-dipolar cycloadditions of azome-
thine ylides[11] with azaaurone. However, to our surprise, de-
spite numerous efforts towards constructing spiro-pseudoin-
doxyl, the catalytic asymmetric approach to valuable spiro-
[pseudoindoxyl-2,3’-pyrrolidine] derivatives is still not available
(Figure 2). This can be attributed to two reasons: 1) the special
structure and low reactivity of azaaurone,[12] which can be rec-
ognized as a combination of electron-donating enamine and
electron-withdrawing a,b-unsaturated ketones; and 2) the diffi-
culty of constructing a spiro quaternary C2 carbon center with
a pyrrolidinyl moiety. In this context, the development of
a new activation strategy for catalytic asymmetric synthesis of
spiro-[pseudoindoxyl-2,3’-pyrrolidine] has been a highly desira-
ble yet challenging task.
Design Plan: Recently, we have developed a series of meth-
ods involving cycloaddition reactions and cascade reactions for
the efficient synthesis of complex molecules and construction
of diverse scaffolds.[13] In continuing with our research pro-
gram, we then questioned whether or not it is possible to de-
velop a direct [3+2] cycloaddition reaction of azaaurone with
azomethine ylides for the synthesis of spiro-[pseudoindoxyl-
2,3’-pyrrolidine]. Indeed, in the presence of representative cat-
alyst, cinchona-thiourea 4a, the reaction of aromatic aldimine
1’ with (Z)-1-acetyl-2-benzylideneindolin-3-one 2a was unsuc-
cessful in toluene at room temperature even after 120 h
(Scheme 1). By considering the special structure and low reac-
tivity of azaaurone, it is expected that the construction of the
pyrrolidinyl moiety incorporated spiro quaternary carbon
center is not trivial. When methanol was used as the solvent,
Figure 1. Indole alkaloids containing the 2,2’-pyrrolidinyl-spirooxindole core
structure.
[a] L.-J. Zhang, X.-Q. Hu, Prof. P.-F. Xu
State Key Laboratory of Applied Organic Chemistry
College of Chemistry and Chemical Engineering
Lanzhou University
Lanzhou 730000 (China)
E-mail: xupf@lzu.edu.cn
[b] Prof. Y. Wang
School of Chemistry and Chemical Engineering
Shandong University
Jinan, 250100 (China)
Supporting information for this article can be found under http://
dx.doi.org/10.1002/asia.201600013.
Chem. Asian J. 2016, 11, 834 – 838
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Figure 2. Different approaches and proposed method for the synthesis of spirooxindole.
To our delight, the corresponding product was ob-
tained as expected with 68% yield and moderate
enantioselectivity in toluene (Table 1, entry 1). En-
couraged by this positive result, we then screened
a series of bifunctional catalysts in toluene at room
temperature. As shown in Table 1, all of the catalysts
4a–4g could give the desired product 3a with mod-
erate-to-high yields. However, only low-to-moderate
enantioselectivities were obtained (Table 1, entries 1–
Scheme 1. Initial attempt.
the desired product was obtained albeit with only 11 % yield.
We realized that extra hydrogen-bonding with the substrate is
critical to the success of the designed [3+2] cycloaddition.[14]
Phenols have been demonstrated to be a unique component
in assembling some supramolecular catalysts, which showed
remarkably new reactivity through noncovalent interactions.[15]
We envisioned that the introduction of a phenol group in the
substrate for assembling a hydrogen-bonding network would
significantly activate the whole catalytic system (Figure 3).
7). Further screening revealed that catalyst 4i could afford the
desired product with 79% yield and good stereoselectivities
(>20:1 dr; 89% ee) (Table 1, entry 9). Upon reducing the cata-
lyst loading to 10 mol%, nearly the same good result was ob-
tained. However, the use of 5 mol% catalyst 4i led to a signifi-
cant decrease in reaction yield (Table 1, entries 10 and 11). The
optimization of solvent showed that 1,2-dichloroethane (DCE)
was the optimal selection (Table 1, entries 12–16). The enantio-
selectivity decreased dramatically when methanol and THF
Figure 3. Design of the reaction.
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Communication
sired products were obtained with
good yields albeit with lower enantio-
selectivities (Table 2, entries 13 and
14). Furthermore, substituted o-hy-
droxy aromatic aldimines could be
applied to this reaction and products
were obtained with good yields and
stereoselectivities (Table 2, entries 17
and 18). The absolute configuration
of the product was determined by X-
ray crystallographic analysis of 3l (see
the Supporting Information).[16]
Table 1. Optimization of the reaction conditions.[a]
As shown in Scheme 2, in the pres-
ence of 20 mol% para-toluenesulfonic
acid (p-TSA), 3a reacted with parafor-
maldehyde to afford polycyclic com-
pound 5 in moderate yield with excel-
lent diastereoselectivity and enantio-
selectivity (64% yield, >20:1 d.r. and
94% ee, see the Supporting Informa-
tion).
When pure (Z)-azaaurone was used
as the substrate, we observed that
there was an equilibrium between
(Z)- and (E)-azaaurone in the reaction
system. Then we performed control
experiments using pure (Z)- and (E)-
azaaurone, respectively (Scheme 3
and see the Supporting Informa-
tion).[17] The results revealed that
under the optimal reaction condi-
tions, the same product 3a was
formed with similar yield and stereo-
selectivity (Scheme 2). In the presence
of catalyst 4i, substarte 1a, and (Z)-
or (E)-azaaurone form complex TS-
1 and TS-2, respectively. Due to steric
interaction, TS-2 is favored and prod-
uct 3a was generated.
Entry
Catalyst
Catalyst [mol%]
Sovlent
t [h]
Yield [%][b]
d.r.[c]
ee [%][d]
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4 f
4g
4h
4i
4i
4i
4i
4i
20
20
20
20
20
20
20
20
20
10
5
10
10
10
10
10
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DCM
72
72
72
72
72
72
72
72
72
72
72
72
72
96
96
48
68
82
23
38
58
81
76
73
79
80
56
78
83
16
26
90
>20:1
13:1
>20:1
>20:1
1:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
4:1
45
51
15
21
9
13
67
87
89
89
90
91
92
45
66
21
9
10
11
12
13
14
15
16
DCE
THF
CH3CN
CH3OH
4i
4i
4i
In summary, by employing a hydro-
gen-bonding network activation strat-
egy, 2,2’-pyrrolidinyl-spirooxindole for
the first time could be directly ob-
tained through a catalytic asymmetric
[a] Unless otherwise noted, all reactions were performed using 1a (0.1 mmol) and 2a (0.12 mmol) in sol-
vent (1.0 mL) at room temperature. [b] Isolated yield. [c] Determined by 1H NMR spectroscopy. [d] Deter-
mined by HPLC using a chiral stationary phase.
were used (Table 1, entries 14 and 16). This observation is con-
sistent with our design plan as these solvents could affect the
hydrogen-bonding network between catalyst and substrates.
Based on the above results, it is the optimal choice to conduct
the reaction in DCE at room temperature in the presence of
catalyst 4i.
[3+2] cycloaddition reaction. The desired products containing
three stereocenters including one spiro quaternary chiral
center at the C2 position of indoles were generally obtained
with high yields and excellent stereoselectivities. This finding is
an important advancement for the synthesis of spirooxindoles.
With the optimized reaction conditions, the substrate scope
of the reaction was then explored (Table 2). In general, good-
to-excellent yields and diastereo- and enantioselectivities were
achieved with a variety of electron-donating or electron-with-
drawing substituents on the phenyl ring (up to 84% yield,
>20:1 d.r. and 95% ee; Table 2, entries 1–12). With azaaurones
possessing a furan or piperonyl aldehyde substituent, the de-
Experimental Section
Chemicals and solvents were either purchased from commercial
suppliers or purified by standard techniques. Analytical thin-layer
chromatography (TLC) was performed on silicycle silica gel plates
with F-254 indicator and the compounds were visualized by irradi-
ation with UV light. Flash chromatography was carried out utilizing
Chem. Asian J. 2016, 11, 834 – 838
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Communication
column delta eluting with DCM and n-hexane or
AD-H column delta eluting with iPrOH and n-
hexane solution. Optical rotation was measured on
the PerkinElmer 341 polarimeter with [a]D values re-
ported in degrees; concentration (c) is reported in
Table 2. Scope of the cycloaddition reaction.[a]
g 100 mLÀ1
.
General procedure for the synthesis of prod-
ucts 3
Entry R1
R2
R3
t [h]
3
Yield [%][b] d.r.[c]
ee [%][d]
The solution of organocatalyst 4i (0.01 mmol,
10 mol%), azaaruone 2 (0.12 mmol, 1.2 equiv), o-hy-
droxy aromatic aldimine 1 (0.1 mmol, 1.0 equiv) in
dry DCE (1.0 mL) was prepared and stirred at room
temperature. After 1 disappeared, as monitored by
TLC, the crude mixture was purified by flash chro-
matography on silica gel to afford product 3.
1
2
3
4
5
6
7
8
H
H
H
H
H
H
H
H
H
H
H
3-BrPh
4-BrPh
4-ClPh
4-FPh
4-CNPh
4-CF3Ph
2-MePh
3-MePh
4-MePh
4-MeOPh
4-C(CH3)3Ph
H
H
H
H
H
H
H
H
H
H
H
68
72
72
72
32
72
72
64
72
72
72
3b
3c
3d
3e
3 f
3g
3h
3i
79
81
81
82
70
79
64
80
81
64
81
>20:1 90
>20:1 95
>20:1 93
>20:1 92
>20:1 92
>20:1 90
>20:1 86
>20:1 91
>20:1 94
>20:1 93
>20:1 90
9
10
11[e]
3j
3k
3l
Synthesis of compound 5
To a solution of 3a (54.2 mg, 0.1 mmol, 1.0 equiv)
and paraformaldehyde (12 mg, 0.4 mmol, 4.0 equiv)
in CH3CN (1.0 mL) was added p-TSA (3.4 mg,
20 mol%) and the reaction was stirred at room tem-
perature for five hours (monitored by TLC). After
evaporation of the solvent in vacuo, the residue
was purified via flash column chromatography
(silica gel, pentane/ethyl acetate=4:1 as eluent) to
afford the product 5.
12
H
H
H
Cl
H
H
H
72
72
72
3m 84
>20:1 93
>20:1 80
>20:1 77
13[f]
14
2-furyl
3n
3o
76
80
15
16
17
18
Ph
H
H
60
58
38
3p
3q
3r
77
84
83
84
>20:1 80
>20:1 92
>20:1 90
>20:1 87
Me Ph
H
H
Ph
Ph
5-Cl
4-MeO 72
3s
Acknowledgements
[a] The reaction was carried out with 1 (0.1 mmol), Z-2 (0.12 mmol), and 4i (10 mol%)
in DCE (1.0 mL) at room temperature. [b] Isolated yield. [c] Determined by 1H NMR spec-
troscopy. [d] Determined by HPLC analysis using a chiral stationary phase. [e] The start-
ing material was used as an Z/E mixture (17:1). [f] The starting material was used as an
Z/E mixture (1:1).
We are grateful to the NSFC (21172097,
21202070, 21302075 and 21372105), the Interna-
tional S&T Cooperation Program of China
(2013DFR70580), the National Natural Science
Foundation from Gansu Province of China (no.
1204WCGA015), and the “111” program from MOE of P. R.
China.
Keywords: [3+2] cycloaddition · azomethine ylides · hydrogen
bonding · quaternary carbons · spiro-pseudoindoxyl
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Manuscript received: January 5, 2016
Accepted Article published: January 26, 2016
Final Article published: February 16, 2016
Chem. Asian J. 2016, 11, 834 – 838
www.chemasianj.org
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