Iridium-Catalyzed Enantioselective and Diastereoselective Allylation of Dioxindoles: A One-Step Synthesis of 3-Allyl-3-hydroxyoxindoles

Article abstract of DOI:10.1021/acs.orglett.8b02655

An iridium-catalyzed asymmetric allylation of dioxindoles, 3-hydroxyoxindoles, regulated by prosthetic groups has been accomplished under mild conditions. The methodology is applicable to a diverse array of 3-hydroxyoxindole and cinnamyl acetate substrates. A range of 3-allyl-3-hydroxyoxindoles containing vicinal tetrasubstituted and trisubstituted stereocenters can be efficiently synthesized in one-step with excellent enantioselectivity (up to >99% enaniomeric excess (ee)) and good diastereoselectivity (up to 11:1 diastereomeric ratio (dr)).

Full text of DOI:10.1021/acs.orglett.8b02655

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Iridium-Catalyzed Enantioselective and Diastereoselective Allylation
of Dioxindoles: A One-Step Synthesis of 3‑Allyl-3-hydroxyoxindoles
†
†
†
†
,‡
,†
Rui He, Shanchao Wu, Haoming Tang, Xiaohong Huo, Zhenliang Sun,* and Wanbin Zhang*
†Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, Shanghai
Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China
‡6
of China
th People’s Hospital South Campus, Shanghai Jiao Tong University, 6600 Nanfeng Road, Shanghai 201499, People’s Republic
*
S Supporting Information
ABSTRACT: An iridium-catalyzed asymmetric allylation of dioxindoles, 3-hydroxyoxindoles, regulated by prosthetic groups
has been accomplished under mild conditions. The methodology is applicable to a diverse array of 3-hydroxyoxindole and
cinnamyl acetate substrates. A range of 3-allyl-3-hydroxyoxindoles containing vicinal tetrasubstituted and trisubstituted
stereocenters can be efficiently synthesized in one-step with excellent enantioselectivity (up to >99% enaniomeric excess (ee))
and good diastereoselectivity (up to 11:1 diastereomeric ratio (dr)).
5
3
-Substituted 3-hydroxy-2-oxindoles have been attracting
molecules, are expected to be effective substrates for use in
transition-metal-catalyzed asymmetric allylic alkylation (AAA),
which is one of the most powerful methodologies for the
increased interest, because of their potential biological activities
and their role as key precursors in the synthesis of bioactive
molecules and natural products. Among those oxindole
derivatives, 3-allyl-3-hydroxyoxindoles as versatile building
blocks have been widely utilized for the preparation of alkaloids
and clinical pharmaceuticals. Accordingly, over the decades,
many useful methods have been reported for the synthesis of
optically active oxindole derivatives containing a single stereo-
center. However, methods to simultaneously form vicinal qua-
1
6
construction of C−C bonds. However, this transformation is
challenging, because dioxindoles possess multiple nucleophilic
sites and a new bond must be formed between two sterically
hindered prochiral carbon atoms with control of both absolute
and relative configurations. Recently, Kesavan et al. have
developed an umpolung strategy that employs a 3-OBoc-
oxindole as a nucleophile in a Pd-catalyzed allylic substitution
2
3
4
4f
ternary and tertiary stereocenters remain largely undeveloped.
reaction to access 3-allyl-3-hydroxyoxindoles (see Scheme 1d).
Recently, Krische et al. divulged the first stereoselective
crotylations of isatin, which was achieved by isopropanol-
mediated transfer hydrogenation (see Scheme 1a); however,
However, 1,3-diaryl-2-propenyl acetates lack substrate diversity
and this approach requires the use of 3-hydroxy-protected
oxindoles. In order to obtain the free tertiary alcohol, removal of
the protecting groups is required, thus adding an additional step
to the synthetic sequence. Considering substrate diversity and
synthetic efficiency, a universally applicable AAA of unprotected
4a
only α-methyl allyl acetate was employed as a crotyl donor.
Oestreich et al. have revealed highly stereoselective BF ·OEt -
3
2
mediated allylations of isatin with an R-chiral allylic stannane
see Scheme 1b). Conversely, allylation of any substituted isatin
using identical reaction conditions proved to exhibit poor
3
3
-hydroxyoxindoles is greatly desired for the preparation of
-allyl-3-hydroxyoxindoles.
Ir-catalyzed AAA of prochiral nucleophiles provides an
(
4b
diastereoselectivities. The most significant limitation of the
methodology is that the specific R-chiral allylic stannane, which is
not particularly stable, must be prepared as the single nucleophile.
Gong et al. have reported a highly diastereoselective Pd-catalyzed
carbonyl allylation of isatins directly using simple acyclic olefins as
effective and reliable method to obtain products with vicinal
7
,8
stereocenters. In recent years, our group has already reported
several Ir-catalyzed reactions between prochiral enolates and
allyl electrophiles for the synthesis of products containing two
8
4e
vicinal stereocenters. Herein, we describe a mild, one-step Ir-
allylating reagents (see Scheme 1c). Regrettably, the enantio-
selectivities of the reactions cannot be controlled. Hence,
identification of suitable nucleophile and electrophile counter-
parts is very important for accessing 3-allyl-3-hydroxyoxindoles.
catalyzed allylation of unprotected dioxindoles to access 3-allyl-
3
-hydroxyoxin-doles containing vicinal tetrasubstituted and
3
-Hydroxyoxindoles, which have been demonstrated to be
Received: August 20, 2018
effective nucleophiles for the synthesis of structurally diverse
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02655
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 1. Synthesisof 3-Allyl-3-hydroxyoxindoles Containing
Vicinal Tetrasubstituted and Trisubstituted Stereocenters
trisubstituted stereocenters in good yields and with excellent
stereoselectivities (see Scheme 1e).
Initially, 3-hydroxy-1-methyloxindole (1a) was selected as the
model substrate for the allylic substitution (Table 1). To test the
effect of the leaving group (LG) on the allyl moiety in this reac-
tion, various cinnamyl alcohol derivatives (entries 1−4 in Table 1)
were subjected to the reaction conditions. The reaction conducted
with cinnamyl acetate (2c) gave 3 with the best stereoselectivity,
with a diastereomeric ratio (dr) of 3:1 and 95% enantiomeric
excess (ee) (entry 3 in Table 1). However, the target compound
was not generated when using cinnamyl bromide (2d) (entry 4
in Table 1). It was reasoned that both the leaving ability of the
LG and the basicity of the resulting groups affected the reactivity
and stereoselectivity of the transformation. Raising the temper-
ature favored greater conversion, but led to a lower stereo-
selectivity (entry 5 in Table 1). When the reaction temperature
was reduced to 0 °C, none of the desired product was detected
(
entry 6 in Table 1). Encouragingly, the presence of additional
bases resulted in good reactivity without reducing the reaction
stereoselectivity (entries 7−11 in Table 1), and TEA was chosen
as the optimal additive. An increase in reactant concentration by
adjusting the volume of solvent led to significantly improved
reaction yield (entries 12 and 13 in Table 1). The yield could be
further increased to 85% or 87% by prolonging the reaction time
to 24 h or 36 h, respectively (entries 14 and 15 in Table 1).
Finally, phosphoramidite ligand L1 was changed to L2 (entry 16
in Table 1), bearing 2-methoxyphenyl substituents, and the
desired product was obtained in 91% yield while retaining
acceptable diastereoselectivity (3:1 dr) and excellent enantio-
9
selectivity (95% ee).
a
Table 1. Optimization of the Reaction Conditions
b
c
d
entry
2
temperature, T (°C)
base
time, t (h)
yield (%)
diastereomeric ratio, dr
enantiomeric excess, ee (%, major/minor)
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
2c
25
25
25
25
50
0
12
12
12
12
12
12
12
12
12
12
12
12
12
24
36
24
44
36
1:1
2:1
3:1
94/86
94/87
95/89
23
e
e
trace
49
ND
ND
2:1
94/89
f
e
e
NR
ND
ND
25
25
25
25
25
25
25
25
25
25
pyridine
DABCO
DMAP
DIPEA
TEA
45
56
43
52
60
74
73
85
87
91
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
3:1
95/85
95/89
94/86
95/87
95/89
95/89
95/89
95/89
95/89
95/89
1
1
0
1
g
1
2
h
TEA
1
3
TEA
g
g
1
1
4
TEA
5
TEA
g
,
i
1
6
TEA
a
Reaction conditions: 1a (0.25 mmol, 1.0 equiv), 2a−2d (0.30 mmol, 1.2 equiv), [Ir(cod)Cl] (2 mol %), L1 (4 mol %), 1 equiv base, 100 mg 4 Å MS,
2
b
c
1
d
THF (2 mL). Overall yield of the diastereoisomers. Ratio of dr determined by H NMR integration. Determined by HPLC analysis using an
IC-3 column. ND = not determined. NR = no reaction. THF (1.5 mL). THF (1 mL). L2 (4 mol %).
e
f
g
h
i
B
DOI: 10.1021/acs.orglett.8b02655
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
In order to acquire higher diastereoselectivity, it is necessary
to adjust the N-prosthetic group (PG) of the dioxindole, which
makes it easier for the chiral allyl-Ir complex to distinguish the
two sides of prochiral enolate intermediate, according to its
asymmetric structure and large steric hindrance. Hence, a range
of different N-substituted substrates (1b−1g) were examined
Scheme 3. Substrate Scope of 3-Hydroxyoxindoles
(
see Scheme 2). Predictably, the dr increased from 2:1 to 6:1 as
a
Scheme 2. Substrate Scope of 3-Hydroxyoxindoles
a
For reaction conditions, please see Table 1, entry 16.
a
For reaction conditions, please see Table 1, entry 16.
a
the steric hindrance became larger. A 1-naphthalene methyl
substituent was selected as the optimal PG for the following
exploration of substrate scopes. Notably, the absolute
configuration of the product (4) without any PG differed from
the others, which indicated that the PG played a role in the
regulation of the diastereoselectivity.
Table 2. Substrate Scope of Allylic Acetates
Notably, a range of 3-hydroxyoxindoles bearing either
electron-donating or electron-withdrawing groups on the
benzene ring, ranging from the 4-posiiton to 7-position,
participated in the allylic substitutions to give the desired
products (8hc−8rc) with high reactivities and good stereo-
selectivities (Scheme 3). 7-Cl-dioxindole afforded the corre-
sponding product (8ic) in 90% yield with excellent stereo-
selectivity (11:1 dr and >99% ee). In contrast, 4-F-dioxindole
was converted to the product (8rc) with low diastereoselectivity
(
2:1 dr) but good enantioselectivity (95% ee).
Next, the scope of a series of cinnamyl acetates (2e−2s) was
investigated (see Table 2). Most products were obtained in high
yields, except for those bearing methyl groups or substituents at
the ortho-position of the arene functionality. The absolute
configuration of 8fk was determined by X-ray crystallographic
1
0
analysis. Although the 2,4-dimethylcinnamyl acetate showed
only moderate reactivity for this reaction, it provided product
8
fo with excellent diastereoselectivity (8:1 dr) and enantiose-
lectivity (95% ee). Furthermore, 3,4-methylenedioxyphenyl,
pyridyl, and n-propyl-substituted allyl acetates were successfully
employed to furnish their desired products (8fq−8fs).
To confirm the scalability of the present methodology, we
performed a larger-scale synthesis of 8, using the standard reaction
conditions, and comparable results were obtained (see Scheme 4).
With the aim of synthesizing bioactive molecules and natural
products, the 3-allyl-3-hydroxyoxindoles that do not possess a
a
For reaction conditions, please see Table 1, entry 16.
C
DOI: 10.1021/acs.orglett.8b02655
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Scale-Up Experiment
Accession Codes
CCDC 1862784 and 1862785 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, U.K.; fax: + 44 1223 336033.
2c,d,11
PG can be utilized more widely.
Therefore, we achieved
the acetylation of the hydroxyl group of 8, followed by
the deprotection of N-naphthylmethylamide 10, using NBS
and AIBN in boiling chlorobenzene to give compound 11
AUTHOR INFORMATION
Corresponding Authors
■
*
*
E-mail: zhenliang6@126.com (Z. Sun).
E-mail: wanbin@sjtu.edu.cn (W. Zhang).
(
see Scheme 5). Furthermore, the addition of terminal alkene
a
Scheme 5. Transformations of Product 8 and ORTEP
Diagram of 13
ORCID
Wanbin Zhang: 0000-0002-4788-4195
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the NSFC (Nos. 21472123, 21620102003, 21831005,
and 81872418), SHMEC (Nos. 201701070002E00030), STCSM
(No. 18ZR1431700), and SMCHFP (No. 2017YQ048) for
financial support.
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ASSOCIATED CONTENT
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(
9) Several types of solvent were also explored. For more details,
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10) The absolute configurations of 8fk and 13 were determined by
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DOI: 10.1021/acs.orglett.8b02655
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