Palladium catalyzed asymmetric allylation of 3-OBoc-Oxindoles: An efficient synthesis of 3-Allyl-3-hydroxyoxindoles

Article abstract of DOI:10.1021/acs.orglett.5b00034

3-Allyl-3-hydroxyoxindoles were synthesized in very good enantio- (up to 97% ee) and diastereoselectivities (dr up to 7.6:1) with contiguous quaternary and tertiary stereogenic centers by employing tartrate derived bi(oxazoline) in Pd-catalyzed allylation of 3-OBoc-oxindole. Synthetic utility of 3-allyl-3-hydroxyoxindole was demonstrated by synthesizing a highly substituted spiro(oxindole-3,2′-tetrahydrofuran) derivative in good yield and stereoselectivity.

Full text of DOI:10.1021/acs.orglett.5b00034

Letter
pubs.acs.org/OrgLett
Palladium Catalyzed Asymmetric Allylation of 3‑OBoc-Oxindoles: An
Efficient Synthesis of 3‑Allyl-3-hydroxyoxindoles
Samydurai Jayakumar, Nandarapu Kumarswamyreddy, Muthuraj Prakash, and Venkitasamy Kesavan*
Laboratory of Chemical Biology, Department of Biotechnology, Bhupat and Jyothi Mehta School of Biosciences, Indian Institute of
Technology Madras, Chennai-600036, India
S
* Supporting Information
ABSTRACT: 3-Allyl-3-hydroxyoxindoles were synthesized in very good enantio- (up to 97% ee) and diastereoselectivities (dr
up to 7.6:1) with contiguous quaternary and tertiary stereogenic centers by employing tartrate derived bi(oxazoline) in Pd-
catalyzed allylation of 3-OBoc-oxindole. Synthetic utility of 3-allyl-3-hydroxyoxindole was demonstrated by synthesizing a highly
substituted spiro(oxindole-3,2′-tetrahydrofuran) derivative in good yield and stereoselectivity.
evelopment of efficient methodologies to access enan-
Dtioenriched 3-substituted-3-hydroxyoxindoles is of great
importance in contemporary organic synthesis, since they can be
exploited as important synthons to synthesize various natural
products and pharmaceutical lead compounds.1 Consequently,
various protocols on nucleophilic addition to isatins 1 have been
developed. Although numerous methods were documented for
the synthesis of 3-substituted-3-hydroxyoxindoles, access to 3-
allyl-3-hydroxyoxindoles is highly warranted.2 3-Allyl-3-hydroxy-
oxindole 5 is a promising intermediate that can be used to
synthesize structurally diverse oxindole derivatives (Scheme 1).
In this context, few metal catalyzed strategies enabling the
enantioselective synthesis of 3-allyl-3-hydroxyoxindoles have
been developed (route I).3 These methods involve expensive Ir-
catalyst or Lewis acids and an excess of allylating reagents such as
allylstannanes or allylsilanes which are difficult to synthesize.
Organocatalytic allylation of isatins 1 also afforded 3-allyl-3-
hydroxyoxindoles with good enantioselectivity (route I).4
Alternatively, hydroxylation of oxindoles 2 using phase transfer
catalysts was also developed (route II).5 However, these
approaches lack substrate diversity as well as the need for
preformed sensitive reagents. Hence, identification of suitable
nucleophile and electrophile counterparts is very important for
accessing 3-allyl-3-hydroxyoxindoles 5.
Pd-catalyzed asymmetric allylic alkylation (AAA) is a bedrock
method to access structurally diverse scaffolds by the employ-
ment of various nucleophiles with allyl acetates/carbonates.6 In
particular, Trost et al. pioneered this method to synthesize
numerous natural products and bioactive molecules using
different types of nucleophiles including 3-alkyl/aryloxindoles.7
Although 3-hydroxyoxindoles are demonstrated as effective
nucleophiles to synthesize structurally diverse molecules,8
exploration of 3-hydroxyoxindole in AAA is yet to be
documented. This intrigued us to develop an Umpolung strategy
by employing a 3-hydroxy protected oxindole (3-OBoc-
oxindole) as a nucleophile9 in a Pd-catalyzed allylic substitution
reaction to access 3-allyl-3-hydroxyoxindoles. Earlier successful
efforts in AAA in our laboratory motivated us to employ tartrate
derived bi(oxazoline) as a ligand.10 To the best of our knowledge,
the AAA strategy is yet to be employed to synthesize substituted
3-allyl-3-hydroxyoxindoles. Herein we wish to report a highly
enantio- and diastereoselective synthesis of 3-allyl-3-hydroxy-
oxindoles 5 via Pd-bi(oxazoline) catalyzed AAA of allyl acetates 4
with 3-OBoc-oxindole 3.
Scheme 1. Strategies for the Synthesis of 3-Allyl-3-
hydroxyoxindoles
Received: October 15, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00034
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Initial efforts were dedicated to identifying suitable reaction
conditions for the model reaction between 3-OBoc-oxindole 3a
and rac-1,3-diphenyl-2-propenyl acetate 4a in dichloromethane
using 2.5 mol % of [Pd(η3-C3H5)Cl]2 with 10 mol % of a chiral
ligand. O-Boc protection of the resultant alkylated product was
deprotected later by subsequent acidic treatment. Identification
of an appropriate ligand was undertaken as a primary task, and
chiral ligands L1−6 were examined under identical conditions.
The observed results are depicted in Table 1.
was obtained using BINAP L6, a slight reduction in stereo-
selectivity was noticed (Table 1, entry 6). These results clearly
imply that ligand L2 is a more suitable chiral counterpart for Pd-
catalyzed asymmetric allylic alkylation of 3-OBoc-N-methyl-
oxindole 3a. Hence, further optimization of the reaction
conditions was carried out using bi(oxazoline) L2 as a chiral
ligand. Changes in other parameters such as solvents, Pd-salts,
and additives improved neither the efficiency nor the stereo-
selectivity of the reaction.12 Hence, 2.5 mol % of [Pd(η3-
C3H5)Cl]2 with 10 mol % of L2 as the catalyst, 10 mol % of
KOAc as an additive, and 3 equiv of BSA in dichloromethane
were identified as the optimal conditions to probe the substrate
scope of the reaction.
a
Table 1. Examination of Suitable Ligands
The effect of different substituents on 1,3-diaryl-2-propenyl
acetates 4a−k was investigated with 3-OBoc-oxindole 3a under
the optimized conditions. The observed results are summarized
in Table 2. Electron-withdrawing ortho-fluoro substituted 1,3-
a
Table 2. Study of Substrate Scope
c
d
e
entry
Ar (4a−k)
C6H5 (a)
5 yield (%)
dr
ee (%) of 5 major/minor
c
d
e
1
(5aa) 92
(5ab) 92
(5ac) 00
(5ad) 63
(5ae) 88
(5af) 80
(5ag) 81
(5ah) 90
(5ai) 85
(5aj) 77
(5ak) 69
6.6:1
7:1
90/90
96/88
−
entry
ligand
5aa yield (%)
dr
ee (%) of 5aa major/minor
2
2-FC6H4 (b)
2-ClC6H4 (c)
3-NO2C6H4 (d)
3-FC6H4 (e)
3-ClC6H4 (f)
3-BrC6H4 (g)
4-FC6H4 (h)
4-ClC6H4 (i)
4-BrC6H4 (j)
2-naphthyl (k)
1
2
3
4
5
6
L1
L2
L3
L4
L5
L6
69
2:1
6.6:1
5:1
nd
44/44
90/90
3
−
92
4
1.5:1
6.5:1
4.4:1
4.8:1
5:1
36/86
90/88
85/84
92/85
82/78
86/76
93/21
80/50
f
67
−74/−63
5
g
trace
82
nd
6
4:1
4:1
80/79
7
87
−88/−79
8
a
The reactions were conducted with substrates 3a (0.10 mmol) and 4a
9
2:1
(0.12 mmol) in dichloromethane with 2.5 mol % of [Pd(η3-C3H5)Cl]2,
10 mol % of appropriate ligand at 25 °C (2 days). BSA = N,O-
Bis(trimethylsilyl)acetamide. Yield of the isolated product. The
diastereomeric ratio was determined by H NMR integration of the
crude alkylated products. Enantiomeric excess values of major and
minor diastereomers were determined by chiral HPLC. The minus
sign signifies the opposite enantiomer. Not determined.
10
11
5:1
b
c
d
3:1
a
1
The reactions were conducted with substrates 3a (0.10 mmol) and
e
4a−k (0.12 mmol) in dichloromethane with 2.5 mol % of [Pd(η3-
C3H5)Cl]2, 10 mol % of ligand L2 at 25 °C (2 days). BSA = N,O-
Bis(trimethylsilyl)acetamide. Yield of the isolated product. The
diastereomeric ratio was determined by H NMR integration of the
crude alkylated products. Enantiomeric excess values of major and
b
f
c
d
g
1
e
The investigation of various bi(oxazoline)s L1−5 clearly
indicates the requirement of an additional chiral appendage,
since bi(oxazoline) L1 which is devoid of an extra chiral
appendage yielded the product 5aa with poor stereoselectivity
(Table 1, entry 1). Allylic alkylation of 3-OBoc-N-methyl-
oxindole 3a occurred with very good diastereo- and enantiose-
lectivities when bi(oxazoline) L2 was employed (Table 1, entry
2). Only the moderate yield and stereoselectivity of the product
5aa was noticed (Table 1, entry 3) when ligand L3
(diastereomeric pair of L2) was used. These observations are
in resonance with our previous results.10b,11 Efforts to increase
the enantioselectivity further by using structurally diverse
bi(oxazoline) ligands L4 and L5 were undertaken. Then inability
of ligand L4 to catalyze the formation of the expected product
can be attributed to the bulky nature of the chiral appendage
which could have prevented the active complex formation (Table
1, entry 4). Use of bi(oxazoline) L5 afforded the product 5aa in
good yield (Table 1, entry 5), though the stereoselectivity is low
when compared to L2. Under the identical conditions BINAP L6
was also investigated. Although a very good yield of product 5aa
minor diastereomers were determined by chiral HPLC.
diaryl-2-propenyl acetate 4b underwent alkylation smoothly to
afford the corresponding product 5ab with good diastereo- (7:1)
and very good enantioselectivity (96/88) (Table 2, entry 2). No
formation of expected product 5ac was observed when ortho-
chloro substituted 1,3-diaryl-2-propenyl acetate 4c was subjected
to AAA (Table 2, entry 3). This can be attributed to the bulkiness
of the chlorine atom which may sterically inhibit the formation of
active π-allyl-palladium species. An electron-withdrawing nitro
substituent at the meta- position hampers the catalytic efficiency
which results in the formation of product 5ad in moderate yield
and stereoselectivity (Table 2, entry 4). On the other hand,
electron-withdrawing as well as bulky meta-halogen substitutions
on 1,3-diaryl-2-propenyl acetates 4e−g did not affect the
formation of desired products. The respective products 5ae−
5ag were isolated in very good yields and stereoselectivities
(Table 2, entries 5−7). 4-Fluoro-substituted allyl acetate 4h
furnished the desired alkylated product 5ah with similar
B
DOI: 10.1021/acs.orglett.5b00034
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
diastereoselectivity as in the case of meta-fluoro-substituted 4e,
but with slightly lowered enantioselectivity (Table 2, entry 8).
The presence of chloro-substitution at the para-position
significantly affected the diastereoselectivity of the product 5ai
(Table 2, entry 9) but had an insignificant effect on the
enantioselectivity. Very poor enantioselectivity was observed in
the case of the minor diastereomer of 5aj when bromine was
present at the para-position; however, the enantioselectivity of
the major diastereomer remained unaffected (Table 2, entry 10).
When more sterically hindered rac-1,3-di(napthalen-2-yl)-
propenyl acetate 4k was employed as a substrate, alkylation
proceeded with good yield and stereoselectivity (Table 2, entry
11).
stereoselectivities (dr up to 6.6:1 and up to 97% ee) (Table S3,
entries 1−4).12
To determine the absolute stereochemistry of alkylated
product 5aa, the derivative 6aa was synthesized and the single
crystal X-ray analysis of 6aa revealed that the quaternary and
tertiary chiral centers possess S,S configurations respectively
(Figure 1).14
Next, the influence of halogen substitutions at the fifth
position of the oxindole ring was studied. Under the identical
reaction conditions, 3-OBoc-5-chloro-N-methyl oxindole 3b was
reacted with rac-1,3-diphenyl-2-propenyl acetate 4a, to furnish
the product 5ba with very good yield as well as diastereo- and
enantioselectivity (Table 3, entry 1). The presence of bromine at
a
Table 3. Effect of Substituents on 3-OBoc-Oxindole
Figure 1. Synthesis and ORTEP diagram of compound 6aa.
c
d
e
entry
3
4
5 yield (%)
dr
ee (%) of 5 major/minor
A spirooxindole comprising a 3,2′-tetrahydrofuran moiety is
an important candidate possessing anticancer activity. Since, only
a few methods describe the synthesis of these scaffolds
enantioselectively, an efficient method to access highly
substituted spiro(oxindole-3,2′-tetrahydrofuran) is highly desir-
ed.8d,15 This intrigued us to demonstrate the synthetic utility of
alkylated product 5ab in constructing this scaffold enantiose-
lectively. The presence of a homo allyl moiety in product 5ab
paved the way to synthesize spiro(oxindole-3,2′-tetrahydro-
furan) derivative 6ab which contains four contiguous stereogenic
centers (Scheme 2). Spirooxindole 6ab was isolated in 83% yield
1
2
3
4
5
6
3b
3c
3d
3e
3f
4a
4a
4b
4b
4b
4b
(5ba) 91
(5ca) 93
(5db) 73
(5eb) 88
(5fb) 85
(5gb) 31
7.6:1
5:1
89/85
43/53
f
5:1
96/nd
3.7:1
4.5:1
3:1
96/51
94/82
57/nd
3g
a
The reactions were conducted with substrates 3b−g (0.10 mmol)
and 4a/b (0.12 mmol) in dichloromethane with 2.5 mol % of [Pd(η3-
b
C3H5)Cl]2, 10 mol % of ligand L2 at 25 °C (2 days). BSA = N,O-
Bis(trimethylsilyl)acetamide. Yield of the isolated product. The
diastereomeric ratio was determined by H NMR integration of the
crude alkylated product. Enantiomeric excess values of major and
minor diastereomers were determined by chiral HPLC. Not
determined.
c
d
1
e
f
Scheme 2. Synthesis of Spiro(oxindole-3,2′-tetrahydrofuran)
Derivative and NOE Correlation of 6ab
the fifth position of the oxindole moiety, although unimpactful to
the yield and diastereoselectivity, lowered the enantioselectivity
for both diastereomers of 5ca (Table 3, entry 2). Since alkylation
of 1,3-bis(2-fluorophenyl)allyl acetate 4b resulted in very good
stereoselectivity, the impact of N1-substitution on oxindole was
similar. Neither the yield nor the stereoselectivity was affected by
the presence of propyl 5db or allyl 5eb or benzyl 5fb substituents
on the N1-position (Table 3, entries 3−5). The labile nature of N-
propargyl protection in the presence of Pd-reagents can be the
reason for the low yield and selectivity of product 5gb (Table 3,
entry 6).13
Encouraged by these results we expanded the substrate scope
to 1,3-unsymmetrically substituted 2-propenyl acetates. Despite
the different substitutions on the C1 and C3 centers of 2-
propenyl acetates, the reaction proceeded to yield both
regioisomers almost equally (up to 1.5:1) with good yields and
by treating 5ab with 1.5 equiv of I2 and 4 equiv of NaHCO3 in
acetonitrile. The relative stereochemistry of the newly generated
chiral centers of 6ab was further confirmed using NOE
correlation by irradiation of Ha, Hb, and Hc protons,
respectively. It is noteworthy that this is the first method which
discloses the synthesis of spirooxindole 6ab which comprises a
tetrahydrofuran ring from 3-allyl-3-hydroxyoxindole with
excellent enantioselectivity.
In summary, we have developed a Pd-catalyzed AAA strategy
to synthesize 3-allyl-3-hydroxyoxindoles 5 by the treatment of 3-
C
DOI: 10.1021/acs.orglett.5b00034
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Tartrate derived bi(oxazoline) L2 provided remarkable asym-
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symmetrically substituted 2-propenyl acetates. Synthetic utility
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structing a highly substituted spiro(oxindole-3,2′-tetrahydro-
furan) 6ab derivative with four consecutive chiral centers in
excellent enantioselectivity.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Complete experimental details and characterization data for the
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
* E-mail: vkesavan@iitm.ac.in.
Notes
The authors declare no competing financial interest.
(12) See Supporting Information.
(13) Pal, M.; Parasuraman, K.; Yeleswarapu, K. R. Org. Lett. 2003, 5,
ACKNOWLEDGMENTS
■
349.
This work was supported by the Department of Science &
Technology, Government of India, New Delhi, IFCPAR (IFC-
4803-4), New Delhi for financial support, Department of
Biotechnology IIT-Madras for the infrastructure, Dr. Sudha
Devi (SAIF IIT-Madras) for X-ray analysis and SAIF IIT-Madras
for the spectral analysis.
(14) CCDC 1046106. The crystal data can be obtained free of charge
via the Internet at www.ccdc.cam.ac.uk/datarequest/cif.
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DOI: 10.1021/acs.orglett.5b00034
Org. Lett. XXXX, XXX, XXX−XXX