Organocatalytic asymmetric Friedel-Crafts reaction of 1-naphthols with isatins: An enantioselective synthesis of 3-aryl-3-hydroxy-2-oxindoles

Article abstract of DOI:10.1016/j.tetlet.2014.02.054

An organocatalytic enantioselective Friedel-Crafts reaction of 1-naphthols with isatins has been developed employing bifunctional thiourea-tertiary amine organocatalysts. A variety of isatin derivatives react well with 1-naphthols in the presence of Cinchona derived thiourea 1a to provide biologically important chiral 3-aryl-3-hydroxy-2-oxindoles (3a-zg) in good yield (70-84%) and moderate to good enantioselectivity (37-83%).

Full text of DOI:10.1016/j.tetlet.2014.02.054

Tetrahedron Letters 55 (2014) 2138–2141
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Organocatalytic asymmetric Friedel–Crafts reaction
of 1-naphthols with isatins: an enantioselective synthesis
of 3-aryl-3-hydroxy-2-oxindoles
⇑
Jasneet Kaur, Akshay Kumar, Swapandeep Singh Chimni
Department of Chemistry, U.G.C. Centre of Advance Studies in Chemistry, Guru Nanak Dev University, Amritsar 143005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
An organocatalytic enantioselective Friedel–Crafts reaction of 1-naphthols with isatins has been devel-
oped employing bifunctional thiourea–tertiary amine organocatalysts. A variety of isatin derivatives react
well with 1-naphthols in the presence of Cinchona derived thiourea 1a to provide biologically important
chiral 3-aryl-3-hydroxy-2-oxindoles (3a–zg) in good yield (70–84%) and moderate to good enantioselec-
tivity (37–83%).
Received 25 November 2013
Revised 11 February 2014
Accepted 17 February 2014
Available online 22 February 2014
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Organocatalysis
Friedel–Crafts reaction
3-Hydroxyoxindoles
Cinchona-thioureas
Isatins
The Friedel–Crafts reaction has undoubtedly been positioned as
one of the most important carbon–carbon bond forming reactions
for the synthesis of valuable building blocks of biologically active
molecules.1 Recently, organocatalytic enantioselective Friedel–
Crafts alkylation reactions2 have attracted much attention due to
advantages associated with organocatalytic transformation and
its significance for the synthesis of optically active aromatic com-
pounds. Various electron rich aromatic compounds have been suc-
cessfully applied in the Friedel–Crafts reaction with diverse
electrophiles and their enantioselective variants have also been
well studied using small organic molecules as catalysts.2 But, most
of the reports in this area are focused on relatively more reactive
heteroarenes–indoles/pyrroles.3 However, electron rich arenes–
naphthols have been demonstrated to be good Friedel–Crafts do-
nors, albeit with a limited range of electrophiles.4 Jørgensen,4a
Chen,4b and Wang4h presented the organocatalytic asymmetric
Friedel–Crafts reaction of 2-naphthol with azodicarboxylates,
reactions.7 The nucleophilic addition to C-3 carbon of isatins pro-
vides biologically relevant 3-substituted 3-hydroxy-2-oxindoles.8
Among them, 3-aryl-3-hydroxy-2-oxindoles are a useful class of
compounds found in several drug candidates.9 A variety of metal
catalyzed asymmetric methods10 have been developed for their
synthesis, but the organocatalytic asymmetric methods11 are lim-
ited to the use of heteroarenes. The organocatalytic asymmetric
Friedel–Crafts reaction of electron rich arenes to isatins has not
been reported.12
As part of our ongoing program in exploring bifunctional Cin-
chona alkaloids as suitable catalysts for asymmetric Friedel–Crafts
reactions of isatins11a, we reasoned that enantioinduction can be
achieved in the Friedel–Crafts reaction of 1-naphthols with isatins
through synergistic activation by bifunctional thiourea–tertiary
amine organocatalysts (Scheme 1). Herein, we report the epiCDT
(I) catalyzed the Friedel–Crafts-type addition of 1-naphthols to
isatins.13
nitroolefins, and
a
,b-unsaturated aldehydes, respectively. Our
Initially, the reaction of 2-naphthol with N-benzylisatin in tet-
rahydrofuran (THF) in the presence of 4 Å molecular sieves using
epiCDT (I) as a catalyst was performed. Unfortunately, there was
no product formation noted even after 48 h of the reaction time
(Scheme 2, Eq. 1). So, we studied the Friedel–Crafts (F–C) reaction
of 1-naphthol (1a) with N-benzylisatin (2a) employing epiCDT
(I, 10 mol %) in THF in the presence of 4 Å molecular sieves
(Scheme 2, Eq. 2). Interestingly, the product 3a was isolated in
78% yield and 77% ee after 6 h of the reaction time (Table 1, entry
1). Encouraged by this result, the catalytic ability of other Cinchona
group5 and Wang and co-workers6 simultaneously reported the
first organocatalytic enantioselective Friedel–Crafts reaction of
naphthols with aldimines employing Cinchona alkaloids.
Recently, isatins have emerged as valuable electrophiles and
have successfully been used in
a variety of organocatalytic
⇑
Corresponding author. Tel.: +91 183 2258809; fax: +91 183 2258820.
E-mail
addresses:
sschimni@yahoo.com,
sschimni.chem@gndu.ac.in
(S.S. Chimni).
http://dx.doi.org/10.1016/j.tetlet.2014.02.054
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

J. Kaur et al. / Tetrahedron Letters 55 (2014) 2138–2141
2139
Table 1
R
S
Chiral
Optimization studya
O
N
Scaffold
OH
X
Ar
N
H
O
N
H
O
Yieldb (%)
eec (%)
∗
Entry
Catalyst
Solvent
Time (h)
N
OH
H
1
2
3
4
5
6
I
THF
THF
THF
THF
THF
THF
THF
THF
Toluene
CHCl3
DCM
MTBE
1,4-Dioxane
Diethyl ether
Ethyl acetate
THF
6
6
6
6
8
8
8
13
7
6
6
6
6
7
6
6
78
71
72
69
46
40
67
68
62
78
76
72
76
74
79
75
77 (+)
68 (+)
20 (+)
40 (À)
40 (À)
0
11 (+)
76 (+)
20 (+)
52 (+)
56 (+)
71 (+)
69 (+)
73 (+)
71 (+)
56 (+)
O
X
II
III
IV
V
VI
VII
I
R1
N
R
R1
7
8d
9
Scheme 1. Proposed dual activation for the thiourea–tertiary amine catalyzed,
I
asymmetric Friedel–Crafts reaction of 1-naphthols with isatins.
10
11
12
13
14
15
16e
I
I
I
I
I
I
I
alkaloids’ derived thiourea derivatives (II–VII) was examined for
the same reaction (Scheme 2, Eq. 2). The thiourea (III) prepared
from cinchonine gave 3a in 72% yield and 20% ee (Table 1, entry
3). The quinine and quinidine derived thiourea epiQNT (II) and
epiQDT (IV) gave complementary enantiomers of adduct 3a in good
yield (71% and 69%), albeit with moderate enantioselectivity (68%
ee and 40% ee) (Table 1, entries 2 and 4). The model reaction cata-
lyzed by thiourea organocatalysts V and VII yielded 3a in 46% and
67% yield; 40% ee and 11% ee, respectively (Table 1, entries 5 and
7). The organocatalysts VI having the thiourea group at a distance
of six bonds from tertiary amine functionality yielded racemic ad-
duct, indicating the importance of close proximity of tertiary
amine and thiourea moiety for enantioselective reaction (Table 1,
entry 6). The reaction catalyzed by epiCDT (I) at À18 °C took 13 h
to provide 3a in 68% yield and without any advantage in terms
of enantioselectivity (Table 1, entry 8). Next, in order to identify
the best solvent for this transformation, different solvents were
screened employing epiCDT (I, 10 mol %) (Table 1, entries 9–15).
The reaction performed in toluene, chloroform, and dichloromethane
a
Reaction conditions: 0.1 mmol N-benzylisatin, 0.1 mmol of 1-naphthol, 4 Å
molecular sieves (50 mg), and catalysts I–VII (10 mol %) in dry THF.
Yield refers to isolated yield after column chromatography.
Enantiomeric excess (ee) determined by chiral HPLC. The sign in parenthesis
indicates enantiomers.
b
c
d
Reaction was performed at À18 °C.
e
10 mol % of benzoic acid.
provided 3a in 20% ee, 52% ee, and 56% ee, respectively (Table 1,
entries 9–11). In etheral solvents such as methyl tert-butyl ether
(MTBE), 1,4-dioxane, diethyl ether, the product 3a was isolated
in 72%, 76%, and 74% yield; 71% ee, 69% ee, and 73% ee, respectively
(Table 1, entries 12–14). Further, the effect of benzoic acid as an
additive on the Friedel–Crafts reaction was examined, which affor-
ded 3a in good yield but with lower enantioselectivity (Table 1,
entry 16). Thus, the best optimized condition consists of 10 mol %
of I, 4 Å molecular sieves, and THF as a solvent at ambient temper-
ature providing Friedel–Crafts adduct 3a in 78% yield and 77% ee;
the optimized condition was used to study the substrate scope of
this reaction.
Once armed with the optimized condition, the substrate scope
was investigated by studying the Friedel–Crafts reaction of
1-naphthols (1a–1b) with different derivatives of isatin (2a–2zg)
(Table 2). The reaction of 1-naphthol (1a) with N-benzyl isatins
substituted with electron withdrawing and donating groups
(2b–2g) was performed. 5-Halogen substituted N-benzyl isatins
(2b–2e) yielded adducts (3b–3e) in 79–84% yield and 65–75% ee,
obviating any predictable trend. The reaction of isatins substituted
with electron donating groups (5-Me and 5-OMe) gave corre-
sponding adducts 3f and 3g in enantiomeric excess of 63% ee and
61% ee; yield of 78% and 80%, respectively. Similarly, no particular
trend was observed in case of reaction of N-allyl isatin derivatives
(2h–2n) with 1-naphthol. 5-Methoxy-N-allyl isatin provided F–C
adduct 3n with the highest level of enantioselectivity (83% ee).
N-Propargylisatin 2o gave product 3o in 80% yield and 58% ee.
The reaction of 4-chloro-1-naphthol (1b) with different deriva-
tives of N-substituted isatin (2p–2ze) afforded corresponding ad-
ducts (3p–3ze) in 72–84% yield and 48–78% ee. The N-H isatins
derivatives 2zf and 2zg react slowly with 4-chloro-1-naphthol to
provide corresponding adducts 3zf and 3zg in 78% and 79% yield;
49% ee and 37% ee, respectively.
O
OH
I
(10 mol%)
1
)...
(No Reaction)
+
O
O
THF 4Å MS
rt, 48 h
N
Bn
HO
O
OH
HO
Catalyst (10 mol%)
solvent, rt
∗
+
O
2
)...
H
N
N
Bn
3a
2a Bn
1a
(R)
X
N
X
(R)
(R)
(S)
N(S)
N
H
(R)
(S)
(R)
H
(S)
(S)
N(S)
(S)
NH
NH
HN
HN
NH
N
S
S
N
S
NH
(S)
N
F3C
CF3 F3C
III
CF3
O
I
) X= H;
epiCDT
) X= H;
epiCNT
V) epiCDT-L-Val
II
IV
) X=OMe; epiQDT
) X = OMe; epiQNT
O
(R)
N
N(S)
H
N
H
N
H
F3C
(S)
(S)
(S)
(S)
NH
S
H
N
H
CF3
In conclusion, we have developed the organocatalytic enantio-
selective Friedel–Crafts-type addition reaction of 1-naphthols with
isatin derivatives employing bifunctional chiral thiourea–tertiary
amine organocatalysts. A wide variety of biologically relevant 3-
aryl-3-hydroxy-2-oxindoles have been synthesized in good yield
(up to 84%) and good enantioselectivity (up to 83% ee).
N
(S)
N
CF3
O
S
Ph
VII) L-Ile-T
VI
) epiCD-L-Phe-T
CF3
Scheme 2. Bifunctional thiourea–tertiary amine catalyzed Friedel–Crafts reaction
of naphthols with isatins.

2140
J. Kaur et al. / Tetrahedron Letters 55 (2014) 2138–2141
Table 2
Substrate scopea,b,c
OH
HO
HO
X
O
R2
epiCDT (I, 10 mol%)
R2
O
+
*
THF, 4Å MS
rt
O
N
R1
N
X
R1
2a-2zg
1a. X = H
1b. X = Cl
3a-3zg
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
Cl
Br
F
I
*
N
*
N
*
N
O
O
O
O
O
N
N
3c
3e
3a
3b
3d
6 hrs
6 hrs
6 hrs
6 hrs
6 hrs
83% yield
75%ee
84% yield
65%ee
76% yield
77%ee
82% yield
74%ee
79% yield
66%ee
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
Me
MeO
Cl
F
*
*
N
*
N
O
O
O
O
O
N
N
N
3h
3j
3i
3g
3f
6 hrs
12 hrs
6 hrs
6 hrs
6 hrs
80% yield
76% yield
48% ee
72% yield
63 % ee
78% yield
63 % ee
83% yield
66% ee
61 % ee
HO
HO
HO
HO
HO
HO
HO
HO
HO
HO
Br
I
Me
MeO
*
N
*
N
*
N
*
N
*
N
O
O
O
O
O
3k
3m
3n
3o
3l
6 hrs
6 hrs
6 hrs
6 hrs
6 hrs
78% yield
83% ee
84% yield
78% ee
77% yield
72% ee
80% yield
58% ee
78 % yield
65 % ee
HO
HO
Cl
HO
HO
Cl
HO
HO
Cl
Cl
Cl
HO
HO
Cl
HO
HO
Cl
Me
I
F
Cl
Cl
Cl
*
N
*
N
*
N
*
N
*
N
O
O
O
O
O
3s
3r
3t
3p
3q
12 hrs
8 hrs
6 hrs
6 hrs
6 hrs
81% yield
56% ee
84% yield
71% ee
79% yield
43% ee
82% yield
50% ee
80% yield
53% ee
HO
HO
Cl
HO
HO
Cl
HO
HO
Cl
HO
HO
HO
HO
Cl
Me
Br
I
Cl
*
*
N
*
N
*
N
*
N
O
O
O
O
O
N
3y
3v
3x
3u
3w
12 hrs
12 hrs
6 hrs
7 hrs
6 hrs
83% yield
60% ee
75% yield
62% ee
81% yield
60% ee
74% yield
60% ee
81% yield
56% ee
HO
HO
Cl
HO
HO
Cl
HO
HO
HO
HO
HO
HO
Cl
Cl
Br
*
N
3za
*
N
3z
*
N
O
*
N
O
*
O
O
O
N
3zc
3zb
3zd
6 hrs
6 hrs
6 hrs
6 hrs
6 hrs
77%yield
52% ee
84% yield
58% ee
77% yield
55% ee
72% yield
63% ee
79% yield
51% ee
HO
HO
Cl
HO
HO
Cl
HO
HO
Cl
Br
Cl
*
N
*
*
O
O
O
N
N
H
H
3ze
3zg
3zf
60 hrs
6 hrs
48 hrs
78% yield
49% ee
70% yield
61% ee
75% yield
37% ee
a
b
c
Reaction conditions: 0.1 mmol of 1-naphthols 1, 0.1 mmol isatins 2, 4 Å molecular sieves (50 mg), and catalysts I (10 mol %) in dry THF.
Yield refers to isolated yield after column chromatography.
Enantiomeric excess (ee) determined by chiral HPLC.

J. Kaur et al. / Tetrahedron Letters 55 (2014) 2138–2141
2141
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Acknowledgments
We are thankful to UGC and CSIR for JRF (NET) and RA
fellowship to J.K. and A.K., respectively. Our research work was
supported by the research project (SR/SI/OC 35/2011) sanctioned
to SSC by the DST. Financial support from the Department of
Science and Technology (DST), India under FIST program and
UGC, India, under CAS-I is gratefully acknowledged.
Supplementary data
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Supplementary data (detailed experimental procedure and
HPLC chromatogram of all compounds) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
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