Catalytic enantioselective synthesis of chiral isatin derivatives by an aldol approach

Article abstract of DOI:10.1055/s-0032-1316551

A catalytic enantioselective aldol reaction of alkenyl esters with isatins was achieved using an (S)-BINOL-derived chiral tin dibromide possessing a 4-tert-butylphenyl group at 3- and 3-positions as the chiral pre-catalyst in the presence of sodium methoxide and methanol. Optically active 3-alkylated 3-hydroxy-2-oxindoles having up to 98% ee were diastereoselectively obtained in high yields not only from cyclic alkenyl esters but also from acyclic ones under the influence of the in situ generated chiral tin bromide methoxide.

Full text of DOI:10.1055/s-0032-1316551

LETTER
▌1783
lCettaertalytic Enantioselective Synthesis of Chiral Isatin Derivatives by an Aldol
Approach
Synthesis of Chiral Isatin Derivatives
Akira Yanagisawa,* Naoyuki Kushihara, Takuya Sugita, Kazuhiro Yoshida
Department of Chemistry, Graduate School of Science, Chiba University, Inage, Chiba 263-8522, Japan
Fax +81(43)2902789; E-mail: ayanagi@faculty.chiba-u.jp
Received: 16.04.2012; Accepted after revision: 23.05.2012
formed into chiral tin enolates in situ, and the chiral tin
Abstract: A catalytic enantioselective aldol reaction of alkenyl es-
catalyst is regenerated from the aldol products under the
ters with isatins was achieved using an (S)-BINOL-derived chiral
influence of an alcohol. The reaction affords the corre-
sponding optically active β-hydroxy carbonyl compounds
tin dibromide possessing a 4-tert-butylphenyl group at 3- and 3′-po-
sitions as the chiral pre-catalyst in the presence of sodium methox-
ide and methanol. Optically active 3-alkylated 3-hydroxy-2- with high enantioselectivities. We considered that if an
oxindoles having up to 98% ee were diastereoselectively obtained
in high yields not only from cyclic alkenyl esters but also from acy-
clic ones under the influence of the in situ generated chiral tin bro-
the above-mentioned chiral tin-catalyzed asymmetric al-
mide methoxide.
isatin or its derivative could exhibit reactivity comparable
to that of an aldehyde, the electrophile would also undergo
dol reaction to provide enantiomerically enriched β-hy-
droxy ketones possessing a 3-hydroxy-2-oxindole moiety.
Thus, we tried to react 6-methoxy-1-tetralone-derived al-
Key words: aldol reaction, alkenyl ester, asymmetric catalysis,
isatin, tin
kenyl trichloroacetate 1a with N-benzyl isatin (2a) using
chiral tin dibromide 4a and sodium methoxide as precata-
lysts, and, as a consequence, expected aldol adduct 3aa
was formed in high yield with remarkable asymmetric in-
duction. For instance, when a mixture of 1a (2 equiv) and
Isatin and its derivatives are key synthetic intermediates
for numerous biologically active compounds.1 For exam-
ple, convolutamydine A shows potent activity in the dif-
ferentiation of HL-60 human promyelocytic leukemia
2a (1 equiv) was treated with chiral tin dibromide 4a (10
cells.2 One of the convenient methods for the construction
mol%) and NaOMe (10 mol%) in the presence of MeOH
of such a nonracemic 3-alkylated 3-hydroxy-2-oxindole
(10 equiv) in toluene at room temperature for 20 minutes,
structure is the catalytic enantioselective aldol reaction of
a 66:34 diastereomeric mixture of 3aa was obtained in al-
most quantitative yield (Table 1, entry 1). The major dia-
isatins. Various chiral organocatalysts3 have been devel-
oped for the asymmetric transformation; however, as far
stereomer of 3aa had 79% ee. Then, we tested sodium
as we know, there are very few examples of the catalytic
ethoxide and ethanol instead of sodium methoxide and
reaction that uses chiral Lewis acid catalysts.4 We report
methanol, and found that the latter combination gave a
here a novel example of the catalytic asymmetric synthe-
better result in terms of enantioselectivity (Table 1, entry
sis of enantiomerically enriched 3-alkylated 3-hydroxy-2-
2). In order to obtain product 3aa with a higher enantio-
oxindoles from isatins via an enantioselective aldol reac-
meric ratio, we investigated the reaction conditions. Low-
tion with alkenyl esters catalyzed by an in situ generated
ering the reaction temperature to –20 °C raised the
chiral tin bromide methoxide (Scheme 1).
enantioselectivity up to 85% ee (Table 1, entry 5). In the
screening for solvent, toluene was found to provide the
highest yield and enantiomeric excess (Table 1, entries 6–
9). We further examined the catalytic activity of chiral tin
precatalysts 4b–e, but 4a was found to be superior to 4b–
e with respect to enantiomeric excess (Table 1, entry 4 vs.
entries 10–13).
..
R2
O
O
HO
cat. chiral tin dibromide
cat. NaOMe
*
OCOCX3
+
R1
O
*
R3
O
R3
R1
MeOH
toluene, 0 °C
N
N
R2
X = F, Cl
Bn
Bn
The protecting group on the nitrogen atom of an isatin de-
rivative is considered to have an effect on the electronic
and/or steric properties. Thus, we examined the potential
use of N-protected isatins 2a–aE in the asymmetric aldol
reaction with alkenyl trichloroacetate 1a at 0 °C in toluene
(Table 2). N-Benzyl isatin (2a) exhibited the highest reac-
tivity and enantioselectivity among the N-protected isat-
ins examined (Table 2, compare entry 1 with entries 2–6).
On the other hand, although N-tosylated derivative 2aE
afforded the highest diastereoselectivity, its reactivity and
enantioselectivity were unsatisfactory (Table 2, entry 6).
Use of sterically bulky trityl group was also effective in
improving the diastereoselectivity (Table 2, compare en-
Scheme 1 The asymmetric aldol reaction of alkenyl esters with isat-
ins catalyzed by an in situ generated chiral tin bromide methoxide
We have already shown that an asymmetric aldol reaction
of aldehydes with alkenyl trichloroacetates proceeds
smoothly in the presence of a catalytic amount of a chiral
tin alkoxide.5 The alkenyl esters are efficiently trans-
SYNLETT 2012, 23, 1783–1788
Advanced online publication: 29.06.2012
DOI: 10.1055/s-0032-1316551; Art ID: ST-2012-U0328-L
© Georg Thieme Verlag Stuttgart · New York
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6

1784
A. Yanagisawa et al.
LETTER
try 5 with entry 1). In order to improve enantioselectivity, Then, we tested the suitability of alkenyl trifluoroacetates
we examined an electronic effect of the p-substituted ben- for the generation of chiral tin enolates. The alkenyl esters
zyl group; however, better results were not obtained (Ta- have been reported to be effective substrates for asymmet-
ble 2, compare entries 2 and 3 with entry 1). Based on ric protonation.6 Thus, the asymmetric aldol reaction of 6-
these results, we concluded that the benzyl group is the methoxy-1-tetralone-derived alkenyl trifluoroacetate 1a′
most appropriate N-protective group for isatins.
with isatins 2a–h was carried out and as a result, targeted
aldol products 3aa–ah were obtained almost quantitative-
ly in the presence of only 2 mol% of a chiral tin catalyst
(Table 3, entries 1–8). In some cases, the reaction was
completed in a shorter time than that using 1a (Table 3,
entries 2–6 and 8). Regarding stereoselectivity of the re-
action, although the enantiomeric excesses of two prod-
ucts increased (Table 3, entries 3 and 6) and two other
products decreased to some extent (Table 3, entries 2 and
7), the reason for the difference is unclear at present.
With the optimal reaction conditions in hand, we exam-
ined the catalytic asymmetric aldol reaction of 1a with
isatins 2a–h (Table 3). High reactivities and enantioselec-
tivities of up to 91% ee as well as good diastereoselectiv-
ities were observed for the reactions of isatins 2b–e, all of
which have an electron-withdrawing group at their 5-po-
sition (Table 3, entries 2–5). In contrast, the electron-
donating-group-substituted isatin 2f gave a product with
relatively low enantiomeric excess (Table 3, entry 6).
Table 1 Optimization of Catalytic Asymmetric Aldol Reaction of Alkenyl Trichloroacetate 1a with Isatin Derivative 2aa
OMe
O
OCOCCl3
+
4 (5 mol%)
NaOMe (5 mol%)
MeOH (30 equiv)
O
O
HO
N
solvent
MeO
Bn
O
1a (2 equiv)
2a
3aa
N
Ar
Bn
4a: Ar = 4-t-BuC6H4
4b: Ar = 4-F3CC6H4
4c: Ar = 4-MeOC6H4
4d: Ar = Ph
Br
Br
Sn
Ar
4e: Ar = H
Entry
4
Solvent
Temp (°C)
Time (h)
Yield (%)b
drc
ee (%)d
1e–g
2e,h
3e–g
4
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
4d
4e
toluene
toluene
toluene
toluene
toluene
toluene
CH2Cl2
THF
r.t.
r.t.
0
0.33
0.33
2
>99
>99
>99
>99
88
66:34
74:26
60:40
60:40
60:40
57:43
60:40
52:48
51:49
53:47
63:37
59:41
58:42
79
68
80
82
85
84
72
45
49
62
80
79
18
0
0.5
24
5e,f
6
–20
–20
–20
–20
–20
0
30
>99
97
7
48
8
48
78
9
Et2O
48
32
10
11
12
13
toluene
toluene
toluene
toluene
0.5
0.5
0.5
0.5
>99
>99
>99
>99
0
0
0
a Unless otherwise specified, the reaction was carried out using chiral tin dibromides 4a–e (5 mol%), NaOMe (5 mol%), alkenyl trichloroacetate
1a (2 equiv), isatin derivative 2a (1 equiv), and MeOH (30 equiv) in the specified solvent.
b Isolated yield of 3aa.
c Determined by 1H NMR analysis.
d The value corresponds to the major diastereomer of 3aa. Determined by HPLC analysis.
e Tin dibromide 4a (10 mol%) was used.
f NaOMe (10 mol%) was used.
g MeOH (10 equiv) was used.
h NaOEt (10 mol%) and EtOH (10 equiv) were used in place of NaOMe and MeOH.
Synlett 2012, 23, 1783–1788
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Chiral Isatin Derivatives
1785
Table 2 Catalytic Asymmetric Aldol Reaction of Alkenyl Trichloroacetate 1a with Various N-Protected Isatins 2a–aEa
OMe
O
OCOCCl3
+
4a (10 mol%)
NaOMe (10 mol%)
MeOH (30 equiv)
O
O
HO
N
toluene, 0 °C
MeO
R
O
1a (2 equiv)
3aa–aaE
2a–aE
N
R
Entry
Isatin
R
Time (h)
Product
Yield (%)b
drc
ee (%)d
1
2
3
4
5
6
2a
Bn
0.5
20
3aa
>99
>99
>99
>99
>99
81
60:40
50:50
60:40
58:42
78:22
89:11
82
76
77
62
80
63
2aA
2aB
2aC
2aD
2aE
4-MeOC6H4CH2
3aaA
3aaB
3aaC
3aaD
3aaE
4-BrC6H4CH2
1
20
20
5
Ph2CH
Tr
Ts
a Unless otherwise specified, the reaction was carried out using chiral tin dibromide 4a (10 mol%), NaOMe (10 mol%), alkenyl trichloroacetate
1a (2 equiv), isatin derivatives 2a–aE (1 equiv), and MeOH (30 equiv) in toluene at 0 °C.
b Isolated yields of 3aa–aaE.
c Determined by 1H NMR analysis.
d The value corresponds to the major diastereomer of 3aa–aaE. Determined by HPLC analysis.
The aforementioned results emboldened us to study the with isatins. The results are shown in Table 4. Not only
utility of numerous alkenyl trichloroacetates or alkenyl cyclic alkenyl esters 1b and 1c but also acyclic ones 1e–i
trifluoroacetates in the catalytic asymmetric aldol reaction could be employed as the precursors of chiral tin enolates
Table 3 Catalytic Asymmetric Aldol Reaction of Alkenyl Trichloroacetate 1a or Alkenyl Trifluoroacetate 1a′ with Various Isatins 2a–ha
OMe
4a (5 or 2 mol%)
NaOMe (5 or 2 mol%)
MeOH (30 equiv)
O
OCOCX3
+
O
R
O
HO
toluene, 0 °C
R
N
MeO
1a (X = Cl, 2 equiv)
1a' (X = F, 1.2 equiv)
Bn
O
3aa–ah
2a–h
N
Bn
Entry
Isatin
R
H
Time (h)b
Product
Yield (%)b,c
drb,d
ee (%)b,e
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
0.5 (1)
3 (1)
3aa
3ab
3ac
3ad
3ae
3af
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
>99 (>99)
60:40 (60:40)
74:26 (74:26)
78:22 (85:15)
82:18 (86:14)
84:16 (84:16)
75:25 (83:17)
69:31 (72:28)
64:36 (64:36)
82 (80)
85 (76)
78 (86)
91 (91)
84 (85)
50 (77)
84 (75)
75 (76)
5-F
5-Cl
5-Br
5-I
3 (1.5)
1.5 (1)
3 (2)
5-MeO
6-Br
7-F
3 (2)
2g
2h
3 (3)
3ag
3ah
3 (1)
a Unless otherwise specified, the reaction was carried out using chiral tin dibromide 4a (5 mol% for 1a, 2 mol% for 1a′), NaOMe (5 mol% for
1a, 2 mol% for 1a′), alkenyl trichloroacetate 1a (2 equiv) or alkenyl trifluoroacetate 1a′ (1.2 equiv), isatin derivatives 2a–h (1 equiv), and
MeOH (30 equiv) in toluene at 0 °C.
b The data for the reaction using 1a′ are shown in parentheses.
c Isolated yields of 3aa–ah.
d Determined by 1H NMR analysis.
e The value corresponds to the major diastereomer of 3aa–ah. Determined by HPLC analysis.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1783–1788

1786
A. Yanagisawa et al.
LETTER
Table 4 Catalytic Asymmetric Aldol Reaction of Various Alkenyl Esters 1b–i with Isatins 2b,d,e,g,ha
O
N
R1
R2
4a (x mol%)
NaOMe (x mol%)
MeOH (30 equiv)
O
OCOCX3
R2
1b–d (X = F, 1.2 equiv)
1e–i (X = Cl, 2 equiv)
HO
R3
+
O
R1
R3
O
toluene, 0 °C
N
Bn
Bn
2b,d,e,g,h
3bd–ib
Entry
Alkenyl ester
2 (R3)
x
2
Time (h)
Product
Yield (%)b
>99
drc
ee (%)d
OCOCF3
1
2d (5-Br)
2
3bd
79:21
89
1b
1c
1d
OCOCF3
2
3
4
5
6
2d (5-Br)
2d (5-Br)
2d (5-Br)
2g (6-Br)
2h (7-F)
2
2
2
5
2
1
3cd
3dd
3ed
3fg
>99
63
64:36
76:24
72:28
70:30
82:18
90
90
83
87
82
OCOCF3
O
24
24
6
OCOCCl3
>99
>99
98
1e (E/Z = 1:4)
OCOCCl3
Ph
1f (E/Z = 1:99)
OCOCCl3
1
3gh
F
1g (E/Z = 1:4)
F
OCOCCl3
7
8
2e (5-I)
5
2
5
3he
3ib
99
77:23
41:59
80
F
1h (E/Z = 1:9)
OCOCCl3
2b (5-F)
24
>99
98e
Ph
1i (E/Z = 1:4)
a Unless otherwise specified, the reaction was carried out using chiral tin dibromide 4a (2 or 5 mol%), NaOMe (2 or 5 mol%), alkenyl trifluo-
roacetates 1b–d (1.2 equiv) or alkenyl trichloroacetates 1e–i (2 equiv), isatin derivative 2b, 2d, 2e, 2g, or 2h (1 equiv), and MeOH (30 equiv)
in toluene at 0 °C.
b Isolated yields of 3bd–ib.
c Determined by 1H NMR analysis.
d The value corresponds to the major diastereomer of 3bd–he. Determined by HPLC analysis.
e The value corresponds to the minor diastereomer of 3ib. Determined by HPLC analysis.
Synlett 2012, 23, 1783–1788
© Georg Thieme Verlag Stuttgart · New York

LETTER
Synthesis of Chiral Isatin Derivatives
1787
without lowering the chemical yields of products ic asymmetric aldol reaction to other substrates and the
3bd,cd,ed–3ib (Table 4, entries 1, 2, and 4–8). On the oth- biological activities of the products are under way.
er hand, a remarkable decrease in reactivity was seen in
the case of 4-chromanone derivative 1d (Table 4, entry 3).
The use of an acyclic aromatic ketone derivative having a
long alkyl substituent as the substrate was effective in in-
creasing the extent of asymmetric induction. In fact, the
highest enantioselectivity (98% ee) was observed in the
reaction of acyclic alkenyl trichloroacetate 1i with 2b,
even if that value was for the minor diastereomer of prod-
uct 3ib (Table 4, entry 8). The origin of the high stereo-
control is not elucidated yet and studies on effect of E/Z
ratio of acyclic alkenyl esters on the stereoselectivity
would be necessary for the elucidation.
Typical Experimental Procedure for the Asymmetric Aldol Re-
action: Synthesis of 1-Benzyl-3-hydroxy-3-(6-methoxy-1-oxo-
1,2,3,4-tetrahydronaphthalen-2-yl)indolin-2-one (3aa, Entry 4
in Table 1, Entry 1 in Table 2, and Entry 1 in Table 3)
To a suspension of chiral tin dibromide 4a5 (20.6 mg, 0.025 mmol)
in dry toluene (3 mL) was added NaOMe in MeOH (25 μL, 0.025
mmol) and MeOH (0.58 mL) at r.t. The resulting mixture was
stirred for 30 min. Then, isatin derivative 2a (118.0 mg, 0.5 mmol)
and alkenyl trichloroacetate 1a (321.5 mg, 1.0 mmol) were added to
the mixture at 0 °C. After being stirred for 30 min at 0 °C, the reac-
tion mixture was treated with MeOH (1 mL), brine (1 mL), and solid
KF (0.5 g) at ambient temperature for 10 min. The resulting precip-
itate was filtered off, the filtrate was dried over Na2SO4, and then
concentrated in vacuo. The residual crude product was purified by
column chromatography on silica gel to give aldol product 3aa
(206.2 mg, >99% yield). The diastereomeric ratio was determined
to be 60:40 by 1H NMR analysis. The enantioselectivity of the ma-
jor diastereomer was determined to be 82% ee by HPLC analysis
using a chiral column [Daicel Chiralpak AD-H, hexane–i-PrOH
(9:1), flow rate = 1.0 mL/min] tR1 = 69.6 min (major), tR2 = 95.2
min (minor).
A catalytic cycle is postulated for the asymmetric aldol re-
action (Scheme 2). First, chiral tin dibromide 4a reacts
with an equimolar amount of sodium methoxide to give
the corresponding chiral tin bromide methoxide, which is
the true catalyst in the present transformation. Subse-
quently, the generated chiral tin bromide methoxide at-
tacks alkenyl ester 1 to form chiral tin enolate 5 and
methyl trihaloacetate. The following aldol reaction be-
tween chiral tin enolate 5 and isatin derivative 2 affords
tin alkoxide of β-hydroxy ketone 6. Lastly, tin alkoxide 6
undergoes protonation with methanol to give optically ac-
tive β-hydroxy ketone 3 with regeneration of the chiral tin
bromide methoxide. The rate of methanolysis of tin alkox-
ide 6 plays a crucial role in the catalytic cycle.
Spectral Data of the Product
1H NMR (500 MHz, CDCl3): δ = 8.10 (d, 1 H, J = 8.9 Hz, ArH),
7.25–7.34 (m, 6 H, ArH), 7.14 (t, 1 H, J = 7.7 Hz, ArH), 6.93 (t, 1
H, J = 7.6 Hz, ArH), 6.86 (dd, 1 H, J = 2.3, 8.9 Hz, ArH), 6.71 (d, 1
H, J = 8.0 Hz, ArH), 6.61 (m, 1 H, ArH), 6.58 (s, 1 H, OH), 5.00 (d,
1 H, J = 15.8 Hz, CH), 4.84 (d, 1 H, J = 15.5 Hz, CH), 3.83 (s, 3 H,
OCH), 3.39 (dd, 1 H, J = 4.6, 13.8 Hz, CH), 3.00 (m, 1 H, CH),
2.68–2.75 (m, 1 H, CH), 1.76–1.88 (m, 1 H, CH), 1.29–1.38 (m, 1
H, CH). 13C NMR (125 MHz, CDCl3): δ = 24.8, 29.0, 44.1, 51.3,
55.7, 79.0, 109.6, 112.5, 113.8, 123.5, 124.9, 125.9, 127.5 (2 C),
127.9, 128.9 (2 C), 129.7, 129.8, 130.2, 135.6, 143.1, 147.2, 164.7,
175.0, 200.6. IR (neat): 3343, 3062, 2939, 2840, 1724, 1669, 1597,
1495, 1466, 1360, 1252, 1178, 1109, 910, 731 cm–1. HRMS (ESI+):
m/z (%) calcd for C26H23NO4Na [M + Na]+: 436.1519; found:
436.1508; [α]D22.9 –33.2 (c 0.95, CHCl3, 82% ee).
O
R3
O
O
R1
R2
N
BrR*2SnO
Bn
2
OSnR*2Br
*
*
R3
O
R1
N
5
R2
Bn
6
Acknowledgment
O
R1
R2
MeOCOCX3
We gratefully acknowledge the financial support from the Iodine
Research Project in Chiba University led by Professor Hideo Togo
and the COE Start-up Program in Chiba University led by Professor
Takayoshi Arai.
MeOH
R3
HO
OCOCX3
*
*
O
R1
R*2SnBr(OMe)
NaBr
N
R2
1 (X = F or Cl)
Bn
3
Supporting Information for this article is available online at
NaOMe
R*2SnBr2 (4a)
http://www.thieme-connect.com/ejournals/toc/synlett.SnuIpofoiprmntgirStanoIuifpog
r
t
iornat
Scheme 2 Plausible catalytic cycle for the asymmetric aldol reaction
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In conclusion, we have developed a novel method for the
catalytic asymmetric synthesis of chiral isatin derivatives
via the enantioselective aldol reaction of alkenyl trihalo-
acetates with achiral isatins. The employment of in situ
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kylated 3-hydroxy-2-oxindoles in a diastereoselective
manner, and high enantioselectivities of up to 98% ee
have been realized even from acyclic ketone-derived alke-
nyl esters. Further studies of the application of the catalyt-
© Georg Thieme Verlag Stuttgart · New York
Synlett 2012, 23, 1783–1788

1788
A. Yanagisawa et al.
LETTER
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