Anionic-anionic asymmetric tandem reactions: One-pot synthesis of optically pure fluorinated indolines from 2-p-tolylsulfinyl alkylbenzenes

Article abstract of DOI:10.1002/anie.200802885

Chirality switch up: A novel strategy has been developed to give optically pure fluorinated indolines with one or two stereogenic centers (see scheme; A_N = nucleophilic addition, PG = protecting group, S_NAr = intramolecular nucleophilic aromatic substitution, Tol = tolyl). Almost complete stereoselectivity and mild reaction conditions are the key features of the title reaction. (Chemical Equation Presented)

Full text of DOI:10.1002/anie.200802885

Angewandte
Chemie
DOI: 10.1002/anie.200802885
Asymmetric Synthesis
Anionic–Anionic Asymmetric Tandem Reactions: One-Pot Synthesis
of Optically Pure Fluorinated Indolines from 2-p-Tolylsulfinyl
Alkylbenzenes**
JosØ Luis García Ruano,* JosØ Alemµn, Silvia Catalµn, Vanesa Marcos, Silvia Monteagudo,
Alejandro Parra, Carlos del Pozo, and Santos Fustero*
The indoline skeleton is a ubiquitous scaffold found in the
structures of several alkaloids[1] and other natural products
which have diverse biological activity.[2] Indolines are consid-
ered to be “privileged structures”[3] which accounts for their
widespread use not only as building blocks in the total
synthesis of natural products, but also as a common motif in
the design of new biologically significant compounds. Indo-
lines have also been successfully employed as chiral auxil-
iaries in asymmetric synthesis.[4] Consequently, a number of
multistep strategies have been developed for synthesizing
these compounds.[5] However, no short and efficient methods
for the preparation of enantiomerically pure substituted
indolines have been developed to date, therefore the search
for new methods for synthesizing them remains a challenge.
Surprisingly, no examples of fluorinated indolines have been
reported despite the fact that the inclusion of fluorinated
fragments, such as the trifluoromethyl group,[6] in organic
molecules has contributed significantly to the development of
new pharmaceuticals.[7]
with complete control of the configuration at the two
[9]
stereogenic centers that are created simultaneously.
We
hypothesized that amines resulting from nucleophilic addition
(AN) could serve as substrates for the synthesis of optically
pure indolines—once the optimal reaction conditions for the
displacement of the sulfinyl group by the nitrogen center are
determined, an intramolecular nucleophilic aromatic substi-
tution (SNAr) could take place (Scheme 1).[10] We also felt that
Scheme 1. Synthesis of fluorinated indolines. RF =CF3 or CF2Cl.
PG=protecting group, Tol=tolyl.
Our research group has recently reported that reactions of
2-(p-tolylsulfinyl) benzylcarbanions with different electro-
philes take place with almost complete control of the
configuration at the benzylic position, thus providing one of
the best methods for creating optically pure benzylic carbon
centers.[8] In particular, the use of imines as electrophiles has
allowed us to synthesize 2-phenylethyl (and propyl) amines
both processes (AN and SNAr) could occur in a tandem
fashion to afford optically pure indolines from 2-(p-tolylsul-
finyl) alkylbenzenes and imines. As part of our continuing
study towards the development of new fluorinated com-
pounds,[11] we set out to determine suitable reaction con-
ditions for the preparation of these previously unreported
optically pure fluorinated indolines, by using fluorinated
imines as electrophiles. Herein, we present optimized nucleo-
philic addition (AN) and the intramolecular nucleophilic
aromatic substitution (SNAr) reactions, each separately and in
a tandem protocol. This approach constitutes a new strategy
for synthesizing enantiomerically pure fluorinated indolines
(Scheme 1).
[*] Prof. Dr. J. L. García Ruano, Dr. J. Alemµn, V. Marcos, A. Parra
Departamento de Química Orgµnica (C-I)
Universidad Autónoma de Madrid
Cantoblanco, 28049 Madrid (Spain)
Fax: (+34)91-497-466
E-mail: joseluis.garcia.ruano@uam.es
Dr. S. Catalµn, S. Monteagudo, Dr. C. del Pozo, Prof. Dr. S. Fustero
Departamento de Química Orgµnica
Universidad de Valencia, Burjassot, 46100 Valencia (Spain)
E-mail: santos.fustero@uv.es
Deprotonation of sulfoxide (S)-1A[8] at its benzylic
position with LDA at ꢀ788C and subsequent treatment
with fluorinated imines 2a–d and then protonation at ꢀ788C
gave mixtures of the two diastereoisomers 3 (major) and 4
(Table 1, entries 1–4). Aldimines 2a and 2b afforded a 70:30
diastereoisomeric mixture (3Aa/3Ab and 4Aa/4Ab, respec-
tively) that were easily separable (Table 1, entries 1 and 2).[12]
The use of ketimines 2c and 2d as electrophiles led to a
significant increase in the selectivity (80% and 92% de). The
major products 3Ac and 3Ad were isolated in 60% and 77%
yield, respectively (Table 1, entries 3 and 4). The results were
Prof. Dr. S. Fustero
Laboratorio de MolØculas Orgµnicas
Centro de Investigación Príncipe Felipe, 46013 Valencia (Spain)
E-mail: sfustero@cipf.es
[**] Financial support of this work by the Ministerio de Educación y
Ciencia (CTQ2006-06741/BQU and CTQ2007-61462) is gratefully
acknowledged. A.P., V.M., S.C., and S.M. express their thanks for
predoctoral fellowships. C.P. acknowledges the Ministerio de
Educación y Ciencia for a Ramón y Cajal contract.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200802885.
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7941

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Table 1: Addition of sulfinyl carbanions derived from (S)-1 to fluorinated
imines 2. LDA=lithium diisopropylamide, PMP=p-methoxyphenyl.
Table 2: Preparation of fluorinated indolines 5. HMDS=hexamethyldi-
silazane.
Entry
R1
R2
Product
Yield [%]
Ent Sulfoxide
(R1)
Imine
d.r.
Product (yield [%])
1
2
3
4
5
6
H
H
Me
Me
Me
Allyl
H
Me
H
Ph
Me
Me
5Aa
5Ad
5Ba
5Bc
5Bd
5Dd
64
67
66
83
71
60
(R2, RF)
3/4[a]
1
2
3
4
5
6
7
8
9
(S)-1A (H)
(S)-1A (H)
(S)-1A (H)
(S)-1A (H)
(S)-1B (Me) 2a (H, CF3)
(S)-1B (Me) 2b (H, CF2Cl)
(S)-1B (Me) 2c (Ph, CF3)
(S)-1B (Me) 2d (Me, CF3)
2a (H, CF3)
2b (H, CF2Cl)
2c (Ph, CF3)
2d (Me, CF3)
69:31
70:30
90:10
96:4
>98:<2 3Ba (71)
>98:<2 3Bb (69)[b]
>98:<2 3Bc (60)
>98:<2 3Bd (86)[b]
3Aa (51)[b] + 4Aa (23)
3Ab (56) + 4Ab (24)
3Ac (60) + 4Ac
3Ad (77) + 4Ad
conditions for the transformation of 3 into 5 involved the use
of KHMDS in THF at 08C.[15] The cyclization of different
aminosulfoxides 3, derived from both aldimines (Table 2,
entries 1 and 3) and ketimines (Table 2, entries 2 and 4–6),
was also studied. Yields ranged from between 60% and 83%.
Asymmetric tandem processes that generate one or more
stereogenic centers in a selective fashion have undeniable
benefits in the field of synthetic organic chemistry.[16] Thus, we
then evaluated the tandem process starting from sulfoxides
(S)-1 and imines 2 without isolation of the addition products 3
(Table 3). Initially KHMDS was used (the base that provided
the best results in the cyclization reaction), but it was
unsuccessful.[17] However, we found that when the reaction
mixture of (S)-1 and 2 was allowed to reach room temper-
ature in the presence of LDA (before protonation), the
intramolecular cyclization took place smoothly in about one
(S)-1B (Me) 2e (2-furyl, CF3) >98:<2 3Be (65)
10 (S)-1C (Et)
11 (S)-1C (Et)
12 (S)-1D (Allyl) 2a (H, CF3)
13 (S)-1D (Allyl) 2d (Me, CF3)
14 (S)-1E (Bn)
15 (S)-1E (Bn)
2a (H, CF3)
2d (Me, CF3)
>98:<2 3Ca (60)
>98:<2 3Cd (40)
>98:<2 3Da (51)
>98:<2 3Dd (52)
>98:<2 3Ea (74)
>98:<2 3Ed (68)
2a (H, CF3)
2d (Me, CF3)
[a] Diastereomeric ratio was determined by 19F and H NMR spectros-
copy. [b] Absolute configuration was determined by X-ray analysis. Bn=
benzyl.
1
much better when substituted benzylcarbanions such as (S)-
1B–E were used.[8,9] The reactions evolved in a completely
stereoselective fashion to yield products 3 as single diaste-
reoisomers with aldimines 2a and 2b (Table 1, entries 5, 6, 10,
12, and 14) and ketimines 2c–e (Table 1, entries 7–9, 11, 13,
and 15). The complete stereoselective control of the quater-
nary centers observed in the reactions with ketimines is
remarkable (even with the enolizable 2d).
Table 3: One pot synthesis of indolines 5 through addition and SNAr.
The absolute configuration of the final products obtained
from sulfoxide (S)-1B was unequivocally established by X-ray
analysis of 3Bb and 3Bd,[13] which were derived from
aldimine 2b and ketimine 2d, respectively. Since these
compounds exhibit the S configuration at the two stereogenic
centers created during the reaction,[14] we have assigned the
same configuration to all the amines that were obtained as
exclusive products in the reactions of (S)-1B (3Ba–3Be), (S)-
1C (3Ca and 3Cd), (S)-1D (3Da and 3Dd), and (S)-1E (3Ea
and 3Ed). The S configuration for the only stereogenic center
of the major isomers obtained in the reaction of (S)-1A was
determined by X-ray analysis of 3Aa,[13] and we again assume
the same stereochemical outcome for 3Ab, 3Ac, and 3Ad.
Next, we focused our attention on the transformation of 3
into indolines in the presence of base. For some of the
experiments listed in Table 1 we isolated small amounts of a
new product, which was identified as the indoline 5 (Table 2).
Detection of this by-product clearly indicated that intra-
molecular aromatic substitution of the sulfinyl group by the
nitrogen center could take place under the right reaction
conditions. After trying different bases (LDA, LiHMDS,
NaHMDS) and temperatures, we found that the best reaction
Ent
Reagents
Indoline (R1, R2)
d.r.[a]
Yield [%][b]
[c]
1
2
3
4
5
6
7
8
(S)-1A + 2a
(S)-1B + 2a
(S)-1C + 2a
(S)-1D + 2a
(S)-1E + 2a
(S)-1B + 2c
(S)-1C + 2c
(S)-1D + 2c
(S)-1A + 2d
(S)-1B + 2d
(S)-1C + 2d
(S)-1D + 2d
(S)-1E + 2d
(S)-1B + 2e
5Aa (H, H)
5Ba (Me, H)
5Ca (Et, H)
5Da (Allyl, H)
5Ea (Bn, H)
5Bc (Me, Ph)
5Cc (Et, Ph)
5Dc (Allyl, Ph)
5Ad (H, Me)
5Bd (Me, Me)
5Cd (Et, Me)
5Dd (Allyl, Me)
5Ed (Bn, Me)
5Be (Me, 2-Furyl)
–
61[d]
61[e]
61
60
71
55
35
46
67
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
[c]
9
–
10
11
12
13
14
>98:<2
>98:<2
>98:<2
>98:<2
>98:<2
70
40[f]
52
55[f]
60
[a] Diastereomeric ratio was determined by 19F and H NMR spectros-
copy. [b] Yield of isolated products. [c] Enantiomers were obtained in this
case. [d] 16% of (S)-1A was recovered. [e] 28% of (S)-1B was recovered.
[f] Cyclization was also possible at ꢀ788C in 90 min.
1
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Chemie
hour to give indolines 5 in moderate yields (35–71%;
Table 3).
vicinal stereogenic centers (one of them quaternary when
starting from a ketimine). Additional interest in the mecha-
nism of this nucleophilic addition stems from the high levels
of selectivity achieved, and deserves further in-depth inves-
tigation.
The reactions were completely stereoselective and thus
fluorinated indolines 5 containing one or two stereogenic
centers were obtained as single compounds, even those
bearing quaternary carbon atoms. Initially, 5Aa and 5Ad
were obtained as indicated in Table 2, entries 1 and 2 (the
starting amines were obtained as a diastereoisomeric mixture
of 3 and 4 which required separation before cyclization),
however, the reaction of (S)-1A with 2a or 2d under the
tandem process reaction conditions afforded 5Aa (61%,
along with 16% of recovery sulfoxide; Table 3, entry 1) or
5Ad (67%; Table 3, entry 9) as the only indolines. Cyclization
Received: June 17, 2008
Published online: September 9, 2008
Keywords: g-sulfinyl carbanions · asymmetric synthesis ·
.
fluorine chemistry · indolines
of sulfoxides where R1 ¼
6
H were faster, and took place at
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ꢀ788C, although they were still slower than the cyclization
required for the preparation of amine 3 (Table 1). Thus,
reactions of 2d with (S)-1B or (S)-1E at ꢀ788C and
subsequent protonation after 10 min afforded 3Bd and 3Ed,
respectively, (Table 1, entries 8 and 15) whereas 5Bd and 5Ed
were obtained exclusively after one hour at low temperature
(Table 3, entries 10 and 13).
The results obtained for the above reactions point to an
intramolecular substitution of the sulfinyl group by the
nitrogenated anion, thus suggesting an SNAr process. How-
ever, since reactions involving the sulfinyl functionality as the
leaving group and a nitrogen center as the nucleophile have
yet to be described[10] we set out to demonstrate unequiv-
ocally that an SNAr process was involved in the conversion of
3 into 5 (most of the references regarding intramolecular
SNAr reactions entail strongly deactivated rings bearing F or
NO2 leaving groups).[18] We carried out several experiments
which resulted in the exclusion of a radical mechanism and
supported the nucleophilic character of the reaction on the
basis of the substituent effects at the aromatic ring. Thus,
electron-donating groups preclude the reaction, whereas
electron-withdrawing groups accelerate it considerably
(Scheme 2; see the Supporting Information for details).
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Scheme 2. Influence of the substituents on the tandem reaction.
In conclusion, we have outlined a new strategy for the
preparation of optically pure fluorinated indolines containing
one or two stereogenic centers. Our approach involves the
direct reaction of N-PMP-fluorinated imines[19] with 2-(p-
toluenesulfinyl) alkylbenzenes in the presence of LDA.
Almost complete stereoselectivity and mild conditions are
the key features of these tandem processes, which include the
unusual intramolecular nucleophilic aromatic substitution of
a p-tolylsulfinyl group by the amide anion as the key reaction.
Moreover, we have demonstrated that reactions of sulfinyl-
ated benzylcarbanions with fluorinated aldimines and ket-
imines lead to the synthesis of fluorinated amines with two
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7943

Communications
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[15] The use of LDA as the base provided lower yields in most cases.
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do not react under similar reaction conditions, which is evidence
for the increase in reactivity provided by the fluorinated
substituent.
[17] Benzylcarbanions are a strong purple colored, but when com-
pounds 1 are treated with KHMDS at ꢀ788C the solutions
remain colorless, which suggests that carbanions are not formed
under these reaction conditions.
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[19] Non-fluorinated indolines can also be obtained in high yields by
the reaction of KHMDS with the amines obtained in reactions of
lithiated (S)-1B with N-PMP derivatives of benzylideneimine
and p-cyanobenzylidene imine under the reaction conditions
outlined in Table 2. Reactions are slower that those of the
fluorinated amines. Direct treatment of (S)-1B with N-PMP-
benzylideneimine under the reaction conditions outlined in
Table 3 afforded the product in low yield (see the Supporting
Information).
[13] CCDC 689575 (3Bb), 689576 (3Bd) and 690716 (3Aa) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[14] When R2 = Ph or 2-furyl, in compounds 3Ac, 3Bc, and 3Be, the
priority of the substituents in the nitrogen containing stereogenic
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