Chiral phosphoric acid-catalyzed enantioselective aza-Friedel-Crafts alkylation of indoles with γ-hydroxy-γ-lactams

Article abstract of DOI:10.1002/adsc.201201008

An enantioselective aza-Friedel-Crafts reaction of indoles with γ-hydroxy-γ-lactams using a chiral phosphoric acid catalyst is reported. The approach described herein provides an efficient access to 5-indolylpyrrolidinones in good to quantitative yields and excellent enantioselectivities (up to >99% ee). The results suggest that the reaction may proceed via N-acyliminium intermediates associated with the chiral phosphoric acid anion. Copyright

Full text of DOI:10.1002/adsc.201201008

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DOI: 10.1002/adsc.201201008
Chiral Phosphoric Acid-Catalyzed Enantioselective Aza-Friedel–
Crafts Alkylation of Indoles with g-Hydroxy-g-lactams
Thibaut Courant,a Sirirat Kumarn,b,c Long He,a Pascal Retailleau,a
and Gꢀraldine Massona,*
a
Centre de Recherche de Gif, Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-Yvette Cedex, France
Fax : (+33)-1-6907-7247; e-mail: masson@icsn.cnrs-gif.fr (http://www.icsn.cnrs-gif.fr/spip.php?article838)
Chulabhorn Research Institute, 54, Kamphaeng Phet 6 Road, Laksi, Bangkok 10210, Thailand
Institute of Molecular Biosciences, Mahidol University, 25/25 Phuttamonthon 4 Road, Salaya, Nakhon Pathom, 73170
b
c
Thailand
Received: November 14, 2012; Revised: January 15, 2013; Published online: March 15, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201201008.
Abstract: An enantioselective aza-Friedel–Crafts
reaction of indoles with g-hydroxy-g-lactams using
a chiral phosphoric acid catalyst is reported. The
approach described herein provides an efficient
access to 5-indolylpyrrolidinones in good to quanti-
tative yields and excellent enantioselectivities (up
to >99% ee). The results suggest that the reaction
may proceed via N-acyliminium intermediates asso-
ciated with the chiral phosphoric acid anion.
TMSCl.[9] Subsequently, Rueping et al.,[10] Zhou
et al.[11] and You et al.[12] described chiral Brønsted
acid-catalyzed FC reactions involving more stable imi-
nium intermediates, including 2-alkyl and non-isomer-
izable iminiums. More recently, Huang and co-work-
ers observed a different regioselectivity for the reac-
tion of indoles with a,b-unsaturated g-lactam-derived
N-acyliminium ions in the presence of a chiral phos-
phoric acid catalyst, exclusively giving chiral N-alky-
lated indoles with good enantioselectivities.[13] There-
fore, the development of novel catalytic enantioselec-
tive methods to construct 5-indolylpyrrolidinones still
remains a challenging and attractive field of research.
Asymmetric counterion-directed catalysis (ACDC)
introduced by List[14] has recently been proved to be
an efficient strategy for enantioselective reactions
that proceed via cationic species and chiral countera-
Keywords: alkylation; asymmetric synthesis; aza-
Friedel–Crafts reaction; chiral phosphoric acids; or-
ganocatalysis
Chiral 5-indolylpyrrolidinones are a class of com- nions.[8–16]The use of chiral phosphoric acids as stereo-
pounds found in a number of natural products and directing counteranions was inspired by the high effi-
synthetic molecules with a wide array of biological ac- ciency of these bifunctional Brønsted acid catalysts
tivities, offering synthetic utility as structural motifs for various enantioselective transformations.[17] Based
and pharmacological potential.[1] The aza-Friedel– on this idea, we hypothesized that the combination of
Crafts (aza-FC) reaction represents one of the most a chiral phosphoric acid and a g-hydroxy-g-lactam
straightforward approaches for the stereoselective would result in an effective catalytic iminium salt[18]
construction of these structures.[2] Although the cata- to undergo enantioselective aza-FC alkylation of in-
lytic asymmetric variant of the transformation has doles.
been demonstrated using a-iminoalkyl-glyoxylate[3]
Our initial studies involved a reaction of indole 1a
and imines derived from aromatic[4] as well as aliphat- and g-hydroxy-g-lactam (5-hydroxy-N-phenyl-2-pyrro-
ic aldehydes,[4a,d,h,5] only limited success was achieved lidinone, 2a) in the presence of a chiral phosphoric
using cyclic aliphatic imines.[1i,6,7] The tendency of acid catalyst (4) in dichloromethane (DCM) at room
cyclic aliphatic imines to isomerize into enamine de- temperature. Catalysts with various substitution pat-
rivatives under acidic conditions renders the catalytic terns at the 3,3’-positions of BINOL were screened,
enantioselective development of this transformation all of which gave the desired C3-alkylated product 3a
much more difficult.[6c,8] The first example of an enan- in excellent yields and no N-alkylation by-product
tioselective intermolecular FC reaction with cyclic N- was observed. The 9-phenanthryl-substituted catalyst
acyliminium ions has been recently developed by Ja- (4e, entry 5) was found to be the best in terms of
cobsen et al. using a thiourea-Schiff base catalyst and yield and enantioselectivity. A solvent screening study
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Chiral Phosphoric Acid-Catalyzed Enantioselective Aza-Friedel–Crafts Alkylation of Indoles
Table 1. Optimization studies the chiral phosphoric acid-catalyzed aza-FC reaction between indole 1a and hydroxylactam
2.[a]
Entry
4a [Ar]
2 [equiv.]
c [mol/L]
Solvent
Yield [%][b]
ee [%][c]
1
4a C6H5
2
0.1
DCM
76
7
2
4b 9-anthracenyl
2
0.1
DCM
69
38
3
4c 2-naphthyl
2
0.1
DCM
80
60
4
4d 1-naphthyl
2
0.1
DCM
83
75
5
4e 9-phenanthryl
2
0.1
DCM
80
82
6
7
8
9
10
11
12
4e
4e
4e
4e
4e
4e
4e
2
2
0.1
0.1
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
87
88
91
95
97
>99
>99
>99
>99
90
1.2
1.2
1.2
1.2
1.2
0.025
0.0167
0.0167
0.0167
0.0167
98[d]
64[d,e]
98[f]
>99
[a]
General conditions: indole 1a (0.10 mmol, 1 equiv.), hydroxylactam 2a and catalyst 4 (10 mol%) in a solvent.
Isolated yields of 3a after chromatography.
Enantiomeric excesses were determined by chiral HPLC analysis.
With 5 mol% of catalyst.
Hydroxylactam 2b was used to give 3b.
[b]
[c]
[d]
[e]
[f]
Without MS.
showed that the reaction proceeded with slightly high enantioselectivity (90% ee, entry 9), the enantio-
higher enantioselectivity in 1,2-dichloroethane selectivity of corresponding product 3l dropped mark-
(entry 7). Interestingly, lowering both the concentra- edly when an aromatic counterpart was used (10% ee,
tion and amount of indole (from 2 to 1.2 equiv.) re- entry 10). This also represents the first successful ex-
sulted in quantitative yield and pleasingly high enan- ample of 2-alkylated indoles in the asymmetric aza-
tioselectivity (entries 7–9). Moreover, the catalyst FC reaction. However, when a C-3-substituted indole
loading could be reduced to 5 mol% to give identical was investigated (entry 11), only the N-alkylated re-
results (entries 10 vs. 11). Further studies revealed gioisomer (3m) was observed with 20% ee (see the
that the protecting group on the amide functionality Supporting Information). The electron demand of the
of the hydroxylactams had an influence on the enan- N-aryl group on hydroxylactams 2 was investigated by
tioselectivity, which was only moderate when 2b was introducing a deactivating substituent at the para po-
used (64% ee, Table 1, entry 11). Conveniently, similar sition. It was pleasing to find that uniformly high
results were observed in the absence of molecular enantioselectivity and reaction yield were observed
sieves (MS) (entries 9 vs. 12).
when a halogen was present (Br or I; entries 12–16).
After establishing the optimal reaction conditions However, when hydroxylactam 2f, which is strongly
(Table 1, entry 12), the scope of the reaction was ex- deactivating due to the presence of the nitro group,
amined (Table 2). It was found that the enantioselec- was used, a double alkylation product with the lactam
tivity was insensitive to the electronic property of the ring being opened was obtained solely (Scheme 1).[19]
indole (Table 2, entries 1–9, 13, 15, and 16). Indeed,
The reaction also failed with hydroxylactams
reactions of indoles substituted by electron-withdraw- having electron-rich aromatic rings such as para-me-
ing and electron-donating groups at the 5-, 6- and 7- thoxyphenyl. Additionally, the use of an N-protected
positions smoothly proceeded with high to excellent indole, where hydrogen bonding with the catalyst was
enantioselectivity. It is noteworthy that while a 2- not possible, was investigated to support our proposed
alkylACHTUNGTRENNUNGindole, namely 2-methylindole, provided 3k with mechanism (Table 2, entry 18). The resulting product
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Table 2. Chiral phosphoric acid-catalyzed enantioselective
Table 2. (Continued)
synthesis of 5-indolylpyrrolidinones 3.[a]
Entry
3
Yield [%][b] ee [%][c]
11
3m 84
20[d]
Entry
1
3
Yield [%][b] ee [%][c]
12
13
14
15
16
3n 62
3o 84
3p 60
3q >99
83
94
93
97
97
19
3c
>99
97
85
92
95
95
98
94
2
3
4
5
6
7
3d 90
3e 80
3f 90
3r
3s
>99
3g
>99
17
38
3h >99
[a]
General conditions: indole 1 (0.10 mol, 1.0 equiv,), hy-
droxylactam 2 (0.12 mmol, 1.2 equiv,) and catalyst 4e (5
mol%).
Isolated yields after chromatography.
Enantiomeric excesses were determined by chiral HPLC
analysis.
[b]
[c]
3i
3j
>99
[d]
The N-alkylated indole was formed instead of the C-3-al-
kylated product.
8
63
>99
9
3k 90
90
10
10
838
3l
90
Scheme 1. Formation of bisindolyl by-product 5.
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Adv. Synth. Catal. 2013, 355, 836 – 840

Chiral Phosphoric Acid-Catalyzed Enantioselective Aza-Friedel–Crafts Alkylation of Indoles
strates with good to excellent enantioselectivities (up
to 99% ee). A representative product has been trans-
formed into a biologically active 3-(2-pyrrolidinyl)in-
dole in two simple steps. Further work to strengthen
the synthetic utility of optically active 5-indolylpyrro-
lidinones is underway.
Experimental Section
General Procedure
To a stirring solution of N-phenylsuccinimidate 2 (1 equiv.,
0.05 mmol) and indole 1 (1.2 equiv., 0.06 mmol), in dry 1,2-
dichloroethane (3 mL) under argon was added catalyst 4e
(0.005 mmol, 0.05 equiv.). The obtained mixture was stirred
under an argon atmosphere at room temperature until com-
pletion of the reaction as judged by TLC monitoring
(AcOEt/heptane 70/30). The solvent was removed under re-
duced pressure and the residue purified by flash chromatog-
raphy over silica gel (AcOEt/n-heptane 40/60) to afford the
corresponding 5-indolylpyrrolidinone 3.
Scheme 2. Proposed mechanism and stereochemical issue.
(3s) was obtained in low yield and as an almost race-
mic mixture. The absolute configuration of compound
3a was determined to be S by X-ray crystallogra-
phy.[20]
Based on these observations, we surmise that the
phosphoric acid-catalyzed FC alkylation may proceed
via elimination of water from 2, to form a chiral ion
pair 6, followed by hydrogen bonding between the
NH site of indole and the Lewis basic phosphoryl
oxygen (Scheme 2). Then, a pseudo-intramolecular Si-
face attack of indole to the cyclic N-acyliminium
carbon could occur via transition state 7, leading to
the isolated product S-3. This work is a good example
of the dual activity of chiral phosphoric acids that
allows: (i) generation of the catalytic iminium inter-
mediate and (ii) the control of the face of attack by
the ionic pair. However, what is remarkable about
this catalytic system is the efficiency of the domino
Acknowledgements
Financial supports from ICSN and CNRS are gratefully ac-
knowledged. T.C. thanks MESR for a doctoral fellowship,
University of Paris XI. S.K. thanks the Embassy of France to
Thailand for a Junior Research Fellowship.
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