Phosphine-catalyzed intramolecular Rauhut-Currier reaction: Enantioselective synthesis of hydro-2: H -indole derivatives

Article abstract of DOI:10.1039/c7ob01820g

A highly enantioselective intramolecular Rauhut-Currier reaction catalyzed by a multifunctional chiral aminophosphine catalyst was reported. A series of hydro-2H-indole derivatives that bear an all-carbon quaternary center were obtained in high yields (up to 94%), and excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). And this reaction could be performed on a gram scale using 2 mol% catalyst loading.

Full text of DOI:10.1039/c7ob01820g

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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Phosphine-Catalyzed Intramolecular Rauhut-Currier Reaction:
Enantioselective Synthesis of Hydro-2H-Indole Derivatives
Hongxing Jin,a Qinglong Zhang,a Erqing Li, a Penghao Jia, a Ning Li a and You Huang∗ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A highly enantioselective intramolecular Rauhut-Currier reaction enantioselective intramolecular RC reaction toward nitrogen-
catalyzed by a multifunctional chiral aminophosphine catalyst was containing skeletons bearing two vicinal chiral centers from
reported. A series of hydro-2H-indole derivatives that bear an all- simple precusors. Most significantly, the catalyst loading could
carbon quaternary center were obtained in high yields (up to be decreased down to 2 mol% without deminishing the
94%), excellent diastereo- and enantioselectivity (up to >20:1 d.r. efficiency.
and >99% ee). And this reaction could be performed on a gram
Previous work:
scale using 2 mol% catalyst loading.
Well established
)
a) C-containing cyclic compound via AIRC
O
(
O
Nitrogen-containing heterocycles are ubiquitous core
structures of various biologically active compounds, natural
products, and pharmaceutical agents. For example, Aporphine
alkaloids telisatin A and B showed cytotoxic activity against
three human cancer cell (GSC-7901, K562, and SPCA-1).
Solanidine mainly present as glycosides, can inhibit
proliferation and exhibit obvious antitumor effect (Fig. 1).1 In
this context, various synthetic strategies have been devised in
recent years for the synthesis of hydro-2H-indole derivatives.2
Despite the fact that much progress has been made in the
construction of nitrogen-containing cores, most of them was
focused on the racemic transformations. The development of
an enantioselective protocol for the synthesis of such skeloton,
employing readily available starting materials and reagents
under user-friendly synthetic conditions, is still a formidable
challenge and in great demand. Herein, we reported our
methodology of organophosphine-catalyzed
O
R2
O
R2
Miller, Gladysz, Christmann,
Wu and Shi
R1
R1
n
Well established
( )
b) O-containing bicyclic compound via AIRC
O
O
R2
R2
O
Sasai, Zhang, Xiao,
Spring and Albrecht
R1
R1
n
n
O
O
O
This work:
c) N-containing bicyclic compound via AIRC (Unknown)
O
O
multifunctional organophosphine
O
O
N
N
R2
R3
R1
R3
R1
R2
Scheme 1. Reported asymmetric intramolecular Rauhut-Currier (AIRC) reaction and our
work.
Over the past decade, enantioselective nucleophilic
phosphine catalysis has captured considerable attention and
been developed as a practical approach to structurally diverse
and synthetically valuable skelotons.3 Among them, the
Rauhut-Currier (RC) reaction, also known as the vinylogous
Morita-Baylis-Hillman reaction, involves the coupling of one
active alkene/latent enolate to another Michael acceptor,
Fig. 1 Nitrogen-containing bioactive molecules.
creating a new C-C bond between the α-position of one
activated alkene and the -position of a second alkene with
a.State Key Laboratory and Institute of Elemento-Organic Chemistry, College of
Chemistry, Nankai University, Tianjin 300071, China.
β
the catalysis of a nucleophilic catalyst, was first reported by
Rauhut and Currier in 1963.4 However, less progress has been
made on this reaction during the past decades due to the lack
of efficient control of the selectivity.5 The pioneering
b.Collaborative Innovation Center of Chemical Science and Engineering (Tianjin)
E-mail: hyou@nankai.edu.cn
† Electronic Supplementary Information (ESI) available: CCDC 1524099 (1e) and
CCDC 1524100 (2b) For ESI and crystallographic data in CIF or other electronic
format See DOI: 10.1039/x0xx00000x
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methodological studies of the intramolecular version of RC within 15 min with 64% yield, and then we tested various
reaction that employed bis-enone substrates were chiral phosphine catalyst 3a 3b 3c and cinchona derived 3d
demonstrated by Krische et al. and Roush et al. respectively, in (Table entries 2-5). The outcome showed that
,
,
1
2002.6 Later on, attractive systems based on achiral catalysis multifunctional phosphine catalyst 3c displayed the best
have been developed for the RC process.7 The first results (99% yield, 95.5% ee). The methylated catalyst 3e was
breakthrough in the enantioselective RC reaction was reported used to elucidate the effect of the phenolic hydroxy group. As
by Aroyan and Miller in 2007.8 And the past decade has seen expected, although the reaction proceeded with good yields,
remarkable advance in enantioselective RC reactions, and the ee value decreased sharply to 58% (Table 1, entry 6), which
many excellent examples describing enantioselective unveiled the importance of hydrogen bonding in stereo-
intermolecular and intramolecular RC reaction have been control. To additionally improve the enantioselectivity of the
reported.9 And it is very meaningful to synthesis fields that reaction, solvent and temperature screening was performed
those tactics had been successfully applied in the synthesis of and toluene was found to be the best solvent. In additon, the
several natural products.10 It is noteworthy that in contrast to enantiometric excess was further improved when conducted
rich literature reports on the construction C-containing and O- at 0 oC.
containing cyclic compounds with the asymmetric
O
O
catalyst (20 mol%)
O
O
intramolecular RC strategies (AIRC) (Scheme 1a, 1b), there
wasn’t an isolated example up to date on the AIRC reaction
involving the construction N-containing cyclic compounds.
During the submitting of this manuscript, Takizawa and Sasai
has reported a very brilliant asymmetric intramolecular RC
work.11 Inspired by these previous work and as part of our
continual interest in asymmetric nucleophilic phosphine
catalysis, we were, therefore, very interested in developing
such a process (Scheme 1c).
toluene, 0 oC, 30 min
N
R2
N
R2
R3 R1
R3 R1
1
2
O
O
O
O
O
N
N
O
Ts
Ts
N
Ts
R
R
R
2a, 94%, 96% ee, R = H
2b, 75%, 96% ee, R = 4-F
2c, 77%, 87% ee, R = 4-Cl
2d, 82%, 96% ee, R = 3-Cl
2e, 68%, 88% ee, R = 4-Br
2f, 66%, 90% ee, R = 4-CF3
2i, 91%, 98% ee, R = 3-Me
2j, 73%, >99% ee, R = 3-OMe
2k, 77%, 92% ee, R = 4-Phenyl
2l, 77%, 72% ee, R = Methyl
2m, 66%, 84% ee, R = Ethyl
2n, 87%, 85% ee, R = Vinyl
2o, 42%, 88% ee, R =
Ethynyltrimethylsilyl
2g, 93%, 95% ee, R = 3,5-2CF3
2h, 83%, 97% ee, R = 3,4,5-3F
Table 1. Optimization of the Reaction Conditionsa
O
O
O
O
O
O
O
O
O
O
N
N
N
N
S
Ts
S
Ts
O
O
Br
Br
2q, 87%, 97% ee
2p, 88%, 95% ee
2r, 79%, 91% ee
2s, 51%, 95% ee
d.r. = 4:1
O
O
O
O
O
O
O
N
O
O
N
N
Cl
S
Ts
Ts
MeO
N
Ts
O
CF3
2u, 33%, 96% ee[b]
2v, 47%, 73% ee[b]
2t, 76%, 86% ee
2w, 49%, 54% ee[c]
entry Cat.
solvent
T
yieldb
[%]
64
eec
[%]
-
Scheme 2. aUnless otherwise specified, all reactions were carried out with 1 (0.1 mmol)
in toluene (1.0 mL) within 30 min. bReaction with 0 oC, 60 min. cReaction with 25 oC, 6 h.
[%]
[oC]
1
PPh3 (20) toluene 20
2
3a (20)
3b (20)
3c (20)
3d (20)
3e (20)
3c (20)
3c (20)
3c (20)
3c (20)
3c (20)
toluene 20
toluene 20
toluene 20
toluene 20
toluene 20
88
95
71
96
78
58
94
93
96
96
95
With the optimized reaction conditions in hand, we
subsequently established the scope of the reaction (Scheme 2).
The reaction was applicable to a wide range of different aryl
substituents on the starting materials. High yields and
excellent enantioselectivities were attained regardless of the
3
59
4
99
5
82
6
73
7
CHCl3
toluene 10
toluene
20
76
electronic properties (Scheme 2, 2a
substituent from methyl to ethyl was tolerated and the
corresponding cyclized products 2l 2m were obtained in good
-2k). Variation of the alkyl
8
82
9d
10d
11e
0
94
,
toluene -10
toluene -20
83
yield with 72-88% ee. Substrates 1n and 1o, which contain
vinyl or ethynyl side chains, also worked well and the desired
products 2n and 2o were isolated in good yield and ee. A
slightly lower yield or ee may due to a smaller steric hindrance
in starting material among 1l to 1n. Furthermore, cyclopentyl
and cyclohexanyl substituted product could be obtained in an
acceptable yield and enantioselectivity. Several different
sulfonyl groups on nitrogen atom also give satisfactory results
84
aUnless otherwise specified, all reactions were carried out with 1a (0.1 mmol) in
solvent (1.0 mL) with 15 min. bYields of isolated products. cDetermined by HPLC
analysis using a chiral stationary phase. dReaction with 30 min. eReaction with
180 min.
We began our investigation by selecting phenyl-substituted
cyclohexadienone 1a as a prototypical substrate. To our
delight, PPh3 can smoothly catalyze the RC reaction in 20 oC
(Scheme 2, 2r
-2t). The chlorine and methoxy-substituted
2 | J. Name., 2012, 00, 1-3
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cyclohexadienones substrate resulted in low yield and good ee
(Scheme 2, 2u 2v). Notably, substrate 1w, which is derived
-
from furfuryl alcohol, offers an opportunity to construct
bicycle hydropyrrole scaffolds with two contiguous chiral
centers, albeit with poor yield and ee value. The relative
configuration of
hydroindole skeleton was determined
unambiguously to be cis configuration by using single crystal X-
12
ray analysis conducted on a representative RC product 2b
should be noted that only one single diastereomer was
obtained in all cases except 2q
. It
.
In order to display the potential applicability of this protocol,
we performed the reaction on a gram scale using 4 mmol of
substrate. Firstly, 20 mol% 3c was used as the catalyst in 40 mL
toluene (the same catalyst concentration as the optimized
reaction conditions) providing 2a in 81% yield and 96% ee
(Table 2, entry 2). We were delighted to find that when the
catalyst loading was decreased to 2 mol%, the reaction still
smoothly proceeded in 5 mL toluene to provide 2a in 82% yield
and 99.5% ee (Table 2, entry 6). The gram-scale reactions were
Scheme 3. Further transformation of products.
The proposed mechanism of the RC reaction using the chiral
catalyst 3c bearing both Brønsted acid (-NHBz and -OH) and
Lewis base (-PPh2) moieties is shown (Scheme 4). First the
Michael addition of the LB moiety to the acrylate unit
generates the phosphonium enolate
A
. Then the enolate
reacts with one of the olefins on the dienone. This second
Michael addition results in the formation of the intermediate
Finally, a proton transfer process results in the formation of
the chiral nitrogen-containing hydro-2H-indole core skeleton
A
also performed in the presence of 2 mol% catalyst to obtain 2b
2d, and 2i in good yields and excellent ee values within
acceptable reaction time from 40 min to 3 h (Table 2, entries
7-9).
,
B
.
2,
along with regeneration of the catalyst through a retro-
Michael reaction. According to our experiment results and
previous research reports,14 we found that the Brønsted acid
unit plays an important role in controlling steric configuration
of the product by intramolecular hydrogen bond interaction,
and the proton transfer step is a crucial process.15
Table 2. RC reactions on a multigram scale.
entr R1
y
x
toluene product
[mL]
yielda eeb
[%]
[%]
96
1c,f
2d,f
3d,g
4d,h
5d,i
6e,j
7e,j
8e,i
9e,k
H
20
20
10
5
1
2a, 37mg 94
2a, 1.28g 81
2a, 1.26g 80
2a, 1.50g 95
2a, 1.35g 86
2a, 1.28g 82
2b, 1.23g 75
2d, 1.49g 87
H
40
20
10
5
96
H
97
H
98
H
2.5
2
97
H
5
99.5
93
4-F
3-Cl
3-
Me
2
5
2
5
91
2
5
2i, 1.43g
88
99.5
Scheme 4. Proposed mechanism of the RC process.
aYields of isolated products. bDetermined by HPLC analysis using
a
chiral
(0.1 mmol), 3c (20 mol%) in
1 (4 mmol), 3c (x mol%) in
stationary phase. cReaction was carried out with
1
toluene at 0 oC. dReaction was carried out with
toluene at 25 oC. fReaction with 30 min. gReaction with 60 min. hReaction with
toluene at 0 oC. eReaction was carried out with
1
(4 mmol), 3c (2 mol%) in
In summary, we have developed a highly enantioselective
intramolecular Rauhut-Currier reaction of cyclohexadienones
120 min. iReaction with 150 min. jReaction with 180 min. kReaction with 40 min.
catalyzed by a multifunctional chiral aminophosphine catalyst.
This present approach could be used to construct nitrogen-
containing hydro-2H-indole core skeletons that bear an all-
carbon quaternary center rapidly (reaction completion within
30 min). Excellent stereoselectivity and yields could be
obtained in most cases (up to 20:1 d.r. and >99% ee and up to
94% yield) under mild conditions. It should be noted that the
reaction undergo smoothly on a gram scale using 2 mol%
catalyst loading. Further investigation of the reaction
mechanism and the application of this method to natural
And then two transformations were performed to
demonstrate the synthetic utility of the highly functionalized
RC product. The Michael addition of dimethyl malonate to 2a
produced 4a (d.r.>20:1) in good yields without decreasing in
the enantioselectivity (Scheme 3, Eq. (1)). Furthermore, the
Lu’s [3+2] annulation process could be easily achieved in the
presence of allenic esters and a new spirocyclo skeleton 4b
was obtained with satisfactory yield and enantioselectivity
results (Scheme 3, Eq. (2)).13
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Chem. 2010, 75, 5784; (c) P. S. Selig and S. J. Miller,
Tetrahedron Lett. 2011, 52, 2148.
product synthesis are underway and will be reported in due
course.
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This work was financially supported by the National Natural
Science Foundation of China (21672109,21472097,21421062)
and the Natural Science Foundation of Tianjin (15JCYBJC20000).
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12 CCDC 1524100
(
2b
)
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. And
the configurations of the products are consistent with the
configurations in Sasai’s system (ref. 11) according to the
value of specific rotation.
4
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