Synthesis of a series of novel chiral Lewis base catalysts and their application in promoting asymmetric hydrosilylation of β-enamino esters

Article abstract of DOI:10.1039/c3ob40430g

A series of novel chiral Lewis base catalysts were synthesized from l-serine and applied in the hydrosilylation of β-enamino esters, in which the optimal one promoted the reactions to afford a wide variety of β-amino esters in good yields with good enantioselectivities. It is noteworthy that several cyclic substrates were hydrosilylated under the optimal conditions to give the cyclic β-amino esters with high yields, good diastereoselectivities as well as good ee values. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ob40430g

Organic &
Biomolecular Chemistry
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Synthesis of a series of novel chiral Lewis base catalysts
and their application in promoting asymmetric
hydrosilylation of β-enamino esters†
Xing Chen,a,b,c Xiao-Yan Hu,b,c Chang Shu,b,c Yong-Hong Zhang,b,c
Yong-Sheng Zheng,b,c Yan Jiang,b,c Wei-Cheng Yuan,b Bo Liu*a and
Xiao-Mei Zhang*b
Cite this: Org. Biomol. Chem., 2013, 11,
3089
Received 1st March 2013,
Accepted 22nd March 2013
DOI: 10.1039/c3ob40430g
www.rsc.org/obc
A series of novel chiral Lewis base catalysts were synthesized from alternative to transition-metal-catalysis in the synthesis of
L-serine and applied in the hydrosilylation of β-enamino esters, in chiral β-amino acids.5e,6 Malkov and co-workers employed an
which the optimal one promoted the reactions to afford a wide L-valine derived formamide as the Lewis base catalyst to
variety of β-amino esters in good yields with good enantio- promote asymmetric hydrosilylation of β-enamino esters to
selectivities. It is noteworthy that several cyclic substrates were afford the corresponding β-amino esters in good yields with
hydrosilylated under the optimal conditions to give the cyclic good enantioselectivities.6a We demonstrated asymmetric
β-amino esters with high yields, good diastereoselectivities as Lewis base organocatalyzed hydrosilylation of β-enamino
well as good ee values.
esters6b as well as α-acetoxy-β-enamino esters.6c Sun and co-
workers reported a highly enantioselective hydrosilylation of
N-alkyl-β-enamino esters catalyzed by an N-sulfinyl L-prolinamide
Lewis base.6d Benaglia and co-workers employed an ephedrine-
derived 4-chloropicolinamide and chiral phosphinamide derived
from proline to accelerate hydrosilylation of β-enamino esters,
respectively.6e,f Jones et al. developed a novel imidazole-based
Lewis base and employed it in enantioselective hydrosilylation of
β-enamino esters.6g Although the present methods delivered a
wide variety of β-amino esters with good yields in good enantio-
selectivities, sometimes they suffered from some harsh con-
ditions, such as low temperature (down to −40 °C) or long
reaction time (up to 2 days). Moreover, reactions of some kinds
of β-enamino esters could not provide satisfactory results.
For example, some cyclic substrates resulted in very poor
enantioselectivities.6b Therefore, it is highly desirable to
develop novel catalytic systems to improve the reaction con-
ditions and adapt them to more types of substrates.
Our group has been actively engaged in research on chiral
Lewis base organocatalyzed hydrosilylation of CvN double
bonds. Recently, we developed a new class of chiral Lewis base
organocatalysts 1 (Fig. 1) which catalyzed asymmetric hydro-
silylation of α-acetoxy-β-enamino esters to afford α-hydroxy-
β-amino esters in high yields with good diastereoselectivities
as well as good enantioselectivities.6c Herein we describe a set
of newly designed analogous catalysts 2 (Fig. 1) and their appli-
cation in asymmetric hydrosilylation of β-enamino esters. To
our delight, the optimal one of 2 promoted asymmetric hydro-
silylation of β-enamino esters successfully. Particularly, some
cyclic substrates underwent the reaction to afford various
cyclic β-amino esters in high yields with good diastereoselectiv-
ities as well as good enantioselectivities.
Introduction
β-Amino acids are key building blocks of peptides, peptidomi-
metics, many natural products and other physiologically active
compounds.1 Notably, peptides based on β-amino acids have
been proven to have secondary structures comparable to their
α-amino acid analogues, but are not vulnerable towards protea-
ses.1b,2 Therefore, the asymmetric synthesis of enantiomeri-
cally pure β-amino acids has been extensively studied.1b,3 In
the past few years, the catalytic asymmetric synthesis of
β-amino acid derivatives using transition metals, organocata-
lysts and biocatalysts has been studied intensively.3i,j Among
various catalytic asymmetric approaches to enantioenriched
β-amino acid derivatives, the most efficient and straightfor-
ward way is catalytic asymmetric reduction of β-enamino
esters. Up to now, many chiral transition metal complexes
have been developed to catalyze asymmetric hydrogenation of
β-enamino esters with high enantioselectivities.4
More recently, Lewis base catalytic asymmetric hydrosilyla-
tion5 of β-enamino esters has emerged as
a powerful
aKey Laboratory of Green Chemistry & Technology of Ministry of Education, College
of Chemistry, Sichuan University, Chengdu, 610041 China
bKey Laboratory for Asymmetric Synthesis and Chiral Technology of Sichuan
Province, Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences,
Chengdu, China. E-mail: xmzhang@cioc.ac.cn; Fax: (+86)28-85257883;
Tel: (+86)28-85257883
cUniversity of Chinese Academy of Sciences, Beijing, China
†Electronic supplementary information (ESI)
available. See DOI:
10.1039/c3ob40430g
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Table 1 Screening of chiral Lewis base catalysts, solvents and temperatures in
the enantioselective hydrosilylation of β-enamino ester 7aa
Entry
Cat.*
Solvent
T (°C)
Yieldb (%)
eec (%)
Fig. 1 Chiral Lewis base organocatalysts 1 and 2.
1
2
3
4
5
6
7
8
2a
2b
2c
2d
2e
2f
2g
2a
2a
2a
2a
2a
2a
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
CH2Cl2
THF
CHCl3
CH3CCl3
ClCH2CH2Cl
ClCH2CH2Cl
−10
−10
−10
−10
−10
−10
−10
−10
−10
−10
−10
0
97
96
97
98
96
96
95
96
95
50
50
96
97
94
92
91
90
88
53
52
92
Cyclic β-amino acids are important building blocks for the
construction of many natural products, biologically active mole-
cules and β-peptides.7 Therefore, the synthesis of enantio-
enriched cyclic β-amino acid derivatives has attracted much
attention.4e,k,8 However, to date, there have been only a few
examples of highly efficient asymmetric catalytic synthesis of
enantioenriched cyclic β-amino acid derivatives. Zhang et al.4e
and Zhou et al.4k have developed transition-metal catalytic
highly diastereoselective and enantioselective hydrogenation of
cyclic β-enamino esters, respectively. We employed a proline
derived chiral Lewis base catalyst to promote the hydrosilylation
of several cyclic β-enamino esters and the reactions gave the pro-
ducts in very high diastereoselectivities, but in very low enantio-
selectivities.6b Gratifyingly, in this study, the newly developed
catalyst 2 made a good compensation for this methodology.
9
87
10
11
12
13
N.D.d
N.D.d
92
−20
93
a Unless specified otherwise, the reaction was carried out with 7a
(0.3 mmol), trichlorosilane (0.6 mmol), catalyst 2 (10 mol%) in 3 mL of
solvent for 10 hours. b Isolated yield based on 7a. c The ee value was
determined by chiral HPLC (Chiralcel OD). d Not detected.
paraformaldehyde or 2,2-dimethoxy propane generates the
novel chiral Lewis base catalysts 2a–2f. As for the synthesis of
2g, it was necessary to protect the amino group of 3 with Boc
before reacting with Grignard reagent.
With these catalysts in hand, we initiated the hydrosilyla-
tion of β-enamino ester 7a in 1,2-dichloroethane at −10 °C.
The results are summarized in Table 1. These sets of new cata-
lysts were found to be highly active and the reactions were
completed in 10 hours to give almost quantitative yields of the
product. First, several diaryl catalysts 2a–2e were tested and
they all exhibited good enantioselection. Among these cata-
lysts, 2a delivered the best ee value of 94% (Table 1, entry 1).
Introduction of electron-donating groups or electron-withdraw-
ing groups at the para positions of the phenyl groups caused
no improvement in enantioselection (Table 1, entries 2–4).
Meanwhile, when catalyst 2e which bears methyl groups at C2
of the six-membered ring of the catalyst was employed, the
product was obtained in excellent yield, but the ee value
decreased obviously (Table 1, entry 5). When the aryl groups
were replaced by alkyl groups, the enantioselectivities
decreased dramatically (Table 1, entries 6 and 7). Apparently
the diaryl substituents of the six-membered ring of the catalyst
were crucial to obtain high enantioselectivity. It suggests that
arene–arene interaction between the catalyst and the substrate
could make a great contribution to the enantioselection.
With the optimal conditions established above, the scope
of this reaction was investigated. In the presence of 10 mol%
of catalyst 2a, a wide variety of β-enamino esters were hydrosi-
lylated in 1,2-dichloroethane at −10 °C. The results are sum-
marized in Table 2. Generally, varying the N-aryl substituent
resulted in marginal changes in enantioselectivity (Table 2,
entries 1–6). In most cases, good enantioselectivities were
observed with β-aryl-N-aryl-β-enamino esters (Table 2, entries
Results and discussion
The catalysts 2a–2g were synthesized from methyl L-serinate
hydrochloride 3 via three or five steps. As depicted in
Scheme 1, first, methyl L-serinate hydrochloride 3 reacted with
various Grignard reagents to afford amino dialcohols 4.9 Con-
densation of picolinic acid with 4 followed by cyclization with
Scheme 1 Preparation of catalysts 2a–2g.
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Table 2 Enantioselective hydrosilylation of β-enamino esters 7a–q promoted
by chiral Lewis base 2aa
Table 3 Asymmetric hydrosilylation of α-substituted-β-enamino esters 7r–y
under the optimal conditionsa
Entry
7
R4
R5
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
7a
7b
7c
7d
7e
7f
7g
7h
7i
7j
7k
7l
7m
7n
7o
7p
7q
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4-FC6H4
97
97
97
97
97
96
98
97
98
96
97
98
96
96
96
96
97
94
93
91
92
4-ClC6H4
4-BrC6H4
4-MeC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-FC6H4
Entry
1
7
Yieldb (%)
97
drc
eed (%)
98
94 (R)d
88
anti : syn
21 : 1
34 (anti)
4-FC6H4
4-BrC6H4
4-MeC6H4
4-MeOC6H4
3-ClC6H4
2-ClC6H4
2-Naphthyl
4-BnOC6H4
3-MeOC6H4
Bn
92
95
94
94
80
96
95
92
9
10
11
12
13
14
15
16
17
2e
90
82
anti : syn
3.2 : 1
90 (anti)
42 (syn)
Ph
4-MeOC6H4
Bn
45
82
3f
syn : anti
10 : 1
90 (syn)
(2R,3S)g
Ph
a Unless specified otherwise, the reaction was carried out with 7
(0.3 mmol), trichlorosilane (0.6 mmol), 2a (10 mol%) in 3 mL of 1,2-
dichloroethane at −10 °C for 10 hours. b Isolated yield based on 7.
c The ee was determined by chiral HPLC (Chiralcel OD, AD or OJ).
d The absolute configuration of the product was determined by a
comparison of its optical rotation value with the literature datum.6b
4
5
98
97
syn : anti
>99 : 1
95 (syn)
96 (syn)
syn : anti
1–11 and 13–15). However, β-ortho-substituted-phenyl substrate
7l gave only a moderate ee value (Table 2, entry 12). β-Alkyl-N-
aryl-β-enamino ester 7p afforded poor enantioselection
(Table 2, entry 16). β-Aryl-N-alkyl-β-enamino ester 7q also
afforded rather low enantioselectivity (Table 2, entry 17).
To further extend the utility of our methodology, several
kinds of α-substituted-β-enamino esters were also subjected to
the titled reaction. The results are summarized in Table 3.
Hydrosilylation of α-phenyl substrates 7r afforded the desired
products 8r with high yield, high diastereoselectivity but poor
enantioselectivity (Table 3, entry 1), while α-allyl substrate 7s
and α-acetyl substrate 7t provided good enantioselectivities,
but inferior diastereoselectivities (Table 3, entries 2 and 3). To
our delight, several cyclic substrates 7u–x reacted smoothy to
generate the corresponding products with excellent yields in
high diastereoselectivities as well as good enantioselectivities
(Table 3, entries 4–7). However, cyclic substrate 7y delivered a
poor enantioselectivity (Table 3, entry 8).
>99 : 1
6
95
98
95
syn : anti
>99 : 1
92 (syn)
90 (syn)
18 (syn)
7
syn : anti
>99 : 1
8e
syn : anti
>99 : 1
a Unless specified otherwise, the reaction was carried out with 7
(0.3 mmol), trichlorosilane (0.6 mmol), 2a (10 mol%) in 3 mL of 1,2-
dichloroethane for 10 hours. b Isolated yield based on 7. c The dr
values were determined by 1H NMR. d The ee values were determined
by chiral HPLC (Chiralcel OD, AD or OJ). e The reaction time is
24 hours. f The yield, dr and ee values were measured after the product
In order to demonstrate the synthetic utility of this trans-
formation, product 8o was hydrolyzed with LiOH followed by
cyclization by PPA to generate the 3-aminoindan-1-one deriva- was hydrolyzed by K2CO3 in MeOH. g The absolute configuration of the
tive 9 (Scheme 2). 3-Aminoindan-1-one derivatives are versatile
synthetic intermediates toward the 1-aminoindane moiety
product was determined by comparison of its optical rotation value
and chiral HPLC with the literature data.6c
embedded in many biologically important compounds includ-
ing HIV protease inhibitors,10 neuroprotective agents,11 and
the drugs used for cocaine abuse treatments.12
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21172217) and the National Basic Research Program of China
(973 Program) (2010CB833300).
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(Scheme 3).8j
Conclusions
In summary, we have developed a highly enantioselective
hydrosilylation of β-enamino esters catalyzed by a novel Lewis
base derived from L-serine. This transformation enables us to
prepare various enantioenriched β-amino esters in good yields
as well as good enantioselectivities under rather moderate con-
ditions. Notably, several cyclic substrates underwent reaction
to provide cyclic β-amino esters in high yields with excellent
diastereoselectivities as well as good enantioselectivities. After-
wards, one of the products was employed in the construction
of a 3-aminoindan-1-one derivative. The absolute configur-
ations of the products were determined by a comparison of
their optical rotation values with the literature data. Further
work is in progress to explore new applications of this novel
Lewis base organocatalyst.
Acknowledgements
We are grateful for the financial support from the National
Natural Science Foundation of China (20972155 and
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