Asymmetric disulfonimide-catalyzed synthesis of δ-amino-β-ketoester derivatives by vinylogous Mukaiyama-Mannich reactions

Article abstract of DOI:10.1002/anie.201407532

An organocatalytic asymmetric synthesis of δ-amino-β-ketoester derivatives has been developed. A chiral disulfonimide (DSI) serves as a highly efficient precatalyst for a vinylogous Mukaiyama-Mannich reaction of readily available dioxinone-derived silylox

Full text of DOI:10.1002/anie.201407532

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DOI: 10.1002/anie.201407532
Organocatalysis
Asymmetric Disulfonimide-Catalyzed Synthesis of d-Amino-b-
Ketoester Derivatives by Vinylogous Mukaiyama–Mannich
Reactions**
Qinggang Wang, Manuel van Gemmeren, and Benjamin List*
Abstract: An organocatalytic asymmetric synthesis of d-
amino-b-ketoester derivatives has been developed. A chiral
disulfonimide (DSI) serves as a highly efficient precatalyst for
a vinylogous Mukaiyama–Mannich reaction of readily avail-
able dioxinone-derived silyloxydienes with N-Boc-protected
imines, delivering products in excellent yields and enantiose-
lectivities. The synthetic utility of this reaction is illustrated in
À
various transformations, including a new C C bond-forming
reaction, which provide useful enantioenriched building
blocks. The methodology is applied in a formal synthesis of
(À)-lasubin.
d-Amino-b-ketoesters are useful building blocks, in par-
Scheme 1. Approach and synthetic potential of d-amino-b-ketoesters.
Boc=tert-butoxycarbonyl, TMS=trimethylsilyl.
ticular for the synthesis of various piperidine and pyrrolidine
alkaloids such as the lasubins.[1,2] Common methods to access
enantiomerically pure d-amino-b-ketoesters rely on the
“chiral pool”[1,3] or on chiral auxiliaries,[4] and catalytic
Table 1: Optimization of the enantioselectivity.
asymmetric approaches are very rare.[5] Here, we report
a general and highly enantioselective approach to the syn-
thesis of d-amino-b-ketoesters by a disulfonimide-catalyzed
vinylogous Mukaiyama–Mannich reaction,[6] utilizing N-Boc
imines 2 and silyloxydienes 3 as substrates (Scheme 1).
Disulfonimides (DSIs) of the general structure 1 (Table 1)
are not only strong Brønsted acids,[7] but also precatalyts,
which become strong Lewis acids upon in situ silylation.[8]
These silicon Lewis acids emerged as powerful catalysts for
the activation of aldehydes in Mukaiyama–aldol reactions[8b]
as well as vinylogous and bisvinylogous variants[8c] and in
hetero-Diels–Alder reactions,[8d] and in methallylations.[8e]
Quite recently it was also found that chiral silylated DSIs
activate alkyloxycarbonyl imines, promoting the correspond-
ing reactions with excellent enantioselectivity.[9] We therefore
hypothesized that our DSIs could potentially catalyze the
enantioselective vinylogous Mukaiyama–Mannich reaction,
thus offering a general approach for the synthesis of
enantiomerically enriched d-amino-b-ketoesters.
We began our investigations using silyloxydiene 3 and N-
Boc imine 2a as model substrates. A systematic screening of
Entry[a]
Catalyst
T [8C]
Conv. [%][b]
e.r.[c]
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1 f
1g
1g
1g
1g
1g
À30
À30
À30
À30
À30
À30
À30
À30
À50
À50
À78
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
<60
89:11
86:14
84:16
86:14
87.5:12.5
73:27
92.5:7.5
89:11
95:5
8[d]
9
[*] Dr. Q. Wang, M. van Gemmeren, Prof. Dr. B. List
Max-Planck-Institut fꢀr Kohlenforschung
10[e]
11[e]
95:5
95:5
Kaiser-Wilhelm-Platz 1, 45470, Mꢀlheim an der Ruhr (Germany)
E-mail: list@mpi-muelheim.mpg.de
[**] Generous support by the Max-Planck-Society, the European
Research Council (Advanced Grant “High Performance Lewis Acid
Organocatalysis, HIPOCAT”), and the Fonds der Chemischen
Industrie (fellowship to M.v.G.) is gratefully acknowledged.
[a] Reactions were carried out with 2a (0.05 mmol) and 3 (0.075 mmol)
in toluene (0.5 mL) for 3 days. [b] Conversions were determined by
1H NMR analysis by comparing the typical signals of products and
hydrolyzed aldehydes. [c] Determined by HPLC analysis on a chiral
stationary phase. [d] TBS-protected nucleophile was used. [e] Catalyst
loading: 1 mol%.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201407532.
13592
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Angew. Chem. Int. Ed. 2014, 53, 13592 –13595

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the reaction conditions was carried out to optimize the
enantioselectivity (Table 1). A solvent screen revealed that
toluene is the optimal solvent.[10] Disulfoniminde 1a could
promote the conversion of 2a to 4a at À308C, delivering the
desired product with > 95% conversion and 89:11 e.r. (enan-
tiomeric ratio) in less than three days (entry 1). Of the
investigated catalysts 1b–g, disulfonimide 1g, with branched
3,3’-substituents, afforded the highest enantioselectivity of
92.5:7.5 e.r. (entries 2–7). The enantioselectivity was slightly
lower (89:11 e.r.) when the TBS-protected silyloxydiene was
employed (entry 8 versus entry 7). By lowering the temper-
ature to À508C, the enantioselectivity could be increased to
95:5 e.r (entry 9). Gratifyingly, when the catalyst loading was
lowered to 1 mol%, > 95% conversion and equally good
enantioselectivity could be achieved in less than three days
(entry 10). Further cooling to À788C did not prove useful, as
the enantioselectivity remained unchanged, and lower con-
version was observed (entry 11).
Table 2: (Continued)
Entry[a,b]
R
Product
Yield [%]
89
e.r.[c]
98:2
11
4k
12
13
4l
96
94
97.5:2.5
95:5
4m
14
15
16
4n
4o
4p
89
90
94
98.5:1.5
95:5
94:6
Under the optimized conditions, the scope of the enan-
tioselective vinylogous Mannich reaction of silyloxydiene 3
catalyzed by disulfoniminde 1g was investigated (Table 2).
From benzaldehyde-derived N-Boc imine 2a, product 4a was
obtained with 96% yield and 95:5 e.r. (entry 1). With
naphthyl-substituted N-Boc imines, excellent yields and
17[e]
18
4q
4r
96
99
90:10
94:6
Table 2: Substrate scope of the vinylogous Mukaiyama–Mannich reac-
tion.
19
20
21
4s
4t
87
80
89
90:10
86.5:13.5
80:20
4u
Entry[a,b]
1
R
Product
Yield [%]
96
e.r.[c]
95:5
22
23
4v
60
60:40
–
4a
4w
<5
2
3
4b
4c
88
91
95:5
99:1
[a] Reactions were carried out with 2 (0.1 mmol) and 3 (0.15 mmol) in
toluene (1 mL) for 3 d. [b] The exclusive formation of g-products was
confirmed by 1H NMR analysis of the crude reaction mixture. [c] Deter-
mined by HPLC analysis on a chiral stationary phase. [d] The absolute
configuration was determined by X-ray crystallographic analysis of 4j
(see the Supporting Information). [e] 1 mol% (S)-1a was used as
catalyst.
4
4d
98
91:9
5
6
4e
4 f
99
93
91:9
enantioselectivities could be obtained (entries 2–4). The
influence of the substitution at different positions was
95.5:4.5
investigated next.
A meta-methyl-substituted substrate
reacted with higher enantioselectivity than the para-methyl-
and ortho-methyl-substituted substrates (entry 6 versus
entries 5 and 7). The meta-methyl product 4 f could be
isolated in 93% yield and with 95.5:4.5 e.r. Substrates bearing
halogen substituents provided the corresponding products
4h–j (entries 8–10) with comparable enantioselectivities. The
meta-methoxy-substituted N-Boc imine proved to be a good
substrate, giving 4k in 89% yield and with 98:2 e.r. (entry 11).
The meta-vinyl N-Boc imine provided the expected product in
96% yield and with 97.5:2.5 e.r. (entry 12). With 3,5-
dimethyl-substituted N-Boc imine, product 4m was obtained
7
4g
98
89.5:10.5
8
4h
4i
93
90
85
95:5
9
94:6
10[d]
4j
93.5:6.5
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in 94% yield and with 95:5 e.r. (entry 13). As alkoxy groups
are quite common substituents in natural products and their
synthesis,[11] several di- and trimethoxy-substituted N-Boc
imines were tested, and the products 4n–q were isolated in
good yields and with excellent enantioselectivities
(entries 14–17). With N-Boc imine 2n, bearing methoxy
groups at the 3- and 5-positions, an e.r. as high as 98.5:1.5
could be obtained. 3,4-Dimethoxyimine 2q reacted to give the
enantiomeric product 4q in 96% yield and with 90:10 e.r.
when (S)-1a served as the catalyst (entry 17). For heterocyclic
substrates, the corresponding products were obtained in
reasonable to good yields and enantioselectivities. With the
quinoline-substituted N-Boc imine 2r, 99% yield and 94:6 e.r.
were observed (entry 18). The reaction of the 1-benzothio-
phene-5-carbaldehyde-derived N-Boc imine gave product 4s
in 87% yield and 90:10 e.r. (entry 19). The 2-thiophenyl
product 4t and 3-furyl product 4u were isolated with lower
e.r. (entries 20 and 21). The aliphatic N-Boc imines 2v and 2w
gave disappointingly low yield and e.r. (entries 22 and 23).
Further explorations towards the development of a catalytic
system suitable for aliphatic substrates are ongoing. The
absolute configuration of 4j was determined to be (R) by X-
ray crystallographic analysis and all further products were
assigned by analogy.
also be generated directly from 4a without any loss of
enantiopurity, by utilizing H2O instead of methanol as
nucleophile under the same conditions. Further versatile
transformations were conducted, by utilizing similar reaction
conditions, but with different nucleophiles including alcohols,
amines, and thiols. The corresponding d-amino-b-ketoester 7,
d-amino-b-ketothioester 8, and d-amino-b-ketoamides 9 and
10 were obtained in excellent yields. Remarkably, when 4a
À
was treated with a silyl ketene acetal, a C C bond-forming
reaction occurred furnishing e-amino-d,b-diketoester 11 in
80% yield. Our methodology was furthermore applied in the
formal synthesis of (À)-lasubin starting from 4q (see the
Supporting Information).
In summary, a versatile organocatalytic asymmetric syn-
thesis of d-amino-b-ketoesters has been developed. Disulfon-
imide 1g, which is converted into a strong Si Lewis acid under
the reaction conditions, serves as a highly efficient precatalyst
for the asymmetric vinylogous Mukaiyama–Mannich reaction
with silyloxydiene 3,[14] delivering products in excellent yields
and enantioselectivities. Various useful transformations of 4a
have also been developed, thus enabling the synthesis of
different enantioenriched amino ketoesters. The method-
ology has been further applied in a formal synthesis of (À)-
lasubin. Our results represent an application of our asym-
metric counteranion-directed catalysis (ACDC) strategy to
Lewis acid catalysis.[15]
We applied the produced chiral dioxinone derivatives in
various versatile transformations giving useful enantiomeri-
cally pure building blocks (Scheme 2).[10,12,13] For example, d-
amino-b-ketoester 5 was generated in quantitative yield by
treatment of 4a with MeOH in toluene at 908C. After
extensive experiments, we concluded that no racemization
occurred during the process, as the enantiomeric ratio was
completely retained even after the further conversion of ester
5 to ketone 6 under basic conditions. b-Aminoketone 6 could
Received: July 23, 2014
Published online: October 27, 2014
Keywords: d-amino-b-ketoesters · disulfonimides ·
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organocatalysis · vinylogous Mukaiyama–Mannich reaction
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