Disulfonimide-catalyzed asymmetric synthesis of β3-amino esters directly from N-Boc-amino sulfones

Article abstract of DOI:10.1021/ja408747m

An asymmetric Mannich reaction of silyl ketene acetals with N-Boc-amino sulfones has been developed. A chiral disulfonimide efficiently catalyzes both the in situ generation of the corresponding N-Boc imines and the asymmetric Mannich reaction with excellent yields and enantioselectivities. Kinetic studies confirm a proposed stepwise mechanism.

Full text of DOI:10.1021/ja408747m

Communication
pubs.acs.org/JACS
Disulfonimide-Catalyzed Asymmetric Synthesis of β3‑Amino Esters
Directly from N‑Boc-Amino Sulfones
Qinggang Wang, Markus Leutzsch, Manuel van Gemmeren, and Benjamin List*
Max-Planck-Institut fur Kohlenforschung, Kaiser Wilhelm-Platz 1, 45470 Mulheim an der Ruhr, Germany
̈
̈
S
* Supporting Information
ABSTRACT: An asymmetric Mannich reaction of silyl
ketene acetals with N-Boc-amino sulfones has been
developed. A chiral disulfonimide efficiently catalyzes
both the in situ generation of the corresponding N-Boc
imines and the asymmetric Mannich reaction with
excellent yields and enantioselectivities. Kinetic studies
confirm a proposed stepwise mechanism.
Figure 1. Disulfonimide (DSI) catalysts 1.
Particularly, we were curious if such Lewis acids could also
catalyze the in situ formation of N-Boc imines from the
corresponding amino sulfones, a reaction that is normally
mediated by a base.12
Our strategy could (1) circumvent this stoichiometric base-
mediated process avoiding an entire step and (2) allow for the
utilization of alkyl N-Boc imines, which are even less stable than
the corresponding aryl derivatives. Encouragement came from
our recent finding that chiral DSIs catalyze the synthesis of
Fmoc-protected homoallylamines from allyl trimethylsilane, the
Fmoc carbamate, and an aldehyde via in situ imine formation.13
Indeed, the Mannich reaction of silyl ketene acetal 4a with
preformed N-Boc imine 2a was readily catalyzed by DSI 1a,
providing product 5a in 88% yield and 90.5:9.5 er (eq 2).
β3-Amino acids have attracted recent attention as constituents
of natural products,1 pharmaceuticals,2 and peptides with
unique properties.3 Their relevance has stimulated the
development of catalytic enantioselective methodologies
leading to the β3-amino acid scaffold, such as additions of
acetate enolate equivalents to imines,4 conjugate additions of
amines to unsaturated carbonyls,5 reductions,6 and others.7
Among these methods, the Mannich reaction of imines with
silyl ketene acetals is considered particularly attractive, as C−C-
bond formation and stereocenter creation coincide.8,9 An
elegant version of this reaction, employing preformed aryl N-
Boc imines, a synthetic silyl ketene acetal, and a chiral thiourea
catalyst was recently reported by Jacobsen et al. (eq 1a).10 We
report here a complementary approach that directly utilizes N-
Boc-amino sulfones as electrophiles, a commercially available
ketene acetal as the nucleophile, and a new member of our
chiral disulfonimide (DSI) catalyst class to deliver high yields of
protected aryl and alkyl β3-amino esters in excellent
enantioselectivity (eq 1b).
Disulfonimides of the general structure 1 (Figure 1) have
emerged as powerful catalysts for the activation of aldehydes in
asymmetric Mukaiyama Aldol reactions,11a as well as vinylogous
and bisvinylogous variants,11b in hetero-Diels−Alder reac-
tions,11c and in methallylations.11d Kinetic, spectroscopic, and
other mechanistic studies have shown that our chiral DSIs are
precatalysts that only upon in situ silylation become powerful
silicon Lewis acid catalysts. We wondered whether or not our
DSIs would also catalyze the Mukaiyama−Mannich reaction.
Remarkably, the reaction proceeded equally well and with
similar enantioselectivity when we used amino sulfone 3a
directly. The reaction can easily be followed by visual
inspection since the almost insoluble amino sulfone disappears
upon full conversion (eq 3).
Encouraged by these preliminary results, a systematic
exploration of different reaction conditions was conducted to
optimize the enantioselectivity (Table 1). A solvent screening
revealed toluene to be optimal.14 Of the investigated catalysts
1a−c, the new disulfonimide 1c, bearing a branched 3,3′-
substituent, afforded the highest enantioselectivity of 92.5:7.5 er
Received: August 29, 2013
© XXXX American Chemical Society
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dx.doi.org/10.1021/ja408747m | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
Table 1. Optimization of the Enantioselectivity
Table 2. Substrate Scope of the Asymmetric Mannich
Reaction
a
b
c
entry
cat.
Ar
R1
conv. (%)
er
1
2
3
4
5
6
7
8
9
1a
1b
1c
1c
1c
1c
1c
1c
1c
1c
1c
1c
Ph
Ph
Ph
Me
Me
Me
Me
Me
Me
Et
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
90:10
85.5:14.5
92.5:7.5
91:9
p-Me-C6H4
p-OMe-C6H4
92:8
p-Cl-C6H4
92:8
Ph
Ph
Ph
Ph
Ph
Ph
89:11
i-Pr
t-Bu
Me
Me
Me
81.5:18.5
89:11
d
10
94.5:5.5
94.5:5.5
95:5
de
,
11
12
ef
,
a
Reactions were carried out with amino sulfone (0.05 mmol) and silyl
b
ketene acetal (0.15 mmol) in toluene (0.5 mL) for 2−72 h. Easily
detectable once full homogeneity of the reaction mixture is observed.
c
d
Determined by HPLC with a chiral stationary phase. The reaction
e
f
was conducted at 10 °C. 2 mol % catalyst. The reaction was
conducted at 8 °C; full conversion after 144 h.
(entry 3). We also investigated different sulfone groups (entries
4−6). As anticipated, the effect on the enantioselectivity was
marginal, suggesting this group not to be involved in the
enantiodiscriminating step. However, in contrast to sulfone 3a
and its electron-poor p-Cl-variant, the reaction with the
electron-rich p-methoxy-substituted sulfone proceeded much
faster. Various alkoxy substituents of the silyl ketene acetals
were investigated, revealing lower enantioselectivities with
increasing bulkiness of the alkoxy group (entry 3 vs entries
7−9). Conveniently, the commercially available silyl ketene
acetal 4a provided the highest enantioselectivity. We also varied
the temperature and found that full conversion and 94.5:5.5 er
could be achieved at 10 °C in less than 72 h (entry 10). Again
full conversion and 95:5 er were obtained at 8 °C after a
reaction time of 6 days (entry 12). Lowering the catalyst
loading to 2 mol % had no influence on yield and
enantioselectivity (entry 11).
The scope of the enantioselective Mannich reaction of the N-
Boc-amino sulfones catalyzed by disulfonimide 1c was next
investigated under optimized conditions (Table 2). With
naphthyl substituted Boc-amino sulfones, excellent yields and
enantioselectivities could be obtained, although the 2-naphthyl
Boc-amino sulfone requires room temperature (entries 2−3).
Electron-donating groups accelerate the reaction while
retaining the high enantioselectivity (entries 4 and 6−10).
Conversely, the electron-withdrawing 6-Br-2-naphthyl substitu-
ent again required room temperature to reach full conversion in
72 h (entry 5). As alkoxy groups are common substituents in
natural substances that can possibly be accessed from our
products,15 several different methoxy-substituted substrates
were tested. In all cases, the reaction proceeded very well
giving the desired products in excellent yields and enantiose-
lectivities (entries 8−10). With heterocyclic substrate 3k, a 3-
furanaldehyde derived Boc-amino sulfone, product 5k was
obtained in 96% yield and with 86:14 er (entry 11). Finally
when applying our optimal conditions to an aliphatic substrate,
excellent yield and 75.5:24.5 er, was obtained. To the best of
our knowledge, this is the first example of a catalytic
a
Reactions were carried out with amino sulfone (0.1 mmol) and silyl
ketene acetal (0.3 mmol) in toluene (1 mL) for 36−72 h.
b
Determined by HPLC with a chiral stationary phase; the absolute
configuration was determined by comparison with literature results.
c
d
The reaction was conducted at 8 °C. Full conversion after 144 h.
The reaction was conducted at rt. After single crystallization from
e
f
MTBE.
asymmetric Mannich reaction with a silyl ketene acetal, which
affords aliphatic Boc-protected β3-amino acid derivatives (entry
12). Nonetheless, further catalyst explorations are expected to
lead to an optimized protocol for aliphatic substrates.
A mechanism that is consistent with our experimental data
and careful NMR spectroscopic studies is proposed in Scheme
1. Accordingly, disulfonimide 1 is initially silylated from ketene
acetal 4a predominantly on oxygen to generate the
corresponding Lewis acid O-TBS-1. Interestingly, this catalyst
B
dx.doi.org/10.1021/ja408747m | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
1
developed in our laboratories. Disulfonimide 1c, which is
transformed into a powerful Si-Lewis acid, serves as an efficient
precatalyst for the asymmetric Mannich reaction with silyl
ketene acetals, giving excellent yields and enantioselectivities.
Our methodology represents an application of asymmetric
counteranion-directed catalysis (ACDC) in Lewis acid
catalysis.17 A stepwise mechanism is proposed based on
experimental observations and a 1H NMR kinetic study.
Attractive features of our approach include (1) use of a
commercially available ketene acetal, (2) easier handling of
stable N-Boc-amino sulfones (including an aliphatic one) in
comparison to the corresponding imines, (3) saving one
normally required step, (4) convenient reaction monitoring by
visually following homogenization with conversion, (5) a low
catalyst loading, and (6) easy to crystallize products. Further
exploration of chiral DSI-catalyzed reactions is currently in
progress in our laboratories.
Scheme 1. Proposed Catalytic Cycle and H NMR Kinetic
Study
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, characterization of all compounds,
and detailed information on our mechanistic studies. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
list@kofo.mpg.de
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Generous support by the Max-Planck-Society, and the Fonds
der Chemischen Industrie (fellowship to M.v.G.) before is
greatfully acknowledged. The project “Sustainable Chemical
Synthesis” (SusChemSys) (co-financed by the European
Regional Development Fund (ERDF) and the state of North
Rhine-Westphalia, Germany, under the Operational Pro-
gramme “Regional Competitiveness and Employment” 2007 -
2013). The European Research Council (Advanced Grant
“High Performance Lewis Acid Organocatalysis, HIPOCAT”)
is gratefully acknowledged.
rapidly silylates sulfone 3a to the corresponding N-silyl
derivative TBS-3a (cycle I). A small but detectable quantity
of N-Boc-imine 2a is then generated via a slow and apparently
rate determining elimination of PhSO2TBS from silylated
sulfone 3a, initiating catalytic cycle II. Expectedly, the rate of
this slow step is influenced by the electronic properties of the
sulfones (entries 3 and 4−6, Table 1). The elimination should
predominantly lead to ion pair equivalent A, which together
with the ketene acetal then assembles transition state B. A 4.5:1
mixture of the two silylated Mannich products N-TBS-5a and
O-TBS-5a results from this reaction at 0 °C in CDCl3. Under
these conditions, no obvious equilibration between the two
isomers takes place, but after two days at room temperature
their ratio will change to >20:1 in CDCl3 and d8-toluene,
suggesting intermediate N-TBS-5a to be thermodynamically
favored.16 A similar product mixture is generated in a 5.7:1 ratio
when product 5a was treated with TBSOTf and NEt3 in CDCl3
at room temperature. The formation of both product isomers
TBS-5a occurred in parallel with the consumption of starting
material 3a. Both intermediate TBS-3a and trace amounts of
imine 2a are only observable while the reaction is in progress.
They completely disappear upon full conversion.14
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