From the Feist-Bénary reaction to organocatalytic domino Michael-alkylation reactions: Asymmetric synthesis of 3(2H)-furanones

Article abstract of DOI:10.1002/chem.201102796

It all adds up! A modified Feist-Bénary reaction employing a domino Michael-alkylation sequence was designed for the enantioselective synthesis of 3(2H)-furanones. L-Threonine-derived tertiary amine/thiourea catalysts were prepared for the first time; such catalysts promoted the designed domino Michael-alkylation reactions in a highly enantioselective manner (see scheme). Copyright

Full text of DOI:10.1002/chem.201102796

COMMUNICATION
DOI: 10.1002/chem.201102796
From the Feist–Bꢀnary Reaction to Organocatalytic Domino Michael–
Alkylation Reactions: Asymmetric Synthesis of 3ACHTUNTRGNEU(GN 2H)-Furanones
Xiaowei Dou, Xiaoyu Han, and Yixin Lu*[a]
3
(2H)-Furanones are structural motifs that are widely
plays a key role in the proton transfer, and the presence of
the halogen atom is crucial for the cyclization step. Since
furanones are furan derivatives, a modified Feist–Bꢀnary re-
action may provide a straightforward method for the synthe-
sis of furanones. By employing a g-halogenated-b-dicarbonyl
compound and a suitable electrophile, a modified Feist–
Bꢀnary reaction through a Michael–alkylation cascade se-
present in natural products and medicinally important
agents (a few examples are illustrated here).[1] Over the past
quence[4] is anticipated to generate
(Scheme 1).
3ACTHUNGTERNNU(G 2H)-furanones
few decades, a number of approaches toward the synthesis
of 3ACHTUNGTRENNUNG(2H)-furanones have been established, including metal-
Scheme 1. Synthesis of furanones through a modified Feist–Bꢀnary reac-
tion.
mediated cyclizations of alkynyl substrates,[2a–e] transforma-
tions from furans,[2f,g] cyclization of dienes or alkynes,[2h–k]
and cycloisomerization of allenes.[2l] However, most of the
above reactions require the employment of specific sub-
strates and the reaction conditions are often harsh. More-
over, to the best of our knowledge, an organocatalytic asym-
metric synthesis of chiral furanone derivatives has not been
reported to date. Thus, there clearly exists a need to devise
an efficient and mild synthetic strategy to access this impor-
tant class of compounds in an optically enriched form.
The Feist–Bꢀnary reaction is a base-catalyzed condensa-
tion between a-halogen ketones and 1,3-dicarbonyl com-
pounds for the preparation of substituted furans.[3] The
Feist–Bꢀnary synthesis can also be viewed as a domino
aldol–alkylation reaction, in which the ketone electrophile
To test the feasibility of our proposal, we chose nitroole-
fins as potential electrophiles, since they are readily avail-
able and versatile electrophiles in conjugate addition.[5,6h,i,l]
For the selection of the catalysts, we focused on bifunctional
amino catalysts containing a Brønsted acid moiety;[6] promo-
tion of the modified Feist–Bꢀnary reaction could be realized
through the interactions of the catalysts with both substrates
in a cooperative manner. The domino reaction between
ethyl 4-bromoacetoacetate (1) and nitrostyrene (2a) was se-
lected as a model reaction,[7] and the results are summarized
in Table 1. When quinidine-derived tertiary amine/thiourea
catalyst 4 was used, the reaction proceeded with very low
conversion (entry 1). This result is not surprising, and it sug-
gested that the catalyst was probably quenched by HBr gen-
erated during the reaction. To circumvent this problem, we
next introduced a stoichiometric amount of base as an addi-
tive to capture HBr released. The inclusion of Et3N or
K2CO3 into the reaction system proved to be beneficial and
the desired products were obtained in very high yields; how-
ever, the enantioselectivities were very poor (entries 2 and
3). Suspecting the above bases were strong enough to
induce undesired background reactions, (NH4)2CO3 was
[a] X. Dou, X. Han, Prof. Dr. Y. Lu
Department of Chemistry & Medicinal Chemistry Program
Life Sciences Institute
National University of Singapore
3 Science Drive 3, Singapore 117543 (Singapore)
Fax : (+65)6779-1691
E-mail: chmlyx@nus.edu.sg
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102796.
Chem. Eur. J. 2012, 18, 85 – 89
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
85

Table 1. Domino Michael–alkylation reaction for the synthesis of 3
furanones.[a]
(2H)-
Table 2. Enantioselective synthesis of 3
ACHTUNGTNER(NUNG 2H)-furanones via an 8a-cata-
lyzed domino Michael–alkylation reaction.[a]
Product
t
Yield
[%][b]
ee
[h]
[%][c]
1
2
24
24
90
89
90
94
Catalyst
Additive
t
Yield
[%][b]
ee
3
4
5
24
24
30
88
83
85
96
96
94
[h]
[%][c]
1
2
3
4
5
6
7
8
9
4
4
4
4
5
6
7
8a
8b
None
Et3N
K2CO3
24
<10
95
86
73
82
60
84
90
85
–
À10
À16
À65
75
0.5
1
16
8
24
24
24
24
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
T
À61
ACHTUNGTRENNUNG
85
N
90
90
C
[a] Reactions were performed with 1 (0.05 mmol), 2a (0.05 mmol), the
additive (0.05 mmol) and the catalyst (0.01 mmol) in toluene (0.5 mL) at
room temperature. [b] Isolated yield. [c] Determined by HPLC analysis
on a chiral stationary phase.
6
24
86
88
then introduced as a HBr scavenger. Gratifyingly, the de-
sired furanone 3a was obtained in good yield and with mod-
erate enantioselectivity in the presence of (NH4)2CO3
(entry 4). To further improve the enantioselectivity of the
reaction, we turned our attention to different types of terti-
ary amine/thiourea catalysts with an amino acid structural
scaffold.[8] Our group recently demonstrated tryptophan-de-
rived tertiary amine/thiourea catalysts[9] and trifunctional
catalysts[10] incorporating amino acids were very useful or-
ganic catalysts. In addition to these catalysts, we also pre-
pared novel tertiary amine/thiourea catalysts based on l-
threonine (catalysts 8a,b, Table 1), a structural scaffold
found to be remarkably powerful in a number of asymmetric
reactions.[11] While cinchonidine-derived trifunctional cata-
lyst 5 and quinidine-derived sulfonamide[12] 6 gave moderate
7
8
24
24
24
30
48
78
84
85
75
72
91
95
91
90
87
9
enantioselectivity (entries 5 and 6), tryptophan-based
7
turned out to be an excellent catalyst, furnishing the desired
product with very good enantioselectivity (entry 7). The l-
threonine-derived tertiary amine/thiourea catalysts 8 proved
to be the most efficient catalysts, and the desired 3ACTHNUTRGNEUG(N 2H)-fur-
anone 3a was obtained in excellent yield and with very high
enantioselectivity (entries 8 and 9).
10
11
The substrate scope for this novel domino Michael–alky-
lation reaction was subsequently examined (Table 2). Con-
sistently high yields and excellent enantioselectivities were
86
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Chem. Eur. J. 2012, 18, 85 – 89

Asymmetric Catalysis
COMMUNICATION
erate yield and with only 50% ee. Meanwhile, furanone 11,
which resulted from the self-cyclization of 9, was obtained
in 30% yield. When 9 was treated with triethylamine alone
(2 equiv), product 11 was isolated in 89% yield, which clear-
ly demonstrated the high tendency of such substrates to un-
dergo rapid cyclization reactions.
The detailed mechanism of the reaction is not investigated
at this stage. A plausible transition state model (TS) is
shown here. The tertiary amine group of the catalyst deprot-
Table 2. (Continued)
Product
t
Yield
[%][b]
ee
[h]
[%][c]
12
48
48
81
78
91
13
93
[a] Reaction were performed with
1
(0.05 mmol),
2
(0.05 mmol),
(NH4)2CO3 (0.05 mmol) and 8a ( 0.01 mmol) in toluene (0.5 mL) at room
temperature. [b] Isolated yield. [c] Determined by HPLC analysis on a
chiral stationary phase.
observed for a wide range of aryl nitroolefins (entries 1–10).
Moreover, alkyl nitroolefins were also suitable substrates,
and high yields and ee values were attained (entries 11–13).
The absolute configurations of the 3ACTHNUTRGNEUNG(2H)-furanones were
determined on the basis of the X-ray crystal structure of 3d
(see the Supporting Information for details).
The 3
(2H)-furanone products 3 are valuable molecules
onates the b-ketoester to form an enolate, which engages in
hydrogen-bonding interactions with the thiourea moiety of
the catalyst. The ionic interaction between the positively
charged ammonium with the nitroolefin moiety is believed
to be important for the substrate binding. Such a proposal is
consistent with the theoretical studies carried out in the lit-
erature for similar systems.[17]
due to the importance of the furanone core in natural prod-
uct and medicinal chemistry.[1] Moreover, such structures are
also valuable synthetic intermediates. As illustrated in
Scheme 2, treatment of furanone 3a with HCl readily gave
In summary, we have designed a modified Feist–Bꢀnary
reaction by employing a domino Michael–alkylation se-
quence for the enantioselective preparation of 3ACTHNUTRGENUG(N 2H)-fura-
nones, synthetically useful structural motifs with significant
biological importance. We have also prepared l-threonine-
derived tertiary amine/thiourea catalysts for the first time;
such catalysts promoted the designed domino Michael–alky-
Scheme 2. Preparation of tetronic acid and g-lactam from furanone 3a.
tetronic acid 3a-1,[13] a core structural skeleton that has
been found in many bioactive natural products.[14] Further-
more, reducing the nitro group of the tetronic acid resulted
in spontaneous formation of a-alkylidenelactam 3a-2,[15] an
intriguing structural motif possessing a wide variety of bio-
logical activities.[16]
lation reactions efficiently, affording 3ACTHUNGTERNNU(G 2H)-furanones in
high yields and with excellent enantioselectivities. Further
development of practical synthetic methods based on the
Feist–Bꢀnary reaction to access biologically significant mole-
cules is currently ongoing in our laboratory.
In an attempt to broaden the substrate scope, bromodike-
tone 9 was examined, as illustrated in Scheme 3. Under the
optimized reaction conditions established in our previous
experiments, the desired furanone 10 was obtained in mod-
Experimental Section
General procedure: Ethyl 4-bromoacetoacetate 1 (10.5 mg, 0.05 mmol)
was added to a mixture of 2a (7.5 mg, 0.05 mmol), (NH4)2CO3 (4.8 mg,
0.05 mmol) and 8a (5.6 mg, 0.01 mmol) in anhydrous toluene (0.5 mL) in
a sample vial. The vial was then sealed and the resulting mixture was
stirred at room temperature for 24 h. The reaction mixture was concen-
trated in vacuo, and the crude was purified by flash column chromatogra-
phy (ethyl acetate/hexane=1/5 to 1/1) to afford furanone 3a (12.5 mg,
90% yield) as a light yellow oil.
Scheme 3. Synthesis of furanone employing bromodiketone instead of
bromoketoester
Chem. Eur. J. 2012, 18, 85 – 89
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
87

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Acknowledgements
We are grateful for the generous financial support from the National
University of Singapore (R-143-000-469-112).
Keywords: asymmetric synthesis · domino reactions · Feist–
Bꢀnary reaction · furanones · organocatalysis
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Received: September 7, 2011
Published online: December 12, 2011
Chem. Eur. J. 2012, 18, 85 – 89
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