Organocatalytic Enantioselective Conjugate Addition of Malonic Acid Half Thioesters to Coumarin-3-carboxylic Acids Using N-Heteroarenesulfonyl Cinchona Alkaloid Amides

Article abstract of DOI:10.1002/adsc.201600040

We disclose herein an efficient enantioselective conjugate addition reaction between coumarin-3-carboxylic acids and malonic acid half thioesters (MAHTs). The reaction was catalyzed by N-heteroarenesulfonyl Cinchona alkaloid amides to afford double-decarboxylative conjugate addition products in good yield with high enantioselectivity. The reaction of various coumarin-3-carboxylic acids with MAHTs gave products in high yield with high enantioselectivity.

Full text of DOI:10.1002/adsc.201600040

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DOI: 10.1002/adsc.201600040
Organocatalytic Enantioselective Conjugate Addition of Malonic
Acid Half Thioesters to Coumarin-3-carboxylic Acids Using N-
Heteroarenesulfonyl Cinchona Alkaloid Amides
Shuichi Nakamura,a,* Ayaka Toda,a Masahide Sano,a Tsubasa Hatanaka,b
and Yasuhiro Funahashib
a
Department of Frontier Materials, Graduate School of Engineering, Nagoya Institute of Technology, Gokiso, Showa-ku,
Nagoya 466-8555, Japan
Fax: (+81)-52-735-5245; e-mail: snakamur@nitech.ac.jp
Department of Chemistry, Graduate School of Science, Osaka University, 1–1 Machikaneyama, Toyonaka, Osaka
b
560-0043, Japan
Received: January 11, 2016; Revised: January 28, 2016; Published online: March 8, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600040.
Abstract: We disclose herein an efficient enantiose-
lective conjugate addition reaction between cou-
marin-3-carboxylic acids and malonic acid half thio-
esters (MAHTs). The reaction was catalyzed by N-
heteroarenesulfonyl Cinchona alkaloid amides to
afford double-decarboxylative conjugate addition
products in good yield with high enantioselectivity.
The reaction of various coumarin-3-carboxylic acids
with MAHTs gave products in high yield with high
enantioselectivity.
of highly enantioselective reactions of a,b-unsaturated
esters with simple ester enolate equivalents still re-
mains a major challenge. From this point of view, ma-
lonic acid half-thioesters (MAHTs) are important
candidates for the generation of ester enolate equiva-
lents under mild reaction conditions, and the develop-
ment of an efficient protocol for the enantioselective
decarboxylative addition of MAHTs to a,b-unsaturat-
ed carbonyl compounds is highly desired. Therefore,
there are several reports on enantioselective decar-
boxylative conjugate additions of MAHTs or b-keto
acids to a,b-unsaturated ketones,[5] aldehydes,[6] sul-
fones,[7] and nitro compounds,[8,9] however, there are
no reports on the enantioselective conjugate addition
of MAHTs to a,b-unsaturated esters. Furthermore,
the conjugate addition of simple ester enolate equiva-
lents with coumarin derivatives as cyclic a,b-unsatu-
rated esters has attracted considerable attention since
À
Keywords: C C bond formation; conjugate addi-
tion; decarboxylative reaction; enantioselectivity;
organocatalysis
The development of mild, catalytic and enantioselec- the reaction gives chiral coumarin derivatives, which
À
tive versions of C C bond-forming processes is often exhibit a broad range of biological activities,
a topic of paramount importance in modern organic such as an aldose reductase inhibitor,[10] antiherpetic
chemistry. In this context, the enantioselective conju- active compounds,[11] estrogenically active com-
gate addition of ester enolates as nucleophiles to a,b- pounds,[12] a sirtuin inhibitor,[13] and other biologically
unsaturated esters as electrophiles using chiral cata- active compounds.[14,15] Recently, Wang and co-work-
lysts continues to attract a great deal of interest, as it ers reported the synthesis of racemic 4-substituted
provides efficient access to chiral 1,5-diesters.[1] There- 3,4-dihydrocoumarin by the conjugate addition of
fore, there are many papers on the enantioselective MAHTs to coumarin-3-carboxylic acids through
conjugate addition of malonates, b-keto esters, and a- a double decarboxylation process.[16] On the other
cyano esters,[2] however, the enantioselective conju- hand, we recently developed the first enantioselective
gate addition of simple ester enolate equivalents has decarboxylative addition of MAHTs to ketones or ke-
been far less explored, probably due to difficulties in timines using bifunctional organocatalysts.[17] Herein
the preparation of enolates using a catalytic amount our ongoing interest was extended to the enantiose-
of chiral activator. Very recently, Kobayashi[3] and lective decarboxylative addition of MAHTs to cou-
Shibasaki[4] reported pioneering studies on the direct marin-3-carboxylic acids using our developed chiral
catalytic asymmetric conjugate addition of amides to organocatalysts (Figure 1).
a,b-unsaturated amides. However, the development
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Shuichi Nakamura et al.
Table 1. Decarboxylative addition of MAHT 2a to coumar-
in-3-carboxylic acid 1a using various organocatalysts 4 and
5.
Figure 1. Enantioselective conjugate addition of MAHTs to
coumarin-3-carboxylic acids.
We first examined the reaction of coumarin-3-car-
boxylic acid 1a (1.2 equiv.) with MAHT 2a
(1.0 equiv.) in the presence of chiral organocatalysts 4
or 5 in THF. The results are shown in Table 1.
The control experiment without any catalyst did
not afford product 3a (Table 1, entry 1). The reaction
using quinine (4a) or 9-amino-epi-quinine (4b) afford-
ed 3a in moderate yield but with low enantioselectivi-
ty (Table 1, entries 2 and 3). Encouraged by these re-
sults, we examined the reaction using various Cincho-
na alkaloid catalysts 5a–d having an 8-quinolinesulfo-
namide group that were developed by our group
(Table 1, entries 4–7). Catalyst 5a emerged as the
most suitable candidate for this reaction, yielding
product 3a in excellent yield and selectivity (Table 1,
entry 4). The reaction of 1a with 2a using 5a in other
solvents such as Et2O, cyclopentyl methyl ether
(CPME), 2-methyl-THF, CH2Cl2, toluene and DMF
afforded product 3a with lower enantioselectivity
than that in THF (Table 1, entries 8–13). The addition
of molecular sieves 5Š enhanced the enantioselectivi-
ty of the product (Table 1, entry 14). The reaction
using 2.0 equivalents of 2a gave improved yield and
enantioselectivity of the product (Table 1, entry 15).
In order to check the efficiency of the heteroarenesul-
fonyl group in a chiral catalyst, 1-naphthalenesulfony-
Entry Catalyst Solvent
Time [h] Yield [%] ee [%][a,b]
1
2
3
4
5
6
7
8
9
10
–
THF
THF
THF
THF
THF
THF
THF
Et2O
CPME
96
24
24
16
24
24
24
20
16
–
–
4a
4b
5a
5b
5c
5d
5a
5a
5a
5a
5a
5a
5a
70
52
91
76
70
62
76
80
91
40
16
60
75
99
76
88
33 (S)
9 (R)
87 (R)
81 (R)
73 (S)
68 (S)
57 (R)
72 (R)
86 (R)
8 (R)
1 (R)
1 (R)
90 (R)
91 (R)
78 (R)
90 (R)
2-MeTHF 16
lated 9-amino-9-deoxy-epi-quinine 5e was used as cat- 11
CH2Cl2
toluene
DMF
THF
THF
THF
168
12
168
16
24
24
24
24
alyst, but a lower yield and enantioselectivity of prod-
uct 3a were obtained than from the reaction using 5a
(Table 1, entry 16). The reaction could be carried out
in an open flask to give product 3a in high yield with
high enantioselectivity (Table 1, entry 17). On the
other hand, the enantiopurity of products could be
improved by recrystallization. For example, single re-
crystallization for 90% ee of 3a from hexane/toluene
afforded enantiomerically pure 3a (99% ee). The ab-
solute configuration of 3a was assigned as (R) by X-
ray crystallographic analysis.
13
14[c]
15[c,d] 5a
16[c,d] 5e
17[c,d,e] 5a
THF
[a]
The ee values were determined by HPLC analysis.
The absolute configuration of 2 is provided in parenthe-
ses.
MS 5Š was added.
1a (1.0 equiv.) and 2a (2.0 equiv.) were used.
The reaction was carried out in an open-flask.
[b]
[c]
[d]
[e]
Having confirmed 5a as the most promising cata-
lyst, the reactions of a series of coumarin-3-carboxylic
acids 1a–j and MAHT 2a were examined (Table 2).
3c–i with high enantioselectivities (Table 2, entries 3–
The reaction of coumarin-3-carboxylic acids bearing 9). The reaction of naphthalene-derived coumarin-3-
an electron-donating methyl or methoxy group gave carboxylic acids 1j also afforded product 3j in moder-
corresponding products 3b, c in moderate yields with ate yield with high enantioselectivity (Table 2,
high enantioselectivities (Table 2, entries 2 and 3). entry 10). To the best of our knowledge, this is the
Electron-deficient coumarin-3-carboxylic acids 1d– first example of not only the enantioselective double-
i having nitro, fluoro, chloro, or bromo groups were decarboxylative addition reaction of MAHTs to a-
tolerated in these reaction conditions, giving products alkoxycarbonyl-a,b-unsaturated carboxylic acids, but
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Organocatalytic Enantioselective Conjugate Addition
Table 2. Decarboxylative addition of MAHT 2a to various
coumarin-3-carboxylic acids 1a–j using catalyst 5a.
Scheme 1. Enantioselective conjugate addition of b-keto car-
boxylic acid 2b and a-cyanoacetic acid 2c to 1a.
Entry
1
3
Time [h]
Yield [%]
ee [%][a]
tion reactions using MAHTs: (i) a decarboxylation-
nucleophilic addition pathway in which MAHT under-
goes decarboxylation to form an ester enolate
(Figure 2, path a), and (ii) a nucleophilic addition-
double decarboxylation pathway in which the nucleo-
philic addition of activated MAHT occurs prior to de-
carboxylation (Figure 2, path b).
In order to check the reaction pathway, we exam-
ined the reaction of a,a-dimethyl MAHT 2d with 1a
in the presence of 10 mol% of 5a. However, the reac-
tion did not afford any product (Scheme 2).
1
2
3
4
5
6
7
8
9
10
1a
1b
1c
1d
1e
1f
1g
1h
1i
3a
3b
3c
3d
3e
3f
3g
3h
3i
24
24
24
48
24
24
48
24
24
48
99
84
99
99
99
99
82
92
66
52
91
94
94
74
90
87
87
91
93
93
1j
3j
[a]
The ee values were determined by HPLC analysis.
also the enantioselective conjugate addition of nucle-
ophiles to coumarin-3-carboxylic acids.
The reaction of 1a with b-keto carboxylic acid 2b
using catalyst 5a and molecular sieves 5Š also afford-
ed product 6 in good yield with good enantioselectivi-
ty (Scheme 1). The absolute stereochemistry of prod-
uct 6 was also assigned as S when compared with the
value of the specific rotation reported in the litera-
Scheme 2. Enantioselective conjugate addition of a,a-di-
methyl MAHT 2d with 1a.
Furthermore, we investigated by ESI-mass spectro-
ture.[18] On the other hand, the reaction of 1a with a- scopic analysis for the mixture of 1a and 5a in a 1:0.1
cyanoacetic acid 2c using catalyst 5a did not afford ratio. The hydrogen-bonding intermediate between 1a
the product.
and 5a was observed in this mixture [cation mode, cal-
We consider two possible mechanistic pathways for culated for C39H36N4NaO7S (1a+5a+Na): 727.2,
À
the organocatalytic decarboxylative C C bond forma- found: 727.3, see the Supporting Information]. In ad-
Figure 2. Proposed reaction pathway for decarboxylative addition of MAHT to ketimines using catalyst 5.
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Shuichi Nakamura et al.
dition, the ESI-mass spectroscopic analysis of the mix-
ture of 2a and 5a also showed an intermediate be-
tween 2a and 5a [cation mode, calculated for
C38H38N4NaO6S2 (2a+5a+Na): 733.2, found: 733.3,
see the Supporting Information].[19] These results sup-
port the reaction proceeding through path b. In addi-
tion, the reaction of coumarins 1k and 1l did not
afford good results, therefore the carboxy group in
coumarin-3-carboxylic acids plays an important role
for in the conjugate addition of MAHT (Scheme 3).
Scheme 4. Transformation of thioester 3a to the correspond-
ing aldehyde 9 under the Fukuyama reduction conditions.
fore MAHTs can be used as efficient surrogates for
the acetaldehyde nucleophile in conjugate addition.
Considering the absolute configurations of products
and the structure of the chiral catalysts,[22] the pro-
posed transition state for the reaction of 2a to cou-
marin-3-carboxylic acid 1a using a chiral sulfonamide
catalyst 5a is shown in Figure 3. The acidic proton of
8-quinolinesulfonamide can activate coumarin-3-car-
boxylic acid by hydrogen bonding, and the quinucli-
dine moiety in 5a makes an enol-form of MAHT. The
reaction of the activated enol of MAHT with coumar-
in-3-carboxylic acid in the coordination sphere of the
chiral sulfonamide catalyst leads to adduct A, which
immediately releases two CO2 molecules to afford
Scheme 3. Enantioselective conjugate addition of MAHT 2a
with 1k and 1l.
We next examined the selective reduction of the product 3a with high enantioselectivity. Since the eno-
thioester moiety in 3a to an aldehyde under the Fu- late approaches the Re-face of coumarin-3-carboxylic
kuyama reduction conditions.[20] Treatment of 90% ee acid, the (R)-isomer is preferably formed. Further
3a with 1.5 equivalents of Et3SiH and 5 mol% of Pd/C studies are required to fully elucidate the mechanistic
in acetone afforded the corresponding aldehyde 9 in detail of the addition reaction.
good yield without any loss of enantiopurity
In conclusion, we have developed a highly enantio-
(Scheme 4). It is well known that acetaldehyde is selective double-decarboxylative conjugate addition
a problematic nucleophile in conjugate addition reac- of MAHTs to coumarin-3-carboxylic acids using
tions to a,b-unsaturated esters due to the reaction of novel 8-quinolinesulfonylated organocatalysts. To the
acetaldehyde with products or acetaldehyde,[21] there- best of our knowledge, this is the first example of the
Figure 3. Assumed transition state for the decarboxylative addition of 2a to 1a using catalyst 5a. H atoms have been omitted
for clarity.
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Organocatalytic Enantioselective Conjugate Addition
Some, J. H. Lee, J.-Y. Kim, M. J. Song, S. Lee, Y. J.
Zhang, C. E. Song, Adv. Synth. Catal. 2011, 353, 3196–
3202.
highly enantioselective decarboxylative addition of
MAHTs to a,b-unsaturated ester derivatives. Further
studies focusing on the scope of the asymmetric syn-
thesis using heteroarenesulfonylated organocatalysts
are currently under investigation and will be reported
in due course.
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For the asymmetric decarboxylative addition of
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M. D. Shair, J. Am. Chem. Soc. 2007, 129, 1032–1033;
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Song, Angew. Chem. 2013, 125, 12365–12369; Angew.
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Experimental Section
Typical Procedure for the Synthesis of 3a using 5a
To a suspension mixture of coumarin-3-carboxylic acid 1a
(0.05 mmol, 9.5 mg), catalyst 5a (0.005 mmol, 2.7 mg), and
molecular sieves 5Š (10.0 mg) in THF (2.0 mL), malonic
acid half thioester 2a (0.010 mmol, 19.6 mg) was added and
the mixture stirred for 24 h. After removal of the solvent,
the residue was purified by silica gel column chromatogra-
phy (eluent: hexane/AcOEt, 90:10) to afford (R)-3a as
a white solid; yield: 16.2 mg (99%).
Acknowledgements
This work was supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas “Advanced Molecular Transfor-
mations by Organocatalysts” from MEXT (26105727).
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thermal unstability of intermediate A.
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asc.wiley-vch.de
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2016, 358, 1029 – 1034