Chiral squaramides as highly enantioselective catalysts for michael addition reacions of 4-hydroxycoumarins and 4-hydroxypyrone to β,γunsaturated α-keto esters

Article abstract of DOI:10.1002/chem.201000094

(Figure Presented) Distance brings forth beauty: The first highly enantioselective organocatalytic Michael addition of 4-hydroxycoumarins and the analogous 4-hydroxy-6methyl-2-pyrone to β,γ-unsaturated α-keto esters by using chiral squaramides as the orga

Full text of DOI:10.1002/chem.201000094

COMMUNICATION
DOI: 10.1002/chem.201000094
Chiral Squaramides as Highly Enantioselective Catalysts for Michael
Addition Reactions of 4-Hydroxycoumarins and 4-Hydroxypyrone to b,g-
Unsaturated a-Keto Esters
Dan-Qian Xu,[a] Yi-Feng Wang,[a] Wei Zhang,[a] Shu-Ping Luo,[a] Ai-Guo Zhong,[b]
Ai-Bao Xia,[a] and Zhen-Yuan Xu*[a]
Coumarin derivatives are an important class of com-
pounds found in a variety of natural products and bioactive
molecules and are used as versatile intermediates in organic
and natural-product synthesis.[1] Consequently, several ap-
proaches have been taken to develop catalytic, enantioselec-
tive modifications of coumarins, which are reported to have
various biological activities, such as antimalarial, antibacteri-
al, anticoagulant, and anti-HIV activities.[2] Recently, the or-
ganocatalytic, asymmetric Michael addition of 4-hydroxy-
coumarin to a,b-unsaturated ketones and aldehydes through
iminum ion catalysis by chiral amines to afford a highly
atom-economic procedure for the formation of optically
active coumarins was reported.[3]
Squaramide derivatives have been intensively investigated
within the area of molecular recognition because of their
strong hydrogen-bonding activity.[4] However, applications
of squaramide derivatives for electrophilic activation by hy-
drogen bonding in enantioselective reactions appear scarce
compared with their corresponding thioureas, which were
recognized as a “privileged” platform for dual hydrogen-
bonding catalysis over the past decade.[5,6] Very recently, the
utility of chiral squaramides as organocatalysts was disclosed
by Rawalꢀs group for the conjugate-addition reaction of 1,3-
dicarbonyl compounds to nitroolefins, which benefit from
squaramideꢀs characteristic dual hydrogen-bonding catalysis,
in which the two hydrogen atoms are further apart than
those in thioureas.[7]
Herein, we describe the development of a squaramide-
promoted, asymmetric Michael addition of 4-hydroxycou-
marins and a 4-hydroxypyrone to b,g-unsaturated a-keto
esters.[8] The corresponding adducts of these chiral coumarin
derivatives are of particular interest, since they not only ex-
hibit highly reactive carbonyl functionalities, but also can be
readily modified to form various functional groups. We
postulated that b,g-unsaturated a-keto esters, which are
electrophiles with two adjacent carbonyl groups connected
by a two-carbon link, could be suitably activated and orien-
tated by squaramide, as its two acidic hydrogen atoms are
also separated by a two-carbon link. Satisfactorily, the orga-
nocatalytic process is effective for both 4-hydroxycoumarins
and the analogous 4-hydroxypyrone to afford highly optical-
ly enriched compounds (up to 99% ee) in good yields by
using easy-to-prepare chiral squaramides as catalysts with
low catalyst loadings.
The addition of 4-hydroxycoumarin 1a to the a-keto ester
2a was used as a test reaction to explore the feasibility of
the enantioselective Michael addition reaction catalyzed by
chiral, tertiary-aminosquaramide-based derivatives I and
IIa–d (Scheme 1). As shown in Table 1, the preliminary re-
sults revealed that, in the presence of the diaminocyclohex-
ane-derived squaramide catalyst I (Table 1, entry 1), the re-
action proceeded smoothly to yield the desired Michael
adduct 3aa in good yield (77%) with moderate ee (75%)
within a short reaction time (3 h). Changing an amine
moiety to a cinchona alkaloid framework to form the epi-
quinine-derived squaramide catalysts IIa and b increased
the enantioselectivities to 90 and 86% ee, respectively
(Table 1, entries 2 and 3). Catalysts also containing chiral
substituents on the other end of the squaramide (IIc and d)
resulted in even higher yields and enantioselectivities
(Table 1, entries 4 and 5). In contrast, the reaction per-
[a] Prof. Dr. D.-Q. Xu, Y.-F. Wang, W. Zhang, Dr. S.-P. Luo, A.-B. Xia,
Prof. Z.-Y. Xu
State Key Laboratory Breeding Base of
Green Chemistry—Synthesis Technology
Zhejiang University of Technology
Hangzhou 310014 (China)
Fax : (+86)571-88320066
E-mail: greenchem@zjut.edu.cn
[b] Dr. A.-G. Zhong
Department of Pharmaceutical and Chemical Engineering
Taizhou College, Linhai Zhejiang 317000 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000094.
Chem. Eur. J. 2010, 16, 4177 – 4180
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4177

The investigation of the Michael addition was then ex-
tended to a variety of b,g-unsaturated a-keto esters with cat-
alyst IId (2.5 mol%) in DCE (Table 2). It was discovered
Table 2. Catalyst IId-promoted enantioselective Michael addition of 4-
hydroxycoumarins
1 or 4-hydroxy-6-methyl-2-pyrone (4) with a-keto
esters 2 to give 3 or 5, respectively.[a]
X
R
3(5)
t
Yield
ee
[h][b]
[%][b,c] [%][b,d]
Scheme 1. Applied squaramide and thiourea organocatalysts.
1
2
3
4
5
6
7
8
9
H(1a)
H(1a)
H(1a)
H(1a)
H(1a)
H(1a)
H(1a)
H(1a)
H(1a)
Ph(2a)
3(5)aa 12(12) 93(85)
3(5)ab 12(12) 88(86)
3(5)ac 12(12) 65(80)
96(94)
93(95)
92(96)
4-MeC6H4(2b)
4-MeOC6H4(2c)
4-ClC6H4(2d)
4-BrC6H4(2e)
4-CF3C6H4(2f)
4-NO2C6H4(2g)
3-BrC6H4(2h)
2, 4-Cl2C6H3(2i)
3(5)ad
3(5)ae
3(5)af
1(1)
1(1)
1(1)
92(89) >99(96)
94(90) 95(96)
95(88) >99(96)
Table 1. The enantioselective Michael addition of 4-hydroxycoumarin
(1a) with 4-phenyl-2-oxo-3-butenoate ethyl ester (2a) using squaramide
and thiourea organocatalysts.[a]
3(5)ag 10(24) 73(68)
91(94)
92(95)
92(92)
92(93)
92(91)
91(92)
86(90)
89(93)
98
3(5)ah 10(8)
81(82)
86(82)
89(83)
3(5)ai
10(8)
10(8)
10 H(1a)
11 H(1a)
12 H(1a)
13 H(1a)
14 H(1a)
15 6-Me(1b) Ph(2a)
16 6-Me(1b) 4-CF3C6H4(2f)
17 6-Cl(1c)
18 6-Cl(1c)
naphthalen-2-yl(2j) 3(5)aj
furan-2-yl(2k)
thien-2-yl(2l)
Me(2m)
3(5)ak 10(12) 86(79)
3(5)al 10(12) 87(75)
3(5)am 10(12) 76(67)
3(5)an 10(12) 71(71)
Pr(2n)
3ba
3bf
3ca
3ce
12
12
12
12
87
82
82
80
Catalyst
[mol%]
Solvent
t
Yield
[%][b]
ee
[h]
[%][c]
97
95
94
Ph(2a)
4-BrC6H4(2e)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
I
20.0
20.0
20.0
20.0
20.0
20.0
20.0
10.0
5.0
2.5
2.5
2.5
2.5
DCM[d]
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCE[e]
CHCl3
Et2O
3
3
3
3
3
12
12
5
77
87
85
90
91
75
69
92
91
92
93
88
76
70
75
90
86
92
93
64
55
94
93
93
96
87
83
81
IIa
IIb
IIc
IId
IIIa
IIIb
IId
IId
IId
IId
IId
IId
IId
[a] Reaction conditions: 1 or 4 (0.1 mmol), 2 (0.1 mmol), IId (2.5 mol%),
DCE (1.0 mL), RT. [b] Results of adducts 5 in parentheses. [c] Yield of
the isolated product. [d] Determined by HPLC analysis using a chiral sta-
tionary phase.
8
that most reactions with a-keto esters containing g-aryl or
-heteroaryl substituents were completed within 12 h in
good-to-excellent yields (73–95%) and excellent enantiose-
lectivities (91–>99% ee;Table 2, entries 1–12). It appears
that the position and electronic properties of substituents on
aromatic rings have a limited effect on the efficiency of this
process. Notably, g-alkyl-substituted a-keto esters also gave
good yields and high enantioselectivities (Table 2, entries 13
and 14). 4-Hydroxycoumarins 1b and c, containing electron-
donating or -withdrawing groups, also afforded excellent re-
sults (Table 2, entries 15–18). Moreover, the scope of the re-
action can be successfully extended utilizing to 4-hydroxy-6-
methyl-2-pyrone (4), a 4-hydroxycoumarin analogue, as the
Michael donor, which leads to differently substituted a-keto
esters 5 with 90–96% ee (Table 2, entries 1–14 in parenthe-
ses). The lower reactivity of compound 2g compared with
2d–f may be attributed to the interference of the nitro
group with the hydrogen-bonding catalysis (Table 2,
entry 7).
12
12
12
12
12
2.5
PhMe
[a] Reaction conditions: 1a (0.1 mmol), 2a (0.1 mmol), solvent (1 mL),
RT. [b] Yield of the isolated product. [c] Determined by HPLC analysis
on a Chiralcel AD-H column. [d] DCM=dichloromethane. [e] DCE=
1,2-dichloroethane.
formed poorly on using the equivalent thioureas IIIa and b
(Table 1, entries 6 and 7), probably due to the lack of effec-
tive activation and orientation of the a-keto esters. Further-
more, the squaramide-based catalyst IId maintained the
high yields and enantioselectivities with a reduced catalyst
loading from 20 to 2.5 mol% with only an extension of the
reaction time (Table 1, entries 5 and 8–10). Of the investi-
gated nonpolar solvents, which are all generally suitable for
hydrogen bonding catalysis, 1,2-dichloroethane (DCE) was
found to be the most effective and the reaction was com-
pleted within 12 h with 93% yield and 96% ee (Table 1, en-
tries 10–14).
To confirm the reasons for the observed differences in the
catalytic activity of squaramide and thiourea, ab initio calcu-
lations at the HF/6-31+G (d,p) level were performed, utiliz-
ing the geometry of initial optimizations by means of the
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Chiral Squaramides
COMMUNICATION
Gaussian 03W program.[9] On the basis of the QST3 proce-
dure, two optimized transition-state structures IV and V that
link the a-keto ester 2a and the simplified catalysts were lo-
cated with only a single imaginary frequency of À1450i and
À1624i cmÀ1, respectively (Figure 1). Two hydrogen bonds
(lengths: H35···O31=2.341 ꢂ, H34···O29=2.045 ꢂ for IV,
and H39···O35=2.451 ꢂ, H38···O33=2.059 ꢂ for V) could
be observed for each transition state. Natural bond orbital
(NBO) analyses[10] were carried out to understand the
nature of the activation of the g-carbon of 2a; this showed
that the NBO value at C29 in V (q
less negative than that of C25 in IV (qAHCUTNGTRENNUNG
ACHTUNGTRENNUNG
Scheme 2. Proposed transition-state model for the Michael addition.
plying that the C29 site of V is more electrophilic than that
of IV, which is consistent with the reaction performance and
results.[11]
ing interactions with the two adjacent carbonyl groups and
increase the electrophilicity of the g-position of the a-keto
ester. Furthermore, the Re face approach of 4-hydroxycou-
marin is induced by the tertiary amine of the catalyst and
leads to the formation of the major stereoisomer, which was
assigned as the R configuration on the basis of the X-ray
structure.[12]
In conclusion, we have disclosed the first highly enantio-
selective organocatalytic Michael addition of 4-hydroxycou-
marins and the analogous 4-hydroxy-6-methyl-2-pyrone to
b,g-unsaturated a-keto esters by using squaramide IId as an
organocatalyst. In this efficient catalytic asymmetric reaction
only 2.5 mol% of the catalyst is sufficient to afford chiral 3-
functionalized 4-hydroxycoumarin and 4-hydroxypyrone de-
rivatives in good yields with excellent enantioselectivities
(up to >99% ee). Given that this is the first example of the
activation of carbonyl esters by a chiral squaramide, this dis-
covery is likely to find immediate synthetic applications.
Considering the high efficiency of the potent transforma-
tions further investigation of the applications of this cataly-
sis system is underway in our group.
Experimental Section
General procedure: A mixture of 4-hydroxycoumarin 1 (0.1 mmol), b,g-
unsaturated a-keto ester 2 (0.1 mmol) and the catalyst IId (0.0025 mmol)
in DCE (1.0 mL) was stirred at room temperature for 1–12 h (monitored
by TLC). The mixture was purified by column chromatography on silica
gel, eluted by petroleum ether/EtOAc (10:1 to 3:1) to give the desired
Michael adduct 3 (e.g. the adduct 3aa in 93% yield and 96% ee; HPLC
on a Chiralcel AD-H column, hexane:iPrOH (70:30) containing 0.15%
TFA at 1.0 mLminÀ1; tmajor =5.8 min, tminor =9.1 min).
Figure 1. Molecular geometry of the hydrogen-bonding interactions in IV
(top) and V (bottom).
Acknowledgements
We are grateful for the financial support of the National Natural Science
Foundation of China (No. 20772110).
To account for the observed stereochemical outcome of
the reaction, a transition state model was proposed and is
depicted in Scheme 2. The NH groups of the squaramide
moiety in the catalyst are believed to form hydrogen-bond-
Keywords: asymmetric catalysis · hydrogen bonds · Michael
addition · organocatalysis · squaramides
Chem. Eur. J. 2010, 16, 4177 – 4180
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4179

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[12] CCDC-756209 (3ad), 756208 (5ad) contains the supplementary crys-
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via www.ccdc.cam.ac.uk/data_request/cif.
Received: January 14, 2010
Published online: March 12, 2010
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