Sequential Michael addition/retro-Claisen condensation of aromatic β-diketones with α,β-unsaturated esters: An approach to obtain 1,5-ketoesters

Article abstract of DOI:10.1039/c5ob02570b

A K₂CO₃-catalyzed one-pot protocol involving sequential C-C bond formation and cleavage of aromatic β-diketones with α,β-unsaturated esters is developed to obtain 1,5-ketoesters. The sequential reaction via Michael addition and retro-Claisen condensation proceeds smoothly under mild conditions in up to 98% isolated yield. The mechanism study disclosed that the cascade process involved C-C bond cleavage of aromatic β-diketone as a phenacyl donor under alcoholic alkalescent conditions.

Full text of DOI:10.1039/c5ob02570b

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Journal Name
COMMUNICATION
Sequential Michael addition/retro-Claisen condensation of
aromatic β-diketones with α, β-unsaturated esters: an approach
to 1, 5-ketoesters
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a,b,
a
a
a
a,
GuiꢀXin Cai *, Jing Wen , TingꢀTing Lai , Xie Dan , ChengꢀHe Zhou *
www.rsc.org/
K
2
CO
3
-catalyzed one-pot protocol involving sequential C-C CꢀC bond cleavage without transition metal catalyst is in its
1
8
bond formation and cleavage of aromatic β-diketones with α, β- infancy . We now demonstrate K
2
3
CO ꢀcatalyzed sequential Michael
unsaturated esters is developed to obtain 1,5-ketoesters. The addition/retroꢀClaisen condensation of aromatic βꢀdiketones with α,
sequential reaction via Michael addition and retro-Claisen βꢀunsaturated esters leading to 1, 5ꢀketoesters (Scheme 1).
condensation proceeds smoothly under mild conditions up to 98%
Scheme 1. Catalytic sequential Michael addition/retroꢀClaisen
isolated yield. The mechanism study disclosed that the cascade
condensation of aromatic βꢀdiketones with α, βꢀunsaturated esters.
process involved C-C bond cleavage of aromatic β-diketone as a
phenacyl donor under alcoholic alkalescent conditions.
O
Michael
addition
Introduction
OR
O
O
Michael sequential reactions such as double Michael,
Michael/Aldol, Michael/Henry and Michael/Conia−Ene improve the
2
K CO
3
1
O
O
Ar
OR
retro-Claisen
condensation
1
ꢀ2
elegance of synthesis . However, sequential Michael/retroꢀClaisen
condensation reaction which makes CꢀC bond formation and
cleavage in one pot is hardly reported. Herein, we presented
sequential Michael/retroꢀClaisen condensation reaction involving Cꢀ
C bond cleavage of βꢀdiketones under mild conditions to obtain 1, 5ꢀ
dicarbonyl compounds, as useful building blocks , which were
generally prepared under harsh conditions
Although selective CꢀC bond cleavage possesses both kinetic and
thermodynamic challenges, significant achievements have been
made in the field . Cook firstly reported CꢀC bond cleavage of βꢀ
Ar1
Ar2
Results and discussion
First of all, treatment of 1, 3ꢀdiphenylpropaneꢀ1, 3ꢀdione (1a) with
ethyl acrylate (2a) was chosen as a model reaction (Table 1). K CO
exhibiting excellent catalytic activity in ethanol at 85 C for 2 h,
astonishingly, afforded ethyl 5ꢀoxoꢀ5ꢀphenylpentanoate (4a) in 98%
isolated yield and a trace of Michael addition product 3a as well as
ethyl benzoate as a side product (Table 1, entry 1). Moreover, the
reaction temperature played an important role in promoting the
transformation (Table 1, entries 2ꢀ3). Experimental results showed
that increasing temperature obviously decreased the energy barrier of
3
2
3
4
ꢀ7
.
o
8
9
diketones, that is, retroꢀClaisen condensation which subsequently
1
0a
10b
was also developed via 6ꢀoxocamphor hydrolase , In(OTf)3
,
1
0c
10d
10e
10f
Fe(OTf)3 , FeCl3
2
, H O2
along with tꢀBuONa , individually.
In particular, CꢀC bond cleavage of aromatic βꢀdiketones, compared
4
a thus promoting the transformation of 3a into 4a. K
catalyst is crucial for this catalytic transformation (Table 1, entries 4ꢀ
2). No product was detected in the absence of the catalyst (Table 1,
entry 4). Furthermore, other carbonates replacing K CO as the
catalyst showed poorer catalytic activities (Table 1, entries 5ꢀ7). This
reaction also proceeded smoothly with KHCO and KOH (Table 1,
entries 8ꢀ9), nevertheless NEt as organic base could not promote
2 3
CO as base
11ꢀ14
15
with that of aliphatic βꢀdiketones
, is a greater challenge. Jiao
1
6
and Song reported copperꢀcatalyzed CꢀC bond cleavage of
1
aromatic βꢀdiketones giving αꢀketoesters and azole amides,
respectively. Rodriguez and Quintard
enantioselective synthesis of 3ꢀalkylpentanols concerning CꢀC bond
cleavage of aromatic βꢀdiketones via dual ironꢀamine catalysis. In
comparison with transition metal catalyzed CꢀC bond cleavage, the
2
3
17
also disclosed the
3
3
+
this process (Table 1, entry 10). Besides, the effect of K ion was
evaluated in the presence of KCl and an equivalent amount additive
of 18ꢀcrownꢀ6, respectively, which showed the proper basicity was
necessary (Table 1, entries 11ꢀ12). In addition, no product was
a
Key Laboratory of Applied Chemistry of Chongqing Municipality, Institute of
Bioorganic & Medicinal Chemistry, School of Chemistry and Chemical Engineering,
2
generated using HOAc or PdCl ; and even the copper catalyst, which
Southwest University, Chongqing 400715, China.
1
1,15
b
usually worked well in CꢀC bond cleavage of 1, 3ꢀdiketones
,
Beijing National Laboratory for Molecular Sciences, Beijing 100190, China
Eꢀmail: gxcai@swu.edu.cn; zhouch@swu.edu.cn.
showed inferior efficiency for this process (Table 1, entries 13ꢀ15).
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DOI: 10.1039/C5OB02570B
COMMUNICATION
Further screening of solvents displayed the production of 4a only in Table 2. K
2
CO
3
ꢀCatalyzed sequential reactions of acrylates 2 with 1,
a
3
ꢀdiphenylpropaneꢀ1, 3ꢀdione 1a
ethanol, which indicated that ethanol played a key role in the
transformation (Table 1, entries 16ꢀ19). Moreover, the pressure in
sealed tube and the inhibitor in commercial acrylates were evaluated,
and experimental results showed that both factors had little effect on
this transformation (Table 1, entry 20). Above systematic studies
indicated that several factors including the proper basicity, alcohol
and the reaction temperature corporately controlled this
transformation.
b
b
Entry
1
2
Yield of 4 (%)
Yield of 5 (%)
-
2
a
4a 98
4a 90
a
Table 1. Optimization of reaction conditions
2
3
4
5
2
b
5a NP (75)
5b 4 (80)
2
c
4a 75
O
Entry
1
Catalyst
Solvent
Yield of
Yield of
b
b
5
O
3
a(%)
4a(%)
2
d
4a 62
4a 87
5c 10 (75)
5d NP (49)
K
K
K
2
CO
2
CO
2
CO
-
3
3
3
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
trace
43
98
d
c
2
NP
e
3
20
40
NP
NP
16
86
93
92
NP
NP
96
NP
NP
15
NP
NP
NP
NP
2e
2f
4
5
6
7
8
9
NP
c
Li
Na
Cs
2
CO
CO
CO
3
trace
29
6
4
a NP
5e 90
5f NP
2
3
2
3
trace
trace
trace
trace
trace
trace
NP
7
KHCO
KOH
3
2
g
4a 94
4a 95
1
1
0
1
NEt
KCl
CO
HOAc
PdCl
CuBr
3
d
8
2h
2i
5g NP (80)
f
1
2
K
2
3
d
1
1
1
1
1
1
1
2
3
4
5
6
7
8
9
0
9
4
4
a 91
a 95
2
NP
5h NP (91)
5i NP (16)
2
NP
d
K
K
K
K
K
2
2
2
2
2
CO
3
3
3
3
3
CH
3
CN
42
10
2
j
CO
CO
CO
CO
Toluene
DCE
10
11
e
11
H
2
O
trace
trace
4a trace
5
j <10
2
k
g
h
EtOH
96 (97)
a
1
a (0.5 mmol), 2a (1 mmol), catalyst (0.05 mmol) and solvent (2 mL) in a
o
b c d o
e
12
-
sealed tube at 85 C for 2 h; Isolated yields; NP = no product; 25 C;
e
o
f
g
6
0
C; 18ꢀcrownꢀ6 (0.5 mmol) as an additive; Atmospheric
2l
4b 30
4c 10
h
pressure in refluxing ethanol; Fresh distilled ethyl acrylate (1
mmol).
e
e
1
3
4
-
2
m
With optimal conditions in hand, the scope of acrylate was
investigated in EtOH and the corresponding alcohol, respectively
(
1
Table 2). Firstly, methyl acrylate (2b) replacing 2a in EtOH gave
2
n
4b 57
5k trace (45)
the complete transesterification product 4a in 90% isolated yield,
while methyl 5ꢀoxoꢀ5ꢀphenylpentanoate (5a) was also obtained in
corresponding MeOH in 75% isolated yield (Table 2, entry 2).
Besides, longꢀchain aliphatic acrylates (2c, 2d) in EtOH led to both
e
e
e
1
5
6
2
2
o
4b 72
4b 35
5l <10
the product 4a in moderate yields and
a little amount of
1
corresponding products (5b, 5c), while 5b and 5c was provided in
corresponding nBuOH and hexylalcohol in 80% and 75% isolated
yields, respectively (Table 2, entries 3ꢀ4). Those results illustrated
that the transesterification process in EtOH preferred shortꢀchain
aliphatic acrylates. Moreover, benzyl acrylate (2e) in EtOH and
corresponding BnOH was wellꢀbehaved with the products of 4a and
5m trace
p
17
2q
4b trace
5n <10
e
1
8
NP
NP
2
r
5d in 87% and 49% yields, respectively (Table2, entry 5). It is
noteworthy that, due to steric hindrance, tertꢀbutyl acrylate (2f) did
not undergo transesterification
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DOI: 10.1039/C5OB02570B
COMMUNICATION
a
o
1
a (0.5 mmol), 2 (1 mmol), K
2
CO
3
(0.05 mmol), EtOH (2 mL), at 85 C,
b
1
ꢀ24 h; Isolated yields. Yields from corresponding alcohol solutions
c
d
3
9
were reported in parentheses; EtOH (0.5 mL); EtOH (2 mL), R OH
(
e
3
0.5 mL); EtOH (0.5 mL), R OH (0.5 mL).
1i
6b 25
a
o
1
(0.5 mmol), 2a (1 mmol), K
2
CO
3
(0.05 mmol), EtOH (2 mL), at 85 C,
in EtOH and completely transferred to the corresponding product 5e
in 90% isolated yield (Table 2, entry 6). Other acrylates (2g-2j) with
b
2
ꢀ48 h; Isolated yields.
functional groups, such as CH
2 2 2 2
CH OH, CH CH OMe,
entries 14ꢀ16). The results of 2q being similar to those of 2k showed
that steric hindrance of the cyclohexyl group played a leading role in
the cascade process compared with that of αꢀposition methyl group
tetrahydrofurfuryl and CH CF , in EtOH afforded the complete
2
3
transesterification product 4a in excellent yields, while those in
corresponding alcohol solutions gave products 5g-5h in moderate to
excellent yields, expect for 5i (Table 2, entries 7ꢀ10). Owing to the
poor nucleophilicity of trifluoroethanol, trifluoroethyl acrylate (2j) in
trifluoroethanol might be limited in the step of retroꢀClaisen
condensation and thus led to desired product 5i in poor yield.
Additionally, cyclohexyl acrylate (2k), due to steric factor, provided
the trace product 5j in EtOH or cyclohexanol (Table 2, entry 11).
Furthermore, it was found that steric hindrance of alkenes obviously
depressed this cascade process. The methyl group presenting at αꢀ
and βꢀposition of ethyl acrylate (2l, 2m, respectively) afforded
corresponding products in low to moderate yields along with a great
deal of acetophenone, which illustrated that steric hindrance of
alkene had the side effect on the step of Michael addition (Table 2,
entries 12ꢀ13, 18). Other methacrylates (2n-2p) containing
functional groups, such as benzyl, trifluoroethyl and glycol, in EtOH
and corresponding alcohol solutions provided the product 4a and the
corresponding products (5k, 5l) in moderate and poor yields,
respectively (Table 2,
(
Table 2, entry 17).
Subsequently, the reactions of various 1, 3ꢀdiketones 1 with ethyl
acrylate (2a) were explored under optimal conditions and the
screening results were displayed in Table 3. To our delight,
symmetrically aromatic βꢀdiketones (1aꢀ1c) containing the methoxyl
group and the pyridyl group were well tolerant under the optimal
conditions affording corresponding products (4a, 4d and 4e) in
moderate to excellent yields (Table 3, entries 1ꢀ3). However, in the
case of asymmetrically aromatic βꢀdiketone 1d, two desired products
both 4f and 4d were obtained in 40% and 57% isolated yields,
respectively, which illustrated that the substituents on the aromatic
rings had a slight effect on the transformation (Table 3, entry 4).
Regarding asymmetrical 1ꢀphenylꢀ1, 3ꢀbutanedione (1e), 4a was
selectively afforded in 89% isolated yield (Table 3, entry 5), which
illustrated that the acetyl group compared with the benzoyl group
was a better leaving group in this transformation. Besides, tested
asymmetrical 1,3ꢀdiketones containing the trifluoromethyl group (1f
and 1g) showed that the trifluoromethyl group made retroꢀClaisen
condensation precede Michael addition and thus the products of CꢀC
bond cleavage 7a and 7b were prior obtained in moderate yields,
respectively (Table 3, entries 6ꢀ7). Furthermore, aliphatic 1, 3ꢀ
diketone 1h, owing to steric hindrance, only afforded Michael
addition product 6a without further CꢀC bond cleavage process
(Table 3, entry 8). Whereas cyclohexaneꢀ1,3ꢀdione (1i) behaved
differently giving bisꢀaddition product 6b, which successfully made
the formation of two CꢀC bonds in one step (Table 3, entry 9).
Table 3 K CO ꢀcatalyzed cascade reactions of 1,3ꢀdicarbonyl
2
3
a
compounds 1 with ethyl acrylate (2a)
b
Entry
1
1
Yield (%)
1
a
4a 98
Control experiments were conducted in Scheme 2. Treatment
of 1a with 2ꢀmethoxyethyl acrylate (2h) under standard
2
3
4
5
6
7
8
conditions for 8 minutes gave a mixture of 6c, 5g and 3a as
1
b
4d 70
4e 75
well as the desired product 4a in 41%, 25%, 8% and trace
isolated yields, respectively (Scheme 2, Eq. 1). However,
extending reaction time provided the desired product 4a in 95%
isolated yield (Scheme 2, Eq. 2). The control experiment
without 2h under standard conditions only obtained a trace of
acetophenone (Scheme 2, Eq. 3). Moreover, the reaction of
acetophenone and 2h under standard conditions resulted in no
product formation (Scheme 2, Eq. 4). Furthermore, 6c smoothly
transferred to 4a in 91% isolated yield under optimal conditions
1
c
1
d
4f 40
4a 89
4d 57
1
e
(
Scheme 2, Eq. 5). Above results showed that the Michael
addition product as the key intermediate participated in the
catalytic cycle, which illustrated that the reaction might firstly
occur through Michael addition and then undergo close two
steps concerning CꢀC bond cleavage and transesterification.
Single retroꢀClaisen process certainly existed in the reaction
system to lead to trace amounts of byproduct.
4
a trace
g trace
7a 65
7b 54
1
f
4
1
g
Scheme 2. Control experiments
1
h
6a 62
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Research Funds for the Central Universities (XDJK2013C112).
Notes and references
1
.
(a) Nicolaou, K. C.; Chen, J. S. Chem. Soc. Rev. 2009, 38,
993ꢀ3099. (b) Grondal, C.; Jeanty, M.; Enders, D. Nature
2
Chem. 2010, 2, 167ꢀ178.
2
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(a) Zeng, X. M. Chem.Rev. 2013, 113, 6864ꢀ6900. (b)
Cichowicz, N. R.; Kaplan, W.; Khomutnyk, Y.; Bhattarai, B.;
Sun, Z.; Nagorny, P. J. Am. Chem. Soc. 2015, 137, 14341ꢀ
1
4348.
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(a) Donohoe, T. J.; Basutto, J. A.; Bower, J. F.; Rathi, A. Org.
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P.; Zhou, Q. L. Angew. Chem. Int. Ed. 2013, 52, 7833ꢀ7836. (c)
Periasamy, M.; Gurubrahamam, R.; Muthukumaragopal, G. P.
Tetrahedron: Asymmetry 2013, 24, 568ꢀ574. (d) Goh, K. K. K.;
Kim, S.; Zard, S. Z. Org. Lett. 2013, 15, 4818ꢀ4821. (e) Tatton,
M. R.; Simpson, I.; Donohoe, T. J. Org. Lett. 2014, 16, 1920ꢀ
2 3
The proposed mechanism of K CO ꢀcatalyzed a pot strategy for
synthesis of 1, 5ꢀketoesters was listed in Scheme 3. In the presence
of K CO , Michael addition between aromatic βꢀdiketones and α, βꢀ
1
923.
(a) Lambert, J. B.; Greifenstein, L. G. J. Am. Chem. Soc. 1973,
5, 6152ꢀ6153. (b) Yasuda, M.; Ohigashi, N.; Shibata, I.; Baba,
2
3
unsaturated carbonyl compounds occurred, which realized the
formation of intermediate I. Subsequently, the CꢀC bond cleavage,
namely retroꢀClaisen condensation, as a key step came true as
followed by releasing ethyl benzoate in the presence of ethanol.
Importantly, the CꢀC bond cleavage and transesterification occurred
almost at the same time regarding to the formation of 1, 5ꢀketoesters.
Besides, alcohol had an unusual part to play in concurrently cleaving
CꢀC and CꢀO bonds.
9
A. J. Org. Chem. 1999, 64, 2180ꢀ2181. (c) Yasuda, M.; Chiba,
K.; Ohigashi, N.; Katoh, Y.; Baba, A. J. Am. Chem. Soc. 2003,
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25, 7291ꢀ7300. (d) Ratnikov, M. O.; Tumanov, V. V.; Smit,
W. A. Angew. Chem. Int. Ed. 2008, 47, 9739ꢀ9742.
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6
7
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Scheme 3. Proposed mechanism
1
619.
(a) Stork, G.; Jung M. E. J. Am. Chem. Soc. 1974, 96, 3682ꢀ
3
2
684. (b) Li, L. Z.; Xiao, B.; Guo, Q. X.; Xue, S. Tetrahedron
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(
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(
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V. J. Am. Chem. Soc. 1982, 104, 318ꢀ321.
9
1
.
In summary, we have disclosed a cascade process involving
Michael addition and retroꢀClaisen condensation in one pot.
Aromatic βꢀdiketones with α, βꢀunsaturated carbonyl compounds
could smoothly access to 1, 5ꢀketoesters in the presence of K CO in
alcohol solutions. Especially, the CꢀC bond cleavage of aromatic βꢀ
diketones supplying the phenacyl group differentiated from reported
0. (a) Grogan, G.; Graf, J.; Jones, A.; Parsons, S.; Turner, N. J.;
Flitsch, S. L. Angew. Chem. Int. Ed. 2001, 40, 1111ꢀ1114. (b)
Kawata, A.; Takata, K.; Kuninobu, Y.; Takai, K. Angew. Chem.
Int. Ed. 2007, 46, 7793ꢀ7795. (c) Biswas, S.; Maiti, S.; Jana, U.
Eur. J. Org. Chem. 2010, 2861ꢀ2866. (d) Rao, C. B.; Rao, D.
C.; Babu, D. C.; Venkateswarlu, Y. Eur. J. Org. Chem. 2010,
2
3
1, 3ꢀdiketones as an acylation reagent. Further mechanical studies
2
855ꢀ2859. (e) Sun, X.; Wang, M.; Li, P.; Zhang, X. L.; Wang,
showed that EtOH played an unprecedented role, which assisted the
cleavages of both CꢀC and CꢀO bonds in one pot. This method
L. Green Chem. 2013, 15, 3289ꢀ3294. (f) Xie, F.; Yan, F. X.;
Chen, M. M.; Zhang, M. RSC Adv. 2014, 4, 29502ꢀ29508.
provides a convenient and practical alternative to 1, 5ꢀdicarbonyl 11. He, C.; Guo, S.; Huang, L.; Lei, A. W. J. Am. Chem. Soc. 2010,
compounds. The synthetic applications in medicinal candidates are
now in progress in our group.
132, 8273ꢀ8275.
1
2. Li, Z. W.; Dong, J. Y.; Chen, X. L.; Li, Q.; Zhou, Y. B.; Yin, S.
F. J. Org. Chem. 2015, 80, 9392ꢀ9413.
This work was supported by the National Natural Science 13. (a) Shrout, D. P.; Lightner, D. A. Synthesis 1990, 1062ꢀ1065.
Foundation of China (21004075, 21372186), Beijing National
Laboratory of Molecular Sciences (BNLMS) (20140130), the
Doctoral Fund of Southwest University (SWU111075) and the
(b) Pierre, M. R.; Markus, M. W.; Dirk, S. L.; Dietmar, F. B.K.;
Andreas, B. B. U.S. Patent 20080242857 (A1), 2008ꢀ8ꢀ2.
This journal is © The Royal Society of Chemistry 20xx
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Orga Pn l iec a s&e Bd oi o nmo to al ed cj uu sl ta mr Ca rhg ei nms istry
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DOI: 10.1039/C5OB02570B
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