A straightforward synthesis of cyclobutenones via a tandem Michael addition/cyclization reaction of 2,3-allenoates with organozincs

Article abstract of DOI:10.1021/ja1108694

An efficient method for synthesis of polysubstituted cyclobutenones, which are not readily available from traditional methods due to the intrinsic ring strain, is described. The reaction of 2,3-allenoates and organozinc reagents proceeds via a tandem Mich

Full text of DOI:10.1021/ja1108694

COMMUNICATION
pubs.acs.org/JACS
A Straightforward Synthesis of Cyclobutenones via a Tandem Michael
Addition/Cyclization Reaction of 2,3-Allenoates with Organozincs
Guobi Chai, Shangze Wu, Chunling Fu, and Shengming Ma*
Laboratory of Molecular Recognition and Synthesis, Department of Chemistry, Zhejiang University, Hangzhou 310027, Zhejiang, P.R. China
S Supporting Information
b
Scheme 1. Known Methods and Concept for the Synthesis of
Cyclobutenones from Allenoates
ABSTRACT: An efficient method for synthesis of poly-
substituted cyclobutenones, which are not readily available
from traditional methods due to the intrinsic ring strain, is
described. The reaction of 2,3-allenoates and organozinc
reagents proceeds via a tandem Michael addition/cyclic 1,2-
addition/elimination mechanism with the functional groups
from the organozinc reagents being introduced to the
3-position of the cyclobutenone products regiospecifically
in moderate to excellent yields. Application to the synthesis
of stereodefined β,γ-unsaturated enones is demonstrated.
yclobutenones and their derivatives are not readily available
Table 1. Effects of Catalyst, Temperature, and Solvent on the
Michael Addition/Cyclization of 2,3-Allenoate with Et2Zn
C
due to their unique structures bearing the carbon-carbon
double bond and carbonyl group in the strained four-membered
rings.1 However, they are important building blocks for the
preparation of R,β-butenolides,2a cyclopentones,2a 1,3-dienes,2b
and a variety of substituted cyclobutene derivatives. In addition,
attention has been focused on their thermal reactivity3 and transition
metal-catalyzed reactions providing diversified ring-expanded
compounds.4 Cyclobutenones derivatives have also served as
selective and orally active COX-2 inhibitors.5 In the past years,
the synthesis of such type of compounds is usually accomplished by
[2þ2] cycloadditions of acetylene with ketene6 or keteniminium
salt7 (Scheme 1), which requires preformation of unstable ketene
precursor and suffers from the limitation of the substituents on the
four-membered ring and the regioselectivity referred to the alkynes.
In principle, intramolecular reaction of the allylic C-M bond with
the ethoxycarbonyl group in the B-type intermediate8 (Scheme 1)
would be a very convenient way to synthesize such compounds.
However, due to the presence of strained cyclic ring and the
difficulty in formation of the stereodefined B intermediate, it would
be a great challenge to realize such a cyclization reaction.9 Here we
present our recent realization of such a concept by using readily
available 2,3-allenoate10 with organozincs as the starting materials.
After many trials and errors, we were pleased to find that the
reaction of methyl 2-butyl-4-propyl-2,3-heptadienoate 1a with
diethyl zinc in toluene at room temperature afforded the desired
product, 2-butyl-3-ethyl-4,4-dipropylcyclobut-2-enone 2a, in 40%
yield, with a recovery of 35% of the starting 1a. The addition of some
transition metal catalysts afforded poor results (entries 2-5,
Table 1). Fortunately, we found that the reaction was significantly
accelerated at an elevated temperature of 100 °C(entry6). Studyon
solvent effect revealed that toluene and n-hexane are the best
(compare entries 7-9, 11 with entries 6 and 10). However, the
reaction in n-hexane is relatively slow (entry 10). Therefore, we
yielda (%)
solvent/
cat.
recovery
of 1aa (%)
entry
temp/time (h)
(5 mol %)
2a
3a
10
1
2
3
4
5
6
7
8
9
toluene/rt/14
-
40
34
53
32
6
35
20
25
39
48
1
toluene/rt/24
Fe(acac)3
7
toluene/rt/24
CuCl
6
toluene/rt/24
CuCl2
5
toluene/rt/13
toluene/100 oC/1.5
Ni(PPh3)2Cl2
9
-
-
-
-
-
-
95
32
0
1.5
Et2O/rt/19
20
48
97
19
0.4
44
CH3CN/60 °C/13
THF/70 °C/18.5
2.5
8
58
96
53
10 n-hexane/60 °C/14
11 dioxane/100 °C/1.5
2
0.8
a Determined by NMR using dibromomethane as internal standard.
defined the reaction of 2,3-allenoates with 3 equiv of diethyl zinc in
toluene at 100 °C as the standard reaction conditions (entry 6).
The scope of this transformation was investigated under the
standard conditions (Table 2). The reaction of a variety of fully
Received: December 3, 2010
Published: February 25, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/ja1108694 J. Am. Chem. Soc. 2011, 133, 3740–3743
|

Journal of the American Chemical Society
COMMUNICATION
Table 2. Tandem Reaction of 2,3-Allenoates 1 with Orangozinc
Reagents Affording Diversified Polysubstituted Cyclobutenones 2a
isolated
entry
R1
R2
R3
R4
time
yield of 2 (%)
1
n-Pr
n-Pr
i-Pr
n-Pr n-Bu (1a)
Et
Et
Et
Et
Et
Et
Et
1.5 h
1.5 h
1.5 h
4 h
70 (2a)
78 (2b)
80 (2c)
74 (2d)
72 (2e)
78 (2f)
72 (2g)
79 (2h)
92 (2i)
88 (2j)
50 (2k)
90 (2l)
82 (2m)
80 (2n)
72 (2o)
74 (2p)
69 (2q)
82 (2r)
89 (2s)
81 (2t)
68 (2a)
Figure 1. ORTEP representation of 2s and trans-4e (right).
Table 3. Reaction of 2,3-Allenoates 1 with Functional Groupsa
2
Me
Me
n-Bu (1b)
n-Bu (1c)
n-Bu (1d)
Ph (1e)
3
4b
t-Bu Me
5
Me
i-Pr
Me
Me
Me
Me
Me
Me
3.5 h
3.5 h
1.5 h
2 h
6
Ph (1f)
7
p-FC6H4 (1g)
8
1-naphthyl (1h) Et
9
-(CH2)4-
-(CH2)5-
t-Bu (1i)
Ph (1j)
Et
Et
Et
Et
1 h
entry
R5
time (min)
isolated yield of 2 (%)
10
11
12
13
14
15c
16c
17
18
19
20
21d
1.5 h
10 min
20 min
n-Bu
H
H
n-Bu (1k)
t-Bu (1l)
1
2
3
4
Br (1m)
90
15
15
15
62 (2u)
73 (2v)
75 (2w)
45 (2x)
n-Bu
n-Pr
n-Bu
Me
CN (1n)
n-Pr n-Bu (1a)
i-Pr 2 h
i-Pr 0.5 h
i-Pr 3.5 h
i-Pr 2 h
CO2CH3 (1o)
H
t-Bu (1l)
COCH3 (1p)
Me
p-FC6H4 (1g)
Ph (1j)
a The reaction was conducted with 0.4 mmol of 2,3-allenoates, 5 mL of
toluene, and 3 equiv of Et2Zn (1.5 M solution in toluene).
-(CH2)5-
n-Pr
n-Pr n-Bu (1a)
Ph
Ph
Ph
Ph
Et
3 h
n-Bu
H
t-Bu (1l)
Ph (1e)
Ph (1j)
0.5 h
1.5 h
2.5 h
2 h
well tolerated under the standard conditions, affording the
corresponding cyclobutenones 2u-2x in 45-75% yields.
To unveil the mechanism, the reaction of substrate 1n with
Et2Zn was carefully studied: the reaction proceeded smoothly even
at room temperature within 5 min, however, affording the conjugate
addition product 3b upon protonolysis in 93% isolated yield.12
Heating this resulting mixture at 100 °C led to the formation of
cyclobutenone 2v in 75% 1H NMR yield, indicating the formation of
2v via the intermediacy of A1-Zn (Scheme2),althoughthecyclic1,2-
addition to the ester group to form the butenone ring is not easy.13
The reaction of 1n with EtMgBr was also examined: after formation
of magnesium 1,3-dienolate A1-Mg under our previous conditions,12
the reaction mixture was heated at 100 °C to afford a complicated
mixture, instead of 2v (Scheme 2), indicating the large difference of
reactivities of the in situ formed Zn and Mg intermediates.
As in our previous study, control experiment showed that the
reaction of 4-phenyl-substituted substrate 1q with Et2Zn at rt
yielded (Z)-5-benzylidenecyclohex-2-enones,13 while the same
reaction at 100 °C afforded a mixture of (Z)-5-benzylidenecyclo-
hex-2-enones13 and naphthols14 (Scheme 3). Here, the phenyl
group on the 4-position may increase the reactivity of B-type
intermediate toward the second molecule of 2,3-allenoate to trigger
the second Michael addition and cyclization at room temperature,
affording the (Z)-5-benzylidenecyclohex-2-enones; for the forma-
tion of the naphthol derivative at 100 °C, the phenyl group on the
4-position is directly involved after conjugate addition.14
Me
Me
-(CH2)5-
n-Pr
n-Pr n-Bu (1a)
a The reaction was conducted with 0.4 mmol of 2,3-allenoates, 5 mL of
toluene, and 3 equiv of R4 Zn (1.5 M solution in toluene for Et2Zn,
2
1.0 M solution in toluene for i-Pr2Zn, or pure Ph2Zn); the formation of
1-9% hydrolysis product 3 was observed by 1H NMR analysis of the
crude reaction mixture. b The reaction was conducted in xylenes at
140 °C for a complete conversion. c The yield of hydrolysis product 3 is
12%. d The reaction was conducted with a 5.0 mmol scale of 1a.
substituted 2,3-allenoates 1a-1j and 4-monosubstituted 2,3-
allenoates 1k-1l with diethyl or diisopropyl zinc reagents
afforded the corresponding cyclobutenones 2a-2p in moderate
to excellent yields (entries 1-16). Furthermore, diphenyl zinc
can also be applied to the reaction affording 2q-2t in good yields
(entries 17-20). The substituents on the 4-position of the
allenoates (R1/R2) can be H, and primary, secondary, and
tertiary alkyl groups; the R3 group could be alkyl and aromatic
groups. When R1 is a bulky alkyl group, i.e., tert-butyl group, the
reaction needs to be conducted in xylenes at 140 °C to get a
complete conversion probably due to the steric effect (entry 4).
However, the reaction of substrates with bulky R3 groups such as
tert-butyl, phenyl, substituted phenyl, or naphthyl groups fin-
ished at 100 °C smoothly (entries 5-10, 12, 14-16, and 18-
20). In addition, the reaction can be easily conducted at a scale of
5.0 mmol of the substrate 1a in a similar yield (entry 21). The
structure of the products was further confirmed by the X-ray
single-crystal diffraction analysis of 2s (Figure 1).11
In order to show new applications of this type of four-
membered products,2-4 the not readily available polysubstituted
β,γ-unsaturated enones 515,16 have been prepared from 2 and
lithium reagents. 1,2-Addition of alkyl lithium reagents with 2a
afforded cyclobutenols 4a and 4b. Subsequent ring-opening of 4
in the presence of LDA afforded β,γ-unsaturated enones 5a and
5b in 83% and 89% yields, respectively (Scheme 4).17
Next, the functional group compatibility of this reaction was
illustrated with substrates 1m-1p. As shown in Table 3, Br and
polar functional groups such as CN, CO2CH3, and COCH3 were
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COMMUNICATION
Scheme 2. Control Experiments of 1n with Et2Zn or EtMgBr
at Different Temperatures
Table 4. 1,2-Addition of Organolithium Reagents with
Cyclobutenones 2a,b
2
isolated yield (%)
entry
R1/R2/R3
R6 time (h)
cis-4
trans-4
1
Pr/Me/Bu (2b)
Bu
1
65 (cis-4c) 25 (trans-4c)
76 (cis-4d) 9 (trans-4d)
70 (cis-4e) 13 (trans-4e)
2
3c
i-Pr/Me/Bu (2c) Bu
i-Pr/Me/Ph (2f) Me
0.5
4
a 3 equiv of n-BuLi (2.5 M solution in n-hexane) or MeLi (1.3 M solution
in Et2O) were used. b In this table, Pr is n-propyl and Bu is n-butyl. c The
reaction was conducted with 4 equiv of n-BuLi (2.5 M solution in n-
hexane) in Et2O for a better conversion of 93% (the conversion in THF
was 69%).
Table 5. LDA-Promoted Ring-Opening Reaction of
Cyclobutenols 4a
Scheme 3. Control Experiments of 1q with Et2Zn under
Different Conditions
entry R1
R3
R6
time (h) isolated yield of 5 (%)
1
n-Pr n-Bu n-Bu (cis-4c)
i-Pr n-Bu n-Bu (cis-4d)
3.5
2
91 (Z-5c)
87 (Z-5d)
85 (Z-5e)
87 (E-5c)
85c (E-5e)
2
3b
i-Pr Ph
n-Pr n-Bu n-Bu (trans-4c)
i-Pr Ph Me (trans-4e)
Me (cis-4e)
36
Scheme 4. SynthesisofPolysubstitutedβ,γ-Unsaturated Enones 5
4
4
4
5
a Single isomer was detected unless otherwise noted. b The reaction was
conducted at -20 °C. c E/Z g 98/2.
Scheme 5. Concerted Retro-4e-cycloaddition of Lithium
2-Cyclobutenoxides with Conrotation: An Explanation for the
Stereochemistry Observed
When R1 ¼
6
R2 (R1 > R2), two stereoisomers, cis-4 and trans-4,
would form from the 1,2-addition reaction with cis-4 being the major
product due to the steric interaction between the incoming R6 group
from the lithium reagent and the bulky R1 group. As expected, a bulkier
substituent of R1 leads to a better stereoselectivity (compare entry 1
with entry 2, Table 4). The configuration of the product was assigned
by the X-ray single-crystal diffraction analysis of trans-4e (Figure 1).18
Excitingly, the LDA-promoted ring-opening reaction of cyclo-
butenols 4 was found to be highly stereoselective: the reaction of
cis-4 in the presence of LDA afforded corresponding Z-5 exclu-
sively in 85-91% yields, while the reaction of trans-4 afforded
corresponding E-5 exclusively in 87% and 85% yields, respectively
(entries 1-5, Table 5). The configuration of the carbon-carbon
double bond was established by NOESY analysis.
The stereochemistry observed here is in accordance with the Wood-
ward-Hoffmann rule, indicating that this opening reaction may
involve a concerted retro-4e-cycloaddition of the lithium 2-cyclobuten-
oxides formed from the reaction of the alcohols with LDA (Scheme 5).
In summary, we have developed an efficient and general method to
synthesize polysubstituted cyclobutenones, which are not readily
available from the traditional methods, in moderate to excellent
yields. The products have been successfully applied to the highly
stereoselective synthesis of poly substituted β,γ-unsaturated enones 5
in the presence of commercially available lithium reagents. Consider-
ing the easy availability and high functional group tolerance of
organozinc reagent19 and 2,3-allenoate,20 this method will be of high
interest in organic chemistry and related disciplines. Two interesting
issues here are the highly stereoselective formation of B-type inter-
mediate and the intramolecular 1,2-addition/elimination affording
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Journal of the American Chemical Society
COMMUNICATION
the highly strained four-membered enones. Further study concerning
on this issue is being conducted in our laboratory.
(7) Hoornaert, C.; Hesbain-Frisque, A. M.; Ghosez, L. Angew.
Chem., Int. Ed. 1975, 14, 569.
(8) For a profile on the reaction of 1,3-dienolates A, see: (a) Fuson,
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McGarvey, G. J.; Kimura, M. J. Org. Chem. 1985, 50, 4655. (b) Aidhen,
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’ ASSOCIATED CONTENT
S
Supporting Information. Spectroscopic data, general pro-
b
cedure, and the 1H/11C NMR spectra of all the products. This material
is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
masm@sioc.ac.cn
(10) For a Co2(CO)8-catalyzed transformation of ethyl 2,4,4-tri-
phenyl-2,3-butadienoate to 3-ethoxy-2,4,4-triphenylcyclobutenone in
12% yield with 20% of conversion, see: Trifonov, L. S.; Orahovats,
A. S.; Heimgartner, H. Helv. Chim. Acta 1987, 70, 1070.
’ ACKNOWLEDGMENT
(11) Crystal data for compound 2s: C18H16O, MW = 248.31,
monoclinic, space group P2(1)/n, final R indices [I > 2σ(I)], R1 =
0.0386, wR2 = 0.1060, R indices (all data) R1 = 0.0432, wR2 = 0.1129,
a = 9.0113(12) Å, b = 9.5129(12) Å, c = 16.236(2) Å, R = 90°, β =
96.700(4)o, γ = 90°, V = 1382.3(3) Å3, T = 296(2) K, Z = 4, reflections
collected/unique: 15462/2435 (Rint = 0.0222), number of observations
[>2σ(I)] 2163, parameters: 172. CCDC 800810.
(12) For our studies on the reaction of 2,3-allenoates with Grignard
reagents affording conjugate addition products, see: (a) Lu, Z.; Chai, G.;
Ma, S. J. Am. Chem. Soc. 2007, 129, 14546. (b) Lu, Z.; Chai, G.; Zhang,
X.; Ma, S. Org. Lett. 2008, 10, 3517. (c) Chai, G.; Lu, Z.; Fu, C.; Ma, S.
Adv. Synth. Catal. 2009, 351, 1946.
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(14) Chai, G.; Lu, Z.; Fu, C.; Ma, S. Chem.—Eur. J. 2009, 15, 11083.
(15) The β,γ-unsaturated enones are important and versatile inter-
mediates in organic synthesis and exist widely as functional groups in
many natural products, see: (a) Thomas, A. F.; Thommen, W.; Will-
halm, B.; Hagaman, E. W.; Wenkert, E. Helv. Chim. Acta 1974, 57, 2055.
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(18) Crystal data for compound trans-4e: C17H24O, MW = 244.36,
triclinic, space group P1, final R indices [I > 2σ(I)], R1 = 0.0852, wR2 =
0.2450, R indices (all data) R1 = 0.1279, wR2 = 0.2840, a = 12.2850(8)
Å, b = 16.5578(10) Å, c = 16.7837(8) Å, R = 74.767(5)o, β = 71.089(5)o,
γ = 70.386(6)o, V = 2996.6(3) Å3, T = 293(2) K, Z = 8, reflections
collected/unique: 23881/10528 (Rint = 0.0541), number of observa-
tions [>2σ(I)] 6270, parameters: 660. CCDC 800811.
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Financial support from the National Basic Research Program
of China (2009CB825300) and the National Natural Science
Foundation of China (20732005) is greatly appreciated. S.W. is
also supported by an undergraduate program issued by the
National Natural Science Foundation of China (J0830413). S.
M. is a Qiu Shi Adjunct Professor at Zhejiang University. We
thank Mr. Suhua Li in our group for reproducing the results
presented in entries 5 and 12 of Table 2, entry 2 of Table 4, and
entry 2 of Table 5, and Prof. J. Zhang for helpful discussion.
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