Three-component assembly and divergent ring-expansion cascades of functionalized 2-iminooxetanes

Article abstract of DOI:10.1002/anie.201004685

A bold diversification strategy: Aromatic alkynes, p-toluenesulfonyl azide, and aromatic 2-oxobut-3-ynoates underwent a copper(I)-catalyzed multicomponent reaction to provide functionalized 2-iminooxetanes 1, which could be converted selectively into five-membered nitrogen-containing heterocycles of two different sorts, depending on the reaction conditions (see scheme; Tf= trifluoromethanesulfonyl; R=Et, iPr). Copyright

Full text of DOI:10.1002/anie.201004685

Communications
DOI: 10.1002/anie.201004685
Heterocycle Synthesis
Three-Component Assembly and Divergent Ring-Expansion Cascades
of Functionalized 2-Iminooxetanes**
Weijun Yao, Lianjie Pan, Yiping Zhang, Gang Wang, Xiaoqin Wang, and Cheng Ma*
In memory of Yaozu Chen
Small-ring heterocycles are of prominent importance because
of their potential as bioactive compounds and synthetic
building blocks. However, considerably less is known about
2-iminooxetanes, not only with respect to their formation but
also in terms of their reactivity profiles. The synthesis of these
compounds has previously been limited to those bearing
electron-donating groups at the nitrogen atom.[1] Further-
more, we were unable to find a report of a ring-expansion
reaction of this type of heterocycle, although many ring-
enlargement processes of other small-ring systems have been
reported to give functionalized molecules efficiently and
expeditiously.[2] Intrigued by their potential synthetic appli-
Scheme 1. Assembly and divergent ring-expansion cascades of func-
tionalized N-sulfonyl-2-iminooxetanes 5. Ts=p-toluenesulfonyl.
cations, especially those based on a ring-opening process, we
therefore focused our efforts on the construction of electron-
deficient 2-iminooxetanes.[3,4] Herein, we present a novel
copper(I)-catalyzed three-component reaction (3-CR) to
Table 1: Effects of the ester moiety of
2
and the base.[a]
produce functionalized N-sulfonyl-2-iminooxetanes 5 by a
[2 + 2] cycloaddition of aromatic 2-oxobut-3-ynoates 2 with
N-sulfonylketenimines I generated in situ.[5,6] These 2-imi-
nooxetanes 5 are well-established substrates for selective
rearrangement to functionalized pyrrolidinones 3 or malei-
mides 4 through a divergent ring-expansion cascade reaction
(Scheme 1).
Initially, we found that, in the presence of triethylamine,
CuI catalyzed the multicomponent reaction of phenylacety-
lene (1a), p-toluenesulfonyl azide (TsN3), and ethyl 2-oxo-4-
phenylbut-3-ynoate (2a)[7] to give the 5-iminopyrrolidinone
3aa and maleimide 4aa in 34 and 11% yield, respectively
(Table 1, entry 1). However, when 2a was replaced with
methyl ester 2b, the same product 4aa was isolated as the
major product along with a small amount of 3ab (Table 1,
entry 2). Realizing that both the structure of the ester moiety
of 2 and the base used might be playing prominent roles in
these processes, we then investigated the reaction of a set of
esters 2a–d under different basic conditions in CH2Cl2 to
improve the reaction selectivity. Interestingly, the use of
K2CO3 instead of NEt3 almost completely suppressed the
Entry
Base/additive
1a [equiv]
2
Yield [%][b]
3
4aa
1
2
3
4
5
6
Et3N
Et3N
2.5
2.5
3.0
3.0
3.0
3.0
3.0
1.5
1.5
2a
2b
2a
2c
2c
2d
2a
2a
2c
34 (3aa)
7 (3ab)
11
33
K2CO3/Et4NI
K2CO3
K2CO3/Et4NI
K2CO3/Et4NI
Cs2CO3
55 (3aa)
58 (3ac)
73 (3ac)
52 (3ad)
<5 (3aa)
<2 (3aa)
<2 (3ac)
<5
<2
<2
<2
77
7[c]
8[c]
9[c]
Cs2CO3
Cs2CO3
75
61
[a] Reaction conditions: 1a/TsN3 1:1, 2 (0.3 mmol), CuI (10 mol%), base
(1.2 equiv), additive (10 mol%), CH2Cl2 (2 mL), reflux, N2. [b] Yield of the
isolated product. [c] After the consumption of 2, trifluoromethanesul-
fonic acid (TfOH; 3 equiv) was added.
[*] W. Yao, L. Pan, Y. Zhang, G. Wang, X. Wang, Prof. C. Ma
Department of Chemistry, Zhejiang University
20 Yugu Road, Hangzhou 310027 (P.R. China)
Fax: (+86)571-879-53-375
formation of 4aa to yield products 3 cleanly (Table 1,
entries 3–6). The addition of Et4NI (10 mol%) accelerated
the formation of 3 and gave the desired product in higher
yield (Table 1, entries 4 and 5). However, the isopropyl ester
2c proved to be the substrate of choice for this transformation
in terms of the yield of products. In sharp contrast, reactions
with cesium carbonate (Cs2CO3) as the base did not enable
access to 3 but furnished [2 + 2] cycloadducts 5, which were
E-mail: mcorg@zju.edu.cn
[**] We thank the National Natural Science Foundation of China
(Grant No. 20772106) and the Fundamental Research Funds
for the Central Universities (2010QNA3010).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201004685.
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Chemie
stable in the reaction mixture and underwent smooth
rearrangement to 4aa upon treatment with TfOH (Table 1,
entries 7–9). Although we expected the adduct 5 to be
converted slowly into 4aa on a silica-gel column at room
temperature, careful chromatography over SiO2 at À108C
enabled the isolation of 5ac in 65% yield (Scheme 2).
and 5). The alkenyl-substituted terminal alkyne 1g did not
undergo this reaction (Table 2, entry 7). Variation of the
aromatic moiety of ketoesters 2 was also possible: aryl
derivatives with different substitution patterns and a hetero-
cycle-substituted substrate underwent the cycloaddition/ring-
expansion cascade efficiently to give the corresponding
products (Table 2, entries 8–10). The structure and relative
configuration of compound 3aa was confirmed unambigu-
ously by single-crystal X-ray diffraction analysis, which
indicated that the two adjacent R1 groups were oriented syn
to one another.[10] In all cases, the 5-imino-2-pyrrolidinone
products 3, which feature two contiguous stereogenic centers
and a chiral allene unit, were obtained as a mixture of only
two diastereomers (d.r. 1.7–1.1:1), as determined by 1H NMR
spectroscopy.
Scheme 2. Formation of 2-iminooxetane 5ac.
We also examined variation of the aryl sulfonyl azide
component for this transformation (Scheme 3). None of the
1H NMR spectroscopy of 5ac revealed that the imine and
enamine tautomers were present in a 1:4 ratio in acetone.[8]
A
possible rationale for the different reaction outcome in the
presence of Cs2CO3 is that this base might enhance the
coordination ability of the enamine form of 5ac to give a
stable complex with cesium or copper ions and thus inhibit the
subsequent ring-opening cascade of 5ac.[9]
Next, we explored the scope of the unique reaction to
form 5-imino-2-pyrrolidinones (Table 2). A variety of aro-
matic alkynes
1 with both electron-donating (Table 2,
Scheme 3. Variation on the aryl sulfonyl azide component in the
synthesis of pyrrolidinones.
entries 2, 3, and 5) and electron-withdrawing (Table 2,
entries 4 and 6) substituents were suitable substrates for this
tandem process in the presence of K2CO3 as the base.
Electron-rich aryl alkynes displayed lower reactivity than
their electron-deficient counterparts; as a result, an extended
reaction time was required for the synthesis of products 3cc
and 3ec in 31 and 41% yield, respectively (Table 2, entries 3
desired product was formed when an azide with a very
strongly electron withdrawing substituent (nitro group) on
the aromatic ring was used. In contrast, 4-acetamidobenze-
nesulfonyl azide, which contains a moderately electron-
donating substituent, underwent the transformation smoothly
to give 7 with d.r. 1.38:1 in 61% yield.
Table 2: Formation of 5-imino-2-pyrrolidinones 3.[a]
Upon treatment with Cs2CO3 and CuI, a variety of
ketoesters 2 reacted readily with tosyl azide and phenyl-
acetylene (1a) to furnish the [2 + 2] cycloadducts 5, which
further rearranged to maleimides 4 in one pot in the presence
of TfOH (Table 3, entries 1–5). Owing to the instability of
intermediates and the product, TsOH (3.0 equiv) was used to
deliver 4ag in 55% yield (Table 3, entry 5). Variation of the
substituent on terminal alkynes 1 was also tolerated (Table 3,
entries 6–9). This protocol offers an alternative, conceptually
new three-component synthetic route to 3,4-disubstituted
maleimides: an important family of natural and synthetic
compounds with valuable pharmacological and photophysical
properties.[11]
We used 2-iminooxetane 5ac to explore the intermolec-
ular cyclization of the unique small-ring system with an aryl
ketene or another N-sulfonylketenimine (Scheme 4). Inter-
estingly, Et3N catalyzed a similar annulation of 5ac with
phenylmethylketene (8) to give a fully substituted succimi-
date derivative 9 with d.r. 1.5:1 in 67% yield in the absence of
CuI; thus, a copper salt was not necessary for the ring opening
and cyclization of 5ac. 2-Iminooxetane 5ac also reacted with
Entry
1, R1
2
t [h]
3
Yield [%][b] (d.r.)[c]
1
2
3
4
5
6
7
8
9
1a, Ph
2c
2c
2c
2c
2c
2c
2c
2e
2 f
2g
4
7
12
4
12
8
24
7
3ac
3bc
3cc
3dc
3ec
3 fc
–
3ae
3af
3ag
73 (1.5:1)
61 (1:1)
31 (1.22:1)
57 (1:1)
41 (2:1)
48 (1.18:1)
–
59 (1.22:1)
51 (1.22:1)
61 (1:1)
1b, 4-MeC6H4
1c, 4-MeOC6H4
1d, 4-ClC6H4
1e, 3-AcNHC6H4
1 f, 3-NO2C6H4
1g, 1-cyclohexenyl
1a, Ph
1a, Ph
1a, Ph
4.5
5.5
10
[a] Reaction conditions: 1 (3 equiv), TsN3 (3 equiv), K2CO3 (1.2 equiv), 2
(0.3 mmol), CuI (10 mol%), Et4NI (10 mol%), CH2Cl2 (2 mL), reflux, N2.
[b] Yield of the isolated product. [c] The diastereomeric ratio was
1
determined by H NMR spectroscopy.
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Table 3: Formation and acid-promoted ring expansion of 2-iminooxe-
tanes 5.[a]
Entry 1, R1
2, R2, R
t[b]
[h]
4
Yield[c]
[%]
1
2
3
1a, Ph
1a, Ph
1a, Ph
2a, Ph, Et
2c, Ph, iPr
2e, 4-FC6H4, iPr
17 4aa 75
12 4ac 61
12 4ae 72
Scheme 5. Gold-catalyzed rearrangement of allenoate 3ac.
4
1a, Ph
1a, Ph
1c, 4-MeOC6H4
1d, 4-ClC6H4
1 f, 3-NO2C6H4
2 f, 2-MeO2CC6H4, iPr 24 4af 41
5[d]
6
2g, 2-thienyl, iPr
2a, Ph, Et
21 4ag 55
13 4ca 35
20 4da 51
12 4 fa 52
12 4ga 57
12, containing furan and pyrrolidinone units, could be
assembled diastereoselectively from three simple acyclic
substrates in only two operations.
7
8
9
2a, Ph, Et
2a, Ph, Et
A postulated mechanism for the present reaction cascade
with Et3N as the base is depicted in Scheme 6. Initially, the
terminal alkyne 1 reacts with the sulfonyl azide upon treat-
ment with CuI and Et3N to give a ketenimine intermediate
I,[13] which undergoes a regioselective [2 + 2] cycloaddition
with a ketoester 2 to yield a 2-iminooxetane 5. Upon
deprotonation by Et3N,[14] 5 is converted into a ring-opened
intermediate II, which undergoes the subsequent cyclization
cascades by two pathways. By path a, a maleimide 4 is formed
through an intramolecular nucleophilic acylation along with
the elimination of an alcohol. This process is sensitive to the
ester moiety of II (Table 1, entries 1 and 2). On the other
hand, when the amidate ion II attacks another unit of the
ketenimine, a formal [3 + 2] cycloaddition furnishes an
enolate III in the trans configuration (path b). The diastereo-
selectivity observed is explained by the favorable p stacking
of an exo-like transition state in contrast to the steric
repulsion in an endo-like transition state. Finally, the enolate
III undergoes alkyne–allene isomerization and protonation to
give the product 3 (path b).[15]
1g, 1-cyclohexenyl 2a, Ph, Et
[a] Reaction conditions:
1
(1.5 equiv), TsN3 (1.5 equiv), Cs2CO3
(1.2 equiv), 2 (0.3 mmol), CuI (10 mol%), CH2Cl2 (2 mL), reflux, N2;
after the consumption of 2, TfOH (3.0 equiv) was added. [b] Reaction
time for the consumption of 2. [c] Yield of the isolated product. [d] TsOH
(3.0 equiv) was used instead of TfOH.
In conclusion, we have developed a novel copper(I)-
catalyzed multicomponent reaction of terminal alkynes,
sulfonyl azides, and aromatic 2-oxobut-3-ynoates to give
functionalized 2-iminooxetanes. Divergent skeleton rear-
rangements of the 2-iminooxetane intermediates could be
controlled well by choosing the appropriate reaction con-
ditions. Thus, functionalized pyrrolidinone and maleimide
derivatives with potential biological and synthetic utility
could be synthesized highly efficiently. Experiments designed
to explore the scope and asymmetric variants of this reaction
as well as other synthetic applications of the unique
2-iminooxetanes are ongoing.
Scheme 4. Cyclization of 2-iminooxetane 5ac.
N-sulfonylketenimines generated in situ from 1c or 1e and
p-toluenesulfonyl azide in the presence of CuI and K2CO3 to
give the target products 10 (d.r. 1.27:1) and 11 (d.r. 1.13:1) in
63 and 57% yield, respectively.
The functionalized pyrrolidinone structure 3 provided a
very useful handle for further structural manipulation. For
example, 3ac was readily transformed into the structurally
complex furan derivative 12 in the presence of methanol by
gold-catalyzed rearrangement of the allenoate moiety
(Scheme 5).[12] Moreover, furan 12 was obtained as a single
diastereomer from two mixtures of the two diastereomers of
3ac with different d.r. values in similar yields. This result
showed that the diastereoisomerism of 3ac was due to the
configuration of the allene group, and that the formation of
the two carbon stereogenic centers in 3ac was completely
diastereoselective. Notably, the heterocyclic architecture of
Experimental Section
3ac: Phenylacetylene (1a, 99 mL, 0.9 mmol) was added to a suspen-
sion of p-toluenesulfonyl azide (177.5 mg, 0.9 mmol), CuI (5.7 mg,
0.03 mmol), K2CO3 (49.7 mg, 0.36 mmol), Et4NI (7.7 mg, 0.03 mmol),
and isopropyl 2-oxo-4-phenylbut-3-ynoate (2c, 64.9 mg, 0.3 mmol) in
CH2Cl2 (2 mL) in a Schlenk tube under N2. The reaction mixture was
stirred at reflux for 4 h and then diluted with CH2Cl2 (20 mL). The
organic layer was washed with aqueous NH4Cl (5 mL) and brines
(5 mL) and then dried over anhydrous MgSO4. The solvent was
removed under vacuum, and the resulting oil was purified by column
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[2] a) B. M. Trost in Small Ring Compounds in
Organic Synthesis (Ed.: A. de Meijere), Springer,
Berlin, 1986, p. 3; b) H. N. C. Wong, K. L. Lau,
K. F. Tam in Small Ring Compounds in Organic
Synthesis (Ed.: A. de Meijere), Springer, Berlin,
1986, p. 83; c) A. Padwa, S. K. Bur, Tetrahedron
2007, 63, 5341; d) Y. Wang, R. L. Tennyson, D.
Romo, Heterocycles 2004, 64, 605; for recent
studies, see: e) Z. Zhang, Q. Zhang, S. Sun, T.
Xiong, Q. Liu, Angew. Chem. 2007, 119, 1756;
Angew. Chem. Int. Ed. 2007, 46, 1726; f) I. Braun,
F. Rudroff, M. D. Mihovilovic, T. Bach, Angew.
Chem. 2006, 118, 5667; Angew. Chem. Int. Ed.
2006, 45, 5541; g) B. M. Trost, J. Xie, N. Maulide, J.
Am. Chem. Soc. 2008, 130, 17258; h) J. Barluenga,
A. Gꢁmez, J. Santamarꢂa, M. Tomꢃs, Angew.
Chem. 2010, 122, 1328; Angew. Chem. Int. Ed.
2010, 49, 1306.
[3] For reviews of ring rearrangements, see: a) N.
Vivona, S. Buscemi, V. Frenna, G. Gusmano, Adv.
Heterocycl. Chem. 1993, 56, 49; b) G. Hajꢁs, Z.
Riedl, G. Kollenz, Eur. J. Org. Chem. 2001, 3405;
c) N. Isambert, R. Lavilla, Chem. Eur. J. 2008, 14,
8444.
Scheme 6. Plausible mechanism for the formation and divergent ring expansion of
functionalized 2-iminooxetanes with Et3N as the base.
[4] For our recent studies on ring transformations of
heterocycles, see: a) C. Ma, H. Ding, Y. Zhang, M.
Bian, Angew. Chem. 2006, 118, 7957; Angew.
Chem. Int. Ed. 2006, 45, 7793; b) H. Ding, Y. Zhang, M. Bian,
W. Yao, C. Ma, J. Org. Chem. 2008, 73, 578; c) H. Ding, Y. Zhang,
W. Yao, D. Xu, C. Ma, Chem. Commun. 2008, 5797; d) C. Ma, Y.
Yang, Org. Lett. 2005, 7, 1343.
chromatography (hexane/EtOAc 3:1) to give 3ac (d.r. 1.5:1, 166.2 mg,
73%) as a white solid (m.p. 190–1918C). The d.r. value of 3ac could
be raised to 23:1 through a single recrystallization from CH2Cl2/
hexane, as determined by 1H NMR spectroscopic analysis.
4aa: CuI (5.7 mg, 0.03 mmol) and Cs2CO3 (117.3 mg, 0.36 mmol)
[5] For elegant [2 + 2] annulations of imines and iminophosphor-
anes, see: a) M. Whiting, V. V. Fokin, Angew. Chem. 2006, 118,
3229; Angew. Chem. Int. Ed. 2006, 45, 3157; b) X. Xu, D. Cheng,
J. Li, H. Guo, J. Yan, Org. Lett. 2007, 9, 1585; c) Y. Shang, X. He,
J. Hu, J. Wu, M. Zhang, S. Yu, Q. Zhang, Adv. Synth. Catal. 2009,
351, 2709; d) S.-L. Cui, J. Wang, Y.-G. Wang, Org. Lett. 2008, 10,
1267.
[6] For reviews on copper(I)-catalyzed multicomponent reactions
that reply on the in situ generation of N-sulfonyl- or N-
phosphorylketenimines, see: a) E. J. Yoo, S. Chang, Curr. Org.
Chem. 2009, 13, 1766; b) P. Lu, Y. G. Wang, Synlett 2010, 165.
[7] For recent references, see: a) Y. Liu, M. Liu, S. Guo, H. Tu, Y.
Zhou, H. Gao, Org. Lett. 2006, 8, 3445; b) H. Li, Y.-Q. Wang, L.
Deng, Org. Lett. 2006, 8, 4063; c) C. Francois-Endelmond, T.
Carlin, P. Thuery, O. Loreau, F. Taran, Org. Lett. 2010, 12, 40;
d) Q. Kang, Z.-A. Zhao, S.-L. You, Org. Lett. 2008, 10, 2031.
[8] For a review of tautomerism, see: A. I. Kolꢄtsov, G. M. Kheifets,
Russ. Chem. Rev. 1972, 41, 452. According to 1H NMR spectro-
scopic analysis, the isolated compound 5ac contained 4aa as an
impurity (ca. 7%). Compound 5ac could be stored under N2 in a
refrigerator at À208C for three weeks without any detectable
change (see the Supporting Information for details).
were added to a solution of p-toluenesulfonyl azide (88.7 mg,
0.45 mmol), phenylacetylene (1a, 50 mL, 0.45 mmol), and ethyl
2-oxo-4-phenylbut-3-ynoate (2a, 60.7 mg, 0.3 mmol) in CH2Cl2
(2 mL) under N2. The mixture was stirred at reflux for 17 h and
then cooled to 0–58C. TfOH (79 mL, 0.9 mmol) was then added, and
the resulting mixture was stirred further at reflux for 4 h. The reaction
was then quenched with saturated aqueous NaHCO3 (5 mL), and the
reaction mixture was extracted with CH2Cl2 (3 ꢀ 10 mL). The
combined organic extracts were washed with brine (10 mL), dried
over anhydrous MgSO4, and then filtered. The filtrate was concen-
trated under vacuum, and the resulting oil was purified by column
chromatography (hexane/CH2Cl2 1:1) to afford 4aa (96.2 mg, 75%)
as a yellow solid (m.p. 198–1998C).
Received: July 29, 2010
Revised: September 16, 2010
Published online: October 20, 2010
Keywords: alkynes · azides · cycloaddition · heterocycles ·
.
ring expansion
[9] For a review of the synthetic application of cesium carbonate,
see: a) T. Flessner, S. Doye, J. Prakt. Chem. 1999, 341, 186; for a
review of the “cesium-ion effect” and macrocyclization, see:
b) C. Galli, Org. Prep. Proced. Int. 1992, 24, 285.
[1] For the formation of 2-iminooxetanes with alkyl groups at the
nitrogen atom, see: a) G. Barbaro, A. Battaglia, P. Giorgianni, J.
Org. Chem. 1988, 53, 5501; b) W. M. F. Fabian, R. Janoschek, J.
Am. Chem. Soc. 1997, 119, 4253; c) L. A. Singer, G. A. Davis,
R. L. Knutsen, J. Am. Chem. Soc. 1972, 94, 1188; d) F. M. Perna,
V. Capriati, S. Florio, R. Luisi, Tetrahedron Lett. 2003, 44, 3477;
e) R. Gandolfi, A. Gamba, M. Presutto, R. Oberti, N. Sardone,
Tetrahedron Lett. 1996, 37, 917; for the ring opening of 2-
iminooxetanes, see: f) G. Barbaro, A. Battaglia, P. Giorgianni, J.
Org. Chem. 1992, 57, 5128; g) G. Barbaro, A. Battaglia, P.
Giorgianni, J. Org. Chem. 1994, 59, 906; h) G. Barbaro, A.
Battaglia, P. Giorgianni, J. Org. Chem. 1995, 60, 1020.
[10] CCDC-779220(3aa),
CCDC-779221(4aa),
and
CCDC-
785681(12) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data request/cif.
[11] a) C. Sꢃnchez, C. Mꢅndez, J. A. Salas, Nat. Prod. Rep. 2006, 23,
1007; b) H.-J. Knꢆlker, K. R. Reddy, Chem. Rev. 2002, 102, 4303;
c) T.-S. Yeh, T. J. Chow, S.-H. Tsai, C.-W. Chiu, C.-X. Zhao,
Chem. Mater. 2006, 18, 832; d) B. K. Kaletas, V. N. Kozhevnikov,
M. Zimine, R. M. Williams, B. Koenig, L. De Cola, Eur. J. Org.
Angew. Chem. Int. Ed. 2010, 49, 9210 –9214
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
9213

Communications
Chem. 2005, 3443; e) L. Bouissane, J. P. Sestelo, L. A. Sarand-
S. Chang, J. Am. Chem. Soc. 2005, 127, 16046; d) M. P. Cassidy, J.
Raushel, V. V. Fokin, Angew. Chem. 2006, 118, 3226; Angew.
Chem. Int. Ed. 2006, 45, 3154; e) E. J. Yoo, I. Bae, S. H. Cho, H.
Han, S. Chang, Org. Lett. 2006, 8, 1347; f) E. J. Yoo, S. H. Park,
S. H. Lee, S. Chang, Org. Lett. 2009, 11, 1155.
eses, Org. Lett. 2009, 11, 1285; f) Y. Li, H. Zou, J. Gong, J. Xiang,
T. Luo, J. Quan, G. Wang, Z. Yang, Org. Lett. 2007, 9, 4057; g) D.
Basavaiah, B. Devendar, K. Aravindu, A. Veerendhar, Chem.
Eur. J. 2010, 16, 2031.
[12] For a review of metal-catalyzed allenoate cyclizations, see: a) S.
Ma, Acc. Chem. Res. 2003, 36, 701; for recent references, see:
b) L.-P. Liu, B. Xu, M. S. Mashuta, G. B. Hammond, J. Am.
Chem. Soc. 2008, 130, 17642; c) A. S. K. Hashmi, C. Lothschꢇtz,
R. Dꢆpp, M. Rudolph, T. D. Ramamurthi, F. Rominger, Angew.
Chem. 2009, 121, 8392; Angew. Chem. Int. Ed. 2009, 48, 8243;
d) Y. Shi, K. E. Roth, S. D. Ramgren, S. A. Blum, J. Am. Chem.
Soc. 2009, 131, 18022.
[14] For the deprotonation of acyclic sulfonylimidates and their use
as nucleophiles, see: a) R. Matsubara, F. Berthiol, S. Kobayashi,
J. Am. Chem. Soc. 2008, 130, 1804; b) S. Kobayashi, R.
Matsubara, Chem. Eur. J. 2009, 15, 10694; c) H. Van Nguyen,
R. Matsubara, S. Kobayashi, Angew. Chem. 2009, 121, 6041;
Angew. Chem. Int. Ed. 2009, 48, 5927.
[15] For 3-alkynoate-to-allenoate isomerizations, see: a) T. Yasokou-
chi, K. Kanematsu, Tetrahedron Lett. 1989, 30, 6559; b) A. K.
Basak, M. A. Tius, Org. Lett. 2008, 10, 4073; c) H. Liu, D. Leow,
K.-W. Huang, C.-H. Tan, J. Am. Chem. Soc. 2009, 131, 7212.
[13] a) E. J. Yoo, M. Ahlquist, I. Bae, K. B. Sharpless, V. V. Fokin, S.
Chang, J. Org. Chem. 2008, 73, 5520; b) I. Bae, H. Han, S. Chang,
J. Am. Chem. Soc. 2005, 127, 2038; c) S. H. Cho, E. J. Yoo, I. Bae,
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