Gold-catalyzed C(sp3)-H/C(sp)-H coupling/cyclization/oxidative alkynylation sequence: A powerful strategy for the synthesis of 3-alkynyl polysubstituted furans

Article abstract of DOI:10.1002/anie.201402475

In sharp contrast to the gold-catalyzed reactions of alkynes/allenes with nucleophiles, gold-catalyzed oxidative cross-couplings and especially C-H/C-H cross-coupling have been under represented. By taking advantage of the unique redox property and carbophilic π acidity of gold, this work realizes the first gold-catalyzed direct C(sp³)-H alkynylation of 1,3-dicarbonyl compounds with terminal alkynes under mild reaction conditions, with subsequent cyclization and in situ oxidative alkynylation. A variety of terminal alkynes including aryl, heteroaryl, alkenyl, alkynyl, alkyl, and cyclopropyl alkynes all successfully participate in the domino reaction. The protocol offers a simple and region-defined approach to 3-alkynyl polysubstituted furans. Pure and simple: The first gold-catalyzed C(sp³)-H/C(sp)-H cross-coupling/cyclization/oxidative alkynylation sequence of 1,3-dicarbonyl compounds reacting with terminal alkynes has been achieved under mild reaction conditions by taking advantage of the unique redox property and carbophilic π acidity of gold. Ultimately, 3-alkynyl polysubstituted furans are synthesized concisely and regiospecifically from simple starting materials.

Full text of DOI:10.1002/anie.201402475

.
Angewandte
Communications
DOI: 10.1002/anie.201402475
Homogeneous Catalysis
3
À
À
Gold-Catalyzed C(sp ) H/C(sp) H Coupling/Cyclization/Oxidative
Alkynylation Sequence: A Powerful Strategy for the Synthesis of
3-Alkynyl Polysubstituted Furans**
Yuanhong Ma, Shuai Zhang, Shiping Yang, Feijie Song,* and Jingsong You*
Abstract: In sharp contrast to the gold-catalyzed reactions of
alkynes/allenes with nucleophiles, gold-catalyzed oxidative
tionalization,[8] we were interested in the gold-catalyzed direct
3
[9]
À
C(sp ) H alkynylation. In combination with the unique
ability of gold to act as a mild carbophilic p acid, we
envisioned that the resulting acetylene-containing molecules
could encounter a one-pot intramolecular cyclization reaction
which enables rapid access to intricate cyclic scaffolds from
À
À
cross-couplings and especially C H/C H cross-coupling
have been under represented. By taking advantage of the
unique redox property and carbophilic p acidity of gold, this
3
À
work realizes the first gold-catalyzed direct C(sp ) H alkyny-
lation of 1,3-dicarbonyl compounds with terminal alkynes
under mild reaction conditions, with subsequent cyclization
and in situ oxidative alkynylation. Avariety of terminal alkynes
including aryl, heteroaryl, alkenyl, alkynyl, alkyl, and cyclo-
propyl alkynes all successfully participate in the domino
reaction. The protocol offers a simple and region-defined
approach to 3-alkynyl polysubstituted furans.
basic chemicals. As a proof of concept, we herein report the
3
À
À
first gold-catalyzed C(sp ) H/C(sp) H cross-coupling/cycli-
zation/oxidative alkynylation sequence of 1,3-dicarbonyl
compounds with terminal alkynes to afford 3-alkynyl poly-
substituted furans under mild reaction conditions (Scheme 1).
T
he homogeneous gold-catalyzed reactions of alkynes/
allenes with nucleophiles have been recognized as one of
the most powerful and useful tools in organic synthesis.[1] The
oxidation state of gold rarely changes in these transforma-
tions. In contrast, the gold-catalyzed oxidative cross-coupling
reactions remain a great challenge because of the relatively
high redox potential of the AuI/AuIII couple (E0 =+ 1.41 V).[2]
Recent research has demonstrated that the use of gold
catalysts goes beyond the simple substitution of the conven-
tional palladium, rhodium, and ruthenium catalysts in oxida-
tive cross-coupling.[3] The representative work of Ball, Lloyd-
Jones, and Russell,[4] and de Haro and Nevado[5] have clearly
3
À
À
Scheme 1. The gold-catalyzed C(sp ) H/C(sp) H coupling/cyclization/
oxidative alkynylation sequence.
Polysubstituted furans are widely distributed in naturally
occurring products, bioactive molecules, and pharmaceuticals,
and are also important building blocks in organic synthesis.[10]
Consequently, a large number of methods have been estab-
lished for the synthesis of such molecules,[11] and they mainly
rely on the intramolecular cyclization of complex acyclic
precursors which may suffer from tedious multistep synthesis
and purification. Undoubtedly, new protocols that can
assemble the furan ring from simple chemicals are highly
desirable. Moreover, the installation of additional functional
groups during the formation of the furan ring is more
attractive from the viewpoint of synthetic efficiency. 3-
Alkynyl furans are among the most important functionalized
furans because the acetylene groups allow a broad range of
transformations including oxidation, reduction, addition,
cyclization, metathesis, etc. However, owing to the inherent
low reactivity of the furan C3-position, the general and
efficient synthesis of 3-alkynyl polysubstituted furans has
been a challenge.[12] The methodology described herein
represents a simple and region-defined approach to 3-alkynyl
polysubstituted furans.
À
shown the great potential of gold catalysis in oxidative C H
À
activation/C C cross-coupling. Despite significant progress,
À
À
the promising oxidative C H/C H cross-coupling reactions
which obviate the prefunctionalization of both coupling
partners have been largely under developed. To the best of
2
À
our knowledge, only the gold-catalyzed oxidative C(sp ) H/
2
[6]
3
3
[7]
À
À
À
C(sp ) H homocoupling, C(sp ) H/C(sp ) H cross-cou-
2
[5]
À
À
pling, and C(sp ) H/C(sp) H cross-coupling have been
realized, and the C(sp ) H/C(sp) H cross-coupling still
remains unexplored. Following our work in C(sp ) H func-
3
À
À
3
À
[*] Y. Ma, S. Zhang, S. Yang, Dr. F. Song, Prof. Dr. J. You
Key Laboratory of Green Chemistry and Technology of Ministry of
Education, College of Chemistry, and State Key Laboratory of
Biotherapy, West China Medical School, Sichuan University
29 Wangjiang Road, Chengdu 610064 (PR China)
E-mail: fsong@scu.edu.cn
jsyou@scu.edu.cn
[**] This work was supported by grants from the National Basic
Research Program of China (973 Program, 2011CB808601), and the
National NSF of China (Nos 21202105, 21025205, 21321061,
J1310008, and J1103315J0104).
To realize the cascade reaction, several obstacles have to
be overcome, including: 1) the identification of a robust
catalyst system that allows the smooth occurrence of all
distinct reactions and avoids mutual interference between
different types of reactions, 2) the realization of the unpre-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201402475.
7870
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 7870 –7874

Angewandte
Chemie
Table 1: Optimization of the reaction conditions.[a]
Table 2: The scope of 1,3-dicarbonyl compounds.[a]
Entry
Catalyst
Ligand
Yield [%][b]
1
2
3
4
4 mol% Ph3PAuCl
4 mol% Ph3PAuCl/AgOTf
4 mol% AuCl3
none
none
none
none
trace
0
trace
17
4 mol% PicAuCl2
5
6
7
8
4 mol% [AuCl2(bipy)]Cl
4 mol% HAuCl4·xH2O
4 mol% HAuCl4·xH2O
4 mol% HAuCl4·xH2O
4 mol% AuCl3
none
44
33
25
22
21
16
70
66
10 mol% bipy
10 mol% L1
10 mol% L2
10 mol% bipy
10 mol% bipy
30 mol% bipy
30 mol% bipy
30 mol% bipy
9
10
11
12
13
4 mol% AuBr3
4 mol% HAuCl4·xH2O
3 mol% HAuCl4·xH2O
2 mol% HAuCl4·xH2O
[a] Reaction conditions: 1 (2.0 equiv), 2a (0.5 mmol), HAuCl4·xH2O
(3 mol%), bipy (30 mol%), KOAc (2.0 equiv), PhI(OAc)2 (2.0 equiv),
and toluene (5.0 mL) under N2 at 408C for 24 h. [b] Yield of the isolated
product. [c] At 808C.
37
[a] Reaction conditions: ethyl acetoacetate (1a; 1.0 mmol), phenylace-
tylene (2a; 0.5 mmol), gold catalyst, ligand, KOAc (2.0 equiv), and
PhI(OAc)2 (2.0 equiv) in toluene (5.0 mL) under N2 at 408C for 24 h.
[b] Yield of isolated product.
the corresponding a-alkyl-substituted substrates. In these
cases, better yields were generally obtained when the
reactions were performed at an elevated temperature of
808C (Table 2, entries 7–10). b-Ketoamides and 1,3-diketones
were also suitable substrates and showed reactivities com-
parable to that of b-ketoesters (Table 2, entries 11–14). The
structure of the product 3l was confirmed by single-crystal X-
ray analysis.[13] Remarkably, when the unsymmetrical 1,3-
diketone 1n was subjected to the standard reaction condi-
tions, the cyclization of the carbonyl group adjacent to the
methyl moiety occurred selectively to afford the 2-methyl-
substituted furan 3n (Table 2, entry 14). It should be empha-
sized that the yields obtained in these reactions were pretty
good considering that three different types of reactions
occurred successively in one pot.
Next, the scope of terminal alkynes was evaluated
(Table 3). It was found that the current protocol tolerated
a wide range of functional groups (e.g., alkyl, methoxy,
methoxymethoxy (MOMO), chloro, iodo, cyano, ester, and
aldehyde) on the phenyl ring of aryl alkynes (4a–i), and offer
the opportunity for further organic transformations. The
compatibility of the iodo group is especially attractive
considering its high reactivity towards Sonogashira coupling
(4e). Besides aryl alkynes, other alkynes including alkyl,
alkenyl, heteroaryl, cyclopropyl, and bulky naphthalenyl
alkynes also underwent the reaction to deliver the desired
furans in moderate to good yields (4j, 4k, and 4m–p).
However, when the terminal 1,3-diyne 2m [1-(buta-1,3-
diynyl)benzene] was employed, the product 4l was obtained
in a low yield of 27%, probably because of the low stability of
2m. These 3-alkynyl furan-based p-conjugated materials (4)
and their derivatives such as the poly(hetero)aryl compound 8
(discussed below) could be expected to exhibit interesting
3
À
cedented gold-catalyzed oxidative C(sp ) H alkynylation
with terminal alkynes, 3) the suppression of the homocou-
pling reaction of terminal alkynes, and 4) the inhibition of the
protodeauration of the gold furyl intermediates. Thus, we first
devoted much effort to optimizing the reaction conditions of
ethyl acetoacetate (1a) and phenylacetylene (2a; Table 1). It
was found that Ph3PAuCl, Ph3PAuCl/AgOTf, AuCl3, and
PicAuCl2 all gave poor results (Table 1, entries 1–4). To our
delight, the desired furan 3a was obtained in 44% yield when
[AuCl2(bipy)]Cl was used as the catalyst (Table 1, entry 5).
This promising result encouraged us to explore the catalysts
generated in situ from commercially available gold(III)
sources (e.g., AuCl3, AuBr3, and HAuCl4·xH2O) and ligands
(e.g. bipy, L1, and L2). After screening various parameters
(see Table S1 in the Supporting Information), 3 mol% of
HAuCl4·xH2O as the catalyst, 30 mol% of bipy as the ligand,
2.0 equivalents of PhI(OAc)2 as the oxidant, and 2.0 equiv-
alents of KOAc as the base in toluene at 408C were chosen as
the optimal reaction conditions (Table 1, entry 12). Under
these reaction conditions, 3a was obtained in 66% yield, and
the protodeaurated product of the gold 3-furyl intermediate
was not observed in the reaction system.
With the optimal reaction conditions in hand, we started
to explore the scope of the 1,3-dicarbonyl compounds
(Table 2). It was found that ethyl, methyl, tert-butyl, benzyl,
and allyl acetoacetates (1a–e) all reacted with 2a to afford the
furans 3a–e in 55–66% yields (Table 2, entries 1–5). The
sterically more hindered 1 f also smoothly underwent the
domino reaction (Table 2, entry 6). However, the reaction of
the a-aryl-substituted 1,3-dicarbonyl compounds 1g–j fur-
nished the desired products 3g–j in slightly lower yields than
Angew. Chem. Int. Ed. 2014, 53, 7870 –7874
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7871

.
Angewandte
Communications
Table 3: The scope of terminal alkynes.[a,b]
3
À
strated that the domino reaction first underwent the C(sp )
À
H/C(sp) H cross-coupling, although many attempts to isolate
the coupled 2-alkynyl-1,3-dicarbonyl compound failed.
It is well-documented that gold intermediates are easily
quenched by protonation.[1] However, the exposure of the C3-
unsubstituted furan 11 and 2a to the standard reaction
conditions failed to deliver any 3a [Eq. (2)], thus suggesting
that the protodeauration of the gold 3-furyl complex could
not be involved in this cascade process. Therefore, the alkynyl
moiety might be integrated into the furan ring through the in
situ oxidative alkynylation of the gold 3-furyl intermedi-
ate.[12a,17]
To get some insight into the mechanism, the stoichiomet-
ric reactions of the gold(I)-acetylide 12 with 1a and 9 were
investigated individually (Scheme 2). It was found that the
[a] Reaction conditions: 1 (2.0 equiv), 2 (0.5 mmol), HAuCl4·xH2O
(3 mol%), bipy (30 mol%), KOAc (2.0 equiv), PhI(OAc)2 (2.0 equiv) and
toluene (5.0 mL) under N2 at 408C for 24 h. [b] Yield of the isolated
product. [c] At 808C.
photophysical properties and find potential applications in
materials science.
The synthetic usefulness of this protocol was subsequently
illustrated (see the Supporting Information). The 3-alkynyl
furan 3a could be readily hydrogenated to afford the
corresponding 3-alkyl furan 5 (ethyl 2-methyl-4-phenethyl-
5-phenylfuran-3-carboxylate) [Eq. (S1)], and especially the
(Z)-3-alkenyl furan 6 ((Z)-ethyl 2-methyl-5-phenyl-4-styryl-
furan-3-carboxylate) which was not easy to access by tradi-
tional methods such as Heck coupling or Wittig reaction
[Eq. (S2)]. In addition, the poly(hetero)aryl compound 8
Scheme 2. Stoichiometric reactions of gold(I) acetylide.
reaction of 1a with 12 could not deliver any products in the
absence of PhI(OAc)2. However, 3a was obtained in 46%
yield with the addition of 2.0 equivalents of PhI(OAc)2
(Scheme 2a). Treatment of 9 with 12 in the absence or
presence of PhI(OAc)2 afforded the cyclization product 13
and cyclization/alkynylation product 10 in 62 and 30% yields,
respectively (Scheme 2b). Based on these results, three
(ethyl
4-(benzofuran-2-yl)-5-(2-hydroxyphenyl)-2-methyl-
furan-3-carboxylate) was easily synthesized from 4i through
the removal of the MOM group and subsequent palladium-
catalyzed cyclization of the resulting 7 (ethyl 5-(2-hydroxy-
phenyl)-4-((2-hydroxyphenyl)ethynyl)-2-methylfuran-3-car-
boxylate) [Eq. (S3)].[14,15]
We then moved on to study the mechanism of this cascade
reaction. It was found that the radical scavenger 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) had a negligible
effect on the reaction of 1a with 2a, thus ruling out the
possibility of a radical pathway (see Table S2 in the Support-
ing Information).[16] The reaction of diphenylacetylene with
1a did not occur under the standard reaction conditions, thus
indicating the critical role of the acetylenic proton [see
Eq. (S4) in the Supporting Information]. Moreover, the
reaction of b-alkynyl ketone 9 with 2a afforded the desired
furan 10 in 55% yield [Eq. (1)]. These observations demon-
scenarios can be envisaged: 1) The gold(III) alkynyl inter-
3
À
mediate may serve as a reactive species; 2) both the C(sp )
À
H/C(sp) H coupling and the in situ oxidative alkynylation
involve AuI/AuIII redox cycle; and 3) the transmetallation
between the gold acetylide and gold furan complex is possible
since there is no free acetylene in this reaction mixture.
On the basis of these observations, a tentative mechanism
was proposed (Scheme 3).[5,17] The domino reaction starts
with the reaction of gold(III) precatalyst and the terminal
alkyne 2 to generate the gold(III) acetylide A. The reaction of
A with 1 and subsequent reductive elimination delivers the 2-
7872
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 7870 –7874

Angewandte
Chemie
111, 1657 – 1712; d) N. Krause, C. Winter, Chem. Rev. 2011, 111,
1994 – 2009.
[2] For selected reviews, see: a) T. C. Boorman, I. Larrosa, Chem.
Soc. Rev. 2011, 40, 1910 – 1925; b) H. A. Wegner, M. Auzias,
Angew. Chem. 2011, 123, 8386 – 8397; Angew. Chem. Int. Ed.
2011, 50, 8236 – 8247; c) M. N. Hopkinson, A. D. Gee, V.
Gouverneur, Chem. Eur. J. 2011, 17, 8248 – 8262; and references
therein.
[3] For selected reviews, see: a) X. Chen, K. M. Engle, D.-H. Wang,
J.-Q. Yu, Angew. Chem. 2009, 121, 5196 – 5217; Angew. Chem.
Int. Ed. 2009, 48, 5094 – 5115; b) D. A. Colby, R. G. Bergman,
J. A. Ellman, Chem. Rev. 2010, 110, 624 – 655; c) C. Liu, H.
Zhang, W. Shi, A. Lei, Chem. Rev. 2011, 111, 1780 – 1824;
d) S. H. Cho, J. Y. Kim, J. Kwak, S. Chang, Chem. Soc. Rev. 2011,
40, 5068 – 5083; e) J. Wencel-Delord, T. Drçge, F. Liu, F. Glorius,
Chem. Soc. Rev. 2011, 40, 4740 – 4761; f) B.-J. Li, Z.-J. Shi, Chem.
Soc. Rev. 2012, 41, 5588 – 5598; g) G. Song, F. Wang, X. Li, Chem.
Soc. Rev. 2012, 41, 3651 – 3678; h) P. B. Arockiam, C. Bruneau,
P. H. Dixneuf, Chem. Rev. 2012, 112, 5879 – 5918.
[4] a) L. T. Ball, G. C. Lloyd-Jones, C. A. Russell, Science 2012, 337,
1644 – 1648; b) L. T. Ball, G. C. Lloyd-Jones, C. A. Russell, J.
Am. Chem. Soc. 2014, 136, 254 – 264.
[5] T. de Haro, C. Nevado, J. Am. Chem. Soc. 2010, 132, 1512 – 1513.
[6] a) A. Kar, N. Mangu, H. M. Kaiser, M. Beller, M. K. Tse, Chem.
Commun. 2008, 386 – 388; b) A. Kar, N. Mangu, H. M. Kaiser,
M. K. Tse, J. Organomet. Chem. 2009, 694, 524 – 537.
Scheme 3. Proposed mechanism.
alkynyl-1,3-dicarbonyl C and a gold(I) species. The alkynyl
moiety of C then coordinates with gold(I) to induce the
intramolecular nucleophilic attack of oxygen atom onto
alkyne to afford the gold(I) complex E. Transmetallation
from E to A leads to the formation of F. The gold(III)-
catalyzed cyclization of C and subsequent transmetallation
between A and the resulting gold(III) intermediate is also
a possible pathway to F. Upon reductive elimination of F, the
desired product 3 or 4 is formed. The reaction of the released
gold(I) complex with terminal alkyne 2 and subsequent
oxidation by PhI(OAc)2 lead to the regeneration of A.
In summary, by taking advantage of the unique redox
[7] J. Xie, H. Li, J. Zhou, Y. Cheng, C. Zhu, Angew. Chem. 2012, 124,
1278 – 1281; Angew. Chem. Int. Ed. 2012, 51, 1252 – 1255.
[8] a) G. Li, S. Qian, C. Wang, J. You, Angew. Chem. 2013, 125,
7991 – 7994; Angew. Chem. Int. Ed. 2013, 52, 7837 – 7840; b) K.
Li, G. Tan, J. Huang, F. Song, J. You, Angew. Chem. 2013, 125,
13180 – 13183; Angew. Chem. Int. Ed. 2013, 52, 12942 – 12945.
property and carbophilic p acidity of gold, we have achieved
for the first time the gold-catalyzed oxidative C(sp ) H/
3
À
[9] For other metal-catalyzed or metal-mediated oxidative C(sp )
3
À
H alkynylation with terminal alkynes, see: a) Z. Li, C.-J. Li, J.
Am. Chem. Soc. 2004, 126, 11810 – 11811; b) C. He, S. Guo, J. Ke,
J. Hao, H. Xu, H. Chen, A. Lei, J. Am. Chem. Soc. 2012, 134,
5766 – 5769.
À
C(sp) H cross-coupling of 1,3-dicarbonyl compounds with
terminal alkynes, a step which is followed by cyclization and
in situ oxidative alkynylation in a cascade process. The
protocol offers an unusually simple approach to 3-alkynyl
polysubstituted furans, and features mild reaction conditions,
readily available starting materials, complete regiocontrol,
wide substrate scope, and high functional-group tolerance. It
[10] a) H. N. C. Wong, X.-L. Hou, K.-S. Yeung, H. Huang in Five-
membered heterocycles: furan, Vol. 1 (Eds.: J. Alvarez-Builla,
J. J. Vaquero, J. Barluenga), Wiley-VCH, Weinheim, 2011,
pp. 533 – 592; b) K.-S. Yeung, Z. Yang, X.-S. Peng, X.-L. Hou,
Prog. Heterocycl. Chem. 2011, 22, 181 – 216; c) C. F. Ingham,
R. A. Massy-Westropp, Aust. J. Chem. 1974, 27, 1491 – 1503;
d) S. Breyer, K. Effenberger-Neidnicht, S. Knauer, R. Schobert,
Bioorg. Med. Chem. 2011, 19, 1264 – 1267; e) D. A. Mulholland,
A. Langlois, M. Randrianarivelojosia, E. Derat, J.-M. Nuzillard,
Phytochem. Anal. 2006, 17, 87 – 90.
[11] For selected reviews, see: a) R. C. D. Brown, Angew. Chem.
2005, 117, 872 – 874; Angew. Chem. Int. Ed. 2005, 44, 850 – 852;
b) S. F. Kirsch, Org. Biomol. Chem. 2006, 4, 2076 – 2080; c) A. V.
Gulevich, A. S. Dudnik, N. Chernyak, V. Gevorgyan, Chem. Rev.
2013, 113, 3084 – 3213; For selected recent elegant examples, see:
d) S. Kramer, T. Skrydstrup, Angew. Chem. 2012, 124, 4759 –
4762; Angew. Chem. Int. Ed. 2012, 51, 4681 – 4684; e) X. Huang,
B. Peng, M. Luparia, L. F. R. Gomes, L. F. Veiros, N. Maulide,
Angew. Chem. 2012, 124, 9016 – 9020; Angew. Chem. Int. Ed.
2012, 51, 8886 – 8890. Also see Ref. [10b].
is worth to noting that the previously reported gold-catalyzed
2
À
À
C H alkynylation reactions are only applicable to C(sp ) H
bonds and employ either electron-deficient terminal
alkynes[5] or alkynyl hypervalent iodine reagents[12a,18] as the
alkynyl source. In the current protocol, a variety of terminal
alkynes including aryl, heteroaryl, alkenyl, alkynyl, alkyl, and
cyclopropyl alkynes all successfully participate in the domino
reaction. This stragegy would broaden the chemistry of gold
and hold great potential in the concise synthesis of complex
molecules.
Received: February 15, 2014
Published online: April 11, 2014
Keywords: cross-coupling · gold · homogeneous catalysis ·
[12] For an elegant recent example, see: a) Y. Li, J. P. Brand, J. Waser,
Angew. Chem. 2013, 125, 6875 – 6879; Angew. Chem. Int. Ed.
2013, 52, 6743 – 6747; for some sporadic examples, see: b) M.
Rawat, V. Prutyanov, W. D. Wulff, J. Am. Chem. Soc. 2006, 128,
11044 – 11053; c) H. Cao, H. Jiang, R. Mai, S. Zhu, C. Qi, Adv.
Synth. Catal. 2010, 352, 143 – 152; d) H. Jiang, W. Yao, H. Cao, H.
Huang, D. Cao, J. Org. Chem. 2010, 75, 5347 – 5350.
.
oxygen heterocycles · synthetic methods
[1] For selected reviews, see: a) A. Fꢀrstner, P. W. Davies, Angew.
Chem. 2007, 119, 3478 – 3519; Angew. Chem. Int. Ed. 2007, 46,
3410 – 3449; b) A. Arcadi, Chem. Rev. 2008, 108, 3266 – 3325;
c) A. Corma, A. Leyva-Pꢁrez, M. J. Sabater, Chem. Rev. 2011,
[13] CCDC 961348 (3l) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
Angew. Chem. Int. Ed. 2014, 53, 7870 –7874
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7873

.
Angewandte
Communications
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[17] M. N. Hopkinson, J. E. Ross, G. T. Giuffredi, A. D. Gee, V.
Gouverneur, Org. Lett. 2010, 12, 4904 – 4907.
[14] J. Liu, Y. Liu, Org. Lett. 2012, 14, 4742 – 4745.
[15] Y. Liang, S. Tang, X.-D. Zhang, L.-Q. Mao, Y.-X. Xie, J.-H. Li,
Org. Lett. 2006, 8, 3017 – 3020.
[16] a) J. Iqbal, B. Bhatia, N. K. Nayyar, Chem. Rev. 1994, 94, 519 –
564; b) R. Yan, J. Huang, J. Luo, P. Wen, G. Huang, Y. Liang,
Synlett 2010, 1071 – 1074.
[18] a) J. P. Brand, J. Charpentier, J. Waser, Angew. Chem. 2009, 121,
9510 – 9513; Angew. Chem. Int. Ed. 2009, 48, 9346 – 9349; b) J. P.
Brand, J. Waser, Angew. Chem. 2010, 122, 7462 – 7465; Angew.
Chem. Int. Ed. 2010, 49, 7304 – 7307; c) J. P. Brand, J. Waser, Org.
Lett. 2012, 14, 744 – 747; d) J. P. Brand, C. Chevalley, R.
Scopelliti, J. Waser, Chem. Eur. J. 2012, 18, 5655 – 5666.
7874
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 7870 –7874