Gold-catalyzed synthesis of furans and furanones from sulfur ylides

Article abstract of DOI:10.1002/anie.201203637

A golden switch: Doubly stabilized sulfonium ylides can be coupled with alkynes in a gold-catalyzed synthesis of heterocycles. This method hinges on a switch in the reactivity of the sulfur ylide resulting from the simple modification of the electron-withdrawing moieties and leads to either furans or furanones bearing a quaternary center (see scheme). Copyright

Full text of DOI:10.1002/anie.201203637

Angewandte
Chemie
DOI: 10.1002/anie.201203637
Gold Catalysis
Gold-Catalyzed Synthesis of Furans and Furanones from Sulfur
Ylides**
Xueliang Huang, Bo Peng, Marco Luparia, Luis F. R. Gomes, Luꢀs F. Veiros, and
Nuno Maulide*
Polysubstituted furan derivatives are fundamental building
blocks in organic synthesis. The plethora of natural products,
agrochemicals, and pharmaceuticals that contain a di-, tri-, or
tetrasubstituted furan moiety attests to this fact (Scheme 1).[1]
A cursory examination of the compounds depicted in
ylides and alkynes.[4–6] We further disclose an intriguing
reactivity switch that allows the synthesis of furanones
containing a quaternary center as well as computational
data on the mechanism of these transformations.[7]
Initial studies focused on the alkynyl sulfonium ylide 1a,
readily available by direct ylide transfer[8] to the correspond-
ing ketoester. Exposure of this compound to diverse gold(I)
promoters led to sharply contrasting results (see the Support-
ing Information for details), and it was found that the simple
combination of commercially available PPh3AuCl and
AgSbF6 led to quantitative conversion into the furofuranone
2a at room temperature within 3 h.[9]
We then examined the scope of this simple yet highly
effective procedure for the intramolecular preparation of
bicyclic furans. Importantly, all the alkynyl sulfonium ylides
employed as substrates could be readily accessed by direct
ylide transfer in very high yields according to our previously
reported procedure.[8] Their stability towards chromatogra-
phy and recrystallization along with their crystallinity renders
them easily handled substrates for subsequent transforma-
tions.
Scheme 1. Selected examples of natural products containing a furan
core.[1]
As shown in Table 1, a variety of bicyclofurans could be
readily prepared by simply stirring the ylide precursors in the
presence of the gold catalyst at room temperature. Impor-
tantly, the preparation of the furopyranone 2i (Table 1,
entry 9) required slightly modified conditions and the use of
a different, more electron-poor phosphine, once again
suggesting that less facile cyclizations of sulfonium ylides
onto alkynes may be favored by the use of more electron-
deficient gold(I) species. Various alkyl and aryl moieties were
tolerated by the procedure, and furopyrrolones such as 2k
could also be prepared by employing an amide-tethered
alkyne. All-carbon tethers are also suitable for this trans-
formation (Table 1, entry 12). To the best of our knowledge,
this constitutes the first intramolecular synthesis of furans
from stabilized sulfonium ylides.[9a]
Scheme 1 reveals a notable incidence of 3-carboxy-function-
alized polysubstituted furans in the natural product cores,
including furofuranone and -pyranone moieties (such as in
angelone).
Herein we report a simple and flexible gold-catalyzed[2,3]
synthesis of densely functionalized 3-carboxyfuran deriva-
tives that hinges on a key cross-coupling between sulfonium
[*] Dr. X. Huang, Dr. B. Peng,[+] Dr. M. Luparia,[+] L. F. R. Gomes,[+]
Dr. N. Maulide
Max-Planck-Institut fꢀr Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mꢀlheim an der Ruhr (Germany)
E-mail: maulide@mpi-muelheim.mpg.de
Homepage: http://www.kofo.mpg.de/maulide
We then turned our interest to an intermolecular version
of the same reaction.[10] The double stabilization of our
sulfonium ylides made some optimization of this process
necessary,[11] and some key results obtained are compiled in
Scheme 2.
The use of tBuXPhos as a ligand was required to obtain
synthetically useful yields of product 5a, particularly with
regard to preventing excessive polymerization of phenyl-
acetylene (4a; Scheme 2a). The very high regioselectivity of
this reaction is noteworthy, as only trace amounts of other
regioisomers could be detected. Additionally, and in comple-
mentary fashion to the recent elegant report by Skyrdstrup
et al. employing singly stabilized sulfonium ylides,[4g] we
Prof. Dr. L. F. Veiros
Centro de Quꢁmica Estrutural, Complexo I
Instituto Superior Tꢂcnico, Universidade Tꢂcnica de Lisboa
Av. Rovisco Pais 1, 1049-001 Lisbon (Portugal)
[+] These authors contributed equally to this work.
[**] This work was funded by the Max-Planck Society, the DFG (grant MA
4861/4-1), the DAAD (project ID 50750793), the Alexander von
Humboldt Foundation (fellowship to M.L.), and the Fundażo para
a CiÞncia e Tecnologia (fellowship SFRH/BD/61220/2009 to L.G.,
PEst-OE/QUI/UI0100/2011 and PTDC/QUI-QUI/099389/2008 to
L.V.).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201203637.
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Table 1: Scope of the intramolecular synthesis of bicyclofurans.
Entry Ylide
t
Product
Yield [%][a]
[h]
1
2
3
4
5
3
3
3
3
4
99
90
87
63
60
Scheme 2. Intermolecular synthesis of furans.
(Scheme 2c). Again, this observation seems to nicely comple-
ment the recent report on an efficient stoichiometric silver-
mediated furan synthesis from active methylene compounds,
for which malonate esters were not competent substrates.[9b]
The ability to access 2-methoxyfuran products is noteworthy
owing to their synthetic versatility.[12]
During our screening of diversely decorated ylide sub-
strates, we hypothesized that modification of one or both
alkoxycarbonyl moieties might unlock new reactivity. In the
event, the reaction of the readily available allyloxyester 3c
with phenylacetylene, promoted by Echavarrenꢀs catalyst 7,[13]
afforded the C-allylated 2-furanone 8a, which contains
a quaternary center, along with the expected furan product.[14]
As polysubstituted g-lactones are common structural motifs
in bioactive natural compounds,[15,16] we decided to inves-
tigate this transformation further.[17]
6[b]
7[b]
8[b]
2 f, R=p-Me, 66
2g, R=p-Cl, 49
2h,
7
R=m-(2-Naph), 46
9[c]
10
2i, n=2, 80
2j, n=3, 0
24
We thus prepared the diallylmalonate-derived ylide 3d
and briefly studied its reaction with phenylacetylene. Under
conditions similar to those described above, the reaction
proceeded to completion within 2 h, giving lactone 8b in high
yield (Table 2, entry 2). Other ligands led to slower reactions
with marginally lower yields (Table 2, entries 1 and 3–6). A
screening of different solvents revealed dichloroethane to be
superior (Table 2, entries 2 and 7–10). Interestingly, switching
to a more coordinating counteranion led to a lower yield of 8b
(Table 2, entry 6). After establishing optimized conditions for
this transformation, we investigated its scope (Scheme 3).
As depicted, a variety of different alkynes can be
employed, and best results are obtained with aromatic
alkynes. The use of an aliphatic alkyne (cf. 8i) leads to
lower yield of the furanone product.
11[b]
12
13
20
50
45
[a] See the Supporting Information for detailed reaction conditions.
Yields refer to isolated products after chromatography. [b] 5 mol% of
Ph3PAuCl/AgSbF6 was employed. [c] 5 mol% of [3,5-(CF3)2C6H3]3PAuCl
was used instead of Ph3PAuCl.
found that arylalkynes are the most successful triple-bond
partners. Best results are obtained when the ylide partner is
the limiting reagent and the alkyne is employed in excess.
We subsequently realized that this intermolecular furan
synthesis is particularly sensitive to the substitution pattern of
both the sulfur ylide and the alkyne partner (Scheme 2b).
While the use of electron-poor alkynes (such as propiolate
derivatives) leads to low yields of product 5b, it was
interesting to note that a malonate-derived sulfonium ylide
provided 2-methoxyfuran 5c in reasonable efficiency
The outcome observed upon exposure of allyloxycarbonyl
substrates to an alkyne and a gold(I) catalyst presumably
results from allylic rearrangement of that ester moiety. To
determine whether the metal catalyst might be involved in
this rearrangement event, we prepared the tetradeuterated
diester 3d and subjected it to the reaction conditions
(Scheme 4). In the event, the rearranged furanone was
formed with complete transfer of the deuterated label to
the vinyl position, a hallmark of highly concerted allylic
transfers characteristic of [3,3]-sigmatropic events.[16]
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Table 2: Optimization of the g-lactone synthesis from sulfonium ylides
and alkynes.[a]
Scheme 4. Labeling experiment for the gold-catalyzed furanone syn-
thesis.
Entry
Cat.
Solvent
Time [h]
Yield [%][b]
1
2
3
4
5
6
7
8
9
7a
7b
7c
7d
7e
7 f
7b
7b
7b
7b
DCE
DCE
DCE
DCE
DCE
DCE
PhMe
PhH
MeCN
MeNO2
7
2
2
5
5
3
3
3
8
2
64
68
65
65
32
63
50
36
45
48
10
[a] Reaction was conducted on a 0.2 mmol scale; ratio of 3d to 4a is 1:5.
[b] Yield of isolated product after column chromatography. DCE=di-
chloroethylene.
Scheme 5. Mechanism calculated for the intramolecular formation of
bicyclofurans.
intramolecular attack of the ylidic carbon on the internal
alkyne C-atom results in the gold–vinyl complex B, in which
the five-membered lactone ring is already formed. In the
subsequent steps, loss of SPh2 and nucleophilic attack of the
acyl oxygen onto the carbon coordinated to the metal result in
complex C. Finally, from C, ligand exchange with release of
the product 2a and addition of a new molecule of ylide
substrate 1a regenerates the p-complex A, thus closing the
catalytic cycle. All steps of the mechanism are thermodynami-
cally favorable (Scheme 5) and the rate-limiting step is the
formation of the furan ring, although the corresponding
barrier is only 1.4 kcalmolÀ1 higher than that calculated for
À
the initial C C bond-formation event.
It is important to stress that a hypothetical gold carbene
intermediate[4g,19] without a neighboring SPh2 substituent
could not be optimized, despite numerous attempts. In all
cases, there was spontaneous formation of the furan ring and
intermediate C.[20] In fact, in the second part of the mechanism
the intermediates formed after SPh2 loss (B1–B3, see Fig-
Scheme 3. Scope of the furanone synthesis. Yields reported are for
pure, isolated compounds.
À
À
ure S1) are stabilized by strong S C or C C interactions
involving that leaving group.
It is conceivable that when an allyloxycarbonyl ester
moiety is present in the substrate, the elimination of gold to
form the 2-allyloxyfuran product is followed by a [3,3]-
sigmatropic rearrangement (see Scheme S2 in the Supporting
Information);[11] this is also in agreement with the labeling
experiment.
In summary, we have developed a gold-catalyzed syn-
thesis of heterocyclic products from doubly stabilized sulfo-
nium ylides. This transformation is amenable to a reactivity
switch triggered by the modification of the electron-with-
The mechanism of the intramolecular formation of
bicyclofurans was studied by exploratory DFT calculations,[18]
using ylide 1a as a case study. The resulting path is depicted in
Scheme 5 along with the free energy values calculated for the
relevant steps (the energy profile for the reaction is given in
Figure S1 in the Supporting Information). The reaction starts
ꢀ
with activation of the C C bond through p-coordination to
the gold catalyst, as shown in intermediate A. From A,
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[6] For reactions of sulfonium ylides with highly activated alkynes to
give furan products, see: a) M. Higo, T, Mukaiyama, Tetrahe-
dron Lett. 1970, 29, 2565; b) Y. Hayasi, M. Kobaysi, H. Nozaki,
Tetrahedron 1970, 26, 4353.
[7] During the preparation of this manuscript, Skrydstrup et al.
reported an efficient gold-catalyzed furan synthesis from singly
stabilized sulfonium ylides and alkynes, prompting us to report
our own results herein. See reference [4g].
drawing moieties of the ylide partner (delivering either furan
products or furanone products having a quaternary center).
Preliminary calculations support the mechanism proposed
and point towards the noninvolvement of a gold carbene
intermediate.
Received: May 10, 2012
Revised: June 8, 2012
Published online: && &&, &&&&
[8] X. Huang, R. Goddard, N. Maulide, Angew. Chem. 2010, 122,
9163; Angew. Chem. Int. Ed. 2010, 49, 8979.
[9] For silver-mediated furan synthesis, see: a) G. Verniest, A.
Padwa, Org. Lett. 2008, 10, 4379; b) C. He, S. Guo, J. Ke, J. Hao,
H. Xu, H. Chen, A. Lei, J. Am. Chem. Soc. 2012, 134, 5766.
[10] For procedures using diazo reagents, see: a) A. Padwa, F. R.
Kinder, J. Org. Chem. 1993, 58, 21; b) H. M. L. Davies, K. R.
Romines, Tetrahedron 1988, 44, 3343; c) L. B. Zhao, Z.-H. Guan,
Y. Han, Y.-X. Xie, S. He, Y.-M. Liang, J. Org. Chem. 2007, 72,
10276; d) W. Pang, S. Zhu, Y. Xin, H. Jiang, S. Zhu, Tetrahedron
2010, 66, 1261; e) L. Zhou, J. Ma, J. Wang, Tetrahedron Lett.
2011, 52, 5486.
Keywords: alkynes · DFT calculations · furans · gold ·
rearrangement · sulfonium ylides
.
[1] a) R. J. Sundberg in Comprehensive Heterocyclic Chemistry, Vol.
5 (Eds.: A. R. Katrzky, C. W. Rees), Pergamon, New York, 1984,
p. 313; b) B. H. Lipshutz, Chem. Rev. 1986, 86, 795; c) X. L. Hou,
H. Y. Cheung, T. Y. Hon, P. L. Kwan, T. H. Lo, S. Y. Tong,
H. N. C. Wong, Tetrahedron 1998, 54, 1955; d) X.-S. Peng, X.-L.
Hou, Prog. Heterocycl. Chem. 2011, 22, 181; e) H. N. C. Wong,
X.-L. Hou, K.-S. Yeung, H. Huang in Five-Membered Hetero-
cycles: Furan, Vol. 1 (Eds.: J. Alvarez-Builla, J. J. Vaquero, J.
Barluenga), Wiley-VCH, Weinheim, 2011, p. 533; f) H. Tan, X.
Chen, Z. Liu, D. Z. Wang, Tetrahedron 2012, 68, 3952; g) M. J.
Piggott, D. Wege, Aust. J. Chem. 2000, 53, 749; h) S. Breyer, K.
Effenberger-Neidnicht, S. Knauer, R. Schobert, Biol. Med.
Chem. 2011, 19, 1264.
[2] For selected reviews of gold catalysis, see: a) D. J. Gorin, F. D.
Toste, Nature 2007, 446, 395; b) A. Fꢁrstner, P. W. Davies,
Angew. Chem. 2007, 119, 3478; Angew. Chem. Int. Ed. 2007, 46,
3410; c) A. S. K. Hashmi, Chem. Rev. 2007, 107, 3180; d) D. J.
Gorin, F. D. Toste, Chem. Rev. 2008, 108, 3351; e) A. Arcadi,
Chem. Rev. 2008, 108, 3266; f) Z. Li, C. Brouwer, C. He, Chem.
Rev. 2008, 108, 3239; g) E. Jimꢂnez-NfflÇez, A. M. Echavarren,
Chem. Rev. 2008, 108, 3326; h) V. Michelet, P. Y. Toullec, J.-P.
GenÞt, Angew. Chem. 2008, 120, 4338; Angew. Chem. Int. Ed.
2008, 47, 4268; i) A. Fꢁrstner, Chem. Soc. Rev. 2009, 38, 3208;
j) A. S. K. Hashmi, Angew. Chem. 2010, 122, 5360; Angew.
Chem. Int. Ed. 2010, 49, 5232; k) C. Aubert, L. Fensterbank, P.
Garcia, M. Malacria, A. Simonneau, Chem. Rev. 2011, 111, 1954;
l) N. Krause, C. Winter, Chem. Rev. 2011, 111, 1994.
[11] See the Supporting Information for details.
[12] For nucleophilic substitution on methoxyfurans, see: M. R.
Iesce, M. L. Graziano, F. Cermola, S. Montella, L. Di Gioia,
Tetrahedron Lett. 2003, 44, 5781.
[13] a) C. Nieto-Oberhuber, S. López, M. P. MuÇoz, D. J. Cµrdenas,
E. BuÇuel, C. Nevado, A. M. Echavarren, Angew. Chem. 2005,
117, 6302; Angew. Chem. Int. Ed. 2005, 44, 6146; b) P. Pꢂrez-
Galµn, N. Delpont, E. Herrero-Gómez, F. Maseras, A. M.
Echavarren, Chem. Eur. J. 2010, 16, 5324; c) V. López-Carrillo,
A. M. Echavarren, J. Am. Chem. Soc. 2010, 132, 9292.
[14] The original observation is shown below:
.
[15] a) E.-i. Negishi, M. Kotora, Tetrahedron 1997, 53, 6707; b) I.
Collins, J. Chem. Soc. Perkin Trans. 1 1999, 1377; c) B. El Ali, H.
Alper, Synlett 2000, 161; d) M. Seitz, O. Reiser, Curr. Opin.
Chem. Biol. 2005, 9, 285; e) S. Gil, M. Parra, P. Rodriguez, J.
Segura, Mini-Rev. Org. Chem. 2009, 6, 345.
[16] See C. Madelaine, V. Valerio, N. Maulide, Angew. Chem. 2010,
122, 1628; Angew. Chem. Int. Ed. 2010, 49, 1583, and references
cited therein.
[3] For seminal work on gold catalysis, see: J. H. Teles, S. Brode, M.
Chabanas, Angew. Chem. 1998, 110, 1475; Angew. Chem. Int. Ed.
1998, 37, 1415.
[17] For selected examples of related transformations, see: a) T.
Shimada, I. Nakamura, Y. Yamamoto, J. Am. Chem. Soc. 2004,
126, 10546; b) I. Nakamura, Y, Mizushima, Y. Yamamoto, J.
Am. Chem. Soc. 2005, 127, 15022; c) A. Fꢁrstner, P. W. Davies, J.
Am. Chem. Soc. 2005, 127, 15024; d) M. Uemura, I. D. G.
Watson, M. Katsukawa, F. D. Toste, J. Am. Chem. Soc. 2009, 131,
3464; e) Y. Shi, K. E. Roth, S. D. Ramgren, S. A. Blum, J. Am.
Chem. Soc. 2009, 131, 18022; f) A. S. K. Hashmi, C. Lothschꢁtz,
R. Dçpp, M. Ackermann, J. D. B. Becker, M. Rudolph, C.
Scholz, F. Rominger, Adv. Synth. Catal. 2012, 354, 133.
[18] a) R. G. Parr, W. Yang, Density Functional Theory of Atoms and
Molecules, Oxford University Press, New York, 1989; b) DFT/
PBE0 calculations were performed using the gaussian 03
package. A complete account of the computational details and
the corresponding list of references are provided in the
Supporting Information.
[4] For selected examples on gold catalysed furan synthesis, see:
a) A. S. K. Hashmi, L. Schwarz, J.-H. Choi, T. M. Frost, Angew.
Chem. 2000, 112, 2382; Angew. Chem. Int. Ed. 2000, 39, 2285;
b) A. W. Sromek, M. Rubina, V. Gevorgyan, J. Am. Chem. Soc.
2005, 127, 10500; c) J. Zhang, H.-G. Schmalz, Angew. Chem.
2006, 118, 6856; Angew. Chem. Int. Ed. 2006, 45, 6704; d) A. S.
Dudnik, A. W. Sromek, M. Rubina, J. T. Kim, A. V. Kelꢀin, V.
Gevorgyan, J. Am. Chem. Soc. 2008, 130, 1440; e) A. S. Dudnik,
Y. Xia, Y. Li, V. Gevorgyan, J. Am. Chem. Soc. 2010, 132, 7645;
f) G. Zhou, F. Liu, J. Zhang, Chem. Eur. J. 2011, 17, 3101; g) S.
Kramer, T. Skrydstrup, Angew. Chem. 2012, 124, 4759; Angew.
Chem. Int. Ed. 2012, 51, 4681.
[5] For selected examples on other metal-catalyzed furan syntheses,
see: a) S. Ma, J. Zhang, J. Am. Chem. Soc. 2003, 125, 12386; b) L.
Peng, X. Zhang, M. Ma, J. Wang, Angew. Chem. 2007, 119, 1937;
Angew. Chem. Int. Ed. 2007, 46, 1905; c) J. Chen, S. Ma, Chem.
Asian J. 2010, 5, 2415; d) K.-D. Umland, A. Palisse, T. T. Haug,
S. F. Kirsch, Angew. Chem. 2011, 123, 10140; Angew. Chem. Int.
Ed. 2011, 50, 9965; e) P. Lenden, D. A. Entwistle, M. C. Willis,
Angew. Chem. 2011, 123, 10845; Angew. Chem. Int. Ed. 2011, 50,
10657.
[19] For a recent minireview on a-oxo gold carbenoids in catalysis,
see: a) J. Xiao, X. Li, Angew. Chem. 2011, 123, 7364; Angew.
Chem. Int. Ed. 2011, 50, 7226. For further recent examples on
gold a-oxo carbenoids in catalysis, see: b) W. He, C. Li, L.
Zhang, J. Am. Chem. Soc. 2011, 133, 8482; c) A. Mukherjee,
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R. B. Dateer, R. Chaudhuri, S. Bhunia, S. N. Karad, R.-S. Liu, J.
Am. Chem. Soc. 2011, 133, 15372; d) Y. Wang, K. Ji, S. Lan, L.
Zhang, Angew. Chem. 2012, 124, 1951; Angew. Chem. Int. Ed.
2012, 51, 1915; e) S. Bhunia, S. Ghorpade, D. B. Huple, R.-S. Liu,
Angew. Chem. 2012, 124, 2993; Angew. Chem. Int. Ed. 2012, 51,
2939. For selected examples of the formation of gold carbenes
through nitrene transfer, see: f) A. Wetzel, F. Gagosz, Angew.
Chem. 2011, 123, 7492; Angew. Chem. Int. Ed. 2011, 50, 7354;
g) B. Lu, Y. Luo, L. Liu, L. Ye, Y. Wang, L. Zhang, Angew. Chem.
2011, 123, 8508; Angew. Chem. Int. Ed. 2011, 50, 8358.
[20] For a previously proposed mechanism of oxazole formation
which also bypasses the formation of gold carbene/carbenoid
intermediates, see: P. W. Davies, A. Cremonesi, L. Dumitrescu,
Angew. Chem. 2011, 123, 9093; Angew. Chem. Int. Ed. 2011, 50,
8931.
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Communications
Gold Catalysis
X. Huang, B. Peng, M. Luparia,
L. F. R. Gomes, L. F. Veiros,
N. Maulide*
&&&& — &&&&
Gold-Catalyzed Synthesis of Furans and
Furanones from Sulfur Ylides
A golden switch: Doubly stabilized sul-
fonium ylides can be coupled with
alkynes in a gold-catalyzed synthesis of
heterocycles. This method hinges on
a switch in the reactivity of the sulfur ylide
resulting from the simple modification of
the electron-withdrawing moieties and
leads to either furans or furanones bear-
ing a quaternary center (see scheme).
6
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