A general synthesis of ynones from aldehydes via oxidative C-C bond cleavage under aerobic conditions

Article abstract of DOI:10.1021/ja506352b

We describe a direct synthesis of various ynones from readily available aldehydes and hypervalent alkynyl iodides. In this method, a gold catalyst and a secondary amine work synergistically to produce the trisubstituted allenyl aldehyde, which can be conv

Full text of DOI:10.1021/ja506352b

Communication
pubs.acs.org/JACS
A General Synthesis of Ynones from Aldehydes via Oxidative C−C
bond Cleavage under Aerobic Conditions
Zhaofeng Wang, Li Li, and Yong Huang*
Key Laboratory of Chemical Genomics, School of Chemical Biology and Biotechnology, Peking University, Shenzhen Graduate
School, Shenzhen, 518055, China
S
* Supporting Information
gold catalyzed dehydrogenative Meyer−Schuster-like rearrange-
ment to generate ynones.19 Despite these contributions to ynone
ABSTRACT: We describe a direct synthesis of various
ynones from readily available aldehydes and hypervalent
alkynyl iodides. In this method, a gold catalyst and a
secondary amine work synergistically to produce the
trisubstituted allenyl aldehyde, which can be converted to
the desired ynone through an in situ C−C bond oxidative
cleavage using molecular oxygen.
synthesis, a mild, practical method to prepare ynones using
readily available materials is highly desirable. Toward this goal,
we report our recent progress on direct synthesis of ynones from
aldehydes using synergistic catalysis (Scheme 1).
Scheme 1. α-Functionalization of Aldehydes Using Amine/
Gold Synergistic Catalysis
he combination of a metal and an organocatalyst in a single
Tpot is a powerful strategy to accomplish previously
unattainable chemical transformations.1 The merge of metal
catalyzed aerobic oxidation and small organic molecule catalyzed
C−C bond formation reactions represents a unique and
refreshing approach to access synthetically valuable intermedi-
ates. Yet, successful metal-organo catalysis synergy and types of
competent reactions remain scarce despite the enormous
advances enjoyed by both metal catalysis and organocatalysis
alone. In particular, catalytic C−C bond cleavage represents a
major challenge for the synergistic catalysis. Although metal
catalyzed C−C bond cleavage has been studied extensively using
transition metals, the reaction substrates often require additional
synthetic efforts.2 In situ assembly of appropriate functionalities
(ready for C−C bond cleavage using metal catalysts) via
organocatalysis enables direct employment of commercially
cheap feedstock for such reactions.
Alkynyl carbonyl compounds, especially ynones, are abundant
structural motifs in natural products. They are often employed as
a key template for rapid structural proliferation toward
sophisticated synthetic targets.3 The direct attachment of a
carbonyl to an acetylene moiety offers numerous synthetic
outlets for structure diversification. For example, ynones are
particularly attractive as precursors to various heterocycles, such
as pyrroles,4 furans,5 furanones,6 pyrazoles,7 isoxazoles,8
pyrimidines,9 flavones,10 quinolones,11 etc. Considerable efforts
have been made to access ynones using various synthetic
approaches.12−16 In most cases, both reaction partners need to be
preactivated or an additional step was required. More recently,
method development for the synthesis of ynones has been
focused on mild conditions, wide functional group tolerance,
readily available starting materials, and practical convenience.
Recently we reported the first direct α-vinylidenation of
aldehydes to access trisubstituted allenyl aldehydes using
synergistic catalysis combing a gold catalyst and a secondary
amine.20 Subsequently, we found that the α-allenyl aldehyde
could further react with a secondary amine to generate the
corresponding ynenamine intermediate, which was reactive
toward various electrophiles. We wondered whether this highly
electrophilic species would be sensitive to in situ oxidation using
molecular oxygen to yield an oxidation product. The
trisubstituted allenyl aldehyde was stirred with 1 equiv of
pyrrolidine and a metal catalyst under an oxygen balloon.
Gratifyingly, AuCl3 was found to promote a clean aerobic
oxidation reaction to give an ynone product 2a in 88% yield.21
This compound contained one less carbon than the starting
materials combined, indicating a C−C bond cleavage process.
Encouraged by this result, we initiated an attempt to synthesize
ynones directly from aldehydes in a one-pot manner. The
reaction conditions survey was carried out by treating a mixture
of aldehyde 1a (1 equiv), pyrrolidine (1 equiv), and 1-
[(triisopropylsilyl)-ethynyl]-1,2-benziodoxol-3(1H)-one
(TIPS-EBX, 1.2 equiv)22 in the presence of a metal catalyst under
Muller et al. reported a series of Sonogashira couplings reactions
̈
between acyl chlorides (and aryl halides in the presence of CO)
and terminal alkynes utilizing Pd−Cu catalysts.9,17 Bolshan et al.
reported a metal-free synthesis of ynones from acyl chlorides and
potassium alkynyltrifluoroborate salts.18 Hashmi et al. reported a
Received: June 24, 2014
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja506352b | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
a
an oxygen balloon at rt (Table 1). Only gold promoted this
Table 2. Substrate Scope of the Ynone Synthesis
aerobic C−C bond cleavage reaction. No ynone product 2a was
Table 1. Conditions Survey for the Direct Conversion of
a
Aldehydes to Ynones
b
entry
solvent
catalyst
ligand
yield (%)
1
toluene
toluene
toluene
toluene
toluene
toluene
toluene
Et2O
Cu
none
none
none
none
none
3a
0
2
Ag
0
3
Fe
0
4
AuCl
AuCl3
AuCl
AuCl3
AuCl
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
AuCl3
trace
13
5
6
27
7
3a
41
8
3a
32
a
Isolated yields.
9
Et2O
3a
53
10
11
12
Et2O
3b
trace
trace
68
Et2O
3c
products 2 were obtained in 60−70% isolated yields. Sterically
congested ynones having a secondary carbon chain directly
attached to the carbonyl could be accessed using β-branched
aldehydes (Table 2, products 2g, 2k, 2l, 2m, 2n). No
deterioration of yield was observed for these substrates.
Et2O
3d
c
13
Et2O
3d
81
14
15
16
17
DMF
3d
trace
32
THF
3d
DME
3d
21
Various substituted EBX reagents were also examined.
Consistent with our previous report, only silyl substituted EBX
reagents were effective for this transformation. When the t-Bu-
EBX reagent was used, the corresponding tert-butyl allenyl
aldehyde was isolated in 50% yield (Scheme 2, eq a). This result
suggested that the formation of the key ynenamine intermediate
was accelerated by the silicon group. Further treating allenyl
aldehyde 3a′ with pyrrolidine did not lead to the ynenamine
formation even at elevated temperature. In sharp contrast, mixing
the allenyl aldehyde 3a with pyrrolidine in CDCl3 at rt resulted
EtOH
3d
trace
a
Reactions were conducted with 0.1 mmol of 1a, 0.1 mmol of
pyrrolidine, and 0.12 mmol of TIPS-EBX in 5.0 mL of solvent under
an O2 balloon. Yields were determined by GC using biphenyl as the
internal standard. 40 °C, 10 h.
b
c
observed when copper, silver, or iron was used, as result of the
ineffective α-vinylidenation by those metals (Table 1, entries 1−
3). No ynone was found in the absence of either gold or an
amine. Without O2, the combination of AuCl/AuCl3 and
pyrrolidine led to a decent conversion to a mixture of α-allenyl
aldehyde and the α-alkynylated aldehyde.20 The desired ynone
product 2a was obtained in 13% yield (GC) when the reaction
was carried out using AuCl3 in toluene (Table 1, entry 5). AuCl
was significantly inferior (Table 1, entry 4). Various secondary
amines were examined as well (for a comprehensive conditions
survey, see Supporting Information (SI)). Pyrrolidine was the
only amine leading to a detectable amount of 2a. For other
amines, the aldol dimerization of the aldehyde was the major side
reaction. The solvent screening quickly identified Et2O as the
optimum reaction media. Ligands had a profound impact on the
conversion to ynones, with 4,5-diazafluorenone being the most
effective. Finally, raising the reaction temperature to 40 °C
afforded ynone 2a in 81% GC yield.
Scheme 2. Mechanistic Studies of the C−C Bond Cleavage by
Aerobic Oxidation
The scope of the aldehydes was studied extensively. For 3-
phenylpropanal analogues, substituents of various electronic
characteristics on the aromatic ring were well tolerated (Table 2,
product 2b, 2c, 2e, 2q). The reaction was also very effective for
various aliphatic aldehydes, especially those bearing useful
functional groups, such as ethers, esters, carbamates, imides,
olefins, hydroxy, sulfonamide, and heterocycles (Table 2,
products 2f, 2h, 2i, 2j, 2o, 2p, 2s−2u). Generally, the ynone
B
dx.doi.org/10.1021/ja506352b | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
quantitative formation of the ynenamine A in 30 min. We reason
the facile ynenamine formation of the TIPS substituted allenyl
aldehyde is likely due to the stabilization of the enyne moiety by
the strong hyperconjugation effect of silicon. The subsequent
C−C bond cleavage of A did not occur in the absence of gold.
Various gold(I) and (III) species catalyzed the ynone formation.
A ligand seemed to have a smaller effect in this step compared to
the allenyl aldehyde formation.
Scheme 4. Diverse Reactivity of Ynones Readily Available
from Aldehydes
An 18O2 experiment confirmed that the oxygen of the ynone
originated from molecular oxygen and this process is indeed
aerobic. When an aldehyde bearing an α-cyclopropane was used
as a radical clock, no ynone product was obtained. Instead,
multiple decomposition products were observed that bear
olefinic protons, indicating the generation of an α-iminium
radical. When 5.0 equiv of TEMPO (2,2,6,6-tetramethyl-1-
oxylpiperidine) was added, the ynone formation was completely
inhibited and a coupling product 2t was isolated in 67% yield,
which also supports a free radical pathway for the oxidative C−C
bond cleavage.
Based on the aforementioned experimental data, we propose
the following mechanism for the ynone formation via C−C bond
cleavage by O2 (Scheme 3).23 Under the standard conditions, a
Scheme 3. Proposed Mechanism for the Aerobic C−C Bond
Cleavage
bisphosphine catalyzed double-Michael reaction to afford 2,5-
disubstituted oxazolidine 8 as a single cis-isomer.27 2,4-
Disubstituted pyrimidine 9 was obtained through an annulation
reaction with amidine.7b Ynone 7 also participated in cyclo-
addition reactions. A Diels−Alder reaction between 7 and 1,3-
diphenylisobenzofuran generated polycyclic product 10.28
Substituted phenol 11 was synthesized by treatment 7 with
Danishefsky’s diene under thermal conditions.29
In summary, we developed a general synthesis of ynones
directly from readily available aldehydes, cheap pyrrolidine, a
gold catalyst, and molecular oxygen. The reaction proceeds
through an α-vinylidenation reaction by gold/amine synergistic
catalysis, followed by in situ aerobic cleavage of the C−C bond.
The relatively broad substrate scope and the aerobic nature of
this method represent a major advance over existing conditions
for the synthesis of the highly valuable ynone functionality.
smooth α-vinylidenation of aldehyde 1 occurs to give α-allenyl
aldehyde 3, which has been verified experimentally. The allenyl
aldehyde continues to react with pyrrolidine to generate the
ynenamine intermediate A. The subsequent aerobic oxidation of
the A is believed to occur through the 1,2-dioxetane
intermediate,24 which leads to the ynone product 2 via C−C
bond cleavage.25 The byproduct pyrrolidine-1-carbaldehyde 4
was detected by HRMS and isolated in good yield. The exact role
of gold in the C−C bond cleavage step is not clear. It is likely that
gold activates molecular oxygen and facilitates single electron
transfer from the eletron-rich ynenamine to O2.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, product characterizations, and copies
of all NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
The TIPS group on the ynone product (such as 2a and 2f) can
be conveniently removed by treatment with TBAF buffered with
acetic acid, furnishing the corresponding terminal alkyne in
quantitative yield (see the SI). The synthetic utilities of ynones
are demonstrated in Scheme 4. Conjugate diene 4 was
synthesized in 82% yield via enyne metathesis from termi-
nalynone 3, which was prepared by desilylation of product 2p
from Table 2. The ynone product 2a was further oxidized to a
diketone product 5 quantitatively using a copper catalyst and
molecular oxygen.26 The ynones showed synthetic versatility
toward various heterocycles. Condensation of 2a with phenyl-
hydrazine generated trisubstituted pyrazole 6 in 85% yield. The
terminal ynone 7 reacted with amino alcohol through a
AUTHOR INFORMATION
Corresponding Author
huangyong@pkusz.edu.cn
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Natural Science
Foundation of China (21372013), Shenzhen Peacock Program
(KQTD201103), and Shenzhen innovation funds
(CXZZ20130517152257071). Y.H. thanks the MOE for the
Program for New Century Excellent Talents in University.
C
dx.doi.org/10.1021/ja506352b | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(18) Taylor, C.; Bolshan, Y. Org. Lett. 2014, 16, 488.
REFERENCES
■
(19) Yu, Y.; Yang, W.; Pflasterer, D.; Hashmi, A. S. K. Angew. Chem., Int.
̈
(1) Reviews: (a) Shao, Z.; Zhang, H. Chem. Soc. Rev. 2009, 38, 2745.
(b) Hashmi, A. S. K.; Hubbert, C. Angew. Chem., Int. Ed. 2010, 49, 1010.
(c) Zhong, C.; Shi, X. Eur. J. Org. Chem. 2010, 2999. (d) Allen, A. E.;
MacMillan, D. W. C. Chem. Sci. 2012, 3, 633. (e) Patil, N. T.; Shinde, V.
S.; Gajula, B. Org. Biomol. Chem. 2012, 10, 211. (f) Han, Z.-Y.; Wang, C.;
Gong, L.-Z. In Science of Synthesis: Asymmetric Organocatalysis; Maruoka,
K., Ed.; Georg Thieme Verlag: Stuttgart, 2012; Vol. 2, p 697. (g) Du, Z.;
Shao, Z. Chem. Soc. Rev. 2013, 42, 1337.
Ed. 2014, 53, 1144.
(20) Wang, Z.; Li, X.; Huang, Y. Angew. Chem., Int. Ed. 2013, 52, 14219.
(21) (a) Hashmi, A. S. K.; Frost, T. M.; Bats, J. W. J. Am. Chem. Soc.
2000, 122, 11553. (b) Hashmi, A. S. K.; Schwarz, L.; Choi, J.-H.; Frost,
T. M. Angew. Chem., Int. Ed. 2000, 39, 2285. (c) Hashmi, A. S. K.;
Lothschutz, C.; Ackermann, M.; Doepp, R.; Anantharaman, S.;
̈
Marchetti, B.; Bertagnolli, H.; Rominger, F. Chem.Eur. J. 2010, 16,
8012.
(2) Reviews on C−C bond cleavage: (a) Bishop, K. C., III. Chem. Rev.
1976, 76, 461. (b) Crabtree, R. H. Chem. Rev. 1985, 85, 245.
(c) Murakami, M.; Ito, Y. Activation of Unreactive Bonds and Organic
Synthesis; Murai, S., Ed.; Springer: Berlin, 1999; p 97. (d) Rybtchinski,
B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870. (e) Crabtree, R. H.
Nature 2000, 408, 415. (f) Jun, C.-H. Chem. Soc. Rev. 2004, 33, 610.
(g) Horino, Y. Angew. Chem., Int. Ed. 2007, 46, 2144. (h) Rubin, M.;
Rubina, M.; Gevorgyan, V. Chem. Rev. 2007, 107, 3117. (i) Gooßen, L.
J.; Rodríguez, N.; Gooßen, K. Angew. Chem., Int. Ed. 2008, 47, 3100.
(j) Park, Y. J.; Park, J.-W.; Jun, C.-H. Acc. Chem. Res. 2008, 41, 222.
(22) Reviews on electrophilic alkynyliodonium salts: (a) Zhdankin, V.
V.; Stang, P. J. Tetrahedron 1998, 54, 10927. (b) Zhdankin, V. V.; Stang,
P. J. Chem. Rev. 2008, 108, 5299. (c) Brand, J. P.; Gonzalez, D. F.;
Nicolai, S.; Waser, J. Chem. Commun. 2011, 47, 102. (d) Brand, J. P.;
Waser, J. Chem. Soc. Rev. 2012, 41, 4165. Recent examples of using EBX
for electrophilic alkynylation reactions: (e) Nicolai, S.; Piemontesi, C.;
Waser, J. Angew. Chem., Int. Ed. 2011, 50, 4680. (f) Brand, J. P.;
Chevalley, C.; Scopelliti, R.; Waser, J. Chem.Eur. J. 2012, 18, 5655.
(g) Brand, J. P.; Waser, J. Org. Lett. 2012, 14, 744. (h) Liu, X.; Wang, Z.;
Cheng, X.; Li, C. J. Am. Chem. Soc. 2012, 134, 14330. (i) Kamlar, M.;
́
(k) Tobisu, M.; Chatani, N. Chem. Soc. Rev. 2008, 37, 300. (l) Najera, C.;
Putaj, P.; Vesely, J. Tetrahedron Lett. 2013, 54, 2097. (j) Li, Y.; Brand, J.
́
Sansano, J. M. Angew. Chem., Int. Ed. 2009, 48, 2452. (m) Winter, C.;
Krause, N. Angew. Chem., Int. Ed. 2009, 48, 2460. (n) Seiser, T.; Cramer,
N. Org. Biomol. Chem. 2009, 7, 2835. (o) Bonesi, S. M.; Fagnoni, M.
Chem.Eur. J. 2010, 16, 13572. (p) Cramer, N.; Seiser, T. Synlett 2011,
449. (q) Murakami, M.; Matsuda, T. Chem. Commun. 2011, 47, 1100.
(r) Ruhland, K. Eur. J. Org. Chem. 2012, 2683. (s) Wang, T.; Jiao, N. Acc.
Chem. Res. 2014, 47, 1137.
P.; Waser, J. Angew. Chem., Int. Ed. 2013, 52, 6743. (k) Aubineau, T.;
Cossy, J. Chem. Commun. 2013, 49, 3303. (l) Frei, R.; Waser, J. J. Am.
́ ̀
Chem. Soc. 2013, 135, 9620. (m) Gonzalez, D. F.; Brand, J. P.; Mondiere,
R.; Waser, J. Adv. Synth. Catal. 2013, 355, 1631. (n) Feng, C.; Loh, T.-P.
Angew. Chem., Int. Ed. 2014, 53, 2722. (o) Xie, F.; Qi, Z.; Yu, S.; Li, X. J.
Am. Chem. Soc. 2014, 136, 4780.
(23) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2010, 49, 5232.
(24) (a) Vanrheen, V. J. Chem. Soc. D 1969, 3, 314. (b) Kaneda, K.; Ito,
T.; Kii, N.; Jitsukawa, K.; Teranishi, S. J. Mol. Catal. 1982, 15, 349.
(c) Ito, T.; Kaneda, K.; Watanabe, I.; Ikeda, S.; Teranishi, S. Chem. Lett.
1976, 227. (d) Ebitani, K.; Nagashima, K.; Mizugaki, T.; Kaneda, K.
Chem. Commun. 2000, 869. (e) Wang, A.; Jiang, H. J. Am. Chem. Soc.
2008, 130, 5030. (f) Lu, B.; Shi, M. Chem.Eur. J. 2011, 17, 9070.
(g) Celaje, J. A.; Zhang, D.; Guerrero, A. M.; Selke, M. Org. Lett. 2011,
13, 4846. (h) Zhang, C.; Xu, Z.; Shen, T.; Wu, G.; Zhang, L.; Jiao, N. Org.
Lett. 2012, 14, 2362.
(3) (a) Clennan, E. L.; Heah, P. C. J. Org. Chem. 1981, 46, 4107.
(b) Kerdesky, F. A. J.; Schmidt, S. P.; Brooks, D. W. J. Org. Chem. 1993,
58, 3516. (c) Adlington, R. M.; Baldwin, J. E.; Pritchard, G. J.; Spencer,
K. Tetrahedron Lett. 2000, 41, 575. (d) Aiguade, J.; Hao, J.; Forsyth, C. J.
Org. Lett. 2001, 3, 979. (e) Forsyth, C. J.; Xu, J.; Nguyen, S. T.; Samdal, I.
A.; Briggs, L. R.; Rundberget, T.; Sandvik, M.; Miles, C. O. J. Am. Chem.
Soc. 2006, 128, 15114. (f) Nicolaou, K. C.; Sarlah, D.; Shaw, D. M.
Angew. Chem., Int. Ed. 2007, 46, 4708. (g) Xu, B. H.; Kehr, G.; Frohlich,
̈
R.; Wibbeling, B.; Schirmer, B.; Grimme, S.; Erker, G. Angew. Chem., Int.
Ed. 2011, 50, 7183.
(25) Examples of gold-catalyzed oxidations: (a) Haruta, M.;
Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 16, 405.
(b) Prati, L.; Rossi, M. J. Catal. 1998, 176, 552. (c) Comotti, M.; Della
Pina, C.; Matarrese, R.; Rossi, M. Angew. Chem., Int. Ed. 2004, 43, 5812.
(d) Guan, B.; Xing, D.; Cai, G.; Wan, X.; Yu, N.; Fang, Z.; Yang, L.; Shi,
Z. J. Am. Chem. Soc. 2005, 127, 18004. (e) Cinellu, M. A.; Minghetti, G.;
Cocco, F.; Stoccoro, S.; Zucca, A.; Manassero, M. Angew. Chem., Int. Ed.
2005, 44, 6892. (f) Liu, Y.; Song, F.; Guo, S. J. Am. Chem. Soc. 2006, 128,
11332. (g) Beltrame, P.; Comotti, M.; Della Pina, C.; Rossi, M. Appl.
Catal., A 2006, 297, 1. (h) Comotti, M.; Della Pina, C.; Falletta, E.;
Rossi, M. Adv. Synth. Catal. 2006, 348, 313. (i) Xing, D.; Guan, B.; Cai,
G.; Fang, Z.; Yang, L.; Shi, Z. Org. Lett. 2006, 8, 693. (j) Hashmi, A. S. K.;
Jaimes, M. C. B.; Schuster, A. M.; Rominger, F. J. Org. Chem. 2012, 77,
6394.
(4) (a) Utimoto, K.; Miwa, H.; Nozaki, H. Tetrahedron Lett. 1981, 22,
4277. (b) Kel’in, A. V.; Sromek, A. W.; Gevorgyan, V. J. Am. Chem. Soc.
2001, 123, 2074.
(5) (a) Jeevanandam, A.; Narkunan, K.; Ling, Y.-C. J. Org. Chem. 2001,
66, 6014. (b) Kel’in, A. V.; Gevorgyan, V. J. Org. Chem. 2002, 67, 95.
(c) Lee, C. G.; Lee, K. Y.; Lee, S.; Kim, J. Tetrahedron 2005, 61, 8705.
(6) Van den Hoven, B. G.; Ali, B. E.; Alper, H. J. Org. Chem. 2000, 65,
4131.
(7) (a) Wang, X.-J.; Tan, J.; Zhang, L. Org. Lett. 2000, 2, 3107.
(b) Grotjahn, D. B.; Van, S.; Combs, D.; Lev, D. A.; Schneider, C.;
Rideout, M.; Meyer, C.; Hernandez, G.; Mejorado, L. J. Org. Chem.
2002, 67, 9200. (c) Mohamed Ahmed, M. S.; Kobayashi, K.; Mori, A.
Org. Lett. 2005, 7, 4487. (d) Kirkham, J. D.; Edeson, S. J.; Stokes, S.;
Harrity, J. P. Org. Lett. 2012, 14, 5354.
(26) (a) Zhang, C.; Jiao, N. J. Am. Chem. Soc. 2010, 132, 28. (b) Zhang,
C.; Xu, Z.; Zhang, L.; Jiao, N. Angew. Chem., Int. Ed. 2011, 50, 11088.
(c) Su, Y.; Sun, X.; Wu, G.; Jiao, N. Angew. Chem., Int. Ed. 2013, 52, 9808.
(27) Sriramurthy, V.; Barcan, G. A.; Kwon, O. J. Am. Chem. Soc. 2007,
129, 12928.
(8) Hojo, M.; Tomita, K.; Hosomi, A. Tetrahedron Lett. 1993, 34, 485.
(9) (a) Karpov, A. S.; Muller, T. J. J. Org. Lett. 2003, 5, 3451.
̈
(b) Karpov, A. S.; Muller, T. J. J. Synthesis 2003, 2815.
̈
(10) Awuah, E.; Capretta, A. Org. Lett. 2009, 11, 3210.
(11) Arcadi, A.; Marinelli, F.; Rossi, E. Tetrahedron 1999, 55, 13233.
(12) (a) Brown, H. C.; Garg, C. P. J. Am. Chem. Soc. 1961, 83, 2952.
(b) Maeda, Y.; Kakiuchi, N.; Matsumura, S.; Nishimura, T.; Kawamura,
T.; Uemura, S. J. Org. Chem. 2002, 67, 6718.
(28) Blitzke, T.; Sicker, D.; Wilde, H. Synthesis 1995, 236.
(29) Khalaf, A.; Gree
Tetrahedron 2011, 67, 3881.
́ ́
, D.; Abdallah, H.; Jaber, N.; Hachem, A.; Gree, R.
(13) Alonso, D. A.; Naj
1615.
́
era, C.; Pacheco, M. C. J. Org. Chem. 2004, 69,
(14) Kroeger, J. W.; Nieuwland, J. A. J. Am. Chem. Soc. 1936, 58, 1861.
(15) Murai, K.; Miyazaki, S.; Fujioka, H. Tetrahedron Lett. 2012, 53,
3746.
(16) Nozaki, K.; Sato, N.; Takaya, H. J. Org. Chem. 1994, 59, 2679.
(17) (a) Karpov, A. S.; Merkul, E.; Rominger, F.; Muller, T. J. J. Angew.
̈
Chem., Int. Ed. 2005, 44, 6951. (b) D’Souza, D. M.; Muller, T. J. J. Nat.
̈
Protoc. 2008, 3, 1660. (c) Boersch, C.; Merkul, E.; Muller, T. J. J. Angew.
̈
Chem., Int. Ed. 2011, 50, 10448.
D
dx.doi.org/10.1021/ja506352b | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX