Versatile oxidative approach to carbazoles and related compounds using MoCl5

Article abstract of DOI:10.1021/ol403304t

The unique oxidizing power of molybdenum pentachloride provides an easy to perform, versatile, and high yielding method to construct carbazoles and the corresponding dibenzo analogues of thiophene, furan, and selenophene. The coupling reaction tolerates a variety of functional groups. The synthesis is highly modular. By this approach a precursor for the naturally occurring carbazole koenigicine was prepared.

Full text of DOI:10.1021/ol403304t

Letter
pubs.acs.org/OrgLett
Versatile Oxidative Approach to Carbazoles and Related Compounds
Using MoCl5
Simon Trosien, Philipp Bottger, and Siegfried R. Waldvogel*
̈
Institute of Organic Chemistry, Duesbergweg 10-14, Johannes Gutenberg University Mainz, 55128 Mainz, Germany
S
* Supporting Information
ABSTRACT: The unique oxidizing power of molybdenum pentachloride
provides an easy to perform, versatile, and high yielding method to construct
carbazoles and the corresponding dibenzo analogues of thiophene, furan,
and selenophene. The coupling reaction tolerates a variety of functional
groups. The synthesis is highly modular. By this approach a precursor for
the naturally occurring carbazole koenigicine was prepared.
he synthesis and application of carbazoles, dibenzothio-
phenes, and dibenzofurans has experienced significant
photocyclization with rearomatization in the presence of
iodine,15,20 and by transition metal catalyzed cyclization21
with leaving functionalities.22−24 Furthermore, a cyclization can
be accomplished by palladium migration.25 Other synthetic
routes involve the formation of the C,O bond.26
Despite the current repertoire for synthesizing carbazoles,
dibenzothiophenes, and dibenzofurans, each method exhibits its
limitation. Therefore, new methods for the versatile preparation
of these heterocycles are highly desired.
Molybdenum pentachloride is a powerful oxidizer with Lewis
acidic properties.27 A variety of labile functionalities as amides,
iodo groups, or acetals are tolerated in the process of the
coupling reaction.28 The performance of the reagent is
influenced by use of additives, e.g. Lewis acids such as TiCl4,
because a preorientation of the substrate is caused29 and side
reactions are avoided by trapping hydrogen chloride.30 During
the reaction, molybdenum salts are coordinated on the
substrates, which leads to unusual consecutive redox reactions31
and controls the stereoselectivity.32 MoCl5 compared to
hypervalent iodo reagents was found to exhibit superior
performance.33
T
attention in the past decades. A variety of bioactive natural
products contain carbazole1 and the dibenzofuran skeleton2
and provide important pharmacological features. A large
number of biologically active compounds involve dibenzothio-
phene moieties.3 Furthermore, carbazoles4 and dibenzothio-
phenes are important building blocks for functional materials as
luminescent polymers,5 photovoltaic devices,6 or transition
metal complexes.7
For the construction of carbazoles, a plethora of synthetic
methods are known.1 Direct oxidative cyclization reactions
starting from diarylamines are feasible by an electrocyclization
reaction induced photochemically8 or thermally.9 The absence
of leaving functionalities makes this approach attractive and was
first reported using palladium(II) acetate in the oxidative
coupling process.10 Despite the moderate yields and the limited
scope of this transformation, the originally reported protocol is
still the best procedure for this kind of cyclization reaction.
Because of the high costs and toxicity of the palladium salt,
catalytic versions have been developed. The first versatile
palladium catalysis using cupric acetate as an oxidant was
Herein, we report a modular synthesis of carbazoles,
dibenzofurans, dibenzothiophenes, and dibenzoselenophenes
involving an intramolecular oxidative coupling step using a
MoCl5/TiCl4 reagent mixture.
reported by Knolker and co-workers.11 Other groups applied
̈
alternative oxidizers, e.g. benzoquinone,12 tert-butyl hydro-
peroxide,13 molecular oxygen,14a or air.14b−e Although this
method provides a wide range of substrates, only the formation
of 9H-carbazoles is viable, whereas N-functionalized carbazoles
are not accessible.14a
Diarylamines serve as precursors for the carbazoles and are
easily accessible by a 2-fold Buchwald−Hartwig amination.34
Starting with an amine and an aryl halide (see Supporting
Information (SI)) using the effective catalytic system Pd(dba)2/
JohnPhos/NaOtBu, the symmetric intermediates were obtained
in 63−87% yield (Scheme 1). If an aryl chloride and an aryl
bromide are present, the unsymmetrically substituted diaryl-
amines are selectively formed under the same conditions as
described above (Scheme 1, method a).35 Alternatively,
sequential addition of different aryl halides can be carried out
(method b). If the arene is equipped with both, chlorine and
For intramolecular C,C bond formation of diaryl sulfides to
form dibenzothiophenes, only the photoinduced electrocycliza-
tion followed by oxidation using iodine is known.15 When
appropriate leaving functionalities are involved, the cyclization
can be accomplished by standard palladium catalysis.16 In
addition, several methods for the arylation of sulfides/sulfoxides
are known wherein cationic species are generated,17 or an
aromatic moiety of dibenzothiophenes is constructed.18
Analogous to the carbazole preparation, the synthetic
approaches to dibenzofurans from the corresponding diaryl
ether are known: Employing a 2-fold C,H activation10,14a,b by
directed ortho metalation and subsequent cyclization,19 by
Received: November 15, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ol403304t | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
subsequent transition metal catalysis. Carbazole 4f exhibits two
chlorine functionalities and is a very useful building block for
polymer synthesis.36 The substitution pattern of the arene
moiety in position 9 of the carbazoles is highly adjustable and
tolerates electron-rich (4g) and electron-poor (4c) substitu-
ents. Even benzyl groups at the heterocyclic nitrogen persist
during the coupling process, and 4b and 4d can be cyclized in
excellent yields (Scheme 2). Additionally, methyl groups in
position 3 are tolerated (4d, 4e), which are important moieties
in various natural products.1
Scheme 1. Synthesis of the Starting Materials
Because of a statistic advantage and a higher electron density
the coupling of trimethoxylated arenes might be favored over
that of dimethoxylated arenes. Despite these facts, amine 3g
cyclizes preferentially at the dimethoxylated arene moieties
when being treated with the MoCl5/TiCl4 mixture (Scheme 3).
Scheme 3. Regioselective Coupling Reaction
bromine as the leaving functionality, the bromine is clearly
preferred in the amination process and chlorinated aryl amines
can be selectively constructed (see SI).
The oxidative cyclization of diarylamines to the correspond-
ing carbazoles is very efficiently performed by the MoCl5/TiCl4
mixture (Scheme 2). Because of the high oxidative power of the
Therefore, a 56% yield of the tetramethoxylated carbazole 4g is
isolated, whereas only 10% of the pentamethoxycarbazole 4h is
obtained. This can be attributed to the sterical repulsion
between a methoxy group in position 4 and C,H in position 5.
The benzyl group of derivative 4d can easily be cleaved off
using KOtBu/oxygen in DMSO/THF to yield carbazole 5
(Scheme 4).37 This molecule can easily be transformed to the
naturally occurring pyranocarbazole koenigicine (6) described
by Kapil and co-workers (Scheme 4).38
Scheme 2. Synthesis of Various Carbazoles (Newly Formed
Bonds in Bold and Red)
Scheme 4. Synthesis of Koenigicine (6)
The oxidative coupling reaction is not limited to the
generation of carbazoles, but is also capable of the formation
of dibenzofurans 8a, dibenzothiophenes 8b, and dibenzosele-
nophenes 8c in very good to excellent yields (Scheme 5).
The starting materials for these compounds are synthesized
by copper-catalyzed Ullmann-type reactions (see SI).
reagent and the electron-rich nature of the substrates, the
reaction is complete within a few minutes. In general,
tetramethoxylated substrates are preferred for the coupling
reaction and can be coupled in excellent isolated yields (4a−c).
If the positions 3 and 6 of the carbazoles are different
substituted, two methoxy groups are sufficient for the
cyclization in very good yields (4e, 4f). If these atoms are
unsubstituted, these positions are too reactive and side
reactions such as intermolecular coupling processes and
chlorination predominate. This cannot be avoided by low
temperature or high dilution. Consequently, di-(3-methoxy-
phenyl)phenylamine is not transformable to the corresponding
carbazole. Fluorine (4c, 4e) and even chlorine (4f) substituents
on the aromatic core are compatible with the reaction
conditions. Chlorine moieties are suitable leaving groups for
Scheme 5. Synthesis of 9-Oxa-, Thia-, and Selenafluorenes
B
dx.doi.org/10.1021/ol403304t | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
To demonstrate that the reactions are not a Nazarov-type
reaction,39 we confirmed that the formation of compounds 4a
and 8b is not feasible with TiCl4 as the sole reagent (entries 4,
9).
ACKNOWLEDGMENTS
■
Support by H.C. Stark GmbH, Goslar, Germany, by a donation
of MoCl5 is highly appreciated.
The unique performance of the MoCl5/TiCl4 mixture is
underlined by comparison with different oxidizing conditions
(Table 1). The combination with the TiCl4 is superior in all
REFERENCES
■
(1) For reviews, see: (a) Knolker, H.-J.; Reddy, K. R. Chem. Rev.
̈
2002, 102, 4303−4428. (b) Schmidt, A. W.; Reddy, K. R.; Knolker, H.-
̈
J. Chem. Rev. 2012, 112, 3193−3328. (c) Bauer, I.; Knolker, H.-J. Top.
̈
Table 1. Comparison of Coupling Conditions and Oxidizing
Mixtures
Curr. Chem. 2012, 309, 203−253.
(2) (a) Huang, H.; Feng, X.; Xiao, Z.; Liu, L.; Li, H.; Ma, L.; Lu, Y.;
Ju, J.; She, Z.; Lin, Y. J. Nat. Prod. 2011, 74, 997−1002. (b) Kaneko,
A.; Tsukada, M.; Fukai, M.; Suzuki, T.; Nishio, K.; Miki, K.; Kinoshita,
K.; Takahashi, K.; Koyama, K. J. Nat. Prod. 2010, 73, 1002−1004.
(c) Lin, C.-H.; Chang, H.-S.; Liao, C.-H.; Ou, T.-H.; Chen, I.-S.; Tsai,
I.-L. J. Nat. Prod. 2010, 73, 1628−1631. (d) Qu, J.; Xie, C.; Guo, H.;
Yu, W.; Lou, H. Phytochemistry 2007, 68, 1767−1774. (e) Shiu, W. K.
P.; Gibbons, S. Phytochemistry 2009, 70, 403−406. (f) Feng, Y.;
Carroll, A. R.; Addepalli, R.; Fechner, G. A.; Avery, V. M.; Quinn, R. J.
J. Nat. Prod. 2007, 70, 1790−1792. (g) Manniche, S.; Sprogøe, K.;
Dalsgaard, P. W.; Christophersen, C.; Larsen, T. O. J. Nat. Prod. 2004,
67, 2111−2112. (h) Carney, J. R.; Krenisky, J. M.; Williamson, R. T.;
Luo, J. J. Nat. Prod. 2002, 65, 203−205.
(3) (a) Queiroz, M.-J. R. P.; Abreu, A. S.; Solange, M.; Carvalho, D.;
Ferreira, P. M. T.; Nazareth, N.; Nascimento, M. S.-J. Bioorg. Med.
Chem. 2008, 16, 5584−5589. (b) Xu, Y.; Qian, X.; Yao, W.; Mao, P.;
Cui, J. Bioorg. Med. Chem. 2003, 11, 5427−5433. (c) Patrick, D. A.;
Hall, J. E.; Bender, B. C.; McCurdy, D. R.; Wilson, W. D.; Tanious, F.
A.; Saha, S.; Tidwell, R. R. Eur. J. Med. Chem. 1999, 34, 575−583.
(d) Wrobel, J.; Sredy, J.; Moxham, C.; Dietrich, A.; Li, Z.; Sawicki, D.
R.; Seestaller, L.; Wu, L.; Katz, A.; Sullivan, D.; Tio, C.; Zhang, Z.-Y. J.
Med. Chem. 1999, 42, 3199−3202.
entry
reagents
reaction conditions
product
yield
1
2
3
4
5
6
7
8
9
MoCl5/TiCl4
MoCl5
0 °C, 10 min
0 °C, 10 min
0 °C, 5 min
0 °C, 5 min
0 °C, 5 min
100 °C, 1 h
0 °C, 20 min
0 °C, 20 min
0 °C, 20 min
4e
4e
4a
4a
4a
4a
8b
8b
8b
72%
55%
95%
MoCl5/TiCl4
TiCl4
a
0%
a
FeCl3
<30%
a
Pd(OAc)2/HOAc
MoCl5/TiCl4
MoCl5
0%
95%
91%
a
TiCl4
0%
a
Determined by GC analysis.
cases (entries 1, 7). Neither ferric chloride (entry 5) nor
Pd(OAc)2/HOAc (2 equiv of palladium salt, entry 6) is capable
of accomplishing this transformation as effectively as the
MoCl5/TiCl4 mixture (entry 3). The use of molybdenum
pentachloride as the sole reagent leads to significantly more
byproducts resulting in a lower amount of the desired product
(entries 2 and 8).
(4) Blouin, N.; Leclerc, M. Acc. Chem. Res. 2008, 41, 1110−1119.
(5) Carbazoles: (a) Grazulevicius, J. V.; Strohriegl, P.; Pielichowski,
J.; Pielichowski, K. Prog. Polym. Sci. 2003, 28, 1297−1353.
(b) Boudreault, P.-L. T.; Blouin, N.; Leclerc, M. Adv. Polym. Sci.
2008, 212, 99−124. Dibenzothiophenes: (c) Dias, F.; Bourdakos, K.
N.; Jankus, V.; Moss, K. C.; Kamtekar, K. T.; Bhalla, V.; Santos, J.;
Bryce, M. R.; Monkman, A. P. Adv. Mater. 2013, 25, 3707−3714.
(d) Waghray, D.; Dehaen, W. Org. Lett. 2013, 15, 2910−2913. (e) Du,
C.; Ye, S.; Chen, J.; Guo, Y.; Liu, Y.; Lu, K.; Liu, Y.; Qi, T.; Gao, X.;
Shuai, Z.; Yu, G. Chem.Eur. J. 2009, 15, 8275−8282.
In summary, the oxidative coupling reaction using MoCl5/
TiCl4 provides a novel synthetic access to 9-heterofluorenes in
very good to excellent yields. Because of the power of the
reagent mixture, the transformations are completed at 0 °C
within a few minutes. The syntheses are easy to perform and
absolutely reliable. A large number of functional groups endure
the coupling conditions, e.g. benzyl groups, chloro, fluoro, and
methyl substituents. This approach is an inexpensive alternative
to oxidative coupling using palladium acetate, provides better
yields, and is capable of synthesizing N-substituted carbazoles.
This method offers new perspectives for the synthesis of
complex carbazole alkaloids and building blocks for functional
materials. This approach is an inexpensive alternative to
oxidative coupling using palladium acetate, provides better
yields, and is capable of synthesizing N-substituted carbazoles.
This method offers new perspectives for the synthesis of
complex carbazole alkaloids and building blocks for functional
materials.
(6) Carbazoles: (a) Lia, J.; Grimsdale, A. C. Chem. Soc. Rev. 2010, 39,
2399−2410. Dibenzothiophenes: (b) Gao, P.; Beckmann, D.; Tsao,
H. N.; Feng, X.; Enkelmann, V.; Pisula, W.; Mullen, K. Chem.
̈
Commun. 2008, 1548−1550. (c) Gao, P.; Feng, X.; Yang, X.;
Enkelmann, V.; Baumgarten, M.; Mullen, K. J. Org. Chem. 2008, 73,
̈
9207−9213. (d) Iwasaki, T.; Kohinata, Y.; Nishide, H. Org. Lett. 2005,
7, 755−758.
(7) Carbazoles: (a) Johnson, K. R. D.; Hayes, P. G. Organometallics
2009, 28, 6352−6361. Dibenzothiophenes: (b) Chantson, J. T.; Lotz,
S.; Ichharam, V. New J. Chem. 2003, 27, 1735−1740. (c) Hirotsu, M.;
Santo, K.; Hashimoto, H.; Kinoshita, I. Organometallics 2012, 31,
7548−7557. (d) Oster, S. S.; Grochowski, M. R.; Lachicotte, R. J.;
Brennessel, W. W.; Jones, W. D. Organometallics 2010, 29, 4923−
4931. (e) Shibue, M.; Hirotsu, M.; Nishioka, T.; Kinoshita, I.
Organometallics 2008, 27, 4475−4483.
ASSOCIATED CONTENT
* Supporting Information
■
S
(8) (a) Grelman, K. A.; Sherman, G. M.; Linschitz, H. J. Am. Chem.
Soc. 1963, 85, 1881−1882. (b) Parker, C. A.; Barnes, W. J. Analyst
1957, 82, 606−618. (c) Bowen, E. J.; Eland, J. H. D. Proc. Chem. Soc.
1963, 202. (d) Bhattacharyya, P.; Jash, S. S.; Dey, A. K. J. Chem. Soc.,
Chem. Commun. 1984, 1668−1669. (e) Hernandez-Perez, A. C.;
Collins, S. K. Angew. Chem., Int. Ed. 2013, 52, 1−6.
Experimental details and analytical data for all isolated
intermediates and products. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
(9) Islam, A.; Bhattacharyya, P.; Chakraborty, D. P. J. Chem. Soc.,
Chem. Commun. 1972, 537−539.
(10) Åkermark, B.; Eberson, L.; Jonsson, E.; Pettersson, E. J. Org.
Chem. 1975, 40, 1365−1367.
*E-mail: waldvogel@uni-mainz.de.
Notes
(11) (a) Knolker, H.-J.; Frohner, W. J. Chem. Soc., Perkin Trans. 1
̈
̈
The authors declare no competing financial interest.
1998, 173−176. (b) Knolker, H.-J.; Reddy, K. R.; Wagner, A.
̈
C
dx.doi.org/10.1021/ol403304t | Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Tetrahedron Lett. 1998, 39, 8267−8270. (c) Krahl, M. P.; Jager, A.;
S. R. Tetrahedron Lett. 2004, 45, 7911−7914. (g) Waldvogel, S. R.
Synlett 2002, 4, 622−624.
̈
Krause, T.; Knolker, H.-J. Org. Biomol. Chem. 2006, 4, 3215−3219.
̈
(28) (a) For review, see: Waldvogel, S. R.; Trosien, S. Chem.
(d) Forke, R.; Krahl, M. P.; Krause, T.; Schlechtingen, G.; Knolker, H.-
̈
Commun. 2012, 48, 9109−9119. (b) Waldvogel, S. R.; Frohlich, R.;
J. Synlett 2007, 268−272. (e) Forke, R.; Jager, A.; Knolker, H. J. Org.
̈
̈
̈
Schalley, C. A. Angew. Chem., Int. Ed. 2000, 39, 2472−2475.
(c) Waldvogel, S. R.; Wartini, A. R.; Rasmussen, P. H.; Rebek, J.
Tetrahedron Lett. 1999, 40, 3515−3517. (d) Waldvogel, S. R.; Aits, E.;
Biomol. Chem. 2008, 6, 2481−2583. (f) Forke, R.; Krahl, M. P.;
Dabritz, F.; Jager, A.; Knolker, H. J. Synlett 2008, 1870−1876.
̈
̈
̈
(f) Knolker, H. J.; O’Sullivan, N. Tetrahedron 1994, 50, 10893−10908.
̈
Holst, C.; Frohlich, R. Chem. Commun. 2002, 1278−1279. (e) Trosien,
̈
(g) Knolker, H. J.; Reddy, K. R. Heterocycles 2003, 60, 1049−1052.
̈
S.; Schollmeyer, D.; Waldvogel, S. R. Synthesis 2013, 45, 1160−1164.
(29) Trosien, S.; Waldvogel, S. R. Org. Lett. 2012, 14, 2976−2979.
(h) Knolker, H. J. Chem. Lett. 2009, 38, 8−13.
̈
(12) Bittner, S.; Krief, P.; Massil, T. Synthesis 1991, 215−216.
(13) Åkermark, B.; Oslob, J. D.; Heuschert, U. Tetrahedron Lett.
1995, 36, 1325−1326.
(30) (a) Kramer, B.; Frohlich, R.; Waldvogel, S. R. Eur. J. Org. Chem.
̈
2003, 3549−3554. (b) Kramer, B.; Waldvogel, S. R. Angew. Chem., Int.
Ed. 2004, 43, 2446−2449.
(14) (a) Hagelin, H.; Oslob, J. D.; Åkermark, B. Chem.Eur. J. 1999,
5, 2413−2416. (b) Liegault, B.; Lee, D.; Huestis, M. P.; Stuart, D. R.;
Fagnou, K. J. Org. Chem. 2008, 73, 5022−5028. (c) Watanabe, T.;
Ueda, S.; Inuki, S.; Oishi, S.; Fujii, N.; Ohno, H. Chem. Commun. 2007,
4516−4518. (d) Watanabe, T.; Oishi, S.; Fujii, N.; Ohno, H. J. Org.
Chem. 2009, 74, 4720−4726. (e) Kumar, V. P.; Gruner, K. K.; Kataeva,
(31) (a) Hackeloer, K.; Schnakenburg, G.; Waldvogel, S. R. Org. Lett.
̈
2011, 13, 916−919. (b) Hackeloer, K.; Waldvogel, S. R. Tetrahedron
̈
Lett. 2012, 53, 1579−1581.
(32) (a) Boshta, N.; Bomkamp, M.; Schnakenburg, G.; Waldvogel, S.
R. Chem.Eur. J. 2010, 16, 3459−3466. (b) Boshta, N.; Bomkamp,
M.; Schnakenburg, G.; Waldvogel, S. R. Eur. J. Org. Chem. 2011,
1985−1992.
O.; Knolker, H.-J. Angew. Chem., Int. Ed. 2013, 52, 11073−11077.
̈
(15) Zeller, K.-P.; Petersen, H. Synthesis 1975, 532−533.
(33) (a) Rempala, P.; Kroulik, J.; King, B. T. J. Am. Chem. Soc. 2004,
́ ́
(16) (a) Sanz, R.; Fernandez, Y.; Castroviejo, M. P.; Perez, A.;
126, 15002−15003. (b) Hackeloer, K.; Waldvogel, S. R. Eur. J. Org.
̈
́
Fananas, F. J. J. Org. Chem. 2006, 71, 6291−6294. (b) Wesch, T.;
̃
Chem. 2011, 6314−6319. (c) Mirk, D.; Willner, A.; Frohlich, R.;
̈
́
Berthelot-Brehier, A.; Leroux, F. R.; Colobert, F. Org. Lett. 2013, 15,
Waldvogel, S. R. Adv. Synth. Catal. 2004, 346, 675−681.
(34) Wolfe, J. P.; Buchwald, S. L. Angew. Chem., Int. Ed. 1999, 38,
2490−2493.
(17) (a) Sirringhaus, H.; Friend, R. H.; Wang, C.; Leuninger, J.;
2413−2416.
Mullen, K. J. Mater. Chem. 1999, 9, 2095−2101. (b) Umemoto, T.;
̈
(35) Harris, M. C.; Buchwald, S. L. J. Org. Chem. 2000, 65, 5327−
Ishihara, S. J. Am. Chem. Soc. 1993, 115, 2156−2164. (c) Shang, X.;
Chen, W.; Yao, Y. Synlett 2013, 24, 851−854.
5333.
̂
(36) Bouchard, J.; Belletete, M.; Durocher, G.; Leclerc, M.
(18) (a) Liebeskind, L. S.; Wang, J. J. Org. Chem. 1993, 58, 3550−
3556. (b) Hashmi, A. S. K.; Yang, W.; Rominger, F. Chem.Eur. J.
2012, 18, 6576−6580. (c) Toguem, S.-M. T.; Malik, I.; Hussain, M.;
Iqbal, J.; Villinger, A.; Langer, P. Tetrahedron 2013, 69, 160−173.
(d) Toguem, S.-M. T.; Knepper, I.; Ehlers, P.; Dang, T. T.; Patonay,
T.; Langer, P. Adv. Synth. Catal. 2012, 354, 1819−1826. (e) Jackson, P.
M.; Moody, C. J. J. Chem. Soc., Perkin Trans. 1 1990, 681−687.
(19) (a) Biland-Thommen, A. S.; Raju, G. S.; Blagg, J.; White, A. J. P.;
Barrett, A. G. M. Tetrahedron Lett. 2004, 45, 3181−3184. (b) Radner,
F.; Eberson, L. J. Chem. Res., Miniprint 1996, 8, 2016−2350.
(20) Oliveira, A. M. A. G.; Raposo, M. M. M.; Oliveira-Campos, A.
M. F.; Griffiths, J.; Machadoc, A. E. H. Helv. Chim. Acta 2003, 86,
2900−2907.
Macromolecules 2003, 36, 4624−4630.
(37) Haddach, A. A.; Kelleman, A.; Deaton-Rewolinski, M. V.
Tetrahedron Lett. 2002, 43, 399−402.
(38) Sharma, R. B.; Verma, R. S.; Kapil, R. S. Experientia 1980, 36,
815.
(39) For a review, see: Santelli-Rouvier, C.; Santelli, M. Synthesis
1983, 429−442.
(21) Carvalho, C. F.; Sargent, M. V. J. Chem. Soc., Perkin Trans. 1
1984, 7, 1613−1620.
(22) (a) Ames, D. E.; Opalko, A. Synthesis 1983, 234−235.
(b) Parisien, M.; Valette, D.; Fagnou, K. J. Org. Chem. 2005, 70,
7578−7584. (c) Campeau, L.-C.; Parisien, M.; Jean, A.; Fagnou, K. J.
Am. Chem. Soc. 2006, 128, 581−590.
(23) (a) Wang, J.; Ferguson, D. M.; Kalyani, D. Tetrahedron 2013,
69, 5780−5790. (b) Nervig, C. S.; Waller, P. J.; Kalyani, D. Org. Lett.
2012, 14, 4838−4841.
(24) Gajera, J. M.; Gopalan, B.; Yadav, P. S.; Patil, S. D.; Gharat, L. A.
J. Heterocycl. Chem. 2008, 45, 797−801.
(25) (a) Huang, Q.; Fazio, A.; Dai, G.; Campo, M. A.; Larock, R. C. J.
Am. Chem. Soc. 2004, 126, 7460−7461. (b) Zhao, J.; Yue, D.; Campo,
M. A.; Larock, R. C. J. Am. Chem. Soc. 2007, 129, 5288−5295.
(c) Huang, Q.; Campo, M. A.; Yao, T.; Tian, Q.; Larock, R. C. J. Org.
Chem. 2004, 69, 8251−8257. (d) Zhao, J.; Larock, R. C. J. Org. Chem.
2006, 71, 5340−5348.
(26) (a) Xiao, B.; Gong, T.-J.; Liu, Z.-J.; Liu, J.-H.; Luo, D.-F.; Xu, J.;
Liu, L. J. Am. Chem. Soc. 2011, 133, 9250−9253. (b) Zhao, J.; Zhang,
Q.; Liu, L.; He, Y.; Li, J.; Li, J.; Zhu, Q. Org. Lett. 2012, 14, 5362−
5365.
(27) (a) Rempala, P.; Kroulik, J.; King, B. T. J. Org. Chem. 2006, 71,
5067−5081. (b) King, B. T.; Kroulik, J.; Robertson, C. R.; Rempala,
P.; Hilton, C. L.; Korinek, J. D.; Gortari, L. M. J. Org. Chem. 2007, 72,
2279−2288. (c) Danz, M.; Tonner, R.; Hilt, G. Chem. Commun. 2012,
48, 377−379. (d) Spurg, A.; Schnakenburg, G.; Waldvogel, S. R.
Chem.Eur. J. 2009, 15, 13313−13317. (e) Kramer, B.; Bergander,
K.; Waldvogel, S. R. Synthesis 2003, 1, 91−96. (f) Mirk, D.; Waldvogel,
D
dx.doi.org/10.1021/ol403304t | Org. Lett. XXXX, XXX, XXX−XXX