Hypervalent iodine catalyzed generation of nitrile oxides from oximes and their cycloaddition with alkenes or alkynes

Article abstract of DOI:10.1021/ol401815n

Hypervalent iodine catalyzed oxidation of aldoximes using oxone as a terminal oxidant generates nitrile oxides, which react with alkenes and alkynes to give the corresponding isoxazolines and isoxazoles in moderate to good yields. This reaction involves active hypervalent iodine species formed in situ from catalytic iodoarene and oxone in the presence of hexafluoroisopropanol in aqueous methanol solution.

Full text of DOI:10.1021/ol401815n

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Hypervalent Iodine Catalyzed Generation
of Nitrile Oxides from Oximes and their
Cycloaddition with Alkenes or Alkynes
Akira Yoshimura,* Kyle R. Middleton, Anthony D. Todora, Brent J. Kastern,
Steven R. Koski, Andrey V. Maskaev, and Viktor V. Zhdankin*
Department of Chemistry and Biochemistry, University of Minnesota;Duluth, Duluth,
Minnesota, 55812, United States
ayoshimu@d.umn.edu; vzhdanki@d.umn.edu
Received June 27, 2013
ABSTRACT
Hypervalent iodine catalyzed oxidation of aldoximes using oxone as a terminal oxidant generates nitrile oxides, which react with alkenes and
alkynes to give the corresponding isoxazolines and isoxazoles in moderate to good yields. This reaction involves active hypervalent iodine
species formed in situ from catalytic iodoarene and oxone in the presence of hexafluoroisopropanol in aqueous methanol solution.
In recent years, hypervalent iodine reagents have
emerged as environmentally friendly and efficient oxidiz-
ing reagents for various synthetically useful oxidative
transformations.1 One of the most impressive recent
achievements in the area of hypervalent iodine chemistry
has been the development of numerous catalytic reactions
utilizing organohypervalent iodine active species in the
iodine(I)/iodine(III) catalytic cycle.1m,2,3 In particular,
several hypervalent iodine catalyzed oxidative cyclization
reactions leading to the important heterocyclic systems of
carbazoles,2a benzimidazoles,2b oxoindoles,2c and numer-
ous γ-lactones2dꢀg have recently been reported. Most of
the iodine(I)/iodine(III) catalytic cycles utilize iodoarenes
as catalysts and m-chloroperoxybenzoic acid as the stoi-
chiometric oxidant.3 Herein, we report the first hyperva-
lent iodine catalyzed synthesis of isoxazole derivatives
(1) For a book and selected reviews on hypervalent iodine chemistry,
see: (a) Hypervalent Iodine Chemistry; Wirth, T., Ed.; Springer-Verlag:
Berlin, 2003. (b) Quideau, S.; Pouysegu, L.; Deffieux, D. Synlett 2008, 467.
(c) Zhdankin, V. V. Sci. Synth. 2007, 31a, 161, Chapter 31.4.1. (d)
Ochiai, M.; Miyamoto, K. Eur. J. Org. Chem. 2008, 4229. (e) Dohi, T.;
Kita, Y. Chem. Commun. 2009, 2073. (f) Yusubov, M. S.; Maskaev,
A. V.; Zhdankin, V. V. ARKIVOC 2011, (i), 370. (g) Liang, H.;
Ciufolini, M. A. Angew. Chem., Int. Ed. 2011, 50, 11849. (h) Uyanik,
M.; Ishihara, K. Aldrichimica Acta 2010, 43, 83. (i) Merritt, E. A.;
Olofsson, B. Synthesis 2011, 517. (j) Zhdankin, V. V. J. Org. Chem. 2011,
76, 1185. (k) Silva, J. L. F.; Olofsson, B. Nat. Prod. Rep. 2011, 28, 1722.
(l) Fernandez Gonzalez, D.; Benfatti, F.; Waser, J. ChemCatChem 2012,
4, 955. (m) Yusubov, M. S.; Zhdankin, V. V. Curr. Org. Synth. 2012, 9,
247–272. (n) Brown, M.; Farid, U.; Wirth, T. Synthesis 2013, 424.
(2) (a) Antonchick, A. P.; Samanta, R.; Kulikov, K.; Lategahn, J.
Angew. Chem., Int. Ed. 2011, 50, 8605. (b) Alla, S. K.; Kumar, R. K.;
Sadhu, P.; Punniyamurthy, T. Org. Lett. 2013, 15, 1334. (c) Fabry, D. C.;
Stodulski, M.; Hoerner, S.; Gulder, T. Chem.;Eur. J. 2012, 18, 10834.
(d) He, Y.; Pu, Y.; Shao, B.; Yan, J. J. Heterocycl. Chem. 2011, 48, 695.
(e) Liu, H.; Tan, C.-H. Tetrahedron Lett. 2007, 48, 8220. (f) Yan, J.;
Wang, H.; Yang, Z.; He, Y. Synlett 2009, 2669. (g) Zhou, Z.-S.; He,
X.-H. Tetrahedron Lett. 2010, 51, 2480.
(3) (a) Ochiai, M.; Takeuchi, Y.; Katayama, T.; Sueda, T.; Miyamoto,
K. J. Am. Chem. Soc. 2005, 127, 12244. (b) Dohi, T.; Maruyama, A.;
Yoshimura, M.; Morimoto, K.; Tohma, H.; Kita, Y. Angew. Chem., Int.
Ed. 2005, 44, 6193. (c) Braddock, D. C.; Cansell, G.; Hermitage, S. A.
Chem. Commun. 2006, 2483. (d) Ochiai, M. Chem. Rec. 2007, 7, 12. (e)
Dohi, T.; Minamitsuji, Y.; Maruyama, A.; Hirose, S.; Kita, Y. Org. Lett.
2008, 10, 3559. (f) Dohi, T.; Maruyama, A.; Takenaga, N.; Senami, K.;
Minamitsuji, Y.; Fujioka, H.; Cammerer, S. B.; Kita, Y. Angew. Chem.,
Int. Ed. 2008, 47, 3787. (g) Altermann, S. M.; Richardson, R. D.; Page,
T. K.; Schidt, R. K.; Holland, E.; Mohammed, U.; Paradine, S. M.;
French, A. N.; Richter, C.; Bahar, A. M.; Witulski, B.; Wirth, T. Eur. J.
Org. Chem. 2008, 5315. (h) Miyamoto, K.; Sei, Y.; Yamaguchi, K.;
Ochiai, M. J. Am. Chem. Soc. 2009, 131, 1382. (i) Ngatimin, M.; Lupton,
D. W. Aust. J. Chem. 2010, 63, 653. (j) Uyanik, M.; Yasui, T.; Ishihara,
K. Angew. Chem., Int. Ed. 2010, 49, 2175. (k) Rodriguez, A.; Moran, W.
Org. Lett. 2011, 13, 2220. (l) Zhou, Z.; He, X. Synthesis 2011, 207. (m)
Miyamoto, K.; Sakai, Y.; Goda, S.; Ochiai, M. Chem. Commun. 2012,
48, 982. (n) Guibault, A.-A.; Basdevant, B.; Wanie, V.; Legault, C. Y.
J. Org. Chem. 2012, 77, 11283. (o) Dohi, T.; Takenaga, N.; Nakae, T.;
Toyoda, Y.; Yamasaki, M.; Shiro, M.; Fujioka, H.; Maruyama, A.;
Kita, Y. J. Am. Chem. Soc. 2013, 135, 4558. (p) Xu, Y.; Yang, Z.; Hu, J.;
Yan, J. Synthesis 2013, 45, 370.
r
10.1021/ol401815n
XXXX American Chemical Society

using iodoarene as a catalyst and oxone (2KHSO5
KHSO4 K2SO4) as an inexpensive and environmentally
safe terminal oxidant.
3
3
Table 1. Optimization of Catalytic Heterocyclizationa
Isoxazolines and isoxazoles represent an important het-
erocyclic system commonly found in natural products,
bioactive compounds, and chiral ligands.4 Previously,
several groups reported the preparation of isoxazolines
and isoxazoles from aldoximes and alkenes or alkynes
using a stoichiometric amount of hypervalent iodine reagents
such as [hydroxy(tosyloxy)iodo]benzene,5 (dichloroiodo)-
benzene,6 iodosylbenzene,7 [bis(trifluoroacetoxy)iodo]-
benzene,8 and (diacetoxyiodo)benzene.9 All these reactions
involve the initial oxidation of aldoximes to nitrile oxides
followed by cycloaddition with the appropriate dipolarophile
to give the corresponding isoxazolines and isoxazoles. Several
other oxidants and catalysts (e.g., hypoiodite species,10a,b
3,3-dimethyldioxirane,10c NaOCl,10d N-chlorosuccinimide,10e
N-bromosuccinimide,10e NaBrO210f) have also been pre-
viously utilized in this reaction.
entry
ArI (equiv)
none
solvents (ratio)
3a (%)b
1
MeOH-HFIP-H2O (10:10:1)
MeOH-HFIP-H2O (10:10:1)
MeOH-H2O (20:1)
0
2
PhI (0.2)
PhI (0.2)
PhI (0.2)
PhI (0.2)
PhI (0.2)
PhI (0.2)
PhI (0.2)
PhI (0.2)
PhI (0.2)
PhI (0.2)
4-MeC6H4I (0.2)
74 (73)
35
3
4
MeOH-TFE-H2O (10:10:1)
40
5
MeOHꢀCH2Cl2ꢀH2O (10:10:1) 12
MeOHꢀMeCN-H2O (10:10:1) 11
6
7
MeOH-AcOEt-H2O (10:10:1)
6
8
MeOH-hexane-H2O (10:10:1) 49
MeOH-THF-H2O (10:10:1)
MeOH-toluene-H2O (10:10:1) 13
9
4
10
11
12
13
14
15
MeOH-HFIP (1:1)
1
MeOH-HFIP-H2O (10:10:1)
71
3,5-Me2C6H3I (0.2) MeOH-HFIP-H2O (10:10:1)
4-ClC6H4I (0.2) MeOH-HFIP-H2O (10:10:1)
4-CF3C6H4I (0.2) MeOH-HFIP-H2O (10:10:1)
85
Recently, our group reported that activated iodine(III)
species, the hydroxy(phenyl)iodonium ion [PhI(OH)]þ,
generated in situ from PhI and oxone, were effective for
the preparation of [bis(trifluoroacetoxy)iodo]arenes and
[bis(trifluoroacetoxy)iodo]perfluoroalkanes and for
Hofmann rearrangement of carboxamides.11 Several other
44
17
16c 3,5-Me2C6H3I (0.2) MeOH-HFIP-H2O (10:10:1)
17c 3,5-Me2C6H3I (0.1) MeOH-HFIP-H2O (10:10:1)
18c,d 3,5-Me2C6H3I (0.2) MeOH-HFIP-H2O (10:10:1)
87 (88)
66
65
a The cyclization of aldoxime and styrene was performed at 40 °C
for 4 h by using styrene 1a (1.2 equiv), aldoxime 2a (1 equiv), ArI (0ꢀ0.2
equiv), and oxone (2ꢀ3 equiv). b Yields of 3a determined from 1H NMR
spectra of reaction mixture (numbers in parentheses show yields of
isolated 3a). c Reaction was performed at rt for 24 h. d 2 equiv of oxone
were used.
(4) Handbook of Heterocyclic Chemistry, 3rd ed.; Katritzky, A. R.,
Ramsden, C. A., Joule, J. A., Zhdankin, V. V.; Elsevier: Oxford, 2010. (b)
Jaeger, V.; Colinas, P. A. Nitrile Oxide. In Synthetic Applications of 1,3-
Dipolar Cycloaddition Chemistry Toward Heterocycles and Natural
Products; Padwa, A., Pearson, W. H., Eds.; Chemistry of Heterocyclic
Compounds; Wiley: Hoboken, NJ, 2002; Vol. 59, p 361. (c) Kozikowski,
A. P. Acc. Chem. Res. 1984, 17, 410. (d) Nair, V.; Suja, T. D. Tetrahedron
2007, 63, 12247. (e) Tapadar, S.; He, R.; Luchini, D. N.; Billadeau,
D. D.; Kozikowski, A. P. Bioorg. Med. Chem. Lett. 2009, 19, 3023. (f)
Takenaka, K.; Nagano, T.; Takizawa, S.; Sasai, H. Tetrahedron:
Asymmetry 2010, 21, 379.
(5) (a) De, S. K.; Mallik, A. K. Tetrahedron Lett. 1998, 39, 2389. (b)
Raihan, M. J.; Kavala, V.; Kuo, C.-W.; Raju, R.; Yao, C.-F. Green
Chem. 2010, 12, 1090.
(6) Radhakrishna, A. S.; Sivaprakash, K.; Singh, B. B. Synth. Com-
mun. 1991, 21, 1625.
(7) (a) Tanaka, S.; Ito, M.; Kishikawa, K.; Khmoto, S.; Yamamoto,
M. Nippon Kagaku Kaishi 2002, 3, 471. (b) Chatterjee, N.; Pandit, P.;
Halder, S.; Patra, A.; Maiti, D. K. J. Org. Chem. 2008, 73, 7775.
(8) (a) Jawalekar, A. M.; Reubsaet, E.; Rutjes, F. P. J. T.; van Delft,
F. L. Chem. Commun. 2011, 47, 3198. (b) Niu, T.-f.; Lv, M.-f.; Wang, L.;
Yi, W.-b.; Cai, C. Org. Biomol. Chem. 2013, 11, 1040.
(9) (a) Das, B.; Holla, H.; Mahender, G.; Banerjee, J.; Reddy, M. R.
Tetrahedron Lett. 2004, 45, 7347. (b) Das, B.; Holla, H.; Mahender, G.;
Venkateswarlu, K.; Bandgar, B. P. Synthesis 2005, 1572. (c) Mendelsohn,
B. A.; Lee, S.; Kim, S.; Teyssier, F.; Aulakh, V. S.; Ciufolini, M. A. Org.
Lett. 2009, 11, 1539. (d) Frie, J. L.; Jeffrey, C. S.; Sorensen, E. J. Org. Lett.
2009, 11, 5394. (e) Turner, C. D.; Ciufolini, M. A. ARKIVOC 2011, (i),
410. (f) Jen, T.; Mendelsohn, B. A.; Ciufolini, M. A. J. Org. Chem. 2011,
76, 728.
(10) (a) Yoshimura, A.; Zhu, C.; Middleton, K. R.; Todora, A. D.;
Kastern, B. J.; Maskaev, A. V.; Zhdankin, V. V. Chem. Commun. 2013,
49, 4800. (b) Minakata, S.; Okumura, S.; Nagamachi, T.; Takeda, Y.
Org. Lett. 2011, 13, 2966. (c) Tanaka, S.; Kohmoto, S.; Yamamoto, M.;
Yamada, K. Nippon Kagaku Kaishi 1992, 420. (d) Kizer, D. E.; Miller,
R. B.; Kurth, M. J. Tetrahedron Lett. 1999, 40, 3535. (e) Gi, H.-J.; Xiang,
Y.; Schinazi, R. F.; Zhao, K. J. Org. Chem. 1997, 62, 88. (f) Moriya, O.;
Nakamura, H.; Kageyama, T.; Urata, Y. Tetrahedron Lett. 1989, 30,
3987.
(11) (a) Zagulyaeva, A. A.; Yusubov, M. S.; Zhdankin, V. V. J. Org.
Chem. 2010, 75, 2119. (b) Yusubov, M. S.; Nemykin, V. N.; Zhdankin,
V. V. Tetrahedron 2010, 66, 5745. (c) Zagulyaeva, A. A.; Banek, C. T.;
Yusubov, M. S.; Zhdankin, V. V. Org. Lett. 2010, 12, 4644. (d)
Yoshimura, A.; Middleton, K. R.; Luedtke, M. W.; Zhu, C.; Zhdankin,
V. V. J. Org. Chem. 2012, 77, 11399.
groups have also reported a hypervalent iodine catalytic
reaction using oxone as the terminal oxidant.12
In the search for the organohypervalent iodine(III)-
catalyzed oxidative cycloaddition of aldoxime and alkene,
we have investigated the reaction of styrene 1a (1.2 equiv),
benzaldoxime 2a (1 equiv), and oxone as a terminal
oxidant in the presence of catalytic amounts of iodoarenes
under different conditions and using various solvents (see
Table 1; for additional details, see Table S1).
In the absence of iodoarene as a precatalyst, the desired
isoxazoline product was not formed from alkene, aldox-
ime, and oxone (Table 1, entry 1). A study of different
solvent systems (Table 1, entries 2ꢀ10) has demonstrated
that the presence of 1,1,1,3,3,3-hexafluoroisopropanol
(HFIP) in the reaction mixture dramatically changes the
results of this cyclization. The accelerating effect of HFIP
on the reactions of hypervalent iodine species was pre-
viously reported by several groups.13 The addition of a
small amount of water is required to increase the solubility
of oxone in the reaction mixture (Table 1, entry 11). Out of
several iodoarenes tested, 3,5-Me2C6H3I was found to be
(12) (a) Thottumkara, A. P.; Bowsher, M. S.; Vinod, T. K. Org. Lett.
2005, 7, 2933. (b) Schulze, A.; Giannis, A. Synthesis 2006, 257. (c)
Uyanik, M.; Akakura, M.; Ishihara, K. J. Am. Chem. Soc. 2009, 131,
251. (d) Ojha, L. R.; Kudugunti, S.; Maddukuri, P. P.; Kommareddy,
A.; Gunna, M. R.; Dokuparthi, P.; Gottam, H. B.; Botha, K. K.;
Parapati, D. R.; Vinod, T. K. Synlett 2009, 117. (e) Uyanik, M.;
Fukatsu, R.; Ishihara, K. Org. Lett. 2009, 11, 3470. (f) Thottumkara,
A. P.; Vinod, T. K. Org. Lett. 2010, 12, 5640. (g) Cui, L.-Q.; Liu, K.;
Zhang, C. Org. Biomol. Chem. 2011, 9, 2258.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 2. Catalytic Heterocyclization of Aldoximes with Alkenes
under Optimized Conditionsa
Table 3. Catalytic Heterocyclization of Aldoximes with Alkynes
under Optimized Conditionsa
a All reactions of alkyne 4 (1.2 equv), aldoxime 2 (1 equiv), 3,5-
Me2C6H3I (0.2 equiv), and oxone (3 equiv) were performed in MeOH/
HFIP/H2Oatrtfor24h. b Isolated yields of products 5. c Performed at 40 °C.
performed at rt for a longer reaction time (Table 1, entry 16).
Decreasing the amount of 3,5-Me2C6H3I from 0.2 to
0.1 equiv and oxone from 3 to 2 equiv leads to decreased
yields of product 3a (Table 1, entries 17, 18).
Using the optimized reaction conditions with 20 mol %
3,5-Me2C6H3I, we have investigated the conversion of
various substituted alkenes 1 to the respective isoxazolines
3 (Table 2). In general, all styrenes with either electron-
donating or -withdrawing substituents afforded corre-
sponding isoxazolines 3 in good yields (Table 2, entries
2ꢀ7). This reaction also gave good yields for aliphatic
alkenes (Table 2, entries 8ꢀ10). However, the reaction of
norbornene 1k with aldoxime 2a gave isoxazoline 3 in only
a 30% yield (Table 2, entry 11). As expected, p-methyl-
benzaldoximes 2b or p-chlorobenzaldoximes 2c also re-
acted smoothly with styrene 1a or 1-octene 1i giving the
respective isoxazolines in moderate to good yields. The
reaction of an aliphatic aldoxime, butylaldoxime 2d, with
styrene 1a gave a low yield of the product (Table 2, entry 16).
Compared to previously reported methods of the oxida-
tive cyclization of alkenes and aldoximes using stoichio-
metric hypervalent iodine reagents,5ꢀ9 the new catalytic
procedure affords isoxazolines in similar yields. Next, we
tried to prepare isoxazoles by replacing alkenes with alkynes.
The reaction of phenylacetylene 4a with benzaldoxime under
the optimized reaction conditions gave the desired isoxazoles
a All reactions of alkene 1 (1.2 equv), aldoxime 2 (1 equiv), 3,5-
Me2C6H3I (0.2 equiv), and oxone (3 equiv) were performed in MeOH-
HFIP-H2O at rt for 24 h. b Isolated yields of products 3.
the best precatalyst in this reaction (Table 1, entries
12ꢀ15). We have also found that this reaction can be
(13) (a) Kita, Y.; Tohma, H.; Hatanaka, K.; Takada, T.; Fujita, S.;
Mitoh, S.; Sakurai, H.; Oka, S. J. Am. Chem. Soc. 1994, 116, 3684. (b)
Miyamoto, K.; Tada, N.; Ochiai, M. J. Am. Chem. Soc. 2007, 129, 2772.
(c) Dohi, T.; Yamaoka, N.; Kita, Y. Tetrahedron 2010, 66, 5775. (d)
Gurard, K. C.; Guerinot, A.; Bouchard-Aubin, C.; Menard, M.-A.;
Lepage, M.; Beaulieu, M. A.; Canesi, S. J. Org. Chem. 2012, 77, 2121. (e)
Zhu, C.; Yoshimura, A.; Wei, Y.; Nemykin, V. V.; Zhdankin, V. V.
Tetrahedron Lett. 2012, 53, 1438. (f) Samanta, R.; Lategahn, J.;
Antonchick, A, P. Chem. Commun. 2012, 48, 3194.
Org. Lett., Vol. XX, No. XX, XXXX
C

5a in 75% yield (Table 3, entry 1). As expected, the reaction of
phenylacetylene 4a and benzaldoximes with p-Me or p-Cl
substituents afforded corresponding isoxazoles in good yields
(Table 3, entries 2, 3). However, the reaction with aliphatic
alkynes and various benzaldoximes required heating and
gave corresponding isoxazoles in moderate yields (Table 3,
entries 4ꢀ6). The reaction with para-substituted arylacety-
lenes and benzaldoximes 2a also gave low yields of isoxazoles
(Table 3, entries 7, 8).
Scheme 2. Proposed Reaction Mechanism
In order to gain additional information on the mechan-
ism of this catalytic reaction, we have performed several
control experiments (Scheme 1). It is known from the
literature that nitrile oxides, generated from aldoximes in
the absence of alkenes or alkynes, undergo cyclodimeriza-
tion or cycloaddition with nitrile oxide and aldoxime.8a,14
We have found that the reaction of 2a in the absence of
alkenes and alkynes under our conditions gave the cy-
cloaddition product, oxadiazole 7, in 38% preparative
yield (Scheme 1, eq 1). This result implies that nitrile oxide
6 can be generatedunderour conditions.14 Asexpected, the
reaction with protected benzaldoxime, O-methyloxime 8,
and 1a did not proceed, and 8 was recovered from the
reaction mixture (Scheme 1, eq 2). This result indicates that
the ligand exchange of aldoxime and hypervalent iodine
active species is important in this reaction.
[hydroxy(aryl)iodonium ion [ArI(OH)]þ 9, which is gen-
erated from ArI and oxone in aqueous HFIP, further
reacts with aldoxime 2 to give the hypervalent alkoxy
iodane 10 via ligand exchange. The intermediate 10 then
undergoes reductive elimination of ArI to give nitrile oxide
11. The regenerated ArI continues the next catalytic cycle.
The intermediate 11reacts with alkene or alkyne togive the
corresponding isoxazoline3 or isoxazole5. The presence of
HFIP may help to generate the highly reactive electron-
deficient hypervalent species 9 or 10, which accelerate
further steps of this catalytic cycle, such as ligand exchange
and oxidation of aldoxime.
Scheme 1. Control Experiments
In summary, we have developed a new procedure for the
cyclization of aldoxime and alkene or alkyne using cataly-
tic hypervalent iodine and oxone as a terminal oxidant in
aqueous HFIP. This reaction proceeds via initial forma-
tion of nitrile oxides, which react with alkenes and alkynes
to give corresponding isoxazolines and isoxazoles. The re-
action mechanism probably involves the electron-deficient
hypervalent iodine species formed from the hydroxy(aryl)-
iodonium ion and HFIP.
Acknowledgment. This work was supported by a re-
searchgrant from the NationalScience Foundation(CHE-
1262479).
From these control experimentsand based on previously
reported cyclizations of aldoximes with alkenes or alkynes
using stoichiometric hypervalent iodine reagents,5ꢀ9 we
propose the catalytic cyclization mechanism outlined
in Scheme 2. The activated hypervalent iodine species,
Supporting Information Available. Experimental pro-
cedures, spectral data for key compounds. This material
is available free of charge via the Internet at http://pubs.
acs.org.
(14) (a) Kelly, D. R.; Baker, S. C.; King, D. S.; de Silva, D. S.; Lord,
G.; Taylor, J. P. Org. Biomol. Chem. 2008, 6, 787. (b) Ghosh, H.; Patel,
B. K. Org. Biomol. Chem. 2010, 8, 384. (c) Schmidt, M. A.; Katipally, K.;
Ramirez, A.; Soltani, O.; Hou, X.; Zhang, H.; Chen, B.-C.; Qian, X.;
Deshpande, R. P. Tetrahedron Lett. 2012, 53, 3994.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX