Splitting a Substrate into Three Parts: Gold-Catalyzed Nitrogenation of Alkynes by C-C and C≡C Bond Cleavage

Article abstract of DOI:10.1002/anie.201508347

A gold-catalyzed nitrogenation of alkynes for the synthesis of carbamides and amino tetrazoles through C-C and C≡C bond cleavages is described. A diverse set of functionalized carbamide and amino tetrazole derivatives were selectively constructed under mild conditions. The chemoselectivity can be easily switched by the selection of the acid additives. The reaction is characterized by its broad substrate scope, direct construction of high value products, easy operation under air, and mild conditions at room temperature. This chemistry provides a way to transform alkynes by splitting the substrate into three parts.

Full text of DOI:10.1002/anie.201508347

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201508347
German Edition: DOI: 10.1002/ange.201508347
À
C C Bond Cleavage
Splitting a Substrate into Three Parts: Gold-Catalyzed Nitrogenation of
À
ꢀ
Alkynes by C C and C C Bond Cleavage
Chong Qin+, Yijin Su+, Tao Shen, Xiaodong Shi,* and Ning Jiao*
Dedicated to Professor Xuelong Hou on the occasion of his 60th birthday
Abstract: A gold-catalyzed nitrogenation of alkynes for the
À
synthesis of carbamides and amino tetrazoles through C C
ꢀ
and C C bond cleavages is described. A diverse set of
functionalized carbamide and amino tetrazole derivatives
were selectively constructed under mild conditions. The
chemoselectivity can be easily switched by the selection of the
acid additives. The reaction is characterized by its broad
substrate scope, direct construction of high value products, easy
operation under air, and mild conditions at room temperature.
This chemistry provides a way to transform alkynes by splitting
the substrate into three parts.
A
s very common, readily available, and reactive functional
molecules, alkynes have been widely used in organic trans-
formations.[1] Transition-metal-catalyzed unstrained C C
À
bond cleavage represents an attractive but challenging
strategy in organic synthesis to discover new transforma-
tions.[2] In the past decades, significant progress has been
À
Scheme 1. The transformation of alkynes through C C bond cleavage.
ꢀ
made on the direct transformation of alkynes through C C
triple bond cleavage, including the alkyne metathesis,[3]
oxidation,[4] and nitrogenation reactions.[5] In these cases,
the original alkyne structure was split and located in two kinds
of products (Scheme 1a). Recently, the groups of Echavar-
ren[6] and Jiao[7] have successfully realized the direct trans-
formation of alkynes to tetrazoles[6] and amides[7] through
À
carbamides and amino tetrazoles through both C C and
C C bond cleavages (Scheme 1c). In this chemistry: 1) The
original alkyne structure was split into three parts and
reassembled in one product molecule. To our knowledge,
this is the first example of the direct transformation of alkynes
ꢀ
À
À
ꢀ
C C single bond cleavage, in which the alkyne structure was
split into two parts but, interestingly, located in one product
molecule (Scheme 1b).
through both C C single bond and C C triple bond cleavage
in one step; 2) The chemoselectivity can be easily controlled
by the reaction conditions. Therefore, the desired carbamides
and amino tetrazoles could be easily switched by the selection
of the acid additives; 3) The reaction is characterized by the
transformation of alkynes, the direct construction of high
value products, easy operation under air, and mild conditions
at room temperature.
Encouraged by these improvements, the discovery of new
À
alkyne transformations through C C bond cleavage is still
attractive but very challenging. Herein, we describe a Au-
catalyzed nitrogenation of alkynes for the synthesis of
[*] C. Qin,[+] T. Shen, Prof. N. Jiao
Gold catalysts have proven to be one of the most effective
catalysts for alkyne activation.[8,9] During the Au-catalyzed
amide synthesis,[7] trace amount of carbamides and amino
tetrazoles were detected in some cases. We therefore started
the investigation by probing the nitrogenation of phenyl-1-
hexyne (1a) with TMSN3 in the presence of Au- and Ag-
catalysts (Table 1). Interestingly, we found that the reaction
was significantly influenced by which acid was used. HOAc
and TFA (trifluoroacetic acid) failed to facilitate the trans-
formation. To our delight, in the presence of PTSA
(p-toluenesulfonic acid, pKa = À2.8 in water), the desired
multiple nitrogenation and hydration products carbamide 2a
and amino tetrazole 3a were obtained in yields of 21% and
13%, respectively (entry 1). The efficiency and selectivity of
this protocol were improved by changing PTSA to MSA
State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences, Peking University
Xue Yuan Rd. 38, Beijing 100191 (China)
Prof. N. Jiao
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences
Shanghai 200032 (China)
E-mail: jiaoning@pku.edu.cn
Dr. Y. Su,[+] Prof. X. Shi
Department of Chemistry, University of South Florida
Tampa, FL 33620 (USA)
E-mail: xmshi@usf.edu
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508347.
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Table 1: Optimization of the reaction conditions.[a]
(pKa = À11.9 in DCE) were added, the reaction generated 3a
in the yields of 48% and 36% respectively, probably owing to
the nucleophilicity of water being inhibited by strong acids.
Furthermore, we found that TCE is also the best medium for
this process, while solvents such as MeCN or PhMe are also
competent (entry 15, 55% yield; see the Supporting Infor-
mation). Similarly, adding TfOH and TMSN3 in two portions
led to a higher yield of amino tetrazoles with good chemo-
selectivity (entry 16, 66%). Control experiments demon-
strated that the reactions in the absence of Au catalyst did
not work (Supporting Information). Although the yields for
the preparation of amino tetrazoles and carbamides are
entry catalyst
(10 mol%)
acid (pKa) additive solvent Yield [%][b]
2a 3a
21 13
1
2
3
PPh3AuCl/Ag2CO3
PPh3AuCl/Ag2CO3
PTSA
(À2.8[c])
MSA
H2O
H2O
H2O
DCE
DCE
DCE
37 <5
28 <5
(À2.6[c])
MSA
(4-CF3-Ph)3PAuCl
/Ag2CO3
À
ꢀ
moderate, considering that both C C and C C bond are
cleaved and nitrogen and/or oxygen atoms are incorporated,
the efficiency for each step is high and acceptable.
4
5
6
7
PPh3AuNTf
PPh3AuCl/AgF
PPh3AuCl/AgSbF6
PPh3AuCl
MSA
MSA
MSA
MSA
MSA
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
–
DCE
DCE
DCE
DCE
DCE
DCE
DCM
TCE
33
42 <5
36 <5
<5
<5
<5
29 <5
58 <5
67 <5
<5 48
0
With optimal reaction conditions for the transformation
of alkynes to carbamides in hand, we explored the generality
of this reaction. As shown in Table 2, a broad range of
0
0
0
8
AgF
9
none
MSA
MSA
10
11
12
13
PPh3AuCl/AgF
PPh3AuCl/AgF
PPh3AuCl/AgF
PPh3AuCl/AgF
Table 2: Transformation of internal alkynes to carbamides.[a,b]
MSA
MSA[d]
TfOH
TCE
DCE
(À14[c],
À11.4[e])
H(NTf)2
(À11.9[e])
TfOH
14
PPh3AuCl/AgF
–
DCE
<5 36
15
16
PPh3AuCl/AgF
PPh3AuCl/AgF
–
–
TCE
TCE
<5 55
<5 66
TfOH[f]
2a 67%
2c 43%
2e 53%
2g 46%
2b 55%
2d 58%
2 f 50%
2h 51%
[a] Reaction conditions: 1a (0.5 mmol), catalyst (0.05 mmol), TMSN3
(2.0 mmol), additive (1.0 mmol) and acid (2.5 mmol) in solvent (2 mL)
at room temperature for 24 h under air. Amide byproducts[7] were
detected in some cases. [b] Yields of isolated products. [c] pKa value of
acid in water. [d] MSA (2.5 mmol) and TMSN3 (2.0 mmol) were added in
two portion in six hours. [e] pKa value of acid in 1,2-dichloroethane.
[f] TfOH (2.5 mmol) and TMSN3 (2.0 mmol) were added in two portion
in six hours. TFA=trifluoroacetic acid, PTSA=p-toluenesulfonic acid,
MSA=methanesulfonic acid.
(methanesulfonic acid, pKa = À2.6 in water), providing 37%
yield of 2a with trace amount of 3a formation (entry 2).
Moreover, evaluation of a number of different gold catalysts
revealed that PPh3AuCl was more efficient than others
(entries 3,4; see the Supporting Information). Next, we
determined that the choice of silver salt had a slight influence
on the reaction outcome (entries 5,6; see the Supporting
Information). In comparison, AgF turned out to be more
effective at mediating this transformation. The required
participation of both the gold catalyst and silver salt was
confirmed in control tests (entries 7–9). The choice of solvent
significantly influenced the transformation, with TCE proving
to be the optimal reaction medium (entries 10,11; see the
Supporting Information). Finally, we found that the addition
of MSA and TMSN3 in two portions provided slightly higher
yields, delivering carbamide 2a in 67% yield and with a good
selectivity with the formation of a trace amount of 3a
(entry 12). In the following investigations, two superacids
(pKa < À12 in water) were employed to change the selectivity
of the transformation (entries 13,14). Fortunately, when
TfOH (pKa = À14 in water and À11.4 in DCE) or H(NTf)2
[a] Reaction conditions: see entry 12, Table 1. [b] Yield of isolated
products.
differently substituted alkynes, bearing both electron-donat-
ing and halo-substitutents at the aromatic ring, reacted with
TMSN3 and H2O to form a series of carbamide molecules.
Electron-rich substrates were suitable for the reaction with
relatively high yields (2b and 2c). Substituents at the meta-
position were tolerated for this transformation (2 f and 2g).
Encouraged by these results with internal alkynes, we then
investigated the nitrogenation and hydration reaction with
a range of terminal alkynes (Table 3). To our delight, although
amide and amino tetrazole byproducts were detected in some
cases, it was noteworthy that the substituted phenylacetylenes
gave the desired products in the presence of TfOH in good
efficiencies (Table 3). Compared with MSA, the usage of
TfOH showed higher efficiencies for terminal alkynes. As
shown in Table 3, electron-donating groups, such as -Me (2k)
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Table 4: Substrate scope of alkynes to amino tetrazoles.[a,b]
Table 3: Transformation of terminal alkynes to carbamides.[a,b]
2i 47%
2l 38%
2j 71%
2k 68%
2n 62%
3a 66%
3c 55%
3e 45%
3g 53%
3b 62%
3d 41%
3 f 63%
3h 67%
2m 69%
2o 38%
2p 51%
2q 64%
[a] Reaction conditions: 1 (0.5 mmol), PPh3AuCl (0.05 mmol), AgF
(0.05 mmol), TMSN3 (2.0 mmol), H2O (1.0 mmol) and TfOH
(2.5 mmol) in TCE (2 mL) at room temperature for 24 hour under air.
Amide byproducts[7] were detected in some cases [b] Yield of isolated
products.
[a] Reaction conditions: 1 (0.5 mmol), PPh3AuCl (0.05 mmol), AgF
(0.05 mmol), TMSN3 (2.0 mmol) and HOTf (2.5 mmol) in TCE (2 mL) at
room temperature for 24 h under air. [b] Yield of isolated product.
and -OMe, (2j), increase the overall efficiency. Notably, this
transformation readily generated halogen-substituted carba-
mides (2l and 2m), offering an opportunity for subsequent
cross-coupling, thereby facilitating expedient synthesis of
complex carbamide derivatives. Substituents at meta- and
ortho-positions also led to the desired products (2n–q).
We next focused on expanding the scope of alkynes for the
amino tetrazole preparation. As illustrated in Table 4, this
protocol for amino tetrazoles generically proceeded at room
temperature in good yields with an extensive range of phenyl
and alkyl substituted acetylenes (Table 4). With respect to the
aryl side, substrates with electron-donating substituents at the
para-position led to good yields (3b–3e). Moreover, meta-
substituted substrates were also accommodated by these
conditions (3 f and 3g). Unfortunately, sterically encumbered
substrates, such as alkynes with ortho-substituted phenyls,
failed to offer the desired products. In terms of the alkyl side,
phenyl was tolerated, furnishing amino tetrazoles in a good
yield (3h).
Scheme 2. Mechanistic studies of cascade reactions.
To gain insight into the mechanism, a proposed inter-
mediate vinyl azide 4 was tested under the standard con-
ditions, resulting in carbamide 2i in a yield of 65%
(Scheme 2a). Besides, in the absence of PPh3AuCl and AgF,
a 62% yield of 2i was obtained, showing that the catalyst was
not necessary in this step of the transformation. To further
explore this reaction, another probable intermediate carbo-
diimide 5 was synthesized and submitted to these trans-
formations (Scheme 2b). In the case of MSA and H2O, an
86% yield of 2a was generated from the hydration of 5.
Similarly, nitrogenation of 5 occurred in the presence of
TfOH and TMSN3, providing an 87% yield of 3a. Moreover,
addition of the gold catalyst had no obvious effect on either of
the reactions.
results strongly suggest that sulfoacid plays an important role
in the reaction mechanism, where the reaction results could
be controlled by the presence and strength of the sulfoacid.
On the basis of the above results, a plausible mechanism
was proposed (Scheme 3). Initially, the alkyne is activated by
cationic gold(I) generated in situ from the gold catalyst and
silver salt. Subsequent nucleophilic attack of TMSN3 on
intermediate A led to alkenyl azide B. Further protonation of
B generated azide cation intermediate C, which could be
transformed to intermediate D through an acid-catalyzed
rearrangement process.[10] Next, cation D was attacked by
TMSN3 once again, producing imino azide E. The nucleo-
philic attack of water was prevented in this step probably
owing to the protonation of H2O to H3O+ by strong or super
acids. The subsequent protonation of E took place in the
presence of sulfuric acid to form a-amino azide carbocation F,
Compound 2a failed to be transformed into 3a in the
presence of TMSN3 and TfOH (Scheme 2c). The above
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À
[2] For reviews of C C cleavage, see: a) R. H. Crabtree, Chem. Rev.
1985, 85, 245; b) B. Rybtchinski, D. Milstein, Angew. Chem. Int.
Ed. 1999, 38, 870; Angew. Chem. 1999, 111, 918; c) C.-H. Jun,
Chem. Soc. Rev. 2004, 33, 610; d) M. Tobisu, N. Chatani, Chem.
Soc. Rev. 2008, 37, 300; e) F. Chen, T. Wang, N. Jiao, Chem. Rev.
2014, 114, 8613; f) A. Dermenci, J. W. Coe, G. Dong, Org. Chem.
Front. 2014, 1, 567; g) H. Liu, M. Feng, X. Jiang, Chem. Asian J.
2014, 9, 3360; h) L. Souillart, N. Cramer, Chem. Rev. 2015, 115,
9410.
[3] For selected reviews, see: a) A. Fürstner, P. W. Davies, Chem.
Commun. 2005, 2307; b) H. Villar, F. Marcus, C. Bolm, Chem.
Soc. Rev. 2007, 36, 55.
[4] For examples of alkynes oxidation to acids, see: a) T. Shimada,
Y. Yamamoto, J. Am. Chem. Soc. 2003, 125, 6646; b) S. Datta, C.-
L. Chang, K.-L. Yeh, R.-S. Liu, J. Am. Chem. Soc. 2003, 125,
9294; c) Y. Liu, F. Song, S. Guo, J. Am. Chem. Soc. 2006, 128,
11332; d) A. Wang, H. Jiang, J. Am. Chem. Soc. 2008, 130, 5030.
[5] a) T. Shen, T. Wang, C. Qin, N. Jiao, Angew. Chem. Int. Ed. 2013,
52, 6677; Angew. Chem. 2013, 125, 6809; b) N. Okamoto, M.
Ishikura, R. Yanada, Org. Lett. 2013, 15, 2571; c) X. Zong, Q.
Zheng, N. Jiao, Org. Biomol. Chem. 2014, 12, 1198; d) X.-R.
Song, B. Song, Y.-F. Qiu, Y.-P. Han, Z.-H. Qiu, X.-H. Hao, X.-Y.
Liu, Y.-M. Liang, J. Org. Chem. 2014, 79, 7616.
Scheme 3. Proposed Mechanism.
which would be converted to G or carbodiimide H by
rearrangement and loss of a proton. Finally, nucleophilic
attack by H3O+ led to the desired carbamide product 2
through the tautomerization of the intermediate I.[11] Alter-
natively, 1,3-dipolar cycloaddition with azide led to the
desired amino tetrazole product 3.[12]
[6] M. Gaydou, A. M. Echavarren, Angew. Chem. Int. Ed. 2013, 52,
13468; Angew. Chem. 2013, 125, 13710.
[7] C. Qin, P. Feng, Y. Ou, T. Shen, T. Wang, N. Jiao, Angew. Chem.
Int. Ed. 2013, 52, 7850; Angew. Chem. 2013, 125, 8004.
[8] For reviews of gold-catalysis, see: a) E. JimØnez-NfflÇez, A. M.
Echavarren, Chem. Commun. 2007, 333; b) E. JimØnez-NfflÇez,
A. M. Echavarren, Chem. Rev. 2008, 108, 3326; c) D. J. Gorin,
B. D. Sherry, F. D. Toste, Chem. Rev. 2008, 108, 3351; d) V.
Michelet, P. Y. Toullec, J.-P. GenÞt, Angew. Chem. Int. Ed. 2008,
47, 4268; Angew. Chem. 2008, 120, 4338; e) A. S. K. Hashmi,
Chem. Rev. 2007, 107, 3180; f) A. Fürstner, P. W. Davies, Angew.
Chem. Int. Ed. 2007, 46, 3410; Angew. Chem. 2007, 119, 3478;
g) L. Zhang, J. Sun, S. A. Kozmin, Adv. Synth. Catal. 2006, 348,
2271; h) D. J. Gorin, F. D. Toste, Nature 2007, 446, 395; i) C.
Obradors, A. M. Echavarren, Acc. Chem. Res. 2014, 47, 902; j) L.
Zhang, Acc. Chem. Res. 2014, 47, 877.
[9] For recent examples of gold-catalyzed alkyne reactions, see:
a) E. P. A. Talbot, J. M. McKenna, F. D. Toste, Adv. Synth. Catal.
2014, 356, 687; b) W. Zi, F. D. Toste, J. Am. Chem. Soc. 2013, 135,
12600; c) J. Carreras, M. Livendahl, P. R. McGonigal, A. M.
Echavarren, Angew. Chem. Int. Ed. 2014, 53, 4896; Angew.
Chem. 2014, 126, 4996; d) E. S. Smirnova, A. M. Echavarren,
Angew. Chem. Int. Ed. 2013, 52, 9023; Angew. Chem. 2013, 125,
9193; e) R. M. P. Veenboer, S. Dupuy, S. P. Nolan, ACS Catal.
2015, 5, 1330; f) S. Shi, T. Wang, V. Weingand, M. Rudolph,
A. S. K. Hashmi, Angew. Chem. Int. Ed. 2014, 53, 1148; Angew.
Chem. 2014, 126, 1166; g) Y. Yu, W. Yang, D. Pflästerer, A. S. K.
Hashmi, Angew. Chem. Int. Ed. 2014, 53, 1144; Angew. Chem.
2014, 126, 1162; h) Z. Y. Yan, Y. Xiao, L. Zhang, Angew. Chem.
Int. Ed. 2012, 51, 8624; Angew. Chem. 2012, 124, 8752; i) Y. Xiao,
L. Zhang, Org. Lett. 2012, 14, 4662; j) Y. Wang, Z. Wang, Y. Li,
G. Wu, Z. Cao, L. Zhang, Nat. Commun. 2014, 5, 3470; k) R. R.
Singh, R.-S. Liu, Chem. Commun. 2014, 50, 15864; l) R. K.
Kawade, R.-S. Liu, Org. Lett. 2013, 15, 4094; m) C. Qi, H. Cong,
K. J. Cahill, P. Müller, R. P. Johnson, Jr., J. A. Porco, Angew.
Chem. Int. Ed. 2013, 52, 8345; Angew. Chem. 2013, 125, 8503;
n) H. Peng, N. G. Akhmedov, Y.-F. Liang, N. Jiao, X. Shi, J. Am.
Chem. Soc. 2015, 137, 8912; o) H. Peng, Y. Xi, N. Ronaghi, B.
Dong, N. G. Akhmedov, X. Shi, J. Am. Chem. Soc. 2014, 136,
13174.
In conclusion, we have developed an efficient gold-
catalyzed double carbon-carbon bond cleavage (both of
À
ꢀ
C C and C C bond are cleaved) of alkynes for the direct
synthesis of amino tetrazoles and carbamides. The original
alkyne structure was split into three parts and reassembled in
one product molecule. The transformation, mild conditions,
and high-value products make this protocol very practical and
attractive. The chemoselectivity can be easily switched by the
selection of the acid additives. Further applications and
mechanistic studies of the applications are ongoing in our
laboratory.
Acknowledgements
Financial support from National Basic Research Program of
China (973 Program) (grant No. 2015CB856600) and National
Natural Science Foundation of China (Nos. 21325206,
21172006), and National Young Top-notch Talent Support
Program are greatly appreciated. We thank Kai Wu in this
group for reproducing the results of 2o and 3h.
À
Keywords: carbamides · C C bond cleavage · gold ·
rearrangements · tetrazoles
How to cite: Angew. Chem. Int. Ed. 2016, 55, 350–354
Angew. Chem. 2016, 128, 358–362
[1] a) Organic Reactions, Wiley, Hoboken, 1994, 46; b) H. Takaya,
R. Noyori, Comprehensive Organic Synthesis (Ed.: B. M. Trost,
I. Fleming), Pergamon, Oxford, 1991; c) S. Nishimura, Hand-
book of Heterogeneous Catalytic Hydrogenation for Organic
Synthesis Wiley-Interscience, New York, 2001; d) A. H. Haines,
Methods for the Oxidation of Organic Compounds. Alkanes,
Alkenes, Alkynes, and Arenes, Academic Press, New York, 1985.
[10] a) A. Hassner, E. S. Ferdinandi, R. J. Isbister, J. Am. Chem. Soc.
1970, 92, 1672; b) R. D. Bach, G. J. Wolber, J. Org. Chem. 1982,
47, 239; c) F. Chen, C. Qin, Y. Cui, N. Jiao, Angew. Chem. Int. Ed.
2011, 50, 11487; Angew. Chem. 2011, 123, 11689; d) C. Qin, W.
Angew. Chem. Int. Ed. 2016, 55, 350 –354
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
353

Angewandte
Communications
Chemie
Zhou, F. Chen, Y. Ou, N. Jiao, Angew. Chem. Int. Ed. 2011, 50,
12595; Angew. Chem. 2011, 123, 12803; For other transforma-
tions with alkenyl azides, see: e) F.-L. Zhang, Y.-F. Wang, G. H.
Lonca, X. Zhu, S. Chiba, Angew. Chem. Int. Ed. 2014, 53, 4390;
Angew. Chem. 2014, 126, 4479; f) Y.-F. Wang, G. H. Lonca, S.
Chiba, Angew. Chem. Int. Ed. 2014, 53, 1067; Angew. Chem.
2014, 126, 1085; g) B. J. Stokes, H. Dong, B. E. Leslie, A. L.
Pumphrey, T. G. Driver, J. Am. Chem. Soc. 2007, 129, 7500; h) Z.
Liu, J. Liu, L. Zhang, P. Liao, J. Song, X. Bi, Angew. Chem. Int.
Ed. 2014, 53, 5305; Angew. Chem. 2014, 126, 5409; i) E. P. Farney,
T. P. Yoon, Angew. Chem. Int. Ed. 2014, 53, 793; Angew. Chem.
2014, 126, 812.
[11] a) A. R. Katritzky, P. -L. Nie, A. Dondoni, D. Tassi, Synth.
Commun. 1977, 7, 387; b) I. V. Kolesnikova, T. D. Petrova, V. E.
Platonov, V. A. Mikhailov, A. A. Popov, V. A. Savelova, J.
Fluorine Chem. 1988, 40, 217.
ˇ
ˇ
´
ˇ
[12] a) J. Svetlík, I. Hrusovsky, A. Martvon, Collect. Czech. Chem.
Commun. 1979, 44, 2982; b) J. Svetlik, A. Martvon, J. Lesko,
Chem. Zvesti 1979, 33, 521; c) O. Tsuge, S. Urano, K. Oe, J. Org.
Chem. 1980, 45, 5130.
Received: September 23, 2015
Published online: October 23, 2015
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