Synthesis of Quinoxaline Derivatives via Tandem Oxidative Azidation/Cyclization Reaction of N -Arylenamines

Article abstract of DOI:10.1021/acs.orglett.6b00148

A new method was developed for the synthesis of quinoxalines. This method employs N-arylenamines and TMSN₃ as the starting materials and implements two oxidative C-N bond-forming processes in a tandem pattern by using (diacetoxyiodo)benzene as the common oxidant. The present reaction conditions are mild and simple and thus are useful in practical synthesis. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.orglett.6b00148

Letter
pubs.acs.org/OrgLett
Synthesis of Quinoxaline Derivatives via Tandem Oxidative
Azidation/Cyclization Reaction of N‑Arylenamines
Haichao Ma, Dianjun Li, and Wei Yu*
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou,
Gansu 730000, China
S
* Supporting Information
ABSTRACT: A new method was developed for the synthesis
of quinoxalines. This method employs N-arylenamines and
TMSN3 as the starting materials and implements two oxidative
C−N bond-forming processes in a tandem pattern by using
(diacetoxyiodo)benzene as the common oxidant. The present
reaction conditions are mild and simple and thus are useful in
practical synthesis.
zide-involved C−N-forming reactions are of great
Recently, Spagnolo et al.,12 Zhang et al.,13 and our group14
Aimportance in the synthesis of nitrogen-containing found that the cyclization of iminyl radicals provides an effective
compounds.1 On the one hand, organic azides can be
means for the synthesis of a quinoxalin-2-one ring system, a sister
conveniently prepared by various types of ionic1,2 and radical
structure of quinoxaline. We envisioned that the quinoxaline
azidation reactions3,4 as well as transition-metal-catalyzed
motif might also be prepared through a similar process. Our
coupling reactions.5 On the other hand, organic azides can
literature survey shows that the quinoxaline-forming radical
cyclization was reported decades ago by McNab15 and Nanni et
al.,16 but the synthetic value of this reaction has not been
demonstrated by further study. To develop a method of practical
usefulness, and also to expand the azide-based C−N-forming
reactions, we designed a new tandem strategy for the synthesis of
a quinoxaline ring system. This strategy uses structurally simple
enamines as the starting material and features two consecutive
C−N-forming reactions with an azide ion as the nitrogen source.
Our synthetic strategy is outlined in Scheme 1. It was expected
that N-arylenamine (1) would undergo oxidative azidation to
react under a variety of conditions to form a new C−N bond.1,6
Recent studies show that organic azides are also reliable
precursors to aminyl and iminyl radicals.7,8 From the viewpoint
of synthetic efficiency, it is highly desirable to combine azidation
reactions with the azidyl-derived C−N-forming process in one
synthetic operation, in a way that two C−N bonds can be
constructed in a tandem or one-pot pattern.9
Quinoxaline is an important heterocyclic skeleton that appears
in many biologically and pharmaceutically significant com-
pounds (Figure 1). The commonly used methods to gain access
to quinoxaline derivatives comprise the Hinsberg−Korner
̈
Scheme 1. Synthetic Design toward a Quinoxaline Ring in
This Work
reaction and other relevant reactions that use 1,2-disubstituted
benzenes, such as 1,2-diaminobenzenes, ortho-nitroanilines, or 2-
fluoro-1-nitrobenzenes, as the substrates,10 whereas those
starting from simpler monosubstituted anilines were still lacking
until now.11
afford vinyl azide intermediate A; further oxidation of A would
result in the formation of iminyl radical B, from which
quinoxaline 2 could be formed via cyclization. Thus, the
realization of this plan hinged on two key steps: azidation of
enamine 1 and conversion of A to iminyl radical B.
Received: January 19, 2016
Figure 1. Several quinoxaline-containing drugs.
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00148
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Organic Letters
Letter
Studies by Chiba et al.17 and others18,19 show that vinyl azides
constitute a reliable source of iminyl radicals. Considering the
electron richness of 2-azidyl enamine A, we assumed that it
would be readily oxidized to iminyl radical B via a single electron
transfer process. The transformation from enamine 1 to A would
also be achieved using an oxidant in combination with an
azidating reagent. Previous studies demonstrate that the
azidation of olefins can be readily achieved under oxidative
conditions.3b,20 Thus, we anticipated that these two oxidative
steps could be combined in a tandem fashion by using one
oxidant. Hypervalent iodine reagents were believed to be suitable
oxidants to meet this requirement because they are not only
capable of oxidizing enamines21,22 but also can effect the
oxidative azidation of olefins including enamines.20h
The optimized reaction conditions (Table 1, entry 10) were
then applied to variously substituted enamine esters (1b−1x),
and the results are illustrated in Scheme 2. Under the indicated
Scheme 2. Preparation of Quinoxalines from Variously
a
Substituted Enamine Esters
Previous investigations by Zhao21 and our group22 show that
enamine esters can be readily oxidized by (diacetoxyiodo)-
benzene (DIB). Thus, at the initial stage of this study, we chose
compound ethyl-3-amino-3-phenyl acrylate (1a) as the substrate
and subjected it to TMSN3 and DIB. The desired reaction took
place readily at room temperature in DCM or CH3CN, giving
rise to quinoxaline product 2a in moderate yield (Table 1, entries
Table 1. Screening of the Reaction Conditions with 1a as the
a
Substrate
equiv of
DIB
equiv of
TMSN3
yield
c
b
entry
cat.
none
solv.
(%)
a
b
Reaction was conducted on a 0.5 mmol scale. Substrates are of Z
1
2
2.0
3.0
3.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
3.0
3.0
1.0
3.0
3.0
3.0
3.0
2.0
2.0
2.0
2.0
2.0
2.0
DCM
DCM
DCM
CH3CN
DCM
DCM
CH3CN
DMF
28
32
37
29
c
d
configuration. Ratio was determined by 1H NMR. Complex mixture
was generated. Structure of 2f was confirmed by single-crystal X-ray
analysis. See ref 23.
none
e
3
none
4
none
d
5
none
14
6
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuCl2
CuBr2
53
53
37
47
69
42
50
41
49
56
conditions, all tested substrates except ethyl 3-((2-methoxy-
phenyl)amino)-3-phenyl acrylate (1m) were converted to the
corresponding quinoxalines 2, albeit with yields that varied
according to the substitution patterns. It is interesting to see that
compounds 1l, 1n, 1o, and 1p, which incorporate an ortho-
substituent at the N-phenyl ring, also reacted to give quinoxa-
lines. This result is in contrast to that of our previous studies on
the cyclization of α-(aminocarbonyl)iminyl radicals, where an
ortho-substituent at the N-phenyl ring would prevent the
formation of a quinoxalin-2-one ring.14 This discrepancy can
be attributed to the relief of steric hindrance in 1l−1p, in which it
is the small hydrogen atom other than an alkyl group that is
attached to the nitrogen atom. The steric repulsion between the
ortho-substituent and the N-alkyl group is believed to be the
reason why the corresponding α-(aminocarbonyl)iminyl radicals
failed to cyclize to afford the quinoxalin-2-one ring. When meta-
substituted 1j and 1k were used as the substrates, the products
were the mixture of two regioisomers, with the sterically
hindered 2j-1 and 2k-1 being the major products. This selectivity
is in accordance with our previous observations.14
7
8
e
9
DMF
10
11
12
13
14
15
DMF
DCM
CH3CN
DMF
Cu(OAc)2
CuPF6
DMF
DMF
a
Reaction was carried out on a 0.2 mmol scale in 2 mL of solvent at
room temperature. Reaction time was 2−4 h unless otherwise
b
specified. With 10 mol % of copper salt used unless otherwise
c
d
e
specified. Isolated yield. Reaction time was 10 h. With 0.5 equiv of
CuCl2. DCM: dichloromethane. CuPF6 is in the form of Cu-
(CH3CN)4PF6.
1−5). Comparable yields were obtained when (ditrifluoro-
acetoxyiodo)benzene was used instead of DIB (not shown in
Table 1). Subsequent experiment indicated that using CuCl2 as
catalyst would improve the yield of 2a. Thus, when the reaction
was carried out in DMF with 10 mol % of CuCl2 as catalyst, 2a
was obtained in a yield of 69% (Table 1, entry 10). Several other
copper salts were also tested for their effectiveness, but they were
found to be inferior to CuCl2 under the current circumstance
(Table 1, entries 13−15).
Cyclization of aryliminoiminyl radicals to a quinoxaline ring
can be realized in two patterns: direct cyclization and initial
formation of a spirodienyl radical followed by 1,2-iminyl
migration (Scheme 3). Cyclization via the second pathway
would lead to the formation of rearrangement products,15 as both
iminyl groups in the spirodienyl radical intermediate could
migrate. In the present reactions, however, no rearrangement
B
DOI: 10.1021/acs.orglett.6b00148
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Proposed Reaction Mechanism
Scheme 6. Test on the Scope of This Method (2)
products were obtained. This result is in accordance with that
reported by Nanni et al.16 and thus supports that the cyclization
of radical B follows exclusively path (a). Apart from this radical
mechanism, there is another possibility that transformation from
A to final product 2 follows the nonradical pathway shown in
Scheme 4. Although we cannot determine which mechanism is
Compound 10 is obviously derived from further oxidation of the
formed azidyl intermediate.24
In summary, we have developed a new and efficient method for
the synthesis of quinoxaline derivatives. This method uses readily
accessible N-arylenamines as the precursors and takes advantage
of a tandem oxidative azidation/cyclization process to build two
C−N bonds consecutively with TMSN3 as the nitrogen source.
As the current reaction can take place rapidly under mild and
simple conditions, it can be applied to practical synthesis. Despite
these advantages, however, the current protocol has some
limitation in scope, and further work is being done in our group
to explore better reaction conditions to solve this problem.
Scheme 4. Alternative Nonradical Mechanism
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00148.
more plausible at the current stage, one thing is certain: oxidation
of compound A must be a fast process because under no
circumstance could it be isolated from the reaction mixture.
To further examine the scope of this method, we applied it
next to compounds 3−6 (Schemes 5 and 6). As shown in Scheme
General experimental procedures, characterization data
for the substrates and products, 1H NMR and 13C NMR
spectra of the substrates and products (PDF)
Crystallographic data for compound 2f (CIF)
AUTHOR INFORMATION
Corresponding Author
■
Scheme 5. Test on the Scope of This Method (1)
*E-mail: yuwei@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The authors thank the National Natural Science Foundation of
China (No. 21372108) for financial support.
■
REFERENCES
■
(1) (a) Scriven, E. F. V.; Turnbull, K. Chem. Rev. 1988, 88, 297−368.
(b) Brase, S.; Gil, C.; Knepper, K.; Zimmermann, K. Angew. Chem., Int.
̈
Ed. 2005, 44, 5188−5240. (c) Organic Azides: Syntheses and Applications;
Brase, S., Banert, K., Eds.; Wiley & Sons, 2010.
̈
(2) (a) Lubriks, D.; Sokolovs, I.; Suna, E. J. Am. Chem. Soc. 2012, 134,
15436−15442. (b) Deng, Q.-H.; Bleith, T.; Wadepohl, H.; Gade, L. H. J.
Am. Chem. Soc. 2013, 135, 5356−5359. (c) Vita, M. V.; Waser, J. Org.
Lett. 2013, 15, 3246−3249. (d) Galligan, M. J.; Akula, R.; Ibrahim, H.
Org. Lett. 2014, 16, 600−603.
5, compound 3 reacted under the indicated conditions to afford
the desired product 7 in a yield of 35%, but similar reaction did
not occur for compounds 4 and 5. In the case of 4, the only
isolable product was α-diazo ester 6; the reaction of 5, on the
other hand, produced a complex mixture.
The conditions shown in Scheme 2 also failed when applied to
compound 6. However, when the reaction was performed under
copper-free conditions, the quinoxaline product 9 was generated
in a moderate yield (Scheme 6), along with compound 10.
(3) (a) Panchaud, P.; Chabaud, L.; Landais, Y.; Ollivier, C.; Renaud, P.;
Zigmantas, S. Chem. - Eur. J. 2004, 10, 3606−3614. (b) Jimeno, C.;
Renaud, P. Radical Chemistry with Azides. In Organic Azides: Syntheses
and Applications; Brase, S., Banert, S. K., Eds.; Wiley & Sons, 2010; pp
̈
239−267. (c) Lapointe, G.; Kapat, A.; Weidner, K.; Renaud, P. Pure
Appl. Chem. 2012, 84, 1633−1641. (d) Song, W.; Kozhushkov, S. I.;
Ackermann, L. Angew. Chem., Int. Ed. 2013, 52, 6576−6578. (e) Vita, M.
V.; Waser, J. Angew. Chem., Int. Ed. 2015, 54, 5290−5292.
C
DOI: 10.1021/acs.orglett.6b00148
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(4) (a) Harschneck, T.; Hummel, S.; Kirsch, S. F.; Klahn, P. Chem. -
Eur. J. 2012, 18, 1187−1193. (b) Tang, C.; Jiao, N. J. Am. Chem. Soc.
2012, 134, 18924−18927. (c) Fan, Y.; Wan, W.; Ma, G.; Gao, W.; Jiang,
H.; Zhu, S.; Hao, J. Chem. Commun. 2014, 50, 5733−5736. (d) Sharma,
A.; Hartwig, J. F. Nature 2015, 517, 600−604. (e) Huang, X.; Bergsten,
T. M.; Groves, J. T. J. Am. Chem. Soc. 2015, 137, 5300−5303. (f) Liu, C.;
Wang, X.; Li, Z.; Cui, L.; Li, C. J. Am. Chem. Soc. 2015, 137, 9820−9823.
(g) Zhu, Y.; Li, X.; Wang, X.; Huang, X.; Shen, T.; Zhang, Y.; Sun, X.;
Zou, M.; Song, S.; Jiao, N. Org. Lett. 2015, 17, 4702−4705.
(h) Shinomoto, Y.; Yoshimura, A.; Shimizu, H.; Yamazaki, M.;
Zhdankin, V. V.; Saito, A. Org. Lett. 2015, 17, 5212−5215. (i) Ren,
R.; Zhao, H.; Huan, L.; Zhu, C. Angew. Chem., Int. Ed. 2015, 54, 12692−
12696.
(16) Camaggi, C. M.; Leardini, R.; Nanni, D.; Zanardi, G. Tetrahedron
1998, 54, 5587−5598.
(17) (a) Chiba, S.; Wang, Y.-F.; Lapointe, G.; Narasaka, K. Org. Lett.
2008, 10, 313−316. (b) Wang, Y.-F.; Toh, K. K.; Chiba, S.; Narasaka, K.
Org. Lett. 2008, 10, 5019−5022. (c) Wang, Y.-F.; Chiba, S. J. Am. Chem.
Soc. 2009, 131, 12570−12572. (d) Wang, Y.-F.; Toh, K. K.; Ng, E. P. J.;
Chiba, S. J. Am. Chem. Soc. 2011, 133, 6411−6421. (e) Ng, E. P. J.;
Wang, Y. F.; Chiba, S. Synlett 2011, 2011, 783−786. (f) Ng, E. P. J.;
Wang, Y. F.; Hui, W.-Q.; Lapointe, G.; Chiba, S. Tetrahedron 2011, 67,
7728−7737. (g) Wang, Y.-F.; Lonca, G. H.; Chiba, S. Angew. Chem., Int.
Ed. 2014, 53, 1067−1071. (h) Wang, Y.-F.; Lonca, G. H.; Le Runigo, M.;
Chiba, S. Org. Lett. 2014, 16, 4272−4275.
(18) Montevecchi, P. C.; Navacchia, M. L.; Spagnolo, P. J. Org. Chem.
1997, 62, 5846−5848.
(5) (a) Zhu, W.; Ma, D. Chem. Commun. 2004, 888−889.
(19) Wang, Q.; Huang, J.; Zhou, L. Adv. Synth. Catal. 2015, 357, 2479−
(b) Andersen, J.; Madsen, U.; Bjorkling, F.; Liang, X. Synlett 2005,
̈
2484.
2209−2213. (c) Monguchi, Y.; Maejima, T.; Mori, S.; Maegawa, T.;
Sajiki, H. Chem. - Eur. J. 2010, 16, 7372−7375. (d) Zhao, H.; Fu, H.;
Qiao, R. J. Org. Chem. 2010, 75, 3311−3316. (e) Markiewicz, J. T.;
Wiest, O.; Helquist, P. J. Org. Chem. 2010, 75, 4887−4890.
(6) Organic azides can undergo intramolecular C−H insertion
reactions by forming nitrenoids with transition metals: (a) Stokes, B.
J.; Dong, H.; Leslie, B. E.; Pumphrey, A. L.; Driver, T. G. J. Am. Chem.
Soc. 2007, 129, 7500−7501. (b) Stokes, B. J.; Richert, K. J.; Driver, T. G.
J. Org. Chem. 2009, 74, 6442−6451. (c) Sun, K.; Sachwani, R.; Richert,
K. J.; Driver, T. G. Org. Lett. 2009, 11, 3598−3601. (d) Stokes, B. J.; Liu,
S.; Driver, T. G. J. Am. Chem. Soc. 2011, 133, 4702−4705. (e) Nguyen,
Q.; Sun, K.; Driver, T. G. J. Am. Chem. Soc. 2012, 134, 7262−7265.
(f) Nguyen, Q.; Nguyen, T.; Driver, T. G. J. Am. Chem. Soc. 2013, 135,
620−623. (g) Kong, C.; Jana, N.; Driver, T. G. Org. Lett. 2013, 15, 824−
827. (h) Hennessy, E. T.; Betley, T. A. Science 2013, 340, 591−595.
(7) (a) Minozzi, M.; Nanni, D.; Spagnolo, P. Chem. - Eur. J. 2009, 15,
7830−7840. (b) Nanni, D.; Spagnolo, P. Unusual Radical Acceptors. In
Encyclopedia of Radicals in Chemistry, Biology and Materials;
Chatgilialoglu, C., Studer, A., Eds.; Wiley & Sons, 2012; Vol. 2, pp
1019−1058.
(20) (a) Matcha, K.; Narayan, R.; Antonchick, A. P. Angew. Chem., Int.
Ed. 2013, 52, 7985−7989. (b) Yuan, Y.; Shen, T.; Wang, K.; Jiao, N.
Chem. - Asian J. 2013, 8, 2932−2935. (c) Wei, X.-H.; Li, Y.-M.; Zhou, A.-
X.; Yang, T.-T.; Yang, S.-D. Org. Lett. 2013, 15, 4158−4161. (d) Zhang,
B.; Studer, A. Org. Lett. 2013, 15, 4548−4551. (e) Yin, H.; Wang, T.;
Jiao, N. Org. Lett. 2014, 16, 2302−2305. (f) Sun, X.; Li, X.; Song, S.; Zhu,
Y.; Liang, Y.-F.; Jiao, N. J. Am. Chem. Soc. 2015, 137, 6059−6066.
(g) Fumagalli, G.; Rabet, P. T. G.; Boyd, S.; Greaney, M. F. Angew.
Chem., Int. Ed. 2015, 54, 11481−11484. (h) Nocquet-Thibault, S.;
Rayar, A.; Retailleau, P.; Cariou, K.; Dodd, R. H. Chem. - Eur. J. 2015, 21,
14205−14210. (i) Yuan, Y.-A.; Lu, D.-F.; Chen, Y.-R.; Xu, H. Angew.
Chem., Int. Ed. 2016, 55, 534−538.
(21) (a) Li, X.; Du, Y.; Liang, Z.; Li, X.; Pan, Y.; Zhao, K. Org. Lett.
2009, 11, 2643−2646. (b) Yu, W.; Du, Y.; Zhao, K. Org. Lett. 2009, 11,
2417−2420.
(22) Wang, J.-Y.; Liu, S.-P.; Yu, W. Synlett 2009, 2009, 2529−2533.
(23) The structure of compound 2f was confirmed by single-crystal X-
ray analysis. CCDC 1441580 contains the supplementary crystallo-
graphic data. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
(24) (a) Shen, T.; Wang, T.; Qin, C.; Jiao, N. Angew. Chem., Int. Ed.
2013, 52, 6677−6680. (b) Zong, X.; Zheng, Q.-Z.; Jiao, N. Org. Biomol.
Chem. 2014, 12, 1198−1202. (c) Huang, X.; Jiao, N. Org. Biomol. Chem.
2014, 12, 4324−4328. (d) Huang, X.; Li, X.; Jiao, N. Chem. Sci. 2015, 6,
6355−6360.
(8) For recent examples using organic azides as iminyl radical
precursors in the synthesis of nitrogen heterocycles, see: (a) Chiba, S.;
Zhang, L.; Lee, J.-Y. J. Am. Chem. Soc. 2010, 132, 7266−7267. (b) Zhang,
L.; Lee, J.-Y.; Yamazaki, N.; Chiba, S. Synlett 2011, 2011, 2167−2170.
(c) Wang, W.-X.; Zhang, Q.-Z.; Zhang, T.-Q.; Li, Z.-S.; Zhang, W.; Yu,
W. Adv. Synth. Catal. 2015, 357, 221−226.
(9) (a) Kim, Y.; Kumar, M. R.; Park, N.; Heo, Y.; Lee, S. J. Org. Chem.
2011, 76, 9577−9583. (b) Goriya, Y.; Ramana, C. V. Chem. Commun.
2013, 49, 6376−6378. (c) Goriya, Y.; Ramana, C. V. Chem. Commun.
2014, 50, 7790−7792. (d) Ou, Y.; Jiao, N. Chem. Commun. 2013, 49,
3473−3475. (e) Jia, F.-C.; Xu, C.; Zhou, Z.-W.; Cai, Q.; Li, D.-K.; Wu,
A.-X. Org. Lett. 2015, 17, 2820−2823. (f) Jia, F.-C.; Zhou, Z.-W.; Xu, C.;
Cai, Q.; Li, D.-K.; Wu, A.-X. Org. Lett. 2015, 17, 4236−4239.
(10) (a) Mamedov, V. A.; Zhukova, N. A. Progress in Quinoxaline
Synthesis (Part 1). Progress in Heterocyclic Chemistry; Elsevier Ltd., 2012;
Vol. 24, pp 55−88. (b) Mamedov, V. A.; Zhukova, N. A. Progress in
Quinoxaline Synthesis (Part 2). Progress in Heterocyclic Chemistry;
Elsevier Ltd., 2013; Vol. 25, pp 1−45.
(11) Yang, Z.-J.; Liu, C.-Z.; Hu, B.-L.; Deng, C.-L.; Zhang, X.-G. Chem.
Commun. 2014, 50, 14554−14557.
(12) Bencivenni, G.; Lanza, T.; Leardini, R.; Minozzi, M.; Nanni, D.;
Spagnolo, P.; Zanardi, G. J. Org. Chem. 2008, 73, 4721−4724.
(13) Li, Z.-S.; Wang, W.-X.; Yang, J.-D.; Wu, Y.-W.; Zhang, W. Org.
Lett. 2013, 15, 3820−3823.
(14) (a) Li, D.; Yang, T.; Su, H.; Yu, W. Adv. Synth. Catal. 2015, 357,
2529−2539. (b) Li, D.; Ma, H.; Yu, W. Adv. Synth. Catal. 2015, 357,
3696−3702.
(15) (a) McNab, H. J. Chem. Soc., Perkin Trans. 1 1980, 2200−2204.
(b) McNab, H.; Smith, G. S. J. Chem. Soc., Chem. Commun. 1982, 996−
997. (c) McNab, H. J. Chem. Soc., Perkin Trans. 1 1982, 1941−1945.
(d) McNab, H. J. Chem. Soc., Perkin Trans. 1 1984, 371−376.
(e) McNab, H. J. Chem. Soc., Perkin Trans. 1 1984, 377−380.
(f) McNab, H.; Smith, G. S. J. Chem. Soc., Perkin Trans. 1 1984, 381−
384.
D
DOI: 10.1021/acs.orglett.6b00148
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