Stereoselective Aminoiodination of Activated Alkynes with Organoiodine(III) Reagents and Amines via Multiple-Site Functionalization: Access to Iodinated Enamines and N-Aryl Indoles

Article abstract of DOI:10.1002/ejoc.201801725

A stereoselective aminoiodination of activated alkynes with PhI(OAc)₂ and amines via multiple-site functionalization to afford (Z)diethyl 2-(diphenylamino)-3-iodomaleate derivatives with superior yields has been described. The key feature of th

Full text of DOI:10.1002/ejoc.201801725

A Journal of
Accepted Article
Title: Stereoselective Aminoiodination of Activated Alkynes with
Organoiodine(III) Reagents and Amines via Multiple-Site
Functionalization: Access to Iodinated Enamines and N-Aryl
Indoles
Authors: Sagar Arepally, Ajoy Chamuah, Narenderreddy Katta, and
Duddu S Sharada
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201801725
Link to VoR: http://dx.doi.org/10.1002/ejoc.201801725
Supported by

10.1002/ejoc.201801725
European Journal of Organic Chemistry
Stereoselective Aminoiodination of Activated Alkynes with Organoiodine(III) Reagents and
Amines via Multiple-Site Functionalization: Access to Iodinated Enamines and N-Aryl
Indoles
Sagar Arepally, Ajoy Chamuah, Narenderreddy Katta, and Duddu S. Sharada*
Catalysis & Chemical Biology Laboratory, Department of Chemistry, Indian Institute of Technology Hyderabad,
Kandi – 502 285, Sangareddy, Telangana, INDIA, Phone: (040) 2301 7058; Fax: (040) 2301 6032,
E-mail: sharada@iith.ac.in , Web: http://drsharadagroup.wixsite.com/sharada
Abstract: A stereoselective aminoiodination of activated alkynes with PhI(OAc)2 and amines via multiple-
site functionalization to afford (Z)diethyl 2-(diphenylamino)-3-iodomaleate derivatives with superior yields
has been described. The key feature of this reaction is the incorporation of iodide and aryl group
concurrently in the same molecule in a stereoselective manner by employing PhI(OAc)2 as electrophilic
reagent as well as iodide and aryl group source. The high stereoselectivity of the reaction can be explained
based on the structure of the possible intermediates, the conformations of which controlled by the hydrogen
bonding, steric hindrance and electrostatic attractions. This reaction proceeds under mild conditions,
providing various dialkyl 2-(diphenylamino)-3-iodomaleates by a single operation starting from activated
alkynes. The robustness of our strategy is revealed by making of bis (dialkyl 2-(diphenylamino)-3-
iodomaleate) derivatives involving formation of four new C-N bonds and two C-I bonds in a single step.
The synthesized inactive 3° enamines (dialkyl 2-(diphenylamino)-3-iodomaleates) could be further
transformed into highly substituted indoles via Pd catalyzed C-H and C-I activation under non-acidic
conditions.
Introduction
Alkyne difunctionalizations have attracted intensive attention in recent years particularly in developing
regio and stereoselective reactions to access multifunctional alkene products with broad synthetic and
biological applications.1 For example the azidativehalo,2 sulfonative,3 aminohalogenative,4 silylzincative 5
iodoacyloxylations6 and perfluoroalkylative7 difunctionalizations of alkynes have been successfully
1
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
realized for the synthesis of various difunctionalization products. In this context, methods to produce
halogenated enamines via aminohalogenation of alkynes are very attractive owing to the utility of these
compounds in medicinal chemistry and organic synthesis.8 Accordingly, considerable efforts have been
made to their syntheses resulting in various synthetic strategies. For example cyclic enaminones,9 2-
alkynyloxycarbonyl azides/amines/O-propargyl carbamates10 have been typically employed as precursors
for halogenated enamines and catalyzed by metal or non-metals with various halogen sources.
Scheme 1. Aminohalogenation of Alkynes and Our Strategy
Furthermore, dehydrogenative aminohalogenation of alkenes via Pd catalysis was developed by
Jiang and co-workers11 for the synthesis of brominated enamines. Later Li et al.12 developed Nickel or
Diacetoxyiodobenzene promoted halogenation of enamines and enamides. However, examples of alkyne
aminohalogenation towards halogenated enamines are scarce (Scheme 1a &b).13 For instance, Headley &
Li and co-workers demonstrated the aminochlorination of arylalkynes with N,N
dichlorobenzenesulfonamide (Scheme 1a).13a In 2014, Liang and Zhang et al. described the
aminohalogenation of alkynes with N-haloimides activated by DBU (Scheme 1b).13b Despite this progress,
developing the multiple-site functionalization of C-C multiple bonds with reagents as both halogen source
as well as promoter in a stereoselective manner remains an intriguing challenge. As part of our ongoing
study on multiple functionalization reactions of alkynes and amines using azides as nitrogen source and
organoiodine compounds as promoter,14 we fancied to synthesize iodinated enamines directly from alkynes
and amines by using organoiodine reagents as both iodine and aryl group source.
2
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
Scheme 2. Plausible Conformations of Intermediates for Stereoselectivity of the Reaction
Organoiodine(III) compounds are usually used as oxidants and electrophilic reagents where only
one ligand of iodine(III) is removed by the substrate or replacement of both ligands with external
nucleophile followed by its decomposition into radicals12,15,16 However, sequential removal of two ligands
from the iodine(III) reagent by active C-H bond of substrate and incorporation of iodobenzene into the same
substrate would indubitably make the reactions atom economic, but such organic transformations are not
much explored.9 On the other hand, compared with known aminohalogenation of alkynes, this multiple-site
functionalization would be of great importance to produce synthetically potential halogenated enamines in
a stereoselective manner. The high stereoselectivity of the reaction can be explained based on the structure
of the possible intermediates. The conformations of intermediates are controlled by the hydrogen bonding,
steric hindrance and electrostatic attractions as shown in scheme 2. These type of interactions have never
been explored for stereoselective alkyne aminohalogenation. Herein we report the first example of a highly
stereoselective multiple-site functionalization of activated alkynes with amines and organoiodine(III)
(Scheme 1c) as the halo and aryl group source for haloenamines. We have also successfully employed these
3
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
relatively inactive 3° enamines in intramolecular cyclizations via Pd catalyzed C-H and C-I activation
resulting in highly functionalized indoles.
Table 1. Optimization reaction of Aminohalogenation of Alkynesa
entry
1
iodine(III) (equiv)
PIDA (1.5)
PIDA (1.5)
PIDA (1.5)
PIDA (1.5)
PIDA (1.5)
PIDA (1.5)
PIDA (1.5)
PIDA (1.5)
PIDA (1.5)
PIDA (1.5)
PIDA (1.5)
PIDA (1.3)
PIDA (2.0)
PIDA (2.5)
PIDA (2.5)
PIDA (1.8)
PIDA (2.5)
PIDA (2.5)
PIDA (2.5)
PIDA (2.5)
base (equiv)
-----
solvent yield (%)b
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCM
MeCN
THF
H2O
30
31c
0d
2
-----
3
-----
4
-----
0e
5
Cs2CO3 (1.0)
Cs2CO3 (1.5)
K2CO3 (1.5)
K3PO4 (1.5)
NaHCO3 (1.5)
Na2CO3 (1.5)
DABCO (1.5)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
Cs2CO3 (2.0)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
60
67
50
36
52
46
ND
55
73
84
84
71
54
61
ND
21
6
7
8
9
10
11
12
13
14
16
17
18
19
20
21
4
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
22
23
24
25
26
PIDA (2.5)
PIDA (2.5)
PIFA (2.5)
PhIO (2.5)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
Cs2CO3 (1.5)
EtOAc
DMF
DCE
DCE
DCE
30
17
ND
ND
ND
PhI(OH)(OTs)
(2.5)
aReaction conditions: Alkyne 1a (1 mmol), Amine 2a (1 mmol), PIDA 3a (2.5 mmol), Cs2CO3 (1.5), dry solvent 3
o
b
c
d
e
mL, rt (27 C) for 8 h; Isolated yield after silica column chromatography; Na2SO4 (4 equiv); TBAI (2 equiv);
NBS (2 equiv); ND = Not detected.
Results and discussion
We choose diethyl acetylenedicarboxylate 1a, aniline 2a and phenyliodine(III) diacetate (PIDA) 3a as the
model substrates to initiates our studies. At the beginning, the reaction was performed at room temperature
in dichloromethane (DCE). To our delight, we observed the formation of desired product 4aaa in 31% yield
with (Z)-configuration (Table 1, entry 1). The structure and stereochemistry of 4aaa was determined by the
X-ray crystallography analysis (Supporting information, SI). With this promising result in hand, we further
optimized the reaction conditions. When reaction was carried out in the presence of 4 equiv of Na2SO4 as
additive, the product 4aaa was yielded in 31 % after 18 h reaction time (Table 1, entry 2). No product was
obtained when the reaction was performed with TBAI and NBS as additives (entries 3&4). We then turned
our attention to the screening of bases to improve the reaction performance. When reaction performed with
Cs2CO3 pleasingly, the product 4aaa was obtained in 67% yield in 12 h (entries 5&6). Other bases such as
K2CO3, K3PO4, NaHCO3, Na2CO3, and DABCO did not improve the yield of the product (entries 7-11).
We then screened the equivalents of PIDA and Cs2CO3 (entries 12-17), 1.5 equiv of Cs2CO3 and 2.5 equiv
of PIDA were found to be the best conditions to afford the product 4aaa in 84 % yield after 8 h (entry 14).
Among the solvents tested, DCE was found to be the best solvent choice (entries 18-23). In our attempts to
further improve the yield, we have screened other organoiodine sources, unfortunately our attempts went
in vain (entries 24-26).
5
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
In order to realize the versatility of this newly developed method, we anticipated to apply it to a
series of amines having neutral, electron donating and withdrawing substituents on phenyl ring. This results
demonstrated that the substrates containing neutral and electron donating groups on phenyl ring afforded
the products (Table 2(A)) in good to very good yield. The weak electron withdrawing substituents on phenyl
ring resulted in relatively low yields (Table 2(B)). Presence of both weak electron withdrawing and
donating groups on the same phenyl ring gave the products fairly in good yields (Table 2, 4fab & 4maa).
Organoiodine with strong electron withdrawing groups (-CO2Me, NO2) gave the products in excellent
yields (Table 2(C)), this may be due to stabilization of intermediate VII (Scheme 4). It is worth to mention
here that, the robustness of our strategy is demonstrated by the synthesis of derivatives (Table 2 (D), 4a”aa
& 4a”ab) involving formation of four new C-N bonds and two C-I bonds in single step. After developing
successful syntheses of halogenated enamines, we envisaged that it would be appropriate to check the
scalability of our protocol for the synthesis halogenated enamines owing to their synthetic utility.
Accordingly, we performed the reaction on gram scale for the synthesis of halogenated enamine resulted in
72% yield (Table 2, 4aaa).
Table 2. Substrate Scope of Aminoiodination Reactiona
6
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
aReaction conditions: Alkyne 1a (1 mmol), Amine 2a (1 mmol), PIDA 3a (2.5 mmol), Cs2CO3 (1.5 mmol), dry DCE
3 mL, rt (27 o C) for 8-14 h; bIsolated yield after silica column chromatography.cCCDC 1579282 for 4aaa
Owing to the importance of halogenated enamines as flexible building blocks in organic synthesis,
we envisioned to employ them in organic transformations. In this direction, we thought to perform
transition-metal catalyzed intramolecular cyclizations by C-I and (sp2)C-H activation. However, less
reactive 3° enamines than 2° enamine homologues in transition-metal catalyzed C-H activation reactions
7
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
are scarce17 and poses a daunting challenge. The conversion of such enamines to indoles was accomplished
by improving the electrophilicity of Pd catalysts under acidic conditions.18
Herein we delighted to employ these iodinated enamines for smooth transformation to N-aryl
indoles via palladium-catalyzed intramolecular cyclization under non-acidic conditions. It was noted that
iodinated enamines containing weak electron withdrawing and electron donating groups on phenyl rings
provided the indoles with good yields than those having electron-withdrawing groups (Table 3). On the
basis of the results and previous reports18, we have proposed a plausible mechanism for palladium-catalyzed
intramolecular cyclization in Scheme S1 (see Supporting Information).
Table 3. N-aryl Indole Synthesis from Iodinated Enamines via Pd-Catalyzed Intramolecular
Cyclizaitona
aReaction conditions: Iodoenamine 4 (1 mmol), Pd(OAc)2 (10% mol), Cu(OAc)2 (1.2 mmol), dry DMF 1.5 mL, at 120 o C for 20
h. bIsolated yield after silica column chromatography. c Ratio estimated by H1 NMR.
8
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
To investigate the reaction mechanism, we performed couple of control experiments (Scheme 3).
The addition of TEMPO uninhibited the formation of 4aaa (Scheme 3(i)), suggesting that reaction might
proceed through ionic pathway. To further rule out the radical pathway, we performed a radical clock
experiment under standard conditions, which afforded only the halogenated enamine product 4rab and no
other ring-opened coupling products (Scheme 3(ii)).
Scheme 3. Control Experiments
Based on the above experiments and literature reports9 we have formulated the following plausible
mechanism (scheme 4). The intermolecular reaction of 1a and 2a generates hydroamination product I
which undergo ligand exchange with PIDA by the loss of acetic acid to give favored intermediate IV, which
would further transformed to intermediate (V) by the loss of one more acetic acid, which followed by
subsequent rearrangement leads to the formation of a possible zwitterion (VII) which then afford the final
product 4aaa. The involvement of ipso-substitution by negative nitrogen (VI) on the phenyl ring through a
five membered cyclic intermediate (VII) would facilitates the transfer of the Aryl group.
Scheme 4. Plausible Mechanism for Aminohalogenation of Alkynes
9
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
Conclusion
In summary, we have developed a novel and facile approach for the stereoselective alkyne
aminoiodination using amines, phenyliodine(III) diacetate (PIDA) as iodide and aryl moiety source as well
as promoter. The key feature of this reaction is the incorporation of iodide and aryl group concurrently in
the same molecule in a stereoselective manner. The advantages of this method are metal-free, mild reaction
conditions and scalability. We have also successfully employed these inactive iodinated 3° enamines in
intramolecular cyclizations via Pd catalyzed C-H and C-I activation resulting in highly functionalized
indoles. Further exploration of iodinated 3° enamines are currently under way in our laboratory.
Experimental Section
Aniline 1a (50 mg, 1 mmol) was taken in a dried round bottom flask, and dialkyl acetylenedicarboxylate
2a (91.3 mg, 1 mmol) was then added slowly with thorough mixing to form a homogeneous paste, then
add 0.5 mL DCE solvent (if required). After confirming the formation of enamine (monitored by TLC),
then added 4 mL DCE solvent, followed by addition of Cs2CO3 (261.959 mg, 1.5 mmol), phenyliodine(III)
diacetate (PIDA) (431.614 mg, 2.5 mmol) in portion wise for 20 min. The progress of the reaction was
monitored by TLC. The reaction mixture was quenched by addition of saturated solution of NaHCO3 and
extracted with ethyl acetate (EtOAc), dried over MgSO4, and concentrated in vacuo. The residue was
purified through a silica gel column chromatography using petroleum ether/ethyl acetate (0.2/9.8) as eluent
to yield (209 mg, 84%). All compounds (4aaa-4rab) were unknown and confirmed by FTIR, 1H NMR, 13C
NMR and HR-MS spectral analyses.
10
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
AUTHOR INFORMATION
Corresponding Author
* E-mail: sharada@iith.ac.in
ACKNOWLEDGMENTS
We gratefully acknowledge the Department of Science and Technology-Science and Engineering Research
Board (DST-SERB-EMR/2016/1000952) New Delhi, India and Indian Institute of Technology Hyderabad
(IITH) for financial support. SA thanks CSIR, NRK thanks DST-SERB, New Delhi, India for the award of
research fellowship.
Keywords: multiple-site functionalization; alkyne aminoiodination; stereoselective; indoles; iodinated 3°
enamines; intramolecular cyclization.
References:
[1] For difunctionalization of alkynes see: (a) N. Iqbal, J. Jung, S. Park, E. J. Cho, Angew. Chem., Int. Ed.
2014, 53, 539-542; Angew. Chem. 2014, 126, 549. (b) P. G. Janson, I. Ghoneim, N. O. Ilchenko, K. J.
Szabo, Org. Lett. 2012, 14, 2882-2885. (c) Y. Xiang, Y. Kuang, J. Wu, Org. Chem. Front. 2016, 3,
901-905. (d) G.-C. Ge, X.-J. Huang, C.-H. Ding, S.-L. Wan, L.-X. Dai, X.-L. Hou, Chem. Commun.
2014, 50, 3048-3051. (e) Y. Lv, W. Pu, Q. Chen, Q. Wang, J. Niu, Q. Zhang, J. Org. Chem. 2017, 82,
8282-8289. (f) J. Sun, G. Zheng, T. Xiong, Q. Zhang, J. Zhao, Y. Li, Q. Zhang, ACS Catal. 2016, 6,
3674-3678. (g) G. Zheng, Y. Li, J. Han, T. Xiong, Q. Zhang, Nat. Commun. 2015, 6, 7011.
[2] (a) N. Okamoto, T. Sueda, H. Minami, Y. Miwa, R. Yanada, Org. Lett. 2015, 17, 1336-1339. (b) G.
Zheng, J. Sun, Y. Liu, S. Yang, Y. Li, H. Sun, Q. Zhang. J. Org. Chem. 2017, 82, 12813-12820.
[3] (a) G. C. Senadi, B.-C. Guo, W.-P. Ju, J.-J. Wang, Chem. Commun. 2016, 52, 11410-11413. (b) K. Sun,
X. Wang, F. Fu, C. Zhang, Y. Chen, L. Liu, Green Chem. 2017, 19, 1490-1493.
[4] (a) R. Wang, H. Zou, Y. Xiong, N. Yi, W. Deng, J. Xiang, Org. Biomol. Chem. 2017, 15, 3964-3967.
(b) S. Li, P. Xiong, L. Zhu, X. Qian, H. Xu, Eur. J. Org. Chem. 2016, 3449-3455.
[5] C. Fopp, E. Romain, K. Isaac, F. Chemla, F. Ferreira, O. Jackowski, M. Oestreich, A. Perez-Luna, Org.
Lett. 2016, 18, 2054-2057.
[6] D. L. Priebbenow, R. W. Gable, J. Baell, J. Org. Chem. 2015, 80, 4412-4418.
[7] (a) X. Wang, A. Studer, Org. Lett. 2017, 19, 2977-2980. (b) Z. Li, A. García-Domínguez, C. Nevado,
J. Am. Chem. Soc. 2015, 137, 11610-11613.
11
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201801725
European Journal of Organic Chemistry
[8] (a) B. Stanovnik, J. Svete, Chem. Rev. 2004, 104, 2433-2480. (b) X-L. Yun, W-Y. Bi, J-H. Huang, Y.
Liu, D. Zhang-Negrerie, Y-F. Du, K. Zhao Tetrahedron Lett. 2012, 53, 5076-5080. (c) E. Kim, C. Lee,
Angew. Chem. Int. Ed. 2012, 51, 12303; Angew. Chem. 2012, 124, 12469. (d) J. Q. Chen, Y. L.
Wei, G. Q. Xu, Y. M. Liang, P. F. Xu, Chem. Commun. 2016, 52, 6455.
[9] (a) Y. Chen, T. Ju, J. Wang, W. Yu, Y. Du, K. Zhao, Synlett 2010, 2, 231-234. (b) D. Bhattacherjee, V.
Thakur, A. Shil, P. Das, Adv. Synth. Catal. 2017, 359, 2201-2208. (c) S. Riedmüller1, B. J.
Nachtsheim, Beilstein J. Org. Chem. 2013, 9, 1202–1209. (d) N. Miralles, R. M. Romero, E.
Ferna´ndez, K. Muniz, Chem. Commun. 2015, 51, 14068-14071. (e) S. G. Modha, M. F. Greaney, J.
Am. Chem. Soc. 2015, 137, 1416-1419.
[10] R. Wang, H. Zou, Y. Xiong, N. Yi, W. Deng, J. Xiang, Org. Biomol. Chem. 2017, 15, 3964-3967.
[11] X. Ji, H. Huang, W. Wu, H. Jiang, J. Am. Chem. Soc. 2013, 135, 5286-5289.
[12] (a) L. Xing, C. Li, J. Org. Chem. 2015, 80, 10000-10008. (b) S. Manna, K. Matcha, A. P. Antonchick,
Angew. Chem. Int. Ed. 2014, 53, 8163-8166; Angew. Chem. 2014, 126, 8302 –8305 (c) A. P.
Antonchick, R. Samanta, K. Kulikov, J. Lategahn, Angew. Chem. Int. Ed. 2011, 50, 8605-8608;
Angew. Chem. 2011, 123, 8764–8767.
[13] (a) S. Karur, S. R. S. S. Kotti, X. Xu, J. F. Cannon, A. Headley, G. Li, J. Am. Chem. Soc. 2003, 125,
13340-13341. (b) M. Li, H. Yuan, B. Zhao, F. Liang, Zhang J. Chem. Commun. 2014, 50, 2360-2363.
[14] (a) S. Arepally, V. N. Babu, M. Bakthadoss, D. S. Sharada, Org Lett. 2017, 19, 5014-5017. (b) A.
Sagar, S. Vidyacharan, A. H. Shinde, D. S. Sharada, Org. Biomol. Chem. 2016, 14, 4018-4022. (c) S.
Arepally, V. N. Babu, A. Polu, D. S. Sharada, Eur. J. Org. Chem. 2018, 5700–5705.
[15] (a) S. Okumura, Y. Takeda, K. Kiyokawa, S. Minakata, Chem. Commun. 2013, 49, 9266-9268. (b)
W.Yu, Y. Du, K. Zhao Org. Lett. 2009, 11, 2417-2420. (c) J. Sun, D. Zhang-Negrerie, Y. Du, K. Zhao,
J. Org. Chem. 2015, 80, 1200-1206. (d) X. Zhang, W. Hou, D. Zhang-Negrerie, K. Zhao, Y. Du, Org.
Lett. 2015, 17, 5252-5255. (e) N. Zhang, R. Cheng, D. Zhang-Negrerie, Y. Du, K. Zhao, J. Org.
Chem. 2014, 79,10581-10587.
[16] (a) S. Manna, P.O. Serebrennikova, I.A. Utepova, A.P. Antonchick, O.N. Chupakhin, Org.
Lett. 2015, 17, 4588-4591. (b) R. Narayan, S. Manna, A. P. Antonchick, Synlett 2015, 26, 1785-1803.
[17] (a) H. Ge, M. J. Niphakis, G. I. Georg, J. Am. Chem. Soc. 2008, 130, 3708-3709. (b) S. Pankajakshan,
Y.-H. Xu, J. K. Cheng, M. T. Low, T.-P. Loh, Angew. Chem., Int. Ed. 2012, 51, 5701; Angew. Chem.
2012, 124, 5799. (c) X. Wang, B. J. Turunen, M. W. Leighty, G. I. Georg, Tetrahedron Lett. 2007, 48,
8811-8814. (d) B. Kranke, H. Kunz, Org. Biomol. Chem. 2007, 5, 349-354. (e) D. I. Comins, S. P.
Joseph, X. Chen, Tetrahedron Lett. 1995, 36, 9141-9144.
[18] X.-L. Lian, Z.-H. Ren, Y.-Y. Wang, Z.-H. Guan, Org. Lett. 2014, 16, 3360-3363.
[19] M. Iinuma, K. Moriyama, H.Togo, Synlett 2012, 23, 2663-2666.
12
This article is protected by copyright. All rights reserved.