Cascade alkylarylation of substituted N-allylbenzamides for the construction of dihydroisoquinolin-1(2H)-ones and isoquinoline-1,3(2H,4H)-diones

Article abstract of DOI:10.3762/bjoc.12.32

An oxidative reaction for the synthesis of 4-alkyl-substituted dihydroisoquinolin-1(2H)-ones with N-allylbenzamide derivatives as starting materials has been developed. The radical alkylarylation reaction proceeds through a sequence of alkylation and intr

Full text of DOI:10.3762/bjoc.12.32

Cascade alkylarylation of substituted N-allylbenzamides for
the construction of dihydroisoquinolin-1(2H)-ones and
isoquinoline-1,3(2H,4H)-diones
Ping Qian1, Bingnan Du1, Wei Jiao1,2, Haibo Mei1, Jianlin Han*1 and Yi Pan*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 301–308.
1School of Chemistry and Chemical Engineering, State Key
Laboratory of Coordination Chemistry, Nanjing University, Nanjing,
210093, China and 2Institute for Chemistry & BioMedical Sciences,
Nanjing University, Nanjing, 210093, China
doi:10.3762/bjoc.12.32
Received: 17 December 2015
Accepted: 04 February 2016
Published: 17 February 2016
Email:
Jianlin Han* - hanjl@nju.edu.cn; Yi Pan* - yipan@nju.edu.cn
Associate Editor: C. Stephenson
* Corresponding author
© 2016 Qian et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
C(sp3)–H bond functionalization; cyclization;
dihydroisoquinolin-1(2H)-one; N-allylbenzamides; oxidation
Abstract
An oxidative reaction for the synthesis of 4-alkyl-substituted dihydroisoquinolin-1(2H)-ones with N-allylbenzamide derivatives as
starting materials has been developed. The radical alkylarylation reaction proceeds through a sequence of alkylation and intramo-
lecular cyclization. The substituent on the C–C double bond was found to play a key role for the progress of the reaction to give the
expected products with good chemical yields. Additionally, N-methacryloylbenzamides were also suitable substrates for the current
reaction and provided the alkyl-substituted isoquinoline-1,3(2H,4H)-diones in good yield.
Introduction
The direct and selective functionalization of an unactivated sp3 developed, such as the Minisci reaction with heteroarenes
C–H bond, which belongs to an effective strategic approach in [16,17], radical addition to unsaturated bonds [18,19],
green and sustainable chemistry, has attracted significant decarboxylative alkenylation of cycloalkanes with aryl vinylic
research attention [1-4]. This fascinating approach has obvious carboxylic acids [20,21], trifluoromethylthiolation [22], thiola-
advantages in functional group transformation and construction tion [23,24], alkenylation [25,26], dehydrogenation−olefination
of biological heterocycles, due to its high efficiency and waste and esterification [27,28], radical addition/1,2-aryl migration
reduction [5-10]. The pioneering works were focused on the [29], cascade alkylation-initiated cyclization [30,31] and other
cross-dehydrogenative coupling (CDC) reactions of alkanes, radical reactions [32-34]. Due to their low polarity and high
which were reported by Li and other groups [11-15]. Recently, bond-dissociation energy, the functionalization of unactivated
several types of reactions with alkanes as substrates have been sp3 C–H bonds in simple alkanes remains as a challenging task.
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Beilstein J. Org. Chem. 2016, 12, 301–308.
The direct cascade, 1,2-alkyarylation of alkenes to construct alkyarylation of substituted N-allylbenzamides with alkanes
multi-substituted heterocycles has been considered as an effi- affording 4-alkyl-substituted dihydroisoquinoline-1(2H)-ones as
cient organic synthetic strategy, which is often featured by a the product.
new ring and dual C–C bond formation in one process [35-42].
Recently, the group of Liu reported a cascade alkylarylation of Results and Discussion
N-alkyl-N-phenylacryamide with simple alkanes resulting in Initially, we selected N-methyl-N-(2-methylallyl)benzamide
alkyl-substituted oxindoles (Scheme 1a) [43]. However, cycli- (4a) and cyclohexane (2a) as model compounds (Table 1). As
zation of N-allylbenzamides initiated by the functionalization of shown in Table 1, we found that the reactions did not happen or
sp3 C–H bonds of simple alkanes remains unexplored. Very gave only a trace amount of the desired product with K2S2O8,
recently, our group developed a metal-free hydroxyalkylation- AIBN, BPO and TBHP as oxidants (Table 1, entries 1, 3–5).
initiated radical six-membered heterocycle formation reaction PhI(OAc)2 and DCP could be used as oxidants, providing a
of N-allylbenzamide with alcohols as radical partners. This pro- slightly better yield (Table 1, entries 2 and 6). Dramatically
vided 4-hydroxyalkyl-substituted 3,4-dihydroisoquinolin- higher chemical yields were found when TBPA and TBPB were
1(2H)-one derivatives (Scheme 1b) [44].
used for this reaction (Table 1, entries 7 and 8). DTBP was the
best oxidant choice, which afforded the highest chemical yield
Based on the knowledge gained from previous reports on the (53%, Table 1, entry 9). Then, a series of transition metal cata-
cyclization of N-allylbenzamide [44], we envisioned that the lysts, including CuI, FeCl2, FeBr2 and FeCl3, were added into
unactivated cycloalkanes (instead of alcohols) could act as the reaction with DTBP as the oxidant. However, no improve-
radical partners for this system. However, the reaction gave a ment was observed at all. Finally, an attempt to shorten the
complex mixture with 15% chemical yield of the expected reaction time to 24 h or to prolong the reaction time to 72 h
product (Scheme 1c). Fortunately, when a methyl substituent resulted in lower yield, thus indicating that 48 h was appro-
was introduced onto the C–C double bond of the N-allylbenz- priate for the completion of the reaction (Table 1, entries 14 and
amide substrate, the cyclization reaction proceeded smoothly 15). Changing the amount of DTBP was also not successful.
(Scheme 1c). Herein, we report a metal-free cascade 1,2- This is shown by the results presented in Table 1, entries 16 and
Scheme 1: Cascade 1,2-difunctionalization and cyclization to construct heterocycles.
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Beilstein J. Org. Chem. 2016, 12, 301–308.
Table 1: Optimization of typical reaction conditions.a
entry
oxidant (equiv)
catalyst (mol %)
temp (°C)
time (h)
yield (%)b
1
K2S2O8 (3.0)
PhI(OAc)2 (3.0)
AIBN (3.0)
BPO (3.0)
–
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
120
100
48
48
48
48
48
48
48
48
48
48
48
48
48
24
72
48
48
48
NR
19
2
–
3c
4c
5c
6c
7c
8c
9c
10
11
12
13
14
15
16
17
18
–
NR
trace
trace
19
–
TBHP (3.0)
DCP ( 3.0 )
TBPA (3.0)
TBPB (3.0)
DTBP (3.0)
DTBP (3.0)
DTBP (3.0)
DTBP (3.0)
DTBP (3.0)
DTBP (3.0)
DTBP (3.0)
DTBP (2.0)
DTBP (4.0)
DTBP (3.0)
–
–
–
45
–
38
–
53
CuI (10)
49
FeCl2 (10)
40
FeBr2 (10)
45
FeCl3 (10)
33
–
–
–
–
–
25
50
30
46
43
aReaction conditions: 4a (0.2 mmol), cyclohexane (2a, 2.0 mL), oxidant, 120 °C, under N2. bIsolated yield based on 4a. cAIBN = azodiisobutyronitrile;
BPO = benzoyl peroxide; TBHP = tert-butyl hydroperoxide, 70% in water; DCP = dicumyl peroxide; TBPA = tert-butyl peracetate; TBPB = tert-butyl
peroxybenzoate; DTBP = di-tert-butyl peroxide.
17 that clearly suggest that 3.0 equiv is the best choice. Finally, cally lower yield was obtained when a substrate with a
the reaction temperature was examined, and a lower chemical N-phenyl group (4j) was used.
yield was found when the reaction was performed at 100 °C
(Table 1, entry 16).
We then carried out another substrate scope examination for the
radical reactions using various cycloalkanes 2 and N-methyl-N-
With the optimized conditions developed, we then carried out a (2-methylallyl)benzamide (4a). As indicated in Scheme 3,
substrate generality study using various types of N-(2-methyl- several cycloalkanes were well-tolerated in this radical reaction
allyl)benzamides 4 to react with cyclohexane (2a). As shown in resulting in the corresponding product. In the case of cyclo-
Scheme 2, these cascade radical cyclization reactions are of pentane (2b), a slightly lower chemical yield was obtained
general use for the preparation of 4-alkyldihydroisoquinolin- (46%, 5ab), while the reactions of seven- and eight-membered
1(2H)-one derivatives 5. The substrates bearing methyl, ring cycloalkanes afforded the same level yield of the corre-
methoxy, halo and trifluoromethyl groups on the aromatic ring sponding product (5ac and 5ad). Finally, methylcyclohexane 2e
all worked well in the reaction, providing the target products was used as the substrate for the investigation of the regioselec-
with 31–65% yield. It should be noted that the reactions of sub- tivity. The reaction almost showed no regio- and stereoselectivi-
strates bearing disubstituted aromatic rings were possible but ty and afforded the corresponding products (5ae1–5ae5) with
resulted in lower yield (5fa and 5ga). On the other hand, the 42% total yield.
variation of the substituent on the nitrogen atom has also been
examined. In the cases of N-ethyl (4h), N-isopropyl (4i), and To extend the synthetic utility of this radical cyclization reac-
N-benzyl (4k), no obvious effect was found and almost the tion, N-methacryloyl-N-methylbenzamide derivatives 6 were
same level of yield was obtained as for 4a. However, a dramati- then tried as substrates for this reaction (Scheme 4). It should be
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Beilstein J. Org. Chem. 2016, 12, 301–308.
Scheme 2: Cyclization of cyclohexane (2a) with substituted N-(2-methylallyl)benzamide (reaction conditions: 4 (0.2 mmol), cyclohexane (2a, 2 mL),
DTBP (0.6 mmol), 120 °C, 48 h under nitrogen atmosphere. Isolated yield based on 4).
Scheme 3: Cyclization of cycloalkanes with N-methyl-N-(2-methylallyl)benzamide (reaction conditions: 4a (0.2 mmol), cycloalkanes 2 (2 mL), DTBP
(0.6 mmol), 120 °C, 48 h under nitrogen atmosphere. Isolated yield based on 4a). aThe total yield of isomers.
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Beilstein J. Org. Chem. 2016, 12, 301–308.
Scheme 4: Cyclization reaction of 6 with cyclohexane 2a (reaction conditions: 6 (0.2 mmol), cyclohexane 2a (2 mL), DTBP (0.6 mmol), 120 °C, 12 h
under nitrogen atmosphere. Isolated yield based on 6).
mentioned that only a few radical precursors, such as the ened to 12 h where all of the starting material 6 is consumed.
TMSCF3 reagent [45] and ethers [46], were developed for such Firstly, the variation of the substituents on the nitrogen atom
a cyclization system. Fortunately, the substrate with the intro- was investigated. We found that by changing the methyl group
duction of a carbonyl group worked well in this system with into one of the bulkier groups, the chemical yield significantly
moderate to good chemical yields (33–86%). Due to the exis- decreased (7ba–7ea). It is worth mentioning that the N-phenyl-
tence of the carbonyl group, the reaction time could be short- N-methacryl-substituted substrate 6d works much better than 4j
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Beilstein J. Org. Chem. 2016, 12, 301–308.
in this system, resulting in higher yield (55%, 5ja). The substit- and the reaction was completely inhibited, affording a cyclo-
uent on the aromatic ring did not greatly affect the reaction effi- hexane radical-trapped compound (Scheme 5b). This implies
ciency, and methyl (7fa and 7ka), methoxy (7ga, 7la and 7ma), that the current transformation is a radical process. Finally, an
chloro (7ia), bromo (7ja), and phenyl (7na) were well-tolerated obvious competing kinetic isotope effect (KIE) was found with
in this system. However, in the case of the strong electron-with- the ratio of 9.3:1 (kH:kD) when the reaction of 6a was per-
drawing group (fluoro, 7ha), the yield clearly decreased, and formed with cyclohexane and [D]-cyclohexane (Scheme 5c).
only 31% yield was obtained. It was noted that the reactions This discloses that the cleavage of the C(sp3)−H bond to form
showed almost no evident regioselectivity, and the ratio of 1:4 the radical may be involved in the turnover-limiting steps of this
(7pa-1:7pa-2) was found.
procedure.
The final study of this reaction was the investigation of the Based on the previous radical cyclization reactions [44,47,48]
mechanism. Firstly, a substrate (8a) bearing a hydrogen atom at and the results obtained above, a plausible mechanism account-
the nitrogen was tried for the current system. The cyclohexane ing for this cascade radical cyclization reaction was proposed
radical addition product 9aa’, instead of a cyclization product, (Scheme 6). Initially, DTBP undergoes homolytic cleavage to
was observed with 35% yield (Scheme 5a). This result is form the tert-butoxy radical A, which reacts with cyclohexane
consistent with our previous report [19], which discloses that (2a) affording intermediate B. Then, intermediate B adds to
the alkylation of the C–C double bond initiates the radical N-methyl-N-(2-methylallyl)benzamide (4a), giving radical
process. Furthermore, a radical-trapping reagent, 2,2,6,6-tetra- intermediate C. Intermediate C proceeds through intramolecu-
methyl-1-piperidinyloxy (TEMPO), was added to the reaction, lar cyclization to give intermediate D. Finally, H-atom abstrac-
Scheme 5: Control experiments for the mechanism studies. a) Reaction with N-unprotected substrate 8a; b) reaction with the addition of radical-trap-
ping reagent TEMPO; c) KIE study.
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Beilstein J. Org. Chem. 2016, 12, 301–308.
Scheme 6: Proposed mechanism.
tion occurs between D and TBPB directly, which gives the solution (1 × 30 mL). After drying over anhydrous Na2SO4, the
product 5aa and regenerates radical A for the next cycle.
solvent was removed. The crude mixture was charged onto
silica gel and purified by flash chromatography to furnish the
corresponding products 5 and 7.
Conclusion
In summary, a metal-free cascade functionalization of unacti-
vated C(sp3)–H bonds and cyclization reactions of N-substi-
tuted allylbenzamides were developed. The reaction involved
cleavage of the C(sp3)–H bond, alkylation and intramolecular
cyclization, affording the 4-alkyl-substituted dihydroiso-
quinolin-1(2H)-one derivatives with moderate to good chemi-
cal yield. The substituent on the C–C double bond was found to
play a key role for the formation of the desired products. Also,
N-methacryloylbenzamides worked well in the current reaction,
which provides an easy way for the preparation of alkyl-subsi-
tuted isoquinoline-1,3(2H,4H)-diones.
Supporting Information
Supporting Information File 1
Experimental details and spectral data.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-32-S1.pdf]
Acknowledgements
We gratefully acknowledge the financial support from the
National Natural Science Foundation of China (Nos. 21102071
and 21472082) and the Fundamental Research Funds for the
Central Universities (020514380018). The Jiangsu 333 program
(for Pan) and the Changzhou Jin-Feng-Huang program (for
Han) are also acknowledged.
Experimental
General procedure for the radical cyclization between
N-allylbenzamide and N-methacryloylbenzamides with
cycloalkanes: Into an oven-dried reaction vial flushed with N2,
substrate 4 or 6 (0.2 mmol), cycloalkanes 2 (2 mL), and DTBP
(0.6 mmol) were added. Then the reaction mixture was
stirred for 12–48 h at 120 °C under nitrogen atmosphere. After
cooling, the reaction was quenched by a saturated NaCl solu-
tion (1 × 5 mL). Ethyl acetate (30 mL) was added to the system,
and the mixture was washed with water (1 × 30 mL) and brine
References
1. Girard, S. A.; Knauber, T.; Li, C.-J. Angew. Chem., Int. Ed. 2014, 53,
74–100. doi:10.1002/anie.201304268
2. Henrion, M.; Ritleng, V.; Chetcuti, M. J. ACS Catal. 2015, 5,
1283–1302. doi:10.1021/cs5014927
307

Beilstein J. Org. Chem. 2016, 12, 301–308.
3. Kumar, R.; Van der Eycken, E. V. Chem. Soc. Rev. 2013, 42,
1121–1146. doi:10.1039/C2CS35397K
32.Wu, C.-J.; Zhong, J.-J.; Meng, Q.-Y.; Lei, T.; Gao, X.-W.; Tung, C.-H.;
Wu, L.-Z. Org. Lett. 2015, 17, 884–887. doi:10.1021/ol503744a
33.Zhou, L.; Tang, S.; Qi, X.; Lin, C.; Liu, K.; Liu, C.; Lan, Y.; Lei, A.
Org. Lett. 2014, 16, 3404–3407. doi:10.1021/ol501485f
34.Ni, S.; Zhang, Y.; Xie, C.; Mei, H.; Han, J.; Pan, Y. Org. Lett. 2015, 17,
5524–5527. doi:10.1021/acs.orglett.5b02356
4. Cho, S. H.; Kim, J. Y.; Kwak, J.; Chang, S. Chem. Soc. Rev. 2011, 40,
5068–5083. doi:10.1039/c1cs15082k
5. Davies, H. M. L.; Morton, D. Chem. Soc. Rev. 2011, 40, 1857–1869.
doi:10.1039/c0cs00217h
6. Engle, K. M.; Mei, T.-S.; Wasa, M.; Yu, J.-Q. Acc. Chem. Res. 2011,
45, 788–802. doi:10.1021/ar200185g
35.Wu, T.; Mu, X.; Liu, G. Angew. Chem., Int. Ed. 2011, 50, 12578–12581.
doi:10.1002/anie.201104575
7. Liu, C.; Zhang, H.; Shi, W.; Lei, A. Chem. Rev. 2011, 111, 1780–1824.
doi:10.1021/cr100379j
36.Meng, Y.; Guo, L.-N.; Wang, H.; Duan, X.-H. Chem. Commun. 2013,
49, 7540–7542. doi:10.1039/c3cc44055a
8. Newhouse, T.; Baran, P. S. Angew. Chem., Int. Ed. 2011, 50,
3362–3374. doi:10.1002/anie.201006368
37.Wang, J.-Y.; Su, Y.-M.; Yin, F.; Bao, Y.; Zhing, X.; Xu, Y.-M.;
Wang, X.-S. Chem. Commun. 2014, 50, 4108–4111.
doi:10.1039/c3cc49315f
9. Zhang, S.-Y.; Zhang, F.-M.; Tu, Y.-Q. Chem. Soc. Rev. 2011, 40,
1937–1949. doi:10.1039/c0cs00063a
38.Tang, X.-J.; Thomoson, C. S.; Dolbier, W. R., Jr. Org. Lett. 2014, 16,
4594–4597. doi:10.1021/ol502163f
10.Rouquet, G.; Chatani, N. Angew. Chem., Int. Ed. 2013, 52,
11726–11743. doi:10.1002/anie.201301451
39.Li, C.-C.; Yang, S.-D. Org. Lett. 2015, 17, 2142–2145.
doi:10.1021/acs.orglett.5b00732
11.Deng, G.; Zhao, L.; Li, C.-J. Angew. Chem., Int. Ed. 2008, 47,
6278–6282. doi:10.1002/anie.200801544
40.Lu, M.-Z.; Loh, T.-P. Org. Lett. 2014, 16, 4698–4701.
doi:10.1021/ol502411c
12.Bettinger, H. F.; Filthaus, M.; Bornemann, H.; Oppel, I. M.
Angew. Chem., Int. Ed. 2008, 47, 4744–4747.
41.Tang, Q.; Xia, D.; Jin, X.; Zhang, Q.; Sun, X.-Q.; Wang, C.
J. Am. Chem. Soc. 2013, 135, 4628–4631. doi:10.1021/ja400020e
42.Chen, Y.-H.; Zhang, Y.-H.; Zhang, H.-J.; Liu, D.-Z.; Gu, M.; Li, J.-Y.;
Wu, F.; Zhu, X.-Z.; Li, J.; Nan, F.-J. J. Med. Chem. 2006, 49,
1613–1623. doi:10.1021/jm050896o
doi:10.1002/anie.200705936
13.Ochiai, M.; Miyamato, K.; Kaneaki, T.; Hayashi, S.; Nakanishi, W.
Science 2011, 332, 448–451. doi:10.1126/science.1201686
14.Tran, B. L.; Li, B.; Driess, M.; Hartwig, J. F. J. Am. Chem. Soc. 2014,
136, 2555–2563. doi:10.1021/ja411912p
43.Li, Z.; Zhang, Y.; Zhang, L.; Liu, Z.-Q. Org. Lett. 2014, 16, 382–385.
doi:10.1021/ol4032478
15.Chuprakov, S.; Malik, J. A.; Zibinsky, M.; Fokin, V. V.
J. Am. Chem. Soc. 2011, 133, 10352–10355. doi:10.1021/ja202969z
16.Antonchick, A. P.; Burgmann, L. Angew. Chem., Int. Ed. 2013, 52,
3267–3271. doi:10.1002/anie.201209584
44.Zhou, W.; Ni, S.; Mei, H.; Han, J.; Pan, Y. Org. Lett. 2015, 17,
2724–2727. doi:10.1021/acs.orglett.5b01140
45.Li, L.; Deng, M.; Zheng, S.-C.; Xiong, Y.-P.; Tan, B.; Liu, X.-Y.
Org. Lett. 2014, 16, 504–507. doi:10.1021/ol403391v
46.Zhang, M.; Xie, P.; Zhao, W.; Niu, B.; Wu, W.; Bian, Z.;
Pittman, C. U., Jr.; Zhou, A. J. Org. Chem. 2015, 80, 4176–4183.
doi:10.1021/acs.joc.5b00158
17.Narayan, R.; Antonchick, A. P. Chem. – Eur. J. 2014, 20, 4568–4572.
doi:10.1002/chem.201400186
18.Zhao, J.; Fang, H.; Qian, P.; Han, J.; Pan, Y. Org. Lett. 2014, 16,
5342–5345. doi:10.1021/ol502524d
19.Zhou, W.; Qian, P.; Zhao, J.; Fang, H.; Han, J.; Pan, Y. Org. Lett. 2015,
17, 1160–1163. doi:10.1021/acs.orglett.5b00088
20.Zhao, J.; Fang, H.; Han, J.; Pan, Y. Beilstein J. Org. Chem. 2013, 9,
1718–1723. doi:10.3762/bjoc.9.197
47.Fang, H.; Zhao, J.; Qian, P.; Han, J.; Pan, Y. Asian J. Org. Chem.
2014, 3, 1266–1269. doi:10.1002/ajoc.201402169
48.Fang, H.; Zhao, J.; Ni, S.; Mei, H.; Han, J.; Pan, Y. J. Org. Chem. 2015,
80, 3151–3158. doi:10.1021/acs.joc.5b00058
21.Cui, Z.; Shang, X.; Shao, X.-F.; Liu, Z.-Q. Chem. Sci. 2012, 3,
2853–2858. doi:10.1039/c2sc20712e
22.Wu, H.; Xiao, Z.; Wu, J.; Guo, Y.; Xiao, J.-C.; Liu, C.; Chen, Q.-Y.
Angew. Chem., Int. Ed. 2015, 54, 4070–4074.
License and Terms
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23.Du, B.; Jin, B.; Sun, P. Org. Lett. 2014, 16, 3032–3035.
doi:10.1021/ol5011449
24.Zhao, J.; Fang, H.; Han, J.; Pan, Y.; Li, G. Adv. Synth. Catal. 2014,
356, 2719–2724. doi:10.1002/adsc.201400032
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25.Zhu, Y.; Wei, Y. Chem. Sci. 2014, 5, 2379–2382.
doi:10.1039/c4sc00093e
26.Gu, H.; Wang, C. Org. Biomol. Chem. 2015, 13, 5880–5884.
doi:10.1039/C5OB00619H
The license is subject to the Beilstein Journal of Organic
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27.Zhao, J.; Fang, H.; Han, J.; Pan, Y. Org. Lett. 2014, 16, 2530–2533.
doi:10.1021/ol5009119
(http://www.beilstein-journals.org/bjoc)
28.Rout, S. K.; Guin, S.; Ali, W.; Gogoi, A.; Patel, B. K. Org. Lett. 2014, 16,
3086–3089. doi:10.1021/ol5011906
The definitive version of this article is the electronic one
which can be found at:
29.Zhao, J.; Fang, H.; Song, R.; Zhou, J.; Han, J.; Pan, Y.
Chem. Commun. 2015, 51, 599–602. doi:10.1039/C4CC07654K
30.Li, Z.; Fan, F.; Yang, J.; Liu, Z.-Q. Org. Lett. 2014, 16, 3396–3399.
doi:10.1021/ol501461u
doi:10.3762/bjoc.12.32
31.Sha, W.; Yu, J.-T.; Jiang, Y.; Yang, H.; Cheng, J. Chem. Commun.
2014, 50, 9179–9181. doi:10.1039/C4CC03304C
308