Gold(III)-catalyzed synthesis of 2,5-disubstituted furans from substituted 5-methoxyhex-3-yn-2-ols—Mechanistic outlook

Article abstract of DOI:10.1080/00397911.2021.1961274

Gold(III)-catalyzed activation of alkynes has been applied for the synthesis of 2,5-disubstituted furans from substituted 5-methoxy-hex-3-yn-2-ols. Mechanistically, the reaction proceeds via an allenyl carbocation intermediate followed by 5-endo-dig cyclization. The high-yielding, open-air, room temperature reaction conditions applied to synthesize a series of alkyl, aryl, and hetero aryl-substituted furans provide uniqueness to the strategy.

Full text of DOI:10.1080/00397911.2021.1961274

Synthetic Communications
An International Journal for Rapid Communication of Synthetic Organic
Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/lsyc20
Gold(III)-catalyzed synthesis of 2,5-disubstituted
furans from substituted 5-methoxyhex-3-yn-2-
ols—Mechanistic outlook
Sagarika Behera, Nabakumar Bera & Debayan Sarkar
To cite this article: Sagarika Behera, Nabakumar Bera & Debayan Sarkar (2021): Gold(III)-
catalyzed synthesis of 2,5-disubstituted furans from substituted 5-methoxyhex-3-yn-2-
ols—Mechanistic outlook, Synthetic Communications, DOI: 10.1080/00397911.2021.1961274
To link to this article: https://doi.org/10.1080/00397911.2021.1961274
View supplementary material
Published online: 06 Aug 2021.
Submit your article to this journal
Article views: 11
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=lsyc20

R
SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2021.1961274
Gold(III)-catalyzed synthesis of 2,5-disubstituted furans
from substituted 5-methoxyhex-3-yn-2-ols—
Mechanistic outlook
Sagarika Behera, Nabakumar Bera, and Debayan Sarkar
Organic Synthesis and Molecular Engineering Lab, National Institute of Technology, Rourkela, India
ABSTRACT
ARTICLE HISTORY
Received 17 June 2020
Gold(III)-catalyzed activation of alkynes has been applied for the
synthesis of 2,5-disubstituted furans from substituted 5-methoxy-hex-
3-yn-2-ols. Mechanistically, the reaction proceeds via an allenyl
carbocation intermediate followed by 5-endo-dig cyclization. The
high-yielding, open-air, room temperature reaction conditions
applied to synthesize a series of alkyl, aryl, and hetero aryl-substi-
tuted furans provide uniqueness to the strategy.
KEYWORDS
Allenyl carbocation; 5-endo-
dig; one-pot; open-air;
propargyl ether
GRAPHICAL ABSTRACT
Among the five-membered heterocyclic compounds, the substituted furans are import-
ant building blocks in many biologically active natural products, which are mostly avail-
able in plants, algae, and microorganisms.[1] Furans have also been the central core of
many pharmaceutical drugs and optoelectronic materials (Figure 1).[2]
Furans are extensively used as important intermediates in industrial organic synthesis
such as flavors, material sciences, agrochemicals, polymers, etc. In addition to that, fur-
ans are employed as useful building blocks for other heterocyclic, acyclic, and macro-
molecules as they undergo facile transformations such as concerted cycloadditions
(Diels–Alder), electrophilic substitutions, metallations, or ring-opening reactions for
synthesizing 1,4-dicarbonyl compounds and thus can be transformed into a variety of
other functionalities.[3]
Classical approaches like Paal–Knorr synthesis, acid-catalyzed intramolecular cycliza-
tion of 1,4-dicarbonyl compounds,[4] Feist–Benary annulations[5] have been widely used
in the synthesis of substituted furans. The metal-catalyzed approach offers several
advantages in terms of efficiency and functional group compatibility as compared to the
CONTACT Debayan Sarkar
sarkard@nitrkl.ac.in
Organic Synthesis and Molecular Engineering Lab, National
Institute of Technology, Rourkela 769008, India.
Supplemental data for this article can be accessed on the publisher’s website
ß 2021 Taylor & Francis Group, LLC

2
S. BEHERA ET AL.
Figure 1. Substituted furans as central core in natural products and pharmaceutical drugs.
classical approach.[6] Organocatalytic approaches have also been described previously
like Krische’s phosphine-mediated reductive condensation of c-acyloxy butynoates and
Jørgensen’s synthesis of 2-hydroxyalkyl and 2-aminoalkyl furans.[7] Being efficient
K-activators, gold catalysis is being extensively used in enormous organic transforma-
tions in the last decade.[8]
In the early 20th century, the first synthesis of furans has been disclosed by the
Hashmi group from allenyl ketones in the presence of a homogenous gold catalyst.[9]
The second report of furan synthesis also came up from the Hashmi group again by the
reaction of gold chloride and allynyl epoxide.[10] In 2011, the Hashmi group again dis-
closed a new gold(I)-carbene cascade toward the synthesis of substituted furans from
alkynylallyl alcohol.[11] An efficient gold(I)-catalyzed cascade reaction that leads to 3-
formylfurans from symmetric and unsymmetric 1,4-diyn3-ols in the presence of an N-
oxide via oxygen transfer results in 1,2-alkynyl migration is favored over a hydride shift
followed by cyclization afford the desired functionalized furan core in good to excellent
yield.[12] The schematic representation of furan synthesis from the Hashmi group by
employing gold catalysts is explained in Scheme 1. Furans were also have been synthe-
sized by different groups via cycloisomerization-elimination reaction cascade.[13]
In the aforesaid reports, but-3-yn-1,2-diols as substrates were used for the synthesis
of furans which resulted in limited substrate scope, also the mechanistic explanations
were based on a theoretical approach. In this report, 5-methoxypent-3-yn-1-ols are
employed toward the synthesis of 2-substituted furans. The present transformation has
been added with mechanistic explanation and experimental evidence which attests to
the formation of a 6-membered pyran ring as a side-product.
It may be highlighted that 5-methoxypent-3-yn-1-ols are easier to be synthesized as
compared to the earlier used but-3-yn-1,2-diols with facile reaction conditions. Another
important outlook is the mechanistic rationale and this demands an acute investigation.
For instance, a typical allenyl intermediate was reported by Zhu[14] which asks for more
experimental support on its generation.
Thus, we disclose an Au-catalyzed activation of substituted propargylic alcohols as an
important strategy toward the synthesis of substituted furans. Generation of the dihy-
dropyran as a minor product exemplifies the mechanistic rationale involving an allenyl
carbocation.

R
SYNTHETIC COMMUNICATIONSV
3
Scheme 1. Gold catalyzed synthesis of furan core via cycloisomerization-elimination mechanism.
Scheme 2. Furan synthesis from propargyl alcohol.
Results and discussion
An initial experiment with propargyl alcohol (1) by employing 3 mol% of AuCl catalyst
at room temperature was nonproductive. The same propargyl alcohol (1) was stirred in
the presence of 3 mol% of HAuCl4.3H2O as a catalyst in THF and delivered furan (2a)
with low yields as in Scheme 2.
In order to achieve better yields of synthesized furan, the propargyl alcohol (1)
was converted to the appropriate propargyl ether (3). The 5-methoxy-1-(p-tolyl)hex-3-
yn-1-ol (3a) with methyl protection, delivered an impressive yield of 93% of furans
(2a) along with the 7% pyran derivative (4a). However, the substituted 5-alkoxy-hex-
4-yn-2-ols bearing other protecting groups result in furans with satisfactory yield
along with a negligible amount of pyran (4) which is summarized in Scheme 3. This
turned out to be important information for us, which clearly clarified the mechanism
of the reaction.
The reaction conditions were optimized in the presence of various solvents, varied
catalyst concentrations, and temperature. The 5-methoxy-1-(p-tolyl)hex-3-yn-1-ol (3a)
was taken for standardization of reaction conditions. With 2 mol% of catalyst, in 8 h at
room temperature, 74% of the yield of furan was obtained. However, increasing the

4
S. BEHERA ET AL.
Entry
3
2a
4
1
2
3
4
3a, R = Methyl
3a1, R = Ethyl
3a2, R = Propyl
3a3, R = Benzyl
93%
91%
87%
84%
4a, 7%a
4b, trace
4c, trace
4d, trace
a
Isolated and confirmed by spectroscopic analysis
Scheme 3. Furan synthesis from 5-alkoxy-hex-4-yn-2-ols.
Table 1. Optimization of reaction condition.
Entry
Catalyst (X mol%)
Solvent
Temp. (ꢀC)
Yield (%)
1
2
3
4
5
6
7
8
3
3
3
3
3
3
3
2
THF
DCM
MeCN
Acetone
H2O
PhMe
DMSO
THF
THF
THF
THF
THF
rt
rt
rt
rt
rt
rt
rt
rt
rt
93
36
10a
15a
NR
30
NR
74b
97
96
55
71
9
5
10
5
10
11
12
rt
0 ꢀC
45 ꢀC
5
aAfter 6 h of reaction time in open-air; NR: No Reaction, Starting material was left; bAfter 8 h reaction time.
catalyst concentration to 5 mol% at room temperature delivered the furan (2a) with
97% of yield. With incremental addition of catalyst concentration from 5 to 10 mol%,
no significant increase of yield was observed. Furthermore, with an increase of tempera-
ture to 45 ꢀC, the yield of the reaction decreased to 71%.
Hence, the optimal condition was maintained with a combination of 5 mol% of catalyst
concentration at room temperature in THF solvent. Further, the reaction was also moni-
tored under an inert atmosphere. It was worth noting that, the same yield was observed in
both open-air and in presence of inert conditions which clearly ruled out the requirement of
inert conditions for the transformation. The standard optimized condition was summarized
in Table 1.
The generalizability of the methodology was checked by employing the reaction
with differently substituted 5-methoxy-hex-3-yn-2-ols with standard conditions. For
cyclic (2o and 2p), acyclic (2i and 2l), aryl (2a, 2b, and 2g), and heteroaryl (2d and
2u) substituted 5-methoxy-hex-3-yn-2-ols, the methodology worked smoothly with
good to excellent yields. The furan derivative 2a was obtained with the maximum
yield of 97% whereas a minimum yield of 68% of 2u was obtained which is sum-
marized in Table 2.

R
SYNTHETIC COMMUNICATIONSV
5
Table 2. Substrate scope of substituted furans.
It may be proposed that the reaction proceeds via a subsequent cycloisomerization-
elimination pathway[15] as depicted in Scheme 1. The isolation of the minor dihydro-
pyran (4a) discloses the vital mechanistic route to this transformation which was not
disclosed in earlier communications by such catalysis. Based on the observation, espe-
cially the non-isolation of the dihydropyran (4b–4d) in Scheme 3 with non-nucleophilic
alkoxides like benzyloxy (3a3), propyloxy (3a2), it can be articulated that the reaction
proceeds through an allenyl carbocation intermediate. The two pathways (a) and (b) are
mutually exclusive. The reaction pathway (a) proceeds through the allenyl carbocation
which undergoes a 5-endo-dig cyclization to deliver the furan, as summarized in
Scheme 4.

6
S. BEHERA ET AL.
Scheme 4. Plausible reaction mechanism.
Experimental section
General procedure (A) for synthesis of 2,5-disubstituted furans (2a–2u)
The methoxy-protected propargylic alcohols, (3a–3t) in THF were taken in an “open-
flask.” 5 mol% of HAuCl4.3H2O in THF was added slowly into the reaction mixture.
The mixture was stirred at room temperature and the product formation was monitored
by TLC. The reaction was stirred approximately for 0.5 to 1 h. After the completion, the
mixture was extracted with EtOAc (2 ꢁ 3 mL) and distilled water, and the organic layer
was filtered over anhydrous Na2SO4 and was being concentrated applying vacuum.
Then, the residue was subjected to flash column chromatography using silica 200:400
mesh size. Elution with the appropriate ratio of Hexane: EtOAc, affords the correspond-
ing 2,5-disubstituted furans, (2a–2u) with an excellent yield.
General procedure (B) for the synthesis of substituted 6-methoxyhept-3-yn-2-
ol (3a–3u)
Diprotected propargylic alcohols, (5a–5t) were taken in THF. The solution of
Tetrabutylammonium fluoride (TBAF, 1.2 equiv.) was added to the well-stirred reaction

R
SYNTHETIC COMMUNICATIONSV
7
mixture. The reaction was performed in an inert atmosphere and monitored by TLC.
After 3–4 h, the reaction was completed and quenched with distilled water and washed
with ethyl acetate. The organic layer was collected over anhydrous Na2SO4 and concen-
trated. The mass was subjected to column-chromatography using silica (200:400 mesh);
elution with Hexane: EtOAc affords the desired mono-protected alcohols (3a–3u).
2-Ethyl-5-(p-tolyl)furan (2a)
According to general procedure A, 2-ethyl-5-(p-tolyl)furan, 2a was synthesized with
97% of yield (65 mg, 0.34 mmol) as yellowish gel. 1H NMR (400 MHz, CDCl3) d
7.5–7.56 (m, 2H), 7.20 (d, J ¼ 8.0 Hz, 2H), 6.53 (d, J ¼ 3.2 Hz, 1H), 6.09–6.08 (m, 1H),
2.75 (q, J ¼ 7.6 Hz, 2H), 2.39 (s, 3H), 1.32 (t, J ¼ 7.6 Hz, 3H). 13C NMR (100 MHz,
CDCl3) d 157.2, 152.4, 136.5, 129.3, 128.6, 123.3, 105.9, 104.9, 21.5, 21.2, 12.3.
IR(neatfilm, KBr): 2961, 2845, 2349, 1651, 1454, 1247, 1033, 936, 831 cmꢂ1. HRMS
(ESI) calcd. for C13H14O: [M þ H]þ: 187.1123, Found 185.1125.
5-Methoxy-6-methyl-2-(p-tolyl)-3,4-dihydro-2H-pyran (4a)
5-methoxy-6-methyl-2-(p-tolyl)-3,4-dihydro-2H-pyran, 4a was obtained by general pro-
cedure A as a side product with low yield of 7% as white semi-solid. 1H NMR
(400 MHz, CDCl3) d 7.28 (d, J ¼ 8 Hz, 2H), 7.18 (d, J ¼ 8 Hz, 2H), 5.14 (t, J ¼ 12 Hz,
1H), 3.33 (s, 3H), 2.35–2.23 (m, 4H), 2.20 (s, 3H), 2.19–2.14 (m, 3H). 13C NMR
(100 MHz, CDCl3) d 138.3, 137.6, 136.2, 129.1, 126.4, 126.3, 84.3, 50.9, 33.3, 25.7, 21.1,
16.3. IR(neatfilm, KBr): 2975, 2831, 2337, 1653, 1441, 1267, 1011, 946, 851 cmꢂ1. HRMS
(ESI) calcd. for C14H18O2: [M þ H]þ: 219.1385, Found 219.1387.
Conclusions
In summary, this report has demonstrated an efficient protocol of Gold(III)-catalyzed
synthesis of 2,5-disubstituted furans from substituted 5-methoxy-hex-3-yn-2-ols via an
elimination-cyclization-aromatization pathway. The substitution of the hydroxyl group
with methoxy in the propargyl alcohols led to a significant improvement in the reactiv-
ity and the formation of furans in good to excellent yield. The one-pot, room tempera-
ture, and open-air reaction conditions and the isolation of side-product make this
protocol highly practical and mechanistically acceptable.
Acknowledgments
SB thanks UGC for the research fellowship. We thank Pratap Chandra Behera for his continued
support in conducting NMR experiments.
Disclosure statement
NO potential conflict of interest was reported by the author(s).

8
S. BEHERA ET AL.
Funding
We sincerely acknowledge Department of Science and Technology (DST), Govt. of India (Grant
no. EEQ/2016/000518; EEQ/2020/000463; TTR/2021/000015).
References
[1] (a) Boto, A.; Alvarez, L. Heterocycles in Natural Product Synthesis; Wiley-VCH:
Weinheim, Germany, 2011; pp 99–152. (b) Kobayashi, J.; Watanabe, D.; Kawasaki, N.;
Tsuda, M. J. Org. Chem. 1997, 62, 9236–9239. DOI: 10.1021/jo9715377.
[2] (a) Tong, X.; Ma, Y.; Li, Y. Appl. Catal. A. 2010, 385, 1–13. DOI: 10.1016/j.apcata.2010.06.
049. (b) Gandini, A. Polym. Chem. 2010, 1, 245–251. DOI: 10.1039/B9PY00233B. (c)
Moreau, C.; Belgacem, M. N.; Gandini, A. Top. Catal. 2004, 27, 11–30. DOI: 10.1023/
B:TOCA.0000013537.13540.0e. (d) Gidron, O.; Dadvand, A.; Sheynin, Y.; Bendikov, M.;
Perepichka, D. F. Chem. Commun. 2011, 47, 1976–1978. DOI: 10.1039/c0cc04699j. (e)
Ramkumar, V.; Kannan, P. J. Lumin. 2016, 169, 204–215. DOI: 10.1016/j.jlumin.2015.09.
020. (f) Gajalakshmi, D.; Boobalan, M. S.; Solomon, R. V.; Tamilmani, V. Comput. Mater.
Sci. 2019, 162, 60–68. DOI: 10.1016/j.commatsci.2019.02.015. (g) Kazantsev, M. S.;
Beloborodova, A. A.; Kuimov, A. D.; Koskin, I. P.; Frantseva, E. S.; Rybalova, T. V.;
Shundrina, I. K.; Becker, C. S.; Mostovich, E. A. Org. Electron. 2018, 56, 208–215. DOI:
10.1016/j.orgel.2018.01.010. (h) Tsuji, H.; Nakamura, E. Acc. Chem. Res. 2017, 50,
396–406. DOI: 10.1021/acs.accounts.6b00595. (i) Kazantsev, M. S.; Beloborodova, A. A.;
Frantseva, E. S.; Rybalova, T. V.; Konstantinov, V. G.; Shundrina, I. K.; Paraschuk, D. Y.;
Mostovich, E. A. CrystEngComm. 2017, 19, 1809–1815. DOI: 10.1039/C6CE02565J. (j)
Cao, H.; Rupar, P. A. Chemistry 2017, 23, 14670–14675. DOI: 10.1002/chem.201703355.
[3] (a) Maier, M. Organic Synthesis Highlights II; VCH: Weinheim, Germany, 1995; pp
231–242. (b) Benassi, R. ComprehensiVe Heterocyclic Chemistry II; Pergamon Press:
Oxford, UK, 1996; pp 259–295. (c) Trost, B. M.; Fleming, I., Eds. ComprehensiVe Organic
Synthesis; Pergamon Press: Oxford, UK, 1991; pp 827–873. (d) Chambers, R. D.; Gilbert,
A. F.; Powell, R. L. J. Fluorine Chem. 2000, 104, 233–237. DOI: 10.1016/S0022-
1139(00)00250-5. (e) Liang, X.; Haynes, B. S.; Montoya, A. Energy Fuels 2018, 32,
4139–4148. ꢂ DOI: 10.1021/acs.energyfuels.7b03239. (f) Alves, A. J. S.; Lopes, S. M. M.;
~
Henriques, M. S. C.; Paixao, J. A.; Pinho e Melo, T. M. V. D. Eur. J. Org. Chem. 2017,
ꢀ €
2017, 4011–4025. DOI: 10.1002/ejoc.201700453. (g) Roscales, S.; Csaky, A. G. Chem.
Commun. 2016, 52, 3018–3021. DOI: 10.1039/c5cc08809g. (h) Cott, D. J.; Ziegler, K. J.;
Owens, V. P.; Glennon, J. D.; Graham, A. E.; Holmes, J. D. Green. Chem. 2005, 7,
105–110. DOI: 10.1039/B408327J. (i) Cheng, Y.-T.; Huber, G. W. Green Chem. 2012, 14,
3114–3125. DOI: 10.1039/c2gc35767d. (j) Zhao, Y.; Snieckus, V. Adv. Synth. Catal. 2014,
356, 1527–1532. DOI: 10.1002/adsc.201400147.
[4] (a) Khaghaninejad, S.; Heravi, M. M. Advances in Heterocyclic Chemistry; Elsevier
Academic Press Inc.: San Diego, CA, 2014; pp 95–146. (b) Knorr, L. Ber. Dtsch. Chem.
Ges. 1884, 17, 2863–2870. DOI: 10.1002/cber.188401702254.
[5] (a) Benary, E. Ber. Dtsch. Chem. Ges. 1911, 44, 489–493. DOI: 10.1002/cber.19110440175.
(b) Feist, F. Ber. Dtsch. Chem. Ges. 1902, 35, 1537–1544. DOI: 10.1002/cber.19020350263.
ꢀ
ꢀ
[6] (a) Barluenga, J.; Riesgo, L.; Vicente, R.; Lopez, L. A.; Tomas, M. J. Am. Chem. Soc. 2008,
130, 13528–13529. DOI: 10.1021/ja8058342. (b) Yang, J.; Wang, C.; Xie, X.; Li, H.; Li, E.;
ꢀ
Li, Y. Org. Biomol. Chem. 2011, 9, 1342–1346. DOI: 10.1039/c0ob00985g. (c) Gonzalez, J.;
ꢀ
ꢀ
ꢀ
Gonzalez, J.; Perez-Calleja, C.; Lopez, L. A.; Vicente, R. Angew. Chem. 2013, 125,
ꢀ
ꢀ
ꢀ
5965–5969. DOI: 10.1002/ange.201301625. (d) Gonzalez, J.; Gonzalez, J.; Perez-Calleja, C.;
ꢀ
Lopez, L. A.; Vicente, R. Angew. Chem. Int. Ed. Engl. 2013, 52, 5853–5857. DOI: 10.1002/
anie.201301625. (e) Xia, Y.; Qu, S.; Xiao, Q.; Wang, Z.-X.; Qu, P.; Chen, L.; Liu, Z.; Tian,
L.; Huang, Z.; Zhang, Y.; et al. J. Am. Chem. Soc. 2013, 135, 13502–13511. DOI: 10.1021/

R
SYNTHETIC COMMUNICATIONSV
9
ꢀ
ja4058844. (f) Corma, A.; Leyva-Perez, A.; Sabater, M. J. Chem. Rev. 2011, 111,
1657–1712. DOI: 10.1021/cr100414u. (g) Hashmi, A. S. K. Chem. Rev. 2007, 107,
3180–3211. DOI: 10.1021/cr000436x. (h) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008,
108, 3239–3265. DOI: 10.1021/cr068434l. (i) Ma, J.; Jiang, H.; Zhu, S. Org. Lett. 2014, 16,
4472–4475. DOI: 10.1021/ol502018d.
[7] Jung, C. K.; Wang, J. C.; Krische, M. J. J. Am. Chem. Soc. 2004, 126, 4118–4119. DOI: 10.
1021/ja049377l.
[8] (a) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem. Int. Ed. Engl. 2006, 45, 7896–7936.
DOI: 10.1002/anie.200602454. (b) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180–3211.
€
DOI: 10.1021/cr000436x. (c) Pflasterer, D.; Hashmi, A. S. K. Chem. Soc. Rev. 2016, 45,
1331–1367. DOI: 10.1039/c5cs00721f.
[9] Hashmi, A. S. K.; Schwarz, L.; Choi, J.-H.; Frost, T. M. Angew. Chem. Int. Ed. 2000, 39,
2285–2288. DOI: 10.1002/1521-3773(20000703)39:13<2285::AID-ANIE2285>3.0.CO;2-F.
[10] Hashmi, A. S. K.; Sinha, P. Adv. Synth. Catal. 2004, 346, 432–438. DOI: 10.1002/adsc.
200303201.
€
[11] Hashmi, A. S. K.; Haffner, T.; Rudolph, M.; Rominger, F. Eur. J. Org. Chem. 2011, 2011,
667–671. DOI: 10.1002/ejoc.201001479.
[12] (a) Wang, T.; Shi, S.; Hansmann, M. M.; Rettenmeier, E.; Rudolph, M.; Hashmi, A. S. K.
Angew. Chem. Int. Ed. Engl. 2014, 53, 3715–3719. DOI: 10.1002/anie.201310146. (b)
Wang, T.; Shi, S.; Rudolph, M.; Hashmi, A. S. K. Adv. Synth. Catal. 2014, 356, 2337–2342.
DOI: 10.1002/adsc.201400356. (c) Wang, T.; Huang, L.; Shi, S.; Rudolph, M.; Hashmi, A.
S. K. Chemistry 2014, 20, 14868–14871. DOI: 10.1002/chem.201404229.
[13] (a) Aponick, A.; Li, C. Y.; Malinge, J.; Marques, E. F. Org. Lett. 2009, 11, 4624–4627.
DOI: 10.1021/ol901901m. (b) Egi, M.; Azechi, K.; Akai, S. Org. Lett. 2009, 11, 5002–5005.
DOI: 10.1021/ol901942t. (c) Blanc, A.; Tenbrink, K.; Weibel, J. M.; Pale, P. J. Org. Chem.
2009, 74, 5342–5348. DOI: 10.1021/jo9008172. (d) Minkler, S. R. K.; Isley, N. A.;
Lippincott, D. J.; Krause, N.; Lipshutz, B. H. Org. Lett. 2014, 16, 724–726. DOI: 10.1021/
ol403402h.
[14] Cheng, C.; Liu, S.; Zhu, G. J. Org. Chem. 2015, 80, 7604–7612. DOI: 10.1021/acs.joc.
5b01182.
[15] (a) Hashmi, A. S. K. Angew. Chem. Int. Ed. Engl. 2010, 49, 5232–5241. DOI: 10.1002/
anie.200907078. (b) Lauterbach, T.; Asiri, A. M.; Hashmi, A. S. K. Adv. Organomet. Chem.
2014, 62, 261–297. DOI: 10.1016/B978-0-12-800976-5.00005-9.