The ketene-surrogate coupling: Catalytic conversion of aryl iodides into aryl ketenes through ynol ethers

Article abstract of DOI:10.1002/anie.201405036

tert-Butoxyacetylene is shown to undergo Sonogashira coupling with aryl iodides to yield aryl-substituted tert-butyl ynol ethers. These intermediates participate in a [1,5]-hydride shift, which results in the extrusion of isobutylene and the generation of aryl ketenes. The ketenes are trapped in situ with multiple nucleophiles or undergo electrocyclic ring closure to yield hydroxynaphthalenes and quinolines.

Full text of DOI:10.1002/anie.201405036

.
Angewandte
Communications
DOI: 10.1002/anie.201405036
Ketenes
Hot Paper
The Ketene-Surrogate Coupling: Catalytic Conversion of Aryl Iodides
into Aryl Ketenes through Ynol Ethers**
Wenhan Zhang and Joseph M. Ready*
Abstract: tert-Butoxyacetylene is shown to undergo Sonoga-
shira coupling with aryl iodides to yield aryl-substituted tert-
butyl ynol ethers. These intermediates participate in a [1,5]-
hydride shift, which results in the extrusion of isobutylene and
the generation of aryl ketenes. The ketenes are trapped in situ
with multiple nucleophiles or undergo electrocyclic ring
closure to yield hydroxynaphthalenes and quinolines.
B
enzyl ketones and aryl acetic acid derivatives are found in
many biologically active natural products and drug candidates
(Scheme 1). For example, benzyl ketones or their derivatives
appear within actinoplanone A, the epilepsy drug oxcarbaze-
pine, and the platelet inhibitor prasugrel. Likewise, aryl acetic
acid derivatives appear in many natural products and
pharmaceutical agents, such as (À)-curvularin and penicil-
lin G. For these reasons, we were motivated to develop a mild
and general catalytic method to prepare these substructures.
Herein we report a novel cross-coupling between aryl iodides
and ynol ethers that provides access to aryl ketenes (1), thus
providing a general catalytic synthesis of benzyl carbonyl
compounds.
Previous catalytic approaches to aryl acetic acid deriva-
tives have relied on arylation of enolates. Nickel and
palladium catalyze couplings of esters and amides with aryl
halides. These protocols have proven relatively general, and,
in some cases, enantioselective.[1] Nonetheless, enolate cou-
plings require strongly basic conditions, elevated temper-
atures, and/or independent preparation of the zinc enolates.
They frequently involve specialized ligands, and achieving
selective monoarylation can be challenging. Alternative
catalytic approaches to the a-arylation of carbonyl com-
pounds include a-arylation with iodonium salts[2] or activated
sulfoxides,[3] addition of aldehydes to in situ generated
quinone derivatives,[4] and a recent oxidative coupling of
ketones with nitroarenes.[5]
Scheme 1. Coupling of ynol ethers with aryl halides to yield aryl
ketenes.
We hypothesized that aryl ketenes (1) could give rise to
esters, amides, carboxylic acids, ketones, and thioesters from
the same intermediate. In contrast, the alternative approaches
outlined above require separate reaction conditions for each
of these products, if they are accessible at all. To date, no
direct methods have been reported for coupling a metalated
ketene (2) with an aromatic ring, and such a reaction
appeared implausible. An alternative presented itself, how-
ever, in the [1,5]-hydride shift of ynol ethers. This process is
accompanied by the sigmatropic extrusion of an olefin, and
occurs under thermal conditions.[6] Finally, the requisite aryl-
substituted ynol ethers 3 could arise from the currently
unknown coupling of an alkoxyacetylene with aryl halides. In
this way, an alkoxyacetylene could serve as a ketene surro-
gate, so we refer to the process as the ketene-surrogate
coupling.[7,8]
[*] W. Zhang, Dr. J. M. Ready
Department of Biochemistry, Division of Chemistry
UT Southwestern Medical Center
5323 Harry Hines Blvd., Dallas, TX 75390-0938 (USA)
E-mail: joseph.ready@utsouthwestern.edu
Homepage: http://www4.utsouthwestern.edu/readylab/index.htm
[**] Funding provided by the NIH (R01GM102403) and the Welch
Foundation (I-1612). We thank Dr. Karen S. MacMillan (UT South-
western) for preliminary experiments related to the synthesis of
anilides and Jibao Xia (UT Southwestern) for independently
repeating the synthesis of 10g, and Prof. Gary Sulikowski for
suggesting the Fries rearrangement to synthesize the ketone 18.
Thermal generation of ketenes from ynol ethers has been
exploited previously in the synthesis of complex molecules,
and indicates that this transformation is reliable and occurs
under mild reaction conditions.[9] In contrast, the cross-
coupling of alkoxyacetylenes remains poorly developed. In
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201405036.
8980
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 8980 –8984

Angewandte
Chemie
this context, Stille coupling of alkoxyethynyl tin reagents has
been described,[10] but Sonogashira coupling could directly
connect the aryl iodides and ynol ethers without necessitating
prefunctionalization of the acetylene motif. While the Sono-
gashira coupling between menthol-derived ynol ethers and
terminal vinyl iodides has been investigated,[11] the only
Sonogashira coupling between an ynol ether and an aryl
halide of which we are aware proceeded in only 11%
yield.[12,13]
To develop the ketene-surrogate coupling, we first inves-
tigated the palladium-catalyzed coupling of alkyl ynol ethers
with aryl iodides. 4-Cyanoiodobenzene (4a) was combined
with ethoxy acetylene, [Pd(PPh3)4], and CuI in triethyl amine
(Table 1, entry 1). No desired product was isolated, and the
arising from tert-butoxyacetylene and the aryl iodide reacting
in a 2:1 ratio.
To minimize the formation of ester and carboxylic acid
side-products, we included molecular sieves in the reaction.
Additionally, the use of a bulkier base, iPr2NEt, suppressed
the generation of the enynol ether 8a (Table 1, entry 3).
Under these reaction conditions, the aryl-substituted ynol
ether 5a was formed in an improved 85% yield. The enynol
8a could arise from either dimerization of tert-butoxyacety-
lene and subsequent coupling with the aryl iodide or
carbometalation of the Sonogashira product 5a. We favor
the former hypothesis because tert-buytoxyacetylene did not
add to the isolated ynol ether 5a under the reaction
conditions. We speculate that a bulky amine prevents the
[(R3N)nCu(acetylide)] complex from reacting with a second
equivalent of tert-butoxyacetylene. Ultimately, the reprodu-
cibility of the reaction could be improved by forming
[Pd(PPh3)n] in situ from [Pd2(dba)3] and PPh3 (entry 4).[16]
Unfortunately, when these reaction conditions were applied
to an electron-neutral substrate (4b), incomplete conversion
was observed (entry 5). Changing to a secondary amine
(iPr2NH) accelerated the coupling (entry 5) without intro-
ducing impurities. Under these reaction conditions, the aryl
iodide 4b was completely consumed, and the aryl-substituted
ynol ether 5b was formed in good yield (entry 6). Reactions
with iPr2NH are generally faster than those containing
iPr2NEt, but electron-poor arenes form small quantities of
the corresponding tertiary amide (e.g. 9a) during the reaction.
Accordingly, we generally recommend iPr2NEt for electron-
poor substrates and iPr2NH for electron-rich and electron-
neutral substrates. Finally, sterically hindered substrates such
as 4c benefited from an even more active catalyst. In
particular, tri(2-furyl)phosphine formed a competent catalyst
in conjunction with [Pd2(dba)3], thus promoting the coupling
of a hindered aryl iodide in good yield (entry 8). To
summarize, three closely related reaction conditions accom-
modate a wide variety of substrate classes: the couplings are
usually more efficient with iPr2NH than with iPr2NEt,
although with electron-deficient substrates, we observed
minor amounts of the amide 9 when iPr2NH was used.
While these substrates perform admirably with inexpensive
PPh3, challenging aryl iodides often necessitate the electron-
deficient phosphine TFP.
Table 1: Optimization of coupling conditions.[a]
Entry R1
R2
Catalyst
Amine Additive Yield
[%][b]
1
2
3
4
5
6
7
8
4-CN (4a) Et
[Pd(PPh3)4]
Et3N
Et3N
–
–
<5
54
4-CN
4-CN
4-CN
tBu [Pd(PPh3)4]
tBu [Pd(PPh3)4]
tBu [Pd2(dba)3]/PPh3
iPr2NEt 4 ꢀ M.S.
iPr2NEt 4 ꢀ M.S.
iPr2NEt 4 ꢀ M.S.
iPr2NH 4 ꢀ M.S.
iPr2NH 4 ꢀ M.S.
85
95
66
89
55
79
[c]
[c]
[c]
[c]
4-Me (4b) tBu [Pd2(dba)3]/PPh3
4-Me tBu [Pd2(dba)3]/PPh3
2-Me (4c) tBu [Pd2(dba)3]/PPh3
2-Me
tBu [Pd2(dba)3]/TFP[c] iPr2NH 4 ꢀ M.S.
[a] Reactions were conducted on a 0.1 mmol scale; 0.25m in 1:1 (v/v)
amine/ynol ether. [b] Yields determined by NMR spectroscopy using
1,3,5-trimethoxybenzene as an internal standard. [c] Used 20 mol%
phosphine. dba=dibenzylideneacetone, M.S.=molecular sieves,
TFP=tri(2-furyl)phosphine.
The coupling tolerates a wide range of electronic proper-
ties and functional groups (see Table 2). In these experiments,
copper, palladium, and amines were removed by rapid
chromatography using neutral Al2O3. In general, electron-
neutral or electron-rich Sonogashira products were isolated in
high purity whereas electron-poor congeners were prone to
hydrolysis. For example, the ynol derived from 4-methoxy-
iodobenzene was stored for more than one year at 48C with
no signs of decomposition. In contrast, the p-CN-substituted
ynol 5a underwent hydrolysis (5–10%) upon attempted
purification. Therefore, after filtration over Al2O3, the crude
aryl-substituted ynol ethers 5 were heated in the presence of
morpholine to generate, consecutively, the aryl ketene (1) and
then the morpholine amides (10; Table 2).[17] These amides
were targeted because of their utility in the synthesis of
ketones. They behave similarly to Weinreb amides, but are
alkyne appeared to have polymerized under the reaction
conditions. Reasoning that a more sterically hindered alkyne
might be less prone to polymerization, we repeated the
experiment using tert-butoxyacetylene.[14] In addition to being
more stable, tert-butoxyacetylenes rearrange to ketenes at
around 808C compared to 1208C, which is required for
conversion of ethoxyacetylenes into ketenes.[15] We were
encouraged by the formation of the alkyne 5a in 54% yield
when tert-butoxyacetylene was used (entry 2). The reaction
also generated products arising from hydration and hydrolysis
of 5a (6a and 7a, respectively), and the enyne 8a, an adduct
Angew. Chem. Int. Ed. 2014, 53, 8980 –8984
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8981

.
Angewandte
Communications
Table 2: Scope of ketene-surrogate coupling.
10j in 74% yield. Additionally 4-fluoroiodobenzene provided
10c in poor yield because of the volatility of the ynol ether
and instability of the amide.
The examples in Table 2 involve trapping the arylketene
with morpholine, but we wanted to explore the reactivity of
this intermediate more broadly. To this end, the ynol ether 5g
was synthesized on a 2.9 g scale and isolated in 88% yield
(Scheme 2). The ketene 1g was then generated at 758C in the
presence of a variety of trapping reagents. Oxygen-based
nucleophiles reacted cleanly and efficiently. Specifically,
water, methanol, and primary, secondary, and tertiary alco-
hols yielded the carboxylic acid or esters 11a–e in high yield.
Likewise, pentafluorophenol and phenol formed the phenolic
esters 11 f and 11g, respectively. The former could serve as
a useful acylating agent while the latter can participate in
a Fries rearrangement. In the special case of allyl alcohol, the
intermediate allyl ester was exposed to soft enolization
conditions to promote a Claisen rearrangement in a two-
step, one-pot procedure to give the C-allyl ester 12.[19] In
related transformations, amines other than morpholine react
with equal facility. Thus, the Weinreb amide 13 emerges from
trapping with (MeO)MeNH, most conveniently free-based in
situ from the hydrochloride salt, and aniline reacted to form
the anilide 14 in nearly quantitative yield. Of note, HI and
isobutylene are the only chemical waste products generated in
these acylations.
À
We next explored strategies to form ketones through C C
bond formation. To this end, the ynol 5g was heated in the
presence of the ylide derived from ethyl 2-bromopropionate
(15), and the trisubstituted allene 16 was isolated in good
yield.[20] Similarly, exposure to alkyl-vinyl ethers initiated
a [2+2] cycloaddition to provide the cyclobutanone products
17 as single trans diastereomers.[21] These last examples
provide compelling evidence for the intermediacy of the
ketene 1g. While, in principle, most of the other products in
Scheme 2 could have arisen from Nu-H addition across the
ynol ether triple bond with subsequent hydrolysis of the tert-
butyl enol ether, the formation of allenes and [2+2] adducts is
more consistent with a ketene intermediate under these
reaction conditions.
[a] Yields of products isolated after two steps. Reactions were conducted
on a 0.3 mmol scale unless otherwise noted; 0.25m in 1:1 (v/v) amine/
ynol ether with 5 mol% [Pd2(dba)3], 20 mol% phosphine, and 13 mol%
of CuI for 12–24 h at RT. [b] 3 mmol scale. [c] 1 mmol scale. [d] 48 h
reaction time.
The facile synthesis of a variety of aryl acetic acid
derivatives provided an entry into several other types of
ketones. For example, the phenol ester 11g underwent Fries
rearrangement to yield the aryl benzyl ketone 18 as a single
regioisomer.[22] The thiol ester 19 was formed in good yield,
and then converted into the ketone 20 under reaction
conditions introduced by Liebeskind and co-workers.[23]
Additionally, we exploited the ability of morpholine amides
to react with hard nucleophiles to provide ketones.[17] In the
event, Grignard reagents proved too basic for these trans-
formations, and enolate formation dominated the reaction.
However, we found that LaCl3·2LiCl promoted these addi-
tions effectively. This additive, introduced by Knochel and
colleagues,[24] assists nucleophilic addition to acidic alde-
hydes,[25] but this is the first report of its use to facilitate
addition to morpholine amides.[26] A primary and a secondary
Grignard reagent performed well, as did a vinyl and phenyl
reagent. No tertiary alcohols were observed from double
addition. Taken together, these examples demonstrate that
more stable and less expensive.[18] We found that ketones,
esters, nitro, cyano, and trifluoromethyl groups were all
compatible with the reaction conditions. Likewise, substrates
featuring electron-donating groups, including 2-, 3-, and 4-
methyl (10q,r,h) and 2-, 3-, and 4-methoxy (10s,t,k) moieties,
provided the corresponding morpholine amides in good yield.
Both 1- and 2-iodonaphthalene were excellent substrates
(10v,m). Moreover, several heterocycles participated in the
reaction, yielding 3- and 4-substituted pyridines (10n,o) and
5- and 7-substitued indoles (10p,w). In the latter cases, the
indole NH did not require protection. The thiazole 10x was
formed in good yield, thus extending the chemistry to five-
membered heterocycles. Aryl bromides do not couple effi-
ciently, but this characteristic allows selective coupling of 4-
bromoiodobenzene to form the 4-bromophenyl acetamide
8982 www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 8980 –8984

Angewandte
Chemie
Scheme 2. Aryl acetic acid derivatives and benzyl ketones by ketene trapping. TC=2-thiophene carboxylate, Tf=trifluoromethanesulfonyl,
TMS=trimethylsilyl.
and then transform into a ketene under mild thermal
conditions. This ketene-surrogate coupling leads to aryl
acetic acid derivatives, ketones, allenes, and cyclobutanone
products in good yield. An advantageous characteristic of the
ketone surrogate coupling is the ability to access a wide range
of carbonyl compounds from a single intermediate. Moreover,
an efficient benzannulation process has been developed to
provide hydroxy naphthylenes and hydroxy quinolines.
Experimental Section
Representative procedure: Aryl iodide (0.3 mmol), [Pd2(dba)3]
(15.6 mg, 0.015 mmol), PPh3 (15.9 mg, 0.06 mmol), CuI (7.5 mg,
0.039 mmol), and 150 mg 4 ꢀ molecular sieves were combined in
a vial and purged with argon. Diisopropylethyl amine (0.6 mL) and
tert-butoxyacetylene (0.6 mL) were added at room temperature. The
reaction was stirred at room temperature until completion as
determined by TLC (12–24 h), and was then directly loaded onto an
Al2O3 plug and eluted with ethyl acetate and hexanes (1:10). The
crude reaction mixture was dissolved in toluene (2.0 mL), and
morpholine (0.2 mL) was added. The reaction mixture was heated
to 758C for 3 h, cooled, and concentrated under reduced pressure.
Pure morpholine amides were isolated after flash chromatography on
silica gel.
Scheme 3. Benzannulation with tert-butoxyacetylene.
the ketene surrogate coupling provides efficient access to aryl,
vinyl, and alkyl ketones.
Finally, taking inspiration from the Danheiser benzannu-
lation,[27] we developed a new benzannulation protocol as
outlined in Scheme 3.[28] 2-Iodostyrenes (22; X = CH) were
found to couple with tert-butoxyacetylene, rearrange to the
aryl ketene, and undergo 6p electrocyclic ring closure to
provide the naphthols 23a–c in good yield. The b-substituted
styrenes leading to 23b and 22c were used as mixture of E and
Z isomers, and both isomers appear to participate in the
sigmatropic rearrangement. The annulation was also success-
ful with 2-vinyl-3-iodopyridine to generate the quinoline 23d,
and even showed modest success with the imine derived from
2-iodoaniline (23e).
Received: May 6, 2014
Published online: June 27, 2014
Keywords: arenes · cross-coupling · ketenes · palladium ·
.
synthetic methods
[1] a) M. Palucki, S. L. Buchwald, J. Am. Chem. Soc. 1997, 119,
11108 – 11109; b) D. A. Culkin, J. F. Hartwig, Acc. Chem. Res.
2003, 36, 234 – 245; c) T. Hama, D. A. Culkin, J. F. Hartwig, J.
Am. Chem. Soc. 2006, 128, 4976 – 4985.
In summary, we have found that tert-butoxyacetylene
serves the role of metalated ketene in cross-coupling reac-
tions. It can undergo Sonogashira coupling with aryl iodides,
Angew. Chem. Int. Ed. 2014, 53, 8980 –8984
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
8983

.
Angewandte
Communications
[2] a) A. E. Allen, D. W. C. MacMillan, J. Am. Chem. Soc. 2011, 133,
4260 – 4263; b) A. L. Bigot, A. E. Williamson, M. J. Gaunt, J.
Am. Chem. Soc. 2011, 133, 13778 – 13781; c) J. S. Harvey, S. P.
Simonovich, C. R. Jamison, D. W. C. MacMillan, J. Am. Chem.
Soc. 2011, 133, 13782 – 13785.
[3] a) X. Huang, M. Patil, C. Farꢁs, W. Thiel, N. Maulide, J. Am.
Chem. Soc. 2013, 135, 7312 – 7323; b) B. Peng, D. Geerdink, C.
Farꢁs, N. Maulide, Angew. Chem. Int. Ed. 2014, 53, 5462 – 5466;
Angew. Chem. 2014, 126, 5566 – 5570.
[4] K. L. Jensen, P. T. Franke, L. T. Nielsen, K. Daasbjerg, K. A.
Jørgensen, Angew. Chem. Int. Ed. 2010, 49, 129 – 133; Angew.
Chem. 2010, 122, 133 – 137.
[5] Q.-L. Xu, H. Gao, M. Yousufuddin, D. H. Ess, L. Kurti, J. Am.
Chem. Soc. 2013, 135, 14048 – 14051.
[6] L. Brandsma, H. J. T. Bos, J. F. Arena in Chemistry of Acetylenes
(Ed.: H. G. Viebe), Marcel Dekker, New York, 1969; pp. 808 –
810.
[7] a-Aryl acetamides from ynamides: S. Bhunia, C.-J. Chang, R.-S.
Liu, Org. Lett. 2012, 14, 5522 – 5525.
[8] Oxidation of aryl acetylenes to aryl acetic acid derivatives: I.
Kim, C. Lee, Angew. Chem. Int. Ed. 2013, 52, 10023 – 10026;
Angew. Chem. 2013, 125, 10207 – 10210.
[9] a) P. A. Magriotis, D. Vourloumis, M. E. Scott, A. Tarli, Tetrahe-
dron Lett. 1993, 34, 2071 – 2074; b) R. M. Moslin, T. F. Jamison, J.
Am. Chem. Soc. 2006, 128, 15106 – 15107.
[10] a) T. Sakamoto, A. Yasuhara, Y. Kondo, H. Yamanaka, Chem.
Pharm. Bull. 1994, 42, 2032 – 2035; b) H. M. Nelson, J. R.
Gordon, S. C. Virgil, B. M. Stoltz, Angew. Chem. Int. Ed. 2013,
52, 6699 – 6703; Angew. Chem. 2013, 125, 6831 – 6835.
[11] P. H. Dussault, D. G. Sloss, D. J. Symonsbergen, Synlett 1998,
1387 – 1389.
[12] a) P. W. Davies, A. Cremonesi, L. Dumitrescu, Angew. Chem.
Int. Ed. 2011, 50, 8931 – 8935; Angew. Chem. 2011, 123, 9093 –
9097; b) J. H. Tatlock, J. Org. Chem. 1995, 60, 6221 – 6223.
[13] An efficient Sonogashira coupling with ynamides has been
reported: M. R. Tracey, Y. Zhang, M. O. Frederick, J. A. Mulder,
R. P. Hsung, Org. Lett. 2004, 6, 2209 – 2212.
[14] J. J. van Daalen, A. Kraak, J. F. Arens, Recl. Trav. Chim. Pays-
Bas 1961, 80, 810 – 818.
[15] a) E. Valenti, M. A. Pericas, F. Serratosa, J. Org. Chem. 1990, 55,
395 – 397; b) A. A. Moyano, M. A. Pericas, F. Serratosa, E.
Valenti, J. Org. Chem. 1987, 52, 5532 – 5538.
[16] We found wide variations in the yields when different batches of
[Pd(PPh3)4] from SigmaAldrich were used, while those from
Strem were more consistent. High reactivity could be rescued by
increasing the copper loading to 20 mol%. It is possible that
higher levels of PPh3 may poison the copper cocatalyst, and can
be overcome with additional copper. Reactions with [Pd2(dba)3]
+ PPh3 were uniformly reproducible.
[17] a) D. I. MaGee, M. Ramaseshan, J. D. Leach, Can. J. Chem.
1995, 73, 2111 – 2118; b) X. Y. Mak, R. P. Ciccolini, J. M.
Robinson, J. W. Tester, R. L. Danheiser, J. Org. Chem. 2009,
74, 9381 – 9387.
[18] a) R. Martꢂn, P. Romea, C. Tey, F. Urpꢂ, J. Vilarrasa, Synlett 1997,
1414 – 1416.
[19] M. Kobayashi, K. Masumoto, E.-i. Nakai, T. Nakai, Tetrahedron
Lett. 1996, 37, 3005 – 3008.
[20] R. W. Lang, H. J. Hansen, Helv. Chim. Acta 1980, 63, 438 – 455.
[21] J. A. Hyatt, P. W. Raynolds, Org. React. 1994, 45, 159 – 646.
[22] I. Jeon, I. K. Mangion, Synlett 2012, 1927 – 1930.
[23] L. S. Liebeskind, J. Srogl, J. Am. Chem. Soc. 2000, 122, 11260 –
11261.
[24] A. Krasovskiy, F. Kopp, P. Knochel, Angew. Chem. Int. Ed. 2006,
45, 497 – 500; Angew. Chem. 2006, 118, 511 – 515.
[25] Y. Wang, C. Wang, J. R. Butler, J. M. Ready, Angew. Chem. Int.
Ed. 2013, 52, 10796 – 10799; Angew. Chem. 2013, 125, 10996 –
10999.
[26] M. Badioli, R. Ballini, M. Bartolacci, G. Bosica, E. Torregiani, E.
Marcantoni, J. Org. Chem. 2002, 67, 8938 – 8942.
[27] a) R. L. Danheiser, S. K. Gee, J. Org. Chem. 1984, 49, 1672 –
1674; b) P. Turnbull, H. W. Moore, J. Org. Chem. 1995, 60, 644 –
649.
[28] a) A. G. Myers, Y. Horiguchi, Tetrahedron Lett. 1997, 38, 4363 –
4366; b) K. Araki, T. Katagiri, M. Inoue, J. Fluorine Chem. 2014,
157, 41 – 47.
8984 www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 8980 –8984