Electrophilic Bromolactonization of Cyclopropyl Carboxylic Acids Using Lewis Basic Sulfide Catalyst

Article abstract of DOI:10.1002/adsc.201500999

A highly facile and efficient electrophilic bromolactonization of cyclopropylcarboxylic acids could be effected by a Lewis basic sulfide catalyst. Mechanistic studies performed revealed that the cyclopropane substrates could undergo radical bromination upon exposure to light, yielding a mixture of regioisomers. In stark contrast, the Lewis basic sulfide catalyst could promote the electrophilic bromolactonization and yield the Markovnikov product exclusively.

Full text of DOI:10.1002/adsc.201500999

COMMUNICATIONS
DOI: 10.1002/adsc.201500999
Electrophilic Bromolactonization of Cyclopropyl Carboxylic
Acids Using Lewis Basic Sulfide Catalyst
Zhihai Ke,+a Ying-Chieh Wong,+b Jie Yang See,b and Ying-Yeung Yeunga,b,*
a
Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong, People’s Republic of China
E-mail: yyyeung@cuhk.edu.hk
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543
b
+
These authors contributed equally in this project.
Received: October 30, 2015; Revised: March 12, 2016; Published online: April 25, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500999.
Abstract: A highly facile and efficient electrophilic
bromolactonization of cyclopropylcarboxylic acids
could be effected by a Lewis basic sulfide catalyst.
Mechanistic studies performed revealed that the cy-
clopropane substrates could undergo radical bromi-
nation upon exposure to light, yielding a mixture of
regioisomers. In stark contrast, the Lewis basic sul-
fide catalyst could promote the electrophilic bromo-
lactonization and yield the Markovnikov product
exclusively.
propylcarboxylic acid 1. However, after examining
the reaction carefully it was found that such a reaction
could not proceed in the absence of light (vide infra,
Table 1). Under illuminated conditions, the cyclization
could proceed to yield the lactone product, suggesting
that, in the absence of a catalyst, cyclization of
1 might proceed through a radical mechanism trig-
gered by light. Alternatively, we report herein the
Table 1. Bromolactonization of cyclopropylcarboxylic acid
1a with or without light.[a]
Keywords: bromolactonization; cyclization; cyclo-
propanes; lactones; Lewis bases
Cyclopropanes are known to possess a strained ring
system. This is a consequence of inefficient orbital
overlapping, leading to three strained s-bonds (with
interorbital angles of 1058) which carry significant p-
character.[1] Thus, the reactivity of cyclopropane re-
sembles that of the carbon-carbon double bond.[2] Re-
cently, multiple research groups have invested consid-
erable efforts in the development of Lewis base-cata-
lyzed electrophilic bromocyclizations of olefinic sub-
strates.[3] On the basis of the above explanation, it can
be rationalized that cyclopropanes could substitute
olefins in the same type of electrophilic bromocycliza-
tions to give Markovnikov-type products.[4] Very re-
cently Hennecke reported the cyclization of 1a using
1,3-dibromo-5,5-dimethylhydantoin (DBH) in the ab-
sence of catalyst.[5] As part of the research interests in
Lewis base-catalyzed halocyclization reactions in our
team, we reported a Lewis basic sulfide-catalyzed
electrophilic bromocyclization of cyclopropylmethyl
amide very recently.[6] In a parallel research program,
we are also studying the bromocyclization of cyclo-
Entry
Br source
Ph3PS [mol%]
Light[b]
Yield [%]
1[c]
2[c]
3
NBS
DBH
NBS
DBH
NBS
NBS
DBH
NBS
DBH
0
0
0
0
off
off
on
on
off
off
off
on
on
0
0
53[e]
57[e]
40[e]
56[f]
88[f]
58[f]
89[f]
4
5[d]
6
0
10
10
10
10
7
8
9
[a]
Reactions were carried out with 1a (0.05 mmol), freshly
recrystallized halogen source (0.06 mmol), and Ph3PS
(0.005 mmol) in CH2Cl2 at 258C in the absence of light.
The yields are of isolated products.
Household 18W fluorescent lamp.
Starting material was recovered quantitatively.
Aged bottle of NBS was used.
The yield of the product mixture with 2a as one of the
products.
2a was isolated exclusively as a clean product.
[b]
[c]
[d]
[e]
[f]
Adv. Synth. Catal. 2016, 358, 1719 – 1724
1719
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

COMMUNICATIONS
asc.wiley-vch.de
cyclization to yield the six-membered lactone 4. The
myriad of possibilities escalates the difficulty of the
cyclopropane cyclization reaction as compared to the
classical bromolactonization of olefinic acids.
The project was initiated with the use of cyclopro-
pylcarboxylic acid 1a as the substrate. It was noted
that in the absence of light, a freshly recrystallized
brominating agent such as N-bromosuccinimide
(NBS) or 1,3-dibromo-5,5-dimethylhydantoin (DBH)
alone could not promote the lactonization and the
starting material 1a was recovered quantitatively after
3 days (Table 1, entries 1 and 2). However, when the
reaction was not shielded from light (standard house-
hold fluorescent lamp at the ceiling), the lactonization
could proceed to give a mixture of cyclized products
(Table 1, entry 3: ca. 53% yield when using NBS;
entry 4: ca. 57% yield when using DBH) which could
not be separated through column chromatography. In
the mixture, 2a was observed as one of the products
(Figure 1a). From the empirical evidence garnered
thus far, it was speculated that light might serve to
catalyze the reaction, presumably through radical
pathways,[7,8] to yield different isomers of lactones 2–4
Scheme 1. Lewis basic sulfide-catalyzed electrophilic bromo-
lactonization of cyclopropylcarboxylic acid 1.
first example of a Lewis basic sulfide-catalyzed elec- (vide infra). Indeed, a similar product mixture could
trophilic bromolactonization of cyclopropylcarboxylic also be obtained in the absence of light when an aged
acids 1 (Scheme 1).
bottle of NBS was used (Table 1, entry 5).
The cyclopropane in 1 contains two types of s-
To our delight, the lactonization of 1a proceeded
bonds with p-character: the sterically hindered but smoothly when 10 mol% of triphenylphosphine sul-
electron-rich bond (a) which is more reactive towards fide was used as the Lewis base catalyst in the ab-
electrophilic co-bromination; and the less substituted sence of light; 2a was furnished in 56% yield (togeth-
bond (b) which has reduced steric hindrance while er with 40% of 1a recovered) when NBS was used as
being comparatively less electron-rich (Scheme 2). the halogen source (Table 1, entry 6). Alternatively,
The bromolactonization of 1 can potentially proceed an 88% yield of 2a was achieved exclusively when the
through multiple pathways: Eq. (1) – bromination at stronger brominating agent DBH was used (Table 1,
bond (a) and the subsequent attack of the carboxylate entry 7). It is noteworthy that a clean product was ob-
at the more substituted carbon I to give the five-mem- tained in both cases and 2a was found to be the only
bered lactone 2; Eq. (2) – bromination at bond (a) constitutional isomer (Figure 1b and c). Unexpectedly,
followed by the attack of the carboxylate at the less the cyclization could still proceed well in the presence
hindered-carbon II to give the seven-membered lac- of both light and the catalyst (10 mol% of triphenyl-
tone 3; Eq. (3) – bromination at bond (b) followed by phosphine sulfide). Reaction performance under
Lewis base catalysis was noted to be identical in the
presence or absence of light (Table 1, entries 6 and 7
Scheme 2. Possible pathways for the bromolactonization of
cyclopropylcarboxylic acid 1.
Figure 1. 1H NMR study of product 2a from the bromolacto-
nization of 1a.
Adv. Synth. Catal. 2016, 358, 1719 – 1724
1720
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

COMMUNICATIONS
asc.wiley-vch.de
vs. entries 8 and 9). This observation suggests that the 9). A brief solvent screening revealed that chlorinated
Lewis basic sulfide-catalyzed electrophilic bromolac- solvents such as CH2Cl2 and 1,2-dichloroethane were
tonization might predominate over the light-triggered suitable reaction media (Table 2, entries 9–13).
pathway.
Next, the substrate scope was investigated
Subsequently, various catalysts were screened and (Table 3). The reactions were generally smooth when
the results are depicted in Table 2. The experimental using electron-rich phenyl-substituted substrates in-
outcomes revealed that tripheylphosphine selenide re- cluding 1b (p-methyl), 1c (m-methyl), 1f (p-methoxy),
sults in a much lower conversion as compared to tri- and 1g (p-tert-butyl). Substrates with sterically bulky
phenylphosphine sulfide (Table 2, entry 2). On the substituents (1d, 1e, 1h) also worked well under this
other hand, tetrahydrothiophene was found to be catalytic protocol to give the corresponding products
a potent Lewis basic catalyst which gave the desired 2d, 2e, and 2h in excellent yields. On the other hand,
lactone 2a in 87% yield (Table 2, entry 3). We also at- substrates 1i–l bearing electron-deficient aryl substitu-
tempted to utilize the organic amine base Et3N or in- ents exhibited lower conversions. Cyclization of 1m,
organic base K2CO3 aiming at generating the carbox- a substrate with a methyl, and 1n, a substrate with
ylate anion as a better nucleophilic partner for the a cyclohexyl substituent, gave the desired products
lactonization. However, the reaction turned out to be 2m and 2n in excellent yields. The reaction was also
sluggish (Table 2, entries 4 and 5). We speculated that
the electrophilic activation of the cyclopropane might
be the main driving force of the bromolactonization
Table 3. Scope of the bromocyclization of 1.[a]
of 1a. Surprisingly, (Æ)-1,1’-binaphthalene-2,2’-diyl hy-
drogen phosphate, a good Brønsted acid activator for
NBS, did not promote the bromolactonization of 1a
(Table 2, entry 6). Several bromine sources were also
evaluated. As compared to NBS, it was realized that
N-bromophthalimide (NBP) and 2,4,4,6-tetrabromo-
2,5-cyclohexadienone (TBCO) were inferior while
DBH returned higher efficiency (Table 2, entries 1, 7–
Table 2. Optimization of the bromolactonization of 1a.[a]
Entry Br
source
Catalyst
Solvent Yield
[%]
1
2
3
4
5
6
NBS
NBS
NBS
NBS
NBS
NBS
Ph3PS
Ph3PSe
tetrahydrothiophene
Et3N
K2CO3
(Æ)-1,1’-binaphthalene-
2,2’-diyl hydrogen phos-
phate
CH2Cl2 58
CH2Cl2 trace
CH2Cl2 87
CH2Cl2
CH2Cl2
CH2Cl2
0
0
0
7
8
9
10
11
12
13
NBP
Ph3PS
CH2Cl2 52
CH2Cl2 28
CH2Cl2 89
(CH2Cl)2 88
CH3NO2 62
TBCO Ph3PS
DBH Ph3PS
DBH Ph3PS
DBH Ph3PS
DBH Ph3PS
DBH Ph3PS
[a]
[b]
Reactions were conducted with 1 (0.05 mmol), DBH
(0.06 mmol), and Ph3PS (0.005 mmol) in CH2Cl2 (2.0 mL)
at 258C in the absence of light.
Yield when using tetrahydrothiophene and NBS instead
of Ph3PS and DBH.
PhCH3
DMF
9
74
[a]
Reactions were carried out with 1a (0.05 mmol), freshly
recrystallized halogen source (0.06 mmol), and Ph3PS
(0.005 mmol) in CH2Cl2 at 258C in the absence of light.
The yields are of isolated products.
[c]
[d]
2.0 mmol scale.
TBCO was used instead of DBH.
Adv. Synth. Catal. 2016, 358, 1719 – 1724
1721
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

COMMUNICATIONS
asc.wiley-vch.de
Scheme 3. Transformations using 2a.
readily scalable without diminishing the reaction
yields (2f and 2k).
The bromide substituent in 2 could be further ma-
nipulated with ease. For instance, 2a could react with
p-nitrobenzenesulfonamide in the presence of AgOTf
to give 5 in 64% yield (Scheme 3). The structure of 5
Scheme 5. Studies on the bromolactonization of 1l.
Other than 1,1-disubstituted cyclopropanes 1, we
was confirmed unambiguously by an X-ray crystallo- also investigated the bromocyclization of trans-1,2-di-
graphic study.[9] In addition, treatment of 2a with substituted cyclopropyl acid 7. To our delight, the de-
DBU at 258C gave rise to the olefin 6, a consequence sired d-lactone 8 was obtained in high yield and dia-
of dehydrobromination, in 72% yield.
stereoselectivity. More importantly, the bromolactoni-
Several control experiments were performed with zation was also found to be enantiospecific
the objective of probing the reaction mechanism. (Scheme 6).
Firstly, it was realized that no bromolactonization oc-
curred for substrate 1k in the absence of both light
and catalyst. A 61% yield of 2k was achieved (with
35% of 1k recovered) when the reaction was conduct-
ed under the optimized conditions. However, a com-
plex reaction mixture was obtained when the reaction
was not shielded from light and no catalyst was ap-
Scheme 6. Studies on the bromolactonization of 7.
plied. Three fractions (regioisomers 2k, 3k, and an in-
separable mixture)[10] were isolated after extensive ef-
forts (Scheme 4). The formation of regioisomers
might be attributed to the uncontrolled radical bromi-
We speculated that the Lewis basic sulfide catalyst
could activate the brominating agent and promote the
nation of cyclopropane.[4,7]
Secondly, it was discovered that bromolactonization electrophilic bromination[11] of the cyclopropane
of the highly electron-deficient substrate 1l did not moiety in 1 to yield bromiranium-like species A
proceed in the absence of a Lewis basic catalyst, even (Scheme 7). Subsequent cyclization following the
after being subjected to illuminated conditions for 3
days (Scheme 5). In stark contrast, 31% of 2l was iso-
lated (together with 65% of 1l recovered) when
10 mol% of triphenylphosphine sulfide was included
in the reaction mixture in the absence of light.
Scheme 7. A plausible mechanism of the lewis base-cata-
lyzed electrophilic bromolactonization of cyclopropylcarbox-
Scheme 4. Studies on the bromolactonization of 1k.
ylic acid 1.
Adv. Synth. Catal. 2016, 358, 1719 – 1724
1722
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

COMMUNICATIONS
asc.wiley-vch.de
Markovnikov rule could give the observed product 2. Experimental Section
On the other hand, the light-triggered conditions
might lead to the radical bromination of the cyclopro- General Procedure for the Bromolactonization of
pane in 1 followed by cyclization to yield lactone Cyclopropylcarboxylic Acids 1
products 2–4 through some plausible pathways as de-
The cyclopropylcarboxylic acid 1 (0.05 mmol, 1.0 equiv.) and
picted in Scheme 7. On the basis of the results in
Ph3PS (0.005 mmol, 0.1 equiv.) were dissolved in dichloro-
Table 1 and Scheme 5, it appears that the Lewis basic
methane (2 mL) and stirred at 258C. Subsequently, DBH
sulfide-catalyzed electrophilic bromination of 1 is
more efficient than the light-triggered radical path-
way. However, we could not rule out the possibility
that a carbocation A’ (the ring-opened species of A)
might be involved in the reaction and a more detailed
mechanistic study is required in order to elucidate the
mechanistic picture.
We also attempted to apply the catalytic protocol
in other reactions (Scheme 8). Preliminary results
show that N-chlorosuccinimide (NCS) could be uti-
lized in this catalytic protocol to give the chlorinated
compound 9. In addition, other than the synthesis of
g-lactone, d-lactone 11 could be furnished using cyclo-
propylcarboxylic acid 10 as the substrate. When using
cyclopropyl 1,3-diol 12 as the substrate, the multi-
functionalized THF 13 could be furnished in good
conversion.
(0.06 mmol, 1.2 equiv.) was added into the reaction mixture.
The mixture was then stirred at 258C in the absence or pres-
ence of light. For the reaction without light, the flask was
wrapped with aluminum foil tightly. For the reaction run in
the presence of light, the flask was exposed under household
florescent lamp (18W). Upon completion, a saturated aque-
ous solution of Na2SO3 (1 mL) was added to quench the re-
action. The mixture was further diluted with DI water
(3 mL) and extracted with CH2Cl2 (3”5 mL). The organic
extracts were combined, dried over sodium sulfate, filtered,
and concentrated under reduced pressure. The residue was
purified over silica gel chromatography with eluent n-
hexane/ethyl acetate (20:1) to yield the corresponding cy-
clized product.
Acknowledgements
We are thankful for the financial support from The Chinese
University of Hong Kong Startup Funding and National Uni-
versity of Singapore Tier 1 Funding (grant no. 143-000-605-
112).
References
[1] a) A. D. Walsh, Nature 1947, 159, 165; b) R. Robinson,
Nature 1947, 159, 400; c) C. A. McDowell, Nature 1947,
159, 508; d) A. D. Walsh, Nature 1947, 159, 712; e) R.
Robinson, Nature 1947, 160, 162; f) J. W. Linnett,
Nature 1947, 160, 162; g) A. D. Walsh, Trans. Faraday
Soc. 1949, 45, 179; h) W. A. Bernett, J. Chem. Educ.
1967, 44, 17; i) A. de Meijere, Angew. Chem. 1979, 91,
867; Angew. Chem. Int. Ed. 1979, 18, 809.
[2] a) H. Zhao, X. Fan, J. Yu, C. Zhu, J. Am. Chem. Soc.
2015, 137, 3490; b) S. Ren, C. Feng, T.-P. Loh, Org.
Biomol. Chem. 2015, 13, 5105; c) J. Kim, M. Sim, N.
Kim, S. Hong, Chem. Sci. 2015, 6, 3611; d) K. S. L.
Chen, H.-Y. Fu, J.-Q. Yu, J. Am. Chem. Soc. 2015, 137,
2042; e) P. D. Pohlhaus, S. D. Sanders, A. T. Parsons, W.
Li, J. S. Johnson, J. Am. Chem. Soc. 2008, 130, 8642;
f) L. Jiao, C. Yuan, Z.-X. Yu, J. Am. Chem. Soc. 2008,
130, 4421.
[3] For selected references on Lewis base-catalyzed bro-
molactonization, see: a) S. Terashima, S.-S. Jew, Tetra-
hedron Lett. 1977, 18, 1005; b) F. Homsi, G. Rousseau,
J. Org. Chem. 1999, 64, 81; c) S. R. Mellegaard, J. A.
Tunge, J. Org. Chem. 2004, 69, 8979; d) S. M. Ahmad,
D. C. Braddock, G. Cansell, S. A. Hermitage, Tetrahe-
dron Lett. 2007, 48, 915; e) W. Zhang, S. Zheng, N. Liu,
J. B. Werness, I. A. Guzei, W. Tang, J. Am. Chem. Soc.
2010, 132, 3664; f) K. Murai, T. Matsushita, A. Naka-
mura, S. Fukushima, M. Shimura, H. Fujioka, Angew.
Chem. 2010, 122, 9360; Angew. Chem. Int. Ed. 2010, 49,
Scheme 8. Halocyclization of 1k, 10, and 12.
In summary, we have developed a facile and highly
efficient electrophilic bromolactonization of cyclopro-
pylcarboxylic acids using a Lewis basic sulfide as the
catalyst. Although light could trigger the reaction to
give a mixture of constitutional isomers, the Lewis
basic sulfide-catalyzed reaction was found to be the
prevailing pathway which led to the desired lactone
as the exclusive isomer. Further studies on the appli-
cations and mechanism of this reaction are underway.
Adv. Synth. Catal. 2016, 358, 1719 – 1724
1723
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

COMMUNICATIONS
asc.wiley-vch.de
9174; g) L. Zhou, C. K. Tan, X. Jiang, F. Chen, Y.-Y.
Yeung, J. Am. Chem. Soc. 2010, 132, 15474; h) S. J.
Balkrishna, C. D. Prasad, P. Panini, M. R. Detty, D.
Chopra, S. Kumar, J. Org. Chem. 2012, 77, 9541. For
recent reviews on halofunctionalization of olefins, see:
i) G. F. Chen, S. M. Ma, Angew. Chem. 2010, 122, 8484;
Angew. Chem. Int. Ed. 2010, 49, 8306; j) S. A. Snyder,
D. S. Treitler, A. P. Brucks, Aldrichimica Acta 2011, 44,
27; k) A. Castellanos, S. P. Fletcher, Chem. Eur. J. 2011,
17, 5766; l) C. K. Tan, L. Zhou, Y.-Y. Yeung, Synlett
2011, 1335; m) S. E. Denmark, W. E. Kuester, M. T.
Burk, Angew. Chem. 2012, 124, 11098; Angew. Chem.
Int. Ed. 2012, 51, 10938; n) U. Hennecke, Chem. Asian
J. 2012, 7, 456; o) K. Murai, H. Fujioka, Heterocycles
2013, 87, 763; p) C. K. Tan, Y.-Y. Yeung, Chem.
Commun. 2013, 49, 7985; q) S. Zheng, C. M. Schiene-
beck, W. Zhang, H.-Y. Wang, W. Tang, Asian J. Org.
Chem. 2014, 3, 366; r) C. B. Tripathi, S. Mukherjee,
Synlett 2014, 25, 163; s) J. R. Wolstenhulme, V. Gouver-
neur, Acc. Chem. Res. 2014, 47, 3560; t) Y. A. Cheng,
W. Z. Yu, Y.-Y. Yeung, Org. Biomol. Chem. 2014, 12,
2333. For selected examples on asymmetric electrophil-
ic bromination catalyzed by Lewis bases, see: u) A. Sa-
kakura, A. Ukai, K. Ishihara, Nature 2007, 445, 900;
v) S. A. Snyder, D. S. Treitler, Angew. Chem. 2009, 121,
8039; Angew. Chem. Int. Ed. 2009, 48, 7899; w) S. A.
Snyder, D. S. Treitler, A. P. Brucks, J. Am. Chem. Soc.
2010, 132, 14303; x) Y. Sawamura, H. Nakatsuji, A. Sa-
kakura, K. Ishihara, Chem. Sci. 2013, 4, 4181; y) Y.
Kawato, A. Kubota, H. Ono, H. Egami, Y. Hamashima,
Org. Lett. 2015, 17, 1244.
[5] C. Rçsner, U. Hennecke, Org. Lett. 2015, 17, 3226.
[6] Y.-C. Wong, Z. Ke, Y.-Y. Yeung, Org. Lett. 2015, 17,
4944.
[7] a) J. M. Tanko, R. H. Mas, N. K. Suleman, J. Am.
Chem. Soc. 1990, 112, 5557; b) J. C. Arnold, G. J.
Gleicher, J. D. Unruh, J. Am. Chem. Soc. 1974, 96, 787;
c) J. M. Tanko, N. K. Suleman, J. F. Blackert, J. Org.
Chem. 1991, 56, 6395; d) X. Creary, J. Org. Chem. 1980,
45, 4653; e) M. Jason, P. Kurzweil, K. Leonard, J. Org.
Chem. 1986, 51, 2550; f) A. Podgorsek, S. Stavber, M.
Zupan, J. Iskra, Tetrahedron Lett. 2006, 47, 7245.
[8] For the light-triggered reaction of 1a, the lactonization
could still proceed but gives a mixture of cyclized prod-
ucts when the light is removed after the reaction is ini-
tiated.
[9] CCDC 1410272 (5) contains the supplementary crystal-
lographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_re-
quest/cif.
[10] MS studies on the inseparable mixture revealed the ex-
istence of a major species having an equal mass to 2k
and 3k. We speculated that the species might be the d-
lactone generated through the cyclization indicated in
Scheme 2 Eq. (3). A more detailed study is underway
to clarify this aspect.
[11] a) S. E. Denmark, M. T. Burk, Proc. Natl. Acad. Sci.
USA 2010, 107, 20655; b) D. W. L. Tay, I. T. Tsoi, J. C.
Er, G. Y. C. Leung, Y.-Y. Yeung, Org. Lett. 2013, 15,
1310; c) F. Chen, C. K. Tan, Y.-Y. Yeung, J. Am. Chem.
Soc. 2013, 135, 1232; d) Z. Ke, C. K. Tan, F. Chen, Y.-
Y. Yeung, J. Am. Chem. Soc. 2014, 136, 5627.
[4] N. V. Zyk, A. Y. Gavrilova, O. B. Bondarenko, O. A.
Mukhina, V. N. Tikhanushkina, Russ. J. Org. Chem.
2011, 47, 340.
Adv. Synth. Catal. 2016, 358, 1719 – 1724
1724
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim