Synthesis of 2-Pyridinemethyl Ester Derivatives from Aldehydes and 2-Alkylheterocycle N-Oxides via Copper-Catalyzed Tandem Oxidative Coupling-Rearrangement

Article abstract of DOI:10.1021/acs.orglett.7b03446

Successful benzylic C(sp³)-H acyloxylation of 2-alkylpyridine, 2-alkylpyrazine, and 2-alkylthiazole compounds was achieved using simple aldehydes. This was carried out via a copper-catalyzed tandem reaction, involving oxidative esterification followed by O-atom transfer of the resultant high yield formed Boekelheide intermediate. The method enables the preparation of functional heterocycles and the desymmetrization of 2,6-dialkylpyridines for efficient synthesis of dissymmetric pincer ligands, thus offering a new life for more practical Boekelheide rearrangement.

Full text of DOI:10.1021/acs.orglett.7b03446

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis of 2‑Pyridinemethyl Ester Derivatives from Aldehydes and
2‑Alkylheterocycle N‑Oxides via Copper-Catalyzed Tandem Oxidative
Coupling−Rearrangement
Chang-Sheng Wang, Thierry Roisnel, Pierre H. Dixneuf, and Jean-Franco̧ is Soule*
́
Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS-Universite
Campus de Beaulieu, 35042 Rennes, France
́ ́ ́
de Rennes “Organometalliques: Materiaux et Catalyse”,
S
* Supporting Information
ABSTRACT: Successful benzylic C(sp3)−H acyloxylation of
2-alkylpyridine, 2-alkylpyrazine, and 2-alkylthiazole com-
pounds was achieved using simple aldehydes. This was carried
out via a copper-catalyzed tandem reaction, involving oxidative
esterification followed by O-atom transfer of the resultant high
yield formed Boekelheide intermediate. The method enables
the preparation of functional heterocycles and the desymmetrization of 2,6-dialkylpyridines for efficient synthesis of dissymmetric
pincer ligands, thus offering a new life for more practical Boekelheide rearrangement.
yridines play an important role in synthetic chemistry as one
P
of the most dominant heteroarenes in pharmaceuticals and
natural products.1 In particular, they are useful in molecular
materials,2 and as ligands in organometallic chemistry,3 including
high-potential photocatalysis.4 One of the most common
synthetic approaches to pyridine-containing targets rely on
cycloaddition or condensation reactions using expensive noble
metals.5 The recent development of crucial pincer ligands based
on functional pyridines for creative pincer-metal catalyzed
reaction requires the discovery of more simple and particle access
tomultifunctionalizedpyridines.6 Amongsyntheticintermediates
of pyridine ligands, 2-pyridinemethyl esters are important and
they are generally obtained via multistep synthesis including
oxidation−reduction sequences.7 In 1954, Boekelheide reported
an efficient pathway to obtain pyridin-2-ylmethanol derivatives
from acyl chloride and 2-methylpyridine N-oxide via the
rearrangement of the 1-acetoxypyridin-1-ium intermediate
(Figure 1a).8 However, the Boekelheide reaction is of limited
utility as (i) it gives a mixture of sp3/sp2-oxylation products,
including chlorinated product and (ii) it shows poor substrate
scope.8 Alternative strategies for the higher yield synthesis of the
2-pyridinemethylestersinvolvetheuseofactivatingreagentssuch
as anhydrides,9 although such protocols suffer from poor atom
efficiency.
Recently, cross-dehydrogenative couplings (CDC) have been
introduced as an eco-friendly substitute protocol allowing
aldehydes to be employed as more convenient acyl sources.10
Indeed, aldehydes are cheap, abundant, water tolerant, easy to
handle, and noncorrosive starting materials, making them ideal
candidatesforindustrialapplicationscomparedtoanhydridesand
acyl chlorides. Since the pioneer work by C.-J. Li on oxidative
amidation and esterification from aldehydes catalyzed by copper
in the presence of peroxide (Figure 1b),11 such CDC couplings
have been widely applied in oxidative amides,12 esters,13 or
ketones synthesis.14 Interestingly, Barbas and co-workers
Figure 1. Cross-dehydrogenative couplings.
reported the metal-free CDC from aldehydes with N-hydroxy-
phthalimide for the preparation of active esters (Figure 1c).15
However, there is no example of CDC reaction from aldehydes
with 2-alkylpyridine N-oxides, althoughitcould allowthe one-pot
synthesis of 2-pyridinemethyl esters. However, such CDC with
heterocycles is very challenging because there are many possible
side-reaction pathways including C(sp2)-acylation.16 Under
CDC conditions, Wu et al. has reported that 2-methylquinoline
N-oxidecanbeacetoxylatedwithoutO-atomtransfer.17 However,
Received: November 6, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03446
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
no reaction occurred with 2-alkylpyridine N-oxides. Here, we
report the first selective C(sp3)−H bond acyloxylation of 2-
alkylpyridine and derivatives via a copper-catalyzed tandem
oxidative process starting from 2-alkylheterocycle N-oxides and
simple aldehydes (Scheme 1d).
Initial optimization was performed using 2-methylpyridine N-
oxide and benzaldehyde with DTBP (di-tert-butyl peroxide) as
the oxidant in tBuOH/DCE at 120 °C (Table 1); such conditions
ligands (L1−L4), and the best yield of 83% was obtained using
1,10-phenanthroline (L3) (Table 1, entries 6−9). Other oxidants
such as TBHP (tert-butyl hydroperoxide) and CHP (cumene
hydroperoxide) were less effective, while PhI(OAc)2 was
completely inactive (Table 1, entries 10−12).
The scope of the reaction proved to be broad, with a wide range
of aryl- and heteroaryl-functionalized aldehydes being suitable for
this copper-catalyzed tandem oxidative reaction, affording the
corresponding pyridin-2-ylmethyl benzoate in good to excellent
yields (Scheme 1). For instance, benzaldehyde substituted at the
para position by an electron-donating group (e.g., methyl, n-
pentyl, and phenyl) underwent a smooth reaction to give the
products 2−4 in good yields, with X-ray diffraction analysis of 4
confirming the structure (CCDC 1564793). Interestingly,
aldehydes bearing an halogen atom (e.g., Cl, Br, and F) at the
para or meta position are well tolerated, as the desired pyridin-2-
ylmethyl substituted benzoates 5−7 were obtained in 62−78%
yields without the cleavage of C−X bonds, allowing further
transformations. 3,4,5-Trimethoxybenzaldehyde displayed good
reactivity, as 8 was isolated in 71% yield. The reaction seems to be
slightly sensitive to the steric factor, as congested 2-tolualdehyde
allowed the formation of pyridin-2-ylmethyl 2-methylbenzoate
(9) in only 63% yield. Heteroaryl benzaldehydes including 5-
thienyl, 2-pyrolyl, and ferrocenecarboxaldehyde were also
evaluated, and the corresponding products 10−12 were obtained
with high efficiency. However, when aliphatic aldehydes are used
no reaction occurred. From DMF as a cheap aldehyde source, the
dimethylcarbamate 13 was obtained in only 33% yield.
Table 1. Optimization of the Reaction Conditions
entry
cat. (x)
L
oxidant
DTBP
yield in 1 (%)
1
−
−
0
2
FeCl3
−
−
−
−
L1
L2
L3
L4
L3
L3
L3
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
TBHP
CHP
38
0
3
[RuCl2(p-cymene)]2
Co(acac)2
4
0
5
CuBr(Me2S)
CuBr(Me2S)
CuBr(Me2S)
CuBr(Me2S)
CuBr(Me2S)
CuBr(Me2S)
CuBr(Me2S)
CuBr(Me2S)
58
70
75
83
76
37
25
0
6
7
8
9
10
11
12
PhI(OAc)2
We next set out to evaluate the reaction scope with respect to
the 2-alkylpyridine N-oxide derivatives in reactions with
benzaldehyde (Scheme 2). Interestingly, this method provided
a
L1 = 2,2′-bipyridine; L2 = 4,4′-di-tert-butyl-2,2′-bipyridine; L3 =
1,10-phenanthroline; L4 = 4,7-dimethyl-1,10-phenanthroline.
Scheme1. Scopeof(Hetero)(Aryl)Aldehydes inCu-Catalyzed
Scheme 2. Scope of 2-Alkyl Heterocycle N-Oxides in Cu-
Catalyzed C(sp3)−H Bond Benzoxylation with Benzaldehyde
C(sp3)−H Bond Benzoxylation of 2-Methylpyridine N-Oxide
the desymmetrization of one methyl group of 2,6-lutidine and
2,4,6-collidine N-oxides with theformation ofmonobenzoxylated
products 14 and 15 in 82% and 76% yield, respectively. The
optimized reaction conditions tolerated a bromo or an ester
substituent on the 2-methylpyridine N-oxide partner, as 16 and
17 were isolated in good yield. Formation of tertiary carbons has
been demonstrated with the synthesis of 2-ethylpyridine N-oxide
18 in 62% yield. 2-Methylquinoline N-oxide reacted via O atom
transfer to deliver 19 in 68% yield. 2-Methyl-pyrazine smoothly
reacted to give the pyrazin-2-ylmethyl benzoate 20 in 35% yield.
Interestingly, the reaction is not limited to 6-membered
are known to generate acyl radical.16a−d No reaction occurred
without catalysts (Table 1, entry 1). FeCl3 proved to be effective
to catalyze tandem acylation−Boekelheide rearrangement
allowing the formation of pyridin-2-ylmethyl benzoate (1) in
38% yield (Table 1, entry 2). No reaction occurred using
[RuCl2(p-cymene)]2 or Co(acac)2, whereas using CuBr(Me2S),
the desired product 1 was obtained in 58% yield (Table 1, entries
3−5). The yield in 1 was improved employing N,N-bidentate
B
DOI: 10.1021/acs.orglett.7b03446
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
heterocycles. For instance, 2-alkylthiazole N-oxides were
successfully benzoxylated to give 21 and 22 in good yields.
To gain insight into the reaction mechanism, several control
experiments were conducted (Scheme 3). First, from 2-
Scheme 4. Proposed Mechanism
Scheme 3. Mechanistic Investigations
should allow the formation of the Cu(III) intermediate E, which
undergoes a reductive elimination to release the desired
Boekelheide intermediate F and the active copper(I) A species.
Then, this unstable intermediate undergoes a [3,3]-sigmatropic
shift to deliver the desired sp3-benzoxylated pyridine derivatives.
Finally, a gram scale reaction of 2,6-lutidine N-oxide with
benzaldehyde was tested; delightfully, this reaction could be
performed on 10 mmol scale and proceeded smoothly to give
product 14 in 83% yield (1.9 g; Scheme 5).
methylpyridine or 3-methylpyridine N-oxide, no benzoxylated
product was generated (Scheme 3a). No reaction occurred using
benzoic acid or tert-butyl benzoperoxoate instead of benzalde-
hyde (Scheme 3a). These results strongly suggested that
acyloxylation of the benzylic C(sp3)−H bond results from an
O-atomtransferoftheN-oxide(internaloxidant)andnotfroman
external oxidant.18 We then carried out a TEMPO radical
quenching experiment; the reaction was stopped, and only the
benzaldehyde−TEMPO adduct was observed, as opposed to the
2-methylpyridine N-oxide−TEMPOadduct. This resultindicates
that only the acyl generation step involves a radical process, not
Scheme 5. Application to the Synthesis of Pincer Ligand
Intermediates
the O-atom transfer. From 2-(methyl-d3)pyridine N-oxide19
a
KIE value of 2.1 was determined from both parallel and
intermolecular completion reactions,20 suggesting that the C−
H bond cleavage occurred during the rate-determining step
(RDS) (Scheme 3b).21 In addition, we conducted two
competitive reactions to probe the substituent preference for
such couplings. From an equimolar ratio of 4-chloro-
benzaldehyde and 3,4,5-trimethoxybenzaldehyde (3 equiv) in
the presence of 2-methylpyridine N-oxide (1 equiv), we observed
the formation of amixture of 5 and 8 in a 34:66 ratio (Scheme 3c).
Electron-donation groups on the pyridine oxide favor the
reaction, as after 2 h the reaction of an equimolar ratio of 2,4,6-
collidine oxide and 3-(ethoxycarbonyl)-2-methylpyridine N-
oxide in the presence of benzaldehyde give 15 and 17 in an
80:20 ratio (Scheme 3d).
On the basis of the above experimental results and literature on
CDC,22 a plausible mechanism is illustrated in Scheme 4. First,
homolytic cleavage of the DTBP peroxide allowed the formation
of the tert-butoxyl radical (tBuO•) which is trapped by the
copper(I) A to generate the copper(II) alkoxide B. σ-Bond
metathesis of B with a 2-alkylpyridine N-oxide regenerates
intermediate C and tert-butoxide.23 Deprotonation of the
benzylic position of C generates the intermediate D in an RDS.
The coordination of the O-heterocycle to the copper center
renders the O-atom particularly reactive toward reaction with C-
based radicals (R•), which are generated by abstraction of a
hydrogen atom from benzaldehyde by a tert-butoxyl radical
(tBuO•). Reaction of the acyl radical (RCO•) with Cu(II) D
Interestingly, wedemonstratedthesyntheticutilityofournovel
method for the synthesis of PNN Milstein-type pincer ligands,24
through the synthesis of the intermediate 23 in 84% overall yield.
The previous method, namely radical bromination of 2,6-lutidine,
required a more toxic media (NBS in CCl4) and gives 23 in poor
25−40% yield.25 From 2-((Boc)amino)-6-methylpyridine N-
oxide, an sp3-benzoxylation reaction allowed the formation of 24
in 61% yield. Notably, the Boc group did not survive the reaction
conditions and the amino group was alkylated with a radical
generating from DTBP. Treatment with acid gave the amino-
alcohol 25, which might be further to the pincer ligands
(PNNP).26 This compound 25 was previously obtained in
three steps from tert-butyl (6-methylpyridin-2-yl)carbamate in
moderate overall yield for the synthesis of multidentate ligands or
pharmaceuticals.27
In summary, a convenient and functional-group tolerant
copper-catalyzed sp3-benxozylation of 2-alkylpyridine N-oxide
C
DOI: 10.1021/acs.orglett.7b03446
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(8) (a) Boekelheide, V.; Linn, W. J. J. Am. Chem. Soc. 1954, 76, 1286.
(b) Boekelheide, V.; Lehn, W. L. J. Org. Chem. 1961, 26, 428. (c) Vozza, J.
F. J. Org. Chem. 1962, 27, 3856. (d) Fontenas, C.; Bejan, E.; Haddou, H.
A.; Balavoine, G. G. A. Synth. Commun. 1995, 25, 629.
(9) (a) Abbate, S.; Bazzini, C.; Caronna, T.; Fontana, F.; Gambarotti,
C.;Gangemi, F.;Longhi,G.;Mele,A.;Sora, I.N.;Panzeri, W. Tetrahedron
2006, 62, 139. (b) Eidamshaus, C.;Reissig, H.-U. Eur. J. Org. Chem. 2011,
2011, 6056. (c) Kawasuji, T.; Johns, B. A.; Yoshida, H.; Weatherhead, J.
G.;Akiyama, T.;Taishi, T.;Taoda, Y.;Mikamiyama-Iwata, M.;Murai, H.;
Kiyama, R.; Fuji, M.; Tanimoto, N.; Yoshinaga, T.; Seki, T.; Kobayashi,
M.; Sato, A.; Garvey, E. P.; Fujiwara, T. J. Med. Chem. 2013, 56, 1124.
(10) (a) Taylor, R. J. K.; Reid, M.; Foot, J.; Raw, S. A. Acc. Chem. Res.
2005, 38, 851. (b) Scheuermann, C. J. Chem. - Asian J. 2010, 5, 436.
(c) Jeena, V.; Robinson, R. S. RSC Adv. 2014, 4, 40720.
(11)(a)Yoo, W.-J.;Li,C.-J. J.Am. Chem.Soc. 2006,128,13064. (b)Yoo,
W.-J.; Li, C.-J. Tetrahedron Lett. 2007, 48, 1033.
(12) (a) Gunanathan, C.; Ben-David, Y.; Milstein, D. Science 2007, 317,
790. (b) Chang, J. W. W.; Chan, P. W. H. Angew. Chem., Int. Ed. 2008, 47,
1138. (c)Ghosh, S.C.;Ngiam, J. S. Y.;Chai, C. L.L.;Seayad, A.M.;Dang,
T. T.; Chen, A. Adv. Synth. Catal. 2012, 354, 1407. (d) Miyamura, H.;
has been developed. The reaction proceeds through an
unprecedented sequence of N-oxide acylation of 2-alkylpyridine
N-oxide with aldehydes followed by a Boekelheide rearrange-
ment. The utility of the method is highlighted by the ability to
carry out the desymmetrization of 2,6-lutidine, which provides a
more efficient route to very useful Milstein-type pincer ligands.
Thisstrategy represents anappealing synthetic methodtoachieve
regio- and chemoselective sp3-functionalization of 2-alkylpyr-
idine.
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b03446.
Experimental procedures, copy of 1H and 13C NMR of all
compounds, X-ray analysis of 4 (PDF)
Accession Codes
́
Min, H.; Soule, J.-F.; Kobayashi, S. Angew. Chem., Int. Ed. 2015, 54, 7564.
CCDC 1564793 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44 1223 336033.
(13) (a) Yoo, W.-J.; Li, C.-J. J. Org. Chem. 2006, 71, 6266.
(b) Gowrisankar, S.; Neumann, H.; Beller, M. Angew. Chem., Int. Ed.
2011, 50, 5139. (c) Liu, C.; Wang, J.; Meng, L.; Deng, Y.; Li, Y.; Lei, A.
Angew. Chem., Int. Ed. 2011, 50, 5144. (d) Rout, S. K.; Guin, S.; Ghara, K.
K.; Banerjee, A.; Patel, B. K. Org. Lett. 2012, 14, 3982.
(14) (a) Basle,
2010, 352, 1145. (b)Shuai, Q.;Yang, L.;Guo, X.;Basle,
Chem. Soc. 2010, 132, 12212.
́
O.; Bidange, J.; Shuai, Q.; Li, C.-J. Adv. Synth. Catal.
́
O.;Li, C.-J. J. Am.
AUTHOR INFORMATION
■
(15) Tan, B.; Toda, N.; Barbas, C. F. Angew. Chem., Int. Ed. 2012, 51,
12538.
Corresponding Author
*E-mail: jean-francois-soule@univ-rennes1.fr.
ORCID
(16) (a) Deng, G.; Ueda, K.; Yanagisawa, S.; Itami, K.; Li, C.-J. Chem. -
Eur. J. 2009, 15, 333. (b) Sun, M.; Hou, L.-K.; Chen, X.-X.; Yang, X.-J.;
Sun, W.;Zang,Y.-S. Adv. Synth. Catal. 2014, 356,3789. (c)Chen, J.;Wan,
M.; Hua, J.; Sun, Y.; Lv, Z.; Li, W.; Liu, L. Org. Biomol. Chem. 2015, 13,
11561. (d) Wang, Y.; Zhang, L. Synthesis 2015, 47, 289. (e) Matcha, K.;
Antonchick, A. P. Angew. Chem. 2013, 125, 2136.
(17) (a) Chen, X.; Zhu, C.; Cui, X.; Wu, Y. Chem. Commun. 2013, 49,
6900. (b) Huang, J.; Li, L.-T.; Li, H.-Y.; Husan, E.; Wang, P.; Wang, B.
Chem. Commun. 2012, 48, 10204.
(18) (a) Xiao, J.; Li, X. Angew. Chem., Int. Ed. 2011, 50, 7226. (b) Zhang,
X.; Qi, Z.; Li, X. Angew. Chem., Int. Ed. 2014, 53, 10794. (c) Sharma, U.;
Park, Y.; Chang, S. J. Org. Chem. 2014, 79, 9899. (d) Barsu, N.; Sen, M.;
Premkumar, J. R.; Sundararaju, B. Chem. Commun. 2016, 52, 1338.
(19) Pavlik, J. W.; Vongnakorn, T.; Tantayanon, S. J. Heterocycl. Chem.
2009, 46, 213.
Jean-Franco̧ is Soule: 0000-0002-6593-1995
́
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
C.-S.W. acknowledges the China Scholarship Council (CSC) for
a PhD grant.
■
REFERENCES
■
(1)(a)Vitaku, E.; Smith, D. T.;Njardarson, J. T. J. Med. Chem. 2014, 57,
10257. (b) Pozharskii, A. F.; Soldatenkov, A. T.; Katritzky, A. R.
Heterocycles in Life and Society; J. Wiley: New York, 1997.
(20) See Supporting Information for more details.
(21) Simmons, E. M.; Hartwig, J. F. Angew. Chem., Int. Ed. 2012, 51,
3066.
(22) (a) Tran, B. L.; Li, B.; Driess, M.; Hartwig, J. F. J. Am. Chem. Soc.
2014, 136, 2555. (b) Zeng, H.-T.; Huang, J.-M. Org. Lett. 2015, 17, 4276.
(2) Kiuru, P.; Yli-Kauhaluoma, J. Pyridine and Its Derivatives. In
Heterocycles in Natural Product Synthesis, Majumdar, K. C.,
Chattopadhyay, S. K., Eds.; Wiley-VCH: Weinheim, 2011; p 267.
(3) (a) Spencer, L. P.; Altwer, R.; Wei, P.; Gelmini, L.; Gauld, J.;
Stephan, D. W. Organometallics 2003, 22, 3841. (b) Gunanathan, C.;
Milstein, D. Chem. Rev. 2014, 114, 12024.
(4) (a) Yoon, T. P.; Ischay, M. A.; Du, J. Nat. Chem. 2010, 2, 527.
(b)Koike,T.;Akita,M. Acc.Chem. Res.2016,49,1937.(c)Twilton, J.;Le,
C.; Zhang, P.; Shaw, M. H.; Evans, R. W.; MacMillan, D. W. C. Nature
Reviews Chemistry 2017, 1, 0052.
(c) Wang, C.-S.; Wu, X.-F.; Dixneuf, P. H.; Soule,
2017, 10, 3075.
́
J.-F. ChemSusChem
(23) Jotham, R. W.; Kettle, S. F. A.; Marks, J. A. J. Chem. Soc., Dalton
Trans. 1972, 1133.
(24) Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc.
2005, 127, 10840.
(25) (a) Gultneh, Y.; Yisgedu, T. B.; Tesema, Y. T.; Butcher, R. J. Inorg.
Chem. 2003, 42, 1857. (b) Rojas, D.; Garcia, A. M.; Vega, A.; Moreno, Y.;
Venegas-Yazigi, D.; Garland, M. T.; Manzur, J. Inorg. Chem. 2004, 43,
6324.
(26) He, L.-P.; Chen, T.; Gong, D.; Lai, Z.; Huang, K.-W.
Organometallics 2012, 31, 5208.
(27) (a) Balkovec, J. M.;Szymonifka, M. J.; Heck, J. V.; Ratcliffe, R. W. J.
Antibiot. 1991, 44, 1172. (b) Ulrich, S.; Petitjean, A.; Lehn, J.-M. Eur. J.
Inorg. Chem. 2010, 2010, 1913.
́
(5) (a) Boger, D. L. Chem. Rev. 1986, 86, 781. (b) Varela, J. A.; Saa, C.
Chem. Rev. 2003, 103, 3787. (c) Hill, M. D. Chem. - Eur. J. 2010, 16,
12052.
(6) (a) Gunanathan, C.; Milstein, D. Acc. Chem. Res. 2011, 44, 588.
(b) Asay, M.; Morales-Morales, D. Dalton Trans. 2015, 44, 17432.
(c) Bauer, G.; Hu, X. Inorg. Chem. Front. 2016, 3, 741.
(7) (a) Gruntz, U.; Katritzky, A. R.; Kenny, D. H.; Rezende, M. C.;
Sheikh, H. J. Chem. Soc., Chem. Commun. 1977, 701. (b) Ko, S.; Lee, C.;
Choi, M.-G.; Na, Y.; Chang, S. J. Org. Chem. 2003, 68, 1607. (c) Shintou,
T.; Kikuchi, W.; Mukaiyama, T. Bull. Chem. Soc. Jpn. 2003, 76, 1645.
(d) Hasnik, Z.; Silhar, P.; Hocek, M. Synlett 2008, 2008, 543. (e) Xie, F.;
Yan, F.; Chen, M.; Zhang, M. RSC Adv. 2014, 4, 29502.
D
DOI: 10.1021/acs.orglett.7b03446
Org. Lett. XXXX, XXX, XXX−XXX