N-Pyridinyl Sulfilimines as a Source for α-Imino Gold Carbenes: Access to 2-Amino-Substituted N-Fused Imidazoles

Article abstract of DOI:10.1021/acs.orglett.9b00140

Gold-catalyzed formal 1,3-dipolar annulation between readily accessible N-pyridinylsulfilimines and ynamides is reported. A diverse set of imidazole derivatives is prepared from the corresponding sulfilimines and ynamides. These functionalized cyclic products can undergo further transformations to afford diverse imidazole frameworks. Moreover, in situ synthesis is feasible and shows good potential in the synthesis of nucleoside analogues.

Full text of DOI:10.1021/acs.orglett.9b00140

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
N‑Pyridinyl Sulfilimines as a Source for α‑Imino Gold Carbenes:
Access to 2‑Amino-Substituted N‑Fused Imidazoles
Xianhai Tian,† Lina Song,† Matthias Rudolph,† Qian Wang,† Xinlong Song,† Frank Rominger,†
and A. Stephen K. Hashmi*,†,‡
†Institut fu
̈
r Organische Chemie, Universitat Heidelberg, Im Neuenheimer Feld 270, Heidelberg 69120, Germany
̈
‡Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
S
* Supporting Information
ABSTRACT: Gold-catalyzed formal 1,3-dipolar annulation between readily accessible N-pyridinylsulfilimines and ynamides is
reported. A diverse set of imidazole derivatives is prepared from the corresponding sulfilimines and ynamides. These
functionalized cyclic products can undergo further transformations to afford diverse imidazole frameworks. Moreover, in situ
synthesis is feasible and shows good potential in the synthesis of nucleoside analogues.
oldcomplexeshaveemergedashighlyeffectivecatalystsfor
the electrophilic activation of alkynes under homogeneous
G
conditions.1 Inparticular,gold-catalyzedreactionsbetweenylides
and alkynes can lead to bioactive compounds, and several ylidic
species were applied in this type of reaction, involving O−−N+,2
O−−S+,3 N−−N+,4 and C−−S+.5 Formation of gold carbenes by
intramolecularorintermolecularatomtransferprocessestoC−C
triple bonds enables efficient transformations. Moreover, the
introduction of functionality adjacent to the electrophilic gold
carbene in such processes offers a wide opportunity for facile
annulations to form nontrivial heterocycles. In this context, gold-
catalyzed dipolar annulations between conjugated ylides, such as
pyridine-N-aminides (N−−N+ ylide),4e acyl sulfonium ylides
(C−−S+ ylide),5a andactivatedalkynesprovidevaluablemethods
for target-directed synthesis, resulting in multisubstituted
oxazoles and furan derivatives.
Figure 1. Bioactive compounds bearing N-fused 2-amino imidazole
cores.
ments, an efficient and short route to this significant compound
class from readily available starting materials under mild
conditions is still highly desirable.
Imidazo[1,2-a]pyridines6 are keystone nitrogen-containing
heterocycles highly important in widespread areas. In particular,
imidazo[1,2-a]pyridines bearing a 2-amino group on the
imidazole ring are key pharmacophores for many biologically
important compounds, exhibiting a broad range of properties,
including antivirus,7 antitumor,8 antiproliferative,9 and smooth-
ened antagonistic10 activities (Figure 1). Hence, synthesis of
imidazo[1,2-a]pyridin-2-aminesisofgreatimportance,andinthe
last two decades, there has been long-standing interest in the
construction of N-fused imidazole frameworks. However,
methods for the preparation of fused 2-amino imidazoles are
rare and are based on classical multistep synthesis. Recently,
Davies’ group4e reported a gold-catalyzed [3 + 2] annulation
between N-pyrimidinylaminide and ynamide for the synthesis of
2-amino-substituted fused imidazole. Later, Chang et al.11
synthesized imidazo[1,2-a]pyridin-2-amine from 2-aminopyr-
idineandnitrileviaSnCl4-promotedcondensationandsequential
I2/KI-mediated oxidative cyclization. Despite these achieve-
Amongsulfur-containingylides,whereassulfoniumylideshave
been widely studied,12 sulfilimines13 are less explored. This
readily available ylidic unit can serve as valuable synthetic
intermediates through a cleavage of the polarized N−S bond.14
This versatile and promising metal−nitrene precursor has been
applied as a gold carbene precusor. Zhang et al.4b employed N-
sulfonylsulfiliminesasnitrenetransferreagentsforthesynthesisof
α,β-unsaturated amidines (two examples). We recently devel-
oped a successful strategy for preparing diverse aza-heterocycles
by utilizing N-phenylsulfilimines as gold carbene precusors.15
This method contains an efficient generation of α-imino gold
carbenes and subsequent divergent intramolecular trappings.
Inspired by the previous studies16 and as a continuous work on
Received: January 12, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b00140
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
sulfilimines, we herein envisioned the construction of 2-amino-
substituted imidazo[1,2-a]pyridine 3a from pyridine-based
sulfilimine 1a and ynamide 2a via a gold carbene intermediate
(Scheme 1).
product, DMS, was not observed (for the detailed investigation,
see Supporting Information).
Undertheoptimizedreactionconditions(Table1,entry4),we
investigated the reaction scope. As shown in Scheme 2, a series of
a b
,
Scheme 2. Scope of Sulfilimines
Scheme 1. Envisioned α-Imino Gold Carbene Transfer
Process via N−S Bond Cleavage
Reaction between S,S-dimethyl-N-(pyridin-2-yl)-sulfilimine
1a and ynamide 2a in toluene was first tested at 80 °C by
employing5mol%ofIPrAuCl/AgNTf2 ascatalyst(Table1,entry
a
Table 1. Optimization of Reaction Conditions
temp
(°C)/
b
entry
RSR
catalyst
solvent
time (h) yield (%)
d
1
2
3
4
5
6
7
8
9
DMS
DMS
DMS
DMS
DMS
DMS
DMS
DMS
DMS
IPrAuCl/AgNTf2
PPh3AuCl/AgNTf2
KAuBr4
PicAuCl2
Au-1
PicAuCl2
PicAuCl2
PicAuCl2
PicAuCl2
toluene
toluene
toluene
toluene
toluene
DCE
80/24
80/24
80/5
80/4
80/8
80/9
80/4
70/4
90/4
29
34
77
95 (97)
81
75
87
91
89
a
Reaction conditions: 1 (0.3 mmol, 1.5 equiv), 2a (0.2 mmol, 1.0
b
g
equiv), PicAuCl2 (3.8 mg, 5 mol %), toluene (2 mL, 0.1 M). Isolated
yields.
THF
sulfiliminesbearingelectron-donatinggroupsonthepyridinering
all smoothly converted to the corresponding imidazopyridines
(3b−f). Electron-deficient sulfilimines 1g−k, bearing a trifluor-
omethyl orhalogen substituentalsodelivered the targets 3g−k in
92−98% yield. A trisubstituted ylide was also well tolerated,
affording product 3l in 76% yield, the structure of which was
further confirmed by single-crystal X-ray structure analysis
(Figure 2). Additionally, this reaction was elaborated to enable
the synthesis of imidazo[1,2-a]pyrimidine 3m, imidazo[1,2-
a]pyrazine3n,imidazo[2,1-b]thiazole3o,benzo[d]imidazo[2,1-
b]oxazole 3p, and benzo[d]imidazo[2,1-b]thiazole 3q from the
corresponding sulfilimines bearing other heteroaromatic units.
toluene
toluene
toluene
toluene
toluene
e
10
11
12
DPS
PicAuCl2
PicAuCl2
100/10
80/10
80/10
61
93
nd
f
THT
DMS
c
a
General reaction conditions: 1 (0.3 mmol, 1.5 equiv), 2a (0.2 mmol,
b
1.0 equiv), catalyst (5 mol %), solvent (2.0 mL, 0.1 M). Isolated
yields of product 3a. Not detected. DMS: dimethylsulfide. DPS:
diphenylsulfide. THT: tetrahydrothiophene. Yield of the 2 mmol
scale reaction (reaction time: 18 h).
c
d
e
f
g
1), resulting in imidazopyridine 3a in 29% yield. Another gold(I)
catalyst did not improve the reaction (entry 2). Gold(III)
catalysts, such as KAuBr4, delivered 3a in good yield (entry 3).
17
Among the Au(III) catalysts tested, PicAuCl2 performed the
best, affording 3a in 95% yield. A 2 mmol scale reaction is even
more efficient (entry 4). DCE and THF are also suitable solvents
(entries 6 and 7). Short temperature screening (entries 8 and 9)
indicated that 80 °C was the best temperature. It is worth
mentioningthatS,S-diphenylsulfilimine1sandTHT-substituted
sulfilimine 1r also performed well, however, in lower yields
(entries 10 and 11). In the absence of any catalyst, no desired
product was achieved, obviously verifying the necessity of a gold
catalyst (entry 12). Poisoning of the gold catalyst by the second
Figure 2. Solid-state molecular structure of 3l.
B
DOI: 10.1021/acs.orglett.9b00140
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Organic Letters
Letter
As a next step, we tested the limitations with respect to the
ynamides (Scheme 3). A wide range of N-Ts-substituted
strongsterichindrance. Dimerizedynamide2aeandbis-ynamide
2af were able to efficiently convert into products 3aw and 3ax
containing two imidazopyridine cores by using 3.0 equiv of ylide
1a.Imidazo[1,2-a]pyridine3av,bycontrast,couldbeachievedby
decreasing the amount of ylide 1a.
a b
,
Scheme 3. Scope of Ynamides
Heterocycles with unprotected amino groups are always
associated with diverse bioactivities, hence a complete depro-
tection strategy was required. With 3w as an example, the
sequentialinsitudetosyl-18 anddebenzylationprocessesgavefree
6-bromo-3-phenylimidazo[1,2-a]pyridin-2-amine4in63%over-
all yield (Scheme 4). Reductive dediazotization of 4 with sodium
a
Scheme 4. Important Postmodifications of 3w
a
Isolated yields.
nitrite in aqueous hypophosphorus acid19 afforded 6-bromo-3-
phenylimidazo[1,2-a]pyridine 5 in 70% yield, which can be
quantitively converted into 2,6-dibromo-3-phenylimidazo[1,2-
a]pyridine 6 via NBS-mediated bromination at the 2-position on
the imidazole ring.20 This compound could serve as a valuable
precursor for cross-coupling strategies.
Saving time and energy needed for purification steps makes in
situ processes important for drug and natural product synthesis.
With sulfilimine 1a, we attempted to probe the feasibility of a
sequential synthesis of imidazopyridine 3a directly from 2-
aminopyridine1a′withoutpurificationoftheintermediate1a.As
depicted in Scheme 5, direct annulation between crude 1a and
a
Scheme 5. Investigations of an in Situ Process
a
Reaction conditions: 1 (0.3 mmol, 1.5 equiv), 2 (0.2 mmol, 1.0
b
equiv), PicAuCl2 (3.8 mg, 5 mol %), toluene (2 mL, 0.1 M). Isolated
yields. 1.0 equiv of ylide 1a was used. 3.0 equiv of ylide 1a was used;
reaction temperature: 85 °C.
c
d
ynamides 2b−j with a variety of R1 substituents, involving alkyl,
allyl, benzyl, and aryl, efficiently participated in this process. For
ynamides2k−n,containingothersulfonylprotectinggroups(Ms,
Ns, PhSO2) on the nitrogen atom, this reaction was found to be
compatible. The reaction proceeded well with various sub-
stituentsinortho,meta,andparapositionsonthebenzeneringsof
starting ynamides 2o−x. The efficiency correlated with the
electron density on the aromatic ring. Electron-rich aryl groups
gave the homologous products 3ah, 3ak, and 3al in good yields,
whereas the reaction was found to be less efficient with electron-
withdrawing groups on the aromatic ring (3af, 3am, and 3ao).
Moreover, 3-(thiophen-3-yl)-substituted (3ap−aq) and 3-
(pyridin-3-yl)-substituted (3ar) imidazo[1,2- a]pyridines were
synthesized from the corresponding heterocyclic-substituted
ynamides.Anynamidewith1,3-enynestructurewasalsofoundto
besuitable, albeitin51%yield(3as). Wethenmovedourfocusto
annulations between sulfilimines and alkyl-substituted ynamides
(R2 = alkyl). Alkyl ynamide 2ac (R2 = 3-phenylpropyl) reacted
well,deliveringthedesiredproduct3atin77%yield,andno1,2-H-
shift product was afforded. In the absence of an α-hydrogen,
ynamide 2ad gave the target compound 3au in 44% yield despite
a
Isolated yields.
ynamide2aaffordedproduct3ainevenhigheryieldthanthetwo-
step reactions using pure 1a. Further application by the in situ
modification of adenine 1r′ demonstrated that this versatile
protocol has great potential for the synthesis of nucleoside
analogues.
In summary, N-pyridinylsulfilimines, as new nitrene transfer
reagents, wereefficientlyappliedforthesynthesisofimidazo[1,2-
a]pyridin-2-amines and related heterocycles via α-imino gold
carbene intermediates. This method features simple reaction
conditions, high efficiency, and excellent functional group
compatibility. Required sulfilimines can be readily prepared
C
DOI: 10.1021/acs.orglett.9b00140
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Organic Letters
Letter
C.-H.;Ruan,P.-P.;Lu, X.;Ye, L.-W.Org.Lett.2016,18,3254. (k)Reddy,
R. J.; Ball-Jones, M. P.; Davies, P. W. Angew. Chem., Int. Ed. 2017, 56,
13310.
(5)(a)Kramer,S.;Skrydstrup,T.Angew.Chem.,Int.Ed.2012,51,4681.
(b)Huang,X.;Peng,B.;Luparia,M.;Gomes,L.F.;Veiros,L.F.;Maulide,
N. Angew. Chem., Int. Ed. 2012, 51, 8886.
(6) (a) Enguehard-Gueiffier, C.; Gueiffier, A. Mini-Rev. Med. Chem.
2007,7,888.(b)Koubachi,J.;ElKazzouli,S.;Bousmina,M.;Guillaumet,
G.Eur.J.Org.Chem.2014,2014,5119.(c)Bagdi,A.K.;Santra,S.;Monir,
from commercially available, inexpensive arylamines, dimethyl
sulfide, and NCS. Furthermore, in situ synthesis is available, and
this in situ process showed great potential in the synthesis of
nucleoside analogues. As a competing reaction, a gold-catalyzed
rearrangement (see Supporting Information) of sulfilimines to
offer a new approach to 3-methylthiomethyl-substituted 2-
aminopyridines was discovered.
ASSOCIATED CONTENT
* Supporting Information
́
K.; Hajra, A. Chem. Commun. 2015, 51, 1555. (d) Dyminska, L. Bioorg.
■
Med. Chem. 2015, 23, 6087. (e) Pericherla, K.; Kaswan, P.; Pandey, K.;
Kumar, A. Synthesis 2015, 47, 887.
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.9b00140.
(7) Hamdouchi, C.; de Blas, J.; del Prado, M.; Gruber, J.; Heinz, B. A.;
Vance, L. J. Med. Chem. 1999, 42, 50.
(8)Jaramillo,C.;deDiego,J.E.;Hamdouchi,C.;Collins,E.;Keyser,H.;
Experimental procedures and compound characterization
(PDF)
́
Sanchez-Martínez, C.; del Prado, M.; Norman, B.; Brooks, H. B.;
Watkins, S. A.; et al. Bioorg. Med. Chem. Lett. 2004, 14, 6095.
(9) Gallud, A.; Vaillant, O.; Maillard, L. T.; Arama, D. P.; Dubois, J.;
Maynadier,M.;Lisowski,V.;Garcia,M.;Martinez,J.;Masurier,N.Eur. J.
Med. Chem. 2014, 75, 382.
Accession Codes
CCDC 1812317 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.
(10) Lu, W.; Geng, D.; Sun, Z.; Yang, Z.; Ma, H.; Zheng, J.; Zhang, X.
Bioorg. Med. Chem. Lett. 2014, 24, 2300.
(11)Tian,X.;Song,L.;Wang,M.;Lv,Z.;Wu,J.;Yu,W.;Chang,J.Chem.
- Eur. J. 2016, 22, 7617.
(12) (a) Trost, B. M.; Melvin, L. S. Sulfur Ylides; Academic Press: New
York, 1975. (b)Ando, W. Acc. Chem. Res. 1977, 10, 179. (c) Aggarwal, V.
K.; Alonso, E.; Hynd, G.; Lydon, K. M.; Palmer, M. J.; Porcelloni, M.;
Studley, J. R. Angew. Chem., Int. Ed. 2001, 40, 1430. (d) Lu, L.-Q.; Chen,
J.-R.; Xiao, W.-J. Acc. Chem. Res. 2012, 45, 1278. (e) Mondal, M.; Chen,
S.; Kerrigan, N. J. Molecules 2018, 23, 738. (f) Neuhaus, J. D.; Oost, R.;
Merad, J.; Maulide, N. Top. Curr. Chem. 2018, 376, 15.
(13) (a) Tsujihara, K.; Furukawa, N.; Oae, K.; Oae, S. Bull. Chem. Soc.
Jpn.1969,42,2631.(b)Gilchrist,T.L.;Moody,C.J.Chem.Rev.1977,77,
409. (c) Oae, S. Sulfoxides and Sulfilimines. Organic Chemistry of Sulfur;
Springer, 1977; p 383.
(14)(a)Marino,J.P.;Zou,N.Org.Lett.2005,7,1915.(b)Grange,R.L.;
Evans,P.A.J.Am.Chem.Soc.2014,136,11870.(c)Grange,R.L.;Clizbe,
E. A.;Counsell, E. J.;Evans,P. A. Chem. Sci. 2015, 6, 777. (d)Yoshida, S.;
Yano, T.;Misawa, Y.;Sugimura, Y.;Igawa, K.;Shimizu, S.;Tomooka, K.;
Hosoya, T. J. Am. Chem. Soc. 2015,137, 14071. (e)Hayashi, R.;Shimizu,
A.; Yoshida, J.-i. J. Am. Chem. Soc. 2016, 138, 8400.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: hashmi@hashmi.de.
ORCID
A. Stephen K. Hashmi: 0000-0002-6720-8602
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
X.T., L.S., Q.W., and X.S. are grateful to the CSC (China
Scholarship Council) for a PhD fellowship.
■
(15) Tian, X.; Song, L.;Rudolph, M.; Rominger, F.;Oeser, T.;Hashmi,
A. S. K. Angew. Chem., Int. Ed. 2019 DOI: 10.1002/anie.201812002.
(16)(a)Zhou,A.-H.;He,Q.;Shu,C.;Yu,Y.-F.;Liu,S.;Zhao,T.;Zhang,
W.;Lu, X.;Ye, L.-W. Chem. Sci.2015, 6, 1265.(b)Jin,H.;Huang, L.;Xie,
J.; Rudolph, M.; Rominger, F.; Hashmi, A. S. K. Angew. Chem., Int. Ed.
2016, 55, 794. (c) Chen, M.; Sun, N.; Chen, H.; Liu, Y. Chem. Commun.
2016, 52, 6324. (d) Jin, H.; Tian, B.; Song, X.; Xie, J.; Rudolph, M.;
Rominger, F.; Hashmi, A. S. K. Angew. Chem., Int. Ed. 2016, 55, 12688.
(e)Zeng,Z.;Jin,H.;Xie,J.;Tian,B.;Rudolph,M.;Rominger,F.;Hashmi,
A.S.K.Org.Lett.2017,19,1020.(f)Sahani,R.L.;Liu,R.-S.Angew.Chem.,
Int. Ed. 2017, 56, 1026. (g) Xu, W.; Wang, G.; Sun, N.; Liu, Y. Org. Lett.
2017, 19, 3307. (h) Shen, W.-B.; Xiao, X.-Y.; Sun, Q.; Zhou, B.; Zhu, X.-
Q.; Yan, J.-Z.; Lu, X.; Ye, L.-W. Angew. Chem., Int. Ed. 2017, 56, 605.
(i)Zhao,Y.;Hu,Y.;Wang,C.;Li,X.;Wan,B.J.Org.Chem.2017,82,3935.
(j)Zeng,Z.;Jin,H.;Sekine,K.;Rudolph,M.;Rominger,F.;Hashmi,A.S.
K. Angew. Chem., Int. Ed. 2018, 57, 6935. (k) Chen, X.; Merrett, J. T.;
HongChan,P.W.Org.Lett.2018,20,1542.(l)Xu,W.;Wang,G.;Xie,X.;
REFERENCES
■
(1)(a)Hashmi,A.S.K.;Hutchings,G.J.Angew.Chem.,Int.Ed.2006,45,
7896. (b) Fu
̈
rstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007, 46,
3410.(c)Gorin,D.J.;Toste,F.D.Nature2007,446,395.(d)Hashmi,A.
S. K. Chem. Rev. 2007, 107, 3180. (e) Arcadi, A. Chem. Rev. 2008, 108,
3266. (f) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2010, 49, 5232.
́
(g)Corma,A.;Leyva-Perez,A.;Sabater,M.J.Chem.Rev.2011,111,1657.
(h) Rudolph, M.; Hashmi, A. S. K. Chem. Soc. Rev. 2012, 41, 2448.
(i) Dorel, R.; Echavarren, A. M. Chem. Rev. 2015, 115, 9028.
(j) Pflaesterer, D.; Hashmi, A. S. K. Chem. Soc. Rev. 2016, 45, 1331.
(k) Day, D. P.; Chan, P. W. H. Adv. Synth. Catal. 2016, 358, 1368.
(2)(a)Yeom,H.-S.;Shin,S.Acc.Chem.Res.2014,47,966.(b)Zhang,L.
Acc. Chem. Res.2014, 47,877. (c)Wang, Y.;Zhang,L. Synthesis2015, 47,
289. (d) Zheng, Z.; Wang, Z.; Wang, Y.; Zhang, L. Chem. Soc. Rev. 2016,
45, 4448. (e) Li, L.; Tan, T.-D.; Zhang, Y.-Q.; Liu, X.; Ye, L.-W. Org.
Biomol. Chem. 2017, 15, 8483.
(3) (a) Li, G.; Zhang, L. Angew. Chem., Int. Ed. 2007, 46, 5156.
(b) Shapiro, N. D.; Toste, F. D. J. Am. Chem. Soc. 2007, 129, 4160.
(4)(a)Gorin,D.;Davis,J.N.R.;Toste,F.D.J.Am.Chem.Soc.2005,127,
11260. (b) Li, C.; Zhang, L. Org. Lett. 2011, 13, 1738. (c) Wetzel, A.;
Gagosz, F. Angew. Chem., Int. Ed. 2011, 50, 7354. (d)Lu, B.; Luo, Y.;Liu,
L.; Ye, L.; Wang, Y.; Zhang, L. Angew. Chem., Int. Ed. 2011, 50, 8358.
(e) Davies, P. W.; Cremonesi, A.; Dumitrescu, L. Angew. Chem., Int. Ed.
2011, 50, 8931. (f) Yan, Z. Y.; Xiao, Y.; Zhang, L. Angew. Chem., Int. Ed.
2012, 51, 8624. (g) Garzon, M.; Davies, P. W. Org. Lett. 2014, 16, 4850.
(h)Shu,C.;Wang,Y.-H.;Zhou,B.;Li,X.-L.;Ping,Y.-F.;Lu,X.;Ye,L.-W.
J. Am. Chem. Soc. 2015, 137, 9567. (i) Wu, Y.; Zhu, L.; Yu, Y.; Luo, X.;
Huang, X. J. Org. Chem. 2015, 80, 11407. (j) Shu, C.; Wang, Y.-H.; Shen,
́
Liu,Y.Org.Lett.2018,20,32.(m)Zhao,Q.;LeonRayo,D.F.;Campeau,
D.; Daenen, M.; Gagosz, F. Angew. Chem., Int. Ed. 2018, 57, 13603.
(17) Hashmi, A. S. K.; Weyrauch, J. P.; Rudolph, M.; Kurpejovic, E.
Angew. Chem., Int. Ed. 2004, 43, 6545.
́
(18) Yoshida, S.; Igawa, K.; Tomooka, K. J. Am. Chem. Soc. 2012, 134,
19358.
(19) Franchetti, P.; Cristalli, G.; Grifantini, M.; Cappellacci, L.; Vittori,
S.; Nocentini, G. J. Med. Chem. 1990, 33, 2849.
(20)Marhadour,S.;Bazin,M.-A.;Marchand,P.TetrahedronLett.2012,
53, 297.
D
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