Synthesis of 5,9-Diaza[5]helicenes

Article abstract of DOI:10.1002/ejoc.201901302

A new method for the synthesis of 5,9-diaza[5]helicenes is presented using 2,3-bis(acylamino)-substituted ortho-terphenyls as precursors. Activation of the amide groups and electrophilic substitution at the ortho positions of the adjacent phenyl groups leads to the 5,9-diaza[5]helicenes. A stepwise reaction including protection of the first amino group, amide formation at the second amino group with subsequent cyclization, followed by deprotection, amide formation and cyclization at the first amino group ensures that both electrophilic substitutions take place at sufficiently activated arenes and allows for the different substituents at the diaza[5]helicenes brought in with the amide groups. The terphenyl precursors are synthesized by two Suzuki couplings of suitably substituted building blocks. Three different 5,9-diaza[5]helicenes with aliphatic, alkenyl and methoxycarbonylalkyl substituents were prepared; the latter would allow to attach further functionalities by ester or amide linkage.

Full text of DOI:10.1002/ejoc.201901302

DOI: 10.1002/ejoc.201901302
Communication
Azahelicenes
Synthesis of 5,9-Diaza[5]helicenes
Aaron Weiß[a] and Joachim Podlech*[a]
Abstract: A new method for the synthesis of 5,9-diaza[5]helice- tutions take place at sufficiently activated arenes and allows for
nes is presented using 2,3-bis(acylamino)-substituted ortho-ter- the different substituents at the diaza[5]helicenes brought in
phenyls as precursors. Activation of the amide groups and elec- with the amide groups. The terphenyl precursors are synthe-
trophilic substitution at the ortho positions of the adjacent sized by two Suzuki couplings of suitably substituted building
phenyl groups leads to the 5,9-diaza[5]helicenes. A stepwise blocks. Three different 5,9-diaza[5]helicenes with aliphatic, alk-
reaction including protection of the first amino group, amide enyl and methoxycarbonylalkyl substituents were prepared; the
formation at the second amino group with subsequent cycliza- latter would allow to attach further functionalities by ester or
tion, followed by deprotection, amide formation and cyclization amide linkage.
at the first amino group ensures that both electrophilic substi-
realized in prior syntheses.[1,7,8] These and other azahelicenes
Helicenes, i.e. polycyclic aromatic compounds with ortho-
have been used as chiral organocatalysts in asymmetric synthe-
sis[9–12] and in kinetic resolutions,[13] as proton sponges,[14–16]
and for the synthesis of helicene-based transition metal com-
plexes.[17] The quite related, positively charges azoniahelicenes
turned out to have a high binding selectivity for triple helix
DNA[18] or show a high thin film conductivity making them pos-
sibly useful for the field of optoelectronics.[19]
fused benzenes, have special optical and electronic properties,
which are similarly present in derivatives bearing heteroatoms
or being constructed from other aromatic or heteroaromatic
moieties.[1–3] According to the IUPAC definition,[4] the helicenes
consist of at least five rings (pentahelicene, [5]helicene). [5]Heli-
cenes are non-planar and thus chiral, albeit with a very low
racemization barrier (Figure 1).[5,6]
The most common syntheses of helicenes and azahelicenes
include the construction of Z-configured stilbene-type com-
pounds (typically by Wittig olefination) and their subsequent
electrocyclization (with concomitant oxidation).[1–3,7,8] Further
occasionally applied methods for the synthesis of aza- and di-
azahelicenes are the transition metal-catalyzed coupling of
ortho,ortho′-dihalobiaryls,[9,20,21] the [2+2+2] cyclization of suit-
able aromatic triynes,[22] or the PtCl4/InCl3-catalyzed cyclization
of ortho-alkynylbiaryls.[23] Herein we present a novel approach
to 5,9-diaza[5]helicenes starting with ortho-terphenyls.
A first considered retrosynthetic approach would cleave the
5,9-diaza[5]helicenes A in two concomitant retro-Friedel-Crafts-
type substitutions into a 2,3′-bis(acylamino)terphenyl B, which
could be traced back in a retro-cross coupling to a suitably
metalated building block D and a 2-halo-3-nitrobiphenyl E. The
latter should again be accessible by cross coupling (Scheme 1).
In the here reported first investigations within this strategy
we considered it useful to aim at first for 3,12-dimethoxy-substi-
tuted diaza[5]helicenes to facilitate the synthetic approach (in-
cluding the final electrophilic acylations), to furthermore im-
prove the solubility of the final products, and to allow for a
later attachment of further functionalities at possibly liberated
hydroxy groups.
Figure 1. Aza- and diaza[5]helicenes.
Seven aza[5]helicenes and 91 diaza[5]helicenes can be con-
structed, where a small fraction of these compounds have been
[a] Institut für Organische Chemie, Karlsruher Institut für Technologie (KIT)
76131 Karlsruhe, Fritz-Haber-Weg 6, Germany
E-mail: Joachim.podlech@kit.edu
http://www.ioc.kit.edu/podlech/
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201901302.
© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
This is an open access article under the terms of the Creative Commons
Attribution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
An electrophilic coupling partner suitable for a Suzuki cou-
pling towards terphenyl derivative 11, the ortho-iodinated bi-
phenyl derivative 7, was on its part prepared by cross coupling
(Scheme 2): 2-Iodo-3-nitroaniline 3, obtained from purchasable
2,6-dinitroaniline (1) by Sandmeyer reaction (→ 2)[24] and re-
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tion with tert-butyl isocyanide in the presence of cobalt(II) acet-
ylacetonate and oxygen (as recently reported by Tobisu et al.
for the synthesis of phenanthridines[30]) did not lead to any
well-defined product. Amide coupling with propionic acid in
the presence of propylphosphonic anhydride (T3P®) to bis-
(amide) 13 was successful, but a subsequent cyclization with
Eaton's reagent (P2O5 in MeSO3H)[31] did only lead to the forma-
tion of phenanthridine derivative 14 (albeit with poor yields).
Obviously, the fastest aromatic substitution is that of the east-
ern amide group at the activated (i.e., OMe-substituted) benz-
ene. The acylamino-substituted benzene seems to be less reac-
tive; its substitution proceeds with lower rates. Once the phen-
anthridine 14 is formed, the second substitution at this elec-
tron-deficient arene is obviously too sluggish – even at elevated
temperatures.
Scheme 1. Retrosynthetic analysis for the synthesis of 5,9-diaza[5]helicenes.
duction of one nitro group,[8] was treated with a borylated anis-
ole 5, prepared from the respective commercially available
bromide 4 by halogen-lithium exchange and subsequent trans-
metalation.[25]
Suzuki coupling using tetrakis(triphenylphosphine)palla-
dium(0)[26] as catalyst furnished 2-aminobiphenyl 6, which was
again subjected to a Sandmeyer reaction[27] and a subsequent
reduction of the nitro group,[28] yielding the iodinated substrate
7 in an overall yield of 21 % over four consecutive steps.
The nucleophilic coupling partner for the cross coupling to
terphenyl 11 was prepared from commercially available 4-
methoxy-2-nitroaniline (8), which was converted into an iodo
derivative 9. It turned out that this could not be transferred to
a boronate (possibly due to the ortho-nitro group), preventing
the planned Suzuki coupling. We alternatively considered a
Stille coupling and prepared aryl stannane 10 by reaction with
bis(tributyltin) in the presence of bis(triphenylphosphine)palla-
dium(II) dichloride as catalyst (57 % over two steps). Stille cross
coupling of building blocks 7 and 10 was achieved with tet-
rakis(triphenylphosphine)palladium(0) and copper(I) iodide as
catalysts and yielded terphenyl derivative 11 (88 %). Reduction
to diamine 12 was successfully performed with potassium boro-
hydride/copper(I) chloride.[29]
Scheme 3. Attempted acylation towards diaza[5]helicenes.
Consequently we changed our strategy and decided to pur-
sue a stepwise strategy (Scheme 4). This would allow to per-
form both electrophilic aromatic substitutions with activated
arenes and would furthermore give access to products F with
different substituents R1 and R2 introduced with the amide moi-
eties. We planned to build the western amide function, where
Nevertheless, all tested protocols for an amide formation the other amino group is still suitably protected (I); electrophilic
with concomitant cyclization towards J failed (Scheme 3). Reac- acylation towards phenanthridine H would now proceed at an
Scheme 2. Synthesis of a terphenyl derivative. Conditions: (a) NaNO2, H2SO4, AcOH, 40 °C, 30 min; then KI, H2O, 0–70 °C, 15 min (79 %); (b) Fe, AcOH, reflux,
2.5 h (44 %); (c) BuLi, –78 °C, 1.5 h; then iPrOB(pin), –78 °C to r.t., 16 h (95 %); (d) cat. Pd(PPh3)4, K2CO3, dioxane/H2O (4:1), 95 °C, 2 d (86 %); (e) NaNO2, conc.
HCl/MeCN (5:3), 0 °C to r.t., 30 min; then KI, H2O, 0–80 °C, 15 min (69 %); (f) NaNO2, H2SO4, H2O, –5 °C, 10 min; then KI, 0 °C, 30 min (78 %); (g) Sn2Bu6, cat.
Pd(PPh3)2Cl2, PPh3, xylene, 100 °C, 66 h (73 %); (h) cat. Pd(Ph3)4, cat CuI, CsF, DMF, 45 °C, 18 h (88 %); (i) KBH4, CuCl, MeOH, r.t., 20 min, 88 %.
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activated substrate. Deprotection and amide formation at the
The nitrobiphenyl 7 (Scheme 2) was reduced to amine 19
eastern amino group would furnish a substrate G, which again by a standard protocol.[28] Cross coupling with boronate 18 to
should be sufficiently reactive towards a substitution.
terphenylamine 20 was achieved in 87 % yield using a proven
method with palladium(II) acetate as precatalyst and SPhos
(2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl) as ligand
(Scheme 6).[36] We considered allyl groups suitable as protect-
ing groups for the amino group,[37] which can be introduced
with standard protocols.[38] They should not deactivate the ar-
ene in the electrophilic substitution, and they can be cleaved
with rhodium catalysis not affecting any of the other functional
groups in the substrates. A double allylation of terphenylamine
20 towards 21 was achieved with 61 % yield.
Scheme 4. Revised retrosynthetic scheme.
For this approach we again started with 4-methoxy-2-nitro-
aniline (8), which was subjected to a Sandmeyer reaction (now
furnishing the respective bromo compound 15[32]) and reduced
to aniline 16.[33] Acylation furnishing acetanilide 17 was here
performed with acetic acid in the presence of propylphos-
phonic anhydride (T3P®) as coupling agent.[34] This transforma-
tion yielded amides with excellent yields (further derivatives
have been prepared but are not part of this publication), but
could in principle be similarly performed with simpler protocols
(e.g. by coupling with the respective acid chlorides). Borylation
to a suitable nucleophile 18 for a pursued Suzuki coupling was
achieved by reaction with bis(pinacolato)diboron in the pres-
ence of [1,1′-bis(diphenylphosphino)ferrocene]dichloropalla-
dium(II) as catalyst (Scheme 5).[35] The rather low yield of 59 %
for this step was due to the poor solubility of the boronate,
which led to losses during the work-up and purification process.
Nevertheless, boronate 18 was thus obtained in four consecu-
tive steps with a total yield of 41 %. A stannylated substrate
which could have been used in a Stille coupling was similarly
synthesized, but since a non-identified side product could not
be separated by conventional methods, this variation was thus
no longer pursued.
Scheme 6. Synthesis of terphenyl derivative 21. Conditions: (n) Fe, HCl, EtOH,
reflux, 3 h (93 %); (o) 18, cat. Pd(OAc)2, SPhos, Cs2CO3, dioxane/H2O (7:1),
70 °C, 18 h (87 %); (p) allyl bromide, Na2CO3, DMF, reflux, 8 h (61 %).
Dehydrating ring closure to phenanthridine 21 and (in the
further course of the synthesis similarly to the diaza[5]helicene
25) was performed with triphenylphosphine oxide and tri-
fluoromethanesulfonic anhydride [Hendrickson reagent,
–
(Ph3P+)2O·2F3CSO3 ],[39] which proved to be superior to a more
conventional protocol using phosphoryl chloride (Scheme 7).
Phenanthridine derivative 22 was thus obtained in 81 % yield.
The second cyclization towards diaza[5]helicenes 25 was
achieved after cleavage of the allyl groups (→ 23)[40] and amide
Scheme 5. Synthesis of the nucleophilic coupling partner for a Suzuki cou-
pling. Conditions: (j) NaNO2, HBr, MeCN, 0 °C to r.t., 2 h; then CuBr, HBr, 80 °C,
10 min (80 %); (k) Fe, NH4Cl, EtOH/H2O (3:2), reflux, 2 h (95 %); (l) AcOH, T3P,
Scheme 7. Synthesis of 5,9-diaza[5]helicenes. Conditions: (q) Ph3PO/Tf2O (2:1),
CH2Cl2, 0 °C, 15 min; then 21, CH2Cl2, 0 °C to r.t., 15 h (22: 81 %); (r) cat.
(PPh3)3RhCl, MeCN/H2O (4:1), 100 °C, 16 h (→ 23: 80 %); (s) RCO2H, T3P,
EtOAc/pyridine (2:1), 0 °C to r.t. (see Table 1).
EtOAc/pyridine (2:1),
0 °C to r.t., 18 h (92 %); (m) B2Pin2, cat.
Pd(dppf)Cl2·CH2Cl2, KOAc, 1,4-dioxane, reflux, 3 h (59 %).
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monomethyl glutarate (58.9 mg, 403 μmol) in EtOAc/pyridine (2:1,
4 mL) placed in a 10 mL flask, the cooling bath was removed, and
formation (→ 24) with T3P® (vide supra). The obtained sub-
strates together with yields for the last two steps are summa-
rized in Table 1.
the mixture was stirred for 25 h at r.t. 1
M HCl (10 mL) was added
and the phases were separated. The aqueous phase was brought
to pH > 7 by addition of NaOH and extracted with Et2O (3 × 5 mL).
The combined organic layers were dried (Na2SO4), concentrated at
reduced pressure and purified by MPLC (silica gel, EtOAc/hexanes,
3:1) to yield the product as a colourless solid (67.1 mg, 142 μmol,
Table 1. Synthesis of 5,9-diaza[5]helicenes.
1
67 %): Rf = 0.58 (EtOAc); H NMR (400 MHz, CDCl3): δ = 1.87 (quint,
3J = 7.2 Hz, 2H, 3-H2), 2.22 (t, 3J = 7.3 Hz, 2H, 4-H2), 2.33 (t, 3J =
7.2 Hz, 2H, 2-H2), 3.03 (s, 3H, 6′-Me), 3.66 (s, 3H, CO2Me), 3.88 (s, 3H,
3
3′-OMe or 4′′-OMe), 3.96 (s, 3H, 4′′-OMe or 3′-OMe), 6.68 (dd, J =
9.4 Hz, 4J = 2.8 Hz, 1H, 2′-H), 7.05 (d, 3J = 9.4 Hz, 1H, 1′-H), 7.15 (bs,
1H, NH), 7.16–7.20 (m, 2H, 2′′-H, 6′′-H or 3′′-H, 5′′-H), 7.20–7.25 (m,
2H, 3′′-H, 5′′-H or 2′′-H, 6′′-H), 7.44 (d, 4J = 2.4 Hz, 1H, 4′-H), 8.25 (d,
3
3J = 9.1 Hz, 2H, 1′-H, 7′-H), 8.67 ppm (d, J = 9.0 Hz, 1H, 8′-H); 13C
[a] Helicene formation yielded product with a non-separable side product.
NMR (100 MHz, CDCl3): δ = 20.5 (CH2), 23.9 (CH3), 33.0 (CH2), 36.8
(CH2), 51.8 (CH3), 55.5 (CH3), 55.6 (CH3), 106.7 (CH), 115.7 (CH), 116.3
(2 × CH), 118.3 (C), 119.6 (CH), 122.9 (C), 126.2 (C), 127.6 (CH), 127.7
(CH), 130.0 (C), 131.2 (2 × CH), 132.3 (C), 138.9 (C), 147.2 (C), 159.2
This strategy was applied to three 5,9-diaza[5]helicenes
(Table 1) with different substituents at positions C-6 and C-10.
To reduce the synthetic effort, we used only one common ter-
phenyl precursor 21 bearing a methyl substituent at the posi-
tion later being C-6 in the diazahelicenes. Different substituents
were introduced with the second amide moiety, an isopropyl,
an (E)-2-propenyl, and a 3-(methoxycarbonyl)propyl group. The
propenyl group would allow for the introduction of further
functionalities, while the latter could be especially useful for
the attachment of structures via an ester or an amide linkage.
Introduction of a 4-heptanyl group was not successful: The re-
spective amide 24 was only isolated with a poor 20 % yield,
possibly due to steric reasons; it was not used in a 5,9-di-
aza[5]helicene synthesis. No amide was obtained, when amine
23 was treated with 4-bromobutanoic acid. Cyclization of amide
24a (R = Et) led to the formation of helicene 25a, but the pres-
ence of unidentified side products and problems during the
purification process prevented an isolation of the product. Its
synthesis was thus not included in Table 1. The 5,9-aza[5]helice-
nes 25 were additionally characterized by their UV/Vis spectra;
they show distinct absorptions around 237 and 310 nm. De-
tailed data and spectra are given in the Supporting Information.
5,9-Aza[5]helicenes 25 were thus obtained in total yields of
2–4 % over 11 linear steps (total 15 steps) starting with com-
mercially available 2,6-dinitroaniline and 4-methoxy-2-nitro-
aniline. The herein presented method allows for variable sub-
stituents at positions C-6 and C-10, but could be applicable
with further substituted starting materials (compounds 25 bear
methoxy groups at positions C-3 and C-12). Slight modifications
of this protocol should furthermore give access to the related
5,10- and 6,10-diaza[5]helicenes (Figure 1); their synthesis is
now investigated in our laboratories, as is the extension of
these methods to the synthesis of higher helicene-type com-
pounds.
(C), 159.3 (C), 160.1 (C), 170.4 (C), 173.4 ppm (C); IR (ATR): ν = 3401
˜
(w), 2998 (w), 2950 (w), 1732 (m), 1691 (m), 1588 (m), 1499 (m),
1026 (m), 829 (m) cm–1; MS (FAB): m/z (%): 474 (35) [M+ + 1], 473
(100) [M+], 472 (24), 345 (19); HRMS (FAB): m/z calcd. for
C28H29N2O5+: 473.2076, found 473.2077.
Methyl
4-(3,12-Dimethoxy-6-methyl-5,9-diaza[5]helicene-10-
yl)butanoate (25d): Tf2O (0.07 mL, 117 mg, 416 μmol) was added
slowly at 0 °C to a solution of Ph3PO (184 mg, 661 μmol) in anhy-
drous CH2Cl2 (4 mL) placed in a 25 mL Schlenk flask and the mixture
was stirred at 0 °C for 10 min. A solution of amide 24d (51.1 mg,
108 μmol) in anhydrous CH2Cl2 (2 mL) was added slowly and the
solution was stirred for 1 h at r.t. Saturated aqueous NaHCO3 solu-
tion (15 mL) was added, the mixture was stirred vigorously, and
the phases were separated. The aqueous phase was extracted with
CH2Cl2 (3 × 5 mL). The combined organic layers were dried
(Na2SO4), concentrated at reduced pressure, and purified by MPLC
(silica gel, cyclohexane/EtOAc, 1:1) to yield the product as a yellow
solid (33.5 mg, 73.7 μmol, 68 %): Rf = 0.41 (cyclohexane/EtOAc, 1:1);
1H NMR (400 MHz, CDCl3): δ = 2.25–2.46 (m, 2H, 3-H2), 2.56–2.66
(m, 2H, 2-H2), 3.12 (s, 3H, 6-Me), 3.38–3.55 (m, 2H, 4-H2), 3.71 (s, 3H,
CO2Me), 3.98 (s, 3H, 3′-OMe), 4.04 (s, 3H, 12′-OMe), 6.92 (dd, 3J =
4
3
4
9.2 Hz, J = 2.7 Hz, 1H, 2′-H), 7.15 (dd, J = 9.2 Hz, J = 2.6 Hz, 1H,
13′-H), 7.52 (d, 4J = 2.7 Hz, 1H, 4′-H), 7.69 (d, 4J = 2.6 Hz, 1H, 11′-H),
3
3
8.03 (d, J = 8.7 Hz, 1H, 7′-H), 8.19 (d, J = 8.7 Hz, 1H, 8′-H), 8.25 (d,
3J = 9.2 Hz, 1H, 1′-H), 8.46 ppm (d, J = 9.3 Hz, 1H, 14′-H); 13C NMR
3
(100 MHz, CDCl3): δ = 23.8 (CH3), 23.8 (CH2), 33.7 (CH2), 35.4 (CH2),
51.1 (CH3), 55.7 (CH3), 55.8 (CH3), 104.9 (CH), 107.9 (CH), 115.9 (CH),
118.9 (C), 119.0 (C), 119.7 (CH), 124.5 (C), 124.9 (CH), 126.9 (C), 128.1
(CH), 128.3 (C), 128.5 (CH), 129.4 (CH), 130.6 (C), 144.8 (C), 146.5 (C),
158.6 (C), 158.8 (C), 160.1 (C), 161.8 (C), 174.1 ppm (C); IR (ATR): ν =
˜
2920 (w), 1729 (m), 1612 (w), 1224 (w), 1166 (m), 1028 (w), 825 (m)
cm–1; UV/Vis (CH2Cl2): λ (log ε) = 236 (4.7), 262 (4.4), 305 (4.4), 315
(4.4), 338 (4.2), 389 (3.4), 410 (3.5) nm (dm3 mol–1 cm–1); MS (FAB):
m/z (%): 455 (15), 133 (100), 91 (15), 89 (21); HRMS (FAB): m/z calcd.
for C28H27N2O4+: 455.1971, found 455.1972.
Acknowledgments
Experimental Section
This work was supported by the Deutsche Forschungsgemein-
schaft (DFG). We are greatly indebted to Victorino Vallejos
González and Maximilian Gutsche for their help during their
Methyl 5-{[3-Methoxy-10-(4-methoxyphenyl)-6-methylphenan-
thridin-9-yl]amino}-5-oxopentanoate (24d): T3P solution (≥ 50 %
in MeCN, 625 mg, corresponds to ≥ 313 mg of T3P, 984 μmol) was
added at 0 °C to a solution of amine 23 (73.8 mg, 213 μmol) and advanced lab courses.
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[22] J. Míšek, F. Teplý, I. G. Stará, M. Tichý, D. Šaman, I. Císařová, P. Vojtíšek, I.
Starý, Angew. Chem. 2008, 120, 3232–3235; Angew. Chem. Int. Ed. 2008,
47, 3188–3191.
[23] J. Storch, J. Čermák, J. Karban, I. Císařová, J. Sýkora, J. Org. Chem. 2010,
75, 3137–3140.
Keywords: Helicenes · ortho-Biphenyls · Suzuki coupling ·
Phenanthridines · Electrophilic aromatic substitution
[24] M. Sienkowska, V. Benin, P. Kaszynski, Tetrahedron 2000, 56, 165–173.
[25] R. Li, T. Nakashima, O. Galangau, S. Iijima, R. Kanazawa, T. Kawai, Chem.
Asian J. 2015, 10, 1725–1730.
[26] B. A. Patel, R. Krishnan, N. Khadtare, K. R. Gurukumar, A. Basu, P. Arora,
A. Bhatt, M. R. Patel, D. Dana, S. Kumar, N. Kaushik-Basu, T. T. Talele,
Bioorg. Med. Chem. 2013, 21, 3262–3271.
[1] Y. Shen, C.-F. Chen, Chem. Rev. 2012, 112, 1463–1535.
[2] M. Gingras, Chem. Soc. Rev. 2013, 42, 968–1006.
[3] M. Gingras, G. Félix, R. Peresutti, Chem. Soc. Rev. 2013, 42, 1007–1050.
[4] G. P. Moss, P. A. S. Smith, D. Tavernier, Pure Appl. Chem. 1995, 67, 1307–
1375.
[5] C. Goedicke, H. Stegemeyer, Tetrahedron Lett. 1970, 11, 937–940.
[6] J. Barroso, J. L. Cabellos, S. Pan, F. Murillo, X. Zarate, M. A. Fernandez-
Herrera, G. Merino, Chem. Commun. 2018, 54, 188–191.
[7] F. Dumitrascu, D. G. Dumitrescu, I. Aron, ARKIVOC 2010, i, 1–32.
[8] I. Starý, I. Stará, Targets Heterocycl. Syst. 2017, 21, 23–53.
[9] N. Takenaka, R. S. Sarangthem, B. Captain, Angew. Chem. 2008, 120,
9854–9956; Angew. Chem. Int. Ed. 2008, 47, 9708–9710.
[10] N. Takenaka, J. Chen, B. Captain, R. S. Sarangthem, A. Chandrakumar, J.
Am. Chem. Soc. 2010, 132, 4536–4537.
[11] M. Šámal, J. Míšek, I. G. Stará, I. Starý, Collect. Czech. Chem. Commun.
2009, 74, 1151–1159.
[12] M. J. Narcis, N. Takenaka, Eur. J. Org. Chem. 2014, 21–34.
[13] M. R. Crittall, H. S. Rzepa, D. R. Carbery, Org. Lett. 2011, 13, 1250–1253.
[14] H. A. Staab, T. Saupe, Angew. Chem. 1988, 100, 895–909; Angew. Chem.
Int. Ed. Engl. 1988, 27, 865–879.
[27] L. Luo, H. Zheng, J. Liu, H. Wang, Y. Wang, X. Luan, Org. Lett. 2016, 18,
2082–2085.
[28] D. Gauthier, R. H. Dodd, P. Dauban, Tetrahedron 2009, 65, 8542–8555.
[29] Y. He, H. Zhao, X. Pan, S. Wang, Synth. Commun. 1989, 19, 3047–3050.
[30] M. Tobisu, K. Koh, T. Furukawa, N. Chatani, Angew. Chem. 2012, 124,
11525–11528; Angew. Chem. Int. Ed. 2012, 51, 11363–11366.
[31] P. E. Eaton, G. R. Carlson, J. T. Lee, J. Org. Chem. 1973, 38, 4071–4073.
[32] K. Tsuchiya, K. Sakaguchi, H. Kasuga, A. Kawakami, H. Taka, H. Kita, K.
Ogino, Polymer 2010, 51, 616–622.
[33] C. Kong, T. G. Driver, Org. Lett. 2015, 17, 802–805.
[34] J. R. Dunetz, Y. Xiang, A. Baldwin, J. Ringling, Org. Lett. 2011, 13, 5048–
5051.
[35] S. Lin, S. J. Danishefsky, Angew. Chem. 2001 113, 2021–2024; Angew.
Chem. Int. Ed. 2001, 40, 1967–1970.
[36] M. Altemöller, T. Gehring, J. Cudaj, J. Podlech, H. Goesmann, C. Feldmann,
A. Rothenberger, Eur. J. Org. Chem. 2009, 2130–2140.
[37] M. Radić Stojković, S. Miljanić, K. Mišković, L. Glavaš-Obrovac, I. Piantan-
ida, Mol. BioSyst. 2011, 7, 1753–1765.
[15] H. A. Staab, M. A. Zirnstein, C. Krieger, Angew. Chem. 1989, 101, 73–75;
Angew. Chem. Int. Ed. Engl. 1989, 28, 86–88.
[16] J. Roithová, D. Schröder, J. Míšek, I. G. Stará, I. Starý, J. Mass Spectrom.
2007, 42, 1233–1237.
[38] J. H. Tidwell, S. L. Buchwald, J. Am. Chem. Soc. 1994, 116, 11797–11810.
[39] J. Xi, Q.-L. Dong, G.-S. Liu, S. Wang, L. Chen, Z.-J. Yao, Synlett 2010, 1674–
1678.
[17] N. Saleh, C. Shen, J. Crassous, Chem. Sci. 2014, 5, 3680–3694.
[18] A. Granzhan, H. Ihmels, ChemBioChem 2006, 7, 1031–1033.
[19] L. Leontie, I. Druta, R. Danac, G. I. Rusu, Synth. Met. 2005, 155, 138–145.
[20] H. A. Staab, M. Diehm, C. Krieger, Tetrahedron Lett. 1994, 35, 8357–8360.
[21] D. C. Harrowven, I. L. Guy, L. Nanson, Angew. Chem. 2006, 118, 2300–
2303; Angew. Chem. Int. Ed. 2006, 45, 2242–2245.
[40] B. C. Laguzza, B. Ganem, Tetrahedron Lett. 1981, 22, 1483–1486.
Received: September 2, 2019
Eur. J. Org. Chem. 2019, 6697–6701
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© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim