Y(OTf)3-Catalyzed Cascade Propargylic Substitution/Aza-Meyer-Schuster Rearrangement: Stereoselective Synthesis of α,β-Unsaturated Hydrazones and Their Conversion into Pyrazoles

Article abstract of DOI:10.1055/s-0034-1381057

A straightforward and concise method for the highly stereoselective synthesis of α,β-unsaturated hydrazones by the Y(OTf)₃-catalyzed cascade propargylic substitution/aza-Meyer-Schuster rearrangement reaction of tertiary propargylic alcohols and

Full text of DOI:10.1055/s-0034-1381057

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© Georg Thieme Verlag Stuttgart · New York
2015, 26, 2170–2174
letter
2170
W. Liu et al.
Letter
Synlett
Y(OTf)3-Catalyzed Cascade Propargylic Substitution/Aza-Meyer–
Schuster Rearrangement: Stereoselective Synthesis of α,β-Unsat-
urated Hydrazones and Their Conversion into Pyrazoles
Wenfei Liu
N
N
R1
R2
Han Wang
R3
Haiying Zhao
Baoguo Li*
Shufeng Chen*
OH
R
1 = aryl, R2 = alkyl
Y(OTf)3 (20 mol%)
MeCN, 80 °C
R2
NNHTs
R3
+
TsNHNH2
R1
R2
R1
H
R3
N
N
R3
Inner Mongolia Key Laboratory of Fine Organic Synthesis,
Department of Chemistry and Chemical Engineering, Inner
Mongolia University, Hohhot 010021, P. R. of China
shufengchen@imu.edu.cn
R1
29 examples
high stereoselectivity
R2
R1 = R2 = aryl
baoguol@sohu.com
through a cascade propargylic substitution/aza-Meyer–Schuster rearrangement
Received: 16.05.2015
Accepted after revision: 03.07.2015
Published online: 12.08.2015
trimethylsilyl-substituted tertiary propargylic alcohols and
p-toluenesulfonyl hydrazide (Scheme 1, eq 2).5 More re-
cently, the same group has also developed a one-pot syn-
thesis of pyrazoles from tertiary propargylic alcohols and p-
DOI: 10.1055/s-0034-1381057; Art ID: st-2015-w0374-l
Abstract A straightforward and concise method for the highly stereo-
selective synthesis of α,β-unsaturated hydrazones by the Y(OTf)3-cata-
lyzed cascade propargylic substitution/aza-Meyer–Schuster rearrange-
ment reaction of tertiary propargylic alcohols and p-toluenesulfonyl
hydrazide under an air atmosphere is developed. A series of α,β-unsatu-
rated hydrazones have been synthesized from simple and readily avail-
able starting materials in good yields. Furthermore, the obtained α,β-
unsaturated hydrazones are converted into pyrazoles in the presence of
LiOt-Bu.
toluenesulfonyl hydrazide through a two-step cascade se-
quence (Scheme 1, eq 3).6 During our recent studies on
propargylic chemistry,7 we have found that tertiary propar-
gylic alcohols with p-toluenesulfonyl hydrazide can be effi-
ciently converted into α,β-unsaturated hydrazones using
Y(OTf)3 [yttrium(III) tifluoromethanesulfonate] as the cata-
lyst through a cascade propargylic substitution/aza-Meyer–
Schuster rearrangement reaction (Scheme 1, eq 4). More-
over, the obtained α,β-unsaturated hydrazones can be fur-
ther transformed into pyrazoles in the presence of LiOt-Bu.
Herein we report the results of this study.
Keywords propargylic alcohols, aza-Meyer–Schuster rearrangement,
Y(OTf)3, α,β-unsaturated hydrazones, pyrazoles
Currently, considerable interest has been focused on the
transition-metal-catalyzed cascade reactions of propargylic
alcohols, since these novel reactions can directly construct
diverse complicated organic molecules from readily avail-
able starting materials under very mild conditions.1 Among
these, the Lewis acid catalyzed carbon–carbon and carbon–
heteroatom bond formations of propargylic alcohols with
various nucleophiles have been extensively investigated
over the last few decades.2 However, compared to the wide
applications of amine nucleophiles in the C–N bond forma-
tion reactions of propargylic alcohols, particularly tertiary
propargylic alcohols, the use of hydrazines as nucleophiles
is barely documented.3 In 2012, Yoshimatsu and co-workers
reported the first convenient method for the preparation of
propargyl hydrazides from tertiary propargylic alcohols
through scandium-catalyzed hydrazination in MeNO2–H2O
(Scheme 1, eq 1).4 Almost at the same time, Zhan and co-
workers reported a novel FeCl3-catalyzed domino regiose-
lective propargylic substitution/aza-Meyer–Schuster rear-
rangement reaction for the synthesis of acrylonitriles from
OH
NHNHTs
Sc(OTf)3 (10 mol%)
Bu4NHSO4 (10 mol%)
(1)
+
TsNHNH2
R1
R2
R1
R2
MeNO2/H2O
R3
R3
OH
R1
R2
CN
FeCl3 (10 mol%)
MeNO2, 60 °C
(2)
+
TsNHNH2
R1
R2
TMS
R1
1) FeCl3 (10 mol%)
PhCl, 60 °C
OH
N
NH
+
TsNHNH2
TsNHNH2
R1
R2
(3)
(4)
R2
2) Cs2CO3 (2 equiv)
100 °C
R3
R3
OH
R2
NNHTs
R3
Y(OTf)3 (20 mol%)
MeCN, 80 °C
+
R1
R2
R1
R3
this work
Scheme 1 The reactions of tertiary propargylic alcohols with p-tolu-
enesulfonyl hydrazide
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2170–2174

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At the outset of this investigation, a series of tertiary
propargylic alcohols 1a–u were prepared by KOt-Bu-pro-
moted alkynylation of arylacetylenes with ketones under
solvent-free conditions recently developed by our group.7b
Next, we employed the propargylic alcohol 1a and p-tolu-
enesulfonyl hydrazide (2) as the substrates to test this reac-
tion. Treatment of compound 1a with 2 in the presence of
10 mol% of Ce(OTf)3 in MeNO2 at 60 °C for 7.5 hours result-
ed in the formation of α,β-unsaturated hydrazone 3a in 34%
isolated yield, as the only stereoisomer (Table 1, entry 1).
Encouraged by this result, we proceeded to optimize the re-
action by first screening the rare-earth-metal catalyst (Ta-
ble 1, entries 1–5). We were delighted to find that Y(OTf)3
could promote the reaction (Table 1, entry 2). Other rare
earth metal salts, such as Yb(OTf)3, La(OTf)3, and Sc(OTf)3,
also worked for this reaction, albeit with lower yields (Table
1, entries 3–5). For the catalyst of Yb(OTf)3, much more by-
products were generated which made the product purifica-
tion more difficult. So we used Y(OTf)3 for the further opti-
mizations. Next, a series of common organic solvents were
examined (Table 1, entries 6–10); MeCN was found to be
the most suitable solvent for this transformation (Table 1,
entry 10), whereas a low yield of 3a was obtained when di-
oxane or DCE was used as the solvent (Table 1, entries 6 and
7). However, the strong polar solvents, such as DMF and
DMSO, were found not effective in this reaction (Table 1,
entries 8 and 9). Furthermore, the reaction temperature
and catalyst loading were examined, and it was found that a
20 mol% of catalyst with 80 °C of reaction temperature led
to the highest yield of 3a in the shortest reaction time (Ta-
ble 1, entry 13). Finally, it is noted that the reaction pro-
ceeded with high stereoselectivity. The structure of α,β-un-
saturated hydrazone 3a was confirmed by X-ray crystallo-
graphic analysis.8
Table 1 Optimization of Reaction Conditionsa
OH
NNHTs
Ph
catalyst
+
TsNHNH2
2
Ph
solvent
Ph
Ph
1a
3a
Entry
Cat.
(mol%)
Solvent
Temp
(°C)
Time
(h)
yield
(%)b
1
2
Ce(OTf)3 (10)
Y(OTf)3 (10)
Yb(OTf)3 (10)
La(OTf)3 (10)
Sc(OTf)3 (10)
Y(OTf)3 (10)
Y(OTf)3 (10)
Y(OTf)3 (10)
Y(OTf)3 (10)
Y(OTf)3 (10)
Y(OTf)3 (10)
Y(OTf)3 (10)
Y(OTf)3 (20)
MeNO2
MeNO2
MeNO2
MeNO2
MeNO2
dioxane
DCE
60
7.5
7.5
6
34
51
48
38
27
27
41
60
60
60
60
60
60
60
60
60
80
100
80
3
4
7.5
7.5
10
7.5
7.5
7.5
7.5
6
5
6
7
8
DMF
trace
9
DMSO
MeCN
MeCN
MeCN
MeCN
trace
60
10
11
12c
13
65
6
62
2.5
71
a All the reactions were carried out with 1a (0.3 mmol) and 2 (0.6 mmol) in
2.0 mL of solvent.
b Yield of isolated product after chromatography.
c The reaction was carried out in a sealed tube.
Interestingly, it was found that the symmetric tertiary
propargylic alcohols (R1 = R2), derived from an aliphatic ke-
tone or an aromatic ketone, were also effective and gave ex-
cellent product yields (Scheme 2, 3q–t). Moreover, it is
noteworthy that the steric effect of ortho substituent was
obviously observed in the formation of 3f and 3i.
With the optimized reaction conditions in hand, the
scope and generality of this transformation was investigat-
ed by using a variety of tertiary propargylic alcohols. As il-
lustrated in Schene 2, the reaction proceeds smoothly over
a wide range of substrates to afford the corresponding α,β-
unsaturated hydrazones in moderate to excellent yields.9
Among the Ar groups of tertiary propargylic alcohols 1,
substituents possessing an electron-rich aromatic ring gave
the desired product 3 in higher yields than those with an
electron-poor ring (Scheme 2, 3a–h). However, when an ali-
phatic alkyne derived (1-hexyne) propargylic alcohol was
used, no desired product was detected. Next, substituent
effects at the propargylic position were investigated
(Scheme 2, 3i–t). In the examples of asymmetric propargyl-
ic alcohols (R1 = Ar, R2 = Alk), the α,β-unsaturated hydra-
zones were obtained in good yields (Scheme 2, 3i–o). Be-
sides, we were pleased to discover that an aliphatic ketone
derived propargylic alcohol 1p is also compatible with this
transformation and with satisfactory yield (Scheme 2, 3p).
The corresponding α,β-unsaturated hydrazones are at-
tractive and can be further converted into organic synthe-
sis.10 First, we were delighted to find that a novel trisubsti-
tuted 3H-pyrazole 4a was obtained in moderate yield when
treatment of α,β-unsaturated hydrazone 3a with LiOt-Bu in
toluene at 80 °C for 1.5 hours. Considering the importance
of pyrazoles in organic chemistry,11 we then proceeded to
investigate this pyrazole formation reaction using various
α,β-unsaturated hydrazones which were synthesized above
(Scheme 3).12 It was noted that a wide range of α,β-unsatu-
rated hydrazone substrates can be employed in this trans-
formation to afford the corresponding 3H-pyrazole in mod-
erate to good yields (Scheme 3, 4a–f).
More interestingly, when α,β-unsaturated hydrazones
3r and 3s were employed in this pyrazole formation reac-
tion, we found that the novel 1H-pyrazoles 5a and 5b rather
than 3H-pyrazoles were obtained in 73% and 56% isolated
yields, respectively (Scheme 4). We attributed the result of
1H-pyrazole formation to a [1,5]-sigmatropic shift as well
as aromatization sequence (Scheme 4). These results are
consistent with Zhan’s report.6
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2170–2174

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OH
R2
NNHTs
Ar
Y(OTf)3 (20 mol%)
MeCN, 80 °C
+
TsNHNH2
R1
R2
R1
Ar
1a–t
2
3a–t
NNHTs
NNHTs
NNHTs
NNHTs
NNHTs
Br
Br
F
Cl
3a, 2.5 h, 71%
NNHTs
3b, 2.5 h, 64%
3c, 3 h, 58%
3d, 3 h, 60%
3e, 3.5 h, 62%
NNHTs
NNHTs
NNHTs
NNHTs
Cl
Cl
3f, 3.5 h, 46%
Cl
OCH3
3g, 3.5 h, 73%
NNHTs
3h, 2.5 h, 80%
3i, 3 h, 48%
3j, 3 h, 60%
NNHTs
NNHTs
NNHTs
NNHTs
Br
I
Br
3k, 3 h, 64%
3l, 3.5 h, 46%
3m, 3 h, 63%
3n, 3 h, 70%
3o, 3 h, 54%
NNHTs
NNHTs
NNHTs
NNHTs
NNHTs
3p, 0.5 h, 64%
3q, 1.5 h, 84%
3r, 2 h, 92%
3s, 1.5 h, 93%
3t, 1 h, 78%
Scheme 2 Y(OTf)3-catalyzed reaction of various tertiary propargylic alcohols with p-toluenesulfonyl hydrazide. All the reactions were carried out with
propargylic alcohols 1a–t (0.3 mmol), p-toluenesulfonyl hydrazide (2, 0.6 mmol), Y(OTf)3 (0.06 mmol) in 2.0 mL of MeCN. Isolated yields are reported.
To further validate whether this reaction can be practi-
cally useful, two gram-scale experiments have been carried
out for propargylic alcohols 1a and 1r under standard reac-
tion conditions. The reaction proceeded smoothly, provid-
ing the corresponding α,β-unsaturated hydrazone products
3a and 3r in 56% and 81% yields, respectively.
On the basis of the above results and related literature,4–
6 two plausible reaction pathways to account for the forma-
tion of α,β-unsaturated hydrazones and pyrazoles from ter-
tiary propargylic alcohols with p-toluenesulfonyl hydrazide
is outlined in Scheme 5. In path a, the Y(OTf)3-catalyzed di-
rect substitution of a propargylic alcohol with a p-toluene-
sulfonyl hydrazide occurred to afford propargyl hydrazide
intermediate A. Next, the intermediate A undergoes a [1,3]-
shift of the NHNHTs group through the aza-Meyer–Schuster
rearrangement, which leads to allene intermediate B. Final-
R1
NNHTs
R3
N
N
R1
R2
t-BuOLi (1.5 equiv)
R3
R2
Ph
toluene, 80 °C
4a–f
3
H
N
N
N
N
N
N
N N
Ph
Ph
NNHTs
t-BuOLi (1.5 equiv)
toluene, 80 °C
Cl
Br
4a, 1.5 h, 54%
4b, 2 h, 64%
4c, 1.5 h, 61%
3r
5a, 73%
Br
N
N
Ph
N
N
N
N
H
N
N
Ph
OCH3
Ph
NNHTs
t-BuOLi (1.5 equiv)
4e, 2 h, 67%
4d, 2.5 h, 71%
4f, 1.5 h, 74%
toluene, 80 °C
Scheme 3 Synthesis of 3H-pyrazoles from the corresponding α,β-un-
saturated hydrazones. All the reactions were carried out with α,β-unsat-
urated hydrazones 3 (0.3 mmol), LiOt-Bu (0.45 mmol) in 2.0 mL of
toluene at 80 °C. Isolated yields are reported.
3s
5b, 56%
Scheme 4 Synthesis of 1H-pyrazoles from α,β-unsaturated hydrazones
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2170–2174

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Letter
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path a
Y3+
H
Y3+
OH
NHNHTs
TsHN
R1
H
Y3+
TsHN
R1
Y(OTf)3
N
R1
R2
N
+
TsNHNH2
R1
R2
propargylic
substitution
R3
R3
R3
R3
R2
R2
B
A
1
2
Y(OTf)3
aza-Meyer–Schuster
rearrangement
path b
Meyer–Schuster
rearrangement
R1
NNHTs
R3
R1
NHNHTs
R3
R1
O
tautomerization
TsNHNH2
H2O
R2
R2
R2
R3
3
C
base
N
R1
N
R1
N2
R2
R3
R2
R3
E
D
H
N
N
N
N
N
N
aromatization
[1,5]-sigmatropic shift
R1, R2 = aryl
R1
R2
R3
R3
R3
R1
R1
R2
H
R2
5
F
4
Scheme 5 Proposed mechanism of the reaction
ly, the intermediate B then isomerizes to give the corre-
sponding α,β-unsaturated hydrazone 3. The product 3 can
be easily converted into the diazo compound C in the pres-
ence of a base.10a,d Next, the 1,5-dipole isomer of the diazo
compound C undergoes an intramolecular 6π-electrocyclic
ring closure; which affords the corresponding 3H-pyrazole
product 4. On the other hand, when R1 and R2 are aryl
groups, the [1,5]-sigmatropic shift occurs and produces the
4H-pyrazoles intermediate E, which subsequently experi-
ence a rapid isomerization to afford the trisubstituted 1H-
pyrazoles 5. However, since the Meyer–Schuster rearrange-
ment of the propargylic alcohol can be catalyzed by Lewis
acid,13 a prior Meyer–Schuster rearrangement followed by
nucleophilic attack of the p-toluenesulfonyl hydrazide can-
not be strictly ruled out. In such case, an α,β-unsaturated
ketone intermediate is generated, which is attacked by the
p-toluenesulfonyl hydrazide and accompanied with the
elimination of water to afford the corresponding α,β-unsat-
urated hydrazone 3 (path b).
Finally, a one-pot transformation of three-step reactions
from ketones with phenylacetylene to pyrazoles directly
was explored. Thus, ketones 6 and 7 were reacted with phe-
nylacetylene in the presence of KOt-Bu under solvent-free
conditions at room temperature. After completion of the
alkynylation, p-toluenesulfonyl hydrazide, Y(OTf)3, and
MeCN were added, respectively, to the reaction mixture.
The resulting mixture was heated at 80 °C for several hours.
After completion of this hydrazone formation reaction, KOt-Bu
was added to the reaction mixture again and continued to
heat at 80 °C for 1.5 hours. After workup, the pyrazoles 4a
and 5a were isolated in 40% and 65% overall yields, respec-
tively (Scheme 6).
1) t-BuOK, neat, r.t.
2) TsNHNH2, Y(OTf)3 (20 mol%)
MeCN, 80 °C, 5 h
O
N
N
Ph
+
Ph
3) t-BuOK
MeCN, 80 °C, 2.5 h
6
4a, 40%
1) t-BuOK, neat, r.t.
2) TsNHNH2, Y(OTf)3 (20 mol%)
MeCN, 80 °C, 2.5 h
H
N
O
N
Ph
+
Ph
Ph
3) t-BuOK
MeCN, 80 °C, 1.5 h
Ph
Ph
5a, 65%
7
Scheme 6 One-pot reactions from ketones
In summary, we have developed a simple and efficient
Y(OTf)3-catalyzed cascade propargylic substitution/aza-
Meyer–Schuster rearrangement reaction of tertiary propar-
gylic alcohols and p-toluenesulfonyl hydrazide for the syn-
thesis of α,β-unsaturated hydrazone derivatives. Moreover,
the obtained α,β-unsaturated hydrazones have been con-
verted into pyrazoles in the presence of LiOt-Bu.
Acknowledgement
This project was generously supported by the National Natural Sci-
ence Foundation of China (21262023) and the Natural Science Foun-
dation of Inner Mongolia of China (No. 2014JQ02).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2170–2174

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Supporting Information
128.8, 128.8, 129.1, 129.7, 131.9, 135.6, 139.5, 144.2, 146.6,
150.9, 162.9, 164.8. ESI-HRMS: m/z calcd for C23H22FN2O2S [M +
H]+: 409.1381; found: 409.1379.
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1381057.
Compound 3q: white solid; mp 114–117 °C. 1H NMR (500 MHz,
CDCl3): δ = 1.29 (s, 6 H), 2.43 (s, 3 H), 3.65 (s, 1 H), 6.07 (s, 1 H),
7.30 (d, J = 7.0 Hz, 5 H), 7.36–7.39 (m, 2 H), 7.83(d, J = 8.0 Hz, 2
H). 13C NMR (125 MHz, CDCl3): δ = 21.6, 27.0, 83.3, 91.6, 122.4,
128.3, 128.3, 128.4, 129.4, 131.7, 135.3, 143.8. Anal. Calcd for
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
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Ding, Z.; Zhan, Z. Org. Biomol. Chem. 2013, 11, 294. (b) Reddy, C.
R.; Vijaykumar, J.; Grée, R. Synthesis 2013, 45, 830.
(4) Yoshimatsu, M.; Ohta, K.; Takahashi, N. Chem. Eur. J. 2012, 18,
15602.
(12) General Procedure for the Synthesis of Pyrazoles from the
Corresponding α,β-Unsaturated Hydrazones
To a solution of α,β-unsaturated hydrazones (0.3 mmol) in
toluene (2.0 mL) was added LiOt-Bu (0.45 mmol) under an air
atmosphere. The resulting mixture was heated at 80 °C for the
indicated time. After completion of the reaction, the solvent
was removed in a vacuum. The resulting residue was purified
on a silica gel column (EtOAc–PE) to provide the desired pyra-
zole products 4 and 5.
Representative Spectroscopic DataCompound 4a: yellow
solid; mp 68–71 °C. 1H NMR (500 MHz, CDCl3): δ = 1.83 (s, 3 H),
7.28–7.36 (m, 4 H), 7.39–7.44 (m, 3 H), 7.48 (t, J = 7.5 Hz, 2 H),
8.12–8.05 (m, 2 H). 13C NMR (125 MHz, CDCl3): δ = 22.2, 100.3,
126.5, 127.3, 128.1, 128.8, 128.9, 129.3, 130.7, 136.34, 136.6,
154.9. ESI-HRMS: m/z calcd for C16H15N2 [M + H]+: 235.1230;
found: 235.1230.
(5) Hao, L.; Wu, F.; Ding, Z.; Xu, S.; Ma, Y.; Chen, L.; Zhan, Z. Chem.
Eur. J. 2012, 18, 6453.
(6) Hao, L.; Hong, J.; Zhu, J.; Zhan, Z. Chem. Eur. J. 2013, 19, 5715.
(7) (a) Chen, S.; Wang, J. J. Org. Chem. 2007, 72, 4993. (b) Chen, S.;
Yuan, F.; Zhao, H.; Li, B. Res. Chem. Intermed. 2013, 39, 2391.
(8) For the crystal structure of 3a, please see the Supporting Infor-
mation.
(9) General Procedure for the Y(OTf)3-Catalyzed Reaction of Ter-
tiary Propargylic Alcohols with p-Toluenesulfonyl Hydrazide
To a solution of propargylic alcohols (0.3 mmol) and p-toluene-
sulfonyl hydrazide (0.6 mmol) in MeCN (2.0 mL) was added
Y(OTf)3 (0.06 mmol) under an air atmosphere. The resulting
mixture was heated at 80 °C for the indicated time. After com-
pletion of the reaction, the mixture was cooled to r.t. The
solvent was removed in a vacuum, and the resulting residue
was purified on a silica gel column (PE–EtOAc) to provide the
desired α,β-unsaturated hydrazone products 3.
Compound 4f: yellow solid; mp 84–87 °C. 1H NMR (500 MHz,
CDCl3): δ = 1.80 (s, 3 H), 7.29 (d, J = 8.5 Hz, 2 H), 7.41–7.51 (m, 5
H), 8.07 (d, J = 8.0 Hz, 2 H). 13C NMR (125 MHz, CDCl3): δ = 22.2,
99.7, 122.1, 127.3, 128.2, 129.0, 129.5, 130.4, 131.9, 135.4,
135.8, 155.3. Anal. Calcd for C16H13BrN2: C, 61.36; H, 4.18; N,
8.94. Found: C, 61.29; H, 4.31; N, 8.95.
(13) For reviews on Meyer–Schuster rearrangement, see:
(a) Cadierno, V.; Crochet, P.; García-Garrido, S. E.; Gimeno, J.
Dalton Trans. 2010, 39, 4015. (b) Engle, D. A.; Dudley, G. B. Org.
Biomol. Chem. 2009, 7, 4149. (c) Swaminathan, S.; Narayanan, K.
V. Chem. Rev. 1971, 71, 429.
Representative Spectroscopic Data
Compound 3b: white solid; mp 128–130 °C. 1H NMR (500 MHz,
CDCl3): δ = 1.86 (s, 3 H), 2.43 (s, 3 H), 6.19 (s, 1 H), 7.00–7.07 (m,
2 H), 7.33 (d, J = 8.0 Hz, 2 H), 7.39–7.44 (m, 3 H), 7.53–7.55 (m, 2
H), 7.63–7.68 (m, 2 H), 7.84 (s, 1 H), 7.89 (d, J = 8.5 Hz, 2 H). 13
C
NMR (125 MHz, CDCl3): δ = 18.2, 21.6, 115.5, 125.9, 128.0,
© Georg Thieme Verlag Stuttgart · New York — Synlett 2015, 26, 2170–2174