α-Selective Allylation of Isatin Imines Using Metallic Barium

Article abstract of DOI:10.1055/s-0035-1561450

The Barbier-type allylation of isatin imines with allylic chlorides was achieved by using metallic barium as the promoter. Various α-allylated 3-amino-2-oxindoles were synthesized from the corresponding allylic chlorides and isatin imines. The double-bond geometry of allylic chlorides was retained throughout the reaction. An arylic bromide or iodide functionality of the products was robust to metalation under the optimum reaction conditions.

Full text of DOI:10.1055/s-0035-1561450

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
A. Yanagisawa et al.
Letter
Synlett
α-Selective Allylation of Isatin Imines Using Metallic Barium
Akira Yanagisawa*
R2
Seiya Yamafuji
NAr
α
Ar NH
Toshiki Sawae
R2
Ba
α
+
X
O
X
O
Department of Chemistry, Graduate School of Science,
Chiba University, Inage, Chiba 263-8522, Japan
ayanagi@faculty.chiba-u.jp
THF, 70 °C
N
Cl
R3
N
R1
γ
stereoretentive!
2 = H, Me, n-C6H13
Me2C=CH(CH2)2;
R1
X = H, 6-Br, 7-F
Ar = Ph, 4-MeC6H4, 4-i-PrC6H4,
4-MeOC6H4, 4-FC6H4,4-ClC6H4, R3 = H, Me, n-C6H13
R
,
22 examples
37–94% yield
α/γ up to >99:1
,
4-BrC6H4, 4-IC6H4, 4-F3CC6H4,
3,5-Me2C6H3, 2,4-F2C6H3
Me2C=CH(CH2)2, Ph
R
1 = Bn, Ph3C, 4-MeOC6H4CH2, Me
Received: 08.03.2016
Accepted after revision: 11.04.2016
Published online: 10.05.2016
NAr
X
O
DOI: 10.1055/s-0035-1561450; Art ID: st-2016-u0166-l
R2
N
α
Ar NH
R1
R3
Abstract The Barbier-type allylation of isatin imines with allylic chlo-
rides was achieved by using metallic barium as the promoter. Various α-
allylated 3-amino-2-oxindoles were synthesized from the correspond-
ing allylic chlorides and isatin imines. The double-bond geometry of al-
lylic chlorides was retained throughout the reaction. An arylic bromide
or iodide functionality of the products was robust to metalation under
the optimum reaction conditions.
Ba
THF, 70 °C
+
X
O
N
R1
R2
α
Cl
R3
γ
Scheme 1 Barbier-type α-allylation of isatin imines using metallic bari-
um
Key words allylation, allylic chlorides, 3-amino-2-oxindoles, barium,
imines
perature.8 A benzylation of azo compounds with benzylic
chlorides has been also achieved by employing a Barbier-
type method that involves the formation of benzylic barium
species in situ in the presence of reactive barium as the
low-valent metal, affording benzylic hydrazines.9 We envi-
sioned that if an allylic barium reagent could be generated
from metallic barium10 and the corresponding allylic halide
under mild reaction conditions and displayed α-selectivity
in the reaction with an isatin imine, the allylation would
provide a practical synthetic procedure for α-allylated 3-
amino-2-oxindoles. Thus, we first selected isatin imine 1a
and prenyl chloride (2a) as the electrophile and the precur-
sor of allylic barium reagent, respectively, and attempted to
carry out a Barbier-type reaction because of the simplicity
of the experimental procedure. When a 2:1 mixture of pre-
nyl chloride (2a, 2 equiv) and isatin imine 1a (1 equiv) was
exposed to metallic barium (2 equiv) in THF at room tem-
perature for 12 hours, the reaction proceeded sluggishly
and targeted prenylated 3-amino-2-oxindole 3aa (α-ad-
duct) was formed in 43% yield. The corresponding γ-adduct
was not observed at all (Table 1, entry 1). The chemical
yield was improved when the amounts of prenyl chloride
(2a) and metallic barium were increased to four equiva-
lents, respectively (Table 1, entry 2). Elevating the reaction
A 3-amino-2-oxindole structure is often seen in biologi-
cally active substances, such as CRTH2 receptor antago-
nists,1 antituberculosis agents,2 and so on.3 Therefore, the
development of useful methods for the synthesis of isatin
derivatives possessing a nitrogen atom at the 3-position has
attracted considerable interest among researchers of organ-
ic synthesis. One of the facile routes to the 3-amino-2-oxin-
dole structure is the allylation of isatin imines.4,5 Allylic in-
dium reagents4b and allylic zinc reagents4d have been uti-
lized for the transformation; however, these reagents show
γ-regioselectivity and as far as we know, there is no exam-
ple of the α-regioselective allylation of isatin imines. We re-
port herein a metallic-barium-promoted Barbier-type α-al-
lylation of isatin imines with allylic chlorides (Scheme 1).
Allylic barium reagents, which can be prepared from re-
active barium (Rieke barium)6 and allylic chlorides, are
known to react α-regioselectively with diverse electro-
philes.7 We have previously reported that an α-selective al-
lylation of azo compounds with allylic barium reagents oc-
curs smoothly at –78 °C, yielding allylic hydrazines (α prod-
ucts). The double-bond geometry of the allylic barium
reagents is retained throughout the reaction at low tem-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

B
A. Yanagisawa et al.
Letter
Synlett
temperature to 70 °C also raised the reactivity (Table 1, en-
try 5) and the yield of 3aa exceeded 80% when four equiva-
lents of those chemicals were used in the reaction at 70 °C
(Table 1, entry 6). Subsequently, we tested solvents other
than THF (Table 1, entries 7–10) and found that THF was the
most suitable solvent with regard to chemical yield (Table 1,
entry 6). Diethyl ether was also a promising solvent (Table
1, entry 7); in contrast, dichloromethane and toluene gave
unsatisfactory results, and the desired adduct 3aa was not
obtained at all in the reaction that used 1,4-dioxane as sol-
vent (Table 1, entries 8–10).
and 1k furnished products in satisfactory yields (Table 2,
entries 9 and 10).
Table 2 Metallic-Barium-Promoted Barbier-Type Prenylation of Vari-
ous Isatin Imines 1b–ka
NAr
O
N
Bn
α
Ar NH
1b–k
Ba (4 equiv)
+
O
THF, 70 °C, 12 h
Table 1 Optimization of Metallic-Barium-Promoted Barbier-Type Pre-
nylation of Isatin Imine 1aa
N
Bn
3ba–ka (α/γ > 99:1)
α
γ
Cl
2a (4 equiv)
NAr
α
Ar NH
Ba (x equiv)
solvent, 12 h
Entry
1 Ar
Product 3
Yield (%)b
α
+
O
O
γ
Cl
1
2
1b Ph
3ba
3ca
3da
3ea
3fa
65
69
82
69
82
93
69
37
91
77
N
N
Bn
3aa (α/γ > 99:1)
2a (x equiv)
Bn
1a (Ar = 4-BrC6H4)
1c 4-MeC6H4
1d 4-i-PrC6H4
1e 4-MeOC6H4
1f 4-FC6H4
3
4
Entry
x (equiv)
Solvent
Temp (°C)
r.t.
Yield (%)b
5
1
2
2
4
6
2
2
4
4
4
4
4
THF
43
66
65
55
74
86
56
4
6
1g 4-ClC6H4
1h 4-IC6H4
3ga
3ha
3ia
THF
r.t.
r.t.
40
70
70
70
70
70
70
7
3
THF
8
1i 4-F3CC6H4
1j 3,5-Me2C6H3
1k 2,4-F2C6H3
4
THF
9
3ja
5
THF
10
3ka
6
THF
a The Barbier-type reaction was carried out using isatin imines 1b–k (1
equiv), metallic barium (4 equiv), and prenyl chloride (2a, 4 equiv) in dry
THF at 70 °C for 12 h.
7
Et2O
CH2Cl2
toluene
8
b The chemical yield was determined by 1H NMR spectroscopy using 1,4-
bis(trimethylsilyl)benzene as an internal standard.
9
8
10
1,4-dioxane
<1
We performed the metallic-barium-promoted Barbier-
type prenylation of isatin imines 1l–p derived from 4-bro-
moaniline and a diversely substituted isatin (Table 3). The
effect of R1 group at 1-position of isatin imines 1n–p on the
chemical yield was worthy of note: an electron-withdraw-
ing group (Ph3C) enhanced the electrophilicity of 1n (Table
3, entry 3), whereas an electron-donating group (4-
MeOC6H4CH2) reduced the reactivity of 1o (Table 3, entry
4). The Me group was also a suitable protective group for
the amide nitrogen of isatin imine 1p (Table 3, entry 5).
To investigate the E/Z stereoselectivity as well as the α-
regioselectivity of the present α-allylation, we executed the
metallic-barium-promoted Barbier-type reaction between
isatin imine 1a and a diverse range of allylic chlorides 2b–i
other than prenyl chloride (2a) under the optimum reac-
tion conditions (Table 4). Not only γ-disubstituted allylic
chlorides 2b and 2c, but also γ-monosubstituted allylic
chlorides 2d–f showed exclusive α selectivity (Table 4, en-
tries 1–5). The γ isomer 4ag was detected as a minor prod-
uct in the reaction of trans-1-chloro-2-butene (2g, Table 4,
a The Barbier-type reaction was carried out using isatin imine 1a (1 equiv),
metallic barium (x equiv), and prenyl chloride (2a, x equiv) in the specified
solvent.
b The chemical yield was determined by 1H NMR spectroscopy using 1,4-
bis(trimethylsilyl)benzene as an internal standard.
With the optimum reaction conditions in hand, we ex-
amined the prenylation of isatin imines 1b–k derived from
various anilines and N-benzyl isatin (Table 2). High reactivi-
ties were observed for the reactions of isatin imines 1f and
1g, which have an electron-withdrawing group at the 4-po-
sition of the N-phenyl group (Table 2, entries 5 and 6). In
contrast, isatin imines 1c,d,e, which have an electron-do-
nating group, afforded products 3ca,da,ea, in higher yields
than isatin imine 1b derived from aniline (Table 2, entries
2–4 vs. entry 1). Employment of 4-trifluoromethylaniline-
derived isatin imine 1i caused a significant decrease in the
yield (37%) of its product 3ia probably due to low solubility
of isatin imine 1i in the reaction solvent (THF) and the con-
comitant reduction of the imine by metallic barium (Table
2, entry 8). Disubstituted aniline-derived isatin imines 1j
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

C
A. Yanagisawa et al.
Letter
Synlett
entry 6). The geometric purity of α products 3ab–ag shows
that the geometric isomerization (E to Z or Z to E) of allylic
chlorides 2b–g can be suppressed to a large extent during
the reaction (Table 4, entries 1–6). Therefore, E-enriched α-
allylated 3-amino-2-oxindoles 3ab,ad,af,ag were selectively
obtained from E-allylic chlorides 2b,d,f,g (Table 4, entries 1,
3, 5, and 6), whereas Z-allylic chlorides 2c and 2e provided
the Z isomers of α products 3ac and 3ae predominantly (Ta-
ble 4, entries 2 and 4). Although the reactivity of a second-
ary allylic chloride was also tested in the present α-allyla-
tion procedure for 3-amino-2-oxindoles, the α/γ ratio given
by the reaction of 3-chloro-1-butene (2h, Table 4, entry 7)
was opposite to that of trans-1-chloro-2-butene (2g, Table
4, entry 6). This reveals that the in situ generating second-
ary allylic barium reagent rapidly isomerizes to the corre-
sponding primary allylic barium reagent before its reaction
with isatin imine 1a at 70 °C. Use of prenyl bromide (2i) in-
stead of prenyl chloride (2a) resulted in a considerable re-
duction in yield of 3aa because of the Wurtz coupling (ho-
mocoupling) of the allylic bromide (Table 4, entry 8).
Table 3 Metallic-Barium-Promoted Barbier-Type Prenylation of Vari-
ous Isatin Imines 1l–pa
NAr
R2
O
N
R1
α
Ar NH
1l–p (Ar = 4-BrC6H4)
Ba (4 equiv)
R2
O
+
THF, 70 °C, 12 h
N
R1
3la–pa (α/γ > 99:1)
α
γ
Cl
2a (4 equiv)
Entry
R1
1 R2
Product 3
Yield (%)b
1
2
3
4
5
Bn
1l 6-Br
1m 7-F
1n H
3la
76
83
94
76
89
Bn
3ma
3na
3oa
3pa
Ph3C
4-MeOC6H4CH2
1o H
1p H
Me
Then, we investigated the tolerance of the arylic halide
functionality in the metallic-barium-promoted Barbier-
type reaction, in comparison with that in the Rieke barium
(Ba*)-promoted reaction (Table 5). In the case of isatin
imine 1a (X = Br), target product 3aa was formed exclusive-
ly and corresponding reduced product 3ba was not detect-
ed at all in the metallic-barium-promoted reaction. In con-
a The Barbier-type reaction was carried out using isatin imines 1l–p (1
equiv), metallic barium (4 equiv), and prenyl chloride (2a, 4 equiv) in dry
THF at 70 °C for 12 h.
b The chemical yield was determined by 1H NMR spectroscopy using 1,4-
bis(trimethylsilyl)benzene as an internal standard.
Table 4 Metallic-Barium-Promoted Barbier-Type α-Selective Allylation of Isatin Imine 1a with Various Allylic Halides 2b–ia
R1 R2
Ar NH
R3
R1
NAr
Ar NH
R3
R1
Ba (4 equiv)
R3
γ
R2
α
O
+
+
O
O
THF, 70 °C, 12 h
X
R2
α
γ
N
N
N
Bn
2b–i (4 equiv)
Bn
Bn
4aa–ah (γ-adduct)
1a (Ar = 4-BrC6H4)
3aa–ah (α-adduct)
Entry
R1
Me
R2
R3
2 X
Product
Yield (%)b
α/γc
>99:1
E/Zc
1
2
3
4
5
6
7
8
Me2C=CH(CH2)2
H
2b Cld
2c Cle
2d Cld
2e Cle
2f Cld
2g Clh
2h Cl
2i Br
3ab + 4ab
3ac + 4acf
3ad + 4ad
3ae + 4aeg
3af + 4af
80
87
92
79
61
69
42
40
96:4
3:97
98:2
5:95
Me2C=CH(CH2)2
Me
n-C6H13
H
H
>99:1
>99:1
>99:1
>99:1
94:6
H
H
n-C6H13
H
H
Ph
H
>99:1
87:13
72:28
–
H
Me
H
H
3ag + 4ag
3ah + 4ahi
3aa + 4aa
H
Me
H
17:83
>99:1
Me
Me
a The Barbier-type reaction was carried out using isatin imine 1a (1 equiv), metallic barium (4 equiv), and allylic halides 2b–i (4 equiv) in dry THF at 70 °C for 12 h.
b The chemical yield was determined by 1H NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene as an internal standard.
c Determined by 1H NMR analysis and HPLC analysis.
d E/Z = >99:1.
e E/Z = <1:99.
f 4ac = 4ab.
g 4ae = 4ad.
h E/Z = 85:15; 25% of 3-chloro-1-butene (2h) was included.
i 3ah = 4ag; 4ah = 3ag.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
A. Yanagisawa et al.
Letter
Synlett
Table 5 Comparison of Metallic Barium with Rieke Barium in the Barbier-Type Prenylation of Isatin Imines 1a and 1h Possessing an Arylic Halide (X = Br, I)a
X
H
X
N
Ba reagent (y equiv)
THF
α
+
+
HN
HN
γ
Cl
α
α
O
2a (x equiv)
N
O
O
N
N
Bn
1a (X = Br) and 1h (X = I)
Bn
Bn
B (3ba)
reduced product
A (3aa or 3ha)
target product
Entry
x
Barium reagent
y
1 X
Temp (°C)
Time (h)
A/Bb
>99:1
Yield (%)c
1
2
3
4
4
2
4
2
metallic barium
Ba*
4
1a Br
1a Br
1h I
70
–78
70
12
2
86
33
69
23
2.2
4
84:16
94:6
metallic barium
Ba*
12
2
2.2
1h I
–78
50:50
a The Barbier-type reaction was carried out using isatin imines 1a and 1h (1 equiv), metallic barium (4 equiv) or reactive barium (2.2 equiv), and prenyl chloride
(2a, 4 or 2 equiv) in dry THF.
b Determined by 1H NMR analysis.
c The chemical yield was determined by 1H NMR spectroscopy using 1,4-bis(trimethylsilyl)benzene as an internal standard.
trast, 3ba was isolated as a minor product in the Rieke bari-
um (Ba*)-promoted reaction (Table 5, compare entry 1 with
entry 2). Moreover, a significant difference between the
Acknowledgment
two procedures in their reactivity toward arylic iodide was
observed in the reaction of isatin imine 1h.
We gratefully acknowledge the financial support from the Iodine Re-
search Project in Chiba University headed by Professor Hideo Togo.
A proposed reaction mechanism is illustrated in Scheme
2. Two pathways are possible for α-allylated homoallylic
amines (α-adducts). An allylic barium reagent generated
from allylic chloride 5 and metallic barium is supposed to
be present in equilibrium between α isomer 6 and γ-isomer
7. Thus, α-adducts are accessible from both isomers 6 and 7
by treating them with imine 8 via transition-state model 9
or 10, although the latter structure 10 is less favorable due
to steric bulkiness of allylic barium species 7. Meanwhile, γ-
allylated homoallylic amines (γ-adducts) can be formed
from 6 by an SE2′-type reaction of 6 with imine 8 through
six-membered cyclic transition state 11. However, 11 is un-
stable due to steric repulsion between an allylic alkyl group
of the barium reagent and an aryl group of the imine. As a
consequence, allylic barium species 6 is anticipated to react
preferentially at the α carbon with imine 8 via four-mem-
bered cyclic transition state 9,7b,c,11 yielding the α-adduct
without E/Z stereoisomerization of starting chloride 5.
In conclusion, we have achieved a novel Barbier-type α-
allylation of isatin imines with allylic barium reagents that
are prepared from allylic chlorides and metallic barium.
The employment of metallic barium as the source of allylic
barium reagents has enabled the synthesis of various α-al-
lylated 3-amino-2-oxindoles in a regio- and stereoselective
manner.12 Further studies of related reactions promoted by
metallic barium are under way.
R1
Cl
γ
α
R2
5
70 °C Ba
BaCl
R1
R1
BaCl
γ
α
α
γ
R2
R2
7
6
Ar2
Ar2
R3
Ar2
R3
N
N
N
slow
fast
slow
Ar1
R3
Ar1
Ar1
8
8
8
R3
R3
Ar2
R1
Ar1
R2
γ
Ar1
R2
N
Ar2
ClBa
R1
Ar1
N
BaCl
Ar2
N
α
R1
Ba
Cl
α
R3
R2
10
9
11
Ar2 NH R3
Ar2 NH R3
R1
α
γ
Ar1
Ar1
R2
R1 R2
α-adducts (major)
γ-adducts (minor)
Scheme 2 Proposed reaction pathways to α- and γ-adducts
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

E
A. Yanagisawa et al.
Letter
Synlett
Supporting Information
(9) Yanagisawa, A.; Sawae, T.; Yamafuji, S.; Heima, T.; Yoshida, K.
Synlett 2015, 26, 1073.
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1561450.
(10) Prof. Miyoshi and co-workers have reported various reactions
promoted by metallic strontium, see: (a) Miyoshi, N. In Science
of Synthesis; Vol. 7; Yamamoto, H., Ed.; Thieme: Stuttgart, 2004,
685. (b) Miyoshi, N.; Ikehara, D.; Matsuo, T.; Kohno, T.; Matsui,
A.; Wada, M. J. Synth. Org. Chem. Jpn. 2006, 64, 845. (c) Miyoshi,
N.; Matsuo, T.; Kikuchi, M.; Wada, M. J. Synth. Org. Chem. Jpn.
2009, 67, 1274. (d) Miyoshi, N.; Kohno, T.; Wada, M.;
Matsunaga, S.; Mizota, I.; Shimizu, M. Chem. Lett. 2009, 38, 984.
(e) Miyoshi, N.; Matsuo, T.; Mori, M.; Matsui, A.; Kikuchi, M.;
Wada, M.; Hayashi, M. Chem. Lett. 2009, 38, 996. (f) Miyoshi, N.;
Asaoka, M.; Miyazaki, Y.; Tajima, T.; Kikuchi, M.; Wada, M.
Chem. Lett. 2012, 41, 35.
(11) Exactly what causes allylic barium reagent 6 to react selectively
at the α-carbon with imine 8 is not clear; however, the unusu-
ally long barium–carbon bond (2.76–2.88 Å) might prevent the
formation of a six-membered cyclic transition-state model 11
leading to the γ-adduct, see: Kaupp, M.; Schleyer, P. v. R. J. Am.
Chem. Soc. 1992, 114, 491.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
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1996, 1. (c) Rieke, R. D.; Hanson, M. V. Tetrahedron 1997, 53,
1925.
(7) For reactions of allylic barium reagents, see: (a) Yanagisawa, A.;
Habaue, S.; Yamamoto, H. J. Am. Chem. Soc. 1991, 113, 8955.
(b) Yanagisawa, A.; Habaue, S.; Yasue, K.; Yamamoto, H. J. Am.
Chem. Soc. 1994, 116, 6130. For reviews, see: (c) Yanagisawa, A.;
Yamamoto, H. In Active Metals: Preparation, Characterization,
Applications; Fürstner, A., Ed.; VCH: Weinheim, 1996, 61.
(d) Yanagisawa, A. In Science of Synthesis; Vol. 7; Yamamoto, H.,
Ed.; Thieme: Stuttgart, 2004, 695. (e) Yanagisawa, A. In Main
Group Metals in Organic Synthesis; Vol. 1; Yamamoto, H.;
Oshima, K., Eds.; Wiley-VCH: Weinheim, 2004, 175.
(12) Typical Experimental Procedure for α-Selective Allylation:
Synthesis
of
1-Benzyl-3-[(4-bromophenyl)amino]-3-(3-
methylbut-2-en-1-yl)indolin-2-one (3aa, Table 1, Entry 6;
Table 4, Entry 8; Table 5, Entry 1)
Freshly cut barium (small pieces, 137.3 mg, 1.0 mmol) was
placed in a Schlenk tube (50 mL) under an argon atmosphere
and covered with dry THF (1 mL). The mixture was ultrasoni-
cated for 30 min, and THF was removed through a cannula
under an argon stream. The resulting barium pieces were vigor-
ously stirred under reduced pressure until they were pulver-
ized. Then, a solution of prenyl chloride (2a, 0.112 mL, 1.0
mmol) and isatin imine 1a (97.8 mg, 0.25 mmol) in THF (4 mL)
was added to the resulting barium powder at room temperature
under an argon atmosphere. After being heated to 70 °C, the
mixture was stirred for 12 h at this temperature and concen-
trated in vacuo after filtration through a Celite pad. The residual
crude product was purified by column chromatography on
silica gel (hexane–EtOAc, 9:1) to afford 3-prenylated 3-amino-
2-oxindole 3aa; mp 136–137 °C. The chemical yield (86%) was
determined by 1H NMR spectroscopy using 1,4-bis(trimethylsi-
lyl)benzene as the internal standard.
Spectral Data of the Product
1H NMR (400 MHz, CDCl3): δ = 7.19–7.31 (m, 7 H, ArH), 6.98–
7.04 (m, 3 H, ArH), 6.78 (d, 1 H, J = 7.9 Hz, ArH), 6.05–6.08 (m, 2
H, ArH), 5.05 (tt, 1 H, J = 1.1, 5.7 Hz, CH), 4.96 (d, 1 H, J = 15.4 Hz,
one H of CH2), 4.87 (d, 1 H, J = 15.4 Hz, one H of CH2), 4.40 (br, 1
H, NH), 2.62–2.73 (m, 2 H, CH2), 1.67 (s, 3 H, CH3), 1.57 (s, 3 H,
CH3). 13C NMR (100 MHz, CDCl3): δ = 177.5, 144.3, 141.9, 138.1,
135.6, 131.7 (2 C), 129.5, 129.0, 128.7 (2 C), 127.7, 127.6 (2 C),
123.8, 122.9, 117.0 (2 C), 115.4, 111.1, 109.6, 64.4, 44.1, 39.0,
26.0, 18.1. IR (neat): 3328, 2917, 1700, 1592, 1486, 1369, 1320,
812, 754, 731 cm–1. ESI-MS: m/z calcd for [C26H25ON2BrNa]+ [M +
Na]+: 483.1042; found: 483.1036.
(8) Yanagisawa, A.; Jitsukawa, T.; Yoshida, K. Synlett 2013, 24, 635.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E