Construction of cyclohepta[ b ]indoles via platinum-catalyzed intermolecular formal [4+3]-cycloaddition reaction of α,β-unsaturated carbene complex intermediates with siloxydienes

Article abstract of DOI:10.1055/s-0033-1338938

Platinum(II)-catalyzed intermolecular formal [4+3]-cycloaddition reaction of α,β-unsaturated carbene complex intermediates with siloxydienes proceeded under mild conditions to give cyclohepta[b]indole derivatives in good yield. The reaction was found to p

Full text of DOI:10.1055/s-0033-1338938

1364▌
LETTER
lCettoernstruction of Cyclohepta[b]indoles via Platinum-Catalyzed Intermolecular
Formal [4+3]-Cycloaddition Reaction of α,β-Unsaturated Carbene Complex
Intermediates with Siloxydienes
Construction of Cyclohepta[b]indoles
Hiroyuki Kusama, Hideyuki Sogo, Kodai Saito, Takuya Suga, Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo, 152-8551, Japan
Fax +81(3)57342931; E-mail: niwasawa@chem.titech.ac.jp
Received: 11.03.2013; Accepted after revision: 16.04.2013
Several conventional methods such as Fischer indole syn-
Abstract: Platinum(II)-catalyzed intermolecular formal [4+3]-cy-
thesis,1f–h intramolecular nucleophilic addition,2 ring-ex-
cloaddition reaction of α,β-unsaturated carbene complex intermedi-
pansion reaction,3 ring-closing metathesis,4 and several
ates with siloxydienes proceeded under mild conditions to give
cyclohepta[b]indole derivatives in good yield. The reaction was
others5 were utilized for the construction of this tricyclic
found to proceed via 1,2-alkyl shift of the carbene complex interme- core, and quite recently, gallium(III)-catalyzed three-
component [4+3]-cycloaddition reaction was reported.6
diates obtained by [4+2]-cycloaddition reaction of the unsaturated
carbenes with dienes.
We already reported a new method for the catalytic gen-
Key words: platinum, alkynes, cycloaddition, siloxy diene, cyclo-
eration of α,β-unsaturated carbene complex intermediate
hepta[b]indole
2 based on the electrophilic activation of alkynes with
Pt(II) catalyst,7–10 and the generated carbene complex fur-
ther reacted with vinyl ethers in a [3+2]-cycloaddition
manner to give tricyclic indoles fused with a five-mem-
bered ring 3. We expected that the cyclohepta[b]indole
Cyclohepta[b]indole derivatives have attracted much at-
tention of organic chemists due to its useful biological ac-
tivities and its unique structure of tricyclic indole fused
skeleton 4 could be constructed by using electron-rich
1,3-dienes instead of vinyl ethers through a formal [4+3]-
with a seven-membered ring (Figure 1).1
cycloaddition reaction of the α,β-unsaturated carbene
Cl
complex intermediate (Scheme 1).11 In this paper the suc-
cessful realization of this approach –along with the clari-
H
N
NC
OH
O
fication of the mechanism of this unique reaction – is
H
described.12
O
H
Cl
OH
H
According to our previous procedure,7 a mixture of ani-
line derivative 1 and 2-triisopropylsiloxyhepta-1,3-diene
(5a) was treated with 5 mol% of [PtCl2(CH2=CH2)]2 (10
mol% based on Pt) and 10 mol% of Ph3P in 1,4-dioxane at
100 °C, and the desired tricyclic compound 6a containing
a seven-membered ring was obtained along with a novel
N
H
CONH2
N
N
CO2H
H
H
actinophyllic acid
ambiguine E
SIRT1 inhibitor
Figure 1 Bioactive compounds containing cyclohepta[b]indole
skeleton
Boc
N
OR
[3+2]
RO
Boc
NHBoc
3
N
[Pt]
previous work
OSiR3
Boc
[Pt]
N
2
OMe
1
[4+3]
OSiR3
4
cyclohepta[b]indole
Scheme 1 The [3+2]- and [4+3]-cycloaddition reaction of α,β-unsaturated carbene complex intermediate
SYNLETT 2013, 24, 1364–1370
Advanced online publication: 15.05.2013
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DOI: 10.1055/s-0033-1338938; Art ID: ST-2013-U0220-L
© Georg Thieme Verlag Stuttgart · New York

LETTER
Construction of Cyclohepta[b]indoles
Table 1 Examination of Phosphine Ligands
1365
tetracyclic compound 7a containing a cyclopropane ring
in 36% and 28% yield,13 respectively (Table 1, entry 1).
Various reaction conditions were examined to improve
the yield and selectivity of the desired tricyclic indole de-
rivative 6a (Table 1). While electron-rich phosphine such
as PCy3 increased the amount of the tetracyclic product 7a
(Table 1, entry 3), the use of electron-deficient phosphine
such as phosphite decreased the formation of 7a (Table 1,
[PtCl2(CH2=CH2)]2 (5 mol%)
phosphine (10 mol%)
NHBoc
OTIPS
Na2CO3 (1 equiv)
+
n-Pr
5a (1 equiv)
5 Å MS, 1,4-dioxane
OMe
1
Boc
Boc
N
N
entry
4).
In
particular,
tris(pentafluorophe-
nyl)phosphine14,15 was found to be the most effective, and
the reaction proceeded smoothly even at room tempera-
ture and completed within one hour to give the desired
product selectively in 80% yield (Table 1, entry 5). More-
over, examination of additives revealed that; 1) the same
results were obtained with or without sodium carbonate,
and 2) the addition of molecular sieves gave slightly better
yield.16
OTIPS
+
H
n-Pr
7a
OTIPS
n-Pr
6a
Entry
Phosphine
Conditions
Yield of
Ratio
6a and 7a (%) 6a/7aa
1
Ph3P
none
PCy3
3 h, 100 °C
5 h, 100 °C
9 h, 100 °C
3 h, 100 °C
1 h, r.t.
64
57:43
90:10
48:52
74:26
100:0
The generality of the reaction under optimized reaction
conditions was examined employing various substituted
2-triisopropylsiloxy dienes, and the desired tricyclic in-
doles 6 were obtained in good yield when 4-aryl- or 4-(n-
propyl)-substituted dienes were employed (Table 2, en-
tries 1–4). However, considerable amounts of C–H inser-
tion products were obtained in the reaction of 4-i-Pr- and
4-SiMe2Ph-substituted 2-triisopropylsiloxy dienes (Table
2, entries 5 and 7). In this case, use of NTs aniline deriva-
tive 8 gave a better result than NBoc derivative 1 (Table
2, entry 6). 4,4-Di-substituted-2-siloxy-1,3-diene 5g and
cyclic diene 5h were also employable (Table 2, entries 8
2
<45
62
3
b
4
P(OR)3
66
5c
P(C6F5)3
80
a Determined by 1H NMR spectroscopy.
b P(OR)3 was tris(2,4-di-tert-butylphenyl)phosphite.
c Conditons: 2 equiv of diene 5a were used.
and 9). The 1.2–2.0 molar amounts of dienes were suffi-
cient to give the products in good yield.
Table 2 Substrate Scope of Siloxydienes22
Boc
N
NHBoc
[PtCl2(CH2=CH2)]2 (5 mol%)
P(C6F5)3 (10 mol%)
diene (X equiv)
+
OMe
4 Å MS, 1,4-dioxane
r.t., 1 h
OTIPS
R
1
6
Entry
1
Diene
x
Product
Yield (%)
80 (83a)
Boc
Boc
N
N
OTIPS
5a
2.0
6a
n-Pr
OTIPS
OTIPS
n-Pr
OTIPS
OTIPS
2
3
5b
5c
1.2
1.2
6b
6c
89
93
Ph
Ph
Boc
N
C6H4-4-OMe
OTIPS
4-MeOC6H4
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1364–1370

1366
H. Kusama et al.
LETTER
Table 2 Substrate Scope of Siloxydienes22 (continued)
Boc
N
NHBoc
[PtCl2(CH2=CH2)]2 (5 mol%)
P(C6F5)3 (10 mol%)
diene (X equiv)
+
OMe
4 Å MS, 1,4-dioxane
r.t., 1 h
OTIPS
R
1
6
Entry
Diene
x
Product
Yield (%)
Boc
N
OTIPS
O
4
5d
1.2
6d
62
O
OTIPS
Boc
N
OTIPS
OTIPS
OTIPS
5
6
7
5e
5e
5f
2.0
2.0
2.0
6e
9
62b
83c
38d
i-Pr
i-Pr
OTIPS
i-Pr
Ts
N
N
OTIPS
OTIPS
i-Pr
Boc
6f
SiMe2Ph
PhMe2Si
Boc
TIPSO
N
8
9
5g
5h
1.2
1.2
6g
6h
73
87
OTIPS
Boc
OTIPS
N
OTIPS
a Reaction conditions: 2.5 mol% of [PtCl2(CH2=CH2)]2 (5 mol% based on Pt) and 5 mol% of P(C6F5)3 were used, and the reaction was run for
1 h at r.t.
b C–H insertion product 7e (Figure 2) was also obtained in 20% yield as an inseparable mixture with 6e, and the yield was based on the inte-
gration of 1H NMR spectra.
c NTs aniline derivative 8 was employed instead of NBoc derivative 1, and the reaction was run for 2 h at r.t.
d C–H insertion product 7f (Figure 2) was also obtained in 30% yield as an inseparable mixture with 6f, and the yield was based on the integra-
tion of 1H NMR spectra.
Boc
Concerning the substituent on the benzene ring, 4-me-
thoxy-, 4-methoxycarbonyl-, or 4-fluoro-substituted ani-
line derivatives also gave the desired products in good
yield (Table 3, entries 1–3). The reaction of an aniline de-
rivative with a methyl substituent at the propargylic posi-
tion 10d also proceeded without problem to give a
methyl-substituted tricyclic indole 11d as a mixture of
diastereomers (Table 3, entry 4).
N
NHTs
OTIPS
H
R
OMe
7e R = i-Pr
7f R = SiMe2Ph
8
Figure 2
Synlett 2013, 24, 1364–1370
© Georg Thieme Verlag Stuttgart · New York

LETTER
Construction of Cyclohepta[b]indoles
1367
mediate A. Then siloxydiene underwent either addition to
the unsaturated carbene complex intermediate in a 1,4-
manner followed by ring closure at the β-position of the
platinum (path a, in Scheme 3) or concerted [4+2]-cyclo-
addition reaction (path b) to give the six-membered spiro-
cyclic carbene complex intermediate B.17 Then 1,2-alkyl
shift occurred at the carbene moiety18 by electron dona-
tion from the nitrogen to give the seven-membered ring
product C19 with regeneration of the Pt(II) catalyst. Selec-
tive migration of the R-substituted carbon is thought to be
due to the electron donation from the silyl enol ether moi-
ety.20 The tetracyclic compound D was produced through
insertion of the carbene intermediate into the neighboring
C–H bond.13,21
Table 3 Substrate Scope of Aniline Derivatives
NHBoc
OTIPS
R1
+
OMe
n-Pr
6a (2 equiv)
R2
10
Boc
N
R2
[PtCl2(CH2=CH2)]2 (5 mol%)
(C6F5)3P (10 mol%)
R1
4 Å MS, 1,4-dioxane
r.t., 1 h
OTIPS
n-Pr
11
Entry
R1
R2
Yield (%)
To support this mechanism, the reaction of a substrate 16
with a tolyl substituent at the propargylic position was ex-
amined. We expected that the tolyl substituent would en-
hance the migrating ability of the carbon substituted with
this tolyl group to give isomeric 1,2-alkyl shifted [4+3]
cycloadduct.
1
2
3
4
OMe
CO2Me
F
H
10a
11a 71
11b 78
11c 88
11d 75a
H
10b
10c
10d
H
H
Me
Treatment of a mixture of an aniline derivative 16 having
a p-tolyl substituent at the propargylic position and si-
loxydiene 5a with Pt(II) catalyst under the similar reaction
conditions gave four isomeric products in a ratio of
17a/17b/18a/18b = 45:9:23:23 in the combined yield of
60% (Scheme 4). These compounds were assigned to be
two regioisomers, both composed of two diastereomers as
shown in Scheme 4. Generation of regioisomers 18a and
18b was an obvious evidence that this [4+3]-cycloaddi-
tion reaction proceeded via 1,2-alkyl shift of the six-mem-
bered carbene complex intermediate E. The tolyl
substituent was thought to promote 1,2-alkyl migration
due to its electron-donating ability.
a Obtained as a 52:48 mixture of diastereomers. C–H insertion prod-
uct 12 (Figure 3) was also obtained in 6% yield.
Me
H
Boc
N
OTIPS
H
n-Pr
12 single isomer
Figure 3
Danishefsky’s diene 14 could also be employed for the
[4+3]-cycloaddition reaction (Scheme 2). By treatment of
crude products with 1 M HCl in THF, enones 15a were
obtained in good yield. Thus, this [4+3]-cycloaddition re-
action showed broad substrate scope.
In conclusion, a unique method for the construction of cy-
clohepta[b]indole skeleton was realized through plati-
num(II)-catalyzed formal [4+3]-cycloaddition reaction of
aniline derivatives ortho-substituted with propargylic
ether moiety with electron-rich dienes. Mechanistic stud-
ies supported that the reaction proceeded through [4+2]
cycloaddition followed by ring expansion to give the
products. Further application of this methodology to the
synthesis of biologically active compounds is now in
progress in our laboratory.
The mechanism of the reaction was thought to be as fol-
lows: Pt(II) catalyst activated the alkyne electrophilically
in a similar manner to the previous reaction, and 5-endo
cyclization of the nitrogen atom followed by methanol
elimination by electron donation from the platinum oc-
curred to generate the unsaturated carbene complex inter-
Boc
R
N
R
[PtCl2(CH2=CH2)]2 (5 mol%)
P(C6F5)3 (10 mol%)
NHBoc
OTIPS
1 M HCl (aq)
+
OMe
14 (X equiv)
4 Å MS, 1,4-dioxane
r.t., 1 h or 23 h
THF
r.t., 4 h
O
OMe
R = H
15a (82%)
(X = 1.2, 1 h)
R
R
1 R = H
13 R = Me
R = Me 15b (57%)
(X = 2.0, 23 h)
Scheme 2 [4+3]-Cycloaddition reaction with Danishefsky’s diene 14
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1364–1370

1368
H. Kusama et al.
LETTER
NHBoc
[PtCl2(CH2=CH2)]2 (5 mol%)
P(C6F5)3 (10 mol%)
OTIPS
+
R
4 Å MS, 1,4-dioxane
r.t., 1 h
OMe
1
Boc
Boc
N
N
OTIPS
+
H
R
OTIPS
R
C
D
Boc
+N
Boc
N
C–H
insertion
[Pt]–
R
[Pt]
OTIPS
A
[4+2]
R
1,2-shift
path a
path b
OTIPS
Boc
Boc
six-membered-
ring formation
N
N
TIPS
O+
OTIPS
R
–[Pt]
[Pt]
H
R
B
Scheme 3 Proposed reaction mechanisms
[PtCl2(CH2=CH2)]2 (5 mol%)
P(C6F5)3 (10 mol%)
NHBoc
OTIPS
+
n-Pr
4 Å MS, 1,4-dioxane
r.t., 67 h
OH
5a (2 equiv)
p-Tol
16
Boc
N
Boc
TIPS
O
n-Pr
p-Tol
N
p-Tol
Boc
N
+
OTIPS
OTIPS
p-Tol
n-Pr
n-Pr
[Pt]
17 (32%, dr = 5:1)
18 (28%, dr = 1:1)
E
1,2-shift of
n-Pr-substituted
carbon
1,2-shift of
p-Tol-substituted
carbon
Scheme 4 [4+3]-Cycloaddition reaction of p-tolyl-substituted aniline derivative 16
Acknowledgment
MS 4 Å (200 mg) were heated by heat gun under reduced pressure
in a test tube just before use. A mixture of [PtCl2(C2H2)]2 (2.9 mg,
0.005 mmol) and P(C6F5)3 (5.3 mg, 0.01 mmol) in 1,4-dioxane (0.5
mL) was added to this, followed by addition of a mixture of 1 (26.1
mg, 0.10 mmol) and 5a (53.5 mg, 0.20 mmol) in 1,4-dioxane (1.5
mL) at r.t. After the reaction mixture was stirred for 1 h, the reaction
was quenched with TMEDA (0.3 mL). After the filtration of MS 4
Å, the filtrate was concentrated under reduced pressure in vacuo.
The crude product was purified by silica gel (FL100DX) column
chromatography (hexane, then toluene–hexane = 20:80) to give 6a
(40.0 mg, 0.08 mmol, 80% yield).
This research was partly supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science,
and Technology of Japan.
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Synlett 2013, 24, 1364–1370
© Georg Thieme Verlag Stuttgart · New York

LETTER
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Construction of Cyclohepta[b]indoles
1369
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Boc
Boc
seven-
membered-
ring formation
N
N
+OTIPS
–[Pt]
OTIPS
R
R
Scheme 5
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Gohain, M.; García-García, P.; Fernández-Rodríguez, M.
(18) For a recent review of 1,2-alkyl migration of carbene
complex intermediates, see: (a) Crone, B.; Kirsch, S. F.
Chem. Eur. J. 2008, 14, 3514. For some examples of 1,2-
alkyl migration of platinum carbene intermediates, see:
(b) Kusama, H.; Ishida, K.; Funami, H.; Iwasawa, N. Angew.
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(d) Kusama, H.; Miyashita, Y.; Takaya, J.; Iwasawa, N. Org.
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Iwasawa, N. Org. Lett. 2004, 6, 605. (f) Li, G.; Huang, X.;
Zhang, L. Angew. Chem. Int. Ed. 2008, 47, 346. (g) Shu,
X.-Z.; Zhao, S.-C.; Ji, K.-G.; Zheng, Z.-J.; Liu, X.-Y.; Liang,
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Zhang, X.; Fu, C.; Ma, S. Adv. Synth. Catal. 2012, 354,
2339.
(19) Tetracyclic compound 7a was not isomerized to the [4+3]-
cycloaddition product 6a under the reaction conditions, and
only 7a was recovered.
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1364–1370

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H. Kusama et al.
LETTER
(20) (a) Kusama, H.; Karibe, Y.; Onizawa, Y.; Iwasawa, N.
Angew. Chem. Int. Ed. 2010, 49, 4269. (b) Kusama, H.;
Karibe, Y.; Imai, R.; Onizawa, Y.; Yamabe, H.; Iwasawa, N.
Chem. Eur. J. 2011, 17, 4839.
(21) For a review of C–H insertion of metal carbenoids, see:
(a) Doyle, M. P.; Duffy, R.; Ratnikov, M.; Zhou, L. Chem.
Rev. 2010, 110, 704. For recent examples of C–H insertion
of platinum carbene intermediates, see: (b) Funami, H.;
Kusama, H.; Iwasawa, N. Angew. Chem. Int. Ed. 2007, 46,
909. (c) Oh, C. H.; Lee, J. H.; Lee, S. J.; Kim, J. I.; Hong, C.
S. Angew. Chem. Int. Ed. 2008, 47, 7505. (d) Oh, C. H.; Lee,
J. H.; Lee, S. M.; Yi, H. J.; Hong, C. S. Chem. Eur. J. 2009,
15, 71.
H), 5.16 (s, 1 H), 6.91 (dt, J = 0.9, 7.4 Hz, 1 H), 7.10–7.15
(m, 2 H), 7.68–7.75 (m, 1 H). 13C NMR (125 MHz, CDCl3,
330 K): δ = 12.8, 14.1, 18.0, 19.8, 24.8, 25.8, 27.8, 28.5,
28.9, 39.7, 55.3, 81.3, 107.3, 115.0, 122.3, 124.6, 126.7,
130.1, 145.9, 150.5, 153.2. HRMS–FAB: m/z [M]+ calcd for
C30H47NO3Si: 497.3325; found: 497.3329.
Compound 6h: colorless oil. IR (neat): 3050, 2942, 2866,
1730, 1655, 1456, 1357, 1132, 1117, 883, 743, 687 cm–1. 1H
NMR (500 MHz in CDCl3): δ = 1.06–1.10 (m, 18 H), 1.13–
1.23 (m, 3 H), 1.66 (s, 9 H), 1.81–1.93 (m, 2 H), 2.03–2.13
(m, 2 H), 2.76–2.80 (m, 1 H), 3.10 (dd, J = 4.6, 18.2 Hz, 1
H), 3.44 (br d, J = 18.2 Hz, 1 H), 3.53–3.57 (m, 1 H), 5.53
(dd, J = 1.8, 7.9 Hz, 1 H), 7.18–7.22 (m, 2 H), 7.42–7.47 (m,
1 H), 8.05–8.10 (m, 1 H). 13C NMR (125 MHz, CDCl3): δ =
12.6, 17.97, 17.99, 25.9, 28.2, 28.3, 31.7, 35.6, 37.8, 83.3,
109.4, 115.4, 116.9, 122.1, 123.0, 123.4, 128.8, 133.9,
134.9, 150.7, 154.0. HRMS–FAB: m/z [M]+ calcd for
C29H43NO3Si: 481.3012; found: 481.3021.
Compound 9: pale yellow oil. IR (neat): 3046, 2945, 2866,
1663, 1598, 1452, 1381, 1367, 1173, 746, 684 cm–1. 1H
NMR (500 MHz, CDCl3): δ = 0.75 (d, J = 6.8 Hz, 3 H), 0.89
(d, J = 6.6 Hz, 3 H), 1.02–1.07 (m, 18 H), 1.08–1.18 (m, 3
H), 1.90–2.00 (m, 1 H), 2.06–2.16 (m, 1 H), 2.32 (s, 3 H),
2.41 (ddd, J = 3.2, 6.4, 16.8 Hz, 1 H), 3.11 (ddd, J = 3.2, 11.7,
16.1 Hz, 1 H), 3.23 (t, J = 7.4 Hz, 1 H), 3.64 (ddd, J = 3.5,
6.4, 16.1 Hz, 1 H), 5.12 (dd, J = 0.7, 7.4 Hz, 1 H), 7.14 (d, J
= 8.2 Hz, 2 H), 7.21–7.29 (m, 2 H), 7.33–7.36 (m, 1 H),
7.51–7.55 (m, 2 H), 8.22–8.26 (m, 1 H). 13C NMR (125
MHz, CDCl3): δ = 12.6, 17.97, 17.99, 20.5, 21.5, 21.7, 22.8,
31.7, 35.2, 39.7, 105.9, 115.4, 118.3, 123.3, 123.8, 124.3,
126.1, 129.6, 131.4, 136.1, 136.3, 136.4, 144.5, 152.0.
HRMS–FAB: m/z [M]+ calcd for C32H45NO3SSi: 551.2889;
found: 551.2881.
(22) Selected Compound Data
Compound 6a: colorless oil. IR (neat): 3049, 2944, 2867,
1730, 1667, 1458, 1369, 1355, 1149, 1119, 742, 683 cm–1.
1H NMR (500 MHz, CDCl3): δ = 0.95 (t, J = 7.3 Hz, 3 H),
1.09–1.13 (m, 18 H), 1.15–1.25 (m, 3 H), 1.38–1.48 (m, 1
H), 1.48–1.57 (m, 1 H), 1.69 (s, 9 H), 1.70–1.79 (m, 2 H),
2.40 (ddd, J = 3.2, 8.2, 15.4 Hz, 1 H), 2.78 (ddd, J = 2.9, 10.3,
15.4 Hz, 1 H), 3.20 (ddd, J = 2.9, 8.2, 16.9 Hz, 1 H), 3.44–
3.53 (m, 2 H), 5.24 (d, J = 8.1 Hz, 1 H), 7.19–7.25 (m, 2 H),
7.38–7.42 (m, 1 H), 8.02–8.07 (m, 1 H). 13C NMR (125 MHz
in CDCl3): δ = 12.6, 14.2, 18.04, 18.05, 21.7, 25.3, 28.3,
32.2, 32.4, 39.2, 83.5, 107.4, 115.2, 117.5, 121.7, 122.2,
123.2, 130.2, 135.1, 136.6, 150.8, 153.1. HRMS–FAB: m/z
[M]+ calcd for C30H47NO3Si: 497.3325; found: 497.3346.
Compound 7a: colorless oil. IR (neat): 3048, 2944, 2867,
1709, 1656, 1478, 1368, 1347, 1224, 1178, 883, 749, 684
cm–1. 1H NMR (500 MHz, CDCl3, 330 K): δ = 0.72 (t, J =
7.4 Hz, 3 H), 0.94–0.99 (m, 2 H), 1.11–1.15 (m, 18 H), 1.15–
1.25 (m, 3 H), 1.25–1.44 (m, 2 H), 1.58 (s, 9 H), 2.12–2.20
(m, 2 H), 2.23–2.31 (m, 1 H), 2.47 (s, 1 H), 2.90–3.04 (m, 1
Synlett 2013, 24, 1364–1370
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