Regioselective synthesis of secondary 1,3-dienamides by successive eliminations

Article abstract of DOI:10.1016/j.tet.2015.11.065

The regioselective synthesis of secondary 1,3-dienamides 3 (1-N-acylamino-1,3-dienes) is successfully demonstrated by regiospecific base-promoted 1,4-elimination of (Z)- or (E)-N,N-di-Boc-4-methoxy-2-buten-1-ylamine 1 followed by mono-Boc elimination in situ.

Full text of DOI:10.1016/j.tet.2015.11.065

Accepted Manuscript
Regioselective synthesis of secondary 1,3-dienamides by successive eliminations
Eiji Tayama, Shun Saito
PII:
S0040-4020(15)30248-9
10.1016/j.tet.2015.11.065
TET 27322
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 9 October 2015
Revised Date: 24 November 2015
Accepted Date: 30 November 2015
Please cite this article as: Tayama E, Saito S, Regioselective synthesis of secondary 1,3-dienamides by
successive eliminations, Tetrahedron (2015), doi: 10.1016/j.tet.2015.11.065.
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Regioselective synthesis of secondary 1,3-
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Eiji Tayama and Shun Saito
Department of Chemistry, Faculty of Science, Niigata University, Niigata 950-2181, Japan

1
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Tetrahedron
journal homepage: www.elsevier.com
Regioselective synthesis of secondary 1,3-dienamides by successive eliminations
a,
a
Eiji Tayama ∗ and Shun Saito
a
Department of Chemistry, Faculty of Science, Niigata University, Niigata, 950-2181, Japan
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The regioselective synthesis of secondary 1,3-dienamides 3 (1-N-acylamino-1,3-dienes) is
successfully demonstrated by regiospecific base-promoted 1,4-elimination (Z)- or (E)-N,N-di-
Boc-4-methoxy-2-buten-1-ylamine 1 followed by mono-Boc elimination in situ.
2
009 Elsevier Ltd. All rights reserved.
Available online
Keywords:
Elimination
1
1
-Amino-1,3-diene
,3-Dienamide
Regioselective
Regiospecific
—
——
∗
Corresponding author. E-mail:tayama@chem.sc.niigata-u.ac.jp

2
Tetrahedron
1
. Introduction
between H and H (J = 13–14 Hz). Even if the reaction was
1 2
ACCEPTED MANUSC
RIPT
performed on a larger scale, an acceptable yield was obtained
entry 5, 70%). The use of a larger amount of LDA (2.2 equiv.)
Enamides (enecarbamates) constitute a class of valuable
(
building blocks in organic synthesis for the synthesis of nitrogen-
containing compounds. Tertiary enamides, which is N-alkylated
derivatives, are less nucleophilic. Thus the synthetic application
of tertiary enamides have been demonstrated in cycloaddition
reactions. In contrast, secondary enamides are reactive toward
common electrophiles in the presence of Lewis or Brønsted
acids. Especially, the N-acylamino substituent (NHCOR), as
found in secondary enamides, can interact with the chiral
Brønsted acids. Various asymmetric reactions using secondary
did not improve the yield of E-3a (entry 6). When the reaction
was performed with 1.0 equiv. of LDA, E-3a (52%) was obtained
with a small amount of E-2a (5%) (entry 7). The use of analog
bases, such as lithium 2,2,6,6-tetramethylpiperidide (LiTMP),
lithium bis(trimethylsilyl)amide (LiHMDS), and sodium
bis(trimethylsilyl)amide (NaHMDS) did not afford the 1,4-
eliminated products 2a and 3a, with Z-1a being recovered
1
2
3-5
(entries 8–10).
The reaction promoted with potassium
bis(trimethylsilyl)amide (KHMDS) afforded E-2a and E-3a in
lower yields (entry 11). The use of 2.2 equiv. of KHMDS gave
complicated products formed by undesirable side reactions (entry
enamides have been developed.
The dienyl derivatives,
secondary 1,3-dienamides (1-N-acylamino-1,3-dienes), have also
attracted much attention from synthetic organic chemists because
they function as reactive dienyl components, such as in Diels–
Alder reactions to construct fused-ring compounds. Their most
standard preparative method is Curtius rearrangement of 2,4-
dienoic acid azides followed by alcolysis under heated conditions
1
2).
6-8
a
Table 1. Base-promoted 1,4-elimination of Z-1a
(
Scheme 1, eq. 1), which was originally developed by Overman
9
et al. This method usually affords the thermodynamically stable
E-regioisomer as the main product. A number of synthetic
10,11
methods of secondary 1,3-dienamides have been reported.
however, stereoselective and E/Z-stereo-controlled synthetic
protocols have never been developed.
b
c
c
Entry
Base (equiv.)
Solvent
THF
Temp.
°C)
E-2a
(%)
E-3a
(%)
(
Our group has studied stereoselective and stereospecific base-
12,13
14
1
2
3
4
n-BuLi (1.5)
n-BuLi (1.5)
LDA (1.5)
LDA (1.5)
LDA (1.5)
LDA (2.2)
LDA (1.0)
LiTMP (1.5)
–78
–78
–78
0
0
0
promoted 1,4-elimination
affording N-Boc-1,3-dienamides;
Et
2
O
0
0
however, this method was limited to preparing tertiary 1,3-
15
dienamides.
Meanwhile, in our study on the regioselective
16
THF
THF
THF
THF
THF
THF
THF
87
0
0
cyclopropanation of 1,3-dienamides, we obtained one example
of the regioselective preparation of a secondary 1,3-dienamide 3
by the 1,4-elimination of N,N-di-Boc-4-methoxy-2-buten-1-
ylamine 1 (Scheme 1, eq. 2). Therefore, we decided to further
investigate the scope and limitations of this reaction. Herein, we
report the regioselective synthesis of secondary 1,3-dienamides
by base-promoted 1,4-elimination followed by mono-Boc
elimination.
78
70
63
52
0
d
5
0
0
6
0
0
e
7
0
5
8
0
0
9
LiHMDS (1.5)
0
0
0
f
1
1
1
0
1
2
NaHMDS (1.5) THF
0
0
5
g
KHMDS (1.5)
KHMDS (2.2)
THF
THF
0
11
30
0
messy
a
Unless otherwise noted, the reactions were performed using
Z-1a (0.30 mmol) and ca. 0.7 M LDA solution (0.64 mL, 0.45
mmol) in THF (2.4 mL).
b
LDA: ca. 0.7 M THF/n-hexane solution prepared from n-
BuLi in n-hexane and diisopropylamine in THF; LiTMP: ca. 0.5
M THF/n-hexane solution prepared from n-BuLi in n-hexane and
2
1
,2,6,6-tetramethylpiperidine in THF; LiHMDS and NaHMDS:
.0 M THF solution (Aldrich); KHMDS: 0.5 M toluene solution
Scheme 1. Preparation of secondary 1,3-dienamides
(
Aldrich).
2
. Results and discussion
c
Isolated yield.
First, we prepared Z-1a as a substrate and investigated the
14
d
e
f
base-promoted 1,4-elimination according to our previous result
Z-1a: 2.14 mmol scale.
(
Table 1). Treatment of Z-1a with n-BuLi in THF or Et O at –78
2
Recovered Z-1a in 24% yield.
Recovered Z-1a in 76% yield.
Recovered Z-1a in 14% yield.
°
C did not afford the expected N,N-di-Boc-1,3-dienamide 2a
without the recovery of Z-1a (entries 1 and 2). The di-Boc group
might be unstable toward strong nucleophiles such as n-BuLi.
Thus, we used a non-nucleophilic base such as LDA. The
desired product 2a was obtained in 87% yield as a single
regioisomer (entry 3). Interestingly, when the reaction was
carried out at 0 °C, one of the N-Boc groups was eliminated to
give the secondary 1,3-dienamide 3a in 78% yield as a single
regioisomer (entry 4). The C –C stereochemistry of 2a and 3a
g
E-3a would be formed by the E-stereoselective 1,4-
elimination of Z-1a followed by the base-promoted elimination
of an N-Boc group, as in E-2a (Scheme 2). The coordination of
excess LDA to one of the Boc-carbonyl oxygen (E-2a + LDA)
accelerates the deprotonation of a tert-butyl proton. This
mechanism required 2 equiv. of LDA for complete conversion;
1
2
1
was assigned as E based on the coupling constant of H NMR

3
however, the best yield of E-3a was obtained
A
in
C
th
C
e
E
ca
P
se
T
o
E
f 1
D
.5 MAin
N
E-
U
1a
S
, p
C
ro
R
ce
I
eded to afford (E)-tert-butyl (4-methoxybut-2-en-
P
T
equiv. of LDA (Table 1, entry 4). Although the exact reason is
presently unclear, undesirable side reactions with LDA might
occur to afford uncharacterized products.
1-yl)carbamate (E-1b) in 37% yield.
a
Table 2. Base-promoted 1,4-elimination of E-1a
b
c
c
Entry Base (equiv.)
Temp. (°C)
Z-2a (%)
Z-3a (%)
1
2
3
4
5
6
LDA (1.5)
LDA (2.2)
LDA (1.5)
LDA (1.5)
LDA (2.2)
–78
–78
0
45
30
0
34
44
71
64
56
29
d
0
0
e
f
0
0
KHMDS (1.5)
0
0
Scheme 2. Proposed mechanism of the base-promoted 1,4-
elimination.
a
Unless otherwise noted, the reactions were performed using
E-1a (0.30 mmol) and ca. 0.7 M LDA solution (0.64 mL, 0.45
mmol) in THF (2.4 mL).
To clarify the mechanism of this stereoselective formation of
the secondary 1,3-dienamide E-3a, we treated E-2a with LDA to
remove one of the Boc groups (Scheme 3). The reaction
proceeded to afford E-3a in 57% yield without isomerization to
Z-3a. Next, we prepared the mono-N-Boc substrate Z-1b and
examined the reaction with 3.0 equiv. of LDA. The 1,4-
eliminated product 3a was obtained as an E/Z = 25/75 mixture in
b
LDA: ca. 0.7 M THF/n-hexane solution (prepared),
KHMDS: 0.5 M toluene solution (Aldrich).
c
Isolated yield.
d
E-1a: 2.18 mmol scale.
6
7% combined yield.
e
E-3a was obtained in 8 % yield.
f
(E)-tert-Butyl (4-methoxybut-2-en-1-yl)carbamate (E-1b)
was obtained in 37 % yield.
To investigate the scope and limitations of this method, we
prepared the 4-substituted (n-butyl) substrate Z-1c and
investigated the reactions (Scheme 4).
Under the same
conditions described above, the corresponding secondary 1,3-
dienamides, di-Boc derivative 1E-2c or mono-Boc derivative 1E-
3
c, were obtained as a mixture of 1E,3E and 1E,3Z regioisomers,
respectively. We attempted the reaction with NaHMDS in Et O–
2
THF (4/1), and those conditions afforded tertiary 1E,3E-1,3-
Scheme 3. Experimental studies on the base promoted 1,4-
elimination.
dienamide via the formation of the chelated intermediate as
14b,17
reported by our group.
However, the reaction was
unsuccessful with the recovery of Z-1c (34%). Elimination of
one Boc group, as in Z-1c, proceeded in 20% yield. Similarly,
the reaction of E-1c gave the secondary 1,3-dienamides 1Z-2c or
1Z-3c as a mixture of 1Z,3E and 1Z,3Z regioisomers by the ‘syn-
With the results in hand, we next investigated the reaction of
the E-counterpart (E-1a) to prepare another regioisomer Z-2a or
Z-3a by the regiospecific and stereoselective 1,4-elimination
18
(
Table 2). When the reaction was carried out under the same
effect’, as proposed by Inomata and Ukaji (Scheme 5).
conditions depicted in Table 1, it afforded Z-2a in 45% and Z-3a
in 34% yield (entry 1). The C –C stereochemistry of 2a and 3a
To demonstrate the sequential, regiospecific, and
regioselective synthetic transformations, we performed the
copper(I)-catalyzed cyclopropanation of E-3a and Z-3a with α-
1
2
1
was assigned as Z based on the coupling constant of H NMR
between H and H (J = 8–10 Hz). One of the Boc groups, as
1
2
H1,H2
16
aryl diazoesters 4 according to our previous report (Scheme 6).
in Z-2a, may be eliminated more easily than from E-2a to form
Z-3a at –78 °C. Complete conversion to the secondary 1,3-
dienamide Z-3a was not accomplished at –78 °C even with the
use of 2.2 equiv. of LDA (entry 2). The elimination proceeded
smoothly at 0 °C, and the desired product Z-3a was obtained in
The reaction of E-3a proceeded at the C –C double bond to give
3
4
the corresponding cyclopropyl enamide E-5 exclusively. On the
other hand, when Z-3a was subjected to the same conditions, the
reaction preferentially proceeded at the C –C double bond to
1
2
afford vinylcyclopropylamine derivative 6 as the main product.
7
1% yield (entry 3). The yield was acceptable if the reaction was
performed on a larger scale (entry 4, 64%). Similarly to the
reaction of Z-1a (Table 1, entry 6), the use of 2.2 equiv. of LDA
did not improve the yield of Z-3a (entry 5) and the stereoisomer
E-3a was isolated in 8% yield as a side product by careful
chromatographic purification. The reaction with KHMDS was
unsuccessful (entry 6). Elimination of one of the Boc groups, as

4
Tetrahedron
ACCEPTED MANIn
U
fra
GX FT-IR spectrometer.
measured on a Varian 400 MHz spectrometer ( H: 400 MHz, C:
00 MHz). The splitting patterns are denoted as follows: s,
S
re
C
d
R
sp
I
e
P
ct
T
ra were recorded on a Perkin Elmer Spectrum
1 13
H and C NMR spectra were
1
13
1
singlet; d, doublet; t, triplet; q, quartet; m, multiplet; and br,
broad peak. High-resolution mass spectra were measured on a
Thermo Fisher Scientific LC/FT-MS spectrometer. Reactions
involving air- or moisture-sensitive compounds were conducted
in appropriate round-bottomed flasks with a magnetic stirring bar
under an argon atmosphere. n-Butyllithium was purchased from
Wako Pure Chemical Industries, Ltd. Diisopropylamine was
were purchased from Aldrich (purified by redistillation, 99.95%)
and used without purification. Lithium bis(trimethylsilyl)amide
solution
(LiHMDS:
1.0
M
in
THF),
sodium
bis(trimethylsilyl)amide solution (NaHMDS: 1.0 M in THF), and
potassium bis(trimethylsilyl) amide solution (KHMDS: 0.5 M in
Scheme 4. 1,4-Elimination of the 4-butyl derivative Z-1c.
toluene)
Tetrakis(acetonitrile)copper(I)
and
were
purchased
from
hexafluorophosphate
(MeCN) CuPF ] was purchased from Tokyo Chemical Industry
Aldrich.
[
(
4
6
TCI) Co., Ltd. Tetrahydrofuran (THF) and diethyl ether (Et O)
2
were purchased from KANTO Chemical Co., Inc., Japan as an
anhydrous solvent. Dichloromethane was distilled from calcium
hydride prior to use. For thin layer chromatography (TLC)
analysis throughout this work, Merck TLC plates (silica gel 60
F254) were used. The products were purified by preparative
column chromatography on silica gel (silica gel 60N, spherical
neutral, KANTO Chemical Co., Inc., Japan).
3
.2. Preparation of ca. 0.7 M LDA THF/n-hexane solution
A dried 100 mL storage flasks with stopcock-equipped
septum-inlet (Aldrich) is charged with diisopropylamine (2.86
mL, 20.4 mmol) in THF (15 mL) under an argon atmosphere. A
Scheme 5. 1,4-Elimination of the 4-butyl derivative E-1c.
1
.61 M n-butyllithium hexane solution (12.7 mL, 20.4 mmol)
was added to the solution of at –78 °C. The mixture was stirred
for 15 min at the same temperature and allowed to warm to 0 °C
2
0
slowly. The resulting pale yellow solution was titrated and
stored in refrigerator.
3
.3. Representative procedure for preparation of E-2a by 1,4-
elimination of Z-1a
A solution of Z-1a (90 mg, 0.30 mmol) in THF (2.4 mL) was
treated with a ca. 0.7 M LDA THF/n-hexane solution (0.64 mL,
0
3
.45 mmol) at –78 °C under an argon atmosphere and stirred for
h at the same temperature. The resulting mixture was poured
into saturated aqueous ammonium chloride and extracted with
ethyl acetate. The combined extractes were washed with brine,
dried over sodium sulfate, and concentrated. The residue was
purified by chromatography on silica gel (n-hexane/ethyl acetate
=
30/1 as the eluent) to afford E-2a (70 mg, 87% yield) as a
colorless oil.
Scheme 6. Regiospecific and regioselective cyclopropanation
of 3a
3
.4. Representative procedure for preparation of Z-3a by 1,4-
elimination of E-1a
In summary, the regioselective synthesis of secondary 1,3-
dienamides 3 (1-N-acylamino-1,3-dienes) was reported. The
reaction proceeds via the regioselective base-promoted 1,4-
elimination of (Z)- or (E)-N,N-di-Boc-4-methoxy-2-buten-1-
A solution of E-1a (90 mg, 0.30 mmol) in THF (2.4 mL) was
treated with a ca. 0.7 M LDA THF/n-hexane solution (0.64 mL,
0.45 mmol) at 0 °C under an argon atmosphere. After stirring for
3 h at the same temperature, the resulting mixture was quenched
with saturated aqueous ammonium chloride. Extractive workup
and purification of the residue by chromatography on silica gel
(n-hexane/ethyl acetate = 20/1 as the eluent) gave Z-3a (36 mg,
71% yield) as a white solid.
19
ylamine 1 followed by mono-Boc elimination.
sequential synthetic transformations were demonstrated by the
Cu-catalyzed regiospecific 3,4- or 1,2-regioselective
cyclopropanation of 3.
Further
3
3
. Experimental
.1. General
3.4.1. (E)-N,N-Di-(tert-butoxycarbonyl)buta-1,3-
dien-1-ylamine (E-2a)
Colorless oil; IR (film) 3088, 2980, 2934, 1755, 1724, 1650,
1
604, 1478, 1457, 1420, 1394, 1369, 1329, 1292, 1259, 1230,

5
-
1
-1 1
1
165, 1135, 1101, 1035, 997, 929, 900, 856, 8
A
15,
C
7
C
84
E
, 7
P
6
T
0 c
E
m
D
;
MA85
N
7,
U
81
S
6,
C
7
R
85
I
,
P
761 cm ; H NMR (400 MHz, DMSO-d ) δ 6.61
6
T
1
H NMR (400 MHz, CDCl ) δ 6.63 (1H, dd, J = 14.2, 0.6 Hz, 1-
(0.45H, d, J = 13.6 Hz, 1-H1E,3Z), 6.48 (0.55H, d, J = 14.3 Hz, 1-
H1E,3E), 6.14-5.94 (1.45H, m, 2-H1E,3Z and 3-H), 5.84 (0.55H, dd,
J = 14.3, 10.6 Hz, 2-H1E,3E), 5.61 (0.55H, dt, J = 15.2, 7.2 Hz, 4-
H1E,3E), 5.36 (0.45H, dt, J = 10.4, 7.8 Hz, 4-H1E,3Z), 2.11-1.99 (2H,
m, 5-H), 1.48 (8.1H, s, t-Bu1E,3Z), 1.47 (9.9H, s, t-Bu1E,3E), 1.38-
1.21 (4H, m, 6- and 7-H), 0.87 (1.65H, t, J = 7.2 Hz, 8-H1E,3E),
3
H), 6.29 (1H, dddd, J = 16.8, 10.4, 10.4, 0.6 Hz, 3-H), 6.00 (1H,
dddd, J = 14.2, 10.4, 0.6, 0.6 Hz, 2-H), 5.12 (1H, ddd, J = 16.8,
.6, 0.6 Hz, 4-H), 5.00 (1H, ddd, J = 10.4, 0.6, 0.6 Hz, 4-H), 1.53
0
(
1
13
18H, s, t-Bu); C NMR (100 MHz, CDCl ) δ 151.1, 134.5,
3
+
27.3, 117.6, 115.2, 83.3, 27.7; HRMS–ESI (m/z): [M+Na]
13
calcd for C H NO Na: 292.1519. Found: 292.1514.
0.86 (1.35H, t, J = 7.2 Hz, 8-H1E,3Z); C NMR (100 MHz,
14
23
4
DMSO-d ) δ 150.7, 150.5, 132.9, 130.1, 127.4, 126.6, 125.9,
6
3
(
.4.2. (E)-tert-Butyl buta-1,3-dien-1-ylcarbamate
E-3a)
White solid; Mp 67–68 °C (lit.
085, 3037, 2982, 2935, 1698, 1658, 1608, 1503, 1460, 1445,
409, 1391, 1368, 1294, 1280, 1252, 1232, 1158, 1051, 1023,
1
2
24.9, 117.3, 111.7, 83.4, 83.2, 31.8, 31.2, 31.0, 27.4, 27.3, 26.7,
1.64, 21.62, 13.8, 13.7; HRMS–ESI (m/z): [M+Na] calcd for
+
9c-9d
67–68 °C); IR (KBr) 3357,
C H NO Na: 348.2145. Found: 348.2137.
18
31
4
3
1
9
3.4.5. tert-Butyl (1E)-octa-1,3-dien-1-ylcarbamate
-
1 1
(
1E-3c)
94, 949, 932, 877, 861, 773, 766, 753 cm ; H NMR (400 MHz,
DMSO-d ) δ 9.33 (1H, br d, J = 10.8 Hz, NH), 6.63 (1H, dd, J =
Colorless oil; 1E,3E/1E,3Z = 30/70; IR (film) 3303, 3135,
6
1
5
4
3.0, 10.8 Hz, 1-H), 6.26 (1H, ddd, J = 17.0, 11.0, 10.4 Hz, 3-H),
.70 (1H, dd, J = 13.0, 11.0 Hz, 2-H), 4.91 (1H, d, J = 17.0 Hz,
3066, 2960, 2929, 2872, 1727, 1703, 1657, 1633, 1621, 1513,
1504, 1455, 1403, 1392, 1367, 1286, 1254, 1233, 1163, 1080,
1047, 1020, 976, 936, 902, 866, 787, 764 cm ; H NMR (400
13
-1
1
-H), 4.76 (1H, d, J = 10.4 Hz, 4-H), 1.42 (9H, s, t-Bu);
C
NMR (100 MHz, CDCl ) δ 152.6, 134.7, 127.4, 112.5, 111.0,
MHz, CD OD) 6.57 (0.7H, d, J = 13.6 Hz, 1-H1E,3Z), 6.49 (0.3H,
d, J = 14.2 Hz, 1-H1E,3E), 6.02-5.82 (1.7H, m, 2-H1E,3Z and 3-H),
5.66 (0.3H, dd, J = 14.2, 10.8 Hz, 2-H1E,3E), 5.43 (0.3H, dt, J =
3
3
+
8
1
0.7, 28.1; HRMS–ESI (m/z): [M+H] calcd for C H NO :
70.1176. Found: 170.1172.
9
16
2
1
2
7
4.8, 7.0 Hz, 4-H1E,3E), 5.19 (0.7H, dt, J = 10.0, 7.6 Hz, 4-H1E,3Z),
.11 (1.4H, dt, J = 7.6, 6.7 Hz, 5-H1E,3Z), 2.05 (0.6H, dt, J = 7.0,
3
.4.1. (Z)-N,N-Di-(tert-butoxycarbonyl)buta-1,3-
dien-1-ylamine (Z-2a)
.0 Hz, 5-H1E,3E), 1.47 (9H, s, t-Bu), 1.40-1.27 (4H, m, 6- and 7-
Colorless oil; IR (film) 3088, 3047, 2980, 2934, 1793, 1751,
718, 1648, 1595, 1478, 1458, 1433, 1393, 1369, 1340, 1283,
246, 1152, 1117, 1035, 995, 911, 883, 852, 824, 790, 769 cm ;
13
H), 0.97-0.85 (3H, m, 8-H); C NMR (100 MHz, CD OD) δ
3
1
1
1
1
1
2
55.5, 155.4, 131.0, 129.6, 128.6, 128.3, 127.9, 126.5, 112.6,
08.0, 81.3, 81.2, 33.7, 33.3, 33.2, 28.8, 28.5, 23.5, 23.4, 14.53,
-
1
1
H NMR (400 MHz, CDCl ) δ 6.30 (1H, ddd, J = 16.9, 10.4, 10.4
+
3
4.49; HRMS–ESI (m/z): [M+H] calcd for C H NO :
1
3
24
2
Hz, 3-H), 5.87 (1H, dd, J = 8.0, 1.0 Hz, 1-H), 5.81 (1H, dd, J =
1
26.1802. Found: 226.1800.
0.4, 8.0 Hz, 2-H), 5.23 (1H, dd, J = 16.9, 1.2 Hz, 4-H), 5.12 (1H,
13
ddd, J = 10.4, 1.2, 1.0 Hz, 4-H), 1.41 (18H, s, t-Bu); C NMR
100 MHz, CDCl ) δ 151.0, 130.0, 125.9, 124.3, 119.3, 82.6,
3.4.6. (1Z)-N,N-Di-(tert-butoxycarbonyl)octa-1,3-
dien-1-ylamine (1Z-2c)
(
2
2
3
+
7.7; HRMS–ESI (m/z): [M+Na] calcd for C H NO Na:
Colorless oil; 1Z,3E/1Z,3Z = 50/50; IR (film) 2978, 2931,
14
23
4
92.1519. Found: 292.1512.
2872, 1790, 1749, 1721, 1656, 1614, 1601, 1479, 1456, 1393,
1
8
368, 1339, 1283, 1246, 1151, 1110, 1035, 975, 926, 885, 853,
3
.4.2. (Z)-tert-Butyl buta-1,3-dien-1-ylcarbamate
-1 1
24, 769 cm ; H NMR (400 MHz, C D ) δ 6.36-6.23 (1H, m, 3-
6
6
(
Z-3a)
White solid; Mp 96–98 °C; IR (KBr) 3276, 3162, 3086, 3006,
979, 2935, 1705, 1678, 1655, 1605, 1517, 1458, 1392, 1363,
314, 1272, 1252, 1165, 1117, 1044, 1022, 996, 939, 921, 890,
H), 6.14 (0.5H, d, J = 8.1, 1.0 Hz, 1-H1Z,3Z), 6.05 (0.5H, ddd, J =
1
5
1
1.4, 8.1, 1.2 Hz, 2-H1Z,3Z), 6.02 (0.5H, d, J = 8.0 Hz, 1-H1Z,3E),
.76 (0.5H, dd, J = 11.0, 8.0 Hz, 2-H1Z,3E), 5.54 (0.5H, dt, J =
4.8, 7.2 Hz, 4-H1Z,3E), 5.40 (0.5H, dtd, J = 10.8, 7.5, 1.0 Hz, 4-
2
1
8
-
1 1
57, 788, 777, 758, 703 cm ; H NMR (400 MHz, CDCl ) (8/2
3
H1Z,3Z), 1.96 (1H, dtd, J = 7.5, 7.1, 1.2 Hz, 5-H1Z,3Z), 1.90 (1H, dt,
J = 7.2, 6.6 Hz, 5-H1Z,3E), 1.394 (9H, s, t-Bu), 1.388 (9H, s, t-Bu),
1
NMR (100 MHz, DMSO-d ) δ 150.9, 150.8, 137.1, 134.5, 124.4,
1
2
mixture of rotamers) δ 7.42 (0.2H, br, NH), 6.80-6.15 (1.8H, br
m, 3-H and NH), 6.62 (1H, br d, J = 10.0 Hz, 1-H), 5.28 (1H, dd,
J = 10.4, 10.0 Hz, 2-H), 5.15 (1H, d, J = 16.8 Hz, 4-H), 5.01 (1H,
d, J = 10.4 Hz, 4-H), 1.48 (9H, s, t-Bu); C NMR (100 MHz,
CDCl ) δ 152.4, 129.0, 123.1, 115.4, 107.8, 80.8, 28.2; HRMS–
13
.25-1.10 (4H, m, 6- and 7-H), 0.80 (3H, t, J = 7.0 Hz, 8-H);
C
13
6
24.0, 123.3, 122.3, 121.4, 119.2, 82.3, 82.2, 31.9, 31.1, 30.7,
+
3
7.4, 26.8, 21.62, 21.61, 13.7; HRMS–ESI (m/z): [M+H] calcd
+
ESI (m/z): [M+Na] calcd for C H NO Na: 192.0995. Found:
1
9
15
2
for C H NO : 326.2326. Found: 326.2315.
18
32
4
92.0994.
3
.4.7. tert-Butyl (1Z)-octa-1,3-dien-1-ylcarbamate
3
.4.3. (E)-tert-Butyl (4-methoxybut-2-en-1-
(
1Z-3c)
yl)carbamate (E-1b)
Colorless oil; 1Z,3E/1Z,3Z = 40/60; IR (film) 3458, 3313,
Colorless oil; IR (film) 3340, 2977, 2930, 2824, 1708, 1694,
3
1
7
1
6
269, 3149, 3096, 2958, 2929, 2872, 1703, 1655, 1611, 1503,
483, 1430, 1366, 1245, 1160, 1062, 1014, 971, 926, 863, 785,
1
1
531, 1519, 1504, 1470, 1454, 1391, 1366, 1338, 1270, 1250,
-1 1
173, 1126, 1053, 1014, 972, 943, 912, 863, 786, 761 cm ; H
-1
1
58 cm ; H NMR (400 MHz, CD OD) δ 6.34 (0.4H, ddd, J =
3
NMR (400 MHz, CDCl ) 5.74 (1H, dt, J = 15.6, 4.4 Hz, 2- or 3-
H), 5.69 (1H, dt, J = 15.6, 4.4 Hz, 2- or 3-H), 4.65 (1H, br, NH),
3
3
CDCl ) δ 155.7, 130.0, 127.9, 79.3, 72.4, 57.9, 41.9, 28.3;
HRMS–ESI (m/z): [M+Na] calcd for C H NO Na: 224.1257.
Found: 224.1253.
3
5.3, 11.2, 1.0 Hz, 3-H1Z,3E), 6.31 (0.6H, d, J = 9.8 Hz, 1-H1Z,3Z),
.23 (0.6H, ddd, J = 11.1, 11.1, 1.2 Hz, 3-H1Z,3Z), 6.16 (0.4H, d, J
9.5 Hz, 1-H1Z,3E), 5.56 (0.4H, dt, J = 15.3, 7.3 Hz, 4-H1Z,3E),
.43 (0.6H, dd, J = 11.1, 9.8 Hz, 2-H1Z,3Z), 5.34 (0.6H, dt, J =
1.1, 7.6 Hz, 4-H1Z,3Z), 5.18 (0.4H, dd, J = 11.2, 9.5 Hz, 2-H1Z,3E),
.19-2.06 (2H, m, 5-H), 1.48 (9H, s, t-Bu), 1.43-1.27 (4H, m, 6-
.90 (2H, dt, J = 4.4, 1.2 Hz, 1- or 4-H), 3.75 (2H, br, 1- or 4-H),
=
13
.33 (3H, s, OCH ), 1.45 (9H, s, t-Bu); C NMR (100 MHz,
3
5
1
2
3
+
10
19
3
and 7-H), 0.913 (1.2H, t, J = 7.0 Hz, 8-H1Z,3E), 0.909 (1.8H, t, J =
7
13
.2 Hz, 8-H1Z,3Z); C NMR (100 MHz, CD OD) δ 155.7, 155.6,
3
.4.4. (1E)-N,N-Di-(tert-butoxycarbonyl)octa-1,3-
3
dien-1-ylamine (1E-2c)
133.7, 131.1, 125.2, 123.8, 123.1, 122.0, 110.2, 105.0, 81.5, 81.3,
3.7, 33.1, 33.0, 28.8, 28.3, 23.50, 23.46, 14.5; HRMS–ESI
3
Colorless oil; 1E,3E/1E,3Z = 55/45; IR (film) 3077, 2979,
931, 2872, 1755, 1723, 1650, 1624, 1611, 1478, 1458, 1393,
369, 1346, 1259, 1226, 1168, 1134, 1108, 1035, 977, 929, 900,
+
(m/z): [M+Na] calcd for C H NO Na: 248.1621. Found:
2
1
13 23 2
2
48.1617.

6
Tetrahedron
+
3
.5. Representative procedure for cyclopropanation of Z-3a
32.9, 28.2; HRMS–ESI (m/z): [M+Na]
calcd for
ACCEPTED MANUSCRIPT
C H BrNO Na: 418.0624. Found: 418.0605.
18
22
4
A solution of Z-3a (50 mg, 0.30 mmol) and 4 (66 mg, 0.26
mmol) in dichloromethane (2.6 mL) was added to
tetrakis(acetonitrile)copper(I) hexafluorophosphate (2.2 mg,
Acknowledgments
0
.006 mmol) in a flask at room temperature under an argon
This work was supported by the Uchida Energy Science
Promotion Foundation (27-1-11).
atmosphere. After stirring for 2 h at the same temperature, the
resulting mixture was quenched with saturated sodium hydrogen
carbonate and extracted with ethyl acetate. The combined
extracts were washed with brine, dried over sodium sulfate, and
concentrated. Purification of the residue by chromatography on
silica gel (n-hexane/ethyl acetate = 10/1 as the eluent) afforded Z-
References and notes
1
.
For reviews: (a) Wang, M.-X. Chem. Commun. 2015, 51, 6039–
049; (b) Courant, T.; Dagousset, G.; Masson, G. Synthesis 2015,
6
5
(15 mg, 16% yield) as a white solid and 6 (66 mg, 64% yield)
47, 1799–1826; (c) Bernadat, G.; Masson, G. Synlett 2014, 25,
2842–2867; (d) Carbery, D. R. Org. Biomol. Chem. 2008, 6,
as a white solid.
3
455–3460; (e) Matsubara, R.; Kobayashi, S. Acc. Chem. Res.
3
.5.1. Methyl 1-(4-bromophenyl)-2-((E)-2-((tert-
2008, 41, 292–301; (f) Dehli, J. R.; Legros, J.; Bolm, C. Chem.
Commun. 2005, 973–986.
butoxycarbonyl)amino)vinyl)cyclo-
propanecarboxylate (E-5)
2
.
(a) Xu, Y.-S, Tang, Y.; Feng, H.-J.; Liu, J.-T.; Hsung, R. P. Org.
Lett. 2015, 17, 572–575; (b) Ko, C.; Hsung, R. P.; Al-Rashid, Z.
F.; Feltenberger, J. B.; Lu, T.; Yang, J.-H.; Wei, Y.; Zificsak, C.
A. Org. Lett. 2007, 9, 4459–4462; (c) Suga, S.; Nishida, T.;
Yamada, D.; Nagaki, A.; Yoshida, J. J. Am. Chem. Soc. 2004, 126,
White solid; Mp 62–64 °C; rel-(1S,2R)/(1R,2R) = 9/1; IR
(
KBr) 3346, 2978, 2951, 1720, 1671, 1513, 1491, 1453, 1434,
1
8
392, 1368, 1252, 1157, 1091, 1070, 1048, 1023, 1011, 946, 859,
-1 1
1
2
4338–14339; (d) Cossey, K. N.; Funk, R. L. J. Am. Chem. Soc.
004, 126, 12216–12217; (e) Shono, T.; Matsumura, Y.; Tsubata,
18, 767, 732, 715 cm ; H NMR (400 MHz, acetone-d ) δ 8.20
6
(0.1H, br d, J = 10.8 Hz, NH), 8.05 (0.9H, br d, J = 11.4 Hz, NH),
K.; Sugihara, Y.; Yamane, S.; Kanazawa, T.; Aoki, T. J. Am.
Chem. Soc. 1982, 104, 6697–6703. See also, ref. 1a.
Examples of the cycloadditions of tertiary enamides (a) Ma, Z.-X.;
Feltenberger, J. B.; Hsung, R. P. Org. Lett. 2012, 14, 2742–2745;
7
7
.48 (2H, d, J = 7.2 Hz, ArH), 7.34 (0.2H, d, J = 7.2 Hz, ArH),
.23 (1.8H, d, J = 7.2 Hz, ArH), 6.75 (0.1H, dd, J = 13.8, 10.8 Hz,
3
.
CH=CHN), 6.60 (0.9H, dd, J = 13.6, 11.4 Hz, CH=CHN), 5.15
0.1H, dd, J = 13.8, 9.2 Hz, CH=CHN), 4.18 (0.9H, dd, J = 13.6,
(b) Feltenberger, J. B.; Hayashi, R.; Tang, Y.; Babiash, E. S. C.;
(
Hsung, R. P. Org. Lett. 2009, 11, 3666–3669; (c) Riches, A. G.;
Wernersbach, L. A.; Hegedus, L. S. J. Org. Chem. 1998, 63,
9
2
1
1
.8 Hz, CH=CHN), 3.57 (3H, s, OCH ), 2.53-2.38 (0.9H, m, 2-H),
3
.30-2.19 (0.1H, m, 2-H), 1.85 (0.9H, dd, J = 9.0, 4.6 Hz, 3-H),
.74 (0.1H, dd, J = 6.8, 4.8 Hz, 3-H), 1.52-1.31 (1H, m, 3-H),
4
691–4696; (d) Hegedus, L. S.; Bates, R. W.; Söderberg, B. C. J.
Am. Chem. Soc. 1991, 113, 923–927.
13
.43 (0.9H, s, t-Bu), 1.39 (8.1H, s, t-Bu); C NMR (100 MHz,
4. Examples of the cyclopropanation of tertiary enamides: (a)
Tayama, E.; Horikawa, K.; Iwamoto, H.; Hasegawa, E.
acetone-d ) [assined only rel-(1S,2R)] δ 174.0, 153.5, 136.8,
6
Tetrahedron Lett. 2014, 55, 3041–3044; (b) Chanthamath, S.;
Nguyen, D. T.; Shibatomi, K.; Iwasa, S. Org. Lett. 2013, 15, 772–
1
2
4
34.8, 131.8, 127.3, 121.5, 107.3, 80.0, 52.7, 34.9, 28.6, 28.5,
+
2.0; HRMS–ESI (m/z): [M+Na] calcd for C H BrNO Na:
18
22
4
7
75; (c) Simone, F. D.; Saget, T.; Benfatti, F.; Almeida, S.; Waser,
18.0624. Found: 418.0618.
J. Chem. Eur. J. 2011, 17, 14527–14538; (d) Lu, T.; Song, Z.;
Hsung, R. P. Org. Lett. 2008, 10, 541–544; (e) Song, Z.; Lu, T.;
Hsung, R. P.; Al-Rashid, Z. F.; Ko, C.; Tang, Y. Angew. Chem.
Int. Ed. 2007, 46, 4069–4072; (f) Jiménez, J. M.; Rifé, J.; Ortuño,
R. M. Tetrahedron: Asymmetry 1996, 7, 537–558; (g) Arenare, L.;
Caprariis, P. D.; Marinozzi, M.; Natalini, B.; Pellicciari, R.
Tetrahedron Lett. 1994, 35, 1425–1426; (h) Tamura, O.;
Hashimoto, M.; Kobayashi, Y.; Katoh, T.; Nakatani, K.; Kamada,
M.; Hayakawa, I.; Akiba, T.; Terashima, S. Tetrahedron Lett.
1992, 33, 3483–3486; (i) Davies, H. M. L.; Saikali, E.; Young, W.
B. J. Org. Chem. 1991, 56, 5696–5700; (j) Wenkert, E.; Hudlický,
T. J. Org. Chem. 1988, 53, 1953–1957.
Other reaction examples of tertiary enamides: (a) Sivaguru, J.;
Saito, H.; Poon, T.; Omonuwa, T.; Franz, R.; Jockusch, S.;
Hooper, C.; Inoue, Y.; Adam, W.; Turro, N. J. Org. Lett. 2005, 7,
2089–2092; (b) Adam, W.; Bosio, S. G.; Wolff, B. T. Org. Lett.
2003, 5, 819–822; (c) Koseki, Y.; Kusano, S.; Ichi, D.; Yoshida,
K.; Nagasaka, T. Tetrahedron 2000, 56, 8855–8865; (d) Xiong,
H.; Hsung, R. P.; Shen, L.; Hahn, J. M. Tetrahedron Lett. 2002,
3
.5.2. rel-(1S,2R)-Methyl 1-(4-bromophenyl)-2-
(
(Z)-2-((tert-butoxycarbonyl)amino)vinyl)cyclopro-
panecarboxylate (Z-5)
White solid; Mp 102–105 °C; IR (film) 3338, 2977, 2929,
2
1
8
8
853, 1722, 1668, 1504, 1488, 1455, 1435, 1393, 1367, 1341,
325, 1256, 1159, 1118, 1088, 1070, 1054, 1011, 988, 952, 864,
-
1 1
25, 772, 755, 736, 716 cm ; H NMR (400 MHz, acetone-d ) δ
6
.41 (1H, br d, J = 10.0 Hz, NH), 7.49 (2H, ddd, J = 8.6, 2.2, 2.2
Hz, ArH), 7.23 (2H, ddd, J = 8.6, 2.2, 2.2 Hz, ArH), 6.41 (1H, dd,
J = 10.0, 10.0 Hz, CH=CHN), 3.55 (3H, s, OCH ), 3.53 (1H, dd,
J = 10.0, 9.6 Hz, CH=CHN), 2.71 (1H, dddd, J = 9.6, 9.0, 6.6,
1
Bu), 1.35 (1H, dd, J = 6.6, 4.2 Hz, 3-H); C NMR (100 MHz,
CDCl ) δ 174.0, 152.6, 134.3, 133.1, 131.3, 124.8, 121.5, 105.0,
8
5
.
3
.0 Hz, 2-H), 1.90 (1H, dd, J = 9.0, 4.2 Hz, 3-H), 1.44 (9H, s, t-
13
3
+
0.9, 52.7, 29.7, 28.2, 25.2, 22.8; HRMS–ESI (m/z): [M+Na]
4
3, 4449–4453.
calcd for C H BrNO Na: 418.0624. Found: 418.0613.
18
22
4
6
7
.
.
For a review: Oppolzer, W. In Comprehensive Organic Synthesis;
Trost, B. M.; Fleming, I., Eds.; Pergamon: Oxford, 1991, Vol. 5,
Chap. 4.1.3.2, pp. 331–333.
Early studies on Diels–Alder reaction of secondary 1,3-
dienamides: (a) Overman, L. E.; Freerks, R. L.; Petty, C. B.;
Clizbe, L. A.; Ono, R. K.; Taylor, G. F.; Jessup, P. J. J. Am. Chem.
Soc. 1981, 103, 2816–2822; (b) Overman, L. E.; Clizbe, L. A.;
Freerks, R. L.; Marlowe, C. K. J. Am. Chem. Soc. 1981, 103,
3
.5.3. rel-(1R,2S,3S)-Methyl 1-(4-bromophenyl)-2-
(
(tert-butoxycarbonyl)amino)-3-vinylcyclopropane-
carboxylate (6)
White solid; Mp 139–142 °C; IR (KBr) 3362, 3274, 2976,
1
1
8
731, 1715, 1688, 1667, 1635, 1530, 1491, 1456, 1434, 1393,
366, 1315, 1237, 1162, 1102, 1071, 1048, 1028, 1012, 968, 922,
2
807–2815.
-
1 1
90, 845, 786, 774, 735 cm ; H NMR (400 MHz, CDCl ) δ 7.47
3
8. Recent examples of Diels–Alder reaction of secondary 1,3-
dienamides: (a) Pous, J.; Courant, T.; Bernadat, G.; Iorga, B. I.;
Blanchard, F.; Masson, G. J. Am. Chem. Soc. 2015, 137, 11950–
(2H, d, J = 8.2 Hz, ArH), 7.15 (2H, d, J = 8.2 Hz, ArH), 5.48 (1H,
dd, J = 16.9, 1.2 Hz, CH=CH ), 5.27 (1H, dd, J = 10.4, 1.2 Hz,
CH=CH ), 5.08 (1H, ddd, J = 16.9, 10.4, 10.0 Hz, CH=CH ),
4
H), 3.60 (3H, s, OCH ), 2.85 (1H, dd, J = 10.0, 9.0 Hz, 3-H),
1
1
2
1
1
1953; (b) Kong, L.; Han, X.; Jiao, P. Chem. Commun. 2014, 50,
4113–14116; (c) Momiyama, N.; Konno, T.; Furiya, Y.;
2
2
.39 (1H, br d, J = 5.2 Hz, NH), 3.75 (1H, dd, J = 9.0, 5.2 Hz, 2-
Iwamoto, T.; Terada, M. J. Am. Chem. Soc. 2011, 133, 19294–
19297; (d) Songis, O.; Géant, P. Y.; Sautrey, G.; Martinez, J.;
Calmès, M. Eur. J. Org. Chem. 2008, 308–318; (e) Singh, R. S.;
Adachi, S.; Tanaka, F.; Yamauchi, T.; Inui, C.; Harada, T. J. Org.
Chem. 2008, 73, 212–218; (f) Huang, J.; Bergmeier, S. C.
3
13
.45 (9H, s, t-Bu); C NMR (100 MHz, CDCl ) δ 172.3, 156.0,
3
34.3, 131.5, 130.6, 130.0, 122.0, 120.7, 80.4, 52.7, 40.7, 37.3,

7
Tetrahedron 2008, 64, 6434–6439; (g) Northrup, A. B.;
MacMillan, D. W. C. J. Am. Chem. Soc. 2002, 124, 2458–2460.
(a) Sparrow, K.; Barker, D.; Brimble, M. A. Tetrahedron 2011,
2014, 55, 2130–2133; (b) Compain, G.; Jouvin, K.; Martin-
ACCEPTED MANUSCRIPT
Mingot, A.; Evano, G.; Marrot, J.; Thibaudeau, S. Chem. Commun.
2012, 48, 5196–5198; (c) Feltenberger, J. B.; Hsung, R. P. Org.
Lett. 2011, 13, 3114–3117; (d) Hayashi, R.; Hsung, R. P.;
Feltenberger, J. B.; Lohse, A. G. Org. Lett. 2009, 11, 2125–2128;
(e) Gourdet, B.; Rudkin, M. E.; Watts, C. A.; Lam, H. W. J. Org.
Chem. 2009, 74, 7849–7858.
9
1
.
6
7, 7989–7999; (b) Jiang, X.; Liu, B.; Lebreton, S.; Brabander, J.
K. D. J. Am. Chem. Soc. 2007, 129, 6386–6387; (c) Jessup, P. J.;
Petty, C. B.; Roos, J.; Overman, L. E. Org. Syn. 1979, 59, 1–7; (d)
Overman, L. E.; Taylor, G. F.; Petty, C. B.; Jessup, P. J. J. Org.
Chem. 1978, 43, 2164–2167; (e) Overman, L. E.; Taylor, G. F.;
Jessup, P. J. Tetrahedron Lett. 1976, 17, 3089–3092.
16. Tayama, E.; Saito, S. Synlett 2015, 26, 1880–1884.
17. In the preparation of tertiary tertiary 4-substituted-1,3-dienamide
(ref. 14b), the 1,4-elimination of Z-substrate with LDA in THF
afforded a mixture of 1E,3E- and 1E,3Z-regioisomers.
0. Copper catalyzed coupling of amides with dienyl iodides
Buchwald protocol): (a) Nicolaou, K. C.; Leung, G. Y. C.; Dethe,
(
D. H.; Guduru, R.; Sun, Y.-P.; Lim, C. S.; Chen, D. Y.-K. J. Am.
Chem. Soc. 2008, 130, 10019–10023; (b) Nicolaou, K. C.; Sun,
Y.-P.; Guduru, R.; Banerji, B.; Chen, D. Y.-K. J. Am. Chem. Soc.
18. For a review and previous studies on “syn-effect”: (a) Inomata, K.
J. Synth. Org. Chem., Jpn. 2009, 67, 1172–1182; (b) Jiménez, J.;
Ramírez, J.; Huelgas, G.; Meléndrez, R.; Cabrera-Vivas, B. M.;
Sansinenea, E.; Ortiz, A. Tetrahedron 2015, 71, 4590–4597; (c)
Kurosawa, F.; Nakano, T.; Soeta, T.; Endo, K.; Ukaji, Y. J. Org.
Chem. 2015, 80, 5696–5703; (d) Nakano, T.; Soeta, T.; Endo, K.;
Inomata, K.; Ukaji, Y. J. Org. Chem. 2013, 78, 12654–12661; (e)
Horii, S.; Ishimaru, I.; Ukaji, Y.; Inomata, K. Chem. Lett. 2011, 40,
521–523; (f) Takeda, N.; Chayama, T.; Takenaka, H.; Ukaji, Y.;
Inomata, K. Chem. Lett. 2005, 34, 1140–1141; (g) Guha, S. K.;
Shibayama, A.; Abe, D.; Sakaguchi, M.; Ukaji, Y.; Inomata, K.
Bull. Chem. Soc. Jpn. 2004, 77, 2147–2157; (h) Hirata, T.; Sasada,
Y.; Ohtani, T.; Asada, T.; Kinoshita, H.; Senda, H.; Inomata, K.
Bull. Chem. Soc. Jpn. 1992, 65, 75–96. See also, ref. 14a.
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however, the reactions resulted in unsuccessful. These N-acyl
groups would be unstable toward LDA or lithium methoxide
generated by 1,4-elimination.
2
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Supplementary Material
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1
Supplementary data associated with this article can be found,
in the online version, at xxx.
(a) Ide, M.; Ohashi, K.; Mihara, S.; Iwasawa, T. Tetrahedron Lett.