The reaction of selenoaldehydes with 2-methoxyfuran using their generation by retro Diels-Alder reaction

Article abstract of DOI:10.1016/S0040-4039(02)02830-7

Selenoaldehydes, regenerated by thermal retro Diels-Alder reaction of anthracene cycloadducts under neutral conditions, reacted with 2-methoxyfuran to give methyl penta-2,4-dienoates along with the deposition of elemental selenium. In a similar reaction w

Full text of DOI:10.1016/S0040-4039(02)02830-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 1179–1182
The reaction of selenoaldehydes with 2-methoxyfuran using their
generation by retro Diels–Alder reaction
Aojia Zhou, Masahito Segi* and Tadashi Nakajima
Department of Applied Science, Graduate School of Natural Science and Technology, Kanazawa University, 2-40-20 Kodatsuno,
Kanazawa 920-8667, Japan
Received 31 October 2002; revised 13 December 2002; accepted 13 December 2002
Abstract—Selenoaldehydes, regenerated by thermal retro Diels–Alder reaction of anthracene cycloadducts under neutral condi-
tions, reacted with 2-methoxyfuran to give methyl penta-2,4-dienoates along with the deposition of elemental selenium. In a
similar reaction with 2-methoxyfuran using thioaldehyde, thiirane compound was isolated. This suggests the formation of
selenirane intermediates in the above reaction. © 2003 Published by Elsevier Science Ltd.
Selenoaldehydes have been generally considered to be
highly reactive species as a dienophile for cycloaddition
reactions because of long bond length and poor overlap
in the carbonꢀselenium p bond, so they are one of the
important intermediates in the synthesis of selenium-
Alder reaction to regenerate the corresponding
selenoaldehydes under neutral conditions, as shown in
5
,6
Scheme 1. Thus 1 can serve as convenient and ‘clean’
precursors of selenoaldehydes. 1,3-Dienes bearing oxy-
gen atom substituents can be tolerated by this retro
Diels–Alder protocol. In this paper we describe the
reaction behavior of selenoaldehydes, generated by
thermal retro Diels–Alder reaction of 1, with 2-
1
containing heterocycles. The interest in their prepara-
tion and reactivities has significantly grown in recent
years, and several synthetic methods of reactive
2
selenoaldehydes have already been reported. We have
7
methoxyfuran which is one of reactive electron-rich
also reported an efficient and simple method for the
direct conversion of a carbonyl group into a selenocar-
bonyl group using selenating reagents, (Me Si) Se-
dienes an8d the formation of methyl penta-2,4-
dienoates.
3
2
3
4
BuLi or (Me Al) Se. Selenoaldehydes generated by
2
2
The toluene solution of 1a (R=Ph) and 2-methoxy-
furan was heated at 90°C for 3 h to give methyl
this method were in situ effectively trapped with conju-
gated dienes to afford the corresponding cycloadducts
via Diels–Alder reaction. Especially, the cycloadducts 1
of selenoaldehydes with anthracene were obtained in
good yields as a stable solid in the most substituents R
5
-phenylpenta-2,4-dienoate (2a) as a mixture of four
geometrical isomers in 88% yield along with the deposi-
tion of elemental selenium (Eq. (1)). Our expectation
was the formation of Diels–Alder adducts 3a and/or 4a.
However, the resulting product contained characteristic
signals of four olefinic protons and methyl ester in the
NMR spectrum. This reaction showed a great depen-
dence on the reaction temperature in the isomer ratio.
The results are summarized in Table 1. The reaction at
(
Scheme 1), and very stable at room temperature in all
5
cases. However, the cycloaddition by this method was
inapplicable to 1,3-dienes having oxygen and/or nitro-
gen atoms because of the coordination of the aluminum
atom in the selenating reagent. On the other hand, 1
decomposed quantitatively via thermal retro Diels–
Scheme 1.
Keywords: selenium and compounds; Diels–Alder reactions; dienoates; seleniranes.
*
Corresponding author. Tel.: +81-234-4787; fax: +81-234-4800; e-mail: segi@t.kanazawa-u.ac.jp
0
040-4039/03/$ - see front matter © 2003 Published by Elsevier Science Ltd.
PII: S0040-4039(02)02830-7

1
180
A. Zhou et al. / Tetrahedron Letters 44 (2003) 1179–1182
(1)
lower temperatures (70 and 90°C) gave 2Z isomers as a
major product, whereas those at higher temperatures
(
140 and 160°C) resulted in the formation of 2E iso-
mers as a main product. This dependence on the reac-
tion temperature can be explained by a thermal
isomerization of 2Z to 2E. Actually when the 2Z,4E
isomer of 2a was heated at 160°C for 2 h in toluene
using a sealed stainless steel vessel, the ratio of 2Z:2E
in 2a after heating was 10:90 (Eq. (2)). Accordingly, the
observed isomer ratio of 2Z:2E at lower temperature is
the kinetically controlled one, while the ratio at higher
temperature is the thermodynamically controlled
one. On the other hand, the geometry of C4ꢀC5
double bond was E with a high selectivity at all temper-
atures.
(2)
Several anthracene cycloadducts (1) with aromatic and
aliphatic substituents were treated with 2-methoxyfuran
in toluene at both 90 and 160°C to afford the corre-
sponding penta-2,4-dienoate derivatives (2) in excellent
yields in all cases, with similar dependency on the
reaction temperature in the isomer ratio. The results are
shown in Table 2. Substituent effects on the aromatic
ring for the isomer ratio were effectively not observed
(entries 1–6). Compound 1 having an aliphatic group
Table 1. Effect of reaction temperature in the treatment of 1a with 2-methoxyfuran in toluene
Entry
Temp. (°C)
Time (h)
Yield (%)a
(2E,4E):(2E,4Z):(2Z,4E):(2Z,4Z)b
1
2
3
4
5
70
90
105
140
160
11
3
3
2
2
84
88
98
77
87
9:1:80:10
11:1:78:10
56:2:39:3
67:3:28:2
88:3:8:1
a
Isolated yield.
b
Determined by 1H NMR analysis.
Table 2. Treatment of 1 with 2-methoxyfuran in toluene
Entry
Substrate
R
Temp. (°C)
Time (h)
Product
Yield (%)a
(2E,4E):(2E,4Z):(2Z,4E):(2Z,4Z)b
1
2
3
4
5
6
7
8
1a
1a
1b
1b
1c
1c
1d
1e
Ph
Ph
90
160
90
160
90
160
160
160
3
2
3
2
3
2
3
3
2a
2a
2b
2b
2c
2c
2d
2e
88
87
91
98
91
91
95
65
11:1:78:10
88:3:8:1
18:3:67:12
89:4:6:1
18:2:72:8
88:2:9:1
51:10:36:3
50:9:39:2
p-MeOC H
p-MeOC H
p-CF C H
p-CF C H
PhCH CH
n-Pr
6
4
6
4
3
6
4
3
6
4
2
2
a
Isolated yield.
b
Determined by 1H NMR analysis.

A. Zhou et al. / Tetrahedron Letters 44 (2003) 1179–1182
1181
Scheme 2. Possible reaction pathway.
did not undergo retro Diels–Alder reaction at 90°C, but
the generation of aliphatic selenoaldehydes from 1 at
way may be as follows. The [4+2] cycloaddition reac-
tion of selenoaldehyde, generated via thermal retro
Diels–Alder reaction of 1, with 2-methoxyfuran affords
cycloadducts 3 and 4 which would be at equilibrium
each other under thermal conditions. Compound 4
having an orthoester structure would be expected to
readily undergo CꢀSe bond cleavage to give the betaine
intermediate 7, followed by the formation of selenirane
compound 8 via intramolecular nucleophilic substitu-
tion of the selenium anion in 7. Subsequently, the
extrusion of the selenium atom from 8 and thermal
isomerization of the resulting 2Z-penta-2,4-dienoate
yields an isomeric mixture of methyl penta-2,4-dienoate
2. To confirm the formation of cycloadducts 3 and 4,
we carried out heating of 1 at 80°C in 2-methoxyfuran
as a solvent. However, 3 and 4 were not detected in a
reaction mixture. Hence there are several other mecha-
nistic possibilities which we cannot rule out. These
involve stepwise mechanisms via ionic or diradical
intermediates.
160°C was efficient. Thus, the corresponding 2,4-
dienoates in the reaction with 2-methoxyfuran were
obtained with a slight preference for the 2E isomer
(
entries 7 and 8). In fact, 2Z-penta-2,4-dienoates substi-
tuted with an aliphatic group at the C5 position isomer-
ized to 2E isomers slower than those with an aromatic
9
group in an independent experiment.
We tried to isolate some intermediates in the above
reaction or to confirm their formation by the NMR
measurements, but failed in our all trials because of the
instability of intermediates containing a selenium atom.
Hence we next carried out a similar reaction using
sulfur analog. Thus when the thioaldehyde-anthracene
cycloadduct 5 was heated with 2-methoxyfuran in tolu-
ene at 90°C for 3 h, thiirane (episulfide) compound 6
was isolated as a mixture of cis and trans isomers
together with 2Z-penta-2,4-dienoate (Eq. (3)). The
geometry of the double bond in 6 was Z in both
In conclusion, we have demonstrated that the reaction
of selenoaldehydes, generated from 1 via thermal retro
1
0
isomers.
(3)
The mechanism for the formation of penta-2,4-
dienoates in the reaction of selenoaldehydes with 2-
methoxyfuran has not been established yet, but taking
into account the above evidence for the formation of
thiirane compound in the analogous reaction using
thioaldehyde, the corresponding selenirane (episelenide)
Diels–Alder reaction, with 2-methoxyfuran gives methyl
penta-2,4-dienoates in high yields with deposition of
elemental selenium. We are currently extending the
scope of cycloadditions of selenoaldehydes with
heterodienes involving oxygen and/or nitrogen
atoms via this thermal retro Diels–Alder protocol using
1. The results of our findings will be reported in due
course.
1
1
intermediate is likely involved in the reaction pathway
Scheme 2). At the present stage the most likely path-
(

1
182
A. Zhou et al. / Tetrahedron Letters 44 (2003) 1179–1182
4. (a) Segi, M.; Koyama, T.; Nakajima, T.; Suga, S.; Murai,
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