Site-selective and regioselective Diels-Alder reaction of allenyl aryl ethers

Article abstract of DOI:10.1007/s00706-010-0406-1

The site-selectivity and regioselectivity of Diels-Alder reactions of allenyl aryl ethers with cyclopentadiene and acrolein were studied. While cyclopentadiene (as an electron-rich diene) only reacted with the external double bond of allenyl aryl ethers t

Full text of DOI:10.1007/s00706-010-0406-1

Monatsh Chem (2010) 141:1333–1337
DOI 10.1007/s00706-010-0406-1
ORIGINAL PAPER
Site-selective and regioselective Diels–Alder reaction
of allenyl aryl ethers
•
Firouz Matloubi Moghaddam Mostafa Kiamehr
Received: 16 March 2010 / Accepted: 16 September 2010 / Published online: 22 October 2010
Ó Springer-Verlag 2010
Abstract The site-selectivity and regioselectivity of
Diels–Alder reactions of allenyl aryl ethers with cyclopenta-
diene and acroleinwerestudied. While cyclopentadiene(as an
electron-rich diene) only reacted with the external double
bond of allenyl aryl ethers to provide the site-selective normal
electron demand Diels–Alder cycloadducts, acrolein (as an
electron-deficient diene) reacted with the C1–C2 p bond of
allenyl aryl ethers to provide the site- and regioselective
hetero-Diels–Alder cycloadducts as exclusive products.
appropriate HOMO–LUMO interaction of the reactants as
rationalized in terms of the frontier molecular orbital
(FMO) theory [6]. This requirement for complementary
interacting moieties is fulfilled in the normal electron
demand Diels–Alder cycloaddition of electron-deficient
allenes with electron-rich dienes [7, 8], and vice versa
in the inverse electron demand reaction of electron-
rich allenes with electron-deficient dienes [9, 10]. On
the other hand, allenes as dipolarophiles [11–18] and
dienophiles [19] have attracted less attention in this type
of reaction. A possible reason could be that unsymmet-
rically substituted allenes possess two different double
bonds capable of undergoing cycloaddition reactions.
Therefore, besides regio-, diastereo-, and enantioselec-
tivity, also site-selectivity of the reaction has to be
considered (Fig. 1). The formation of 16 isomeric prod-
ucts is conceivable if unsymmetrical dienes react with
monosubstituted allenes.
Keywords Acrolein Á Allenes Á Cycloadditions Á
Cyclopentadiene Á Hetero-Diels–Alder Á Solvent-free
Introduction
Diels–Alder reactions of allenes as dienophiles provide a
valuable approach to cyclic systems with a reactive exo-
alkylidene moiety, which is advantageous for further
transformations often required in the synthesis of natural
products [1–4]. Unfortunately, allene cycloadditions are
often hampered by a low periselectivity allowing the
[2 ? 2] cycloaddition, which competes with the Diels–
Alder reaction [5]. In addition the classical variant of this
reaction, which is usually conducted under thermal con-
ditions, is restricted in its scope by the necessity of an
Important subgroups of allenes are those containing
heteroatom substitutions such as allenol ethers, allenam-
ines, and allenamides (Fig. 2). The electron-donating
ability of the oxygen or nitrogen atom renders them even
more synthetically attractive because of the electronic bias
imposed by the heteroatoms, allowing site- and regiose-
lective transformations of these molecules.
A review of the literature showed that there have been
very limited reports on the development of effective allenyl
ether-based methods for Diels–Alder reaction [19]. To the
best of our knowledge there is no report on the site- and
regioselective reaction of both double bonds of allenyl aryl
ethers. It is worth noting that these are the first such pro-
cesses that employ allenyl aryl ethers in the reaction with
cycopentadiene and that the opposite site-selectivity is
produced preferentially as compared with reaction of
electron-deficient dienes such as acrolein.
F. M. Moghaddam (&) Á M. Kiamehr
Laboratory of Organic Synthesis and Natural Products,
Department of Chemistry, Sharif University of Technology,
P. O. Box 11155-9516, Tehran, Iran
e-mail: matloubi@sharif.edu
123

1334
F. M. Moghaddam, M. Kiamehr
regioisomers
obtained. When the reaction was carried out on the surface
of activated silica gel in solvent-free conditions (entry 10),
shorter reaction times and an enhancement of the overall
yield were observed. No evidence for other adducts besides
a
b
R
a
b
c
R
c
a
b
d
a
d
a
R
R
Diels-Alder reaction
site-
.
1
c
isomers
two adducts was found by TLC and H NMR inspection.
d
b
c
b
c
R
The ratio of adducts (major/minor 55:45) was determined
by 1H NMR inspection. The products were isolated in 86%
yield (entry 10). This site-selectivity can be reasonably
understood by considering the preferential interaction
between the HOMO of cyclopentadiene and the LUMO of
the external double bond of the allenyl aryl ether 3a. These
results imply that the enol–ether moiety of allenyl aryl
ethers simply acts as an electron-withdrawing group.
The conditions of entry 10 (Table 1) were used with the
other prepared allenes, and the results are shown in
Scheme 2. When the phenyl group of allenyl aryl ether is
substituted in the para position by a nitro group (as an
electron-withdrawing group) and/or a methoxy group (as
an electron-donating group), the yield of the reaction is
increased. A significant stereoselectivity was not observed
under these conditions.
d
d
Fig. 1 Site- and regioselectivity in Diels–Alder reaction of 1,3-
dienes with monosubstituted allenes (stereoisomers are not shown)
O
RO
R1
R2N
R1
N
R2
R3
R2
R3
R2
R3
•
•
•
R1
allenamines
allenamides
allenol ethers
Fig. 2 Allenol ethers, allenamines, and allenamides as important
subgroups of allenes
Results and discussion
The structures of the products were established from
their NMR spectroscopic data (1H NMR, 13C NMR, and
DEPT) and elemental analysis. The site-selectivity of the
A number of activated allenes, such as dialkyl 2,3-pentadi-
enedioates, alkyl 2,3-butadienoates, various 1,2-propadienyl
sulfones[20], and diphenyl(1,2-propadienyl)phosphane oxide
[21] have been successfully employed in thermal [4 ? 2]
cycloadditions with various 1,3-dienes. Surprisingly, the
cycloaddition reaction of allenes carrying an electron-donat-
ing aryloxy substituent seems tobeunknown. The synthesis of
allenyl aryl ethers 3a–3c was performed by classical strate-
gies. The functionalized phenols 1a–1c were propargylated
under basic conditions affording the corresponding propargyl
derivatives 2a–2c. These compounds were converted into the
corresponding allenes 3a–3c in good overall yields by treat-
ment with t-BuOK in anhydrous DMSO at ambient
temperature (Scheme 1) [22, 23].
1
Diels–Alder reaction was confirmed by the H NMR and
13C NMR analysis of 4a–4c. The characteristic signal for
4a–4c in the 1H NMR spectra is a singlet for the =CHOAr
group between 6.18 and 6.62 ppm; also the 13C NMR
spectra of compounds 4a–4c showed two CH2 groups at
30.6–31.24 and 49.9–51.0 ppm that are in agreement with
the proposed structure. The ratio (major/minor) of the
products 4a–4c is based on the integration of their 1H NMR
peaks for the (E/Z) isomers.
We also investigated the intermolecular inverse electron
demand Diels–Alder cycloaddition reaction of allenyl aryl
ethers 3a–3c with acrolein under thermal and Lewis acid
conditions like the previous reactions. When the allenyl
aryl ether 3a was heated with acrolein on activated silica
gel under solvent-free conditions at 70 °C for 3 h, the
corresponding cycloadduct 5a was obtained in optimal
yield (45%), and is the only regioisomer found in this
reaction (Scheme 3). Similar cycloadducts have been
reported from the reaction of 1-ethoxy-1,2-propadiene [19],
allenamines [24], and allenamides [25].
The Diels–Alder reaction of compound 3a with cyclo-
pentadiene was carried out under several sets of reaction
conditions. The results are summarized in Table 1. Heating
the allene 3a with cyclopentadiene in toluene at 100 °C
(sealed tube) for 5 h afforded a mixture of three com-
pounds. After chromatographic separation, the (E/Z) Diels–
Alder adducts mixture and the [2 ? 2] cycloadduct were
Furthermore, we also studied the effect of substituents in
the para position of the phenyl group on the reactivity of
allenyl aryl ethers. As shown in Scheme 3, the allenyl aryl
ether 3c with an electron-withdrawing –NO2 group was
much less reactive than the allenyl aryl ether 3b under the
same reaction conditions. These reactivity differences
suggest that the electron-withdrawing substitution in the
para position of the phenyl group in allenyl aryl ether 3c
could considerably decrease delocalization of the oxygen
R
R
K2CO3 / 8-12 h
acetone / reflux
R
t-BuOK
DMSO
O
•
O
OH
Br
1a 1c
-
3a 3c
-
2a 2c
-
Yield (%)
R
3a
54
75
65
H
OMe
3b
3c NO2
Scheme 1
123

Diels–Alder reaction of allenyl aryl ethers
1335
Table 1 Diels–Alder reaction of allenyl aryl ether 3a with cyclopentadiene and ratio of products under different conditions
O
O
+
O
O
•
O
O
O
Exo
3a
E
Endo
Dimer
Z
Entry
Lewis acid
Conditionsa
Productsb
Ratioc
Overall yield (%)d
1
2
3
4
5
6
7
8
9
10
a
–
Toluene, 100 °C, 5 h
CH3CN, 80 °C, 5 h
CH3CN, 60 °C, 5 h
CH3CN, 60 °C, 5 h
CH3CN, 60 °C, 5 h
CH3CN, 0 °C to RT, 10 h
80 °C, 3 h
60 °C, 8 h
E/Z/dimer
E/Z/dimer
E/Z
30:48:22
38:50:12
45:55
53
58
40
46
35
30
23
10
65
86
–
ZnO
ZnCl2
CuI
E/Z
43:57
E/Z
40:60
Et2OÁBF3
SiOe2
SiO2
SiO2
E/Z
46:54
E/Z/dimer
E/Z
30:42:28
42:58
70 °C, 5 h
70 °C, 3 h
E/Z
45:55
Activated SiOf2
E/Z
45:55
1 equiv. of allene 3a, 2.5–3 equiv. of cyclopentadiene, and 1 equiv. of Lewis acid were used
1
Based on H NMR spectra of crude products
b
c
d
e
f
Determined by column chromatography
Cumulative yield of isolated mixture of products
Silica gel 2.5–3 weight equiv. was used
Merck silica gel 70–230 mesh was activated at 150 °C in vacuum for 5 h
R
O
O
O
SiO2 / solvent free
70 °C
+
O
•
R
5a 5c
-
3a 3c
-
Yield (%)
Time (h)
R
5a
5b
45
H
3
3
6
OMe
56
5c NO2
trace
Scheme 3
Scheme 2
ethers 3a–3c undergoes the Diels–Alder reaction with
electron-rich 1,3-dienes (cyclopentadiene), the internal
double bond of the allenyl aryl ethers 3a–3c undergoes the
Diels–Alder reaction with electron-deficient 1,3-dienes
(acrolein) as a dienophile. The reactions proceed in a
highly site- and regioselective manner. Further develop-
ment of the synthetic utility of these reactions is in progress
in our laboratory.
lone pairs of the ether, thereby reducing the ability to
donate electron density towards the allene moiety and
diminishing their reactivity. However, the allenyl aryl ether
3b was the most reactive among the three allenyl aryl
ethers, providing the cycloadduct 5b in good yield in 3 h.
Conclusion
Experimental
The present results have demonstrated an interesting pe-
riselectivity in cycladdition reaction of allenyl aryl ethers
3a–3c. While the external double bond of allenyl aryl
Diels–Alder reactions were carried out under nitrogen using
flame-dried glassware in sealed tubes. Cyclopentadiene was
123

1336
F. M. Moghaddam, M. Kiamehr
obtained by cracking of the dimer (Sigma–Aldrich) directly
before use. Acrolein was dried over CaSO4 and then distilled
before use. Merck silica gel 70–230 mesh was activated
(dried) at 150 °C in vacuum for 5 h. SiO2 TLC plates 60 F254
were purchased from Merck. Chromatography columns were
equiv.) was added, and the reaction vessel was shaken
vigorously. The mixture was allowed to stand at the tem-
perature and time indicated for each compound. Work-up
method A: a suitable solvent such as CHCl3 was added and
the silica gel was removed by filtration. The filtrate was
washed with additional solvent. The combined filtrates
were evaporated and the residue was separated by column
chromatography. Work-up method B: the volatile materials
were removed under reduced pressure, and the solid pow-
der was separated by column chromatography to afford the
pure compounds.
1
prepared from Merck silica gel 70–230 mesh. H and 13C
NMR spectra were recorded by using a Bruker DRX-500
Avance spectrometer operating at 500 MHz for 1H and
125 MHz for 13C. In the 13C NMR spectra signals corre-
sponding to CH, CH2, or CH3 groups were assigned from
DEPT. Chemical shifts are given in ppm relative to Me4Si as
internal standard. CDCl3 was used as solvent at ambient
temperature. Elemental analysis (C, H, N) was performed on a
Perkin Elmer 2004 (II) CHN analyzer. Results agreed within
standard errors with calculated values.
(5E/Z)-5-(Phenoxymethylene)bicyclo[2.2.1]hept-2-ene
(4a, C14H14O)
T = 70 °C, t = 1.5 h, work-up method B, eluent: petro-
leum ether/ethyl acetate (20:1); Rf = 0.69; pale yellow oil;
yield 0.34 g (86%).
Preparation of propargyl ethers 2a–2c
Major isomer (55%): 1H NMR (500 MHz, CDCl3):
d = 1.48 (d, J = 8.1 Hz, 1H), 1.67–1.69 (m, 1H), 1.94 (dt,
J = 15.1, 2.3 Hz, 1H), 2.43 (dm, J = 14.5 Hz, 1H), 3.09
(s, 1H), 3.33 (s, 1H), 6.14–6.17 (m, 1H), 6.20–6.25 (m,
1H), 6.59 (s, 1H), 7.01–7.07 (m, 3H), 7.30–7.36 (m, 2H)
ppm; 13C NMR (125 MHz, CDCl3): d = 31.04 (CH2),
42.22 (CH), 46.70 (CH), 51.03 (CH2), 116.40 (CH), 122.34
(CH), 127.53 (C), 129.95 (CH), 132.55 (CH), 134.61 (CH),
137.00 (CH), 158.45 (C) ppm.
Minor isomer (45%): 1H NMR (500 MHz, CDCl3):
d = 1.45 (d, J = 8.2 Hz, 1H), 1.63–1.65 (m, 1H), 1.87 (d,
J = 13.9 Hz, 1H), 2.43 (dm, J = 14.5 Hz, 1H), 3.09 (s,
1H), 3.80 (s, 1H), 6.14–6.17 (m, 1H), 6.20–6.25 (m, 1H),
6.25 (s, 1H), 7.01–7.07 (m, 3H), 7.30–7.36 (m, 2H) ppm;
13C NMR (125 MHz, CDCl3): d = 30.80 (CH2), 42.11
(CH), 44.55 (CH), 50.02 (CH2), 116.30 (CH), 122.27 (CH),
127.89 (C), 129.94 (CH), 132.05 (CH), 134.65 (CH),
136.88 (CH), 158.78 (C) ppm.
A mixture of phenol derivative 1a–1c (50 mmol) and
5.6 cm3 propargyl bromide (75 mmol) was stirred in
40 cm3 acetone in the presence of 8.3 g K2CO3 (60 mmol)
for 8–10 h at reflux temperature. After completion of the
reaction as monitored by TLC, 120 cm3 ice-cold water was
added to the reaction mixture. The reaction mixture was
extracted with diethyl ether (3 9 40 cm3), and the ether
layer was washed with brine and dried with anhydrous
MgSO4. Evaporation of the solvent gave propargyl ethers
2a–2c. The propargyl ethers were used without further
purification for the next stage.
Isomerisation of propargyl ethers 2a–2c into allenyl
aryl ethers 3a–3c
To 5 mmol of the prepared propargyl ether 2 was added
2.5 mmol of finely powdered potassium tert-butoxide (t-
BuOK) in 10 cm3 anhydrous DMSO at ambient tempera-
ture using a flame-dried flask. The reaction was slightly
exothermic. The reaction mixture was stirred for 1.5 h.
Subsequently the brown reaction mixture was poured on
ice water. The reaction mixture was extracted with
3 9 20 cm3 CH2Cl2 or CHCl3 and the combined extracts
were dried with MgSO4. Evaporation of the solvent and
column chromatography of the product on silica gel gave
the pure products. The allenyl aryl ethers 3a–3c were
identified by comparison of their spectra with those of
authentic samples [23].
(5E/Z)-5-[(4-Methoxyphenoxy)methylene]bicy-
clo[2.2.1]hept-2-ene (4b, C15H16O2)
T = 70 °C, t = 1.5 h, work-up method B, eluent: petro-
leum ether/ethyl acetate (5:1); Rf = 0.42; pale yellow oil;
yield 0.42 g (92%).
Major isomer (55%): 1H NMR (500 MHz, CDCl3):
d = 1.44 (d, J = 8.2 Hz, 1H), 1.61–1.66 (m, 1H), 1.84 (d,
J = 13.9 Hz, 1H), 2.40–2.44 (dm, J = 14.1 Hz, 1H), 3.08
(s, 1H), 3.79 (s, 1H), 3.82 (s, 3H), 6.12–6.20 (m, 2H), 6.18
(s, 1H), 6.86–6.89 (m, 2H), 6.94–6.97 (m, 2H) ppm; 13C
NMR (125 MHz, CDCl3): d = 30.64 (CH2), 41.99 (CH),
44.34 (CH), 49.93 (CH2), 56.15 (CH3), 115.03 (CH),
117.37 (CH), 126.82 (C), 132.79 (CH), 134.59 (CH),
136.78 (CH), 152.70 (C), 155.14 (C) ppm.
General procedure for Diels–Alder reaction of allenyl
aryl ethers 3a–3c with cyclopentadiene and acrolein
Minor isomer (45%): 1H NMR (500 MHz, CDCl3):
d = 1.46 (d, J = 8.2 Hz, 1H), 1.61–1.66 (m, 1H), 1.92 (d,
J = 15.1 Hz, 1H), 2.40–2.44 (dm, J = 14.1 Hz, 1H), 3.08
(s, 1H), 3.30 (s, 1H), 3.82 (s, 3H), 6.12–6.20 (m, 2H), 6.50
A mixture of 2.5 equiv. (5 mmol) of cyclopentadiene (or
acrolein) and 1 equiv. (2 mmol) of the allenyl aryl ether 3
was prepared at room temperature. Silica gel (2.5–3 weight
123

Diels–Alder reaction of allenyl aryl ethers
1337
(s, 1H), 6.86–6.89 (m, 2H), 6.94–6.97 (m, 2H) ppm;
13C NMR (125 MHz, CDCl3): d = 30.87 (CH2), 42.13
(CH), 46.53 (CH), 50.91 (CH2), 56.15 (CH3), 115.00 (CH),
117.21 (CH), 126.31 (C), 133.40 (CH), 134.59 (CH),
136.86 (CH), 152.40 (C), 155.08 (C) ppm.
(s, 3H), 4.94–4.96 (m, 1H), 5.16 (d, J = 1.6 Hz, 1H), 5.22
(d, J = 1.7 Hz, 1H), 5.75 (s, 1H), 6.30–6.31 (m, 1H), 6.89
(d, J = 9.0 Hz, 2H), 7.08 (d, J = 9.0 Hz, 2H) ppm; 13C
NMR (125 MHz, CDCl3): d = 25.90 (CH2), 56.08 (CH3),
99.22 (CH), 101.92 (CH), 113.02 (CH2), 114.98 (CH),
119.09 (CH), 139.29 (C), 140.16 (CH), 150.99 (C), 155.59
(C) ppm.
(5E/Z)-5-[(4-Nitrophenoxy)methylene]bicyclo[2.2.1]-
hept-2-ene (4c, C14H13NO3)
T = 70 °C, t = 1.5 h, work-up method B, eluent: petro-
leum ether/ethyl acetate (5:1); Rf = 0.53; yellow oil; yield
0.46 g (95%).
Acknowledgments We are grateful to Dr. S. Taheri and Miss Z.
Karimnia for recording NMR spectra. Also, we would like to
acknowledge the Islamic Development Bank (IDB) for granting a
loan in 1993 for purchasing a 500 MHz Bruker NMR spectrometer.
Major isomer (70%): 1H NMR (500 MHz, CDCl3):
d = 1.48 (d, J = 8.3 Hz, 1H), 1.69–1.71 (m, 1H), 1.94 (dt,
J = 15.3, 2.3 Hz, 1H), 2.36–2.40 (dm, J = 15.2 Hz, 1H),
3.09 (s, 1H), 3.37 (s, 1H), 6.12–6.14 (m, 1H), 6.22–6.24
(m, 1H), 6.62 (dd, J = 2.0, 1.6 Hz, 1H), 7.04–7.10 (m,
2H), 7.21–7.26 (m, 2H) ppm; 13C NMR (125 MHz,
CDCl3): d = 31.24 (CH2), 41.98 (CH), 46.64 (CH),
50.96 (CH2), 115.87 (CH), 126.31 (CH), 130.80 (CH),
131.53 (C), 134.19 (CH), 137.62 (CH), 142.60 (C), 163.09
(C) ppm.
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3-Methylene-2-phenoxy-3,4-dihydro-2H-pyran
(5a, C12H12O2)
T = 70 °C, t = 3 h, work-up method A, eluent: petroleum
ether/ethyl acetate (10:1); Rf = 0.52; colorless oil; yield
0.17 g (45%); 1H NMR (500 MHz, CDCl3): d = 2.78
(ddd, J = 18.9, 4.7, 0.6 Hz, 1H), 3.23 (dm, J = 18.9 Hz,
1H), 4.95–4.97 (m, 1H), 5.18 (d, J = 2.5 Hz, 1H), 5.25
(d, J = 2.3 Hz, 1H), 5.90 (s, 1H), 6.30–6.32 (m, 1H), 7.09
(t, J = 7.3 Hz, 1H), 7.16 (d, J = 7.7 Hz, 2H), 7.36 (t,
J = 7.5 Hz, 2H) ppm; 13C NMR (125 MHz, CDCl3):
d = 25.89 (CH2), 98.15 (CH), 101.91 (CH), 113.72 (CH2),
117.49 (CH), 122.79 (CH), 129.90 (CH), 139.11 (C),
140.20 (CH), 157.08 (C) ppm.
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T = 70 °C, t = 3 h, work-up method A, eluent: petroleum
ether/ethyl acetate (7:1); Rf = 0.45; colorless oil; yield
0.24 g (56%); 1H NMR (500 MHz, CDCl3): d = 2.78 (dd,
J = 18.9, 4.5 Hz, 1H), 3.23 (dm, J = 18.9 Hz, 1H), 3.82
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