Electron-transfer-induced intermolecular [2 + 2] cycloaddition reactions based on the aromatic "redox tag" strategy

Article abstract of DOI:10.1021/jo200490q

Novel electron-transfer-induced intermolecular [2 + 2] cycloaddition reactions between an aliphatic cyclic enol ether and several unactivated olefins have been demonstrated on the basis of the aromatic "redox tag" strategy. The aromatic "redox tag" was oxidized during the formation of the cyclobutane ring, affording the relatively long-lived aromatic radical cation, which was then reduced to complete the overall reaction that constructed the corresponding [2 + 2] cycloadducts. The aromatic "redox tag" was also found to facilitate electron-transfer-induced cycloreversion reactions of cyclobutane rings.

Full text of DOI:10.1021/jo200490q

ARTICLE
pubs.acs.org/joc
Electron-Transfer-Induced Intermolecular [2 þ 2] Cycloaddition
Reactions Based on the Aromatic “Redox Tag” Strategy
Yohei Okada,† Asaki Nishimoto,† Ryoichi Akaba,‡ and Kazuhiro Chiba*,†
†Department of Applied Life Science, Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan
‡Department of Chemistry, Gunma College of Technology, 580 Toriba-machi, Maebashi, Gunma 371-8530, Japan
S Supporting Information
b
ABSTRACT: Novel electron-transfer-induced intermolecular [2 þ 2] cycloaddition
reactions between an aliphatic cyclic enol ether and several unactivated olefins have
been demonstrated on the basis of the aromatic “redox tag” strategy. The aromatic
“redox tag” was oxidized during the formation of the cyclobutane ring, affording the
relatively long-lived aromatic radical cation, which was then reduced to complete the
overall reaction that constructed the corresponding [2 þ 2] cycloadducts. The
aromatic “redox tag” was also found to facilitate electron-transfer-induced cyclo-
reversion reactions of cyclobutane rings.
’ INTRODUCTION
Scheme 1. Electron-Transfer-Induced Intermolecular [2 þ 2]
Cycloaddition Reaction between 3,4-Dihydro-2H-pyran (1)
Cycloaddition reactions are intriguing both synthetically and
mechanistically. To date, numerous elegant synthetic strategies
and 4-Allylanisole (2)
based on the cycloaddition reactions have been established to
construct a wide variety of ring systems.1 One can also access the
vast number of clear-cut mechanistic studies on the cycloaddition
reactions.2 Furthermore, cycloreversion reactions have also been
widely investigated because of their practical and theoretical
potentials.3 In particular, electron-transfer-induced [2 þ 2]
cycloaddition reactions have extensively been studied.4 In order
to generate radical ion intermediates, photochemical processes
cation,12 an excess of 2 was essential to trap the radical cation of 1
to afford 3a,b, which should also be a driving force of the reaction.
are widely employed.5 One-electron oxidants and reductants can
also be used to produce radical ions.6 In this context, electro-
chemical processes have been utilized to trigger either one or two
electron transfers that afford radical ion intermediates in unique
ways.7 The notable insights have also been made into [2 þ 2]
cycloaddition reactions, providing a powerful tool for the rational
design of novel cycloaddition reactions.8 In this respects, electron
transfer-induced cycloreversion reactions of four-membered ring
have been represented through radical ion intermediates,9 which
appear to be involved in DNA lesions, allowing biological
processes to be probed.10
ox
The oxidation potential of 1 (Ep = 1.41 V vs Ag/AgCl) was
ox
lower than that of 2 (Ep = 1.51 V vs Ag/AgCl), enabling the
selective anodic oxidation of 1. Since no [2 þ 2] cycloadduct was
obtained when allylbenzene was used in place of 2, even in the
presence of a large excess of anisole (4), the alkoxyphenyl group
was proven to be essential for the reaction, serving as a “redox
tag” (Scheme 2). Thus, the alkoxyphenyl group was oxidized
during the formation of the cyclobutane ring, affording the
relatively long-lived alkoxyphenyl radical cation, which was then
reduced to complete the overall reaction (Scheme 3).13 With
these results in hand, we sought to find new electron-transfer-
induced intermolecular [2 þ 2] cycloaddition reactions based on
the aromatic “redox tag” strategy (Scheme 4).
We have previously developed electron-transfer-induced in-
termolecular [2 þ 2] cycloaddition reactions between enol
ethers and unactivated olefins.11 For example, the anodically
generated radical cation of 3,4-dihydro-2H-pyran (1) was
trapped by 4-allylanisole (2) to give the corresponding [2 þ 2]
cycloadducts 3a,b in excellent yield as diastereomeric mixtures
(Scheme 1). Because of the short lifetime of the enol ether radical
Received: March 5, 2011
Published: April 05, 2011
r
2011 American Chemical Society
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Scheme 2. Anodic Oxidation of 3,4-Dihydro-2H-pyran (1) in
the Presence of an Excess of Allylbenzene (3) and Anisole (4)
Scheme 5. Electron-Transfer-Induced Intermolecular [2 þ 2]
Cycloaddition Reaction between 3,4-Dihydro-2H-pyran (1)
and 3-Allylanisole (5)
Scheme 6. Electron-Transfer-Induced Intermolecular [2 þ 2]
Cycloaddition Reactions between 3,4-Dihydro-2H-pyran (1)
and Alkenylanisoles 7 and 8
Scheme 3. Plausible Function of Alkoxyphenyl “Redox Tag”
Scheme 7. Electron-Transfer-Induced Intermolecular [2 þ 2]
Cycloaddition Reactions between 3,4-Dihydro-2H-pyran (1)
and Alkoxyallylbenzenes 11 and 12
Scheme 4. New Electron-Transfer-Induced Intermolecular
[2 þ 2] Cycloaddition Reactions Based on Aromatic “Redox
Tag” Strategy
alkoxyphenyl group functioned as a “redox tag” even though it
was positioned remotely from the cyclobutyl moiety (Scheme 6).
Moreover, both 4-allyl-2-methylanisole (11) and 1-allyl-4-phe-
noxybenzene (12) effectively trapped the radical cation of 1 to
afford the corresponding [2 þ 2] cycloadducts 13a,b and 14a,b,
respectively, in excellent yields as diastereomeric mixtures
(Scheme 7). However, no corresponding [2 þ 2] cycloadducts
were obtained when 4-allyl-N,N-dimethylaniline (15) or 4-al-
lylthioanisole (16) were used in place of 2, suggesting that
neither aminophenyl group nor thiophenyl group could function
as aromatic “redox tag” (Scheme 8). On the other hand,
alkylphenyl group was found to facilitate the electron transfers
through the reactions, constructing the corresponding [2 þ 2]
cycloadducts. The efficiencies of the alkylphenyl “redox tags”
significantly relied on the number of the alkyl substituent
(Scheme 9). These results could be rationalized based on the
oxidation potentials of the substrates (Table 1). Thus, there was
’ RESULTS AND DISCUSSION
The present work began with the preparation of 3-allylanisole
(5).14 The anodic oxidation of 1 was attempted in the presence of
an excess of 5 to give the corresponding [2 þ 2] cycloadducts 6a,
b in excellent yield as diastereomeric mixtures (Scheme 5). This
observation indicated that there were little positional effects of
the substituent on the aromatic “redox tag.” In addition, both
4-(but-3-enyl)anisole (7) and 4-(pent-4-enyl)anisole (8) also
efficiently trapped the radical cation of 1 to give the correspond-
ing [2 þ 2] cycloadducts 9a,b and 10a,b, respectively, in
excellent yields as diastereomeric mixtures, indicating that the
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Scheme 8. Anodic Oxidation of 3,4-Dihydro-2H-pyran (1) in
the Presence of an Excess of 4-Allyl-N,N-dimethylaniline (15)
or 4-Allylthioanisole (16)
Scheme 9. Electron-Transfer-Induced Intermolecular [2 þ 2]
Cycloaddition Reactions between 3,4-Dihydro-2H-pyran (1)
and Alkylallylbenzenes 17ꢀ19
an appropriate value of the oxidation potentials for the aromatic
ring to function as a “redox tag”; namely, the lower oxidation
potentials were favorable, as long as higher than that of 1
(Figure 1). In principle, it was perhaps fair to say that novel
electron-transfer-induced intermolecular [2 þ 2] cycloaddition
reactions could be designed through simply measuring oxidation
potentials, enabled by the aromatic “redox tag” strategy.
The electron impact mass spectrum of 3a (trans-isomer)
showed a fragmentation pattern that displayed a base peak at
m/z 148, which could be assigned to the radical cation of 2
(Figure S1, Supporting Information). This result suggested that
the high energy radical cation of 3a that was produced in the mass
spectrometer participated in the cycloreversion reaction of the
cyclobutane ring. The oxidation potential of 3a (Epox = 1.54 V vs
Ag/AgCl) was close to that of 4-propylanisole (26) (Epox = 1.49
V vs Ag/AgCl), indicating that 3a and 26 could each be oxidized
on the alkoxyphenyl group to generate the corresponding radical
cations (Scheme S1, Supporting Information).
With these results in hand, the anodic oxidation of 3a was
attempted to give the cycloreversion product 2 in moderate yield
(Scheme 10). Although the reaction was highly chemoselective,
the isolated yield was moderate because the anodic oxidation of 2
must occur in competition with that of 3a under reaction condi-
tion, leading to its decomposition (Scheme S2, Figures S2 and S3,
Supporting Information). Through careful examination of the
reaction mixture, we also found that the cycloreversion reaction of
3a was accompanied by the formation of a small amount of 3b
(cis-isomer), strongly verifying that the radical cation generated
on the alkoxyphenyl group contributed the cleavage of the
carbonꢀcarbon bond that constitutes the cyclobutane ring
(Scheme 11). This reaction was suspected to be accompanied
by the formation of radical cation of 1, possibly resulting in
decomposition. The cycloreversion reactions of 9a and 10a were
also anodically induced (Scheme 12). The oxidation potentials of
13a (Epox = 1.50 V vs Ag/AgCl) and 14a (Epox = 1.45 V vs Ag/
AgCl) were both similar to that of 26, indicating that the
alkoxyphenyl group was serving as the “redox tag” to facilitate
the generation of the corresponding radical cation through
anodic oxidation. Moreover, the anodic oxidation of 22a was
attempted to give the cycloreversion product 19 in moderate
yield, suggesting that the trimethylphenyl group was also serving
as the “redox tag” to trigger the cycloreversion reaction through
the production of the corresponding radical cation (Scheme 13).
We subsequently carried out the anodic oxidation of 3a in the
presence of an excess of 7 that led to the formation of 9a,b,
accompanied by the formation of 2. This result suggested that the
radical cation of 1 was released from 3a, which was then trapped
by 4 in a crossover reaction (Scheme 14). In this case, the amount
of 2 derived from the cycloreversion reaction of 3a was not
expected to be sufficient to retrap the radical cation of 1.
Figure 1. Relationship between the oxidation potentials of the sub-
strates and the yields of the corresponding [2 þ 2] cycloadducts.
Scheme 10. Electron-Transfer-Induced Cycloreversion
Reaction of the [2 þ 2] Cycloadduct 3a
Finally, the spin distribution of the radical cation of 3a was
calculated. It was observed that the spin was not localized entirely
on the alkoxyphenyl group, but was also distributed onto the
cyclobutyl moiety (Figure 2). This observation was contrasting
with the spin distribution of the radical cation of the [2 þ 2]
cycloadduct 27.13 Indeed, the anodic oxidation of 27 gave 2 only
in low yield and might be accompanied by the formation of the
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Table 1. Oxidation Potentials of the Substrates Used
substrates
oxidation potentials (vs Ag/AgCl)
yielda (%)
24 (R1 = H, R2 = MeO, R3 = MeO, R4 = MeO, R5= H)
23 (R1 = H, R2 = MeO, R3 = MeO, R4 = H, R5= H)
1
Epox = 1 18 V
Epox = 1 33 V
Epox = 1 41 V
Epox = 1 46 V
Epox = 1 51 V
Epox = 1 69 V
Epox = 1 76 V
Epox = 1 83 V
Epox = 1 89 V
Epox = 2 12 V
trace
10
11 (R1 = Me, R2 = H, R3 = MeO, R4 = H, R5 = H)
2 (R1 = H, R2 = H, R3 = MeO, R4 = H, R5 = H)
12 (R1 = H, R2 = H, R3 = PhO, R4 = H, R5 = H)
19 (R1 = Me, R2 = H, R3 = Me, R4 = H, R5 = Me)
18 (R1 = Me, R2 = H, R3 = Me, R4 = H, R5 = H)
17 (R1 = Me, R2 = H, R3 = H, R4 = H, R5 = H)
25 (R1 = H, R2 = H, R3 = H, R4 = H, R5 = H)
a Yields of the corresponding [2 þ 2] cycloadducts determined by NMR.
87
94
94
84
60
13
n.d.
Scheme 11. Plausible Reaction Mechanism of the Electron-Transfer-Induced Cycloreversion Reaction of the [2 þ 2]
Cycloadduct 3a
radical cation of 28 (Scheme 15). Apparently, the ring strain of
the bicyclic structure was also responsible for driving the
cycloreversion reactions.
through anodic oxidation, which then induced electron-transfer-
induced cycloreversion reactions of cyclobutane rings.
’ EXPERIMENTAL SECTION
’ CONCLUSION
[2 þ 2] Cycloaddition Reactions. Olefins possessing an alkoxy-
phenyl group (4.0 mmol) and aliphatic enol ethers (0.20 mmol) were
added to 1.0 M LiClO4/CH3NO2 (20 mL). The undivided reaction cell
was capped with a septum equipped with the carbon felt anode (20 mm
ꢁ 20 mm), carbon felt cathode (20 mm ꢁ 20 mm), and the Ag/AgCl
reference electrode. The electrolysis was then performed at 1.0 V (vs.
Ag/AgCl). After the reaction was completed, the reaction mixture was
poured into EtOAc, and the EtOAc solution was successively washed
with brine. The organic layer was dried over anhydrous MgSO4. After
filtration and evaporation under reduced pressure, the residue was
purified by silica gel column chromatography using n-hexaneꢀEtOAc
to give cycloadducts. The products yields were determined by NMR.
In conclusion, we have demonstrated novel electron-transfer-
induced intermolecular [2 þ 2] cycloaddition reactions based on
the aromatic “redox tag” strategy. The anodically generated
radical cation of an aliphatic cyclic enol ether was effectively
trapped by several unactivated olefins possessing aromatic “redox
tag” to construct the corresponding [2 þ 2] cycloadducts. The
aromatic “redox tag” was oxidized during the formation of the
cyclobutane ring, affording the relatively long-lived aromatic
radical cation, which was then reduced to complete the overall
reaction. Furthermore, aromatic “redox tag” was also found to
facilitate the generation of the corresponding radical cations
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Scheme 12. Electron-Transfer-Induced Cycloreversion
Reactions of the [2 þ 2] Cycloadducts 9aꢀ10a
Scheme 13. Electron-Transfer-Induced Cycloreversion
Reaction of the [2 þ 2] Cycloadduct 22a
Figure 2. Spin distributions of the radical cation of 3a (left) and that of
27 (right).
Scheme 15. Electron-Transfer-Induced Cycloreversion
Reaction of the [2 þ 2] Cycloadduct 27
Scheme 14. Electron-Transfer-Induced Crossover [2 þ 2]
Cycloaddition Reaction between the [2 þ 2] Cycloadduct 3a
and 4-(But-3-enyl)anisole (7)
(NaCl, cmꢀ1) 2933, 2849, 1612, 1512, 1245, 1176, 1040, 833; MS (rel
int) m/z 232 (Mþ, 12), 148 (90), 133 (47), 121 (100), 105 (32), 84
(68); HRMS calcd for C15H20O2 232.1463 (M þ H 233.1542), found
233.1539.
8-(3-Methoxybenzyl)-2-oxabicyclo[4.2.0]octane (6a) (trans, major):
1H NMR (CDCl3, 600 MHz) δ 7.19 (1H, t, J = 8.1 Hz), 6.77 (1H, d, J =
8.1 Hz), 6.75ꢀ6.69 (2H, m), 3.89 (1H, t, J = 6.6 Hz), 3.79 (3H, s),
3.74ꢀ3.66 (1H, m), 3.65ꢀ3.56 (1H, m), 2.95ꢀ2.82 (2H, m),
2.68ꢀ2.55 (1H, m), 2.43ꢀ2.30 (1H, m), 1.98ꢀ1.84 (1H, m),
1.75ꢀ1.63 (1H, m), 1.62ꢀ1.44 (3H, m), 1.44ꢀ1.35 (1H, m); 13C
NMR (CDCl3, 150 MHz) δ 159.5, 142.5, 129.2, 121.0, 114.3, 111.1,
74.7, 63.0, 55.1, 39.8, 39.0, 30.5, 26.9, 25.7, 23.3; IR (NaCl, cmꢀ1) 2937,
2845, 1733, 1600, 1489, 1262, 1105, 779; MS (rel int) m/z 232 (Mþ, 1),
148 (100), 133 (14), 117 (33), 105 (11), 84 (32); HRMS calcd for
C15H20O2 232.1463 (M þ H 233.1542), found 233.1538.
Cycloreversion Reactions. [2 þ 2] Cycloadducts (0.10 mmol)
were added to 1.0 M LiClO4/CH3NO2 (20 mL). The undivided reaction
cell was capped with a septum equipped with the carbon felt anode (20
mm ꢁ 20 mm), carbon felt cathode (20 mm ꢁ 20 mm), and the Ag/
AgCl reference electrode. The electrolysis was then performed at 1.2 V
(vs. Ag/AgCl). After the reaction was completed, the products yields
were determined by GCꢀMS.
8-[2-(4-Methoxyphenyl)ethyl]-2-oxabicyclo[4.2.0]octane (9a) (trans,
major): 1H NMR (CDCl3, 600 MHz) δ 7.09 (2H, d, J = 8.8 Hz), 6.82
(2H, d, J = 8.8 Hz), 3.82 (1H, t, J = 7.3 Hz), 3.79 (3H, s), 3.72ꢀ3.57 (2H,
m), 2.67ꢀ2.48 (3H, m), 2.40ꢀ2.29 (1H, m), 1.99ꢀ1.86 (1H, m),
1.85ꢀ1.74 (1H, m), 1.73ꢀ1.61 (2H, m), 1.56ꢀ1.43 (3H, m),
1.35ꢀ1.24 (1H, m); 13C NMR (CDCl3, 150 MHz) δ 157.6, 134.6,
129.3, 113.7, 75.5, 62.9, 55.3, 37.5, 36.2, 32.8, 30.5, 27.0, 25.9, 23.4 IR
(NaCl, cmꢀ1) 2932, 2851, 1611, 1513, 1247, 1111, 1038, 822; MS (rel
int) m/z 246 (Mþ, 1), 162 (67), 121 (100), 84 (27), 69 (19), 55 (11);
HRMS calcd for C16H22O2 246.1620 (M þ H 247.1698), found
247.1703.
8-[2-(4-Methoxyphenyl)ethyl]-2-oxabicyclo[4.2.0]octane (9b) (cis,
minor): 1HNMR(CDCl3, 600 MHz) δ7.09 (2H, d, J= 8.1 Hz), 6.81 (2H,
d, J = 8.1 Hz), 4.16ꢀ4.08 (1H, m), 3.89ꢀ3.81 (1H, m), 3.78 (3H, s),
3.24ꢀ3.15 (1H, m), 2.57ꢀ2.42 (2H, m), 2.21ꢀ2.11 (1H, m), 1.94ꢀ1.85
(2H, m), 1.85ꢀ1.72 (2H, m), 1.71ꢀ1.61 (1H, m), 1.57ꢀ1.47 (2H, m),
1.46ꢀ1.37 (1H, m); 13C NMR (CDCl3, 150 MHz) δ 157.5, 135.0, 129.3,
113.6, 75.1, 64.6, 55.3, 37.0, 32.6, 30.4, 30.2, 29.0, 23.5, 21.8; IR
(NaCl, cmꢀ1) 2932, 2852, 1613, 1513, 1246, 1177, 1048, 826; MS (rel
int) m/z 162 (28), 134 (1), 121 (100), 84 (15), 69 (8), 55 (7); HRMS
calcd for C16H22O2 246.1620 (M þ H 247.1698), found 247.1698.
8-[3-(4-Methoxyphenyl)propyl]-2-oxabicyclo[4.2.0]octane (10a) (trans,
major): 1H NMR (CDCl3, 600 MHz) δ 7.09 (2H, d, J = 8.1 Hz), 6.82
8-(4-Methoxybenzyl)-2-oxabicyclo[4.2.0]octane (3a) (trans, major):
1H NMR (CDCl3, 600 MHz) δ 7.08 (2H, d, J = 8.8 Hz), 6.82 (2H, d, J =
8.8 Hz), 3.87 (1H, t, J = 6.6 Hz), 3.78 (3H, s), 3.73ꢀ3.65 (1H, m),
3.63ꢀ3.56 (1H, m), 2.88ꢀ2.78 (2H, m), 2.58 (1H, dd, J = 15.4, 10.3
Hz), 2.40ꢀ2.30 (1H, m), 1.96ꢀ1.86 (1H, m), 1.72ꢀ1.64 (1H, m),
1.60ꢀ1.51 (1H, m), 1.51ꢀ1.42 (2H, m), 1.42ꢀ1.34 (1H, m); 13C NMR
(CDCl3, 150 MHz) δ 157.7, 132.9, 129.5, 113.7, 74.7, 63.0, 55.2, 39.4,
38.8, 30.4, 26.8, 25.7, 23.3; IR (NaCl, cmꢀ1) 2933, 2845, 1612, 1513,
1247, 1109, 1037, 837; MS (rel int) m/z 232 (Mþ, 12), 148 (83), 133
(53), 121 (100), 105 (35), 84 (65); HRMS calcd for C15H20O2
232.1463 (M þ H 233.1542), found 233.1527
8-(4-Methoxybenzyl)-2-oxabicyclo[4.2.0]octane (3b) (cis, minor):
1H NMR (CDCl3, 600 MHz) δ 7.11 (2H, d, J = 8.8 Hz), 6.80 (2H, d, J =
8.8 Hz), 4.11 (1H, dd, J = 8.1, 4.4 Hz), 3.92ꢀ3.87 (1H, m), 3.78 (3H, s),
3.25ꢀ3.18 (1H, m), 2.82 (1H, dd, J = 13.9, 8.1 Hz), 2.54 (1H, dd, J =
13.9, 8.1 Hz), 2.34ꢀ2.24 (1H, m), 2.18ꢀ2.09 (1H, m), 1.99 (1H, dd, J =
20.5, 10.3 Hz), 1.91ꢀ1.84 (1H, m), 1.84ꢀ1.77 (1H, m), 1.60ꢀ1.49
(2H, m), 1.47ꢀ1.40 (1H, m); 13C NMR (CDCl3, 150 MHz) δ 157.5,
133.8, 129.5, 113.6, 75.0, 64.7, 55.2, 39.3, 33.7, 30.2, 29.1, 23.4, 21.7; IR
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(2H, d, J = 8.1 Hz), 3.79 (3H, s), 3.76 (1H, t, J = 7.3 Hz), 3.72ꢀ3.65
(1H, m), 3.64ꢀ3.56 (1H, m), 2.59 (1H, sext, J = 7.3 Hz), 2.54 (2H, t,
J = 7.3 Hz), 2.35ꢀ2.26 (1H, m), 1.97ꢀ1.87 (1H, m), 1.70ꢀ1.62
(1H, m), 1.62ꢀ1.44 (2H, m), 1.44ꢀ1.34 (1H, m), 1.32ꢀ1.21 (1H,
m); 13C NMR (CDCl3, 150 MHz) δ 157.6, 134.8, 129.2, 113.6, 75.4,
62.9, 55.2, 37.9, 35.0, 33.6, 30.5, 29.6, 27.0, 25.9, 23.4; IR
(NaCl, cmꢀ1) 2932, 2851, 1613, 1512, 1246, 1111, 1038, 827; MS
(rel int) m/z 260 (Mþ, 1), 176 (65), 134 (100), 121 (66), 84 (37), 69
(9); HRMS calcd for C17H24O2 260.1776 (M þ H, 261.1855),
found 261.1866.
8-[3-(4-Methoxyphenyl)propyl]-2-oxabicyclo[4.2.0]octane (10b) (cis,
minor): 1H NMR (CDCl3, 600 MHz) δ 7.10 (2H, d, J = 8.1 Hz), 6.82
(2H, d, J = 8.1 Hz), 4.09 (1H, q, J = 4.4 Hz), 3.87ꢀ3.81 (1H, m), 3.79
(3H, s), 3.18 (1H, t, J = 11.0 Hz), 2.61ꢀ2.45 (2H, m), 2.20ꢀ2.12 (1H,
m), 2.12ꢀ2.04 (1H, m), 1.94ꢀ1.82 (2H, m), 1.82ꢀ1.72 (1H, m),
1.56ꢀ1.46 (5H, m), 1.44ꢀ1.37 (2H, m); 13C NMR (CDCl3, 150 MHz)
δ 157.5, 135.2, 129.2, 113.6, 75.3, 64.6, 55.2, 37.8, 35.2, 30.5, 29.7, 29.1,
28.1, 23.5, 21.8; IR (NaCl, cmꢀ1) 2929, 2853, 1613, 1512, 1246, 1177,
1040, 828; MS (rel int) m/z 260 (Mþ, 1), 176 (46), 134 (100), 121 (74),
84 (27), 69 (7); HRMS calcd for C17H24O2 260.1776 (M þ H
261.1855), found 261.1871.
8-(4-Methoxy-2-methylbenzyl)-2-oxabicyclo[4.2.0]octane (13a) (trans,
major): 1H NMR (CDCl3, 400 MHz) δ 7.03 (1H, d, J = 8.2 Hz), 6.71
(1H, d, J = 1.4 Hz), 6.68 (1H, dd, J = 8.2, 1.4 Hz), 3.87 (1H, t, J = 6.4 Hz),
3.78 (3H, s), 3.71 (1H, t, J = 8.8 Hz), 3.64ꢀ3.58 (1H, m), 2.91ꢀ2.80
(2H, m), 2.57 (1H, dd, J = 13.9, 8.1 Hz), 2.42ꢀ2.34 (1H, m), 2.28 (3H,
s), 1.97ꢀ1.88 (1H, m), 1.75ꢀ1.68 (1H, m), 1.57ꢀ1.46 (3H, m), 1.39
(1H, dt, J = 11.0, 7.3 Hz); 13C NMR (CDCl3, 100 MHz) δ 157.7, 137.3,
131,1, 129.7, 115.8, 110.8, 74.7, 63.0, 55.2, 38.1, 35.8, 30.5, 26.9, 25.7,
23.3, 19.8; IR (NaCl, cmꢀ1) 2936, 2858, 1609, 1502, 1254, 1112, 1052,
863; MS (rel int) m/z 246 (Mþ, 2), 162 (100), 147 (48), 135 (13), 91
(12), 55 (5); HRMS calcd for C17H24O2 246.1620 (M þ H 247.1698),
found 247.1687.
21.0; IR (NaCl, cmꢀ1) 2933, 2849, 2362, 1516, 1452, 1220, 1108, 1056;
MS (rel int) m/z 216 (Mþ, 1), 132 (19), 117 (34), 105 (10), 84 (100),
55 (13); HRMS calcd for C15H20O 216.1514 (M þ H 217.1592), found
217.1601.
8-(2,4-Dimethylbenzyl)-2-oxabicyclo[4.2.0]octane (21a) (trans,
major): 1H NMR (CDCl3, 600 MHz) δ 7.00 (1H, d, J = 8.1 Hz), 6.96
(1H, s), 6.93 (1H, d, J = 8.1 Hz), 3.87 (1H, t, J = 6.6 Hz), 3.78ꢀ3.67 (1H,
m), 3.66ꢀ3.60 (1H, m), 2.95ꢀ2.82 (2H, m), 2.68ꢀ2.55 (1H, m),
2.44ꢀ2.35 (1H, m), 2.28 (3H, s), 2.26 (3H, s), 2.00ꢀ1.86 (1H, m),
1.78ꢀ1.67 (1H, m), 1.62ꢀ1.45 (3H, m), 1.39 (1H, dt, J = 11.0, 7.3 Hz);
13C NMR (CDCl3, 100 MHz) δ 135.8, 135.8, 135.3, 130.9, 128.8, 126.4,
74.7, 63.0, 37.9, 36.2, 30.5, 27.0, 25.7, 23.3, 20.9, 19.5; IR (NaCl, cmꢀ1
)
3294, 2937, 2860, 2358, 1505, 1451, 1218, 1112; MS (rel int) m/z 230
(Mþ, 1), 146 (66), 131 (100), 115 (24), 84 (55), 71 (76); HRMS calcd
for C16H22O 230.1671 (M þ H 231.1749), found 231.1722.
8-(2,4-Dimethylbenzyl)-2-oxabicyclo[4.2.0]octane (21b) (cis, minor):
1H NMR (CDCl3, 600 MHz) δ 7.05 (1H, d, J = 7.3 Hz), 6.94 (1H, s),
6.91 (1H, d, J = 7.3 Hz), 4.13 (1H, q, J = 4.4 Hz), 3.97ꢀ3.85 (1H, m),
3.21 (1H, t, J = 11.0 Hz), 2.80 (1H, dd, J = 13.9, 7.3 Hz), 2.60 (1H, dd, J =
13.9, 7.3 Hz), 2.39ꢀ2.28 (1H, m), 2.27 (6H, s), 2.21ꢀ2.10 (1H, m),
2.02 (1H, q, J = 10.3 Hz), 1.97ꢀ1.86 (1H, m), 1.86ꢀ1.75 (1H, m),
1.55ꢀ1.50 (1H, m), 1.49ꢀ1.39 (1H, m), 1.26 (1H, s); 13C NMR
(CDCl3, 100 MHz) δ 136.7, 135.8, 134.9, 130.8, 128.8, 126.3, 75.3, 64.6,
37.9, 31.1, 30.3, 29.4, 23.5, 21.8, 20.9, 19.4; IR (NaCl, cmꢀ1) 2923, 2854,
2361, 1505, 1449, 1071, 1046, 829; MS (rel int) m/z 230 (Mþ, 1), 146
(78), 131 (100), 119 (4), 84 (44), 55 (7); HRMS calcd for C16H22O
230.1671 (M þ H 231.1749), found 231.1742.
8-(2,4,6-Trimethylbenzyl)-2-oxabicyclo[4.2.0]octane (22a) (trans,
major): 1H NMR (CDCl3, 400 MHz) δ 6.83 (2H, s), 3.86 (1H, t, J =
7.3 Hz), 3.71ꢀ3.58 (1H, m), 2.92ꢀ2.77 (2H, m), 2.71 (1H, dd, J = 14.7,
9.6 Hz), 2.40ꢀ2.31 (1H, m), 2.28 (6H, s), 2.24 (3H, s), 2.00ꢀ1.83 (1H,
m), 1.70ꢀ1.58 (1H, m), 1.55ꢀ1.42 (2H, m), 1.38 (1H, dt, J = 10.5, 7.3
Hz); 13C NMR (CDCl3, 100 MHz) δ 136.2, 134.9, 134.9, 128.9, 74.3,
62.8, 38.1, 32.1, 30.7, 27.2, 25.6, 23.5, 20.8, 20.3; IR (NaCl, cmꢀ1) 2937,
2861, 2359, 1455, 1375, 1216, 1103, 852; MS (rel int) m/z 244 (Mþ, 1),
160 (71), 145 (100), 133 (13), 84 (20), 55 (15); HRMS calcd for
C17H24O 244.1827 (M þ H 245.1905), found 245.1911.
8-(4-Methoxy-2-methylbenzyl)-2-oxabicyclo[4.2.0]octane (13b) (cis,
minor): 1H NMR (CDCl3, 600 MHz) δ 7.08 (1H, d, J = 8.8 Hz), 6.69
(1H, d, J = 2.1 Hz), 6.66 (1H, dd, J = 8.8, 2.1 Hz), 4.12 (1H, q, J = 4.4
Hz), 3.95ꢀ3.86 (1H, m), 3.76 (3H, s), 3.21 (1H, t, J = 11.0 Hz), 2.78
(1H, dd, J = 13.9, 7.3 Hz), 2.57 (1H, dd, J = 13.9, 8.1 Hz), 2.39ꢀ2.25
(4H, m), 2.23ꢀ2.11 (1H, m), 2.01 (1H, q, J = 10.3 Hz), 1.95ꢀ1.86 (1H,
m), 1.86ꢀ1.77 (1H, m), 1.65ꢀ1.50 (2H, m), 1.49ꢀ1.39 (1H, m); 13C
NMR (CDCl3, 150 MHz) δ 158.5, 137.7, 133.8, 130.7, 115.1, 110.7,
8-(2,4,6-Trimethylbenzyl)-2-oxabicyclo[4.2.0]octane (22b) (cis,
minor): 1H NMR (CDCl3, 400 MHz) δ 6.81 (2H, m), 4.11 (1H, q, J =
3.7 Hz), 3.96ꢀ3.86 (1H, m), 3.28ꢀ3.17 (1H, m), 2.90ꢀ2.77 (1H, m),
2.77ꢀ2.67 (1H, m), 2.30 (6H, s), 2.28 (1H, s), 2.24 (1H, s), 2.23 (3H,
s), 2.17ꢀ2.06 (2H, m), 1.96ꢀ1.79 (2H, m), 1.60ꢀ1.49 (1H, m),
1.49ꢀ1.41 (1H, m); 13C NMR (CDCl3, 100 MHz) δ 136.4, 135.9,
134.6, 128.7, 76.0, 64.7, 38.2, 30.6, 30.1, 27.7, 23.4, 21.7, 20.8, 20.2; IR
(NaCl, cmꢀ1) 2923, 2858, 2361, 2343, 1449, 1064, 1049, 850; MS (rel
int) m/z 244 (Mþ, 1), 160 (61), 145 (100), 133 (13), 84 (11), 71 (99);
HRMS calcd for C17H24O 244.1827 (M þ H 245.1905), found
245.1918
1-(2-Ethoxy-3-methylcyclobutylmethyl)-4-methoxybenzene (27):
1H NMR (CDCl3, 600 MHz) δ 7.07 (2H, d, J = 8.1 Hz), 6.81 (2H, d,
J = 8.1 Hz), 3.78 (3H, s), 3.43 (2H, m), 3.16 (1H, t, J = 7.3 Hz), 2.88 (1H,
dd, J = 13.9, 5.1 Hz) 2.55 (1H, dd, J = 13.9, 9.5 Hz), 2.22 (1H, m), 2.02
(1H, sept, J = 7.3 Hz) 1.94 (1H, dd, J = 14.7, 8.7 Hz), 1.17 (3H, t, J = 7.3
Hz), 1.10 (3H, d, J = 6.6 Hz), 0.77 (1H, dd, J = 14.7, 10.3 Hz); 13C NMR
(CDCl3, 150 MHz) δ 157.7, 132.8, 129.5, 113.6, 85.5, 64.1, 55.2, 42.1,
40.1, 35.7, 27.0, 19.8, 15.5; IR (NaCl, cmꢀ1) 2952, 2930, 2867, 1512,
1245, 1177, 1119, 1038; MS (rel int) m/z 234 (Mþ, 5), 192 (66), 148
(100), 121 (90), 91 (48), 77 (54); HRMS calcd for C15H22O2 234.1620,
found 234.1630.
75.2, 64.7, 55.2, 38.0, 30.3, 29.7, 29.3, 23.4, 21.8, 20.1; IR (NaCl, cmꢀ1
)
2918, 2850, 2359, 1608, 1489, 1289, 1239, 1050; MS (rel int) m/z 246
(Mþ, 3), 162 (100), 147 (63), 135 (18), 91 (19), 55 (8); HRMS calcd
for C17H24O2 246.1620 (M þ H 247.1698), found 247.1673.
8-(4-Phenoxybenzyl)-2-oxabicyclo[4.2.0]octane (14a) (trans, major):
1H NMR (CDCl3, 600 MHz) δ 7.40ꢀ7.29 (2H, m), 7.13 (2H, d, J = 8.1
Hz), 7.07 (1H, t, J = 7.3 Hz), 6.99 (2H, d, J = 8.8 Hz), 6.96ꢀ6.89 (2H,
m), 3.89 (1H, t, J = 6.6 Hz), 3.78ꢀ3.67 (1H, m), 3.66ꢀ3.55 (1H, m),
2.96ꢀ2.81 (2H, m), 2.73ꢀ2.58 (1H, m), 2.47ꢀ2.31 (1H, m),
2.04ꢀ1.87 (1H, m), 1.79ꢀ1.66 (1H, m), 1.60ꢀ1.46 (2H, m), 1.40
(1H, dt, J = 11.0, 7.3 Hz); 13C NMR (CDCl3, 150 MHz) δ 157.6, 155.1,
135.8, 129.8, 129.6, 122.9, 118.5, 74.7, 63.0, 39.2, 39.0, 30.5, 26.9, 25.7,
23.3; IR (NaCl, cmꢀ1) 3302, 2935, 2851, 1592, 1505, 1238, 1108, 868;
MS (rel int) m/z 294 (Mþ, 1), 210 (11), 117 (28), 91 (21), 77 (100), 51
(39); HRMS calcd for C20H22O2 294.1620 (M þ H 295.1698), found
295.1718.
8-(4-Methylbenzyl)-2-oxabicyclo[4.2.0]octane (20a) (trans, major):
1H NMR (CDCl3, 400 MHz) δ 7.07 (4H, dd, J = 11.0, 8.7 Hz), 3.87
(1H, t, J = 6.4 Hz), 3.77ꢀ3.64 (1H, m), 3.64ꢀ3.52 (1H, m), 2.98ꢀ2.73
(2H, m), 2.60 (1H, dt, J = 10.5, 5.0 Hz), 2.42ꢀ2.32 (1H, m), 2.31 (3H,
s), 2.00ꢀ1.83 (1H, m), 1.72ꢀ1.62 (1H, m), 1.54ꢀ1.43 (2H, m), 1.39
(1H, dt, J = 11.0, 7.3 Hz), 1.26 (1H, s); 13C NMR (CDCl3, 100 MHz) δ
137.7, 135.1, 128.9, 128.4, 74.7, 63.0, 39.3, 39.2, 30.4, 26.8, 25.7, 23.3,
’ ASSOCIATED CONTENT
S
Supporting Information. Additional schemes and fig-
b
ures, coordinates, general information, experimental details,
3475
dx.doi.org/10.1021/jo200490q |J. Org. Chem. 2011, 76, 3470–3476

The Journal of Organic Chemistry
ARTICLE
1
characterization data, and copies of H NMR and 13C NMR
spectra of new compounds This material is available free of
charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*Tel: (þ81)-42-367-5667. Fax: (þ81)-42-360-7167. E-mail:
chiba@cc.tuat.ac.jp.
’ ACKNOWLEDGMENT
This work was partially supported by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture,
Sports, Science and Technology. Moreover, we thank the
anonymous reviewer for his valuable suggestions and comments
regarding the mechanism of the reactions.
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