Cationic palladium-catalyzed [5 + 2] annulation of 2-acylmethoxyarylboronic acids and allenoates: Synthesis of 1-benzoxepine derivatives

Article abstract of DOI:10.1021/jo200672w

The 1-benzoxepine derivatives were synthesized conveniently by cationic palladium-catalyzed [5 + 2] annulation reaction of 2-acylmethoxyarylboronic acids with allenoates in high yields. This annulation involves the intramolecular nucleophilic addition to ketones without the formation of π-allylpalladium species.

Full text of DOI:10.1021/jo200672w

NOTE
pubs.acs.org/joc
Cationic Palladium-Catalyzed [5 + 2] Annulation of
2-Acylmethoxyarylboronic Acids and Allenoates: Synthesis of
1-Benzoxepine Derivatives
Xufen Yu and Xiyan Lu*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, China
S Supporting Information
b
ABSTRACT: The 1-benzoxepine derivatives were synthesized
conveniently by cationic palladium-catalyzed [5 + 2] annulation
reaction of 2-acylmethoxyarylboronic acids with allenoates in
high yields. This annulation involves the intramolecular nucleo-
philic addition to ketones without the formation of π-allylpal-
ladium species.
ransition-metal-catalyzed reactions of allenes have attracted
Scheme 1. Reaction of the πꢀAllylpalladium Complex
T
increasing interest during the last two decades due to the
existence of two orthogonal π-bonds.1ꢀ5 Various kinds of
reactions using organoboronic acids and transition metals have
been developed to construct carbonꢀcarbon bonds.6,7
Transition-metal-catalyzed reactions of organometallic re-
agents provide powerful approaches toward the synthesis of
useful organic compounds. Special attention was concentrated to
palladium-based catalysts as it provides the versatile possibilities
for the carbonꢀcarbon bond formation.8ꢀ11 Recently, our group
and others have developed a series of addition reactions of
arylboronic acids to carbonꢀheteroatom multiple bonds cata-
lyzed by Pd(II) species.12ꢀ22 In all of these systems, Pd(II)
species were used as the catalyst without the use of any redox
system that is indispensable in many Pd(0)-catalyzed reactions.
Among the Pd(II) catalysts, cationic Pd(II) complexes have great
advantages in exhibiting high reactivity in the transmetalation
step due to the following reasons: (1) vacant coordination site for
substrates or reagents; (2) higher Lewis acidity.23
The benzoxepine moiety is an important structural unit in
many natural products,24ꢀ26 biologically active molecules,25 and
natural herbicides.27 The simple and efficient synthesis of
benzoxepine derivatives is attractive in synthetic organic chem-
istry and medicinal chemistry.28ꢀ30 Our group previously devel-
oped the synthesis of 1-benzoxepines by cationic palladium
complex [Pd(dppp)(H2O)2]2+(TfOꢀ)2 catalyzed tandem cycli-
zation of 2-aroylmethoxyarylboronic acids and alkynes involving
the addition of vinylpalladium species to ketones.17 Inspired by
this reaction, we set out to investigate the tandem reaction of
2-acylmethoxyarylboronic acids and allenes.
react with nucleophiles.34 In the previous reports of Pd-catalyzed
allylation of the carbonyl groups, a process called “umpolung” was
generallyused to change the electrophilicity of the π-allylpalladium
species by adding low-valent metals or some reducing agents,
making the addition reaction possible.35ꢀ42 Unfortunately, this
umpolung process is not suitable for the Pd(II)-catalyzed reactions
because Pd(0) will be formed in the presence of reducing agents,
making the regeneration of the Pd(II) catalyst impossible. Mal-
inakova realized the intermolecular three-component coupling of
an arylboronic acid with allenes and aldehydes using a particular
β-pinene-derived π-allylpalladium dimer as the catalyst, suggesting
that η1-allylpalladium species was formed which can act as a
nucleophile.43 We recently reported the cationic palladium com-
plex catalyzed diastereo- and enantioselective tandem annulation
of 2-formylphenylboronic acids with substituted allenoates
According to the literature, an allene can insert rapidly into a
palladiumꢀcarbon bond, generated from oxidative addition of an
organic halide to the palladium catalyst or transmetalation process,
to give π-allylpalladium species (Scheme 1).31ꢀ33 It is widely
accepted that a π-allylpalladium complex is electrophilic and will
Received: March 31, 2011
Published: July 11, 2011
r
2011 American Chemical Society
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dx.doi.org/10.1021/jo200672w J. Org. Chem. 2011, 76, 6350–6355
|

The Journal of Organic Chemistry
NOTE
Scheme 2. Tandem Annulation Reaction of the 2-Formyl-
phenylboronic Acids with Substituted Allenoates
Table 1. Optimization of the Tandem Annulation Reaction
Conditionsa
involving the intramolecular nucleophilic addition without the
formation of π-allylpalladium species, which solved the problems
mentioned above (Scheme 2).19 Herein, we report a new, efficient
method to synthesize the 1-benzoxepine derivatives by cationic
palladium complex [Pd(dppp)(H2O)2]2+(BF4ꢀ)2 catalyzed tan-
dem cyclization of 2-acylmethoxyarylboronic acids and allenoates.
We initiated our studies by investigating the tandem annula-
tion reaction of 2-acylmethoxyarylboronic acid 1a with allenoate
2a under a variety of conditions. A reaction conducted in dioxane
at 80 °C catalyzed by [Pd(dppp)(H2O)2]2+(TfOꢀ)2 led to the
formation of the intramolecular cyclization product 3aa0 in 73%
yield (Table 1, entry 1). By changing the solvent to toluene, the
reaction gave a complex mixture of products. Gratifyingly, the
desired product 3aa could be obtained in 87% yield within 25
yield (%)b
entry
catalyst (3 mol %)
solvent t (h)
3aa
4aa
1
2
3
4
5
6
[Pd(dppp)(H2O)2]2+(TfOꢀ)2 dioxane 0.5 3aa0(73)c
[Pd(dppp)(H2O)2]2+(TfOꢀ)2 toluene 0.5 complex mixture
ꢀ
[Pd(dppp)(H2O)2]2+(BF4
[Pd(dppp)(H2O)2]2+(SbF6
[Pd(dppe)(H2O)2]2+(BF4
[Pd(bpy)(H2O)2]2+(BF4
)
toluene 0.4
toluene
87
ꢀ
trace
ꢀ
ꢀ
ꢀ
2
ꢀ
)
1
2
ꢀ
)
toluene 12
toluene 12
toluene 72
toluene 0.4
trace
2
ꢀ
)
NR
2
7d [Pd(dppp)(H2O)2]2+(BF4ꢀ
)
trace
84
ꢀ
trace
ꢀ
2
2
2
2
2
min when the reaction was conducted in toluene under 80 °C in
8e [Pd(dppp)(H2O)2]2+(BF4ꢀ
)
)
)
)
ꢀ
the presence of [Pd(dppp)(H2O)2]2+(BF4
) as a catalyst
2
9
[Pd(dppp)(H2O)2]2+(BF4ꢀ
THF
DCE
1
complex mixture
70
(Table 1, entry 3). Various cationic palladium complexes were
evaluated in order to improve the yield of the desired product 3aa
and prevent the formation of dꢀeboronated product. The cationic
palladium complex with SbF6 as the counteranion could not
furnish the tandem annulation reaction (Table 1, entry 4).
Therefore, the counteranions of cationic palladium catalysts are
particularly important in this reaction. The difference of the
reactivity is probably due ꢀto their coordinating ability to the
transition metals, and BF4 is less coordinating than TfOꢀ.44
Thus, the cationic palladium complex [Pd(dppp)(H2O)2]2+-
(BF4ꢀ)2 has more vacant sites for coordinating with the sub-
strates to accelerate the process. At present, the reason for poor
performance of SbF6ꢀ cannot be explained. Meanwhile, it should
be noted that the ligand of the cationic palladium has an obvious
10 [Pd(dppp)(H2O)2]2+(BF4ꢀ
11 [Pd(dppp)(H2O)2]2+(BF4
12 Pd(OAc)2/dppp
12
ꢀ
dioxane 0.5 complex mixture
toluene 12
NR
13 Pd(TFA)2/dppp
toluene 15
NR
a Reaction conditions: 1a (0.18 mmol), 2a (0.15 mmol), and catalyst (3
mol %) were stirred at 80 °C. b Cited yields refer to pure product after
c
column chromatography. 3aa0 was the product of intramolecular
cyclization. d The reaction was stirred at room temperature. e The ratio
of substrates was increased to 2:1.
investigation, the optimal condition for this tandem annulation
reaction was as follows: [Pd(dppp)(H2O)2]2+(BF4ꢀ)2 (3 mol %),
arylboronic acid 1a (1.2 equiv), and allenoate 2a (1 equiv) in
toluene at 80 °C.
effect on the reaction. When [Pd(dppe)(H2O)2]2+(BF4ꢀ)2 and
ꢀ
[Pd(bpy)(H2O)2]2+(BF4
) were used as the catalyst, the
2
With the optimized reaction conditions in hand, a number of
other readily available arylboronic acids including relatively
electron-rich and electron-poor substrates were allowed to react
with the allenoate 2a. As summarized in Table 3, the correspond-
ing annulation products 3 were obtained in moderate to good
yields with excellent diastereoselectivities (Table 3, entries 1ꢀ4).
When aromatic ketone 1e or 1f reacted with allenoate 2a,
moderate amounts of the corresponding products were isolated
(Table 3, entries 5 and 6). When the naphthylboronic acid 1g was
subjected to reaction conditions, only the deboronated product
4ga was formed. The tert-butyl-substituted ketone 1h (Figure 1)
did not give the corresponding annulation product presumably
due to the high steric hindrance. When the boronic acid 1i
bearing a cyclic ketone moiety was treated with 2a in the
presence of [Pd(dppp)(H2O)2]2+(BF4ꢀ)2 (3 mol %) in toluene
at 80 °C, a tricyclic product 3ia was formed in relatively low yield
and the intramolecular cyclizationꢀdehydration product 5ia was
obtained in moderate yield (Scheme 3). The scope of the tandem
annulation with regard to the allenoate was also investigated.
A number of allenoates could react with boronic acid 1a smoothly
to yield the expected products 3 in good to excellent yields
reactions were inhibited (Table 1, entries 5 and 6). Furthermore,
the [5 + 2] annulation was not catalyzed by Pd(OAc)2/dppp or
Pd(TFA)2/dppp under the same reaction conditions. The reac-
tion was also affected by the solvent. While toluene led to the best
result, the reaction was complicated in THF or dioxane, and large
amounts of deboronated product 4aa were obtained when using
DCE as the solvent (Table 1, entries 9ꢀ13). When the mixture
was lowered to room temperature, the reaction was totally
inhibited, even after extending the time to 3 days (Table 1, entry
7). Increasing the ratio of substrates to 2:1 did not play a
pronounced role in this tandem cyclization (Table 1, entry 8).
Additives are well-known to play an important role in the
transmetalationbetween arylboronic acids andpalladium.45 There-
fore, we examined the base effect on this tandem annulation
reaction. Adding K3PO4 (2 equiv) and H2O (2 equiv) resulted
in complicated reaction, and none of the annulation product
was obtained (Table 2, entries 1 and 2). The addition of
Amberlite IRA-400 (OH) could completely suppress the for-
mation of the undesired product 4aa, but the yield of 3aa was
low (Table 2, entry 5) as compared with the condition without
adding any additive (Table 1, entry 3). On the basis of the above
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NOTE
Table 2. Effect of the Additivea
Table 3. Substrate Scope for [Pd(dppp)(H2O)2]2+(BF4
)
ꢀ
2
Catalyzed [5 + 2] Annulation of 2-Acylmethoxyarylboronic
Acids with Allenoatesa
entry solvent
additive (equiv)
t (h)
yield (%)b
1
2
3
4
5
6
7
8
toluene K3PO4 (2), H2O (2)
dioxane K3PO4 (2), H2O (2)
toluene K3PO4 (2)
12
12
10
12
complex mixture
complex mixture
complex mixture
complex mixture
entry
substrate 1 (R1, R2)
allenoates (R3)
yield (%)b
1
2
R1 = H, R2 = Me (1a)
R1 = Me, R2 = Me (1b)
R1 = tBu, R2 = Me (1c)
R1 = Cl, R2 = Me (1d)
R1 = H, R2 = Ph (1e)
R1 = H, R2 = p-OMeC6H4 (1f)
Et (2a)
Et (2a)
Et (2a)
Et (2a)
Et (2a)
Et (2a)
Me (2b)
iPr (2c)
nBu (2d)
Bn (2e)
Ph (2f)
3aa (87)
3ba (72)
3ca (80)
3da (64)
3ea (34)
3fa (49)
3ab (93)
3ac (87)
3ad (83)
3ae (80)
3af (87)
toluene Ba(OH)2 (2)
toluene Amberlite IRA-400 (OH) (1.5) 0.5 58
3
DCE
dioxane Amberlite IRA-400 (OH) (1.5) 10
DME Amberlite IRA-400 (OH) (1.5) 10
Amberlite IRA-400 (OH) (1.5) 10
NR
4
complex mixture
NR
5
6
a Reaction conditions: 1a (0.18 mmol), 2a (0.15 mmol), and catalyst
(3 mol %) were stirred at 80 °C. b Cited yields refer to pure product after
column chromatography.
7
1a
1a
1a
1a
1a
8
9
10
11
(Table 3, entries 7ꢀ11). All of the reactions listed in Table 3
proceeded with excellent diastereoselectivities, giving 3 as the
only cyclization products. However, the multiply substituted
allenoates (not shown) could not finish the annulation reaction,
and the substrate scope was limited. In order to achieve the
synthesis of molecules with large rings, the reactions of substrates
1jꢀ1l with allenoate 2a under the optimal conditions were also
tested, but all of them resulted in the recovery of the starting
materials (Figure 1).
a Reaction conditions: 1a (0.18 mmol), 2a (0.15 mmol) and catalyst
(3 mol %) were stirred at 80 °C using toluene (2 mL) as the solvent.
b Cited yields refer to pure product after column chromatography with
dr >99.9%.
Subsequently, our attention turned to the asymmetric versꢀion
of the reaction. Initially, using the Pd2+(CH3CN)4(BF4
)
2
combined with various chiral ligands (such as chiral diphosphine
or monophosphine ligands) as the catalyst, the (2S,4S)-bis-
(diphenylphosphinyl)pentane ((2S,4S)-bdpp) was found to be
superior, providing the expected product in 81% yield, but the
enantioselectivity was poor. In addition, we also optimized this
asymmetric reaction by employing chiral palladium catalysts
{Pd[(R)-binap](H2O)2}2+(TfOꢀ)2 and {Pd[(S,S)-bdpp]-
(H2O)2}2+(BF4ꢀ)2, but only the latter catalyzed formation of the
product, which was formed in 72% yield with very low enantiomeric
excess value.
Figure 1. Limitation of the substrates.
Scheme 3. Reaction of 1i and Allenoate 2a
The proposed [5 + 2] annulation reaction mechanism is
described in Scheme 4. The palladium complex A would generate
the Pd hydroxo complex C, which is supposed to be the active
catalytic species10,46ꢀ48 and enables smooth transmetalation
with the substrate 1 without any assistance of bases.23,48 Mean-
while, the allenoate will coordinate to the palladium center to
give intermediate D. Then, η1-allylpalladium complex E is
formed when the allenoate is inserted into the palladiumꢀcarbon
bond. While the substrates with oxygen linkage lead to successful
reactions, the substrate with carbon linkage (1j) does not provide
the desired product, a result that may be explained by the
coordination of the oxygen atom with cationic palladium, which
makes the intermediate E more stable, leading the successful
reactions. High Lewis acidity of cationic palladium species E may
activate the carbonyl group by coordination, and the high
nucleophilic property of cationic η1-allylpalladium complex E
may result in intramolecular 1,2-addition to furnish the seven-
membered ring intermediate F. The subsequent protonation of F
by B(OH)3 or boronic acid would afford product 3 and
regenerate the catalytically active species C to complete the
catalytic cycle. From the structure of the product 3 of this
reaction, it is worth noting that the more stable η3-allyl complex
cannot be formed immediately due to the orthogonal orbitals of
the allene. The η1-allyl complex lacking the conjugation with
other double bond is always formed first, and then η3-coordina-
tion forms after a rotation around the σ-CꢀC bond.2 The highly
active cationic η1-allylpalladium complex will react with the
carbonyl group immediately once it was formed without the
formation of the η3-allyl complex.49 The success for the tandem
cyclization of allenes without the use of “umpolung” reagents
showed the advantage of the use of cationic palladium complex as
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NOTE
Scheme 4. Proposed Mechanism of the Tandem Reaction
2-iodophenol (4.845 g, 22 mmol) in acetone (13 mL), and the resulting
mixture was heated under reflux for 4 h. The mixture was then allowed
to cool to room temperature and poured into water (100 mL). The
precipitate was collected by filtration to afford crude product 1-(2-
iodophenoxy)-propan-2-one, which was used for the next step without
purification. The crude product was added to a stirred solution of
benzene (150 mL), ethylene glycol (5.5 g, 89 mmol), and p-toluene-
sulfonic acid (120 mg, 0.66 mmol), fitted with a water trap and refluxed
overnight. The reaction mixture was cooled and washed with saturated
solution of NaHCO3 (20 mL) and NaCl (20 mL). The organic portion
was dried with Na2SO4, concentrated in vacuo, and the residue was
purified by flash column chromatography to obtain the product 2-(2-
iodophenoxymethyl)-2-methyl-1,3-dioxolane (6.687 g, 64% for two
steps). To 17.6 mmol of compound 2-(2-iodophenoxymethyl)-2-
methyl-1,3-dioxolane dissolved in the mixture of Et2O (30 mL) and
THF (60 mL) in a flame-dried 250 mL round-bottom flask was added
at ꢀ78 °C 13.1 mL (21 mmol) of a 1.6 M solution of n-BuLi in hexanes.
The mixture was stirred at ꢀ78 °C for 20 min followed by addition of
4 mL (35 mmol) of B(OMe)3 in one portion via syringes. The resulting
mixture was allowed to stir at ꢀ78 °C for 0.5 h, warmed to ambient
temperature, and stirred for further 2 h. Twenty milliliters of aq 1 N
HCl was added, and the mixture was stirred for an additional 0.5 h. The
organic layer was separated, and the aqueous layer was extracted with
EtOAc (2 ꢁ 30 mL). The organic layers were combined, washed with
water, 10% aqueous Na2S2O3, then brine, dried with Na2SO4, and
concentrated. The crude product was added to MeOH (20 mL) plus
2 mL of 8 N H2SO4, and the mixture was allowed to stand at room
temperature overnight. After dilution with water and being extracted
with EtOAc (2 ꢁ 20 mL), the organic layers were combined, dried with
Na2SO4, and concentrated. The pure compound 1a (2.591 g, 55% for
two steps) was obtained by recrystillization in a mixture of EtOAc/
petroleum ether.
a catalyst for allenes. On the other hand, another pathway
involving the reaction of the palladium enolate intermediate E0
formed from the η1-allylpalladium species E with the carbonyl
group could not be excluded. In addition, it is supposed that the
tunable role of allenes may arise from the stronger affinity of the
palladium complex with the carbonꢀcarbon double bond rather
than with the intramolecular carbonyl group in intermediate D,
facilitating the insertion of allenes into the carbonꢀpalladium
bond to form intermediate E. Therefore, the intramolecular
addition product could be suppressed.
In summary, we have uncovered a new and highly efficient way
to achieve the synthesis of benzoxepine derivatives from 2-acyl-
methoxyarylboronic acids and allenoates in the presence of a
catalytic amount of [Pd(dppp)(H2O)2]2+(BF4ꢀ)2. The use of
cationic palladium species is crucial for this reaction. This tandem
annulation involves the intramolecular nucleophilic addition of
η1-allylpalladium species to ketones, and no redox reagent for the
catalytic system is involved. Current studies are aimed at the
asymmetric version of this reaction.
1
Substrate 1a: white solid; yield 55%; mp 129ꢀ130 °C; H NMR
(300 MHz, CDCl3) δ 7.89ꢀ7.87 (m, 1H), 7.46ꢀ7.40 (m, 1H), 7.10
(t, J = 7.2 Hz, 1H), 6.79 (d, J = 8.1 Hz, 1H), 4.79ꢀ4.74 (m, 2H), 2.26
(s, 3H); 13C NMR (75 MHz, DMSO-d6) δ 204.2, 162.3, 136.0, 132.0,
121.1, 111.9, 72.5, 26.0; IR (KBr) ν 3337 (br), 1732, 1604 cmꢀ1; MS
(70 eV, EI) m/z (%) 194 (M+), 151, 121, 107, 43 (100). Anal. Calcd for
C9H11BO4: C, 55.72; H, 5.72. Found: C, 55.66; H, 5.73.
1
Substrate 1b: white solid; yield 45%; mp 129ꢀ130 °C; H NMR
(300 MHz, DMSO-d6) δ 7.94 (s, 2H), 7.46 (s, 1H), 7.19 (d, J = 8.4 Hz,
1H), 6.83 (d, J = 8.4 Hz, 1H), 4.88 (s, 2H), 2.23 (s, 3H), 2.16 (s, 3H);
13C NMR (75 MHz, DMSO-d6) δ 204.2, 160.3, 136.5, 132.3, 129.5,
111.8, 72.7, 25.9, 20.1; IR (KBr) ν 3425 (br), 1732, 1608 cmꢀ1; MS (70
eV, EI) m/z (%) 208 (M+), 165, 121 (100), 91. Anal. Calcd for
C10H13BO4: C, 57.74; H, 6.30. Found: C, 57.78; H, 6.32.
Substrate 1c: white solid; yield 74%; mp 122ꢀ123 °C; 1H NMR (300
MHz, CDCl3) δ7.94 (d, J= 2.4 Hz, 1H), 7.43 (dd, J= 2.4and8.4 Hz, 1H),
6.71 (d, J = 8.7 Hz, 1H), 4.72 (s, 2H), 2.23 (s, 3H), 1.31 (s, 9H);13C NMR
(75 MHz, CDCl3) δ 202.8, 160.5, 144.6, 134.0, 129.4, 110.8, 72.9, 34.1,
31.4, 25.9; IR(KBr) ν3444, 3469, 2963, 1726, 1067 cmꢀ1; MS(70eV, EI)
m/z (%) 252 (M+ + 2), 235, 206, 191 (100), 135. Anal. Calcd for
C13H19BO4: C, 62.43; H, 7.66. Found: C, 62.16; H, 7.89.
’ EXPERIMENTAL SECTION
General. All solvents were dried and distilled before use according
to the standard procedures. All melting points were uncorrected. The
cationic palladium complexes [Pd(dppp)(H2O)2]2+(TfOꢀ)2,48 [Pd(dppp)-
(H2O)2]2+(BF4ꢀ)2,19 [Pd(dppp)(H2O)2]2+(SbF6ꢀ)2,19 {Pd[(R)-
1
Substrate 1d: white solid; yield 63%; mp 140ꢀ141 °C; H NMR
(300 MHz, CDCl3) δ 8.07 (br, 2H), 7.60 (d, J = 2.7 Hz, 1H), 7.45ꢀ7.41
(m, 1H), 6.98 (d, J = 8.7 Hz, 1H), 4.93 (s, 2H), 2.19 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ 204.0, 160.8, 134.9, 131.3, 125.1, 114.0, 72.9, 26.0;
IR (KBr) ν 3423, 3320, 1731, 1484 cmꢀ1; MS (70 eV, EI) m/z (%) 228
(M+), 210, 168, 155, 43 (100). Anal. Calcd for C9H10ClBO4: C, 47.32;
H, 4.41. Found: C, 47.35; H, 4.26.
ꢀ
20
binap](H2O)2}2+(TfOꢀ)2,50 and {Pd[(S,S)-bdpp](H2O)2]2+(BF4
)
2
were synthesized following the literature procedure. The boronic acids
1e, 1f, and 1jꢀ1l were all synthesized according to the published
procedures.12 Amberlite IRA 400 (Cl) were purchased from Lancaster.
Before use, they were treated with NaOH solution (2 N), then washed
with water until neutrality and filtered under vacuum.
Experimental Procedure for Synthesis of Substrates.
Typical Procedure for the Preparation of 1a: K2CO3 (3.036 g, 22 mmol)
was added to a solution of 1-bromopropan-2-one (3.0 g, 22 mmol) and
1
Substrate 1g: white solid; yield 50%; mp 122ꢀ123 °C; H NMR
(300 MHz, DMSO-d6) δ 8.38 (br, 2H), 7.86ꢀ7.79 (m, 3H), 7.46 (t, J =
6.9 Hz, 1H), 7.35(t, J = 7.2 Hz, 1H), 7.23(t, J = 9.0Hz, 1H), 4.84 (s, 2H),
2.22 (s, 3H); 13C NMR (75 MHz, DMSO-d6) δ 205.9, 157.5, 135.8,
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NOTE
129.6, 128.8, 127.9, 127.5, 126.0, 123.4, 114.5, 73.7, 26.5; IR (KBr) ν
3383, 3290, 1733, 1620, 1509 cmꢀ1; MS (70 eV, EI) m/z (%) 244 (M+),
182 (100), 153, 115. Anal. Calcd for C13H13BO4: C, 63.98; H, 5.37.
Found: C, 63.90; H, 5.33.
Compound 3ea: oil; 1H NMR (400 MHz, CDCl3) δ 7.68 (d, J = 5.7
Hz, 2H), 7.43ꢀ7.35 (m, 3H), 7.31ꢀ7.29 (m, 1H), 7.25ꢀ7.21 (m, 1H),
7.06ꢀ7.04 (m, 1H), 7.02ꢀ7.00 (m, 1H), 5.64 (s, 1H), 5.32 (s, 1H), 5.19
(s, 1H), 4.32 (s, 1H), 4.23ꢀ4.15 (m, 2H), 4.03 (q, J = 7.2 Hz, 2H), 1.05
(t, J = 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ 172.7, 157.9, 144.6,
140.0, 129.1, 128.9, 128.4, 128.2, 127.5, 125.1, 122.7, 119.9, 115.2, 79.9,
76.5, 61.2, 55.7, 13.7; IR (neat) ν 3473 (br), 2980, 1709, 1603, 1482,
764 cmꢀ1; MS (70 eV, EI) m/z (%) 324 (M+), 306, 278, 145, 105 (100);
HRMS calcd for C20H20O4 (M+) 324.1364, found 324.1362.
Compound 3fa: oil; 1H NMR (300 MHz, CDCl3) δ 7.59 (d, J = 8.4
Hz, 2H), 7.42 (d, J = 7.5 Hz, 1H), 7.22 (t, J = 8.1 Hz, 1H), 7.06ꢀ6.97
(m, 2H), 6.91 (d, J = 8.4 Hz, 2H), 5.63 (s, 1H), 5.31 (s, 1H), 5.17 (s, 1H),
4.28 (s, 1H), 4.22ꢀ4.11 (m, 2H), 4.05 (q, J = 7.2 Hz, 2H), 3.80 (s, 3H),
1.09 (t, J = 7.5 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 172.8, 158.9, 157.9,
140.1, 136.8, 130.3, 129.0, 128.9, 126.3, 122.6, 119.9, 115.0, 113.6, 80.1,
76.3, 61.1, 55.7, 55.2, 13.7; IR (neat) ν 3480 (br), 2980, 2937, 1709, 1610,
1512, 1184, 733 cmꢀ1; MS (70 eV, EI) m/z (%) 354 (M+), 337, 308, 150,
135 (100), 121,107; HRMS calcd for C21H22O5 (M+) 354.1470, found
354.1467.
1
Substrate 1h: white solid; yield 25%; mp 123ꢀ125 °C; H NMR
(300 MHz, CDCl3) δ 7.92 (d, J = 7.5 Hz, 1H), 7.43ꢀ7.38 (m, 1H), 7.08
(t, J = 7.5 Hz, 1H), 6.81 (d, J = 8.4 Hz, 1H), 4.92 (s, 2H), 1.25 (s, 9H);
13C NMR (75 MHz, CDCl3) δ 209.5, 162.8, 137.2, 132.5, 121.9, 111.3,
68.5, 42.7, 26.2; IR (KBr) ν 3351 (br), 1720, 1608, 1579, 1455,
749 cmꢀ1; MS (70 eV, EI) m/z (%) 218 (M+ ꢀ H2O), 192, 174, 159,
131, 77, 57 (100); HRMS calcd for C12H15BO3 (M+ ꢀ H2O) 218.1114,
found 218.1117.
Substrate 1i: white solid; yield 51%; mp 103ꢀ105 °C (lit.12 103 °C);
1H NMR (300 MHz, CDCl3) δ 7.89 (dd, J = 1.8 and 7.5 Hz, 1H),
7.41ꢀ7.34 (m, 1H), 7.06ꢀ7.01 (m, 1H), 6.89 (s, 2H), 6.79 (d, J = 8.1
Hz, 1H), 4.82ꢀ4.76 (m, 1H), 2.69ꢀ2.60 (m, 2H), 2.47ꢀ2.41 (m, 1H),
2.17ꢀ2.04 (m, 2H), 1.89ꢀ1.71 (m, 3H); 13C NMR (75 MHz, CDCl3)
δ 207.5, 162.5, 137.2, 132.4, 121.8, 111.9, 81.8, 40.7, 34.7, 27.4, 23.6; IR
(KBr) ν 3378 (br), 2955, 1722, 1600, 774 cmꢀ1; MS (70 eV, EI) m/z
(%) 234 (M+), 138, 120 (100), 96.
1
Compound 3ia: oil; H NMR (300 MHz, CDCl3) δ 7.36ꢀ7.34
Experimental Procedure for [Pd(dppp)(H2O)2]2+(BF4ꢀ)2 Catalyzed
Synthesis of 1-Benzoxepine Derivatives (3). A dried schenk tube was
charged with 1a (35 mg, 0.18 mmol), 2a (16.8 mg, 0.15 mmol), and
2 mL of toluene. [Pd(dppp)(H2O)2]2+(BF4ꢀ)2 (3.3 mg, 3 mol %) was
then added to the mixture, and the solution was stirred at 80 °C. The
progress of the reaction was monitored by TLC (EtOAc/petroleum
ether = 1:4). Upon completion, the solution was purified by flash
column chromatography (pure petroleum ether, then ethyl acetate/
petroleum ether = 1:4) to obtain the product 3aa.
(m, 1H),7.20ꢀ7.15 (m, 1H), 6.99ꢀ6.92 (m, 2H), 5.52 (s, 1H), 5.14 (s, 1H),
4.47 (s, 1H), 4.22ꢀ4.16 (m, 2H), 3.98 (s, 1H), 3.59 (s, 1H), 1.84ꢀ1.57
(m, 8H), 1.27 (t, J = 6.9 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ 173.8,
158.1, 140.0, 130.2, 128.8, 128.7, 121.9, 119.7, 112.9, 81.2, 73.8, 61.2,
56.5, 34.2, 27.7, 20.8, 20.0, 13.9; IR (neat) ν 3485 (br), 2936, 1708,
1479, 1450, 1019, 755 cmꢀ1; MS (70 eV, EI) m/z (%) 302 (M+), 284,
229, 171, 160 (100), 131; HRMS calcd for C18H22O4 (M+) 302.1516,
found 302.1518.
1
Compound 3ab: oil; H NMR (300 MHz, CDCl3) δ 7.33ꢀ7.30
1
(m, 1H),7.21ꢀ7.16 (m, 1H), 7.02ꢀ6.91 (m, 2H), 5.49 (s, 1H), 5.20 (s, 1H),
4.61 (s, 1H), 4.03ꢀ3.93 (m, 2H), 3.69ꢀ3.66 (m, 4H), 1.48 (s, 3H); 13C
NMR (75 MHz, CDCl3) δ 173.6, 157.6, 140.7, 129.8, 129.2, 129.1, 122.7,
119.8, 115.7, 78.6, 73.4, 56.8, 52.2, 25.8; IR (neat) ν 3501 (br), 2975, 2955,
1716, 1603, 1483, 1207, 734 cmꢀ1; MS (70 eV, EI) m/z (%) 248 (M+), 230,
216, 145 (100), 131, 115, 43; HRMS calcd for C14H16O4 (M+) 248.1049,
found 248.1048.
Compound 3aa: oil; H NMR (300 MHz, CDCl3) δ 7.33ꢀ7.31
(m, 1H),7.20ꢀ7.16 (m, 1H), 7.00ꢀ6.91 (m, 2H), 5.48 (s, 1H), 5.21 (s, 1H),
4.80 (s, 1H), 4.16ꢀ4.02 (m, 2H), 3.99ꢀ3.94 (m, 2H), 3.63 (s, 1H), 1.47
(s, 3H), 1.17ꢀ1.13 (m, 3H); 13C NMR (100 MHz, CDCl3) δ 173.2, 157.6,
140.9, 129.9, 129.4, 129.0, 122.6, 119.7, 115.8, 78.8, 73.5, 61.2, 57.0, 25.6, 13.8;
IR (neat) ν 3491 (br), 3068, 2978, 2935, 1711, 1603, 1570, 759 cmꢀ1; MS
(m/z, EI) 262 (M+), 244, 216 (100), 189, 145, 115, 91, 43; HRMS-EI calcd
for C15H18O4 (M+) 262.1203, found 262.1205.
Compound 3ac: oil; 1H NMR (300 MHz, CDCl3) δ 7.34ꢀ7.30 (m,
1H), 7.20ꢀ7.15 (m, 1H), 7.01ꢀ6.90 (m, 2H), 5.48 (s, 1H), 5.21 (s, 1H),
5.02ꢀ4.98 (m, 2H), 3.99ꢀ3.94 (m, 2H), 3.59 (s, 1H), 1.46 (s, 3H), 1.21
(d, J = 6.6 Hz, 3H), 1.05 (d, J = 6.0 Hz, 3H); 13C NMR (75 MHz,
CDCl3) δ 172.8, 157.6, 141.1, 129.9, 129.6, 129.0, 122.5, 119.7, 115.9,
78.9, 73.5, 68.8, 57.2, 25.5, 21.6, 21.1; IR (neat) ν 2982, 2938, 1706,
1630, 1483, 1220, 1106 cmꢀ1; MS (70 eV, EI) m/z (%) 276 (M+), 258,
216, 173, 145, 131, 91, 45 (100); HRMS calcd for C16H20O4 (M+)
276.1362, found 276.1364.
Compound 3ba: oil; 1H NMR (400 MHz, CDCl3) δ 7.11 (d, J = 2.0
Hz, 1H), 6.99ꢀ6.97 (m, 1H), 6.82 (d, J = 8.0 Hz, 1H), 5.47 (s, 1H),
5.19 (s, 1H), 4.74 (s, 1H), 4.19ꢀ4.09 (m, 2H), 3.98ꢀ3.91 (m, 2H), 3.61
(s, 1H), 2.29 (s, 3H), 1.46 (s, 3H), 1.18 (t, J = 7.2 Hz, 3H); 13C NMR
(100 MHz, CDCl3) δ 173.3, 155.6, 141.0, 131.9, 129.7, 129.6, 119.5,
115.6, 78.7, 73.4, 61.1, 57.1, 25.7, 20.5, 13.8; IR (neat) ν 3489 (br), 2978,
2934, 1710, 1631, 1035, 734 cmꢀ1; MS (70 eV, EI) m/z (%) 276 (M+),
230, 175, 159 (100), 115, 91, 43; HRMS calcd for C16H20O4 (M+)
276.1352, found 276.1362.
Compound 3ad: oil; 1H NMR (300 MHz, CDCl3) δ 7.25ꢀ7.22 (m,
1H), 7.13ꢀ7.07 (m, 1H), 6.94ꢀ6.83 (m, 2H), 5.41 (s, 1H), 5.13 (s, 1H),
4.76 (s, 1H), 4.03ꢀ3.95 (m, 2H), 3.92ꢀ3.86 (m, 2H), 3.56 (s, 1H),
1.45ꢀ1.39 (m, 5H), 1.23ꢀ1.15 (m, 2H), 0.82 (t, J = 7.5 Hz, 3H); 13C
NMR (75 MHz, CDCl3) δ 173.3, 157.6, 140.9, 129.9, 129.4, 129.0,
122.6, 119.8, 115.8, 78.8, 73.4, 65.0, 57.0, 30.3, 25.7, 18.9, 13.5; IR (neat)
ν 3492, 2963, 1710, 1631, 1604, 1483, 1218, 759 cmꢀ1; MS (70 eV, EI)
m/z (%) 290 (M+), 272, 232, 216, 173, 161, 145 (100), 131, 115; HRMS
calcd for C17H22O4 (M+) 290.1518, found 290.1520.
Compound 3ca: oil; 1H NMR (400 MHz, CDCl3) δ 7.30 (d, J = 2.1
Hz, 1H), 7.22 (dd, J = 8.4 and 1.8 Hz, 1H), 6.87 (d, J = 8.4 Hz, 1H), 5.47
(s, 1H), 5.22 (s, 1H), 4.80 (s, 1H), 4.15ꢀ4.11 (m, 2H), 3.97ꢀ3.92 (m,
2H), 3.63 (s, 1H), 1.46 (s, 3H), 1.31 (s, 9H), 1.14 (t, J = 7.2 Hz, 3H); 13C
NMR (100 MHz, CDCl3) δ 173.3, 155.4, 145.3, 141.5, 129.2, 126.2,
126.0, 119.3, 115.8, 78.7, 73.5, 61.1, 57.2, 34.2, 31.4, 25.6, 13.9; IR (neat)
ν 3494 (br), 2964, 2872, 1712, 1035 cmꢀ1; MS (70 eV, EI) m/z (%) 318
(M+), 303, 272, 245, 229 (100), 187, 173; HRMS calcd for C19H26O4
(M+) 318.1827, found 318.1831.
Compound 3ae: oil; 1H NMR (300 MHz, CDCl3) δ 7.32ꢀ7.14 (m,
7H), 6.98ꢀ6.90 (m, 2H), 5.46 (s, 1H), 5.17 (s, 1H), 5.09 (s, 2H), 4.69
(s, 1H), 4.03ꢀ3.93 (m, 2H), 3.69 (s, 1H), 1.46 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ 173.0, 157.6, 140.6, 135.1, 129.7, 129.4, 129.1,
128.4, 128.3, 128.1, 122.7, 119.8, 116.1, 78.8, 73.5, 66.9, 57.1, 25.6; IR
(neat) ν 3500, 3066, 1716, 1483, 910, 734 cmꢀ1; MS (70 eV, EI) m/z
(%) 324 (M+), 233, 216, 173, 145, 91 (100); HRMS calcd for C20H20O4
(M+) 324.1362, found 324.1364.
Compound 3da: oil; 1H NMR (300 MHz, CDCl3) δ 7.29 (d, J = 2.4
Hz, 1H), 7.15 (dd, J = 8.7 and 2.4 Hz, 1H), 6.87 (d, J = 8.4 Hz, 1H), 5.49
(s, 1H), 5.25 (s, 1H), 4.78 (s, 1H), 4.21ꢀ4.10 (m, 2H), 3.96 (s, 2H),
3.60 (s, 1H), 1.46 (s, 3H), 1.20 (t, J = 7.2 Hz, 3H); 13C NMR (100 MHz,
CDCl3) δ 172.9, 156.3, 139.9, 131.4, 128.9, 128.8, 127.5, 121.2, 116.9,
78.8, 73.3, 61.3, 56.8, 25.6, 13.8; IR (neat) ν 3492 (br), 2980, 2939, 1712,
1478, 1032, 824 cmꢀ1; MS (70 eV, EI) m/z (%) 296 (M+), 278, 250,
223, 179, 165, 115, 43 (100); HRMS calcd for C15H17ClO4 (M+)
296.0820, found 296.0815.
1
Compound 3af: oil; H NMR (300 MHz, CDCl3) δ 7.41ꢀ7.20
(m, 5H),7.06ꢀ6.96 (m, 2H), 6.86ꢀ6.83 (m, 2H), 5.59 (s, 1H), 5.36 (s, 1H),
6354
dx.doi.org/10.1021/jo200672w |J. Org. Chem. 2011, 76, 6350–6355

The Journal of Organic Chemistry
NOTE
4.51 (s, 1H), 4.06ꢀ4.04 (m, 2H), 3.91 (s, 1H), 1.55 (s, 3H); 13C NMR
(75 MHz, CDCl3) δ 171.9, 157.6, 150.0, 140.8, 129.6, 129.5, 129.4,
129.3, 126.2, 122.8, 121.2, 120.0, 116.7, 78.8, 73.7, 57.8, 25.4; IR (neat) ν
3529, 2979, 1744, 1633, 1602, 1484, 1199, 734 cmꢀ1; MS (70 eV, EI)
m/z (%) 310 (M+), 230, 216, 173 (100), 161, 145, 131, 115; HRMS
calcd for C19H18O4 (M+) 310.1205, found 310.1204.
(23) Mikami, K.; Hatano, M.; Akiyama, K. Top. Organomet. Chem.
2005, 14, 279 and references therein.
(24) Macías, F. A.; Varela, R. M.; Torres, A.; Molinillo, J. M. G.
Tetrahedron Lett. 1999, 40, 4725.
(25) Wijnberg, J. B. P. A.; van Veldhuizen, A.; Swarts, H. J.; Frankland,
J. C.; Field, J. A. Tetrahedron Lett. 1999, 40, 5767.
(26) Asakawa, Y.; Hashimoto, T.; Takikawa, K.; Tori, M.; Ogawa, S.
Phytochemistry 1991, 30, 235.
(27) Vyvyan, J. R.; Looper, R. E. Tetrahedron Lett. 2000, 41, 1151.
(28) Hoberg, J. O. Tetrahedron 1998, 54, 12631.
(29) Synder, N. L.; Haines, H. M.; Peczuh, M. W. Tetrahedron 2006,
62, 9301.
Compound 5ia: oil; 1H NMR (300 MHz, CDCl3) δ 7.40ꢀ7.36 (m,
2H), 7.21ꢀ7.16 (m, 2H), 2.74ꢀ2.70 (m, 2H), 2.63ꢀ2.58 (m, 2H),
1.96ꢀ1.80 (m, 4H); 13C NMR (75 MHz, CDCl3) δ 154.2, 153.9, 128.8,
122.9, 122.0, 118.3, 112.8, 110.7, 23.4, 22.9, 22.6, 20.4; IR (neat) ν 2934,
2847, 1477, 1454, 743 cmꢀ1; MS (70 eV, EI) m/z (%) 172 (M+), 144
(100), 128, 115; HRMS calcd for C12H12O (M+) 172.0892, found
172.0888.
(30) Yamaguchi, S.; Tsuchida, N.; Miyazawa, M.; Hirai, Y. J. Org.
Chem. 2005, 70, 7505.
(31) Besson, L.; Gorꢀe, J.; Cazes, B. Tetrahedron Lett. 1995, 36, 3853.
(32) Yamamoto, Y.; Al-Masum, M.; Asao, N. J. Am. Chem. Soc. 1994,
116, 6019.
(33) Larock, R. C.; He, Y.; Leong, W. W.; Han, X.; Refvik, M. D.;
Zenner, J. M. J. Org. Chem. 1998, 63, 2154.
(34) Tsuji, J. In Handbook of Organopalladium Chemistry for Organic
Synthesis; Negish, E., Ed.; Wiley: New York, 2000; p 1669.
(35) Trost, B. M.; Herndon, J. W. J. Am. Chem. Soc. 1984, 106, 6835.
(36) Tabuchi, T.; Inanaga, J.; Yamaguchi, M. Tetrahedron Lett. 1986,
27, 1195.
’ ASSOCIATED CONTENT
Supporting Information. Copies of H NMR and 13C
1
S
b
NMR spectra. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: xylu@mail.sioc.ac.cn.
(37) Masuyama, Y.; Kinugawa, N.; Kurusu, Y. J. Org. Chem. 1987,
52, 3702.
(38) Ha, Y.-H.; Kang, S.-K. Org. Lett. 2002, 4, 1143.
(39) Kang, S.-K.; Lee, S.-W.; Jung, J.; Lim, Y. J. Org. Chem. 2002,
67, 4376.
(40) Kang, S.-K.; Ha, Y.-H.; Ko, B.-S.; Lim, Y.; Jung, J. Angew. Chem.,
Int. Ed. 2002, 41, 343.
(41) Yu, C.-M.; Youn, J.; Lee, M.-K. Org. Lett. 2005, 7, 3733.
(42) Anwar, U.; Grigg, R.; Rasparini, M.; Savic, V.; Sridharan, V.
Chem. Commun. 2000, 645.
’ ACKNOWLEDGMENT
We thank the National Basic Research Program of China
(2009CB825300), National Natural Science Foundation of
China (20732005), and Chinese Academy of Sciences for
financial support.
(43) Hopkins, C. D.; Malinakova, H. C. Org. Lett. 2004, 6, 2221 and
references cited therein.
(44) Phan, D. H. T.; Kim, B.; Dong, V. M. J. Am. Chem. Soc. 2009,
131, 15608.
’ REFERENCES
(1) Ma, S. Chem. Rev. 2005, 105, 2829.
(2) Schuster, H. F.; Coppola, G. M. Allenes in Organic Synthesis; John
(45) Suzuki, A. In Metal-Catalyzed Cross-Coupling Reactions; Dieder-
ich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, Germany, 1998; pp
49ꢀ97.
Wiley & Sons: New York, 1988.
(3) Hashmi, A. S. K. Angew. Chem., Int. Ed. 2000, 39, 3590.
(4) Walkup, R. D.; Kim, S. W.; Wagy, S. D. J. Org. Chem. 1993,
58, 6486.
(5) Zimmer, R.; Dinesh, U.; Nadanan, E.; Khan, F. A. Chem. Rev.
2000, 100, 3067.
(6) Hall, D. G., Ed. Boronic Acids: Preparation and Applications in
Organic Synthesis and Medicine; Wiley-VCH: Weinheim, Germany, 2005.
(7) Suzuki, A. Organoboranes in Organic Synthesis; Hokkaido Uni-
versity: Sapporo, 2004.
(8) Tsuji, J. Palladium Reagent and Catalysis: Innovations in Organic
Synthesis; John Wiley: Chichester, UK, 1995.
(9) Soderberg, B. C. In Comprehensive Organometallic Chemistry II;
Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford,
1995; Vol. 12, p 241.
(46) Pisano, C.; Consigilio, G.; Sironi, A.; Moret, M. J. Chem. Soc.,
Chem. Commun. 1991, 421.
(47) Nishikata, T.; Yamamoto, Y.; Miyaura, N. Organometallics
2004, 23, 4317 and references therein.
(48) Stang, P. J.; Cao, D. H.; Poulter, G. T.; Arif, A. M. Organome-
tallics 1995, 14, 1110.
(49) Grigg, R.; Cooper, I. R.; Cleghorn, L. T.; William, S, M.;
Sridharan, V. Chem. Commun. 2000, 645.
(50) Hagiwara, E.; Fujii, A.; Sodeoka, M. J. Am. Chem. Soc. 1998,
120, 2474.
(10) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
(11) Soai, K.; Shibata, T. In Comprehensive Asymmetric Catalysis;
Jacobsen, E. N., Pfaltz, A., Yamamoto, H., Eds.; Springer-Verlag: Berlin,
2004; Suppl. 1, pp 95ꢀ106.
(12) Liu, G.; Lu., X. J. Am. Chem. Soc. 2006, 128, 16504.
(13) Zhao, B.; Lu, X. Tetrahedron Lett. 2006, 47, 6765.
(14) Zhao, B.; Lu, X. Org. Lett. 2006, 8, 5987.
(15) Song, J.; Shen, Q.; Xu, F.; Lu, X. Org. Lett. 2007, 9, 2947.
(16) Dai, H.; Lu, X. Org. Lett. 2007, 9, 3077.
(17) Liu, G.; Lu., X. Adv. Synth. Catal. 2007, 349, 2247.
(18) Yang, M.; Zhang, X.; Lu., X. Org. Lett. 2007, 9, 5131.
(19) Yu, X.; Lu, X. Org. Lett. 2009, 11, 4366.
(20) Han, X.; Lu, X. Org. Lett. 2010, 12, 108.
(21) Zhou, C.; Larock, R. C. J. Am. Chem. Soc. 2004, 126, 2302.
(22) Zhou, C.; Larock, R. C. J. Org. Chem. 2006, 71, 3551.
6355
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