Preparation of polysubstituted dihydrofurans through a PhI(OAc)2-promoted haloenolcyclization of olefinic dicarbonyl compounds

Article abstract of DOI:10.1039/c8ob02161a

A metal-free cyclization of olefinic dicarbonyl compounds for the synthesis of various 5-halomethyl-4,5-dihydrofurans is presented. Using (diacetoxyiodo)benzene as the reaction promoter and halotrimethylsilane as the halogen source, the intramolecular hal

Full text of DOI:10.1039/c8ob02161a

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Takeharu Haino et al.
Solvent-induced emission of organogels based on tris(phenylisoxazolyl)
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ARTICLE
Preparation of polysubstituted dihydrofurans through a
PhI(OAc) -promoted haloenolcyclization of olefinic dicarbonyl
compounds
2
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
a
DOI: 10.1039/x0xx00000x
Ji Liu, Qing-Yun Liu, Xing-Xiao Fang, Gong-Qing Liu* and Yong Ling*
www.rsc.org/
A metal-free cyclization of olefinic dicarbonyl compounds for the synthesis of various 5-halomethyl-4,5-dihydrofurans is presented.
Using (diacetoxyiodo)benzene as the reaction promoter and halotrimethylsilane as the halogen source, the intramolecular
haloenolcyclization of the 2-allyl-1,3-diketones smoothly proceeded, leading to the corresponding 5-halomethyl-4,5-dihydrofurans
in good to excellent isolated yields. Moreover, the resulting 5-iodomethyl products could be converted to functionalized furans in
almost quantitative yields by treatment with DBU followed by acid-catalyzed rearrangement. The reactions could be carried out on
a gram scale and did not require harsh reaction conditions. The good isolated yields, mild conditions, and operational simplicity
make this reaction a viable method for the construction of different dihydrofuran and furan structures.
allyl-substituted 1,3-dicarbonyl compounds followed by
5
3
cleavage of the selenium linkers with CH I–NaI in DMF.
Introduction
Terent’ev et al. developed a one-pot procedure for the
Dihydrofurans are key structural motifs in several biologically
interesting skeletons and are important subunits in different
construction of bicyclic compounds containing 1,2-dioxolane
1
natural products. Moreover, these functional groups are also
and tetrahydrofuran ring by the reaction of 2-allyl-1,3-
6
system. In addition, other
diketones with the I
2
/H
2
O
2
frequently employed as synthons in natural product synthesis,
2
medicinal chemistry and diversity-oriented synthesis since the
7
substrates such as α-allyl-substituted β-keto sulfones, α-allyl-
8
substituted β-ketoesters, β-enamino esters, β-enamino
9
reactive enol ether moiety in dihydrofurans can be further
manipulated by various reactions such as aromatization to
furans or ring opening to generate linear chain compounds. To
this end, a substantial amount of effort has been devoted to
the development of novel methods for the synthesis of these
1
0
11
ketones, and β-phosphonate ketones have also been
employed in this type of reaction to prepare dihydrofurans.
OH
O
O
R
3
compounds.
R
R
R
R
E
R
O
R
O
O
I
O
IV
2-Allyl-1,3-diketones
I
, which contain an acidic proton alpha to
II
the carbonyl functionality due to their strong electron-
withdrawing effect, could isomerize to enol II and undergo an
electrophile-mediated enolcyclization, furnishing highly
functionalized dihydrofurans III (Scheme 1, left). Therefore,
E
E+
X
O
III
R
this strategy has been widely applied to for the construction of Scheme 1. Schematic representation of the cyclization of 2-allyl-1,3-diketone.
dihydrofuran rings due to its step economy. For example,
Antonioletti
and
coworkers
reported
the
iodoenoletherification of 2-alkenyl-substituted 1,3-dicarbonyl Despite these advances, general methods for the preparation
4
compounds, leading to the iodo-dihydrofurans in high yields.
Huang et al. prepared compounds of this class via polymer- dihydrofurans in particular, are still limited. This can be
of various halogenated dihydrofurans, bromomethyl
supported, selenium-induced electrophilic cyclizations of
α
-
attributed to the use of olefinic 1,3-dicarbonyl compounds
I,
which are highly susceptible to halogenation α to the carbonyl
1
groups (Scheme 1, right). The resulting α-halogenated
2
dicarbonyl compound IV could not be converted into the
1
3
corresponding
dihydrofurans.
Furthermore,
a
superstoichiometric amount of electrophile is usually needed
for most reactions of this type, even for well-developed
4
, 7, 8c, 8g
iodocyclization.
These drawbacks more or less limited
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the application of these methods in the preparation of atom and the ether oxygen atom in 2k, is a consequence of the
structurally diverse dihydrofuran compounds. Nonetheless, cyclization proceeding through a kinetically controlled trans-
1
7
herein we report an efficient, practical and scalable diaxial addition to the double bond. Preparation of 3,4-
halocyclization of olefinic 1,3-dicarbonyl compounds to afford dihydro-2H-pyrans 2l was also possible using a homoallylic
to 2,3,5-trisubstituted dihydrofurans as a continuation of our diketone as the starting material. Finally, alkenyl-substituted β-
1
4
investigation of the cyclization of unfunctionalized olefins.
ketoesters were good candidates for this transformation (2m),
further highlighting the generality and great potential of the
developed method.
Results and Discussion
Recently, we have shown that (diacetoxyiodo)benzene can be
used to promote haloheterocyclizations of different
unfunctionalized olefins (Scheme 2). In the presence of
2
Table 1. Substrate scope of Ph(OAc) -mediated iodoenolcyclization
PhI(OAc)
2
and suitable halogen sources, a variety of
halocyclizations could be realized, giving the corresponding
1
4b,
halogenated products in good to excellent isolated yields.
1
4d
2
Scheme 2. PhI(OAc) -promoted intramolecular haloheterocyclizations.
As an extension of our ongoing work toward the cyclization of
unactivated olefins and in analogy to the reactions mentioned
above, we envisage that haloenolcyclization of olefinic 1,3-
dicarbonyl compounds could be realized by the utilization of a
previously developed protocol. Based on our previous results,
2
we first examined a PhI(OAc) /TMSI system for the cyclization
of 2-allyl-1,3-diphenylpropane-1,3-dione (1a). To our delight,
under standard conditions, the reaction proceeded readily and
provided desired dihydrofuran 2a a 91% yield. No reaction
occurred in the absence of PhI(OAc)
2
or TMSI. Encouraged by
1m were then
these results, other dicarbonyl compounds 1b
−
subjected to the same conditions to test the scope of the
reaction, and the results are summarized in Table 1. As these
results showed, all reactions occurred in a clean fashion, giving
exclusively the expected Markovnikov products. Dicarbonyl
substrates with either electron-rich substituents or electron-
deficient aryl substituents all gave the expected products in
good isolated yields (2a
worked well in the current reaction system, and corresponding
,5-dihydrofuran 2g was obtained in good yield. Notably, the
cyclization of 2-allyl-1-phenyl-1,3-butanedione led to a mixture
of two isomers (2h 2h’) in a 2:3 ratio. The same cyclization in
-2f). Alkyl diketones such as 1g also
4
/
Huang’s report led to exclusive incorporation of the benzoyl
group into the furan ring to form 3-acetyl-5-iodomethyl-2-
5
phenyl-4,5-dihydrofuran (2h) in 79% yield. Interestingly, α-
allylcyclohexane-1,3-diones
iodocyclization, affording 2-iodomethyltetrahydrobenzofuran-
-one (2i) in 73%, and no further aromatization to the
benzofuran was observed, probably due to the mild reaction
could
smoothly
undergo
4
Reaction conditions: The reaction was carried out in an open air system with 0.5
mmol of substrate, 1.0 mmol of PhI(OAc) and 1.0 mmol of TMSI in 5 mL of DCM.
15
2
conditions. Substituents on the C=C double bonds showed
little impact on the reaction outcome, and the products could
be isolated in satisfactory yields (2j and 2k). Bicyclic product 2k
The scalability of the developed methodology was
demonstrated by the gram-scale synthesis of dihydrofuran 2a
1
6
exhibited a cis-fused ring. The cis relative configuration of the
rings, as well as the trans relationship between the iodine
.
2
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Iodocyclization of 2-allyl-1,3-diphenylpropane-1,3-dione (1a
ARTICLE
)
Table 2. Synthesis of polysubstituted furans
was carried out on a 10-mmol scale, and product 2a was
obtained in 83% isolated yield (Scheme 3). Moreover, the C−I
bond in 2a provides easy access to a variety of useful
functional groups. For example, the iodine atom in 2a could be
O
O
1) DBU, reflux,
benzene
I
R
R
2) H+
O
O
replaced by different nucleophiles, such as azide (
phthalimide (
), under mild conditions. Furthermore, the C−I
bond in 2a could also be easily cleaved via tributylstannane
reduction. When 2a was treated with AIBN/nBu SnH at 100 °C
could be obtained in an
3) and
R
R
2
a-2f, 2i, 2k and 2n
6a-6i
4
Entry
Substrate
O
Product
O
Isolated Yield
3
I
for 12 h in toluene, compound
almost quantitative isolated yield.
5
R
R
O
R
O
R
1
2
3
4
5
6
R = Ph (2a)
6a
6b
6c
6d
6e
6f
89%
93%
90%
95%
94%
95%
O
R = 4-MeOPh (2b)
R = 4-MePh (2c)
R = 4-FPh (2d)
R = 4-ClPh (2e)
R = 4-BrPh (2f)
Ph
Ph
O 1a 10 mmol
.24g
3%
3
8
a
O
O
I
Ph
Ph
O
Ph
O
7
8
N3
H
I
O
O
O
O
b
Ph
d
2
i
6g
90%
O
O
Ph
O
Ph
O
O
2a
Ph
Ph
Ph
5
95%
3
81%
H
H
c
Ph
O
O
6h
O
2
k
96%
96%
I
Ph
N
I
EtO2C
O
EtO2C
Ph
O
9
O
4
76%
O
Me
Me
2
n
6i
Conditions: (a) 1a (10 mmol), PhI(OAc)
r.t., 24 h. (b) 2a, NaN (2.0 equiv.), DMF, r.t, 8 h;. (c) 2a, phthalimide (1 1.0 mmol of DBU in 5 mL of benzene.
equiv.), K CO (2 equiv.), DMF, 80 °C, 12 h. (c) 2a, AIBN (0.05 equiv.),
nBu SnH (3 equiv.), toluene, 100 °C, 12 h.
Scheme 3. Gram-scale iodocyclization and derivatization of dihydrofurans 2a to
2
(10 mmol), TMSI (10 mmol), DCM, Reaction conditions: The reaction was carried out with 0.5 mmol of substrate and
3
2
3
3
Compared
to
the
well-documented
intramolecular
different bioactive structures.
iodoenolcyclizations of different 2-alkenyl-1,3-dicarbonyl
compounds, the bromine variant of this type of reaction is less
To further demonstrate the synthetic value of the investigated, and only a few examples have been reported. For
PhI(OAc)
/TMSI system in the construction of diversely instance, Mphahlele et al. discovered that -allylcyclohexane-
functionalized furan derivatives, selected iodomethyl 1,3-diones could undergo pyridinium tribromide-promoted
α
2
dihydrofurans were dehydroiodinated by treatment with a bromocyclization
nonnucleophilic, hindered tertiary amine (DBU) at reflux bromomethyltetrahydrobenzofuran-4-ones
and
afford
a
mixtures
and
of
2-
3-
1
5
condition in benzene. The reaction gave the corresponding bromotetrahydrobenzopyran-5-ones. Yeung and coworkers
alkylidenedihydrofurans, which could be converted to furan recently reported an enantioselective bromocyclization of
derivatives through an acid-catalyzed isomerization. As shown olefinic 1,3-dicarbonyl compounds, but the substrates were
1
3
in Table 2, furans
6
were obtained in almost quantitative limited to substituted alkenes. Motivated by the scant
isolated yields. It was noted that bicyclic substrate 2k gave a attention paid in the literature to the bromoenolcyclization of
different result. In this case, 2k produced exclusively the olefinic dicarbonyl compounds, the bromine variant of the
product of the anti-elimination. Resulting product 6h cannot reaction was next studied using the same method. As shown in
4
b, 4e, 18
aromatize to the furan under acidic conditions.
Scheme 4, different bromomethyl dihydrofurans could be
obtained in acceptable isolated yields using TMSBr as the
1
9
bromine source. Resulting dihydrofurans
7 are valuable
building blocks and can be converted to synthetically valuable
keto-benzoates and 1,2-diketo products via simple
2
0
derivatizations.
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Scheme 4. Ph(OAc)
compounds.
2
-promoted bromoenolcyclization of olefinic dicarbonyl
On the basis of the relative configuration of 2k and previous
literature reports, preliminary reaction pathway was
proposed (Scheme 5, pathway a). First, the substrate 1k
formed an iodonium species by electrophilic addition of IOAc,
which was generated in situ by oxidation of Ph(OAc) with
a
A
Scheme 5. Plausible mechanism for iodoenolcyclization
2
2
1
TMSI. Subsequently, an intramolecular nucleophilic attack of
oxygen atom on the iodinium three-membered ring produced
2k. The trans relative configuration between the iodine atom Conclusions
and the ether oxygen atom in 2k arose from the anti-attack of
4
The results shown in this work extend the utility of the
e, 8f, 9, 17
oxygen atom on the double bond.
An alternative
(diacetoxyiodo)benzene-promoted
cyclizations
of
pathway (Scheme 5, pathway b) involving activation of alkene
unfunctionalized olefins. Using 1.0 equiv. of PhI(OAc)
2
as the
by PhI(OAc) followed by intramolecular nucleophilic attack of
2
reaction promoter and 1.0 equiv. of TMSX (X = I and Br) as the
halogen source, 5-halomethyl-4,5-dihydrofuran products could
be obtained in good to excellent isolated yields. The reaction
could be carried out on a gram scale, and the iodide
substituent could be easily converted to other functional
groups via conventional methods. Moreover, the resulting
iodomethyl dihydrofurans could be dehydroiodinated by
treatment with DBU in refluxing benzene to afford
functionalized furans in almost quantitative yields. Thus, we
believe that the methodology described here will provide an
effective route to a variety of functionalized dihydrofurans and
furans. Efforts towards an asymmetric variant of this
transformation are currently underway in our laboratory.
oxygen atom and nucleophilic substitution by TMSI seemed
unlikely since two S 2 processes would lead to cis relative
N
configuration of the iodine atom and the oxygen atom in
product. In addition, NMR experiments were also carried out
to study the possible interaction between PhI(OAc) and the
2
olefinic part of the substrate. However, no significant chemical
shift change was observed when 1a was allowed to mix with
2
Ph(OAc)2. This results indicated that the substrate interaction
2
with the hypervalent iodine reagent generating species
less possible.
B/C is
Experimental section
General information: Reagents were used as received without
further purification unless otherwise indicated. Solvents were
dried and distilled prior to use. Reactions were monitored with
thin layer chromatography using silica gel GF254 plates. Organic
solutions were concentrated in vacuo with a rotavapor. Flash
column chromatography was performed using silica gel
(
200−300 meshes). Petroleum ether used had a boiling point
range of 60−90 °C. Melting points were measured on a digital
melting point apparatus without correction of the
thermometer. Nuclear magnetic resonance spectra were
recorded at ambient temperature (unless otherwise stated) at
1
3
4
3
00 MHz (100 MHz for C) in CDCl . Chemical shifts were
reported in ppm (δ) using TMS as internal standard, and
4
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spin−spin coupling constants (J) were given in Hz. High 130.0, 129.8, 129.6, 128.9, 127.7, 123.5, 111.5, 78.6, 41.7, 36.2.
+
resolution mass spectrometry (HRMS) analyses were carried HRMS–ESI: calc. for [C26
out on an FTICR HR-ESI-MS. found:410.1384.
General procedure for the preparation of 5-iodomethyl-4,5- (5-Methyl-2-phenyl-4,5-dihydrofuran-3-
dihydrofurans: The reaction was carried out in an open air yl)(phenyl)methanone ). To a 100 mL flask was added
system. In a 20 mL sealed tube were added olefinic 1,3- tributyltinhydride (1.5 mmol), AIBN (0.025 mmol), 2a (0.5
dicarbonyl compounds (0.5 mmol), PhI(OAc) (0.5 mmol), and mmol) and 20 mL of toluene. The resulting solution was
TMSI (0.5 mmol) in dry CH Cl (5 mL). The reaction mixture was refluxed at 100 °C for 12 h. The residue obtained by
stirred at room temperature for 12 h. CH Cl (10 mL) was then concentration was purified by flash column chromatography to
added, and the mixture was washed with aqueous Na give the as a colorless oil (88 mg, 92% yield) after flash
The combined organic layer was dried (Na SO and chromatography. H NMR (400 MHz, CDCl
4
H19NO +H] : m/z = 410.1392,
(
5
2
2
2
2
2
2
S
2
O
3
.
5
1
2
4
)
3
): δ/ppm= 7.34 (d, J
concentrated to give crude residue, which was purified by = 8.4 Hz, 2H), 7.15 - 7.03 (m, 4H), 7.00 – 6.94 (m, 4H), 4.97 -
flash column chromatography to give the corresponding 4.80 (m, 1H), 3.31 (dd, J = 14.7, 9.5 Hz, 1H), 2.87 (dd, J = 14.7,
1
3
products.
5-(Iodomethyl)-2-phenyl-4,5-dihydrofuran-3-
3
8.2 Hz, 1H), 1.46 (d, J = 6.2 Hz, 3H). C NMR (100 MHz, CDCl ):
(
δ/ppm= 188.9, 161.2, 134.5, 126.2, 125.6, 125.2, 124.6, 124.1,
yl)(phenyl)methanone
(
2a). Compound 2a was prepared 122.9, 107.3, 74.3, 35.3, 16.8. Spectral data are in agreement
2
3
according to the general procedure and isolated as a colorless with literature values.
solid (178 mg, 91% yield) after flash column chromatography General procedure for the preparation of furans: The mixture
1
(
petroleum ether/ethyl acetate = 15/1); mp = 97.5–99.5 °C. H of iodomethyl dihydrofurans (0.5 mmol) and DBU (2 mmol)
NMR (400 MHz, CDCl ): δ/ppm= 7.48 (d, J = 8.4 Hz, 2H), 7.28 – was stirred under nitrogen in 5 mL of benzene for 12 h at 60 C.
.18 (m, 4H), 7.11 (d, J = 7.5 Hz, 2H), 7.07 (d, J = 7.6 Hz, 2H), CH Cl (10 mL) was then added, and the mixture was washed
.94 – 4.87 (m, 1H), 3.55 – 3.49 (m, 3H), 3.11 (dd, J = 15.5, 7.0 with diluted HCl. A few drops of H SO (10 M) were added and
): δ/ppm= 193.2, 164.8, the solution was stirred at room temperature until the
38.8, 131.3, 130.1, 129.8, 129.3, 128.9, 127.7, 111.7, 80.0, completion of the reaction (TLC monitoring, usually 5 minutes).
9.2, 8.7. Spectral data are in agreement with literature Then the solution was diluted with CH Cl and washed with
SO ) and concentrated
to give crude residue, which was purified by flash column
3
°
7
4
2
2
2
4
1
3
Hz, 1H). C NMR (100 MHz, CDCl
3
1
3
2
2
5
values.
brine. The organic layer was dried (Na
2
4
General procedure for nucleophilic substitution of 2a
.
Compound 2a was dissolved in 2 mL of DMF, the nucleophile chromatography to give the corresponding products.
of interest was added, and the reaction mixture was stirred for (5-Methyl-2-phenylfuran-3-yl)(phenyl)methanone
(
6a).
a given time. Then 20 mL of CH
reaction mixture was washed with water, dried over Na
and concentrated under reduced pressure. The crude product after flash column chromatography (petroleum ether/ethyl
2
Cl
2
was added, and the Compound 6a was prepared according to the general
2
SO procedure and isolated as a colorless oil (116 mg, 89% yield)
4
,
1
3
was purified by silica gel column chromatography to give the acetate = 15/1). H NMR (400 MHz, CDCl ): δ/ppm= 7.73 (d, J =
corresponding product.
5-(Azidomethyl)-2-phenyl-4,5-dihydrofuran-3-
3
yl)(phenyl)methanone ). Compound 3 was prepared (d, J = 1.0 Hz, 3H). C NMR (100 MHz, CDCl ): δ/ppm= 190.9,
8.4 Hz, 2H), 7.57 (d, J = 8.1 Hz, 2H), 7.40 – 7.36 (m, 1H), 7.28 –
(
7.24 (m, 1H), 7.23 – 7.13 (m, 3H), 6.20 (d, J = 1.0 Hz, 1H), 2.29
1
3
(
3
according to the general procedure and isolated as a colorless 153.5, 150.1, 137.2, 131.6, 129.0, 128.6, 127.5, 127.2, 126.2,
oil (124 mg, 81% yield) after flash column chromatography 120.7, 108.7, 12.4. Spectral data are in agreement with
1
24
(
petroleum ether/ethyl acetate = 15/1). H NMR (400 MHz, literature values.
): δ/ppm= 7.48 (d, J = 8.2 Hz, 2H), 7.26 – 7.19 (m, 4H), General procedure for the preparation of 5-bromomethyl-
CDCl
3
7
.13 – 7.07 (m, 4H), 5.09 – 5.02 (m, 1H), 3.62 (d, J = 5.0 Hz, 2H), 4,5-dihydrofurans: The reaction was carried out in an open air
.44 (dd, J = 15.3, 10.4 Hz, 1H), 3.13 (dd, J = 15.3, 7.5 Hz, 1H). system. In a 20 mL sealed tube were added olefinic 1,3-
3
1
3
C NMR (100 MHz, CDCl
3
): δ/ppm = 193.2, 164.7, 138.7, 131.4, dicarbonyl compounds (0.5 mmol), PhI(OAc)
Cl (5 mL). The reaction mixture
+H] : m/z =306.1243, found: was stirred at room temperature for 12 h. CH Cl (10 mL) was
then added, and the mixture was washed with aqueous
Na . The combined organic layer was dried (Na SO ) and
was concentrated to give crude residue, which was purified by
2
(0.5 mmol), and
130.2, 129.6, 129.3, 128.9, 127.7, 111.7, 80.3, 54.5, 35.9. TMSBr (0.5 mmol) in dry CH
2
2
+
HRMS–ESI: calc. for [C18
H
15
N
3
O
2
2
2
3
06.1241.
-((4-Benzoyl-5-phenyl-2,3-dihydrofuran-2-
). Compound
2
2
S
2
O
3
2
4
yl)methyl)isoindoline-1,3-dione
(
4
4
prepared according to the general procedure and isolated as a flash column chromatography to give the corresponding
colorless solid (155 mg, 76% yield) after flash column products.
chromatography (petroleum ether/ethyl acetate = 60/1) ; mp = (5-(Bromomethyl)-2-phenyl-4,5-dihydrofuran-3-
1
32-134 °C. H NMR (400 MHz, CDCl
1
3
): δ/ppm= 7.99 – 7.85 (m, yl)(phenyl)methanone
(
7a). Compound 7a was prepared
4H), 7.77 (d, J = 8.4 Hz, 2H), 7.45 (d, J = 8.4 Hz, 2H), 7.22 – 7.15 according to the general procedure and isolated as a colorless
(
m, 3H), 7.11 – 7.03 (m, 3H), 5.21 – 7.17 (m, 1H), 4.30 (dd, J = oil (138 mg, 80% yield) after flash column chromatography
1
4.2, 9.0 Hz, 1H), 3.88 (dd, J = 14.2, 3.8 Hz, 1H), 3.58 (dd, J = (petroleum ether/ethyl acetate = 15/1). H NMR (400 MHz,
1
1
5.3, 10.3 Hz, 1H), 3.06 (dd, J = 15.3, 5.3 Hz, 1H). C NMR (100 CDCl
3
1
3
): δ/ppm= 7.47(d, J = 8.4 Hz, 2H), 7.27 – 7.18 (m, 4H),
3
MHz, CDCl ): δ/ppm= 193.3, 168.3, 165.4, 132.7, 132.0, 131.2, 7.11 (d, J = 7.2 Hz, 2H), 7.07 (d, J = 7.3 Hz, 2H), 5.13 – 5.16 (m,
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Journal Name
1
1
1
1
3
3
H), 3.71 (dd, J = 10.3, 5.5 Hz, 1H), 3.68 (dd, J = 10.3, 4.4 Hz,
1995, 36, 9019; (c)R. Antonioletti, F. D'Onofrio and B.
Rossi, Synth. Commun., 2001, 31 911; (d)R.
Antonioletti, S. Malancona and P. Bovicelli,
Tetrahedron, 2002, 58, 8825; (e)R. Antonioletti, S.
Malancona, F. Cattaruzza and P. Bovicelli, Tetrahedron,
,
H), 3.52 (dd, J = 15.4, 10.2 Hz, 1H), 3.21 (dd, J = 15.4, 7.0 Hz,
1
3
H). C NMR (100 MHz, CDCl
3
): δ/ppm= 193.2, 165.0, 138.8,
31.3, 130.1, 129.6, 129.3, 128.9, 127.7, 127.7, 111.7, 79.8,
+
7.5, 34.6. HRMS–ESI: calc. for [C18
43.0334, found: 343.0323.
2
H15BrO +H] : m/z =
2
003, 59, 1673.
E. Tang, X. Huang and W.-M. Xu, Tetrahedron, 2004, 60
963.
5
6
.
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,
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A. T. Zdvizhkov, A. O. Terent’ev, P. S. Radulov, R. A.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This study was supported by the Natural Science Foundation of
Jiangsu Province in China (BK20170439), Advanced Research
Foundation for Natural Science of Nantong University
(
16ZY15), Start-Up Funding for Research of Nantong University
17R40), Lager Instruments Open Foundation of Nantong
(
University (KFJN1853), and Science and Technology Plan
Projects of Nantong (MS12017023-7).
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| J. Name., 2012, 00, 1-3
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