A simple synthesis of fluoroalkyl substituted dihydrofurans by rhodium(II)-catalyzed 1,3-dipolar reactions

Article abstract of DOI:10.1016/S0040-4020(01)00183-1

The rhodium(II)-catalyzed reactions of ethyl 2-diazo-fluoroalkylacetoacetate 1 with vinyl ethers have been studied. The reactions of 1a-e with isobutyl vinyl ethers afforded dihydrofuroates 3a-e in good to excellent yields. Further transformation of the dihydrofuroates by acid-catalyzed alcohol elimination could give α-fluoroalkyl-β-furoates readily. Similarly, stable diazo compounds 1a and b reacted with cyclic vinyl ethers to give 1,3-dipolar cycloaddition products. Interestingly, only vinyl C-H insertion compound was obtained concerning the reaction of 1a with 2,5-dimethylfuran. The reaction mechanism was also discussed.

Full text of DOI:10.1016/S0040-4020(01)00183-1

TETRAHEDRON
Pergamon
Tetrahedron 57 %2001) 3383±3387
A simple synthesis of ¯uoroalkyl substituted dihydrofurans by
rhodiumꢀII)-catalyzed 1,3-dipolar reactions
Yanli Wang and Shizheng Zhup
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Lu, Shanghai 200032, People's Republic of China
Received 28 November 2000; revised 17 January 2001; accepted 8 February 2001
AbstractÐThe rhodium%II)-catalyzed reactions of ethyl 2-diazo-¯uoroalkylacetoacetate 1 with vinyl ethers have been studied. The
reactions of 1a±e with isobutyl vinyl ethers afforded dihydrofuroates 3a±e in good to excellent yields. Further transformation of the
dihydrofuroates by acid-catalyzed alcohol elimination could give a-¯uoroalkyl-b-furoates readily. Similarly, stable diazo compounds 1a
and b reacted with cyclic vinyl ethers to give 1,3-dipolar cycloaddition products. Interestingly, only vinyl C±H insertion compound was
obtained concerning the reaction of 1a with 2,5-dimethylfuran. The reaction mechanism was also discussed. q 2001 Elsevier Science Ltd.
All rights reserved.
1. Introduction
alkyl-containing molecules with high regio- and chemo-
selectivity.
Introduction of ¯uoroalkyl, especially tri¯uoromethyl
group, into organic molecules often results in dramatic
modi®cation of their physical and chemical properties as
well as their biological activity pro®le.1 In recent years, an
increasing need for ¯uorine-containing molecules both for
bio-active and for functional materials has prompted the
development of methods for the preparation of versatile
¯uoro-containing synthons. Although ¯uoroalkylation of
aromatics could be achieved rather readily with a variety
of methods, most notably by using tri¯uoromethyl copper
and related oganometallics, there also exists a lack of appro-
priate means for constructing various ¯uoroalkyl-containing
aliphatic compounds.2 During our development of new
versatile ¯uoroalkyl-containing building blocks, we were
attracted by the fact that an enormous number of trans-
formations %e.g. cyclopropanation, Wolff rearrangement,
dipolar addition, dimerization, oxidation and ylide forma-
tion followed by sigmatropic rearrangement) occurred with
the diazo carbonyl compounds, thus making them extremely
versatile reactants.3 Furthermore, the fast development of
new chiral catalysts for diazo compounds makes the diazo-
carbonyls more useful in organic synthesis.3 Due to the
former reasons, in line with our search for new ¯uoro-
alkyl-containing synthons, our strategy has been to prepare
appropriate ¯uoroalkyl-substituted 2-diazo-1,3-dicarbonyls
and utilize them as precursors for carbenoid intermediates,
which would enable us to synthesize a variety of ¯uoro-
We have recently reported the 1,3-dipolar reactions of ethyl
2-diazo-¯uoroalkylacetoacetate 1 with alkyl and aromatic
nitriles to synthesize a series of 5-¯uoroalkyl substituted
oxazoles.4 In an extension of this study, we now describe
herein further synthetic utility of 1 in its rhodium%II)-
catalyzed decomposition to produce, in situ, a ¯uoroalkyl-
containing carbenoids which reacts with alkyl vinyl ethers
to give ¯uoroalkyl-substituted dihydrofuroates. Since many
molecules which contain one or more of a furan or dihydro-
furan ring could be extensively used as agricultural and
industrial organic chemicals,5synthesis of a series of ¯uoro-
alkyl-substituted dihydrofurans would be meaningful.
2. Results and discussions
Diazo compounds 1a±e were easily prepared through
simple diazo transfer reaction from 1,3-dicarbonyls.4 1a±c
were stable enough for storage at room temperature while
1d and e would decompose partly during puri®cation and
storage,4 so they must be freshly prepared before use. The
different stability should be attributed to the different elec-
tron-withdrawing ability of the ¯uoroalkyl. Previously,
Hoffmann et al. have reported that decomposition of 1a in
the presence of a catalytic amount of Rh2%OAc)4 in ethyl
vinyl ether at 1008C in a sealed tube gave rise to ethyl
5-ethoxy-2-tri¯uoromethyl-4,5-dihydro-3-furancarboxylate
in 92% yield.6 However, the reactions of ¯uorinated 1,3-
dicarbonyls with vinyl ethers have not been studied compre-
hensively. To demonstrate the general applicability of this
approach, we have extended this rhodium-catalyzed reac-
tion to the ¯uoroalkyl-substituted 2-diazo-1,3-dicarbonyls
Keywords: ¯uorinated diazo compounds; vinyl ethers; carbenoids; rhodium
acetate; 1,3-dipolar cycloaddition.
p
Corresponding author. Tel.: 186-21-6416-3300/3525; fax: 186-21-
6416-6128; e-mail: zhusz@pub.sioc.ac.cn
0040±4020/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020%01)00183-1

3384
Y. Wang, S. Zhu / Tetrahedron 57 -2001) 3383±3387
and e gave slightly lower yields than those of 1a±c because
of their instability.
The dihydrofuroic esters 3a and c could be further converted
to a-¯uoroalkyl-b-furoates 4a and c by re¯uxing in carbon
tetrachloride catalyzed by sulfuric acid in 45 and 38%
yields, respectively %Scheme 1). Previously, the 3-ethoxy-
carbonyl-4-tri¯uoromethylfuran had been prepared through
two different methods from 2-ethoxycarbonyl-3-¯uoro-
alkyl-7-oxabicyclo[2.2.1]hepta-2,5-diene A %Fig. 1): one
method reported by Cambon et al. was by thermolysis at
1508C in xylene of the Diels±Alder cycloaddition of
compound A and tetracyclone in low yield %38%);8a the
other method reported by Tipping et al. was controlled
hydrogenation of compound A followed by the pyrolysis
of the product at 4008C to give 3-ethoxycarbonyl-4-
tri¯uoromethylfuran and ethylene,8b but these two
methods required extremely severe conditions and the
synthesis of 2-¯uoroalkyl-3-ethoxycarbonylfurans had
not been reported till now, so we had to provide a
practical and direct method for the preparation of
a-¯uoroalkyl-b-furoates.
Scheme 1.
Figure 1.
After success with the former reactions, we tried the
rhodium%II)-catalyzed reactions of 1a±c with cyclic enol
ethers, such as dihydrofuran and 3,4-dihydro-2H-pyran.
With 1a and b, only 1,3-dipolar cycloaddition products 6
and 8 were isolated after the diazo compound was heated in
neat cyclic enol ether in the presence of 1 mol% of
Rh2%OAc)4 for about 20 h at 608C. The reaction results are
summarized in Table 1. The structures of 6 and 8 assigned to
these substituted dihydrofurans were based on the 1H NMR
spectra, which were identical with those of their analogues
%Scheme 2).7 However, in the case of 1c, bromodi¯uoro-
methyl acetyl diazoacetate, only a sticky polymeric
substrate was obtained, but no expected cycloaddition
compound was detected. We assumed that with heating 1c
might undergo a radical polymerization reaction with cyclic
enol ethers initiated by the bromodi¯uoromethyl group.
Nevertheless, when some radical inhibitors, such as hydro-
quinone and nitrosobenzene, were added to the reaction
system to improve the reaction results, no encouraging
results were observed. Further attempts by conducting the
reaction in different solvents such as chloroform, hexane,
and benzene also failed.
Table 1. Reaction results of ¯uorinated diazo compounds 1 with vinyl
ethers
Entry
Diazo compounds
Vinyl ethers
Products
Yield %%)a
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1a
1b
1a
1b
2
2
2
2
2
5
5
7
7
2a
2b
2c
2d
2e
6a
6b
8a
8b
95
95
88
72
83
62
64
70
68
a
Isolated yields of pure compounds.
1a±e. Similarly, the reactions of 1a±e with isobutyl vinyl
ethers catalyzed by 0.5 mol% of Rh2%OAc)4 gave dihydro-
furoates 3a±e in good to excellent yields %72±95%). The
results indicated that different ¯uoroalkyl substituents on the
diazo compounds, either tri¯uoromethyl or per¯uoropentyl,
exhibited little effect on the reaction yields. Meanwhile, 1d
Scheme 2.

Y. Wang, S. Zhu / Tetrahedron 57 -2001) 3383±3387
3385
Scheme 3.
Earlier studies indicated that the copper- or rhodium%II)-
catalyzed reaction of furan or mono-substituted furan with
acyclic a-diazo carbonyl compounds underwent a cyclo-
propanation followed by a ring-opening process, to afford
the Z±E diene adducts.9 Meanwhile, the cyclic metal car-
benoids, such as 2-diazo-1,3-cyclohexanedione, showed a
distinct departure from the reactivity of acyclic diazo
compounds by producing 1,3-dipolar cycloadducts rather
than dienals. In addition, allylic or vinyl C±H insertion
products were also detected by the reactions of a cyclic
carbenoid with 2-methylfuran.10 However, we observed a
different result in our experiments. After 1a was re¯uxed
with 5 equiv. of 2,5-dimethylfuran in benzene or hexane in
the presence of 1 mol% of Rh2%OAc)4 for 14 h, the only
product isolated was 3-position vinyl C±H insertion product
10 %Scheme 3).
Scheme 4.
groups in the 1H NMR spectrum, suggesting an intra-
molecular interaction of one of its OH groups with the
ethoxycarbonyl group, as shown in Fig. 2.
The structure assigned as 10 was based on its analytical and
spectroscopic data: 19F NMR spectrum, 1H NMR spectrum,
IR absorptions and the electron impact mass spectrum.
Firstly, the 19F NMR spectrum exhibited a single peak at
15.9 ppm %with TFA as external standard, and with up®eld
positive), which suggested the signal of a hydrated tri¯uoro-
acetyl.11 Secondly, the peaks of chemical shift 4.79 and 4.65
The mechanism of 1,3-dipolar ketocarbene addition with
vinyl ethers had been represented as initial cyclopropana-
tion followed by catalytic rearrangement.12 However, we
were prone to another interpretation of the accumulated
results outlined in Scheme 4 to accommodate our experi-
mental results.13 Charge development in the metal
carbenoid derived from 1, followed by addition to the
electron-rich alkenes, afforded a reaction intermediate 11
capable of reacting with the nucleophilic carbonyl oxygen
to form dihydrofurans %path a) or by forming vinyl C±H
insertion products by a 1,2-hydrogen transfer reaction
%path b). According to this mechanism, when the carbenoid
reacted with 2,5-dimethylfuran, the hydrogen transfer
surpassed the dihydrofurans formation due to the steric
factors, so only vinyl C±H insertion product 10 was yielded.
1
in the H NMR were easily exchanged by D2O, which
further suggested the presence of an active hydroxyl
group. In addition, IR spectra also showed strong hydroxyl
absorption at 3376 cm21, but no carbonyl groupabsorption
was detected. Another proof was that the electron impact
mass spectrum showed a molecular ion peak at 296, which
matched with the molecular mass of 10 exactly. Moreover,
there were no vinyl proton signals in the 1H NMR spectrum,
but only a distinctive peak of furan proton at chemical shift
6.03, which further con®rmed the structure of 10 as vinyl
hydrogen insertion product of carbenoid from 1a to 2,5-
dimethylfuran followed by hydration of the tri¯uoroacetyl.
Previously, some aryltri¯uoromethyl ketone was reported to
form stable gem-diols, both for the inductive effects of the
tri¯uoromethyl, and also for the intramolecular H-bond
formation.11 The stable 10 showed two nonequivalent OH
3. Conclusions
In this paper, we have studied the 1,3-dipolar reaction of
2-¯uoroalkyl-substituted 1,3-dicarbonyls with vinyl ethers
to synthesize a series of ¯uoroalkyl-containing dihydrofuro-
ates. Moreover, ethyl 2-¯uoroalkyl-3-furoates could be
obtained easily by the acid-catalyzed alcohol elimination.
Since a very important derivatization in the synthesis of
biological active compounds consists of the introduction
of per¯uoroalkyl, especially tri¯uoroalkyl group, these
easily available ¯uoroalkyl-containing diazo compounds
will play an important role in the synthesis of ¯uoroalkyl-
containing heterocycles.
Figure 2.

3386
Y. Wang, S. Zhu / Tetrahedron 57 -2001) 3383±3387
4. Experimental
%CDCl3): 5.65 %1H, dd, Jˆ7.2, 2.7 Hz), 4.22 %2H, q,
Jˆ7.0 Hz), 3.33±3.64 %2H, m), 2.89±3.25 %2H, m), 1.89
%1H, m), 1.30 %3H, t, Jˆ7.0 Hz), 0.90±0.96 %6H, m). dF
%CDCl3): 224.3 %s). nmax %neat)/cm21: 1714s, 1651m,
1331±1152vs. m/z 299 %M1, 68.88), 254 %M12OEt,
17.06), 225 %M1212CO2Et, 67.55), 213 %M12CF2Cl,
80.31), 57 %C4H91, 100). %Found: C, 48.27; H, 5.91%.
Calcd for C12H17ClO4F2: C, 48.24; H, 5.70%.)
4.1. General
Melting points were measured in Temp-Melt apparatus. 1H
and 19F NMR spectra were recorded on Varian-360L or
Bruker AM-300 instruments with Me4Si and TFA %with
up®eld positive) as internal and external standards, respec-
tively. NMR spectra were recorded in chloroform-d unless
otherwise stated. IR spectra were obtained with a Perkin
Elmer 983G spectrophotometer using KBr disks of the
compounds. Low mass spectrum was obtained with HP
5989a instrument. Elemental analyses were performed by
this Institute. All reactions as well as column chromato-
graphy were monitored routinely with the aid of TLC or
19F NMR spectroscopy. Benzene and hexane were dried
over sodium wire; reagents were puri®ed before use according
to the standard methods. 1a±c were prepared by diazo transfer
reaction of ethyl ¯uoroalkylacetoacetate with tosyl azide. 1d,e
were prepared according to the procedures reported.4
4.3.3. Ethyl 2-bromodi¯uoromethyl-5-iso-butoxy-4,5-
dihydro-3-furancarboxylate 3c. Colorless liquid: dH
%CDCl3): 5.65 %1H, dd, Jˆ7.2, 2.7 Hz), 4.21 %2H, q,
Jˆ7.0 Hz), 3.26±3.69 %2H, m), 2.90±3.25 %2H, m), 1.91
%1H, m), 1.29 %3H, t, Jˆ7.0 Hz), 0.87±0.92 %6H, m). dF
%CDCl3): 228.3 %s). nmax %neat)/cm21: 1715s, 1649m,
1373±1154vs. m/z 345/343 %M112/M1, 4.03/4.75), 298
%M12OEt, 3.51), 269 %M1212CO2Et, 3.43), 57 %C4H9
,
1
100). %Found: C, 42.33; H, 4.90%. Calcd for C12H17O4F2Br:
C, 41.98; H 4.96%.)
4.3.4. Ethyl 2-ꢀ3-chloro-1,1,2,2,3,3-hexa¯uoropropyl)-5-
iso-butoxy-4,5-dihydro-3-furancarboxylate 3d. Colorless
liquid: dH %CDCl3): 5.63 %1H, dd, Jˆ7.2, 2.4 Hz), 4.24 %2H,
q, Jˆ7.0 Hz), 3.24±3.62 %2H, m), 2.9±3.2 %2H, m), 1.95
%1H, m), 1.25 %3H, t, Jˆ7.0 Hz), 0.82±0.94 %6H, m). dF
4.2. General procedure for the reaction of per¯uoro-
alkanesulfonyl azide with ethyl per¯uoroalkylaceto-
acetate to prepare 2-diazo ¯uoroalkylacetoacetates 1d,e
%CDCl3): 210.1 %t), 34.2 %m), 43.1 %m). nmax %neat)/cm21
:
A solution of ethyl ¯uoroalkylacetoacetate %ca. 1 mmol,
2 ml) and an equimolar amount of poly¯uoroalkanesulfonyl
azide in anhydrous diethyl ether at 08C was treated dropwise
with an equimolar amount of TEA. TLC analysis showed
that the reaction was complete within 0.5 h. The solvent was
evaporated and the residue was chromatographed on a silica
column using petroleum ether±ether %6:1) as eluant; two
products, ethyl 2-diazo-3-oxo-¯uoroalkylacetoacetate and
ethyl diazoacetate, were obtained, evaporated under vacuum
and gave the pure compounds 1d,e as light yellow oil.
1708s, 1654m, 1358±1122vs. m/z 399 %M1, 7.02), 325
1
%M1212CO2Et, 23.81), 267 %M1212CO2Et±C4H10
,
20.05), 57 %C4H91, 100), 41 %C3H51, 45.13). %Found: C,
42.46; H, 4.67%. Calcd for C14H17ClO4F6: C, 42.16; H,
4.27%.)
4.3.5. Ethyl 2-per¯uoropentyl-5-iso-butoxy-2,5-dihydro-
3-furancarboxylate 3e. Colorless liquid: dH %CDCl3): 5.62
%1H, dd, Jˆ7.2, 2.0 Hz), 4.20 %2H, q, Jˆ7.0 Hz), 3.26±3.56
%2H, m), 2.9±3.2 %2H, m), 1.86%1H, m), 1.24 %3H, t,
Jˆ7.0 Hz), 0.89±0.93 %6H, m). dF %CDCl3): 3.40 %s), 35.2
%m), 55.2 %m), 59.0 %m). nmax %neat)/cm21: 1705s, 1650m,
4.3. General procedure for the reactions of diazo
compounds with isobutyl vinyl ether
1350±1153vs. m/z 482 %M1, 1.06), 436 %M1112OEt,
7.92), 409 %M12CO2Et, 5.23), 351 %M12CO2Et±C4H9
,
1
A solution of 1a %0.420 g, 2.0 mmol) in isobutylvinyl ether
%2 ml) was added over a 10 min period to a stirring, re¯ux-
ing solution of Rh2%OAc)4 %4 mg, 1 mmol) in isobutylvinyl
ether %2 ml), and the mixture then heated for another 20 min
until 19F NMR indicated completion of the reaction. The
reaction mixture was concentrated under vacuum and chro-
matographed. Elution with petroleum ether±diethyl ether
%15:1 v:v) yielded the ester 3a %0.536 g, 1.9 mmol, 95%)
as a colorless liquid.
44.34), 57 %C4H91, 100), 41 %C3H51, 28.93). %Found: C,
39.81; H, 3.55%. Calcd for C16H17O4F11: C, 39.83; H 3.53%.)
4.3.6. Ethyl 3-tri¯uoromethyl-2-furancarboxylate 4a. A
solution of 3a %0.8 g, 2.8 mmol) in carbon tetrachloride
%5 ml) was added dropwise to a stirring mixture of concen-
trated sulfuric acid %0.2 g, 4 mmol) in carbon tetrachloride
%65 ml) at re¯ux over a 3 h period, and the heating was
continued for 2 h. Then the mixture was washed with
water, 5% sodium bicarbonate solution and water, again
dried with Na2SO4, and evaporated %temperature of water
bath ,308C). Chromatography of the residue on silica gel
and elution with 30:1 petroleum ether %30±608C)±ether
produced a light yellow liquid 4a %0.239 g, 1.1 mmol) in
41% yield. dH %CDCl3): 7.40 %1H, m), 6.70 %2H, m), 4.25
%2H, q, Jˆ7.0 Hz), 1.23 %3H, t, Jˆ7.0 Hz). dF %CDCl3):
216.2 %s). nmax %neat)/cm21: 1730s, 1601m, 1324±1149vs.
m/z 208 %M1, 1.06), 180 %M1112Et, 43.18), 163
%M1112OEt, 100), 135 %M1112CO2Et, 10.33). %Found:
208.03147. HRMS calcd for C8H7O3F3: 208.03310.)
4.3.1. Ethyl 2-tri¯uoromethyl-5-iso-butoxy-4,5-dihydro-
3-furancarboxylate 3a. Colorless liquid: dH %CDCl3): 5.65
%1H, dd, Jˆ7.3, 3.0 Hz), 4.23 %2H, q, Jˆ7.0 Hz), 3.24±3.61
%2H, m), 2.90±3.15 %2H, m), 1.90 %1H, m), 1.28 %3H, t,
Jˆ7.0 Hz), 0.93±0.97 %6H, m). dF %CDCl3): 212.0 %s).
nmax %neat)/cm21: 1715s, 1661m, 1377±1153vs. m/z 282
%M1, 0.66) 236 %M12HOEt, 7.50), 197 %M12C4H9OC1,
1
27.95),
181
%M12C4H9CO2
,
15.58),
151
%M1112CO2Et±C4H91, 12.91), 69 %CF31, 3.94), 57
%C4H91, 100). %Found: C, 50.88; H, 6.20%. Calcd for
C12H17O4F3: C, 51.06; H, 6.03%.)
4.3.7. Ethyl 3-bromodi¯uoromethyl-2-furancarboxylate
4c. The same reaction and workupon
4.3.2. Ethyl 2-chlorodi¯uoromethyl-5-iso-butoxy-4,5-
dihydro-3-furancarboxylate 3b. Colorless liquid: dH
3c %0.6 g,

Y. Wang, S. Zhu / Tetrahedron 57 -2001) 3383±3387
3387
1.7 mmol) yielded a light yellow liquid 4c %0.175 g,
0.65 mmol) in 38% yield. dH %CDCl3): 7.35 %1H, m), 6.73
%2H, m), 4.25 %2H, q, Jˆ7.0 Hz), 1.25 %3H, t, Jˆ7.0 Hz). dF
%CDCl3): 216.2 %s). nmax %neat)/cm21: 1724s, 1592m,
1498m, 1313±1167vs. m/z 270/268 %M112/M1, 10.95),
223 %M1212OEt, 15.44), 195 %M1212CO2Et, 8.64),
189 %M12Br, 60.22), 161 %M1112C2H5±Br, 71.02), 141
%M1122BrCF2, 100). %Found: C, 35.57; H, 2.79%. Calcd
for C8H7BrO3F2: C, 35.70; H, 2.60%.)
46.85; H, 4.66%. Calcd for C11H13O4F2Cl: C, 46.72; H,
4.60%.)
4.4.5. Ethyl 2-ꢀ2,2,2-tri¯uoro-1,1-dihydroxyethyl)-3-
furanacetate 10. A solution of 1a %0.210 g, 1 mmol) in
benzene %1.5 ml) was added over a 6 h period to a stirring,
re¯uxing solution of Rh2%OAc)4 %4 mg, 1 mmol) and 2,5-
dimethylfuran %1.5 ml), and the stirring continued for
another 13 h. The mixture was concentrated. Chromato-
graphy and elution with 4:1 petroleum ether±ethyl acetate
gave the ester 10 %0.157 g, 0.62 mmol, 62%) as a light
yellow liquid. dH %CDCl3): 6.03 %1H, s), 4.79 %1H, s), 4.65
%1H, s), 4.32 %1H, q, Jˆ7.0 Hz), 3.90 %1H, s), 2.25 %6H, s),
1.25 %3H, t, Jˆ7.0 Hz). dF %CDCl3): 15.9 %s). nmax %KBr)/
cm21: 3376vs, 1699s, 1466m, 1438m, 1375±1020vs. m/z
296 %M1, 8.18), 279 %M1112H2O, 29.51), 205
%M12H2O±CO2Et, 23.82), 182 %M12CF3CO2H, 25.03).
%Found: C, 48.81; H, 5.11%. Calcd for C12H15O5F3: C,
48.65; H, 5.07%.)
4.4. General procedure for the reactions of diazo
compounds with cyclic vinyl ethers
A solution of 1a %0.210 g, 1 mmol) in 2,3-dihydrofuran
%1.5 ml) was added over a 6 h period to a stirring, re¯uxing
solution of Rh2%OAc)4 %4 mg, 1 mmol) and 2,3-dihydro-
furan %1.5 ml), and the stirring continued for another 13 h.
Then the mixture was concentrated. Chromatography and
elution with petroleum ether±ethyl acetate %8:1 v:v) gave
the ester 6a %157 mg, 0.62 mmol, 62%) as a light yellow
liquid.
Acknowledgements
4.4.1. Ethyl 3-tri¯uoromethyl-2,8-dioxabicyclo[3.3.0]oct-
3-ene-4-carboxylate 6a. dH %CDCl3): 6.25 %1H, d, Jˆ
6.0 Hz), 4.23 %2H, q, Jˆ7.0 Hz), 3.90±4.20 %2H, m,
OCH2), 3.75 %1H, m), 2.21 %2H, m), 1.30 %3H, t, Jˆ
7.0 Hz). dF %CDCl3): 213.3 %s). nmax %neat)/cm21: 1715s,
1652m, 1381±1161vs. m/z 252 %M1, 6.26), 206
%M1212OEt, 100), 179 %M12CO2Et, 28.81), 109
%M1212CO2Et±CF3, 33.24), 81 %C2F31, 23.94), 69
%CF31, 23.67). %Found: C, 47.64; H, 4.53%. Calcd for
C10H11O4F3: C, 47.62; H, 4.37%.)
The authors thank the National Natural Science Foundation
of China %NNSFC) %No. 20072049) and the Innovation
Foundation of the Chinese Academy of Sciences for
®nancial support.
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[3.3.0]oct-3-ene-4-carboxylate 6b. dH %CDCl3): 6.24 %1H,
dd, Jˆ6.0, 3.0 Hz), 4.25 %2H, q, Jˆ7.0 Hz), 3.91±4.16 %2H,
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Jˆ7.0 Hz). dF %CDCl3): 224.0 %s). nmax %neat)/cm21
:
17145s, 1635m, 1319±1144vs. m/z 269 %M1, 40.73), 224
%M12OEt, 40.19), 183 %M12CF2Cl, 28.81), 155
%M12CF2Cl±OEt, 18.82), 109 %M1212CF2Cl±CO2Et,
28.26). %Found: C, 44.89; H, 4.36%. Calcd for
C10H11O4F2Cl: C, 44.69; H, 4.10%.)
4. Wang, Y. L.; Zhu, S. Z. J. Fluorine Chem. 2000, 103, 139±144.
5. Katritzky, A. R.; Rees, C. W.; Scriven, E. F. V. In Compre-
hensive Heterocyclic Chemistry, Vol. II; Pergamon: Oxford,
1996; pp. 395±413.
6. Hoffmann, M. G.; Wenkert, E. Tetrahedron 1993, 49, 1057±
1062.
4.4.3. Ethyl 3-tri¯uoromethyl-2,9-dioxabicyclo[4.3.0]-
non-3-ene-4-carboxylate 8a. dH %CDCl3): 5.94 %1H, d,
Jˆ7.5 Hz), 4.12 %2H, q, Jˆ7.0 Hz), 3.85 %2H, m), 3.05±
3.18 %1H, m), 1.6±2.1 %4H, m), 1.25 %3H, t, Jˆ7.0 Hz). dF
%CDCl3): 212.6 %s). nmax %neat)/cm21:1715s, 1647m, 1377±
1111vs. m/z 266 %M1, 9.65), 220 %M1212OEt, 100), 192
%M1212CO2Et, 65.86), 169 %M1212CO2Et±CF3, 66.75),
69 %CF31, 39). %Found: C, 49.34; H, 4.78%. Calcd for
C11H13O4F3: C, 49.62; H, 4.89%.
7. Wenkert, E.; Ananthanaravan, T. P.; Ferreira, V. F.; Hoffmann,
M. G.; Kim, H. J. Org. Chem. 1990, 55, 4975±4975.
8. %a) Nezis, A.; Fagn, J.; Cambon, A. J. Fluorine Chem. 1991,
53, 297±299. %b) Barlow, M.; Suliman, G.; Nadia, N. E.;
Tipping, A. E. J. Fluorine Chem. 1995, 70 %1), 59±69.
9. Ye, T.; McKervey, M. A. Chem. Rev. 1994, 94, 1124±1125.
10. Pirrung, M. C.; Zhang, J. C.; Lackey, K.; Sternbach, D. D.;
Brown, F. J. Org. Chem. 1995, 60, 2112±2124.
11. %a) Salvador, R. L.; Saucier, M. T. Tetrahedron 1971, 27,
1221. %b) Guthrie, J. P. Can. J. Chem. 1975, 53, 898.
12. %a) Alonso, M. E.; Jano, P.; Hernandez, M. I.; Greenberg, R. S.;
Wenkert, E. J. Org. Chem. 1983, 48, 3047±3050. %b) Alonso,
M. E.; Morales, A.; Chitty, A. W. J. Org. Chem. 1982, 47,
3747±3754.
4.4.4. Ethyl 3-chlorodi¯uoromethyl-2,9-dioxabicyclo-
[4.3.0]non-3-ene-4-carboxylate 8b. dH %CDCl3): 5.94
%1H, d, Jˆ7.5 Hz), 4.25 %2H, q, Jˆ7.0 Hz), 3.88 %2H, m),
3.17±3.30 %1H, m), 1.62±2.13 %4H, m), 1.29 %3H, t,
Jˆ7.0 Hz). dF %CDCl3): 226.9 %s). nmax %neat)/cm21
:
13. Doyle, M. P.; McKervey, M. A.; Ye, T. Modern Catalytic
Methods for Organic Synthesis with Diazo Compounds,
From Cyclopropanes to Ylides; Wiley: New York, 1998; pp.
206 %and references therein).
1715s, 1647m, 1377±1111vs. m/z 285/283 %M112/M1,
18.52/6.07), 238 %M12OEt, 38.00), 211 %M1112CO2Et,
11.98), 123 %M1212CO2Et±CF2Cl, 14.39. %Found: C,