Convenient synthesis of 3-fluoro-4,5-diphenylfuran-2(5H)-one from benzoin ethers. Novel and efficient Z-E isomerisation and cyclisation of 2-fluoroalkenoate precursors, substitution of vinylic fluorine

Article abstract of DOI:10.1016/j.jfluchem.2006.05.012

Mixtures of ethyl (E)- and (Z)-4-alkoxy-2-fluoro-3,4-diphenylbut-2-enoates (6-8) prepared from benzoin ethers and ethyl 2-(diethoxyphosphoryl)-2-fluoroacetate were transformed in high yields to the target 3-fluoro-4,5-diphenylfuran-2(5H)-one (14) using bromine in tetrachloromethane at room temperature. The non-cyclisable Z-isomers 6b-8b were gradually isomerised to the cyclisable E-isomers 6a-8a during the process. The reaction of the (E)-butenoates 6a-8a with boron trifluoride led to furanone 14, while in Z-isomers 6b-8b both alkoxy group and vinylic fluorine were substituted with bromine during the reaction. Mechanisms for both complex reactions have been proposed. Furanone 14 was transformed to 2-[tert-butyl(dimethyl)silyloxy]-3-fluoro-4,5-diphenylfuran (18) as a novel building block.

Full text of DOI:10.1016/j.jfluchem.2006.05.012

Journal of Fluorine Chemistry 127 (2006) 1390–1397
www.elsevier.com/locate/fluor
Convenient synthesis of 3-fluoro-4,5-diphenylfuran-2(5H)-one
from benzoin ethers
Novel and efficient Z–E isomerisation and cyclisation of
2-fluoroalkenoate precursors, substitution of vinylic fluorine§
*
´ˇ ˇ
Karel Pomeisl, Jaroslav Kvıcala, Oldrich Paleta
´
Department of Organic Chemistry, Prague Institute of Chemical Technology, Technicka 5, 16628 Prague 6, Czech Republic
Received 17 April 2006; received in revised form 11 May 2006; accepted 16 May 2006
Available online 22 May 2006
ˇ
Dedicated to Prof. Boris Zemva on the occasion of his receiving the 2006 ACS Award for Creative Work in Fluorine Chemistry.
Abstract
Mixtures of ethyl (E)- and (Z)-4-alkoxy-2-fluoro-3,4-diphenylbut-2-enoates (6–8) prepared from benzoin ethers and ethyl 2-(diethoxypho-
sphoryl)-2-fluoroacetate were transformed in high yields to the target 3-fluoro-4,5-diphenylfuran-2(5H)-one (14) using bromine in tetrachlor-
omethane at room temperature. The non-cyclisable Z-isomers 6b–8b were gradually isomerised to the cyclisable E-isomers 6a–8a during the
process. The reaction of the (E)-butenoates 6a–8a with boron trifluoride led to furanone 14, while in Z-isomers 6b–8b both alkoxy group and
vinylic fluorine were substituted with bromine during the reaction. Mechanisms for both complex reactions have been proposed. Furanone 14 was
transformed to 2-[tert-butyl(dimethyl)silyloxy]-3-fluoro-4,5-diphenylfuran (18) as a novel building block.
# 2006 Elsevier B.V. All rights reserved.
Keywords: 2-(Diethoxyphosphoryl)-2-fluoroacetate; 4-Alkoxy-2-fluoro-3,4-diphenylbut-2-enoates; 3-Fluoro-4,5-diphenylfuran-2(5H)-one; 2-(Trialkylsilyloxy)-3-
fluoro-4,5-diphenylfuran; Z–E isomerisations; Vinylic fluorine substitution; Ether bond cleavage
1. Introduction
We have also found significant cytotoxic properties for presently
prepared 4,5-dialkylated 3-fluorofuran-2(5H)-ones [10].
Our research on fluorine-containing substituted 3-fluoro-
furan-2(5H)-ones (but-2-enolides) has been stimulated by the
fact that but-2-enolide ring is a component of several classes of
bioactive natural compounds and that the fluoro substituents,
such as F, CF3, or OCF3 are generally powerful modifiers of
chemical and biological properties of organic compounds [1,2].
Some natural but-2-enolides exhibit antitumor activity [3],
cytotoxic properties to human tumor cells [4], intestinal
carcinogenicity inhibition [5], significant activity against
lymphotic leukemia system [6], and also HIV-1 protease-
inhibitor activity [7]. Some analogues of natural butenolides
provedtohaveantitumor[8] andantitumor-promoteractivity[9].
As several phenyl groups in one structure can be powerful
pharmacophores, e.g. in antitumor drugs Tamoxiphen and
Droloxifene or coronary vasodilator Amotriphene, we directed
our research to the synthesis of 3-fluoro-4,5-diphenylfuran-
2(5H)-one (14) which is presented in this publication. The
formation of the 3-fluorofuran-2(5H)-one moiety has usually
been accomplished by a deprotection of the hydroxyl group
followed by the usually spontaneous closure of the lactone ring
in the intermediate 2-fluoro-4-hydroxyalk-2-enoates [11]. A
simple synthetic pathway leading to these hydroxyalkenoates
has appeared to be the Horner–Wadsworth–Emmons (HWE)
synthesis involving the reaction of 2-(diethoxyphosphoryl)-2-
fluoroacetate with an appropriate oxo compound [11,12].
This reaction that we published recently [11(c)] afforded
4-substituted 2-fluorobut-2-enoate with the desired configura-
tion and we therefore followed this route. The intermediate
4-hydroxybutenoates were released by the cleavage of the ether
functionality in 4-alkoxybutenoates: boron tribromide or
§
Part 10 in the series, Fluorinated butanolides and butenolides. Part 9,
J. Fluorine Chem. 113 (2002) 177.
* Corresponding author. Fax: +420 2 2431 1082.
E-mail address: oldrich.paleta@vscht.cz (O. Paleta).
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2006.05.012

K. Pomeisl et al. / Journal of Fluorine Chemistry 127 (2006) 1390–1397
1391
Table 1
Reactions of 1–3 with fluorinated HWE reagenta
radical E–Z isomerisation followed by in situ-formed-hydro-
bromide cleavage were applied to (E)- and (Z)-4-alkoxy-2-
fluoro-3,4-diphenylbut-2-enoates. We would like to report here
about these attempts to obtain the target furanone 14.
Starting compound
Productb
E (% rel)
Z (%)
Yield (%)
1
2
3
4
5
6a (27)
7a (6)
8a (5)
9a (12)
12a (82)
6b (73)
7b (94)
8b (95)
9b (88)
12b (18)
60
77
80
40
65
2. Results and discussion
For the synthesis of the target furanone 14, we have followed
the above described methodology exploiting HWE reaction
synthetic sequence and hydrolysis of the resulting intermediate.
Benzoin ethers 1–3, which are industrially available, benzoin
acetate (4) and 2-methoxycyclohexan-1-one (5) were selected
as the starting oxo compounds. The precursor of the HWE
reagent, ethyl 2-(diethoxyphosphoryl)-2-fluoroacetate, was
prepared from chlorotrifluoroethene using our optimised
synthetic protocol [11(d)]. The reaction of benzoin derivatives
1–4 and the compound 5 with the HWE reagent prepared in situ
from ethyl 2-(diethoxyphosphoryl)-2-fluoroacetate [11(d)]
(Scheme 1) afforded the corresponding products 6–9 and 12
as E–Z mixtures (Table 1). Unfortunately, the undesired Z-
isomers (6b–9b) were highly prevailing in the stereoisomeric
mixtures in amounts of 72–96%. Only 2-methoxycyclohex-
anone afforded the desired E-isomer 12a in a good yield
(Table 1). To compare the stereochemical results, we carried out
an analogous HWE reaction of benzoin ethers 1–2 and 5 with a
non-fluorinated HWE reagent prepared from ethyl 2-(diethox-
yphosphoryl)acetate. The stereochemical results for the starting
1 and 2, which afforded cyclisable Z-isomers 10a and 11a,
respectively, in relative amount of 77–79% were reverse to
those for fluorinated HWE reagent (products 6–7, Table 1).
The results show that fluorine atom at the place of hydrogen
atom in 2-(diethoxyphosphoryl)-2-fluoroacetate can influence
dramatically the stereochemistry of the HWE reaction.
a
Ratio of products E/Z Ph(OR)CH–C(Ph) CH–COOEt using (EtO)2P(O)-
CH2CO2Et was as follows—1: 10a/10b = 88/12; 2: 11a/11b = 76/24; 5: 13a/
13b = 65/35.
The E/Z ratio was determined by 19F NMR spectroscopy.
b
2.1. Cleavage of the ether bond by boron tribromide
The transformation of 4-alkoxybut-2-enoates 6a–8a and 12a
to the target fluorofuranone 14 via intermediate 4-hydroxybut-
2-enoate 17e (Schemes 2 and 3) required a cleavage of the ether
bond. The reaction of fluorinated 4-alkoxybutenoates in boiling
hydrobromic acid was unsuccessful [13]. Therefore, we applied
boron tribromide as a more efficient agent that has led to good
results in the preparation of 2,3-difluorobuten-2-olides [13].
Our cyclisable (E)-butenoates 6a–8a were transformed to
furanone 14, while non-cyclisable Z-isomers 6b–8b were
transformed to dibromobutenoate 15 (Scheme 2). It could be
expected that the yields of furanone 14 would correspond to the
content of the E-butenoates 6a–8a in the starting E–Z mixtures
(Tables 1 and 2). However, the yields of furanone 14 were
Scheme 1.
Scheme 2.

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K. Pomeisl et al. / Journal of Fluorine Chemistry 127 (2006) 1390–1397
Scheme 3.
higher (Table 2). It can be deduced that the starting non-
cyclisable Z-isomers 6b–8b were partly isomerised to
cyclisable E-isomers during the process. A proposed mechan-
ism is depicted in Scheme 3 where intermediate 17a is
isomerised by a ionic process to 17c via carbanion 17b.
Cyclohexylidenefluoroacetate 12a did not cyclise probably
due to the rigidity of the bond system around the double bond.
Instead, the methoxy group was substituted with bromine atom
during the process. The stereoisomer 12b reacted analogously.
The ratio of the products 16a/16b was the same as that in the
starting compound 12, i.e. no isomerisation occurred.
During the formation of dibromobutenoate 15, the a-
fluorine atom attached to the double bond was substituted with
a bromine atom (Schemes 2 and 3) in Z-isomers 6b–8b. A
driving force for this substitution of vinylic fluorine can be
attributed to the high affinity of the boron(III) central atom to
fluoride ion [14]. The substitution could proceed as proposed in
Scheme 3 (intermediates 17f and 17g). This vinylic substitution
did not occur in the rigid structures 12a and b (Scheme 2).
Acetoxybutenoate 9 as a mixture of E–Z isomers was
transformed to the target furanone 14 by basic hydrolysis of
both ester groups and subsequent acidification. The yield of the
product 14 corresponded exactly the content of the E-isomer in
the starting mixture (12%), i.e. no isomerisation ocurred during
the hydrolysis.
2.2. E–Z isomerisation and cleavage of the ether bond
The unfavourable Z-configuration of the products 6b–8b can
potentially be transformed to the E-configuration (6a–8a),
suitable for cyclisation (Scheme 1), by an appropriate Z–E
isomerisation,during which the ester group and more bulky
hydroxyalkyl group are placed at the mutual cis-position (4a–
6a). Such Z-to-E-isomerisation of some 2-alkenoates was
carried out by several methods including photochemical and
thermal processes. Photochemical Z–E isomerisations have
been applied to cinnamates [15] and Z-2-fluoro-3-phenylprop-
2-enoate [16]. The photosensitised Z–E isomerisation of our
HWE precursors 6–8 using UV light of the wavelength above
280 nm (selected by a Simax1 filter) or in the presence of
acetone as a sensitiser was only partly efficient affording the
desired E-isomers in maximum relative yield only 20%.
Irradiation of the E–Z mixtures of the precursors 6–8 using
unfiltered UV light from a medium-pressure mercury lamp led
to a complex mixture of products.
Thermal isomerisation can be carried out by a halogen when
it forms an unstable adduct-radical with a substrate and does not
add to the double bond of an olefin. Iodine was usually used as a
catalyst in the isomerisation reaction affording thermodyna-
mically more stable non-fluorinated E-isomers [17]. An E-to-Z-
isomerisation was also observed using iodine [18]. Unfortu-
nately, iodine has appeared to be inefficient for the isomerisa-
tion of 6–8 even under irradiation with visible light. Bromine
usually adds to the double bond in fluorinated alk-2-enoates
[19]. On the other hand, bromine in tetrachloromethane has also
been used as a catalyst for the E–Z isomerisation of ethyl
2-fluoro-3-phenylprop-2-enoate resulting in the mixture con-
taining major thermodynamic Z-isomer [20]. The application of
this last protocol to the precursors 6–8 has led to surprising
Table 2
The results of the reaction of a-fluoroalkenoates 6–8 and 12 with BBr3
Starting compound
BBr3
Yield (%/mmol)
(mmol)
A
Content of
isomers (%)
14
15
16a
1
a (E)
b (Z)
positive results: the monitoring of the reaction mixture by H
and/or 19F NMR showed that in the mixture appeared the target
furanone (but-2-enolide) 14 which amount was increasing with
reaction time. This result confirms that in the reaction mixture a
series of reactions including Z–E isomerisation occurred
(Scheme 4). The conversion of 6 or 8 to the target furanone
6
7
8
Me
Et
27
6
73
94
95
18
1.3
1.3
1.3
2.8
44/0.44
17/0.17
11/0.11
–
14/0.14
11/0.11
31/0.31
–
–
–
iPr
Me
5
–
12
82
77/0.77
a
The ratio of Z/E-isomers 16a and 16b (82:18) was determined by 19F NMR.

K. Pomeisl et al. / Journal of Fluorine Chemistry 127 (2006) 1390–1397
1393
only 4-chloro-3-fluoro-2-(trimethylsilyloxy)furan has been
reported [23], but it appeared to be unstable above ca.
À30 8C. We carried out the transformation of furanone 14 to
furan 18 by applying the usual protocol [23], viz. low
temperature lithiation and subsequent silylation, to obtain furan
18 in a yield of 72%. It was stable at 0–4 8C (check by 19F NMR
for 2 weeks).
3. Experimental
3.1. General comments
Melting points were determined on a Kofler block and are
uncorrected. NMR spectra were measured on a Bruker 400
AM (FT, 19F at 376.5 MHz) and Gemini 300 (1H at
Scheme 4.
1
300.1 MHz and 13C at 75.5 MHz using H decoupling) in
14 was 100% for these esters and 64% for ester 7. Isolated
yields of 14 were dependent on the isolation technique and were
in the range of 48–85%. The reactions were carried out at room
temperature in order to avoid bromination as a side reaction.
A proposal of the potential mechanism for the above
transformation is depicted in Scheme 5. The Z–E isomerisation
can occur via adduct-radicals 19a and 19b, but could also
proceed analogously in the hydroxy ester (21) stage. The
isomerisation is followed by deprotection of the hydroxy group
by the cleavage of the ether bond caused by hydrogen bromide
(intermediate 20) and finally by cyclisation of the intermediate
hydroxy ester 21. Hydrogen bromide is formed by the reaction
of bromine with ethanol [21] liberated during the mechanism.
Fluorinated furanone 14 can be transformed to the
corresponding 3-fluoro-4,5-diphenyl-2-(trialkyl-silyloxy)furan
18 as a novel building block. Non-fluorinated silyloxyfurans
have been employed in electrophilic reactions [22] and in
pheromone synthesis [22(c)]. Among fluorinated analogues,
CDCl3. Chemical shifts were relative to tetramethylsilane and
fluorotrichloro-methane. IR spectra were recorded on a FTIR
Spec Nicolet 740 in CHCl3. Mass spectra were scanned on an
Autospec Ultima (Micromass) using GC (HP 6890, ionization
with electron impact at 70 eV). E- and Z-configurations were
1
assigned on the basis of 19F-{1H} NOEDIF NMR and H-
{1H} NOEDIF NMR experiments for stereoisomeric mixtures
carried out on a Bruker Avance DRX 500.1 (at 470.4 MHz)
apparatus.
Chemicals used were as follows: ethyl 2-(diethoxypho-
sphoryl)acetate was prepared according to Ref. [24]; ethyl 2-
(diethoxyphosphoryl)-2-fluoroacetate was synthesized from
trifluorochloroethylene by our improved procedure [11(d),25];
benzoin acetate (4) was prepared by acetylation of benzoin
according to Ref. [26]. Benzoin ethers 1–3 as well as other
chemicals were purchased from Sigma–Aldrich. Tetrahydro-
furan and dichloromethane were dried and purified according to
standard procedures.
Scheme 5.

1394
K. Pomeisl et al. / Journal of Fluorine Chemistry 127 (2006) 1390–1397
3.2. Synthesis of alkenoates 6–9 and 12: general procedure
129.3 (4Â s); the signals of CF, C CH, C CF and COO were
not found due to their very low intensity. 19F NMR (CDCl3): d
À124.1 (s, 1F).
A solution of butyllithium (2.5 M, 9.25 mmol) in hexane
was added through septum to a solution of ethyl 2-
(diethoxyphosphoryl)-2-fluoroacetate (9.23 mmol) in THF
(15 mL) cooled to ca. À75 8C under nitrogen while stirring.
The mixture was stirred for 20 min and then benzoin ether (1–3)
or acetate 4 (4.4 mmol) in THF (10 mL) was syringed dropwise
into the flask. The mixture was stirred for 15 min, then allowed
to warm to r.t. and stirred for another 10 h. THF was removed
on rotary evaporator under reduced pressure and the oily
residue chromatographed to afford products 6–9 or 12 as
mixtures of E/Z-isomers.
3.2.3. Ethyl 2-fluoro-4-isopropoxy-3,4-diphenylbut-2-
enoate (8)
Yield 1.2 g (80.3%) of the mixture 8a/8b (5:95), which was
purified by microdistillation, bp 136–138 8C/133 Pa. Column
chromatography on silica gel (hexane–dichloromethane 10:3).
Anal. calcd for C21H23FO3: C, 74.76; H, 6.87. Found: C, 74.42;
H, 6.93. IR (CHCl3) n cmÀ1: 3085 (w), 3060 (w), 2975 (m),
2934 (w), 2875 (w), 1731 (s), 1649 (w), 1601 (w), 1495 (w),
1450 (m), 1370 (m), 1246 (m), 1149 (s), 1093 (m), 1062 (m),
1
1029 (m), 702 (s). 8a (E): H NMR (CDCl3): d 0.94 (t, 3H,
3.2.1. Ethyl 2-fluoro-4-methoxy-3,4-diphenylbut-2-enoate
(6)
J = 7.1 Hz), 1.20 (d, 3H, J = 6.0 Hz), 1.23 (d, 3H, J = 6.0 Hz),
3.83 (m, 1H, J = 6.0 Hz), 3.99 (q, 2H, J = 7.1 Hz), 5.96 (s, 1H),
6.77 (d, 2H, J = 7.1 Hz), 7.14–7.29 (m, 8H). 13C NMR
(CDCl3): d 13.4, 21.8, 21.4 (2Â s), 61.3, 69.8, 73.7 (J = 6),
126.6–129.5 (7Â s), 132.6 (J = 5 Hz), 134.3 (J = 12 Hz), 139.4,
144.8 (J = 255 Hz), 160.6 (J = 37 Hz). 19F NMR (CDCl3): d
À119.9 (s, 1F). 8b (Z): 1H NMR (CDCl3): d 1.49 (t, 3H,
J = 6.9 Hz), 1.20 (d, 3H, J = 6.0 Hz), 1.23 (d, 3H, J = 6.0 Hz),
3.83 (m, 1H, J = 6.0 Hz), 4.42 (q, 2H, J = 7.1 Hz), 6.70 (d, 1H,
J = 2.8 Hz), 6.95 (d, 2H, J = 8.8 Hz), 7.14–7.29 (m, 8H). 13C
NMR (CDCl3): the signals of this stereoisomer were not
sufficiently separated from those of 8a to enable unequivocal
assignment; d 14.1, 21.8, 21.4, 61.9, 69.5, 73.3, 126.6–129.5
(4Â s); the signals of CF, C CH, C CF and (COO) were not
found due to their very low intensity. 19F NMR (CDCl3): d
À124.4 (s, 1F).
Yield 828 mg (59.6%) of the mixture 6a/6b (27:73), which
was purified by microdistillation, bp 163 8C/0.2 kPa. Column
chromatography on silica gel (benzene). Anal. calcd for
C19H19FO3: C, 72.37; H, 6.18. Found: C, 72.39; H, 6.18. IR
(CHCl3) n cmÀ1: 3069 (w), 2998 (w), 2828 (w), 1725 (s), 1650
(w), 1495 (w), 1451 (w), 1371 (w), 1284 (m), 1098 (m), 1075
(w), 704 (s). 6a (E): 1H NMR (CDCl3): d 1.42 (t, 3H,
J = 7.1 Hz), 3.48 (s, 3H), 4.42 (q, 2H, J = 7.1 Hz), 6.45 (d, 1H,
J = 2.8 Hz), 6.99 (d, 2H, J = 7.7 Hz), 7.12–7.42 (m, 8 H). 13C
NMR (CDCl3): d 14.1, 57.5, 62.6, 78.7 (J = 3 Hz), 127.2–130.0
(6Â s), 132.4, 139.8, 134.7 (J = 10 Hz), 145.9 (J = 257 Hz),
162.1 (J = 35 Hz). 19F NMR (CDCl3): À118.4 (s, 1F). 6b (Z):
1H NMR (CDCl3): d 0.94 (t, 3H, J = 7.1 Hz), 3.49 (s, 3H), 3.99
(q, 2H, J = 7.1 Hz), 5.71 (s, 1H), 6.84 (d, 2H, J = 7.7), 7.12–
7.42 (m, 8H). 13C NMR (CDCl3): d 14.1, 57.7, 62.0, 79.3
(J = 6 Hz), 127.2–130.0 (5Â s), 132.9 (J = 5 Hz), 133.8
(J = 12 Hz), 139.0, 145.9 (J = 257 Hz), 161.1 (J = 37 Hz).
19F NMR (CDCl3): d À123.6 (s, 1F).
3.2.4. Ethyl 4-acetoxy-2-fluoro-3,4-diphenylbut-2-enoate
(9)
Yield 598 mg (39.7%) of the mixture 9a/9b (12:88). Column
chromatography on silica gel (toluene). Anal. calcd for
C20H19FO4: C, 70.16; H, 5.59. Found: C, 69.65; H, 5.67. IR
(CHCl3) n cmÀ1: 1732 (s), 1654 (w), 1495 (w), 1371 (m), 1233
(s), 704 (s). 9a (E): 1H NMR (CDCl3): d 0.94 (t, 3H,
J = 7.1 Hz), 2.06 (s, 3H), 3.97 (q, 2H, J = 7.2 Hz), 6.87 (m, 2H),
7.01 (s, 1H), 7.21–7.39 (m, 8H). 13C NMR (CDCl3): d 13.9,
61.9, 72.6 (J = 6 Hz), 127.0–129.9 (10Â s), 132.5 (J = 12 Hz),
132.8 (J = 5 Hz), 137.2, 145.6 (J = 262 Hz), 160.8 (J = 37 Hz),
3.2.2. Ethyl 4-ethoxy-2-fluoro-3,4-diphenylbut-2-enoate
(7)
Yield 1.1 g (77.0%) of the mixture 7a/7b (4:96), which was
purified by microdistillation, bp 129–130 8C/160 Pa. Column
chromatography on silica gel (hexane followed by hexane–
dichloromethane 10:3). Anal. calcd for C20H21FO3: C, 73.15;
H, 6.24. Found: C, 73.15; H, 6.45. IR (CHCl3) n cmÀ1: 3085
(w), 3061 (w), 2979 (w), 2931 (w), 2875 (w), 1725 (s), 1649
(w), 1494 (w), 1450 (w), 1190 (m), 1072 (m), 703 (m). 7a (E):
1H NMR (CDCl3): d 0.99 (t, 3H, J = 7.1 Hz), 1.25 (dd, 3H,
J = 7.1, 6.6 Hz), 3.68 (m, 2H, J = 37.9, 7.1, 2.2 Hz), 3.98 (q,
2H, J = 7.1 Hz), 5.82 (s, 1H), 6.76 (d, 2H, J = 7.1 Hz), 7.12–
7.32 (m, 8H). 13C NMR (CDCl3): d 13.4, 15.0, 61.3, 64.7, 76.0
(J = 4.8 Hz), 126.4–129.3 (6Â s), 132.3 (J = 5 Hz), 133.6
(J = 13 Hz), 138.8, 145.0 (J = 257 Hz), 160.5 (J = 38 Hz). 19F
1
170.4. 19F NMR (CDCl3): d À118.4 (s, 1F). 9b (Z): H NMR
(CDCl3): d 1.40 (t, 3H, J = 7.1 Hz), 2.06 (s, 3H), 4.43 (q, 2H,
C
J = 7.1 Hz), 6.89 (m, 1H), 6.89 (m, 2H), 7.21–7.39 (m, 8H). 13
NMR (CDCl3): the signals of this stereoisomer were not
sufficiently separated from those of 9a to enable unequivocal
assignment; d 14.5, 21.2, 62.7, 71.7 (J = 1 Hz), 127.0–129.9
(10Â s); the signals of C CF, C CH and CFCOO were not
found due to their very low intensity. 19F NMR (CDCl3): d
À120.5 (s, 1F).
1
NMR (CDCl3): d À119.0 (s, 1F). 7b (Z): H NMR (CDCl3):
1.41 (t, 3H, J = 7.1 Hz), 1.24 (dd, 3H, J = 6.6, 6.9 Hz), 3.68 (m,
2H, J = 37.9, 7.1, 2.2 Hz), 4.41 (q, 2H, J = 7.1 Hz), 6.54 (d, 1H,
J = 2.2 Hz), 6.93 (d, 2H, J = 8.2 Hz), 7.12–7.32 (m, 8H). 13C
NMR (CDCl3): the signals of this stereoisomer were not
sufficiently separated from those of 7a to enable unequivocal
assignment; d 14.1, 15.0, 61.9, 64.3, 76.6 (J = 5 Hz); 126.4–
3.2.5. Ethyl 2-fluoro-2-(2-methoxycyclohexylidene)acetate
(12)
Yield 618 mg (65.0%) of the mixture 12a/12b (82:18),
which was purified by microdistillation, bp 74 8C/40 Pa.
Column chromatography on silica gel (benzene–ethyl acetate

K. Pomeisl et al. / Journal of Fluorine Chemistry 127 (2006) 1390–1397
1395
5:1). Anal. calcd for C11H17FO3: C, 61.10; H, 7.92. Found: C,
61.36; H, 8.23. IR (CHCl3) n cmÀ1: 3027 (w), 2986 (w), 2940
(m), 2864 (w), 1721 (m), 1464 (w), 1370 (w), 1295 (s), 1258
(CDCl3): d 1.21 (t, 3H, J = 7.1 Hz), 1.34 (t, 3H, J = 7.2 Hz),
3.62 (dq, 2H, J = 6.9, 1.4 Hz), 4.27 (q, 2H, J = 7.1 Hz), 6.22 (s,
1H), 6.75 (s, 1H), 7.16–7.31 (m, 8H), 7.40 (d, 2H, J = 7.7 Hz).
13C NMR (CDCl3): d 14.2, 15.2, 60.3, 64.6, 76.7, 121.4, 126.3,
127.0, 127.7, 128.0, 129.36, 128.43, 138.2, 140.1, 158.2, 166.2.
11b (E): 1H NMR (CDCl3): d 1.05 (t, 3H, J = 7.1 Hz), 1.26 (t,
3H, J = 6.9 Hz), 3.51 (dq, 2H, J = 6.9, 1.4 Hz), 3.98 (q, 2H,
J = 7.1 Hz), 4.93 (s, 1H), 6.41 (d, 1H, J = 1.1 Hz), 6.87 (m, 2H),
7.12–7.31 (m, 3H), 7.33 (m, 2H), 7.48 (m, 2H), 8.02 (d, 1H,
J = 7.2 Hz). 13C NMR (CDCl3): d 13.9, 15.2, 59.9, 64.7, 85.3,
116.9, 127.5, 127.9, 127.3, 128.6, 128.8, 128.2, 137.7, 138.6,
157.9, 166.2.
1
(m), 1144 (w), 1094 (w). 12a (E): H NMR (CDCl3): d 1.28–
1.43 (m, 2H); 1.33 (t, 3H, J = 7.1 Hz), 1.48 (dm, 1H,
J = 12.6 Hz), 1.74 (m, 1H), 1.85 (dm, 1H, J = 12.6 Hz), 2.08
(m, 2H), 2.76 (d, 1H, J = 13.2), 3.22 (s, 3H), 4.27 (m, 2H), 5.10
(d, 1H, J = 2.8 Hz). 13C NMR (CDCl3): d 14.7, 20.4, 23.2
(J = 8 Hz), 27.4, 33.5, 56.2, 62.0, 72.6 (J = 6 Hz), 135.2
(J = 10 Hz), 143.7 (J = 254 Hz), 162.0 (J = 37 Hz). 19F NMR
(CDCl3): d À125.9 (d, 1F, J = 2.0 Hz). 12b (Z): 1H NMR
(CDCl3): d 1.28–1.43 (m, 2H), 1.33 (t, 3H, J = 7.1 Hz), 1.48
(dm, 1H, J = 12.6 Hz), 1.74 (m, 1H), 1.85 (dm, 1H,
J = 12.6 Hz), 2.08 (m, 2H), 3.34 (d, 1H, J = 13.2 Hz), 4.27
(m, 2H); 4.45 (s, 1H). 13C NMR (CDCl3): the signals of this
stereoisomer were not sufficiently separated from those for 12a
to enable unequivocal assignment; d 14.7, 20.6, 23.5, 27.7, 33.2,
56.5, 62.0, 72.3 (J = 9 Hz); the signals of C CF, CF and COO
were not found for their very low intensity. 19F NMR (CDCl3): d
À128.9 (s, 1F).
3.3.3. Ethyl 2-(2-methoxycyclohexylidene)acetate (13)
Yield 572 mg (65.7%) of the mixture 13a/13b (65:35).
Column chromatography on silica gel (benzene–ethyl acetate
10:1). MS EI: 198 (M+, 34), 183 (9), 169 (35), 166 (30), 153
(37), 137 (100), 125 (97), 121 (18), 110 (13), 97 (7), 93 (33),
81 (17), 67 (16), 55 (18), 45 (20), 41 (27), 32 (10), 28 (47). IR
(CHCl3) n cmÀ1: 3025 (w), 2986 (w), 2941 (m), 1709 (s),
1653 (m-w), 1464 (w), 1447 (w), 1381 (w), 1198 (w), 1167
(s), 1143 (m), 1095 (m), 1036 (w). 13a (Z): 1H NMR (CDCl3):
d 1.27 (t, 3H, J = 7.1 Hz), 1.31–2.02 (m, 6H), 2.54 (m, 1H),
3.05 (m, 1H), 3.29 (s, 3H), 4.15 (q, 2H, J = 7.2 Hz), 5.79 (dd,
1H, J = 11, 0.8 Hz). 13C NMR (CDCl3): d 14.3, 20.1, 27.2,
3.3. Preparation of compounds 10, 11 and 13: general
procedure
The procedure was analogous to that for the preparation of
compounds 6–9 using ethyl 2-(diethoxyphosphoryl)acetate.
1
27.7, 32.8, 56.6, 59.8, 72.5, 116.5, 160.9, 166.1. 13b (E): H
NMR (CDCl3): d 1.27 (t, 3H, J = 7.1 Hz), 1.31–2.02 (m, 6H),
2.54 (m, 1H), 3.24 (s, 3H), 3.56 (m, 1H), 4.14 (q, 2H,
J = 7.2 Hz), 5.24 (t, 1H, J = 6.8 Hz). 13C NMR (CDCl3): d
14.3, 20.1, 27.2, 27.7, 32.8, 56.6, 59.8, 72.5, 116.5, 160.9,
166.1.
3.3.1. Ethyl-3,4-diphenyl-4-methoxybut-2-enoate (10)
Yield 804 mg (61.7%) of the mixture 10a/10b (88:12).
Column chromatography on silica gel (benzene–ethyl acetate
10:1). Anal. calcd for C19H20O3: C, 77.00; H, 6.80. Found: C,
76.54; H, 6.82. IR (CHCl3) n cmÀ1: 3088 (w), 3073 (w), 2979
(w), 2935 (w), 2907 (w), 1709 (s), 1625 (w), 1601 (w), 1495
(w), 1451 (w), 1444 (w), 1371 (w), 1180 (s), 1097 (m), 1024
(w), 700 (m). MS EI: 296 (M+, 74), 267 (8), 251 (9), 235 (42),
223 (12), 207 (16), 191 (21), 163 (8), 121 (100), 105 (50), 91
(10), 77 (33), 51 (6), 43 (7). 10a (Z): 1H NMR (CDCl3): 1.34 (t,
3H, J = 7.1 Hz), 3.42 (s, 3H), 4.26 (q, 2H, J = 7.3 Hz), 6.23 (s,
1H), 6.63 (s, 1H), 7.25 (m, 8H), 7.37 (d, 2H, J = 7.6 Hz). 13C
NMR (CDCl3): d 14.3, 57.0, 60.5, 78.8, 121.9, 126.2–128.5 (6Â
3.4. Reaction of alkenoates 6–8 and 12 with boron
tribromide: general procedure
Boron tribromide (1.3 or 2.8 mmol, see Table 2) was
syringed dropwise under nitrogen into the flask charged with a
mixture of E/Z-alkenoates 6–8 or 12 (1 mmol) and dry
dichloromethane (5 mL) cooled to À50 8C. The resulting
mixture was allowed to warm to r.t. and then stirred for 2–4 h.
The mixture was washed and neutralized with a water solution
of NaHCO3, while the used water layer was extracted with
dichloromethane (2Â 10 mL). Organic solutions were com-
bined and dried over MgSO4. Solvents were removed (rotary
evaporator) and the residue chromatographed on silica gel
column (petroleum ether) to obtain products 14 and 15.
Compound 16 was purified by microdistillation (vide infra).
1
s), 138.0, 139.7, 157.7, 166.2. 10b (E): H NMR (CDCl3): d
1.04 (t, 3H, J = 7.3 Hz), 3.39 (s, 3H), 3.98 (q, 2H, J = 7.3 Hz),
4.81 (s, 1H), 6.38 (s, 1H), 7.25–7.37 (m, 10H). 13C NMR
(CDCl3): the signals of this stereoisomer were not sufficiently
separated from those for 10a to enable unequivocal assignment;
d 13.9, 57.0, 60.0, 87.2, 117.0, 127.5–128.3 (3Â s); 137.5,
157.4; the signals of CH–C CH and COO were not found due
to their very low intensity.
3.4.1. 4,5-Diphenyl-3-fluorofuran-2(5H)-one (14)
3.3.2. Ethyl 3,4-diphenyl-4-ethoxybut-2-enoate (11)
Slightly yellow solid, for yield see Table 2. Its part was
crystallised (hexane–acetone) for analytical purposes to afford
white crystals, mp 114–116 8C. Anal. calcd for C16H11FO2: C,
Yield 1.2 g (91.0%) of the mixture 11a/11b (76:24). Column
chromatography on silica gel (benzene–ethyl acetate 10:1).
Anal. calcd for C20H22O3: C, 77.39; H, 7.14. Found: C, 77.09;
H, 7.14. IR (CHCl3) n cmÀ1: 3088 (w), 3062 (w), 2982 (w),
2931 (m), 2902 (m), 2875 (w), 1708 (s), 1625 (w), 1599 (w),
1494 (w), 1450 (w), 1371 (w-m), 1252 (w), 1179 (s), 1097 (m),
75.58; H, 4.36. Found: C, 75.66; H, 4.39. IR (CHCl3) n cmÀ1
:
1778 (s), 1680 (w). 1H NMR (CDCl3): d 6.24 (d, 1H,
J = 5.2 Hz), 7.32–7.43 (m, 8H), 7.45–7.52 (m, 2H). 13C NMR
(CDCl3): d 79.7 (J = 7 Hz), 127.1 (J = 6 Hz), 127.9, 128.1,
128.2, 129.0, 129.2, 130.0, 130.7 (J = 2 Hz), 134.0 (J = 2 Hz),
1
1074 (m), 1024 (w-m), 725 (w), 700 (s). 11a (Z): H NMR

1396
K. Pomeisl et al. / Journal of Fluorine Chemistry 127 (2006) 1390–1397
135.5, 143.5 (J = 280 Hz), 164.4 (J = 31 Hz). 19F NMR
(CDCl3): d À143.5 (d, 1H, J = 5.5 Hz).
3.5.1. The reaction of alkenoate 6 with bromine
Yield 18 mg (71.7%) of 14. The complete conversion of 6a/
6b (25:73) was checked by 19F NMR.
3.4.2. Ethyl (Z)-2,4-dibromo-2-fluoro-3,4-diphenylbut-2-
enoate (15)
3.5.2. The reaction of alkenoate 7 with bromine
Yield 20 mg (48.0%) of 14. The conversion ꢀ64% of 7a/7b
(6:94) was checked by 19F NMR.
Yellow solid, for yield see Table 2, mp 62–64 8C. Anal. calcd
for C18H16Br2O2: C, 50.97; H, 3.80; Br, 37.68. Found: C, 51.29;
H, 4.00; Br, 37.23. IR (CHCl3) n cmÀ1: 1725 (s), 1494 (w),
1
1449 (w), 1368 (w), 1304 (m-w), 1233 (s), 700 (s). H NMR
3.5.3. The reaction of alkenoate 8 with bromine
Yield 32 mg (84.9%) of 14. The complete conversion of 8a/
8b (5:95) was checked by 19F NMR.
(CDCl3): d 0.81 (t, 3H, J = 7.1 Hz), 3.87 (q, 2H, J = 7.1 Hz),
6.65 (s, 1H), 6.84 (d, 2H, J = 7.7 Hz), 7.15–7.24 (m, 8H). 13C
NMR (CDCl3): d 14.0, 55.0, 62.7, 113, 127.9, 128.7, 129.1,
129.2, 129.4, 130.8, 135.2, 136.9, 147.3, 164.7.
3.6. Synthesis of 2-[tert-butyl(dimethyl)silyloxy]-4,5-
diphenyl-3-fluorofuran (18)
3.4.3. Ethyl 2-(2-bromocyclohexylidene)-2-fluoroacetate
(16)
Yield 204 mg (77.0%) of the mixture 16a/16b (82:18),
which was purified by microdistillation, bp 97–98 8C/106 Pa.
Anal. calcd for C10H14BrO2: C, 45.30; H, 5.32; Br, 30.14.
Found: C, 45.39; H, 5.72; Br, 27.23. IR (CHCl3) n cmÀ1: 2986
(w), 2944 (w), 1721 (m), 1651 (w), 1447 (w), 1371 (w), 1295
A solution of butyllithium (2.5 M, 81 mL, 0.203 mmol) was
syringed dropwise under nitrogen into the flask charged with
diisopropylamine (21 mg, 0.203 mmol) and dry THF (5 mL)
cooled to 0 8C and the mixture was stirred for 20 min and then
cooled to À78 8C. Furanone 14 (47 mg, 0.184 mmol) in THF
(2 mL) was then syringed dropwise while stirring. After
45 min, a solution of tert-butyl(dimethyl)silyl chloride
(31 mg, 0.203 mmol) in THF (2 mL) was syringed, the
mixture stirred for 20 min and then allowed to warm to r.t. The
solvent was removed under reduced pressure at r.t. (rotary
evaporator) and the residue flash chromatographed on silica
gel column (petroleum ether), the yield of 18 was 33 mg
(48.4%) as a colorless oil. No decomposition of 18 was found
1
(s), 1257 (m), 1136 (w). 16a (E): H NMR (CDCl3): d 1.22–
1.44 (m, 2H), 1.36 (t, 3H, J = 7.1 Hz), 1.66, 2.20 (2Â dm, 2H,
J = 14.3 Hz), 1.78–2.06 (m, 2H), 2.45 (m, 1H), 2.85 (d, 1H,
J = 14.8 Hz), 4.32 (q, 2H, J = 7.1 Hz), 6.36 (s, 1H). 13C NMR
(CDCl3): d 14.1, 20.4, 25.7, 35.5, 22.2 (J = 8 Hz), 47.9
(J = 6 Hz), 61.6, 134.0 (J = 16 Hz), 141.8 (J = 259 Hz), 160.0
(J = 34 Hz). 19F NMR (CDCl3): d À126.7 (s, 1F). 16b (Z): 1H
NMR (CDCl3): d 1.22–1.44 (m, 2H), 1.36 (t, 3H, J = 7.1 Hz);
1.66, 2.20 (2Â dm, 2H, J = 14.3 Hz), 1.78–2.06 (m, 2H), 2.45
(m, 1H), 3.47 (d, 1H, J = 14.8 Hz), 4.28 (q, 2H, J = 7.1 Hz);
5.52 (s, 1H). 13C NMR (CDCl3): the signals of this stereoisomer
were not sufficiently separated from those 16a to enable
unequivocal assignment; d 14.1, 20.3, 22.4, 26.0, 35.1, 45.9
(J = 13 Hz); 61.6; the signals of CF and COO were not found
due their very low intensity. 19F NMR (CDCl3): d À123.9 (d,
1F, J = 2 Hz).
1
during 2 weeks keeping at 0–4 8C (check by 19F NMR). H
NMR (CDCl3): d 0.35 (s, 6H), 1.06 (s, 9H), 7.13–7.29 (m, 3H),
7.34–7.49 (m, 7H). 13C NMR (CDCl3): d À4.6, 18.2, 25.4,
115.4 (J = 15 Hz), 125.0, 126.8, 127.8, 128.3, 128.7, 129.4,
128.5, 131.7, 130.5 (J = 3 Hz), 130.9 (J = 2 Hz), 134.6
(J = 6 Hz). 19F NMR (CDCl3): d À187.65 (s, 1F). MS EI:
368 (M+, 34), 254 (12), 209 (4), 189 (3), 105 (11), 77 (18), 73
(100), 59 (5), 45 (4).
3.5. The reaction of alkenoates 6–8 with bromine: general
procedure
Acknowledgements
The research was supported by the Ministry of Education of
the Czech Republic (project MSM 6046137301) and Institute
of Chemical Technology, Prague.
A dry flask (10 mL) equipped with septum and magnetic
spinbar was charged with dry tetrachloromethane (1 mL) and
alkenoate 6–8 (0.16 mmol). A solution of bromine (2.52 mmol)
in tetrachloromethane (1 mL) was syringed into the flask and
the mixture stirred at r.t. for 6 h after that the conversion of
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