Electrolytic partial fluorination of organic compounds. Part 29. Anodic mono- and difluorination of 2-benzoxazolyl sulfides

Article abstract of DOI:10.1016/S0022-1139(98)00292-9

Anodic fluorination of 2-benzoxazolyl sulfides using various fluoride salts and dimethoxyethane as a supporting electrolyte and a solvent, respectively, resulted in the formation of the corresponding mono- and difluorinated products in reasonable yields. The monofluorination took place selectively at the position α to the sulfur atom while the difluorination resulted at both the α-position and the benzene ring while the expected α,α-difluorination did not take place at all. Moreover, anodic fluorination of the electrolytic solvent, DME, which has never been reported so far, was also observed as a side-reaction and this fluorination was further confirmed by carrying out anodic fluorination of DME itself in the absence of a substrate sulfide.

Full text of DOI:10.1016/S0022-1139(98)00292-9

Journal of Fluorine Chemistry 93 (1999) 159±164
Electrolytic partial ¯uorination of organic compounds. Part 291.
Anodic mono- and di¯uorination of 2-benzoxazolyl sul®des
Kamal M. Dawood2, Seiichiro Higashiya, Yankun Hou, Toshio Fuchigami*
Department of Electronic Chemistry, Tokyo Institute of Technology, 4259, Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
Received 28 May 1998; received in revised form 17 August 1998; accepted 28 September 1998
Abstract
Anodic ¯uorination of 2-benzoxazolyl sul®des using various ¯uoride salts and dimethoxyethane as a supporting electrolyte and a solvent,
respectively, resulted in the formation of the corresponding mono- and di¯uorinated products in reasonable yields. The mono¯uorination
took place selectively at the position ꢀ to the sulfur atom while the di¯uorination resulted at both the ꢀ-position and the benzene ring while
the expected ꢀ,ꢀ-di¯uorination did not take place at all. Moreover, anodic ¯uorination of the electrolytic solvent, DME, which has never
been reported so far, was also observed as a side-reaction and this ¯uorination was further con®rmed by carrying out anodic ¯uorination of
DME itself in the absence of a substrate sul®de. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Selective anodic ¯uorination; Benzoxazolyl sul®des; Fluoride salt; Dimethoxyethane
1. Introduction
toward various ¯uoride salts and electrolytic solvents. Ano-
dic mono¯uorination at ꢀ-position to the sulfur atom took
place preferentially and by-products di¯uorinated at both a
side-chain and a benzene ring were also formed. Interest-
ingly, anodic ¯uorination of 1,2-dimethoxyethane (DME),
solvent used for the electrolysis, was also observed.
Benzoxazole derivatives are well-known to exhibit anti-
bacterial [2], antimicrobial [3], antifungal [4], antiin¯am-
matory and analgesic [5,6] activities. On the other hand,
¯uoroorganic compounds have attracted a great deal of
interest due to their considerable importance as medicines
and agrochemicals [7±10]. Therefore, introduction of ¯uor-
ine atom into the benzoxazoles 2a±d or their side chains may
enhance their biological activities. However, direct ¯uorina-
tion is not always straightforward and very often requires
hazardous or costly ¯uorinating agents. Recently, anodic
¯uorination proved to be more convenient because of its
selectivity and safety [11±22]. For example, anodic ¯uor-
ination of some heterocyclic sul®des bearing electron-with-
drawing groups at their ꢀ-position was ®rstly explored by
our research group and the ¯uorine atom has been intro-
duced selectively to the ꢀ-position but the heterocyclic ring
¯uorination did not take place at all [23±29].
2. Results and discussion
2.1. Preparation of 2-benzoxazolyl sulfides 2a±c
Treatment of 2-mercaptobenzoxazole (1) with ꢀ-chloro-
acetone, ethyl ꢀ-chloroacetate, and ꢀ-chloroacetonitrile in
re¯uxing THF, in the presence of K2CO3, afforded the
corresponding 2-benzoxazolyl sul®des 2a±c, respectively,
in good yields, as shown in Scheme 1.
2.2. Oxidation potentials of 2-benzoxazolyl sulfides
In our continuous studies on the anodic ¯uorination of a
variety of heterocyclic sul®des [23±29], we disclose herein
the anodic behavior of 2-mercaptobenzoxazoles 2a±d
The oxidation potentials of 2-benzoxazolyl sul®des 2a±d
and the benzoxazole (6) were measured by cyclic voltam-
metry in an anhydrous acetonitrile solution containing
Bu4NmÁBF4 (0.1 M) using a divided cell and platinum
electrodes. All benzoxazoles 2a±d and 6 displayed irrever-
sible oxidation peaks. Among all compounds under study,
the benzoxazole (6) has the highest oxidation potential
(2.32 V) and 2-methylthiobenzoxazole (2d) has the lowest
*Corresponding author. Fax: +81-45-924-5406; e-mail:
fuchi@echem.titech.ac.jp
1
For Part 28, see [1].
2UNESCO Research Fellow (1997±1998), on leave from Department of
Chemistry, Faculty of Science, Cairo University, Cairo, Egypt.
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
P I I : S 0 0 2 2 - 1 1 3 9 ( 9 8 ) 0 0 2 9 2 - 9

160
K.M. Dawood et al. / Journal of Fluorine Chemistry 93 (1999) 159±164
taneously. Therefore, this electrolytic system does not seem
to be preferable for anodic ¯uorination. On the other hand, a
large cathodic shift was observed after addition of 2a to the
electrolytic acetonitrile solution as shown in Fig. 1(A).
Compared with the former case, the latter electrolytic
solution gave a much larger faradaic current due to the
discharge of 2a. Therefore, acetonitrile seemed to be sui-
table for anodic ¯uorination. Experimental results showed
however, the reverse trend: DME was found to be a suitable
electrolytic solvent.
Scheme 1.
Table 1
Oxidation potentials of 2-benzoxazolyl sulfides
2.4. Anodic fluorination of 2-S-substituted
mercaptobenzoxazoles 2a±d
Anodic ¯uorination of 2-acetonylmercaptobenzoxazole
(2a), as a model example, was studied in details under
various conditions and the results are summarized in
Table 2.
Instead of selective ¯uorination at the ꢀ-position to the
electron-withdrawing group as reported, previously by us
for the cases of 2-pyridyl [24,27] and 2-benzothiazolyl
sul®des [26], the 2-benzoxazolyl sul®de (2a) differed to
some extent and the ¯uorination took place at the position ꢀ
to the sulfur atom as well as at the benzene ring as depicted
in Scheme 2.
When the reaction was monitored by GC±MS during the
electrolysis, it was found that ¯uorination took place ®rstly
at the a-position till 5 F mol 1, then di¯uorination started to
take place at the benzene ring. However, the expected ꢀ,ꢀ-
di¯uorinated product 5a, like the case of the corresponding
phenyl sul®des [21], was not formed at all during the
electrolysis. Among the ¯uoride salts used, Et4NFÁ4HF
was the best ¯uorinating agent in DME and the ꢀ-¯uori-
nated product 3a was obtained in reasonable yield as a major
product. Regardless of supporting electrolytes, a small
amount of di¯uorinated product 4a was obtained as a by-
product when DME was used as a solvent (Scheme 2). The
position of the additional ¯uorination could not be deter-
Sulfide
Epox (V vs. SSCE)a
No
EWG
2a
2b
COCH3
COOEt
CN
2.08
1.96
2.22
1.90
2.32
2c
2d
H
Benzoxazole
1
aIn 0.1 M Bu4NÁBF4/MeCN. Anode: Pt plate. Sweep rate: 100 mVs
.
one (1.90 V). Substitution of an electron-withdrawing group
at the position ꢀ to the sulfur atom increases the oxidation
potentials in the order COOEt<COCH3<CN as shown in
Table 1.
2.3. Current±potential curves
Before carrying out the anodic ¯uorination, the current±
potential curves for acetonitrile and DME in the presence
and absence of 2a in 0.3 M Et4NFÁ4HF were measured in
order to select the proper electrolytic solvent. As shown in
Fig. 1(B), addition of 2-acetonylmercaptobenzoxazole (2a)
to the DME electrolytic solution led to an anodic shift. This
indicated that both 2a and DME should be oxidized simul-
Fig. 1. Current±potential curves of the electrolytic solutions: (A) 0.3 M Et4NFÁ4HF/MeCN (20 ml) and (B) 0.3 M Et4NFÁ4HF/DME (20 ml), in the presence
(~) and the absence (&) of substrate 2a (1 mmol).

K.M. Dawood et al. / Journal of Fluorine Chemistry 93 (1999) 159±164
161
Table 2
Anodic fluorination of 2-acetonylmercaptobenzoxazole (2a)
Run
Supporting electrolyte
Solvent
Charge passed (F/mol)a
Yield (%)b
3a
4a
Others
c,e
1
2
3
4
5
6
7
8
9
Et4NFÁ4HF
Et4NFÁ4HF
Et4NFÁ3HF
Et4NFÁ3HF
Et3NÁ3HF
DME
DME
9.5
18
7
17
39
10
34
35
0
3
8
2
7
7
0
0
0
0
‡
c
‡
c,e
DME
‡
c
DME
18
18
7
‡
c
DME
‡
d
Et4NFÁ4HF
Et3NFÁ3HF
Et4NFÁ4HF
Et4NFÁ4HF
MeCN
MeCN
CH2Cl2
(Neat)
‡
d
7
2
‡
e
8
2
<1
‡
e
8
‡
aConstant current electrolysis (6 mA cm 2). Anode and cathode were Pt plates (3Â2 cm2).
bDetermined by 19F-NMR spectra.
cCH3OCH2CH2OCH2F was formed.
dUnidentified solid product was obtained.
eStarting material was recovered.
Scheme 2.
mined due to its extremely low yield, however, it seems to be
the 5- or 6-position of the benzoxazole ring like the case of
benzoxazole (6) (Scheme 7).
On the other hand, acetonitrile and dichloromethane were
not suitable as a solvent even when Et4NFÁ4HF was used as
a supporting electrolyte. Thus, DME gave much better
results than acetonitrile, which cannot be explained from
I±E curves of 2a and the solvents, DME and acetonitrile.
This can be explained in terms of the ability of DME to
solvate the cationic part of the ¯uoride salts, due to its higher
donor number (23.9) than acetonitrile (14), leaving a naked
¯uoride anion which can consequently attack easily the
anodically generated cationic intermediate of the substrate.
A plausible mechanism is outlined in Scheme 3.
Although Et4NFÁ4HF/DME was effective for the anodic
¯uorination, Et4NFÁ4HF without any solvent was not effec-
tive at all (run 9). This is quite different from the anodic
¯uorination of arenes using neat Et4NFÁ4HF [30].
It is notable that anodic ¯uorination of DME (Scheme 4)
was observed as a side reaction, in all cases using DME as an
electrolytic solvent, which may re¯ect the poor current
ef®ciency in runs 1±5.
Scheme 3.
Fluorinated DME was established based on 19F NMR and
mass spectrum of the reaction product. A triplet signal
(JHFˆ56 Hz) was observed at ꢁ
74.61 ppm in the
Scheme 4.
19F NMR and molecular ion peak of mono¯uorinated
DME, m/e 108 was also detected by the mass spectrum
although the molecular ion peak was rather weak. The
formation of mono¯uorinated DME was further con®rmed
by carrying out the anodic ¯uorination of DME itself
without substrate 2a and the same results were obtained.
A possible mechanism is depicted in Scheme 5.
Next, we extended the anodic ¯uorination to other 2-
substituted mercaptobenzoxazoles 2b±d using DME as an
electrolytic solvent (Scheme 6). As shown in Table 3,
mono- and di¯uorinated products 3b,c and 4b,c were
obtained in reasonable yields, respectively, regardless of

162
K.M. Dawood et al. / Journal of Fluorine Chemistry 93 (1999) 159±164
Scheme 7.
Scheme 5.
the electron-withdrawing groups. In contrast to these cases,
2-methylthiobenzoxazole (2d) devoid of an electron-with-
drawing group gave an extremely low yield of ¯uorinated
products 3d and 4d. The ¯uorination of DME was observed
in all cases and particularly in the case of 2d, the di¯uor-
ination of DME took place considerably. However the ꢀ,ꢀ-
di¯uorinated products were not detected at all regardless of
sul®des.
Scheme 8.
3. Experimental
Anodic ¯uorination of benzoxazole (6) was also per-
formed. The formation of two mono¯uorinated products
7 and 8 were detected and con®rmed by 19F NMR [ꢁ 38.02
(ddd, Jˆ8.27, 4.60, 3.67 Hz) and 48.18 (dd, Jˆ8.25,
3.1. General
Caution. Et3NÁ3HF and Et4NFÁnHF (nˆ3,4) are toxic and
if in contact with skin cause a serious burn, so proper safety
precautions should be taken at all times. It is therefore
recommended to protect the hands with rubber gloves.
1H NMR, 19F NMR and 13C NMR spectra were recorded
at 270, 254 and 68 MHz, respectively, in CDCl3 as a solvent
using a JEOL EX-270 spectrometer. The chemical shifts for
1H and 13C NMR are given in ꢁ ppm down®eld from internal
TMS and 19F NMR are given in ꢁ ppm down®eld from
external per¯uorobenzene. Mass spectra were obtained with
Shimadzu GCMS-QP2000A and JEOL JMS-700 mass spec-
trometers. Cyclic voltammetry was performed using a
Hokuto Denko Potentiostat/Galvanostat HA 6-151 at a scan
speed of 100 mV s 1 in 0.1 M Bu4NÁBF4/CH3CN. Prepara-
tive electrolysis experiments were carried out using a
Metronix Corp. (Tokyo) constant-current power supply.
‡
‡
4.60 Hz)] and GC±MS spectra [m/z 137 (M ), 110 (M ±
HCN)], respectively, although their yields were extremely
low. In this case, ¯uorination took place at the benzene ring
but the oxazole ring was not ¯uorinated at all (Scheme 7).
This trend is quite similar to the anodic ¯uorination of
benzothiazole [26] and oxindole [25].
N-Fluoropyridinium salts are known to be a good ¯uor-
inating reagents [31,32]. Therefore, the chemical ¯uorina-
tion of compound 2a, as a model compound, was also
attempted. However, treatment of 2a with various N-¯uor-
opyridinium tri¯ates 9a±c in dichloromethane at either
room temperature or under re¯uxing resulted in no forma-
tion of any desired ¯uorinated products as shown in Scheme
8. Therefore, the electrochemical ¯uorination proved to be
superior to the conventional chemical method.
Scheme 6.
Table 3
Anodic fluorination of 2-benzoxazolyl sulfides 2a±d.
Sulfide
EWG
Supporting electrolyte
Charge passed (F/mol)a
Yield (%)b
3
4
Othersc
2a
2b
2b
2c
2c
2d
COCH3
COOEt
COOEt
CN
Et4NFÁ4HF
Et4NFÁ4HF
Et3NÁ3HF
Et4NFÁ4HF
Et3NÁ3HF
Et4NFÁ4HF
18
10
10
18
18
18
3a(39)
3b(35)
3b(29)
3c(17)
3c(12)
3d(2.2)
4a(8)
4b(13)
4b(8)
4c(6)
4c(8)
4d(2)
‡
‡
‡
‡
CN
‡
H
‡‡
aConstant current electrolysis (6 mA cm 2). Anode and cathode were Pt plates (3Â2 cm2).
bDetermined by 19F-NMR spectra.
cCH3OCH2CH2OCH2F was formed.

K.M. Dawood et al. / Journal of Fluorine Chemistry 93 (1999) 159±164
163
‡
‡
‡
3.2. Preparation of 2-S-substituted mercaptobenzoxazoles
2a±c: general procedure
(M ±CH2=C=O), 163 (M ±CH3COF), 150 (M ±
CHFCOCH3); HRMS: Calcd. for C10H8FNO2S: m/e
225.0260; Found 225.0263.
To a stirred solution of 2-mercaptobenzoxazole (1)
(3.02 g, 20 mmol) in THF (40 ml), in the presence of
potassium carbonate (3.4 g, 25 mmol), was added the appro-
priate ꢀ-chloro compound (20 mmol). The reaction mixture
was heated under re¯ux for 1 h, then left to cool. The
inorganic salts were ®ltered off and the ®ltrate was evapo-
rated under vacuum. The remaining product was crystal-
lized from hexane/isopropanol (5:1) or methanol to give the
desired products 2a±c.
Ethyl ꢀ-(2-benzoxazolylthio)-ꢀ-¯uoroacetate (3b):
1H NMR ꢁ 1.35 (t, Jˆ7.26 Hz, 3H), 4.37 (q, Jˆ7.26 Hz,
2H), 6.94 (d, Jˆ50.56 Hz, 1H), 7.33 (m, 2H), 7.50 (m, 1H),
7.67 (m, 1H); 13C NMR ꢁ 13.90, 63.31, 91.66 (dd, Jˆ238,
6.1 Hz), 110.29, 119.30, 124.88 (d, Jˆ9.5 Hz), 141.33,
152.16, 159.15 (d, Jˆ2.4 Hz), 164.48 (d, Jˆ25.7 Hz);
‡
19F NMR ꢁ 85.52 (d, Jˆ50.56 Hz); MS m/e 255 (M ),
‡
‡
182 (M ±COOEt), 150 (M ±CHFCOOEt); HRMS: Calcd.
for C11H10FNO3S: m/e 255.0365; Found 255.0364. Anal.
Calcd. for C11H10FNO3S: C, 51.76%; H, 3.95%; N, 5.49%.
Found C, 51.75%; H, 3.82%; N, 5.30%.
1-(2-Benzoxazolylthio)-2-propanone (2a) [33]: 78%
yield, m.p. 68±698C; 1H NMR ꢁ 2.40 (s, 3H), 4.23 (s,
2H), 7.27 (m, 2H), 7.44 (dt, Jˆ7.26, 1.65 Hz, 1H), 7.57
ꢀ-(2-Benzoxazolylthio)-ꢀ-¯uoroacetonitrile
(3c):
‡
‡
(dt, Jˆ6.93, 1.65 Hz, 1H); MS m/e 207 (M ), 164 (M ±
COCH3); Anal. Calcd. for C10H9NO2S: C, 57.96%; H,
4.38%; N, 6.76%. Found C, 58.14%; H, 4.22%; N, 6.53%.
Ethyl ꢀ-(2-benzoxazolylthio)acetate (2b) [34]: 78%
yield, m.p. 468C; 1H NMR ꢁ 1.29 (t, Jˆ7.26 Hz, 3H),
4.12 (s, 2H), 4.24 (q, Jˆ7.26 Hz, 2H), 7.28 (m, 2H), 7.44
(dt, Jˆ5.94, 1.65 Hz, 1H), 7.60 (dt, Jˆ5.94, 1.65 Hz, 1H);
1H NMR ꢁ 7.22 (d, Jˆ49.16 Hz, 1H), 7.37 (m, 2H),
7.53(m, 1H), 7.71 (m, 1H); 19F NMR ꢁ 81.13 (d,
‡
‡
Jˆ48.72 Hz); MS m/e 208 (M ), 191 (M ±HCN), 150
‡
(M ±CHFCN); HRMS Calcd. for C9H5FN2OS: m/e
208.0106; Found 208.0114.
1-[5(6)-Fluoro-2-benzoxazolylthio]-1-¯uoro-2-propa-
1
none (4a): H NMR ꢁ 2.51 (d, Jˆ3.62 Hz, 3H), 6.76 (d,
‡
‡
MS m/e 237 (M ), 164 (M ±COOEt); Anal. Calcd. for
C11H11NO3S: C, 55.68%; H, 4.67%; N, 5.90%. Found C,
55.78%; H, 4.59%; N, 5.78%.
Jˆ50.48 Hz, 1H), 7.10 (m, 1H), 7.22 (dd, Jˆ7.91, 2.64 Hz,
1H), 7.58 (dd, Jˆ8.91, 4.95 Hz, 1H); 19F NMR ꢁ 38.29
(m, 1F), 85.89 (dq, Jˆ50.55, 3.68 Hz, 1F); MS m/e 243
‡
‡
‡
ꢀ-(2-Benzoxazolylthio)acetonitrile (2c) [35]: 84% yield,
m.p. 958C; 1H NMR ꢁ 4.11 (s, 2H), 7.34 (m, 2H), 7.48 (m,
1H), 7.66 (m, 1H); MS m/e 190 (M ), 164 (M ±HCN), 150
(M ), 201 (M ±CH2=C=O), 168 (M ±CHFCOCH3);
HRMS: Calcd. for C10H7F2NO2S, m/e 243.0165; Found
243.0165.
‡
‡
‡
(M ±CH2CN); Anal. Calcd. for C9H6N2OS: C, 56.83%; H,
3.18%; N, 14.73%. Found C, 56.84%; H, 2.99%; N, 14.81%.
Ethyl ꢀ-[5(6)-Fluoro-2-Benzoxazolylthio]-ꢀ-¯uoroace-
tate (4b): 1H NMR ꢁ 1.35 (t, Jˆ7.26 Hz, 3H), 4.37 (q,
Jˆ7.26 Hz, 2H), 6.88 (d, Jˆ50.48 Hz, 1H), 7.09 (m, 1H),
7.24 (dd, Jˆ7.91, 2.31 Hz, 1H), 7.61 (dd, Jˆ8.91, 4.95 Hz,
1H); 19F NMR ꢁ 38.23 (m, 1F), 85.50 (d, Jˆ50.56 Hz,
3.3. Anodic fluorination of 1-(2-benzoxazolylthio)-2-
propanone (2a)
‡
‡
‡
1F); MS m/e 273 (M ), 200 (M ±COOEt), 168 (M ±
CHFCOOEt); HRMS Calcd. for C11H9F2NO3S: m/e
273.0271; Found 273.0269.
Electrolysis was carried out with platinum electrodes
(3Â2 cm2) in 0.3 M Et4NFÁ4HF/DME (15 ml) to which
the substrate 2a (1 mmol) was added, using an undivided
cell under nitrogen atmosphere at ambient temperature.
Constant current (6 mA cm 2) was applied until the starting
material 2a was almost consumed. The course of the reac-
tion was monitored by TLC and GC±MS. After complete
electrolysis, the electrolytic solution was neutralized with
10% NaHCO3 solution followed by extraction with ether
several times. The combined extracts were dried over
anhydrous MgSO4, then evaporated under vacuo and the
product yields were estimated by 19F NMR spectroscopy.
The solvent was removed by evaporation and the oily
residue was puri®ed by passing through column chromato-
graphy on silica gel using hexane/ethyl acetate (5:1) as an
eluent.
ꢀ-[5(6)-Fluoro-2-Benzoxazolylthio]-ꢀ-¯uoroacetonitrile
1
(4c): H NMR ꢁ 7.15 (d, Jˆ49.15 Hz, 1H), 7.15 (m, 1H),
7.30 (dd, Jˆ7.92, 2.64 Hz, 1H), 7.65 (dd, Jˆ8.91, 4.95 Hz,
1H); 19F NMR ꢁ 37.55 (m, 1F), 81.11 (d, Jˆ48.72 Hz,
‡
‡
‡
1F); MS m/e 226 (M ), 199 (M ±HCN), 168 (M ±
CHFCN); HRMS Calcd. for C9H4F2N2OS: m/e 226.0012;
Found 226.0022.
Acknowledgements
We are grateful to Morita Chemical Industries Co., Ltd.
and Chichibu±Onoda Cement Co. for their kind gifts of
Et4NFÁ3HF and N-¯uoropyridinium salts, respectively.
This typical electrolysis procedure was also carried out
for the other benzoxazoles 2b±d and 6 as well as DME.
References
1-(2-Benzoxazolylthio)-1-¯uoro-2-propanone
(3a):
1H NMR ꢁ 2.50 (d, Jˆ3.63 Hz, 3H), 6.81 (d, Jˆ50.48 Hz,
[1] Y. Hou, S. Higashiya, T. Fuchigami, Synlett (1998), 973.
[2] I. Yalcin, I. Oren, E. Sener, A. Akin, N. Ucarturk, Eur. J. Med. Chem.
27 (1992) 401.
1H), 7.33 (m, 2H), 7.49 (m, 1H), 7.65 (m, 1H); 19F NMR ꢁ
‡
85.89 (dq, Jˆ50.55, 3.68 Hz); MS m/e 225 (M ), 183

164
K.M. Dawood et al. / Journal of Fluorine Chemistry 93 (1999) 159±164
[3] I. Yalcin, E. Sener, Int. J. Pharm. 98 (1993) 1.
[21] A. Konno, T. Fuchigami, J. Org. Chem. 62 (1997) 8579.
[22] A. Konno, M. Shimojo, T. Fuchigami, J. Fluorine Chem. 87 (1998)
137.
[4] E. Sener, I. Yalcin, E. Sungur, Quant. Struct.-Act. Relatsh. 10 (1991)
223.
[5] F. Russo, G. Romeo, N.A. Santagati, A. Caruso, V. Cutuli, D. Amore,
Eur. J. Med. Chem. 29 (1994) 569.
[23] S. Narizuka, T. Fuchigami, J. Org. Chem. 58 (1993) 4200.
[24] A.W. Erian, A. Konno, T. Fuchigami, J. Org. Chem. 60 (1995) 7654.
[25] Y. Hou, S. Higashiya, T. Fuchigami, J. Org. Chem. 61 (1996)
8773.
[6] C. Safak, R. Simsek, K. Erol, K. Vural, Pharmazie 51 (1996) 180.
[7] R. Filler, Y. Kobayashi (Eds.), Biomedicinal Aspects of Fluorine
Chemistry, Kodansha and Elsevier Biomedical, Tokyo, 1982.
[8] J.T. Welch, Tetrahedron 43 (1987) 3123.
[26] Y. Hou, S. Higashiya, T. Fuchigami, J. Org. Chem. 62 (1997) 9173.
[27] S. Higashiya, T. Sato, T. Fuchigami, J. Fluorine Chem. 87 (1998)
203.
[9] T. Welch, S. Eswarakrishnan (Eds.), Fluorine in Bioorganic
Chemistry, Wiley, New York, 1991.
[28] T. Fuchigami, S. Narizuka, A. Konno, K. Momota, Electrochim.
Acta 43 (1998) 1985.
[10] H. Yoshioka, C. Nakayama, N. Matsuo, J. Synth. Org. Chem. Jpn. 42
(1984) 809.
[29] W.V. Childs, T. Fuchigami (Eds.), in: The Electrochemical Society,
Electrochemistry in the Preparation of Fluorine and Its Compounds,
Pennington, NJ, 1997, pp. 65±73.
[11] T. Fuchigami, Rev. Heteroatom. Chem. 10 (1994) 155.
[12] S. Narizuka, H. Koshiyama, A. Konno, T. Fuchigami, J. Fluorine
Chem. 73 (1995) 121.
[30] K. Momota, T. Yonezawa, K. Mukai, M. Morita, J. Fluorine Chem.
87 (1998) 173.
[13] T. Fuchigami, T. Fujita, A. Konno, Tetrahedron Lett. 35 (1994) 4153.
[14] T. Fuchigami, M. Sano, J. Electroanal. Chem. 369 (1994) 255.
[15] T. Fuchigami, T. Fujita, J. Org. Chem. 59 (1994) 7190.
[16] T. Fuchigami, A. Konno, K. Nakagawa, M. Shimojo, J. Org. Chem.
59 (1994) 5937.
[31] T. Umemoto, G. Tomizawa, Bull. Chem. Soc. Jpn. 59 (1986) 3625.
[32] T. Umemoto, G. Tomizawa, J. Org. Chem. 60 (1995) 8563.
[33] V. Kalcheva, M. Tosheva, P. Hadjieva, Lieb. Ann. Chem. (1993)
1319.
[17] T. Fuchigami, M. Shimojo, A. Konno, J. Org. Chem. 60 (1995) 3459.
[18] S. Narizuka, T. Fuchigami, Bioorg. Med. Chem. Lett. 5 (1995) 1293.
[19] A. Konno, T. Fuchigami, J. Appl. Electrochem. 25 (1995) 173.
[20] T. Fuchigami, M. Sano, J. Electroanal. Chem. 414 (1996) 81.
[34] V.A. Petrosyan, M.E. Niyazymbetov, L.D. Konyushkin, V.P.
Litvinov, Synthesis. (1990) 841.
[35] V.P. Ahluwalia, M. Alauddin, C.H. Khanduri, V.D. Mehta, Hetero-
cycles 29 (1989) 1729.