Photochromism of bis(2-alkyl-1-benzofuran-3-yl)perfluorocyclopentene derivatives

Article abstract of DOI:10.1021/jo051372z

Photochromic diarylethene derivatives having benzofuran heteroaryl groups, bis(2-methyl-1-benzofuran-3-yl)perfluorocyclopentene and bis(2-butyl-1- benzofuran-3-yl)perfluorocyclopentene, were synthesized, and their photochromic performance was examined in hexane solution as well as in the single-crystalline phase. The compounds exhibited photochromic reactivity even in the single-crystalline phase.

Full text of DOI:10.1021/jo051372z

Photochromism of
Bis(2-alkyl-1-benzofuran-3-yl)perfluorocyclopentene Derivatives
Tadatsugu Yamaguchi* and Masahiro Irie*
Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University,
6
-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan
tyamagu@cstf.kyushu-u.ac.jp; irie@cstf.kyushu-u.ac.jp
Received July 2, 2005
Photochromic diarylethene derivatives having benzofuran heteroaryl groups, bis(2-methyl-1-
benzofuran-3-yl)perfluorocyclopentene and bis(2-butyl-1-benzofuran-3-yl)perfluorocyclopentene, were
synthesized, and their photochromic performance was examined in hexane solution as well as in
the single-crystalline phase. The compounds exhibited photochromic reactivity even in the single-
crystalline phase.
Introduction
and their photochromic performance is superior to that
16
of other derivatives having pyrrole or indole groups. The
latter derivatives undergo thermally reversible photo-
chromic reactions and cannot be used for optical memo-
ries and switches.
Photochromic compounds have attracted considerable
attention because of their potential application to pho-
tonic devices, such as optical memories and switches.
1
,2
Among various thermally irreversible photochromic com-
pounds, diarylethene derivatives are the most promising
candidates for the application because of their thermally
irreversible and fatigue-resistant photochromic perfor-
1,2-Bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclo-
pentene 3a undergoes a thermally irreversible and
1
7
fatigue-resistant photochromic reaction in solution.
Although the benzothiophene heteroaryl moiety can
provide the fatigue-resistant property to a diarylethene,
the compound is photochemically inactive in the crystal-
line phase. The photochromic reactivity of diarylethene
derivatives in the crystalline phase is controlled by their
conformation and distance between reactive carbon at-
oms. An antiparallel conformation and a distance shorter
than 0.42 nm are necessary conditions for the reaction
3
mance. Although many diarylethene derivatives have
been synthesized, most of them are composed of thiophene
or benzothiophene heteroaryl groups.4
-15
These com-
pounds can be prepared by simple synthesis procedures,
(
1) D u¨ rr, H.; Bouas-Laurent, H. Photochromism Molecules and
System; Elsevier: Amsterdam, 1990.
2) Irie, M. Photo-Reactive Materials for Ultrahigh-Density Optical
(
Memory; Elsevier: Amsterdam, 1994.
18
to take place. Although 3a is packed in the antiparallel
(
(
(
3) Irie, M. Chem. Rev. 2000, 100, 1685.
conformation in the crystal, the distance between reactive
4) Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 6136.
5) Hanazawa, M.; Sumiya, R.; Horikawa, Y.; Irie, M. J. Chem. Soc.,
Chem. Commun. 1992, 206.
(12) Rath, S.; Heilig, M.; Al-Khalisy, E.; Klingler, T.; Port, H. J.
Lumin. 2004, 108, 401.
(
6) Yamaguchi, T.; Uchida, K.; Irie, M. J. Am. Chem. Soc. 1997, 119,
6
066.
(13) Jukes, R. T. F.; Adamo, V.; Hartl, F.; Belser, P.; Cola, L. De.
Inorg. Chem. 2004, 43, 2779.
(
7) Nakayama, Y.; Hayashi, K.; Irie, M. J. Org. Chem. 1990, 55,
2
592.
(14) Krayushkin, M. M.; Shirinyan, V. Z.; Bere n˜ kil, L. I.; Shirnkin,
A. A.; Martynkin, A. Y.; Uzhinov, B. M. Russ. J. Org. Chem. 2002, 38,
1335.
(
8) Gilat, S. L.; Kawai, S. H.; Lehn, J.-M. J. Chem. Soc., Chem.
Commun. 1993, 1439.
(
(
9) Peters, A.; Branda, N. R. J. Am. Chem. Soc. 2003, 125, 3404.
10) Takeshita, M.; Kato, N.; Kawauchi, S.; Imase, T.; Watanabe,
(15) Matsuda, K.; Irie, M. J. Am. Chem. Soc. 2000, 122, 7195.
(16) Uchida, K.; Matsuoka, T.; Sayo, K.; Iwamoto, M.; Hayashi, S.;
Irie, M. Chem. Lett. 1999, 835.
(17) Kobatake, S.; Yamada, M.; Yamada, T.; Irie, M. J. Am. Chem.
Soc. 1999, 121, 8450.
J.; Irie, M. J. Org. Chem. 1998, 63, 9306.
(11) Yokoyama, Y.; Shiraishi, H.; Tani, T.; Yokoyama, Y.; Yamagu-
chi, Y. J. Am. Chem. Soc. 2003, 125, 7194.
1
0.1021/jo051372z CCC: $30.25 © 2005 American Chemical Society
Published on Web 11/04/2005
J. Org. Chem. 2005, 70, 10323-10328
10323

Yamaguchi and Irie
carbon atoms is as long as 0.435 nm. When nitro groups
are substituted at the 6-position of the two benzothiophene
rings, the distance decreases to as short as 0.385 nm and
the compound shows a photochromic reactivity in the
crystalline phase.17
Furyl fulgide, which has a furan ring, undergoes
photochromism in solution.1
9,20
This suggests that di-
arylethene derivatives with furan rings are also expected
to undergo photochromism. Theoretical calculation for
1
,2-bis(3-furyl)ethene21 predicts that the derivative un-
dergoes a thermally irreversible photochromic reaction.
In this work, we have synthesized 1,2-bis(2-alkyl-1-benzo-
furan-3-yl)perfluorocyclopentene derivatives and exam-
ined their photochromic reactivity in hexane as well as
in the single-crystalline phase. Bis(2-methylbenzofuryl)-
ethene and bis(2-butylbenzofuryl)ethene were chosen
because the starting chemicals are readily obtained from
the commercial products.
FIGURE 1. (a) Absorption spectra of 1a (solid line) and 1b
(
dashed line) and in the photostationary state (dotted line)
-
5
under irradiation with 313 nm light in hexane (2.43 × 10
mol/L). (b) Absorption spectra of 2a (solid line) and 2b (dashed
Results and Discussion
line) and in the photostationary state (dotted line) under
-
5
Preparation of Diarylethene Derivatives. 1a and
irradiation with 313 nm light in hexane (3.16 × 10 mol/L).
2
a were synthesized using the procedure shown in
Scheme 1. 2-Methyl-1-benzofuran (1.0 equiv) in THF was
for 1 h at -78 °C, treated with water, extracted with
ether, and purified by column chromatography (eluent/
hexane) to give 1a in 46% yield. The synthesis route for
SCHEME 1
2
a was the same as that for 1a. 6 (1.0 equiv) was used
instead of 5. Compound 4a, which has two 2-butyl-1-
benzothiophene rings, was also synthesized as a refer-
ence.
Photochromism in Solution. Figure 1a shows the
absorption spectral change of 1a induced by photoirra-
diation in hexane. Upon irradiation with 313 nm light, a
new absorption band due to the closed-ring isomer 1b
appeared at 469 nm. In the photostationary state, under
irradiation with 313 nm light, 47% of the open-ring
isomer 1a converted to the closed-ring isomer 1b. The
closed-ring isomer has an absorption coefficient as large
4
-1
-1
as 1.44 × 10 M cm (469 nm), which is 1.6 times as
reacted with N-bromosuccinimide (1.1 equiv) at room
temperature to give 3-bromo-2-methyl-1-benzofuran 5 in
3
-1
-1
large as that of 3b (ꢀ517 9.1 × 10
M
cm ). Upon
irradiation with light of a wavelength longer than 550
8
5% yield. A THF solution containing 5 (1.0 equiv) was
nm, the closed-ring isomers reverted to 1a.
cooled to -78 °C, and then 1.6 M n-BuLi hexane solution
(
1.1 equiv) was added to the solution dropwise at -78
As observed for 1a, compound 2a also underwent
photochromism upon irradiation with UV and visible
light in hexane solution. Figure 1b shows the spectral
change of 2a in hexane. In the photostationary state,
upon irradiation with 313 nm light, 66% of the open-ring
isomer 2a converted to the closed-ring isomer 2b, which
has an absorption maximum at 489 nm. The absorption
maximum of the closed-ring isomer showed a bathochro-
matic shift of as much as 20 nm by replacing methyl
substituents with butyl substituents. Upon irradiation
°
°
C. The reaction mixture was stirred for 30 min at -78
C, and then octafluorocyclopentene (0.5 equiv) was
added to the solution. The reaction mixture was stirred
(
18) Kobatake, S.; Uchida, K.; Tsuchida, E.; Irie, M. Chem. Commun.
002, 2804.
19) Darcy, P. J.; Heller, H. G.; Strydom, P. J.; Whittall, J. J. Chem.
2
(
Soc., Perkin Trans. 1 1981, 202.
(
20) Yokoyama, Y.; Goto, T.; Inoue, T. Chem. Lett. 1988, 1049.
21) Nakamura, S.; Irie, M. J. Org. Chem. 1988, 53, 6136.
(
10324 J. Org. Chem., Vol. 70, No. 25, 2005

Photochromism of Diarylethene Derivatives
TABLE 1. Absorption Characteristics and
Photochromic Reactivities of Diarylethene Derivatives
1
-4 in Hexane
/10 dm mol cm-1
4
3
-1
quantum yield
cyclization cycloreversion
ꢀ
(
wavelength/nm)
compound
a
b
1
2
3
4
1.00 (274) 1.44 (469)
1.14 (275) 1.46 (489)
1.41 (258) 0.91 (517)
1.71 (258) 0.93 (536)
0.38 (313)
0.49 (313)
0.35 (313)
0.44 (313)
0.35 (469)
0.24 (489)
0.35 (517)
0.35 (536)
TABLE 2. Maximum Wavelengths (nm) of Closed-Ring
Isomers 1-4 in Various Solvents
FIGURE 2. Thermal stability of closed-ring isomers 1b
circles) and 2b (squares) at 70 °C.
solvent
(
(
dielectric constant)
1
2
3
4
hexane (1.88)
ethyl acetate (6.02)
THF (7.58)
EtOH (24.55)
acetonitrile (37.50)
469
476
479
475
476
489
495
498
495
496
517
522
523
521
521
535
542
545
542
544
calculation21 predicted that thermal stability is dependent
on the aromatic stabilization energy of the aryl group. It
is of interest to examine the thermal stability of di-
arylethene derivatives having benzofuran heteroaryl
groups.
with light of a wavelength longer than 550 nm, the
closed-ring isomers reverted to 2a.
To determine the effect of substituents on the absorp-
tion maximum of the closed-ring isomer (1b and 2b),
compounds 3a and 4a were also synthesized and their
absorption spectral characteristics were examined. Table
Figure 2 shows the thermal stability of the closed-ring
isomers 1b and 2b in toluene at 70 °C. Both isomers are
stable, and the absorption intensity remained constant
even after 175 h. Fatigue resistance was also measured.
The cyclization and cycloreversion could be repeated more
than 1000 times in toluene in the presence of oxygen.
Both 1 and 2 have high fatigue resistance comparable to
that of 1,2-bis (2-methylbenzo-1-thiophen-3-yl)perfluoro-
cyclopentene.
1
shows the absorption maxima of 1-4 in hexane. The
absorption maximum of 4b (536 nm) also showed a
bathochromatic shift of as much as 19 nm upon replace-
ment of methyl substituents with butyl substituents. The
tendency was similar in all solvents examined, such as
ethyl acetate, THF, ethanol, and acetonitrile, as shown
in Table 2. The bulky butyl substituents are effective in
shifting the absorption band of the closed-ring isomers
to longer wavelengths. Similar behavior was reported for
the closed-ring isomer of bis(2-tert-butyl-1-benzothiophen-
X-ray Crystallographic Analysis. Single crystals of
1a, 1b, 2a, and 2b were obtained by recrystallization in
hexane. Figure 3 shows the ORTEP drawings based on
the results of the X-ray crystallographic analyses. Both
1a and 2b are packed in the antiparallel conformation,
and the distances between reactive carbons of 1a and 2a
are 0.356 and 0.372 nm, respectively, which are suf-
ficiently short for the molecules to undergo the photo-
chromic reaction in the crystalline phase. As described
in the Introduction, the distance between the reactive
carbon atoms of 3a is 0.435 nm, and 3a does not show
3
-yl)perfluorocyclopentene.22
Theoretical calculation for the absorption bands of the
closed-ring isomers 1b and 2b was carried out with the
Gaussian 98 and GAMESS program packages.23 The
calculated wavelengths of 1b and 2b are 492.06 nm (f )
1
3
any photochromic reactivity in the crystalline phase.
0
.3131) and 503.09 nm (f ) 0.2701), respectively. The
The difference in the distances between the reactive
carbon atoms is anticipated to affect the reactivity of
these two crystals. The ORTEP drawings of 1b and 2b
indicate that conrotatory photocyclization reactions take
place in these compounds.
absorption wavelengths correlate well with those of
experimental ones. Bulky substituents are considered to
strain the π-conjugated structure and cause the batho-
chromatic shift of the absorption band of the closed-ring
2
4
isomer.
It was not easy to carry out the X-ray crystallographic
analysis of 2b because the 2b crystal decomposes below
-60 °C. Therefore, X-ray crystallographic measurement
was carried out at room temperature. This is the reason
why extremely large thermal disorder is observed, par-
ticularly in butyl and cyclopentene parts.
The difference between 1b and 2b is the angle of
methyl(butyl)-carbon-oxygen. The angles of methyl and
butyl derivatives are 111° and 116°, respectively. Bulky
substituents increase the angles. As described in the
calculation of the absorption spectrum of 1b and 2b, the
constrained structure of 2b shifts the absorption maxi-
mum to a longer wavelength.
Thermal Stability and Fatigue Resistance. The
thermal stability of both open- and closed-ring isomers
is an indispensable property for the application to optical
memory media. The closed-ring isomers of diarylethene3
derivatives having thiophene groups are thermally stable,
whereas diarylethene derivatives having pyrrol rings,
such as 1,2-bis(2-cyano-1,5-dimethyl-4-pyrrolyl)perfluo-
2
5
rocyclopentene, are thermally unstable and return to
the open-ring isomers even in the dark. Theoretical
(
22) Uchida, K.; Tsuchida, E.; Aoi, Y.; Nakamura, S.; Irie, M. Chem.
Lett. 1999, 63.
23) Morimitsu, K.; Kobatake, S.; Nakamura, S.; Irie, M. Chem. Lett.
003, 32, 858.
24) Kobatake, S.; Morimoto, M.; Asano, Y.; Murakami, A.; Naka-
mura, S.; Irie, M. Chem. Lett. 2002, 1224.
25) Uchida, K.; Matsuoka, T.; Sayo, K.; Iwamoto, M.; Hayashi, S.;
Irie, M. Chem. Lett. 1999, 835.
(
2
Photochromism in the Single-Crystalline Phase.
(
1
a and 2a showed photochromism in the single-crystal-
line phase. Figure 4 shows the photographs of the single-
crystalline photochromism of 2a observed under polarized
(
J. Org. Chem, Vol. 70, No. 25, 2005 10325

Yamaguchi and Irie
FIGURE 3. ORTEP drawings of (a) 1a, (b) 1b, (c) 2a, and (d) 2b showing 50% probability displacement ellipsoids.
FIGURE 4. Photographs of single crystals of 2a under polarized light before ((a) θ ) 0°; (b) θ ) 90°) and after ((c) θ ) 0°; (d) θ
90°) irradiation with 365 nm light. θ is the rotation angle of the crystal.
)
light. The crystal surface is (010) face. Before photoirra-
diation, the crystal was colorless (Figure 4a,b). Upon
irradiation with 365 nm light, the crystal turned red at
a certain angle (Figure 4c). When the crystal was rotated
by as much as 90°, it turned to pale red. The red color
reappeared at 270°. The clear dichroism indicates that
the photochromic reaction proceeds in the crystal lattice.
Upon irradiation with 400 nm light, the crystal became
colorless.
Figure 5 shows the polarized absorption spectra of the
UV-irradiated single crystal of 2 at (a) 0° and (b) 90° and
the polar plots of the absorbance at 507 nm. The
absorption maximum of the colored crystal 2b is 507 nm.
The absorption band of the colored isomer showed a
10326 J. Org. Chem., Vol. 70, No. 25, 2005

Photochromism of Diarylethene Derivatives
Experimental Section
-Bromo-2-methy-1-benzofuran (5). To a stirred THF
3
solution (30 mL) containing 2-methyl-1-benzofuran (1.0 g, 7.68
mmol) was slowly added 1.65 g of N-bromosuccinimide (9.30
mmol) at 5 °C. The reaction mixture was stirred for 15 h at
room temperature. The reaction mixture was poured into
sodium thiosulfate solution and extracted with diethyl ether.
The organic phase was dried over anhydrous magnesium
sulfate, and the solution was evaporated in vacuo. The crude
product was purified by column chromatography on silica gel
(
hexane) to give 0.632 g of 5 in 39% yield.
1
5
: colorless liquid; H NMR (200 MHz) δ 2.50 (s, 3H), 7.27-
FIGURE 5. (a) Polarized absorption spectra of a photogener-
ated colored crystal of 2 and (b) polar plots of the absorbance
of 2 at 507 nm.
+
7
.34 (m, 2H), 7.40-7.49 (m, 2H); MS (EI) m/z [M ] 210. Anal.
Calcd for C
9
H
7
BrO: C, 51.22; H, 3.34%. Found: C, 51.21; H,
3
.30%.
,2-Bis(2-methyl-1-benzofuran-3-yl)perfluorocyclopen-
tene (1a). To a stirred THF solution (40 mL) containing 5
0.632 g, 2.99 mmol) was slowly added 1.97 mL of 1.6 M
1
(
butyllithium hexane solution (3.14 mmol) at -78 °C, and the
solution was stirred for 15 min at -78 °C. Then the octafluoro-
cyclopentene (0.199 mL, 1.50 mmol) was added slowly to the
reaction mixture at -78 °C and left to stand with stirring at
-
78 to 30 °C for 12 h. The reaction mixture was poured into
concentrated sodium chloride solution and extracted with
diethyl ether. The organic phase was dried over anhydrous
magnesium sulfate and evaporated in vacuo. The crude
product was purified by column chromatography on silica gel
(
hexane) to give 0.299 g of 1a in 46% yield.
1
1
a: colorless crystals; mp 102-103 °C; H NMR (200 MHz)
FIGURE 6. (a) Polarized absorption spectra of a photogener-
ated colored crystal of 1 and (b) polar plots of the absorbance
of 1 at 504 nm.
δ 2.07 (s, 3H), 2.16 (s, 3H), 7.14-7.30 (m, 4H), 7.36-7.48 (m,
+
2
H), 7.59-7.77 (m, 2H); MS (EI) m/z (M ) 436; Anal. Calcd
for C23 : C, 63.31; H, 3.23%. Found: C, 63.35; H, 3.19%.
-Bromo-2-butyl-1-benzofuran (6). The bromination of
14 6 2
H F O
3
2
-butyl-1-benzofuran (1.00 g, 5.74 mL) and NBS (1.30 g, 7.30
bathochromic shift in comparison with the spectrum in
hexane solution.
mmol) was performed as described for compound 5. The crude
product was purified by column chromatography on silica gel
1a also showed photochromism in the single-crystalline
(hexane) to give 1.17 g of 6 in 81% yield.
1
6
: colorless liquid; H NMR (200 MHz) δ 0.95 (t, J ) 7 Hz,
phase. Upon irradiation with 365 nm light, the colorless
single crystal turned orange. Upon irradiation with 400
nm light, the crystal turned colorless again. The photo-
graphs of 1 under polarized light are shown in the
Supporting Information.
3
2
2
H), 1.31-1.52 (m, 2H), 1.66-1.81 (m, 2H), 2.83 (t, J ) 7 Hz,
+
H), 7.24-7.30 (m, 2H), 7.35-7.46 (m, 2H); MS (EI) m/z [M ]
52. Anal. Calcd for C12H13BrO: C, 56.94; H, 5.18%. Found:
C, 56.84; H, 5.20%.
1,2-Bis(2-butyl-1-benzofuran-3-yl)perfluorocyclopen-
tene (2a). The coupling of 3-bromo-2-butyl-1-benzofuran (1.17
g, 4.64 mmol) and octafluorocyclopentene (0.307 mL, 2.32
mmol) was performed as described for compound 1a. The crude
product was purified by column chromatography on silica gel
Figure 6 shows the polarized absorption spectra of the
closed-ring form of the isomer of 1 at (a) 0° and (b) 90°
and the polar plots of the absorbance at 504 nm. The
crystal surface is (001) face, which is a well-developed
face. The absorption maximum of the colored isomer was
(
hexane) to give 0.351 g of 2a in 29% yield.
1
2
a: colorless crystals; mp 71-72 °C; H NMR (200 MHz) δ
2
2
.07 (s, 3H), 2.16 (s, 3H), 7.14-7.30 (m, 4H), 7.36-7.48 (m,
5
04 nm at 0°. The color change by rotation was not as
+
H), 7.59-7.77 (m, 2H); MS (EI) m/z (M ) 520. Anal. Calcd
remarkable as that observed for single-crystal 2. The low-
order orientation of 1 is estimated by X-ray crystal-
lographic analysis (the data are shown in the Supporting
Information). Compound 3, which has two benzothiophene
units, did not show any photochromism in the single-
crystalline phase. This result suggests that the difference
in the nature between oxygen and sulfur atoms affects
the conformation and photoreactivity of the molecules in
the single-crystalline phase.
for C H F O : C, 66.92; H, 5.03%. Found: C, 67.02; H, 5.07%.
29
26
6
2
3-Bromo-2-butyl-1-benzothiophene (7). The bromination
reaction of 2-butyl-1-benzothiophene (4.96 g, 26.1 mmol) and
NBS (5.61 g, 31.3 mmol) was performed as described for
compound 5. The crude product was purified by column
chromatography on silica gel (hexane) to give 5.28 g of 7 in
75% yield.
1
7: colorless liquid; H NMR (200 MHz) δ 0.96 (t, J ) 7 Hz,
3
2
2
H), 1.35-1.53 (m, 2H), 1.65-1.80 (m, 2H), 2.94 (t, J ) 8 Hz,
+
H), 7.28-7.45 (m, 2H), 7.71-7.76 (m, 2H); MS (EI) m/z [M ]
68. Anal. Calcd for C12H13BrS: C, 53.54; H, 4.87%. Found:
C, 53.57; H, 4.88%.
,2-Bis(2-butyl-1-benzothien-3-yl)perfluorocyclopen-
tene (4a). The coupling of 2-butyl-3-bromo-1-benzothiophene
1.50 g, 5.57 mmol), 1.6 M n-BuLi (3.83 mL, 6.1 mmol), and
octafluorocyclopentene (0.369 mL, 2.78 mmol) was performed
as described for compound 1a. The crude product was purified
by column chromatography on silica gel (hexane) to give 0.508
1
Conclusion
(
New photochromic compounds with benzofuran hetero-
aryl groups were synthesized, and their photochromic
reactivity was examined in hexane solution as well as in
the single-crystalline phase. The compounds underwent
photochromic reactions even in the single-crystalline
phase.
g of 4a in 33% yield.
1
4
a: colorless crystals; mp 109-110 °C; H NMR (200 MHz)
δ 0.70 (t, J ) 7 Hz, 6H), 0.82-1,12 (m, 4H), 1.27-1.50 (m,
4H), 2.18-2.31 (m, 1H), 2.55-2.70 (m, 1H), 7.14-7.42 (m, 6H),
J. Org. Chem, Vol. 70, No. 25, 2005 10327

Yamaguchi and Irie
+
7
.54-7.74 (m, 2H); MS (FAB) m/z [M ] 552. Anal. Calcd for
Supporting Information Available: Photographs of
single crystals of 1 before and after irradiation with UV light
and crystal structures of 1a and 2a. This material is available
free of charge via the Internet at http://pubs.acs.org.
29 26 6 2
C H F S
: C, 63.03; H, 4.74%. Found: C, 63.02; H, 4.78%.
Acknowledgment. This work was supported by a
Grant-in-Aid for Scientific Research (S) (No. 15105006)
from the Japan Society of the Promotion of Science.
JO051372Z
10328 J. Org. Chem., Vol. 70, No. 25, 2005