Diastereospecific photocyclization of a isopropylbenzophenone derivative in crystals and the morphological changes

Article abstract of DOI:10.1021/jo071005i

(Chemical Equation Presented) Reaction of crystals of 2,4,6- triisopropylbenzophenone derivative with the (S)-phenylethylamide group caused diastereospecific Norrish type II photocyclization by UV irradiation to give (R,S)-cyclobutenol as a sole product.

Full text of DOI:10.1021/jo071005i

Diastereospecific Photocyclization of a Isopropylbenzophenone
Derivative in Crystals and the Morphological Changes
Hideko Koshima,*,† Michitaro Fukano,† and Hidehiro Uekusa‡
Department of Materials Science and Biotechnology, Ehime UniVersity, Matsuyama 790-8577, Japan, and
Department of Chemistry and Materials Science, Tokyo Institute of Technology, O-Okayama, Meguro-ku,
Tokyo 152-8551, Japan
koshima@eng.ehime-u.ac.jp
ReceiVed May 12, 2007
Reaction of crystals of 2,4,6-triisopropylbenzophenone derivative with the (S)-phenylethylamide group
caused diastereospecific Norrish type II photocyclization by UV irradiation to give (R,S)-cyclobutenol
as a sole product. In contrast, the solution photolysis gave an almost 1:1 mixture of (R,S)- and (S,S)-
cyclobutenol. The specific diastereodifferentiation in the crystalline state is attributed to the smooth
transformation with minimum molecular motion due to the very similar molecular shapes as well as the
2-fold helical arrangements between the reactant crystal and the product (R,S)-cyclobutenol crystal. UV
irradiation of the bulk crystals led to cracking and breaking into small fragments. In contrast, the
microcrystals maintained the single-crystalline morphology in the course of photocyclization, suggesting
the single-crystal-to-single-crystal transformation.
Introduction
rise to cracks and deformation.7 Atomic force microscopic
(AFM) study made possible the observation that the photo-
dimerizations of trans-cinnamic acids and anthracenes in the
crystalline state induced surface morphological changes at tens
and hundreds of nanometers level by the transportation and
rebuilding of the surface molecules.8 Surface relief gating (SRG)
formation was demonstrated on the single crystal of 4-(dim-
ethylamino)azobenzene by repeated irradiation with two coher-
ent laser beams.9 Reversible shape changes were also repor-
ted on a photochromic diarylethene single crystal by UV
irradiation.10
Since the study of [2 + 2] photocyclization of trans-cinnamic
acids in 1964,1 a large number of crystalline-state reactions
have been reported.2-6 The relationship between the crystal
structures and the reactions has been elucidated so far. In
contrast, the morphological changes on the crystals by the
reactions have been scarcely studied. It was reported that the
topochemical photopolymerization of diolefin crystals gave
* To whom correspondence should be addressed.
† Ehime University.
‡ Tokyo Institute of Technology.
(1) Cohen, M. D.; Schmidt, G. M. J. Chem. Soc. 1964, 1996-2000.
(2) Organic Solid State Chemistry; Desiraju, G. R., Ed.; Elsevier: New
York, 1987.
(3) ReactiVity in Molecular Crystals; Ohashi, Y., Ed.; Kodansha-VCH:
Tokyo, 1993.
(4) Tanaka, K.; Toda, F. Chem. ReV. 2000, 100, 1025-1074.
(5) Organic Solid-State Reactions; Toda, F., Ed.; Kluwer Academic
Publishers: Dordrecht, 2002.
(7) (a) Nakanishi, H.; Hasegawa, M.; Sasada, Y. J. Polym. Sci. Polym.
Chem. Ed. 1972, 10, 1537-1553. (b) Nakanishi, H.; Hasegawa, M.;
Kirihara, H.; Yurugi, T. Nippon Kagaku Zasshi 1977, 1046-1050. (c)
Hasegawa, M. Chem. ReV. 1983, 83, 507-518.
(8) (a) Kaupp, G. Angew. Chem., Int. Ed. Engl. 1992, 31, 592-595. (b)
Kaupp, G. Angew. Chem., Int. Ed. Engl. 1992, 31, 595-598. (c) Kaupp,
G.; Plagmann, M. J. Photochem. Photobiol. A: Chem. 1994, 80, 399-
407.
(6) Chiral Photochemistry; Inoue, Y., Ramamurthy, V., Eds.; Marcel
Dekker, Inc.: New York, 2004.
(9) Nakano, H.; Tanino, T.; Shirota, Y. Appl, Phys. Lett. 2005, 87,
061910.
10.1021/jo071005i CCC: $37.00 © 2007 American Chemical Society
Published on Web 08/10/2007
6786
J. Org. Chem. 2007, 72, 6786-6791

Photocyclization of a Isopropylbenzophenone DeriVatiVe
TABLE 1. Selected Crystal Data
SCHEME 1
(S)-1
(R, S)-2
(S, S)-2
crystal system
space group
a, Å
b, Å
c, Å
orthorhombic
P212121 (#19)
21.341(2)
9.6458(7)
13.273(1)
90.0
2732.2(4)
4
1.108
monoclinic
P21 (#4)
10.119(2)
9.494(1)
14.032(2)
97.056(8)
1337.8(3)
2
tetragonal
P41(#76)
11.1250(8)
11.1250(8)
22.9386(19)
90.0
2839.0(4)
4
1.066
â, deg
V, Å3
Z
D, g cm-3
1.131
The molecular conformation of (S)-1 is very similar to that of
(R,S)-2 but different from that of (S,S)-2. In the crystal (S)-1, a
2-fold helical chain is formed among the amide groups through
the N-H---OdC hydrogen bond (2.17 Å) along the b axis
(Figure 1b). The helical chains are alternatively arranged in
opposite directions and a half translation along the c-axis
(Figure 1c).
In the product crystal (R,S)-2, a 2-fold helical chain similar
to that of (S)-1 is formed among the amide groups through the
N-H---OdC hydrogen bond (2.35 Å) along the b axis (Figure
1e). An intramolecular O-H---OdC hydrogen bond (1.61 Å)
is also formed between the hydroxyl group and the carbonyl
group. The helical chains are arranged in a direction along the
b axis (Figure 1f). Despite the fact that the crystals (S)-1 and
(R,S)-2 belong to different crystal systems, orthorhombic and
monoclinic, respectively, the lengths of the b and c axes of the
unit cell are very similar and the length of the a axis of (R,S)-2
is almost half of that of (S)-1 (Table 1). Therefore, the cell
volume of (R,S)-2 is almost half of that of (S)-1.
In contrast, the crystal system of (S,S)-2 is cubic. A 4-fold
helical chain in the clockwise direction is formed between the
hydroxyl group and the amide group through the intermolecular
H-O---H-N hydrogen bond (2.05 Å) along the c axis (Figures
1h and 1i). Intramolecular O-H---OdC hydrogen bond (1.64
Å) is also formed between the hydroxyl group and the carbonyl
group. Thus, the molecular arrangement is different from those
of (S)-1 and (R,S)-2.
Irradiation of the crystal (S)-1 with high-pressure mercury
lamp through Pyrex glass causes n,π* excitation of the carbonyl
O1 oxygen atom of the (S)-1 molecule (Figure 1a). The excited
O1 atom has the possibility of abstraction of both the methine
H1 and H2 hydrogen atoms of two o-isopropyl groups to
produce (R)- and (S)-ketyl radicals, respectively, and corre-
sponding methine radicals. Then the radicals approach each other
and finally couple to afford (R)- and (S)-cyclobutenol, i.e., both
the diastereomeric products (R,S)-2 (Figure 1d) and (S,S)-2
(Figure 1g). However, only (R,S)-2 was obtained by the
crystalline-state photocyclization; (S,S)-2 was not at all
produced. Hence, we compared all the geometrical parameters
for the two possible hydrogen atom abstractions formulated by
Scheffer.15 The both distances of O1---H1 (2.83 Å) and
O1---H2 (2.97 Å) are enough short and similar to each other.
The angles of C1-O1-H1 (57.3°) and C1-O1-H2 (53.7°)
are similar. Furthermore, the angles defined as the degree to
which the abstracted H1 and H2 hydrogen atoms lie outside
the mean plane of the carbonyl group are similar: 53.7° and
52.9°, respectively. This means that the diastereospecific
O1---H1 hydrogen abstraction could not be explained from the
It is known that 2,4,6-triisopropylbenzophenone derivatives
undergo Norrish type II photocyclization to give the cy-
clobutenol in the solid state and in the solution phase.11 In our
previous papers, it was revealed that the highly enantioselective
photocyclization of the carboxylic acid derivatives was achieved
in the salt crystals with chiral and achiral amines, and the
reactions proceeded via single-crystal-to-single-crystal transfor-
mation.12-14 Herein, we report the diastereospecific photocy-
clization of 2-(2,4,6-triisopropybenzoyl)((S)-1-phenylethyl)ben-
zamide (S)-1 in the crystalline state and the morphological
changes of single crystals.
Results and Discussion
UV irradiation of the pulverized crystals of (S)-1 with a 400
W high-pressure mercury lamp through Pyrex glass under argon
at 20 °C for 8 h caused photocyclization to afford R-cyclobutenol
(R,S)-2 as the sole product in 100% chemical yield and 100%
de (Scheme 1). No diastereomer (S,S)-2 was obtained by silica
gel column chromatography or detected by HPLC analysis,
confirming the diastereospecific photocyclization in the crystal-
line state. In contrast, the solution photolysis of (S)-1 in
acetonitrile gave a mixture of (R,S)-2 and (S,S)-2 in 24% and
21% chemical yield, respectively, revealing the low diastereo-
selectivity of only 6% de.
We carried out the X-ray crystallographic analyses of the
reactant as well as the products in order to elucidate the
mechanism for the diastereospecific photocyclization in the
crystal (S)-1. The single crystals of (R,S)-2 and (S,S)-2 were
prepared by slow evaporation of the acetonitrile solutions. The
absolute structures were determined on the basis of the S
configuration of phenylethylamide group of the molecules. The
selected crystal data are summarized in Table 1.
ORTEP drawings of the reactant (S)-1 and the products
(R,S)-2 and (S,S)-2 are shown in Figure 1a,d,g, respectively.
(10) (a) Irie, M.; Kobatake, S.; Horichi, M. Science 2001, 291, 188-
191. (b) Kobatake, S.; Takami, S.; Muto, H.; Ishikawa, T.; Irie, M. Nature
2007, 446, 778-781.
(11) (a) Ito, Y.; Nishimura, H.; Umehara, Y.; Yamada, Y.; Tone, M.;
Matsuura, T. J. Am. Chem. Soc. 1983, 105, 1590-1597. (b) Ito, Y.;
Matsuura, T. Tetrahedron Lett. 1988, 29, 3087-3090.
(12) (a) Koshima, H.; Maeda, A.; Matsuura, T.; Hirotsu, K.; Okada, K.;
Mizutani, H.; Ito, Y.; Fu, T. Y.; Scheffer, J. R.; Trotter, J. Tetrahedron:
Asymmetry 1994, 5, 1415-1418. (b) Hirotsu, K.; Okada, K.; Mizutani, H.;
Koshima, H.; Matsuura, T. Mol. Cryst. Liq. Cryst. 1996, 277, 99-106.
(13) Koshima, H.; Matsushige, D.; Miyauchi, M. CrystEngComm 2001,
33, 1-3.
(14) Koshima, H.; Kawanishi, H.; Nagano, M.; Yu, H.; Shiro, M.;
Hosoya, T.; Uekusa, H.; Ohashi, Y. J. Org. Chem. 2005, 70, 4490-4497.
(15) Scheffer, J. F. In Organic Solid State Chemistry; Desiraju, G. R.,
Ed.; Elsevier: New York, 1987; pp 1-45.
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Koshima et al.
FIGURE 1. ORTEP drawings of (a) (S)-1, (d) (R,S)-2, and (g) (S,S)-2; the thermal ellipsoids are plotted at the 15% probability level. Molecular
arrangements (b), (c) in (S)-1, (e), (f) in (R,S)-2, and (h), (i) in (S,S)-2; the hydrogen atoms are omitted for clarity. The green dotted lines in (b),
(e), and (h) represent the hydrogen bonds.
geometrical parameters between the carbonyl oxygen atom and
the two γ-hydrogen atoms.
Next, the diastereospecific photocyclization was discussed
based on a cavity, which concept was introduced by Ohashi
and co-workers.16 The void space around the o-isopropyl groups
and the phenone group of the reactant (S)-1 is important because
the moieties are accompanied with large motion by the hydrogen
abstraction by the excited carbonyl oxygen. The surroundings
are well represented by a cavity, which is defined as a space
limited by a concave surface of the spheres of inter- and
intramolecular atoms in the neighborhood. Figure 2 shows the
stereoscopic drawings of cavities for the o-isopropyl groups and
the phenone group of the reactant (S)-1. The cavities have the
volumes of 66.3 (Figure 2a) and 59.0 Å3 (Figure 2b), corre-
sponding to produce (R,S)-2 and (S,S)-2, respectively. The
difference of cavities by 7.3 Å3 is relatively large, and therefore,
the formation of (R,S)-2 in higher priority than (S,S)-2 is
reasonable. However, no formation of (S,S)-2 cannot be
explained from the smaller cavity size.
Recent intensive studies have revealed that crystalline state
reactions proceed generally in the minimum molecular motion
in the crystal lattice because the molecules are arranged at close
positions in three-dimensional regularity and the motion is very
restricted.2-6 As shown in Figure 1a-f, the resemblance of
molecular shapes and 2-fold helical arrangements in the crystals
(S)-1 and (R,S)-2 suggests that the transformation from the
molecule (S)-1 to (R,S)-2 can smoothly proceed within the
FIGURE 2. Stereoviews of the cavities for the o-isopropyl groups
and the phenone group in the crystal (S)-1.
helical chains by UV irradiation due to the minimum motion.
In contrast, the molecular conformation and the 4-fold helical
(16) Ohashi, Y.; Yanagi, K.; Kurihara, T.; Sasada, Y.; Ohgo, Y. J. Am.
Chem. Soc. 1981, 103, 5805-5812.
6788 J. Org. Chem., Vol. 72, No. 18, 2007

Photocyclization of a Isopropylbenzophenone DeriVatiVe
Figure 4a shows the change in the IR spectrum under UV
irradiation. The intensities of the reactant (S)-1 band at 1678
and 1640 cm-1 for the stretching vibration due to the benzophe-
none carbonyl group and the amide carbonyl group, respectively,
decreased with an increasing UV irradiation time. Conversely,
the intensity of the product (R,S)-2 band at 1609 cm-1 for the
stretching vibration due to the amide carbonyl group increased.
Irradiation for longer than 10 min did not change the IR
spectrum, revealing that the photocyclization was completed
within 10 min. The conversion was calculated based on the
intensity change in the absorption at 1678 cm-1 to give the
almost linear relationship with the irradiation time (Figure 4b).
The morphological changes of the microcrystals during the
photocylization were observed by AFM. Figure 5b shows the
AFM image of a piece of microcrystal before irradiation. The
top surface is the (100) plane. In contrast to the bulk crystals,
surprisingly, the microcrystal maintained the single-crystalline
phase in the course of the photocyclization (Figure 5c,d). No
cracks were recognized during and after the completion of
reaction. The shape and the size of microcrystal were the same
as the reactant crystal. We could not understand the reason for
no change of the morphology because the crystal structures
between the reactant (S)-1 and the product (R,S)-2 are different
(Table 1).
FIGURE 3. Morphology of bulk crystals of (S)-1 before and after
UV irradiation: crystals in (b) and (d) were obtained from the crystals
in (a) and (c) after UV irradiation with a high-pressure mercury lamp
under argon at 15 °C for 7 and 20 min, respectively.
arrangement in the crystal (S,S)-2 (Figures 1g-i) are very
different from those of (S)-1 (Figure 1a-c). For the formation
of (S,S)-2, the phenylethylamide group of molecule (S)-1 must
turn to the opposite side. As a matter of course, such the large
motion within the restricted lattice is difficult. This is most
probably the reason why (S,S)-2 is not at all obtained as one of
the diastereomeric products.
We observed the changes of surface morphology of the
crystals of (S)-1 during the photocyclization. UV irradiation of
the bulk crystals led to cracking and finally breaking into small
fragments (Figure 3). We understood the phenomenon unques-
tionably because the crystal structures of the reactant (S)-1 and
the product (R,S)-2 were different, and therefore, the strain was
accumulated to break the single crystals.
Next, the microcrystals of (S)-1 were submitted to UV
irradiation. The microcrystals were prepared by vaporizing the
powder samples on a heater at slightly lower temperature than
the melting point (142 °C), collecting the vapor on a quartz
plate, and crystallizing at room temperature (Figure 5a). The
photocyclization process was monitored by FT-IR spectroscopy.
Hence, powder X-ray diffraction profiles were recorded to
elucidate the reaction process. When the reactant crystals of
(S)-1 were submitted to UV irradiation (Figure 6a), several new
peaks appeared and oppositely the intensities of reactant peaks
decreased (Figure 6b). It reveals that the photocyclization
proceeds with accompanying the phase separation between the
reactant crystals and the product crystals. The profiles changed
continuously and irradiation for 500 min completed the reaction,
giving the product (R,S)-2 (Figure 6c). Interestingly, recrystal-
lization of the product crystals from acetonitrile changed to the
crystal structure (Figure 6d), which was completely co-incident
with the diffraction profile from the X-ray crystallographic
analysis using the single crystal (R,S)-2 obtained by recrystal-
lization from acetonitrile (Figure 6e).
The relatively resemblance of diffraction profiles between
the reactant (Figure 6a) and the product (Figure 6c) suggests
that the single-crystal-to-single-crystal reaction might occur.
However, the photocyclization proceeds from the surface of
single crystal by UV irradiation, and therefore, the strain is
FIGURE 4. (a) IR spectral change and (b) conversion of the photocyclization of (S)-1 in the microcrystals under UV irradiation.
J. Org. Chem, Vol. 72, No. 18, 2007 6789

Koshima et al.
in the microcrystals, suggesting the single-crystal-to-single-
crystal transformation, even the breaking of bulk crystals. X-ray
crystallographic analysis of the product single crystal by careful
UV irradiation of the reactant crystal is now underway. When
complete, the results will be published elsewhere.
Experimental Section
Preparation of (S)-1. 2,4,6-Triisopropyl-2′-carboxybenzophe-
none (0.201 g, 0.57 mmol) and (S)-phenylethylamine (0.20 g, 1.8
mmol) were dissolved in chloroform (10 mL) and THF (1 mL)
followed by addition of triethylamine (2.0 mL, 14 mmol) and BOP
(0.50 g, 1.2 mmol). The solution was stirred at room temperature
for 18 h, and the solvent was evaporated in vacuum. The crude
product was purified via silica gel column chromatography (hexane/
ethyl acetate, 10:1) to afford white powder (S)-1 (0.232 g, 0.51
mmol, 90% yield). Characterization: mp (acetonitrile) 141.2-
143.8 °C (uncorrected); 1H NMR (300 MHz, CDCl3, TMS) δ 1.10
(d, J ) 6.30 Hz, 12H), 1.27 (d, J ) 6.60 Hz, 6H), 1.69 (d, J )
6.60 Hz, 3H), 2.74 (septet, J ) 6.60 Hz, 2H), 2.93 (septet, J )
6.60 Hz, 1H), 5.40-5.49 (m, 1H), 6.18 (d, J ) 8.10 Hz, 1H), 7.05
(s, 2H), 7.27-7.56 (m, 9H); IR (KBr) 3279, 1680, 1640 cm-1. Anal.
Calcd for C31H37NO2: C, 81.72; H, 8.18; N, 3.07. Found: C, 81.85;
H, 8.22; N, 3.16.
FIGURE 5. Microcrystals of (S)-1 and the AFM images of the
morphological change under UV irradiation: (a) and (b) before UV
irradiation, (c) 50% conversion after 4 min, and (d) 100% conversion
after 10 min.
Solid-State Photolysis of (S)-1. Pulverized crystals of (S)-1
(0.200 g, 0.44 mmol) were placed between two Pyrex plates and
irradiated with a 400 W high-pressure mercury lamp under argon
at 20 °C for 8 h to give cyclobutenol (R,S)-2 (0.200 g, 0.44 mmol,
100% yield) as the sole product. Characterization of (R,S)-2: white
1
powder; mp (acetonitrile) 146.2-148.3 °C; H NMR (300 MHz,
CDCl3, TMS) δ 0.75 (s, 3H), 1.22-1.26 (m, 9H), 1.29 (s, 3H),
1.33 (d, J ) 7.02 Hz, 3H), 1.66 (d, J ) 6.90 Hz, 3H), 2.88 (septet,
J ) 6.90 Hz, 1H), 3.03 (septet, J ) 6.90 Hz, 1H), 5.26-5.38 (m,
1H), 6.32 (d, J ) 7.20 Hz, 1H), 6.78 (s, 1H), 7.05 (s, 1H), 7.20-
7.55 (m, 9H); IR (KBr) 3316, 1611 cm-1. Anal. Calcd for C31H37-
NO2: C, 81.72; H, 8.18; N, 3.07. Found: C, 81.76; H, 8.17; N,
3.18.
Solution Photolysis of (S)-1. A solution of (S)-1 (2.51 g, 5.51
mmol) in acetonitrile (100 mL) was internally irradiated with a
100 W high-pressure mercury lamp at room temperature for 54 h
under the bubbling of argon. The solvent was evaporated from the
irradiated solution, and the residue was submitted to silica gel
column chromatography (hexane/ethyl acetate, 10:1) to give (R,S)-2
(0.60 g, 1.34 mmol, 24% yield) and (S,S)-2 (0.536 g, 1.18 mmol,
21% yield) as diastereoisomeric products and remaining starting
(S)-1 (0.851 g, 1.87 mmol, 66% conversion). Characterization of
(S,S)-2: white powder; mp (acetonitrile) 137.2-139.0 °C; 1H NMR
(300 MHz, CDCl3) δ 0.98 (s, 3H), 1.22-1.30 (m, 9H), 1.33 (d, J
) 6.90 Hz, 3H), 1.51 (s, 3H), 1.66 (d, J ) 6.90 Hz, 3H), 2.90
(septet, J ) 6.90 Hz, 1H), 3.07 (septet, J ) 6.90 Hz, 1H), 5.31-
5.42 (m, 1H), 6.30 (d, J ) 7.80 Hz, 1H), 6.83 (s, 1H), 7.07 (s,
1H), 7.25-7.48 (m, 9H); IR (KBr) 3319, 1634 cm-1. Anal. Calcd
for C31H37NO2: C, 81.72; H, 8.18; N, 3.07. Found: C, 81.77; H,
8.18; N, 3.13.
X-ray Crystallograhic Analysis. X-ray diffractions were col-
lected on an imaging plate two-dimensional area detector using
graphite-monochromatized Cu KR radiation. All the crystallographic
calculations were performed by using teXsan18 or CrystalStructure
crystallographic software.19 The structure was solved by direct
methods and expanded using Fourier techniques. The non-hydrogen
atoms were refined anisotropically, and hydrogen atoms were not
refined. Hydrogen atoms attached to carbon atoms were located in
the calculated positions. Absolute structure was determined by the
FIGURE 6. Powder X-ray diffraction profiles: (a) before irradiation
of (S)-1, (b) after irradiation for 20 min, (c) product (R,S)-2 obtained
by irradiation for 500 min, (d) recrystallized (R,S)-2, and (e) profile
from the X-ray crystallographic analysis of the recrystallized
(R,S)-2.
generated within the crystal lattice to lead to crack and break
into the polycrystals. On the other hand, the strain accumulated
in the microcrystals is so small that the reaction can be
completed in keeping the initial single-crystalline shape.
Recently, Takahashi et al. reported that in the topochemical
polymerization by successive [2 + 2] photocyclization of
diolefin derivatives the bulk crystal was broken into frag-
ments during polymerization but the nanocrystals main-
tained the single-crystalline phase in the course of
polymerization.17
In conclusion, the diastereospecific photocyclization of (S)-1
was found to proceed without any change of the morphology
(18) TeXsan, X-ray Structure Analysis Package, Molecular Structure
Corporation, The Woodlands, TX, 1985.
(19) CrystalStructure, Ver. 3.7, Rigaku Corp., Tokyo, 2005.
(17) Takahashi, S.; Miura, H.; Okada, S.; Oikawa, H.; Nakanishi, H. J.
Am. Chem. Soc. 2002, 124, 10944-10945.
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Photocyclization of a Isopropylbenzophenone DeriVatiVe
S configuration of phenylethylamide group of the molecule. The
crystal data are summarized in Table 1.
Supporting Information Available: Details for the general
experimental methods and NMR spectral data and crystal-
lographic information for (S)-1, (R,S)-2, and (S,S)-2. This ma-
terial is available free of charge via the Internet at http://
pubs.acs.org.
Acknowledgment. This work was supported by a Grant-
in-Aid for Scientific Research (KAKENHI) in Priority Area
“Molecular Nano Dynamics” from the Ministry of Education,
Culture, Sports, Science and Technology of Japan and the Asahi
Glass Foundation.
JO071005I
J. Org. Chem, Vol. 72, No. 18, 2007 6791