Chiral helicenoid diarylethene with large change in specific optical rotation by photochromism

Article abstract of DOI:10.1021/jo0620213

A diarylethene possessing one [4]thiaheterohelicene and one benzothiophene, the latter with a chiral methoxymethoxyethyl group on its C-3 position, was proved to work as a switch of specific optical rotation at a wavelength at which both colored and colorless forms have no absorption in solution. The difference of the specific optical rotation was 1300° between the open form and the photostationary state. The specific optical rotation of one of the isolated optically active major colored forms was -4680°. The conversion to the colored form was 64%, and the diastereomeric excess of photocyclization was 47%.

Full text of DOI:10.1021/jo0620213

Chiral Helicenoid Diarylethene with Large Change in Specific
Optical Rotation by Photochromism1,2
Tomoyuki Okuyama, Yutaka Tani, Kentaro Miyake, and Yasushi Yokoyama*
Department of AdVanced Materials Chemistry, Graduate School of Engineering, Yokohama National
UniVersity, Hodogaya, Yokohama 240-8501, Japan
yyokoyam@ynu.ac.jp
ReceiVed September 30, 2006
A diarylethene possessing one [4]thiaheterohelicene and one benzothiophene, the latter with a chiral
methoxymethoxyethyl group on its C-3 position, was proved to work as a switch of specific optical
rotation at a wavelength at which both colored and colorless forms have no absorption in solution. The
difference of the specific optical rotation was 1300° between the open form and the photostationary
state. The specific optical rotation of one of the isolated optically active major colored forms was -4680°.
The conversion to the colored form was 64%, and the diastereomeric excess of photocyclization
was 47%.
Introduction
helical conformation of the open form by the action of the well-
known allylic 1,3-strain,8 which resulted in the generation of
one diastereomer of the colored C-form in up to 94% de.9
When several aromatic rings are fused contiguously next to
each other onto the spatially narrower peripheral, the resulting
polyaromatic compound can no longer be planar and forms a
helical structure. These compounds are called helicenes.10
Helicenes and their closely related compounds can take two
mirror-imaged helical structures: P (plus; right-handed screw)
Diarylethenes3 form one of the thermally irreversible pho-
tochromic families, which also include fulgides,4 arylbuta-
dienes,5 and aryloxynaphthacenequinones.6 Because of the
fatigue resistivity and easy accessibility to the specially designed
compounds, diarylethenes have attracted much attention as
molecular-level switches.7 We have recently reported that one
stereogenic carbon atom located at the end of the peripheral of
the photoreacting hexatriene moiety of a diarylethene molecule
1O (open form, O-form) worked as the screw-bolt to fix the
(8) (a) Johnson, F. Chem. ReV. 1968, 68, 375. (b) Hoffmann, R. W. Chem.
ReV. 1989, 89, 1841. (c) Yokoyama, Y.; Kawashima, H.; Masaki, H. Chem.
Lett. 1989, 453. (d) Yokoyama, Y.; Kawashima, H. Kohno, M.; Ogawa,
Y.; Uchida, S. Tetrahedron Lett. 1991, 32, 1479. (e) Yokoyama, Y.;
Tsuchikura, K. Tetrahedron Lett. 1992, 33, 2823. (f) Moritani, Y.;
Fukushima, C.; Miyagishima, T.; Ohmizu, H.; Iwasaki, T. Bull. Chem. Soc.
Jpn. 1996, 69, 2281. (g) Adam, W.; Alsters, P. L.; Neumann, R.; Saha-
Moeller, C. R.; Sloboda-Rozner, D.; Zhang, R. J. Org. Chem. 2003, 68,
1721.
(9) (a) Yokoyama, Y.; Shiraishi, H.; Tani, Y.; Yokoyama, Y.; Yamaguchi,
Y. J. Am. Chem. Soc. 2003, 125, 7194. (b) Kose, M.; Shinoura, M.;
Yokoyama, Y.; Yokoyama, Y. J. Org. Chem. 2004, 69, 8403. (c) Yokoyama,
Y. Chem. Eur. J. 2004, 10, 4389.
* To whom correspondence should be addressed. Phone/Fax: +81-45-339-
3934.
(1) Studies on Helicenoid Diarylethenes, Part 1.
(2) Part of this work was presented at the 21st International Conference
on Photochemistry, Nara, Japan, Abstract p 155, 5A05, July, 2003.
(3) Irie, M. Chem. ReV. 2000, 100, 1685.
(4) Yokoyama, Y. Chem. ReV. 2000, 100, 1717.
(5) Shrestha, S. M.; Nagashima, H.; Yokoyama, Y.; Yokoyama, Y. Bull.
Chem. Soc. Jpn. 2003, 76, 363.
(6) (a) Barachevsky, V. A. In Organic Photochromic and Thermochromic
Compounds; Crano, J. C., Guglielmetti, R. J., Eds.; Plenum Press: New
York, 1999; Vol. 1, Main Photochromic Families, Chapter 7, p 267. (b)
Yokoyama, Y.; Fukui, S.; Yokoyama, Y. Chem. Lett. 1996, 355.
(7) Irie, M. In Molecular Switches; Feringa, B. L. Ed.; Wiley-VCH:
Weinheim, Germany, 2001; Chapter 2, p 37.
(10) (a) Iwasaki, T.; Nishide, H. Curr. Org. Chem. 2005, 9, 1665. (b)
Grimme, S.; Haren J.; Sobanski, A.; Voegtle, F. Eur. J. Org. Chem. 1998,
1491.
10.1021/jo0620213 CCC: $37.00 © 2007 American Chemical Society
Published on Web 02/08/2007
1634
J. Org. Chem. 2007, 72, 1634-1638

Helical Diarylethene with Large Optical Rotation Change
SCHEME 1. Structure and Synthetic Route of 3O and 6O
FIGURE 1. Diarylethene 1O and its hypothetical extension to the
helicenoid structure (2O).
and M (minus; left-handed screw). When the ring number of
the helicene is more than six, the interconversion between the
helicities requires high temperature because of the steric
crowding.11 Helicenes are known to have large optical rotation
values.12 Therefore, if an enantiomerically pure (or at least
enriched) helicenoid structure is incorporated in the diarylethene
skeleton of the C-form by using both aromatic wings, then the
helicenoid can be generated and destroyed by photoirradiation
with a large change in optical rotation values.
Although the introduction of a helicene-like structure into
diarylethene by extending the array of the aromatic rings of
1O can generate the double-helicene-like structure 2O whose
helicity might be controlled by the stereogenic carbon atom, it
was proved that the steric repulsion between the helicene wings
and the hexafluorocyclopentene ring prevented the synthetic
reaction of the corresponding O-form (Figure 1). Therefore we
decided to construct the allylic-strain-controlled chiral helicenoid
structure beneath the hexafluorocyclopentene. Although race-
mic13 and chiral14 helicenoid diarylethenes have been nicely
constructed by Branda and co-workers, we intended to control
the helicity by only one stereogenic carbon atom. We here show
the synthesis, photochromism, and optical properties of a chiral
helicenoid diarylethene that shows a large change in specific
optical rotation induced by photochromism.
Results and Discussion
Molecular Design. The basic structure of the compound we
synthesized belongs to the diarylethenes, which form dihydro-
helicene-like structures by photoirradiation. The initial dia-
rylethene can be restored by irradiation of light of another,
always longer, wavelength to the dihydrohelicene derivative.
When a diarylethene has two condensed aromatic rings on the
perfluorocyclopentene and each wing is not long enough to form
a helicene by itself, irradiation of UV light to induce the
photochemical electrocyclization may give the dihydrohelicene
of which the aromatic-ring array is long enough to prohibit the
change in its helicity. It is advantageous that the dihydrohelicene
skeleton enhances the helicity of the whole molecular structure
because the hybridization of the bond-forming carbon atoms
changes from planar sp2 to tetrahedral sp3 by the photoreaction.
When an intramolecular regulation to determine the helical
sense of the hexatriene moiety would work in the open form,
UV irradiation would produce the dihydrohelicene in a stereo-
controlled manner. We employed the allylic strain that success-
fully controlled the stereochemistry of the hexatriene moiety
in 1O and related compounds.9 We therefore designed dia-
rylethene 3O. As for the condensed aromatic arrays, we chose
4 and 5, because they are too short to generate the helical
structure alone, but after the photoreaction, they would coop-
eratively form a long-enough dihydrohelicene structure.
We also synthesized 6O with two longer wings 4 possessing
no stereogenic carbon atom, for the comparison of the photo-
reactivity.
Synthesis and Structure Determination. As shown in
Scheme 1, the longer wing 4 was synthesized via photochemical
oxidative ring closure of 7, which was prepared from 3-meth-
ylthiophene and 2-formylbenzothiophene. The shorter wing 5
was synthesized from 3-acetylbenzothiophene, which was
obtained as the inseparable mixture with 2-acetylbenzothiophene
by Friedel-Crafts reaction of benzothiophene and acetyl
chloride. The regioisomers were separated after lithiation of the
mixture of 2/3-(1-methoxymethoxyethyl)benzothiophene with
butyllithium followed by the reaction with octafluorocyclopen-
tene, because the hydrogen only at C-2 (only the desired
compound (5) possesses) could be abstracted by butyllithium
to generate the carbanion. Successive introduction of 4 gave
3O. The ORTEP drawing of the X-ray crystallographic analysis
of racemic 3O is shown in Figure 2. Detailed crystallographic
(11) (a) Martin, R. H.; Marchant, M. J. Tetrahedron 1974, 30, 347. (b)
Borkent, J. H.; Laarhoven, W. H. Tetrahedron 1978, 34, 2565.
(12) (a) Newman, M. S.; Lutz, W. B.; Lednicer, D. J. Am. Chem. Soc.
1955, 77, 3420. (b) Newman, M. S.; Lednicer, D. J. Am. Chem. Soc. 1956,
78, 4765.
(13) Norsten, T. B.; Peters, A.; McDonald, R.; Branda, N. R. J. Am.
Chem. Soc. 2001, 123, 7447.
(14) Wigglesworth, T. J.; Sud, D.; Norsten, T. B.; Lekhi, V. S.; Branda,
N. R. J. Am. Chem. Soc. 2005, 127, 7272.
J. Org. Chem, Vol. 72, No. 5, 2007 1635

Okuyama et al.
FIGURE 3. UV-vis spectra of 6O, 6C (calculated spectrum), and
the photostationary state (PSS) of 405-nm light irradiation to 6O in
toluene.
FIGURE 2. ORTEP drawing of 3O with thermal ellipsoids drawn at
the 30% probability level. All hydrogen atoms except H(14) on the
stereogenic carbon atom (C(29)) are omitted for clarity.
information is described in Supporting Information. The struc-
ture clearly shows the effect of the allylic strain. The smallest
substituent (H) on the stereogenic carbon atom on C-3 of the
benzothiophene comes close to the hexafluorocyclopentene ring
because of the steric repulsion between the hexafluorocyclo-
pentene and other groups (methyl and methoxymethoxyl
(MOMO) groups) on the stereogenic carbon atom. Although
the quadruple-ring wing is located close to the rather smaller
methyl group in the crystal, it is not the only conformation in
solution (vide infra).
FIGURE 4. Diarylethenes 8O and 9O reported by Branda et al.13
TABLE 1. Absorption Spectral Data and 6O/6C Ratio of
Photostationary State (PSS) with 405-nm Light Irradiation in
Toluene
λ
max/nm (ꢀ/mol-1dm3cm-1
)
Introduction of two molecules of 4 to octafluorocyclopentene
gave 6O. The ORTEP drawing of 6O, together with the detailed
crystallographic information, is shown in Supporting Informa-
tion.
O-form
C-form
6O/6C at PSS
374 (17770)
501 (3550)
20/80
Unfortunately, all C-forms of 3 and 6, either as the racemic
mixture, diastereomeric mixture, or isolated enantiomer or
diastereomer, did not give crystals suitable for X-ray crystal-
lographic analysis.
1H NMR spectra of 3O and 6O are shown in Supporting
Information. Both spectra show that the interconversion between
antiparallel and parallel conformations, the former cyclizable
and the latter cyclization impossible upon photoirradiation, is
so fast that only the averaged signals can be observed. The
conformation issue of the related diarylethenes is discussed in
the following paper.15
Photochromism. Irradiation of 405-nm light to a yellow
solution of 6O in toluene gave the red solution of the
photostationary state of 6O and 6C. The absorption spectra of
6O, 6C, and the photostationary state are shown in Figure 3.
The absorption maximum wavelengths, molar absorption coef-
ficients, and the ratio of 6O and 6C at the photostationary state
of 405-nm light irradiation are listed in Table 1. Branda and
co-workers examined the photochromic reactions of helicenoid
diarylethenes 8O and 9O (Figure 4).13 The diarylethene 9O,
without the methyl groups on the ring-closing carbon atoms on
the thiophene moiety, yielded the photocyclized compound,
which was isolated as the fully aromatized dehydro compound
9C′ in good yield. On the other hand, 8O with methyl groups
on the ring-closing carbon atoms did not give the cyclized
compound with UV light irradiation at any wavelength and at
the elevated temperature. Contrary to the results for 8O, our
compound 6O, with the same number of the aromatic rings as
8O, gave the cyclized compound. It came from the difference
in the nature of the third ring counting from the ring-closing
thiophene ring: six-membered benzene ring of 8O and five-
membered thiophene ring of 6O. The tight winding of the helical
structure in 8O caused by the six-membered ring compared with
the rather loose winding of 6O, in view of the steric repulsion
between both ends of the helicenoid, might be one of the reasons
for the lack of photoreactivity of 8O.
Next we focused on the photochromism of 3O. First we
resolved the enantiomers of 3O (3Of and 3Os, moving more
quickly (3Of) and more slowly (3Os) in the chiral column (Daicel
Chiralpac OD-H) of HPLC).
Absorption spectral change by 405-nm light irradiation to a
toluene solution of 3Of is shown in Figure 5. The absorption
maximum at 485 nm is attributed to the formation of 3C. HPLC
analysis proved that a major C-form (3Cf/major: the major
C-form generated from 3Of) and a minor C-form (3Cf/minor
:
the minor C-form generated from 3Of) were formed. The ratio
of the isomers 3Of, 3Cf/major, and 3Cf/minor at the photostationary
(15) Tani, Y.; Ubukata, T.; Yokoyama, Y.; Yokoyama, Y. J. Org. Chem.
2007, 72, 1639.
1636 J. Org. Chem., Vol. 72, No. 5, 2007

Helical Diarylethene with Large Optical Rotation Change
1
FIGURE 6. Assignment of methyl groups of H NMR measurement
in Table 3.
TABLE 3. 1H NMR (300 MHz) Signals of Methyl Groups of 3O in
Different Solvents
chemical shift of methyl
protons/δ (coupling data)
secondary
methyl
(1)
aromatic
methyl
(2)
MOM
methyl
(3)
deuterated solvent
(diastereomer excess)
FIGURE 5. UV-vis spectra of 3Of, 3Cf (calculated spectrum), and
the photostationary state (PSS) of 405-nm light irradiation to 3Of in
toluene.
cyclohexane-d12 (32% dea)
toluene-d8 (41% de)
CDCl3b (-)
0.946 (d, 6.6 Hz)
0.919 (d, 6.6 Hz)
0.949 (d, 6.6 Hz)
0.878 (d, 6.8 Hz)
2.62 (s)
2.49 (s)
2.65 (s)
2.58 (s)
3.10 (s)
2.97 (s)
3.22 (s)
3.15 (s)
TABLE 2. Conversion Ratio and Diastereomeric Excess (de)
Values of 3O in Several Solvents with the Indices of Solvent
Polarity, ET(30)
CD3OD (41% de)
a Data in hexane. b 270 MHz.
entry
solvent (ET(30))
3O/3C
de (%)
1
2
3
4
hexane (31.0)
toluene (33.9)
methanol (55.4)
ethyl acetate (38.1)
37/63
33/67
35/65
36/64
32
41
41
47
were measured. We chose toluene-d8, methanol-d4, and cyclo-
hexane-d12, as well as the common solvent CDCl3. In all
solvents, only a set of signals were observed even though the
existence of the antiparallel and parallel conformers as well as
the combination of the chirality of the stereogenic center and
the helicity of the whole molecular shape were expected. We
concluded that it is because of the fast exchange of the
conformers. Chemical shifts of signals of doublet and singlet
methyl groups around the hexatriene array as well as the methyl
signal on the MOM group (Figure 6) in different deuterated
solvents are listed in Table 3.
In all cases, the conformational isomers were not observed.
This means that all possible conformational isomers exchange
rapidly in solution at room temperature. In toluene and in
methahol, the diastereomeric excess is higher than in hexane.
The chemical shifts of the doublet methyl group in methanol
and in toluene are located in the higher magnet field compared
with that in cyclohexane. It may be the result of close proximity
of the right side wing to the secondary methyl group when 3O
takes the stable cyclization-possible conformation, which will
increase the diastereoselectivity. The same tendency can be seen
for the aromatic methyl group. The chemical shifts in toluene
and in methanol are at a higher field than that in cyclohexane.
The shielding effect of the magnetic field by the aromatic rings
worked more strongly to the methyl group in toluene and in
methanol than in cyclohexane. To the contrary, no such
influence was observed for the methyl group on the MOM group
because the MOM group suffers the solvent effect strongly as
it is located outside of the molecule.
state was determined to be 33/47/20 by HPLC. The conversion
ratio (O/C) was 33/67. The diastereomeric excess was 41%.
The solvent effects on the ratio of isomers were then
examined. We chose three other solvents (hexane, methanol,
and ethyl acetate) in addition to the toluene commonly used in
our laboratory. The conversion ratios, the diastereomeric excess
values, and the indices of polarity of the solvents are shown in
Table 2. The best diastereomeric excess (47% de) was observed
in ethyl acetate.
The reason for the lower diastereomeric excess of 3O (47%
de), compared with that of 1O and the related compounds (88-
94% de), was explained as follows. It is apparent that the allylic
strain worked well to fix the conformation of the O-form around
the stereogenic center with regard to the double bond on the
benzothiophene. The diastereomeric excess is determined mainly
by the ratio of the conformational isomers in which the larger
right wing comes closer to either the sterically and electronically
less repulsive methyl group on the stereogenic center (more
stable conformation) or closer to the sterically and electronically
more repulsive methoxymethoxyl group (less stable conforma-
tion). The only difference in the arrangement of the molecular
structure around the hexatriene moiety between 1O and 3O is
the position of the sulfur atom. In 1O, the sulfur atom on the
other benzothiophene is close to the methoxymethoxyl group
when it takes the unfavorable conformation. On the other hand,
the corresponding sulfur atom in 3O is now far from the
methoxymethoxyl group even in the unfavorable conformation.
Therefore the repulsion between the sulfur atom on the aromatic
ring close to the perfluorocyclopentene and the oxygen atoms
on the methoxymethoxyl group in 1O and 3O played a crucial
role in determining the diastereoselectivity of the photocycliza-
tion.
Change in Specific Optical Rotation. We resolved the
enantiomers of 6C by HPLC. The optical rotation of one of the
enantiomers of 6C (6Cf; faster-moving enantiomer on Daicel
Chiralpac OD-H with 1 v/v % 2-propanol in hexane as the
eluent) in toluene was -4510° at 633 nm and -1170° at 820
nm. Although 6Cf is chiral, it will generate the non-chiral 6O
when it is irradiated with visible light because its O-form exists
as a mixture of racemic conformers. Although the change in
optical rotation may be repeated when the resolved 6C is
In order to obtain further information about the conformation
of 3O in solution, 1H NMR spectra of 3O in different solvents
J. Org. Chem, Vol. 72, No. 5, 2007 1637

Okuyama et al.
TABLE 4. Specific Optical Rotation Values of 3Of and PSS in
Several Solvents
the effect of the ring number (7 for 3Cf/major and 9 for 6Cf) is
not so large.
[R]D (deg)
entry
solvent
hexane
toluene
methanol
ethyl acetate
de (%)
3Of
PSS
Conclusion
1
2
3
4
32
41
41
47
-64
-74
-66
-69
-804
-1320
-1170
-1370
Two helicenoid diarylethenes, 3O with a stereogenic center
and 6O without a chiral unit, were synthesized. Photochromic
ring closure of 6O afforded racemic 6C, which was resolved
into enantiomers. The optical rotation value of the resolved 6C
was 4510° at 633 nm. On the other hand, 3O generated a pair
of diastereomers. Its largest diastereomeric excess was observed
in ethyl acetate (47% de), which showed, after optical resolution
of its O-form, a large change in specific optical rotation at a
wavelength where both O- and C-forms have no absorption
(1300° at 589 nm). Thus, the photochromic 3O/3C system was
proved to work as a switch of the optical rotation, which can
be detected without inducing photochromic reaction. The fact
that the diastereomeric excess of 3O was not as high as that of
1O and the analogs (88-94% de) was attributed to the lack of
the electronic repulsion between the oxygen atoms of meth-
oxymethoxyl group and the sulfur atom in the heteroaromatic
rings. Efforts to restore the electronic repulsive nature to attain
high diastereoselectivity are described in the following paper.15
introduced to a rigid matrix such as a high-Tg polymer film,
the helical chirality is loose and unreliable when the compound
experiences the hot ground state just after the photochemical
isomerization. In order to repeat the change in optical rotation
by photoirradiation securely, an enantiomerically pure (or highly
biased) chiral compound with a rigid chirality such as the
optically resolved 3O is required.
We resolved 3O into enantiomers in a manner similar to the
resolution of 6C, and observed the change in specific optical
rotation values by the photochromic reaction of 3Of. The results
are listed in Table 4. As expected, the largest difference in
optical rotation values (PSS -1370°, 3Of -69°, difference
1300°) at 589 nm (sodium D-line) was recorded in ethyl acetate.
The difference of specific optical rotation outside the absorption
band between O-form and PSS is comparable to the data of
Branda et al.14 Although we can observe the change in optical
rotation values between the O-form and the photostationary state
(PSS), the optical rotation value of the resolved 3C is scientifi-
cally interesting. Thus, we separated 3Cf/major from the PSS
mixture. Unfortunately, as 3Cf/minor was not completely sepa-
rated from 3Of by the preparative scale HPLC, the specific
optical rotation of only 3Cf/major was measured. The specific
optical rotation of 3Cf/major at 589 nm was -4680° in ethyl
acetate. This result shows that the magnitude of specific optical
rotation of 3Cf/major is larger than that of 6Cf, though the number
of the rings of 3Cf/major is smaller.
Comparison of the optical rotation values of different
compounds is difficult because (i) specific optical rotation value
is not directly linked to the rotatory power per mole (or per
molecule) because the normalization is done by the mass-based
concentration of the solution, and (ii) specific optical rotation
value is larger when the measuring wavelength is closer to the
absorption band of the compound. In addition, as for the
photochromic compounds, the measuring wavelength should be
outside of the absorption band in order not to change the
photochromic state. This problem is quite serious because the
absorption maximum of the colored form is usually in the visible
region, and the default wavelength to measure optical rotation
is 589 nm (sodium D-line). In spite of these difficulties, we
would compare the optical rotatory power by the molar optical
rotation value [Φ]λ, at the wavelength close to but outside of
the longest absorption band. The molar optical rotation is
correlated to the specific optical rotation values [R]λ by the
equation
Experimental Section
Details of synthesis, purification, optical resolution, and structure
identification including measurement of optical rotation and X-ray
crystallographic analysis of compounds are described in Supporting
Information.
Photochemical reactions at 405 nm in solvents, such as ethyl
acetate (1.50 × 10-4 M), were carried out in a 10 mm path length
quartz cell, using a 500 W high-pressure mercury lamp that was
separated by filters (5-cm water filter, UV-35 and V-44 glass filters,
and a KL-40 interference glass filter). Photochemical reactions at
517 nm were carried out using a 500 W xenon lamp that was
separated by filters (IRA-25S and Y-52 glass filters and a KL-50
interference glass filter). During the photoreaction, solutions in the
cell were stirred continuously.
Change in component concentration as a function of irradiation
time during photoreaction was followed by a high-pressure liquid
chromatography using as eluent a mixture of ethyl acetate and
hexane for the silica gel column and a mixture of 2-propanol and
hexane for the chiral column.
Acknowledgment. The authors thank Dr. Yoshitaka Yamagu-
chi for X-ray crystallographic analyses of diarylethenes and
Zeon Corp. for the generous gift of octafluorocyclopentene. This
work was supported by a Grant-in-Aid for Scientific Research
(A) (no. 16205025) and a Grant-in-Aid for Scientific Research
on Priority Area (404) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. The authors thank the
Analytical Instrument Center of Yokohama National University
1
for the use of H NMR and MS facilities.
[Φ]λ ) [R]λ × MW/100
Supporting Information Available: Experimental details of
synthetic procedures, H NMR spectra, ORTEP drawing of 6O,
characterization data of all new compounds, experimental details
of X-ray crystallographic analysis, crystallographic data of 3O and
6O, and CIF files of 3O and 6O. This material is available free of
charge via the Internet at http://pubs.acs.org.
where MW is the molecular weight and λ is the measuring
wavelength. The molar optical rotation value of 3Cf/major are
-30380° in ethyl acetate at 589 nm and -30700° in toluene at
633 nm, respectively. While the value of 3Cf/major was measured
at a wavelength just a little longer than where the absorption
band reached the baseline, that for 6Cf was measured a little
far from the absorption band. This should be the reason why
1
JO0620213
1638 J. Org. Chem., Vol. 72, No. 5, 2007