Naphthalene- and anthracene-based aromatic foldamers with iminodicarbonyl linkers: Their stabilities and application to a chiral photochromic system using retro [4 + 4] cycloaddition

Article abstract of DOI:10.1021/jo0611317

A circular dichroism (CD) spectral study on chiral aromatic chain imides possessing anthracene and naphthalene moieties with bulky N substituents showed that their helical chirality based on folding remained for a reasonably long time without racemization

Full text of DOI:10.1021/jo0611317

Naphthalene- and Anthracene-Based Aromatic Foldamers with
Iminodicarbonyl Linkers: Their Stabilities and Application to a
Chiral Photochromic System Using Retro [4 + 4] Cycloaddition
Hyuma Masu,† Ikuko Mizutani,‡ Takako Kato,† Isao Azumaya,† Kentaro Yamaguchi,†
Keiki Kishikawa,‡ and Shigeo Kohmoto*,‡
Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Chiba UniVersity, 1-33,
Yayoi-cho, Inage-ku, Chiba 263-8522, Japan and Faculty of Pharmaceutical Sciences at Kagawa Campus,
Tokushima Bunri UniVersity, 1314-1, Shido, Sanuki, Kagawa 769-2193, Japan
kohmoto@faculty.chiba-u.jp
ReceiVed June 2, 2006
A circular dichroism (CD) spectral study on chiral aromatic chain imides possessing anthracene and
naphthalene moieties with bulky N substituents showed that their helical chirality based on folding remained
for a reasonably long time without racemization in solution. Racemization due to conformational
equilibration occurred very slowly, requiring over 1 week at ambient temperature. Their CD spectra both
in solution and in the solid state gave similar CD signals, suggesting retention of helicity observed in the
solid state even after dissolving. As an application of this novel chiral folding of aromatic chain imides,
a chiral photochromic system was investigated based on the photo [4 + 4] cycloaddition and its thermal
cycloreversion of an anthracene-naphthalene system. The foldamer possessing an anthracene moiety in
the center connected with two naphthalene moieties below and above it by iminodicarbonyl linkers was
prepared for this purpose. Induced CD was observed for the foldamer with (S)-1-(1-naphthyl)ethyl
substituents at the imide nitrogen atoms. Chiral photochromic cycles were monitored by CD spectral
measurement.
Introduction
forces. By applying these interactions, a variety of foldamers
with unique structural features have been prepared.2 Our interest
is to develop a helically chiral folding building block with
conformational stability, which contributes in designing chiral
self-assembled foldamers.3 To the best of our knowledge, there
is no report on a simple chiral foldamer without an asymmetric
In recent years, much work has been done to explore new
types of artificial higher order structures in order to understand
and mimic the secret of elaborate biological systems.1 One of
the key structural elements of a unique three-dimensional
structure of biological macromolecules is folding governed by
hydrogen bonds, electrostatic interactions, and hydrophobic
(2) (a) Gellman, S. H. Acc. Chem. Res. 1998, 31, 173-180. (b) Hill, D.
J.; Mio, M. J.; Prince, R. B.; Hughes, T. S.; Moore, J. S. Chem. ReV. 2001,
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Chem. Res. 2006, 39, 11-20.
† Tokushima Bunri University.
‡ Chiba University.
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Chem. Res. 2000, 33, 745-754. (b) Nakano, T.; Okamoto, Y. Chem. ReV.
2001, 101, 4013-4038. (c) Lan, T.; McLaughlin, L. W. Biochemistry 2001,
40, 968-976. (d) Brooks, C. L., III Acc. Chem. Res. 2002, 35, 447-454.
(e) Straub, J. E.; Guevara, J.; Huo, S.; Lee, J. P. Acc. Chem. Res. 2002, 35,
473-481. (f) Wang, W.; Wan, W.; Zhou, H.-H.; Niu, S. Li, A. D. Q. J.
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10.1021/jo0611317 CCC: $33.50 © 2006 American Chemical Society
Published on Web 09/20/2006
J. Org. Chem. 2006, 71, 8037-8044
8037

Masu et al.
center or the aid of a chiral auxiliary. It is known that the folded
conformations of aromatic amide oligomers are more stable than
those of aliphatic amides.4 Considering this, we examined the
stability of the folded conformation of aromatic chain imides.
Two aromatic moieties connected at the iminodicarbonyl linker
may have a conformation in which they face each other due to
the planarity and preferred cis conformation of an amide unit.5
It is also known that optically active atropisomeric aromatic
amides have a long half-life for racemization in solution.6
Aromatic chain imides can be considered where two units of
atropisomeric aromatic amides are connected. Therefore, a
conformationally stable concave-shaped folding unit can be
created. In our previous study, we found that aromatic chain
imides 1 and 2 gave chiral crystals upon recrystallization though
the molecules have no asymmetric carbon.7 They had concave-
shaped folded conformations in crystals examined by single-
crystal X-ray analysis. We have been interested in the stability
of their folded conformations and whether the helical chirality
observed in the crystalline state can be preserved even in
solution.8 In the first part of this paper we report on the simplest
chiral concave-shaped foldamer (or a unit of foldamer) which
shows reasonably stable helical chirality even in solution at
ambient temperature. In the last part of the paper, we report on
the application of this concave-shaped aromatic foldamer to a
chiral photochromic system which is used as a potent tool for
nanotechnology,9 especially for optical data storage and process-
ing.10 Generally, bistable molecular systems are used for this
purpose. The cis-trans photoisomerization of sterically over-
crowded alkenes is among the most extensively studied ex-
ample.11 The cis-trans photoisomerization of pendant azoben-
zenes is frequently utilized in liquid crystalline polymers for
this purpose.12 Recently, the reaction was applied to molecular
shuttles.13 In addition to these applications, the 1,6-electrocy-
clization of diarylethenes14 and fulgides,15 typical photochromic
systems, have been applied as well. A few examples employ
redox systems.16 In light of the progress in this area, an entry
of a novel chiral photochromic system based on a new concept
using hitherto unexplored reactions is highly desirable. Fol-
damers containing chromophoric functionalities are of current
interest.17 Cyclodimers of coumarin18 and anthracene19 are well
known to show their retro cycloreversion. A difficulty associated
with applying an aromatic [4 + 4] cycloadduct for a chiral
photochromic system is control of molecular chirality after retro
cycloreversion. To keep the molecular chirality to manifest its
induced CD in solution, the geometrical relation of two aromatic
moieties regenerated by cycloreversion should be the same as
the original one with the same helicity. This is hard to
accomplish with a linearly tethered molecule due to its flex-
ibility. In view of this, we considered that the conformational
stability of aromatic chain imides is useful for creation of a
new chiral photochromic system.
Herein, we report the construction of a simple helically chiral
foldamer unit and its application to a novel chiral photochromic
system based on aromatic [4 + 4] photocycloaddition.7
Results and Discussion
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(20) The known chiral aromatic chain imides were prepared according
to the previously reported method (refs 5c and 7).
8038 J. Org. Chem., Vol. 71, No. 21, 2006

Naphthalene- and Anthracene-Based Aromatic Foldamers
FIGURE 1. Chiral aromatic chain imides possessing anthracene and
naphthalene moieties.
FIGURE 3. CD spectra of (P)-2 and (M)-2 in solution (CH3CN) and
in the solid state (KBr). Absorption spectrum of 2 in the solid state
(KBr) is indicated by dashed lines.
A different CD spectral pattern is observed for (P)-(S)-4. The
region from 250 to 315 nm is composed of both induced CD
as well as intrinsic CD signal by the chiral naphthyl substituent,
while the region between 315 and 400 nm is solely due to
exciton coupling of the anthryl and naphthyl chromophores. The
sign of Cotton effect of its long-wavelength band is the same
as those of (P)-1 and (P)-(R)-3. The helicity of (P)-(S)-4 is
determined based on this long-wavelength band. Induced CD
signals originate in the exciton coupling between the anthryl
and naphthyl chromophores.21 The chiral auxiliaries regulate
their chiral orientation, but they are not the chromophore
responsible for the induced CD signals. The bulkiness of the N
substituent seems to affect significantly the orientation of these
two chromophores in the solid state. While (P)-(S)-4 possesses
P helicity, the CD signal corresponding to M helicity has been
observed for (M)-(S)-3. Thus, the observed CD signals are not
consistent to the absolute configuration of the chiral auxiliaries
at the imide nitrogen atoms.
FIGURE 2. Solid-state CD spectra of (P)-1, (P)-(R)-3, (M)-(S)-3, and
(P)-(S)-4 (KBr). Due to noticeable noise, the spectra could not be
measured precisely in the region below 250 nm.
Only (P)-2, (M)-2, and (P)-(S)-4 showed CD spectra in
solution, whereas no CD signal was observed for (P)-1,
(P)-(R)-3, and (M)-(S)-3 in solution. Bulkiness of the N
substituents could play a crucial role in displaying CD in
solution. Because of a lack of steric hindrance by the N
substituents, (P)-1, (P)-(R)-3, and (M)-(S)-3 could have a flexible
conformation in solution. Therefore, immediate racemization
or epimerization occurs to result in the observation of no CD
signal. As shown in Figure 3, enantiomeric chiral crystals of
(P)-2 and (M)-2 gave CD signals of mirror images in solution
as well as in the solid state. Slightly red-shifted CD spectra
were observed in the solid state compared to those in solution.
The retention of helicity in the solid state even after dissolving
in solution was confirmed by employing a relatively large single
crystal of (P)-2. One single crystal of (P)-2 (ca. 8 mg) was cut
into two pieces. The CD spectral measurement was carried out
in the solid state for one piece and in solution for the other
piece. Identical signs of Cotton effect and the similar shape of
CD spectra were observed for both pieces. The results indicate
that their folded conformations in crystals are retained after
dissolving in solution.
(S)-3, and (P)-(S)-4 (Figure 1), were examined in the solid state
and CHCl3 (Figures 2 and 3). Although imides 1 and 2 have no
chiral center, they gave enantiomeric chiral crystals upon
recrystallization. Imides (P)-(R)-3, (M)-(S)-3, and (P)-(S)-4
possess chiral substituents at the imide nitrogen atoms. Their
helicities were determined based on the single-crystal X-ray
structure of 3 reported previously5c with the (S)-1-phenylethyl
substituent at the imide nitrogen atom. Its helicity was deter-
mined to be (M).
Consequently, the helicity of the other diastereomer of 3 with
the (R)-1-phenylethyl substituent was assigned to be (P). The
helicity of other carboxamides were assigned based on their
CD signals in comparison with those of (P)-(R)-3 and (M)-(S)-3.
The solid-state CD spectra were recorded on their KBr pellets.
The pellets were prepared by grinding ca. 1 mg of imide and
ca. 10 mg of dry KBr powder followed by pressing under
vacuum. Unlike in solution, all compounds showed CD signals
in the solid state. Figure 2 shows the solid-state (KBr) CD
spectra of (P)-1, (P)-(R)-3, (M)-(S)-3, and (P)-(S)-4. A similar
CD spectral pattern is observed for (P)-1 and (P)-(R)-3, which
indicates that they have the same helicity. Almost mirror images
of CD spectra were observed between (P)-(R)-3 and (M)-(S)-3.
(21) Snatzke, G. Angew. Chem., Int. Ed. Engl. 1979, 18, 363-377.
J. Org. Chem, Vol. 71, No. 21, 2006 8039

Masu et al.
SCHEME 1. Racemization of 2 in Solution
disappear. A distinct downfield shift should be observed24 for
the most shielded proton (H3 at δ 6.30 in CDCl3). Its 1H NMR
spectra recorded after the disappearance of CD were identical
to those before heating. Moreover, the characteristic fluorescence
of the naphthalene-anthracene exciplex (λem 482 nm in CH3-
CN) due to the folding conformation remained without change.
Therefore, we conclude that disappearance of the CD signal of
2 is caused by racemization and its conformation is kept folded.
Unlike atropisomers, in general, it is hard to keep the optical
activity of imides based on the molecular chirality without
racemization after dissolving it in solution since they possess
multiple flexible single bonds. To keep the helically folded
conformation in solution, introduction of noncovalent interac-
tions is generally required, such as intramolecular hydrogen
bonding commonly employed in â-peptides.25 In our case,
electrostatic repulsion between carbonyls settles the folded
conformation with the assistance of π-π interaction and steric
hindrance due to the bulky groups substituted at the imide
nitrogen atom, which may be the driving force to prevent
racemization.
Chiral Photochromic System based on Reversible [4 +
4] Photocycloaddition. During the course of our study on
aromatic imide foldamers with multiple units of naphthalene
moieties we found that induced CDs were generated by the
helical arrangement of folding naphthalene moieties.26 Unlike
the concave-shaped molecules 1-3 with two units of aromatic
moieties, these foldamers with more than three aromatic moieties
connected with iminodicarbonyl linkers could keep their folding
conformations in solution without change observed in their CD
spectra. From these findings we considered that aromatic
foldamers can be applied to a chiral photochromic system. As
the photochromic foldamer capable of [4 + 4] cycloaddition
and its cycloreversion, we prepared compounds 5a, 5b, and 5c
which possessed an anthracene moiety in the center and two
naphthalene moieties below and above the anthracene moiety
(Scheme 2).
FIGURE 4. CD spectral changes of (P)-2 standing at 70 °C in CH3-
CN.
Though 2 and 4 possess a fairly stable helical conformation
in solution, they gradually lose their CD signals after standing
at room temperature for a long time. Figure 4 shows the CD
spectral change of (P)-2 at 70 °C in CH3CN. Its induced CD
corresponding to the exciton coupling based on the absorption
(ca. 250 nm) along the long axes of the anthracene and
naphthalene moieties indicated that the molecule has P helicity.
It can be confirmed also from the fact that (P)-2 has a similar
pattern for the CD spectrum to those of (P)-1, (P)-(R)-3, and
(P)-(S)-4. The results indicated that (P)-2 was gradually losing
its CD signal. The kinetic study of the disappearance of CD
signal of (P)-2 was carried out by measuring the decrease of
∆ꢀ at 263 nm in CH3CN. A fairly large Ea (27.9 kcal/mol) of
the disappearance of CD was observed. Recently, the frozen
chirality of a similar type of compounds was reported on chiral
crystals of an achiral asymmetrically substituted imide with a
tetrahydronaphthyl group on the nitrogen atom.22 In that case,
the CD signal observed in its crystalline state quickly disap-
peared in THF with a half-life of 7.8 min at -20 °C. Even
though its half-life is very short, it is exceptionally stable for
this type of compound. There was an indication of such a frozen
chirality of aromatic amides in cold solution.8a Chiral hosts could
assist fixation of chiral conformation of amides at low temper-
ature.23 Our example is extremely stable compared with those
of similar types of compounds. Because of the bulky substituent
at the nitrogen atom, the CD signal of (P)-(S)-4 remained
without change even after refluxing in CH3CN for 2 weeks in
a sealed tube. The results show a marked contrast to (P)-(R)-3
and (M)-(S)-3, which showed no CD in solution. The results
clearly show that the bulkiness of the N substituent governs
the stability of helical conformation.
Two possible explanations could be considered for the
disappearance of the CD signals of 2 after standing for a long
time: (1) racemization while keeping their folding conforma-
tions intact and (2) conformational changes due to rotation
around the single bonds, which results in unfolding of its
conformation (Scheme 1). To investigate the cause of this, we
carried out their 1H NMR spectral study before and after heating.
If no spectral change is observed, racemization is the origin. If
unfolding of their conformations takes place, shielding caused
by the overlapping of naphthalene and anthracene moieties will
The S-shaped folding structure was confirmed by single-
crystal X-ray analysis of 5a and 5b (Figure 5). 5a and 5b gave
achiral single crystals, although 1 and 2 afforded chiral crystals.
For 5c, suitable single crystals for X-ray analysis could not be
obtained. The naphthalene and anthracene moieties are twisted
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Am. Chem. Soc. 2002, 124, 2466-2473. (b) Bernardi, F.; Garavelli, M.;
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8040 J. Org. Chem., Vol. 71, No. 21, 2006

Naphthalene- and Anthracene-Based Aromatic Foldamers
SCHEME 2. Reversible [4 + 4] Cycloaddition of S-Shaped
Foldamers 5a-5c
and 4.52 Å for 5b as shown in Figure 5. This indicates that a
facile intramolecular [4 + 4] photocycloaddition can be attained.
Because of this overlapping, some of the protons of the
naphthalene moiety are expected to be shielded by the an-
thracene moiety resulting in the upfield shifts if the molecule
has the similar folded conformation in solution as in the solid
state. Indeed, several protons were highly upfield shifted in their
1H NMR spectra.
1
To confirm the folded conformation in solution, H NMR
spectral study of 5a was carried out. Table 1 shows the
assignment of the protons of 5a based on its HH-COSY
spectrum. Protons b, c, and d are highly upfield shifted.
According to its single-crystal X-ray structure, these protons
are located in the shielding zone of the anthracene moiety.
Thereby, it is expected that the conformation of 5a in solution
is similar to that in the solid state. In addition, NOE difference
spectral study also supported the folded conformation in
solution. Irradiation of protons h caused NOEs on protons c
and d, and vice versa. It is difficult to assign the anthracene
protons responsible for these NOEs since all anthracene protons
appeared together as protons h. However, it is quite natural to
consider that these NOEs are caused by the nearest anthracene
protons. The distance between proton c and the nearest
anthracene proton is 2.91 Å, and that of proton d and the nearest
anthracene proton is 3.41 Å from its single-crystal X-ray data.
These distances are reasonable to consider the occurrence of
TABLE 1. Chemical Shift Assignment of Protons of 5a
proton
chemical shifta
a
b
c
d
e
f
g
h
i
5.48 (s, 4H)
1
NOE enhancement. Due to the complex H NMR spectra it is
6.15 (t, J ) 7.6, 2H)
6.32 (d, J ) 6.4, 2H)
6.41 (d, J ) 8.2, 2H)
6.63 (t, J ) 7.6, 2H)
6.85 (d, J ) 8.2, 2H)
7.09 (t, J ) 7.6, 2H)
7.11 (m, 8H)
difficult to assign the proton chemical sifts of 5b and 5c.
However, a similar tendency of upfield shift was also observed
for 5b and 5c.
Irradiation of 5a, 5b, and 5c with a high-pressure mercury
lamp in CHCl3 through a Pyrex filter caused an immediate color
change of their solution from yellow to colorless. Standing the
irradiated solution of them at ambient temperature caused
gradual recovery of their yellow color with a half-life of ca.
8.5 h at ca. 25 °C, 32 h at ca. 15 °C, and 50 min at ca. 25 °C
for 5a, 5b, and 5c, respectively. The 1H NMR spectrum of the
sample recorded immediately after irradiation at ca. 0 °C showed
the peaks characteristic to the [4 + 4] cycloadducts. Their
structures were identified by comparison of their 1H NMR
spectral patterns with those of the similar [4 + 4] adducts 77
and 8,5c reported previously. Figure 6 shows a comparison of
7.15 (m, 2H)
j
k
l
7.41 (t, J ) 7.3, 2H)
7.48 (t, J ) 7.3, 4H)
7.77 (d, J ) 7.3, 4H)
1
the characteristic H NMR chemical shifts of cycloadducts 6a
and 6c. Similar patterns were observed for two vinylic protons
H2 and H3 and the bridgehead proton H4. In cycloadduct 6a,
the benzylic protons changed their chemical shifts from δ 5.48
(s) in 5a to two sets of doublets at δ 5.18 (d, J ) 13.8) and
5.24 (d, J ) 13.8) and δ 4.00 (d, J ) 14.0) and 4.05 (d, J )
14.0). The former corresponds to the N-benzylic protons of the
pyrrolidine 2,5-dione ring which is almost identical to those of
7. The remaining benzylic protons shifted upfield largely due
to disappearance of the deshielding effect caused by the
anthracene moiety before cycloaddition. A similar tendency was
observed for 6b and 6c. Cycloadduct 6c was obtained as a
diastereomeric mixture in a ratio 95:5 at 0 °C in CDCl3. The
diastereoselectivity was improved slightly to 97:3 when the
reaction was carried out at -40 °C. The diastereoselectivity was
similar to that observed in our previous study of the photocy-
FIGURE 5. Single-crystal X-ray structure of 5a (side (a) and top (b)
views) and 5b (side (c) and top (d) views). Distances are indicated in
Angstroms.
against each other with an angle of 43° and 41° for 5a and 5b,
respectively, creating the folding structures. The folding occurs
in a zigzag way. Satisfactory overlapping of the anthracene and
naphthalene moieties was observed. The distances between the
adjacent carbons of them are 2.87 and 4.61 Å for 5a and 2.85
1
cloaddition of 8.7 Since the H NMR spectra after irradiation
of 5c were complicated due to formation of the [4 + 4]
cycloadduct 6c (major) together with the minor diastereomeric
cycloadduct and 5c generated by retro cycloreversion due to
J. Org. Chem, Vol. 71, No. 21, 2006 8041

Masu et al.
1
FIGURE 6. Comparison of the H NMR chemical shifts (CDCl3) of
cycloadducts 6a and 6c with those of the known cycloadducts 7 and 8.
FIGURE 7. (a) Absorption spectral changes of 5c (5.1 × 10-5 M)
upon irradiation (>290 nm), changes from top to bottom, and thermal
recovery of 5c at ambient temperature (ca. 25 °C) in CHCl3. (b) CD
spectral changes of 5c to 6c after irradiation (>290 nm), changes from
top to bottom, and thermal recovery of 5c at ambient temperature (ca.
25 °C) in CHCl3. †Irradiation was carried out with a high-pressure
mercury lamp through a slit to slow the speed of photocycloaddition.
‡Irradiation was carried out for 25 s for completion of the cycloaddition
without a slit.
instability of the cycloadduct, assignment of the peaks was
carried out based on the HH-COSY spectrum of the irradiated
mixture. Due to the instability of 6c, isolation was difficult.
When the CDCl3 solution of 5a (1.1 × 10-2 M) in a NMR
tube was irradiated externally by a high-pressure mercury lamp
at -40 °C, its yellow color disappeared completely. However,
1
after measurement of the H NMR spectrum, the color of the
solution turned slightly yellow. The degree of the retro cyclo-
reversion during the measurement was 7% based on integration.
Therefore, generation of 5c is due to its thermal instability and
not because of the photochemical retro process. The photo-
chemical reaction was a complete conversion of 5c to 6c, and
no photostationary state was involved. Figure 7a shows the UV-
vis absorption spectral changes of 5c during irradiation.
Complete conversion of 5c was attested by the complete
disappearance of the absorption band around 420 nm which was
responsible for the yellow color. This disappearance is due to
destruction of the overlapped p system. Since no crossing point
exists in the absorption spectra of 5c and 6c, no isosbestic point
is observed for conversion of 5c to 6c even though conversion
proceeds in a first-order process. Thermally, the cycloadduct
6c was completely converted to 5c. Its CD and absorption
spectra were changed to the identical spectra to those of 5c after
standing for 5.5 h at ambient temperature (ca. 25 °C) or heating
for 20 min at 60 °C. The T-type photochromism (one of the
reversible processes is thermal and the other is photochemical)
took place. Since 5c possesses chiral auxiliaries, cycloaddition
may afford two diastereomers.
than that of 5c calculated by AM1 calculation,27 indicating that
6c is thermally less stable than 5c.
Unlike the usual thermal retro [4 + 4] cycloreversion, that
of 6c occurred at moderate temperature, resulting in 5c having
recovery of the CD signal identical to that before the irradia-
tion-heating cycle. The results imply that the chiral photo-
chromic system can be created by utilizing reversible [4 + 4]
cycloaddition of 5c. No spectral change in the CD spectrum of
5c was observed, even after heating in CHCl3 at 60 °C for 1
day. When the CD spectrum of 5c was measured at -45 °C,
its signal intensity was slightly (ca. 20%) increased, probably
due to a volumetric change of the solvent, and no significant
change of its shape was observed. If the conformation of 5c is
the zigzag type as observed in the solid state of 5a and 5b, the
helical chirality generated between the anthracene moiety and
the naphthalene moieties above and down cancels each other
out (Figure 8). It is essential to have helical structures to generate
induced CD. We tentatively postulate that introduction of a large
chiral auxiliary at the imide nitrogen atom may change the
pattern of folding from a symmetrical zigzag type to an
asymmetric helical type.
After irradiation for 25 s with a high-pressure mercury lamp,
the CD signal of 5c disappeared almost completely (Figure 7b).
The [4 + 4] photocycloaddition destroyed the overlapped π
system of 5c to afford 6c, which results in annihilation of the
induced CD as well as disappearance of yellow color. Thermal
There was a large difference in stability among cycloadducts.
In contrast to 6a, 6b, and 6c, cycloadducts without an extra
naphthoyl moiety, 7 and 8, were stable at room temperature. It
required heating at higher temperature for their retro [4 + 4]
cycloaddition. For example, 7 have a half-life of 3.9 h in DMSO-
1
d6 at 130 °C measured by H NMR study. We considered that
(27) AM1 calculation was carried out by WinMOPAC Ver. 3 (Fujitsu,
Ltd.). Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209-220 and 221-
264.
this difference might arise from the steric constraint caused by
the additional naphthyl moiety. ∆H of 6c is 17.2 kcal/mol larger
8042 J. Org. Chem., Vol. 71, No. 21, 2006

Naphthalene- and Anthracene-Based Aromatic Foldamers
Experimental Section
Synthesis of Aromatic Imides 5. N-(Benzyl){10-[N-(benzyl)-
N-(naphthylcarbonyl)carbamoyl](9-anthryl)}-N-(naphthylcar-
bonyl)carboxamide 5a. Thionyl chloride (2.75 mL, 37.9 mmol)
was added to 9,10-anthracenedicarboxylic acid (500 mg, 1.88 mmol)
in dry toluene (35 mL). After the solution was refluxed with stirring
for 17 h, excess thionyl chloride and toluene was evaporated.
Benzylamine (0.431 mL, 3.95 mmol), DMAP (483 mg, 3.95 mmol),
and dry toluene (40 mL) were added to the flask, and then the
solution was refluxed with stirring for 6 h. After cooling to room
temperature, the precipitate was filtered and the filtrate washed with
ethyl acetate to give anthracene-9,10-dicarboxylicacid bis-benzy-
lamide as a white powder. No further purification was necessary.
Yield: 369 mg (44%); mp 247-249 °C. This diamide (350 mg,
0.787 mmol) and DMAP (211 mg, 1.73 mmol) were dissolved into
dry pyridine (20 mL) and toluene (30 mL) under reflux. To the
resulting mixture, 1-naphthoyl chloride (0.300 mL, 1.99 mmol) was
added. The resulting solution was refluxed for an additional 39 h
with stirring. After cooling to room temperature, drops of concen-
trated HCl were added to the solution until the pH of the solution
became 4-5. Then, the solution was washed with diluted HCl and
brine. The organic layer was dried over MgSO4, filtered, and
evaporated. The residues were recrystallized from chloroform-
hexane to give 5a as yellow crystals (202 mg, 34%): mp 220-
222 °C. UV-vis (CHCl3) 268 (12 000), 326 (3200), 402 (2700),
FIGURE 8. Two possible conformations, zigzag and helical conforma-
tions, of S-shaped foldamers.
1
421 (3000) nm. H NMR (CDCl3, 400 MHz) δ 7.77 (d, J ) 7.3,
4H), 7.48 (t, J ) 7.3, 4H), 7.41 (t, J ) 7.3, 2H), 7.24-7.14 (m,
10H), 7.09 (t, J ) 7.6, 2H), 6.85 (d, J ) 8.2, 2H), 6.63 (t, J ) 7.6,
2H), 6.41 (d, J ) 8.2, 2H), 6.32 (d, J ) 6.4, 2H), 6.15 (t, J ) 7.6,
2H), 5.48 (s, 4H). 13C NMR (CDCl3, 100 MHz) δ 173.3, 170.6,
137.0, 132.0, 131.8, 131.7, 130.3, 129.2, 128.8, 128.2, 128.0, 126.7,
126.5, 126.4, 125.6, 125.1, 124.7, 122.9, 122.3, 47.9. MS (FAB)
m/z 753 [MH+]. Anal. Calcd for C52H36N2O4‚H2O: C, 81.02; H,
4.97; N, 3.63. Found: C, 80.73; H, 4.76; N, 3.65. Crystallographic
data for 5a: triclinic, P-1, a ) 9.0732(10) Å, b ) 10.0821(11) Å,
c ) 10.6590(12) Å, R ) 87.530(2)°, â ) 85.689(2)°, γ ) 70.442-
(2)°, V ) 916.03(18) Å3, Z ) 1, Dc ) 1.365 mg m-3, T ) 150 K,
µ ) 0.086 mm-1, GOF on F2 ) 1.061, R1 ) 0.0454, wR2 ) 0.1105
([I > 2σ(I)]).
FIGURE 9. CD spectral changes (∆ꢀ) observed at 271 (a) and 341
nm (b) in chiral photochromic cycles between 5c and 6c. †The solution
of 5c (5.1 × 10-5 M) was irradiated (>290 nm) for 25 s and then
‡heated for 20 min at 60 °C in an alternate way.
reversion (heating for 20 min at 60 °C) of 6c revealed complete
recovery of the CD spectrum of the starting 5c. The results
indicated that the original helical conformation of 5c was
regenerated completely after heating. Figure 9a and 9b shows
reversible photochromic cycles for 5c-6c (irradiation and
heating were carried out alternately) presented by the CD
spectral changes (∆ꢀ) at 341 and 271 nm, respectively. Good
reversibility was observed. Some sterically constrained mol-
ecules have been known to undergo ring opening. The norbon-
adiene-quadricyclane cycle28 is a good example of a ring-strain-
driven T-type photochromic system. The driving force for our
cycloreversion is possibly similar to this, the steric constraint
generated by cycloaddition. The [4 + 4] photocycloaddition of
5c gives bi-planemer-29type adduct 6c which thrusts its planes
up- and downward, causing steric compression on the remaining
naphthoyl moiety. Due to this steric constraint, the retro
cycloaddition of cycloadducts might occur to release the steric
energy. The bulky substituents at the imide nitrogen atom might
regulate conformational equilibrium between the two helical
foldamers.
N-(2-Methylbenzyl){10-[N-(2-methylbenzyl)-N-(naphthylcar-
bonyl)carbamoyl](9-anthryl)}-N-(naphthylcarbonyl)carboxam-
ide 5b. In a similar way as for the synthesis of 5a, 5b was prepared
in 37% yield as yellow crystals (chloroform-hexane): mp 265-
267 °C. UV-vis (CHCl3) 270 (24 601), 322 (6962), 400 (5258),
1
421 (5641) nm. H NMR (CDCl3, 500 MHz) δ 7.73 (d, J ) 6.7,
2H), 7.32-7.24 (m, 17H), 7.3 (t, J ) 7.3, 2H), 6.89 (d, J ) 8.2,
2H), 6.48 (d, J ) 6.7, 2H), 6.43 (d, J ) 8.55, 2H), 6.20 (t, J ) 7.6,
2H), 5.53 (s, 4H), 2.65 (s, 6H). 13C NMR (CDCl3, 125 MHz) δ
173.2, 170.5, 136.9, 135.1, 132.0, 131.8, 131.7, 130.7, 130.0, 129.5,
128.0, 127.9, 126.6, 126.5, 126.4, 126.2, 125.5, 125.2, 124.8, 122.7,
122.3, 44.8, 19.8. MS (FAB) m/z 780 [M+]. Anal. Calcd for
C53H38N2O4‚0.5H2O: C, 82.04; H, 5.07; N, 3.61. Found: C, 82.33;
H, 5.09; N, 3.55. Crystallographic data for 5b: triclinic, P-1, a )
9.0414(7) Å, b ) 9.4957(7) Å, c ) 13.2099(10) Å, R ) 103.9160-
(10)°, â ) 97.8330(10)°, γ ) 110.0190(10)°, V ) 1004.17 (13)
Å3, Z ) 1, Dc ) 1.295 mg m-3, T ) 150 K, µ ) 0.081 mm-1
,
GOF on F2 ) 1.050, R1 )0.0585, wR2 ) 0.0.1486 ([I > 2σ(I)]).
N-((1S)-1-Phenylethyl){10-[N-((1S)-1-phenylethyl)-N-(naph-
thylcarbonyl)carbamoyl]-(9-anthryl)}-N-(naphthylcarbonyl)car-
boxamide 5c. In a similar way as for the synthesis of 5a, 5c was
prepared in 75% yield as yellow crystals (from chloroform-
hexane): mp 255-257 °C. UV-vis (CHCl3) 273 (13 000), 329
We demonstrated a novel T-type chiral photochromic system
utilizing [4 + 4] photocycloaddition reaction of sterically
constrained aromatic foldamers.
1
(3700), 400 (2700), 421 (3000) nm. H NMR (CDCl3, 500 MHz)
(28) (a) Dauben, W. G.; Cargill, R. L. Tetrahedron 1961, 15, 197-201.
(b) Garman, A. A.; Leyland, R. L. Tetrahedron Lett. 1972, 13, 5345-5348.
(c) Franceschi, F.; Guardigli, M.; Solari, E.; Floriani, C.; Chiesi-Villa, A.;
Rizzoli, C. Inorg. Chem. 1997, 36, 4099-4107.
(29) Kimura, M.; Sirasu, K.; Okamoto, H.; Satake, K.; Morosawa, S.
Tetrahedron Lett. 1992, 33, 6975-6978.
δ 8.67 (br d, J ) 7.1, 2H), 7.93 (br s, 2H), 7.89 (br d, J ) 7.9,
2H), 7.81 (br t, J ) 6.7, 2H), 7.75 (br d, J ) 6.4, 2H), 7.59 (ddd,
J ) 8.3, 7.3, 0.8, 4H), 7.48 (br s, 2H), 7.44 (br s, 2H), 7.40 (br s,
2H), 7.26 (br s, 2H), 7.21 (br q, J ) 6.7, 2H), 7.18 (br s, 2H), 7.00
(br t, J ) 6.4, 2H), 6.78 (br s, 2H), 6.50 (br t, J ) 7.2, 2H), 6.24
J. Org. Chem, Vol. 71, No. 21, 2006 8043

Masu et al.
(br d, J ) 6.5, 2H), 5.95 (br d, J ) 5.5, 2H), 5.79 (br s, 2H), 2.43
(br d, J ) 4.9, 6H). 13C NMR (CDCl3, 125 MHz) δ 173.5, 168.8,
133.8, 132.0, 131.8, 131.6, 129.39, 129.36, 128.7, 128.3, 127.83,
127.79, 126.5, 126.5, 126.2, 126.0, 125.8, 125.6, 125.4, 125.2,
125.1, 124.8, 123.4, 122.5, 122.4, 121.8, 51.0, 18.2. HRMS (FAB)
calcd for C62H44N2O4 [M+] 880.3375, found 880.3301.
Structural Elucidation of [4 + 4] Photocycloadducts. Since
the [4 + 4] photocycloadducts were thermally unstable due to the
retro cycloreversion to the original imides, structural elucidation
was carried out on the irradiated sample immediately after irradia-
tion by 1H NMR spectroscopy. Their spectra were compared with
those of the similar reported system. In a typical run irradiation
was carried out externally with a 400 W high-pressure mercury
lamp for 10 min in an ice-water bath on the sample of carboxamide
5 (ca. 20 mg) in CDCl3 (ca. 0.6 mL).
Since the 1H NMR spectrum after irradiation of 5c was
complicated due to formation of the [4 + 4] cycloadduct 6c (major)
together with the minor diastereomeric cycloadduct and 5c gener-
ated by retro cycloreversion, assignment of the peaks of the major
isomer 6c was carried out based on the COSY spectrum of the
irradiated mixture. The diastereomeric ratio of cycloadducts was
95:5 based on integration of the methyl groups. 1H NMR (CDCl3,
500 MHz) spectra of 6c (major diastereomer): δ 8.47 (d, J ) 8.6,
1H), 8.38 (d, J ) 8.3, 1H), 8.09 (d, J ) 7.3, 1H), 8.00 (t, J ) 8.4,
2H), 7.95 (d, J ) 8.2, 1H), 7.90 (d, J ) 8.2, 1H), 7.8-7.1 (m,
18H), 6.63 (q, J ) 7.0, 1H), 6.49 (br m, 2H), 6.35 (br m, 1H), 6.26
(br d, J ) 8.2, 1H), 5.97 (d, J ) 7.4, 1H), 5.86 (t, J ) 7.3, 1H),
5.69 (d, J ) 7.7, 1H), 5.56 (t, J ) 7.8, 1H), 5.54 (d, J ) 8.3, 1H),
5.41 (t, J ) 7.4, 1H), 5.22 (q, J ) 6.7, 1H), 4.97 (d, J ) 7.3, 1H),
2.22 (d, J ) 7.1, 3H), 1.45 (d, J ) 6.7, 3H).
1H NMR (CDCl3, 500 MHz) spectra of 6a: δ 8.43 (d, J ) 8.6,
1H), 7.99 (d, J ) 8.0, 1H), 7.92 (d, J ) 8.3, 1H), 7.68-7.24 (m,
17H), 7.05-6.88 (m, 4H), 6.74-6.65 (m, 4H), 6.37 (d, J ) 7.4,
1H), 5.98 (t, J ) 7.7, 1H), 5.75 (d, J ) 7.7, 1H), 5.24 (d, J ) 7.4,
1H), 5.17 (d, J ) 13.8, 1H), 5.09 (d, J ) 13.8, 1H), 4.04 (d, J )
14.0, 1H), 4.00 (d, J ) 14.0, 1H).
Absorption and CD Spectral Measurements after Irradiation.
The solution of 5c (5.1 × 10-5 M) in a cuvette with a stopcock
was irradiated externally with a 100 W high-pressure mercury lamp
with a Pyrex filter (<290 nm) at ambient temperature (ca. 25 °C)
in CHCl3. Absorption and CD spectra of the irradiated sample were
measured at certain intervals.
1H NMR (CDCl3, 500 MHz) spectra of 6b: δ 8.31 (d, J ) 8.4,
1H), 8.05 (d, J ) 8.2, 1H), 7.98 (d, J ) 8.2, 1H), 7.88 (d, J ) 6.8,
1H), 7.68 (m, 2H), 7.61 (t, J ) 7.2, 1H), 7.44 (m, 1H), 7.34 (d, J
) 7.0, 1H), 7.28 (m, 4H), 7.18-7.00 (m, 5H), 6.92 (t, J ) 7.2,
1H), 6.83 (t, J ) 7.5. 2H), 6.77-6.67 (m, 6H), 6.46 (m, 1H), 6.01
(t, J ) 7.9, 1H), 5.78 (d, J ) 8.2, 1H), 5.18 (d, J ) 14.5, 1H), 5.11
(d, J ) 14.5, 1H), 4.13 (d, J ) 16.6, 1H), 4.07 (d, J ) 16.6, 1H),
2.61 (s, 3H), 1.40 (s, 3H).
Supporting Information Available: Copies of 1H NMR
spectroscopic data of 5a-c and 6a-c (PDF) and crystallographic
information files (CIF) and ORTEP diagrams (PDF) of 5a and 5b.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JO0611317
8044 J. Org. Chem., Vol. 71, No. 21, 2006