Peptidehelicenes in solution and gel: chiral discrimination through interactions between two types of helixes

Article abstract of DOI:10.1016/j.tetasy.2010.03.040

Helical [5]thiaheterohelicene 5HM, which rapidly interconverts between P and M enantiomers in solution, was connected to helical l-phenylalanine oligomers with an ester linkage to give peptidehelicenes (5Fn, where n: number of bonded phenylalanines). The

Full text of DOI:10.1016/j.tetasy.2010.03.040

Tetrahedron: Asymmetry 21 (2010) 659–664
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Peptidehelicenes in solution and gel: chiral discrimination through
interactions between two types of helixes
a
a
a
a
Hiroko Nakagawa a, , Kumiko Ohtsuka , Katsuhito Sugahara , Chika Kobayashi , Yuichi Masuoka ,
*
Koh-ichi Yamada a, Masami Kawase b
a Faculty of Pharmaceutical Sciences, Josai University, 1-1 Keyakidai, Sakado, Saitama 350-0295, Japan
b Faculty of Pharmaceutical Sciences, Matsuyama University, 4-2 Bunkyo-cho, Matuyama, Ehime 790-8578, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Helical [5]thiaheterohelicene 5HM, which rapidly interconverts between P and M enantiomers in solu-
tion, was connected to helical L-phenylalanine oligomers with an ester linkage to give peptidehelicenes
(5Fn, where n: number of bonded phenylalanines). The characteristics of 5F4 and 5F5 with two types
of helixes in a molecule were investigated, particularly in comparison with those of 5F1–5F3 with an
Received 1 February 2010
Accepted 16 March 2010
Available online 14 May 2010
incomplete coil of a peptide moiety. L-Phenylalanine peptide chains induced a shift in the equilibrium
between the P and M helixes of 5HM toward the P side for all the 5Fns examined. The enantiomeric
excess (ee) of the P form increased with a decrease in temperature, together with an elongation of the
peptide chains. 5F4 and 5F5 in hot solutions of some solvents formed a gel at room temperature, whereas
5F1–5F3 showed no such behavior. In this gel, the stable helical form of the 5HM moiety in 5F4 and 5F5
was observed to be the M form in contrast to that in their solutions.
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
([5]thiaheterohelicene, 5HM). The interaction between these two
helixes has aroused our interest, particularly, as regard to the dif-
When a helical structure is formed in a molecule or a system, it
is interesting to learn how the sense of a helix is determined, in
connection with the formation of natural helical substances. Along
this line, much research has been reported in recent years. For var-
ious molecules and substances, such as artificial polymers,1,2
molecular aggregates,3,4 and supramolecules,5,6 a major group of
a large achiral portion is controlled by a minor group of a small chi-
ral portion, to form a large chiral environment (Sergeants and Sol-
ference between the interactions of 5HM with tripeptides having
an incomplete helix and with tetrapeptides having a complete he-
lix. The peptides used here consist of
omers with its N-terminal being protected by
butoxycarbonyl (Boc) group. A peptide chain produces a complete
-helix coil with approximately 3.6 amino acid residues. We tenta-
L-phenylalanine (Phe, F) olig-
a
tert-
a
tively call the present molecules ‘peptidehelicenes’ in which a Phe
oligomer is bonded to a 5HM moiety with an ester linkage. These
peptidehelicenes are indicated as 5Fn (n = 1–5), where n is the
number of Phes as shown in Scheme 1. Thus, 5F4 and 5F5 possess
both one complete peptide helix and a 5HM helix in each molecule.
5HM, belonging to the helicene family, has a helical shape aris-
ing from the steric repulsion between its terminal hydrogen
diers effects). Furthermore, when
a slight excess of either
enantiomer is produced in a racemic system, the chirality of the
entire system is changed to the same sense as that of the enantio-
mer (Majority Rules7). In these phenomena, local chirality spreads
out over its circumference through the interaction between chiral
molecules and achiral or other chiral molecules, fixing the whole
chirality sense in a system. These types of interaction concerning
chirality control have found several practical applications such as
to the synthesis of liquid crystal materials,8 the optical active
films,9 the fabrication of electronic biodevices,10 the helical nano-
tubes,11 and the development of new methods for asymmetric syn-
theses12 using small amounts of enantiomeric catalysts.
Boc
O
H
S
N
O
CH3
CH3
O
CH
H
2 O
CH3
H
H
S
Herein, we describe the synthesis and properties of molecules
with two types of helixes; a peptide helix and a flexible helicene
n
S
5HM
Fn
* Corresponding author. Tel.: +81 49 271 7686; fax: +81 49 271 7714.
E-mail address: hnaka@josai.ac.jp (H. Nakagawa).
Scheme 1. Structures of 5Fn (n = 1–5).
0957-4166/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2010.03.040

660
H. Nakagawa et al. / Tetrahedron: Asymmetry 21 (2010) 659–664
atoms.13 However, such repulsion is too weak in solution to main-
tain its helical skeleton, causing a rapid inversion of its helix.
Therefore, 5HM in solution shows an equilibrium between right-
handed (P) and left-handed (M) configurations without inducing
any circular dichroism (CD) absorption. Once 5HM is placed in chi-
ral environments, such as micelles,14 membranes,15 and proteins,16
the equilibrium shifts to either side of the P or M enantiomer,
inducing CD absorption. The direction and extent of the equilib-
rium shift can be estimated from the sign and intensity of Cotton
effects in CD absorptions, respectively. The induced CD based on
5HM is considerably intense and appears in long-wavelength re-
gions, a characteristic of its widely conjugated chromophore,
where the absorption seldom overlaps with those of peptides.
of the helicene family have been assigned to the
tions by Clar.18 The negative Cotton effect of the
a
, b and p transi-
a
band along with
the positive Cotton effects of the p and b bands indicates that the P
form dominates over its antipode, as determined from the compar-
ison of those Cotton effects with the Cotton effects of P [7]hetero-
helicene with no helix inversion.19 Therefore, it is concluded that
L-5F1 preferentially induces the P form, and that D-5F1 reversely
induces the M form. In other words, L-Phe in 5F1 promotes the
helicity of 5HM with a rapid interconversion even at À20 °C20 to
the right-handed one at room temperature, and vice versa.
2. Results and discussion
2.1. Synthesis of 5Fn
The synthetic routes of 5Fn are summarized in Scheme 2. The
key step in the synthesis of 5F1 and 5F2 was the ester formation
between 5HM and Boc-(Phe)n (n = 1, 2) by the actions of 4-dimeth-
ylaminopyridine (DMAP)17 and dicyclohexylcarbodiimide (DCC). In
the course of the synthesis of 5F3, 5F4, and 5F5, it was important
to prevent the racemization of the stereogenic carbons. Thus, Boc
was first removed from 5Fn (n = 1, 2) to be transformed to hydro-
gen chloride salts which were combined with Boc-(Phe)2 or Boc-
(Phe)3 in the presence of DCC.
Figure 1. CD spectra of 5-L-F1 (a) and 5-D-F1 (b) in THF at rt.
2.3. Effect of peptide length of 5Fn on configuration
2.2. Configuration of 5HM moiety in L-5F1 and D-5F1
In 5F1, the predominant chirality of the helicene moiety was
Since a mutual, rapid conversion between P and M enantiomers
takes place in solution, 5HM in principle induces no CD absorption.
determined by the chirality of Phe used in 5F1:
L-Phe induced
the P-configuration and -Phe induced the M-configuration. Next,
D
However, when an
L
-Phe molecule is linked to 5HM, a pair of dia-
we examined how the extent of configuration of the helicene moi-
ety is affected by an increase in the length of the peptide chain of
5Fn (n = 2–5). In particular, we were interested in 5F4 and 5F5
having a coil longer than one helix. Figure 2 shows the CD spectra
of 5F1 to 5F5 in THF at À100 °C. The intensity of each CD absorp-
tion increased markedly with an increase in the length of peptides.
All the spectra demonstrated similar shapes, with the signs of Cot-
ton effects being the same. This indicates that the helicity of the
helicene moiety in 5Fn is controlled by the chirality of Phe, with
similar wavelengths of peaks and troughs, regardless of the length
of peptides.
stereoisomers, P-5F1 and M-5F1, becomes feasible; the two com-
pounds have different stabilities. This may cause a shift in the
equilibrium of the 5HM moiety to the more stable side, that is,
to either the P or M side; therefore CD absorption may be induced
depending on the excess of P or M form. The attachment of L-phen-
ylalanine to 5HM (5-L-F1) induced CD absorptions, though weak,
in THF (Fig. 1), suggesting a small shift in the equilibrium. It is
interesting that the spectra of both D-5F1 and L-5F1 are almost
complete mirror images (Fig. 1). This implies that both compounds
exist as enantiomers in solution. The absorption bands of members
O
H
O
S
N
O
CH3
CH3
S
H
N
O
OH
O
CH3
CH3
HO
DCC, DMAP
CH2
O
CH3
H
H
CH2
O
CH3
+
S
S
n
S
S
n
5HM
Boc-(Phe)n (n=1,2)
5Fn (n=1,2)
O
S
NH3
O
Cl
CH2
4 M HCl
1) 4M HCl 2) Et3N
3) Boc-(Phe)3, DCC
1) Et3N
5F3, 5F4
5F2
5F5
5F1
S
2) Boc-(Phe)n,
DCC (n=2,3)
S
Scheme 2. Synthesis of 5Fn (n = 1–5).

H. Nakagawa et al. / Tetrahedron: Asymmetry 21 (2010) 659–664
661
Figure 2. CD spectra of 5Fn (n = 1–5) in THF at À100 °C. (a) 5F1, (b) 5F2, (c) 5F3, (d)
Figure 4. Relative intensities of
5F5, monitored at approximately 350 nm.
D
e
/
e
of 5Fn. (a) 5F1, (b) 5F2, (c) 5F3, (d) 5F4, and (e)
5F4, (e) 5F5. [5F1] = 5.2 Â 10À5 M, [5F2] = 5.7 Â 10À5 M, [5F3] = 5.2 Â 10À5 M,
[5F4] = 5.5 Â 10À5 M, [5F5] = 5.2 Â 10À5 M.
small fluctuations from approximately 1 with varying tempera-
2.4. Effect of temperature on configuration
ture. However, the present case seems very different.
creased rapidly with decreasing temperature and increasing
length of the peptide chains in 5Fn. The variations of with
De/e in-
Figure 3 shows the temperature effects on the CD and UV spec-
tra of 5F4 at 20 to À100 °C in THF. With a decrease in temperature,
CD intensity markedly increased (Fig. 3A), indicating the equilib-
rium shift to the P-configuration side. On the other hand, the
absorption bands of the UV spectra became sharp with decreasing
temperature, because of a change of electron populations on the
vibrational levels in solution. The contributions of the equilibrium
shift to the increase in CD intensity with decreasing temperature
De/e
temperature were relatively small for 5F1 and 5F2, but relatively
large for 5F3 with almost one helical coil of peptide, and for 5F4
and 5F5 with peptides longer than one helical coil. These imply
that the lower the temperature and the longer the peptide chain,
the more enhanced the extent of configuration of the helicene moi-
ety. Namely, the reduced temperature and the long peptide chain
effectively prevent the inversion from the P form to the M form
of the helicene moiety, thereby inducing intense CD absorption.
can be estimated by alterations of anisotropy factors (
(=el d) is the circular dichroism and is the molecular
absorptivity in UV absorption.21 The temperature dependence of
at kmax of approximately 350 nm (p band) is plotted in
Fig. 4, in which the values in the ordinate are normalized in such
a way that = 1 at 20 °C. Generally, for the system in which
the structure of a stereogenic center does not change with temper-
ature, such normalized values as mentioned above show
De/e), where
D
e
À
e
e
2.5. Gel formation and chiral discrimination
De/e
When a hot solution of 5F4 in CH3CN (ca. 2.2 Â 10À3 M, 0.22 w/
v%) was slowly cooled to purify it by recrystallization, gelation took
place suddenly to give a transparent gel (Fig. 5). The gel was stable
at least over one day at room temperature. Among several solvents
examined, only CH3CN solution of 5F4 gave such a gel. In order to
measure the UV and CD spectra of this gel, a hot CH3CN solution
was put in a combination cell with a 0.2 mm path length, forming
a thin gel film. Figure 6 shows the CD and UV spectra of the 5F4 gel
at room temperature together with the spectra of the 5F4 solution
at À100 °C. The UV spectrum of the gel became broader than that
of the solution, because of lower molecular mobility. On the other
hand, the CD spectra revealed that the enhanced intensity of the
gel was over 8 times larger than that in the solution. Of particular
interest was the reversal of the signs of Cotton effects. These imply
De/e
De/e
Figure 3. Effect of temperature on CD spectra (A) and UV–vis spectra (B) of 5F4 in
THF. [5F4] = 5.4 Â 10À5 M. (a) 20, (b) À20, (c) À60, (d) À100 °C.
Figure 5. Gel of 5F4 in a sample vial. [5F4] = 2.2 Â 10À3 M in CH3CN at rt.

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H. Nakagawa et al. / Tetrahedron: Asymmetry 21 (2010) 659–664
2.6. Effect of temperature on gelation
When the temperature of 5F4 in CH3CN (5.2 Â 10À4 M; 10 times
more concentrated than that in the case shown in Fig. 3) was grad-
ually reduced from 20 to À10 °C, the Cotton effects of CD spectrum
gradually attenuated, and at approximately 4 °C the signs of Cotton
effects reversed, followed by an increase in the absolute value of
intensity with further decrease in temperature. In contrast, an in-
crease in temperature from À10 to 20 °C caused a decrease in
the absolute value of intensity and no reversal of the signs was ob-
served up to approximately 20 °C. These changes in CD absorption-
al intensity monitored at 340 nm are shown in Fig. 8. This figure
clearly shows the hysteresis effect of solution/gel transition: the
gel state at low temperatures and the solution state at high tem-
peratures. As generally noted for gelation phenomena, the transi-
tion of a solution to a gel occurs at temperatures (approximately
4 °C) lower than that of a gel to a solution (approximately 20 °C).
Although the transition temperatures of solution/gel have been
measured by various methods, in the present case it is interesting
that the transition temperatures were determined on the basis of
the difference in chiral discrimination between the solution and
the gel by CD measurement.
Figure 6. UV–vis (A) and CD (B) spectra of 5F4. Solution spectra (black) were
measured in THF at À100 °C and [5F4] = 5.4 Â 10À5 M. CH3CN gel spectra (red) were
measured at rt and [5F4] = 2.2 Â 10À3 M.
that (1) the equilibrium between the P and M diastereomers of the
helicene moiety shifts by gelation, the M-configuration being over-
whelmingly predominant over the P form and that (2) the environ-
ments surrounding the helicene moiety of 5F4 are different
between the solution and the gel in terms of chiral discrimination.
As for the other 5Fn compounds, 5F1, 5F2, and 5F3 with rela-
tively shorter peptide chains showed no gelation. For 5F5, gelation
similar to that observed in 5F4 took place. Therefore, it is under-
stood that such gelation requires a relatively longer peptide chain.
5F5 had a poor solubility in CH3CN, that is, it does not set as a gel in
this solvent. However, it showed an ability to form a gel in various
kinds of solvents, such as CHCl3, CH2Cl2, CHCl3/CH3OH, CHCl3/
C2H5OH, CHCl3/C6H6, THF/C2H5OH, and 1,4-dioxane/C6H14 at con-
centrations of 1 – 3 Â 10À3 M. The CD and UV–vis spectra of 5F5
in the gel (CHCl3) together with those in CHCl3 solution are shown
in Fig. 7. The UV–vis spectrum of 5F5 in gel (red line) showed a
sharper absorption than that of 5F4 in gel, suggesting that the heli-
cene moiety of 5F5 is in a more mobile environment. The Cotton
effects and absorptional wavelengths in the CD spectrum were al-
most the same as those of 5F4, in agreement with the predominant
M-configuration of the helicene moiety. However, the intensity
was much weaker than that of 5F4 for some solvent systems and
almost comparable to that in solution (black line).
Figure 8. Hysteresis of gel/solution transition of 5F4. [5F4] = 5.2 Â 10À4 M. Inten-
sities were monitored at 340 nm. The solid line shows
De when temperature
decreased and the broken line shows
De when temperature increased.
3. Conclusion
Chiral discrimination was investigated for new compounds,
namely, peptidehelicenes (5Fn), consisting of an -phenylalanine
L
peptide and a mobile helicene (5HM). The helical sense of the
5HM moiety was determined by the chirality of the applied phen-
ylalanine, which was commonly observed in all the 5Fn com-
pounds in solution. 5F4 and 5F5, which have a helical coil of
peptide in their molecules, formed a stable organogel in some sol-
vents. The helical sense of the 5HM moiety in the gel state was
completely opposite to that of the 5HM moiety in the solution state
at room temperature. The difference in helical sense between both
states may be mainly attributable to the difference in the mode of
action against 5Fn molecules between a solution and a gel, that is,
intramolecular interaction in a solution and intermolecular inter-
action in a gel.
Figure 7. UV–vis (A) and CD (B) spectra of 5F5. Solution spectra (black) were
measured in THF at À100 °C and [5F5] = 5.6 Â 10À5 M. CHCl3 gel spectra (red) were
measured at rt and [5F5] = 3.3 Â 10À3 M.
In the gel, it can be tentatively presumed that the helicene moi-
ety of 5Fn (n = 4, 5) is encapsulated within a cavity formed with
hydrogen-bonding networks by solvent molecules and the peptide
chains of 5Fn. The gel of 5F5 having a longer peptide chain formed
a lager cavity than the gel of 5F4. Therefore, the helicene moiety of
5F5 is more loosely encapsulated in the cavity than that of 5F4,
resulting in a smaller equilibrium shift between the P- and M-con-
figurations. This may be the reason why 5F5 demonstrated a weak-
er CD absorption than 5F4.
4. Experimental
4.1. General
Unless otherwise noted, all starting materials and solvents were
obtained from commercial suppliers and used without further
purification. tert-Butyloxycarbonyl-
H-Phe-OMeÁHCl, and Boc- -Phe-OH were purchased from Kokusan
Chemical co. (Tokyo). Boc-Phe-Phe-OH was purchased from
L-phenylalanine (Boc-Phe-OH),
D

H. Nakagawa et al. / Tetrahedron: Asymmetry 21 (2010) 659–664
663
BACHEM co. (Torrance). 2-Hydroxymethyl[5]thiaheterohelicene
4.4. Preparation of Boc-(phenylalanine)3 methyl[5]
thiaheterohelicene ester 5F3
(5HM) was prepared according to the procedures described in
our previous article.16 UV–vis spectra were performed on a JASCO
V-560 spectrophotometer equipped with an ETC-505T tempera-
ture controller. CD spectra were measured on a JASCO J720W spec-
5F1 (60.8 mg, 0.1 mmol) was dissolved in 4 mol/l HCl 1,4-diox-
ane solution (0.6 ml) at 0 °C, then the solution was left at room
temperature for 2 h. The solvent was evaporated under 30 °C. A
small portion of anhydrous 1,4-dioxane was added and evaporated
again. This procedure was repeated three times. After washing
with anhydrous ethyl ether three times, 5F1–NH2ÁHCl was ob-
tained. 5F1–NH2ÁHCl was dissolved in 0.3 ml anhydrous DMF, neu-
tralized with 5% trimethylamine in 1,4-dioxane (0.2 ml, 0.1 mmol)
at 0 °C, and then Boc-Phe-Phe-OH (49.4 mg, 0.12 mmol) was
added. The same procedure as that for 5F2 was done using
0.1 mol/l DCC 1,4-dioxane solution at 0 °C to give orange solid.
Recrystallization from acetonitrile produced pale yellow solid of
5F3 (29.9 mg, 34.4%). Mp 209–210 °C and m/z 890 (M++Na+) (calcd
868). Anal. Calcd for C49H45N3O6S3: C, 67.80; H, 5.22; N, 4.84.
Found: C, 67.30; H, 5.59; N, 4.42. 1H NMR (270 Hz, CDCl3):
d = 8.31 ppm (s, 1H), 8.25 (d, 1H, J = 5.6 Hz), 8.03 (d, 1H,
J = 8.6 Hz), 7.95 (d, 1H, J = 8.6 Hz), 7.90 (d, 1H, J = 8.6 Hz), 7.88 (d,
1H, J = 8.4 Hz), 7.74 (d, 1H, J = 5.6 Hz), 6.89–7.30 (m, 15H), 6.37
(d, 1H, J = 7.8 Hz), 6.22 (br, 1H), 5.49 (ABq, 2H, J = 15.8 Hz), 4.75–
4.82 (m, 2H), 4.55 (q, 1H, J = 6.8 Hz), 4.27 (q, 1H, J = 6.8 Hz),
2.87–3.11 (m, 6H) and 1.35 (s, 9H).
tropolarimeter equipped with
a
PTC-348WI temperature
controller. UV–vis and CD spectra were measured using 10, 0.5,
and 0.2 mm light-path length quartz cells. Spectra at low temper-
atures were obtained using Oxford cryostats DN704 for UV–vis
spectra and an Optistat DN for CD spectra with an ITC502 temper-
ature controller. 1H NMR was measured on a JEOL JNM-EX270FT
(270.05 MHz) and a JEOL JNM-A500 (500.00 MHz). Mass spectral
data were measured on a JEOL JMS-700 by means of EI and FAB
techniques.
4.2. Preparation of Boc-phenylalanine methyl[5]
thiaheterohelicene ester 5F1
5HM (130.6 mg, 0.4 mmol), Boc-L-Phe (123.0 mg, 0.48 mmol),
and DMAP (4-dimethylaminopyridine) (12.2 mg, 0.1 mmol) were
placed in a screw-capped glass vial, and 1.5 ml of anhydrous 1,4-
dioxane was poured to dissolve the mixture completely. 1 mol/l
DCC (dicyclohexylcarbodiimide) dioxane solution (0.4 ml,
0.4 mmol) was added to the above solution with stirring, and con-
tinuously stirred overnight. After confirming that no 5HM re-
mained in the solution by TLC (benzene/ethanol = 10/1), 10%
acetic acid dioxane solution (0.2 ml, 0.4 mmol) was added to the
solution and stirred for 15 min to convert DCC into dicyclohexyl
urea (DCurea). DCurea was removed by filtration, and the filtrate
was extracted with 20 ml of ethyl acetate. The ethyl acetate layer
was washed with 3% HCl, water, 3% sodium hydrogen carbonate,
and water, and then dried over anhydrous sodium sulfate. When
the solvent was evaporated under reduced pressure, orange solid
was obtained. This residue was purified by silica-gel column chro-
matography (dichloromethane/acetonitrile = 30/1) to give pale yel-
low amorphous solid of 5F1 (193.5 mg, yield 84.3%). Mp 58.3–
61.5 °C and m/z 573 (calcd 573); Anal. Calcd for C31H27NO4S3: C,
64.90; H, 4.74; N, 2.44. Found: C, 64.71; H, 4.90; N, 2.31. 1H NMR
(270 Hz, CDCl3): d = 8.31 ppm (s, 1H), 8.25 (d, 1H, J = 5.6 Hz), 8.02
(d, 1H, J = 8.6 Hz), 7.94 (d, 1H, J = 8.6 Hz), 7.90 (d, 1H, J = 8.6 Hz),
7.88 (d, 1H, J = 8.6 Hz), 7.73 (d, 1H, J = 5.6 Hz), 7.02–7.08 (m, 5H),
5.60 (d, 1H, J = 12.9 Hz), 5.56 (d, 1H, J = 12.9 Hz), 5.00 (d, 1H,
J = 8.3 Hz), 4.68 (q, 1H), 3.11 (2H) and 1.41 (s, 9H).
4.5. Preparation of Boc-(phenylalanine)3-OMe F3
H-Phe-OMeÁHCl (537.7 mg, 2.5 mmol) and triethylamine
(0.35 ml, 2.5 mmol) were dissolved in 5 ml of anhydrous dichloro-
methane. Boc-Phe-Phe-OH (1034 mg, 2.5 mmol) and DCC
(583.7 mg, 3 mmol) were added to the mixture at 0 °C, which
was stirred for one night at room temperature. The same procedure
as that for 5F1 was performed, giving white solid. The solid was
recrystallized from acetone/water to give white solid of F3
(861.2 mg, yield 60.2%). Mp 174–175 °C and m/z 574 (M+H) (calcd
573). Anal. Calcd for C33H39N3O6: C, 69.09; H, 6.85; N, 7.32. Found:
C, 69.01; H, 6.91; N, 7.38. 1H NMR (270 Hz, CDCl3): d = 6.99–
7.16 ppm (m, 15H), 6.37 (d, 1H, J = 7.4 Hz), 4.79 (br, 1H), 4.69 (q,
1H, J = 7.4 Hz), 4.53 (q, 1H, J = 7.3 Hz), 4.29 (q, 1H, J = 6.8 Hz),
3.67 (s, 3H), 2.87–3.09 (m, 6H) and 1.37 (s, 9H).
4.6. Preparation of Boc-(phenylalanine)3-OH
Boc-(phenylalanine)3-OMe F3 (581.6 mg, 1.0 mmol) was dis-
solved in 10 ml of warm methanol. 1 M sodium hydroxide
(1.2 ml, 1.2 mmol) was added to the solution, which was cooled
to room temperature with stirring for 5 h. After confirming no
5HM remained by TLC, the solution was made neutral by 10% citric
acid and then the solvent was removed. The residue was dissolved
in ethyl acetate and washed with 3% HCl and water twice. Perform-
ing the same procedure as that for the synthesis of 5F1, white solid
was obtained. The solid was recrystallized from ethyl acetate/hex-
ane to give white solid of Boc-(phenylalanine)3-OH (369.2 mg,
60.1%). Mp 194–196 °C and m/z 560 (M+H) (calcd 559). Anal. Calcd
for C32H37N3O6: C, 68.68; H, 6.66; N, 7.51. Found: C, 68.51; H, 6.80;
N, 7.38. 1H NMR (270 Hz, CDCl3): d = 7.03–7.29 ppm (m, 15H), 6.48
(d, 1H, J = 6.9 Hz), 4.92 (br, 1H), 4.58–4.68 (m, 2H), 4.38 (br, 1H),
3.20 (d, 1H, J = 5.6 Hz), 3.14 (d, 1H, J = 5.6 Hz, 2.86–3.03 (m, 6H)
and 1.36 (s, 9H).
D-5F1 was prepared with the same procedure as that for L-5F1
using Boc- -Phe. D-5F1 (162.9 mg, yield 94.6%); mp 60–61 °C.
D
4.3. Preparation of Boc-(phenylalanine)2 methyl[5]
thiaheterohelicene ester 5F2
5HM (99.0 mg, 0.3 mmol), Boc-Phe-Phe-OH (152 mg,
0.36 mmol), and DMAP (9.2 mg, 0.07 mmol) were dissolved in
1 ml of anhydrous dichloromethane. DCC (85.6 mg, 0.36 mmol)
was added to the mixture at 0 °C. The mixture was continuously
stirred until no residual 5HM was detected by silica-gel TLC (ethyl
acetate/hexane = 1/2). The same procedure for 5F1 synthesis gave
orange solid. Recrystallization of this solid from ethanol gave pale
yellow solid of 5F2 (116.7 mg, yield 53.3%). Mp 174–175 °C and m/
z 743 (M++Na+) (calcd 720). Anal. Calcd for C40H36N2O5S3: C, 66.64;
H, 5.03; N, 3.89. Found: C, 66.68; H, 5.22; N, 3.87. 1H NMR (270 Hz,
CDCl3): d = 8.31 ppm (s, 1H), 8.24 (d, 1H, J = 5.4 Hz), 8.03 (d, 1H,
J = 8.6 Hz), 7.96 (d, 1H, J = 8.6 Hz), 7.91 (d, 1H, J = 8.4 Hz), 7.89 (d,
1H, J = 8.6 Hz), 7.73 (d, 1H, J = 5.6 Hz), 6.86–7.22 (m, 10H), 6.30
(d, 1H, J = 7.1 Hz), 5.49 (ABq, 2H, J = 18.5 Hz), 4.85–4.88 (m, 2H),
3.01–3.08 (m, 4H) and 1.37 (s, 9H).
4.7. Preparation of Boc-(phenylalanine)4-OMe F4
H-Phe-OMeÁHCl (44.2 mg, 0.2 mmol), triethylamine (0.028 ml,
0.2 mmol), Boc-(phenylalanine)3-OH (115.0 mg, 0.2 mmol), DCC
(51.1 mg, 0.24 mmol), and 1.2 ml of anhydrous dichloromethane
were used according to the same procedures of the synthesis of

664
H. Nakagawa et al. / Tetrahedron: Asymmetry 21 (2010) 659–664
Boc-(phenylalanine)3-OMe to give white solid of F4 (69.7 mg,
47.2%). Mp 183–184 °C and m/z 721 (M+H) (calcd 720). Anal. Calcd
for C42H48N4O7: C, 69.98; H, 6.71; N, 7.77. Found: C, 69.96; H, 6.80;
N, 7.15. 1H NMR (270 Hz, CDCl3): d = 6.98–7.26 ppm (m, 20H), 6.56
(br, 1H), 6.34 (br, 1H), 4.74–4.76 (m, 2H), 4.50 (q, 1H, J = 6.4 Hz),
4.17 (q, 1H, J = 5.8 Hz), 4.03 (br, 1H), 3.69 (s, 3H), 2.84–3.12 (m,
8H) and 1.34 (s, 9H).
5.52; N, 6.04. 1H NMR (270 Hz, CDCl3): d = 8.33 ppm (s, 1H), 8.29
(d, 1H, J = 5.8 Hz), 8.02 (d, 1H, J = 8.6 Hz), 7.94 (d, 1H, J = 8.6 Hz),
7.88 (d, 1H, J = 8.4 Hz), 7.77 (d, 1H, J = 5.6 Hz), 6.87–7.29 (m,
26H), 6.59 (br, 1H), 6.29 (d, 1H, J = 5.6 Hz), 5.53 (ABq, 2H,
J = 11.7 Hz), 4.90 (q, 1H, J = 7.4 Hz), 4.69 (m, 2H), 4.58 (br, 1H),
4.30 (q, 1H, J = 5.8 Hz), 4.07–4.14 (m, 2H), 2.70–3.22 (m, 10H)
and 1.27 (s, 9H).
4.8. Preparation of Boc-(phenylalanine)4 methyl[5] thiahet-
erohelicene ester 5F4
References
1. Ben, T.; Furusyo, Y.; Goto, H.; Miwa, K.; Yashima, E. Org. Biomol. Chem. 2009, 7,
2509–2512.
2. Deng, J.; Luo, X.; Zhao, W.; Yang, W. J. Polym. Sci., Part A: Polym. Chem. 2008, 46,
4112–4121.
3. Yamamoto, T.; Fukushima, T.; Kosaka, A.; Jin, W.; Yamamoto, Y.; Ishii, N.; Aida,
T. Angew. Chem., Int. Ed. 2008, 47, 1672–1675.
According to the same procedures for 5F3 using 5F1 (143.7 mg,
0.25 mmol), 1 ml of 4 M HCl in 1,4-dioxane, 5% triethylamine in
1,4-dioxane
(0.5 ml,
0.25 mmol),
Boc-(phenylalanine)4-OH
(168.8 mg, 0.03 mmol), 0.1 M DCC in 1,4-dioxane (3.0 ml,
0.3 mmol), and 0.6 ml of anhydrous DMF, orange solid was ob-
tained. The solid was recrystallized from acetonitrile/ethanol, giv-
ing pale yellow solid of 5F4 (40.3 mg, 15.9%). Mp 218–219 °C and
m/z 1037 (M++Na+) (calcd 1015). Anal. Calcd for C58H54N4O7S3: C,
68.62; H, 5.36; N, 5.52. Found: C, 67.02; H, 5.31; N, 5.18. 1H NMR
(270 Hz, CDCl3): d = 8.33 ppm (s, 1H), 8.29 (d, 1H, J = 5.4 Hz), 8.03
(d, 1H, J = 8.4 Hz), 7.96 (d, 1H, J = 8.6 Hz), 7.89 (d, 1H, J = 8.4 Hz),
7.88 (d, 1H, J = 8.6 Hz), 7.76 (d, 1H, J = 5.6 Hz), 6.92–7.26 (m,
20H), 6.66 (br, 1H), 6.48 (br, 1H), 6.21 (d, 1H, J = 6.9 Hz), 5.52
(ABq, 2H, J = 11.2 Hz), 4.84 (q, 1H, J = 6.3 Hz), 4.68 (m, 2H), 4.44
(q, 1H, J = 6.8 Hz), 4.12 (q, 1H, J = 6.3 Hz), 2.76–3.18 (m, 8H) and
1.31 (s, 9H).
4. Kaiser, T.; Stepanenko, V.; Wurthner, F. J. Am. Chem. Soc. 2009, 131, 6719–6732.
5. Wilson, A. J.; Masuda, M.; Sijbesma, R. P.; Meijer, E. W. Angew. Chem., Int. Ed.
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10. Giorgi, T.; Lena, S.; Mariani, P.; Cremonini, M. A.; Masiero, S.; Pieraccini, S.;
Rabe, J. P.; Samori, P.; Spada, G. P.; Gottarelli, G. J. Am. Chem. Soc. 2003, 125,
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1984, 40, 1039–1041.
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4.9. Preparation of Boc-(phenylalanine)5 methyl[5]thiahet-
erohelicene ester 5F5
According to the same procedures for 5F3 using 5F2 (74.1 mg,
0.1 mmol), 0.6 ml of 4 M HCl in 1,4-dioxane, 5% triethylamine in
1,4-dioxane
(0.2 ml,
0.1 mmol),
Boc-(phenylalanine)3-OH
(67.8 mg, 0.12 mmol), 0.1 M DCC in 1,4-dioxane (1.2 ml,
0.12 mmol), and 0.4 ml of anhydrous DMF, orange solid was ob-
tained. The solid was purified by HPLC (silica-gel column, chloro-
form), yielding pale yellow solid of 5F5 (66.1 mg, 55.3%). Mp
222–223 °C and m/z 1184 (M++Na+) (calcd 1162). Anal. Calcd for
C67H63N5O8S3: C, 69.23; H, 5.46; N, 6.02. Found: C, 68.43; H,