Synthesis and evaluation of α-helix mimetics based on a trans-fused polycyclic ether: sequence-selective binding to aspartate pairs in α-helical peptides

Article abstract of DOI:10.1016/j.tetlet.2006.05.170

Inspired by the topological similarity between ladder-like cyclic ether skeletons and α-helical peptides, a trans-fused 6/6/6/6 tetracyclic ether containing two hydroxyl groups separated by a distance of 4.8 A? was designed as a scaffold for a nonpeptidic

Full text of DOI:10.1016/j.tetlet.2006.05.170

Tetrahedron Letters 47 (2006) 5801–5805
Synthesis and evaluation of a-helix mimetics based on a
trans-fused polycyclic ether: sequence-selective binding to
aspartate pairs in a-helical peptides
Hiroki Oguri,*, Shintaro Tanabe, Akifumi Oomura,
Mitsuo Umetsuà and Masahiro Hirama*
Department of Chemistry, Graduate School of Science, Tohoku University, SORST,
Japan Science and Technology Corporation (JST), Sendai 980-8578, Japan
Received 3 April 2006; revised 26 May 2006; accepted 26 May 2006
Available online 21 June 2006
Abstract—Inspired by the topological similarity between ladder-like cyclic ether skeletons and a-helical peptides, a trans-fused 6/6/
˚
6/6 tetracyclic ether containing two hydroxyl groups separated by a distance of 4.8 A was designed as a scaffold for a nonpeptidic a-
helix mimetic. Two alkyl guanidinium groups were attached to the hydroxyl groups to develop a synthetic receptor for the specific
recognition of i + 4 spaced aspartate pairs on the surface of an a-helical peptide. A circular dichroism (CD) titration showed that
this mode of molecular recognition stabilizes a-helical structures of peptides containing i + 4 spaced aspartate pairs.
ꢀ 2006 Elsevier Ltd. All rights reserved.
˚
The rational design of small molecules that recognize
protein surfaces and subsequently disrupt protein–pro-
tein interactions is an ongoing challenge.1 One approach
to meeting this challenge is the design of molecular scaf-
folds that mimic the surface functionality projected
along one face of an a-helix. As well as investigations
of cross-linked interfacial peptides,2 b-peptides,3 and
oligoamide foldamers,4 synthetic approaches to devel-
oping nonpeptidic molecules of a-helix mimetics have
been reported.5 In this context, we recently designed a
structurally defined polycyclic ether scaffold as an a-
helix mimetic,6 inspired by the topological similarity
between ladder-like cyclic ether marine toxins and
a-helical peptides.7 In the 6/6/6/6 trans-fused polycyclic
ether system shown in Figure 1a, the distance between
skeletal oxygen atoms on the same side (4.8 A) of the
cyclic ethers is almost identical to the interval between
the side-chains of a-helical peptides in the canonical i,
˚
i + 4 relationship (ca. 5 A). In addition, it is thought
that incorporation of the oxygen atoms on the skeleton
results in moderate aqueous solubility. Despite extensive
efforts toward the total synthesis of complex marine tox-
ins,8 attempts to design functional molecules based on a
trans-fused cyclic ether framework, harnessing the
potential of privileged molecular properties, have been
limited to date.9 In the present report, we describe the
synthesis of a 6/6/6/6 tetracyclic ether scaffold 1 possess-
ing two equatorial hydroxyl groups (C4 and C10) sepa-
˚
rated by a distance of 4.8 A (Fig. 1b). An a-helix
mimetic 2, which has two guanidinium groups linked
to 1 through two hydroxyl groups, has also been devel-
oped. By exploiting the ability of alkylguanidinium
groups to bind strongly to carboxylates in polar
solvents (Fig. 1a),10 the sequence selective binding of
the synthetic receptor 2 to aspartate pairs in a-helical
peptides were evaluated by circular dichroism (CD)
spectroscopy.
Keywords: Polycyclic ether; a-Helix mimetic; Scaffold; Molecular
recognition.
*
Corresponding authors. Tel.: +81 22 217 6563; fax: +81 22 217 6566
(H.O.); e-mail addresses: oguri@sci.hokudai.ac.jp; hirama@ykbsc.
chem.tohoku.ac.jp
Graduate School of Science, Hokkaido University.
à Institute of Multidisciplinary Research for Advanced Materials,
Tohoku University.
To construct the trans-fused 6/6/6/6 tetracyclic ether
skeleton, we planned to assemble two tetrahydropyrans
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.05.170

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H. Oguri et al. / Tetrahedron Letters 47 (2006) 5801–5805
diketones to CSA in MeOH/CH2Cl2 (4:1) effected the
removal of the TBS groups and in situ bishemiacetal
formation to afford tetracyclic products 13–15 in good
yields.16 It was anticipated that the Et3SiH/TMSOTf-
mediated reduction of bishemiacetals 13–15 to form
16–18 would be affected by participation of the neigh-
boring C10 hydroxyl group. In fact, the reduction of
13 (R = OH) failed to provide the desired tetracyclic
ether 16; instead, triol 19 was obtained in 89% yield.17
The C10 hydroxyl group was thus protected as a benzyl
ether, inhibiting the ring-opening to form 19. Reduction
of 14 under identical conditions afforded the desired
tetracyclic ether 17 in moderate yield (61%) along with
substantial amounts of diol 20 (26%).17 After consider-
able experimentation, the optimized result was attained
by reduction of 15, which contains a benzoyl ester
group at C10. Upon treatment of 15 with Et3SiH–
TMSOTf at 0 ꢁC, the desired tetracyclic ether 18 was
obtained in 91% yield in a stereocontrolled manner
without the formation of products having C9 hydroxyl
groups corresponding to 19 and 20.18,19 It is likely that
hydrogen bond formation between the carbonyl oxygen
and the C10 hemiacetal group, along with the electron-
withdrawing nature of the benzoyl group, stabilize the
hemiacetal (A) and facilitate reduction leading to 18
(Scheme 2).20 Protecting group manipulation of 18
and subsequent allylation of the resulting secondary
alcohols gave 21. The allyl groups were subjected to
hydroboration, followed by reaction with N,N0-bis(benz-
yloxycarbonyl)guanidine 22 under Mitsunobu condi-
tions to give 23 in 77% yield.21 Hydrogenolysis of the
benzyloxycarbonyl and the benzylidene acetal groups
liberated the hydrochloride salt of 2 in 88% yield (2
steps).
Figure 1. (a) Illustration of the binding of a cyclic ether-based
synthetic receptor to an a-helical peptide through simultaneous
hydrogen bondings between guanidinium and carboxylate groups.
(b) Design of a cyclic ether scaffold 1, a synthetic receptor 2 containing
two guanidinium groups, and a half-receptor 3.
After obtaining the tetracycle 2, interactions with a-heli-
cal peptides were evaluated by CD spectroscopy. We
prepared a family of 16-mer peptides with two aspartate
groups at different positions (i + 3, i + 4, i + 5, and
i + 11) along the chain, which included C- and N-termi-
nal capping (Fig. 2).22 All peptides were used as their
bistetramethylammonium salts in 10% H2O/90%
MeOH at 25 ꢁC and were designed to possess significant
a-helical character under the conditions used.23 Upon
addition of increasing amounts of receptor 2 (up to
4 equiv: 1.23 mM) into the solution of the peptide
i + 4, the CD spectrum showed a marked increase
(5%) in a-helicity (minimum at 222 nm), as shown in
Figure 3. In comparison, the increase in helicity for
the i + 3 peptide was far less obvious, and no substan-
tial alterations of the peptide structures of i + 5 and
i + 11 were observed upon addition of 4 equiv of recep-
tor 2 to the peptides (Fig. 3b). To verify the effect of sin-
gle-point binding on peptide conformational stability, a
similar CD titration of the i + 4 peptide was carried out
using the half-receptor 3. The change in the peptide
structure was not clear. Thus, these results suggest that
receptor 2 binds preferentially to the helically oriented
i + 4 peptide, in which the rigid cyclic ether scaffold ori-
ents two guanidinium groups to interact simultaneously
based on the convergent strategy developed by Fujiwara
et al.11 Nakata and co-workers,12 and Mori et al.13 As
shown in Scheme 1, this assembly involves (i) acetyl-
ide-mediated connection of two tetrahydropyrans
(6 + 7 ! 8), (ii) oxidation of the alkyne group to an
a-diketone (8 ! 10), (iii) double cyclization to tetra-
cyclic dihemiacetal (10 ! 13), and (iv) stereocontrolled
reduction of the dihemiacetal, leading to the tetracyclic
ether framework (13 ! 16). To incorporate the two
equatorial hydroxyl groups into the C4 and C10 posi-
tions of the tetracyclic ether skeleton, the coupling com-
ponent, acetylene 6 containing the C4 hydroxyl group
was synthesized from tri-O-benzyl-D-glucal 4. Epoxida-
tion of 4 using Spilling’s method, followed by addition
of propargyl Grignard reagent HCCCH2MgBr, yielded
6 in 53% yield (3 steps).14 The C10 hydroxyl group was
installed by stereoselective addition of the lithium acet-
ylide of 6 to aldehyde 7,15 resulting in a mixture of sep-
arable diastereomers (8:9 = 2.7:1.0) in quantitative
yield. Protection of the resulting hydroxyl group of 8
and subsequent oxidation of the internal acetylene with
RuO2–NaIO4 gave diketones 10–12. Exposure of the

H. Oguri et al. / Tetrahedron Letters 47 (2006) 5801–5805
5803
Scheme 1. Reagents and conditions: (a) NBS, THF/H2O (10/1), 0 ꢁC; (b) KN(SiMe3)2, toluene, ꢀ78 ꢁC; (c) HCCCH2MgBr, 53% (3 steps); (d)
TBSCl, imidazole, DMF, 60 ꢁC, 89%; (e) n-BuLi, THF, ꢀ80 ꢁC, then 7, ꢀ80 to ꢀ20 ꢁC, 8 (73%), 9 (27%); (f) TBSCl, imidazole, DMF, 40 ꢁC, 86%,
then RuO2, NaIO4, CCl4/MeCN/H2O (1/1/1.5), 10 (61%); (g) benzyl bromide, NaH, THF/DMF (3/1), 99%, then RuO2, NaIO4, CCl4/MeCN/H2O
(1/1/1.5), 11 (70%); (h) benzoyl chloride, DMAP, pyridine, 96%, then RuO2, NaIO4, CCl4/MeCN/H2O (1/1/1.5), 12 (73%); (i) CSA, MeOH/CH2Cl2
(4/1), 13 (73%), 14 (90%) or 15 (96%); (j) Et3SiH, TMSOTf, MeCN/CH2Cl2 (3/1), ꢀ40 to 0 ꢁC; (k) H2, Pd(OH)2, AcOEt/MeOH (5/1);
(l) PhCH(OMe)2, CSA, DMF, 95% (2 steps); (m) K2CO3, MeOH; (n) allyl bromide, NaH, THF/DMF (4/1), 85% (2 steps); (o) (Sia)2BH, THF,
0 ꢁC, then H2O2, NaOH 71%; (p) 22, DEAD, PPh3, THF, 77%; (q) H2, Pd(OH)2/C, MeOH/AcOEt (1/3); (r) MeOH, concd HCl (10/1), 88% (2
steps).
˚
with two carboxylates spaced by 4–5 A in an approxi-
mately parallel arrangement, as illustrated in Figure
1a. The resulting binding curves of the i + 4 peptide
with 2 were fitted by a 1:1 binding model,25 and the
association constants were calculated as Ka = 1.00 ·
103 Mꢀ1
.
26
In conclusion, a small-molecule receptor for specific rec-
ognition of i + 4 spaced aspartate pairs on the surface
of a-helical peptides has been developed. We also
showed that this mode of molecular recognition pro-
motes a-helicity of the peptide in aqueous media. Fur-
ther efforts will be directed to exploit the structurally
defined cyclic ether scaffold for the development of
cell-permeable small molecules that bind to a larger area
of the protein surface and disrupt protein–protein
interactions.
Scheme 2.
Figure 2. Substrate peptide sequences; target aspartates are shown in bold.

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H. Oguri et al. / Tetrahedron Letters 47 (2006) 5801–5805
Figure 3. (a) CD spectra of peptide i + 4 (0.314 mM) in the absence or presence of the receptor 2 (0–1.23 mM (4 equiv)) in 10% water/methanol at
25 ꢁC. (b) Ellipticity at 222 nm for each peptide in the presence of increasing amounts of receptor 2: ( ) i + 4 (0.314 mM), ( ) i + 3 (0.341 mM), ( )
i + 5 (0.299 mM), and ( ) i + 11 (0.273 mM) in 10% water/methanol at 25 ꢁC. Control titration of i + 4 (0.314 mM) with the half-receptor 3 is also
shown ( ). Uncertainties in all CD measurements were estimated to be 10%.
(b) Yin, H.; Hamilton, A. D. Bioorg. Med. Chem. Lett.
2004, 14, 1375.
Acknowledgments
5. Selected examples for nonpeptide based a-helix mimetics,
see: (a) Albert, J. S.; Peczuh, M. W.; Hamilton, A. D.
Bioorg. Med. Chem. 1997, 5, 1455; (b) Haack, T.; Peczuh,
M. W.; Salvatella, X.; Sanchez-Quesada, J.; de Mendoza,
J.; Hamilton, A. D.; Giralt, E. J. Am. Chem. Soc. 1999,
121, 11813; (c) Xuereb, H.; Maletic, M.; Gildersleeve, J.;
Pelczer, I.; Kahne, D. J. Am. Chem. Soc. 2000, 122, 1883;
(d) Yin, H.; Lee, G.-I.; Sedey, K. A.; Kutzki, O.; Park, H.
S.; Orner, B. P.; Ernst, J. T.; Wang, H.-G.; Sebti, S. M.;
Hamilton, A. D. J. Am. Chem. Soc. 2005, 127, 10191; For
reviews, see: (e) Peczuh, M. W.; Hamilton, A. D. Chem.
Rev. 2000, 100, 2479.
The authors thank Professor M. Inoue (Tohoku Univer-
sity) for valuable suggestions about the synthesis of 18
and Professor K. Tsumoto (The University of Tokyo) for
insightful discussions about interactions of trans-fused
cyclic ethers with proteins. This work was supported
by the Core Research for Evolutional Science and Tech-
nology (CREST) and Solution Oriented Research for
Science and Technology (SORST) programs of the
Japan Science and Technology Agency (JST), and by a
Grant-in-Aid for Scientific Research (S) from the Japan
Society for the Promotion of Science (JSPS).
6. Oguri, H.; Oomura, A.; Tanabe, S.; Hirama, M. Tetra-
hedron Lett. 2005, 46, 2179.
References and notes
7. Murata et al. recently reported the interaction of the
ladder-shaped polycyclic ether with the a-helix that
induces dissociation of glycopholin oligomers, see: Mori,
M.; Oishi, T.; Matsuoka, S.; Ujihara, S.; Matsumori, N.;
Murata, M.; Satake, M.; Oshima, Y.; Matsusita, N.;
Aimoto, S. Bioorg. Med. Chem. 2005, 13, 5099.
8. For recent reviews and accounts, see: (a) Fujiwara, K.;
Murai, A. Bull. Chem. Soc. Jpn. 2004, 77, 2129; (b) Sasaki,
M.; Fuwa, H. Synlett 2004, 1851; (c) Inoue, M.; Hirama,
M. Acc. Chem. Res. 2004, 37, 961; (d) Kadota, I.;
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T. Chem. Rev. 2005, 105, 4314; (f) Inoue, M. Chem. Rev.
2005, 105, 4379; (g) Nicolaou, K. C. J. Org. Chem. 2005,
70, 7007.
1. For recent reviews, see: (a) Pagliaro, L.; Felding, J.;
Audouze, K.; Nielsen, S. J.; Terry, R. B.; Krog-Jensen, C.;
Butcher, S. Curr. Opin. Chem. Biol. 2004, 8, 442; (b) Zhao,
L.; Chmielewski, J. Curr. Opin. Struct. Biol. 2005, 15, 31;
(c) Yin, H.; Hamilton, A. D. Angew. Chem., Int. Ed. 2005,
44, 4130.
2. (a) Andrews, M. J. I.; Tabor, A. B. Tetrahedron 1999, 55,
11711; (b) Venkatraman, J.; Shankaramma, S. C.; Bala-
ram, P. Chem. Rev. 2001, 101, 3131.
3. Kritzer, J. A.; Stephens, O. M.; Guarracino, D. A.;
Reznik, S. K.; Schepartz, A. Bioorg. Med. Chem. 2005, 13,
11, and references cited therein.
4. (a) Ernst, J. T.; Becerril, J.; Park, H. S.; Yin, H.;
Hamilton, A. D. Angew. Chem., Int. Ed. 2003, 42, 535;
9. (a) Rodr´ıguez, R. M.; Morales, E. Q.; Delgado, M.;
Esp´ınola, C. G.; Alvarez, E.; Pe´rez, R.; Mart´ın, J. D. Org.

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5805
´
´
´
Lett. 1999, 1, 725; (b) Espınola, C. G.; Perez, R.; Martın,
19. Mori et al. reported that similar reduction of the tetra-
cyclic bishemiacetal without the C10 hydroxyl group
resulted in the formation of a complex mixture or an
unexpected skeletal rearrangement yielding a spiroketal,
see Ref. 13a.
20. Reduction of the corresponding acetate in place of
benzoate 15 resulted in the formation of the desired
tetracyclic ether in a lower yield (36%). This suggests that
the electron-withdrawing nature of the benzoate group
plays an important role in this conversion.
J. D. Org. Lett. 2000, 2, 3161.
10. Schug, K. A.; Lindner, W. Chem. Rev. 2005, 105, 67, and
references cited therein.
11. Fujiwara, K.; Morishita, H.; Saka, K.; Murai, A. Tetra-
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12. (a) Matsuo, G.; Hinou, H.; Koshino, H.; Suenaga, T.;
Nakata, T. Tetrahedron Lett. 2000, 41, 903; (b) Suzuki, K.;
Nakata, T. Org. Lett. 2002, 4, 2739; (c) Suzuki, K.;
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13. (a) Mori, Y.; Mitsuoka, S.; Furukawa, H. Tetrahedron
Lett. 2000, 41, 4161; (b) Mori, Y.; Hayashi, H. Tetra-
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H.; Noyori, R. J. Org. Chem. 2003, 68, 9050.
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22. The peptide concentration was determined by the BCA
(bicinchoninic acid) protein assay (Pierce) using BSA
(bovine serum albumin) as a standard.
23. Ellipticity is reported as the mean residue ellipticity. The
degree (fraction, f) of helicity was calculated from
f = ꢀ(H222 + 2340)/30300.24 The percentages of helicity
of the peptides were determined as i + 3 (63%), i + 4
(56%), i + 5 (71%), and i + 11 (64%) by monitoring the
ellipticity at 222 nm using the average of 120 points
collected over 60 s.
16. Attempts to convert bishemiacetal 14 into bismethylketals
by treatment with CSA and CH(OMe)3 in MeOH/CH2Cl2
were unsuccessful.
17. The stereochemistries of 19 and 20 were unambiguously
determined by NMR analysis after acetylation. The
coupling constants between H9 and H10 for the triacetate
of 19 and the diacetate of 20 were J = 6.0 and 9.0 Hz,
respectively. A plausible explanation for the stereochem-
ical outcome at C9 is as follows.
24. Chen, Y.-H.; Yang, J. T.; Martinez, H. M. Biochemistry
1972, 11, 4120.
25. To ensure that the mode of binding was 1:1, a Job CD
titration was conducted with peptide i + 4 and receptor
2 in 10% water/methanol at 25 ꢁC. Maximum signal
change was observed when the molar composition of i + 4
and 2 were equivalent, indicative of the 1:1 complex
formation.
0.3
0.25
0.2
0.15
0.1
0.05
0
18. The stereochemistry of 18 was unambiguously determined
by NOE experiments.
0
0.2
0.4
0.6
0.8
1
mole ratio χ
26. (a) Connors, K. A. Binding Constants; John Wiley and
Sons: New York, 1987; (b) Tabet, M.; Labroo, V.;
Sheppard, P.; Sasaki, T. J. Am. Chem. Soc. 1993, 115,
3866, and see: Refs. 5a and 5b.