Design and synthesis of simplified polycyclic ethers and evaluation of their interaction with an α-helical peptide as a model of target proteins

Article abstract of DOI:10.1016/j.tetlet.2007.03.041

Two simplified pentacyclic ethers, having 6/7/6/6/7 and 6/7/6/7/7 ring systems, were synthesized. A convergent route based on the Suzuki-Miyaura cross-coupling strategy was applied to synthesize these two compounds. Interactions between α-helical peptide,

Full text of DOI:10.1016/j.tetlet.2007.03.041

Tetrahedron Letters 48 (2007) 3181–3186
Design and synthesis of simplified polycyclic ethers and
evaluation of their interaction with an a-helical peptide as a
model of target proteins
Masato Sasaki and Kazuo Tachibana*
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku 113-0033, Japan
Received 10 February 2007; revised 3 March 2007; accepted 8 March 2007
Available online 12 March 2007
Abstract—Two simplified pentacyclic ethers, having 6/7/6/6/7 and 6/7/6/7/7 ring systems, were synthesized. A convergent route
based on the Suzuki–Miyaura cross-coupling strategy was applied to synthesize these two compounds. Interactions between a-heli-
cal peptide, melittin, and synthesized pentacyclic ethers were evaluated by circular dichroism (CD) spectroscopy. Interestingly, only
the polycyclic ether having a 6/7/6/7/7 ring stabilized the a-helical structure of melittin. This result indicated that a ring fusion man-
ner of the polycyclic structure is important to recognize membrane proteins.
ꢀ
2007 Elsevier Ltd. All rights reserved.
Since the structure of brevetoxin B, a redtide toxin of
Florida red tides, was first reported by Nakanishi and
co-workers in 1981, a large number of fused polycyclic
result, we hypothesize a general recognition mechanism
between polycyclic ethers and membrane proteins.
9
1
Recently, Murata, Oishi, and co-workers reported that
a natural polycyclic ether, yessotoxin, interacts with a
ether marine toxins have been isolated and character-
2
10
ized. The intriguing structures of these marine toxins,
transmembrane protein, glycophorin A. They also re-
along with their potent and diverse biological activities,
have stimulated the interest of both chemists and biolo-
gists. Because of the scarcity of polycyclic ethers from
natural sources, the target receptor protein has been
ported the interaction between a synthesized tetracyclic
ether and the same protein but there was no study about
a ring fusion system.
9
c
3
4
identified only for brevetoxins and ciguatoxins. These
toxins share a common binding site of voltage-sensitive
The importance of continuous oxepane rings of breve-
1
1
toxin B was reported and this ring fusion system is
thought to give flexibility to the molecule. Recently,
Mart ´ı n and co-workers reported that two fused oxepane
rings provide milli-second scale conformational
5
sodium channels (VSSCs), designated as site 5. The toxi-
city of these molecules is thought to be associated with
2
b
its conformational flexibility but detailed mechanism
of these molecular recognitions remains unclear.
1
2
change. Based on these facts, we focused on the con-
tinuous oxepane rings to evaluate the skeletal factor of
polycyclic ethers and designed two simplified pentacyclic
ether compounds, 1 (6/7/6/6/7) and 2 (6/7/6/7/7)
(Fig. 1). Polycyclic ether 2 has flexible two fused oxe-
pane rings. An alkyl chain and a benzyl ether in a left
hemisphere were designed to give hydrophobicity con-
sidering the amphiphilic nature of natural polycyclic
ethers. In this Letter, we report convergent syntheses
of 1 and 2, and their interaction with the a-helix part
of melittin.
Previously, our group reported that brevetoxins A and B
2
+
inhibit the Ca influx of C6 glioma cells induced by the
6
,7
polycyclic ether toxin, maitotoxin.
Although this
inhibition needed high concentration of brevetoxins, this
result suggests that brevetoxins weakly bind to maito-
toxin-target molecules, which are thought to be mem-
8
brane proteins different from VSSCs. Considering this
Keywords: Polycyclic ether; Simplified ring system; Suzuki–Miyaura
cross coupling; Convergent synthesis; a-Helix.
Corresponding author. Tel./fax: +81 3 5841 4366; e-mail: ktachi@
chem.s.u-tokyo.ac.jp
Numerous efforts have been directed toward the synthe-
sis of polycylclic ethers, among which Sasaki and
others of our group developed the Suzuki–Miyaura
1
3
*
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.03.041

3182
M. Sasaki, K. Tachibana / Tetrahedron Letters 48 (2007) 3181–3186
Figure 1. Common structural feature of polycyclic ether toxins and the structure of simplified polycyclic ethers 1 (6/7/6/6/7) and 2 (6/7/6/7/7).
cross-coupling strategy.14–16 Two pentacyclic ethers
ment with KOt-Bu in THF furnished the left segment 4
were synthesized by combination of three segments, 4–
in 93% yield for the two steps.
6
, using this strategy (Scheme 1). To expedite the synthe-
sis, these segments were prepared from the same oxe-
Synthesis of another right segment 6 also began with the
same oxepane 3 (Scheme 3). Protection of a hydroxy
group as the MPM ether followed by hydroboration–
oxidation led to alcohol 13 in 96% for the two steps.
pane 3, which could derive the following known
1
7
scheme.
2
0
The synthesis of 4 and 5 is outlined in Scheme 2. The
side chain of oxepane 3 was homologated to unsaturated
ester 7 by ozonolysis and Wittig reaction. Hydrogena-
tion of 7 followed by lactonization gave lactone 8. These
Swern oxidation of 13 followed by Wittig reaction
led to unsaturated ester 14 in 94% for the two steps. Re-
moval of the MPM ether by DDQ gave rise to hydroxy
ester 15 in 94% yield and hydrogenation and saponifica-
tion furnished hydroxy acid 16 in 91% yield for the two
steps. Lactonization following the Yamaguchi proto-
1
8
steps followed the known procedure. Treatment of 8
with KHMDS, HMPA, and (PhO) P(O)Cl gave ketene
2
2
1
22
acetal phosphate 5 (73% yield), as one of the right
col gave lactone 17 88% yield. Finally, the treatment
segments.
of lactone 17 with KHMDS, HMPA, and (PhO) P(O)Cl
2
furnished ketene acetal phosphate 6 (96% yield) as
Preparation of the left segment 4 was started from the
same hydroxyester 7. Simultaneous hydrogenation and
removal of benzylidene were achieved by treatment of
another right segment.
The synthesis of 1 and 2 is outlined in Scheme 4. Stereo-
selective hydroboration of exocyclic enol ether 4 with 9-
7
with 10% Pd/C in THF under hydrogen. Following
2
3
reprotection and lactonization gave a lactone (86%
yield) and the treatment of the lactone with KHMDS,
BBN produced alkylborane, which was in situ reacted
with ketene acetal phosphate 5 in the presence of aque-
ous Cs CO and a catalytic amount of PdCl (dppf)Æ
HMPA, and (PhO) P(O)Cl gave ketene acetal phos-
2
2
3
2
phate 9 in 70% yield. Suzuki–Miyaura cross-coupling
CH Cl , giving rise to cross-coupling product 18 in
2 2
1
9
of 9 with alkylborane derived from olefin 10, followed
by stereoselective hydroboration–oxidation, provided
alcohol 11 in 77% for the two steps. The stereochemistry
89% yield from 4. Stereoselective hydroboration–oxida-
tion of 18 furnished alcohol 19 in 88% yield. Oxidation
2
4
of the resultant hydroxyl group with TPAP/NMO led
to ketone 20 in 98% yield. The relative configuration of
20 was unambiguously confirmed by NOE analysis as
shown. Removal of the MPM group followed by treat-
of 11 was confirmed by proton coupling constant analy-
3
sis of the corresponding acetate derivative ( J
=
9
,10
3
3
9
.6 Hz, J8eq,9 = 4.8 Hz, J8ax,9 = 11.2 Hz). Protec-
tion of 11 as the benzyl ether followed by regioselective
cleavage of p-methoxybenzylidene acetal with DIBAL–
H gave alcohol 12 in 76% yield for the two steps. Iodin-
ation of 12 under standard conditions followed by treat-
ment with EtSH and Zn(OTf) furnished mixed thio-
acetal 21 in 75% for the two steps. Finally, radical
2
2
5
reduction with Ph SnH in the presence of AIBN gave
3
2
6
rise to pentacyclic ether 1 in high yield.
Scheme 1. Synthetic plan of simplified polycyclic ethers.

M. Sasaki, K. Tachibana / Tetrahedron Letters 48 (2007) 3181–3186
3183
Scheme 2. Reagents and conditions: (a) O
PPTS, benzene, reflux, 84%; (d) KHMDS, (PhO)
benzene, reflux, 86% (two steps); (g) KHMDS, (PhO)
PdCl (dppf)ÆCH Cl , DMF, 45 ꢁC, 87%; (i) thexylborane, THF, 0 ꢁC; then aq NaOH, 30% H
9%; (k) DIBAL–H, CH Cl , PPh , imidazole, benzene, rt, 95%; (m) KOt-Bu, THF, 0 ꢁC, 98%.
, À40 ꢁC, 85%; (l) I
3
, CH
2
Cl
P(O)Cl, HMPA, THF, À78 ꢁC, 73%; (e) H
P(O)Cl, HMPA, THF, À78 ꢁC, 70%; (h) 10, 9-BBN, THF, 0 ꢁC; then aq Cs
, 0 ꢁC, 88%, (j) KOt-Bu, THF, 0 ꢁC; then BnBr,
2
, À78 ꢁC; then SMe
2
; then Ph
3
PCHCO
2
Me, rt, 98%; (b) H
2
, 10% Pd/C, EtOAc, rt, 93%; (c)
2
2
, 10% Pd/C, THF, rt; (f) p-MeOC
6
H
4
CH(OMe)
2
, CSA,
, 9,
2
2
CO
3
2
2
2
2 2
O
8
2
2
2
3
Another pentacyclic ether 2 was synthesized by an analo-
gous process. Hydroboration of 4 with 9-BBN followed
by coupling with 6 proceeded in 91% yield from 4 and
hydroboration–oxidation of the coupling product pro-
ceeded stereoselectively to yield alcohol 22 in 88% yield.
Oxidation of the resultant hydroxyl led to ketone 23 in
81% yield. The relative configuration of 23 was unam-
biguously confirmed by NOE analysis in the same man-
ner as shown. Three-step conversion provided mixed
thioacetal 24 in 81% overall yield. Finally, radical reduc-
tion under the above conditions gave rise to another
2
7
pentacyclic ether 2 in 94% yield. The relative configu-
ration of 1 and 2 was unambiguously confirmed by NOE
analysis of acetate derivatives 25 and 26 (Fig. 2).
Because the only known target molecule of polycyclic
ethers, VSSCs, is comprised 24 transmembrane helices,
we assumed that the interacting motifs of them are
membrane-integrated a-helices. Therefore, interaction
of two simplified polycyclic ethers, 1 and 2, with an a-
helical peptide, melittin, was evaluated by CD spectro-
Scheme 3. Reagents and conditions: (a) KOt-Bu, THF, 0 ꢁC; then
MPMCl, 99%; (b) 9-BBN, THF, 0 ꢁC; then aq NaOH, 30% H
ꢁC, 97%; (c) (COCl) , DMSO, CH Cl , À78 ꢁC; then Et N, À78 ꢁC
to rt; (d) Ph PCHCO Et, THF, rt, 94% (two steps); (e) DDQ, CH Cl
buffer (pH 7.0), rt, 94%; (f) H , 10% Pd/C, EtOAc, rt; (g) LiOHÆH O,
THF–H O, 45 ꢁC, 91% (two steps); (h) 2,4,6-trichlorobenzoyl chloride,
Et N, THF, rt; then DMAP, toluene, 100 ꢁC, 88%; (i) KHMDS,
PhO)
P(O)Cl, HMPA, THF, À78 ꢁC, 96%.
2
2
O ,
2
8
scopy. Melittin is the principal venom component of
0
2
2
2
3
2
9
the European honey bee, Apis meliffella. This peptide
is composed of 26 amino acid residues and is known that
the secondary structure of melittin mainly consists of
3
2
2
2
-
2
2
2
3
0
rather unstable a-helix in methanol. So we used this
3
(
2
solvent as the medium to investigate change in the CD

3184
M. Sasaki, K. Tachibana / Tetrahedron Letters 48 (2007) 3181–3186
Scheme 4. Reagents and conditions: (a) 4, 9-BBN, THF, 0 ꢁC; then aq Cs
2
CO
, 0 ꢁC, 88%; (c) TPAP, NMO, 4 A molecular sieves, rt, 98%; (d) DDQ, CH
, rt, 75% (two steps); (f) Ph SnH, AIBN, toluene, reflux, quant; (g) 4, 9-BBN, THF, 0 ꢁC; then aq Cs
DMF, 45 ꢁC, 91%; (h) thexylborane, THF, 0 ꢁC; then aq NaOH, 30% H
THF, rt, 88%; (k) DDQ, CH Cl -buffer (pH 7.0), rt; (l) EtSH, Zn(OTf)
3
, 5, PdCl
2
(dppf)ÆCH
2
Cl
2
, DMF, 45 ꢁC, 89%; (b) BH
Cl -buffer (pH 7.0), rt; (e) EtSH,
CO , 6, PdCl (dppf)ÆCH Cl
, 0 ꢁC, 81%; (i) TPAP, NMO, 4 A molecular sieves, rt, 94%; (j) TBAF,
, CH Cl , rt, 93% (two steps); (m) Ph SnH, AIBN, toluene, reflux, 94%.
3
ÆTHF, THF,
˚
0
ꢁC; then aq NaOH, 30% H
2
O
2
2
2
Zn(OTf) , CH Cl
2
2
2
3
2
3
2
2
2
,
˚
2
O
2
2
2
2
2
2
3
Figure 2. Stereochemical confirmation of pentacyclic ethers 25 and 26.
spectra intensity of melittin coexisting 1 and 2. As
shown in Figure 3, the CD spectrum showed little con-
formational change of melittin with the existence of 1,
whereas, the existence of 2 provides a marked increase
of a-helicity in melittin (minimum at 208 and
3
1
2
22 nm). Figure 3b shows the relationship between
the concentration of simplified polycyclic ethers and a-
helicity of melittin. The a-helicity of melittin depended
on the concentration of 2. These results suggest that
there is mutual interaction between the a-helix and the
polycyclic ether structures. Thus, the flexibility of poly-
cyclic ether is thought to be an important factor of these
interactions.
Figure 3. (a) CD spectra of melittin (10 lM) in the absence or the
presence of 1 and 2 (100 lM) in methanol at 25 ꢁC; (b) ellipticity
change at 208 nm for melittin (10 lM) in the presence of various
amounts of 1 and 2 (10–100 lM).
In conclusion, two pentacyclic ethers 1 and 2 were
designed and synthesized using the Suzuki–Miyaura
cross-coupling strategy with the aim to evaluate the skel-
etal factor of their interaction with a-helical peptides. As
a result, the stronger interaction between pentacyclic
ether 2 and a-helical peptide, melittin, was observed.
Structural difference between 1 and 2 is only the size
of the ether ring. Thus, this result indicates the

M. Sasaki, K. Tachibana / Tetrahedron Letters 48 (2007) 3181–3186
3185
importance of the ring arrangement of the polycyclic
ether core to interact with proteins. In order to accumu-
late further knowledge about the mechanism of these
interactions, the synthesis of elongated polycyclic ethers
is in progress.
8. Kobayashi, M.; Ochi, R.; Ohizumi, Y. Br. J. Pharmacol.
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1
9
. The distance between skeletal oxygen atoms on the same
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2
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Acknowledgments
Ujihara, S.; Matsumori, N.; Murata, M.; Oishi, T. Bioorg.
Med. Chem. Lett. 2006, 16, 6355.
We are grateful to Professor M. Satake in our labora-
tory for a valuable discussion. Mr. S. Motoi contributed
to this study in an earlier stage. This work was finan-
cially supported by CREST, Japan Science and Tech-
nology Agency (JST) and 21st century COE program.
A fellowship (to M.S.) from the Ajinomoto Scholarship
Foundation is also gratefully acknowledged.
10. Mori, M.; Oishi, T.; Matsuoka, S.; Ujishima, S.; Matsu-
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1
2
6. Data for 1: H NMR (500 MHz, CDCl ) d 7.35–7.26 (m,
3
5
3
3
H), 4.63 (d, J = 11.4 Hz, 1H), 4.42 (d, J = 11.4 Hz, 1H),
.85–3.80 (m, 1H), 3.68 (dd, J = 11.2, 4.2 Hz, 1H), 3.65–
.53 (m, 3H), 3.49–3.44 (m, 1H), 3.32–3.24 (m, 2H),

3186
M. Sasaki, K. Tachibana / Tetrahedron Letters 48 (2007) 3181–3186
3
2
1
1
1
7
2
.23–3.12 (m, 5H), 3.09–3.04 (m, 1H), 3.01–2.91 (m, 2H),
.55 (dt, J = 11.8, 3.9 Hz, 1H), 2.32–2.26 (m, 2H), 2.07–
138.0, 128.4, 127.8 (·2), 86.5, 84.9, 82.8, 81.7, 81.6, 81.2,
80.8, 80.4, 79.3, 79.0, 76.3, 71.9, 70.9, 64.7, 62.9, 38.8, 36.9,
.94 (m, 3H), 1.94–1.71 (m, 9H), 1.70–1.62 (m, 2H), 1.51–
30.2, 29.9, 29.2, 29.1, 28.7, 28.6 (·2), 28.4; HRMS (FAB)
13
+
.35 (m, 4H); C NMR (125 MHz, CDCl
3
) d 137.9, 128.5,
calcd for C31
585.3058.
H
46
O
9
Na [(M+Na) ] 585.3040. Found:
27.9, 127.8, 86.2, 82.4, 81.9, 81.6, 81.4, 80.8, 79.3, 79.1,
6.6 (·2), 76.2, 71.2, 70.9, 64.7, 63.0, 37.0 (·2), 36.9, 30.0,
28. (a) Chen, Y.-H.; Yang, J. T.; Chau, K. H. Biochem. 1974,
13, 3350; (b) Greenfield, N. J. Anal. Biochem. 1996, 235, 1.
29. Hambermann, E. Science. 1972, 177, 314.
30. (a) Bazzo, R.; Tappin, M. J.; Pastore, A.; Harvey, T. S.;
Carver, J. A.; Campbell, I. D. Eur. J. Biochem. 1988, 173,
139; (b) Hirota, N.; Mizuno, K.; Goto, Y. Protein Sci.
1997, 6, 416.
9.2, 29.1 (·2), 28.6, 26.5; HRMS (FAB) calcd for
+
C
30
H
44
O
9
Na [(M+Na) ] 571.2883. Found: 571.2872.
1
2
7. Data for 2: H NMR (500 MHz, CDCl
3
) d 7.35–7.26 (m,
5H), 4.62 (d, J = 11.4 Hz, 1H), 4.42 (d, J = 11.4 Hz, 1H),
3.72 (ddd, J = 11.3, 8.0, 4.2 Hz, 1H), 3.67–3.51 (m, 4H),
3.48–3.35 (m, 3H), 3.27–3.12 (m, 6H), 3.09 (dt, J = 9.3,
6.1 Hz, 1H), 3.00 (dt, J = 8.9, 3.9 Hz, 1H), 2.52 (dt,
31. The degree (fraction, f) of helicity was calculated from
2
8a
J = 11.8, 3.9 Hz, 1H), 2.31 (t, J = 5.9 Hz, 1H), 2.28 (dt,
J = 11.9, 4.3 Hz, 1H), 2.09–1.74 (m, 14H), 1.71–1.62 (m,
3
f = À(h + 2340)/30,300.
The percentages of helicity
2
22
of melittin were increased from 62% to 73% with the
1
3
H), 1.52–1.34 (m, 3H); C NMR (125 MHz, CDCl
3
) d
existence of 2.