Solid-phase synthesis of dehydropeptide, AM-toxin II, using a novel selenyl linker by side-chain tethered strategy

Article abstract of DOI:10.1016/S0040-4039(01)01790-7

The cyclic dehydrodepsipeptide, AM-toxin II, was efficiently synthesized by a solid-phase method using a novel selenyl linker. The cleavage from the resin with formation of a double bond is successfully achieved under mild oxidative conditions with TBHP.

Full text of DOI:10.1016/S0040-4039(01)01790-7

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 8337–8339
Solid-phase synthesis of dehydropeptide, AM-toxin II, using a
novel selenyl linker by side-chain tethered strategy
Eiji Horikawa,a,b Masato Kodaka,a Yoshiaki Nakahara,c Hiroaki Okunoa and
Kazuhiko Nakamuraa,*
aNational Institute of Advanced Industrial Science and Technology (AIST), Central 6, Higashi 1-1-1 Tsukuba 305-8566, Japan
bFaculty of Industrial Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan
cDepartment of Industrial Chemistry, Tokai University, Kitakaname 1117, Hiratsuka, Kanagawa 259-1292, Japan
Received 17 August 2001; revised 17 September 2001; accepted 21 September 2001
Abstract—The cyclic dehydrodepsipeptide, AM-toxin II, was efficiently synthesized by a solid-phase method using a novel selenyl
linker. The cleavage from the resin with formation of a double bond is successfully achieved under mild oxidative conditions with
TBHP. © 2001 Elsevier Science Ltd. All rights reserved.
A number of dehydroamino acids are found from natu-
ral sources as a component of bioactive peptides.1 It is
reported that dehydroamino acid residues possess the
activities of a Michael-acceptor, and it is presumed that
various biological activities are due to such reactivity.
In addition, based on structural considerations, they
are strong inducers of folded conformations in
peptides.2
dehydroalanine (DAla). Synthetic studies employing
solution-phase cyclization are already reported.6–9
Recently, we presented a side-chain tethered strategy
for a solid-phase synthesis using a novel linker, and the
synthesis of glycopeptides and Ser/Thr containing pep-
tide, a silyl ether-type linker, was used to bind a side-
chain hydroxyl group onto the solid support.10–12 This
approach has several advantages due to the orthogonal
protection of the N- and C-terminal of amino acids to
allow bi-directional peptide chain elongation after selec-
tive deprotection.
AM-toxins are known as members of this class of
molecules (Fig. 1), as well as host-specific phytotoxins
produced by Alternaria alternata, the fungus causing
leafspot disease of apple trees.3–5 The structure of AM-
toxins is that of cyclic depsipeptides containing a,b-
Based on this study, we designed a selenated alanine as
an anchoring residue of peptide for the synthesis of
dehydropeptides.13 A key feature of this approach, an
oxidative cleavage from the solid support, will give the
corresponding dehydroalanine residue after peptide
elongation and cyclization. The solid-phase synthesis of
cyclic compounds may provide a possible solution to
these problems by anchoring molecules to prevent
oligomerization,14 along with the feasibility of provid-
ing libraries for effective screening of new bioactive lead
compounds.
O
R
NH
O
HN
O
O
HN
O
R = OMe
R = H (1)
R = OH
I
II
III
AM-Toxins
Our synthesis started with the preparation of a sele-
nated alanine. N-Fmoc-L-serine allyl ester was reacted
Figure 1. Structure of AM-toxins.
with o-nitro phenyl selenocyanide with Bu3P in
pyridine15 to give the corresponding selenoether16 in
83% yield (Scheme 1). The resulting compound 2 was
successively reduced with Zn–AcOH to give the aniline
derivative 3 (82%), which was treated with succinic
Keywords: AM-toxin; solid-phase synthesis; dehydropeptide; cyclic
peptide; selenyl linker.
* Corresponding author. Tel.: +81-298-61-6186; fax: +81-298-61-6123;
e-mail: kazuhiko.nakamura@aist.go.jp
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)01790-7

8338
E. Horikawa et al. / Tetrahedron Letters 42 (2001) 8337–8339
H
O
NO2
NH2
N
OH
a
b
Se
c
O
Se
Fmoc-Ser-OAll
Se
FmocHN
COOAll
FmocHN
COOAll
FmocHN
COOAll
2
3
4
H
H
N
O
O
N
Gly
Gly
e
d
O
O
Se
Se
FmocHN
COOAll
FmocHN
COOH
6
5
Scheme 1. Reagents and conditions: (a) o-NO2-C6H4-SeCN, Bu3P/pyridine, rt, 2 h (83%); (b) Zn, AcOH/THF, rt, 2 h (82%); (c)
succinic anhydride, DIEA/CH2Cl2, rt, 3 h (81%); (d) Gly-Wang resin (0.65 mmol/g), HATU, DIEA/DMF, voltex, rt, overnight,
98% in Keiser test; (e) Pd(PPh3)4, dimedone/THF, voltex, rt, 3 h.
anhydride to afford succinanilic acid16 in 81% yield.
The carboxylic acid was immobilized to the commer-
cially available Gly-Wang resin (Novabiochem, Gly
content 0.65 mmol/g) with HATU and DIEA. The final
seleno amino acid content (0.17 mmol/g) was calculated
by the weight of carboxylic acid after acid cleavage by
the action of TFA. Then, the C-terminal protecting
group was removed by palladium chemistry (cat.
Pd(PPh)4 and excess dimedone in THF) to give immo-
bilized free acid 6.
The C-terminal fragment was synthesized by the usual
liquid-phase procedure. Boc-Ala and -2-hydroxy-3-
methylbutanoic acid ( -Hmb) allyl ester (prepared from
-valine by the van Slyke reaction in 76% yield; fol-
L
L
L
lowed by allylation with Cs2CO3 and allyl bromide in
65%) were condensed with DCC–DMAP to give the
corresponding ester (73%), which was deprotected by
palladium chemistry (cat. Pd(PPh)4 and excess pipe-
ridine in THF, 93%) to give carboxylic acid 7 (Scheme
2). The other unusual amino acid,
L
-2-amino-5-
H
N
O
O
O
O
H
Gly
g
H2N
N
a - d
e, f
BocNH
OH
O
O
OAll
O
L-Val
Se
O
O
O
O
H
H
N
N
7
FmocNH
O
OAll
8
O
O
C3H6Ph
9
H
O
H
O
N
N
Gly
Gly
O
O
Se
O
Se
O
O
NH HN
O
h - j
k
AM-toxin II
NH HN
O
1
O
O
HN
O
C3H6Ph
O
O
HN
O
C3H6Ph
10
Scheme 2. Reagents and conditions: (a) NaNO2/AcOH aq., rt, 30 min (76%); (b) Cs2CO3 then allyl bromide, rt, overnight (65%);
(c) Boc-Ala, DCC, DMAP/DMF, rt, overnight (73%); (d) cat. Pd(PPh3)4, piperidine/THF, rt, 2 h (93%); (e) -App-OAll (see text),
L
HATU, DIEA/DMF, rt, 6 h (88%); (f) 50% TFA/CH2Cl2, rt, 1 h (quant.); (g) 6, HATU, DIEA/DMF, voltex, rt, overnight; (h)
cat. Pd(PPh3)4, dimedone/THF, voltex, rt, 2 h; (i) 40% piperidine/DMF, voltex, rt, 1 h; (j) FDPP, DIEA/DMF, voltex, rt, 2 days;
(k) TBHP/CH2Cl2–TFE, voltex, rt, 7 h.

E. Horikawa et al. / Tetrahedron Letters 42 (2001) 8337–8339
8339
phenylpentanoic acid (App), was synthesized by a simi-
lar protocol described by Shirahama and co-workers.8
The known racemic acetyl amino acid was prepared
from 3-phenyl propanol in four steps (1. PBr3, 2.
diethyl acetoamidomalonate–NaOEt/EtOH, 3. NaOH,
7. Ueda, K.; Waki, M.; Teruya, Y.; Izumiya, N. Bull. Chem.
Soc. Jpn. 1989, 62, 1635–1638.
8. Hashimoto, K.; Sakai, M.; Okuno, T.; Shirahama, H.
Chem. Commun. 1996, 1139–1140.
9. Miyashita, M.; Nakamori, T.; Murai, T.; Miyagawa, H.;
Akamatsu, M.; Ueno, T. Biosci. Biotechnol. Biochem.
1998, 62, 1799–1801.
10. Nakamura, K.; Hanai, H.; Kanno, M.; Kobayashi, A.;
Ohnishi, Y.; Ito, Y.; Nakahara, Y. Tetrahedron Lett.
1999, 40, 515–518.
4. decarboxylation).6 The enantiomerically pure17
L-
App was prepared by enzymatic hydrolysis with amino
acylase (from Aspergius genus), which was converted
into an allyl ester via an Na-Boc derivative (1. Boc2O,
2. Cs2CO3 then allyl bromide, 3. TFA) in 21% yield (in
four steps including optical resolution). The coupling
between Boc-Ala-Hmb (7) and App-OAll was carried
out by treatment with HATU and DIEA in 88% yield,
followed by deprotection with TFA to give free amine
8 (quant.).
11. Nakamura, K.; Ishii, A.; Ito, Y.; Nakahara, Y. Tetra-
hedron 1999, 55, 11253–11266.
12. Ishii, A.; Hojo, H.; Kobayashi, A.; Nakamura, K.; Naka-
hara, Y.; Ito, Y.; Nakahara, Y. Tetrahedron 2000, 56,
6235–6243.
13. An application of the specially prepared selenated resin
for the synthesis of benzopyranes by a solid-phase
methodology has been reported. See: Nicolaou, K. C.;
Pastor, J.; Barluenga, S.; Winssinger, N. Chem. Commun.
1998, 1947–1948.
14. This effect is known as the pseudo-dilution. See: Bourne,
G. T.; Meutermans, W. D. F.; Alewood, P. F.; McGeary,
R. P.; Scanlon, M.; Watson, A. A.; Smythe, M. L. J. Org.
Chem. 1999, 64, 3095–3101.
The C-terminal condensation was achieved by the
tripeptide allyl ester 8 using HATU–DIEA to give the
protected dihydro seco-acid on the solid support (9).
After deprotection with Pd(0) followed by piperidine
treatment, cyclization was carried out just on the resin
by using FDPP (pentafluorophenyl diphenylphosphi-
nate)8,18 and DIEA.
15. Grieco, P. A.; Gilman, S.; Nishizawa, M. J. Org. Chem.
Finally, oxidative cleavage via selenoxide from resin
was performed by treatment with TBHP in CH2Cl2–
TFE to give successfully AM-toxin II (1) with concomi-
tant formation of an unsaturated bond. The yield (ca.
10%)19 was estimated on the basis of the calculated
loading amount of selenated amino acid 5. The syn-
thetic compound was identical to a natural sample with
spectrum aspects4 and Rf values of TLC (four different
solvent systems).16
1976, 41, 1485–1486.
16. Selected data of the key compounds. Compound 2: [h]D26
+6.1 (c 1, CHCl3); lH (CDCl3) 3.34 (1H, dd, J=5.8, 12.8
Hz), 3.48 (1H, dd, J=5.8, 12.8 Hz), 4.19 (1H, t, J=7.0
Hz), 4.38 (2H, d, J=7.0 Hz), 4.55 (2H, ddd, J=6.3, 6.3,
13.3 Hz), 4.80 (1H, br.q, J=ca. 6 Hz), 5.23 (1H, br.d,
J=10.4 Hz), 5.35 (1H, br.d, J=17.4 Hz), 5.65 (1H, br.d,
J=7.6 Hz, NH), 5.95 (1H, m), 7.20–8.22 (ꢀ12H, arom.).
Compound 4: [h]D26 +16.1 (c 1, CHCl3); lH (CDCl3) 2.73
(2H, br.t, J=ca. 7 Hz), 2.77 (2H, dist.t), 3.14 (1H, dd,
J=5.0, 12.8 Hz), 3.35 (1H, dd, J=5.0, 12.8 Hz), 4.20
(1H, t, J=7.0 Hz), 4.33 (2H, m), 4.35 (1H, m), 4.55 (1H,
dd, J=6.0, 13.3 Hz), 4.67 (1H, br.dd, J=ca. 6, 6 Hz),
5.18 (1H, br.d, J=ca. 10 Hz), 5.24 (1H, br.d, J=ca. 17
Hz), 5.76 (1H, m), 5.87 (1H, d, J=7.6 Hz, NH), 7.0–7.7
(ꢀ12H, arom.), 8.26 (1H, d, J=7.6 Hz, NH). Synthetic
AM-toxin II (1): [h]D28 −18.7 (c 0.17, DMSO) (lit. [h]2D7
−14.2 (c 0.55, DMSO)); HRMS calcd for C22H29N3O5
m/z 415.2105, found m/z 415.2096; lH (DMSO-d6, 45°C)
0.89 and 0.90 (each 3H, d, J=ca. 7 Hz, overlapped), 1.35
(3H, d, J=7.5 Hz), 1.53 (1H, m), 1.62 (2H, m), 1.86 (1H,
m), 1.98 (1H, m), 2.56 (1H, m), 2.66 (1H, m), 4.32 (2×1H,
m, overlapped), 4.68 (1H, d, J=6.5 Hz), 5.36 (1H, br.s),
5.40 (1H, s), 7.16–7.28 (5H, arom.), 7.99 (1H, br.s, NH),
8.11 (1H, br.d, J=9.0 Hz, NH), 9.05 (1H, br.s, NH);
TLC (Merck silica gel 60) Rf=0.56 (PhH:acetone=2:1;
lit.6 0.57), Rf=0.28 (EtOAc:CHCl3=1:1; lit.7 0.30), Rf=
0.28 (CHCl3:MeOH:AcOH=95:5:1; lit.7 0.30), Rf=0.84
(n-BuOH:AcOH:H2O=4:1:1; lit.7 0.83).
In conclusion, we have demonstrated the potential of
the newly developed synthetic strategy to dehydropep-
tide. AM-toxin has been successfully synthesized by
C-terminal peptide elongation and cyclization, followed
by oxidative cleavage by formation of the double bond.
We have also demonstrated that this methodology is
very useful to synthesize unsaturated compounds using
solid-phase chemistry.
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19. Degradation products derived from the seco-acid were
obtained as by-products (structures not determined).