Total synthesis of cis,cis-ceratospongamide, a bioactive thiazole-containing cyclic peptide from marine origin

Article abstract of DOI:10.1055/s-2001-14641

The first total synthesis of cis,cis-ceratospongamide (1a), isolated from marine source, was accomplished via thiazole synthesis using CMD methodology, DEPC-mediated peptide coupling, macrolactamization, and cyclodehydration. Comparison of the cyclization sites and coupling reagents in the macrolactamization step was also investigated.

Full text of DOI:10.1055/s-2001-14641

986
LETTER
Total Synthesis of cis,cis-Ceratospongamide, a Bioactive Thiazole-Containing
Cyclic Peptide from Marine Origin
Fumiaki Yokokawa,* Hirofumi Sameshima, Takayuki Shioiri*
Faculty of Pharmaceutical Sciences, Nagoya City University, Tanabe-dori, Mizuho-ku, Nagoya 467-8603, Japan
Fax +81 (52) 8344172; E-mail: shioiri@phar.nagoya-cu.ac.jp
Received 30 January 2001
Dedicated to Professor Ryoji Noyori in recognition of his remarkable contributions to modern asymmetric synthesis
L-Phe
L-Pro
Thz
S
Abstract: The first total synthesis of cis,cis-ceratospongamide (1a),
isolated from marine source, was accomplished via thiazole synthe-
sis using CMD methodology, DEPC-mediated peptide coupling,
macrolactamization, and cyclodehydration. Comparison of the cy-
clization sites and coupling reagents in the macrolactamization step
was also investigated.
O
N
cyclodehydration
O
N
N
H
O
N
ceratospongamide (1)
NH
HO
H
N
H
N
O
O
L-Phe
L-Pro
Key words: ceratospongamide, chemical manganese dioxide
(CMD), macrolactamization, cyclic peptide, total synthesis
O
L-aThr
L-Ile
2
Boc-L-Ile-L-aThr-L-Phe-L-Pro-
Thz-OMe (3)
1) segment condensation
2) macrocyclization
Ceratospongamide is a bioactive thiazole-containing cy-
clic heptapeptide isolated by Gerwick and co-workers
from the Indonesian red alga Ceratodictyon spongiosum
and symbiotic sponge Sigmadocia symbiotica.1 This pep-
tide consists of two L-phenylalanine residues, one (L-iso-
leucine)-L-methyloxazoline residue, one L-proline
residue, and one (L-proline)thiazole residue. Interestingly,
ceratospongamide is able to be isolated as two stable iso-
mers, which are assigned as cis,cis-and trans,trans-iso-
mers (1a, 1b) of the two proline amide bonds by Gerwick
and co-workers (Figure 1). Both compounds show moder-
ate potency in the brine shrimp toxicity assay (LD50 = 13-
19 M). In addition, trans,trans-ceratospongamide (1b)
exhibits potent inhibition of the expression of a key en-
zyme in the inflammatory cascade, secreted phospholi-
+
Boc-L-Phe-L-Pro-OMe (4)
Scheme 1 Retrosynthetic analysis of ceratospongamide
pase A2 (sPLA2) with an ED50 of 32 nM, whereas the
cis,cis-conformer (1a) is inactive. As a part of our pro-
gram toward the total synthesis of marine natural prod-
ucts,2 we have embarked on the total synthesis of
ceratospongamide.3
Our retrosynthetic analysis of ceratospongamide is shown
in Scheme 1. As the oxazoline ring is sensitive under acid-
ic conditions, we used allo-L-threonine as a precursor of
the oxazoline and postponed the cyclodehydration of the
threonine residue to the final stage of the total synthesis
for reducing the risk of racemization at the chiral center
attached to the heterocycle.4 For the synthesis of the mac-
rocycle 2, a [5+2] convergent strategy was adopted to give
the pentapeptide segment 3 and dipeptide segment 4. Be-
cause the thiazole and L-proline residues located on the C-
terminus in peptide coupling would be tolerant of racem-
ization, the segment condensation and macrocyclization
at the thiazole/L-phenylalanine and L-proline/L-isoleucine
amide bonds were chosen among the six amide bonds.
O
S
N
O
N
N
H
O
NH
N
N
H
N
O
O
O
cis,cis-Ceratospongamide (1a)
For the preparation of the thiazole amino acid fragment 8,
we applied our CMD (chemical manganese dioxide) oxi-
dation for the conversion of thiazolidine to thiazole
(Scheme 2).3b,5 Coupling of Boc-protected L-proline with
N,O-dimethylhydroxylamine using DEPC (diethyl phos-
phorocyanidate)6 afforded the amide 5 in 88% yield. After
reduction of the amide 5 with lithium aluminum hydride,7
the resulting aldehyde 6 was condensed with L-cysteine
methyl ester to give the thiazolidine 7 as a diastereomeric
O
S
N
O
N
N
H
NH
O
N
N
H
N
O
O
O
trans,trans-Ceratospongamide (1b)
Figure
Synlett 2001, SI, 986–988 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Total Synthesis of cis,cis-Ceratospongamide
987
Boc-L-Phe-OH
DEPC, Et3N, DMF
97 %
Boc-L-Phe-L-Pro-OMe (4)
HCl·H-L-Pro-OMe
HCl-dioxane
Boc-L-Pro-Thz-OMe (8)
DEPC, Et3N, DMF
HCl-dioxane
Boc-L-Phe-L-Pro-Thz-OMe (9)
DEPC, Et3N, DMF
94 %
HCl-dioxane
94 %
Boc-L-aThr-L-Phe-
L-Pro-Thz-OMe (10)
Boc-L-Phe-OH
DEPC, Et3N, DMF
75 %
Boc-L-aThr-OH
Boc-L-Ile-OH
Boc-L-Ile-L-aThr-L-Phe-L-Pro-Thz-OMe (3)
S
CO2Me
N
1. aq. LiOH, THF
2. HCl-dioxane
HCl-dioxane
O
Boc-L-Ile-L-aThr-L-Phe-
L-Pro-Thz-OMe (3)
N
HN
DEPC, Et3N, DMF
94 %
O
2
Boc
H
N
NH
HO
3. a) DPPA, Et3N, DMF (0.002 M),4 °C, 144 h
31 %
N
b) FDPP, i-Pr2NEt, DMF (0.002 M), 4 °C, 144 h 47 %
63 %
H
N
Boc-L-Phe-L-Pro-OMe (4)
aq. LiOH, THF
c) FDPP, i-Pr2NEt, DMF (0.002 M), rt, 50 h
O
d) HATU, i-Pr2NEt, DMF (0.002 M), 4 °C, 144 h 18 %
O
O
11
O
S
N
aq. LiOH, THF
O
1. aq. LiOH, THF
2. HCl-dioxane
N
N
H
Boc-L-Ile-L-aThr-L-Phe-
L-Pro-Thz-OMe (3)
O
DEPC, Et3N, DMF
99 %
2
NH
HO
H
N
HCl-dioxane
N
3. a) DPPA, Et3N, DMF (0.002 M),rt, 74 h
b) FDPP, i-Pr2NEt, DMF (0.002 M), rt, 48 h
c) FDPP, i-Pr2NEt, DMF (0.002 M), rt, 74 h
d) HATU, i-Pr2NEt, DMF (0.002 M), rt, 137 h
23 %
24 %
27 %
0 %
Boc
Boc-L-Phe-L-Pro-OMe (4)
NH
O
O
CO2Me
12
F
F
F
F
N
O
P
-
N
Ph
Ph
PF6
O
S
+
(PhO)2P(O)N3
DPPA
O
F
N
O
N
N
NMe2
O
Deoxo-Fluor
2
N
N
H
O
NMe2
FDPP
HATU
NH
CH2Cl2, -30 ~ -20 °C
N
N
H
N
54 %
O
Deoxo-Fluor
(CH3OCH2CH2)2NSF3
O
O
cis,cis-ceratospongamide (1a)
Scheme 3 Total synthesis of cis,cis-ceratospongamide (1a)
mixture in 73% yield. Subsequent CMD oxidation of the ative removal of the Boc protective group followed by
thiazolidine 7 provided the thiazole fragment 8 in 51% peptide coupling with the Boc derivatives of L-allo-threo-
yield.
nine and L-isoleucine led to the pentapeptide segment 3.
After acidic deprotection of the Boc group of 3, segment
condensation with the acid derived from the dipeptide 4
proceeded smoothly to give the fully protected heptapep-
tide 11 in 94% yield (Scheme 3). Similarly, deprotection
of the Boc group of 4 and segment condensation with the
acid derived from 3 also afforded the other linear precur-
sor 12 in 99% yield. The stage was now set for the com-
parison of macrolactamization step at two sites (Thz/Phe
vs. Pro/Ile). After deprotection of the Boc group in 11 and
12 with hydrogen chloride in dioxane and saponification
of the methyl ester with lithium hydroxide, macrolactam-
izations of the resulting two free linear precursors were
performed in a DMF solution (0.002 M) by using DPPA
(diphenyl phosphorazidate),8 FDPP (pentafluorophenyl
diphenylphosphinate),9 and HATU [O-(7-azabenzotriaz-
ol-1-yl)-N,N,N’,N’-tetramethyluronium hexafluorophos-
phate],10 respectively. The cyclization at the Thz/Phe site
proceeded smoothly and the yield by FDPP was higher
than those by DPPA and HATU. In contrast, the cycliza-
tion at Pro/Ile site was sluggish to proceed, and especially
Me
N
HCl·HN(OMe)Me
DEPC, Et3N
LiAlH4
Et2O
CHO
CO2H
OMe
N
Boc
6
N
Boc
N
Boc
5
DMF
88 %
O
HCl·H-L-Cys-OMe
Et3N
S
S
CMD
N
N
Boc
8
toluene
benzene
51 %
Boc
N
H
N
CO2Me
CO2Me
73 % (2 steps)
7
H
Scheme 2 Synthesis of Boc-L-Pro-Trz-OMe
The dipeptide segment 4 was obtained by coupling of
Boc-protected L-phenylalanine with L-proline methyl es-
ter using DEPC in 97% yield (Scheme 3). For the prepa-
ration of the pentapeptide segment 3, removal of the Boc
protective group of the thiazole fragment 8 with hydrogen
chloride in dioxane followed by condensation with Boc-
protected L-phenylalanine using DEPC in 94% yield. Iter-
Synlett 2001, SI, 986–988 ISSN 0936-5214 © Thieme Stuttgart · New York

988
F. Yokokawa et al.
LETTER
Colsen, K.; Asano, T.; Yokokawa, F.; Shioiri, T.; Gerwick, W.
H. J. Am. Chem. Soc. 2000, 122, 12041-12042. (d) Yokokawa,
F.; Asano, T.; Shioiri, T. Org. Lett. 2000, 2, 4169-4172.
(e) For a review, see Shioiri, T.; Hamada, Y. Synlett 2001,
184-201.
no cyclized product was obtained by use of HATU. Ac-
cordingly, we carried out the macrolactamization at the
Thz/Phe site with 1.5 equivalents of FDPP and 4 equiva-
lents of i-Pr2NEt at room temperature to give the cyclic
peptide 2 in 63% yield from the corresponding linear pre-
cursor 11. Finally, cyclodehydration of the allo-threonine
residue with bis(2-methoxyethyl)aminosulfur trifluoride
(Deoxo-fluor) to the oxazoline11 produced the target com-
pound, which was completely identical with natural
(3) A part of this work was presented at 37th Japanese Peptide
Symposium, Nagoya, Oct 18-20, 2000. See Yokokawa, F.;
Sameshima, H.; Shioiri, T. Peptide Science 2000, 2001, 55-
58.
(4) See: Shioiri, T.; Hamada, Y. Studies in Natural Products
Chemistry; Atta-ur-Rahman, Ed.; Elsevier: Amsterdam 1989;
Vol. 4 (Part C), pp 83-110.
(5) (a) Hamada, Y., Shibata, M., Sugiura, T., Kato, S., Shioiri, T.
J. Org. Chem. 1987, 52, 1252-1255. (b) Aoyama, T., Sonoda,
N., Yamauchi, M., Toriyama, K., Anzai, A., Ando, A., Shioiri,
T. Synlett 1998, 35-36. (c) Fujiwara, H., Tojiki, K.,
Yokokawa, F., Shioiri, T. Peptide Science 1999 2000, 9-12.
(6) Takuma, S., Hamada, Y., Shioiri, T. Chem. Pharm. Bull.
1982, 30, 3147-3153 and references therein.
1
cis,cis-ceratospongamide (1a) as judged by H and 13C
NMR spectra, HPLC, and TLC Rf values (Scheme 3).
In conclusion, we have developed an efficient strategy to-
ward the total synthesis of cis,cis-ceratospongamide. The
conversion of synthetic cis,cis-ceratospongamide (1a) to
its trans,trans-isomer (1b) is currently underway.
(7) Fehrentz, J.-A., Castro, B. Synthesis 1983, 676-678.
(8) Shioiri, T., Ninomiya, K., Yamada, S. J. Am. Chem. Soc.
1972, 94, 6203-6205.
(9) (a) Chen, S., Xu, J. Tetrahedron Lett. 1991, 32, 6711-6714. (b)
Dudash, J., Jr., Jiang, J., Mayer, S. C., Joullie, M. M. Synth.
Commun. 1993, 23, 349-356. (c) Deng, J.; Hamada, Y.;
Shioiri, T.; Matsunaga, S.; Fusetani, N. Angew. Chem. Int. Ed.
Engl. 1994, 33, 1729-1731. (d) Deng, J.; Hamada, Y.; Shioiri,
T. Synthesis 1998, 627-638.
Acknowledgement
We thank Prof. W. H. Gerwick and Mr. L. T. Tan for comparison of
our synthetic cis,cis-ceratospongamide with the natural sample.
This work was financially supported in part by Grant-in-Aid for Re-
search in Nagoya City University (to F.Y.), Uehara Memorial Foun-
dation (to F.Y.), and Grant-in-Aids from the Ministry of Education,
Science, Sports and Culture, Japan.
(10) Ehrlich, A., Heyne, H.-U., Winter, R., Beryermann, M.,
Haber, H., Carpino, L. A., Bienert, M. J. Org. Chem. 1996, 61,
8831-8838.
(11) Phillips, A. D., Uto, Y., Wipf, P., Reno, M. J., Williams, D. R.
Org. Lett. 2000, 2, 1165-1168.
References and Notes
(1) Tan, L. T., Williamson, R. T., Gerwick, W. H., Watts, K. S.,
McGough, K., Jacobs, R. J. Org. Chem. 2000, 65, 419-425.
(2) (a) Yokokawa, F.; Fujiwara, H.; Shioiri, T. Tetrahedron 2000,
56, 1759-1775. (b) Sugiyama, H.; Yokokawa, F.; Shioiri, T.
Org. Lett. 2000, 2, 2149-2152. (c) Wu, M.; Okino, T.; Nogle,
L. M.; Marquez, B. L.; Williamson, R. T.; Sitachitta, N.;
Berman, F. W.; Murray, T. F.; McGough, K.; Jacobs, R.;
Article Identifier:
1437-2096,E;2001,0,SI,0986,0988,ftx,en;Y03501ST.pdf
Synlett 2001, SI, 986–988 ISSN 0936-5214 © Thieme Stuttgart · New York