Soluble tag-assisted peptide head-to-tail cyclization: Total synthesis of mahafacyclin B

Article abstract of DOI:10.1021/ol4003477

A soluble tag-assisted liquid-phase method was successfully applied to peptide head-to-tail cyclization, leading to the total synthesis of antimalarial cyclic heptapeptide, mahafacyclin B (1). The cyclization was carried out in the liquid phase with the tag remaining, which allowed rapid reaction workup and product isolation.

Full text of DOI:10.1021/ol4003477

ORGANIC
LETTERS
2013
Vol. 15, No. 6
1155–1157
Soluble Tag-Assisted Peptide Head-to-Tail
Cyclization: Total Synthesis
of Mahafacyclin B
Yuko Fujita,† Shuji Fujita,‡ Yohei Okada,† and Kazuhiro Chiba*,†
Department of Applied Biological Science, Tokyo University of Agriculture and
Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183-8509, Japan, and Research and
Development Division, JITSUBO Co., Ltd., 2-24-16 Naka-cho, Koganei,
Tokyo 184-0012, Japan
chiba@cc.tuat.ac.jp
Received February 5, 2013
ABSTRACT
A soluble tag-assisted liquid-phase method was successfully applied to peptide head-to-tail cyclization, leading to the total synthesis of
antimalarial cyclic heptapeptide, mahafacyclin B (1). The cyclization was carried out in the liquid phase with the tag remaining, which allowed rapid
reaction workup and product isolation.
Since the discovery of gramicidin S,1 cyclic peptides have
offered promising medicinal candidates that preorganize
their amino acid sequences into rigid conformations that
are more resistant to enzymatic degradation than their
linearvariants.2 Thousands of cyclic peptides from a variety
of natural sources have been isolated, and they and their
analogues have been synthesized, leading to potential
therapeutic agents. As represented by octreotide, cyclic
peptides account for an increasing portion of the pharma-
ceutical market.3 A particular advantage of peptides is
a fully established method of chemical synthesis based on
solid-phase techniques, whichalsohavecombinedwellwith
automatedandcombinatorialstrategies, enablingextensive
and rapid screening for improved specificity and/or bio-
availability. Among the most challenging syntheses is the
preparation of rigid small-to-medium sized cyclic peptides
because such ring closures are often slow processes and are
prone to side reactions, typically, cyclodimerizations.4
When it comes to head-to-tail cyclized peptides, their
preparation must be creative, because current solid-phase
and tag-assisted liquid-phase5 synthetic approaches have
(4) White, C. J.; Yudin, A. K. Nat. Chem. 2011, 3, 509–524 and
references therein.
(5) (a) Shemyakin, M. N.; Ovchinnikov, Y. A.; Kiryushkin, A. A.;
Kozhevnikovd, I. V. Tetrahedron Lett. 1965, 6, 2323–2327. (b) Geckeler,
K.; Pillai, V. N. R.; Mutter, M. Adv. Polym. Sci. 1981, 39, 65–94. (c)
Narita, M.; Hirata, M.; Kusano, K.; Itsuno, S.; Ue, M.; Okawara, M.
Pept. Chem. 1980, 17, 107–112. (d) Mutter, M.; Hagenmaier, H.; Bayer,
E. Angew. Chem., Int. Ed. Engl. 1971, 10, 811–812. (e) Bayer, E.; Mutter,
M. Nature 1972, 237, 512–513. (f) Bayer, E. Angew. Chem., Int. Ed. Engl.
1991, 30, 113–129. (g) Gravert, D. J.; Janda, K. D. Chem. Rev. 1997, 97,
489–510. (h) Dickerson, T. J.; Reed, N. N.; Janda, K. D. Chem. Rev.
2002, 102, 3325–3344. (i) Bergbreiter, D. E. Chem. Rev. 2002, 102, 3345–
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530–582.
† Tokyo University of Agriculture and Technology.
‡ JITSUBO Co., Ltd.
(1) Gause, G. F.; Brazhnikova, M. G. Nature 1944, 154, 703–703.
(2) Tyndall, J. D. A.; Nall, T.; Fairlie, D. P. Chem. Rev. 2005, 105,
973–999.
(3) Sewald, N.; Jakubke, H.-D. Peptides: Chemistry and Biology,
2nd ed.; Wiley-VCH: Weinheim, 2009.
r
10.1021/ol4003477
2013 American Chemical Society
Published on Web 03/05/2013

been dominated by the use of C-terminal protecting
supports. This means that head-to-tail cyclizations must
becarried out after removal ofsuchsupports, complicating
reaction workup and product isolation. In this context,
sophisticated backbone amide linker strategies have been
well-establishedbased on a solid-phase technique torealize
on-resin peptide head-to-tail cyclization.6 Such linker
strategies should find further application in combination
with a tag-assisted liquid-phase technique that generally
can yield purer peptides on a larger scale. We have been
developing soluble tag-assisted liquid-phase methods
using hydrophobic benzyl alcohols as support groups for
versatile production of bioactive peptides.7 However, since
our supports are also C-terminal protecting, head-to-tail
cyclization is still a matter of demand. Described herein
is the application of soluble tag-assisted methods toward
head-to-tail cyclization using antimalarial cyclic heptapep-
tide, mahafacyclin B (1),8 as a model.
Scheme 1. Retrosynthetic Strategy for Mahafacyclin B (1)
The present work began with the preparation of hydro-
phobically tagged Gly-OMe (3) (Scheme 1). H-Gly-OMe
(4) was introduced into hydrophobic benzaldehyde (5)
through reductive amination to afford desired tagged
Gly-OMe (3) (Scheme 2). Several amino acid methyl
esters, including H-Phe-OMe and H-Thr(tBu)-OMe, can
be introduced into hydrophobic benzaldehyde (5) under
the same reaction conditions (Table S1, Supporting
Information), providing an amide nitrogen as a tagging
site. To avoid commonly occurring epimerization, the
glycine residue was selected as a C-terminus. Fmoc-Phe-
OH (6) was then coupled to give the tagged dipeptide (7).
In this case, the use of HATU in combination with HOAt
was found to be effective for the coupling, while HBTU
was used in combination with HOBt for peptide synthesis
unless otherwise mentioned.
Basic deprotection of the hydrophobically tagged dipep-
tide (7), however, did not give the desired product (8),
instead leading to the formation of diketopiperazine (9)
(Scheme S1, Supporting Information). It is well-known
that basic deprotection induces diketopiperazine forma-
tion when the C-terminal residue is a proline. The
N-alkylation of peptides also has a significant impact on
their conformations (Table S2, Supporting Information).
When a Boc-protected variant was used instead, both
the Boc-group and the tag were removed through acidic
deprotection (Scheme S2, Supporting Information). This
Scheme 2. Preparation of the Hydrophobically Tagged
Gly-OMe (3) and the Hydrophobically Tagged Dipeptide (7)
could be avoided through the coupling of Fmoc-Phe-
Phe-OH to the hydrophobically tagged Gly-OMe (3)
(Scheme 3). This step was evaluated by HPLC, and only a
trace amount of epimerized variant was observed (Figure S1,
Supporting Information). Basic deprotection of the hydro-
phobically tagged tripeptide (10) gave the desired product
(11) predominantly, accompanied by the formation of only
a trace amount of cyclized compound (12) (Scheme 4). This
could be simply rationalized based on the resulting ring sizes;
thus, the formation of six-membered rings was much more
favorable than that of nine-membered rings.
Scheme 3. Preparation of the Hydrophobically Tagged
Tripeptide (10)
(6) (a) Boas, U.; Brask, J.; Jensen, K. J. Chem. Rev. 2009, 109, 2092–
2118. (b) Alsina, J.; Jensen, K. J.; Albericio, F.; Barany, G. Chem.;Eur.
J. 1999, 5, 2787–2795. (c) Jensen, K. J.; Alsina, J.; Songster, M. F.;
Vagner, J.; Albericio, F.; Barany, G. J. Am. Chem. Soc. 1998, 120, 5441–
5452. (d) Boojamra, C. G.; Burow, K. M.; Thompson, L. A.; Ellman,
J. A. J. Org. Chem. 1997, 62, 1240–1256. (e) Boojamra, C. G.; Burow,
K. M.; Ellman, J. A. J. Org. Chem. 1995, 60, 5742–5743.
(7) (a) Kitada, S.; Fujita, S.; Okada, Y.; Kim, S.; Chiba, K. Tetra-
hedron 2013, 69, 2555–2559. (b) Okada, Y.; Suzuki, H.; Nakae, T.;
Fujita, S.; Abe, H.; Nagano, K.; Yamada, T.; Ebata, N.; Kim, S.; Chiba,
K. J. Org. Chem. 2013, 78, 320–327. (c) Kitada, S.; Fujita, S.; Okada, Y.;
Kim, S.; Chiba, K. Bioorg. Med. Chem. Lett. 2011, 21, 4476–4479.
(d) Tana, G.; Kitada, S.; Fujita, S.; Okada, Y.; Kim, S.; Chiba, K. Chem.
Commun. 2010, 46, 8219–8221.
(8) (a) Sayyadi, N.; Skropeta, D.; Jolliffe, K. A. Org. Lett. 2005, 7,
5497–5499. (b) Baraguey, C.; Blond, A.; Cavelier, F.; Pousset, J.-L.;
Bodo, B.; Auvin-Guette, C. J. Chem. Soc., Perkin Trans. 1 2001, 2098–
2103.
Withthe hydrophobicallytaggedtripeptide(11) inhand,
thedesiredsequence(13) wasthenelaboratedusing general
Fmoc-chemistry followed by basic deprotection, which
removed both the Fmoc-group and the C-terminal methyl
ester to give the desired product (2) in one step (Scheme 5).
All reactions took place efficiently in less polar solvents
followed by dilution with poor solvents to give the product
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Org. Lett., Vol. 15, No. 6, 2013

Scheme 4. Basic Deprotection of the Hydrophobically Tagged
Tripeptide (10)
Scheme 6. Preparation of the Linear Precursor (17) Using
Conventional C-Terminal Protecting Tag (15)
as a precipitate. Generally, a high content of hydrophobic
residues tends to cause peptide aggregation in polar sol-
vents, which might be a crucial drawback for peptide
cyclizations, because folding of their linear precursors is
impeded. Indeed, when conventional C-terminal hydro-
phobic benzyl alcohol (15) was usedinstead as asupport for
construction of the same sequence (16), acidic deprotection
of the tag required to form the linear precursor (17) induced
severe gelation (Scheme 6 and Scheme S3, Supporting
Information). In sharp contrast, the linear precursor (2)
was highly soluble in less polar solvents due to the hydro-
phobic tag. Finally, the head-to-tail cyclization took place
effectively to give the cyclic version (14) with the tag
remaining without any gelation (Scheme 7). This facilitated
the reaction workup and product isolation significantly,
and acidic deprotection gave mahafacyclin B (1).
Scheme 7. Synthesis of Mahafacyclin B (1)
Scheme 5. Preparation of the Linear Precursor (2)
Acknowledgment. This work was partially supported
by a Grant-in-Aid for Scientific Research from the Minis-
try of Education, Culture, Sports, Science and Technology
of Japan.
In conclusion, we successfully applied a soluble tag-assisted
liquid-phase method to peptide head-to-tail cyclization,
leading to the total synthesis of mahafacyclin B (1). The
cyclization was carried out in the liquid phase with the tag
remaining, which allowed rapid reaction workup and pro-
duct isolation. Because this strategy employs an amide nitro-
gen as the tagging site instead of the C-terminus, it should find
further application in the preparation of several head-to-tail
cyclized peptides in a sequence-independent manner.
Supporting Information Available. Additional schemes,
tables, figures, general information, experimental infor-
mation, spectral information, and copies of 1H and
13C NMR. This material is available free of charge via
the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
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