Preparation of peptide thioesters through fmoc-based solid-phase peptide synthesis by using amino thioesters

Article abstract of DOI:10.1002/ejoc.201300927

An effective procedure for the synthesis of peptide alkyl thioesters by 9-fluorenylmethoxycarbonyl (Fmoc) solid-phase peptide synthesis was developed. The free C terminus of a fully protected peptide was coupled in solution with the free amino group of an

Full text of DOI:10.1002/ejoc.201300927

Job/Unit: O30927
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Date: 16-07-13 19:15:12
Pages: 6
SHORT COMMUNICATION
Peptide Thioesters
N. Stuhr-Hansen, T. S. Wilbek,
K. Strømgaard* ................................ 1–6
Preparation of Peptide Thioesters through
Fmoc-Based Solid-Phase Peptide Synthesis
by Using Amino Thioesters
The synthesis of peptide alkyl thioesters by
9-fluorenylmethoxycarbonyl (Fmoc) solid-
phase peptide synthesis (SPPS) is devel-
oped. The free C terminus of a fully pro-
tected peptide is coupled in solution with
the free amino group of an amino thioester.
This furnishes the fully protected peptide
thioester, which can be globally depro-
tected to afford the desired unprotected
peptide thioester.
Keywords: Amino acids / Peptides / Protein
modifications / Glycosylation / Peptide
thioesters
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N. Stuhr-Hansen, T. S. Wilbek, K. Strømgaard
SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300927
Preparation of Peptide Thioesters through Fmoc-Based Solid-Phase Peptide
Synthesis by Using Amino Thioesters
Nicolai Stuhr-Hansen,[a][‡] Theis S. Wilbek,[a][‡] and Kristian Strømgaard*[a]
Keywords: Amino acids / Peptides / Protein modifications / Glycosylation / Peptide thioesters
An effective procedure for the synthesis of peptide alkyl thio-
esters by 9-fluorenylmethoxycarbonyl (Fmoc) solid-phase
peptide synthesis was developed. The free C terminus of a
fully protected peptide was coupled in solution with the free
amino group of an amino thioester. This furnished the fully
protected peptide thioester, which was globally deprotected
to afford the desired unprotected peptide thioester. The
method is compatible with labile groups such as phosphoryl
and glycosyl moieties.
native methods that are based upon the milder 9-fluorenyl-
methoxycarbonyl (Fmoc) SPPS methodology.[9]
Introduction
Peptide and protein ligation strategies, such as native
chemical ligation (NCL)[1] and expressed protein ligation
(EPL),[2] are versatile methods for the construction of larger
peptides and proteins from peptide fragments. Central to
these methodologies is the formation of an amide bond be-
tween unprotected peptides and proteins with a C-terminal
peptide thioester and an N-terminal cysteine, respec-
tively.[1a,3] Sequential ligation of peptide fragments can then
facilitate fully chemically synthesized proteins that are nei-
ther restricted by the general limitations of recombinant ex-
pression nor to accumulation of impurities during solid-
phase peptide synthesis (SPPS).[1,2] Prominent examples of
the application of NCL includes the total synthesis and
subsequent determination of the X-ray crystal structure of
the 203 amino acid HIV-1 protease,[4] as well as the 166
amino acid polypeptide erythropoietin (EPO) including a
very recent synthesis of its glycosylated version.[5] EPL has
been used in extensive studies of complex proteins such as
K+ channels[6] and histones.[7]
Peptide thioesters are typically prepared by tert-butoxy-
carbonyl (Boc) SPPS, in which the thioester functionality is
released upon cleavage from the resin by treatment with
HF.[8] Owing to the strong acidic cleavage conditions, thio-
ester peptides containing acid-labile groups, including im-
portant post-translational modifications (PTMs) such as
phosphates and glycosylates, cannot generally be prepared
by Boc-SPPS. This, combined with the challenges in hand-
ling HF, has spurred substantial interest in developing alter-
The current Fmoc-SPPS methods for the preparation of
peptide thioesters were recently reviewed by Mende and
Seitz[9] and include side-chain or backbone anchoring and
subsequent C-terminal functionalization,[10] application of
tris(thioorthoesters) generated by treatment of the peptide
with ethanethiol and AlMe3,[11] thiolysis of peptides bound
to safety-catch sulfonamide linkers,[12] and, finally, several
versions of intramolecular OǞS or NǞS acyl shifts.[13]
These methodologies each contain inherent challenges such
as epimerization and hydrolysis or formation of diketopip-
erazines,[9] and a general method based on Fmoc-SPPS
seems yet to be established. Peptide thioesters can also be
synthesized directly by adding a C-terminal amino acid sur-
rogate thioester or indirectly through a masked surrogate
thioester;[5b,9] the latter was introduced by Danishefsky and
co-workers.[14] This can be achieved by coupling the free C
terminus of a fully protected peptide with amino thioesters
followed by global deprotection to afford the peptide thio-
ester. However, this principle has so far been restricted to
the introduction of, for example, Gly- and Ala-terminated
peptide thioesters[10a,15] or side-chain anchoring of Thr.[16]
Herein, we explore this methodology for the synthesis of
peptide alkyl thioesters by Fmoc SPPS, which allows the
introduction of any proteinogenic amino acid as the C-ter-
minal thioester. Importantly, we demonstrate that this
method can be used in the preparation of peptide thioesters
containing PTMs such as phosphorylations and glycosyl-
ations and that these are conveniently used in NCL.
[a] Department of Drug Design and Pharmacology
University of Copenhagen
Results and Discussion
Universitetsparken 2, 2100 Copenhagen, Denmark
E-mail: kristian.stromgaard@sund.ku.dk
Homepage: www.drug.ku.dk/chembiol
To develop a general method based on Fmoc SPPS for
the synthesis of peptide thioesters, we envisioned that an
amino acid and a thiol would first be combined to form the
[‡] These authors contributed equally to this work
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300927.
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Preparation of Peptide Thioesters by Fmoc-Based SPPS
C-terminal amino thioester building block, which could HPLC analysis, which showed no sign of racemization
then be coupled to a protected peptide in solution. This (Ͼ99%ee, see the Supporting Information).
would generate a protected peptide thioester, which sub-
sequently could be deprotected to provide the desired pept-
ide thioester (Figure 1).
Scheme 1. Ethyl amino thioesters from (a) standard N-Boc-pro-
tected amino acids and (b) appropriately protected standard N-
Fmoc-protected amino acids.
As initial proof-of-principle of introducing the amino
thioester, we prepared a dipeptide thioester. The amino
thioester building block H-Val-SEt was coupled with Boc-
Val-OH in solution by using HBTU. The crude dipeptide
thioester was purified by normal-phase chromatography to
provide Boc-Val-Val-SEt (1, Scheme 2) in high yield.
Figure 1. Outline of the principle for the generation of peptide thio-
esters containing acid-labile PTMs. The peptide is prepared by
standard Fmoc SPPS and released as a fully protected peptide,
which is subsequently coupled with an amino thioester. Global de-
protection provides the desired peptide thioester.
A central requirement was the generation of the C-ter-
minal amino thioester building blocks for all 20 proteinog-
enic amino acids, which would be the amino thioesters with
Scheme 2. Synthesis of the Boc-Val-Val-SEt dipeptide.
Having all the C-terminal amino thioester building
a free N terminus and appropriate side-chain protecting blocks available and having demonstrated the feasibility of
groups. The free carboxy termini of the protected amino the coupling of these to a free C terminus, we wanted to
acids were treated with ethanethiol in the presence of apply the methodology to the synthesis of peptide thio-
O-benzotriazole-N,N,NЈ,NЈ-tetramethyluronium hexafluo- esters. We prepared a model 15 amino acid peptide,
rophosphate (HBTU) and N,N-diisopropylethylamine FYWTSRQPNKHEDCV, by standard Fmoc SPPS that in-
(DIPEA)[17] to provide fully protected amino thioesters (see cluded all side-chain protecting groups and a bulky C ter-
the Supporting Information).
minus. A Boc-protected amino acid was introduced as the
For the eight amino acids without side-chain protecting N-terminal amino acid, which could be deprotected by TFA
groups (Gly, Ala, Val, Leu, Ile, Phe, Pro, Met), N-Boc-pro- treatment concomitant with the side-chain protecting
tected amino thioesters were smoothly converted into the groups. All proteinogenic amino acids fulfilling these
corresponding free amino thioesters with trifluoroacetic requirements are commercially available. The peptide was
acid (TFA) in quantitative yields (Scheme 1a). For the cleaved from the 2-chlorotrityl resin by using 1,1,1,3,3,3-
amino acids with acid-labile side-chain protecting groups hexaflouroisopropanol (HFIP),[19] which furnished the fully
used in Fmoc SPPS [Ser(tBu), Thr(tBu), Tyr(tBu), Cys(Trt), protected peptide with a free carboxy terminus (i.e., 2;
Lys(Boc), Asp(tBu), Asn(Trt), Glu(tBu), Gln(Trt), His(Trt), Scheme 3a). The crude peptide was then treated with the
Arg(Pbf), Trp(Boc)], another strategy was required, and the free amino thioesters by using different coupling reagents,
corresponding Fmoc-protected amino thioesters were there- knowing that epimerization of the C-terminal residue of 2
fore generated. Knowing that thioesters are generally prone (i.e., Val) might be a challenge (Table S2, Supporting Infor-
to degradation upon treatment with secondary amines, we mation). Indeed, under the standard coupling conditions
found that treating Fmoc-protected amino thioesters with with the use of HBTU, a mixture of isomers was obtained
piperidine (2 equiv.) for 2 min followed by quenching in in which the d-Val isomer was present in 35%, and this
aqueous acetic acid smoothly afforded the desired side- indicated that a large degree of epimerization had occurred.
chain-protected amino thioesters (Scheme 1b), analogously The same degree of epimerization was observed by using
to a previous report.[18] Notably, the reaction was free of bromotris(dimethylamino)phosphonium hexafluorophos-
any amide side products, and the amino thioesters were phate (BroP)[20] (34% d isomer), whereas the use of the
shelf-stable, as no byproducts were observed upon storage pentafluorophenyl ester reduced the extent of epimerization
at room temperature for several days. We also verified that (11% d isomer). Gratifyingly, if the coupling was carried
the chirality was maintained during thioester synthesis and out with 4-(1-cyano-2-ethoxy-2-oxoethylidenaminooxy)di-
subsequent removal of the N-protecting groups by chiral methylaminomorpholinocarbenium hexafluorophosphate
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N. Stuhr-Hansen, T. S. Wilbek, K. Strømgaard
SHORT COMMUNICATION
(COMU)[21] in dichloromethane, the d isomer content was model 14 amino acid peptide with a phosphorylated threo-
only approximately 1%, and these conditions were therefore nine, Fmoc-O-(benzylphospho)-l-threonine [Fmoc-Thr-
used throughout this work.
(PO3BnH)-OH],[25] by standard Fmoc SPPS. The protected
phosphorylated peptide was cleaved with HFIP and cou-
pled to H-Ala-SEt to give the corresponding protected thio-
ester 7. Global deprotection including debenzylation of the
phosphoryl group gratifyingly provided the phosphorylated
peptide thioester 8 (Scheme 4a).
Scheme 3. Synthesis of peptide thioesters. (a) Coupling of a pro-
tected peptide in solution with an amino thioester building block.
(b) Coupling of a protected peptide in solution with a side-chain-
protected amino thioester.
By using these conditions, peptide 2 was coupled with H-
Val-SEt to afford fully protected peptide thioester 3. After
normal-phase purification of 3, treatment with TFA pro-
vided the desired unprotected 16 amino acid peptide thioes-
ter 4 (Scheme 3a). Normal-phase chromatography, that is,
standard silica purification, of fully protected peptide inter-
mediate thioester 3 is crucial to the success of this ap-
proach.[22] Direct deprotection of crude 3 did not provide
the peptide thioester, and, likewise, coupling of fully pro-
tected peptide 3 directly with ethanethiol provided a com-
plex reaction mixture.
Scheme 4. Synthesis of peptide thioesters with PTMs. (a) Synthesis
of a phosphorylated peptide thioester. (b) Synthesis and ligation of
a glycosylated peptide thioester.
To examine the feasibility of introducing a C-terminal
amino thioester with a side-chain protecting group, we
Analogously, a glycosylated model 14 amino acid peptide
prepared
a
protected 14 amino acid peptide, was synthesized by Fmoc SPPS to examine if glycosyl moie-
FYWTSRQPNKHEDV, analogously to peptide 2. H- ties are compatible with the developed methodology. A pro-
Ser(tBu)-SEt and H-Arg(Pbf)-SEt were used as amino thio- tected, glycosylated asparagine, N2-[(9H-fluoren-9-yl-
ester building blocks, and their respective thioester peptides, methoxy)carbonyl]-N-[3,4,6-tri-O-acetyl-2-(acetylamino)-2-
5
and 6, respectively, were conveniently obtained deoxy-β-d-glucopyranosyl]-l-asparagine
[Fmoc-Asn-
(Scheme 3b).
(Ac3AcNH-β-Glc)-OH],[26] was introduced, and the fully
One of the advantages of both NCL and EPL is the abil- protected glycopeptide was cleaved from the resin to pro-
ity for site-specific introduction of unnatural moieties and vide 9 (Scheme 4b). Then, H-Ala-SEt was coupled to the
PTMs,[23] and ligation-based strategies have been used to free C terminus of 9, and TFA cleavage allowed deprotec-
generate proteins endowed with PTMs at specific sites that tion of the amino acid side chains while keeping the protect-
are not attainable by conventional recombinant methods.[24] ing groups of the glycosyl moiety. This provided the desired
One of the primary shortcomings of Boc SPPS is that the glycosylated peptide thioester 10 (Scheme 4b). To demon-
preparation of phosphorylated and glycosylated peptide strate that peptide 10 could be conveniently used in NCL,
thioesters is generally not possible owing to the strong it was ligated to an N-terminal Cys peptide, CYTQPKHEV,
acidic (HF) treatment. Thus, a particular challenge for any by using standard NCL conditions. The ligated product was
general Fmoc SPPS method is to allow easy access to pept- purified and subsequently treated with sodium methoxide
ide thioesters decorated with phosphorylations and glycosy- to remove the O-acetyl protecting groups of the glycol moi-
lations, and we therefore examined our methodology for ety to furnish unprotected, glycosylated 24 amino acid
generating such peptide thioesters.
First, we investigated the compatibility for generating access to both phosphorylated and glycosylated peptide
phosphorylated peptide thioesters and synthesized thioesters on the basis of conventional Fmoc SPPS.
peptide 11. Thus, the methodology conveniently provides
a
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Preparation of Peptide Thioesters by Fmoc-Based SPPS
72, 249; c) R. David, M. P. O. Richter, A. G. Beck-Sickinger,
Conclusions
Eur. J. Biochem. 2004, 271, 663; d) V. Muralidharan, T. W.
Muir, Nat. Methods 2006, 3, 429; e) M. Vila-Perelló, T. W.
Muir, Cell 2010, 143, 191.
a) T. Wieland, E. Bokelmann, L. Bauer, H. U. Lang, H. Lau,
Justus Liebigs Ann. Chem. 1953, 583, 129; b) E. C. B. Johnson,
S. B. H. Kent, J. Am. Chem. Soc. 2006, 128, 6640.
A general Fmoc SPPS based method for the synthesis of
peptide thioesters was developed, and the applied amino
thioester building blocks were synthesized for all the corre-
sponding 20 proteinogenic amino acids. An advantage of
using amino ethyl thioester building blocks in the synthesis
of peptide thioesters is that direct handling of foul-smelling
thiols is avoided. It is believed that an important feature
of the described method is that the fully protected peptide
thioesters are purified by normal-phase chromatography to
remove the byproducts from the coupling reagents, as these
significantly hamper the generation of the subsequent un-
protected peptide thioester. We also attempted direct cou-
pling of free thiols with the C termini of the peptides, as
previously reported,[17b,27] but this did not provide the de-
sired peptide thioesters.
A primary objective was to generate peptide thioesters
with acid-labile PTMs such as glycosylations and phos-
phorylations, which are generally not attainable even by
Boc SPPS. This was demonstrated by the straightforward
synthesis of two peptide thioesters containing these impor-
tant PTMs. The glycopeptide thioesters were subsequently
used in a NCL to provide a 24-mer glycopeptide. Fully pro-
tected peptide thioesters can easily be solubilized in organic
solvents, which allows easy handling and purification of the
peptide derivatives in organic media.
[3]
[4]
[5]
V. Y. Torbeev, S. B. H. Kent, Angew. Chem. 2007, 119, 1697;
Angew. Chem. Int. Ed. 2007, 46, 1667.
a) G. G. Kochendoerfer, S. Y. Chen, F. Mao, S. Cressman, S.
Traviglia, H. Shao, C. L. Hunter, D. W. Low, E. N. Cagle, M.
Carnevali, V. Gueriguian, P. J. Keogh, H. Porter, S. M. Strat-
ton, M. C. Wiedeke, J. Wilken, J. Tang, J. J. Levy, L. P. Mi-
randa, M. M. Crnogorac, S. Kalbag, P. Botti, J. Schindler-Hor-
vat, L. Savatski, J. W. Adamson, A. Kung, S. B. H. Kent, J. A.
Bradburne, Science 2003, 299, 884; b) P. Wang, S. Dong, J. A.
Brailsford, K. Iyer, S. D. Townsend, Q. Zhang, R. C. Hendrick-
son, J. Shieh, M. A. Moore, S. J. Danishefsky, Angew. Chem.
2012, 124, 11744; Angew. Chem. Int. Ed. 2012, 51, 11576.
a) F. I. Valiyaveetil, M. Sekedat, T. W. Muir, R. MacKinnon,
Angew. Chem. 2004, 116, 2558; Angew. Chem. Int. Ed. 2004,
43, 2504; b) F. I. Valiyaveetil, M. Leonetti, T. W. Muir, R.
MacKinnon, Science 2006, 314, 1004.
a) R. K. McGinty, J. Kim, C. Chatterjee, R. G. Roeder, T. W.
Muir, Nature 2008, 453, 812; b) B. Fierz, C. Chatterjee, R. K.
McGinty, M. Bar-Dagan, D. P. Raleigh, T. W. Muir, Nat.
Chem. Biol. 2011, 7, 113; c) B. Fierz, T. W. Muir, Nat. Chem.
Biol. 2012, 8, 417.
a) H. Hojo, S. Aimoto, Bull. Chem. Soc. Jpn. 1991, 64, 111; b)
H. Hojo, Y. Kwon, Y. Kakuta, S. Tsuda, I. Tanaka, K. Hikichi,
S. Aimoto, Bull. Chem. Soc. Jpn. 1993, 66, 2700; c) T. M. Hack-
eng, J. H. Griffin, P. E. Dawson, Proc. Natl. Acad. Sci. USA
1999, 96, 10068.
[6]
[7]
[8]
[9]
F. Mende, O. Seitz, Angew. Chem. 2011, 123, 1266; Angew.
Chem. Int. Ed. 2011, 50, 1232.
Experimental Section
[10]
a) J. Alsina, T. S. Yokum, F. Albericio, G. Barany, J. Org.
Chem. 1999, 64, 8761; b) K. Nakamura, N. Hanai, M. Kanno,
A. Kobayashi, Y. Ohnishi, Y. Ito, Y. Nakahara, Tetrahedron
Lett. 1999, 40, 515.
a) D. Swinnen, D. Hilvert, Org. Lett. 2000, 2, 2439; b) J. Brask,
F. Albericio, K. J. Jensen, Org. Lett. 2003, 5, 2951.
R. Ingenito, E. Bianchi, D. Fattori, A. Pessi, J. Am. Chem. Soc.
1999, 121, 11369.
a) B. Wu, J. H. Chen, J. D. Warren, G. Chen, Z. H. Hua, S. J.
Danishefsky, Angew. Chem. 2006, 118, 4222; Angew. Chem. Int.
Ed. 2006, 45, 4116; b) R. Okamoto, K. Morooka, Y. Kajihara,
Angew. Chem. 2012, 124, 195; Angew. Chem. Int. Ed. 2012, 51,
191.
a) J. D. Warren, J. S. Miller, S. J. Keding, S. J. Danishefsky, J.
Am. Chem. Soc. 2004, 126, 6576; b) G. Chen, J. D. Warren, J.
Chen, B. Wu, Q. Wan, S. J. Danishefsky, J. Am. Chem. Soc.
2006, 128, 7460.
a) S. Futaki, K. Sogawa, J. Maruyama, T. Asahara, M. Niwa,
H. Hojo, Tetrahedron Lett. 1997, 38, 6237; b) S. Ficht, R. J.
Payne, R. T. Guy, C. H. Wong, Chem. Eur. J. 2008, 14, 3620.
D. Lelièvre, P. Barta, V. Aucagne, A. F. Delmas, Tetrahedron
Lett. 2008, 49, 4016.
a) J. W. Wehner, T. K. Lindhorst, Synthesis 2010, 3070; b) A. C.
Nagalingam, S. E. Radford, S. L. Warriner, Synlett 2007, 2517.
General Procedure for the Synthesis of Peptide Thioesters: A fully
protected peptide with a free carboxy terminus (0.25 mmol) was
dissolved in a solution of a free amino thioester (H-AA-Set,
0.3 mmol) and DIPEA (0.105 mL, 0.6 mmol) in dichloromethane
(0.7 mL), and the reaction vessel was placed in a freezer at –18 °C.
COMU (0.128 g, 0.3 mmol) was added, and vigorous shaking was
maintained at room temperature for 1 h. The reaction mixture was
separated directly on silica by means of ethyl acetate. An inhomo-
geneous mixture of TFA/TIPS/H2O (18:1:1, 4 mL) was added, and
shaking was maintained for 1 h. After concentration in a gentle
stream of nitrogen, the crude peptide thioester was washed with
cold diethyl ether (3ϫ 5 mL) to give the crude product, which was
successively separated by HPLC to afford the peptide thioester.
[11]
[12]
[13]
[14]
[15]
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, characterization data, and 1H and
13C NMR spectra of the products.
Acknowledgments
[16]
[17]
[18]
[19]
[20]
[21]
We thank B. Nielsen, University of Copenhagen, for assistance
with chiral HPLC. The Lundbeck Foundation is thanked for finan-
cial support to N. S.-H.
K. N. Barlett, R. V. Kolakowski, S. Katukojvala, L. J. Williams,
Org. Lett. 2006, 8, 823.
R. Bollhagen, M. Schmiedberger, K. Barlos, E. Grell, J. Chem.
Soc., Chem. Commun. 1994, 2559.
J. Coste, M.-N. Dufour, A. Pantaloni, B. Castro, Tetrahedron
Lett. 1990, 31, 669.
a) A. El-Faham, R. S. Funosas, R. Prohens, F. Albericio,
Chem. Eur. J. 2009, 15, 9404; b) A. El-Faham, F. Albericio, J.
Pept. Sci. 2010, 16, 6.
[1] a) P. E. Dawson, T. W. Muir, I. Clark-Lewis, S. B. H. Kent, Sci-
ence 1994, 266, 776; b) P. E. Dawson, S. B. H. Kent, Annu. Rev.
Biochem. 2000, 69, 923; c) B. L. Nilsson, M. B. Soellner, R. T.
Raines, Annu. Rev. Biophys. Biomol. Struct. 2005, 34, 91; d)
S. B. H. Kent, Chem. Soc. Rev. 2009, 38, 338.
[2] a) T. W. Muir, D. Sondhi, P. A. Cole, Proc. Natl. Acad. Sci.
USA 1998, 95, 6705; b) T. W. Muir, Annu. Rev. Biochem. 2003,
Eur. J. Org. Chem. 0000, 0–0
© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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SHORT COMMUNICATION
[22] A. Bach, J. N. N. Eildal, N. Stuhr-Hansen, R. Deeskamp, M. [26]
M. Meldal, K. Bock, Tetrahedron Lett. 1990, 31, 6987.
[27] a) R. von Eggelkraut-Gottanka, A. Klose, A. G. Beck-Sick-
inger, M. Beyermann, Tetrahedron Lett. 2003, 44, 3551; b) S.
Flemer, J. Pept. Sci. 2009, 15, 693.
Gottschalk, S. W. Pedersen, A. S. Kristensen, K. Strømgaard,
J. Med. Chem. 2011, 54, 1333.
[23] I. S. Carrico, Chem. Soc. Rev. 2008, 37, 1423.
[24] M. Shogren-Knaak, H. Ishii, J. M. Sun, M. J. Pazin, J. R. Da-
vie, C. L. Peterson, Science 2006, 311, 844.
[25] T. Vorherr, W. Bannwarth, Bioorg. Med. Chem. Lett. 1995, 5,
2661.
Received: June 24, 2013
Published Online: ᭿
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