Ring-closing alkyne metathesis approach toward the synthesis of alkyne mimics of thioether A-, B-, C-, and DE-ring systems of the lantibiotic nisin Z

Article abstract of DOI:10.1021/ol0508781

(Chemical Equation Presented) Ring-closing alkyne metathesis toward the synthesis of the alkyne-brigded A-, B-, C-, and (D)E-ring mimics of the peptide antibiotic nisin Z is described. We have successfully synthesized alkyne-bridged cyclic peptides containing 4-7 amino acid residues in yields ranging from 18 to 82%.

Full text of DOI:10.1021/ol0508781

ORGANIC
LETTERS
2005
Vol. 7, No. 14
2961-2964
Ring-Closing Alkyne Metathesis
Approach toward the Synthesis of
Alkyne Mimics of Thioether A-, B-, C-,
and DE-Ring Systems of the Lantibiotic
Nisin Z
Nourdin Ghalit,† Alex J. Poot,† Alois Fu
1
rstner,‡ Dirk T. S. Rijkers,† and
Rob M. J. Liskamp*,†
Department of Medicinal Chemistry, Utrecht Institute for Pharmaceutical Sciences,
Utrecht UniVersity, P.O. Box 80082, 3508 TB Utrecht, The Netherlands, and
Max-Planck-Institut fu¨r Kohlenforschung, D-45470 Mu¨lheim/Ruhr, Germany
r.m.j.liskamp@pharm.uu.nl
Received April 21, 2005
ABSTRACT
Ring-closing alkyne metathesis toward the synthesis of the alkyne-brigded A-, B-, C-, and (D)E-ring mimics of the peptide antibiotic nisin Z
is described. We have successfully synthesized alkyne-bridged cyclic peptides containing 4−7 amino acid residues in yields ranging from 18
to 82%.
Replacement of natural conformational constraints such as
thioether (sulfide) or disulfide bridges by unnatural con-
straints is an attractive approach to obtain mimics with
increased metabolic stability.1 Moreover, these constraints
might also lead to a better control of the three-dimensional
shape that is essential for biological activity. The antimi-
crobial peptide nisin Z poses a particular challenge as a target
for this aim since it contains five consecutive thioether
bridges (Figure 1) that play an important role in the dual
Figure 1. Structure of the lantibiotic nisin Z.
mode of action of nisin as a bactericide.2 Nisin binds via its
† Utrecht University.
N-terminus, comprising the ABC-ring system, to lipid II,
which is an essential precursor for cell wall biosynthesis.
‡ Max-Plank-Institut.
(1) (a) Miller, S. J.; Blackwell, H. E.; Grubbs, R. H. J. Am. Chem. Soc.
1996, 118, 9606. (b) Williams, R. M.; Lui, J. Org. Chem. 1998, 63, 2130.
(c) Gao, Y.; Lane-Bell, P.; Vederas, J. C. J. Org. Chem. 1998, 63, 2133.
(d) Stymiest, J. L.; Mitchell, B. F.; Wong, S.; Vederas, J. C. Org. Lett.
2003, 5, 47.
(2) (a) Jung, G. Angew. Chem., Int. Ed. Engl. 1991, 30, 1051. (b)
Chatterjee, C.; Paul, M.; Xie, L.; van der Donk, W. A. Chem. ReV. 2005,
105, 633.
10.1021/ol0508781 CCC: $30.25
© 2005 American Chemical Society
Published on Web 06/04/2005

As a result, the C-terminus, comprising the knotted DE-ring,
can form pores in the phospholipid membrane. This ulti-
mately leads to cell leakage and causes a collapse of the
vital ion gradients across the membrane.3,4
Scheme 1. Derivatization of (S)-2-Amino-4-hexynoic Acid 1
Recently, we communicated the preorganization-induced
synthesis of a crossed alkene-bridged mimic of the knotted
DE-ring system by ring-closing metathesis (RCM).5 We now
wish to report our results on obtaining the even more rigid
triple bond, containing alkyne mimics of the A-, B-, C-, and
(D)E-thioether rings of nisin Z, by ring-closing alkyne
metathesis (RCAM).6
In the macrocyclic series, ring-closing metathesis (RCM)
leads in most cases to a mixture of cis and trans double-
bond-containing alkene derivatives. However, after applica-
tion of ring-closing alkyne metathesis (RCAM),7 the triple
bond can be selectively reduced to either the cis isomer via
Lindlar’s catalyst8 or to the trans isomer by trans-selective
hydrosilylation followed by protodesilylation.9
The synthesis of the required (S)-2-amino-4-hexynoic acid
(2-butyne-glycine: “Bug”, 1)10 for incorporation in the
RCAM precursors was carried out according to the method
of Belokon,11,12 subjecting the Ni(II) complex of the Schiff
base derived from glycine and (S)-2-(N′-(N-benzylprolyl)-
amino)benzophenone to alkylation with 1-bromo-2-butyne
in the presence of base. After column chromatography
separation of the addition products (95% yield, diastereo-
metric ratio 96:4) and treatment with acid, (S)-2-amino-4-
hexynoic acid 1 was obtained as its hydrochloride salt in
80% yield with an enantiomeric excess of >98%. (deter-
mined by Chiral HPLC of 4). Acid 1 was converted to the
required derivatives for solution- and solid-phase peptide
synthesis (Scheme 1). Thus, methyl ester 2 (quantitative),
the N-R-Boc derivative 3 (95%), and the N-R-Fmoc deriva-
tive 4 (88%) were prepared. The latter compound was
coupled to plain ArgoGel resin using Sieber conditions13 to
give 5 (Scheme 1).
Linear RCAM-precursor peptide 6, corresponding to the
sequence of the A-ring in nisin, was synthesized in solution
starting from ester 2 in seven steps with an overall yield of
73%. RCAM of 6 (0.04 mM) was performed in the presence
of the tungsten-alkylidyne complex (tBuO)3WtCtBu as a
catalyst14 in toluene at 80 °C to give alkyne bridged cyclic
peptide 7 in a yield of 42% (Scheme 2).15
Scheme 2. Synthesis of Alkyne Mimic A (7)
(3) Hasper, H. E.; de Kruijff, B.; Breukink, E. Biochemistry 2004, 43,
11567.
(4) Hsu, S.-T.; Breukink, E.; Tischenko, E.; Lutters, M. A. G.; de Kruijff,
B.; Kaptein, R.; Bonvin, A. M. J. J.; van Nuland, N. A. J. Nat. Struct. Mol.
Biol. 2004, 11, 963.
(5) Ghalit, N.; Rijkers, D. T. S.; Kemmink, J.; Versluis, C.; Liskamp,
R. M. J. Chem. Commun. 2005, 192.
(6) RCAM on peptides, see: (a) Aguilera, B.; Wolf, L. B.; Nieczypor,
P.; Rutjes, F. P. J. T.; Overkleeft, H. S.; van Hest, J. C. M.; Schoemaker,
H. E.; Wang, B.; Mol, J. C.; Fu¨rstner, A.; Overhand, M.; van der Marel, G.
A.; van Boom, J. H. J. Org. Chem. 2001, 66, 3584. (b) IJsselstijn, M.;
Aguilera, B.; van der Marel, G. A.; van Boom, J. H.; van Delft, F. L.;
Schoemaker, H. E.; Overkleeft, H. S.; Rutjes, F. P. J. T.; Overhand, M.
Tetrahedron Lett. 2004, 45, 4379. (c) IJsselstijn, M.; Kaiser, J.; van Delft,
F. L.; Schoemaker, H. E.; Rutjes, F. P. J. T. Amino Acids 2003, 24, 263.
(7) See for example: (a) Fu¨rstner, A.; Seidel, G. Angew. Chem., Int.
Ed. 1998, 37, 1734. (b) Fu¨rstner, A. Angew. Chem., Int. Ed. 2000, 39, 3012.
(c) Fu¨rstner, A.; Davies, P. Chem. Commun. 2005, 2307.
(8) Campos, K. R.; Cai, D.; Journet, M.; Kowal, J. J.; Larsen, R. D.;
Reider, P. J. J. Org. Chem. 2001, 66, 3634.
(9) (a) Fu¨rstner, A.; Radkowski, K. Chem. Commun. 2002, 2182. (b)
Lacombe, F.; Radkowski, K.; Seidel, G.; Fu¨rstner, A. Tetrahedron 2004,
60, 7315.
(10) For other methods for the synthesis of this amino acid, see: (a)
Wolf, L. B.; Sonke, T.; Tjen, K. C. M. F.; Kaptein, B.; Broxterman, Q. B.;
Schoemaker, H. E.; Rutjes, F. P. J. T. AdV. Synth. Catal. 2001, 343, 662.
(b) Ref 6c.
When this reaction was carried out at a higher concentra-
tion, lower yields were obtained and oligomerization was a
dominant side reaction. The synthesis of 7 is the first example
(11) Belokon, Y. N. Janssen Chim. Acta 1992, 10, 4.
(12) Collet, S.; Bauchat, P.; Danion-Bougot, R.; Danion, D. Tetra-
hedron: Asymmetry 1998, 9, 2121.
(14) (a) Akiyama, M.; Chisholm, M. H.; Cotton, F. A.; Extine, M. W.;
Haitko, D. A.; Little, D.; Fanwick, P. E. Inorg. Chem. 1979, 18, 2266. (b)
Listermann, M. L.; Schrock, R. R. Organometallics 1985, 4, 74.
(13) Sieber, P. Tetrahedron Lett. 1987, 28, 6147.
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Org. Lett., Vol. 7, No. 14, 2005

of RCAM of a peptide without any preorganization of the
backbone, as was the case for proline or â-turn motif-
containing sequences.6b,c
Scheme 4. Synthesis of Alkyne Mimic C (13)
RCAM precursor (9) of the alkyne mimic of ring B was
synthesized in solution starting from Boc-protected (S)-2-
amino-4-hexynoic acid 3 in an overall yield of 86% (Scheme
3). Treatment of 9 with the tungsten-alkylidyne catalyst
Scheme 3. Synthesis of Alkyne Mimic B (10)
the presence of second-generation Grubbs Ru catalyst.16
Nevertheless, treatment of 12 with the tungsten-alkylidyne
catalyst resulted in cyclic heptapeptide 13 in only 18% yield.
The main product that was isolated consisted of insoluble
oligomers.
resulted in cyclic tetrapeptide 10 in a yield of 82%. This
increased yield, as compared to that of 7, can be explained
by a certain degree of preorganization induced by the proline
residue, in agreement with literature data.6b,c Moreover, the
alkyne ring-closure leading to 10 was also faster (45 min)
than that affording 7 (2 h).
Starting from pentapeptide methylester 11, we synthesized
the RCAM-precursor 12 for the alkyne mimic of fragment
C by fragment coupling of Boc-Bug-Gly-OH, in an
overall yield of 63% (Scheme 4). The isolated yield was
somewhat lower than that of 9 and 6, mainly due to the low
solubility of 12 and its precursors. On the basis of previous
experience, we decided to replace the central leucine with a
D-leucine residue, to favor a turn-like conformation, since it
was known that the sequence with all L-amino acid residues
did not cyclize under ring-closing metathesis conditions in
Our first approach to synthesize the crossed DE-ring
system (Scheme 5) was based on the preorganization-induced
synthesis of both ring systems in a single RCM reaction step
as was recently reported for the synthesis of bisalkene DE-
ring mimic 18.5 Therefore, the RCAM precursor with the
amino acid sequence comprising the DE-ring with four
alkyne moieties was synthesized on the solid-phase starting
from solid-phase resin derivative 5 and using 2-butyne
glycine derivatives 3 and 4. After treatment with KCN in
MeOH, protected hexapeptide methyl ester 14 was obtained.
Unfortunately, upon RCAM, no monocyclic intermediates
or bicyclic 15 were formed. After 24 h, the formation of
polymeric material was observed; the conversion was still
incomplete, and starting material was recovered.
Therefore, we changed our strategy by synthesizing peptide
16 containing two allylglycine residues and two 2-butyne-
glycine residues. Since the ruthenium alkene metathesis
catalyst and the tungsten alkyne metathesis catalyst can
operate orthogonally to each other, it was envisioned to first
treat 16 with the tungsten-alkylidyne catalyst in order to
synthesize the monocyclic alkyne-bridged peptide 17. This
might be immediately followed by a RCM reaction in the
presence of second-generation Grubbs catalyst16 or a RCM
reaction after first reducing the triple bond to the corre-
(15) General Procedure for Ring-Closing Alkyne Metathesis. All
RCAM reactions were carried out under Ar in flame-dried glassware using
Schlenk techniques. Precursor peptide 6 (46 mg, 70 µmol) was dissolved
in dry toluene (200 mL), and the tungsten-alkylidyne catalyst (tBuO)3Wt
CtBu (4 mg, 9 µmol) was added. The obtained reaction mixture was heated
to 80 °C and stirred for 2 h. The reaction was monitored by TLC until no
changes in product distribution could be observed. Then, H2O (1 mL) was
added to quench the catalyst, and the solvent was removed by evaporation.
The residue was purified by column chromatography with DCM/MeOH
97.5:2.5 v/v as the eluent. Cyclic pentapeptide 7 was obtained as an off-
white powder in 42% yield (18 mg). Rf ) 0.56 (DCM/MeOH 9:1 v/v).
ESMS calcd for C29H47N5O8 594.4, found 594.7 [M + H]+, 538.6 [(M -
C4H8) + H]+, 494.5 [(M - C2H5O2) + H]+. HRMS calcd for C29H47N5O8-
Na 616.33218, found 616.33223. NMR assignments: see Supporting
Information.
(16) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999, 1,
953.
Org. Lett., Vol. 7, No. 14, 2005
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Scheme 5. Approach to the Synthesis of the Crossed Alkyne DE-Mimic
sponding double bond. These procedures would then lead
from 18 to 82%. These cyclic peptides are rigid mimics of
the thioether rings of nisin. The resulting mimics will be
used to assemble AB- and ABC-alkyne mimics of nisin Z.
These fragments will be tested for their potency to bind to
lipid II as possible antibiotics. Furthermore, stereoselective
reduction of the triple bonds may lead to derivatives of which
the influence of the geometry of the resulting double bonds
on the affinity toward lipid II can be evaluated.
to a DE mimic with the double bond originating from the
triple bond with a defined (cis or trans) geometry.
After RCAM of precursor peptide 16 under high dilution
(250 µM) conditions for 90 min, the reaction mixture was
quenched by the addition of H2O. HPLC analysis showed
that two products were formed in a ratio of 3:2, which could
be separated by preparative HPLC. According to NMR and
LC-MS/MS, 17 was the major product and was obtained in
18% isolated yield (5 mg). As a side product, a dimer
(connected through Bug3 and Bug6 residues) was identified
by LC-MS/MS. Unfortunately, due to the extremely low
solubility of 17, the reduction of the triple bond and the
subsequent alkene ring-closing metathesis reaction were
unsuccessful.
Acknowledgment. We thank Cees Versluis for measuring
the LC-MS/MS spectra.
Supporting Information Available: Experimental pro-
1
cedures, H/13C NMR data, mass analyses, and HPLC data
for compounds 1-4, 6-14, 16, and 17. This material is
available free of charge via the Internet at http://pubs.acs.org.
In conclusion, we have successfully applied ring-closing
alkyne metathesis for the synthesis of alkyne-bridged pep-
tides of up to 4-7 amino acid residues in yields ranging
OL0508781
2964
Org. Lett., Vol. 7, No. 14, 2005