Cu(I)- and Ru(II)-mediated "click" cyclization of tripeptides toward vancomycin-inspired mimics

Article abstract of DOI:10.1021/ol201184b

Structural mimics comprising 1,4- and 1,5-disubstituted triazole-containing cyclic tripeptides with excellent resemblance toward the DE-ring of vancomycin are conveniently accessible using Cu(I)- or Ru(II)-assisted "click" cyclization.

Full text of DOI:10.1021/ol201184b

ORGANIC
LETTERS
2011
Vol. 13, No. 13
3438–3441
Cu(I)- and Ru(II)-Mediated “Click”
Cyclization of Tripeptides Toward
Vancomycin-Inspired Mimics
Jinqiang Zhang, Johan Kemmink, Dirk T. S. Rijkers, and Rob M. J. Liskamp*
Division of Medicinal Chemistry & Chemical Biology, Utrecht Institute for
Pharmaceutical Sciences, Department of Pharmaceutical Sciences, Faculty of Science,
Utrecht University, P.O. Box 80082, 3508 TB Utrecht, The Netherlands
r.m.j.liskamp@uu.nl
Received May 4, 2011
ABSTRACT
Structural mimics comprising 1,4- and 1,5-disubstituted triazole-containing cyclic tripeptides with excellent resemblance toward the DE-ring of
vancomycin are conveniently accessible using Cu(I)- or Ru(II)-assisted “click” cyclization.
Reduction of conformational flexibility is important to
increase the affinity of a peptide for its natural receptor.1 A
first convenient approach toward achieving this goal is
head-to-tail cyclization. However, Nature has found many
other covalent constraints to reduce the flexibility of a
peptide and even beyond this in the creation of cavity or
shell-like structures. The cavity of the vancomycin anti-
biotics induced by the biaryl ether bridge is an outstanding
example in this respect.2
There is a great challenge in the development of ap-
proaches to control the conformation and shape of pep-
tides, which are usually very flexible. Therefore, next to our
goal toward the synthetic accessibility of small cyclic pep-
tides, the presented work is also central in our ongoing quest
toward uncovering promising alternatives for the biaryl
ether bridge in the vancomycin antibiotics possibly leading
to attractive mimics.
In our previous approaches we have used ring-closing
metathesis for a side-chain knotted pentapeptide 13a in-
spired by vancomycin as well as the Sonogashira reaction
for the preparation of alkyne bridged cyclic tripeptides like
23b toward constrained mimics of vancomycin (Figure 1).
As a new attractive alternative for the biaryl ether bridge
in vancomycin, we report here on the successful introduc-
tion of the triazole ring system in cyclic tripeptides4 toward
the synthesis of vancomycin-inspired peptidomimetics.
Inprinciple, this triazole ring can be introduced verycon-
veniently by the Cu(I)-catalyzed azideÀalkyne cycloaddi-
tion (CuAAC) reaction.5 However, one has to realize that
the resulting cyclic peptides are (highly) constrained, and
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H. W.; Liskamp, R. M. J. Org. Biomol. Chem. 2004, 2, 2658. (b) ten
Brink, H. T.; Rijkers, D. T. S.; Liskamp, R. M. J. J. Org. Chem. 2006, 71,
1817.
1172.
r
10.1021/ol201184b
Published on Web 05/26/2011
2011 American Chemical Society

iodine in the presence of silver trifluoracetate.9 Next, the
desired alkyne moiety was introduced via a Pd-catalyzed
Sonogashira cross-coupling using TBDMS-protected
acetylene to afford 5 in an excellent yield (94%). Origin-
ally, we used TMS-protected acetylene, but it was found
that the TMS group was not completely impervious to-
ward acid treatment needed for removal of the Boc
group.10 Dipeptide 6 was obtained after treatment with
TFA, followed by BOP-mediated coupling with Boc-D-
Ala-OH in a good yield (80%). At this point, the optimal
length of the side chain of the azido-amino acid needed for
cyclization by CuAAC was not known. Therefore, after
removal of the Boc-group, four different azido-amino
acids were introduced, which were conveniently accessible
in a diazotransfer reaction using imidazole-1-sulfonyl
azide.11 Finally, the TBDMS group was removed by
TBAF and the click precursors 8aÀd were obtained in
high yields (Scheme 1).
Figure 1. Design of triazole-containing vancomycin mimics.
therefore, dimers or oligomers may be formed as side
products.6 The triazole ring can be introduced intramole-
cularly as the final macrocyclization step or intermolecu-
larly at the beginning of the synthesis, followed by a macro-
lactamization step by means of peptide coupling reagents.7
In the synthesis of the required precursors for both
routes, the sensitivity toward racemization of the hydro-
xyphenyl glycine core is a critical issue. Therefore, after
introduction of the Boc group, the carboxylic acid was
converted to the methyl amide to give 3 instead of an ester
to suppress racemization during subsequent peptide cou-
pling steps (Scheme 1).8 Methylation of the phenolic
hydroxyl group in 3 with methyl iodide in DMF led to
the methyl ether and was followed by iodination to 4 using
Scheme 1. Synthesis of the Linear Precursor Peptides 8aÀda
(4) (a) Angell, Y.; Burgess, K. J. Org. Chem. 2005, 70, 9595.
(b) Billing, J. F.; Nilsson, U. J. J. Org. Chem. 2005, 70, 4847. (c) van
Maarseveen, J. H.; Horne, W. S.; Ghadiri, M. R. Org. Lett. 2005, 7,
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A.; Ghadiri, M. R. Angew. Chem., Int. Ed. 2009, 48, 4718. (i) Beierle,
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W.; Workman, P.; Moody, C. J. Chem.;Eur. J. 2010, 16, 2758.
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J. T.; Galloway, W. R. J. D.; Bender, a.; Welch, M.; Spring, D. R. Proc.
Natl. Acad. Sci. U.S.A. 2011, 108, 6793. (m) Jacobsen, Ø.; Maekawa, H.;
a Xaa: Dap, (S)-2,3-diaminopropanoic acid; Dab, (S)-2,4-diamino-
butanoic acid; Orn, ornithine; Lys, lysine.
€
Ge, N.-H.; Gorbitz, C. H.; Rongved, P.; Ottersen, O. P.; Amiry-
Moghaddam, M.; Klaveness, J. J. Org. Chem. 2011, 76, 1228.
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Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596. (c) Meldal, M.;
Tornøe, C. W. Chem. Rev. 2008, 108, 2952.
In the CuAAC reactions, 1.5 equiv of the soluble Cu(I)
catalyst Cu(CH3CN)4PF6 was used at a diluted (1 mM)
(6) (a) Punna, S.; Kuzelka, J.; Wang, Q.; Finn, M. G. Angew. Chem.,
Int. Ed. 2005, 44, 2215. (b) Jagasia, R.; Holub, J. M.; Bollinger, M.;
Kirshenbaum, K.; Finn, M. G. J. Org. Chem. 2009, 74, 2964.
(c) Reference 4a.
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48, 4509.
(10) Treatment with TFA/CH2Cl2, HCl/Et2O, or p-TosOH/toluene
led to removal of the TMS group and/or addition of the conjugated base
to the triple bond.
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Org. Lett., Vol. 13, No. 13, 2011
3439

substrate concentration (Scheme 2). Under these con-
ditions, the “click” macrocyclization proceeded with com-
plete conversion after 24 h at room temperature. Not
unexpectedly, cyclization of the precursor containing the
shortest azido side chain 8a led to formation of dimer 10a
as a major product and even its trimer (not shown), was
obtained in an appreciable yield as was judged by HPLC
and MALDI-TOF (Table 1). Apparently, the length of the
ornithine azide was not sufficient for formation of the
desired click macrocycle 9c, but now the corresponding
cyclic trimer was not formed and dimer 10c was only
obtained in a low yield (15%), partly due to its poor
solubility. Gratifyingly, the precursor containing the lysine
derived azide 8d led to formation of the desired macrocycle
9d as the major product (46%) together with dimer 10d as
the minor product (15%).
Scheme 2. Cu(I)-Catalyzed Cyclization and Macrolactamiza-
tion of the 1,4-Triazoles
As was found previously in the preparation of alkyne
bridged cyclic tripeptides toward constrained mimics of
vancomycin, macrolactamization after introduction of
triazole ring might be an attractive alternative,3b,7 since a
variety of coupling reagents are available. To evaluate this
approach, the precursors with the shortest azide contain-
ing side chain 14a and with the longest azide containing
side chain 14b were prepared (Scheme 2). After this, BOP-
mediated cyclizations were carried out at diluted (0.5 mM)
conditions in DMF. Different from the results above, the
macrocyclization precursor with the shortest azide chain
led to the formation of the dimer 16a, but also the desired
monocycle 15a was obtained albeit present in a complex
reaction mixture. The ratio between 15a:16a was found to
be 1:3 based on LCMS analysis. However, macrocycliza-
tion of 14b gave only the cyclic dimer 16b (in an isolated
yield of 7%) as judged by HPLC, 1H NMR, and MALDI-
TOF analysis.
seemed a very attractive for application on click precursors
8aÀd (Scheme 3).
Scheme 3. Ru(II)-Catalyzed Cyclization and Macrolactamiza-
tion of the 1,5-Triazoles
These results are indicative of the scope of the prepara-
tion of small cyclic peptides containing the 1,4-disubsti-
tuted triazole ring system (Scheme 2). Access to a 1,5-
disubstituted triazole system via RuAAC12 in which the
substituents are positioned under a smaller angle, might be
beneficial for obtaining a more easily obtainable bent site
in the said macrocycles. Fortunately, Marcaurelle and co-
workers recently described the Ru(II)-catalyzed synthesis
of 1,5-disubstituted triazole N-methyl lactams,13 which
(12) (a) Zhang, L.; Chen, X. G.; Xue, P.; Sun, H. H. Y.; Williams,
I. D.; Sharpless, K. B.; Fokin, V. V.; Jia, G. C. J. Am. Chem. Soc. 2005,
127, 15998. (b) Majireck, M. M.; Weinreb, S. M. J. Org. Chem. 2006, 71,
8680. (c) Rasmussen, L. K.; Boren, B. C.; Fokin, V. V. Org. Lett. 2007, 9,
5337. (d) Boren, B. C.; Narayan, S.; Rasmussen, L. K.; Zhang, L.; Zhao,
H. T.; Lin, Z. Y.; Jia, G. C.; Fokin, V. V. J. Am. Chem. Soc. 2008, 130,
8923. J. Am. Chem. Soc. 2008, 130, 14900 (correction).
ꢀ
(13) (a) Kelly, A. R.; Wei, J.; Kesavan, S.; Marie, J.-C.; Windmon,
N.; Young, D. W.; Marcaurelle, L. A. Org. Lett. 2009, 11, 2257. (b)
Marcaurelle, L. A.; Comer, E.; Dandapani, S.; Duvall, J. R.; Gerard, B.;
Kesavan, S.; Lee, M. D., IV; Liu, H.; Lowe, J. T.; Marie, J.-C.;
Mulrooney, C. A.; Pandya, B. A.; Rowley, A.; Ryba, T. D.; Suh,
B.-C.; Wei, J.; Young, D. W.; Akella, L. B.; Ross, N. T.; Zhang,
Y.-L.; Fass, D. M.; Reis, S. A.; Zhao, W.-N.; Haggarty, S. J.; Palmer,
M.; Foley, M. A. J. Am. Chem. Soc. 2010, 132, 16962.
(14) The crystal structure of balhimycin (a vancomycin-related gly-
copeptide antibiotic) in complex with Lys-D-Ala-D-Ala (Protein Data
ꢀ
bank accession code: 1GO6) was used, see:Lehmann, G.; Bunkoczi, G.;
[Cp*RuCl]4 was selected as a catalyst since it leads to a
good regioselectivity of formation of the 1,5-disubstituted
triazole moiety. Attempts with the cyclization precursor
containing the shortest azide-containing side chain 8a were
already successful after some optimization. Indeed, com-
plete conversion was achieved after 24 h at 50 °C using a
ꢀ
Vertesy, L.; Sheldrick, G. M. J. Mol. Biol. 2002, 318, 723.
3440
Org. Lett., Vol. 13, No. 13, 2011

substrate concentration of 5 mM in the presence of 15 mol
% catalyst. In addition to the dimer 18a (31%) and trimer
(not shown, 8%), the desired monocycle 17a was obtained
in an encouraging yield of 14% (Table 1). Using this
protocol, precursors 8bÀd were subjected to Ru(II)-cata-
lyzed cyclization, which resulted in exclusive formation of
the desired monocycles 17bÀd in attractive yields for these
relatively small macrocyclic tripeptides, without the detec-
tion of any dimers 18bÀd (Table 1).
Table 1. Macrocycles Containing the 1,4- and 1,5-Disubstituted
Triazole by CuAAC and RuAAC, Respectively
yielda
(%)
monomer/dimer/trimerb
(%)
compd approach c (mM)
8a
CuAAC
1
76
27
15
61
9a (0)/10a (57)/19
9b (0)/10b (19)/8
9c (0)/10c (15)/0
9d (46)/10d (15)/0
15a (À)/16a (À)/0d
15b (0)/16b (7)/0
17a (14)/18a (31)/8
17b (68)/18b (0)/0
17c (59)/18c (0)/0
17d (55)/18d (0)/0
20a (47)/21a (28)/0
20b (70)/21b (0)/0
Figure 2. Superimposition of balhimycin (in red) with 1,4-triazole
9d (left) and 1,5-triazole 17d (right).15 The carbon atoms RC1,
RC2, RC3, arom-C4, and triazole-C5 have been used as fixed
coordinates for superimposition. Note: the atom coordinates of
balhimycin have been mirror-imaged, since 9d and 17d represent
the enantiomers of vancomycin’s backbone stereochemistry.
8b
CuAAC
1
8c
CuAAC
1
8d
CuAAC
1
14a
14b
8a
Cu(I)/MLc
Cu(I)/ML
RuAAC
0.5
0.5
5
7
53
68
59
55
75
70
8b
RuAAC
5
8c
RuAAC
5
8d
RuAAC
5
package. Structures 9d and 17d were energy minimized
using the simulated annealing protocol employing the
AMBER99 force field. Molecules were superimposed by
minimizing the rmsd between the five selected atoms in the
ring. Although all 1,5-disubstituted triazole-containing
cyclic monomers 17aÀd were accessible by Ru(II)-
catalyzed cycloaddition, only 17d was superimposed on
balhimycin (Figure 2) since the number of atoms in its ring
system equals that of the DE-ring (16 atoms, Figure 1).
This superimposition indicated a high structural resem-
blance of 17d with the DE-ring system, while superimposi-
tion of 9d showed a more distorted appearance.
In conclusion, the aim of this research was the develop-
ment of a versatile approach toward the synthesis of
relatively small (and constrained) cyclic peptide systems as
are especially prominent in the vancomycin structure.
Although this aim was achieved successfully using RuAAC,
since the CuAAC click cyclization mostly led to the forma-
tion of cyclic dimers. These peptide derivatives are also
of considerable interest, since symmetrical relatively small
dimeric peptides form the backbone of potent antibiotic
peptides like gramicidin.
19a
19b
Ru(II)/ML
Ru(II)/ML
0.5
0.5
a Isolated yield. b Yield as determined by LCMS. c ML: macrolacta-
mization. d Ratio of 15a:16a was 1:3 as determined by LCMS.
For completion of this study, macrocyclization of pre-
cursors derived from the shortest azide 19a and longest
azide-containing chain 19b was carried out. It was found
that BOP-mediated coupling of the shortest side chain
containing precursor also gave 28% dimer 21a next to the
desired monocycle 20a (47%). The precursor with the
longest side chain even led to a slightly higher yield of the
monocycle 20b (70%) as compared to formation of the 1,
5-triazole as the “click” cyclization step to give 17d (55%),
while formation ofdimer 21b was completely absent. These
results demonstrated clearly that the geometry of the
triazole moiety determines to a large extent the outcome
of both cyclization approaches. Since the 1,4-disubstituted
triazole moiety is extended, predominantly cylic dimers
were formed. Only when the ring size is 17 atoms, a cyclic
monomer was formed. Because of its β-turn-like confor-
mation, the 1,5-disubstituted triazole moiety favored
the formation of cyclic monomers and allowed the forma-
tion of a highly constrained ring structure of 13 atoms (17a
and 20a).
Acknowledgment. J.Z. acknowledges the China Scho-
larship Council for a Ph.D. scholarship, Hans W. Hilbers
(Medicinal Chemistry & Chemical Biology, Utrecht
University) is thanked for perfoming the MALDI-TOF
measurements.
Cyclic monomer 9d which was obtained by CuAAC was
superimposed with the right half of the vancomycin-re-
lated balhimycin antibiotic14 comprising the DE-ring sys-
tem (Figure 2). Modeling of the structures was accompli-
shed using the YASARA Structure 10.5.2.1 software
Supporting Information Available. Experimental proce-
dures, spectroscopic data, and HPLC and MALDI-TOF
analysis for compounds 8aÀd, 9d, 10aÀd, 14a,b, 16a,b, 17a,
b, 18a, 19a,b, 20a,b, and 21a. This material is available free
of charge via the Internet at http://pubs/acs.org.
(15) Modeling has been performed with the YASARA software
(www.yasara.org).
Org. Lett., Vol. 13, No. 13, 2011
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