Backbone amide linker strategy for the synthesis of 1,4-triazole-containing cyclic tetra- and pentapeptides

Article abstract of DOI:10.1002/ejoc.200800143

A backbone amide linker strategy was chosen for the solid-phase synthesis of triazole-containing cyclic tetra- and pentapeptides. An alkyne-substituted linker derived from 4-hydroxy-2-methoxybenzaldehyde was elongated by using standard Fmoc-based solid phase chemistry and terminated by coupling of an azido acid. In solution, the peptides were cyclized by a Cu ^I-catalyzed azide-alkyne cycloaddition reaction. The solid-supported synthesized linear peptides had to be cleaved prior to cyclization. As an example of cyclic tetrapeptides, a triazole analog of cyclo-[Pro-Val-Pro-Tyr] (2) was prepared by the solid-phase/solution-phase method. For the cyclic pentapeptides, segetalin B (3) was chosen as a model compound to show the applicability of this method. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800143

FULL PAPER
DOI: 10.1002/ejoc.200800143
Backbone Amide Linker Strategy for the Synthesis of 1,4-Triazole-Containing
Cyclic Tetra- and Pentapeptides
Jasper Springer,[a] Kimberly R. de Cuba,[a] Sandrine Calvet-Vitale,[a] Jan A. J. Geenevasen,[a]
Pedro H. H. Hermkens,[b] Henk Hiemstra,[a] and Jan H. van Maarseveen*[a]
Keywords: BAL strategy / Cyclic peptides / Click chemistry / Triazole
A backbone amide linker strategy was chosen for the solid-
phase synthesis of triazole-containing cyclic tetra- and
pentapeptides. An alkyne-substituted linker derived from 4-
hydroxy-2-methoxybenzaldehyde was elongated by using
standard “Fmoc-based” solid phase chemistry and termin-
ated by coupling of an azido acid. In solution, the peptides
were cyclized by a CuI-catalyzed azide-alkyne cycloaddition
reaction. The solid-supported synthesized linear peptides
had to be cleaved prior to cyclization. As an example of cyclic
tetrapeptides, a triazole analog of cyclo-[Pro-Val-Pro-Tyr] (2)
was prepared by the solid-phase/solution-phase method. For
the cyclic pentapeptides, segetalin B (3) was chosen as a
model compound to show the applicability of this method.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
all- cyclic tetrapeptide cyclo-[Pro-Val-Pro-Tyr][8] (1, Fig-
ure 1), which could not be prepared by traditional lactami-
zation strategies, could be ring-closed easily as a triazole-
containing analog.[9] Substitution of the C-terminus by an
alkyne and the N-terminus by an azide gave the linear pre-
cursor for the 1,3-dipolar cycloaddition reaction, that was
carried out subsequently to give the cyclic pseudopeptide in
an excellent 70% yield.[9a] Moreover, the tyrosinase inhibi-
tory activity was retained,[9b] showing that the triazole acts
in this case as a mimic for the replaced amide bond. These
findings are in line with results reported by Ghadiri et al.
who have shown the similarity between a 1,4-connected tri-
Introduction
Small cyclic peptides comprise a priviliged class of com-
pounds,[1] and many – such as gramicidin S,[2] tentoxin[3]
and apicidin[4] – show relevant biological activities. A li-
brary of analogs of these compounds would be a valuable
tool not only for the identification of the pharmacophoric
properties on existing targets, but also on new biological
targets and to carefully investigate the interactions with
these proteins. Because of the vast array of available natural
and unnatural amino acids carrying diverse functional
groups, cyclic peptide properties can be easily varied.
The synthesis of small cyclic peptides may be hampered
by the cyclization step.[5] Small peptide lactamization is
often low-yielding and leads to the formation of unwanted
oligomers and polymers. These difficulties are due to the
nature of the amide bonds of the linear peptide, which pref-
erentially adopt a transoid character. This places the mutu-
ally reactive C- and N-termini of the linear peptide far
apart,[6] providing difficulties in the final lactamization re-
action.
It has been shown earlier that the CuI-catalyzed 1,3-di-
polar cycloaddition reaction between an alkyne and an
azide to give a 1,4-connected 1,2,3-triazole moiety provides
a powerful tool for the cyclization of such peptides.[7] We
have shown recently that the natural tyrosinase inhibitor
[a] Van ’t Hoff Institute for Molecular Sciences, Faculty of Science,
University of Amsterdam,
Nieuwe Achtergracht 129, 1018 WS Amsterdam, The Nether-
lands
Fax: +31-20-5255670
E-mail: jvm@science.uva.nl
[b] NV Organon, Part of Schering-Plough,
P. O. Box 20, 5340 BH Oss, The Netherlands
Figure 1. Cyclic peptides 1 and 3 and their triazole analogs 2
and 4.
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1,4-Triazole-Containing Cyclic Tetra- and Pentapeptides
azole and a trans amide bond in peptides in terms of elec- attachment of 13 onto the solid support, the benzyl ester
tronic, steric and hydrogen-bond-donating and -accepting was saponified with lithium hydroxide in MeOH. Subse-
capacities.[10]
quent protection of the amine with Fmoc chloride and
This paper describes an extension of our work to enable DIPEA furnished 15 in 76% yield which was loaded on
parallel access to triazole-containing cyclic pseudopeptides. polystyrene resin functionalized with primary amine (PS-
Since the introduction of solid-phase peptide synthesis AM-NH2, 16) by coupling using HATU/DIPEA in DMF
(SPPS) by Merrifield in 1963,[11] this field has emerged and in 51% yield, as determined by the Fmoc-loading test.[17]
has led to several strategies for the combinatorial synthesis Treatment of the resin with piperidine liberated the propar-
of cyclic peptides.[12] For the solid-phase synthesis of tri- gylic amine 17 ready for acylation.
azole-containing cyclic pseudopeptides we selected a so-
called backbone amide linker (BAL) for anchoring of the
linear cyclization precursors, thereby introducing a tertiary
amide bond inducing a turn in the peptide chain facilitating
cyclization.[13,14] The use of an acid-labile backbone amide
linker (5, Scheme 1) in combination with Fmoc-SPPS en-
sures a facile synthesis of the linear peptide precursors 6
after which the cyclization can be performed on the solid
support (route A) or in solution after cleavage (route B).
Scheme 1. Strategy for the combinatorial synthesis of triazole-con-
taining cyclic peptides 9.
Two model cyclic peptides were selected to develop the
new chemistry, i.e. cyclo-[Pro-Val-Gly-ψ(triazole)-Tyr] (2),
derived from the naturally occurring cyclic tetrapeptide 1
which was isolated from L. helveticus and shows tyrosinase
inhibition activity,[9] and cyclo-[Trp-Ala-Gly-ψ(triazole)-
Val-Ala] (4) which is the triazole analog of the cyclic penta-
peptide segetalin B (3), isolated from the seeds of Vaccaria
segetalis (Figure 1).[15] The site for the introduction of the
ψ(triazole) moiety in 2 and 4 was selected by the presence
of a trans amide bond configuration at these positions in
the natural congeners.
Scheme 2. Synthesis of the solution-phase linker 13 and the solid-
phase linker 17.
The synthesis of the solution-phase linear precursor 20a
started from 13 by coupling of Fmoc-Val-OH using HATU
as the coupling reagent to give 18a in 95% yield (Scheme 3).
Fmoc removal of 18a with piperidine followed by coup-
ling with Fmoc-Pro-OH using similar conditions provided
the pseudo-tripeptide 19a in 95% overall yield. To suppress
epimerization, the final azido acid N3-Tyr(OBn)-OH[18] was
coupled using EDCI/HOBt at 0 °C to give the desired linear
tetrapeptide precursor 20a in 62% yield.
The solid-phase synthesis of the linear precursor 20b
started by coupling of Fmoc-Val-OH to the resin 17 using
Results and Discussion
The benzyl 4-(4-formyl-3-methoxyphenoxy)butanoate similar reagents as mentioned above in excess, to afford the
linker (12, Scheme 2) which was chosen is based on anchor- product in 98% yield (Scheme 3). Fmoc removal and coup-
ing of a backbone amide bond by an acid-labile benzylic ling of Fmoc-Pro-OH afforded the tripeptide 19b. Final
bond. The synthesis of the cyclic pseudopeptides with the coupling of N3-Tyr(OBn)-OH using DIC and HOBt at 0
linker system was first validated in solution from the benzyl °C afforded the cyclization precursor 20b in 87% yield.
ester 13. The aldehyde linker could be prepared in two steps
Now the stage was set for the CuI-catalyzed cyclization
from commercially available starting materials (Scheme 2). reaction. Although several conditions for the macrocycliza-
Protection of the acid moiety of 4-bromobutanoic acid (10) tion were evaluated, the conditions of choice were similar
yielded the benzyl ester 11 in 98% yield.[16] Alkylation of 4- as described previously (Scheme 4). Thus, treatment of 20a
hydroxy-2-methoxybenzaldehyde with the bromide 11 pro- in solution with CuBr and an excess of DBU in refluxing
vided 12 in 70% yield. The linker 12 was loaded with pro- toluene afforded the cyclic peptide 21a in 40% isolated
pargylamine by a reductive amination to afford 13. To allow yield. However, subjection of the immobilized linear precur-
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J. H. van Maarseveen et al.
FULL PAPER
Scheme 1) to afford cyclic peptide 23 in 43% isolated yield.
Final hydrogenolytic removal of the tyrosine benzylic ether
protective group furnished the target peptide cyclo-[Pro-
Tyr-ψ(triazole)-Gly-Val] 2 in 73% yield.
To show the applicability of our method in the cyclic
pentapeptide series, the triazole analog 4 of segetalin B
{cyclo-[Ala-Val-Gly-Ala-Trp] (3)} was chosen (Scheme 5) as
a model pentapeptide. Starting from the solution-phase
linker 13, the proper Fmoc-protected amino acids were cou-
pled in good yields (67–71%) using EDCI/HOBt, and fi-
nally N3-Val-OH was attached to furnish the linear penta-
peptide 27 in 80% yield. Cyclization was performed using
CuBr and a pybox-type ligand in acetonitrile at room tem-
perature[19] to afford the cyclic peptide 28 in 37% yield
along with 10% of the dimer, which could be easily sepa-
rated by flash column chromatography. Final removal of
the linker and the protective group of the tryptophan resi-
due afforded the triazole analog 4 of segetalin B in 65%
isolated yield.
Scheme 3. Synthesis of the linear precursors in solution 20a from
13 and on the solid phase 20b from 17.
sor 20b to the cycloaddition conditions and subsequent
cleavage of the cyclic peptide from the resin with TFA (cf.
route A, Scheme 1) only afforded unidentified side prod-
ucts. IR spectroscopy revealed complete disappearance of
the azide signal on the resin.
Scheme 5. Synthesis of the triazole analog 4 of segetalin B in solu-
tion from 13.
To investigate the influence of substitution of an amide
group for a 1,4-connected triazole moiety on the conforma-
tion of the cyclic pentapeptide, NMR studies were per-
formed. All proton and carbon signals could be assigned
Scheme 4. Cyclization of the linear precursors 20a and 20b to af-
ford the cyclic peptides 21a and 2, respectively.
As an alternative, the immobilized cyclization precursor by COSY, HMBC and HMQC NMR experiments. Both
20b was cleaved off by treatment with TFA to liberate the proton and carbon NMR spectra indicated the presence of
linear precursor 22 (53% crude overall yield) to which CuBr only one stable conformer in solution, as only sharp and
and DBU in refluxing toluene were added (cf. route B, single peaks were observed.
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1,4-Triazole-Containing Cyclic Tetra- and Pentapeptides
To determine whether intramolecular hydrogen bonds
The NOE correlation spectra also provide information
are present, the temperature dependence of the amide pro- about whether the amide bonds are cisoid or transoid. In
ton chemical shifts was examined by NMR spectroscopy.[20] the case of cisoid amide bonds, NOE correlations between
The chemical shifts of the amide NH protons in [D6]DMSO adjacent CαH can be expected. As no CαH-CαH corre-
were monitored during elevation of the temperature from lations were observed, it is likely that in the triazole analog
300 to 330 K. The temperature coefficients which were ob- 4 all amide bonds are in the transoid form. Further ongoing
tained are shown in Table 1.
conformational studies in combination with molecular
modeling and biological testing should provide more re-
Table 1. Temperature coefficients (–dδ/dTϫ10–3 ppm/K) of the fined conclusions on the exact conformation of the triazole
amide protons in cyclic peptide 4.
analog.
Solvent
Gly1
Ala3
7.0
Trp4
0.8
Ala5
8.9
[D6]DMSO
6.4
Conclusions
This research has shown the syntheses of the triazole-
containing analog of cyclic tetrapeptide cyclo-[Pro-Val-Gly-
ψ(triazole)-Tyr] (2) by means of a backbone amide linkage
strategy in solution and on a solid phase. It was found that
cleavage of the linear precursor from the resin was neces-
sary prior to cyclization by an azide/alkyne 1,3-dipolar cy-
cloaddition in solution. The triazole analog of the cyclic
pentapeptide segetalin B, cyclo-[Trp-Ala-Gly-ψ(triazole)-
Val-Ala] (4) was prepared in solution by applying the back-
bone amide linkage strategy and was used to study the im-
pact of the replacement of an amide bond by a triazole
linkage by extensive NMR analysis. Although a different
hydrogen-bond pattern was observed, the NOE correlations
in the specific β-turn were similar as in the parent com-
pound. Biological testing and molecular modeling are still
in progress. Moreover, a small library of triazole-containing
cyclic tetrapeptides will be constructed by using the devel-
oped method.
On raising the temperature, all intermolecular hydrogen
bonds will be broken, while intramolecular hydrogen bonds
remain intact. When the temperature coefficient for the NH
signals is larger than 4ϫ10–3 ppm/K (in [D6]DMSO), it is
considered to indicate external NH orientation. In the case
of intramolecular hydrogen bonds using DMSO as the sol-
vent, values will be lower then 2ϫ10–3 ppm/K. As can be
seen in Table 1, only the amide-NH of Trp seems to be in-
volved in intramolecular hydrogen bonding, in contrast to
the parent natural cyclic peptide segetalin B where none of
the amide NH bonds are involved in intramolecular hydro-
gen bonding.
Comparison of 2D NOE spectra of the cyclic peptide
analog 4 with NOE correlations found for segetalin B,
could provide information whether the conformation was
retained upon introduction of the triazole linkage (Fig-
ure 2). The most important NOE correlations of segetalin B
and the triazole analog 4 are shown. Segetalin B is thought
to adopt a type-II β-turn in the Ala-Trp-Ala-Gly region of
the natural peptide, as can be seen by specific NOE en-
hancements of CαH of the Trp, the CαH of the Ala and the
NH of the Ala residue.[21] In the region which is expected
to have a type-II β-turn conformation, the NOE corre-
lations found for the cyclic peptide analog 4, correspond to
the NOE correlations given for the natural product segeta-
lin B (Figure 2).[15a] NOESY spectra show correlations be-
tween the Ala5 CαH and the Ala5 NH. Also the correlation
between the Ala5 NH with the Trp CαH is seen. These NOE
enhancements are also found in the natural product and
suggest that the triazole-containing analog adopts the same
type-II β-turn as the natural product.
Experimental Section
General: All reactions were conducted under nitrogen unless stated
otherwise and monitored by TLC on silica gel coated aluminium
sheets. Flash column chromatography was performed on silica gel
300–400 mesh using the indicated solvent mixtures. All solvents
were distilled from sodium benzophenone ketyl (THF, Et2O) or
CaH2 (DMF, CH2Cl2). The NMR spectra were determined in
CDCl3 solutions, using a Bruker ARX 400 and a Varian Inova 500
spectrometer unless indicated otherwise. Spectra are reported in δ
units (ppm) and J values (Hz) with Me4Si as the internal standard.
HRMS spectra (FAB+) were recorded with a JEOL JMS SX/SX
102A four-sector mass spectrometer. Infrared (IR) spectra were ob-
tained with a Bruker IFS 28 FTIR spectrometer and are reported
in wavenumbers (cm–1). Melting points were recorded with a
Büchi B-545 melting point apparatus and are uncorrected. HPLC
analyses were measured with an Agilent HPLC system equipped
with a C-18 column (Varian chrompack, inertsil-ODS-3, 3µ,
50ϫ4.6 mm), a maximum flowspeed of 2.0 mL/min, the UV/Vis
detector at λ = 220 nm or 254 nm as indicated, and a gradient of
100% A to 100% B [A: H2O/CH3CN/HCOOH (95:5:0.04); B: H2O/
CH3CN/HCOOH (5:95:0.04)] in 5 min as the eluent. Optical rota-
tions were measured with a Perkin–Elmer 241 polarimeter in a 1-
dm cell (2 mL) in the indicated solvent at the indicated concentra-
tion, temperature and wavelenght.
Abbreviations: DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene, DIC =
N,NЈ-diisopropylcarbodiimide, DIPEA = diisopropylethylamine,
Figure 2. NOE correlations of cyclic peptide segetalin B (3) and its
analog 4.
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EDCI = N-(3-dimethylaminopropyl)-NЈ-ethylcarbodiimide, HATU
= O-(7-azabenzotriazol-1-yl)-N,N,NЈ,NЈ-tetramethyluronium hexa-
fluorophosphate, HOBt = 1-hydroxybenzotriazole, pybox = (S,S)-
2,2Ј-(2,6-pyridinediyl)bis(4-isopropyl-2-oxazoline).
128.4, 128.1, 128.0, 119.9, 104.2, 98.8, 82.2, 71.2, 66.5, 66.1, 55.1,
47.3, 37.2, 30.6, 24.5 ppm. RP-HPLC: tr = 6.68 min (λ = 254 or
220 nm).
4-{3-Methoxy-4-[(prop-2-ynylamino)methyl]phenoxy}butanoic Acid
(14): The linker 13 (0.63 g, 1.73 mmol, 1 equiv.) was dissolved in
wet MeOH (15 mL). LiOH (0.08 g, 3.46 mmol, 2 equiv.) was added
and the reaction mixture stirred at 50 °C for 3 d. Upon completion,
the solvents were evaporated, and 0.44 g (92%) of product was ob-
tained after flash column chromatography [silica gel, CH2Cl2/
Benzyl 4-Bromobutanoate (11): The reaction was carried out using
a Dean–Stark trap. 4-Bromobutyric acid (8.51 g, 51 mmol, 1 equiv.)
was dissolved in cyclohexane (70 mL). Benzyl alcohol (6.8 mL,
66 mmol, 1.3 equiv.) and pTSA (0.97 g, 5.1 mmol, 0.1 equiv.) were
added to the solution. The reaction mixture was heated to reflux
and stirred overnight. Upon completion, the reaction mixture was
cooled to room temperature and washed with saturated aqueous
sodium hydrogen carbonate solution (40 mL) and brine (30 mL).
The organic layer was dried with Na2SO4 and the solvent evapo-
rated to provide the crude product. The purified product was ob-
tained by vacuum distillation (1 Torr, 110 °C) as clear oil (12.84 g,
MeOH, 8:2]. IR (neat): ν = 2938, 1686, 1611, 1586, 1421, 1292,
˜
1134, 1034, 979, 835 cm–1. 1H NMR (400 MHz, CDCl3): δ = 8.10–
7.90 (m, 2 H), 7.04 (d, J = 8.2 Hz, 1 H), 6.34 (s, 1 H), 6.29 (d, J =
8.2 Hz, 1 H), 3.93–3.76 (m, 4 H), 3.70 (m, 3 H), 3.41 (m, 2 H), 2.31
(t, J = 6.8 Hz, 2 H), 2.25 (s, 1 H), 1.95 (t, J = 6.5 Hz, 2 H) ppm.
13C NMR (100 MHz, MeOD): δ = 180.4, 162.9, 160.4, 133.0, 114.8,
106.7, 99.9, 77.0, 68.8, 56.1, 47.1, 40.0, 36.9, 34.1, 26.8 ppm.
HRMS (FAB+): calcd. for C15H20NO4 [MH+] 278.1394; found
278.1397.
98%). IR (neat): ν = 1735, 1637, 1496, 1449, 1385, 1311, 1202,
˜
1
1163, 1125, 966 cm–1. H NMR (400 MHz, CDCl3): δ = 7.41–7.33
(m, 5 H), 5.18 (s, 2 H), 3.47 (t, J = 6.4 Hz, 2 H), 2.58 (t, J = 7.2 Hz,
2 H), 2.2 (m, 2 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 172.4,
135.9, 128.7, 128.4, 128.3, 66.5, 32.8, 32.5, 27.8 ppm. HRMS
(FAB+): calcd. for C11H14BrO2 [MH+] 257.0179; found 257.0177.
4-(3-Methoxy-4-{[(9H-fluoren-9-ylmethoxycarbonyl)(prop-2-ynyl)-
amino]methyl}phenoxy)butanoic Acid (15): The amine 14 (0.44 g,
1.6 mmol, 1 equiv.) was dissolved in a 1:1 mixture of CH2Cl2/ace-
tone (10 mL). DIPEA (0.53 mL, 3.2 mmol, 2 equiv.) and Fmoc-Cl
(0.50 g, 1.76 mmol, 1.1 equiv.) were added. The reaction mixture
was stirred at room temperature overnight. The solvents were evap-
orated, and the resulting residue was dissolved in EtOAc and water.
The water layer was acidified until the pH was Ͻ7.0. The organic
layer was washed with a 1  solution of potassium hydrogen sul-
fate. The organic layer was dried with Na2SO4, filtered, and con-
centrated in vacuo. Purification by flash column chromatography
[silica gel, CH2Cl2/MeOH, 95:5] yielded the desired product (0.60 g,
Benzyl 4-(4-Formyl-3-methoxyphenoxy)butanoate (12): The bro-
mide 11 (5.14 g, 20 mmol, 1 equiv.) and 4-hydroxy-2-methoxybenz-
aldehyde (3.04 g, 20 mmol, 1 equiv.) were dissolved in DMF
(60 mL). K2CO3 (2.77 g, 20 mmol, 1 equiv.) and KI (0.33 g,
2 mmol, 0.1 equiv.) were added, and the reaction mixture was
stirred at 70 °C for 3 d. After cooling to room temperature, water
(100 mL) was added and the mixture extracted with Et2O
(2ϫ100 mL). The combined organic layers were dried with
Na2SO4 and concentrated in vacuo. Pentane (70 mL) and iPrOH
(70 mL) were added to induce precipitation, after which the prod-
uct was filtered off and obtained as a white solid (4.56 g, 70%);
76%). IR (neat): ν = 3290, 3066, 2941, 1704, 1613, 1507, 1451,
˜
1418, 1291, 1239, 1201, 1133, 1036, 910, 835 cm–1 1H NMR
.
(400 MHz, CDCl3): δ = 9.79 (br. s, 1 H), 7.79–7.28 (m, 9 H), 6.47
(m, 2 H), 4.58–4.95 (m, 4 H), 4.16–4.02 (m, 5 H), 3.80 (s, 3 H),
2.62 (m, 2 H), 2.38 (br. s, 1 H), 2.13 (m, 2 H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 178.1, 159.6, 158.4, 156.2, 143.9, 141.2,
127.6, 127.5, 127.0, 124.8, 120.0, 117.3, 104.7, 98.8, 79.9, 71.6, 66.6,
65.0, 55.3, 47.2, 44.5, 36.2, 30.5, 24.4 ppm. HRMS (FAB+): calcd.
for C30H30NO6 [MH+] 500.2075; found 500.2077.
m.p. 71–73 °C. IR (neat): ν = 1734, 1676, 1602, 1500, 1462, 1392,
˜
1264, 1205, 1165, 1110, 1029, 974 cm–1. 1H NMR (400 MHz,
CDCl3): δ = 10.30 (s, 1 H), 7.81 (d, J = 8.7 Hz, 1 H), 7.38–7.34 (m,
5 H), 6.53 (dd, J = 8.7, J = 2.0 Hz, 1 H), 6.50 (d, J = 2.1 Hz, 1
H), 5.16 (s, 2 H), 4.11 (t, J = 6.1 Hz, 2 H), 3.90 (s, 3 H), 2.61 (t, J
= 7.2 Hz, 2 H), 2.19 (m, 2 H) ppm. 13C NMR (100 MHz, CDCl3):
δ = 183.4, 172.8, 165.4, 163.6, 135.8, 130.8, 128.8, 128.3, 128.2,
119.0, 98.9, 98.3, 67.1, 66.4, 55.6, 30.6, 24.4 ppm. HRMS (FAB+):
calcd. for C19H21O5 [MH+] 329.1391; found 329.1389.
Resin-Bonded 4-{3-Methoxy-4-[(prop-2-ynylamino)methyl]phenoxy}-
butanamide (17): Compound 15 (0.502 g, 1 mmol, 1 equiv.) was dis-
solved in CH2Cl2 (5 mL). HOBt (0.202 g, 1.5 mmol, 1 equiv.) and
DIC (0.24 mL, 1.5 mmol, 1.5 equiv.) were added. The resulting
mixture was added to AM-NH2 resin (1.00 g, 0.98 mmolg–1) which
had previously been washed with CH2Cl2 (3ϫ5 mL). The mixture
was stirred at room temperature for 1.5 h. The resin was then fil-
tered and washed with CH2Cl2 and MeOH. The coupling step was
repeated with compound 15 (0.7 mmol). The remaining amine
groups were capped by suspension of the resin in a solution of
Ac2O (2 mL), pyridine (3 mL) and CH2Cl2 (5 mL) for 30 min. The
resin was suspended in 20 vol.-% piperidine/DMF solution
(15 mL). The mixture was stirred at room temperature for 1.5 h.
TheresinwasfilteredandwashedwithDMF,CH2Cl2,MeOHandDMF.
Kaiser test indicated the absence of free amine. Final loading
(0.401 mmolg–1) and yield (51%) of the resin were calculated ac-
cording to the UV-Fmoc test.
Benzyl 4-{3-Methoxy-4-[(prop-2-ynylamino)methyl]phenoxy}-
butanoate (13): The aldehyde linker 12 (2.00 g, 6.1 mmol, 1 equiv.)
was dissolved in THF (60 mL). Na2SO4 (13.9 g, 98 mmol,
16 equiv.) and propargylamine (0.48 mL, 6.8 mmol, 1.1 equiv.) were
added. The reaction mixture was stirred at room temperature over-
night. NaBH(OAc)3 (7.83 g, 37 mmol, 6 equiv.) was added and the
resulting mixture stirred at room temperature overnight. The reac-
tion mixture was quenched with saturated potassium carbonate
solution (100 mL). The aqueous layer was extracted with EtOAc
(2ϫ125 mL). The combined organic layers were washed with brine,
dried with Na2SO4 and concentrated in vacuo. Purification was
carried out by flash column chromatography [silica gel, ethyl ace-
tate/petroleum ether (boiling range 40–65 °C), 1:1] to yield a light
yellow oil (1.81 g, 80%). IR (neat): ν = 3292, 2939, 1733, 1614,
˜
1584, 1506, 1458, 1290, 1257, 1201, 1163, 1094, 1037, 974,
832 cm–1. 1H NMR (400 MHz, CDCl3): δ = 7.38–7.36 (m, 5 H), Benzyl 4-(4-{[{(2S)-2-[(9H-Fluoren-9-ylmethoxycarbonyl)amino]-3-
7.16 (d, J = 8.2 Hz, 1 H), 6.46 (d, J = 2.3 Hz, 1 H), 6.43 (dd, J =
8.2, J = 2.4 Hz, 1 H), 5.16 (s, 2 H), 4.03 (t, J = 6.1 Hz, 2 H), 3.82
methylbutanoyl}(prop-2-ynyl)amino]methyl}-3-methoxyphenoxy)but-
anoate (18a): Fmoc-Val-OH (0.32 g, 0.94 mmol, 1.25 equiv.) was
(s, 5 H), 3.41 (d, J = 2.4 Hz, 2 H), 2.61 (t, J = 7.3 Hz, 2 H), 2.25 dissolved in CH2Cl2 (4 mL). HATU (0.44 g, 1.13 mmol, 1.5 equiv.)
(t, J = 2.4 Hz, 1 H), 2.17–2.13 (m, 2 H), 1.68 (br. s, 1 H) ppm. 13
NMR (100 MHz, CDCl3): δ = 172.8, 159.3, 158.5, 135.8, 130.5,
C
and DIPEA (0.19 mL, 1.32 mmol, 1.5 equiv.) were added, and the
solution was stirred at room temperature for 20 min. The solution
2596
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 2592–2600

1,4-Triazole-Containing Cyclic Tetra- and Pentapeptides
was added to a flask charged with the linker 13 (0.27 g, 0.75 mmol,
1 equiv.) in CH2Cl2 (4 mL). The reaction mixture was stirred at
room temperature overnight. Upon completion, the reaction mix-
ture was diluted with Et2O and washed with water (3ϫ20 mL).
The combined organic layers were dried with Na2SO4, filtered, and
concentrated in vacuo. Flash column chromatography [silica gel,
ethyl acetate/petroleum ether (boiling range 40–65 °C), 1:2] yielded
the desired product as colorless oil (0.50 g, 95%). [α]D = –19.0 (c
13C NMR (100 MHz, CDCl3, mixture of rotamers): δ = 174.9,
173.0, 171.7, 160.3, 159.0, 131.0, 130.4, 129.1, 128.7, 128.6, 128.3,
128.2, 128.0, 127.2, 127.1, 127.0, 126.7, 125.5, 124.4, 124.3, 124.0,
121.0, 119.7, 115.7, 107.8, 104.8, 104.4, 98.9, 71.4, 66.7, 66.3, 60.7,
60.5, 55.3, 45.0, 47.4, 47.3, 45.9, 43.4, 36.8, 33.2, 32.0, 31.4, 31.3,
31.1, 30.8, 26.3, 26.2, 26.2, 26.6, 24.5, 19.8, 17.1 ppm. HRMS
(FAB+): calcd. for C47H52N3O8 [MH+] 786.3756; found 786.3754.
RP-HPLC: tr = 6.68 min (λ = 254 or 220 nm).
= 1.0, CH Cl ). IR (neat): ν = 3299, 2966, 1734, 1646, 1614, 1585,
˜
2
2
Resin-Bonded 4-[4-({[(2S)-2-{[(2S)-1-(9H-Fluoren-9-ylmethoxy-
carbonyl)pyrrolidin-2-ylcarbonyl]amino}-3-methylbutanoyl](prop-2-
ynyl)amino}methyl)-3-methoxyphenoxy]butanamide (19b): Resin 18b
was suspended in 20 vol.-% piperidine/DMF solution (15 mL). The
mixture was stirred at room temperature for 1.5 h. The resin was
filtered and washed with DMF, CH2Cl2, MeOH and DMF. Fmoc-
Pro-OH (0.506 g, 1.5 mmol, 3 equiv.) was dissolved in DMF
(8 mL). HATU (0.855 g, 2.25 mmol, 4.5 equiv.) and DIPEA
(0.4 mL, 2.25 mmol, 4.5 equiv.) were added, and the mixture was
stirred for 30 min. The solution was added to the resin and the
mixture stirred for 3 h. The resin was filtered and washed with
DMF, CH2Cl2 and MeOH. The coupling step was then repeated
to ensure that the reaction was completed. Kaiser test indicated
the absence of free amine. Final loading (0.334 mmolg–1) and yield
(91%) of the resin were calculated according to the UV-Fmoc test.
1506, 1449, 1290, 1208, 1164, 1125, 1095, 1033, 974, 835 cm–1. H
NMR (400 MHz, CDCl3): δ = 7.78 (d, J = 7.5 Hz, 2 H), 7.65–7.60
(m, 2 H), 7.43–7.30 (m, 8 H), 7.15 (d, J = 8.2 Hz, 0.3 H), 7.10 (d,
J = 8.2 Hz, 0.7 H), 6.40–6.36 (m, 2 H), 5.77 (d, J = 8.9 Hz, 0.7 H),
5.60 (d, J = 8.9 Hz, 0.3 H), 5.14 (s, 2 H), 4.99–4.95 (m, 1 H), 4.81–
4.72 (m, 1 H), 4.59–4.55 (m, 1 H), 4.45–4.31 (m, 2 H), 4.25–4.22
(m, 1 H), 3.96 (t, J = 6.0 Hz, 2 H), 3.77 (s, 1 H), 3.72 (s, 3 H),
3.65–3.60 (m, 1 H), 2.57 (t, J = 7.2 Hz, 2 H), 2.13–2.06 (m, 2 H),
1.61 (br. s, 3 H), 1.02 (d, J = 6.7 Hz, 3 H), 0.95 (d, J = 6.6 Hz, 3
H) ppm. 13C NMR (100 MHz, CDCl3, mixture of rotamers): δ =
172.9, 171.6, 160.4, 159.0 (major), 158.8 (minor), 156.3, 144.0,
144.0, 141.3, 141.3, 135.9, 131.2, 130.6, 128.6, 128.3, 128.2, 127.7,
127.1, 125.3, 125.2, 125.2, 120.0, 120.0, 115.5, 104.8, 104.4, 99.1,
98.8, 71.7, 66.9, 66.7, 66.3, 56.0, 55.7, 55.3, 47.2, 47.2, 46.0, 33.2,
32.2, 31.6, 30.8, 30.8, 24.6; 24.6, 19.6, 17.0 ppm. HRMS (FAB+):
calcd. for C42H45N2O7 [MH+] 689.3227; found 689.3226. RP-
HPLC: tr = 6.67 min (λ = 254 or 220 nm).
1
Benzyl 4-[4-({[(2S)-2-{[(2S)-1-{2-Azido-3-[4-(benzyloxy)phenyl]-
propanoyl}pyrrolidin-2-ylcarbonyl]amino}-3-methylbutanoyl](prop-2-
ynyl)amino}methyl)-3-methoxyphenoxy]butanoate (20a): Com-
pound 19a (0.62 g, 0.78 mmol, 1 equiv.) was dissolved in 20 vol.-%
piperidine/DMF solution (3 mL). The mixture was stirred at room
temperature for 1 h. The solvent was evaporated to yield the depro-
tected intermediate which was used without further purification. It
was dissolved in CH2Cl2 (5 mL) and the solution cooled to 0 °C.
N3-Tyr-OH (0.276 g, 0.93 mmol, 1.2 equiv.), HOBt (0.114 g,
0.85 mmol, 1.1 equiv.) and EDCI (0.163 g, 0.85 mmol, 1.1 equiv.)
were added, and the mixture was stirred overnight. The solution
was diluted with Et2O and the organic layer subsequently washed
with water, saturated aqueous sodium hydrogen carbonate solution
and a 1  solution of potassium hydrogen sulfate. The organic layer
was dried with Na2SO4, filtered, and concentrated in vacuo. The
desired product, a light yellow oil (0.407 g, 62%), was obtained by
flash column chromatography [silica gel, CH2Cl2/MeOH, 98:2].
Resin-Bonded 4-(4-{[{(2S)-2-[(9H-Fluoren-9-ylmethoxycarbonyl)-
amino]-3-methylbutanoyl}(prop-2-ynyl)amino]methyl}-3-methoxy-
phenoxy)butanamide (18b): Fmoc-Val-OH (0.611 g, 1.8 mmol,
3 equiv.) was dissolved in DMF (8 mL). HATU (1.05 g, 2.7 mmol,
4.5 equiv.) and DIPEA (0.45 mL, 2.7 mmol, 4.5 equiv.) were added,
and the mixture was stirred for 30 min. The solution was added to
resin 17, and the mixture was stirred for 3 h. The resin was filtered
and washed with DMF, CH2Cl2 and MeOH. The coupling step
was then repeated to ensure that the reaction was completed.
Kaiser test indicated the absence of free amine. Final loading
(0.380 mmolg–1) and yield (98%) of the resin were calculated ac-
cording to the UV-Fmoc test.
Benzyl 4-[4-({[(2S)-2-{[(2S)-1-(9H-Fluoren-9-ylmethoxycarbonyl)-
pyrrolidin-2-ylcarbonyl]amino}-3-methylbutanoyl](prop-2-ynyl)-
amino}methyl)-3-methoxyphenoxy]butanoate (19a): Compound 18a
(0.557 g, 0.81 mmol, 1 equiv.) was dissolved in 20 vol.-% piperidine/
DMF solution (3 mL). The mixture was stirred at room tempera-
ture for 1 h. The solvent was evaporated to yield the deprotected
intermediate which was used without further purification. It was
dissolved in CH2Cl2 (15 mL). Fmoc-Pro-OH (0.412 g, 1.23 mmol,
1.5 equiv.), HATU (0.478 g, 1.23 mmol, 1.5 equiv.) and DIPEA
(0.20 mL, 1.23 mmol, 1.5 equiv.), premixed for 10 min in CH2Cl2
(5 mL), were added, and the mixture was stirred at room tempera-
ture overnight. The solution was diluted with Et2O and the organic
layer subsequently washed with water, saturated aqueous sodium
hydrogen carbonate solution and a 1  solution of potassium hy-
drogen sulfate. The organic layer was dried with Na2SO4, filtered,
and concentrated in vacuo. The desired product (0.612 g, 95%) was
obtained by flash column chromatography [silica gel, CH2Cl2/
MeOH, 98:2] as colorless oil. [α]D = –39.0 (c = 1.0, CH2Cl2). IR
[α]D = –43.5 (c = 1.0, CH Cl /MeOH, 95:5). IR (neat): ν = 2105,
˜
2
2
1734, 1648, 1510, 1242 cm–1. 1H NMR (400 MHz, CDCl3): δ =
7.44–7.37 (m, 10 H), 7.23–7.05 (m, 4 H), 6.98–6.94 (m, 4 H), 6.43
(m, 2 H), 5.16 (s, 2 H), 5.08 (d, J = 5.6 Hz, 3 H), 4.73–4.53 (m, 2
H), 4.01–4.3.91 (m, 4 H), 3.80–3.52 (m, 4 H), 3.43–3.06 (m, 3 H),
2.60–2.58 (m, 3 H), 2.20–2.13 (m, 7 H), 0.99–0.92 (m, 6 H) ppm.
13C NMR (100 MHz, CDCl3, mixture of rotamers): δ = 173.1,
171.6, 171.4, 170.9, 170.8, 169.0, 168.4, 160.5, 159.1, 158.0, 136.0,
130.4, 129.4, 129.0, 128.9, 128.7, 128.4, 128.1, 127.6, 127.4, 127.1,
126.8, 125.6, 121.1, 119.9, 119.7, 115.7, 115.3, 115.2, 105.0, 104.6,
99.1, 79.4, 71.7, 70.1, 66.9, 66.5, 61.9, 61.9, 60.8, 56.9, 55.4, 55.3,
55.0, 54.0, 47.3, 46.0, 43.5, 36.2, 33.2, 32.3, 30.8, 28.7, 25.1, 24.6,
19.7, 17.3 ppm. HRMS (FAB+): calcd. for C48H55N6O8 [MH+]
843.4081; found 843.4072. RP-HPLC: tr = 6.50 min (λ = 254 or
220 nm).
(neat): ν = 3298, 2966, 1733, 1644, 1507, 1449, 1292, 1205, 1164, Resin-Bonded 4-[4-({[(2S)-2-{[(2S)-1-{2-Azido-3-[4-(benzyloxy)phen-
˜
1
1124, 1033, 975 cm–1. H NMR (400 MHz, CDCl3): δ = 7.38–7.69 yl]propanoyl}pyrrolidin-2-ylcarbonyl]amino}-3-methylbutanoyl]-
(m, 5 H), 7.43–7.32 (m, 8 H), 6.73–7.10 (m, 2 H), 6.11–6.37 (m, 2
H), 5.10–5.18 (m, 1 H), 5.15 (s, 2 H), 4.45–4.72 (m, 2 H), 4.25–4.43
(prop-2-ynyl)amino}methyl)-3-methoxyphenoxy]butanamide (20b):
Resin 19b was suspended in 20 vol.-% piperidine/DMF solution
(m, 4 H), 4.12–4.22 (m, 1 H), 4.03–4.11 (m, 1 H), 3.72–3.87 (m, 2 (15 mL). The mixture was stirred at room temperature for 1.5 h.
H), 3.55–3.69 (m, 4 H), 3.15–3.37 (m, 1 H), 2.38–2.50 (m, 2 H), The resin was filtered and washed with DMF, CH2Cl2, MeOH and
1.80–2.17 (m, 6 H), 1.46–1.57 (m, 2 H), 0.72–0.90 (m, 6 H) ppm. DMF. N3-Tyr(OBn)-OH (0.297 g, 1.0 mmol, 2 equiv.) was dis-
Eur. J. Org. Chem. 2008, 2592–2600
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2597

J. H. van Maarseveen et al.
FULL PAPER
solved in CH2Cl2 (8 mL) and cooled to 0 °C. HOBt (0.304 g,
2.25 mmol, 4.5 equiv.) and DIC (0.35 mL, 2.25 mmol, 4.5 equiv.)
were added. The solution was added to the resin and the mixture
stirred at 0 °C for 3 h. The resin was filtered and washed with
CH2Cl2, MeOH and Et2O. The coupling step was then repeated to
ensure that the reaction was completed. Kaiser test indicated the
absence of free amine. Final loading (0.332 mmol g–1) and yield
= 8.18 (s, 1 H), 7.47–7.28 (m, 5 H), 7.14 (d, J = 8.6 Hz, 2 H), 6.94
(d, J = 8.6 Hz, 2 H), 6.76 (m, 1 H), 6.55 (d, J = 9.2 Hz, 1 H), 5.82
(t, J = 7.6 Hz, 1 H), 5.16–5.02 (m, 1 H), 5.01 (s, 2 H), 4.94 (dd, J
= 7.6, J = 15.2 Hz, 1 H), 4.58–4.48 (m, 1 H), 4.31–4.22 (m, 1 H),
4.14 (t, J = 8.0 Hz, 1 H), 3.73–3.61 (m, 1 H), 3.60–3.25 (m, 2 H),
2.21–1.55 (m, 5 H), 0.95 (d, J = 6.8 Hz, 3 H), 0.78 (d, J = 6.4 Hz,
3 H) ppm. 13C NMR (100 MHz, CDCl3): δ = 171.1, 170.7, 167.7,
158.2, 136.8, 130.0, 128.7, 128.6, 128.0, 127.5, 125.4, 115.5, 69.9,
62.8, 61.8, 58.5, 48.0, 37.8, 35.5, 29.7, 29.5, 28.7, 24.8, 19.7,
18.0 ppm. HRMS (FAB+): calcd. for C29H35N6O4 [MH+] 531.2722;
found 531.2723. RP-HPLC: tr = 5.5 min (λ = 254 or 220 nm).
(87 %) of the resin were calculated by weighing. IR (neat): ν =
˜
2104 cm–1.
cyclo-[Pro-Val-N-linker-Gly-Ψ(triazole)-Tyr(OBn)] (21a): Com-
pound 20a (0.084 g, 0.1 mmol, 1 equiv.) was dissolved in toluene
(100 mL). DBU (0.05 mL, 0.3 mmol, 3 equiv.) was added and the
solution degassed with argon for 30 min. The reaction mixture was
heated to reflux while flushing with argon. At reflux, CuBr
(0.003 g, 0.02 mmol, 0.2 equiv.) was added and the reaction mixture
stirred at reflux overnight. The reaction mixture was cooled to
room temperature and filtered through a Celite pad. The Celite pad
was washed with MeOH. The filtrate was concentrated in vacuo to
yield the crude product. Purification was carried out by flash col-
umn chromatography [silica gel, CH2Cl2/MeOH, 95:5] to provide
cyclo-[Pro-Val-Gly-Ψ(triazole)-Tyr] (2): The benzyl-protected cyclic
peptide 23 (0.01 mmol, 1 equiv.) was dissolved in MeOH (0.5 mL)
and CH2Cl2 (4.5 mL). 5% Pd/C (50 wt.-%, 0.003 g) was added and
the resulting mixture subjected to a three-cycle of vacuum and H2
and stirred at room temperature under H2 (balloon) overnight. The
catalyst was removed by filtration through a pad of Celite and the
filtrate concentrated in vacuo to afford the cyclic peptide (0.003 g,
73%) as a white solid. M.p. 188–193 °C. [α]D = –33.0 (c = 0.2,
CH Cl /MeOH, 95:5). IR (neat): ν = 3390, 2925, 2853, 1653, 1560,
˜
2
2
1
1518, 1456, 1263, 1131, 1007, 984, 830 cm–1. H NMR (400 MHz,
CDCl3/MeOD, 955): δ = 8.20 (s, 1 H), 7.68 (br. s, 1 H), 7.13 (br. s,
1 H), 6.97 (d, J = 8.4 Hz, 2 H), 6.71 (d, J = 7.6 Hz, 2 H), 5.77 (t,
J = 7.6 Hz, 1 H), 4.88 (d, J = 15 Hz, 1 H), 4.47 (t, J = 5.6 Hz, 1
H), 4.17 (d, J = 15.6 Hz, 2 H), 4.05 (d, J = 8.0 Hz, 1 H), 3.78–3.30
(m, 4 H), 2.18–1.50 (m, 5 H), 0.93 (d, J = 6.8 Hz, 3 H), 0.79 (d, J
= 6.8 Hz, 3 H) ppm. 13C NMR (100 MHz, MeOD): δ = 173.4,
173.4, 168.6, 157.6, 147.5, 131.2, 127.9, 127.0, 116.5, 65.4, 62.8,
61.0, 38.5, 33.0, 31.1, 27.8, 26.0, 23.7, 19.7, 19.1 ppm. HRMS
(FAB+): calcd. for C22H29N6O4 [MH+] 441.2252; found major peak
at 881.4 (as dimeric artefact).[22]
the desired product as light yellow oil (0.032 g, 40%). IR (neat): ν
˜
= 2924, 1739, 1645, 1510, 1455, 1260, 1022 cm–1 1H NMR
.
(400 MHz, CDCl3): δ = 7.45–7.25 (m, 13 H), 7.01–6.82 (m, 5 H),
6.52–6.49 (m, 1 H), 5.16–5.00 (m, 7 H), 4.12–4.02 (m, 2 H), 3.74–
3.51 (m, 7 H), 3.00–2.96 (m, 2 H), 2.42 (m, 1 H), 2.20–2.01 (m, 2
H), 1.87–1.53 (m, 4 H), 1.43–1.28 (m, 4 H), 0.88–0.76 (m, 6 H)
ppm. HRMS (FAB+): calcd. for C48H55N6O8 [MH+] 843.4083;
found 843.4083. RP-HPLC: tr = 6.01 min (λ = 254 or 220 nm).
(2S)-1-[(2S)-2-Azido-3-(4-benzyloxyphenyl)propanoyl]-N-{(1S)-2-
methyl-1-[(prop-2-ynylamino)carbonyl]propyl}pyrrolidine-2-carbox-
amide (22): Resin 20b (0.124 g) was suspended in TFA (5 mL) and
anisole (0.5 mL). The solution was stirred at room temperature for
2 h. The solvents were filtered off, and the resin was washed with
TFA (5 mL), CH2Cl2 (3ϫ5 mL) and MeOH (3ϫ5 mL). The wash-
ings were collected and concentrated to yield the crude product
(0.023 g, 53%). [α]D = –37.6 (c = 0.66, CH2Cl2/MeOH, 95:5). IR
Benzyl 4-(4-{[{(2S)-2-[(9H-Fluoren-9-ylmethoxycarbonyl)amino]pro-
panoyl](prop-2-ynyl)amino]methyl}-3-methoxyphenoxy)butanoate
(24): Compound 13 (0.551 g, 1.5 mmol, 1 equiv.) was dissolved in
CH2Cl2 (15 mL). Fmoc-Ala-OH (0.584 g, 1.88 mmol, 1.25 equiv.),
HOBT (0.431 g. 2.25 mmol, 1.5 equiv.) and EDCI (0.304 g,
2.25 mmol, 1.5 equiv.) were added to the solution. The reaction
mixture was stirred at room temperature overnight. The reaction
mixture was diluted with EtOAc (30 mL) and the organic layer
washed with water, saturated aqueous sodium hydrogen carbonate
solution and a 1  solution of potassium hydrogen sulfate. The
organic layer was dried with Na2SO4, filtered, and concentrated in
vacuo. Flash column chromatography [silica gel, CH2Cl2/MeOH,
98:2] yielded the pure compound as colorless oil (0.699 g, 70%).
(neat): ν = 3292, 2964, 2106, 1655, 1512, 1451, 1177 cm–1. 1H NMR
˜
(CDCl3): δ = 7.51–7.31 (m, 5 H), 7.17 (d, J = 8.8 Hz, 2 H), 7.01
(d, J = 8.4 Hz, 1 H), 6.94 (d, J = 8.6 Hz, 2 H), 6.01 (t, J = 5.6 Hz,
1 H), 4.61 (m, 1 H), 4.21 (m, 1 H), 4.05 (m, 2 H), 3.93 (m, 1 H),
3.61–3.38 (m, 2 H), 3.26–3.04 (m, 3 H), 2.29–1.91 (m, 6 H), 0.93
(d, J = 6.8 Hz, 3 H), 0.89 (d, J = 6.8 Hz, 3 H) ppm. 13C NMR
(100 MHz, MeOD, mixture of rotamers): δ = 174.0, 173.6, 173.4,
170.9, 155.6, 155.4, 143.0, 142.8, 142.6, 132.5, 132.4, 131.3, 130.7,
129.9, 129.5, 129.3, 128.9, 128.6, 127.6, 126.8, 116.4, 116.2, 72.3,
72.2, 64.9, 63.5, 62.2, 61.6, 60.3, 55.6, 46.2, 38.4, 37.2, 36.6, 35.7,
32.8, 32.3, 30.8, 30.4, 25.9, 23.3, 19.7, 19.0, 18.9 ppm. HRMS
(FAB+): calcd. for C29H35N6O4 [MH+] 531.2722; found 531.2720.
RP-HPLC: tr = 4.78 min (λ = 254 or 220 nm).
[α]D = –13.9 (c = 1.0, CH Cl /MeOH, 95:5). IR (neat): ν = 3299,
˜
2
2
2943, 1732, 1651, 1614, 1507, 1454, 1295, 1256, 1208, 1165, 1032,
1
975, 834 cm–1. H NMR (400 MHz, CDCl3, mixture of rotamers):
δ = 7.80 (d, J = 7.5 Hz, 2 H), 7.65 (d, J = 7.0 Hz, 2 H), 7.44–7.32
(m, 9 H), 7.18 (d, J = 8.2 Hz, 0.3 H), 7.12 (d, J = 7.9 Hz, 0.7 H),
6.43–6.40 (m, 2 H), 5.93 (m, 1 H), 5.16 (s, 2 H), 5.10–5.06 (m, 1
H), 4.74–4.66 (m, 1 H), 4.50–4.46 (m, 2 H), 4.41–4.38 (m, 2 H),
4.27–4.23 (m, 1 H), 4.02–3.98 (m, 2 H), 3.80–3.76 (m, 4 H), 2.60
(t, J = 7.2 Hz, 2 H), 2.25–2.23 (m, 1 H), 2.22–2.11 (m, 2 H), 1.46–
1.41 (m, 3 H) ppm. 13C NMR (100 MHz, CDCl3, mixture of rota-
mers): δ = 172.9, 172.5, 172.4, 160.4, 159.8, 158.9, 158.8, 155.5,
155.4, 144.0, 143.9, 141.3, 135.9, 131.1, 130.3, 128.6, 128.3, 128.2,
127.7, 127.0, 125.2, 120.0, 116.8, 115.3, 104.9, 104.5, 99.1, 98.9,
78.7, 72.8, 71.9, 67.0, 66.8, 66.8, 66.7, 66.3, 55.3, 55.2, 53.8, 47.2,
45.7, 43.8, 36.6, 33.4, 30.8, 30.7, 29.3, 24.6, 24.6, 19.7, 19.4 ppm.
HRMS (FAB+): calcd. for C40H41N2O7 [MH+] 661.2916; found
661.2917. RP-HPLC: tr = 6.46 min (λ = 254 or 220 nm).
cyclo-[Pro-Val-Gly-Ψ(triazole)-Tyr(OBn)] (23): To a solution of 22
(0.039 mmol, 1 equiv.) in toluene (40 mL) was added DBU
(0.12 mmol, 3 equiv.). The solution was degassed with argon for
30 min and then heated to reflux while flushing with argon. At
reflux, CuBr (0.031 mmol, 0.8 equiv.) was added and the solution
stirred at reflux under argon overnight. The mixture was cooled to
room temperature and filtered through a pad of Celite. The Celite
pad was washed with CH2Cl2 (3ϫ30 mL) and the filtrate concen-
trated in vacuo to provide a brown oil. Flash column chromatog-
raphy [silica gel, CH2Cl2/MeOH, 97:3] eluted the product (0.009 g,
43%) as a white amorphous solid. [α]D = –22.9 (c = 0.32, CH2Cl2/
MeOH, 95:5). IR (neat): ν = 3307, 2922, 2851, 1659, 1512, 1451,
Benzyl 4-[4-({[(2S)-2-({[(2S)-3-[1-(tert-Butoxycarbonyl)-1H-indol-3-
yl]-2-[(9H-fluoren-9-ylmethoxycarbonyl)amino]propanoyl}amino)pro-
˜
1
1242, 1179, 1110, 1025, 822 cm–1. H NMR (400 MHz, CDCl3): δ
2598
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© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 2592–2600

1,4-Triazole-Containing Cyclic Tetra- and Pentapeptides
panoyl](prop-2-ynyl)amino}methyl)-3-methoxyphenoxy]butanoate 130.4, 128.6, 128.3, 128.3, 127.7, 127.1, 125.2, 124.6, 122.7, 120.0,
(25): Compound 24 (0.660 g, 1.0 mmol, 1 equiv.) was dissolved in
a 20 vol.-% piperidine/DMF solution (2.5 mL). The mixture was
stirred at room temperature for 2 h. The solvents were evaporated
to yield the deprotected intermediate which was used without fur-
ther purification. It was dissolved in dry CH2Cl2 (10 mL). Fmoc-
Trp(Boc)-OH (0.658 g, 1.25 mmol, 1.25 equiv.), HOBt (0.287 g,
1.5 mmol, 1.5 equiv.) and EDCI (0.203 g, 1.5 mmol, 1.5 equiv.)
were added to the solution. The reaction mixture was stirred at
room temperature overnight. The mixture was diluted with Et2O
(20 mL) and the organic layer washed with water, saturated aque-
ous sodium hydrogen carbonate solution and a 1  solution of po-
tassium hydrogen sulfate. The organic layer was dried with Na2SO4,
filtered, and concentrated in vacuo. Flash column chromatography
[silica gel, CH2Cl2 followed by CH2Cl2/MeOH, 98:2] yielded the
pure compound as colorless oil (0.654 g, 69%). [α]D = –12.9 (c =
119.1, 119.0, 116.7, 115.3, 115.2, 115.1, 104.9, 104.5, 99.1, 98.8,
83.7, 78.7, 72.9, 71.9, 67.1, 66.8, 66.4, 55.4, 55.1, 53.3, 50.7, 47.1,
46.9, 46.0, 45.9, 44.6, 42.8, 39.8, 35.6, 32.4, 29.9, 27.3, 27.0, 25.7,
24.2, 23.9, 23.7, 18.2, 18.0 ppm. HRMS (FAB+): calcd. for
C59H64N5O11 [MH+] 1018.4602; found 1018.4598. RP-HPLC: tr =
6.50 min (λ = 254 or 220 nm).
Benzyl 4-[4-({[(2S)-2-{[(2S)-3-[1-(tert-Butoxycarbonyl)-1H-indol-3-
yl]-2-{[(2S)-2-{[(2R)-2-azido-3-methylbutanoyl]amino}propanoyl]-
amino}propanoyl]amino}propanoyl](prop-2-ynyl)amino}methyl)-3-
methoxyphenoxy]butanoate (27): Compound 26 (0.408 g, 0.4 mmol,
1 equiv.) was dissolved in 20 vol.-% piperidine/DMF solution
(2.0 mL). The mixture was stirred at room temperature for 1 h. The
solvents were evaporated to yield the deprotected intermediate
which was used without further purification. It was dissolved in
CH2Cl2 (7.5 mL). The solution was cooled to 0 °C. N3-Val-OH
(0.072 g, 0.5 mmol, 1.25 equiv.), HOBt (0.115 g, 0.6 mmol,
1.5 equiv.) and EDCI (0.081 g, 0.6 mmol, 1.5 equiv.) were added to
the solution. The reaction mixture was stirred overnight. The mix-
ture was diluted with Et2O and the organic layer washed with
water, saturated aqueous sodium hydrogen carbonate solution and
a 1  solution of potassium hydrogen sulfate. The organic layer was
dried with Na2SO4, filtered, and concentrated in vacuo. Purifica-
tion was carried out by flash column chromatography [silica gel,
CH2Cl2 followed by CH2Cl2/MeOH, 98:2]. The desired compound
was obtained as light yellow oil (0.293 g, 80%). [α]D = –4.6 (c =
1.0, CH Cl /MeOH, 95:5). IR (neat): ν = 3299, 1729, 1639, 1541,
˜
2
2
1506, 1452, 1371, 1260, 1161, 1087, 1038, 831 cm–1 1H NMR
.
(400 MHz, CDCl3): δ = 8.16 (m, 1 H), 7.79 (d, J = 7.5 Hz, 2 H),
7.66–7.56 (m, 4 H), 7.53–7.21 (m, 11 H), 7.09–7.06 (m, 2 H), 6.43–
6.41 (m, 2 H), 5.57–5.55 (m, 1 H), 5.21 (m, 1 H), 5.16–5.15 (m, 2
H), 4.69–4.65 (m, 2 H), 4.49–4.37 (m, 4 H), 4.23–4.22 (m, 1 H),
4.03–3.96 (m, 2 H), 3.77–3.65 (m, 4 H), 3.22–3.18 (m, 2 H), 2.63–
2.56 (m, 2 H), 2.21–2.12 (m, 3 H), 1.67 (m, 9 H), 1.35–1.31 (m, 3
H) ppm. 13C NMR (100 MHz, CDCl3, mixture of rotamers): δ =
173.0, 171.8, 171.6, 169.9, 169.7, 160.4, 159.8, 158.8, 155.9, 149.5,
143.9, 143.7, 141.3, 135.9, 135.5, 130.4, 130.3, 128.6, 128.3, 128.2,
127.7, 127.1, 125.2, 124.6, 124.5, 122.8, 120.0, 119.0, 119.0, 116.6,
115.4, 115.3, 115.1, 104.8, 104.5, 99.1, 98.8, 83.6, 78.6, 72.9, 71.9,
67.3, 66.7, 66.7, 66.4, 66.3, 55.3, 55.1, 47.2, 47.1, 46.0, 45.9, 45.5,
43.7, 36.5, 33.3, 30.8, 28.2, 24.6, 19.2, 18.9 ppm. HRMS (FAB+):
calcd. for C56H59N4O10 [MH+] 947.4231; found 947.4229. RP-
HPLC: tr = 6.92 min (λ = 254 or 220 nm).
0.28, CH Cl /MeOH, 95:5). IR (neat): ν = 3296, 2968, 2099, 1732,
˜
2
2
1508, 1454, 1369, 1256, 1160 cm–1. 1H NMR (CDCl3): δ = 8.11 (m,
1 H), 7.60–7.59 (m, 1 H), 7.49–7.48 (m, 1 H), 7.36–7.16 (m, 9 H),
7.07–7.05 (m, 3 H), 6.41–6.39 (m, 2 H), 5.21 (m, 1 H), 5.15 and
5.13 (s, 2 H), 4.85 (m, 1 H), 4.67–4.37 (m, 3 H), 4.01–3.97 (m, 2
H), 3.76 (s, 1 H), 3.69–3.65 (m, 4 H), 3.23–3.19 (m, 2 H), 2.61–2.58
(m, 2 H), 2.31–2.28 (m, 1 H), 2.21–2.08 (m, 3 H), 1.62–1.54 (m, 9
H), 1.38–1.27 (m, 6 H), 1.03 (d, J = 6.8 Hz, 3 H), 0.89 (d, J =
6.4 Hz, 3 H) ppm. 13C NMR (CDCl3, mixture of rotamers): δ =
173.0, 171.7, 169.4, 168.8, 160.8, 160.4, 159.8, 158.8, 158.7, 149.6,
135.9, 130.9, 130.4, 128.6, 128.3, 128.2, 124.6, 124.5, 122.6, 119.1,
119.0, 116.6, 115.2, 115.2, 115.1, 104.9, 104.5, 99.1, 98.8, 83.6, 78.7,
72.8, 71.9, 69.8, 66.8, 66.3, 55.3, 55.1, 53.3, 48.7, 46.8, 46.0, 45.8,
45.5, 43.6, 40.6, 36.4, 33.31, 28.2, 27.8, 26.6, 25.1, 24.7, 19.6, 19.5,
18.9, 18.5, 16.8 ppm. HRMS (FAB+): calcd. for C49H61N8O10
[MH+] 921.4511; found 921.4509. RP-HPLC: tr = 6.50 min (λ =
254 or 220 nm).
Benzyl 4-[4-({[(2S)-2-({(2S)-3-[1-(tert-Butoxycarbonyl)-1H-indol-3-
yl]-2-({(2S)-2-[(9H-fluoren-9-ylmethoxycarbonyl)amino]propanoyl}-
amino)propanoyl}amino)propanoyl](prop-2-ynyl)amino}methyl)-3-
methoxyphenoxy]butanoate (26): Compound 25 (0.615 g,
0.65 mmol, 1 equiv.) was dissolved in 20 vol.-% piperidine/DMF
solution (2.0 mL). The mixture was stirred at room temperature for
2 h. The solvents were evaporated to yield the deprotected interme-
diate which was used without further purification. It was dissolved
in dry CH2Cl2 (10 mL). Fmoc-Ala-OH (0.253 g, 0.81 mmol,
1.25 equiv.), HOBt (0.132 g, 0.98 mmol, 1.5 equiv.) and EDCI
(0.132 g, 0.98 mmol, 1.5 equiv.) were added to the solution. The
reaction mixture was stirred at room temperature overnight. The
mixture was diluted with Et2O and the organic layer washed with
water, saturated aqueous sodium hydrogen carbonate solution and
a 1  solution of potassium hydrogen sulfate. The organic layer
was dried with Na2SO4, filtered, and concentrated in vacuo. Flash
column chromatography [silica gel, CH2Cl2 followed by CH2Cl2/
MeOH, 98:2] yielded the pure compound as a white foam (0.444 g,
cyclo-[Trp(Boc)-Ala-N-linker-Gly-Ψ(triazole)-Val-Ala] (28): Ligand
(S,S)-bis(4-isopropyl)pybox (0.003 g, 0.01 mmol, 0.4 equiv.) and
CuBr (0.0008 g, 0.005 mmol, 0.2 equiv.) were dissolved in acetoni-
trile (5 mL), and the mixture was stirred at room temperature for
10 min. The linear peptide 27 (0.023 g, 0.025 mmol, 1 equiv.) was
dissolved in acetonitrile (20 mL). DIPEA (0.013 mL, 0.075 mmol,
3 equiv.) and the pybox/CuBr solution were added, and the reac-
tion mixture was stirred at room temperature overnight. After con-
sumption of the starting material, the solvents were evaporated to
67%). [α]D = –17.1 (c = 1.0, CH Cl /MeOH, 95:5). IR (neat): ν =
˜
2
2
3295, 2931, 1729, 1653, 1506, 1450, 1376, 1256, 1208, 1160, yield a crude product. Flash column chromatography [silica gel,
1
1083 cm–1. H NMR (100 MHz, CDCl3): δ = 8.12 (m, 1 H), 7.78 CH2Cl2/MeOH, 95:5] yielded 0.007 g (32%) of the desired product
(d, J = 7.2 Hz, 2 H), 7.63–7.58 (m, 4 H), 7.41–7.28 (m, 8 H), 7.26– as light yellow oil. [α]D = –7.0 (c = 0.75, CH2Cl2/MeOH, 95:5). IR
7.16 (m, 1 H), 7.08–7.05 (m, 2 H), 6.87 (br. s, 1 H), 6.42–6.40 (m,
(neat): ν = 2670, 1731, 1665, 1614, 1150, 1371, 1259, 1159, 1084,
˜
1
2 H), 5.47 (br. s, 1 H), 5.16 (s, 2 H), 4.82 (m, 1 H), 4.48–4.21 (m, 1022 cm–1. H NMR (400 MHz, CDCl3): δ = 8.14 (d, J = 7.9 Hz,
4 H), 4.01–3.99 (m, 3 H), 3.76 (s, 1 H), 3.68 (s, 3 H), 3.31–3.27 (m, 1 H), 7.87 (br. s, 1 H), 7.64 (d, J = 7.7 Hz, 1 H), 7.47–7.10 (m, 9
2 H), 2.62–2.59 (m, 2 H), 2.20–2.14 (m, 3 H), 1.86–1.70 (m, 2 H),
1.66–1.56 (m, 9 H), 1.40 (m, 3 H), 1.29 (br. s, 3 H), 1.29 (d, J =
6.4 Hz, 3 H) ppm. 13C NMR (100 MHz, CDCl3, mixture of rota-
mers): δ = 173.0, 172.2, 171.7, 169.5, 169.3, 160.4, 159.8, 158.9,
158.8, 155.9, 149.6, 143.9, 143.8, 141.3, 135.9, 135.4, 131.0, 130.4,
H), 7.12 (d, J = 8.8 Hz, 1 H), 6.92 (br. s, 1 H), 6.46 (m, 2 H), 5.16
(s, 2 H), 4.98–4.93 (m, 2 H), 4.69 (m, 1 H), 4.15–4.13 (m, 2 H),
4.03 (t, J = 6.1 Hz, 2 H), 3.90 (m, 1 H), 3.78 (s, 3 H), 3.57–3.49
(m, 1 H), 3.32–3.23 (m, 1 H), 2.63–2.59 (m, 3 H), 2.19–2.12 (m, 3
H), 1.66 (m, 9 H), 1.37 (d, J = 6.9 Hz, 3 H), 1.27 (d, J = 7.1 Hz, 5
Eur. J. Org. Chem. 2008, 2592–2600
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
2599

J. H. van Maarseveen et al.
FULL PAPER
H), 1.14 (d, J = 6.6 Hz, 3 H), 0.89 (d, J = 6.5 Hz, 3 H) ppm. 13C
NMR (100 MHz, CDCl3): δ = 173.2, 172.5, 170.5, 167.8, 160.4,
158.9, 149.6, 136.1, 135.7, 130.4, 130.0, 128.8, 128.5, 128.4, 125.0,
124.7, 123.0, 119.2, 116.7, 115.9, 115.6, 104.8, 99.4, 83.9, 71.6, 67.0,
66.6, 57.2, 55.5, 50.8, 47.3, 47.1, 45.7, 40.9, 30.9, 30.7, 29.9, 29.5,
28.4, 26.8, 26.0, 25.3, 24.9, 24.8, 19.4, 19.1, 18.2, 17.0 ppm. HRMS
(FAB+): calcd. for C49H61N8O10 [MH+] 921.4511; found 921.4512.
RP-HPLC: tr = 6.20 min (λ = 254 or 220 nm).
Dombrowski, J. D. Polishook, D. M. Schmatz, Proc. Natl.
Acad. Sci. USA 1996, 93, 13143–13147; b) W. Kuriyama, T.
Kitahara, Heterocycles 2001, 55, 1–4.
[5] a) K. D. Kopple, J. Pharm. Sci. 1972, 61, 1345–1356; b) J. S.
Davies, J. Pept. Sci. 2003, 9, 471–501.
[6] U. Schmidt, J. Langner, J. Pept. Res. 1997, 49, 67–73.
[7] a) V. V. Rostovtsev, L. G. Green, V. V. Fokin, K. B. Sharpless,
Angew. Chem. Int. Ed. 2002, 41, 2596–2599; b) C. W. Tornøe,
C. Christensen, M. Meldal, J. Org. Chem. 2002, 67, 3057–3064;
c) for a recent review, see: V. D. Bock, H. Hiemstra, J. H.
van Maarseveen, Eur. J. Org. Chem. 2006, 51–68.
[8] H. Kawagishi, A. Somoto, J. Kuranari, A. Kimura, S. Chiba,
Tetrahedron Lett. 1993, 34, 3439–3440.
[9] a) V. D. Bock, R. Perciaccante, T. P. Jansen, H. Hiemstra, J. H.
van Maarseveen, Org. Lett. 2006, 8, 919–922; b) V. D. Bock, D.
Speijer, H. Hiemstra, J. H. van Maarseveen, Org. Biomol.
Chem. 2007, 5, 971–975.
[10] a) W. S. Horne, C. D. Stout, M. R. Ghadiri, J. Am. Chem. Soc.
2003, 125, 9372–9376; b) for a review, see: H. C. Kolb, K. B.
Sharpless, Drug Discovery Today 2003, 8, 1128–1137; c) W. S.
Horne, M. K. Yadav, C. D. Stout, M. R. Ghadiri, J. Am. Chem.
Soc. 2004, 126, 15366–15367; d) J. H. van Maarseveen, W. S.
Horne, M. R. Ghadiri, Org. Lett. 2005, 7, 4503–4506.
[11] R. B. Merrifield, J. Am. Chem. Soc. 1963, 85, 2149–2154.
[12] D. A. Horton, G. T. Bourne, M. L. Smythe, Mol. Diversity
2000, 5, 289–304.
cyclo-[Trp-Ala-Gly-Ψ(triazole)-Val-Ala] (4): The cyclic pentapep-
tide 28 (0.017 g, 0.018 mmol) was dissolved in TFA (2.5 mL) and
anisole (0.25 mL). The reaction mixture was stirred at room tem-
perature overnight. The solvents were evaporated. Cold Et2O was
added to the residue and the precipitate filtered off. The desired
product was obtained as a white solid (0.006 g, 65%). [α]D = –5.9
(c = 0.31, CH Cl /MeOH, 95:5). IR (neat): ν = 3283, 1668, 1532,
˜
2
2
1
1454, 1203, 1138, 1056 cm–1. H NMR (400 MHz, [D5]pyridine): δ
= 11.90 (s, 1 H), 9.75 (d, J = 7.6 Hz, 1 H), 9.53 (d, J = 7.2 Hz, 1
H), 9.32 (d, J = 7.6 Hz, 1 H), 9.15 (m, 1 H), 8.66 (s, 1 H), 7.94 (d,
J = 8.0 Hz, 1 H), 7.54 (d, J = 8.1 Hz, 1 H), 7.41 (d, J = 2.1 Hz, 1
H), 7.25 (m, 1 H), 7.09 (t, J = 7.4 Hz, 1 H), 5.48 (AX part of ABX
system, JAB = 15.1, JAX = 8.0, JBX = 3.3 Hz, 1 H), 5.27 (d, J =
10.9 Hz, 1 H), 4.86 (m, 1 H), 4.64 (t, J = 7.4 Hz, 1 H), 4.53 (AB
part of ABX system, JAB = 15.1, JAX = 8.0, JBX = 3.3 Hz, 1 H),
[13] a) C. G. Boojamra, K. M. Burow, J. A. Ellman, J. Org. Chem.
1995, 60, 5742–5743; b) M. F. Songster, J. Vagner, G. Barany,
Lett. Pept. Sci. 1996, 2, 265–270; c) D. Sarantakis, J. J. Bicksler,
Tetrahedron Lett. 1997, 38, 7325–7328; d) K. J. Jensen, J. Al-
sina, M. Songster, J. Vagner, F. Albericio, G. Barany, J. Am.
Chem. Soc. 1998, 120, 5441–5452; e) J. Alsina, K. J. Jensen, F.
Albericio, G. Barany, Chem. Eur. J. 1999, 5, 2787–2795; f) G. T.
Bourne, W. D. F. Meutermans, P. F. Alewood, R. P. McGeary,
M. Scanlon, A. A. Watson, M. L. Smythe, J. Org. Chem. 1999,
64, 3095–3101.
4.37 (t, J = 7.0 Hz, 1 H), 3.82 (AB part of ABX system, JAB
=
13.9, JAX = 7.2, JBX = 7.1 Hz, 2 H), 2.95 (m, 1 H), 1.73–1.71 (m,
6 H), 1.67 (d, J = 6.6 Hz, 3 H), 0.78 (d, J = 6.6 Hz, 3 H) ppm. 13C
NMR (100 MHz, [D5]pyridine): δ = 173.3, 172.5, 172.4, 168.9,
148.3, 137.8, 128.7, 125.1, 122.3, 122.2, 119.7, 112.3, 111.2, 72.5,
57.1, 53.5, 51.1, 36.1, 30.6, 29.2, 19.9, 19.3, 18.2, 16.9 ppm. HRMS
(FAB+): calcd. for C25H33N8O4 [MH+] 509.2627; found 509.2628.
Acknowledgments
[14] a) R. E. Austin, C. A. Waldraff, F. Al-Obeidi, Tetrahedron Lett.
2002, 43, 3555–3556; b) J. Beythien, P. D. White, Tetrahedron
Lett. 2005, 46, 101–104.
The Council for Chemical Sciences of the Netherlands Organiza-
tion for Scientific Research (NWO-CW) is kindly acknowledged
for the financial support of this project. Also we thank NV Orga-
non, part of Schering-Plough, for the financial support.
[15] a) H. Morita, Y. S. Yun, K. Takeya, H. Itokawa, K. Yamada,
Tetrahedron 1995, 51, 6003–6014; b) P. Sonnet, S. da Nasci-
mento, D. Marty, N. Franceschini, J. Guillon, J. D. Brion, J.
Rochette, Tetrahedron Lett. 2003, 44, 3293–3296.
[16] S. Isomura, P. Wirsching, K. D. Janda, J. Org. Chem. 2001, 66,
[1] a) H. Kessler, Angew. Chem. Int. Ed. Engl. 1982, 21, 511–523;
b) J. N. Lambert, J. P. Mitchell, K. D. Roberts, J. Chem. Soc.
Perkin Trans. 1 2001, 1, 471–484; c) P. Li, P. P. Roller, J. Xu,
Curr. Org. Chem. 2002, 6, 411–440; d) M. Katsara, T. Tselios,
S. Deraos, G. Deraos, M. T. Matsoukas, E. Lazoura, J. Mat-
soukas, V. Apostolopoulos, Curr. Med. Chem. 2006, 13, 2221–
2232.
4115–4121.
[17] G. B. Fields, Z. Tian, G. Barany in Synthetic Peptides – A
User’s Guide (Ed.: G. A. Grant), W. H. Freeman, New York,
1992, pp. 77–183.
[18] For the preperation of azido acids from amino acids, see: J. T.
Lundquist, J. C. Pelletier, Org. Lett. 2001, 3, 781–783.
[19] J. Meng, V. V. Fokin, M. G. Finn, Tetrahedron Lett. 2005, 46,
4543–4546.
[2] R. J. Consden, A. H. Gordon, A. J. P. Martin, R. D. M. Synge,
Biochem. J. 1947, 41, 596–602.
[20] H. Kessler, Angew. Chem. Int. Ed. Engl. 1982, 21, 511–523.
[3] a) W. L. Meyer, G. E. Templeton, C. I. Grable, C. W. Sigel, R. [21] A. Perczel, M. Hollosi, B. M. Foxman, G. D. Fasman, J. Am.
Jones, S. H. Woodhead, C. Sauer, Tetrahedron Lett. 1971, 25,
2357–2360; b) D. H. Rich, P. Mathiaparanam, Tetrahedron
Lett. 1974, 46, 4037–4040.
Chem. Soc. 1991, 113, 9772–9784.
[22] A similar effect was observed by: S. A. Dietrich, L. Banfi, A.
Basso, G. Damonte, G. Guanti, R. Riva, Org. Biomol. Chem.
2005, 3, 97–106.
[4] a) S. J. Darkin-Rattray, A. M. Gurnett, R. W. Myers, P. M.
Dulski, T. M. Crumley, J. J. Allocco, C. Cannova, P. T. Meinke,
S. L. Colletti, M. A. Bednarek, S. B. Singh, M. A. Goetz, A. W.
Received: February 5, 2008
Published Online: April 4, 2008
2600
www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 2592–2600