Conformational behavior of symmetrical and unsymmetrical mono(alkynylpeptide)-tungsten complexes

Article abstract of DOI:10.1016/j.jorganchem.2012.03.021

A series of N-protected amino acid alkynylesters were prepared by reaction of the amino acid carboxylate group with either propargyl alcohol (to yield the asymmetric esters 2a-c) or with 1, 4-but-2-yne diol (to yield the symmetric esters 3a-d). The alkynylesters were reacted with W(CO)₃(dmtc) ₂ to yield monoalkyne complexes having the general formula W(CO)(dmtc)₂(alkynyl ester). The monoalkyne complexes 6a-f were unstable in the presence of oxygen and had to be kept under an inert atmosphere. Analysis of the NMR spectra of the monoalkyne complexes showed that two diastereomers were formed in the synthesis, and that there was rapid rotation of the alkyne about the tungsten center at 23 °C and above with both diastereomers. At lower temperatures alkyne rotation is significantly slowed. Symmetric alkynylesters of a dipeptide (4) and tripeptide (5) were also prepared and reacted with W(CO)₃(dmtc)₂ to yield monoalkyne complexes. The resulting complexes (6g and 6h) also formed two diastereomers and displayed rapid rotation of the alkyne about the tungsten center at 23 °C and above, and slow rotation at lower temperatures. The amide NH protons in 6g and 6h were probed by DMSO titration to see if they were involved in intramolecular hydrogen bonds; they were not, which indicates that the peptide portions of 6g and 6h adopt an extended conformation in solution.

Full text of DOI:10.1016/j.jorganchem.2012.03.021

Journal of Organometallic Chemistry 711 (2012) 15e24
Contents lists available at SciVerse ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Conformational behavior of symmetrical and unsymmetrical
mono(alkynylpeptide)-tungsten complexes
*
Timothy P. Curran , Whitney E. Smith, Peter C. Hendrickson
Department of Chemistry, Trinity College, Hartford, CT 06106-3100, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of N-protected amino acid alkynylesters were prepared by reaction of the amino acid carboxylate
group with either propargyl alcohol (to yield the asymmetric esters 2aec) or with 1, 4-but-2-yne diol (to
yield the symmetric esters 3aed). The alkynylesters were reacted with W(CO)3(dmtc)2 to yield mono-
alkyne complexes having the general formula W(CO)(dmtc)2(alkynyl ester). The monoalkyne complexes
6aef were unstable in the presence of oxygen and had to be kept under an inert atmosphere. Analysis of
the NMR spectra of the monoalkyne complexes showed that two diastereomers were formed in the
synthesis, and that there was rapid rotation of the alkyne about the tungsten center at 23 ꢀC and above
with both diastereomers. At lower temperatures alkyne rotation is significantly slowed. Symmetric
alkynylesters of a dipeptide (4) and tripeptide (5) were also prepared and reacted with W(CO)3(dmtc)2 to
yield monoalkyne complexes. The resulting complexes (6g and 6h) also formed two diastereomers and
displayed rapid rotation of the alkyne about the tungsten center at 23 ꢀC and above, and slow rotation at
lower temperatures. The amide NH protons in 6g and 6h were probed by DMSO titration to see if they
were involved in intramolecular hydrogen bonds; they were not, which indicates that the peptide
portions of 6g and 6h adopt an extended conformation in solution.
Received 13 July 2011
Received in revised form
6 March 2012
Accepted 9 March 2012
Keywords:
Tungsten
Alkynes
Peptide
Conformation
Bioorganometallic
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
Our initial studies with these bis(alkynylpeptide) complexes
have shown that the alkynylpeptide ligands can and do assume
In recent work from our laboratory we have been exploring the
possibility that tungsten-bis(alkyne) complexes [1] can be used to
constrain peptides to specific three-dimensional conformations
[2e4]. Specifically, we have discovered that alkynes can be readily
appended to peptides at either their N-terminus, C-terminus or
side chain, and that the resulting alkynylpeptides can be reacted
with W(CO)3(dmtc)2 (dmtc ¼ N,N-dimethyldithiocarbamate) [5] to
yield tungsten-bis(alkynylpeptide) complexes [2]. We have also
discovered that peptides bearing two alkynes, one at the N-
terminus and the other at the C-terminus, will also react with
W(CO)3(dmtc)2 to yield cyclic tungsten-bis(alkynylpeptide)
complexes [3,4]. We have termed these cyclic complexes metal-
lacyclicpeptides, since the metal atom is part of the macrocyclic
ring structure. Analysis of a metallacyclictripeptide and a metal-
lacyclictetrapeptide by variable temperature NMR spectroscopy
indicated the presence of intramolecular hydrogen bonds in the
peptide, which is consistent with the conclusion that the peptides
in these complexes assume turn conformations [4].
a variety of orientations around the tungsten, and that, at room
temperature, these different conformational isomers are slow to
interconvert. The relatively large number of different conforma-
tional isomers is manifested in the NMR spectra of these
complexes; the result is that the 1H NMR spectra of these species
present an overlay of all the possible isomers. Thus, each proton in
one of these complexes can and does appear as multiple signals; in
the more crowded parts of the spectrum, there is severe overlap
of the different signals. It is this overlap of signals that makes
conformational analysis of the peptide using NMR methods
difficult.
Our initial work has focused on tungsten-bis(alkynylpeptide)
complexes [2e4]. An unexplored area is the behavior of
tungsten-mono(alkynylpeptide) complexes. In the present work
we have prepared and examined the dynamic behavior of
tungsten-mono(alkynylpeptide) complexes [1]. The alkynylpep-
tides used here are symmetric and asymmetric. In the asym-
metric species one peptide is linked to one end of a terminal
alkyne, while in the symmetric species two identical peptides
are covalently attached to both ends of one alkyne. The advan-
tage of examining the mono(alkynylpeptide) complexes is that
* Corresponding author. Tel.: þ1 860 297 5276.
E-mail address: timothy.curran@trincoll.edu (T.P. Curran).
0022-328X/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2012.03.021

16
T.P. Curran et al. / Journal of Organometallic Chemistry 711 (2012) 15e24
Scheme 1.
these species should have fewer conformational isomers open to
them than the bis(alkynylpeptide) complexes; in addition, the
use of a symmetrically substituted alkynylpeptide as the ligand
prevents formation of conformational isomers resulting from
different alkyne orientations. It was hypothesized that the
coordination of the alkyne in these species might force the two
peptides on either end of the alkyne into close proximity, leading
to the adoption of intramolecular hydrogen bonds between the
two peptides. Peptide derivatives of 1,4-diamino-2-butyne, very
similar to the peptide derivatives of 1,4-dihydroxy-2-butyne
2.2. Synthesis of symmetrical alkynylpeptides
A symmetrical dipeptide (4) and a symmetrical tripeptide (5)
having alkynylesters were also prepared. The synthetic route used
to prepare 4 and 5 is shown in Scheme 3. Reaction of 3d with tri-
fluoroacetic acid in CH2Cl2 removed the Boc protecting group and
generated the N-terminal amine as its trifluoroacetate salt. Subse-
quent reaction of this salt with the succinimidyl ester Boc-Ala-OSu
under basic conditions yielded dipeptide 4. Similarly, reaction of 4
under the same, two step reaction procedure yielded the tripeptide
5. Both 4 and 5 were purified by flash chromatography and
characterized by MS and 1H NMR spectroscopy.
examined here, adopt
a C2-symmetric turn conformation
where the two peptides on either side of the alkyne fold
back to form intramolecular hydrogen bonds [6]. If the
symmetrical tungsten complexes under investigation here do
generate similar intramolecular hydrogen, the resulting struc-
2.3. Preparation of tungsten-mono(alkynylamino acid) complexes
ture would resemble a
these studies.
b-sheet. This paper reports the results of
The tungsten-mono(alkyne) complexes 6aeh were prepared by
adaptation of literature procedures [7,8]. One equivalent of the
alkynylamino acid or alkynylpeptide (2aec, 3aec, 4, or 5) was
reacted with one equivalent of W(CO)3(dmtc)2 in CH2Cl2, under N2,
for 0.5e1 h (Scheme 4). During the course of the reaction the
solution went from an orange to a green color. Following evapo-
ration of the solvents, the crude product was purified using flash
chromatography. Purity was assessed using HPLC. The complexes
were characterized using MS and 1H NMR spectroscopy. The puri-
fied tungsten-mono(alkyne) complexes would decompose over
time if left exposed to the air, so they were stored under N2. All of
the complexes studied here were amorphous solids; repeated
attempts to induce crystal formation did not produce crystalline
solid.
The ESMS spectra of the monoalkyne complexes 6aeh display
a unique pattern of peaks that arises primarily from the four major
isotopes of tungsten. This unique isotope pattern is proof for the
successful preparation of the desired complex [2e4]. In addition,
these complexes also undergo fragmentation in the mass spec-
trometer with loss of CO. Accordingly along with the M þ Na ion,
one also finds the (MeCO) þ Na ion.
2. Results and discussion
2.1. Synthesis of alkynylamino acids
To introduce the alkyne an amino acid with its amine group
acylated with either a carbobenzyloxy (Cbz) or t-butyloxycarbonyl
(Boc) protecting group was reacted with the coupling reagent
dicyclohexylcarbodiimide (DCC), the acylation catalyst 4-
dimethylaminopyridine (DMAP) and an alkynylalcohol. Thus, 1
equivalent of the amino acid derivative (1aec) was reacted with 1
equivalent of DCC and 1 equivalent of propargyl alcohol to yield the
alkynylesters 2aec (Scheme 1). Compounds 2aec were character-
ized by MS and 1H NMR spectroscopy.
Similarly, the corresponding symmetrical alkynylesters 3aed
were prepared by reacting 2 equivalents of 1aed with two equiv-
alents of DCC, a catalytic amount of DMAP and one equivalent of
1,4-dihydroxy-2-butyne (Scheme 2). Compounds 3aed were
characterized by MS and 1H NMR spectroscopy.
Scheme 2.

T.P. Curran et al. / Journal of Organometallic Chemistry 711 (2012) 15e24
17
Scheme 3.
2.4. Dynamic behavior of 6aec
likely to be very similar, a nearly equal mixture of 6a1 and 6a2
should be obtained when 2a is reacted with W(CO)3(dmtc)2.
Because 6a1 and 6a2 are diastereomers, each species will have
its own unique NMR spectrum. Thus, the NMR spectrum of 6a
should be an overlay of the spectrum of 6a1 and the spectrum of
6a2. Given the close structural similarity between 6a1 and 6a2, large
differences in chemical shifts between resonances in 6a1 and 6a2
are unlikely. So, the overlay of the spectra of 6a1 and 6a2 could yield
complete overlap of similar resonances, or very close overlap of
similar resonances.
Another source of isomerism in 6a is rotation of the alkyne
about the tungsten [8]. Thus 6a1 can also adopt conformation 6a3.
Similarly, 6a2 can adopt the conformation 6a4.
In prior work by Ward and Templeton on the monoalkyne
acetylene complex it was found that rotation of the alkyne ligand is
rapid at 23 ꢀC [8]. If rotation is also rapid for 6aec, then one would
Before analyzing the conformational behavior of the tungsten-
mono(alkynylpeptide) complexes that were made, it is important
to consider the possible conformational isomers of these species.
First, reaction of 2aec with W(CO)3(dmtc)2 will generate 6aec as
a mixture of two diastereomers. Consider for example the forma-
tion of 6a. When complex 6a is formed there are two possible ways
to arrange the dmtc ligands, as shown in structures 6a1 and 6a2
(Fig. 1). These two possible orientations for the dmtc ligands make
the tungsten a stereocenter. 6a1 and 6a2 also possess a second
stereocenter located at Ca on the alanine residue. 6a1 and 6a2
superimpose at the alanine stereocenter, but do not superimpose at
the tungsten stereocenter. This means that 6a1 and 6a2 are dia-
stereomers, since they are not mirror images and are not super-
imposable. Since the steric interactions for either diastereomer are
Scheme 4.

18
T.P. Curran et al. / Journal of Organometallic Chemistry 711 (2012) 15e24
More significantly, the alkyne hydrogen resonance appears as
a single peak. This behavior differs markedly from the behavior of
the corresponding bis alkyne complexes, which produce multiple
singlets for each alkyne hydrogen [2e4]. The appearance of a single
peak for the alkyne hydrogen indicates that the chemical shifts for
this proton in each diastereromer are the same. The single peak also
suggests that rotation about the alkyne-tungsten bond is relatively
facile at 23 ꢀC, which is in accord with previous work in this area [8].
Another resonance in the spectra of 6aec that is of interest is the
urethane NH proton. Typically, this proton in the NMR spectrum of
an amino acid will appear as a doublet. In compounds 6aec this
proton appears as two doublets that have very similar chemical
shifts. The urethane NH for 6c can be seen in Fig. 2A at 5.4 ppm.
That the urethane NH shows up as two doublets indicates that
there are two isomeric forms of 6aec in solution. This indicates that
rotation of the alkyne ligand is relatively fast at 23 ꢀC.
Fig. 1. The four possible isomers for complex 6a.
Finally, another resonance that provides a window onto the
solution conformation of 6aec is the methylene adjacent to the
alkyne carbon. Because 6aec possess a stereocenter in the amino
acid portion of the molecule, these methylenes are prochiral.
Hence, each proton in the methylene will have its own unique
chemical shift in the NMR spectrum. This usually means that for
each diastereomer the methylene protons would appear as two
doublets; for two diastereomers, four doublets. Indeed, the NMR
spectra of 6aec show that this methylene appears as four doublets.
The resonances for the methylene in 6c are shown in Fig. 2A at
6.1 ppm. This shows that the two diastereomers of 6aec are present
and it indicates that rotation of their alkyne ligands is relatively fast.
Overall, the appearance of the alkyne hydrogen, the urethane
NH, and the methylene adjacent to the alkyne shows that both of
the expected diastereomers in 6aec are present. Since only two
sets of peaks for each resonance were observed, interconversion
between the rotational isomers is likely to be very rapid at 23 ꢀC. To
confirm this, samples of 6aec were examined by 1H NMR over
a range of elevated temperatures. In order to access higher
temperatures d6-DMSO was used as the solvent. As the
expect the NMR spectrum to consist of the overlay of the spectrum
of each diastereomer. For 6a this would mean the averaging of the
spectra of 6a1 and 6a3, and the averaging of the spectra of 6a2 and
6a4. The result would be the appearance of two separate signals for
each unique proton in 6a. In contrast, if rotation of the alkyne
ligands is not rapid at 23 ꢀC, then the NMR spectrum of 6a will
consist of the overlay of the individual spectra of 6a1, 6a2, 6a3 and
6a4. This overlay could produce four separate signals for each
unique proton in 6a.
The conformational behavior of monoalkyne complexes 6aec
was examined using 1H NMR spectroscopy. The complexes 6aec
were studied first in CDCl3 because this solvent is not an aggres-
sive hydrogen bond donor or acceptor, so it should have minimal
interaction with 6aec. The spectra of 6aec in CDCl3 all show
a resonance for the alkyne hydrogen located between 12.8 and
13.0 ppm. The alkyne hydrogen undergoes a significant chemical
shift change when it is complexed to the tungsten, leading to a very
large movement downfield. This chemical shift has been noted in
other tungsten monoalkyne complexes [7,8] and is not remarkable.
Fig. 2. The NMR spectra of (A) 6c and (B) 6f in the region between 4 and 8 ppm. The resonances visible here are, from left to right, the phenyl protons (7.3 ppm), the methylene
adjacent to the alkyne (6.4e6.0 ppm in A; 6.2e5.8 ppm in B), the urethane NH (5.4e5.2 ppm in A; 5.7e5.4 ppm in B), the Cbz methylene (5.1 ppm), and the CaH proton (4.5 ppm).

T.P. Curran et al. / Journal of Organometallic Chemistry 711 (2012) 15e24
19
Fig. 3. Portions of the 1H NMR spectrum of 6a at temperatures ranging from 23 ꢀC to ꢁ27 ꢀC. (A) The resonances derived from the alkyne hydrogen in 6a. (B) The resonances derived
from the methylene adjacent to the alkyne in 6a. As the temperature is lowered these resonances broaden due to slowing of rotation of the alkyne about the tungsten center.
temperature of the sample was raised, no significant change in the
appearance of the spectrum was observed. Since no change was
observed, it can be concluded that at 23 ꢀC and higher the rotational
isomers are rapidly equilibrating, as illustrated in Fig. 1 for 6a.
To confirm this conclusion, the behavior of 6a at lower
temperatures was examined using CDCl3 as the solvent. If rotation
is rapid at 23 ꢀC, then lowering the temperature is likely to slow
down that rotation, and this should be evident in the NMR spec-
trum. Shown in Fig. 3 are the resonances for the alkyne hydrogen
(Fig. 3A) and the methylene adjacent to the alkyne (Fig. 3B) for 6a.
As the temperature of the sample was lowered from 23 ꢀC
to ꢁ27 ꢀC, the signals for the alkyne hydrogen and the methylene
broaden significantly. This broadening can be attributed to slowing
of the rotation of the alkyne about the tungsten center. This
confirms that the alkyne ligand in these complexes is rapidly
rotating at 23 ꢀC.
The alkyne hydrogen resonance in 6a was not the only one to
broaden as the temperature was lowered. The resonance for the
methylene adjacent to the alkyne broadened as the temperature
was lowered to ꢁ27 ꢀC. Because this methylene is in close prox-
imity to the alkyne, it is not surprising that these resonances are
affected by a slowing of alkyne rotation. Protons further removed
from the alkyne, for example, the resonances for the urethane NH,
CaH and the methylene adjacent to the lysine side chain nitrogen
stayed sharp even down to ꢁ27 ꢀC. That these resonances remained
sharp is further evidence for slowing of alkyne rotation as the
temperature is lowered.
Overall, the NMR data supports the conclusion that for 6aec
there are two diastereomers present in solution. For each diaste-
reomer there are two rotational isomers. At 23 ꢀC both rotational
isomers are present, and interconversion between the two is rapid
on the NMR time scale, indicating that the barrier to interconver-
sion is relatively small, consistent with past findings [8]. Knowing
how 6aec behave sets the stage for understanding the dynamic
behavior of the symmetrical complexes 6def.
2.5. Dynamic behavior of 6def
Similar to 6aec, the syntheses of 6def produces two diaste-
reomers. The two diastereomers for 6d are shown in Fig. 4; 6d1 and
6d2 differ in the arrangement of the two dmtc ligands around the
tungsten. Because the alkynylpeptide ligand is symmetrical there
are no rotational isomers possible for 6d1 and 6d2. However, the
two halves of the alkynylpeptides in 6def are not situated in
identical environments, so they are not NMR equivalent. However,
due to signal averaging they would appear identical in the NMR
spectrum if the alkyne ligand is rapidly rotating about the tungsten.
Because 6def comes as a mixture of two diastereomers, if there is
rapid rotation at 23 ꢀC, then the NMR spectrum would show two
resonances for each unique proton in the alkynylpeptide, one
Fig. 4. The two possible isomers for complex 6d.

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T.P. Curran et al. / Journal of Organometallic Chemistry 711 (2012) 15e24
Fig. 5. The NMR spectrum of 6f at 23 ꢀC and lower between 6.5 and 4.2 ppm. This region shows resonances for the methylene adjacent to the alkyne (6.1 and 5. 9 ppm), the
urethane NH proton (5.6 and 5.5 ppm), the Cbz methylene (5.1 ppm) and the CaH proton (4.5 ppm). As the temperature is lowered, the methylene adjacent to the alkyne broadens,
indicating that rotation of the alkyne about the tungsten is slowing.
derived from each diastereomer. In contrast, if the alkyne ligand in
6def is slow to rotate at 23 ꢀC, then the two halves of the alky-
nylpeptide would not be equivalent, and the NMR spectrum should
show four resonances for each unique proton.
6g and 6h, and the methylene appears as four doublets. This
behavior indicates that 6g and 6h, like 6def, exist as two diaste-
reomers in solution, and that rotation about the tungstenealkyne
bond is rapid at room temperature.
The 1H NMR spectra of 6def show only two resonances for each
proton at 23 ꢀC. For example, the NH protons in 6def appear as two
closely spaced doublets. Likewise, the methylenes adjacent to the
alkyne appear as four doublets. This can be seen in Fig. 2B, which
shows the methylene adjacent to the alkyne (5.9 ppm) and the
urethane NH (5.5 ppm) for 6f. In fact, comparison of the spectra of
the unsymmetrical alkyne complexes 6aec with their corre-
sponding symmetrical alkyne relatives 6def shows that they are
very similar; the only significant difference is the presence of the
alkyne hydrogen in 6aec and its absence in 6def. The similarity of
the spectra is illustrated in Fig. 2, which shows the 4e8 ppm region
of the 1H NMR spectra of 6c and 6f.
To determine if intramolecular hydrogen bonds were present in
6geh, these species were subjected to a DMSO titration [6,9e13].
Unlike chloroform, DMSO is an aggressive hydrogen bond acceptor.
Accordingly, the chemical shifts of the NH protons in a molecule
will differ greatly between chloroform and DMSO if they interact
with the solvent. If, however, an NH proton in a molecule is
Like the spectra of 6aec, the NMR spectra of 6def show that
rotation about the tungsten-alkyne bond is fast on the NMR time-
scale, indicating that there is a low energetic barrier to this inter-
conversion [8]. This conclusion was supported by the behavior of
6def in variable temperature NMR experiments. Heating of 6def
in d6-DMSO to various elevated temperatures did not lead to any
significant changes in the NMR spectra. And cooling of 6f in CDCl3
down to ꢁ53 ꢀC caused broadening of the methylene adjacent to
the alkyne and the urethane NH (Fig. 5), indicating that the alkyne
ligand is no longer rapidly rotating about the tungsten.
2.6. Dynamic behavior of 6geh
Compounds 6def have one amino acid residue bonded to each
end of the alkyne moiety. In order to determine if intramolecular
hydrogen bonds could form between the two peptide chains
bonded to the alkyne, a dipeptide complexes (6g) and a tripeptide
complex (6h) were prepared and studied. The NMR spectra of 6g
and 6h were similar to the spectra of 6def, except for the appear-
ance of extra resonances arising from the additional amino acid
residues. In particular, the resonances for the urethane NH protons
and the methylene adjacent to the alkyne had nearly identical
chemical shifts. Also, the urethane NH appears as two doublets in
Fig. 6. Illustration of the extended conformation adopted by complex 6h. Only one of
the two diastereomers is shown.

T.P. Curran et al. / Journal of Organometallic Chemistry 711 (2012) 15e24
21
protected from the solvent by being involved in an intramolecular
hydrogen bond, then the chemical shift of that proton will not
change much in going from chloroform to DMSO. All of the NH
protons in 6g and 6h showed large chemical shift changes as the
solvent composition was changed from CDCl3 to d6-DMSO. This
indicates that none of the NH protons in 6g and 6h are involved in
intramolecular hydrogen bonds.
(CH2eC^C, 2H, m), 4.3 ppm (CaH, 1H, m), 3.2 ppm (CH2eN, 2H, m),
2.5 ppm (C^CeH, 1H, s), 1.8e1.2 ppm (3 CH2, 6H, m), 1.4 ppm (3
CH3, 9H, s); ESMS: M þ Na ion calculated for C22H30N2O6Na, 441 m/
z; found, 441 m/z.
3.3. Preparation of Cbz-Ala-OCH2CCH, 2b
To a solution of 605 mL (10.2 mmol, 1.0 equiv.) propargyl alcohol
2.7. Conclusions
and 2.279 g (10.2 mmol, 1.0 equiv.) Cbz-Ala-OH (1b) in 30 mL
CH2Cl2 and 10 mL THF at 0 ꢀC was added 2.109 g (10.2 mmol, 1.0
equiv.) DCC and 0.154 g (1.02 mmol, 0.1 equiv.) DMAP. The resulting
solution slowly warmed to 25 ꢀC over the course of 3 h. A white
precipitate formed during this time. The reaction solution was
chilled in an ice bath for 5 min, after which the solid was removed
by filtration. The filtrate was evaporated and the residue resus-
pended in 20 mL EtOAc. After being chilled in an ice bath for 5 min,
the insoluble white solid was removed by filtration. Again, the
filtrate was evaporated to yield a crude solid. Recrystallization of
this solid from hexanes produced 1.574 g (59%) of pure 2b: TLC, Rf
The monoalkyne complexes (6aeh) prepared and studied here
all exhibit fast rotation of the alkyne about the tungsten center at
23 ꢀC [8]. With the symmetrical complexes (6deh) the data shows
that they adopt an extended conformation (shown below in Fig. 6)
in which the two peptide chains extend away from each other; the
two peptides are not held together by intramolecular hydrogen
bonds.
3. Experimental section
0.48 (1:1 EtOAc/hexanes); 1H NMR (CDCl3):
d 7.3 ppm (C6H5, 5H,
3.1. General procedures
m), 5.3 ppm (NH, 1H, d, J ¼ 6.4 Hz), 5.1 ppm (PheCH2, 2H, s),
4.7 ppm (CH2eC^C, 2H, m), 4.4 ppm (Ala CaH, 1H, m), 2.5 ppm
(C^CeH, 1H, m), 1.4 ppm (Ala CH3, 3H, d, J ¼ 7.3 Hz); ESMS: M þ Na
ion calculated for C14H15NO4Na, 284 m/z; found, 284 m/z.
Amino acid derivatives were purchased from ChemImpex
International. Propargyl alcohol, 1, 4-dihydroxy-2-butyne, anisole,
and DMF (dimethylformamide) were purchased from
SigmaeAldrich. TFA, DIEA, and THF were purchased from Acros
Organics. CDCl3, d6-acetone and d6-DMSO were purchased from
Cambridge Isotope Labs. W(CO)3(dmtc)2 was prepared according to
a literature procedure [5] starting from W(CO)6, which was
obtained from Strem Chemical. Silica gel for flash chromatography
was purchased from Silicycle. NMR spectra were obtained on a GE
Omega 300 MHz instrument or a Bruker Avance III 400 MHz
instrument. Electrospray mass spectra were obtained on a LCQ
APCI/Electrospray LC MSeMS. Samples for mass spectral analysis
were dissolved in MeOH (approximately 1 mg/mL) in borosilicate
glass test tubes. Theoretical mass spectral isotope patterns were
calculated using the Sheffield Chemputer [14]. HPLC analyses were
performed on a Hitachi Elite LaChrom HPLC system equipped with
L-2400 detector, an L-2200 autosampler and an L-2130 pump. Each
run was monitored primarily at 410 nm, which is the wavelength of
maximum absorbance for the monoalkyne complexes. A Vydac C18
Peptide and Protein column was used as the stationary phase. The
mobile phase involved a linear gradient program using two
solvents, 0.1% trifluoroacetic acid and acetonitrile. The gradient
program started at a 50:50 mixture of the two solvents and
changed to 100% acetonitrile over the course of 12 min. The solvent
was then held at 100% acetonitrile for an additional 8 min.
3.4. Preparation of Cbz-Ile-OCH2CCH, 2c
To a solution of 605
mL (10.2 mmol, 1.0 equiv.) of propargyl
alcohol and 2.717 g (10.2 mmol, 1.0 equiv.) of Cbz-Ile-OH (1c) in
30 mL CH2Cl2 and 10 mL THF at 0 ꢀC was added 2.119 g (10.2 mmol,
1.0 equiv.) DCC and 0.154 g (1.02 mmol, 0.1 equiv.) DMAP. The
resulting solution slowly warmed to 25 ꢀC over the course of 3 h. A
white precipitate formed during this time. The reaction solution
was chilled in an ice bath for 5 min, after which the solid was
removed by filtration. The filtrate was evaporated and the residue
resuspended in 20 mL EtOAc. After being chilled in an ice bath for
5 min, the insoluble white solid was removed by filtration. Again,
the filtrate was evaporated to yield a crude solid. The residue was
purified using flash chromatography (1:1 EtOAc/hexanes) to yield
2.256 g (73%) of pure 2c as a clear, colorless oil: TLC, Rf 0.79 (1:1
EtOAc/hexanes); 1H NMR (CDCl3)
d 7.3 ppm (C6H5, 5H, m), 5.3 ppm
(NH, 1H, d, J ¼ 8.3 Hz), 5.1 ppm (OeCH2, 2H, m), 4.7 ppm
(CH2eC^C, 2H, m), 4.4 ppm (CaH, 1H, m), 2.5 ppm (C^CeH, 1H, s),
1.9 ppm (CbH, 1H, m), 1.3 ppm (Ile CH2, 1H, m), 1.1 ppm (Ile CH2, 1H,
m), 0.9 ppm (Ile 2 CH3, 6H, m); ESMS: M þ Na ion calculated for
C17H21NO4Na, 326 m/z; found, 326 m/z.
3.5. Preparation of (Boc-Lys(Cbz)-OCH2C)2, 3a
3.2. Preparation of Boc-Lys(Cbz)-OCH2CCH, 2a
To a solution of 0.283 g (3.29 mmol,1.0 equiv.) of 1, 4-dihydroxy-
2-butyne and 2.410 g (6.33 mmol, 2.0 equiv.) Boc-Lys(Cbz)-OH (1a)
in 30 mL CH2Cl2 and 10 mL THF at 0 ꢀC was added 1.323 g
(6.41 mmol, 2.0 equiv.) DCC and 0.049 g (0.40 mmol, 0.1 equiv.)
DMAP. The resulting solution slowly warmed to 25 ꢀC over the
course of 3 h. A white precipitate formed during this time. The
reaction solution was chilled in an ice bath for 5 min, after which
the solid was removed by filtration. The filtrate was evaporated and
the residue resuspended in 20 mL EtOAc. After being chilled in an
ice bath for 5 min, the insoluble white solid was removed by
filtration. Again, the filtrate was evaporated to yield a crude solid.
The crude product was purified using flash chromatography (1:1
EtOAc/hexanes) to yield 2.25 g (86%) of pure 3a as a clear, colorless
oil: TLC (1:1 EtOAc/hexanes): Rf 0.39; 1H NMR (300 MHz, CDCl3):
To a solution of 405
mL (6.81 mmol, 1.0 equiv.) of propargyl
alcohol and 2.593 g (6.81 mmol, 1.0 equiv.) of Boc-Lys(Cbz)-OH (1a)
in 30 mL CH2Cl2 and 10 mL THF at 0 ꢀC was added 1.410 g
(6.81 mmol, 1.0 equiv.) DCC and 0.106 g (0.68 mmol, 0.1 equiv.)
DMAP. The resulting solution slowly warmed to 25 ꢀC over the
course of 3 h. A white precipitate formed during this time. The
reaction solution was chilled in an ice bath for 5 min, after which
the solid was removed by filtration. The filtrate was evaporated and
the residue resuspended in 20 mL EtOAc. After being chilled in an
ice bath for 5 min, the insoluble white solid was removed by
filtration. Again, the filtrate was evaporated to yield a crude oil. The
residue was purified using flash chromatography (1:1 EtOAc/
hexanes) to yield 2.68 g (94%) of pure 2a as clear, colorless oil: TLC,
Rf 0.87 (1:1 EtOAc/hexanes); 1H NMR (CDCl3)
m), 5.3 ppm (NH, 1H, m), 5.1 ppm (OeCH2 and NH, 3H, m), 4.7 ppm
d
7.3 ppm (C6H5, 5H,
d
7.3 ppm (Cbz C6H5, 10H, m), 5.2 ppm (Cbz CH2, Cbz NH, Lys NH,
8H, m), 4.7 ppm (CH2eCCeCH2, 4H, 2 d, J ¼ 15 Hz), 4.3 ppm (CaH,

22
T.P. Curran et al. / Journal of Organometallic Chemistry 711 (2012) 15e24
2H, m), 3.2 ppm (CH2eNHeCbz, 4H, m), 1.8 ppm (CbHeCH2, 6H),
1.4 ppm (Boc CH3, CbHeCH2eCH2, 22H, m); ESMS: M þ Na ion
calculated for C42H58N4O12Na, 833 m/z; found, 833 m/z.
4H, m), 0.9 ppm (Ile CH3, 12H, m); ESMS, M þ Na ion calculated for
C26H44N2O8Na, 535 m/z; found, 535 m/z.
3.9. Preparation of (Boc-Ala-Ile-OCH2C)2, 4
3.6. Preparation of (Cbz-Ala-OCH2C)2, 3b
To a solution of 1.200 g (2.34 mmol, 1.0 equiv.) of 3d in 5 mL
CH2Cl2 and 1.2 mL anisole at 0 ꢀC was added 5 mL TFA. After stirring
for 1 h at 25 ꢀC, the solvents were evaporated; the residue that
remained had a light pink color. After redissolving the residue in
10 mL CH2Cl2, 1.389 g (4.69 mmol, 2.0 equiv.) of Boc-Ala-OSu and
12 mL of DIEA was added. After stirring for 3e4 h, the solvents were
evaporated. The crude product was redissolved in 25 mL CH2Cl2.
The organic layer was washed: 3 ꢂ 50 mL 0.1 M HCl, 3 ꢂ 50 mL
NaHCO3, and 1 ꢂ 50 mL NaCl. The organic layer was then dried with
MgSO4, filtered, and evaporated. The residue was purified using
flash chromatography (1:2 EtOAc/hexanes) to yield 852 mg (56%) of
To a solution of 0.407 g (4.728 mmol, 1.0 equiv.) 1, 4-dihydroxy-
2-butyne and 2.091 g (9.367 mmol, 2.0 equiv.) Cbz-Ala-OH (1b) in
30 mL CH2Cl2 and 10 mL THF was added 1.946 g (9.431 mmol, 2.0
equiv.) DCC. The resulting solution slowly warmed to 25 ꢀC over the
course of 3 h. A white precipitate formed during this time. The
reaction solution was chilled in an ice bath for 5 min, after which
the solid was removed by filtration. The filtrate was evaporated and
the residue resuspended in 20 mL EtOAc. After being chilled in an
ice bath for 5 min, the insoluble white solid was removed by
filtration. Again, the filtrate was evaporated to yield a crude solid.
The remaining residue was purified using flash chromatography
(1:1 EtOAc/hexanes) to yield 664 mg (28%) of pure 3b as a white
solid: TLC (1:1 EtOAc/hexanes): Rf 0.62; 1H NMR (300 MHz, CDCl3):
pure 4: TLC, Rf 0.26 (1:1 EtOAc/hexanes); 1H NMR (CDCl3)
(Ile NH, 2H, m), 4.9 ppm (Ala NH, 2H, d, J ¼ 5.7 Hz), 4.7 ppm
(CH2eC^CeCH2, 4H, 2 d, J ¼ 15.0 Hz), 4.6 ppm (Ile C H, 2H, m),
4.2 ppm (Ala C H, 2H, m), 1.8 ppm (Ile CbH, 2H, m), 1.5 ppm (Boc
d 6.7 ppm
a
d
7.2 ppm (2Ph, 10H, m), 5.3 ppm (NH, 2H, d, J ¼ 6.3 Hz), 5.1 ppm
a
(Cbz CH2, 4H, s), 4.7 ppm (CH2eC^CeCH2, 4H, s), 4.4 ppm (CaH, 2H,
m), 1.4 ppm (Ala CH3, 6H, d); ESMS, M þ Na ion calculated for
C32H40N2O8Na, 519 m/z; found, 519 m/z.
CH3, Ala CH3, 24H, m), 1.2 ppm (CbHeCH2, 4H, m), 0.9 ppm (Ile CH3,
12H, m); ESMS, M þ Na ion calculated for C32H54N4O10Na, 677 m/z;
found, 677 m/z.
3.7. Preparation of (Cbz-Ile-OCH2C)2, 3c
3.10. Preparation of (Boc-Val-Ala-Ile-OCH2C)2, 5
To a solution of 0.515 g (5.98 mmol, 1.0 equiv.) of 1, 4-dihydroxy-
2-butyne and 3.058 g (11.54 mmol, 2.0 equiv.) Cbz-Ile-OH (1c) in
30 mL CH2Cl2 and 10 mL THF at 0 ꢀC was added 2.412 g (11.69 mmol,
2.0 equiv.) DCC and 0.090 g (0.60 mmol, 0.1 equiv.) DMAP. The
resulting solution slowly warmed to 25 ꢀC over the course of 3 h. A
white precipitate formed during this time. The reaction solution
was chilled in an ice bath for 5 min, after which the solid was
removed by filtration. The filtrate was evaporated and the residue
resuspended in 20 mL EtOAc. After being chilled in an ice bath for
5 min, the insoluble white solid was removed by filtration. Again,
the filtrate was evaporated to yield a crude solid. The residue was
purified using flash chromatography (1:1 EtOAc/hexanes) to yield
3.05 g (92%) of pure 3c as a white solid: m.p. 84.5e86.0 ꢀC; TLC, Rf
To a solution of 398 mg (0.608 mmol, 1.0 equiv.) of 4 in 1 mL
CH2Cl2 and 265 m
L anisole (2.43 mmol, 4.0 equiv.) at 0 ꢀC was added
1 mL TFA. After stirring for 1 h at 25 ꢀC, the solvents were evapo-
rated. The crude product was redissolved in 2 mL CH2Cl2, then
treated with 385 mg (1.22 mmol, 2.0 equiv.) of Boc-Val-OSu and
2.5 mL of DIEA. After stirring for 4 h, the solvents were evaporated.
The crude product was redissolved in 5 mL CH2Cl2. The organic
layer was washed 3 ꢂ 10 mL 0.1 M HCl, 3 ꢂ 10 mL NaHCO3, and
1 ꢂ 10 mL NaCl. The organic layer was then dried with MgSO4,
filtered, and evaporated to yield 292 mg (56%) of pure 5 as an
amorphous white solid: 1H NMR (d6-DMSO)
d 8.2 ppm (Ile NH or
Ala NH, 2H, d, J ¼ 7.3 Hz), 8.0 ppm (Ile NH or Ala NH, 2H, d,
0.40 (1:1 EtOAc/hexanes); 1H NMR (300 MHz, CDCl3):
d 7.3 ppm (2
J ¼ 6.8 Hz), 6.7 ppm (Val NH, 2H, d, J ¼ 8.8 Hz), 4.8 ppm
(CH2eC^CeCH2, 4H, m), 4.4 ppm (C
a
H, 2H, t, J ¼ 6.8 Hz), 4.2 ppm
Ph, 10H, m), 5.3 ppm (NH, 2H, d, J ¼ 6.4 Hz), 5.1 ppm (Cbz CH2, 4H,
s), 4.7 ppm (CH2eC^CeCH2, 4H, m), 4.4 ppm (CaH, 2H, m), 1.9 ppm
(CbH, 2H, m), 1.4 ppm (Ile CH2, 2H, m), 1.2 ppm (Ile CH2, 2H, m),
0.9 ppm (Ile 4 CH3, 12 H, m); ESMS, M þ Na ion calculated for
C32H40N2O8Na, 603 m/z; found, 603 m/z.
(C
a
H, 2H, t, J ¼ 6.5 Hz), 3.8 ppm (C
aH, 2H, t, 7.5), 1.9 ppm (CbH, 2H,
q, J ¼ 7.0), 1.7 ppm (CbH, 2H, m), 1.5 ppm (Boc CH3, 18H, s), 1.2 ppm
(Ile CbHeCH2, 4H, m), 0.9 ppm (Ile, Ala and Val CH3, 30H, m); ESMS,
M þ Na ion calculated for C42H72N6O12Na, 875 m/z; found, 875 m/z.
3.8. Preparation of (Boc-Ile-OCH2C)2, 3d
3.11. Preparation of W(CO)(dmtc)2(Boc-Lys(Cbz)-OCH2CCH), 6a
To a solution of 0.513 g (5.96 mmol, 1.0 equiv.) of 1, 4-dihydroxy-
2-butyne and 2.704 g (11.7 mmol, 2.0 equiv.) of Boc-Ile-OH (1d) in
30 mL CH2Cl2 and 10 mL THF at 0 ꢀC was added 2.406 g (11.7 mmol,
2.0 equiv.) of DCC and 0.092 g (0.61 mmol, 0.1 equiv.) of DMAP. The
resulting solution slowly warmed to 25 ꢀC over the course of 3 h. A
white precipitate formed during this time. The reaction solution
was chilled in an ice bath for 5 min, after which the solid was
removed by filtration. The filtrate was evaporated and the residue
resuspended in 20 mL EtOAc. After being chilled in an ice bath for
5 min, the insoluble white solid was removed by filtration. Again,
the filtrate was evaporated to yield a crude solid. The residue was
purified using flash chromatography (1:3 EtOAc/hexanes) to yield
2.152 g (73%) of pure 3d as a clear, colorless liquid: TLC, Rf 0.30 (1:3
To a solution of 42 mg (0.083 mmol, 1.0 equiv.) W(dmtc)2(CO)3
in 10 mL degassed CH2Cl2 under a N2 atmosphere at 23 ꢀC was
added 35 mg (0.083 mmol, 1.0 equiv.) 2a. The solution color
changed from orange to green after 30 min of stirring. The solvents
were evaporated. The remaining residue was purified using flash
chromatography (1:1 EtOAc/hexanes) to yield 37 mg (51%) of pure
6a as an amorphous green solid: TLC, Rf 0.39 (1:1 EtOAc/hexanes);
HPLC, Rt 12.40 min; 1H NMR (CDCl3)
d 12.8 ppm (C^CeH, 1H, s),
7.3 ppm (Ph, 5H, m), 6.2 ppm (CH2eC^C, 2H, 4d, J ¼ 17.8 Hz),
5.2 ppm (PheCH2 and NH, 3H, m), 4.8 ppm (NH, 1H, m), 4.5 ppm
(CaH, 1H, m), 3.2 ppm (NeCH3, CH2eN, 14H, m), 1.9 ppm (CbHeCH2,
2H, m), 1.9, 1.5 ppm (CbHeCH2eCH2, Boc CH3, 14H, m); ESMS,
M þ Na ion pattern calculated for WC29H42N4O7S4Na: 891 m/z
(62.0), 892 (56.3), 893 (100), 894 (39.9), 895 (89.5), 896 (31.5), 897
(19.8), 898 (5.8); found: 891 m/z (74.8), 892 (69.7), 893 (100), 894
(34.8), 895 (56.4), 896 (39.0), 897 (21.7), 898 (10.3).
EtOAc/hexanes); 1H NMR (CDCl3)
4.8 ppm (CH2eC^CeCH2, 4H, m), 4.3 ppm (C
1.8 ppm (CbH, 2H, m), 1.5 ppm (Boc CH3, 18H, s), 1.2 ppm (CbHeCH2,
d
5.1 ppm (NH, 2H, d, J ¼ 9.3 Hz),
a
H, 2H, q, J ¼ 4.9 Hz),

T.P. Curran et al. / Journal of Organometallic Chemistry 711 (2012) 15e24
23
3.12. Preparation of W(CO)(dmtc)2(Cbz-Ala-OCH2CCH), 6b
6.0 ppm (CH2eC^CeCH2, 4H, 4d, J ¼ 16.6 Hz), 5.5 ppm (NH, 2H, m),
5.1 ppm (Cbz CH2, 4H, m), 4.5 ppm (C H, 2H, m), 3.2 ppm (NeCH3,
a
To a solution of 42 mg (0.083 mmol, 1.0 equiv.) W(dmtc)2(CO)3
in 10 mL degassed CH2Cl2 under a N2 atmosphere at 23 ꢀC was
added 21 mg (0.083 mmol, 1.0 equiv.) 2b. The solution changed
from orange to green after 30 min of stirring. The solvent was
evaporated, and the remaining residue was purified using flash
chromatography (1:1 EtOAc/hexanes) to yield 46 mg (78%) of pure
6b as an amorphous green solid: TLC, Rf 0.41 (1:1 EtOAc/hexanes);
12H, m), 1.5 ppm (Ala CH3, 6H, m); ESMS, M þ Na ion pattern
calculated for WC33H40N4O9S4Na: 969 (60.3), 970 (57.3), 971 (100),
972 (43.4), 973 (89.2), 974 (34.6), 975 (21.0); found: 969 (78.1), 970
(38.8), 971 (100), 972 (20.8), 973 (65.2), 974 (22.9), 975 (10.7).
3.16. Preparation of W(CO)(dmtc)2((Cbz-Ile-OCH2C)2), 6f
HPLC, Rt 7.75 min (410 nm); 1H NMR (CDCl3)
d
12.8 ppm (C^CeH,
To a solution of 42 mg (0.083 mmol, 1.0 equiv.) W(dmtc)2(CO)3
in 10 mL degassed CH2Cl2 under a N2 atmosphere at 23 ꢀC was
added 48 mg (0.083 mmol, 1.0 equiv.) of 3c. The solution changed
from orange to green after 30 min of stirring. The solvents were
evaporated, and the remaining residue was purified using flash
chromatography (1:1 EtOAc/hexanes) to yield 60 mg (71%) of pure
6f as an amorphous green solid: TLC, Rf 0.51 (1:1 EtOAc/hexanes);
1H, s), 7.3 ppm (Ph, 5H, m), 6.2 ppm (CH2eC^CeCH2, 2H, 4d,
J ¼ 17.1 Hz), 5.3 ppm (NH, 1H, m), 5.1 ppm (PheCH2, 2H, m),
4.5 ppm (CaH, 1H, m), 3.2 ppm (NeCH3, 12H, m), 1.5 ppm (Ala CH3,
3H, m); ESMS,
M
þ
Na ion pattern calculated for
WC21H27N3O5S4Na: 734 (65.4), 735 (53.2), 736 (100), 737 (32.4),
738 (90.6), 739 (24.5), 740 (17.8); found: 734 (63.5), 735 (62.0), 736
(100), 737 (71.2), 738 (85.2), 739 (70.3), 740 (35.0).
HPLC, Rt 10.95 min (410 nm); 1H NMR (CDCl3)
m), 6.0 ppm (CH2eC^CeCH2, 4H, 4d, J ¼ 16.0 Hz), 5.6 ppm (NH, 2H,
H, 2H, m),
d 7.3 ppm (2 Ph, 10H,
3.13. Preparation of W(CO)(dmtc)2(Cbz-Ile-OCH2CCH), 6c
2d, J ¼ 9.0 Hz), 5.1 ppm (PheCH2, 4H, m), 4.5 ppm (C
a
3.2 ppm (4 NeCH3, 12H, m), 2.0 ppm (CbH, 2H, m), 1.4 ppm (Ile CH2,
2H, m), 1.2 ppm (Ile CH2, 2H, m), 0.9 ppm (Ile 2 CH3, 12H, m); ESMS,
M þ Na ion pattern calculated for WC39H52N4O9S4Na: 1053 (58.0),
1054 (59.0), 1055 (100), 1056 (48.2), 1057 (88.8), 1058 (39.1), 1059
(22.5); found: 1053 (68.2), 1054 (42.2),1055 (100),1056 (45.6),1057
(92.1), 1058 (31.1), 1059 (20.6).
To a solution of 42 mg (0.083 mmol, 1.0 equiv.) W(dmtc)2(CO)3
in 10 mL degassed CH2Cl2 under a N2 atmosphere at 23 ꢀC was
added 35 mg (0.115 mmol, 1.4 equiv.) 2c. The solution changed from
orange to green after 30 min of stirring. The solvents were evapo-
rated, and the remaining residue was purified using flash chro-
matography (3:2 EtOAc/hexanes) to yield 16 mg (25%) of pure 6c.
TLC, Rf 0.46 (1:1 EtOAc/hexanes); HPLC, Rt 8.97 min (410 nm); 1H
3.17. Preparation of W(CO)(dmtc)2((Boc-Ala-Ile-OCH2C)2), 6g
NMR (CDCl3)
6.0 ppm (CH2eC^C, 2H, 4d, J ¼ 16.4 Hz), 5.6 ppm (NH, H, m),
5.1 ppm (PheCH2, 2H, m), 4.5 ppm (C H, 1H, m), 3.2 ppm (NeCH3,
12H, m), 2.0 ppm (CbH, 1H, m), 1.4 ppm (Ile CH2, 1H, m), 1.2 ppm (Ile
CH2, 1H, m), 0.9 ppm (Ile 2 CH3, 6H, m); ESMS, M þ ion pattern
calculated for WC24H33N3O5S4Na: 776 (64.2), 777 (54.5), 778 (100),
779 (35.1), 780 (90.1), 781 (27.0), 782 (18.4); found: 776 (81.9), 777
(56.8), 778 (100), 779 (43.2), 780 (64.1), 781 (28.2), 782 (20.1).
d 12.8 ppm (C^CeH, 1H, s), 7.3 ppm (Ph, 5H, m),
To a stirred solution of 43 mg (0.083 mmol, 1.0 equiv.) of
W(dmtc)2(CO)3 in 10 mL degassed CH2Cl2 under a N2 atmosphere at
23 ꢀC was added 55 mg (0.083 mmol, 1.0 equiv.) of 4. The solution
changed from orange to green after 30 min of stirring. The solvents
were evaporated, and the remaining residue was purified using
flash chromatography (1:1 EtOAc/hexanes) to yield 63 mg (68%) of
pure 6g as an amorphous green solid: TLC, Rf 0.19 (1:1 EtOAc/
a
hexanes); HPLC, Rt 9.3 min (410 nm); 1H NMR (CDCl3)
NH, 2H, m), 6.0 ppm (CH2eC^CeCH2, 4H, 4d, J ¼ 16.6 Hz), 5.2 ppm
(Ala NH, 2H, m), 4.7 ppm (Ile C H, 2H, m), 4.2 ppm (Ala C H, 2H, m),
d 6.8 ppm (Ile
3.14. Preparation of W(CO)(dmtc)2((Boc-Lys(Cbz)-OCH2C)2), 6d
a
a
To a solution of 42 mg (0.083 mmol,1.0 equiv.) W(dmtc)2(CO)3 in
10 mL degassed CH2Cl2 under a N2 atmosphere at 23 ꢀC was added
67 mg (0.083 mmol, 1.0 equiv.) 3a. The solution changed from
orange to green after 30 min of stirring. The solvents were evapo-
rated, and the remaining residue was purified using flash chroma-
tography (1:1 EtOAc/hexanes) to yield 57 mg (54%) of pure 6d as an
amorphous green solid: TLC, Rf 0.35 (1:1 EtOAc/hexanes); HPLC, Rt
3.3 ppm (4 NeCH3, 12H, m), 2.0 ppm (Ile CbH, 2H, m), 1.5 ppm (Boc
CH3, Ala CH3, 24H, m), 1.2 ppm (Ile CbHeCH2, 4H, m), 0.9 ppm (Ile 2
CH3, 12H, m); ESMS,
M
þ
Na ion pattern calculated for
WC39H66N6O11S4Na: 1127 (57.5), 1128 (59.1), 1129 (100), 1130
(49.0), 1131 (89.0), 1132 (39.8), 1133 (23.1); found: 1127 (26.5), 1128
(54.4), 1129 (100), 1130 (54.1), 1131 (71.0), 1132 (27.8), 1133 (8.9).
11.7 min (410 nm); 1H NMR (CDCl3):
6.0 ppm (CH2eC^CeCH2, 4H, 4d, J ¼ 16.6 Hz), 5.3 ppm (NH, 2H, m),
5.1 ppm (Cbz CH2 and NH, 6H, m), 4.5 ppm (C H, 2H, m), 3.2 ppm
d
7.3 ppm (2 Ph, 10H, m),
3.18. Preparation of W(CO)(dmtc)2((Boc-Val-Ala-Ile-OCH2C)2), 6h
a
To a stirred solution of 28 mg (0.055 mmol, 1.0 equiv.) of
W(dmtc)2(CO)3 in 10 mL degassed CH2Cl2 under a N2 atmosphere at
23 ꢀC was added 47 mg (0.055 mmol, 1.0 equiv.) of 5. The solution
changed from orange to green after 60 min of stirring. The solvents
were evaporated, and the remaining residue was purified using
flash chromatography (1:1 EtOAc/hexanes) to yield 32 mg (44%) of
pure 6h as an amorphous green solid: TLC, Rf 0.32 (2:1 EtOAc/
(NeCH3, CH2eN, 16H, m), 1.9 ppm (CbHeCH2, 2H, m), 1.7 ppm
(CbHeCH2, 2H, m), 1.4 ppm (Boc CH3, CbHeCH2eCH2, 26H, m);
ESMS, M þ Na ion pattern calculated for WC49H70N6O13S4Na: 1283
(53.8), 1284 (61.1), 1285 (100), 1286 (56.5), 1287 (89.1), 1288 (46.7),
1289 (26.6),1290 (10.3); found: 1283 (49.5),1284 (86.1),1285 (100),
1286 (47.4), 1287 (78.7), 1288 (47.4), 1289 (25.1), 1290 (21.4).
hexanes); HPLC, Rt 10.1 min (410 nm); 1H NMR (CDCl3)
(Ile NH and Ala NH, 4H, m), 6.0 ppm (CH2eC^CeCH2, 4H, 4d,
J ¼ 16.6 Hz), 5.5 ppm (Val NH, 2H, d, J ¼ 8.3 Hz), 4.6 ppm (Ile C
and Ala C H, 4H, m), 4.0 ppm (Val C H, 2H, m), 3.5 ppm (4 NeCH3,
d 7.1 ppm
3.15. Preparation of W(CO)(dmtc)2((Cbz-Ala-OCH2C)2), 6e
a
H
To a solution of 42 mg (0.083 mmol, 1.0 equiv.) W(dmtc)2(CO)3
in 10 mL degassed CH2Cl2 under a N2 atmosphere at 23 ꢀC was
added 41 mg (0.083 mmol, 1.0 equiv.) of 3b. The solution changed
from orange to green after 30 min of stirring. The solvents were
evaporated, and the remaining residue was purified using flash
chromatography (1:1 EtOAc/hexanes) to yield 65 mg (83%) of pure
6e as an amorphous green solid: TLC, Rf 0.37 (1:1 EtOAc/hexanes);
a
a
12H, m), 2.1 ppm (Ile CbH or Val CbH, 2H, q, J ¼ 6.5 Hz), 1.9 ppm (Ile
CbH or Val CbH, 2H, m), 1.5 ppm (Boc CH3, 18H, m), 1.2 ppm (Ile
CbHeCH2, 4H, m), 0.9 ppm (Ile CH3, Ala CH3, Val CH3, 30H, m);
ESMS, M þ Na ion pattern calculated for WC49H84N8O13S4Na: 1325
(53.4), 1326 (61.3), 1327 (100), 1328 (57.1), 1329 (89.2), 1330 (47.2),
1331 (26.9), 1332 (10.5); found: 1325 (84.3), 1326 (79.5), 1327 (100),
1328 (58.6), 1329 (23.2), 1330 (40.8), 1331 (14.5), 1332 (11.1).
HPLC, Rt 8.1 min (410 nm); 1H NMR (CDCl3)
d 7.3 ppm (Ph, 10H, m),

24
T.P. Curran et al. / Journal of Organometallic Chemistry 711 (2012) 15e24
[3] T.P. Curran, R.S.H. Yoon, B.R. Volk, J. Organomet. Chem. 689 (2004) 4837.
Acknowledgements
[4] T.P. Curran, A.B. Lesser, R.S.H. Yoon, J. Organomet. Chem. 692 (2007)
1243.
This work was supported by a grant from the National Science
Foundation (NSF-RUI CHE-0305325), and from a Student Research
Assistant Grant from Trinity College. The Bruker Avance III NMR
was obtained with a National Science Foundation MRI grant (NSF-
MRI CHE-0619275), and the final experiments were performed in
a laboratory that was renovated with a National Science Foundation
ARI grant (NSF-ARI CHE-0963165). We thank David Henderson
(Trinity College) for his assistance with LC/MS. We also thank Adam
Boynton, Thomas McTeague and William McCarthy for technical
support.
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