γ2-, γ3-, and γ2,3,4-amino acids, coupling to γ-hexapeptides: CD spectra, NMR solution and X-ray crystal structures of γ-peptides

Article abstract of DOI:10.1002/1521-3765(20020201)8:3<573::AID-CHEM573>3.0.CO;2-H

There are numerous possible γ-amino acids with different degrees of substitution and with various constitutions and configurations. Of these the γ⁴-and the like- and unlike-γ^2,4-amino acids have been previously used as building block

Full text of DOI:10.1002/1521-3765(20020201)8:3<573::AID-CHEM573>3.0.CO;2-H

FULL PAPER
g2-, g3-, and g2,3,4-Amino Acids, Coupling to g-Hexapeptides:
CD Spectra, NMR Solution and X-rayCrystal Structures of g-Peptides
Dieter Seebach,* Meinrad Brenner, Magnus Rueping, and Bernhard Jaun[a]
Abstract: There are numerous possible
g-amino acids with different degrees of
substitution and with various constitu-
tions and configurations. Of these the g4-
and the like- and unlike-g2,4-amino acids
have been previously used as building
blocks in g-peptides. The synthesis of g2-,
g3-, and g2,3,4-peptides is now described.
The corresponding amino acids have
been prepared by Michael addition of
chiral N-acyl-oxazolidinone enolates to
nitro-olefins, with subsequent reduction
of the NO2 to NH2 groups. Such addi-
tions to E-2-methyl-nitropropene pro-
coupling lead to g-di-, tri-, and hexapep-
tides (12 23), which were fully charac-
terized. The crystal structures of one of
the g-amino acids (2,3-dimethyl-4-ami-
linear conformation (sheet-like), a novel
(M) helixbuilt of nine-membered hy-
drogen-bonded rings, and (M) 2.614 heli-
ces have thus been identified. NMR
measurements at different temperatures
(298 393 K) and H/D-exchange rates
obtained for the g2,3,4-hexapeptide are
interpreted as evidence for the stability
of the 2.614 helix(no ™melting∫) and for
its non-cooperative folding mechanism.
CD Spectra of the g-peptides have been
measured in MeOH and CH3CN, indi-
cating that only the protected and un-
protected g2,3,4-hexapeptide is present as
the 2.614 helixin solution. The structures
of the g2- and g3-hexapeptides (1, 2)
could not be determined.
no-pentanoic acid ¥ HCl, 9a), of a g2,3,4
-
di- and a g2,3,4-tetrapeptide (20, 22) are
described, and the NMR solution struc-
ture in MeOH of a g2,3,4-hexapeptide (3)
has been determined (using TOCSY,
COSY, HSQC, HMBC and ROESY
measurements and a molecular dynam-
ics simulated-annealing protocol). A
Keywords: amino acids
¥ circular
vide
(2R,3R,4R)-2-alkyl-3-methyl-4-
dichroism ¥ helical structures ¥ pep-
tides
amino-pentanoic acid derivatives (9,
10, 11). Stepwise coupling and fragment
This feature, together with the possibility of designing the
folding by employing appropriate b-amino acid residues, will
allow for the rational construction of novel, complexprotein-
like structures.
Introduction
The biological functions of proteins are based on the property
of the peptide chain to fold into definite three-dimensional
structures.[2] Different secondary structural motifs such as
helices, sheets, and turns are found in these biomacromole-
cules, although they all consist of only l-a-amino acid
residues. Among synthetic oligomers investigated so far, only
a few show a similar diversity of secondary structures. An
outstanding position in this regard occupy oligomers consist-
ing entirely of b-amino acids.[3] These so-called b-peptides
have been extensively investigated over the last few years.
Most interestingly, they do not only form secondary structures
similar to those found in proteins, but these structures are also
more stable, and they are adopted with short chain lengths.
Introduction of a further carbon atom into the backbone of
each residue of b-peptides leads to g-peptides. This kind of
molecules has not been studied as intensively as b-peptides, so
far. Still, various secondary structures of g-peptidic chains
have already been identified. Schreiber and co-workers have
reported on oligomers consisting of a,b-unsaturated g-amino
acids, so-called vinylogous polypeptides.[4] They have ob-
served parallel sheet structures for g-peptides of type A
(Figure 1) and antiparallel sheet structures for g-peptides of
type B in the crystalline state.[5] Hanessian×s[6] and our group[7]
have found that oligomers of type C form stable 2.614 helices
in solution, at chain lengths as short as four residues.[8] This
finding was rather unexpected, since the propylene unit of
each residue is a conformationally flexible element, which
makes the formation of a specific conformer entropically
unfavorable. On the other hand, 2,4-disubstituted g-amino
acid residues are known to have pronounced conformational
preferences.[9] Thus, these building blocks can be used to
stabilize or destabilize certain secondary structures of g-
peptides. In fact, oligomers of type D form even more stable
[a] Prof. Dr. D. Seebach, M. Brenner, M. Rueping, Prof. Dr. B. Jaun
Laboratorium f¸r Organische Chemie
der Eidgenˆssischen Technischen Hochschule Z¸rich
ETH-Zentrum, Universit‰tstrasse 16
8092 Z¸rich (Switzerland)
Fax: ( ‡41) 1-632-1144
E-mail: seebach@org.chem.ethz.ch
Supporting information for this article is available on the WWW under
http://www.wiley-vch.de/home/chemistry/ or from the author.
Chem. Eur. J. 2002, 8, No. 3
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573

D. Seebach et al.
FULL PAPER
formation of 2.614-helical structures without massive van der
Waals interactions. While g-peptide C' forms an (M) 2.614
helix, g-peptides 2 and 3 would fit into a (P) 2.614 helixand g-
peptide 1 into both, an (M)- and a (P)- 2.614 helix.
O
O
H
H
N
N
R
R
A
B
O
O
O
H
N
H
N
H
N
R1 R2
R1 R2
R
Results and Discussion
C
E
D
Synthesis of g-amino acids: The g-amino acid derivatives 4a
c and 5a c, which are required for the preparation of the g-
peptides 1 and 2, respectively, were prepared as previously
described by us (Figure 3).[14] The building blocks for g-
peptide 3 were obtained following a related route (see
Scheme 1). Since we originally envisaged the introduction of
a residue with an iPr side chain into g-peptide 3, we also
prepared the corresponding g-amino acid derivatives.
OH
O
O
H
H
N
N
R
R
OH
G
F
R2
O
O
R
O
H
N
H
H
N
N
R1 R3
R
J
H
I
O
Figure 1. Types of g-peptides investigated so far (A G) and described in
this paper (H J). For details and references, see text.
BocNH
OBn
O
BocNH
OMe
4a
5a
5b
2.614-helical structures than those of type C, while g-peptides
of type E cannot form such helices at all.[6, 10] Hanessian has
described a turn structure for a g-tetrapeptide of type E,[10]
and we have recently reported on a stable turn structure of an
N-acyl-g-dipeptide amide consisting of two residues of type E
with opposite chirality.[11] In order to investigate the influence
of heteroatom substituents on the conformation of g-peptides,
we have also prepared oligomers of type F and G.[12]
O
O
O
O
BocNH
BocNH
BocNH
OH
OH
OH
4b
BocNH
OH
Here, we report on the synthesis and structural character-
ization of g-peptides with the substitution pattern H, I, and J.
To facilitate comparisons between the different g-peptides, we
chose compounds 1 and 2, which are isomers of the known
helixforming g-peptide C', and 3 for this study (Figure 2).[13]
The substitution patterns of these g-peptides tolerate the
4c
5c
Figure 3. g-Amino acid derivatives[14] used for the preparation of peptides
1 and 2.
Addition of the Ti-enolates of the chiral acyloxazolidinones
6a c to (E)-2-nitro-but-2-ene yielded mixtures of nitro
compounds 7a c and their respective diastereoisomers
(4'-epi-7a c) in ratios ranging from 4:1 to 6:1.[15] After
separation by column chromatography or crystallization, the
major stereoisomers 7a c were obtained in 50 60% yield.
The nitro compounds were then subjected to catalytic hydro-
genation over neutral Raney-Ni, a reaction, which was
accompanied by some epimerization, and mixtures of pyrro-
lidinones 8a c and 5-epi-8a c (drˆ 91:9 86:14) were iso-
lated.[16] These mixtures were used in the next step without
separation of the diastereoisomers. Treatment of the N-Boc
derivatives of 8a and 5-epi-8a with LiOH, following a
procedure by Grieco,[17, 18] did not furnish 10a. Rather,
hydrochlorides 9a and 9c (of dr > 98:2) had to be prepared
in a first step, by heating the corresponding pyrrolidinones in
refluxing 6n HCl and recrystallizing the crude products.[19]
Even under these strongly acidic conditions we observed an
equilibrium between the pyrrolidinones and the g-amino acid
hydrochlorides, depending on the substituent R in the
3-position of the g-lactam. Thus, while hydrolysis of 8a/5-
epi-8a went almost to completion, the equilibrium was
reached at 80% conversion for 8c/5-epi-8c and at 65%
conversion for 8b/5-epi-8b. N-Boc Protection finally furnish-
ed acids 10a and 10c. Compound 10b was obtained in only
O
O
O
O
O
O
O
O
O
H
N
H
N
H3N+
OH
OH
OH
OH
N
H
N
H
N
H
O
O
O
O
O
O
O
O
O
O
O
O
–
CF3CO2
C'
O
H
N
H
N
H3N+
N
H
N
H
N
H
–
CF3CO2
1
O
H
N
H
N
H3N+
N
H
N
H
N
H
–
CF3CO2
2
O
H
N
H
N
H3N+
N
H
N
H
N
H
–
CF3CO2
3
Figure 2. Formulae of the known helix-forming g-peptide C'[7] and of g-
peptides 1 3 described herein.
574
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g-Hexapeptides
573 584
the free g-peptide 1 was ob-
tained by deprotection and pu-
rification by HPLC. Again, pyr-
rolidinone formation was ob-
served as a side reaction during
hydrolysis of the methyl ester.
Thus, we decided to use a
benzyl ester as C-terminal pro-
tecting group for the prepara-
tion of the other g-peptides.
O
O
O
O
R
NO2
1) TiCl4, (iPr)2NEt
Raney-Ni / H2
O
N
O
N
NO
R
Ph
2)
2 , TiCl4
Ph
O
Ph
Ph
3 ) aq. workup
6a - c
7a - c
+ 4'-epi-7a - c
R
O
HCl / H2O
Cl– H2N+
BocNH
NaOH / (Boc)2O
OH
OH
O
N
H
R
R
9a - c
10a - c
8a - c
+ 5-epi-8a - c
For the synthesis of 2 with the
side chains in 3-position of the
amino acids, we first coupled
compound 5a with acid 5b to
the g-dipeptide derivative 16,
which was then coupled with
acid 5c to g-tripeptide 17. Hy-
O
BocNH
Cs2CO3
BnBr
a R = Me
b R = CHMe2
c R = CH2CHMe2
OBn
10c
11
Scheme 1. Preparation of 2,3-disubstiuted-4-amino-pentanoic acid derivatives.
drogenolysis of 17 furnished acid 18 in quantitative yield. g-
Hexapeptide 19 was obtained by coupling of g-tripeptide 17
with acid 18. Deprotection of 19 and purification by HPLC
finally yielded g-peptide 2.
moderate yield from 8b/5-epi-8b without isolation of the
intermediate 9b. Finally, 10c was converted to the benzyl
ester 11.
The coupling of g-amino acids having side chains in the 2-,
3-, and 4-position, turned out to be more difficult than
coupling of mono-substituted g-amino acids. Aminoester 11
was coupled with acid 10a to the g-dipeptide 20, using HATU
(1-[bis-(dimethylamino)methyliumyl]-1H-1,2,3-triazolo[4,5-b]-
pyridin-3-oxide hexafluorophosphate) as coupling reagent,
since this gave better yields than EDC/HOBt. All our
attempts to couple acid 10b with g-dipeptide 20 were
unsuccessful. On the other hand, acid 21 could by coupled
with 20 in good yield, using EDC/DMAP as reagents. Finally,
the protected g-tetrapeptide 22 and g-dipeptide 21 were used,
by appropriate deprotection and coupling steps, to prepare
the fully protected g-hexapeptide 23, from which compound 3
was obtained.
Synthesis of the g-peptides: g-Peptides 1 and 2 were prepared
from the corresponding g-amino acids in solution by using
EDC (1-[3-dimethylamino]propyl-3-ethylcarbodiimide hy-
drochloride) and HOBt (1-hydroxy-1H-benzotriazole) as
coupling reagents. For the synthesis of g-peptide 1, a methyl
ester was used as C-terminal protecting group. Thus, com-
pound 4a was coupled with acid 4b to yield g-dipeptide 12,
which was coupled with acid 4c to g-tripeptide 13 (Figure 4).
Methyl ester hydrolysis yielded acid 14. It turned out that the
basic conditions used for methyl ester hydrolysis also led to
cyclization of the C-terminal g-amino acid, resulting in an N-
acyl pyrrolidinone. Coupling of g-tripeptides 13 and 14
furnished the fully protected g-hexapeptide 15, from which
X-rayCrsytal structures : The X-ray crystal structure of
hydrochloride 9a shows an extended conformation of the g-
amino acid backbone (Figure 5). Each Cl ion forms three
O
BocHN
OMe
N
H
12
‡
O
hydrogen bonds to NH3 groups (Cl ¥¥¥ N distances: 3.18, 3.31
and 3.22 ä), and one hydrogen bond to a COOH group
(Cl ¥¥¥ O distance: 3.06 ä). Despite the absence of hydrogen
bonds between the amino acids, the crystal structure resem-
bles that of a parallel pleated sheet.
In the crystal structure of g-dipeptide 20 a different
backbone conformation of the residues is found. Rather than
forming a sheet-like structure, left-handed helices with nine-
membered H-bonded rings are observed (Figure 6). Dado and
Gellman have previously identified such intramolecular
hydrogen bonds of g-amino acid derivatives in CH2Cl2
solution.[20] The crystal packing of 20 shows stacks of helices,
connected by hydrogen bonds between the ester CO and the
carbamate NH of neighboring molecules.
13 n = 1, R = Me
14 n = 1, R = H
OR
15 n = 2, R = Me
O
O
H
N
BocHN
N
H
O
n
O
BocHN
OBn
O
16
N
H
O
O
H
N
17 n = 1, R = Bn
18 n = 1, R = H
19 n = 2, R = Bn
BocHN
N
H
OR
O
n
We also obtained crystals suitable for single-crystal X-ray
structure analysis of the g-tetrapeptide 22. Again, a helical
structure was found, but with two 14-membered H-bonded
rings (Figure 7). The formation of such a ring is not possible
for g-dipeptide 20. Nevertheless, the backbone conformation
of residues 1 3 in the crystal structure of the tetrapeptide
20 n = 1, R = Bn
21 n = 1, R = H
22 n = 2, R = Bn
23 n = 3, R = Bn
O
BocHN
OR
N
H
O
n
Figure 4. g-Peptide intermediates in the syntheses of 1 3.
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575

D. Seebach et al.
FULL PAPER
Figure 7. X-ray crystal structure of g-tetrapeptide 22; left: side view of the
helical structure; right: view along the helixaxis; for color coding (atom
specification) see caption of Figure 5.
and NH of residue 2 of another neighbor. The H-bonding
pattern and the backbone conformation of residues 1 3 fit
perfectly into the pattern of a 2.614 helix, similar to the one
found in solution for g-peptide C', but–as expected–with
opposite sense of chirality.
Figure 5. X-ray crystal structure of g-amino acid hydrochloride 9a; C
atoms in gray, H atoms in white, N atoms in blue, O atoms in red, and Cl
ions in green.
Remarkably, in all three crystal structures described here,
only two distinguished conformations of the g-amino acid
residue backbones were observed. In fact, only these two
conformations do not suffer from unfavorable syn-pentane
interactions.[21]
NMR spectroscopy: g-Peptides have been shown to fold into
stable secondary structures in solution.[6, 7, 10] The g-peptides
1
3, soluble in a variety of organic solvents, were investigated
1
by NMR experiments. Two-dimensional H NMR measure-
ments were performed in methanol and pyridine solutions in
order to allow for comparison of these peptides with the
already known structures determined in the same solvents.
Complete proton resonance assignment of the individual
residues was achieved using TOCSY, COSY and HSQC
measurements. Sequence-specific assignment was accom-
plished by HMBC measurements and a combination of
TOCSY and ROESY experiments analyzing short range
NOEs between H(a)i and NH(i‡1). To determine the
three-dimensional structure, ROESY spectra at different
mixing times (150 ms, 300 ms) were recorded. However, for
g-peptides 1 and 2, limited dispersion of the chemical shifts
and resonance overlap made the assignment of the NOEs
impossible. This was also true for pyridine solutions, which in
earlier measurements of other g-peptides had shown greater
dispersion.
For g-peptide 3 a complete assignment of the NOEs in
methanol solution was achieved. The NOEs obtained were
classified into three distance categories: strong, medium and
weak based upon their volume. These NOE-derived distances
as well as the dihedral angles derived from coupling constants
and NOEs were used in a restrained molecular dynamics
simulated-annealing protocol, yielding 25 structures that
could be clustered to a left-handed helix. The structures show
Figure 6. X-ray crystal structure of g-dipeptide 20; left: side view of two
molecules showing the formation of stacks; right: view of a molecule along
the helixaxis; for color coding (atom specification) see caption of Figure 5.
derivative 22 is very similar to the one found in the crystal
structure of the dipeptide analogue 20, while residue 4 adopts
an extended conformation, similar to the one found in the
crystal structure of hydrochloride 9a. Since the C-terminal
ester group cannot act as a H-bond donor, the formation of a
third 14-membered H-bonded ring is not possible. Thus, the
conformation of residue 4 may mainly be determined by
interactions with neighboring molecules. In fact, the ester CO
is involved in an intermolecular hydrogen bond to the
carbamate NH of the neigboring molecule. A second inter-
molecular hydrogen bond is formed between CO of residue 3
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g-Hexapeptides
573 584
3
a well-defined 2.6-helical structure with 14-membered hydro-
The backbone JHN,HCg-coupling constants listed in Table 1
decrease only slowly upon increase of temperature;[22] this is
an indication that the helical structure is still present at
363 K.[23] The observed temperature coefficients (Table 1)
show a linear relationship of d versus T for all residues. As
anticipated for a helical structure, the C-terminal residue has a
high dd/dT value (ca. 8 ppbKÀ1) which is characteristic of
solvent exposed groups. Residues 2 5 have a low dd/dT
value, suggesting that they are part of a stable secondary
structure, which is in agreement with the H/D exchange
experiment,[24] carried out by dissolving the peptide in
CD3OD:[25] the residues 3 5 exchange at lower rates than
the terminal residues (Table 1).
ˆ
gen-bonded rings from C O of residue i to NH of residue i‡3
(Figure 8).
Table 1. Characteristic 1H NMR parameters for g-peptide 3. For further
details see Supporting Information.
Residue of
g-peptide 3
NH
d[a]
3JHN,HCg
[Hz][a]
dd/dT
[ppbKÀ1
Half life
values [min]
]
Ala1
Leu2
Ala3
Leu4
Ala5
Leu6
8.20[b]
8.12
7.41
6.90
7.65
8.18
9.2
9.6
8.9
8.2
8.8
À 6.0
À 6.0
À 1.7
À 5.8
À 7.9
< 5
240
453
197
35
[a] Solvent: CD3OH, Tˆ 298 K. [b] Assigned by COSY measurements.
All of these data indicate that 2,3,4-substituted g-hexapep-
tides with the relative configuration of the residues as in 3
adopt stable secondary structures that are maintained even at
high temperatures. As with b-peptides, there is no cooperative
melting of the helix.[26] In contrast to g-peptides 1 and 2,
carrying one substituent only in each residue, and showing
intensive resonance overlap, g-peptide 3 contains three
aliphatic side chains in each residue and shows a much larger
dispersion of signals. This may be a consequence of the helical
secondary structure. The conformational constraint imposed
by the three substituents sets the conformation of the
individual amino acid residue in the backbone[9, 11] towards
helixformation.
Figure 8. NMR solution structure of g-peptide 3 in methanol. The peptide
forms a left-handed 2.6-helical structure with 14-membered hydrogen-
ˆ
bonded rings from C O of residue i to NH of residue i‡3.
In order to probe the stability of the helix, temperature-
dependent NMR measurements in methanol were performed
at intervals of 10 K up to 393 K. The experiments were carried
out in a sealed NMR tube and the solvent was suppressed by
presaturation. Figure 9 shows the temperature dependence of
the chemical shifts of the amide protons.
CD Spectroscopy: Circular di-
chroism (CD) spectroscopy is a
widely used method for the
structural characterization of
oligopeptides consisting of a-
amino acids.[27] This technique
has also proven to be useful for
the investigation of unnatural
oligomers forming secondary
structures.[28]
The CD spectra of g-peptides
C', 1, and 2 in methanol solution
show only weak intensities at
wavelengths longer then 210 nm
(Figure 10, left). It is obvious,
that such uncharacteristic spec-
tra are not useful for structural
assignments. The g-peptides C'
and 2 have very similar CD
Figure 9. Temperature-dependent 1H NMR spectra (NH region) of g-peptide 3 in methanol. Measurements
were performed at intervals of 10 K up to 393 K. The J values remain large, the dispersion of the chemical shifts is
maintained, the temperature coefficients are linear (Table 1) and the NH signals are not disappearing by
exchange with the solvent. See also Supporting Information.
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577

D. Seebach et al.
FULL PAPER
spectra. The CD spectrum of the g-peptide 1 shows a weak
minimum around 212 nm. In contrast, the CD spectrum of g-
peptide 3 has a pronounced maximum near 213 nm, but,
interestingly, it has no resemblance to the CD spectrum of the
g-peptide C', despite the fact that we know from NMR
investigations that both peptides form a 2.614 helixin solution
(see above).
tetrapeptide 22 is present (Figure 10, right). Again, the CD
spectra of the g-hexapeptides 3 and 23 show a couplet for the
p ! p* transition, but with higher intensities than in methanol
solution, while the intensities for the maxima at 215 nm are
similar in both solvents. In the spectrum of 3, the long
wavelength component of the p ! p* band at ꢀ200 nm is
most intense.
Even though it turned out to be difficult to gain structural
information by comparing CD spectra of different g-peptides,
comparisons of CD spectra within one type of g-peptides may
be useful. Thus, we measured the CD spectra of the fully
protected g-peptides 20, 22 and 23, as well as of the
deprotected g-peptide 3 in methanol solution (Figure 10,
middle). All of these g-peptides consist of the same two
residues. The spectra of g-dipeptide 20 and g-tetrapeptide 22
are almost superimposable. Both have a maximum near
215 nm and a zero-crossing at 208 nm (the more intensive
Cotton effect of 22 reflects the larger number of residues, as
compared to 20). Upon going from g-tetrapeptide 22 to g-
hexapeptide 23, a drastic change of the CD pattern occurs.
The maximum at 215 nm becomes much more intense and the
p ! p* transition is split into a couplet, leading to a shoulder
at 203 nm–caused by the low-energy component of the
couplet–and a zero-crossing at 197 nm. The CD spectrum of
the unprotected g-peptide 3 is similar to the one of its
protected precursor 23, but the intensity is reduced.
Conclusion
We have shown, that a g-hexapeptide consisting of residues
with side chains in the 2, 3, and 4-position form stable 2.614-
helical structures in methanol solution. A g-tetrapeptide
consisting of the same residues forms a 2.614 helixin the solid
state, but probably not to a larger extend in methanol or
acetonitrile solution. A new kind of helical structure with a
nine-membered H-bonded ring was found in the crystalline
state for a g-dipeptide.
Structural assignments of g-peptides consisting of mono
substituted residues with the side chains in the 2- or in the
3-positions were hampered by strong overlap of the NMR
signals and uncharacteristic CD spectra.
Experimental Section
With due care, we draw the following conclusions from
these CD spectra: i) both, the protected (23) and the
unprotected (3) g-hexapeptides are present as 2.614-helical
structures in methanol solutions (unambiguously proven by
NMR analysis of 3); ii) the g-tetrapeptide 22, which was found
as a 2.614 helixin crystals does not appear to fold to such a
helixto a any larger extend in methanol solution; iii) rather,
the g-tetra- (22) and the g-dipeptides (20) might have similar
structures in methanol, which can not be 2.614-helical, because
the latter can not possibly from a single turn of such a helix(it
adopts a nine-membered ring with a next-neighbor hydrogen
bond in the crystal).
In aprotic solvents, such as acetonitrile, secondary struc-
tures with intramolecular hydrogen bonds are stabilized,
relative to methanol, which competes strongly for hydrogen
bonding. Nevertheless, the CD spectra in acetonitrile indicate
that only a minor population of a 2.614-helical structure for g-
General: Starting materials and reagents: Acyl-oxazolidinones 6a c,[29]
(E)-2-nitro-but-2-ene[30] and g-amino acids 4a
c
and 5a c[14] were
prepared according to literature procedures. THF was distilled from Na
under an Ar atmosphere prior to use. Solvents for chromatography and
workup procedures were distilled from Sikkon (anhydrous CaSO4; Fluka
Chemie AG) or KOH (Et2O). All other solvents and reagents were used as
received from Fluka. Acronyms: Boc2O ˆ di(tert-butyl) dicarbonate,
DMAP ˆ dimethyl-pyridin-4-yl-amine, drˆ diastereoisomer ratio (deter-
1
mined by H NMR), EDC ˆ 1-[3-(dimethylamino)propyl]-3-ethylcarbodi-
imide hydrochloride, HATU ˆ (1-[bis-(dimethylamino)methyliumyl]-1H-
1,2,3-triazolo[4,5-b]pyridin-3-oxide hexafluorophosphate), HOBt ˆ 1-hy-
droxy-1H-benzotriazole, NMM ˆ 4-methylmorpholine.
Equipment: Thin-layer chromatography (tlc): silica gel 60 F254 glass plates
(Merck); visualization by UV254 light and development with KMnO4
solution (NaOH (12 g), KMnO4 (1.5 g), H2O (300 mL)) or anisaldehyde
solution (anisaldehyde (9.2 mL), AcOH (3.75 mL), H2SO4 conc. (12.5 mL),
EtOH (340 mL)). Flash column chromatography: silica gel 60 (40 63 mm,
Fluka), pressure 0.2 0.3 bar. Preparative HPLC: Knauer HPLC system
(pump type 64, programmer 50, UV detection (variable-wavelength
Figure 10. CD Spectra of g-peptides; left: fully deprotected g-hexapeptides C' and 1 3 in methanol; middle: protected g-di-, g-tetra-, and g-hexapeptides
(20, 22, and 23), as well as deprotected g-hexapeptide 3 in methanol; right: g-peptides 20, 22, 23, and 3 in acetonitrile. The spectra were recorded with 0.2m
peptide solutions and are not normalized to the number of residues.
578
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0803-0578 $ 17.50+.50/0
Chem. Eur. J. 2002, 8, No. 3

g-Hexapeptides
573 584
monitor)); column: Nucleosil 100-7C8 (250 Â 21 mm, Macherey Nagel).
M.p.: B¸chi-510 apparatus, uncorrected. Optical rotations: Perkin-Elmer
241 polarimeter (10 cm, 1 mL cell) at rt. Circular dichroism (CD) spectra:
Jasco J-710 recording from 190 to 250 nm at 208C; 1 mm cell; average of
five scans, corrected for the baseline; peptide concentration 0.2mm;
smoothing by Jasco software. IR Spectra: Perkin Elmer-782 spectropho-
tometer. NMR Spectra: Bruker AMX 500 (1H 500 MHz, 13C 125 MHz),
AMX 400 (1H 400 MHz, 13C 100 MHz); d in ppm downfield from internal
TMS (d ˆ 0); MS: VG Tribrid (EI), VG ZAB2-SEQ (FAB, in a 3-nitro-
benzyl alcohol matrix), Finnigan-MAT-TSQ 7000 (ESI) and InoSpec
Ultima (MALDI FT-MS, high resolution MS (HRMS), in a 2,5-dihydroxy-
benzoic acid matrix). Elemental analyses were performed by the Micro-
analytical Laboratory of the Laboratorium f¸r Organische Chemie, ETH-
Z¸rich.
H2 (1 bar) for 12 h. Subsequent filtration through Celite and concentration
under reduced pressure yielded the crude carboxylic acid which was used
without further purification.
Reversed-phase (RP) HPLC purification of g-peptides–General proce-
dure 7 (GP 7): Crude products were purified by preparative RP-HPLC
using a gradient of A (0.1% CF3CO2H in H2O) and B (CH3CN) at a flow
rate of 20 mLminÀ1 with UV detection at 220 nm. The products were
isolated and dried by lyophilization.
(S)-4-Isopropyl-3-((2R,3R,4R)-2,3-dimethyl-4-nitro-pentanoyl)-5,5-di-
phenyl-oxazolidin-2-one (7a) and 4'-epi-7a: Reaction of propionyl-oxazo-
lidinone 6a (13.5 g, 40.0 mmol) with (E)-2-nitro-but-2-ene according to
GP 1 yielded after trituration (Et2O) a mixture of 7a and 4'-epi-7a (9.5 g,
54%, drˆ 92:8) as a white solid (dr of the crude product ˆ 4:1). Rf ˆ 0.38
(pentane/Et2O 3:1); 1H NMR (400 MHz, CDCl3): 7a: d ˆ 0.77 (d,
J(H,H) ˆ 6.9 Hz, 3H; CH3), 0.82 (d, J(H,H) ˆ 6.7 Hz, 3H; CH3), 0.91 (d,
J(H,H) ˆ 7.0 Hz, 3H; CH3), 0.94 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 1.59 (d,
J(H,H) ˆ 6.8 Hz, 3H; CH3), 1.99 (app sept.d, J(H,H) ˆ 6.9, 3.2 Hz, 1H;
CH(CH3)2), 2.39 2.47 (m, 1H; CHCHNO2), 3.70 3.77 (m, 1H; CHCO),
4.59 4.65 (m, 1H; CHNO2), 5.41(d, J(H,H) ˆ 3.2 Hz, 1H; CHNCO),
7.26 7.48 (m, 10H; 2Ph); 4'-epi-7a: 0.77 (d, J(H,H) ˆ 6.8 Hz, 3H; CH3),
0.77 (d, J(H,H) ˆ 6.9 Hz, 3H; CH3), 0.87 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3),
0.90 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 1.47 (d, J(H,H) ˆ 6.8 Hz, 3H; CH3),
1.95 2.03 (m, 1H; CH(CH3)2), 2.79 (dqd, J(H,H) ˆ 8.3, 7.0, 5.3 Hz, 1H;
CHCHNO2), 3.63 (dq, J(H,H) ˆ 8.3, 6.9 Hz, 1H; CHCO), 4.62 (qd,
J(H,H) ˆ 6.8, 5.3 Hz, 1H; CHNO2), 5.34 (d, J(H,H) ˆ 3.6 Hz, 1H;
CHNCO), 7.25 7.40 (m, 10H, 2Ph); 13C NMR (100 MHz, CDCl3): d ˆ
10.7, 12.8, 16.3, 17.2, 21.8, 29.8, 38.8, 39.1, 64.6, 86.1, 89.5, 125.6, 125.9, 128.0,
128.4, 128.7, 128.9, 137.9, 142.3, 152.6, 175.1; 4'-epi-7a: 10.6, 13.2, 14.4, 16.6,
29.5, 38.7, 39.4, 65.0, 84.3, 89.7, 125.6, 125.8, 128.1, 128.5, 128.7, 137.7, 142.1,
152.8, 175.1; IR (CHCl3): nÄ ˆ 2974 (m), 1781 (s), 1709 (s), 1550 (s), 1494 (w),
1450 (m), 1391 (m), 1363 (s), 1316 (m), 1093 (w), 1052 (w), 990 cmÀ1 (w);
Addition of 3-acyl-4-isopropyl-5,5-diphenyl-oxazolidin-2-ones to (E)-2-
nitro-but-2-ene–General procedure 1 (GP 1): TiCl4 (1.1 equiv) was added
at À788C to a solution of the respective 3-acyl-4-isopropyl-5,5-diphenyl-
oxazolidin-2-one in CH2Cl2 (0.25m). After stirring for 5 min, (iPr)2NEt
(1.2 equiv) was added at À788C and the resulting dark red solution was
stirred at 08C for 30 min. A solution of (E)-2-nitro-but-2-ene in CH2Cl2
(1.1 equiv, 1.5m) and TiCl4 (1.1 equiv) were successively added at À788C.
The mixture was stirred for 5 h at À788C and treated with aq. NH4F
solution (20%). The organic layer was separated and washed with aq.
NH4F solution (1 Â ). To the resulting yellow solution was added 1n NaOH
(5 equiv). After stirring the emulsion for 10 min, sat. NaHCO3 solution
(10 mL per 1 mmol oxazolidinone) was slowly added under vigorous
stirring over a period of 2 h. The organic layer was separated, washed with
H2O, dried over MgSO4 and concentrated under reduced pressure. The
resulting crude product was purified by flash column chromatography.
Reduction of the nitro compounds to pyrrolidin-2-ones–General proce-
dure 2 (GP 2): The respective nitro compound was added to a suspension of
W2 Raney/nickel (freshly prepared from 600 mg Al/Ni alloy per 1 mmol
nitro compound[31]) in EtOH/AcOEt 1:2 (15 mL1 mmolÀ1). The mixture
was stirred in an autoclave for 3 d at 508C under H2 (30 bar). The
precipitated oxazolidinone auxiliary was dissolved by adding CH2Cl2. The
catalyst was removed by filtration through Celite and the filtrate was
evaporated to yield a white solid residue. Trituration in boiling Et2O
followed by filtration yielded pure recovered oxazolidinone auxiliary as a
white solid. The filtrate was concentrated under reduced pressure and the
resulting crude product was purified by flash column chromatography.
‡
MS (FAB): m/z (%): 439 (100) [M‡H] , 392 (47), 348 (32), 238 (43);
elemental analysis calcd (%) for C25H30N2O5 (438.52): C 68.47, H 6.90, N
6.39; found: C 68.39, H 6.98, N 6.42.
(S)-4-Isopropyl-3-((2R,3R,4R)-2-isopropyl-3-methyl-4-nitro-pentanoyl)-
5,5-diphenyl-oxazolidin-2-one (7b): Reaction of acyl-oxazolidinone 6b
(11.0 g, 30.0 mmol) with (E)-2-nitro-but-2-ene according to GP 1 yielded
after purification by flash column chromatography (pentane/Et2O 7:1 !
3:1) compound 7b (6.7 g, 47%, dr > 97:3) as a white solid (dr of the crude
product ˆ 4:1). Rf ˆ 0.45 (pentane/Et2O 3:1); m.p. 196 1678C; [a]DRT
ˆ
À146.1 (c ˆ 0.98 in CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 0.37 (d,
J(H,H) ˆ 7.0 Hz, 3H; CH3), 0.48 (d, J(H,H) ˆ 6.8 Hz, 3H; CH3), 0.85 (d,
J(H,H) ˆ 6.8 Hz, 3H; CH3), 0.96 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 1.04 (d,
J(H,H) ˆ 7.0 Hz, 3H; CH3), 1.59 (d, J(H,H) ˆ 6.7 Hz, 3H; CH3), 1.77 1.89
(m, 1H; (CH3)2CHCHCO), 2.04 (app sept.d, J(H,H) ˆ 6.9, 3.0 Hz, 1H;
(CH3)2CHCHN), 2.28 2.34 (m, 1H; CHCHNO2), 3.92 (dd, J(H,H) ˆ 8.1,
6.1 Hz, 1H; CHCO), 4.65 (qd, J(H,H) ˆ 6.7, 5.0 Hz, 1H; CHNO2), 5.47 (d,
J(H,H) ˆ 3.0 Hz, 1H; CHNCO), 7.23 7.56 (m, 10H; 2Ph); 13C NMR
(100 MHz, CDCl3): d ˆ 11.9, 16.2, 18.0, 18.3, 19.5 21.9, 28.4, 30.0, 38.4, 49.4,
65.4, 85.8, 88.8, 125.4, 125.7, 127.9, 128.4, 128.5, 128.8, 137.9, 142.7, 152.7,
174.0; IR (CHCl3): nÄ ˆ 2970 (m), 1782 (s), 1709 (m), 1549 (s), 1494 (w), 1450
(m), 1395 (m), 1363 (s), 1316 (m), 1103 (w), 1052 (w), 1002 cmÀ1 (w); MS
Boc Deprotection of g-amino acids and g-peptides–General procedure 3
(GP 3): To a solution of the respective g-amino acid (or g-peptide) in
CH2Cl2 (0.5m) was added an equal volume of CF3CO2H at 08C. The
mixture was allowed to warm to rt and stirred for further 1.5 h.
Concentration under reduced pressure yielded the crude CF3CO2H salt,
which was used without further purification.
Peptide coupling with EDC/HOBt–General procedure 4 (GP 4): The
respective CF3CO2H salt (1 equiv) was dissolved in THF, CH2Cl2 or CHCl3
(0.5m) and cooled to 08C. To the resulting solution was added successively
NMM (5 equiv), HOBt (1.2 equiv),
a solution of the Boc-protected
fragment (1 equiv) in THF, CH2Cl2 or CHCl3 (0.5m) and EDC (1.2 equiv).
The mixture was allowed to warm to rt and stirred until TLC indicated
complete reaction. After dilution with AcOEt the mixture was washed with
1m HCl (3 Â ), sat. aq. NaHCO3 solution (3 Â ) and sat. aq. NaCl solution
(3 Â ). The organic layer was dried over MgSO4 and concentrated under
reduced pressure. The resulting crude product was purified by flash column
chromatography.
‡
‡
(FAB): m/z (%): 933 (5) [2M‡H] , 467 (100) [M‡H] , 420 (29), 376 (16);
elemental analysis calcd (%) for C27H34N2O5 (466.58): C 69.51, H 7.34, N
6.00; found: C 69.28, H 7.44, N 6.02.
(S)-3-((2R,3R,4R)-2-Isobutyl-3-methyl-4-nitro-pentanoyl)-4-isopropyl-
5,5-diphenyl-oxazolidin-2-one (7c): Reaction of acyl-oxazolidinone 6c
(17.5 g, 46.0 mmol) with (E)-2-nitro-but-2-ene according to GP 1 yielded
after purification by flash column chromatography (pentane/Et2O 10:1)
compound 7c (11.1 g, 50%, dr > 97:3) as a white solid (dr of the crude
product ˆ 4:1). Rf ˆ 0.44 (pentane/Et2O 5:1); [a]RDT ˆ À102.7 (c ˆ 1.03 in
CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 0.38 (d, J(H,H) ˆ 6.5 Hz, 3H;
CH3), 0.60 (d, J(H,H) ˆ 6.6 Hz, 3H; CH3), 0.67 0.74 (m, 1H;
CHHCH(CH3)2), 0.77 0.89 (m, 1H; CH2CH(CH3)2), 0.79 (d, J(H,H) ˆ
6.8 Hz, 3H; CH3), 0.85 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 0.93 (d, J(H,H) ˆ
6.9 Hz, 3H; CH3), 1.64 (d, J(H,H) ˆ 6.7 Hz, 3H; CH3), 1.80 (ddd, J(H,H) ˆ
13.4, 11.8, 4.4 Hz, 1H; CHHCH(CH3)2), 2.02 (app sept.d, J(H,H) ˆ 6.9,
3.6 Hz, 1H; CHCH(CH3)2), 2.49 (dqd, J(H,H) ˆ 9.6, 6.9, 3.0 Hz, 1H;
CHCHNO2), 3.69 3.74 (m, 1H; CHCO), 4.49 (dq, J(H,H) ˆ 9.5, 6.7 Hz,
1H; CHNO2), 5.39 (d, J(H,H) ˆ 3.6 Hz, 1H; CHNCO), 7.25 7.51 (m, 10H;
2Ph); 13C NMR (100 MHz, CDCl3): d ˆ 11.4, 16.5, 17.7, 21.2, 21.7, 23.3, 26.2,
Peptide coupling with EDC/DMAP–General procedure 5 (GP 5): The
respective CF3CO2H salt was dissolved in CH2Cl2 (0.3m) and cooled to 08C.
The Boc-protected fragment (1 equiv), DMAP (3 equiv) and EDC
(1.2 equiv) were added and the resulting mixture was stirred for 3 h at
08C. The mixture was allowed to warm to rt and stirring was continued until
TLC indicated complete reaction. The mixture was diluted with AcOEt
and washed with 1m HCl (3 Â ), sat. aq. NaHCO3 solution (3 Â ) and sat. aq.
NaCl solution (3 Â ). The organic layer was dried over MgSO4 and
concentrated under reduced pressure. The resulting crude product was
purified by flash column chromatography.
Benzyl ester deprotection–General procedure 6 (GP 6): The respective
benzyl ester was dissolved in MeOH (0.1m) and 10% (m/m) Pd/C (10%)
was added. The resulting mixture was stirred at rt under an atmosphere of
Chem. Eur. J. 2002, 8, No. 3
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0803-0579 $ 17.50+.50/0
579

D. Seebach et al.
FULL PAPER
29.6, 33.4, 38.6, 41.2, 65.1, 88.0, 89.8, 125.4, 125.8, 128.1, 128.5, 128.6, 128.9,
137.6, 142.2, 152.8, 173.5; IR (CHCl3): nÄ ˆ 2961 (m), 1777 (s), 1698 (m), 1553
(s), 1494 (w), 1450 (m), 1390 (m), 1364 (m), 1119 (w), 1053 (w), 1036 (w),
(m, 1H; CHO), 2.49 2.56 (m, 1H; CHCHN), 3.54 3.61 (m, 1H; CHN);
13C NMR (100 MHz, CD3OD): d ˆ 11.6, 12.1, 19.6, 28.8, 41.6, 42.3, 50.3,
79.9, 158.2, 180.0; IR (CHCl3): nÄ ˆ 3439 (m), 2980 (s), 1705 (s), 1510 (s),
1454 (m), 1392 (m), 1368 (m), 1083 (m), 1020 (w), 857 cmÀ1 (w); MS (ESI
pos.): m/z (%): 513 (48) [2M‡Na] , 268 (100) [M‡Na] ; MS (ESI neg.):
m/z (%): 511 (17) [2M‡Na À 2H]À, 489 (33) [2M À H]À, 280 (6) [M‡Cl]À,
244 (100) [M À H]À; elemental analysis calcd (%) for C12H23NO4 (245.32):
C 58.75, H 9.45, N 5.71; found: C 58.77, H 9.61, N 5.77.
‡
988 (w), 900 cmÀ1 (w); MS (FAB): m/z (%): 961 (12) [2M‡H] , 481 (100)
‡
‡
‡
[M‡H] , 434 (50), 390 (23); elemental analysis calcd (%) for C28H36N2O5
(480.60): C 69.98, H 7.55, N 5.83; found: C 70.16, H 7.52, N 5.66.
(3R,4R,5R)-3,4,5-Trimethyl-pyrrolidin-2-one (8a) and 5-epi-8a: Com-
pound 7a (9.47 g, 21.6 mmol) was hydrogenated according to GP 2. After
purification by flash column chromatography (Et2O/MeOH 60:1 ! 25:1) a
mixture of 8a and 5-epi-8a (2.59 g, 94%, dr ˆ 87:13) was obtained as a
(2R,3R,4R)-4-{[(tert-Butoxy)carbonyl]amino}-2-isopropyl-3-methyl-pen-
tanoic acid (10b): Lactam 8b (1.09 g, 7.0 mmol, drˆ 86:14,) was dissolved
in 6n HCl (50 mL) and stirred under refluxfor 5 h. The solvent was
evaporated and the residue (light yellow oil consisting of a 2:1 mixture of
8b and 9b) was dissolved in H2O (20 mL). The resulting solution was
cooled to 08C. 1n NaOH (7 mL) and a solution of Boc2O (1.09 g, 5.0 mmol)
in THF (1 mL) was added. The resulting mixture was stirred at rt for 18 h,
with pH being retained at 10 12 by adding small amounts of 1n NaOH. 1n
NaOH was then added until pH 14 was reached and the reaction mixture
was washed with Et2O (5 Â ). The organic layers were combined, dried over
MgSO4 and concentrated under reduced pressure to yield recovered lactam
8b (730 mg, 67%, dr ˆ 84:16). AcOEt was added to the aqueous layer and
the resulting mixture was acidified at 08C with aq. KHSO4 solution (10%)
to pH 2 3. The aqueous layer was separated and extracted with AcOEt
(5 Â ). Concentration of the aqueous layer yielded a mixture of 9b and
inorganic salts. The organic layers were combined, dried over MgSO4 and
evaporated. Purification by trituration (Et2O) yielded acid 10b (279 mg,
15% from 8b, dr > 98:2) as a white solid. M.p. 171 1728C (decomp);
[a]RDT ˆ ‡12.1 (c ˆ 0.62 in MeOH); 1H NMR (400 MHz, CD3OD): d ˆ 0.90
(d, J(H,H) ˆ 7.1 Hz ; 3 H ; CH3), 0.91 (d, J(H,H) ˆ 6.8 Hz, 3H; CH3), 1.01 (d,
J(H,H) ˆ 6.9 Hz, 3H; CH3), 1.11 (d, J(H,H) ˆ 6.8 Hz, 3H; CH3), 1.43 (s,
9H; tBu), 1.93 2.07 (m, 2H; CH(CH3)2, CHCHN), 2.24 (dd, J(H,H) ˆ
10.4, 4.6 Hz, 1H; CHCO), 3.72 (qd, J(H,H) ˆ 6.8, 3.2 Hz, 1H; CHN);
13C NMR (100 MHz, CD3OD): d ˆ 11.4, 17.4, 19.8, 22.5, 28.0, 28.8, 38.7, 49.9,
55.2, 79.9, 158.1, 180.0; IR (CHCl3): nÄ ˆ 3442 (m), 2973 (s), 1698 (s), 1504
(m), 1456 (m), 1392 (m), 1368 (s), 1088 (m), 1055 cmÀ1 (w); MS (ESI pos.):
1
white solid. Rf ˆ 0.34 (AcOEt); H NMR (400 MHz, CDCl3, 8a): d ˆ 1.04
(d, J(H,H) ˆ 6.8 Hz, 3H; CH3), 1.07 (d, J(H,H) ˆ 6.6 Hz, 3H; CH3), 1.16 (d,
J(H,H) ˆ 6.8 Hz, 3H; CH3), 2.00 2.14 (m, 2H; CHCHCO), 3.62 3.67 (m,
1H; CHN), 6.82 (brs, NH); 13C NMR (100 MHz, CDCl3, 8a): d ˆ 13.5, 13.6,
16.5, 41.0, 41.1, 51.1, 180.1; IR (CHCl3): nÄ ˆ 3431 (m), 3007 (m), 2970 (m),
2932 (w), 2877 (w), 1693 (s), 1457 (m), 1417 (m), 1382 (m), 1343 (w), 1295
(w), 1086 (w), 1056 (w), 1011 (w), 968 cmÀ1 (w); MS (EI): m/z (%): 127 (16)
‡
[M] , 112 (93), 84 (6), 69 (100), 56 (47), 55 (16), 44 (79), 42 (16), 41 (36), 28
(16); elemental analysis calcd (%) for C7H13NO (127.19): C 66.11, H 10.30,
N 11.01; found: C 66.27, H 10.26, N 11.15.
(3R,4R,5R)-3-Isopropyl-4,5-dimethyl-pyrrolidin-2-one (8b) and 5-epi-8b:
Compound 7b (8.21 g, 17.6 mmol) was hydrogenated according to GP 2.
After purification by flash column chromatography (CH2Cl2/MeOH 50:1
! 35:1) a mixture of 8b and 5-epi 8b (2.58 g, 94%, drˆ 86:14) was
obtained as a white solid. Rf ˆ 0.43 (AcOEt); 1H NMR (400 MHz, CDCl3,
8b): d ˆ 0.99 (d, J(H,H) ˆ 6.9 Hz, 3H; CH3), 1.03 (d, J(H,H) ˆ 7.0 Hz, 3H;
CH3), 1.03 (d, J(H,H) ˆ 7.1 Hz, 3H; CH3), 1.08 (d, J(H,H) ˆ 6.6 Hz, 3H;
CH3), 1.97 (dd, J(H,H) ˆ 7.6, 3.9 Hz, 1H; CHCO), 2.11 2.22 (m, 1H;
CH(CH3)2), 2.33 2.42 (m, 1H; CHCHN), 3.64 3.71 (m, 1H; CHN), 6.34
(brs, 1H; NH); 13C NMR (100 MHz, CDCl3, 8b): d ˆ 15.8, 16.9, 19.1, 20.2,
27.8, 33.9, 51.1, 53.8, 179.0; IR (CHCl3): nÄ ˆ 3426 (m), 3000 (m), 2965 (m),
2933 (m), 2875 (m), 1687 (s), 1465 (m), 1417 (m), 1385 (m), 1279 (w), 1084
‡
(w), 1007 cmÀ1 (w); MS (EI): m/z (%): 155 (0.6) [M] , 113 (32), 98 (100), 69
(35), 55 (21), 44 (23), 42 (14), 41 (37), 39 (19), 28 (43); elemental analysis
calcd (%) for C9H17NO (155.24): C 69.63, H 11.04, N 9.02; found: C 69.76,
H 11.21, N 8.92.
‡
‡
m/z (%): 569 (100) [2M ‡ Na] , 296 (28) [M‡Na] ; MS (ESI neg.): m/z
(%): 567 (6) [2M‡Na À 2H]À, 545 (100) [2M À H]À, 308 (10) [M‡Cl]À, 272
(50) [M À H]À; elemental analysis calcd (%) for C14H27NO4 (273.37): C
61.51, H 9.95, N 5.12; found: C 61.27, H 9.80, N 4.97.
(3R,4R,5R)-3-Isobutyl-4,5-dimethyl-pyrrolidin-2-one (8c) and 5-epi-8c:
Compound 7c (11.0 g, 22.8 mmol) was hydrogenated according to GP 2.
After purification by flash column chromatography (Et2O/MeOH 100:1 !
30:1) a mixture of 8c and 5-epi-8c (3.50 g, 91%, dr ˆ 91:9) was obtained as
a white solid. Rf ˆ 0.36 (Et2O/MeOH 50:1); 1H NMR (400 MHz, CDCl3,
8c): d ˆ 0.92 (d, J(H,H) ˆ 6.6 Hz, 3H; CH3), 0.93 (d, J(H,H) ˆ 6.6 Hz, 3H;
CH3), 1.04 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 1.09 (d, J(H,H) ˆ 6.6 Hz, 3H;
CH3), 1.26 1.33 (m, 1H; CHH), 1.62 (ddd, J(H,H) ˆ 13.8, 7.9, 5.8 Hz, 1H;
CHH), 1.83 1.93 (m, 1H; CH(CH3)2), 2.04 (app td, J(H,H) ˆ 7.9, 5.8 Hz,
1H; CHCO), 2.14 2.23 (m, 1H; CHCHN), 3.65 3.72 (m, 1H; CHN); 7.28
(brs, 1H; NH); 13C NMR (100 MHz, CDCl3, 8c): d ˆ 14.6, 16.7, 22.3, 23.0,
25.9, 39.1, 39.4, 45.2, 50.9, 180.3; IR (CHCl3): nÄ ˆ 3429 (m), 2963 (m), 1691
(s), 1466 (m), 1416 (m), 1384 (m), 1086 cmÀ1 (m); MS (EI): m/z (%): 170
(2R,3R,4R)-4-{[(tert-Butoxy)carbonyl]amino}-2-isobutyl-3-methyl-penta-
noic acid (10c): Lactam 8c (2.20 g, 13.0 mmol, drˆ 91:9,) was dissolved in
6n HCl (80 mL) and stirred for 24 h under reflux. The solvent was
evaporated and the residue coevaporated with acetone (2 Â ) and triturated
in Et2O. The hydrochloride 9c (1.76 g, 61%, dr > 98:2) was obtained as a
white solid. Repeating the same procedure with the residue of the mother
liquid yielded a second amount of 9c (462 mg, 16%). To a solution of 9c
(2.13 g, 9.5 mmol) in 1n NaOH (20 mL) was added a solution of Boc2O
(3.12 g, 14.3 mmol) in dioxane (8 mL). The resulting mixture was stirred at
rt for 18 h, with pH being retained at 10 12 by adding small amounts of 1n
NaOH. 1n NaOH was added until pH 14 was reached and the resulting
reaction mixture was washed with Et2O (2 Â ). AcOEt was added to the aq.
layer and the mixture was acidified at 08C with aq. KHSO4 solution (10%)
to pH 2 3. The aq. layer was separated and extracted with AcOEt (3 Â ).
The organic layers were combined, dried over MgSO4 and evaporated. The
residue was purified by recrystallisation (AcOEt/hexane) to yield acid 10c
(2.34 g, 86% from 9c) as a white solid. M.p. 126 1278C; [a]RDT ˆ ‡13.5
(c ˆ 0.81 in MeOH); 1H NMR (400 MHz, CDCl3/CD3OD 4:1): d ˆ 0.88 (d,
J(H,H) ˆ 6.6 Hz, 3H; CH3), 0.91 (d, J(H,H) ˆ 7.1 Hz, 3 H ; CH3), 0.91 (d,
J(H,H) ˆ 6.5 Hz, 3H; CH3), 1.12 1.18 (m, 1H; CHH), 1.14 (d, J(H,H) ˆ
6.7 Hz, 3H; CH3), 1.44 (s, 9H; tBu), 1.51 1.64 (m, 2H; CHHCH(CH3)2),
1.80 1.95 (m, 1H; CHCHN), 2.40 2.50 (m, 1H; CHCO), 3.60 3.70 (m,
1H; CHN); 13C NMR (100 MHz, CDCl3/CD3OD 4:1): d ˆ 12.0, 19.0, 21.4,
24.0, 26.6, 28.4, 37.3, 41.0, 46.2, 79.7, 156.4, 178.7; IR (CHCl3): nÄ ˆ 3440 (m),
2959 (m), 1707 (s), 1511 (s), 1455 (m), 1368 (s), 1111 cmÀ1 (m); MS (ESI
‡
(31) [M‡H] , 154 (13), 126 (23), 113 (65), 112 (25), 98 (100), 69 (8);
elemental analysis calcd (%) for C10H19NO (169.27): C 70.96, H 11.31, N
8.27; found: C 70.95, H 11.32, N 8.28.
(2R,3R,4R)-4-{[(tert-Butoxy)carbonyl]amino}-2,3-dimethyl-pentanoic
acid (10a): Lactam 8a (1.53 g, 12.0 mmol, drˆ 87:13,) was dissolved in 6n
HCl (90 mL) and stirred for 2 h under reflux. The solvent was evaporated
and the residue was triturated (acetone). After filtration hydrochloride 9a
(1.55 g, 71%, dr > 98:2) was obtained as a white solid. A solution of
hydrochloride 9a (1.24 g, 6.8 mmol) in 1n NaOH (14 mL) was cooled to
08C and a solution of Boc2O (1.92 g, 8.8 mmol) in dioxane (5 mL) was
added. The resulting mixture was stirred at rt for 18 h, with pH being
retained at 10 12 by adding small amounts of 1n NaOH. 1n NaOH was
then added until pH 14 was reached and the reaction mixture was washed
with Et2O (2 Â ). AcOEt was added to the aqueous layer and the resulting
mixture was acidified at 08C with aq. KHSO4 solution (10%) to pH 2 3.
The aqueous layer was separated and extracted with AcOEt (3 Â ). The
organic layers were combined, dried over MgSO4 and concentrated under
reduced pressure. Purification of the crude product by recrystallisation
(AcOEt/hexane) yielded acid 10a (1.41 g, 84% from 9a) as a white solid.
M.p. 143 1448C; [a]RDT ˆ À8.4 (c ˆ 0.52 in MeOH); 1H NMR (400 MHz,
CD3OD): d ˆ 0.86 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 1.06 (d, J(H,H) ˆ 7.1 Hz,
3H; CH3), 1.12 (d, J(H,H) ˆ 6.7 Hz, 3H; CH3), 1.43 (s, 9H; tBu), 1.89 1.97
‡
‡
pos.): m/z (%): 597 (4) [2M‡Na] , 310 (100) [M‡Na] ; MS (ESI neg.): m/z
(%): 595 (8) [2M‡Na À 2H]À, 286 (100) [M À H]À; elemental analysis calcd
(%) for C15H29NO4 (287.4): C 62.69, H 10.17, N 4.87; found: C 62.90, H
10.18, N 4.58.
(2R,3R,4R)-4-{[(tert-Butoxy)carbonyl]amino}-2-isobutyl-3-methyl-penta-
noic acid benzyl ester (11): Similar to a reported procedure,[32] amino acid
10c (1.44 g, 5.00 mmol) was dissolved in MeOH (50 mL). Aq. Cs2CO3
solution (20%) was added until a pH of 7 8 was reached. The solvent was
580
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0803-0580 $ 17.50+.50/0
Chem. Eur. J. 2002, 8, No. 3

g-Hexapeptides
573 584
‡
evaporated and the residue coevaporated with DMF (2 Â ) and dissolved in
DMF (50 mL). To the resulting solution BnBr (0.65 mL, 5.5 mmol) was
added. After stirring for 1 h at rt the solvent was evaporated and the
residue purified by flash column chromatography (pentane/Et2O 5:1).
Ester 11 (1.80 g, 95%) was obtained as a colorless oil. Rf ˆ 0.32 (pentane/
Et2O 4:1); [a]RDT ˆÆ 0.0 (c ˆ 0.98 in CHCl3); 1H NMR (400 MHz, CDCl3):
d ˆ 0.85 (d, J(H,H) ˆ 6.5 Hz, 3H; CH3), 0.86 (d, J(H,H) ˆ 6.6 Hz, 3H;
CH3), 0.88 (d, J(H,H) ˆ 7.1 Hz, 3H; CH3), 1.11 (d, J(H,H) ˆ 6.8 Hz, 3H;
CH3), 1.17 (ddd, J(H,H) ˆ 13.4, 10.1, 3.3 Hz, 1H; CHHCH), 1.41 (s, 9H;
tBu), 1.38 1.48 (m, 1H; CH(CH3)2), 1.61 1.68 (m, 1H; CHHCH), 1.67
1.95 (m, 1H; CHCHN), 2.49 (ddd, J(H,H) ˆ 11.6, 7.4, 3.3 Hz, 1H; CHO),
3.63 3.73 (m, 1H; CHN), 4.28 4.38 (brd, 1H; NH), 5.11 (d, J(H,H) ˆ
12.5 Hz, 1H; CHHPh), 5.14 (d, J(H,H) ˆ 12.5 Hz, 1H; CHHPh), 7.28 7.37
(m, 5H; Ph); 13C NMR (100 MHz, CDCl3): d ˆ 12.5, 18.6, 21.4, 23.8, 26.4,
28.4, 37.5, 40.6, 45.6, 49.3, 66.1, 79.1, 128.0, 128.1, 128.4, 136.1, 155.3, 175.8;
IR (CHCl3): nÄ ˆ 3620 (m), 3445 (w), 3008 (m), 2975 (s), 1709 (s), 1499 (m),
1454 (m), 1391 (m), 1368 (m), 1046 (s), 877 cmÀ1 (m); MS (MALDI): m/z
(MALDI): m/z (%): 508 (10) [M‡Na] , 408 (21), 390 (22), 231 (100); HR-
‡
‡
MS: calcd for [C25H47N3O6Na] : 508.3357; found: 508.3356 [M‡Na] .
Hexapeptide (15): Tripeptide 13 (225 mg, 0.450 mmol) was Boc-depro-
tected according to GP 3 and coupled with 14 (219 mg, 0.450 mmol) in THF
according to GP 4. After purification by flash column chromatography
(CH2Cl2/MeOH 15:1) hexapeptide 15 (221 mg, 57%) was obtained as a
colorless glass. Rf ˆ 0.47 (CH2Cl2/MeOH 10:1); m.p. 122 1238C; [a]DRT
ˆ
À1.9 (c ˆ 0.52 in MeOH); 1H NMR (500 MHz, CDCl3): d ˆ 0.86 1.01 (m,
30H; 10CH3), 1.09 1.26 (m, 4H, 2CH2CH(CH3)2), 1.43 (s, 9H; tBu), 1.63
1.72 (m, 4H), 1.75 1.90 (m, 2H), 2.03 2.37 (m, 16H), 3.12 3.39 (m, 12H;
6CH2N), 3.66 (s, 3H; OCH3), 4.98 (brt, 1H; NHBoc), 6.95 (brt, 1H; NH),
7.03 (brt, 1H; NH), 7.37 (brt, 1H; NH), 7.49 (brt, 1H; NH), 7.70 (brt, 1H;
NH); 13C NMR (125 MHz, CDCl3): d ˆ 18.2, 18.3, 18.4, 19.4, 19.7, 19.9, 20.1,
20.2, 22.6, 22.7, 22.8, 25.1, 25.2, 28.4, 29.1, 29.3, 31.3, 31.4, 31.5, 33.3, 34.0,
36.4, 36.9, 37.0, 37.5, 39.3, 40.8, 41.4, 41.6, 41.7, 41.9, 42.1, 42.3, 43.1, 44.2, 44.6,
44.7, 51.6, 79.6, 156.8, 172.5, 173.1, 173.6, 173.7, 173.8; IR (CHCl3): nÄ ˆ 3448
(w), 3321 (m), 2961 (s), 1699 (m), 1654 (s), 1518 (m), 1466 (m), 1387 (w),
1368 (m), 1100 (m), 1016 cmÀ1 (m); MS (FAB): m/z (%): 890 (11)
‡
(%): 400 (100) [M‡Na] , 344 (78), 300 (44), 278 (31), 273 (28), 192 (39);
elemental analysis calcd (%) for C22H35NO4 (377.52): C 69.99, H 9.34, N
3.71; found: C 69.93, H 9.50, N 3.69.
‡
‡
[M‡Na] , 868 (100) [M‡H] , 768 (40); HR-MS: calcd for
‡
‡
[C46H86N6O9Na] : 889.6349; found: 889.6344 [M‡Na] .
Dipeptide 12: Compound 4a (1.23 g, 4.50 mmol) was Boc-deprotected
according to GP 3 and coupled with acid 4b (0.98 g, 4.50 mmol) in THF
according to GP 4. After purification by flash column chromatography
(AcOEt/pentane 5:2) dipeptide 12 (1.57 g, 94%) was obtained as a white
solid. Rf ˆ 0.50 (AcOEt/pentane 5:1); m.p. 53 548C; [a]RDT ˆ ‡3.6 (c ˆ
0.61 in CHCl3); 1H NMR (500 MHz, CDCl3): d ˆ 0.88 (d, J(H,H) ˆ 6.6 Hz,
3H; CH3), 0.90 (d, J(H,H) ˆ 6.6 Hz, 3H; CH3), 0.95 (d, J(H,H) ˆ 6.7 Hz,
3H; CH3), 1.08 1.16 (m, 1H; CHHCH(CH3)2), 1.20 1.27 (m, 1H;
CHHCH(CH3)2), 1.44 (s, 9H; tBu), 1.62 1.70 (m, 1H; CH(CH3)2), 2.02
2.22 (m, 4H, CH2CO, 2CHCH2CO), 2.25 2.35 (m, 2H, CH2CO), 3.00
3.35 (m, 4H, 2CH2N), 3.68 (s, 3H; OCH3), 4.90 (brt, 1H; NHBoc), 6.72
(brt, 1H; NH); 13C NMR (125 MHz, CDCl3): d ˆ 18.0, 22.6, 22.7, 25.2, 28.4,
31.9, 33.1, 37.4, 41.1, 41.6, 43.1, 45.5, 51.6, 79.5, 156.8, 172.6, 173.8; IR
(CHCl3): nÄ ˆ 3452 (w), 2959 (m), 1704 (s), 1660 (m), 1513 (s), 1438 (w), 1368
Dipeptide 16: Compound 5a (1.22 g, 3.50 mmol) was Boc-deprotected
according to GP 3 and coupled with acid 5b (760 mg, 3.50 mmol) in CH2Cl2
according to GP 4. After purification by flash column chromatography
(Et2O/pentane 3:1 ! 5:1) dipeptide 16 (1.47 g, 94%) was obtained as a
colorless oil. Rf ˆ 0.29 (Et2O/pentane 5:1); [a]RDT ˆ À26.7 (c ˆ 0.93 in
CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 0.85 (d, J(H,H) ˆ 6.5 Hz, 3H;
CH3), 0.87 (d, J(H,H) ˆ 6.5 Hz, 3H; CH3), 1.11 (d, J(H,H) ˆ 6.9 Hz, 3H;
CH3), 1.24 1.31 (m, 1H; CHHCH(CH3)2), 1.43 (s, 9H; tBu), 1.43 1.57 (m,
1H; CH(CH3)2), 1.59 1.66 (m, 1H; CHHCH(CH3)2), 1.68 1.83 (m, 4H;
2CH2CH2N), 2.14 2.29 (m, 1H; CH(CH3)CO); 2.53 2.60 (m, 1H;
CHiBu), 2.96 3.03 (m, 1H; CHHN), 3.18 3.50 (m, 3H; 3CHHN), 4.71
(brt, 1H; NHBoc), 5.12 (app s, 2H; CH2Ph), 6.35 (brt, 1H; NH), 7.30 7.39
(m, 5H; Ph); 13C NMR (100 MHz, CDCl3): d ˆ 18.1, 22.0, 22.9, 26.1, 28.4,
32.3, 35.2, 37.5, 38.3, 38.6, 41.4, 41.5, 66.3, 79.4, 128.2, 128.3, 128.6, 136.0,
156.6, 175.8, 176.1; IR (CHCl3): nÄ ˆ 3452 (w), 2961 (m), 1706 (s), 1665 (m),
1512 (s), 1455 (w), 1368 (m), 1167 cmÀ1 (m); MS (FAB): m/z (%): 471 (91)
‡
(m), 1171 cmÀ1 (m); MS (FAB): m/z (%): 395 (27) [M‡Na] , 373 (97)
‡
[M‡H] , 371 (16), 317 (17), 273 (100), 256 (31), 142 (29); elemental
‡
‡
analysis calcd (%) for C19H36N2O5 (372.50): C 61.26, H 9.74, N 7.52; found:
C 61.39, H 9.74, N 7.49.
[M‡Na] , 449 (100) [M‡H] , 349 (81); elemental analysis calcd (%) for
C25H40N2O5 (448.60): C 66.94, H 8.99, N 6.24; found: C 66.93, H 8.78, N 6.27.
Tripeptide 13: Dipeptide 12 (1.44 g, 3.86 mmol) was Boc-deprotected
according to GP 3 and coupled with acid 4c (947 mg, 3.86 mmol) in THF
according to GP 4. After purification by flash column chromatography
(CH2Cl2/MeOH 20:1) tripeptide 13 (1.61 g, 83%) was obtained as a
Tripeptide 17: Dipeptide 16 (1.30 g, 2.90 mmol) was Boc-deprotected
according to GP 3 and coupled with acid 5c (711 mg, 2.90 mmol) in CHCl3
according to GP 4. After purification by flash column chromatography
(AcOEt/hexane 2:1 ! AcOEt) tripeptide 17 (1.05 g, 63%) was obtained as
colorless glass. Rf ˆ 0.51 (CH2Cl2/MeOH 10:1); m.p. 64 658C; [a]DRT
ˆ
a white solid. Rf ˆ 0.35 (CH2Cl2/MeOH 20:1); m.p. 129 1308C; [a]DRT
ˆ
1
À23.5 (c ˆ 1.04 in CHCl3); 1H NMR (500 MHz, CDCl3): d ˆ 0.85 (d,
J(H,H) ˆ 6.6 Hz, 3H; CH3), 0.87 (d, J(H,H) ˆ 6.5 Hz, 3H; CH3), 0.90 (d,
J(H,H) ˆ 6.4 Hz, 3H; CH3), 0.92 (d, J(H,H) ˆ 6.5 Hz, 3H; CH3), 1.11 (d,
J(H,H) ˆ 6.7 Hz, 3H; CH3), 1.26 1.31 (m, 1H; CHHCH(CH3)2), 1.43 (s,
9H; tBu), 1.47 1.55 (m, 2H), 1.55 1.64 (m, 2H), 1.68 1.86 (m, 6H), 2.28
2.53 (m, 1H; CH(CH3)CO), 2.54 2.59 (m, 1H; CHiBu), 2.98 3.08 (m,
1H; CHHN), 3.08 3.19 (m, 2H; 2CHHN), 3.19 3.30 (m, 2H; 2CHHN),
3.32 3.42 (m, 1H; CHHN), 4.74 (brt, 1H; NHBoc), 5.09 5.14 (m, 2H;
CH2Ph), 6.34 (brt, 1H; NH), 6.80 (brt, 1H; NH), 7.30 7.43 (m, 5H; Ph);
13C NMR (125 MHz, CDCl3): d ˆ 18.0, 20.3, 21.0, 22.0, 23.0, 26.1, 28.4, 30.4,
30.8, 32.4, 34.4, 37.4, 37.5, 38.2, 38.9, 41.4, 41.5, 51.9, 66.2, 79.5, 128.2, 128.6,
136.0, 156.7, 175.3, 175.9, 176.0; IR (CHCl3): nÄ ˆ 3448 (w), 2962 (m), 1703
(s), 1662 (s), 1514 (s), 1454 (m), 1391 (w), 1368 (m), 1166 cmÀ1 (m); MS
À1.4 (c ˆ 0.60 in CHCl3); H NMR (500 MHz, CDCl3): d ˆ 0.87 1.00 (m,
15H; 5CH3), 1.10 1.16 (m, 1H; CHHCH(CH3)2), 1.18 1.24 (m, 1H;
CHHCH(CH3)2), 1.43 (s, 9H; tBu), 1.63 1.71 (m, 2H), 1.80 1.90 (m, 1H),
2.04 2.36 (m, 8H), 3.07 3.35 (m, 6H; 3CH2N), 3.67 (s, 3H; OCH3), 4.99
(brt, 1H; NHBoc), 6.72 (brt, 1H, NH), 6.77 (brt, 1H; NH); 13C NMR
(125 MHz, CDCl3): d ˆ 18.4, 19.5, 20.0, 22.7, 22.7, 25.2, 28.5, 29.3, 31.2, 33.2,
36.7, 37.5, 41.4, 41.7, 42.0, 42.2, 43.1, 44.7, 51.6, 79.3, 156.7, 172.4, 173.4, 173.8;
IR (CHCl3): nÄ ˆ 3449 (w), 2962 (m), 1702 (s), 1657 (s), 1514 (s), 1438 (w),
À1
‡
1368 (m), 1171 cm
(m); MS (FAB): m/z (%): 500 (100) [M‡H] , 400
(42); elemental analysis calcd (%) for C26H49N3O6 (499.69): C 62.50, H 9.88,
N 8.41; found: C 62.56, H 9.77, N 8.36.
Tripetide (14): A solution of tripeptide 13 (375 mg, 0.750 mmol) in MeOH
(1 mL) was treated with 0.75n NaOH (1 mL). After stirring for 5.5 h at rt
the mixture was diluted with H2O and AcOEt and acidified to pH 2 with 1n
HCl. The aq. layer was separated and extracted with AcOEt (2 Â ). The
organic layers were combined, dried over MgSO4 and concentrated under
reduced pressure. After purification by flash column chromatography
(CH2Cl2/MeOH 10:1) peptide 14 (345 mg, 95%) was obtained as a white
‡
‡
(FAB): m/z (%): 598 (24) [M‡Na] , 576 (100) [M‡H] , 476 (76);
elemental analysis calcd (%) for C32H53N3O6 (575.79): C 66.75, H 9.28, N
7.30; found: C 66.87, H 9.39, N 7.38.
Tripeptide 18: Debenzylation of tripeptide 17 (461 mg, 0.800 mmol)
according to GP 6 yielded tripeptide 18 (387 mg, 99%) as a white solid.
M.p. 108 1108C; [a]RDT ˆ ‡0.2 (c ˆ 1.00 in CHCl3); 1H NMR (400 MHz,
CD3OD): d ˆ 0.89 (d, J(H,H) ˆ 6.6 Hz, 3H; CH3), 0.90 (d, J(H,H) ˆ 6.3 Hz,
3H; CH3), 0.91 (d, J(H,H) ˆ 6.4 Hz, 3H; CH3), 0.94 (d, J(H,H) ˆ 6.7 Hz,
3H; CH3), 1.11 (d, J(H,H) ˆ 6.9 Hz, 3H; CH3), 1.23 1.31 (m, 1H), 1.43 (s,
9H; tBu), 1.50 1.94 (m, 10H), 2.36 2.50 (m, 2H; 2CHCO), 2.71 3.08 (m,
3H; 3CHHN), 3.14 3.28 (m, 3H; 3CHHN); 13C NMR (100 MHz,
CD3OD): d ˆ 18.4, 20.7, 21.3, 22.5, 23.5, 27.4, 28.9, 31.0, 31.8, 33.5, 34.6,
38.4, 38.6, 39.5, 40.0, 42.7, 42.9, 52.7, 80.0, 158.5, 177.7, 178.7, 180.0; IR
(CHCl3): nÄ ˆ 3447 (w), 2963 (m), 1699 (s), 1660 (s), 1516 (s), 1453 (m), 1392
solid. Rf ˆ 0.40 (CH2Cl2/MeOH 7:1); m.p. 173 1748C (decomp); [a]DRT
ˆ
‡9.9 (c ˆ 0.98 in MeOH); 1H NMR (500 MHz, CD3OD): d ˆ 0.88 0.97
(m, 15H; 5CH3), 1.10 1.30 (m, 2H; CH2CH(CH3)2), 1.43 (s, 9H; tBu),
1.63 1.80 (m, 2H; 2CH(CH3)2), 1.89 1.97 (m, 1H; CHCH2N), 2.04 2.26
(m, 8H; 3CH2CO, 2CHCH2N), 2.97 3.07 (m, 3H; CH2N, CHHN), 3.10
3.14 (m, 1H; CHHN), 3.21 3.26 (m, 2H; 2CHHN); 13C NMR (125 MHz,
CD3OD): d ˆ 18.2, 19.0, 19.9, 23.0, 23.5, 26.4, 28.9, 29.6, 32.4, 35.0, 36.5,
40.2, 41.7, 42.7, 42.9, 43.1, 44.3, 46.1, 80.2, 158.7, 175.2, 176.0, 180.8; IR
(KBr): nÄ ˆ 3333 (m), 2976 (s), 1686 (s), 1654 (s), 1541 (s), 1458 (m), 1389
(m), 1367 (m), 1249 (m), 1170 (s), 1076 (w), 1021 (w), 668 cmÀ1(w); MS
‡
(m), 1368 (m), 1168 cmÀ1 (m); MS (FAB): m/z (%): 508 (100) [M‡Na] ,
Chem. Eur. J. 2002, 8, No. 3
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0803-0581 $ 17.50+.50/0
581

D. Seebach et al.
FULL PAPER
‡
‡
486 (15) [M‡H] , 386 (22); HR-MS: calcd for [C25H47N3O6Na] : 508.3357;
Tetrapeptide 22: Dipeptide 20 (298 mg, 0.590 mmol) was Boc-deprotected
according to GP 3 and coupled with dipeptide 21 (245 mg, 0.590 mmol)
according to GP 5. After purification by flash column chromatography
(hexane/AcOEt 3:2) tetrapeptide 22 (404 mg, 85%) was obtained as a
white solid. Rf ˆ 0.31 (hexane/AcOEt 1:1); m.p. 160 1618C; [a]RDT ˆ ‡3.8
(c ˆ 0.53 in CHCl3); CD (0.2m in MeOH): ‡1.2  104 degcm2 dmolÀ1
‡
found: 508.3349 [M‡Na] .
Hexapeptide 19: Tripeptide 17 (288 mg, 0.500 mmol) was Boc-deprotected
according to GP 3 and coupled with 18 (243 mg, 0.500 mmol) in CH2Cl2
according to GP 4. After purification by flash column chromatography
(CH2Cl2/MeOH 30:1 ! 7:1) hexapeptide 19 (401 mg, 85%) was obtained
1
(215 nm); H NMR (400 MHz, CDCl3): d ˆ 0.82 (d, J(H,H) ˆ 6.6 Hz, 6H;
as a white solid. Rf ˆ 0.56 (CH2Cl2/MeOH 7:1); m.p. 199 2018C; [a]DRT
ˆ
2CH3), 0.82 (d, J(H,H) ˆ 6.5 Hz, 3H; CH3), 0.85 (d, J(H,H) ˆ 7.2 Hz, 3H;
CH3), 0.85 (d, J(H,H) ˆ 6.6 Hz, 6H; 2CH3), 0.88 (d, J(H,H) ˆ 7.1 Hz, 3H;
CH3), 0.98 (d, J(H,H) ˆ 6.8 Hz, 3H; CH3), 1.01 (d, J(H,H) ˆ 7.0 Hz, 3H;
CH3), 1.04 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 1.05 (d, J(H,H) ˆ 6.9 Hz, 3H;
CH3), 1.06 (d, J(H,H) ˆ 6.8 Hz, 3H; CH3), 1.08 1.22 (m, 2H;
2CHHCH(CH3)2), 1.10 (d, J(H,H) ˆ 6.9 Hz, 3H; CH3), 1.14 (d, J(H,H) ˆ
6.6 Hz, 3H; CH3), 1.27 1.49 (m, 2H; 2CH(CH3)2), 1.42 (s, 9H; tBu), 1.52
1.59 (m, 1H; CHHCH(CH3)2), 1.61 1.90 (m, 5H; CHHCH(CH3)2,
4CHCHN), 2.01 2.09 (m, 1H; CHCO), 2.17 2.29 (m, 2H; 2CHCO),
2.60 2.66 (m, 1H; CHCO), 3.75 3.85 (m, 1H; CHN), 4.00 4.10 (m, 2H;
2CHN), 4.15 4.21 (m, 1H; CHN), 4.30 (d, J(H,H) ˆ 9.6 Hz, 1H; NHBoc),
5.00 (d, J(H,H) ˆ 12.3 Hz, 1H; CHHPh), 5.21 (d, J(H,H) ˆ 12.3 Hz, 1H;
CHHPh), 6.13 (d, J(H,H) ˆ 8.5 Hz, 1H; NH), 6.83 (d, J(H,H) ˆ 8.4 Hz,
1H; NH); 7.25 7.36 (m, 5H; Ph), 7.83 (d, J(H,H) ˆ 8.7 Hz, 1H; NH);
13C NMR (100 MHz, CDCl3): d ˆ 9.7, 10.0, 10.1, 11.2, 15.5, 16.3, 19.7, 19.7,
19.9, 20.7, 21.3, 21.9, 23.9, 24.3, 26.0, 26.6, 28.4, 36.9, 39.4, 40.4, 40.5, 40.6,
40.9, 43.4, 44.5, 46.4, 47.1, 47.4, 47.7, 47.9, 48.1, 66.2, 79.3, 127.9, 128.4, 128.5,
136.4, 156.0, 175.6, 175.9, 176.1, 176.7; IR (CHCl3): nÄ ˆ 3306 (m), 2974 (s),
1702 (s), 1659 (s), 1506 (s), 1454 (m), 1368 (m), 1166 (s), 1129 (w), 1077 cmÀ1
1
À47.3 (c ˆ 0.81 in CF3CH2OH); H NMR (500 MHz, CDCl3/CD3OD 1:1):
d ˆ 0.86 (d, J(H,H) ˆ 6.6 Hz, 3H; CH3), 0.87 (d, J(H,H) ˆ 6.5 Hz, 3H;
CH3), 0.88 (d, J(H,H) ˆ 6.5 Hz, 3H; CH3), 0.90 (d, J(H,H) ˆ 6.7 Hz, 9H;
3CH3), 0.92 (d, J(H,H) ˆ 6.7 Hz, 3H; CH3), 0.94 (d, J(H,H) ˆ 6.6 Hz, 3H;
CH3), 1.09 (d, J(H,H) ˆ 6.9 Hz, 3H; CH3), 1.11 1.19 (m, 1H), 1.13 (d,
J(H,H) ˆ 6.9 Hz, 3H; CH3), 1.26 1.32 (m, 1H; CHHCH(CH3)2), 1.45 (s,
9H; tBu), 1.45 1.63 (m, 16H), 1.68 1.86 (m, 12H), 1.96 2.02 (m, 1H),
2.30 2.39 (m, 3H; 3CHCO), 2.55 2.61 (m, 1H; CHCO), 2.97 3.40 (m,
12H; 6CH2N), 5.10 (d, J(H,H) ˆ 12.3 Hz, 1H; CHHPh), 5.15 (d, J(H,H) ˆ
12.3 Hz, 1H; CHHPh), 7.28 7.38 (m, 5H; Ph); 13C NMR (125 MHz,
CDCl3/CD3OD 1:1): d ˆ 17.7, 18.4, 20.3, 20.4, 20.8, 21.0, 22.1, 22.5, 23.1, 23.4,
26.4, 26.4, 28.6, 29.6, 30.3, 30.8, 30.9, 32.6, 33.1, 34.0, 34.1, 37.4, 37.5, 37.7, 37.9,
38.2, 39.3, 41.8, 41.9, 42.3, 42.4, 51.3, 52.1, 66.7, 79.7, 128.5, 128.6, 128.8, 136.3,
157.3, 176.5, 176.5, 176.7, 177.0, 177.6, 177.7; IR (KBr): nÄ ˆ 3298 (s), 3089 (w),
2960 (s), 1734 (m), 1685 (s), 1639 (s), 1552 (s), 1451 (m), 1388 (m), 1367 (m),
1248 (m), 1208 (m), 1171 (m), 991 (w), 872 (w), 698 cmÀ1 (m); MS (FAB):
‡
‡
m/z (%): 965 (100) [M‡Na] , 943 (92) [M‡H] , 843 (70); HR-MS: calcd
‡
‡
for [C52H90N6O9Na] : 965.6662; found: 965.6657 [M‡Na] .
Dipeptide 20: Amino acid 11 (1.09 g, 2.90 mmol) was Boc-deprotected
according to GP 3. The resulting CF3CO2H salt was dissolved in CH2Cl2
(28 mL). The solution was cooled to 08C and amino acid 10a (711 mg,
2.90 mmol), NMM (1.6 mL, 14.5 mmol) and HATU (1.33 g, 3.50 mmol)
were added. The mixture was stirred for 2 h at 08C and for 16 h at rt. After
dilution with AcOEt the mixture was washed with 1m HCl (3 Â ), sat. aq.
NaHCO3 solution (3 Â ) and sat. aq. NaCl solution (3 Â ). The organic layer
was dried over MgSO4 and concentrated under reduced pressure.
Purification by flash column chromatography (pentane/Et2O 2:1) yielded
peptide 20 (1.23 g, 84%) as a white solid. Rf ˆ 0.44 (pentane/Et2O 2:3); m.p.
133 1348C; [a]RDT ˆ À0.86 (c ˆ 1.01 in CHCl3); 1H NMR (400 MHz,
CDCl3): d ˆ 0.75 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 0.81 0.86 (m, 9H;
3CH3), 1.01 (d, J(H,H) ˆ 7.1 Hz, 3H; CH3), 1.05 (d, J(H,H) ˆ 6.9 Hz, 3H;
CH3), 1.13 (d, J(H,H) ˆ 6.6 Hz, 3H; CH3), 1.19 1.29 (m, 1H; CHHCH),
1.36 1.46 (m, 1H; CH(CH3)2), 1.42 (s, 9H; tBu), 1.55 1.69 (m, 2H;
CHHCH(CH3)2, CHCHN), 1.83 1.92 (m, 1H; CHCHN), 2.05 (dq,
J(H,H) ˆ 10.3, 6.8 Hz, 1H; CH(CH3)CO), 2.73 2.79 (m, 1H; CHiBu),
3.89 3.96 (m, 1H, CHN), 4.00 4.08 (m, 1H; CHN), 4.27 (d, J(H,H) ˆ
9.2 Hz, 1H; NHBoc); 5.00 (d, J(H,H) ˆ 12.3 Hz, 1H; CHHPh), 5.24 (d,
J(H,H) ˆ 12.3 Hz, 1H; CHHPh), 7.05 (d, J(H,H) ˆ 8.0 Hz, 1H; NH), 7.27
7.37 (m, 5H; Ph); 13C NMR (100 MHz, CDCl3): d ˆ 9.9, 10.3, 15.3, 19.8,
20.0, 21.1, 24.0, 26.5, 28.4, 38.7, 40.5, 41.1, 43.8, 46.6, 47.1, 48.2, 66.2, 79.3,
128.0, 128.4, 128.6, 136.3, 156.2, 175.9, 176.3; IR (CHCl3): nÄ ˆ 3619 (m),
3443 (w), 2976 (s), 1701 (s), 1654 (m), 1506 (m), 1454 (m), 1391 (m), 1368
(m), 1166 (m), 1046 (s), 877 cmÀ1 (m); MS (MALDI): m/z (%): 543 (10)
‡
(w); MS (MALDI): m/z (%): 824 (34) [M‡Na] , 724 (100); elemental
analysis calcd (%) for C46H80N4O7 (801.16): C 68.96, H 10.06, N 6.99; found:
C 68.87, H 10.24, N 7.03.
Hexapeptide 23: Tetrapeptide 22 (160 mg, 0.200 mmol) was Boc-depro-
tected according to GP 3 and coupled with dipeptide 21 (91 mg,
0.220 mmol) according to GP 5. After flash column chromatography
(CH2Cl2
!
CH2Cl2/MeOH 20:1) hexapeptide 23 (181 mg, 82%) was
obtained as a white solid. Rf ˆ 0.55 (CH2Cl2/MeOH 20:1); m.p. 95 1158C;
[a]RDT ˆ ‡22.1 (c ˆ 0.56 in CHCl3); CD (0.2m in MeOH): ‡4.9 Â
104 degcm2 dmolÀ1 (215 nm);
CD
(0.2m
in
CH3CN):
4.9 Â
104 degcm2 dmolÀ1
(205 nm),
À5.6 Â 104 degcm2 dmolÀ1
(193 nm);
1H NMR (500 MHz, CDCl3): d ˆ 0.82 0.92 (m, 33H; 11CH3), 0.99 1.02
(m, 9H; 3CH3), 1.05 1.20 (m, 24H; 7CH3, 3CHH), 1.44 (s, 9H; tBu),
1.44 1.56 (m, 5H; 3CHMe2, 2CHH), 1.66 1.73 (m, 1H; CHH), 1.75 1.92
(m, 6H; 6CHCHN), 2.00 2.06 (m, 1H; CHCO), 2.18 2.36 (m, 4H;
4CHCO), 2.62 2.66 (m, 1H; CHCO), 3.70 3.85 (m, 1H; CHN), 3.94
4.07 (m, 3H; 3CHN), 4.15 4.23 (m, 2H; 2CHN), 4.30 (d, J(H,H) ˆ 9.9 Hz,
1H; NHBoc), 5.00 (d, J(H,H) ˆ 12.3 Hz, 1H; CHHPh), 5.18 (d, J(H,H) ˆ
12.3 Hz, 1H; CHHPh), 5.89 (d, J(H,H) ˆ 8.6 Hz, 1H; NH), 6.34 (d,
J(H,H) ˆ 8.4 Hz, 1H; NH), 6.43 (d, J(H,H) ˆ 9.4 Hz, 1H; NH), 7.26 7.36
(m, 5H; Ph), 7.39 (d, J(H,H) ˆ 8.1 Hz, 1H; NH), 8.01 (d, J(H,H) ˆ 8.7 Hz,
1H; NH); 13C NMR (125 MHz, CDCl3): 9.2, 9.5, 9.5, 9.8, 10.0, 11.6, 16.1,
16.6, 17.1, 19.4, 19.5, 19.8, 20.5, 20.9, 21.0, 21.4, 21.8, 22.0, 23.9, 24.3, 24.5,
25.8, 25.9, 26.6, 28.4, 35.8, 39.3, 39.7, 40.1, 40.2, 40.3, 40.4, 40.9, 43.5, 43.7,
44.7, 46.0, 46.5, 46.9, 47.2, 47.3, 47.4, 47.7, 47.9, 48.4, 66.1, 79.3, 127.9, 128.4,
128.5, 136.4, 155.9, 175.1, 175.6, 175.8, 176.4, 176.6, 177.0; IR (CHCl3): nÄ ˆ
3311 (w), 2972 (m), 1710 (s), 1650 (s), 1506 (m), 1452 (m), 1366 (m), 1166
‡
‡
[M‡K] , 527 (15) [M‡Na] , 427 (100); elemental analysis calcd (%)
for C29H48N2O5 (504.71): C 69.01, H 9.59, N 5.55; found: C 68.89, H 9.52, N
5.46.
‡
‡
(m); MS (MALDI): m/z (%): 1120 (93) [M‡Na] , 1098 (19) [M‡H] , 1020
Dipeptide 21: Debenzylation of dipeptide 20 (606 mg, 1.20 mmol) accord-
ing to GP 6 yielded dipeptide 21 (498 mg, quantitative) as a white solid.
M.p. 77 818C; [a]RDT ˆ À11.4 (c ˆ 0.95 in MeOH); 1H NMR (400 MHz,
CD3OD): d ˆ 0.84 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 0.89 (d, J(H,H) ˆ 6.8 Hz,
3H; CH3), 0.91 (d, J(H,H) ˆ 6.9 Hz, 3H; CH3), 1.01 (d, J(H,H) ˆ 6.9 Hz,
3H; CH3), 1.01 (d, J(H,H) ˆ 7.0 Hz, 3H; CH3), 1.09 (d, J(H,H) ˆ 6.8 Hz,
3H; CH3), 1.14 (d, J(H,H) ˆ 6.7 Hz, 3H; CH3), 1.18 1.28 (m, 1H;
CH(CH3)2), 1.44 (s, 9H; tBu), 1.49 1.60 (m, 2H; CH2), 1.67 1.75 (m,
1H; CHCHO), 1.80 1.88 (m, 1H; CHCHO), 2.22 2.30 (m, 1H;
CH(CH3)CO), 2.66 2.71 (m, 1H; CHiBu), 3.81 3.91 (m, 1H; CHN),
3.91 3.98 (m, 1H; CHN), 6.29 (d, J(H,H) ˆ 9.0 Hz, 1H; NHBoc), 7.87 (d,
J(H,H) ˆ 7.6 Hz, 1H; NH); 13C NMR (100 MHz, CD3OD): d ˆ 10.8, 11.4,
15.0, 19.5, 20.1, 21.9, 24.4, 27.8, 28.9, 39.1, 41.7, 42.4, 44.7, 47.5, 48.8, 49.7, 79.8,
158.4, 178.9, 179.9; IR (KBr): nÄ ˆ 3316 (m), 2963 (s), 1686 (s), 1654 (s), 1560
(s), 1458 (m), 1368 (m), 1254 (w), 1183 (m), 1141 (m), 836 (w), 722 cmÀ1
‡
(100); HR-MS: calcd for [C63H112N6O9Na] : 1119.8383; found: 1119.8385
‡
[M‡Na] .
Hexapeptide 1: 5n NaOH (1.5 mL) was added to a solution of hexapeptid
15 (70 mg, 0.081 mmol) in CF3CH2OH (0.6 mL). After stirring at rt for 12 h
the mixture was acidified with 1n HCl to pH 2 and extracted with AcOEt
(3 Â ). The organic layers were combined, dried over MgSO4 and
concentrated under reduced pressure. The resulting crude product was
Boc-deprotected according to GP 3. After purification by preparative
HPLC (30 ! 40% B in 15 min, tR ˆ 13.0 min) according to GP 7, peptide 1
(37 mg, 53%) was obtained as a white solid. CD (0.2m in MeOH) ˆ À7 Â
103 degcm2 dmolÀ1 (212 nm); 1H NMR (500 MHz, CD3OD): d ˆ 0.87 1.00
(m, 30H; 10CH3), 1.08 1.25 (m, 4H; 2CH2CH(CH3)2), 1.63 1.85 (m, 4H;
4CH(CH3)2), 1.93 2.34 (m, 17H), 2.45 2.51 (m, 1H), 2.90 2.94 (m, 1H),
3.00 3.03 (m, 1H), 3.09 3.15 (m, 7H), 3.19 3.27 (m, 3H); 13C NMR
(125 MHz, CD3OD): d ˆ 18.1, 18.2, 18.7, 19.1, 19.7, 19.9, 23.1, 23.2, 23.3,
26.4, 26.4, 29.8, 31.1, 32.5, 32.7, 34.7, 35.2, 36.8, 37.3, 38.4, 40.7, 41.1, 41.7, 42.1,
42.2, 42.7, 42.8, 42.9, 43.3, 44.0, 46.1, 46.3, 175.1, 175.2, 175.2, 175.3, 175.8,
‡
(w); MS (MALDI): m/z (%): 437 (28) [M‡Na] , 337 (100), 301 (60), 298
‡
(56), 210 (80); HR-MS: calcd for [C22H42N2O5Na] : 437.2986; found:
‡
437.2985 [M‡Na] .
582
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0803-0582 $ 17.50+.50/0
Chem. Eur. J. 2002, 8, No. 3

g-Hexapeptides
573 584
‡
‡
176.7; MS (FAB): m/z (%): 791 (27) [M‡K] , 775 (42) [M‡Na] , 753 (100)
Vˆ 2580.1(12) ä3, Z ˆ 2, 1calcd ˆ 1.031 gcmÀ3, m ˆ 0.543mmÀ1, crystal size
0.30 Â 0.20 Â 0.02 mm. A total of 4479 unique reflections (3.32 < 2V<
66.238) were processed of which 1140 were considered significant with Inet
> 3s(Inet). Final residuals were R ˆ 0.0898 and wR2 ˆ 0.1961 (GOF ˆ 1.525)
‡
‡
[M‡H] ; HR-MS: calcd for [C40H77N6O7] : 753.5848; found: 753.5837
‡
[M‡H] .
Hexapeptide 2: Hexapeptide 18 (132 mg, 0.140 mmol) was debenzylated
according to GP 6. The resulting carboxylic acid was Boc-deprodected
according to GP 3. After purification by preparative HPLC (25 ! 35% B
in 35 min, tR ˆ 15.8 min) according to GP 7 hexapeptide 2 (98 mg, 81%)
was obtained as a white solid. [a]RDT ˆ À17.6 (c ˆ 0.68 in MeOH); 1H NMR
(500 MHz, CD3OD): d ˆ 0.88 0.95 (m, 21H; 7CH3), 0.97 (d, J(H,H) ˆ
6.7 Hz, 3H; CH3), 1.10 (d, J(H,H) ˆ 6.9 Hz, 3H; CH3), 1.12 (d, J(H,H) ˆ
6.9 Hz, 3H; CH3), 1.16 1.19 (m, 1H; CHHCH(CH3)2), 1.20 1.30 (m, 1H;
CHHCH(CH3)2), 1.48 2.00 (m, 20H), 2.29 2.50 (m, 4H; 4CHCO), 2.77
2.89 (m, 2H; CH2N), 3.03 3.29 (m, 10H; 5CH2N); 13C NMR (125 MHz,
CD3OD): d ˆ 18.3, 18.4, 20.4, 20.8, 21.2, 21.2, 22.5, 22.6, 23.5, 23.7, 27.2, 27.4,
28.4, 30.6, 31.6, 31.9, 33.5, 33.6, 34.1, 34.6, 34.9, 38.4, 38.6, 38.7, 38.8, 39.2,
39.5, 40.0, 42.6, 42.9, 43.3, 43.8, 52.4, 52.6, 176.4, 177.4, 178.0, 178.5, 178.8,
179.7; IR (KBr): nÄ ˆ 3303 (s), 3085 (m), 2963 (s), 1646 (s), 1559 (s), 1458
(m), 1388 (m), 1202 (s), 1139 (m), 836 (w), 799 (w), 721 cmÀ1 (w); MS
for 243 parameters. D1 (max, min) ˆ 0.242, À0.256 eAÀ3
.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge Crystallo-
graphic Data Centre as supplementary publication no. CCDC-167172 (9b),
CCDC-167171 (20) and CCDC-182/1874 (22). Copies of the data can be
obtained free of charge on application to CCDC, 12 Union Road,
Cambridge CB21EZ, UK (fax: (‡44)1223-336-033; e-mail: deposit@
ccdc.cam.ac.uk).
Acknowledgements
We thank Dr. B. Schweizer of the Crystallography Service Division for
determining the X-ray crystal structures and Mrs. B. Brandenberger for
recording NMR spectra. Mr. F. Luck is thanked for performing the high-
pressure hydrogenations. We gratefully acknowledge the Novartis Stipen-
dienfonds for a scholarship granted to M.B. and Novartis Pharma AG
(Basel) for ongoing financial support.
‡
‡
‡
(FAB): m/z (%): 791 (15) [M‡K] , 775 (30) [M‡Na] , 753 (100) [M‡H] ;
‡
‡
HR-MS: calcd for [C40H76N6O7Na] : 775.5679; found: 775.5672 [M‡Na] .
Hexapeptide 3: Hexapeptide 22 (60 mg, 0.055 mmol) was debenzylated
according to GP 6. The resulting carboxylic acid was Boc-deprodected
according to GP 3. Purification by preparative HPLC (35 ! 75% B in
30 min, tR ˆ 12.7 min) according to GP 7 yielded hexapeptide 3 (47 mg,
84%) as a white solid. [a]RDT ˆ ‡8.5 (c ˆ 0.42 in MeOH); CD (0.2m in
MeOH): ‡3.0  104 degcm2 dmolÀ1 (213 nm); CD (0.2m in CH3CN): 5.8 Â
104 degcm2 dmolÀ1 (202 nm); 1H NMR (500 MHz, CD3OD): d ˆ 0.87 1.24
(m, 63H; 20CH3, 3CHH), 1.34 (d, J(H,H) ˆ 6.8 Hz, 3H; CH3), 1.42 1.57
(m, 4H; CHH, 3CH(CH3)2), 1.60 1.66 (m, 1H; CHH), 1.71 1.96 (m, 7H;
[1] Part of the Ph.D. thesis of M.B. (ETH Dissertation No. 14409) and of
the projected Ph.D. thesis of M.R.
[2] T. E. Creighton, Proteins: Structures and Molecular Properties, 2nd
ed., Freeman, New York, 1993.
[3] For reviews on b-peptides see: D. Seebach, J. L. Matthews, Chem.
Commun. 1997, 2015 2022; S. H. Gellman, Acc. Chem. Res. 1998, 31,
173 180; K. Gademann, T. Hintermann, J. V. Schreiber, Curr. Med.
Chem. 1999, 6, 905 924; W. F. DeGrado, J. P. Schneider, Y. Hamuro,
J. Pept. Res. 1999, 54, 206 217.
6
CHCHN, CHH), 2.15 2.21 (m, 1H; CHCO), 2.39 2.46 (m, 2H;
2CHCO), 2.49 2.54 (m, 1H; CHCO), 2.56 2.64 (m, 1H; CHCO), 2.64
2.69 (m, 1H; CHCO), 3.52 3.54 (m, 1H; CHN), 3.91 3.98 (m, 3H;
3CHN), 4.05 4.10 (m, 1H; CHN), 4.22 4.26 (m, 1H; CHN); 13C NMR
(125 MHz, CD3OD): d ˆ 10.7, 10.9, 11.0, 11.9, 12.5, 13.1, 16.0, 16.3, 16.8,
18.4, 19.6, 19.9, 20.0, 20.3, 21.2, 22.0, 22.1, 22.9, 24.3, 24.4, 24.9, 27.3, 27.7, 27.9,
37.3, 37.5, 40.5, 40.8, 41.4, 41.4, 41.7, 42.0, 42.8, 44.6, 45.1, 45.9, 47.1, 47.5, 47.9,
176.5, 178.0, 178.3, 178.9, 179.0, 179.4; IR (KBr): nÄ ˆ 2969 (m), 1645 (s),
1541 (s), 1458 (m), 1388 (m), 1203 (m), 1136 cmÀ1 (m); MS (ESI pos.): m/z
[4] M. Hagihara, N. J. Anthony, T. J. Stout, J. Clardy, S. L. Schreiber, J.
Am. Chem. Soc. 1992, 114, 6568 6570.
[5] Sheet-like structures are also found in some polymeric materials
consisting of g-amino acid building blocks: a) nylon 4: R. J. Freder-
icks, T. H. Doyne, R. S. Sprague, J. Polym. Sci. Part A-2 1966, 4, 899
911; R. J. Fredericks, T. H. Doyne, R. S. Sprague, J. Polym. Sci. Part
A-2 1966, 4, 913 922; M. A. Bellinger, A. J. Waddon, E. D. T. Atkins,
W. J. MacKnight, Macromolecules 1994, 27, 2130 2135; b) alternate
copolymer of 4-aminobutyric acid and a-isobutyl-l-glutamic acid:
M. T. Casas, J. PuiggalÌ, Polymer 2000, 41, 5437 5441; c) poly(g-
(%): 930 (6) [M‡Na] , 908 (38) [M‡H] , 466 (100) [M‡Na‡H]2‡; MS
‡
‡
(ESI neg.): m/z (%): 1020 (66) [M‡CF3CO2]À, 942 (33) [M‡Cl]À, 906 (100)
[M À H]À; HR-MS: calcd for [C51H97N6O7Na2] : 951.7209; found: 951.7163
‡
‡
[M À H‡2Na] .
X-ray crystal structure analysis of 9a, 20, and 22: The reflections were
measured on an Enraf Nonius CAD-4 Diffractometer with CuKa radiation
(graphite monochromator, l ˆ 1.54184 ä). Structures of 9a and 20 was
solved by direct method with SIR97.[33] The non-H atoms were refined
anisotropically with SHELXL-97[34] (full-matrixlast-squares on F2). Part of
the structure of 22 was solved by direct methods with SIR97, the remaining
non hydrogen atoms were found from a difference Fourier map. The non-H
atoms were refined isotropically with SHELXL-97. The number of
observed reflections did not allow anisotropic refinement. Hydrogen
atoms were calculated at idealised positions and included in the structure
factor calculation with fixed isotropic displacement parameters.
ƒ
glutamic acid)esters: J. PuiggalÌ, S. Munoz-Guerra, A. RodrÌguez-
¬
Galan, C. Alegre, J. A. Subirana, Makromol Chem. Macromol. Symp.
1988, 20/21, 167 182.
[6] S. Hanessian, X. Luo, R. Schaum, S. Michnick, J. Am. Chem. Soc. 1998,
120, 8569 8570.
[7] T. Hintermann, K. Gademann, B. Jaun, D. Seebach, Helv. Chim. Acta
1998, 81, 983 1002.
[8] Helical structures have also been proposed for polymeric materials
consisting of g-amino acid building blocks: a) poly(g-glutamic acid)-
esters: see ref. [5c]; b) unionized poly(g-glutamic acid): H. N. Rydon,
¬
ƒ
J. Chem. Soc. 1964, 1328 1333; D. Zanuy, C. Aleman, S. Munoz-
Crystal data for 9a (C7H16ClNO2): M ˆ 181.66, Tˆ 293(2) K, orthorhom-
bic, space group P212121, a ˆ 6.318(2), b ˆ 11.889(4), c ˆ 13.944(4) ä, Vˆ
1047.4(6) ä3, Z ˆ 4, 1calcd ˆ 1.152 gcmÀ3, m ˆ 2.926mmÀ1, crystal size 0.40 Â
0.40 Â 0.25 mm. A total of 1805 reflections were collected (4.89 < 2V<
66.928) of which 1529 were independent and 1336 were considered
significant with Inet > 3s(Inet). Final residuals were R ˆ 0.0486 and wR2 ˆ
0.1358 (GOF ˆ 1.270) for 105 parameters. D1 (max, min) ˆ 0.316,
Guerra, Int. J. Biol. Macromol. 1998, 23, 175 184; D. Zanuy, C.
¬
ƒ
Aleman, S. Munoz-Guerra, Macromol. Theory Simul. 2000, 9, 543
549.
[9] R. W. Hoffmann, M. A. Lazaro, F. Caturla, E. Framery, I. Valancogne,
C. A. G. N. Montalbetti, Tetrahedron Lett. 1999, 40, 5983 5986; R. W.
Hoffmann, F. Caturla. M. A. Lazaro, E. Framery, M. C. Bernabeu, I.
Valancogne, C. A. G. N. Montalbetti, New J. Chem. 2000, 24, 187 194.
[10] S. Hanessian, X. Luo, R. Shaum, Tetrahedron Lett. 1999, 40, 4925
4929.
[11] M. Brenner, D. Seebach, Helv. Chim. Acta 2001, 84, 2155 2166.
[12] M. Brenner, D. Seebach, Helv. Chim. Acta 2001, 84, 1181 1189.
[13] Preliminary communication: D. Seebach, M. Brenner, M. Rueping, B.
Schweizer, B. Jaun, Chem. Commun. 2001, 207 208.
À0.199 eAÀ3
.
Crystal data for 20 (C29H48N2O5): M ˆ 504.71, Tˆ 293(2) K, monoclinic,
space group P21, a ˆ 9.320(2), b ˆ 12.176(5), c ˆ 13.426(4) ä, b ˆ 93.93(2)8,
Vˆ 1521.6(6) ä3, Z ˆ 2, 1calcd ˆ 1.102 gcmÀ3, m ˆ 0.592mmÀ1, crystal size
0.30 Â 0.10 Â 0.10 mm. A total of 2590 unique reflections (3.30 < 2V<
64.828) were processed of which 2006 were considered significant with Inet
>
3s(Inet). Final residuals were R ˆ 0.0398 and wR2 ˆ 0.1119 (GOF ˆ
[14] M. Brenner, D. Seebach, Helv. Chim. Acta 1999, 82, 2365 2379.
[15] Following the workup procedure in ref. [14] we obtained larger
amounts of the corresponding ketones, presumably derived from a
Nef reaction, with intramolecular assistance by the acylcarbonyl
1.036) for 326 parameters. D1 (max, min) ˆ 0.175, À0.124eAÀ3. Crystal
data for 22 (C46H80N4O7): M ˆ 801.14, Tˆ 293(2) K, monoclinic, space
group P21, a ˆ 9.462(2), b ˆ 20.472(6), c ˆ 13.866(4) ä, b ˆ 106.14(2)8,
Chem. Eur. J. 2002, 8, No. 3
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
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583

D. Seebach et al.
FULL PAPER
group. This had not been observed for primary nitro olefin adducts
and was negligible with a secondary nitro compound described in
ref. [14]. After modification of the workup procedure, the formation
of ketones was substantially reduced (for details see Experimental
Section).
peptide 2 and 3 (half life values ca. 110 min for the two least solvent
exposed NH protons).
[26] K. Gademann, B. Jaun, D. Seebach, R. Perozzo, L. Scapozza, G.
Folkers, Helv. Chim. Acta 1999, 82, 1.
[27] For a recent book on CD spectroscopy see: Circular Dichroism:
Principles and Applications, 2nd ed. (Eds.: N. Berova, K. Nakanishi,
R. W. Woody), Wiley-VCH, New York, 2000.
[28] K. D. McReynolds, J. Gervay-Hague, Tetrahedron: Asymmetry 2000,
11, 337 362.
[16] The relative configurations in 8a and 8c were confirmed by NOE
measurements. The characteristic 1H NMR chemical shifts for H-C(5)
(d ˆ 3.6 3.7 for 8a c and 3.2 for 5-epi-8a c) was used for stereo-
chemical assignment of 8b.
[17] D. L. Flynn, R. E. Zelle, P. A. Grieco, J. Org. Chem. 1983, 48, 2424
2426.
[18] This procedure was successfully applied to the preparation of 4a
and 5a c. See ref. [14].
[19] The stereochemical assignment of 9a was confirmed by single crystal
X-ray crystallography.
[20] G. P. Dado, S. H. Gellmann, J. Am. Chem. Soc. 1994, 116, 1054 1062.
[21] See the extensive discussion in our paper on g-peptidic turns ref. [11].
[22] For a more detailed list see Supporting Information.
[23] K. W¸thrich, NMR of Proteins and Nucleic Acids, Wiley, New York,
1986.
[24] J. N. S. Evans, Biomolecular NMR Spectroscopy, Oxford University
Press, Oxford, 1995.
[25] In contrast to g-peptide 3, g-peptide 2 shows faster H/D exchange
values (half life < 20 min for the least solvent exposed NH proton).
The H/D exchange rates of g-peptide C' are in between those of g-
[29] T. Hintermann, D. Seebach, Helv. Chim. Acta 1998, 81, 2093 2126.
[30] J. Melton, J. E. McMurry, J. Org. Chem. 1975, 40, 2138 2139.
[31] H. G. O. Becker, W. Berger, G. Domschke, E. Fangh‰nel, J. Faust, M.
Fischer, F. Gentz, K. Gewald, R. Gluch, R. Mayer, K. M¸ller, D. Pavel,
H. Schmidt, K. Schollberg, K. Schwetlick, E. Seiler, G. Zeppenfeld,
Organikum, 20th ed., Barth Verlag, H¸tig, Heidelberg/Leipzig, 1996,
pp. 703 704.
[32] S.-S. Wang, B. F. Gisin, D. P. Winter, R. Makofske, I. D. Kuleska, C.
Tzougraki, J. Meienhofer, J. Org. Chem. 1977, 42, 1286 1290.
[33] A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C. Giaco-
vazzo, A. Guagliardi, A. G. G. Moliterni, G. Polidori, R. Spagna, J.
Appl. Crystallogr. 1999, 32, 115 119.
c
[34] G. M. Sheldrick, SHELXL-97, Program for the Refinement of Crystal
Structures, University of Gˆttingen, Germany, 1997.
Received: July 23, 2001 [F3434]
584
¹ WILEY-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002
0947-6539/02/0803-0584 $ 17.50+.50/0
Chem. Eur. J. 2002, 8, No. 3