Synthesis of conformationally restricted mimetics of γ-turns and incorporation into desmopressin, an analogue of the peptide hormone vasopressin

Article abstract of DOI:10.1002/(SICI)1521-3765(19990802)5:8<2241::AID-CHEM2241>3.0.CO;2-L

Mimetics of tripeptides adopting inverse and classical γ-turn conformations have been designed and synthesized by an enantioselective approach and then incorporated in an analogue of the hormone vasopressin. In the mimetics one of the amide bonds of the γ

Full text of DOI:10.1002/(SICI)1521-3765(19990802)5:8<2241::AID-CHEM2241>3.0.CO;2-L

FULL PAPER
Synthesis of Conformationally Restricted Mimetics of g-Turns and
Incorporation into Desmopressin, an Analogue of the Peptide Hormone
Vasopressin
Kay Brickmann,[a] ZhongQing Yuan,[a] Ingmar Sethson,[a] Peter Somfai,*[b] and
Jan Kihlberg*[a]
Abstract: Mimetics
of
tripeptides
blocks: azido epoxides, a-bromo acids
and b-amino alcohols. Of these, the
stereochemistry of the azido epoxide
determines whether a classical or an
inverse g-turn is mimicked. The key
azido epoxide building blocks were
prepared in six steps and approximately
40% overall yield, whereas the a-bromo
acids and b-amino alcohols were either
commercially available or readily pre-
pared from amino acids. The building-
block approach allowed substantial var-
iation of the side chains of the mimetics,
together with complete stereocontrol, as
well as use of uniform conditions for
preparation of both classical and inverse
g-turn mimetics. Conformational studies
based on ab initio calculations and
1H NMR spectroscopy revealed that
the minimum-energy conformations of
the mimetics closely resembled inverse
or classical g-turns.
adopting inverse and classical g-turn
conformations have been designed and
synthesized by an enantioselective ap-
proach and then incorporated in an
analogue of the hormone vasopressin.
In the mimetics one of the amide bonds
of the g-turn has been exchanged for a
Y[CH2O] isostere and the hydrogen
bond between residues i and i‡2 of the
turn has been replaced by a methylene
bridge to give a six-membered, morpho-
lin-3-one ring. The turn mimetics were
assembled from three types of building
Keywords: azides
analysis g-turn
peptidomimetics
´
conformation
hormones
´
´
´
bond between residues i and i‡3 to give a pseudo-ten-
membered ring. Similarly, in g-turns a hydrogen bond may be
Introduction
ˆ
Turns, defined as regions where a peptide chain reverses its
overall direction, constitute an important class of polypeptide
secondary structure.[1] When direction reversal occurs over
four residues in such a way that the carbonyl oxygen atom of
the first residue (i) and the amide NH proton of the fourth
residue (i‡3) come close in space a b-turn is formed. In most
cases this involves formation of an intramolecular hydrogen
formed between the C O of residue i and the NH of residue
i‡2, thereby generating a pseudo-seven-membered ring.
Turns may account for as many as one third of the residues
of globular proteins.[1b] They are often located at the surface of
proteins where they can undergo posttranslational modifica-
tion and serve as recognition sites for interactions with
receptors and antibodies.[1, 2] In addition, structural studies
revealed that a large number of small peptides functioning as
hormones or neurotransmitters, or having other regulatory
roles in organisms, may adopt b- or g-turn conformations.
Examples include the hormones oxytocin,[3] which induces
labor and lactation in mammals, vasopressin,[4] which regu-
lates water readsorption in the kidneys, and LHRH,[5] which
releases sex-specific hormones from the testes and ovaries.
Furthermore, turns have been found in the endogenous
morphinelike substance Leu-enkephalin,[6] as well as angio-
tensin II[7] and bradykinin,[8] which are involved in regulation
of blood pressure. b-Turns are more frequent than g-turns in
these biologically active peptides, but vasopressin, Leu-
enkephalin, angiotensin II and bradykinin have been found
to populate g-turn conformations.
[a] Prof. Dr. J. Kihlberg, Dr. K. Brickmann, M.Sc. Z. Yuan,
Assoc. Prof. Dr. I. Sethson
Ê
Department of Organic Chemistry, Umea University
Ê
SE-90187 Umea (Sweden)
Fax : (‡ 46)90-138885
E-mail: jan.kihlberg@chem.umu.se
[b] Assoc. Prof. Dr. P. Somfai
Department of Organic Chemistry, Arrhenius Laboratory
Stockholm University
SE-10691 Stockholm (Sweden)
Fax : (‡ 46)8-154908
E-mail: peter.somfai@organ.su.se
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. 1999, 5, No. 8
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P. Somfai, J. Kihlberg et al.
FULL PAPER
Peptides display a multitude of diverse and important
biomedicinal activities, as illustrated by the above examples.
Moreover, biological evaluation of libraries of peptides
prepared by combinatorial chemistry continuously provides
further peptide leads for drug development. Unfortunately,
the poor pharmacokinetic properties of peptides, namely low
uptake on oral administration, rapid enzymatic degradation
and facile excretion, often restrict their use as drugs.[9] In
addition, conformational flexibility may reduce the biological
activity and receptor selectivity of peptides, and unfavourable
solubility often imposes restrictions on their use under
physiological conditions. To circumvent these problems large
efforts have been devoted to the design and preparation of
peptidomimetics, compounds that have chemical structures
different from those of peptides but are nevertheless ligands
at the same receptor as the parent peptide.[10]
Ideally, several requirements should be fulfilled by mim-
etics of peptide secondary structure, such as b- and g-
turns.[10b, c] First, the conformation of the mimetic should
accurately resemble that of the peptide backbone in the turn.
Since it is well-known that the side chains of biologically
active peptides have crucial roles in receptor recognition it is
also essential that these can be introduced at desired positions
in the mimetic with correct stereochemistry. Finally, the
synthetic route to the mimetic should be short and efficient.
Peptidomimetics that meet the first two criteria can also be
expected to suffer from fewer pharmacokinetic problems than
peptide drug candidates because the replacement of amide
bonds with isosteric groups[11] both decreases enzymatic
degradation and improves uptake[12] across membranes.[13]
Owing to their rigid nature, mimetics are also useful tools
for establishing the bioactive conformation of peptides.
Furthermore, since rigid analogues pay a lower entropy cost
upon binding to a receptor, they should be more potent and
more selective for a specific type of receptor.[10a, 14]
synthetic analogue of the neurohypophyseal peptide hormone
vasopressin (2).[18] Compared with vasopressin, desmopressin
has a prolonged antidiuretic effect, but does not cause
vasoconstriction and contraction of smooth muscles, and is
therefore used for treatment of diabetes insipidus.[19] It is also
useful in treatment of patients with mild haemophilia A,
von Willebrandꢁs disease and thrombocyte dysfunction prior
to surgery.[20] The conformation of desmopressin, when bound
to its cellular membrane receptor, has not been determined
because of the difficulty of isolating intact receptor molecules.
However, the conformations adopted by desmopressin in
different solutions,[4c, 21] as well as when bound to the carrier
protein neurophysin,[22] have recently been calculated based
on NMR data. It was thus found that desmopressin populates
different b-turn conformations when bound to neurophysin[22]
and in solutions containing trifluoroethanol,[21] whereas an
inverse g-turn encompassing residues Phe3-Asn5 is formed in
aqueous solution under physiological conditions.[4c]
Results and Discussion
Design and molecular modelling of g-turn mimetics: g-Turns
are stabilized by an intramolecular hydrogen bond between
residues i and i‡2 and are classified either as inverse or
classical depending on the values of the backbone torsional
angles F and Y for the second (i‡1) residue of the turn
(Figure 2).[1] g-Turns have been termed open when no hydro-
gen bond is formed and the F, Yangles are within 308 of those
Ri
H2N
6
O
2
Ri+1
HO2C
Ri+2
N
4
O
Ri
3
H
Ri+1 equatorial: Mimetic of
Herein we describe a novel approach which provides access
to conformationally restricted mimetics of both inverse and
classical g-turns. In contrast to most,[7, 8, 15] but not all,[16] of the
previous routes to g-turn mimetics, our approach does not
suffer from drawbacks such as lack of possibilities for
introducing side chains at appropriate positions of the
mimetic or to control their stereochemistry. To illustrate the
scope of our approach, several g-turn mimetic building blocks
with different side chains were prepared. One of these was
then incorporated into the drug desmopressin (1; [1-desami-
no-8-d-arginine]vasopressin DDAVP, Figure 1),[17] which is a
N
H
N
Φi+1
O
inverse γ-turn
Ri+1
O
H
N
Ψi+1
Ri
Ri+1
2
O
Ri+2
6
O
H2N
HO2C
N
inverse γ-turn:
Φi+1 = -70 to -85°
Ψi+1 = 60 to 70°
4
O
Ri+2
4
classical γ-turn:
Φi+1 = 70 to 85°
Ψi+1 = -60 to -70°
Ri+1 axial: Mimetic of
classical γ-turn
Figure 2. g-Turns are divided into inverse and classical g-turns depending
on the Fand Y torsional angles of the i‡1 residue of the turn. Morpholin-3-
ones having substituents in positions 2, 4, and 6 are able to mimic both
inverse and classical g-turns depending on the stereochemistry at C-6.
Mpa-Tyr-Phe-Gln-Asn-Cys-Pro-D-Arg-Gly-NH2
1: Desmopressin
given in Figure 2.[1] In inverse g-turns, the side chain of the i‡1
residue assumes a pseudoequatorial orientation. In contrast, it
is pseudoaxial in classical g-turns. The distance between the
carbonyl carbon atom of residue i and the nitrogen atom of
the amino group of residue i‡2 in ideal g-turns is 3.0 Š,[23]
suggesting that the intramolecular hydrogen bond between
these functionalities could be replaced by a methylene bridge.
Simultaneous replacement of the amide bond between
Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-Gly-NH2
2: Vasopressin
Figure 1. Structures of the drug desmopressin (1) and the hormone
vasopressin (2). Desmopressin was developed[17] from vasopressin by
structural modifications.
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Chem. Eur. J. 1999, 5, No. 8

g-Turn Mimetics
2241±2253
1
residues i and i‡1 with a methylene ether isostere revealed
that morpholin-3-ones, with appropriate substituents in posi-
tions 2, 4 and 6, are g-turn mimetics (cf. 3 and 4, Figure 2).
Moreover, construction of molecular models indicated that
the morpholin-3-ones are able to mimic either inverse or
classical g-turns depending on the stereochemistry at C-6.
Ab initio calculations performed on the simplified inverse
g-turn mimetic 5, which has three methyl group substituents
on the morpholin-3-one ring (Figure 3A), revealed that it
found to be a local energy minimum located 2 kcalmol
above the global minimum. In analogy with inverse g-turn
mimetic 5, the torsional angles of mimetic 6 (F ˆ 528, Yˆ
418) were close to those of a classical g-turn, and mimetic 6
superimposed well on a tripeptide oriented as a classical g-
turn (Figure 3D).
A retrosynthetic analysis suggested that bromo acids 9,
diastereomeric azido epoxides 10 and 11, and protected b-
amino alcohols 12 were suitable building blocks for synthesis
of g-turn mimetics 3 and 4 in enantiomerically pure form
(Scheme 1). In these building blocks the azido group in 10 and
Ri
Ri
H2N
O
H2N
OH
Br
Ri+1
Ri+1
+
N
NH
Ri+2
HO2C
HO2C
HO2C
O
Ri+2
9
7 and 8
3 and 4
Ri
10
11
2
2
N3
O
NH2
Ri+2
or
Ri
tBuPh2SiO
+
12
N3
O
Scheme 1. Retrosynthetic analysis reveals that mimetics of inverse and
classical g-turns can be prepared from a-bromo acids 9, azido epoxides 10
or 11, and b-amino alcohols 12.
11 serves as a precursor for the amino group of the first
residue of the turn, while the silylated hydroxymethyl group in
12 corresponds to the carboxylic acid group of the third
residue of the turn. The success of this approach relies on a
route that provides ready access to both 10 and 11 in
enantiomerically pure form, since the stereochemistry at C-2
of the epoxides determines which type of g-turn, inverse or
classical, will be prepared. Protected b-amino alcohols 12 may
be prepared in a few steps from amino acids and then used for
opening of 10 or 11 to give 7 or 8. Acylation of 7 or 8 by (R)-2-
bromo acids 9, which are commercially available or prepared
by diazotization of d-a-amino acids,[25] and intramolecular
substitution of the bromine atom then complete the synthesis
of g-turn mimetics 3 and 4. Because of our interest in
replacing the Phe3-Gln4-Asn5 inverse g-turn in desmopressin
with turn mimetics, we focused on preparation of azido
epoxides in which Ri is a benzyl group. Since glutamine at
position 4 in desmopressin can be replaced by alanine, a-
aminobutyric acid or valine with little decrease in or even
enhanced activity,[26] we chose methyl, ethyl and isopropyl
groups for the Ri‡1 position of the turn mimetics. Asparagine
is required at position 5 for the activity of desmopressin and
therefore amino alcohol 12 was prepared from asparagine.
Figure 3. A) The substituent corresponding to the i‡1 side chain in the
minimum energy conformation of the simplified mimetic 5 occupies a
pseudoequatorial orientation and 5 thereby mimics an inverse g-turn; B)
energy minimization of 6 revealed that the i‡1 side chain is located in a
pseudoaxial orientation and 6 thus mimics a classical g-turn; C) and (D)
superimpositions of mimetics 5 and 6 on Ala-Ala-Ala tripeptides oriented
as an ideal inverse g-turn (F ˆ 77.58 and Yˆ 658) and an ideal classical g-
turn (F ˆ 77.58 and Yˆ 658), respectively. Oxygen atoms are grey and
nitrogen atoms black.
adopted a half-chair minimum-energy conformation with the
C-2 substituent in a pseudoequatorial orientation, and that it
thereby mimicked an inverse g-turn.[24] Furthermore, the
torsional angles for the i‡1 residue of energy-minimized
conformation of mimetic 5 (F ˆ 578, Yˆ 458) were found to
be close to those of an inverse g-turn (cf. values given in
Figure 2). Consequently, superimposition of energy-mini-
mized 5 on an Ala-Ala-Ala tripeptide oriented as an ideal
inverse g-turn revealed an excellent overlap with deviations
of only 0.1 ± 0.3 Š between the a-carbon atoms of the
tripeptide and the corresponding atoms in the mimetic
(Figure 3C). Ab initio calculations performed on the diaster-
eomeric, classical g-turn mimetic 6 showed that it adopted a
minimum energy conformation with the substituent corre-
sponding to the side chain of residue i‡1 in a pseudoaxial
orientation (Figure 3B). In this case a boatlike conformation
with the i‡1 substituent in an equatorial orientation was
Synthesis of mimetics of inverse and classical g-turns: The two
key diastereomeric azido epoxides 17 and 18 were prepared by
slight modifications of a published procedure (Scheme 2).[27]
Wittig olefination of phenylacetaldehyde (13) and subsequent
Chem. Eur. J. 1999, 5, No. 8
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P. Somfai, J. Kihlberg et al.
FULL PAPER
inverse g-turn mimetic, but only in a 22% yield. Furthermore,
this one-step procedure failed with more hindered a-halo
esters such as esters of (R)-(‡)-2-bromopropionic acid. Thus,
preparation of g-turn mimetics having side chains other than a
hydrogen atom at the i‡1 position of the turn was not possible
by this route.
In efforts to improve the yield in the conversion of 20 to
morpholin-3-ones, and to allow introduction of side chains at
C-2 of the morpholinone ring, we turned our attention to a
two-step acylation ± intramolecular ring closure sequence
(Scheme 4); a method which has been applied previously for
Ph
Ph
O
Ph
a)
b)
c)
CHO
OH
OH
13
14
15
Ph
Ph
N3
Ph
d)
OH
N3
N3
O
O
18
16
17
OH
e)
Scheme 2. a) i) Ph3PCHCO2Et, CH2Cl2, RT, 2 h, 81%; ii) DIBAL-H,
CH2Cl2, 788C, 1 h, 81%; b) Ti(OiPr)4, tBuOOH, d-( )-diethyl tartrate,
CH2Cl2, 208C, 24 h, 87%; c) [Ti(OiPr)2(N3)2], benzene, reflux, 30 min,
95%; d) i) TsCl, DMAP (cat.), pyridine, 08C, 2 h, 92%; ii) NaH, DMF, 08C,
1 h, 76%; e) i) BzCl, collidine, CH2Cl2, 788C !RT, 18 h, then MsCl,
08C !RT, 24 h, 91%; ii) NaOMe, THF, RT, 15 min, 80%.
Ph
N3
Ph
N3
OH
O
OSiEt3
NH
Me
22 R = tBuPh2Si
Ph
N3
a)
b)
20
Me
Br
N
RO
RO
Me
reduction with diisobutylaluminium hydride gave allylic
alcohol 14. This was subjected to Katsuki ± Sharpless asym-
metric epoxidation to give epoxide 15 (>95% ee based on
1H NMR analysis of a Mosher ester derivative),[28, 29] followed
by regioselective nucleophilic opening of the epoxide with
Ti(OiPr)2(N3)2[30] to furnish azido diol 16. Both azido epoxides
were then prepared in enantiomerically pure form from this
intermediate. Tosylation of the primary hydroxyl group of 16,
followed by sodium hydride induced intramolecular substitu-
tion, gave azido epoxide 17 in 38% yield from 13.[27, 31] The
preparation of azido epoxide 18 (39% yield from 13) was
accomplished by benzoylation of the primary hydroxyl group
of 16, mesylation of the secondary hydroxyl group and sodium
methoxide mediated intramolecular ring closure.[27, 31]
Mimetics of inverse g-turns corresponding to the tripep-
tides Phe-Gly-Ala and Phe-Ala-Ala, which have nonreactive
side chains, were first chosen as synthetic targets in order to
establish suitable conditions for conversion of 3-azido epox-
ides 17 and 18 into g-turn mimetics. Regioselective nucleo-
philic attack at the primary position of epoxide 18 with
silylated (S)-(‡)-2-amino-1-propanol (19) was achieved in
refluxing EtOH to give amino alcohol 20 (68%) (Sche-
me 3).[32] 2-Amino alcohols have previously been converted
23 R = tBuPh2Si
Ph
O
N3
O
c)
e)
Me
Me
N
N
HO2C
RO
O
O
Me
Me
24 R = tBuPh2Si
25 R = H
26
d)
Scheme 4. a) Et3SiCl, imidazole, CH2Cl2, 08C !RT, 4 h, 73%; b) (R)-(‡)-
2-bromopropionic acid, diisopropylcarbodiimide, CH2Cl2, 08C, 4 h, then
aqueous workup followed by 2m aqueous HCl/THF (3:2), RT, 96 h, 67%;
c) KH, THF/DMF (3:1), 08C, 55 min, 86%; d) TBAF, THF, RT, 2 h, 100%;
e) NaIO4, RuCl3 ´ H2O (cat.), H2O/CCl4/CH3CN (3:2:2), RT, 2 h, 86%.
preparation of stereochemically well-defined Y[CH2O] iso-
steres.[34] All attempts to acylate the amino group of 20
regioselectively in the presence of the hydroxyl group were
unsuccessful. Therefore, the secondary hydroxyl group was
first protected by silylation with triethylsilyl chloride to give
22 (73%). Then the amino group was acylated with an excess
of (R)-(‡)-2-bromopropionic acid activated with diisopropyl-
carbodiimide. In spite of substantial efforts the desired amide
could not be obtained completely free from diisopropylurea,
the side-product of the coupling. However, removal of the
triethylsilyl group by treatment with aqueous HCl allowed
remaining diisopropylurea to be removed by flash column
chromatography and hydroxyamide 23 was isolated in 67%
yield from 22. Inclusion of additives during the coupling such
as 1-hydroxybenzotriazole[35a] (HOBt) or 1-hydroxy-7-aza-
benzotriazole[35b] (HOAt), which are used to suppress race-
mization during synthesis of peptides, or use of other coupling
reagents (1-ethyl-3-(3'-dimethylaminopropyl)carbodiimide
hydrochloride (EDC) or the HOAt-derived HATU (O-(7-
azabenzotriazol-1-yl)-N,N,N',N'-tetramethyluronium hexa-
fluorophosphate[36]) did not result in any further improve-
ments in yield for these two steps. The key cyclization of 23
was then achieved by converting 23 into the corresponding
alkoxide with potassium hydride in a mixture of THF and
DMF at 08C. This resulted in a spontaneous intramolecular
Ph
Ph
N3
Ph
N3
N3
OH
NH
Me
O
O
b)
a)
18
N
+
RO
RO
O
Me
NH2
Me
RO
21
20
19
(R = tBuPh2Si)
Scheme 3. a) EtOH, reflux, 96 h, 68%; b) NaH, THF, RT, 30 min, then add
ethyl bromoacetate, RT!reflux, 5 h, 22%.
into morpholin-3-ones by treatment with ethyl chloroacetate
and sodium hydride as part of a synthetic route to Y[CH2O]
dipeptide isosteres.[33] Attempted cyclization of 20 with the
more reactive ethyl bromoacetate under basic conditions did
indeed give morpholine-3-one 21, which is a Phe-Gly-Ala
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Chem. Eur. J. 1999, 5, No. 8

g-Turn Mimetics
2241±2253
ring closure to give morpholin-3-one 24 in high yield (86%).
Formation of side-products due to b-elimination or epimeri-
zation of the stereocentre a to the carbonyl group in 24 could
not be detected in the ring-closure reaction (cf. discussion for
compound 36 below). Finally, deprotection of the primary
hydroxyl group in 24 with tetrabutylammonium fluoride
(quantitative) and subsequent oxidation of 25 to the carbox-
ylic acid stage with the biphasic RuCl3 ± NaIO4 system gave
the desired inverse g-turn mimetic 26 (86%),[37] which has
side-chain functionalities corresponding to those of the
tripeptide Phe-Ala-Ala.[38]
After establishing this synthetic approach to inverse g-turn
mimetics it was important to investigate whether it was
compatible with incorporation of different side chains at
positions i‡1 and i‡2 of the turn. As our aim was to prepare
turn mimetics replacing the Phe3-Gln4-Asn5 segment of
desmopressin (1), we chose Asn for the i‡2 position, and the
side chains of alanine, a-aminobutyric acid and valine were
selected for the i‡1 position. The required Asn building block
was prepared from commercially available Fmoc-Asn(Trt)-
OH (Trt ˆ triphenylmethyl) by conversion into a mixed
anhydride followed by reduction with sodium borohydride[39]
and subsequent protection of alcohol 28 as a tert-butyldiphe-
nylsilyl ether to give 29 (Scheme 5). Deprotection of the Fmoc
Ph
N3
NH2
tBuPh2SiO
a)
+
O
CONHTrt
30
18
Ph
N3
Ph
N3
OH
O
OR
c)
d)
R
N
NH
tBuPh2SiO
tBuPh2SiO
Br
CONHTrt
CONHTrt
31 R = H
33 R = Me
34 R = Et
35 R = iPr
b)
32 R = SiEt3
Ph
Ph
N3
O
N3
O
e)
R
R
N
N
tBuPh2SiO
HO
O
O
CONHTrt
CONHTrt
39 R = Me
40 R = iPr
36 R = Me
37 R = Et
38 R = iPr
Ph
N3
O
NHFmoc
CONHTrt
NHFmoc
CONHTrt
b)
a)
O
f)
HO
HO
R
N
HO2C
O
27
28
CONHTrt
NHFmoc
CONHTrt
NH2
41 R = Me
42 R = iPr
c)
tBuPh2SiO
tBuPh2SiO
CONHTrt
Scheme 6. a) EtOH, reflux, 96 h, 66%; b) Et3SiCl, imidazole, CH2Cl2,
08C !RT, 4 h, 91%; c) (R)-(‡)-2-bromopropionic acid or (R)-2-bromo-
butyric acid or (R)-(‡)-2-bromo-3-methylbutyric acid, diisopropylcarbo-
diimide, CH2Cl2, 08C, 4 ± 21 h, then aqueous workup followed by 2m
aqueous HCl/THF (3:2), RT, 48 ± 96 h, 67% for 33, 31% for 34, 40% for
35; d) KH, THF/DMF (3:1), 08C, 40 ± 60 min; 84% for 36, 63% for 37, 69%
for 38; e) TBAF, THF, RT, 3 h, 98% for both 39 and 40; f) NaIO4, RuCl3 ´
H2O (cat.), H2O/CCl4/CH3CN (3:2:2), RT, 3 h; 81% for 41, 72% for 42.
29
30
Scheme 5. a) N-methylmorpholine, isobutyl chloroformate, THF, 108C,
10 min, then NaBH4, 15 min, followed by MeOH, 08C, 15 min, 70%; b) tert-
butylchlorodiphenylsilane, imidazole, CH2Cl2, 08C !RT, 1.5 h, 77%;
c) morpholine, RT, 1 h, 88%.
group with morpholine gave the asparaginol derivative 30
with a free amino group (52% yield over 3 steps). The
triphenylmethyl group was chosen for protection of the side-
chain amide functionality because of its compatibility with the
conditions used in Fmoc solid-phase peptide synthesis. Of the
required a-bromo acids that constitute building blocks for the
i‡1 residue of the turn, (R)-(‡)-2-bromopropionic acid and
(R)-(‡)-2-bromo-3-methylbutyric acid are commercially
available, whereas (R)-2-bromobutyric acid was prepared by
diazotization of d-a-aminobutyric acid.[25] Three mimetics of
inverse g-turns corresponding to the sequences Phe-Ala-Asn,
Phe-gAba-Asn and Phe-Val-Asn were then assembled from
these building blocks by the sequence of transformations used
in the preparation of Phe-Ala-Ala mimetic 26 (Scheme 6).
Regioselective nucleophilic opening of azido epoxide 18 with
protected asparaginol 30 was performed under the conditions
employed in the reaction between 18 and alaninol 19, and
gave amino alcohol 31 in an almost identical yield (66%). The
secondary hydroxyl group in 31 was then protected by silylation
to give amine 32 (91%), which was acylated with (R)-(‡)-2-
bromopropionic acid, (R)-2-bromobutyric acid and (R)-(‡)-
2-bromo-3-methylbutyric acid by means of diisopropylcarbo-
diimide as a coupling reagent. After removal of the triethyl-
silyl protective group the three hydroxyamides 33, 34 and 35
were obtained in 67, 31 and 40% yields, respectively.[40]
Potassium hydride induced cyclization of 33 ± 35, which is
the key step of the synthetic route, furnished morpholin-3-
ones 36 ± 38 in 63 ± 84% yields. Competing b-elimination was
not observed in any of these cyclizations, including cyclization
of sterically hindered 35. Moreover, this base-induced cycli-
zation was shown to proceed without epimerization of the
stereocentre a to the carbonyl group in the morpholin-3-one
ring (C-2). This was demonstrated by acylation of amine 32
with racemic 2-bromopropionic acid and subsequent cycliza-
tion, which gave 36 as an inseparable mixture with the
1
corresponding C-2 epimer. The H NMR spectrum of this
mixture contained doublets at d ˆ 1.47 and 1.38 derived from
the methyl group in each of the two diastereomers. Of these
Chem. Eur. J. 1999, 5, No. 8
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2245

P. Somfai, J. Kihlberg et al.
FULL PAPER
doublets, the one at d ˆ 1.47 matched the sole doublet from
the methyl group in 36. Finally, the tert-butyldiphenylsilyl
ether in 36 and 38 was cleaved and the resulting primary
alcohols 39 and 40 were oxidized to give acids 41 and 42 (79%
and 71% yields respectively, over two steps).
building blocks under standard conditions for Fmoc solid-
phase peptide synthesis (Scheme 8). Synthesis of 49 was
performed on a polystyrene resin grafted with polyethylene-
glycol chains functionalized with the Rink linker.[41] Na-Fmoc
Mimetics of classical g-turns can be prepared by an analogous
route (Scheme 7). Thus, compound 48, which is a mimetic of a
classical Phe-Ala-Asn g-turn, was prepared from azido
HO
O
O
N
H
Ph
N3
N
Ph
N3
NH2
H
O
tBuPh2SiO
a)
Me
S
O
S
N
O
CONHTrt
+
O
CH3
O
O
Solid-phase synthesis
N
N
HO2C
H
17
30
N
O
CONH2
O
O
CONHTrt
HN
NH2
Ph
N3
N
41
H
Ph
N3
O
+
NH2
OR
OH
O
N
H2N
c)
49
H
CH3
Br
NH
CONHTrt
Scheme 8. Incorporation of mimetic 41 in the desmopressin analogue 49 in
tBuPh2SiO
N
tBuPh2SiO
23% overall yield by solid-phase synthesis.
CONHTrt
amino acids carrying standard side-chain protecting groups
were coupled to the resin as benzotriazolyl (HOBt) esters,[35a]
whereas mimetic 41 was activated as a more reactive HOAt
ester.[35b] This allowed complete acylation of the resin-bound
Cys-Pro-d-Arg-Gly tetrapeptide with only 1.3 equivalents of
the valuable 41 relative to the capacity of the resin. After
attachment of 41 to the solid phase the azido group was
reduced by treatment with tin(ii) chloride in the presence of
thiophenol and triethylamine.[42] The reduction could be
monitored conveniently by the disappearance of the N3
stretch in the IR spectrum obtained from a few resin beads.[43]
The azido group had thus served as a masked amino group
throughout the synthesis of 41 and was unmasked at the latest
possible stage of the synthesis. After completion of the
synthesis, the peptide was cleaved from the resin, and the
amino acid side chains were simultaneously deprotected by
treatment with trifluoroacetic acid. Disulfide bond formation
was effected by oxidation with iodine in methanol,[44] and
purification by reversed-phase HPLC gave the desmopressin
analogue 49 in 23% overall yield, based on the resin capacity.
The structure of analogue 49 was confirmed by means of fast
atom bombardment mass spectroscopy, amino acid analysis,
43 R = H
45
b)
44 R = SiEt3
Ph
N3
Ph
N3
O
O
f)
d)
CH3
CH3
N
N
HO2C
RO
O
O
CONHTrt
CONHTrt
48
46 R = tBuPh2Si
47 R = H
e)
Scheme 7. a) EtOH, reflux, 43 h, 72%; b) Et3SiCl, imidazole, CH2Cl2,
08C !RT, 1.5 h, 95%; c) (R)-(‡)-2-bromopropionic acid, diisopropylcar-
bodiimide, CH2Cl2, 08C, 1 h, then aqueous workup followed by 2m aqueous
HCl/THF (3:2), RT, 96 h, 58%; d) KH, THF/DMF (3:1), 08C, 4 h, 84%;
e) TBAF, THF, RT, 2 h, 100%; f) NaIO4, RuCl3 ´ H2O (cat.), H2O/CCl4/
CH3CN (3:2:2), RT, 3 h; 96%.
epoxide 17, the asparagine building block 30 and (R)-(‡)-2-
bromopropionic acid by the same transformations as were
used for synthesis of the inverse g-turn mimetics. The yields of
the steps leading to mimetic 48 closely resembled those
observed in the synthesis of the diastereomeric inverse g-turn
mimetic 41. This allowed mimetic 48 to be obtained in an
overall yield of 32% from azido epoxide 17. As for inverse g-
turn mimetics, the diastereomeric purity of morpholin-3-one
46 was probed in a control experiment starting from racemic
1
and H NMR spectroscopy (Table 1).[45]
The conformations of the inverse g-turn mimetics 26, 41
and 42 were investigated in chloroform solution by NOESY
spectroscopy. For each of them a strong NOE was observed
between the protons on C-2 and C-6 in the morpholin-3-one
ring (Figure 4). This observation supported the prediction that
the inverse g-turn mimetics adopt the calculated half-chair
conformation, with the substituents on the morpholin-3-one
ring in pseudoequatorial or equatorial orientations and the
C-2 and C-6 protons in pseudoaxial and axial positions
(Figure 3A). Furthermore, a strong NOE was observed
between the C-2 and C-6 protons of the morpholin-3-one
ring in an aqueous solution of the desmopressin analogue 49.
Thus, the conformation of the morpholin-3-one inverse g-turn
mimetic appears to be unaffected both by the incorporation in
the macrocyclic peptide and by the solvent. In comparison,
1
2-bromopropionic acid. Once again, H NMR spectroscopy
confirmed that the stereocentre a to the carbonyl group in the
ring of 46 had not undergone detectable epimerization during
the base-promoted cyclization of 45.
Incorporation of an inverse g-turn mimetic in desmopressin
and preliminary conformational studies: The Phe-Ala-Asn
inverse g-turn mimetic 41 was incorporated in the desmo-
pressin analogue 49 by means of solid-phase synthesis,
thereby revealing that the g-turn mimetics can be used as
2246
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Chem. Eur. J. 1999, 5, No. 8

g-Turn Mimetics
2241±2253
Table 1. 1H NMR data (d) for the desmopressin analogue 49 in aqueous
solution.[a]
Experimental Section
General methods and materials: All reactions were carried out under an
inert atmosphere with dry solvents under anhydrous conditions, unless
otherwise stated. CH2Cl2 and THF were distilled from calcium hydride and
sodium benzophenone, respectively. DMF was distilled and then dried over
two portions of 3 Š molecular sieves. TLC was performed on silica gel
60F254 (Merck) with detection by UV light and charring with phosphomo-
lybdic acid in EtOH (26 mmoll 1). Flash column chromatography (eluents
given in brackets) was performed on silica gel (Matrex, 60 Š, 35 ± 70 mm,
Grace Amicon).
1H and 13C NMR spectra were recorded on a Bruker DRX-360, a Bruker
DRX-400 or a Bruker ARX-500 spectrometer for solutions in CDCl3
(residual CHCl3 (dH ˆ 7.26) or CDCl3 (dC ˆ 77.0) as internal standard) at
298 K. The NMR sample of the desmopressin analogue 49 was prepared by
dissolving 49 (4 mg, 3.95 mmol) in 0.05 mL phosphate buffer (200mm in
D2O, pD 6.41) and 0.45 mL of a 0.3mm solution of NaN3 in doubly distilled
water (concentration of the sample: ca. 8mm, pH 6.70). Spectra of 49 were
Residue NH H-a
H-b
H-g
Others
Mpa1
Tyr2
2.61[b]
8.33 4.11
3.30, 2.82
2.40[b]
6.72 and 6.67 (H arom)
Mimetic
Phe3
7.72 4.24
4.29
2.97, 2.88
1.43
2.90, 2.65
7.34 ± 7.17 (H arom), 3.90 (CHO)
Ala4
Asn5
bridge
Cys6
4.28
7.71 and 6.91 (CONH2)
3.23 and 3.68 (CH2N)
8.11 4.63
4.37
3.14, 2.87
Pro7
2.24, 1.83 1.99[b] 3.83 and 3.56 (H-d)
1.83, 1.70 1.58[b] 3.12 (H-d),[b] 7.23 (NH-e)
8.33 3.86, 3.81 6.88 and 6.42 (CONH2)
d-Arg8 8.89 4.20
Gly9
[a] Obtained at 600 MHz, 278 K, and pH ˆ 6.70 (phosphate buffer) for an ꢀ8mm
solution of 49 in water containing 10% D2O [HDO (d ˆ 4.98) as internal
standard]. [b] Degeneracy has been assumed.
recorded on a Bruker Avance DRX-600 spectrometer with HDO (dH
ˆ
4.98) as internal standard at 278 K. First-order chemical shifts and coupling
constants were obtained from one-dimensional spectra and proton
resonances were assigned from appropriate combinations of COSY,[46a]
TOCSY,[46b] ROESY[46c] and NOESY[46d±f] experiments. Ratios of diaster-
eomeric or rotameric mixtures were determined by integration of 1H NMR
resonances. The index A refers to the major, B to the minor isomer.
Ph
NOE
Ph
H
CH3
N3
6
6
O
2
Ri+1
HO2C
O
N3
N
4
2
N
H
4
Ri+2
H
O
O
HO2C
IR spectra were recorded on an ATI Mattson Genesis Series FTIR
spectrometer. Optical rotations were measured with a Perkin ± Elmer 343
polarimeter. Positive fast atom bombardment mass spectra (FAB MS) were
recorded on a JEOL SX102 A mass spectrometer. Ions for FAB MS were
produced by a beam of xenon atoms (6 keV) from a matrix of glycerol and
thioglycerol. Combustion analyses were performed by F. Hambloch,
Institute of Organic Chemistry, University of Göttingen (Germany).
NOE
CONHTrt
48
26 Ri+1 = Me, Ri+2 = Me
41 Ri+1 = Me, Ri+2 = CH2CONHTrt
42 Ri+1 = iPr, Ri+2 = CH2CONHTrt
Figure 4. NOEs which support the calculated conformations for inverse
Compounds 17[27, 31] and 18[27, 31] were prepared essentially as outlined in the
cited references. Experimental procedures describing the syntheses of 17
and 18 are provided as Supporting Information. (R)-(‡)-2-Bromopropionic
acid and (R)-(‡)-2-bromo-3-methylbutyric acid were purchased from
Fluka (Switzerland) and Aldrich (USA), respectively, whereas (R)-(‡)-2-
bromobutyric acid was prepared by diazotization of (R)-2-aminobutyric
acid as described previously.[25] (S)-Alaninol and Fmoc-Asn(Trt)-OH were
purchased from Aldrich (USA) and Bachem (Switzerland), respectively.
(26, 41, and 42) and classical (48) g-turn mimetics.
the NOESY spectrum of the classical g-turn mimetic 48
displayed a strong NOE between the methyl group at C-2 and
the proton at C-6 of the six-membered ring (Figure 4). Again,
this confirms predominant population of the half-chair
calculated minimum-energy conformation, in which the
methyl group is positioned pseudoaxially and the substituents
at C-6 and N-4 occupy equatorial and pseudoequatorial
orientations (Figure 3B).
Molecular modelling: Energy minimization of the simplified g-turn
mimetics 5 and 6 was performed by ab initio molecular orbital calculations
with the Spartan RHF 4.1 program[47] using the STO-3G[48] basis sets.
Superimposition of energy-minimized conformations of 5 and 6 on ideal
inverse and classical g-turns, respectively, was performed using the
MacMimic 3 program.[49] In the superimpositions atoms C-6, C-2, and
N-4 in the morpholinone ring of each mimetic were superimposed on the
Conclusions
ˆ
C O, C-a and NH atoms, respectively, of residues i, i‡1, and i‡2 of the
We have developed an enantioselective synthetic approach to
mimetics of both inverse and classical g-turns and shown that
the mimetics can be employed in solid-phase synthesis of
peptides using the Fmoc strategy. The turn mimetics were
assembled from three types of building blocks: azido epox-
ides, a-bromo acids and b-amino alcohols; of these, the
stereochemistry of the azido epoxide determines if a classical
or an inverse g-turn is mimicked. The flexible synthetic
approach allows introduction of different side chains in the
turn mimetics with full stereochemical control. In addition,
this approach may allow preparation of libraries of g-turn
mimetics, since i) the synthetic route to the key azido epoxides
allows preparation of >10 g amounts in less than two weeks,
and ii) a-bromo acids and b-amino alcohols are either
commercially available or readily prepared from amino acids.
Finally, conformational studies based on ab initio calculations
and 1H NMR spectroscopy revealed that the minimum-energy
conformations of the mimetics closely resembled inverse or
classical g-turns.
corresponding g-turn.
(2S)-1-(tert-Butyldiphenylsilyloxy)propyl-2-amine (19): Imidazole (1.216 g,
17.87 mmol) was added to (S)-alaninol (0.610 g, 8.12 mmol) in CH2Cl2
(40 mL) at room temperature. After cooling of the mixture to 08C, tert-
butylchlorodiphenylsilane (3.99 mL, 4.23 g, 17.1 mmol) was added; the
reaction mixture was stirred for 3 h, poured into satd. aqueous NaHCO3
and extracted with CH2Cl2 (3 Â 30 mL). The combined organic phases were
dried with Na2SO4 and concentrated. Flash chromatography (heptane/ethyl
acetate 2:1, then EtOH) of the residue yielded 19 (2.419 g, 95%): [a]D20
ˆ
‡3.3 (c ˆ 1.9 in CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 7.74 ± 7.65 (m,
4H; Ph), 7.46 ± 7.34 (m, 6H; Ph), 3.55 (ABX-type dd, J ˆ 9.8 and 4.3 Hz,
1H; CH2OSi), 3.36 (ABX-type dd, J ˆ 9.8 and 7.5 Hz, 1H; CH2OSi), 3.10 ±
3.00 (m, 1H; CHNH2), 2.17 (brs, 2H; NH2), 1.06 (s, 9H; tBu), 1.00 (d, J ˆ
6.5 Hz, 3H; CH3); 13C NMR (100 MHz, CDCl3): d ˆ 135.5 (2C), 129.6,
127.6, 70.4, 48.5, 26.8, 26.7, 19.2; IR (neat): nÄ ˆ 1425 cm 1 (C O); HR FAB
MS calcd for C19H28NOSi [M‡H] 314.1940, found 314.1945; C19H27NOSi
(313.5): calcd C 72.79, H 8.68, N 4.47; found C 72.67, H 8.87, N 4.42.
(2S,3S)-3-Azido-1-[(1S)-2-(tert-butyldiphenylsilyloxy)-1-methylethylami-
no]-4-phenylbutan-2-ol (20): Azido epoxide 18[27, 31] (0.151 g, 0.798 mmol)
and the O-silylated alaninol 19 (0.238 g, 0.760 mmol) were refluxed in
EtOH (5 mL) for 96 h. The solvent was evaporated and flash chromatog-
raphy (heptane/ethyl acetate 1:1, then EtOH) of the residue yielded 20
Chem. Eur. J. 1999, 5, No. 8
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2247

P. Somfai, J. Kihlberg et al.
FULL PAPER
(0.260 g, 68%): [a]D20
ˆ
1.0 (c ˆ 0.4 in CHCl3); 1H NMR (400 MHz,
the residue yielded 23 (56 mg, 67% over 2 steps): [a]D20
ˆ
4.0 (c ˆ 0.9 in
CDCl3): d ˆ 7.68 ± 7.62 (m, 4H; Ph), 7.46 ± 7.23 (m, 11H; Ph), 3.63 ± 3.57 (m,
2H; CH2OSi and CHOH), 3.50 (ABX-type dd, J ˆ 10.2 and 6.4 Hz, 1H;
CH2OSi), 3.36 ± 3.30 (m, 1H; CHN3), 3.08 (ABX-type dd, J ˆ 13.6 and
6.6 Hz, 1H; CH2Ph), 3.00 (ABX-type dd, J ˆ 13.7 and 8.4 Hz, 1H; CH2Ph),
2.83 ± 2.75 (m, 1H; CH2NH), 2.74 ± 2.61 (m, 3H; CH2NH and OH), 1.06 (s,
9H; tBu), 1.02 (d, J ˆ 6.5, 3H; CH3); 13C NMR (100 MHz, CDCl3): d ˆ
137.5, 135.5 (2C), 133.3, 133.2, 129.7, 129.3, 128.6, 128.3, 127.7, 126.7, 70.1,
67.0, 65.7, 54.6, 49.3, 36.8, 26.8, 19.2, 17.1; IR (neat): nÄ ˆ 3305 (OH),
CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 7.61 ± 7.56 (m, 4H; Ph), 7.50 ± 7.36
(m, 6H; Ph), 7.34 ± 7.22 (m, 5H; Ph), 4.55 (q, J ˆ 6.5 Hz, 1H; CHBr), 4.12 ±
4.02 (m, 2H; CHMeN and OH), 3.91 ± 3.84 (m, 1H; CHOH), 3.59 (ABX-
type dd, J ˆ 11.1 and 4.6 Hz, 1H; CH2OSi), 3.51 ± 3.38 (m, 3H; CH2OSi,
CH2N and CHN3), 3.22 (ABX-type dd, J ˆ 14.4 and 2.5 Hz, 1H; CH2N),
3.07 (ABX-type dd, J ˆ 14.0 and 4.9 Hz, 1H; CH2Ph), 2.93 (ABX-type dd,
J ˆ 13.9 and 9.5 Hz, 1H; CH2Ph), 1.75 (d, J ˆ 6.5 Hz, 3H; CHBrCH3), 1.12
(d, J ˆ 6.7 Hz, 3H; CH3), 1.04 (s, 9H; tBu); 13C NMR (100 MHz, CDCl3):
d ˆ 172.5, 137.7, 135.5, 135.4, 132.7, 132.4, 130.1 (2C), 129.2, 128.6, 127.9
1
2105 cm (N3); HR FAB MS calcd for C29H39N4O2Si [M‡H] 503.2842,
found 503.2841; C29H38N4O2Si (502.7): calcd C 69.29, H 7.62, N 11.14; found
C 69.31, H 7.33, N 11.35.
(2C), 126.7, 72.8, 66.1, 64.8, 55.2, 46.1, 38.9, 36.4, 26.8, 21.6, 19.0, 14.6; IR
1
ˆ
(neat): nÄ ˆ 3380 (OH), 2105 (N3), 1630 cm (C O); HR FAB MS calcd for
C32H42BrN4O3Si [M‡H] 637.2210, found 637.2206; C32H41BrN4O3Si (637.7):
(6S)-6-[(1S)-1-Azido-2-phenylethyl]-4-[(1S)-2-(tert-butyldiphenylsilyl-
oxy)-1-methylethyl]morpholin-3-one (21): Amino alcohol 20 (55 mg,
0.11 mmol) in THF (3 mL) was added to NaH (50% dispersion in mineral
oil, 4.8 mg, 0.12 mmol) and the suspension was stirred at room temperature
for 30 min. After addition of ethyl bromoacetate (12.0 mL, 18.0 mg,
0.110 mmol) stirring was continued for 2 h at room temperature and finally
under reflux for 3 h.[33a] The reaction was quenched with satd. aqueous
NH4Cl (5 mL) and extracted with EtOAc (3 Â 10 mL). The combined
organic phases were dried with Na2SO4 and concentrated. Flash chroma-
calcd C 60.27, H 6.48, N 8.79; found C 60.06, H 6.34, N 8.99.
(2S,6S)-6-[(1S)-1-Azido-2-phenylethyl]-4-[(1S)-2-(tert-butyldiphenylsilyl-
oxy)-1-methylethyl]-2-methylmorpholin-3-one (24): Bromo alcohol 23
(72 mg, 0.11 mmol) in THF (0.5 mL) was added dropwise to KH (5.9 mg,
0.15 mmol) suspended in THF (1 mL) and DMF (0.3 mL) at 08C. After
55 min at 08C the reaction mixture was poured into satd. aqueous NaHCO3
(5 mL) and extracted with CH2Cl2 (3 Â 5 mL). The combined extracts were
dried with Na2SO4 and concentrated. Flash chromatography (heptane/ethyl
acetate 10:1 !5:1) of the residue gave the morpholinone derivative 24
tography (heptane/ethyl acetate 5:1 !1:1) of the residue yielded 21 (13 mg,
22%): [a]2D0
ˆ
30.1 (c ˆ 1.7 in CHCl3); H NMR (400 MHz, CDCl3): d ˆ
(54 mg, 86%): [a]D20
ˆ
30.9 (c ˆ 1.2 in CHCl3); 1H NMR (400 MHz,
1
7.58 ± 7.53 (m, 4H; Ph), 7.46 ± 7.39 (m, 2H; Ph), 7.38 ± 7.18 (m, 9H; Ph),
4.81 ± 4.72 (m, 1H; CHMeN), 4.38 (AB-type d, J ˆ 16.4 Hz, 1H; CH2CO),
4.10 (AB-type d, J ˆ 16.4 Hz, 1H; CH2CO), 3.70 (ABX-type dd, J ˆ 10.8
and 4.5 Hz, 1H; CH2OSi), 3.62 ± 3.54 (m, 2H; CH2OSi and CHO), 3.49
(ABX-type dd, J ˆ 11.0 and 11.0 Hz, 1H; CH2N), 3.31 (ddd, J ˆ 7.6, 7.6 and
2.9 Hz, 1H; CHN3), 3.14 (ABX-type dd, J ˆ 11.4 and 2.3 Hz, 1H; CH2N),
3.05 (ABX-type dd, J ˆ 13.4 and 7.6 Hz, 1H; CH2Ph), 2.98 (ABX-type dd,
J ˆ 13.4 and 7.7 Hz, 1H; CH2Ph), 1.16 (d, J ˆ 7.1 Hz, 3H; CH3), 0.96 (s, 9H,
tBu); 13C NMR (100 MHz, CDCl3): d ˆ 166.0, 136.5, 135.5 (2C), 133.0,
CDCl3): d ˆ 7.64 ± 7.59 (m, 4H; Ph), 7.50 ± 7.44 (m, 2H; Ph), 7.43 ± 7.23 (m,
9H; Ph), 4.82 ± 4.72 (m, 1H; CHMeN), 4.23 (q, J ˆ 6.8 Hz, 1H; OCHCH3),
3.75 (ABX-type dd, J ˆ 10.7 and 4.5 Hz, 1H; CH2OSi), 3.71 ± 3.61 (m, 2H;
CH2OSi and CHO), 3.55 (ABX-type dd, J ˆ 11.1 and 11.1 Hz, 1H; CH2N),
3.36 (ddd, J ˆ 7.6, 7.6 and 3.2 Hz, 1H; CHN3), 3.21 (ABX-type dd, J ˆ 11.5
and 2.6 Hz, 1H; CH2N), 3.10 (ABX-type dd, J ˆ 13.5 and 7.5 Hz, 1H;
CH2Ph), 3.02 (ABX-type dd, J ˆ 13.5 and 7.7 Hz, 1H; CH2Ph), 1.59 (d, J ˆ
6.8 Hz, 3H; OCHCH3), 1.20 (d, J ˆ 7.1 Hz, 3H; CH3), 1.02 (s, 9H; tBu);
13C NMR (100 MHz, CDCl3): d ˆ 169.2, 136.8, 135.6, 133.1, 129.9, 129.2,
132.9, 129.9, 129.8, 129.1, 128.9, 127.8, 127.2, 73.4, 68.0, 65.0, 63.0, 49.3, 43.4,
129.0, 127.9, 127.2, 74.7, 73.1, 65.2, 63.2, 49.7, 44.2, 36.2, 26.9, 19.2, 18.6, 13.6;
1
1
ˆ
ˆ
36.0, 26.7, 19.1, 13.4; IR (KBr): nÄ ˆ 2105 (N3), 1650 cm (C O); HR FAB
IR (neat): nÄ ˆ 2110 (N3), 1650 cm (C O); HR FAB MS calcd for
MS calcd for C31H39N4O3Si [M‡H] 543.2791, found 543.2786; C31H38N4O3Si
C32H41N4O3Si [M‡H] 557.2948, found 557.2942.
(542.8): calcd C 68.60, H 7.06, N 10.32; found C 68.37, H 6.86, N 10.07.
(2S,6S)-6-[(1S)-1-Azido-2-phenylethyl]-4-[(1S)-2-hydroxy-1-methylethyl]-
2-methylmorpholin-3-one (25): Tetrabutylammonium fluoride hydrate
(32 mg, 10 mmol) was added to the protected alcohol 24 (52 mg, 93 mmol)
in THF (3.5 mL) at room temperature. After stirring of the mixture for 2 h
the solvent was evaporated. Flash chromatography (heptane/ethyl acetate
(2S,3S)-N-[(1S)-2-(tert-Butyldiphenylsilyloxy)-1-methylethyl]-3-azido-4-
phenyl-2-(triethylsilyloxy)butylamine (22): Imidazole (36 mg, 0.53 mmol)
was added to amino alcohol 20 (0.115 g, 0.229 mmol) in CH2Cl2 (6 mL) at
room temperature. The solution was cooled to 08C and chlorotriethylsilane
(85 mL, 76 mg, 0.50 mmol) was added. After 4 h the reaction mixture was
poured into satd. aqueous NaHCO3 and extracted with CH2Cl2 (3 Â 5 mL).
The combined extracts were dried with Na2SO4 and concentrated. Flash
chromatography (heptane/ethyl acetate 10:1 !2:1) of the residue furnish-
1:1, then EtOH) of the residue furnished 25 (30 mg, 100%): [a]D20
ˆ
46.2
(c ˆ 0.4 in CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 7.45 ± 7.18 (m, 5H; Ph),
4.68 ± 4.57 (m, 1H; CHMeN), 4.21 (q, J ˆ 6.8 Hz, 1H; OCHCH3), 3.78
(ddd, J ˆ 10.7, 3.1 and 3.1 Hz, 1H; CHO), 3.66 (ABX-type dd, J ˆ 11.6 and
4.1 Hz, 1H; CH2OH), 3.58 ± 3.49 (m, 2H; CH2OH and CH2N), 3.46 (ddd,
J ˆ 7.6, 7.6 and 3.3 Hz, 1H; CHN3), 3.20 ± 3.01 [m, 3H; CH2N (1H) and
CH2Ph (2H)], 1.54 (d, J ˆ 6.8 Hz, 3H; OCHCH3), 1.16 (d, J ˆ 7.1 Hz, 3H;
CH3); 13C NMR (100 MHz, CDCl3): d ˆ 171.1, 136.6, 129.2, 128.9, 127.2,
ed 22 (0.103 g, 73%): [a]D20
ˆ
1.9 (c ˆ 0.5 in CHCl3); 1H NMR (400 MHz,
CDCl3): d ˆ 7.73 ± 7.68 (m, 4H; Ph), 7.48 ± 7.24 (m, 11H; Ph), 3.88 ± 3.79 (m,
1H; CHOSi), 3.70 ± 3.49 (m, 3H; CH2OSi and CHN3), 2.97 (ABX-type dd,
J ˆ 13.9 and 4.2 Hz, 1H; CH2Ph), 2.93 ± 2.70 [m, 4H; CHMeNH, CH2NH
and CH2Ph (1H)], 1.09 (s, 9H; tBu), 1.07 ± 1.00 (m, 12H; SiCH2CH3 and
CH3), 0.70 (q, J ˆ 7.9 Hz, 6H; SiCH2CH3); 13C NMR (100 MHz, CDCl3):
d ˆ 138.3, 135.5, 133.5, 129.6 (2C), 129.1, 128.5, 127.6, 126.5, 67.9, 65.2, 54.8,
49.8, 36.6, 26.8, 19.2, 17.2, 6.9, 5.1; IR (neat): nÄ ˆ 2105 cm 1 (N3); HR FAB
MS calcd for C35H53N4O2Si2 [M‡H] 617.3707, found 617.3713; C35H52N4O2-
Si2 (617.0): calcd C 68.13, H 8.49, N 9.08; found C 67.75, H 8.39, N 9.04.
74.6, 73.1, 64.5, 63.0, 51.7, 44.0, 36.2, 18.3, 13.2; IR (neat): nÄ ˆ 3365 (OH),
1
ˆ
2110 (N3), 1630 cm (C O); HR FAB MS calcd for C16H23N4O3 [M‡H]
319.1770, found 319.1768; C16H22N4O4 (318.4): calcd C 60.17, H 7.26, N
17.54; found C 60.32, H 7.37, N 17.38.
(2S)-2-{(2S,6S)-6-[(1S)-1-Azido-2-phenylethyl]-2-methylmorpholin-3-one-
4-yl}propionic acid (26): A catalytic amount of RuCl3 ´ H2O (2.2 mol%)
was added to a biphasic solution of alcohol 25 (30 mg, 94 mmol) and NaIO4
(60 mg, 0.28 mmol) in CCl4 (0.2 mL), acetonitrile (0.2 mL), and H2O
(0.3 mL). The black mixture was stirred vigorously for 2 h at room
temperature. Then CH2Cl2 (5 mL) was added and the phases were
separated. The aqueous phase was extracted with CH2Cl2 (4 Â 3 mL). The
combined organic extracts were dried with MgSO4, filtered and concen-
trated. The resulting residue was triturated with diethyl ether (5 mL),
(2R)-N-[(2S,3S)-3-Azido-2-hydroxy-4-phenylbutyl]-N-[(1S)-2-(tert-butyl-
diphenylsilyloxy)-1-methylethyl]-2-bromopropionamide (23): 1,3-Diiso-
propylcarbodiimide (DIC, 0.140 mL, 0.116 g, 0.917 mmol) was added to a
solution of (R)-(‡)-2-bromopropionic acid (97.3 mL, 0.160 g, 1.05 mmol) in
CH2Cl2 (2 mL) at 08C. The reaction mixture was stirred at 08C for 20 min,
after which amine 22 (81 mg, 0.13 mmol) in CH2Cl2 (2 mL) was added and
stirring was continued for 4 h at 08C. Then diisopropylurea was filtered off
and the filtrate was extracted with satd. aqueous NaHCO3 (2 Â 5 mL), 10%
citric acid (2 Â 5 mL) and brine (5 mL). The organic solution was dried over
Na2SO4 and concentrated. The residue was flash chromatographed
(heptane/ethyl acetate 10:1) to give the desired triethylsilylated amide
(0.136 g) contaminated by coeluting diisopropylurea. The silylated amide
(0.136 g) was dissolved in a mixture of 2m aqueous HCl (3 mL) and THF
(2 mL) and stirred at room temperature for 96 h.[50] The reaction mixture
was poured into satd. aqueous NaHCO3 (5 mL) and extracted with CH2Cl2
(3 Â 5 mL). The combined organic extracts were dried over Na2SO4 and
concentrated. Flash chromatography (heptane/ethyl acetate 10:1 !5:1) of
filtered through a Celite pad and concentrated to yield the acid 26 (27 mg,
1
86%): [a]2D0
ˆ
41.5 (c ˆ 0.4 in CHCl3); H NMR (500 MHz, CDCl3): d ˆ
7.21 ± 7.36 (m, 5H; Ph), 5.13 (q, J ˆ 7.4 Hz, 1H; CHMeN), 4.25 (q, J ˆ
6.8 Hz, 1H; OCHCH3), 3.80 (ddd, J ˆ 10.7, 3.2 and 3.2 Hz, 1H; CHO),
3.63 (ABX-type dd, J ˆ 10.9 and 10.9 Hz, 1H; CH2N), 3.45 (ddd, J ˆ 7.9, 7.0
and 3.4 Hz, 1H; CHN3), 3.12 (ABX-type dd, J ˆ 11.3 and 2.7 Hz, 1H;
CH2N), 3.06 (ABX-type dd, J ˆ 13.8 and 6.9 Hz, 1H; CH2Ph), 3.02 (ABX-
type dd, J ˆ 13.6 and 8.0 Hz, 1H; CH2Ph), 1.52 (d, J ˆ 6.8 Hz, 3H;
OCHCH3), 1.46 (d, J ˆ 7.5 Hz, 3H; CH3); 13C NMR (100 MHz, CDCl3):
d ˆ 175.0, 170.5, 136.5, 129.2, 128.9, 127.2, 74.6, 73.3, 62.9, 51.6, 45.4, 36.2,
2248
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0508-2248 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 8

g-Turn Mimetics
2241±2253
1
ˆ
ˆ
18.2, 13.7; IR (neat): nÄ ˆ 2110 (N3), 1730 (C O), 1615 cm (C O); HR
FAB MS calcd for C16H21N4O4 [M‡H] 333.1563, found 333.1571;
C16H20N4O4 (332.4): calcd C 57.82, H 6.07, N 16.86; found C 58.02, H 6.30,
N 16.69.
9.4 Hz, 1H; CH2NH), 2.57 ± 2.40 (m, 3H; CH2CO and CH2NH), 1.10 (s,
9H; tBu); 13C NMR (100 MHz, CDCl3): d ˆ 170.9, 144.8, 137.2, 135.5 (2C),
132.9 (2C), 129.9, 128.7, 128.6, 127.9, 127.8, 126.9, 126.8, 71.2, 70.2, 66.0, 63.9,
56.3, 49.9, 39.2, 37.0, 26.8, 19.2; IR (KBr): nÄ ˆ 3310 (OH), 2100 (N3),
1
ˆ
1655 cm (C O); C49H53N5O3Si (788.1): calcd C 74.68, H 6.78, N 8.89;
N-Triphenylmethyl (3S)-3-(9-fluorenylmethoxycarbonylamino)-4-hydroxy-
butyramide (28). N-Methylmorpholine (0.10 mL, 93 mg, 0.92 mmol) fol-
lowed by isobutyl chloroformate (0.12 mL, 0.13 g, 0.92 mmol) were added
to a stirred solution of Fmoc-Asn(Trt)-OH (27, 0.546 g, 0.915 mmol) in
THF (5 mL) at 108C. After 10 min, NaBH4 (0.103 g, 2.75 mmol) was
added in one portion. After a further 15 min MeOH (10 mL) was added
dropwise to the mixture over a period of 5 min at 08C. The solution was
stirred for an additional 10 min, and then neutralized with 2m aqueous HCl
(2 mL). The organic solvents were evaporated and the residue was
extracted with EtOAc (3 Â 10 mL). The combined organic phases were
dried with Na2SO4 and the solvent was removed. Flash chromatography
(toluene/EtOH 20:1) furnished alcohol 28 (0.373 g, 70%) as a white solid:
m.p. 168 ± 1708C; [a]2D0 ˆ ‡21.6 (c ˆ 0.7 in CHCl3); 1H NMR (400 MHz,
CDCl3): d ˆ 7.81 ± 7.72 (m, 2H; Ar), 7.60 ± 7.53 (m, 2H; Ar), 7.44 ± 7.37 (m,
2H; Ar), 7.32 ± 7.14 (m, 17H; Ar), 5.87 (d, J ˆ 7.6 Hz, 1H; OCONH), 4.40 ±
4.26 (m, 2H; CH2OCO), 4.15 (t, J ˆ 6.8 Hz, 1H; fluorenyl-9-H), 3.96 ± 3.85
(m, 1H; CHNFmoc), 3.71 ± 3.48 (m, 3H; CH2OH), 2.71 (ABX-type dd, J ˆ
14.5 and 5.8 Hz, 1H; CH2CO), 2.60 (ABX-type dd, J ˆ 14.4 and 4.3 Hz, 1H;
CH2CO); 13C NMR (100 MHz, CDCl3): d ˆ 170.7, 156.4, 144.1, 143.7, 141.2,
found C 74.92, H 7.01, N 8.94.
N-Triphenylmethyl (3S)-3-[(2S,3S)-3-azido-4-phenyl-2-triethylsilyloxybu-
tylamino]-4-(tert-butyldiphenylsilyloxy)butyramide (32): Amino alcohol
31 was silylated (0.501 g, 0.637 mmol) in CH2Cl2 (20 mL) with chlorotri-
ethylsilane (0.24 mL, 0.21 g, 1.4 mmol) in the presence of imidazole
(0.100 g, 1.47 mmol), as described for 22. After workup, flash chromatog-
raphy (heptane/ethyl acetate 10:1 !1:1) of the residue furnished 32
(0.489 g, 91%): m.p. 56 ± 578C; [a]D20
ˆ
23.1 (c ˆ 0.2 in CHCl3); 1H NMR
(400 MHz, CDCl3): d ˆ 9.44 (s, 1H; NHTrt), 7.66 ± 7.61 (m, 4H; Ph), 7.46 ±
7.19 (m, 26H; Ph), 3.85 (ABX-type dd, J ˆ 10.5 and 3.2 Hz, 1H; CH2OSi),
3.72 (ddd, J ˆ 8.3, 4.0 and 4.0 Hz, 1H; CHN3), 3.49 (ABX-type dd, J ˆ 10.5
and 4.0 Hz, 1H; CH2OSi), 3.39 (ddd, J ˆ 11.1, 3.6 and 3.6 Hz, 1H; CHOSi),
3.06 ± 3.00 (m, 1H; CHNH), 3.00 (ABX-type dd, J ˆ 14.1 and 2.9 Hz, 1H;
CH2NH), 2.79 (ABX-type dd, J ˆ 11.1 and 8.5 Hz, 1H; CH2Ph), 2.64
(ABX-type dd, J ˆ 10.8 and 4.2 Hz, 1H; CH2Ph), 2.63 (ABX-type dd, J ˆ
14.1 and 10.8 Hz, 1H; CH2NH), 2.54 (ABX-type dd, J ˆ 16.2 and 9.2 Hz,
1H; CH2CO), 2.42 (ABX-type dd, J ˆ 16.2 and 2.8 Hz, 1H; CH2CO),
2.10 ± 1.87 (brs, 1H; NH), 1.07 (s, 9H; tBu), 0.95 (t, J ˆ 7.9 Hz, 9H;
SiCH2CH3), 0.55 (q, J ˆ 8.0 Hz, 6H; SiCH2CH3); 13C NMR (90 MHz,
CDCl3): d ˆ 171.3, 145.1, 138.1, 135.6, 135.5, 133.1, 132.8, 129.9 (2C), 129.0,
128.8, 127.9, 127.8, 126.8 (2C), 74.1, 70.1, 66.5, 63.6, 56.8, 48.3, 39.8, 35.7,
128.5, 128.3, 127.9, 127.7, 127.1, 127.0, 125.0, 119.9, 70.8, 66.8, 64.1, 50.1, 47.0,
39.0; IR (neat): nÄ ˆ 3315 (OH), 1705 cm (C O); HR FAB MS calcd for
1
ˆ
C38H35N2O4 [M‡H] 583.2597, found 583.2591.
26.9, 19.3, 7.0, 5.0; IR (KBr): nÄ ˆ 2105 (N3), 1680 (C O), 1110 cm 1 (C O);
ˆ
N-Triphenylmethyl (3S)-4-(tert-butyldiphenylsilyloxy)-3-(9-fluorenylmeth-
oxycarbonylamino)butyramide (29): Imidazole (63 mg, 0.93 mmol) was
added to alcohol 28 (0.271 g, 0.465 mmol) in CH2Cl2 (5 mL) at room
temperature. After cooling to 08C tert-butylchlorodiphenylsilane (0.14 mL,
0.15 g, 0.62 mmol) was added and stirring was continued for 1.5 h. Then the
reaction mixture was poured into brine (10 mL) and extracted with CH2Cl2
(3 Â 10 mL). The combined organic phases were dried with Na2SO4 and
concentrated. Flash chromatography (heptane/ethyl acetate 10:1 !1:1) of
the residue yielded 29 (0.294 g, 77%) as a white solid: m.p. 84 ± 868C;
C55H67N5O3Si2 (902.3): calcd C 73.21, H 7.48, N 7.76; found C 73.44, H 7.59,
N 7.99.
N-Triphenylmethyl (3S)-3-{N'-[(2S,3S)-3-azido-2-hydroxy-4-phenylbutyl]-
N'-[(1R)-1-bromoethylcarbonyl]amino}-4-(tert-butyldiphenylsilyloxy)bu-
tyramide (33): Acylation of amine 32 (0.666 g, 0.791 mmol) with (R)-(‡)-2-
bromopropionic acid (0.570 mL, 0.968 g, 6.33 mmol) and DIC (0.860 mL,
0.698 g, 5.54 mmol), followed by removal of the triethylsilyl group with 2m
aqueous HCl (15 mL) and THF (10 mL), as described for 23, gave 33
(0.487 g, 67% over 2 steps) after purification by flash chromatography
[a]2D0
ˆ
7.4 (c ˆ 0.6 in CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 7.78 ± 7.72
(heptane/ethyl acetate 5:1). Compound 33 had: m.p. 808C; [a]D20
ˆ
38.1
(m, 2H; Ar), 7.65 ± 7.59 (m, 4H; Ar), 7.57 ± 7.50 (m, 2H; Ar), 7.45 ± 7.30 (m,
9H; Ar), 7.29 ± 7.22 (m, 10H; Ar), 7.20 ± 7.15 (m, 6H; Ar), 6.87 (s, 1H;
NHTrt), 5.73 (d, J ˆ 7.7 Hz, 1H; OCONH), 4.29 ± 4.22 and 4.15 ± 4.06 (2m,
4H; CH2OCO, fluorenyl-9-H, CHNFmoc), 3.84 ± 3.77 (m, 1H; CH2OSi),
3.72 ± 3.64 (m, 1H; CH2OSi), 2.87 ± 2.78 (m, 1H; CH2CO), 2.68 ± 2.58 (m,
1H; CH2CO), 1.05 (s, 9H; tBu); 13C NMR (100 MHz, CDCl3): d ˆ 170.0,
155.9, 144.5, 143.9 (2C), 141.2, 135.4, 133.0, 129.9, 129.8, 128.6, 128.3, 128.0,
(c ˆ 0.08 in CHCl3); 1H NMR (400 MHz, CDCl3, 1:1 mixture of rotamers A
and B): d ˆ 7.65 ± 7.51 (m, 4H; Ph, A ‡ B), 7.47 ± 7.13 (m, 26H; Ph, A ‡ B),
6.72 (s, 0.5H; NHTrt, A), 6.68 (s, 0.5H; NHTrt, B), 4.80 (q, J ˆ 6.6 Hz,
0.5H; CHBr, A), 4.58 ± 4.44 [m, 1.5H; CHN (A ‡ B, 1H) and CHBr (B,
0.5H)], 4.25 ± 4.15 [m, 1.5H; CH2OSi (A, 0.5H) and OH (A ‡ B)], 3.99 ±
3.92 (m, 0.5H; CHOH, A), 3.82 ± 3.72 [m, 1H; CH2OSi (A, 0.5H) and
CHOH (B, 0.5H)], 3.69 ± 3.62 (m, 0.5H; CH2OSi, B), 3.53 ± 3.34 [m, 1H;
CHN3 (A, 0.5H) and CH2OSi (B, 0.5H), 3.33 ± 3.22 [m, 1.5H; CH2NH
(A ‡ B, 1H) and CHN3 (B, 0.5H)], 3.18 ± 3.14, 3.09 ± 2.80, 2.75 ± 2.68 and
2.40 ± 2.28 [4m, 5H; CH2NH (A ‡ B, 1H), CH2Ph (A ‡ B, 2H) and
CH2CO (A ‡ B, 2H)], 1.73 (d, J ˆ 6.6 Hz, 1.5H; CH3, A), 1.72 (d, J ˆ
6.6 Hz, 1.5H; CH3, B), 1.04 (s, 4.5H; tBu, A), 1.00 (s, 4.5H; tBu, B);
13C NMR (100 MHz, CDCl3): d ˆ 172.4, 170.8, 169.9, 168.1, 144.4, 144.3,
137.4 (2C), 135.6, 135.4, 132.5, 132.4, 132.2, 130.1, 130.0 (2C), 129.3, 129.2,
129.1, 129.0, 128.7, 128.6, 128.5, 128.0 (2C), 127.9, 127.8, 127.2, 127.1, 126.9,
126.8, 71.7, 71.5, 70.9, 70.7, 66.2, 64.8, 64.0, 63.4, 56.3, 54.7, 47.6, 40.8, 39.7,
127.8 (2C), 127.7, 127.6, 127.1, 127.0, 125.2, 119.9, 70.8, 66.8, 64.5, 50.1, 47.1,
37.7, 26.9, 19.3; IR (neat): nÄ ˆ 1700 cm (C O); C54H52N2O4Si (821.1): calcd
1
ˆ
C 78.99, H 6.38, N 3.41; found C 79.22, H 6.58, N 3.32.
N-Triphenylmethyl
(3S)-3-amino-4-(tert-butyldiphenylsilyloxy)butyr-
amide (30): Compound 29 (0.294 g, 0.358 mmol) was dissolved in morpho-
line (5 mL) at room temperature. After 1 h the solution was coevaporated
with toluene (3 Â 5 mL). Flash chromatography (heptane/ethyl acetate
10:1 !1:1, then toluene/EtOH 1:1 !EtOH) of the residue gave 30 (0.189 g,
88%) as a white solid: m.p. 1258C; [a]D20
ˆ
7.4 (c ˆ 0.8 in CHCl3); 1H NMR
(360 MHz, CDCl3): d ˆ 9.26 (s, 1H; NHTrt), 7.66 ± 7.62 (m, 3H; Ph), 7.49 ±
7.34 (m, 5H; Ph), 7.32 ± 7.20 (m, 17H; Ph), 3.60 (ABX-type dd, J ˆ 10.1 and
4.2 Hz, 1H; CH2OSi), 3.49 (ABX-type dd, J ˆ 10.0 and 6.4 Hz, 1H;
CH2OSi), 3.31 ± 3.21 (m, 1H; CHNH2), 2.35 ± 2.29 (m, 2H; CH2CO), 1.08 (s,
9H; tBu); 13C NMR (90 MHz, CDCl3): d ˆ 170.7, 144.9, 135.3, 132.9, 129.7,
128.5, 127.7, 127.6, 126.6, 70.1, 68.6, 50.4, 40.2, 26.8, 19.1; IR (KBr): nÄ ˆ 1655
37.9, 37.1, 36.1, 35.9, 26.8, 21.8, 19.1, 19.0; IR (neat): nÄ ˆ 3360 (OH), 2105
1
ˆ
(N3), 1650 cm (C O); HR FAB MS calcd for C52H57BrN5O4Si [M‡H]
922.3363, found 922.3366; C52H56BrN5O4Si (923.0): calcd C 67.66, H 6.12, N
7.59; found C 68.01, H 6.39, N 7.51.
N-Triphenylmethyl (3S)-3-{N'-[(2S,3S)-3-azido-2-hydroxy-4-phenylbutyl]-
N'-[(1R)-1-bromopropylcarbonyl]amino}-4-(tert-butyldiphenylsilyloxy)-
butyramide (34): Acylation of amine 32 (0.111 g, 0.132 mmol) with (R)-2-
bromobutyric acid[25] (0.176 g, 1.05 mmol) and DIC (0.143 mL, 0.117 g,
0.924 mmol), followed by removal of the triethylsilyl group using 2m
aqueous HCl (3 mL) and THF (2 mL), as described for 23, gave 34
(38 mg, 31% over 2 steps) after purification by flash chromatography
1
ˆ
(C O), 1105 cm (C O); C39H42N2O2Si (598.9): calcd C 78.22, H 7.07, N
4.68; found C 78.52, H 7.32, N 4.53.
N-Triphenylmethyl (3S)-3-[(2S,3S)-3-azido-2-hydroxy-4-phenylbutylami-
no]-4-(tert-butyldiphenylsilyloxy)butyramide (31): Opening of epoxide 18
(0.210 g, 1.11 mmol) with amine 30 (0.637 g, 1.06 mmol) as described for 20,
followed by concentration and flash chromatography (heptane/ethyl
acetate 5:1 !1:1) of the residue, gave 31 (0.548 g, 66%) as a white solid:
(heptane/ethyl acetate 8:1 !3:1). Compound 34 had: m.p. 73 ± 758C;
m.p. 63 ± 658C; [a]D20
ˆ
9.4 (c ˆ 0.2 in CHCl3); 1H NMR (400 MHz,
[a]2D0
ˆ
28.0 (c ˆ 0.6 in CHCl3); H NMR (400 MHz, CDCl3, 7:3 mixture
1
CDCl3): d ˆ 8.13 (s, 1H; NHTrt), 7.71 ± 7.61 (m, 4H; Ph), 7.50 ± 7.19 (m,
26H; Ph), 3.76 (ABX-type dd, J ˆ 10.5 and 4.2 Hz, 1H; CH2OSi), 3.61
(ABX-type dd, J ˆ 10.4 and 4.6 Hz, 1H; CH2OSi), 3.40 (ddd, J ˆ 9.3, 3.1
and 3.1 Hz, 1H; CHOH), 3.28 (ddd, J ˆ 8.9, 6.1 and 3.3 Hz, 1H; CHN3),
3.01 ± 2.84 (m, 3H; CHNH and CH2Ph), 2.67 (ABX-type dd, J ˆ 11.7 and
of rotamers A and B): d ˆ 7.66 ± 7.53 (m, 4H; Ph, A ‡ B), 7.48 ± 7.13 (m,
26H; Ph, A ‡ B), 6.69 (s; NHTrt, B), 6.67 (s; NHTrt, A), 4.65 (dd, J ˆ 8.1
and 5.5 Hz; CHBr, B), 4.61 ± 4.54 (m; CHN), 4.28 ± 4.17 [m; CH2OSi and
CHBr (B)], 4.00 ± 3.92 (m; CHOH, A), 3.80 ± 3.73 (m; CHOH, B), 3.73 ±
3.70 (m; CH2OSi), 3.65 (ABX-type dd, J ˆ 10.0 and 5.3 Hz; CH2OSi),
Chem. Eur. J. 1999, 5, No. 8
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0508-2249 $ 17.50+.50/0
2249

P. Somfai, J. Kihlberg et al.
FULL PAPER
3.56 ± 3.46 (m; CH2OSi), 3.45 ± 3.39 (m; CHN3, A), 3.38 ± 3.23 [m; CH2NH
and CHN3 (B)], 3.21 ± 3.13 (m; CH2NH), 3.08 ± 2.56 (m; CH2Ph and
CH2CO), 2.41 ± 2.32 (m; CH2CO), 2.08 ± 1.90 (m, 2H; CH2CH3), 1.05 (s,
9H; tBu, A ‡ B), 1.04 ± 1.00 (m, 3H; CH3, A ‡ B); 13C NMR (100 MHz,
CDCl3): d ˆ 172.1, 170.5, 169.9, 168.1, 144.4, 144.3, 137.5, 137.3, 135.6, 135.5
(2C), 135.4, 132.5, 132.4, 132.3, 132.2, 130.1, 130.0, 129.9 (2C), 129.3, 129.2,
128.7, 128.6, 128.5, 128.4, 128.0 (2C), 127.9 (2C), 127.8 (2C), 127.2, 127.1,
126.9, 126.8, 71.7, 71.6, 70.9, 70.7, 66.2, 64.6, 64.2, 63.6, 56.2, 48.0, 48.1, 47.8,
CH2CO), 2.07 ± 1.96 (m, 1H; CH2CH3), 1.85 ± 1.72 (m, 1H; CH2CH3), 1.09 ±
0.99 (m, 12H; CH3 and tBu); 13C NMR (100 MHz, CDCl3): d ˆ 169.7, 169.3,
144.5, 136.9, 135.5, 135.4, 133.0, 132.9, 129.8, 129.1, 128.8, 128.5, 127.9, 127.8
(2C), 127.0, 78.7, 73.5, 70.4, 63.1, 62.8, 60.9, 51.8, 36.9, 36.0, 26.7, 25.4, 19.1,
1
ˆ
ˆ
9.7; IR (KBr): nÄ ˆ 2110 (N3), 1685 (C O), 1635 cm (C O); HR FAB MS
calcd for C53H58N5O4Si [M‡H] 856.4258, found 856.4266; C53H58N5O4Si
(856.2): calcd C 74.35, H 6.71, N 8.18; found C 74.23, H 7.07, N 8.11.
N-Triphenylmethyl (3S)-3-{(2S,6S)-6-[(1S)-1-azido-2-phenylethyl]-2-iso-
propylmorpholin-3-one-4-yl}-4-(tert-butyldiphenylsilyloxy)butyramide
(38): Intramolecular cyclization of bromo alcohol 35 (0.106 g, 0.115 mmol)
with KH (13 mg, 0.27 mmol) as described for 24, followed by workup and
purification of the residue by flash chromatography (heptane/ethyl acetate
47.7, 37.8, 37.1, 36.0, 35.8, 29.0, 28.4, 27.0, 26.8, 19.0 (2C), 12.1, 11.9; IR
1
ˆ
(neat): nÄ ˆ 3345 (OH), 2105 (N3), 1650 cm (C O); HR FAB MS calcd for
C53H59BrN5O4Si [M‡H] 936.3520, found 936.3530; C53H58BrN5O4Si (937.1):
calcd C 67.93, H 6.24, N 7.47; found C 67.89, H 6.25, N 7.51.
3:1) gave 38 (67 mg, 69%): m.p. 88 ± 908C; [a]D20
ˆ
33.1 (c ˆ 0.3 in
N-Triphenylmethyl (3S)-3-{N'-[(2S,3S)-3-azido-2-hydroxy-4-phenylbutyl]-
N'-[(1R)-1-bromo-2-methylpropylcarbonyl]amino}-4-(tert-butyldiphenyl-
silyloxy)butyramide (35): Acylation of amine 32 (0.316 g, 0.375 mmol) with
(R)-(‡)-2-bromo-3-methylbutyric acid (0.543 g, 3.00 mmol) and DIC
(0.406 mL, 0.331 g, 2.63 mmol), followed by removal of the triethylsilyl
group using 2m aqueous HCl (4.5 mL) and THF (3 mL), as described for
23, gave 35 (0.143 g, 40% over 2 steps) after purification by flash
chromatography (heptane/ethyl acetate 5:1). Compound 35 had: m.p.
CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 7.64 ± 7.58 (m, 4H; Ph), 7.45 ± 7.11
(m, 26H; Ph), 6.72 (s, 1H; NHTrt), 4.15 ± 4.06 (m, 1H; CH2OSi), 3.81 (d,
J ˆ 2.0 Hz, 1H; CHiPr), 3.78 ± 3.67 [m, 3H; CHN, CH2N (1H) and CH2OSi
(1H)], 3.50 ± 3.42 (m, 1H; CHO), 3.31 ± 3.13 [m, 3H; CHN3, CH2N (1H)
and CH2CO (1H)], 2.98 ± 2.93 (m, 2H; CH2Ph), 2.54 ± 2.39 [m, 2H;
CH(CH3)2 and CH2CO], 1.13 (d, J ˆ 7.1 Hz, 3H; CH3), 1.03 (s, 9H; tBu),
0.92 (d, J ˆ 6.8 Hz, 3H; CH3); 13C NMR (100 MHz, CDCl3): d ˆ 169.5,
169.3, 144.4, 136.9, 135.5, 135.4, 133.0, 132.9, 129.8, 129.1, 128.8, 128.5, 127.9,
78 ± 808C; [a]2D0
ˆ
35.1 (c ˆ 1.0 in CHCl3); 1H NMR (400 MHz, CDCl3,
127.8, 127.7, 127.0, 81.6, 73.3, 70.3, 63.2, 62.7, 37.0, 35.9, 30.6, 26.7, 19.4, 19.0,
4:1 mixture of rotamers A and B): d ˆ 7.65 ± 7.52 (m, 4H; Ph, A ‡ B), 7.47 ±
7.12 (m, 26H; Ph, A ‡ B), 6.67 (s, 0.2H; NHTrt, B), 6.65 (s, 0.8H; NHTrt,
A), 4.61 ± 4.53 [m, 0.4H; CHBr (B, 0.2H) and CHN (B, 0.2H)], 4.24 (brs,
1H; OH, A ‡ B), 4.06 ± 3.88 [m, 1H; CHBr (A, 0.8H) and CHOH (B,
0.2H)], 3.80 ± 3.67 [m, 1H; CHOH (A, 0.8H) and CH2OSi (B, 0.2H)],
3.64 ± 3.50 (m, 1.8H; CH2OSi, A ‡ B), 3.44 ± 3.20 [m, 4.6H; CH2NH (A ‡
B, 2H), CHN3 (A ‡ B, 1H), CHN (A, 0.8H) and CH2CO (A, 0.8H)],
3.09 ± 3.01 and 2.94 ± 2.83 (2m, 2H; CH2Ph, A ‡ B), 2.75 ± 2.68 (m, 0.2H;
CH2CO, B), 2.49 ± 2.42 (m, 0.2H; CH2CO, B), 2.36 ± 2.28 (m, 0.8H; CH2CO,
A), 2.27 ± 2.16 [m, 1H; CH(CH3)2, A ‡ B], 1.04 (s, 9H; tBu, A ‡ B) 1.02 ±
0.77 (m, 6H; CH(CH3)2, A ‡ B); 13C NMR (100 MHz, CDCl3): d ˆ 171.9,
170.6, 170.0, 168.1, 144.5, 144.4, 137.6, 137.4, 135.6 (2C), 135.5 (2C), 132.7,
132.5, 132.4, 130.1, 130.0, 129.3 (2C), 129.0, 128.7 (2C), 128.3, 128.0 (2C),
127.9, 127.8, 127.2, 127.1, 126.8, 72.2, 71.2, 71.0, 70.8, 66.3, 64.6, 64.3, 64.2,
1
ˆ
ˆ
15.8; IR (KBr): nÄ ˆ 2105 (N3), 1685 (C O), 1635 cm (C O); HR FAB MS
calcd for C54H60N5O4Si [M‡H] 870.4415, found 870.4404.
N-Triphenylmethyl
(3S)-3-{(2S,6S)-6-[(1S)-1-azido-2-phenylethyl]-2-
methylmorpholin-3-one-4-yl}-4-hydroxybutyramide (39): Deprotection of
36 (97 mg, 0.12 mmol) with tetrabutylammonium fluoride hydrate (33 mg,
0.13 mmol) as described for 25 followed by concentration and purification
of the residue by flash chromatography (heptane/ethyl acetate 1:1, then
EtOH) furnished 39 (68 mg, 98%): m.p. 93 ± 948C; [a]D20
ˆ
35.7 (c ˆ 0.2 in
CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 7.45 ± 7.24 (m, 20H; Ph), 4.11 (q,
J ˆ 6.8 Hz, 1H; CHCH3), 4.05 ± 3.93 (m, 1H; CHN), 3.82 ± 3.63 [m, 3H;
CH2O (2H) and CH2N (1H)], 3.58 (ddd, J ˆ 10.8, 3.1 and 3.1 Hz, 1H;
CHO), 3.48 ± 3.38 (m, 1H; CHN3), 3.32 (ABX-type dd, J ˆ 11.4 and 2.5 Hz,
1H; CH2N), 3.11 (ABX-type dd, J ˆ 15.2 and 9.3 Hz, 1H; CH2CO), 3.08 ±
2.96 (m, 2H; CH2Ph), 2.75 (ABX-type dd, J ˆ 15.3 and 5.8 Hz, 1H;
CH2CO), 1.52 (d, J ˆ 6.8 Hz, 3H; CH3); 13C NMR (100 MHz, CDCl3): d ˆ
170.9, 169.5, 144.3, 136.6, 129.1, 128.7, 128.5, 127.8, 127.0, 126.9, 74.1, 73.4,
56.3, 54.5, 53.6, 38.0, 37.1, 36.0, 35.9, 32.0, 31.8, 27.0, 26.9, 20.7, 20.3, 20.2, 19.1
1
ˆ
(2C); IR (neat): nÄ ˆ 3340 (OH), 2105 (N3), 1645 cm (C O); HR FAB MS
calcd for C54H61BrN5O4Si [M‡H] 950.3676, found 950.3657; C54H61BrN5O4-
Si (951.1): calcd C 68.19, H 6.36, N 7.36; found C 67.97, H 6.68, N 7.16.
70.4, 63.0, 62.7, 59.0, 49.9, 36.1, 36.0, 18.0; IR (KBr): nÄ ˆ 3410 and 3300
1
ˆ
ˆ
(N H and O H), 2110 (N3), 1660 (C O), 1635 cm (C O); HR FAB MS
N-Triphenylmethyl
(3S)-3-{(2S,6S)-6-[(1S)-1-azido-2-phenylethyl]-2-
calcd for C36H37N5O4 [M‡H] 604.2924, found 604.2931.
methylmorpholin-3-one-4-yl}-4-(tert-butyldiphenylsilyloxy)butyramide
(36): Intramolecular cyclization of bromo alcohol 33 (0.446 g, 0.483 mmol)
with KH (44.6 mg, 1.11 mmol) as described for 24, followed by workup and
purification of the residue by flash chromatography (heptane/ethyl acetate
N-Triphenylmethyl (3S)-3-{(2S,6S)-6-[(1S)-1-azido-2-phenylethyl]-2-iso-
propylmorpholin-3-one-4-yl}-4-hydroxybutyramide (40): Deprotection of
38 (59 mg, 68 mmol) with tetrabutylammonium fluoride hydrate (23 mg,
88 mmol) as described for 25 followed by concentration and purification of
the residue by flash chromatography (heptane/ethyl acetate 1:1, then
5:1 !1:1) gave 36 (0.341 g, 84%): m.p. 75 ± 788C; [a]D20
ˆ
18.6 (c ˆ 0.1 in
CHCl3); 1H NMR (500 MHz, CDCl3): d ˆ 7.64 ± 7.58 (m, 4H; Ph), 7.45 ± 7.13
(m, 26H; Ph), 6.76 (s, 1H; NHTrt), 4.09 (ABX-type dd, J ˆ 9.3 and 9.3 Hz,
1H; CH2OSi), 4.04 (q, J ˆ 6.7 Hz, 1H; CHCH3), 3.95-3.82 (m, 1H; CHN),
3.73 (ABX-type dd, J ˆ 11.2 and 11.2 Hz, 1H; CH2N), 3.68 (ABX-type dd,
J ˆ 10.0 and 5.1 Hz, 1H; CH2OSi), 3.54 ± 3.46 (m, 1H; CHO), 3.33 ± 3.26
(m, 1H; CHN3), 3.25 ± 3.16 (m, 2H; CH2N and CH2CO), 2.99 ± 2.88 (m,
2H; CH2Ph), 2.50 (ABX-type dd, J ˆ 15.3 and 4.1 Hz, 1H; CH2CO), 1.47
(d, J ˆ 6.6 Hz, 3H; CH3), 1.04 (s, 9H; tBu); 13C NMR (100 MHz, CDCl3):
d ˆ 170.2, 169.3, 144.5, 136.9, 135.5 (2C), 133.1, 132.9, 129.8, 129.2, 128.8,
EtOH) furnished 40 (42 mg, 98%): [a]D20
ˆ
46.6 (c ˆ 0.2 in CHCl3);
1H NMR (400 MHz, CDCl3): d ˆ 7.40 ± 7.14 (m, 20H; Ph), 7.07 (s, 1H;
NHTrt), 4.18 ± 3.99 (m, 1H; OH), 3.90 (d, J ˆ 2.3 Hz, 1H; CHiPr), 3.83 ±
3.71 (m, 3H; CHN and CH2OH), 3.61 (ABX-type dd, J ˆ 11.1 and 11.1 Hz,
1H; CH2N), 3.45 (ddd, J ˆ 10.7, 2.9 and 2.9 Hz, 1H; CHO), 3.25 (ABX-type
dd, J ˆ 11.7 and 2.6 Hz, 1H; CH2N), 3.20 (ddd, J ˆ 7.4, 7.4 and 3.1 Hz, 1H;
CHN3), 3.17 ± 3.08 (m, 1H; CH2CO), 3.04 ± 2.95 (m, 2H; CH2Ph), 2.84 ±
2.76 (m, 1H; CH2CO), 2.43 [septet of d, J ˆ 6.9 and 2.4 Hz, 1H;
CH(CH3)2], 1.14 (d, J ˆ 7.0 Hz, 3H; CH3), 0.92 (d, J ˆ 6.8 Hz, 3H; CH3);
13C NMR (100 MHz, CDCl3): d ˆ 170.6, 169.5, 144.3, 136.7, 129.1, 128.8,
128.6, 127.9, 127.8, 127.0, 74.3, 73.6, 70.4, 63.1, 62.9, 60.1, 51.6, 36.8, 36.1, 26.7,
1
ˆ
19.1, 18.0; IR (KBr): nÄ ˆ 2100 (N3), 1640 (C O), 1120 cm (C O);
C52H55N5O4Si (842.1): calcd C 74.17, H 6.58, N 8.32; found C 74.37, H 6.54, N
8.33.
128.5, 127.9, 127.0, 81.4, 73.2, 70.5, 63.9, 62.5, 60.4, 50.7, 36.1, 35.9, 30.9, 19.2,
1
ˆ
15.6; IR (neat): nÄ ˆ 3295 (OH), 2110 (N3), 1630 cm (C O); HR FAB MS
calcd for C38H42N5O4 [M‡H] 632.3237, found 632.3244.
N-Triphenylmethyl (3S)-3-{(2S,6S)-6-[(1S)-1-azido-2-phenylethyl]-2-ethyl-
morpholin-3-one-4-yl}-4-(tert-butyldiphenylsilyloxy)butyramide (37): In-
tramolecular cyclization of bromo alcohol 34 (38 mg, 41 mmol) with KH
(3.7 mg, 93 mmol) as described for 24, followed by workup and purification
of the residue by flash chromatography (heptane/ethyl acetate 3:1), gave 37
(2S)-2-{(2S,6S)-6-[(1S)-1-Azido-2-phenylethyl]-2-methylmorpholin-3-one-
4-yl}-3-(N-triphenylmethylcarbamoyl)propionic acid (41): Oxidation of
alcohol 39 (62 mg, 0.10 mmol) in a mixture of CCl4 (0.4 mL), acetonitrile
(0.4 mL), and H2O (0.6 mL) with NaIO4 (66 mg, 0.31 mmol) and a catalytic
amount of RuCl3 ´ H2O (2.2 mol%) as described for 26, followed by
(22 mg, 63%): m.p. 71 ± 738C; [a]D20
ˆ
31.5 (c ˆ 0.03 in CHCl3); 1H NMR
(400 MHz, CDCl3): d ˆ 7.64 ± 7.58 (m, 4H; Ph), 7.46 ± 7.12 (m, 26H; Ph),
6.75 (s, 1H; NHTrt), 4.10 (ABX-type dd, J ˆ 10.0 and 8.7 Hz, 1H; CH2OSi),
4.04 (dd, J ˆ 7.5 and 3.5 Hz, 1H; CHC2H5), 3.87 ± 3.79 (m, 1H; CHN), 3.75
(ABX-type dd, J ˆ 11.5 and 11.0 Hz, 1H; CH2N), 3.67 (ABX-type dd, J ˆ
10.2 and 5.1 Hz, 1H; CH2OSi), 3.49 (ddd, J ˆ 11.1, 3.4 and 3.4 Hz, 1H;
CHO), 3.29 ± 3.16 [m, 3H; CHN3, CH2N (1H) and CH2CO (1H)], 3.00 ±
2.88 (m, 2H; CH2Ph), 2.49 (ABX-type dd, J ˆ 15.1 and 4.6 Hz, 1H;
purification, gave 41 (51 mg, 81%): m.p. 106 ± 1088C; [a]D20
ˆ
44.0 (c ˆ 0.3
in CHCl3); 1H NMR (360 MHz, CDCl3): d ˆ 8.84 ± 8.15 (brs, 1H; COOH),
7.39 ± 7.15 (m, 20H; Ph), 4.20 (q, J ˆ 6.7 Hz, 1H; CHCH3), 4.18 ± 4.11 (m,
1H; CHN), 3.69 (ABX-type dd, J ˆ 10.9 and 10.9 Hz, 1H; CH2N), 3.50
(ddd, J ˆ 10.6, 3.2 and 3.2 Hz, 1H; CHO), 3.38 ± 3.25 (m, 2H; CHN3 and
CH2N), 2.99 ± 2.79 (m, 4H; CH2Ph and CH2CO), 1.50 (d, J ˆ 6.8 Hz, 3H;
CH3); 13C NMR (90 MHz, CDCl3): d ˆ 172.6, 170.6, 169.5, 144.4, 136.8,
2250
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0508-2250 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 8

g-Turn Mimetics
2241±2253
129.1, 128.8, 128.7, 127.8, 127.0 (2C), 73.9, 70.5, 63.0, 58.6, 51.4, 36.2, 36.1,
CDCl3, 7:3 mixture of rotamers A and B): d ˆ 7.57 (m, 4H; Ph, A ‡ B),
7.47 ± 7.13 (m, 26H; Ph, A ‡ B), 6.72 (s, 0.7H; NHTrt, A), 6.57 (s, 0.3H;
NHTrt, B), 4.79 (q, 0.7H, J ˆ 6.4 Hz; CHBr, A), 4.67 (q, 0.3H, J ˆ 6.4 Hz;
CHBr, B), 4.53 ± 4.37 (m, 1H; CH2OSi, A ‡ B), 3.86 ± 3.49 [3 m, 5.4H;
CH2OSi (1H), A ‡ B; CHN3 (1H), A ‡ B; CHOH (1H), A ‡ B; CHN
(1H), A ‡ B; CH2N, (0.7H, 2 C), A], 3.33 [m, 1H; CH2CO (0.7H), A;
CH2N (0.3H), B], 3.09 [m, 0.6H; CH2N (0.3H), B; CH2Ph (0.3H), B], 2.84
(ABX-type dd, J ˆ 14.1 and 4.6 Hz, 0.7H; CH2Ph, A), 2.70 (ABX-type dd,
J ˆ 14.1 and 9.3 Hz, 0.3H; CH2Ph, B), 2.65 (ABX-type dd, J ˆ 14.1 and
9.6 Hz, 0.7H; CH2Ph, A), 2.56 (ABX-type dd, J ˆ 14.8 and 3.5 Hz, 0.3H;
CH2CO, B), 2.34 (ABX-type dd, J ˆ 15.3 and 3.1 Hz, 0.7H; CH2CO, A),
2.17 (ABX-type dd, J ˆ 14.8 and 9.8 Hz, 0.3H; CH2CO, B), 1.83 (d, J ˆ
6.5 Hz, 2.1H; CHCH3, A), 1.72 (d, J ˆ 6.5 Hz, 0.9H; CHCH3, B), 1.04 (s,
6.3H; tBu, A), 1.02 (s, 2.7H; tBu); 13C NMR (100 MHz, CDCl3): d ˆ 173.7,
171.2, 169.6, 167.2, 144.3, 144.2, 137.3, 137.2, 135.6, 135.5, 135.4, 135.3, 132.4
(2C), 130.2, 130.1, 130.0, 129.2, 129.2, 128.7 (2C), 128.6 (2C), 128.0 (2C),
127.9 (2C), 127.2, 127.1, 126.9, 126.8, 73.9, 70.9, 70.8, 69.3, 67.0, 66.4, 63.8,
62.6, 56.4, 46.6, 40.5, 39.1, 37.6, 37.1, 36.8, 26.9 (3C), 26.8 (3C), 21.6 (2C),
ˆ
29.6, 17.5; IR (KBr): nÄ ˆ 3345 (COOH), 2115 (N3), 1735 (C O), 1675
1
ˆ
ˆ
(C O), 1650 cm (C O); HR FAB MS calcd for C36H35N5NaO5 [M‡Na]
640.2536, found 640.2523; C36H35N5O5 (617.7): calcd C 70.0, H 5.71, N 11.34;
found C 69.93, H 5.83, N 11.46.
(2S)-2-{(2S,6S)-6-[(1S)-1-Azido-2-phenylethyl]-2-isopropylmorpholin-3-
one-4-yl}-3-(N-triphenylmethylcarbamoyl)propionic acid (42): Oxidation
of alcohol 40 (42 mg, 67 mmol) in a mixture of CCl4 (0.4 mL), acetonitrile
(0.4 mL), and H2O (0.6 mL) with NaIO4 (43 mg, 0.20 mmol) and a catalytic
amount of RuCl3 ´ H2O (2.2 mol%) as described for 26, followed by
purification, gave 42 (31 mg, 72%): m.p. 118 ± 1198C; [a]D20
ˆ
50.3 (c ˆ 0.5
in CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 7.39 ± 7.06 (m, 21H; Ph and
NHTrt), 4.18 ± 4.04 (m, 1H; CHN), 4.01 ± 3.97 (m, 1H; CHiPr), 3.70 (ABX-
type dd, J ˆ 11.1 and 11.1 Hz, 1H; CH2N), 3.53 ± 3.44 (m, 1H; CHO), 3.31 ±
3.23 (m, 1H; CH2N), 3.22 ± 3.13 (m, 1H; CHN3), 2.97 ± 2.84 [m, 3H; CH2Ph
(2H) and CH2CO], 2.81 ± 2.68 (m, 1H; CH2CO), 2.51 ± 2.37 [m, 1H;
CH(CH3)2], 1.16 (d, J ˆ 6.9 Hz, 3H; CH3), 0.95 (d, J ˆ 6.7 Hz, 3H; CH3);
13C NMR (100 MHz, CDCl3): d ˆ 173.0, 169.9, 169.5, 144.5, 137.0, 129.2,
129.1, 129.0, 128.8, 128.7, 128.3, 127.9, 127.8, 127.7, 127.1, 127.0 (2C), 81.4,
ˆ
ˆ
19.1, 18.9; IR (KBr): nÄ ˆ 3415 (OH), 2110 (N3), 1685 (C O), 1655 (C O),
1
1110 cm (C O); C52H56BrN5O4Si (923.0): calcd C 67.66, H 6.12, N 7.59;
73.9, 70.5, 62.9, 58.9, 51.1, 36.2, 35.9, 30.8, 19.4, 15.6; IR (neat): nÄ ˆ 3345
1
found C 67.96, H 6.31, N 7.60.
ˆ
ˆ
ˆ
(COOH), 2110 (N3), 1735 (C O), 1685 (C O), 1645 cm (C O); HR FAB
MS calcd for C38H40N5O5 [M‡H] 646.3029, found 646.3033; C38H39N5O5
(645.8): calcd C 70.68, H 6.09, N 10.85; found C 70.79, H 6.38, N 10.91.
N-Triphenylmethyl
(3S)-3-{(2S,6R)-6-[(1S)-1-azido-2-phenylethyl]-2-
methylmorpholin-3-one-4-yl}-4-(tert-butyldiphenylsilyloxy)butyramide
(46): Intramolecular cyclization of bromo alcohol 45 (0.290 g, 0.315 mmol)
with KH (28 mg, 0.72 mmol) as described for 24, followed by workup and
purification of the residue by flash chromatography (heptane/ethyl acetate
N-Triphenylmethyl
(3S)-3-[(2R,3S)-3-Azido-2-hydroxy-4-phenylbutyl-
amino]-4-(tert-butyldiphenylsilyloxy)butyramide (43): Opening of epoxide
17[27, 31] (0.127 g, 0.671 mmol) with amine 30 (0.402 g, 0.671 mmol) as
described for 20, followed by concentration and flash chromatography
10:1 !3:1), gave 46 (0.219 g, 84%) as a white solid: m.p. 74 ± 768C; [a]D20
ˆ
‡3.8 (c ˆ 0.7 in CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 7.63 ± 7.61 (m,
4H; Ph), 7.43 ± 7.22 (m, 20H; Ph), 7.19 ± 7.16 (m, 6H; Ph), 6.93 (s, 1H;
NHTrt), 4.36 (q, J ˆ 6.0 Hz, 1H; CHCH3), 4.16 ± 4.00 (m, 2H; CHN and
CH2OSi), 3.81 (ddd, J ˆ 10.0, 7.3 and 3.3 Hz, 1H; CHO), 3.70 (ABX-type
dd, J ˆ 9.9 and 8.2 Hz, 1H; CH2OSi), 3.60 (ddd, J ˆ 9.6, 6.5 and 3.2 Hz, 1H;
CH N3), 3.50 (ABX-type dd, J ˆ 12.2 and 10.5 Hz, 1H; CH2N), 3.35 (ABX-
type dd, J ˆ 12.3 and 3.2 Hz, 1H; CH2N), 3.15 (m, 1H; CH2CO), 3.03
(ABX-type dd, J ˆ 14.1 and 3.2 Hz, 1H; CH2Ph), 2.69 (ABX-type dd, J ˆ
14.1 and 9.4 Hz, 1H; CH2Ph), 2.45 (ABX-type dd, J ˆ 14.9 and 4.2 Hz, 1H;
CH2CO), 1.40 (d, J ˆ 7.0 Hz, 3H; CHCH3), 1.04 (s, 9H; tBu); 13C NMR
(100 MHz, CDCl3): d ˆ 170.5, 169.1, 144.5, 136.9, 135.5, 135.4, 132.9 (2C),
129.8, 129.3, 128.6 (2C), 127.9 (2C), 127.8 (2C), 127.0, 126.9, 72.3, 70.6, 69.6,
(heptane/ethyl acetate 10:1 !1:1) of the residue gave 43 (0.382 g, 72%) as
1
a white solid: m.p. 125 ± 1268C; [a]D20
ˆ
2.1 (c ˆ 0.9 in CHCl3); H NMR
(400 MHz, CDCl3): d ˆ 7.96 (s, 1H; NHTrt), 7.64 ± 7.59 (m, 4H; Ph), 7.44 ±
7.19 (m, 26H; Ph), 3.72 (ABX-type dd, J ˆ 10.4 and 4.5 Hz, 1H; CH2OSi),
3.63 (ABX-type dd, J ˆ 10.5 and 5.0 Hz, 1H; CH2OSi), 3.47 (ddd, J ˆ 9.6,
5.6 and 4.0 Hz, 1H; CHN3), 3.33 (ddd, J ˆ 8.8, 5.8 and 3.0 Hz, 1H; CHOH),
3.05 (m, 1H; CHNH), 2.87 (ABX-type dd, J ˆ 14.1 and 4.1 Hz, 1H;
CH2Ph), 2.72 (ABX-type dd, J ˆ 12.5 and 3.0 Hz, 1H; CH2NH), 2.66
(ABX-type dd, J ˆ 14.0 and 9.4 Hz, 1H; CH2Ph), 2.46 [m, 3H; CH2NH
(1H) and CH2CO], 1.07 (s, 9H; tBu); 13C NMR (100 MHz, CDCl3): d ˆ
170.6, 144.7, 137.5, 135.5 (2C), 133.0, 132.9, 129.9, 129.3, 128.7 (2C), 128.6,
127.9 (2C), 127.8, 127.0, 126.7, 72.2, 70.4, 66.5, 64.5, 56.6, 48.1, 39.0, 36.8, 26.9,
64.9, 63.3, 59.8, 50.5, 36.8, 36.4, 26.8 (3C), 19.1, 17.2; IR (KBr): nÄ ˆ 2110
ˆ
19.2; IR (KBr): nÄ ˆ 3410 (OH), 3315 (NH), 2100 (N3), 1655 (C O),
1
1
ˆ
ˆ
(N3), 1690 (C O), 1640 (C O), 1110 cm (C O); C52H55N5O4Si (842.1):
calcd C 74.17, H 6.58, N 8.32; found C 74.04, H 6.78, N 8.09.
1110 cm (C O); C49H53N5O3Si (788.1): calcd C 74.68, H 6.78, N 8.89;
found C 74.46, H, 6.94, N 8.81.
N-Triphenylmethyl
(3S)-3-{(2S,6R)-6-[(1S)-1-azido-2-phenylethyl]-2-
N-Triphenylmethyl (3S)-3-[(2R,3S)-3-azido-4-phenyl-2-triethylsilyloxybu-
tylamino]-4-(tert-butyldiphenylsilyloxy)butyramide (44): Silylation of ami-
no alcohol 43 (0.144 g, 0.183 mmol) in CH2Cl2 (6 mL) with chlorotriethyl-
silane (68 mL, 61 mg, 1.4 mmol) in the presence of imidazole (29 mg,
0.42 mmol) was performed as described for 22. After workup, flash
chromatography (heptane/ethyl acetate 15:1 !3:1) of the residue furnish-
methylmorpholin-3-one-4-yl}-4-hydroxybutyramide (47): Deprotection of
46 (0.200 g, 0.238 mmol) with tetrabutylammonium fluoride hydrate
(72 mg, 0.27 mmol) as described for 25 followed by concentration and
purification of the residue by flash chromatography (heptane/ethyl acetate
1:1, then EtOH) furnished 47 (0.144 g, 100%) as a white solid: m.p. 91 ±
938C; [a]2D0
ˆ
1.0 (c ˆ 0.8 in CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ
ed 44 (0.146 g, 95%) as a white solid: m.p. 47 ± 498C; [a]D20
ˆ
5.0 (c ˆ 1.0 in
CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 8.73 (s, 1H; NHTrt), 7.64 ± 7.61
(m, 4H; Ph), 7.43 ± 7.14 (m, 26H; Ph), 3.70 ± 3.64 (m, 3H; CHOSi and
CH2OSi), 3.57 (m, 1H; CHN3), 3.04 (m, 1H; CHNH), 2.85 (ABX-type dd,
J ˆ 14.1 and 3.5 Hz, 1H; CH2Ph), 2.81 (ABX-type dd, J ˆ 11.8 and 5.1 Hz,
1H; CH2NH), 2.68 (ABX-type dd, J ˆ 11.9 and 4.9 Hz, 1H; CH2NH), 2.60
(ABX-type dd, J ˆ 13.9 and 10.4 Hz, 1H; CH2Ph), 2.38 (d, J ˆ 5.5 Hz, 2H;
CH2CO), 1.06 (s, 9H; t Bu), 0.92 (t, J ˆ 7.9 Hz, 9H; SiCH2CH3 ), 0.54 (q, J ˆ
7.8 Hz, 6H; SiCH2CH3); 13C NMR (100 MHz, CDCl3): d ˆ 170.3, 144.9,
137.9, 135.5, 135.4, 133.0, 132.9, 129.8, 129.1, 128.7, 128.6 (2C), 127.8 (2C),
7.35 ± 7.19 (m, 20H; Ph), 4.34 (q, J ˆ 6.8 Hz, 1H; CHCH3), 4.03 (m, 1H;
CHN), 3.75 ± 3.57 (m, 4H; CHO and CHN3 and CH2O), 3.48 (ABX-type
dd, J ˆ 12.4 and 10.2 Hz, 1H; CH2N), 3.30 (ABX-type dd, J ˆ 12.4 and
3.0 Hz, 1H; CH2N), 3.07 (ABX-type dd, J ˆ 14.0 and 3.6 Hz, 1H; CH2Ph),
3.03 (ABX-type dd, J ˆ 15.0 and 9.5 Hz, 1H; CH2CO), 2.73 (ABX-type dd,
J ˆ 14.0 and 9.1 Hz, 1H; CH2Ph), 2.58 (ABX-type dd, J ˆ 15.0 and 5.6 Hz,
1H; CH2CO), 1.37 (d, J ˆ 7.0 Hz, 3H; CHCH3); 13C NMR (100 MHz,
CDCl3): d ˆ 171.3, 169.2, 144.3, 136.6, 129.3, 128.6, 128.5 (2C), 127.8 (2C),
126.9, 126.8, 72.0, 70.4, 69.4, 64.5, 63.0, 59.4, 49.4, 36.7, 35.8, 17.3; IR (KBr):
1
ˆ
ˆ
nÄ ˆ 3415 (OH), 2110 (N3), 1670 (C O), 1640 cm (C O); HR FAB MS
126.7, 126.6, 74.3, 70.2, 66.6, 64.9, 57.0, 48.7, 37.8, 36.6, 26.9, 19.2, 6.9, 4.8; IR
1
ˆ
calcd for C36H37N5O4 [M‡H] 604.2924, found 604.2925.
(KBr): nÄ ˆ 3315 (NH), 2100 (N3), 1665 (C O), 1110 cm
(C O);
C55H67N5O3Si2 (902.3): calcd C 73.21, H 7.48, N 7.76; found C 73.64, H
7.67, N 7.75.
(2S)-2-{(2S,
6R)-6-[(1S)-1-azido-2-phenylethyl]-2-methylmorpholin-3-
one-4-yl}-3-(N-triphenylmethylcarbamoyl)propionic acid (48): Oxidation
of alcohol 47 (36 mg, 59 mmol) in a mixture of CCl4 (0.4 mL), acetonitrile
(0.4 mL), and H2O (0.6 mL) with NaIO4 (38 mg, 0.18 mmol) and a catalytic
amount of RuCl3 ´ H2O (2.2 mol%) as described for 26, followed by
N-Triphenylmethyl
(3S)-3-{N'-[(2R,3S)-3-azido-2-hydroxy-4-phenylbu-
tyl]-N'-[(1R)-1-bromoethylcarbonyl]amino}-4-(tert-butyldiphenylsilyloxy)-
butyramide (45): Acylation of amine 44 (0.400 g, 0.447 mmol) with (R)-
(‡)-2-bromopropionic acid (0.327 mL, 0.546 g, 3.57 mmol) and DIC
(0.622 mL, 0.507 g, 3.13 mmol), followed by removal of the triethylsilyl
group using 2m aqueous HCl (3 mL) and THF (1.5 mL), as described for
23, gave 45 (0.357 g, 58% over 2 steps) after purification by flash
chromatography (heptane/ethyl acetate 15:1 !5:1). Compound 45 had:
purification, gave 48 (35 mg, 96%): m.p. 103 ± 1058C; [a]D20
ˆ
13.8 (c ˆ 0.8
in CHCl3); 1H NMR (400 MHz, CDCl3): d ˆ 10.50 (brs, 1H; COOH),
7.36 ± 7.18 (m, 20H; Ph), 4.53 (q, J ˆ 7.0 Hz, 1H; CHCH3), 4.23 (m, 1H;
CHN), 3.84 (ddd, J ˆ 9.8, 6.5 and 3.2 Hz, 1H; CHO), 3.56 (ddd, J ˆ 9.8, 6.5
and 3.3 Hz, 1H; CHN3), 3.41 (ABX-type dd, J ˆ 12.5 and 10.1 Hz, 1H;
CH2N), 3.29 (ABX-type dd, J ˆ 12.5 and 3.1 Hz, 1H; CH2N), 3.06 (ABX-
m.p. 76 ± 788C; [a]D20
ˆ
38.0 (c ˆ 0.7 in CHCl3); 1H NMR (400 MHz,
Chem. Eur. J. 1999, 5, No. 8
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0508-2251 $ 17.50+.50/0
2251

P. Somfai, J. Kihlberg et al.
FULL PAPER
type dd, J ˆ 14.1 and 3.2 Hz, 1H; CH2Ph), 2.89 (ABX-type dd, J ˆ 16.3 and
5.7 Hz, 1H; CH2CO), 2.82 (ABX-type dd, J ˆ 16.1 and 8.3 Hz, 1H;
CH2CO), 2.69 (ABX-type dd, J ˆ 14.1 and 9.6 Hz, 1H; CH2Ph), 1.45 (d, J ˆ
6.9 Hz, 3H; CHCH3); 13C NMR (100 MHz, CDCl3): d ˆ 172.5, 170.8, 169.4,
144.4, 137.7, 136.9, 129.2, 128.9, 128.7 (2C), 128.6, 128.5, 128.1, 127.8 (2C),
Supporting information: Supporting information for this article, available
on the WWW under http://www.wiley-vch.de/home/chemistry/ or from the
author, describes the synthetic procedures for the preparation of azido
epoxides 17 and 18; 13C NMR spectra for compounds 24, 28, 38, 39, 40, and
47, 1H NMR spectra, including COSY and TOCSY spectra, for compound
49, are also included.
126.9, 126.8, 71.9, 70.6, 69.3, 64.5, 59.8, 51.7, 36.8, 35.8, 16.7; IR (KBr): nÄ ˆ
1
ˆ
ˆ
ˆ
3415 (COOH), 2110 (N3), 1735 (C O), 1685 (C O), 1650 (C O), 1260 cm
(C O); C36H35N5O5 (617.7): calcd C 70.00, H 5.71, N 11.34; found C 69.83, H
5.62, N 11.23.
Acknowledgment
Procedure for solid-phase peptide synthesis of 49: Peptide 49 was
synthesized with DMF as a solvent in a mechanically agitated reactor. A
polystyrene resin grafted with aminated polyethyleneglycol chains (Tenta-
Gel S NH2, Rapp Polymere, Germany; 0.309 g, 0.26 mequivg 1, 80 mmol)
was used for the synthesis. The resin was functionalized with the linker p-
[(R,S)-a-[1-(9H-fluoren-9-yl)-methoxyformamido]-2,4-dimethoxybenzyl]-
phenoxyacetic acid[41] (Novabiochem, Switzerland). Na-Fmoc amino acids
(Bachem, Switzerland) with the following protective groups were used:
2,2,5,7,8-pentamethylchroman-6-sulfonyl (Pmc) for d-Arg, triphenylmethyl
(Trt) for Cys and 3-mercaptopropionic acid, and tert-butyl (tBu) for Tyr.
Reagent solutions and DMF for washing were added manually to the
reactor.
This work was funded by a grant from the TMR programme of the
European Commission to K.B. and by grants from the Swedish Natural
Ê
Science Research Council, the Biotechnology Fund at Umea University,
and the Swedish National Board for Industrial and Technical Development.
[1] a) J. A. Smith, L. G. Pease, CRC Crit. Rev. Biochem. 1980, 8, 315 ±
399; b) G. D. Rose, L. M. Gierasch, J. A. Smith, in Turns in Peptides
and Proteins, Vol. 37 (Eds.: C. B. Anfinsen, J. T. Edsall, F. M.
Richards), Academic Press, Orlando, Florida, 1985, pp. 1 ± 109.
[2] a) R. L. Stanfield, T. M. Fieser, R. A. Lerner, I. A. Wilson, Science
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Philos. Trans. R. Soc. London B 1990, 327, 625 ± 654; c) G. Lippens, K.
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Russell, D. D. Smith, V. J. Hruby, Biochemistry 1993, 32, 9423 ± 9434;
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42, 181 ± 193; b) F. Guarnieri, H. Weinstein, J. Am. Chem. Soc. 1996,
118, 5580 ± 5589.
The linker, the Na-Fmoc amino acids and mercaptopropionic acid were
coupled to the peptide resin as 1-benzotriazolyl (HOBt) esters.[35a] These
were prepared, in situ, from the appropriate acid (0.32 mmol, 4 equiv),
HOBt (65 mg, 0.48 mmol, 6 equiv), and DIC (48.5 mL, 0.314 mmol,
3.9 equiv) in DMF (1 mL), and added to the reactor after 60 min. The
inverse g-turn mimetic 41 (67 mg, 0.11 mmol, 1.3 equiv) was coupled in
DMF (1 mL) containing HOAt[35b] (21 mg, 0.16 mmol, 2.0 equiv) and DIC
(14.9 mL, 96.5 mmol, 1.2 equiv). Acylations were monitored by addition of
bromophenol blue[51] (0.05% of the resin capacity) to the reactor and,
additionally, by the ninhydrin test for the coupling of 41.[52] Na-Fmoc
deprotection was performed by treatment with 20% piperidine in DMF
(2 Â 8 min). After incorporation of mimetic 41 into the peptide, the azido
group[42, 53] was reduced by sequential addition of triethylamine (0.20 mL,
1.5 mmol), thiophenol (0.121 mL, 1.18 mmol) and SnCl2 (49 mg,
0.26 mmol) to the suspended resin (0.317 g, 59 mmol) in THF (1 mL).
After 19 h at room temperature the resin was washed with THF (5 Â 1 mL)
and CH2Cl2 (5 Â 1 mL) and dried in vacuo. A positive ninhydrin test and
the disappearance of the N3 stretch in the IR spectrum (recorded on a few
resin beads[43]
) indicated successful reduction. The resin was then
suspended in DMF (1 mL) and synthesis of 49 was continued as described
above.
[6] a) G. D. Smith, J. F. Griffin, Science 1978, 199, 1214 ± 1216; b) A.
Aubry, N. Birlirakis, M. Sakarellos-Daitsiotis, C. Sakarellos, M.
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Thörnwall, B. Synnergren, A. Nilsson, C. J. Welch, M. Sohtell, C.
After completion of the synthesis, the resin was washed with CH2Cl2 (5 Â
5 mL) and dried in vacuo to give 216 mg peptide resin. The peptide was
then cleaved from the resin (133 mg resin, 26.5 mmol), and the amino acid
side chains were deprotected by treatment with trifluoroacetic acid ± wa-
ter± thioanisole ± ethanedithiol (87.5:5:2.5, 13.3 mL) for 2 h, followed by
filtration. Acetic acid (6.5 mL) was added to the filtrate, the solution was
concentrated, and acetic acid (6.5 mL) was added again followed by
concentration. The residue was triturated with diethyl ether (6.5 mL) which
gave a solid, crude peptide which was dissolved in acetic acid ± water (1:9,
10 mL) and freeze-dried. Cyclization[44] was performed by alternating
additions of portions of the crude peptide in acetic acid and 0.1m I2 in
methanol to 10% acetic acid in methanol (2 mLmg 1 cleaved resin). After
the final addition of I2, a light brown solution was obtained which was
neutralized and decolourized by stirring with Dowex 1 Â 8 anion exchange
resin (Fluka, 50 ± 100 mesh, converted from Cl to acetate form by washing
with 1m aqueous NaOH, water, acetic acid, water, and methanol), filtered
and concentrated. The residue was then dissolved in water and freeze-
dried.
Â
Westerlund, F. Nyberg, A. Karlen, A. Hallberg, J. Med. Chem. 1997,
40, 903 ± 919.
[8] M. Sato, J. Y. H. Lee, H. Nakanishi, M. E. Johnson, R. A. Chrusciel,
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[10] Reviewed in: a) J. B. Ball, P. F. Alewood, J. Mol. Recog. 1990, 3, 55 ±
64; b) G. Hölzemann, Kontakte (Darmstadt) 1991, 3 ± 12, 55 ± 63; c) M.
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Conformational Constraints, and Related Backbone Replacements,
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pp. 267 ± 357; b) G. A. Patani, E. J. LaVoie, Chem. Rev. 1996, 96,
3147 ± 3176.
Peptide 49 was analyzed on a Beckman System Gold HPLC with a
Kromasil C-8 column (100 Š, 5 mm, 4.6  250 mm) and a linear gradient of
1
0 ± 80% of B in A over 60 min with a flow rate of 1.5 mLmin and
detection at 214 nm (solvent systems: A, 0.1% aqueous trifluoroacetic acid
and B, 0.1% trifluoroacetic acid in acetonitrile). Purification of crude 49
(33 mg, from 26.5 mmol resin) was performed with the same HPLC system
on a Kromasil C-8 column (100 Š, 5 mm, 20  250 mm) with a flow rate of
11 mLmin 1 (retention time: 26 min). This gave 49 (6.1 mg, 23%): FAB MS
calcd for C45H63N12O11S2 [M‡H] 1011, found 1011. Amino acid analysis:
Arg 1.03 (1), Cys 0.88 (1), Gly 1.05 (1), Pro 1.03 (1), Tyr 1.00 (1). 1H NMR
data for 49 are given in Table 1.
[12] C. A. Lipinski, F. Lombardo, B. W. Dominy, P. J. Feeney, Advanced
Drug Delivery Rev. 1997, 23, 3 ± 25.
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2252
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
0947-6539/99/0508-2252 $ 17.50+.50/0
Chem. Eur. J. 1999, 5, No. 8

g-Turn Mimetics
2241±2253
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by using more concentrated solutions in the coupling step and
prolonged coupling times (Z. Yuan and J. Kihlberg, unpublished
results).
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Â
Â
[19] I. Vavra, A. Machova, V. Holecek, J. H. Cort, M. Zaoral, F. Sorm,
Lancet 1968, 948 ± 952.
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[45] Pharmacological studies with peptide 49 are in progress and will be
reported separately when completed.
[46] a) A. E. Derome, M. P. Williamson, J. Magn. Reson. 1990, 88, 177 ±
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residue i‡1. An ideal classical g-turn has F ˆ 77.58 and Yˆ 658 for
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Received: December 14, 1998 [F1494]
Chem. Eur. J. 1999, 5, No. 8
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