Synthesis of Z-protected Aib- and Phe(2Me)-containing pentapeptides and their crystal structures

Article abstract of DOI:10.1002/hlca.201400084

A series of pentapeptide derivatives containing α,α- disubstituted α-amino acids have been prepared by a combination of the 'azirine/oxazolone method' and segment condensations. X-Ray crystal-structure determinations of the molecular structures confirmed

Full text of DOI:10.1002/hlca.201400084

Helvetica Chimica Acta – Vol. 97 (2014)
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Synthesis of Z-Protected Aib- and Phe(2Me)-Containing Pentapeptides and
Their Crystal Structures
by Franziska S. Arnhold1), Anthony Linden, and Heinz Heimgartner*
Institut fꢀr Chemie der Universitꢁt Zꢀrich, Winterthurerstrasse 190, CH-8057 Zꢀrich
(phone: þ 41446354282; fax: þ 41446356812; e-mail: heinz.heimgartner@chem.uzh.ch)
A series of pentapeptide derivatives containing a,a-disubstituted a-amino acids have been prepared
by a combination of the ꢂazirine/oxazolone methodꢃ and segment condensations. X-Ray crystal-structure
determinations of the molecular structures confirmed the presence of helical conformations stabilized by
b-turns of type III or III’. Pentapeptides containing (R)-Phe(2Me) form a right-handed helix, whereas
those containing (S)-Phe(2Me) adopt a left-handed helical structure.
1. Introduction. – The ꢂazirine/oxazolone methodꢃ, established as a convenient
protocol for the synthesis of Aib-containing peptides [1 – 3], has been used successfully
for the preparation of model peptides [2c][3 – 6], sterically congested oligopeptides
[7][8], and peptaibols [2d][9 – 11]. It has also been shown that this procedure can be
adapted to solid-phase methodology [12]. Another modification allowed the synthesis
of endothiopeptides containing Aib units [13 – 16].
In the present study, a series of Z-protected (Z ¼ (benzyloxy)carbonyl) pentapep-
tides containing a,a-disubstituted a-amino acids were prepared according to this
method. All a,a-disubstituted a-amino acids were introduced via coupling of amino or
peptide acids with 2,2-disubstituted 2H-azirin-3-amines. The synthon for Phe(2Me) was
the racemic 2-benzyl-2,N-dimethyl-N-phenyl-2H-azirin-3-amine leading to mixtures of
(R)- and (S)-Phe(2Me)-peptides2). In the case of diastereoisomers, they were
separated chromatographically.
Based on the studies of Ramachandran et al. [18], it is well-known that the
introduction of a,a-disubstituted a-amino acids3) into peptides stabilizes b-turn and
310-helical conformations [19][20]. For the crystalline state, this has been established by
X-ray crystallography (cf., e.g., [21][22]). In spite of this knowledge, current interest in
conformations of small peptides is documented by several recent reports (e.g., for
pentapeptides, [23][24]). For this reason, we determined the crystal structures of some
of the prepared pentapeptides.
2. Results and Discussion. – 2.1. Synthesis of Pentapeptides. The syntheses of the Z-
protected pentapeptides were carried out via combinations of the coupling of amino or
1
)
In part from the Ph.D. thesis of F. S. A., Universitꢁt Zꢀrich, 1997. Present address: Bachem AG,
Hauptstrasse 144, CH-4416 Bubendorf
Synthons for enantiomerically pure (R)- and (S)-Phe(2Me) are now also available [4a][17].
The prototype is a-aminoisobutyric acid (Aib).
2
3
)
)
ꢄ 2014 Verlag Helvetica Chimica Acta AG, Zꢀrich

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peptide acids with 2H-azirin-3-amines, and standard coupling with amino acid esters or
peptide segments. Two examples are shown in Scheme 1: as described in [11][25], Z-
Gly-OH in THF was reacted with 2,2,N-trimethyl-N-phenyl-2H-azirin-3-amine (1a;
Aib synthon [26]) leading to dipeptide amide 2, followed by selective hydrolysis to give
Z-Gly-Aib-OH (3). Reaction of the latter with 2-benzyl-2,N-dimethyl-N-phenyl-2H-
azirin-3-amine (1b) [26b] afforded the tripeptide amide 4a [25a], which was hydrolyzed
(! 5a), coupled with 1a (!6a), and again hydrolyzed (! 7a). The coupling of 7a with
H-Gly-OMe and H-Phe-OBn, respectively, was carried out in CH2Cl2 using PyBOP/
DIEA4), as the coupling reagent, leading to pentapeptide 8a5) and 8b, respectively.
The latter was obtained as a 1:1 mixture of diastereoisomers.
Similar to the preparation of 8a and 8b, pentapeptide 8c containing 1-amino-
cyclobutanecarboxylic acid (Acb) was synthesized (Scheme 2). Coupling of 3 with 2-
Scheme 1
4
)
PyBOP ¼ [(Benzotriazol-1-yl)oxy]tripyrrolidinophosphonium hexafluorophosphate; DIEA ¼
ethyl(diisopropyl)amine (EtNiPr2, Hꢀnig base).
The coupling of 7 with H-Gly-OMe was also achieved by treatment with N,N’-dicyclohexylcarbo-
diimide (DCC), ZnCl2, and Et3N in DMF [27][28] leading to 8a in 70% yield.
5
)

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Scheme 2
(dimethylamino)-1-azaspiro[2.3]hex-1-ene (1c) [2b] gave the tripeptide amide 4b in
87% yield. The hydrolysis of the terminal amide group was achieved in 3n HCl (THF/
H2O) at 408 leading to 5b (96%). Subsequent azirine coupling with 1a (!6b), selective
hydrolysis at room temperature (! 7b), and coupling with H-Gly-OMe by treatment
with PyBOP/DIEA gave 8c.
The Pro-containing pentapeptide 8d was obtained by segment coupling of 3 and H-
Pro-Aib-Aib-N(Me)Ph (9) in CH2Cl2 with HBPyU6)/DIEA as the coupling reagent
[29] (Scheme 3). The tripeptide 9 [25b] was prepared by subsequent coupling of Z-Pro-
OH with azirine 1a, selective hydrolysis, again coupling with 1a, and deprotection of the
N-terminus by hydrogenolysis. Selective hydrolysis of 8d under the usual conditions
gave the pentapeptide acid 10 in 70% yield.
Scheme 3
6
)
HBPyU ¼ O-[(Benzotriazol-1-yl)oxy]dipyrrolidinocarbenium hexafluorophosphate.

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The pentapeptides 8e and 8f were prepared via coupling of the dipeptide segment
11 [25a] with the tripeptide segment 12 and the corresponding H-Gly-Aib-Aib-
N(Me)Ph [25a], respectively (Scheme 4). The coupling was performed by treatment
with PyBOP/DIEA in CH2Cl2 at room temperature and led to 8e as a 1:1 mixture of
diastereoisomers in almost quantitative yield. The corresponding 8f was isolated in
82% yield as a racemate. The segment 12 was obtained by coupling of 3 with H-Phe-
OMe (PyBOP/DIEA; 85%), followed by hydrogenolytic deprotection of the NH2
group (98%). The diastereoisomers of 8e were separated chromatographically (SiO2,
AcOEt/MeOH), and the pure isomers were isolated in 47 ((S,S)-8e) and 44% ((R,S)-
8e) yield. The configuration of (S,S)-8e was determined by X-ray crystallography (see
Sect. 2.3).
Scheme 4
Finally, the dipeptide segment 11 was coupled with the tripeptides H-Pro-Aib-Phe-
OMe (13) and the corresponding H-Pro-Aib-Aib-N(Me)Ph [25b], respectively, leading
to pentapeptides 8g and 8h, respectively (Scheme 5). These couplings proved to be
rather difficult. In the case of 8g, the reaction with PyBOP/DIEA in CH2Cl2 for 19.5 h
gave the product in only 39% yield, whereas with TBTU/HOBt/DIEA7), after 19.5 h,
8g was obtained in 74% yield. Also in the case of 8h, TBTU/HOBt gave slightly better
results than PyBOP/DIEA. The tripeptide 13 was prepared by coupling of Z-Pro-Aib-
OH [25b] with H-Phe-OMe by treatment with PyBOP/DIEA.
In both cases, 8g and 8h, mixtures of diastereoisomers were formed, which could be
separated by column chromatography (SiO2; AcOEt/MeOH). The pure epimers
7
)
TBTU ¼ O-(1H-Benzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium tetrafluoroborate; HOBt ¼ 1-hy-
droxybenzotriazole.

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623
Scheme 5
(R,S,S)-8g and (S,S,S)-8g were obtained in a 1.6 :1 ratio8). As the starting dipeptide 11
was a racemate, the coupling of (R)-11 with 13 was obviously more efficient than that of
(S)-11. A similar result was obtained in the case of 8h: the epimers (R,S)-8h and (S,S)-
8h8) were obtained in a 2 :1 ratio.
2.2. Conformation of the Pro-Peptide Bonds in the Pentapeptides in Solution.
Whereas in general the trans-conformation of the peptide bond is highly preferred, cis-
and trans-conformations of Xaa-Pro peptide bonds show comparable thermodynamic
stability. This was shown first for short peptides such as (S-Bn)-Cys-Pro-Leu-Gly-NH2
1
on the basis of their H-NMR spectra [30]. Later, it has been demonstrated that
13C-NMR spectroscopy is a convenient method for distinguishing between the cis- and
trans-conformation of the Xaa-Pro bond [31]. Typically, the chemical shifts of
C(b)(Pro) and C(g)(Pro) in cis-Xaa-Pro peptides are ca. 31.3 and 22.5 ppm,
respectively, whereas the corresponding values for trans-Xaa-Pro peptides are 29.5
and 24.2 ppm [31c]. Obviously, the chemical shift difference, Dd, is a reliable indicator
for the conformation, with values of ca. 8.8 for the cis- and 5.3 ppm for the trans-
conformation. For this reason, we analyzed the 13C-NMR spectra of the prepared Pro-
containing pentapeptides 8d, (R,S,S)- and (S,S,S)-8g, (R,S)- and (S,S)-8h, and 10
(Table 1). In all cases, only one conformation was observed with Dd values between 2.3
and 2.7 ppm, indicating the trans-conformation. This result is in accordance with the
crystal-structure determination (cf. Sect. 2.3), i.e., the Xaa-Pro peptide bond adopts the
same conformation in solution and in the crystal.
2.3. Crystal Structures of Pentapeptides. Suitable crystals of the tetrapeptide Z-Gly-
Aib-(RS)-Phe(2Me)-Aib-N(Me)Ph (6a) were obtained from CH2Cl2/Et2O/hexane.
8
)
The configurations of (R,S,S)-8g and (R,S)-8h were established by X-ray crystallography (see
Sect. 2.3).

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Table 1. 13C-NMR Chemical Shifts [ppm] of C(b)- and C(g)-Atoms of the Pro Residue in Pro-Containing
Pentapeptides
Peptide
Solvent
C(b)
C(g)
Z-Gly-Aib-Pro-Aib-Aib-N(Me)Ph (8d)
CDCl3
CDCl3
CDCl3
CDCl3
CDCl3
(D6)DMSO
28.8
28.5
28.3
28.7
28.6
28.2
26.2
26.0
26.0
26.3
26.2
25.5
Z-Gly-(R)-Phe(2Me)-Pro-Aib-Phe-OMe ((R,S,S)-8g)
Z-Gly-(S)-Phe(2Me)-Pro-Aib-Phe-OMe ((S,S,S)-8g)
Z-Gly-(R)-Phe(2Me)-Pro-Aib-Aib-N(Me)Ph ((R,S)-8h)
Z-Gly-(S)-Phe(2Me)-Pro-Aib-Aib-N(Me)Ph ((S,S)-8h)
Z-Gly-Aib-Pro-Aib-Aib-OH (10)
The crystals are racemic, and the asymmetric unit contains one peptide molecule plus
one half of a disordered molecule of hexane, which sits across a center of inversion. In
Fig. 1, the peptide molecule with the (S)-configured amino acid Phe(2Me) is shown.
The molecule forms a b-turn of type III’ [32] with the Phe(2Me) unit in position (i þ 3)
and an intramolecular H-bond N(4)ꢀH ··· O(11) (H ··· O 2.13(3) ꢅ, N ··· O 2.963(3) ꢅ,
NꢀH ··· O 169(3)8). This interaction has a graph set motif [34] of S(10). A second
intramolecular H-bond is formed between N(10)ꢀH and N(13). The other two NH
groups are involved in intermolecular H-bonds with an adjacent molecule (N(7)ꢀ
H ··· O(2’), N(13)ꢀH ··· O(5’)); these intermolecular interactions link the molecules
into extended chains which run parallel to the [010] direction and can be described by
graph set motifs of C(8) and C(11), respectively.
All peptide bonds show the trans-conformation, but the Phe(2Me)-Aib bond
deviates significantly from planarity (w 155.5(2)8). The values of the torsion angles
w(Gly-Aib), w(Aib-Phe(2Me)), and w(Aib-N(Me)Ph) are close to 1808 (175.0(2),
ꢀ 176.6(2), and ꢀ 177.5(2)8, resp.). The torsion angles f and y of Aib(2) (61.0(4) and
25.3(4)8) and Phe(2Me) (60.2(3) and 23.2(3)8) correspond well with the typical values
Fig. 1. ORTEP Plot [33] of the molecular structure of the tetrapeptide 6a (50% probability ellipsoids,
arbitrary numbering of atoms, H-atoms bonded to C-atoms omitted for clarity, and only the major
conformation of the disordered central benzyl ring is shown; solvent molecule not included).

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625
Fig. 2. ORTEP Plot [33] of the molecular structure of the pentapeptide 8a (50% probability ellipsoids,
arbitrary numbering of atoms, H-atoms bonded to C-atoms omitted for clarity).
Table 2. Torsion Angles f, y, and w [8] of the Backbone of Compounds 8a, (S,S)-8e, 8f, (R,S,S)-8g,
(R,S)-8h, and 10 in the Crystal (amino acid (i) ¼ Gly(1))
8a
(S,S)-8e
8f
(R,S,S)-8g
(R,S)-8h
10
f(i)
y(i)
w(i)
53.7(2)
32.4(2)
ꢀ 178.7(1)
ꢀ 96.3(3)
ꢀ 5.6(5)
173.2(3)
ꢀ 88.7(5)
ꢀ 33.9(6)
178.7(4)
70.6(5)
179.0(4)
179.4(4)
64(1); 62(1)
82.2(6)
155.3(5)
ꢀ 169.2(4)
ꢀ 170.3(7); ꢀ 172.3(7)
175.1(7); 177.9(7)
f(iþ1)
y(iþ1)
w(iþ1) ꢀ 178.7(1)
55.8(2)
21.8(2)
53.9(4)
34.3(5)
179.5(3)
49.4(6)
42.2(6)
173.1(4) ꢀ 173.7(4)
ꢀ 49.6(5)
ꢀ 43.4(5)
ꢀ 51.5(9); ꢀ 51.0(9)
ꢀ 36(1); ꢀ 40.5(9)
ꢀ 53.8(6)
ꢀ 37.8(6)
ꢀ 178.9(7); ꢀ 177.8(6) ꢀ 176.9(4)
f(iþ2)
y(iþ2)
w(iþ2)
51.9(2)
29.6(2)
178.6(1) ꢀ 170.8(3)
61.3(5)
13.5(5)
61.3(6)
29.4(6)
ꢀ 177.5(4)
ꢀ 66.2(5)
ꢀ 13.6(6)
171.5(4)
ꢀ 55 (1); ꢀ 57(1)
ꢀ 61.2(6)
ꢀ 31.7(6)
178.7(4)
ꢀ 30(1); ꢀ 30 (1)
ꢀ 179.0(7); ꢀ 178.1(7)
f(iþ3)
y(iþ3)
w(iþ3)
57.0(2)
30.4(2)
166.3(1)
55.8(4)
30.6(5)
163.6(4)
59.4(6)
32.4(6)
ꢀ 55.6(5)
ꢀ 32.0(5)
ꢀ 64.8(9); ꢀ 58.4(9)
ꢀ 54.1(7)
ꢀ 39.5(5)
175.4(4)
ꢀ 30(1); ꢀ 32.6(9)
ꢀ 170.7(4) ꢀ 172.5(4)
ꢀ 47.4(7) ꢀ 111.0(5)
ꢀ 162.1(8); ꢀ 154.6(7)
f(iþ4)
y(iþ4)
ꢀ 83.0(2)
ꢀ 81.0(5)
ꢀ 57(1); ꢀ 52(1)
49.4(6)
45.2(6)
162.8(1) ꢀ 179.1(4)
ꢀ 57.4(7)
179.4(4)
ꢀ 50(1); ꢀ 54(1)

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Table 3. Intramolecular H-Bonds of Compounds 8a, (S,S)-8e, 8f, (R,S,S)-8g, (R,S)-8h, and 10 (atom
numbering refers to Figs. 1 – 7)
Compound N(1)ꢀH ··· O(8)
N(4)ꢀH ··· O(11)
N(7)ꢀH ··· O(14)
N ··· O [ꢅ] NꢀH ··· O [8] N ··· O [ꢅ] NꢀH ··· O [8] N ··· O [ꢅ] NꢀH ··· O [8]
8a
(S,S )-8e
8f
(R,S,S)-8g
(R,S)-8h
2.928(2)
3.007(4)
2.976(5)
2.960(5)
2.974(8);
3.055(7)
3.034(5)
163(2)
165(4)
130
162(4)
163; 168
3.052(2)
2.872(4)
2.977(5)
3.001(5)
3.036(8);
3.067(8)
3.000(5)
171(2)
166(4)
141
161(4)
149; 147
3.013(2)
166(1)
–
–
–
–
10
126
140
–
of þ 608 and þ 308 for a left-handed helical structure (b-turn of type III’) (e.g.
[7][35]).
The crystals of the pentapeptide Z-Gly-Aib-(RS)-Phe(2Me)-Aib-Gly-OMe (8a),
obtained from CH2Cl2/hexane, are also racemic. The extension of the peptide chain of
6a by an additional C-terminal Gly-OMe results in the formation of a 310-helix, i.e.,
three consecutive b-turns of type III/III’. The molecule containing (S)-Phe(2Me) again
forms a left-handed helix with b-turns of type III’ (Fig. 2 and Table 2). Three
intramolecular H-bonds stabilize the 310-helix: N(1)ꢀH ··· O(8), N(4)ꢀH ··· O(11), and
N(7)ꢀH ··· O(14) (Table 3). All these interactions form formal ten-membered rings
(graph set motif S(10)), the last one involving the C¼O group of the Z group. The other
two NH groups (Gly(1) and Aib(2)) are involved in intermolecular H-bonds with the
same O(5’)-atom of Phe(2Me) of an adjacent molecule. This O(5’)-atom, therefore,
accepts two H-bonds, although one of these is quite weak. The intermolecular H-bonds
link the molecules into extended chains which run parallel to the [101] direction and
can be described by graph set motifs of C(11) and C(8) for the interactions involving
the donors Gly(1) and Aib(2), respectively.
Racemic crystals of Z-Gly-(RS)-Phe(2Me)-Gly-Aib-Aib-N(Me)Ph (8f) were
grown from AcOEt/hexane. In Fig. 3, the molecule containing (S)-Phe(2Me) is
depicted. The asymmetric unit contains one molecule of 8f and approximately one
third of a highly disordered hexane molecule, which sits across a center of inversion.
Four of the five NH groups act as donors for H-bonds. Two of them, N(1)ꢀH and
N(4)ꢀH, form intramolecular H-bonds with amide O-atoms that are seven atoms along
the peptide backbone to give a graph set motif of S(10) in each case (Table 3). These
interactions form two b-turns of type III’ (Table 2), leading to a left-handed helical
structure for the molecule containing (S)-Phe(2Me). Surprisingly, Gly(3) does not
continue this helical structure, and the Z group is turned away from the helix. Rather,
N(7)ꢀH of Gly(3) forms an intermolecular H-bond with the terminal amide O(24) of a
neighboring molecule. The N(13)ꢀH group of Gly(1) forms an intermolecular H-bond
to O(2) of the same neighboring molecule. These intermolecular interactions link the
peptide molecules into extended chains, which run parallel to the [010] direction (graph
set motifs C(11) and C(14), resp.). Interestingly, N(10)ꢀH is not involved in any H-
bonding. Finally, N(1)ꢀH, N(4)ꢀH, N(7)ꢀH, and N(10)ꢀH exhibit the usual weak
ꢂsidewaysꢃ contacts with one of their neighboring N-atoms. All peptide bonds are in the

Helvetica Chimica Acta – Vol. 97 (2014)
627
Fig. 3. ORTEP Plot [33] of the molecular structure of the pentapeptide 8f (50% probability ellipsoids,
arbitrary numbering of atoms, H-atoms bonded to C-atoms omitted for clarity; solvent molecule not
included).
trans-conformation and differ only slightly from planarity, but the terminal amide
group deviates significantly from planarity (torsion angle w ꢀ 153.1(8)8; Table 2).
The crystals of Z-Gly-(S)-Phe(2Me)-Gly-Aib-Phe-OMe ((S,S)-8e) are enantio-
merically pure; however, the absolute configuration of the molecule has not been
determined. The enantiomer used in the refinement was based on the (S)-configuration
at C(23), resulting from (S)-configured H-Phe-OMe used in the synthesis. Based on
this, the configuration at C(9) of Phe(2Me) is also (S) (Fig. 4). The Ph ring of the
Fig. 4. ORTEP Plot [33] of the molecular structure of the pentapeptide (S,S)-8e (50% probability
ellipsoids, arbitrary numbering of atoms, H-atoms bonded to C-atoms omitted for clarity, and only the
major conformation of the disordered terminal Bn group is shown; H2O molecules not included).

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terminal Bn group is disordered over two orientations, with a ratio of the conformers of
0.33 :0.67. The crystal lattice also contains one disordered H2O molecule per peptide
molecule. All five peptide NH groups act as donors for H-bonding. Two of the H-bonds
are intramolecular interactions with, in each case, the amide O-atom two peptide units
further along the peptide chain, forming two consecutive b turns of type III’ (graph set
motif: S(10); Table 3). As a result of the presence of (S)-Phe(2Me), a left-handed
helical structure is formed (see, e.g., [36]). The presence of (S)-Phe, an amino acid
which usually forms right-handed helices, probably has no influence because of its
position at the C-terminus of the peptide. As in 8f, the helix is not continued by Gly(3),
and the Z group is turned away. The NH group of Gly(3) forms an intermolecular H-
bond with O(2’) of Aib(4) of an adjacent molecule. A second intermolecular H-bond,
N(13)ꢀH ··· O(5’), is formed with the same neighboring molecule. These two
interactions link the peptide molecules into extended chains, which run parallel to
the [100] direction (graph set motifs C(8) and C(11), resp.). The fifth interaction, from
N(10)ꢀH of Phe(2Me), involves a H2O molecule acting as an acceptor. Furthermore,
analogous to 8f, N(1)ꢀH, N(4)ꢀH, N(7)ꢀH, and N(10)ꢀH are close enough to adjacent
N-atoms in the peptide chain to form weak interactions.
The asymmetric unit of Z-Gly-(R)-Phe(2Me)-Pro-Aib-Phe-OMe ((R,S,S)-8g)
contains one peptide molecule, one molecule of AcOEt, and one site for a H2O
molecule, which is partially occupied (50%). The crystals are enantiomerically pure;
however, the absolute configuration of the molecule has not been determined. The
enantiomer used in the refinement was based on the known (S)-configuration at C(6)
and C(23) of the Pro and Phe residues, respectively. Based on this assumption, C(9) of
Phe(2Me) has the (R)-configuration. The molecule forms a right-handed 310-helix, as
expected for a peptide containing two protein amino acids and (R)-Phe(2Me) (Fig. 1
and Table 2). Again, the Z group is turned away from the helix, in this case because of
the presence of Pro in position 3 of the peptide. The N(1)ꢀH and N(4)ꢀH groups of
Phe(5) and Aib(4), respectively, form intramolecular H-bonds with amide O-atoms
that are seven atoms along the peptide backbone (graph set motif S(10) for each
interaction, Table 3). The NH groups of Phe(2Me)(2) and Gly(1), N(10)ꢀH and
N(13)ꢀH, respectively, form intermolecular H-bonds with the amide O-atoms, O(5’)
and O(2’’), respectively, of different neighboring molecules. The N(10)ꢀH interaction
links the molecules into extended chains, which run parallel to the [100] direction
(graph set motif C(8)), while the N(13)ꢀH interaction links the molecules into a second
type of extended chains, which also run parallel to the [100] direction (graph set motif
C(14)). The H2O molecule acts as a donor for two H-bonds to amide O-atoms of two
different peptide molecules (graph set motif D for each interaction). As a result, the
amide O-atoms O(2) of Aib(4) and O(11) of Gly(1) are each acceptors of two H-bonds.
Neither the H2O molecule nor the AcOEt molecule acts as an acceptor for H-bonds.
The combination of all intermolecular interactions links the molecules into extended
sheets, which lie parallel to the (010) plane. Finally, N(1)ꢀH and N(4)ꢀH exhibit the
usual weak ꢂsidewaysꢃ contacts with one of their neighboring N-atoms.
The crystals of Z-Gly-(R)-Phe(2Me)-Pro-Aib-Aib-N(Me)Ph ((R,S)-8h) are enan-
tiomerically pure; however, the absolute configuration of the molecule has not been
determined. The enantiomer used in the refinement was based on the known (S)-
configuration of the Pro residue (C(6)). The configuration of Phe(2Me), at (C(9)), is

Helvetica Chimica Acta – Vol. 97 (2014)
629
Fig. 5. ORTEP Plot [33] of the molecular structure of the pentapeptide (R,S,S)-8g (50% probability
ellipsoids, arbitrary numbering of atoms, H-atoms bonded to C-atoms omitted for clarity, solvent
molecules not included).
therefore (R) (Fig. 6). The asymmetric unit contains two independent peptide
molecules, one hexane molecule, and a H2O molecule, which lies on a twofold axis.
The two peptide molecules have generally similar conformations with the major
difference being in the orientation of the Ph rings at each end of the peptide chain. One
of the peptide molecules (molecule B) has disorder in the Ph ring of the protecting
group Z. In molecule A, the NH groups of the two Aib units, N(1)ꢀH and N(4)ꢀH,
form intramolecular H-bonds with O(8) of Phe(2Me) and O(11) of Gly, respectively,
resulting in two b-turns of type III (graph set motifs of S(10); Table 3). The same
interactions occur in molecule B, involving N(51)ꢀH and N(54)ꢀH. These are the
normal intramolecular interactions found for peptides of this type and help to form a
right-handed 310-helical structure (Table 2). These NH groups also have weak
ꢂsidewaysꢃ interactions with the nearest neighboring N-atom in the chain. The other
NH groups and the H2O molecule act as donors for intermolecular H-bonds: N(10)ꢀH
in molecule A forms an H-bond with the amide O-atom, O(74), at the opposite end of
molecule B. The corresponding group, N(60)ꢀH, in molecule B, however, interacts with
a different O-atom in another molecule A, namely O(2). These two interactions form
spiralling ··· A ··· B ··· A ··· B ··· chains of molecules which run parallel to the [001]

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Fig. 6. ORTEP Plot [33] of the molecular structure of one of the two symmetry-independent molecules of
the pentapeptide (R,S)-8h (50% probability ellipsoids, arbitrary numbering of atoms, H-atoms bonded to
C-atoms omitted for clarity; solvent molecules not included).
direction and have a binary graph set motif of C22(25). Furthermore, N(13)ꢀH of
molecule A forms an intermolecular H-bond with the amide O-atom of Pro(3), O(55),
of molecule B and the same interaction, involving N(63)ꢀH, occurs to link molecule B
to atom O(5) of a different molecule A, thus forming extended ··· A ··· B ··· A ··· B ···
chains of molecules, which run parallel to the [010] direction (graph set motif C22(22)).
The H2O molecule sits on a two-fold axis and, therefore, acts as a donor for two
equivalent H-bonds to the amide O-atom O(52) of Aib(4) in two different B molecules
(graph set motif D). The combination of the intermolecular interactions links the
molecules into three-dimensional supramolecular frameworks.
The crystals of Z-Gly-Aib-Pro-Aib-Aib-OH (10) are enantiomerically pure.
However, the absolute configuration has not been determined. The enantiomer used
in the refinement was based on the known (S)-configuration of Pro (C(6)) (Fig. 7). The
asymmetric unit contains one peptide and one disordered MeOH molecule. Two
intramolecular H-bonds, N(1)ꢀH ··· O(8) and N(4)ꢀH ··· O(11), form two b-turns of
type III (graph set motif S(10); Table 3), i.e., a right-handed 310-helix, as expected for a
peptide containing (S)-Pro as the only chiral amino acid. The other two NH groups,
N(10)ꢀH and N(13)ꢀH, as well as the carboxy OH group, O(25)ꢀH, form
intermolecular H-bonds with C¼O O-atoms in three different neighboring mole-
cules. The O(25)ꢀH ··· O(5’) interaction links the molecules into extended chains
which run parallel to the [100] direction and have a graph set motif of C(10). The

Helvetica Chimica Acta – Vol. 97 (2014)
631
Fig. 7. ORTEP Plot [33] of the molecular structure of the pentapeptide 10 (50% probability ellipsoids,
arbitrary numbering of atoms, H-atoms bonded to C-atoms omitted for clarity; solvent molecule not
included).
N(10)ꢀH ··· O(2’’) interaction links the molecules into extended chains which run
parallel to the [010] direction (graph set motif C(11)), while the N(13)ꢀH ··· O(8’’’)
interaction links the molecules into extended chains which also run parallel to the [100]
direction (graph set motif C(8)). The combination of these intermolecular interactions
links the molecules into a two-dimensional network, which lies parallel to the (001)
plane. The O(8)-atom acts as an acceptor for two H-bonds, an intra- and an
intermolecular one. Both positions for the disordered MeOH molecule indicate the
presence of a H-bond to the C¼O O(14)-atom.
3. Conclusions. – The presented results confirm once more the usefulness of the
ꢂazirine/oxazolone methodꢃ, combined with traditional segment coupling, in the
synthesis of peptides containing a,a-disubstituted a-amino acids. All of the Aib and
Phe(2Me) residues, as well as the 1-aminocyclobutane carboxylic acid, have been
added to N-protected amino acids or peptides via the reaction with the corresponding
2,2-disubstituted 2H-azirin-3-amine 1. As reported previously [7][8][25][37], the
coupling of two a,a-disubstituted a-amino acids does not suffer from steric hindrance.
This is of importance, because coupling reactions with N-terminal Aib residues are
known to be difficult (see, e.g., [38]). As the synthon for Phe(2Me), 1b was used as a
racemate2), racemic pentapeptides such as 8a or mixtures of epimers such as 8b were
formed. The pure epimers were obtained after chromatographic separation in the cases
of 8e, 8g, and 8h.
It is also important to note that the segment coupling with dipeptides with a C-
terminal Aib or Phe(2Me) residue could be performed in good yields, even in the case

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of tripeptides 9 and 13 with a N-terminal Pro. Surprisingly, the coupling with TBTU/
HOBt yielded the pentapeptides 8g and 8h in significantly higher yields than with
PyBOP, which was the preferred coupling reagent in the cases without Pro.
The X-ray crystal-structure determinations revealed that all of the prepared
pentapeptides adopt 310-helical conformations, as expected for Aib-containing peptides
[39]. As Aib is achiral, the handedness of the helix is determined by the chiral amino
acid present. It has been shown that a right-handed helix is preferred, when the poly-
Aib peptide contains a N-terminal l-amino acid or a l-amino acid within the peptide
chain [40]. On the other hand, a C-terminal l-amino acid leads to the inverse
relationship, i.e., a left-handed helix. Furthermore, it was found that the chiral a,a-
disubstituted a-amino acid Phe(2Me) has the opposite effect, i.e., an internal l-
Phe(2Me) residue induces a left-handed helix [36a][41]. Recently, a similar reversal of
the screw-sense induction was described for the amino acids Val and Val(2Me): the
hexapeptides Ac-l-Xaa-(Aib)4-Gly-NH2 and pentapeptides Ac-l-Xaa-(Aib)4-OR
form left-handed helices with Xaa ¼ Val, but right-handed helices with Xaa ¼
Val(2Me) [42]. The study of the influence of the type and the position of l- and d-
amino acids on the helical screw sense of Aib-containing peptides is a topic of current
interest (see, e.g., [43]).
The crystal structures of the prepared pentapeptides are in full accordance
with the results reported in [36][41]. Whereas the peptide 10 with (S)-Pro as the only
chiral amino acid adopts a right-handed 310-helical structure, as is ꢂnormalꢃ for protein
amino acids, the enantiomers of the racemic peptides 8a and 8f, containing (S)-
Phe(2Me) in position 3 and 2, respectively, exist in a left-handed 310-helical
conformation in the crystal. Furthermore, in the crystal of the racemic tetrapeptide
6a, the enantiomer with (S)-Phe(2Me) forms a b-turn of type III’, i.e., a turn with a left-
handed helical structure. On the other hand, both (R,S,S)-8g and (R,S)-8h form right-
handed 310-helices, supported by the presence of (R)-Phe(2Me) as well as (S)-Pro in
position 3.
It is worth mentioning that in 8a three intramolecular (1 4) H-bonds are formed,
stabilizing the 310-helix (Table 3). One of them is the interaction of N(7)ꢀH of
Phe(2Me) with C¼O(14) of the Z protecting group. Surprisingly, only two intra-
molecular (1 4) H-bonds are present in the analogous pentapeptide 8f, and the Z
group is not involved in an intramolecular H-bond. The analogous H-bond pattern with
only two (1 4) interactions, is observed in the enantiomerically pure (S,S)-8e. In both
cases, N(7)ꢀH of Gly(3) does not form intramolecular H-bonds. In the two remaining
pentapeptides, (R,S,S)-8g and (R,S)-8h, which possess a Pro(3) unit, only two
intramolecular (1 4) H-bonds, N(1)ꢀH ··· O(8) and N(4)ꢀH ··· O(11), are possible.
We thank the analytical sections of our institute for spectra and analyses, and the Stipendienfonds der
Basler Chemischen Industrie and F. Hoffmann-La Roche AG, Basel, for financial support.
Experimental Part
1. Abbreviations. Acb, 1-Aminocyclobutane carboxylic acid; Aib, 2-aminoisobutyric acid (2-
methylalanin); CSA, camphor-10-sulfonic acid; DCC, N,N’-dicyclohexylcarbodiimide; DEPC, diethyl
cyanophosphonate; DIEA, ethyl(diisopropyl)amine (EtNiPr2; Hꢀnig base); DPPA, diphenyl phosphor-

Helvetica Chimica Acta – Vol. 97 (2014)
633
azidate; HBPyU, O-[(benzotriazol-1-yl)oxy]dipyrrolidinocarbenium hexafluorophosphate; HOBt, 1-
hydroxybenzotriazole; PyBOP ¼ [(benzotriazol-1-yl)oxy]tripyrrolidinophosphonium hexafluorophos-
phate; TBTU, O-(1H-benzotriazol-1-yl)-N,N,N’,N’-tetramethyluronium tetrafluoroborate; TFA, tri-
fluoroacetic acid; Z, (benzyloxy)carbonyl.
2. General. See [11][25]. Amino acids were purchased from Novabiochem and Bachem, and all of
them were l-configured; other reagents and solvents were from Aldrich, Bachem, Fluka, and Merck,
resp. Solvents were purified by standard procedures. TLC: Merck TLC glass plates, silica gel 60 F254. Prep.
TLC: Merck glass plates, silica gel 60 F254. Column chromatography (CC): Uetikon-Chemie, silica gel C-
560 (0.04 – 0.063 mm). M.p.: Mettler-FP-5 apparatus; uncorrected. [a]D Values: PerkinElmer-241
polarimeter at 218. IR Spectra: PerkinElmer-781 spectrometer, in KBr. 1H- and 13C-NMR spectra:
Bruker AC-300 or Bruker ARX-300 spectrometer at 300 (1H) and 75.5 MHz (13C), in CDCl3, CD3OD, or
(D6)DMSO; multiplicities of 13C signals determined by the DEPT technique. ESI-MS: Finnigan TSQ-
700 instrument; and CI-MS (with NH3 or isobutane): Finnigan MAT-90 or SSQ-700 instrument; m/z (rel.
%).
General Procedure 1 (GP 1; Coupling with 2H-Azirin-3-amines). To a soln. of an N-protected amino
acid or N-protected peptide (1 mmol) in abs. THF (5 ml) at 08, 1.1 equiv. of the corresponding 2H-azirin-
3-amine in THF were added, and the mixture was stirred at r.t. for several h (under Ar). After
completion of the reaction, the solvent was evaporated, and the product was purified by crystallization
from CH2Cl2/hexane or AcOEt/hexane. In the case of incomplete reactions, the residue was dissolved in
CH2Cl2, the remaining acid was extracted with 1n aq. NaOH, the solvent was evaporated, and the residue
was crystallized.
General Procedure 2 (GP 2; Selective Hydrolysis of Peptide N-Methyl-N-phenylamides). A soln. of
the peptide amide (1 mmol) in 3n HCl (THF/H2O 1:1; 10 ml) was stirred at r.t. for 1 – 60 h. Then, aq. 2n
HCl was added, and the mixture was extracted with Et2O. The org. layers were combined and dried
(Na2SO4), and the solvent was evaporated.
General Procedure 3 (GP 3; Hydrogenolytic Deprotection). The Z-protected peptide (1 mmol) was
dissolved in MeOH, and 10 weight-% Pd/C (10%) was added to the soln. The mixture was stirred at r.t.
under H2 overnight. Then, the soln. was filtered through a pad of Celite, and the solvent was evaporated.
The product was dried in high vacuum (i.v.).
General Procedure 4 (GP 4; Deprotection via Catalytic Hydrogen Transfer [44]). To a soln. of Z-
protected peptide (1 mmol) in MeOH were added 10 weight-% Pd/C (10%) and HCO2NH4 (5 mmol).
The mixture was heated at reflux (10 min) and filtered through a pad of Celite, and washed with MeOH,
and the solvent was evaporated.
General Procedure 5 (GP 5; Coupling of Amino Acids). To a soln. of a Z-protected peptide
(1 mmol), an amino acid ester (1.1 mmol), and a coupling reagent (1 mmol) in CH2Cl2 (1 ml) was added
dropwise DIEA (2 mmol; 3 mmol by using the hydrochloride of the amino acid ester) at r.t. The soln. was
stirred for 1 h, and then the solvent was evaporated. The residue was dissolved in AcOEt (20 ml), washed
with 5% aq. KHSO4 (3ꢁ), 5% aq. NaHCO3 (3ꢁ), and sat. aq. NaCl soln., and purified by CC.
3. Synthesis of the Pentapeptides 8. 3.1. Z-Gly-Aib-(RS)-Phe(2Me)-Aib-Gly-OMe (8a). 3.1.1. Z-Gly-
Aib-(RS)-Phe(2Me)-N(Me)Ph (4a). According to GP 1, to a soln. of Z-Gly-Aib-OH (3 [25]; 4.49 g,
15.3 mmol) in abs. THF (70 ml) at 08 was added 2-benzyl-2,N-dimethyl-N-phenyl-2H-azirin-3-amine (1b
[26b]; 15.1 g of a ca. 3 :4 mixture of 1b and 2-benzyl-N-methyl-N-phenylpropanamide (ca. 25.9 mmol
1b)); stirring at r.t. for 70 h. The solvent was evaporated, and the product was purified by CC (CH2Cl2/
MeOH 97:3): 5.95 g (72%) of 4a. Colorless foam. M.p. 72.2 – 73.18. IR (KBr): 3320m, 3060m, 3030m,
2980m, 2940m, 1640s, 1590s, 1515s, 1495s, 1450s, 1380m, 1365m, 1260m, 1240m, 1190m, 1155m, 1105m,
1080m, 1050m, 770m, 735m, 700s. 1H-NMR (CDCl3): 7.46 (s, NH); 7.40 – 7.20 (m, 11 arom. H); 7.15 – 7.05
(m, 4 arom. H); 6.85 (s, NH); 5.62 (br. s, NH); 5.11 (s, PhCH2O); 3.77 (d, J ¼ 5.4, CH2(Gly)); 3.48, 3.06
(AB, JAB ¼ 14.2, PhCH2); 3.31 (s, MeN); 1.42 (s, Me2C); 1.33 (s, Me(Phe(2Me))). 13C-NMR (CDCl3):
172.6, 172.5, 168.1 (3s, 3 CO(amide)); 156.5 (s, CO(urethane)); 144.1, 136.4, 136.3 (3s, 3 arom. C); 129.8,
129.5, 128.5, 128.2, 128.1, 128.0, 127.0 (7d, 15 arom. CH); 67.0 (t, PhCH2O); 63.2, 57.5 (2s, C(2)(Aib),
C(2)(Phe(2Me))); 44.9 (t, CH2(Gly)); 42.3 (t, PhCH2); 41.8 (q, MeN); 24.9, 24.6, 23.5 (3q, Me2C,
Me(Ph(2Me))). CI-MS: 545 (7, [M þ 1]þ), 439 (13), 438 (53, [M ꢀ Ph(Me)N]þ), 437 (100, [M ꢀ

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Helvetica Chimica Acta – Vol. 97 (2014)
PhCH2O]þ), 412 (8), 411 (15). Anal. calc. for C31H36N4O5 (544.65): C 68.36, H 6.66, N 10.29; found: C
67.96, H 6.90, N 10.35.
3.1.2. Z-Gly-Aib-(RS)-Phe(2Me)-OH (5a). According to GP 2, a soln. of 4a (5.86 g, 10.8 mmol) in
3n HCl was stirred at r.t. for 7 h. The solvent was evaporated, and the precipitate was filtered: 4.50 g
(92%) of 5a. Colorless crystals. M.p. 192.6 – 193.88. IR (KBr): 3310m, 3040m, 2980m, 2970m, 1720s,
1660s, 1650s, 1560m, 1525s, 1460m, 1450m, 1440m, 1285m, 1275m, 1245s, 1195m, 1165m, 1130m, 700m.
1H-NMR ((D6)DMSO): 12.6 (br. s, COOH); 7.96 (s, NH); 7.49 (t, J ¼ 5.8, NH(Gly)); 7.35 – 7.15 (m, 8
arom. H, NH); 7.10 – 7.05 (m, 2 arom. H); 5.00 (s, PhCH2O); 3.58 (d, J ¼ 6.0, CH2(Gly)); 3.17 (s, PhCH2);
1.39, 1.35, 1.30 (3s, Me2C, Me(Phe(2Me))). 13C-NMR ((D6)DMSO): 175.2, 173.5, 169.0 (3s,
2 CO(amide), COOH); 156.8 (s, CO(urethane)); 137.4, 137.0 (2s, 2 arom. C); 130.7, 128.7, 128.1, 128.0,
126.8 (5d, 10 arom. CH); 65.8 (t, PhCH2O); 59.5, 56.6 (2s, C(2)(Aib), C(2)(Phe(2Me))); 44.2 (t,
CH2(Gly)); 41.0 (t, PhCH2); 25.4, 24.8, 22.7 (3q, Me2C, Me(Phe(2Me))). CI-MS: 457 (6), 456 (24, [M þ
1]þ), 439 (28), 438 (100, [M ꢀ H2O þ 1]þ), 348 (12), 330 (8), 312 (12), 295 (47), 277 (15, [M ꢀ
Phe(2Me)]þ, 251 (18), 180 (30), 134 (26). Anal. calc. for C24H29N3O6 (455.51): C 63.28, H 6.42, N
9.22; found: C 62.99, H 6.31, N 9.05.
3.1.3. Z-Gly-Aib-(RS)-Phe(2Me)-Aib-N(Me)Ph (6a). According to GP 1, 5a (931 mg, 2.04 mmol)
was reacted with 2,2,N-trimethyl-N-phenyl-2H-azirin-3-amine (1a [26]; 395 mg, 2.27 mmol) in THF/DMF
(20 :1; 21 ml) for ca. 70 h: 1.21 g (94%) of 6a. Colorless crystals. M.p. 91.3 – 93.08. IR (KBr): 3420m,
3320m, 3280m, 1720s, 1680s, 1650s, 1635s, 1590m, 1530s, 1495s, 1455m, 1395m, 1380m, 1365m, 1280m,
1240s, 1215m, 1195m, 1170m, 1100m, 1090m, 1050m, 740m, 705s. 1H-NMR (CDCl3): 7.66 (s, NH); 7.35 –
7.15 (m, 13 arom. H); 7.10 – 7.05 (m, 2 arom. H); 6.87, 6.56, 6.18 (3s, 3 NH); 5.03, 4.98 (AB, JAB ¼ 12.2,
PhCH2O); 3.65 – 3.55 (m, CH2(Gly), 1 H of PhCH2); 3.35 (s, MeN); 3.09 (d, JAB ¼ 13.7, 1 H of PhCH2);
1.46 (s, Me2C); 1.42, 1.38, 1.27 (3s, Me2C, Me(Phe(2Me))). 13C-NMR (CDCl3): 174.0, 173.5, 172.9, 170.3
(4s, 4 CO(amide)); 157.1 (s, CO(urethane)); 145.6, 136.8, 136.2 (3s, 3 arom. C); 130.9, 129.2, 128.5, 128.2,
128.0, 127.9, 127.5, 127.0, 126.7 (9d, 15 arom. CH); 67.0 (t, PhCH2O); 60.2, 57.6, 57.2 (3s, 2 C(2)(Aib),
C(2)(Phe(2Me))); 45.3 (t, CH2(Gly)); 40.5 (q, MeN); 40.3 (t, PhCH2); 25.8, 25.5, 24.3 (3q, 2 Me2C,
Me(Phe(2Me))). ESI-MS: 668 (29, [M þ K]þ, 652 (100, [M þ Na]þ, 523 (47, [M ꢀ N(Me)Ph]þ). Anal.
calc. for C35H43N5O6 (629.76): C 66.75, H 6.88, N 11.12; found: C 66.55, H 6.67, N 11.00.
Suitable crystals for the X-ray crystal-structure determination were grown from CH2Cl2/Et2O/
hexane by slow evaporation of the solvent.
3.1.4. Z-Gly-Aib-(RS)-Phe(2Me)-Aib-OH (7a). According to GP 2, 6a (4.12 g, 6.55 mmol) was
hydrolyzed for 3.5 h: 3.21 g (91%) of 7a. Colorless solid. M.p. 193.5 – 195.78. IR (KBr): 3400m, 3300m,
2980m, 1760s, 1680s, 1650s, 1530s, 1500m, 1470m, 1455m, 1390m, 1315m, 1280m, 1240m, 1160m, 700m.
1H-NMR ((D6)DMSO): 8.27 (s, NH); 7.51 (t, J ¼ 5.5, NH(Gly)); 7.47 (s, NH); 7.40 – 7.20 (m, 8 arom. H);
7.15 – 7.10 (m, 2 arom. H); 6.97 (s, NH); 5.01, 4.98 (AB, JAB ¼ 13.4, PhCH2O); 3.70 – 3.50 (ABX, JAB ¼ 16.3,
CH2(Gly)); 3.40, 2.99 (AB, JAB ¼ 13.4, PhCH2); 1.36, 1.35, 1.33, 1.30, 1.22 (5s, 2 Me2C, Me(Phe(2Me))).
13C-NMR (CDCl3): 175.5, 172.82, 172.78, 169.9 (4s, 3 CO(amide), COOH); 156.6 (s, CO(urethane));
137.0, 136.7 (2s, 2 arom. C); 130.7, 128.2, 127.73, 127.68, 127.5, 126.1 (6d, 10 arom. CH); 65.6 (t, PhCH2O);
58.7, 56.1, 54.7 (3s, 2 C(2)(Aib), C(2)(Phe(2Me))); 43.6 (t, CH2(Gly)); 39.2 (t, PhCH2); 25.6, 24.9, 24.3,
24.2, 23.2 (5q, 2 Me2C, Me(Phe(2Me))). ESI-MS: 563 (100, [M þ Na]þ). Anal. calc. for C28H36N4O7
(540.62): C 62.21, H 6.71, N 10.36; found: C 62.15, H 6.70, N 10.34.
3.1.5. Z-Gly-Aib-(RS)-Phe(2Me)-Aib-Gly-OMe (8a). According to GP 5, 7a (2.46 g, 4.54 mmol)
was coupled with H-Gly-OMe · HCl (639 mg, 5.09 mmol) by treatment with PyBOP (2.22 g, 4.63 mmol)
and DIEA (1.81 g, 13.97 mmol) in CH2Cl2 (7 ml); 1 h. CC (AcOEt) gave 2.54 g (91%) of 8a. Colorless
foam. M.p. 160.3 – 161.38. IR (KBr): 3340m, 3320s, 3000s, 2960m, 1755s, 1705s, 1675s, 1650s, 1530s, 1460m,
1385m, 1365m, 1305m, 1290m, 1260m, 1240m, 1215m, 1195m, 1180m, 1160m, 975m, 740m, 710m, 700m.
1H-NMR (CDCl3): 7.81 (t, NH(Gly)); 7.41 (s, NH); 7.35 – 7.20 (m, 8 arom. H); 7.10 – 7.05 (m, 2 arom. H);
6.90, 6.82 (2s, 2 NH); 6.08 (t-like, NH(Gly)); 4.97 (s, PhCH2O); 3.95 (d, J ¼ 5.9, CH2(Gly)); 3.71 – 3.70 (m,
CH2(Gly)); 3.56 (s, MeO); 3.23, 3.03 (AB, JAB ¼ 13.7, PhCH2); 1.49, 1.46, 1.39, 1.37 (4s, 2 Me2C,
Me(Phe(2Me))). 13C-NMR (CDCl3): 176.2, 174.8, 173.8, 171.5, 170.5 (5s, 4 CO(amide), COOMe); 157.3
(s, CO(urethane)); 136.1, 136.0 (2s, 2 arom. C); 130.8, 128.6, 128.3, 128.1, 127.9, 127.0 (6d, 10 arom. CH);
67.1 (t, PhCH2O); 59.9, 57.3, 57.0 (3s, 2 C(2)(Aib), C(2)(Phe(2Me))); 52.2 (s, MeO); 45.4, 41.4, 41.1 (3t,

Helvetica Chimica Acta – Vol. 97 (2014)
635
2 CH2(Gly), PhCH2); 26.2, 25.4, 24.5, 23.4 (4q, 2 Me2C, Me(Phe(2Me))). ESI-MS: 650 (5, [M þ K]þ),
634 (100, [M þ Na]þ], 612 (3, [M þ 1]þ). Anal. calc. for C31H41N5O8 (611.69): C 60.87, H 6.76, N 11.45;
found: C 60.78, H 6.63, N 11.44.
Suitable crystals for the X-ray crystal-structure determination were grown from CH2Cl2/hexane by
slow evaporation of the solvent.
3.2. Z-Gly-Aib-(RS)-Phe(2Me)-Aib-Phe-OBn (8b). According to GP 5, 7a (432 mg, 0.779 mmol)
was coupled with H-Phe-OBn · HCl (250 mg, 0.857 mmol) by treatment with PyBOP (373 mg,
0.779 mmol) and DIEA (307 mg, 2.205 mmol) in CH2Cl2 (3 ml); 5.5 h. CC (AcOEt/hexane 3 :1)
furnished 527 mg (85%) of 8b (mixture of diastereoisomers). Colorless solid. M.p. 134.0 – 136.58. [a]D ¼
ꢀ20.3 (c ¼ 0.5, EtOH). IR (KBr): 3300m, 1740m, 1700m, 1660s, 1535s, 1500m, 1455m, 1275m, 1240m,
700m. 1H-NMR (CDCl3): 7.78 (s, NH); 7.46, 7.36 (2s, 1 NH); 7.25 – 7.15 (m, 18 arom. H); 7.10 – 7.05 (m, 2
arom. H); 6.69, 6.93 (2s, 1 NH); 6.75, 6.71 (2s, 1 NH); 6.22, 6.17 (2 br. s, 1 NH); 5.01 (s, PhCH2O); 5.00 –
4.90 (m, PhCH2O); 4.75 – 4.65 (m, CH(Phe)); 3.65 – 3.60 (m, CH2(Gly)); 3.37, 3.20 – 3.10, 3.02 (t,m,d, J ¼
15.6, 13.7, 1:2 :1, 2 PhCH2); 1.46, 1.44, 1.40, 1.37, 1.34, 1.33 (6s, 2 Me2C; Me(Phe(2Me))). 13C-NMR
(CDCl3): 175.7, 174.1, 173.0, 172.1, 170.3 (5s, 4 CO(amide), COOBn); 157.3 (s, CO(urethane)); 137.1,
136.1, 135.5 (3s, 3 arom. C); 130.9, 129.3, 129.2, 128.5, 128.4, 128.3, 128.12, 128.09, 128.0, 126.9, 126.6 (11d,
20 arom. CH); 67.1, 66.8, 66.7 (3t, 2 PhCH2O); 59.9, 57.0 (2s, 2 C(2)(Aib), C(2)(Phe(2Me))); 54.5 (d,
CH(2)Phe); 45.4, 37.5 (2t, CH2(Gly), 2 PhCH2); 25.7, 25.0, 24.5, 24.1, 23.4 (5q, 2 Me2C, Me(Phe(2Me))).
ESI-MS: 801 (100, [M þ Na]þ). Anal. calc. for C44H51N5O8 (777.92): C 67.94, H 6.61, N 9.00; found: C
67.79, H 6.88, N 9.00.
3.3. Z-Gly-Aib-Acb-Aib-Gly-OMe (8c). 3.3.1. Z-Gly-Aib-Acb-NMe2 (4b). According to GP 1, 3
(1.68 g, 5.72 mmol) was reacted with 2-(dimethylamino)-1-azabicyclo[2.3]hex-1-ene (1c [2b]; 782 mg,
6.30 mmol) in THF (16 ml); ca. 1 h. The precipitate was filtered and washed with Et2O: 1.02 g of 8c. CC
(CH2Cl2/MeOH 10 :1) of the filtrate gave another 1.06 g of 8c. Total yield: 2.08 g (87%). Colorless solid.
M.p. 212.3 – 213.78. IR (KBr): 3340m, 3270s, 3050m, 2980m, 2940m, 1730s, 1710m, 1680m, 1660s, 1630s,
1540m, 1530m, 1395m, 1270m, 1245m, 1230m, 750w, 700w. 1H-NMR ((D6)DMSO): 7.90, 7.89 (2s, 2 NH);
7.56 (t-like, NH); 7.35 – 7.30 (m, 5 arom. H); 5.05 (s, PhCH2O); 3.61 (d, J ¼ 5.8, CH2(Gly)); 3.33 (s, Me2N);
2.60 – 2.50, 2.20 – 2.10, 1.76 – 1.74, 1.60 – 1.55 (4m, 2 :2 :1:1, 3 CH2(Acb)); 1.36 (s, Me2C). 13C-NMR
((D6)DMSO): 175.2, 170.6, 168.4 (3s, 3 CO(amide)); 156.6 (s, CO(urethane)); 136.9 (s, 1 arom. C); 128.3,
127.7, 127.5 (3d, 5 arom. CH); 65.4 (t, PhCH2O); 58.2, 55.7 (2s, C(2)(Aib), C(2)(Acb)); 44.0 (t,
CH2(Gly)); 36.8, 36.0 (2q, Me2N); 30.9 (t, 2 CH2(Acb)); 24.8 (q, Me2C), 14.3 (t, CH2(Acb)). ESI-MS: 441
(100, [M þ Na]þ).
3.3.2. Z-Gly-Aib-Acb-OH (5b). According to GP 2, 4b (372 mg, 0.890 mmol) was hydrolyzed for
70 h at 408: 334 mg (96%) of 5b. Colorless crystals. M.p. 180.5 – 182.38. IR (KBr): 3340s, 3290s, 3060m,
2980m, 2950m, 1720s, 1680s, 1670s, 1650s, 1565m, 1550s, 1540s, 1525m, 1470m, 1430s, 1385m, 1280s, 1250s,
1235m, 1165m, 1110m, 750m. 1H-NMR (CD3OD): 7.40 – 7.25 (m, 5 arom. H); 5.11 (s, PhCH2O); 3.72 (s,
CH2(Gly)); 2.65 – 2.50, 2.40 – 2.30, 2.05 – 1.85 (3m, 3 CH2(Acb)); 1.44 (s, Me2C). 13C-NMR (CD3OD):
175.5, 174.8, 170.1 (3s, 2 CO(amide), COOH); 157.8 (s, CO(urethane)); 136.6 (s, 1 arom. C); 128.0, 127.5,
127.3 (3d, 5 arom. CH); 66.3 (t, PhCH2O); 58.2, 56.2 (2s, C(2)(Aib), C(2)(Acb)); 44.0 (t, CH2(Gly)); 30.5
(t, 2 CH2(Acb)); 23.8 (q, Me2C), 14.8 (t, CH2(Acb)). ESI-MS: 430 (51, [M þ K]þ), 414 (100, [M þ Na]þ).
Anal. calc. for C19H25N3O6 · 0.5 H2O (400.43): C 56.99, H 6.54, N 10.49; found: C 57.25, H 6.77, N 10.76.
3.3.3. Z-Gly-Aib-Acb-Aib-N(Me)Ph (6b). According to GP 1, 5b (840 mg, 2.15 mmol) was reacted
with 1a (411 mg, 2.36 mmol) in THF/DMF 28 :1 (29 ml). CC (CH2Cl2/MeOH 10 :1) gave 1.14 g (94%) of
6b. Colorless solid. Phase transitions at 137 and 1678. M.p. 187.8 – 188.38. IR (KBr): 3300s, 3060m, 2990m,
2940m, 1710s, 1680s, 1675s, 1670s, 1660s, 1650s, 1645s, 1630s, 1605m, 1590m, 1550s, 1540s, 1530s, 1520s,
1505s, 1495s, 1470s, 1395m, 1275m, 1265m, 1240s, 1190m, 1155m, 1090m, 700m. 1H-NMR (CD3OD): 7.51
(br. s, 2 NH); 7.40 – 7.20 (m, 10 arom. H); 6.58 (br. s, NH); 6.44 (s, NH); 5.15 (s, PhCH2O); 3.70 (d, J ¼ 5.2,
CH2(Gly)); 3.30 (s, MeN); 2.70 – 2.55, 2.15 – 1.95, 1.95 – 1.80 (3m, 2 :3 :1, 3 CH2(Acb)); 1.44, 1.28 (2s,
2 Me2C). 13C-NMR (CDCl3): 174.0, 173.7, 173.5, 170.7 (4s, 4 CO(amide)); 157.6 (s, CO(urethane)); 146.0,
136.5 (2s, 2 arom. C); 129.2, 128.5, 128.2, 127.8, 127.3, 127.0 (6d, 10 arom. CH); 67.0 (t, PhCH2O); 59.2,
56.8, 56.6 (3s, 2 C(2)(Aib), C(2)(Acb)); 45.7 (t, CH2(Gly)); 40.5 (q, MeN); 31.6, 31.3 (2t, 2 CH2(Acb));
25.7, 24.9 (2q, 2 Me2C); 16.0 (t, CH2(Acb)). ESI-MS: 588 (100, [M þ Na]þ), 459 (5, [M ꢀ N(Me)Ph]þ),
374 (26, [M ꢀ Aib-N(Me)Ph]þ), 277 (3, [M ꢀ Acb-Aib-N(Me)Ph)]þ).

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3.3.4. Z-Gly-Aib-Acb-Aib-OH (7b). According to GP 2, 6b (1.033 g, 1.83 mmol) was hydrolyzed,
and the product was extracted with AcOEt: 828 mg (95%) of 7b. Colorless solid. M.p. 201.4 – 203.68. IR
(KBr): 3350m, 3320s, 3270s, 3000m, 2960m, 1730s, 1705s, 1670s, 1660s, 1650s, 1635s, 1605m, 1545s, 1540s,
1530s, 1470m, 1465m, 1395m, 1330m, 1310m, 1290m, 1260s, 1245s, 1215m, 1170m, 1150m, 970m, 745m.
1H-NMR (CD3OD): 8.40 (s, 0.3 NH); 7.98 (s, NH); 7.34 (s, 0.7 NH); 7.40 – 7.25 (m, 10 arom. H); 5.08 (s,
PhCH2O); 3.71 (s, CH2(Gly)); 2.70 – 2.60, 2.25 – 2.10, 2.00 – 1.90 (3m, 2 H each, 3 CH2(Acb); 1.47, 1.44
(2s, 2 Me2C). 13C-NMR (CD3OD): 178.2, 176.3, 175.3, 172.5 (4s, 3 CO(amide), COOH); 159.5 (s,
CO(urethane)); 138.1 (s, 1 arom. C); 129.5, 129.1, 128.9 (3d, 5 arom. CH); 68.1 (t, PhCH2O); 60.5, 57.7,
57.1 (3s, 2 C(2)(Aib), C(2)(Acb)); 45.3 (t, CH2(Gly)); 31.9 (t, 2 CH2(Acb)); 25.4 (q, 2 Me2C); 16.5 (t,
CH2(Acb)). ESI-MS: 499 (100, [M þ Na]þ). Anal. calc. for C23H32N4O7 (476.53): C 57.97, H 6.77, N 11.76;
found: C 57.76, H 6.84, N 11.31.
3.3.5. Z-Gly-Aib-Acb-Aib-Gly-OMe (8c). According to GP 5, 7b (268 mg, 0.653 mmol) and H-Gly-
OMe · HCl (79 mg, 0.629 mmol) were treated with HBPyU (243 mg, 0.564 mmol) and DIEA (220 mg,
1.702 mmol) for 0.5 h. CC (AcOEt/MeOH 20 :1) gave 255 mg (83%) of 8c. Colorless solid. M.p. 160.0 –
161.48. IR (KBr): 3340s, 3280s, 3060m, 2980m, 2950m, 1740s, 1710s, 1685s, 1650s, 1630s, 1550s, 1540s,
1530s, 1505m, 1470m, 1465m, 1455m, 1440m, 1380m, 1365m, 1310m, 1275s, 1240s, 1220m, 1195m, 1170m,
1155m, 1050m, 700m. 1H-NMR (CDCl3): 7.52 (s, NH); 7.46 (t, J ¼ 5.8, NH(Gly)); 7.40 – 7.35 (m, 5 arom.
H, 1 NH); 6.90 (s, NH); 6.31 (t-like, NH); 5.14 (s, PhCH2O); 3.98 (d, J ¼ 5.8, CH2(Gly)); 3.75 (d, J ¼ 5.2,
CH2(Gly)); 3.65 (s, MeO); 2.75 – 2.70, 2.05 – 1.85 (2m, 1:2, 3 CH2(Acb); 1.55, 1.46 (2s, 2 Me2C).
13C-NMR (CDCl3): 176.5, 175.3, 173.6, 171.0, 170.6 (5s, 4 CO(amide), COOMe); 157.8 (s, CO(ur-
ethane)); 136.4 (s, 1 arom. C); 128.6, 128.2, 127.6 (3d, 5 arom. CH); 67.2 (t, PhCH2O); 59.2, 57.0, 56.6 (3s,
2 C(2)(Aib), C(2)(Acb)); 52.1 (q, MeO); 45.8, 41.4 (2t, 2 CH2(Gly)); 30.9 (t, 2 CH2(Acb)); 25.3, 24.7 (2q,
2 Me2C); 15.4 (t, CH2(Acb)). ESI-MS: 570 (100, [M þ Na]þ).
3.4. Z-Gly-Aib-Pro-Aib-Aib-N(Me)Ph (8d). According to GP 5, 3 (1.18 g, 4.00 mmol) and H-Pro-
Aib-Aib-N(Me)Ph [25b] (1.65 g, 4.40 mmol) in CH2Cl2 (20 ml) were treated with HBPyU (1.72 g,
4.00 mmol) and DIEA (1.03 g, 8.00 mmol) for 15 h. CC (CH2Cl2/MeOH 20 :1) gave 2.15 g (82%) of 8d.
Colorless solid. M.p. 107.6 – 108.78. [a]D ¼ þ34.5 (c ¼ 1.04, EtOH). IR (KBr): 3420m, 3300s, 3060m,
2980m, 2930m, 1725s, 1660s, 1625s, 1595m, 1540s, 1520s, 1495s, 1470m, 1455m, 1410m, 1395m, 1365m,
1280m, 1240s, 1215m, 1170m, 1090m, 1050m, 705m, 700m. 1H-NMR (CDCl3): 7.57 (s, NH); 7.41 (s, NH);
7.40 – 7.15 (m, 10 arom. H, 1 NH); 6.14 (br. s, NH); 5.10, 5.08 (AB, JAB ¼ 12.0, PhCH2O); 4.20 – 4.15 (m,
CH2(2)(Pro)); 3.80 – 3.55 (m, CH2(Gly), 1 H of CH2(5)(Pro)); 3.38 (s, MeN); 3.35 – 3.20 (m, 1 H of
CH2(5)(Pro)); 2.35 – 2.20, 1.95 – 1.80, 1.75 – 1.65 (3m, CH2(3), CH2(4)(Pro)); 1.60, 1.58, 1.49, 1.43, 1.28
(5s, 1:1:1:2:1, 3 Me2C). 13C-NMR (CDCl3): 175.1, 174.3, 173.3, 171.7, 169.7 (5s, 5 CO(amide)); 156.9 (s,
CO(urethane)); 145.5, 136.5 (2s, 2 arom. C); 129.1, 128.5, 128.2, 128.0, 127.2, 126.6 (6d, 10 arom. CH);
66.9 (t, PhCH2O); 64.2 (d, CH(2)(Pro)); 57.1, 57.0, 56.5 (3s, 3 C(2)(Aib)); 48.7 (t, CH2(5)(Pro)); 44.1 (t,
CH2(Gly)); 40.1 (s, MeN); 28.8, 26.2 (2t, CH2(3), CH2(4)(Pro)); 27.8, 26.1, 25.8, 25.6, 23.9, 23.6 (6q,
3 Me2C). ESI-MS: 673 (100, [M þ Na]þ).
3.5. Z-Gly-Aib-Pro-Aib-Aib-OH (10). According to GP 2, 8d (1.03 g, 1.58 mmol) was hydrolyzed for
4.5 h. The precipitate was filtered (549 mg of 10), and the filtrate was purified by CC (CH2Cl2/MeOH
10 :1). Total yield of 10: 629 mg (70%). Colorless solid. M.p. 192.3 – 193.68. [a]D ¼ þ22.4 (c ¼ 1.02,
MeOH). IR (KBr): 3460m, 3400m, 3310s, 3290s, 3040m, 2990m, 2940m, 1715s, 1665s, 1650s, 1600s, 1540s,
1520s, 1470m, 1450m, 1425m, 1410m, 1365m, 1310m, 1290m, 1280m, 1250m, 1215m, 1170m, 695m.
1H-NMR ((D6)DMSO): 8.53 (s, NH); 7.55 – 7.30 (m, 5 arom. H, 2 NH); 7.17 (s, NH); 5.04, 5.00 (AB, JAB
¼
12.6, PhCH2O); 4.09 (t, J ¼ 7.8, CH2(2)(Pro)); 3.85 – 3.70, 3.70 – 3.55, 3.55 – 3.54 (3m, 1:2 :1, CH2(Gly),
CH2(5)(Pro)); 2.15 – 2.00, 1.95 – 1.70, 1.65 – 1.45 (3m, 1:2 :1, CH2(3), CH2(4)(Pro)); 1.37, 1.34, 1.32, 1.29
(4s, 1:2 :2 :1, 3 Me2C). 13C-NMR ((D6)DMSO): 175.4, 173.4, 172.3, 171.1, 168.9 (5s, 4 CO(amide),
COOH); 156.5 (s, CO(urethane)); 136.9 (s, 1 arom. C); 128.2, 127.7, 127.6 (3d, 5 arom. CH); 65.4 (t,
PhCH2O); 62.8 (d, CH(2)(Pro)); 55.6, 54.6 (2s, 3 C(2)(Aib)); 47.8 (t, CH2(5)(Pro)); 42.7 (t, CH2(Gly));
28.2, 25.5 (2t, CH2(3), CH2(4)(Pro)); 25.9, 25.4, 24.9, 24.1, 24.0, 23.9 (6q, 3 Me2C). ESI-MS: 584 (100,
[M þ Na]þ). Anal. calc. for C27H39N5O8 · 1.5 H2O (588.66): C 55.09, H 7.19, N 11.90; found: C 54.99, H
7.23, N 11.88.
Suitable crystals for the X-ray crystal-structure determination were grown from MeOH.

Helvetica Chimica Acta – Vol. 97 (2014)
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3.6. Z-Gly-(RS)-Phe(2Me)-Gly-Aib-Phe-OMe (8e). 3.6.1. Z-Gly-Aib-Phe-OMe. According to GP 5,
a mixture of Z-Gly-Aib-OH (3) [25] (1.03 g, 3.51 mmol) and H-Phe-OMe · HCl (834 mg, 3.87 mmol) in
CH2Cl2 (5 ml) was treated with PyBOP (1.68 g, 3.52 mmol) and DIEA (1.34 g, 10.65 mmol). CC (AcOEt/
hexane 3 :1) gave 1.36 g (85%) of Z-Gly-Aib-Phe-OMe. Colorless solid. M.p. 150 – 158.78. [a]D ¼ þ6.6
(c ¼ 1.01, EtOH). IR (KBr): 3380m, 3260m, 3070m, 1745s, 1710s, 1680s, 1635s, 1555s, 1525m, 1380m,
1275m, 1250m, 1215m, 1205m, 1180m, 1160m, 1110w, 1080w, 1030w, 695m. 1H-NMR (CDCl3): 7.35 – 7.20
(m, 8 arom. H); 7.15 – 7.10 (m, 2 arom. H); 6.67 (br. d, J ¼ 6.7, NH); 6.54, 5.37 (2 br. s, 2 NH); 5.13 (s,
PhCH2O); 4.90 – 4.80 (ABX, CH(2)(Phe)); 3.78 (d, J ¼ 5.6, CH2(Gly)); 3.72 (s, MeO); 3.20 – 3.05 (ABX,
JAB ¼ 13.9, PhCH2); 1.48 (s, Me2C). 13C-NMR (CDCl3): 173.7, 171.9, 168.6 (3s, 2 CO(amide), COOMe);
156.7 (s, CO(urethane)); 136.1, 135.9 (2s, 2 arom. C); 129.3, 128.52, 128.48, 128.2, 128.0, 127.0 (6d, 10
arom. CH); 67.2 (t, PhCH2O); 57.2 (s, C(2)(Aib)); 53.4 (q, MeO); 52.3 (d, CH(2)(Phe)); 44.9 (t,
CH2(Gly)); 37.7 (t, PhCH2); 25.0, 24.9 (2q, Me2C). ESI-MS: 478 (7, [M þ Na]þ). Anal. calc. for
C24H29N3O6 (455.50): C 63.29, H 6.42, N 9.23; found: C 62.99, H 6.64, N 8.99.
3.6.2. H-Gly-Aib-Phe-OMe (12). According to GP 3, Z-Gly-Aib-Phe-OMe (158.6 mg, 0.348 mmol)
was deprotected: 110 mg (98%) of 12. Colorless oil. IR (CDCl3): 3420m, 3310m, 2930m, 1740m, 1685s,
1660s, 1635s, 1600m, 1540s, 1510s, 1495s, 1450m, 1435m, 1385m, 1360m, 1330m, 1275m, 1175m, 690m.
1H-NMR (CDCl3): 7.64 (s, NH); 7.30 – 7.10 (m, 5 arom. H, 1 NH); 4.90 – 4.80 (ABX, CH(2)(Phe)); 3.71 (s,
MeO); 3.26 (s, CH2(Gly)); 3.20 – 3.05 (ABX, JAB ¼ 13.9, PhCH2); 1.51, 1.49 (2s, Me2C). 13C-NMR
(CDCl3): 174.0, 172.8, 172.0 (3s, 2 CO(amide), COOMe); 136.0 (s, 1 arom. C); 129.3, 128.3, 126.9 (3d, 5
arom. CH); 56.8 (s, C(2)(Aib)); 53.3 (d, CH(2)(Phe)); 52.2 (q, MeO); 44.9 (t, CH2(Gly)); 37.6 (t,
PhCH2); 25.0 (q, Me2C). ESI-MS: 665 (46, [2M þ Na]þ), 643 (100, [2M þ 1]þ), 633 (82, [2M ꢀ MeOH þ
Na]þ), 611 (26, [2M ꢀ OMe]þ), 344 (88, [M þ Na]þ), 322 (21, [M þ 1]þ).
3.6.3. Z-Gly-(RS)-Phe(2Me)-Gly-Aib-Phe-OMe (8e). According to GP 5, a mixture of Z-Gly-(RS)-
Phe(2Me)-OH (11) [25a] (734 mg, 1.98 mmol) and 12 (700 mg, 2.18 mmol) in CH2Cl2 (10 ml) was treated
with PyBOP (949 mg, 1.98 mmol) and DIEA (533 mg, 4.12 mmol). CC (2 ꢁ , AcOEt/MeOH 20 :1) and
PLC (AcOEt/MeOH 10 :1) gave 623 mg (S,S)-8e and 586 mg (R,S)-8e; total yield: 1.21 mg (91%).
Z-Gly-(S)-Phe(2Me)-Gly-Aib-Phe-OMe ((S,S)-8e). Colorless solid. M.p. 162.5 – 164.18. [a]D
¼
ꢀ69.0 (c ¼ 1.01, EtOH). IR (KBr): 3410m, 3280s, 1745s, 1730s, 1670s, 1660s, 1535s, 1455m, 1385m,
1
1360m, 1325m, 1280m, 1230s, 1190m, 1185m, 1165m, 1150m, 1110w, 1080w, 700m. H-NMR (CDCl3):
7.40 – 7.05 (m, 15 arom. H, 3 NH); 6.49 (s, NH); 5.80 (t-like, NH); 5.05, 4.98 (AB, JAB ¼ 12, PhCH2O);
4.80 – 4.70 (ABX, CH(2)(Phe)); 3.90 – 3.65 (m, 2 CH2(Gly)); 3.60 (s, MeO); 3.29, 3.03 (AB, JAB ¼ 13.6,
PhCH2); 3.20 – 3.05 (m, PhCH2); 1.48, 1.44, 1.40 (3s, Me2C, Me(Phe(2Me))). 13C-NMR (CDCl3): 175.3,
175.0, 173.2, 170.7, 170.0 (5s, 4 CO(amide), COOMe); 157.3 (s, CO(urethane)); 136.8, 136.1, 135.3 (3s, 3
arom. C); 130.5, 129.1, 128.6, 128.5, 128.4, 128.3, 128.0, 127.2, 126.8 (9d, 15 arom. CH); 67.3 (t, PhCH2O);
59.6, 57.0 (2s, C(2)(Aib), C(2)(Phe(2Me))); 54.1 (d, CH(2)(Phe)); 52.5 (q, MeO); 45.4, 44.3 (2t,
2 CH2(Gly)); 36.9, 30.7 (2t, 2 PhCH2); 25.9, 24.3, 22.7 (3q, Me2C, Me(Phe(2Me))). ESI-MS: 696 (100,
.
[M þ Na]þ). Anal. calc. for C36H43N5O8 H2O (691.78): C 62.50, H 6.56, N 10.12; found: C 62.43, H 6.30, N
10.07.
Suitable crystals for the X-ray crystal-structure determination were grown from AcOEt/hexane by
slow evaporation of the solvent.
Z-Gly-(R)-Phe(2Me)-Gly-Aib-Phe-OMe ((R,S)-8e). Colorless solid. M.p. 116.4 – 118.28. [a]D ¼
þ67.5 (c ¼ 1.05, EtOH). IR (KBr): 3380m, 3280s, 1730s, 1680s, 1670s, 1660s, 1540s, 1530s, 1455m,
1440m, 1410m, 1390m, 1365m, 1345m, 1310m, 1275m, 1230m, 1195m, 1175m, 1150m, 1120w, 1080w,
1
1045m, 700m. H-NMR (CDCl3): 8.35 (br. s, NH); 7.65 (s, NH); 7.42 (d, J ¼ 7.9, NH); 7.40 – 7.05 (m, 15
arom. H); 7.00 (s, NH); 6.18 (br. s, NH); 5.12, 5.00 (AB, JAB ¼ 12.3, PhCH2O); 4.65 – 4.55 (m,
CH(2)(Phe)); 4.00 – 3.60 (m, 2 CH2(Gly)); 3.47, 3.10 (AB, JAB ¼ 13.7, PhCH2); 3.24 (s, MeO); 3.25 – 2.95
(m, PhCH2); 1.43, 1.38, 1.26 (3s, Me2C, Me(Phe(2Me))). 13C-NMR (CDCl3): 176.9, 176.4, 172.3, 171.0,
169.9 (5s, 4 CO(amide), COOMe); 157.0 (s, CO(urethane)); 136.7, 136.4, 136.2 (3s, 3 arom. C); 131.1,
129.0, 128.5, 128.4, 128.2, 128.1, 126.8, 126.6 (8d, 15 arom. CH); 67.1 (t, PhCH2O); 59.4, 57.0 (2s,
C(2)(Aib), C(2)(Phe(2Me))); 54.1 (d, CH(2)(Phe)); 52.0 (q, MeO); 45.5, 44.3 (2t, 2 CH2(Gly)); 39.5,
37.2 (2t, 2 PhCH2); 27.0, 23.5 (2q, 1:2, Me2C, Me(Phe(2Me))). ESI-MS: 696 (100, [M þ Na]þ). Anal.
calc. for C36H43N5O8 (673.76): C 64.18, H 6.43, N 10.39; found: C 63.91, H 6.33, N 10.53.

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3.7. Z-Gly-(RS)-Phe(2Me)-Gly-Aib-Aib-N(Me)Ph (8f). According to GP 5, a mixture of 11
(100 mg, 0.270 mmol) and H-Gly-Aib-Aib-N(Me)Ph [25a] (99 mg, 0.296 mmol) in CH2Cl2 (2 ml) was
treated with PyBOP (129 mg, 0.270 mmol) and DIEA (85 mg, 0.658 mmol): 152 mg (82%) of 8f.
Colorless solid. M.p. 191.9 – 192.88. IR (KBr): 3410m, 3300m, 1725m, 1680s, 1665s, 1650s, 1640s, 1595m,
1
1535s, 1495m, 1455m, 1395m, 1375m, 1365m, 1270m, 1250m, 1240m, 1095m, 710m. H-NMR (CDCl3):
7.86 (br. s, NH); 7.40 – 7.05 (m, 15 arom. H, 2 NH); 6.83, 6.54 (2br. s, 2 NH); 5.08, 5.02 (AB, JAB ¼ 12.2,
PhCH2O); 3.80 – 3.60 (m, 2 CH2(Gly)); 3.23 (s, MeN); 3.23, 2.98 (AB, JAB ¼ 13.6, PhCH2); 1.49, 1.46, 1.27
(3s, 3 :1 :1, 2 Me2C, Me(Phe(2Me))). 13C-NMR (CDCl3): 175.6, 175.2, 174.0, 170.8, 169.2 (5s,
5 CO(amide)); 157.3 (s, CO(urethane)); 145.3, 136.2, 135.6 (3s, 3 arom. C); 130.5, 129.1, 128.4, 128.2,
128.1, 127.9, 127.0, 126.9 (8d, 15 arom. CH); 67.0 (t, PhCH2O); 59.3, 57.1 (2s, 2 C(2)(Aib),
C(2)(Phe(2Me))); 45.4, 44.8, 40.7 (3t, 2 CH2(Gly), PhCH2); 40.3 (q, MeN); 25.91, 25.88, 24.9, 22.7
(4q, 1:2 :1:1, 2 Me2C, Me(Phe(2 Me))). ESI-MS: 725 (8, [M þ K]þ), 709 (100, [M þ Na]þ), 580 (5,
[M ꢀ N(Me)Ph]þ). Anal. calc. for C37H46N6O7 (686.81): C 64.71, H 6.75, N 12.24; found: C 64.90, H 6.89,
N 12.40.
Suitable crystals for the X-ray crystal-structure determination were grown from AcOEt/hexane by
slow evaporation of the solvent.
3.8. Z-Gly-(RS)-Phe(2Me)-Pro-Aib-Phe-OMe (8g). 3.8.1. Z-Pro-Aib-Phe-OMe. According to GP 5,
a mixture of Z-Pro-Aib-OH [25b] (1.14 g, 3.40 mmol) and H-Phe-OMe · HCl (808 mg, 3.75 mmol) in
CH2Cl2 (6 ml) was treated with PyBOP (1.63 g, 3.40 mmol) and DIEA (1.32 g, 10.23 mmol). CC (AcOEt/
hexane 3 :1) gave 1.33 g (79%) of Z-Pro-Aib-Phe-OMe. Colorless solid. M.p. 169.7 – 170.48. [a]D ¼ ꢀ23.8
(c ¼ 0.817, EtOH). IR (KBr): 3380m, 3280m, 1745s, 1690s, 1665s, 1640s, 1550m, 1525m, 1455m, 1435s,
1355m, 1240m, 1215m, 1205m, 1180m, 1170m, 1120m, 700m. 1H-NMR (CDCl3): 7.35 – 7.10 (m, 10 arom.
H, 0.8 NH); 6.86 (br. s, NH); 6.40 (br. s, 0.2 NH); 5.20 – 5.10 (m, PhCH2O); 4.80 – 4.75, 4.25 – 4.20 (2m,
CH(2)(Phe), CH(2)(Pro)); 3.68 (s, MeO); 3.60 – 3.40 (m, CH2(5)(Pro)); 3.20 – 3.00 (ABX, JAB ¼ 13.9,
PhCH2); 2.25 – 1.25 (m, CH2(3), CH2(4)(Pro)); 1.44, 1.26 (2br. s, Me2C). 13C-NMR (CDCl3): 173.7, 171.9
(2s, 2 CO(amide), COOMe); 156.1 (s, CO(urethane)); 136.3 (s, 2 arom. C); 129.2, 128.4, 128.3, 128.1,
127.8, 126.8 (6d, 10 arom. CH); 67.2 (t, PhCH2O); 61.0 (d, CH(2)(Pro)); 57.2 (s, C(2)(Aib)); 53.6 (d,
CH(2)(Phe)); 52.0 (s, MeO); 47.0 (t, CH2(5)(Pro)); 37.8 (t, PhCH2); 28.4, 24.8 (2t, CH2(3),
CH2(4)(Pro)); 25.5, 24.8 (2q, Me2C). ESI-MS: 518 (100, [M þ Na]þ). Anal. calc. for C27H33N3O6
(495.57): C 65.44, H 6.71, N 8.48; found: C 65.49, H 6.97, N 8.54.
3.8.2. H-Pro-Aib-Phe-OMe (13). According to GP 3, Z-Pro-Aib-Phe-OMe (1.15 g, 2.32 mmol) in
MeOH (25 ml) in the presence of Pd/C (113 mg) was deprotected (2 h): 842 mg (84%) of 13. Colorless
oil. [a]D ¼ þ3.6 (c ¼ 1.24, CHCl3). IR (CHCl3): 3280m, 3020m, 3000s, 2980m, 1740s, 1670s, 1510s, 1455m,
1440m, 1390m, 1360m, 1215m, 1180m, 850m, 700m. 1H-NMR (CDCl3): 7.89 (s, NH); 7.41 (d, J ¼ 7.5, NH);
7.30 – 7.15 (m, 5 arom. H); 4.80 – 4.75, 3.70 – 3.65 (2m, CH(2)(Pro), CH(2)(Phe)); 3.69 (s, MeO); 3.20 –
2.90 (m, CH2(5)(Pro), PhCH2, NH(Pro)); 2.15 – 2.05, 1.90 – 1.85, 1.75 – 1.65 (3m, 1:1:2, CH2(3),
CH2(4)(Pro)); 1.47, 1.45 (2s, Me2C). 13C-NMR (CDCl3): 175.2, 174.2, 172.1 (3s, 2 CO(amide), COOMe);
136.3 (s, 1 arom. C); 129.3, 128.4, 126.9 (3d, 5 arom. CH); 60.7 (d, CH(2)(Pro)); 56.9 (s, C(2)(Aib)); 53.5
(d, CH(2)(Phe)); 52.2 (s, MeO); 47.2 (t, CH2(5)(Pro)); 37.7 (t, PhCH2); 30.6, 26.0 (2t, CH2(3),
CH2(4)(Pro)); 25.2, 25.1 (2q, Me2C). CI-MS: 363 (21), 362 (100, [M þ 1]þ).
3.8.3. Z-Gly-(RS)-Phe(2Me)-Pro-Aib-Phe-OMe (8g). According to GP 5, a mixture of 11
(374.5 mg, 1.01 mmol) and 13 (400 mg, 1.11 mmol) in CH2Cl2 (10 ml) was treated with TBTU
(326.2 mg, 1.02 mmol) and DIEA (301.8 mg, 2.34 mmol) for 16 h. CC (4 ꢁ , AcOEt/MeOH 20 :1) gave
193.9 mg of (S,S)-8g and 319.8 mg of (R,S)-8g; total yield: 513.7 mg (71%).
Z-Gly-(S)-Phe(2Me)-Pro-Aib-Phe-OMe ((S,S,S)-8g). Colorless lacquer. M.p. 59.1 – 60.78. [a]D ¼
ꢀ17.6 (c ¼ 1.03, EtOH). IR (KBr): 3300m, 2940m, 1745s, 1740s, 1730s, 1720s, 1670s, 1660s, 1635s,
1625s, 1565m, 1540s, 1520s, 1505s, 1460m, 1455m, 1410m, 1385m, 1240s, 1170m, 1050m, 700m. 1H-NMR
(CDCl3): 7.49 (d, J ¼ 8.1, NH); 7.45 – 7.05 (m, 15 arom. H, NH); 6.92 (s, NH); 5.71 (t-like, NH); 5.16, 5.10
(AB, JAB ¼ 12.2, PhCH2O); 4.80 – 4.70 (m, CH(2)(Phe)); 4.41 (t, CH(2)(Pro)); 3.90 – 3.70 (AB of ABX,
CH2(Gly)); 3.61 (s, MeO); 3.40 – 3.10, 3.10 – 2.95 (2m, 1:1, 2 PhCH2), CH2(5)(Pro)); 2.25 – 2.10, 1.85 –
1.65, 1.65 – 1.50 (3m, 1:2 :1, CH2(3), CH2(4)(Pro)); 1.52, 1.50, 1.34 (3s, Me2C, Me(Phe(2Me))).
13C-NMR (CDCl3): 175.1, 172.2, 171.6, 171.1, 169.0 (5s, 4 CO(amide), COOMe); 156.8 (s, CO(ur-
ethane)); 137.1, 136.0, 134.5 (3s, 3 arom. C); 130.2, 129.2, 128.9, 128.6, 128.5, 128.2, 127.7, 126.5 (8d, 15

Helvetica Chimica Acta – Vol. 97 (2014)
639
arom. CH); 67.4 (t, PhCH2O); 63.6 (d, C(2)(Pro)); 60.1, 57.1 (2s, C(2)(Aib), C(2)(Phe(2Me))); 53.9 (d,
CH(2)(Phe)); 52.0 (q, MeO); 48.6 (t, CH2(5)(Pro)); 44.8 (t, CH2(Gly)); 43.0, 37.9 (2t, 2 PhCH2); 28.3,
26.0 (2t, CH2(3), CH2(4)(Pro)); 26.7, 24.4, 24.0 (3q, Me2C, Me(Phe(2Me))). ESI-MS: 736 (100, [M þ
Na]þ).
Z-Gly-(R)-Phe(2Me)-Pro-Aib-Phe-OMe ((R,S,S)-8g). Colorless solid. M.p. 165.5 – 166.58. [a]D ¼
þ68.9 (c ¼ 1.04, EtOH). IR (KBr): 3400s, 3060m, 3030m, 2980m, 2940m, 1750s, 1730s, 1655s, 1625s,
1545s, 1535s, 1500m, 1455m, 1445m, 1405m, 1385m, 1375m, 1365m, 1305m, 1280m, 1240s, 1170m, 1160m,
1130w, 1100m, 1080m, 1045m, 1030m, 1000m, 740m, 700m. 1H-NMR (CDCl3): 7.50 (d, J ¼ 8.1, NH); 7.39
(s, NH); 7.40 – 7.00 (m, 15 arom. H); 6.85 (s, NH); 5.70 (br. s, NH); 5.08 (s, PhCH2O); 4.75 – 4.65 (m,
CH(2)(Phe)); 4.37 (t-like, CH(2)(Pro)); 4.00 – 3.80 (AB of ABX, CH2(Gly)); 3.75 – 3.65, 3.35 – 3.20 (2m,
CH2(5)(Pro)); 3.61 (s, MeO); 3.42, 3.17 (AB, JAB ¼ 13.8, PhCH2); 3.25 – 3.10, 3.10 – 2.95 (2m, PhCH2);
2.20 – 2.05, 1.95 – 1.80, 1.80 – 1.60 (3m, 1:1:2, CH2(3), CH2(4)(Pro)); 1.46, 1.39, 1.26 (3s, Me2C,
Me(Phe(2Me))). 13C-NMR (CDCl3): 175.3, 172.5, 172.4, 171.2, 169.3 (5s, 4 CO(amide), COOMe); 156.8
(s, CO(urethane)); 136.8, 136.6, 136.1 (3s, 3 arom. C); 131.1, 129.1, 128.6, 128.3, 128.22, 128.17, 128.1,
126.8, 126.6 (9d, 15 arom. CH); 67.2 (t, PhCH2O); 63.7 (d, CH(2)(Pro)); 59.1, 57.1 (2s, C(2)(Aib),
C(2)(Phe(2Me))); 53.6 (d, CH(2)(Phe)); 52.0 (q, MeO); 48.7 (t, CH2(5)(Pro)); 44.4 (t, CH2(Gly)); 40.9,
37.8 (2t, 2 PhCH2); 28.5, 26.0 (2t, CH2(3), CH2(4)(Pro)); 27.0, 24.1, 21.1 (3q, Me2C, Me(Phe(2Me))). ESI-
MS: 736 (100, [M þ Na]þ), 714 (10, [M þ 1]þ), 535 (6, [M ꢀ PheOMe]þ).
Suitable crystals for the X-ray crystal-structure determination were grown from AcOEt/MeOH by
slow evaporation of the solvent.
3.9. Z-Gly-(RS)-Phe(2Me)-Pro-Aib-Aib-N(Me)Ph (8h). According to GP 5, a mixture of 11 (1.25 g,
3.36 mmol) and H-Pro-Aib-Aib-N(Me)Ph [25b] (1.40 g, 3.73 mmol) in CH2Cl2 (30 ml) was treated with
TBTU (1.20 g, 3.74 mmol), HOBt (910 mg, 6.73 mmol), and DIEA (916 mg, 7.09 mmol) for 18 h. CC
(CH2Cl2/MeOH 15 :1) gave 1.76 g (72%) of 8h. Separation of 815.5 mg of the mixture of the
diastereoisomers by PLC (AcOEt/MeOH 20 :1, 5 ꢁ developed) led to 224.6 mg of (S,S)-8h and 470.6 mg
of (R,R)-8h.
Z-Gly-(S)-Phe(2Me)-Pro-Aib-Aib-N(Me)Ph ((S,S)-8h). Colorless solid. M.p. 92.4 – 93.98. [a]D ¼
þ14.0 (c ¼ 0.57, EtOH). IR (KBr): 3310s, 2980m, 2930m, 1730s, 1680s, 1660s, 1630s, 1595m, 1545s,
1540s, 1530s, 1495s, 1465m, 1455m, 1405m, 1390s, 1360m, 1280m, 1240s, 1200m, 1160m, 1090m, 1050m,
740m, 700s. 1H-NMR (CDCl3): 7.52 (s, NH); 7.35 – 7.05 (m, 15 arom. H, 2 NH); 6.29 (br. s, NH); 5.12, 5.08
(AB, JAB ¼ 12.3, PhCH2O); 4.22 (t, J ¼ 8.3, CH(2)(Pro)); 3.80 – 3.55 (AB of ABX, JAB ¼ 17.2, CH2(Gly));
3.38 (s, MeN); 3.20 – 3.00 (m, CH2(5)(Pro)); 2.91 (s, PhCH2); 2.30 – 2.15, 1.85 – 1.70, 1.70 – 1.40 (3m,
1:1:2, CH2(3), CH2(4)(Pro)); 1.60, 1.57, 1.47, 1.45, 1.40 (5s, 2 Me2C, Me(Phe(2Me))). 13C-NMR
(CDCl3): 174.7, 174.1, 171.8, 171.6, 169.8 (5s, 5 CO(amide)); 157. (s, CO(urethane)); 146.1, 136.4, 134.6
(3s, 3 arom. C); 130.1, 129.0, 128.8, 128.6, 128.3, 128.1, 127.6, 127.2, 126.6 (9d, 15 arom. CH); 67.0 (t,
PhCH2O); 64.4 (d, C(2)(Pro)); 60.3, 57.2, 57.0 (3s, 2 C(2)(Aib), C(2)(Phe(2Me))); 48.6, 44.6, 43.6 (3t,
CH2(Gly), CH2(5)(Pro), PhCH2); 40.1 (q, MeN); 28.6, 26.2 (2t, CH2(3), CH2(4)(Pro)); 27.8, 26.4, 25.4,
24.3, 23.7 (5q, 2 Me2C, Me(Phe(2Me))). ESI-MS: 749 (100, [M þ Na]þ). Anal. calc. for C40H50N6O7 ·
0.5H2O (735.88): C 65.29, H 6.99, N 11.42; found: C 65.28, H 7.17, N 11.15.
Z-Gly-(R)-Phe(2Me)-Pro-Aib-Aib-N(Me)Ph ((R,S)-8h). Colorless solid. M.p. 168.7 – 170.28.
[a]D ¼ þ114 (c ¼ 0.51, EtOH). IR (KBr): 3300s, 3060w, 3030w, 2980w, 2940w, 1725s, 1660s, 1625s,
1595m, 1540s, 1495m, 1465m, 1455m, 1410m, 1395m, 1375m, 1360m, 1280m, 1240m, 1160m, 1135m,
1090m, 1050m, 705s. 1H-NMR (CDCl3): 7.44 (s, NH); 7.35 – 7.05 (m, 15 arom. H, 2 NH); 5.95 (br. s,
NH); 5.09 (s, PhCH2O); 4.23 (t, J ¼ 8.3, CH(2)(Pro)); 3.90 – 3.60 (AB of ABX, JAB ¼ 17, CH2(Gly));
3.70 – 3.60, 3.30 – 3.10 (2m, CH2(5)(Pro)); 3.37, 3.08 (AB, JAB ¼ 13.7, PhCH2); 3.29 (s, MeN); 2.30 – 2.15,
1.95 – 1.85, 1.85 – 1.60 (3m, 1:1:2, CH2(3), CH2(4)(Pro)); 1.54, 1.45, 1.39, 1.17 (4s, 1:1:2 :1, 2 Me2C,
Me(Phe(2Me))). 13C-NMR (CDCl3): 174.8, 173.9, 173.1, 171.3, 169.6 (5s, 5 CO(amide)); 156.8 (s,
CO(urethane)); 145.9, 136.5, 136.2 (3s, 3 arom. C); 131.2, 128.9, 128.6, 128.3, 128.2, 127.2, 126.8, 126.6 (8d,
15 arom. CH); 67.1 (t, PhCH2O); 64.4 (d, CH(2)(Pro)); 58.9, 57.1 (2s, 2 C(2)(Aib), C(2)(Phe(2Me)));
48.8 (t, CH2(5)(Pro)); 44.2 (t, CH2(Gly)); 40.7 (t, PhCH2); 40.1 (q, MeN); 28.7, 26.2 (2t, CH2(3),
CH2(4)(Pro)); 27.7, 26.3, 25.6, 23.7, 20.9 (5q, 2 Me2C, Me(Phe(2Me))). ESI-MS: 749 (100, [M þ Na]þ),
620 (7, [M ꢀ N(Me)Ph]þ).

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Helvetica Chimica Acta – Vol. 97 (2014)
Table 4. Crystallographic Data for Compounds 6a, 8a, (S,S)-8e, 8f, (R,S,S)-8g, (R,S)-8h, and 10
6a
8a
(S,S)-8e
Crystallized from
Empirical formula
Formula weight
Crystal color, habit
Crystal dimensions [mm]
Temp. [K]
Crystal system
Space group
Z
CH2Cl2/Et2O/hexane
C35H43N5O6 · 0.5 C6H14 C31H41N5O8
672.84
colorless, prism
CH2Cl2/hexane
AcOEt/hexane
C36H43N5O8 · H2O
691.78
611.69
colorless, prism
colorless, irregular prism
0.23 ꢁ 0.25 ꢁ 0.45
0.33 ꢁ 0.41 ꢁ 0.43 0.25 ꢁ 0.27 ꢁ 0.50
173(1)
173(1)
monoclinic
P21/n
173(1)
orthorhombic
P212121
4
monoclinic
P21/n
4
4
Reflections for cell determination
22
25
25
2q Range for cell determination [8] 32 – 39
38 – 40
13.698(3)
13.979(4)
17.409(3)
90
23 – 33
15.347(4)
24.232(4)
10.028(4)
90
Unit cell parameters a [ꢅ]
11.642(3)
b [ꢅ]
c [ꢅ]
a [8]
15.135(3)
21.307(2)
90
b [8]
g [8]
97.294(12)
90
109.133(14)
90
90
90
V [ꢅ3]
3724(1)
1.200
0.0817
w/2q
3150(1)
1.290
0.0940
w/2q
3729(2)
1.232
0.0892
w
Dx [g cmꢀ3
]
m(MoKa) [mmꢀ1
Scan type
]
2q(max) [8]
55
60
55
Transmission factors (min; max)
Total reflections measured
–
–
9898
–
5535
9296
Symmetry-independent reflections
Reflections used [I > 2s(I)]
Parameters refined; restraints
8539
4362
484; 164
9182
6222
5389
3892
423; 0
0.0454
0.1262
0.0525; 0.4262
–
548; 202
0.0549
0.1682
0.0886; 0.8267
–
Final R(F) [I > 2s (I) reflections] 0.0577
wR(F2) (all data)
Weighting parameter (a; b)a)
Secondary extinction coefficient
Goodness-of-fit
0.1671
0.0830; 0
0.0022(6)
0.927
1.034
1.058
Final Dmax/s
D1 (max; min) [e ꢅꢀ3
0.000
0.29; ꢀ 0.24
0.000
0.37; ꢀ 0.18
0.000
0.79; ꢀ 0.26
]
a) wꢀ1 ¼ s2 (F2o ) þ (aP)2 þ bP, where P ¼ (Fo2 þ 2F2c )/3.
Suitable crystals for the X-ray crystal-structure determination were grown from AcOEt/MeOH/
hexane by slow evaporation of the solvent.
4. X-Ray Crystal-Structure Determination of 6a, 8a, (S,S)-8e, 8f, (R,S,S)-8g, (R,S)-8h, and 10 (see
Table 4 and Figs. 1 – 7)9). The measurements were performed using graphite-monochromated MoKa
radiation (l 0.7107 ꢅ) on a Rigaku AFC5R diffractometer fitted to a 12-kW rotating-anode generator.
The intensities were corrected for Lorentz and polarization effects. Empirical absorption corrections
based on azimuthal scans of several reflections [45] were applied in the cases of 8f and (R,S,S)-8g.
9
)
CCDC-989272 – 989278 contain the supplementary crystallographic data for this article. These data
can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.

Helvetica Chimica Acta – Vol. 97 (2014)
641
Table 5.
8f
(R,S,S)-8g
MeOH/AcOEt
(R,S)-8h
10
AcOEt/hexane
AcOEt/MeOH/hexane
MeOH
C37H46N6O7 · 0.33C6H14 C39H47N5O8 · C4H8O2 · 0.5 H2O C40H50N6O7 · 0.5 C6H14 · 0.25 H2O C27H39N5O8 · CH4O
715.33
colorless, plate
0.10 ꢁ 0.38 ꢁ 0.42
173(1)
orthorhombic
Pbca
810.94
colorless, plate
0.20 ꢁ 0.45 ꢁ 0.50
173(1)
orthorhombic
P212121
4
774.46
593.67
colorless, prism
0.18 ꢁ 0.33 ꢁ 0.50
173(1)
orthorhombic
P212121
4
colorless, hexagonal prism
0.36 ꢁ 0.36 ꢁ 0.38
173(1)
trigonal
P3121
8
12
25
25
25
17
21 – 25
25.465(4)
20.729(6)
14.536(5)
90
24 – 35
16.601(5)
26.785(8)
9.720(6)
90
20 – 27
15.706(6)
15.706(6)
60.971(6)
90
20 – 26
12.983(9)
10.249(5)
23.918(7)
90
90
90
90
90
90
90
120
90
7673(4)
1.239
4322(3)
1.246
13026(11)
1.185
3183(3)
1.239
0.0860
w
0.0895
w
0.0813
w
0.0928
w/2q
50
50
50
55
0.806; 1.000
8431
0.903; 1.000
5016
–
–
4118
29374
6768
4874
15416
4090
2204
3299
8853
2798
457; 0
554; 0
1087; 187
0.0787
0.2097
0.0641; 16.507
0.0014(2)
1.057
407; 36
0.0611
0.1975
0.0808; 2.3471
–
0.0701
0.1842
0.0781; 0
–
0.0446
0.1104
0.0392; 0.9522
–
0.790
1.013
1.085
0.000
0.000
0.002
0.000
0.32; ꢀ 0.25
0.19; ꢀ 0.21
0.42; ꢀ 0.38
0.33; ꢀ 0.36
Equivalent reflections, other than Friedel pairs for structures in non-centrosymmetric space groups, were
merged. The data collection and refinement parameters are compiled in Table 4, and views of the
molecules are shown in Figs. 1 – 7. The structures were solved by direct methods using SHELXS86 [46]
for 6a, 8a, (S,S)-8e, 8f, (R,S,S)-8g, and 10, which revealed the positions of all non-H-atoms, and SnB
[47]10) for (R,S)-8h, which revealed the positions of most of the non-H-atoms. In the latter case, the
remaining non-H-atoms were located in a subsequent difference electron-density map. For all structures,
10
)
We are grateful to R. Miller and C. Weeks of the Hauptman-Woodward Medical Research Institute,
Buffalo, who carried out the solution of the structure using what was, at the time in 1996, a new,
unreleased version of their program, SnB. Without their assistance, this structure determination
would not nave been successful.

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Helvetica Chimica Acta – Vol. 97 (2014)
the non-H-atoms were refined anisotropically. Unless indicated otherwise for individual structures below,
H-atoms on heteroatoms were generally placed in the positions indicated by difference electron-density
maps and their positions were allowed to refine together with individual isotropic displacement
parameters. All other H-atoms were placed in geometrically calculated positions and refined by using a
riding model, with Uiso(H) ¼ 1.2 Ueq(parent atom) (1.5 Ueq for Me groups).
In the case of 6a, the Ph ring of the middle Bn group of the molecule is disordered over two
orientations. The site occupation factor for the major conformation refined to 0.645(19). Similarity
restraints were applied to the chemically equivalent bond lengths and angles involving all disordered C-
atoms, while neighboring atoms within and between each conformation of the disordered ring were
restrained to have similar atomic displacement parameters. There are disordered solvent molecules
present in the crystal lattice, and these lie across centres of inversion, so that the ratio of peptide to
solvent molecules is 2 :1. The solvent appears to be a hexane molecule which has two orientations,
although the end atoms in one orientation may be further disordered. As the disorder could not be
modelled adequately, the contribution of the solvent molecules to the intensity data was removed by
using the SQUEEZE routine [48] of the PLATON program [49]. Omission of the solvent molecules
leaves two cavities of 217 ꢅ3 per unit cell. The number of electrons contributing to each void in the
structure was calculated by the SQUEEZE routine to be ca. 41 e, although this may be an underbound.
Allowing for one hexane molecule per cavity yields 50 e and this assumption was used in the subsequent
calculation of the empirical formula, formula weight, density, linear absorption coefficient and F(000).
In the case of (S,S)-8e, the terminal Bn group is disordered over two orientations with a site
occupation factor for the major conformation of 0.669(5). Similarity restraints were applied to the
chemically equivalent bond lengths and angles involving all disordered C-atoms, while neighboring
atoms within and between each conformation of the disordered group were restrained to have similar
atomic displacement parameters. The terminal COOMe group might also be slightly disordered, because
the atomic displacement parameters of these atoms, particularly O(24), are large. However, the disorder
could not be resolved reasonably and average positions were used for the atoms of this region of the
molecule. As a result, the C(24)¼O(24) bond appears to be somewhat shorter than it should be. The
asymmetric unit also contains two sites which are partially occupied by H2O molecules. Initial refinement
of the site occupation factors of these molecules gave values close to 0.5 and in the final refinement the
site occupation factors of the H2O molecules were set to 0.5. H-Atoms were not included for the H2O
molecules.
The crystals of 8f rapidly lost included solvent on standing in air. The asymmetric unit contains one
peptide molecule plus approximately one third of a highly disordered hexane molecule which sits across a
centre of inversion. As the disorder could not be modelled adequately, the contribution of the solvent
molecules to the intensity data was removed by using the SQUEEZE routine of the PLATON program.
Omission of the solvent molecules leaves four cavities of 128 ꢅ3 per unit cell. The number of electrons
contributing to each void in the structure was calculated by the SQUEEZE routine to be ca. 31 e.
Allowing for two thirds of a hexane molecule per cavity yields 30 e, which leads to a peptide/hexane ratio
in the structure of 3 :1, and this assumption was used in the subsequent calculations. The amide H-atoms
were placed in geometrically calculated positions and refined as described above for riding H-atoms.
In the case of (R,S,S)-8g, the asymmetric unit contains one peptide molecule, one AcOEt molecule,
and one partially occupied site for a H2O molecule. The site occupation factor for the O-atom of H2O was
initially refined and then fixed at 0.5. The H-atoms of the H2O molecule were included in the positions
obtained from a difference electron-density map and then constrained to ride on the parent O-atom with
Uiso(H) ¼ 1.5 Ueq(O).
The asymmetric unit of (R,S)-8h contains two peptide, one hexane, and one half of a H2O molecule,
where the latter sits on a two-fold axis. One of the peptide molecules has a disordered Ph ring, in the Z
group. Two sets of positions were defined for the atoms of this Ph ring and the site occupation factor of
the major conformation of the ring refined to 0.504(1). Similarity restraints were applied to the
chemically equivalent bond lengths and angles involving all disordered C-atoms, the CꢀC bonds were
restrained to 1.395(5) ꢅ, the two conformations of the ring were restrained to be planar, and neighboring
atoms within and between each conformation of the disordered Ph ring were restrained to have similar
atomic displacement parameters. The amide H-atoms were placed in geometrically calculated positions

Helvetica Chimica Acta – Vol. 97 (2014)
643
and refined as described above for riding H-atoms. The position of the symmetry-unique water H-atom
was refined with Uiso(H) ¼ 1.5 Ueq(O).
In the case of 10, the asymmetric unit contains one molecule of the peptide plus a disordered
molecule of MeOH. The site occupation factor of the major orientation of the MeOH molecule refined to
0.659(14). Neighboring atoms within and between each conformation of the disordered MeOH molecule
were restrained to have similar atomic displacement parameters. The amide and peptide OH H-atoms
were placed in geometrically calculated positions and refined as described above for riding H-atoms; for
the OH H-atom Uiso(H) ¼ 1.5 Ueq(O). The OH H-atoms of the MeOH molecules were not included in
the model.
The refinement of each structure was carried out on F2 by using full-matrix least-squares procedures,
which minimized the function Sw(F2o ꢀ Fc2 )2. A correction for secondary extinction was applied in the
case of 6a and (R,S)-8h. Neutral atom-scattering factors for non-H-atoms were taken from [50], and the
scattering factors for H-atoms were taken from [51]. Anomalous dispersion effects were included in Fc
[52]; the values for f’ and f’’ were those of [53]. The values of the mass attenuation coefficients are those
of [54]. All refinements were performed using SHELXL-2013 [55].
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Received March 11, 2014