Synthesis of a Derivative of the Peptaibol-Antibiotic Trichovirin I 1B by Means of the 'Azirine/Oxazolone Method'

Article abstract of DOI:10.1002/hlca.200390339

According to the earlier published synthesis of the C-terminal nonapeptide of Trichovirin I 1B, Z-Ser(^tBu)-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (5), the complete tetradecapeptide Z-Aib-Asn(Trt)-Leu-Aib-Pro-Ser( ^tBu)-Val-Aib-Pro-Aib-Leu-

Full text of DOI:10.1002/hlca.200390339

Helvetica Chimica Acta Vol. 86 (2003)
4093
Synthesis of a Derivative of the Peptaibol-Antibiotic Trichovirin I 1B by
Means of the −Azirine/Oxazolone Method×
by RoelandT. N. Luykx 1), Anthony Linden, and Heinz Heimgartner*
Organisch-chemisches Institut der Universit‰t Z¸rich, Winterthurerstrasse 190, CH-8057 Z¸rich
(phone: ‡4116354282; fax: ‡4116356836; e-mail: heimgart@oci.unizh.ch)
Dedicated to Professor Duilio Arigoni on the occasion of his 75th birthday
According to the earlier published synthesis of the C-terminal nonapeptide of Trichovirin I 1B, Z-Ser(tBu)-
Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (5), the complete tetradecapeptide Z-Aib-Asn(Trt)-Leu-Aib-Pro-Ser(t-
Bu)-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (11b), a protected Trichovirin I 1B, has now been prepared by means
of the −azirine/oxazolone method×. With the exception of the N-terminal Aib(1), all Aib residues were
introduced by the coupling of the corresponding amino or peptide acids with 2,2-dimethyl-2H-azirine-3-(N-
methyl-N-phenylamine) (1a) and methyl N-(2,2-dimethyl-2H-azirin-3-yl)-l-prolinate (3a) as the Aib and Aib-
Pro synthons, respectively. Single crystals of two segments, i.e., the N-terminal hexapeptide Z-Aib-Asn(Trt)-
Leu-Aib-Pro-Ser(tBu)-OMe (23) and the C-terminal octapeptide Z-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (17),
were obtained and their structures have been established by X-ray crystallography. Following the same strategy,
the C-terminal nonapeptide of Trichovirin I 4A, Z-Ala-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (26), was also
synthesized and characterized by X-ray crystallography.
1. Introduction. In the last couple of years, we have shown that 2,2-disubstituted
2H-azirin-3-amines of type 1 are ideal reagents for the introduction of a,a-disubstituted
glycines into peptide backbones [2 5]. The coupling occurs by reacting 1 with an N-
protected amino acid or peptide acid, leading to the amides of the corresponding di- or
oligopeptides of type 2 (Scheme 1). Selective hydrolysis of the terminal amide group
gives the elongated peptide acid, which is further used for the next azirine coupling or
for segment coupling by means of common peptide-synthesis methodology. Similarly,
the analogous reactions with N-(2H-azirin-3-yl)-l-prolinates of type 3 give peptides of
type 4 [6 9] (cf. also [10 12]). These results demonstrate that, e.g., 2H-azirin-3-
amines of type 1 and 3 with R1 ˆ R2 ˆ Me (1a and 3a, resp.) are suitable synthons for
Aib (a-aminoisobutyric acid, 2-methylalanine) and Aib-Pro, respectively, in peptide
synthesis.
A group of peptide antibiotics, containing a high proportion of Aib are the
peptaibols [13]. These membrane-active, linear peptides with 11 20 amino acid
residues are produced by some filamentous fungi and possess an N-acetylated terminus
and an a-amino alcohol at the C-terminus. The most-prominent member of this family
is Alamethicin F-30 [14]. Nowadays, it is well documented that peptaibols adopt helical
structures [15 17] essential for their biological activity [18]. Their relatively stable
1
)
From the Ph.D. thesis of R. T. N. L., Universit‰t Z¸rich, 2000; presented at the − Herbstversammlung 1999×
of the Swiss Chemical Society [1].

4094
Helvetica Chimica Acta Vol. 86 (2003)
Scheme 1
conformation is the result of the presence of a,a-disubstituted a-amino acids, as these
amino acids induce b-turns in short oligopeptides [5][6][19 23].
For many years, the chemical synthesis of peptaibols was a difficult task, as it was
hampered by the steric hindrance in the coupling steps of Aib and Aib-containing
peptide segments [24]. With the elaboration of new protocols and more-efficient
coupling reagents [25], enzymatic coupling [26], and solid-phase peptide synthesis with
Fmoc-protected Aib fluoride [27], the main drawbacks have been surmounted (for a
more-detailed survey, see [28]). In our laboratory, we have developed the so-called
−azirine/oxazolone method× to introduce Aib and Iva (isovaline, 2-ethylalanine)
residues into segments of Alamethicin F-30 [29], Trichotoxin A-50G [30], Antiamoe-
bin I [30][31], Trichovirin I 1B [6], and Zervamicin II-2 [8].
Recently, Br¸ckner and Koza [28] reported in detail the solution-phase preparation
of the 14-residue peptaibol antibiotic Trichovirin I 4A, which is one of the major
components of the microheterogeneous mixture of Trichovirin I [28]. These results
prompted us to publish the synthesis of a derivative of Trichovirin I 1B by the −azirine/
oxazolone method×. Trichovirins I 1B and I 4A differ in residue 6, which is serine in the
former and alanine in the latter case.
2. Results andDiscussion. According to our original strategy for the synthesis of
Trichovirin I 1B, the previously prepared C-terminal nonapeptide Z-Ser(tBu)-Val-Aib-
Pro-Aib-Leu-Aib-Pro-Leuol (5) [6] should be deprotected at the NH2 group ( ! 6) and
coupled sequentially with the tripeptide Z-Leu-Aib-Pro-OH (7) and the dipeptide Ac-
Aib-Asn-OH (8; Scheme 2).
The pentapeptide Z-Ser(tBu)-Val-Aib-Pro-Aib-OH [6], which by coupling with H-
Leu-Aib-Pro-Leuol (TBTU/HOBt) yielded 5, has been prepared by a modified
synthesis with a significantly improved yield of 67.1% [32] compared to 16.1% in [6].
The reaction of Z-Val-OH with 3a, base-catalyzed hydrolysis, and coupling with 1a gave
Z-Val-Aib-Pro-Aib-N(Me)Ph in 75.2% yield. The N-terminus was deprotected (H2/Pd/
C, MeOH), and coupling with Z-Ser(tBu)-OH (TBTU/HOBt), followed by hydrolysis
of the C-terminal amide group (6n HCl/THF 1:1; r.t.), yielded the desired

Helvetica Chimica Acta Vol. 86 (2003)
4095
Scheme 2
pentapeptide (89.2%). The coupling of 6 with 7 to give 9 and the deprotection to 10
proceeded in very good yields. Unfortunately, the final coupling of 10 and Ac-Aib-Asn-
OH with TBTU/HOBt to give 11a failed2).
Therefore, we elaborated a different route in which the N-terminal hexapeptide and
the C-terminal octapeptide should be combined in the final step3). The corresponding
coupling protocol is shown in Scheme 3. It is worth mentioning that cleavage of the
acid-labile AibÀPro bond was detected neither during saponification of the methyl
prolinate residues nor in the acid-catalyzed hydrolysis of the terminal amide group of
the tetrapeptide Z-Val-Aib-Pro-Aib-N(Me)Ph (14).
2.1. Synthesis of Z-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (17). The coupling of Z-
Val-OH with 3a gave the desired tripeptide 12 in 86% yield. Subsequent basic
hydrolysis with LiOH ¥ H2O led to Z-Val-Aib-Pro-OH (13) in 95% yield (Scheme 4).
The coupling of the following Aib residue was achieved by reaction with azirine 1a,
yielding the tetrapeptide Z-Val-Aib-Pro-Aib-N(Me)Ph (14; 92%). The cleavage of the
terminal N-methylanilido group with 3n HCl in THF/H2O 1:1 was carried out without
any problems, and the peptide acid 15 was obtained in 89% yield. The segment coupling
of 15 with H-Leu-Aib-Pro-Leuol (16) [6] by the use of the coupling reagent TBTU in
2
)
)
The corresponding coupling of the analogue of 10 with Ala instead of Ser(tBu) in position 4 was achieved
in 29 37% yield using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC)/HOBt in
DMF [28].
For other approaches, see [32].
3

4096
Helvetica Chimica Acta Vol. 86 (2003)
Scheme 3
Scheme 4
the presence of HOBt gave the crystalline octapeptide 17 in 66% yield4). By
crystallization from AcOEt/MeOH, single crystals suitable for X-ray analysis were
obtained. The crystal structure of 17 is shown in Fig. 15).
The OH group and each NH group of 17 are involved mostly in intramolecular H-
bonds that form a regular pattern along the peptide chain. The OH group and the
N(4)ÀH to N(16)ÀH groups interact with the amide O-atom seven atoms further
along the peptide backbone, i.e., the (i ‡ 3)th amino acid. Each of these interactions has
the graph-set motif [35] of S(10). This serves to maintain a fairly rigid helical
conformation. The remaining two NH groups, N(22)ÀH and N(25)ÀH, which are
unable to form such intramolecular interactions because of their position in the
backbone, form intermolecular H-bonds with the amide and hydroxy O-atoms,
4
)
The same coupling has been carried out by Br¸ckner and Koza in 72% yield with EDC/HOBt; the
segments had been prepared by stepwise coupling of the amino acids [28].
The crystal structure of 17 has also been published by Gessmann et al. [34] as its hemimethanol dihydrate.
5
)

Helvetica Chimica Acta Vol. 86 (2003)
4097
Fig. 1. ORTEP Plot [33] of the molecular structure of 17 (50% probability ellipsoids; arbitrary atom numbering;
thin lines represent intramolecular H-bonds)
respectively, at the opposite end of a neighboring molecule (graph sets: C(20) and
C(26), resp.). These interactions link the molecules into extended chains running
parallel to the y-axis (Table 1, Fig. 2). Each NH group from N(4)ÀH to N(16)ÀH also
forms a close contact to the next N-atom in the chain, i.e., the (i ‡ 1)th amino acid. These
weak H-bonds form in a −sideways× fashion (acute NÀH ¥¥¥ N angle) and might result
from the geometrical constraints placed on the conformation of the molecule by the
other H-bonding interactions. Such close contacts are often found in peptide chains.
Table 1. Intra- and Intermolecular H-Bonds in the Crystal Structure of 17 (D ˆ donor, A ˆ acceptor)
DÀH
Aa)
DÀH [ä]
H ¥¥¥ A [ä]
D ¥¥¥ A [ä]
DÀH ¥¥¥ A [8]
O(1)ÀH(1)
O(8)
0.85(4)
0.88(3)
0.87(3)
0.84(3)
0.86(3)
0.83(3)
0.87(3)
0.88(3)
0.87(3)
0.84(3)
0.86(3)
1.97(4)
2.09(3)
2.35(3)
2.29(3)
2.19(3)
2.18(3)
2.03(3)
2.38(3)
2.41(3)
2.34(3)
2.42(3)
2.809(3)
2.893(3)
3.138(4)
3.048(4)
2.921(3)
2.973(3)
2.892(4)
2.800(4)
2.767(4)
2.739(4)
2.839(4)
169(5)
153(3)
150(3)
151(3)
142(3)
160(3)
169(4)
110(2)
105(3)
110(2)
111(3)
N(4)ÀH(4)
O(11)
O(17)
O(20)
O(23)
O(5')
O(1')
N(7)
N(13)
N(16)
N(19)
N(10)ÀH(10)
N(13)ÀH(13)
N(16)ÀH(16)
N(22)ÀH(22)
N(25)ÀH(25)
N(4)ÀH(4)
N(10)ÀH(10)
N(13)ÀH(13)
N(16)ÀH(16)
3
2
1
2
a) Symmetry operator for primed atoms: À x, À ‡ y, 1 À z.
2.2. Synthesis of Z-Aib-Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-OMe (24). The coupling of
tripeptide Z-Leu-Aib-Pro-OH (18) [6] and the side-chain protected amino acid H-
Ser(tBu)-OMe by use of TBTU/HOBt led to the tetrapeptide Z-Leu-Aib-Pro-Ser(tBu)-
OMe (19) in a relatively low yield of 63% (Scheme 5). A possible explanation could be
the steric hindrance of the t-Bu group in the side chain of serine and the reduced

4098
Helvetica Chimica Acta Vol. 86 (2003)
Fig. 2. Crystal packing of 17 showing H-bonding interactions (uninvolved H-atoms omitted for clarity)
Scheme 5

Helvetica Chimica Acta Vol. 86 (2003)
4099
Fig. 3. ORTEP Plot [33] of the molecular structure of 23 (50% probability ellipsoids; arbitrary atom
numbering; thin lines represent intramolecular H-bonds)
conformational freedom of proline6). The following hydrogenolysis gave 20 in
quantitative yield, and the coupling with Z-Asn(Trt)-OH under standard conditions
led to the pentapeptide Z-Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-OMe (21) in 80% yield.
Steric interactions with the bulky trityl group in the side chain of asparagine are
believed to be responsible for the lower yield of the coupling.
The deprotection of the N-terminus of 21 by hydrogenolysis led to 22 in
quantitative yield. The subsequent coupling of 22 and Z-Aib-OH with TBTU/HOBt
under standard conditions resulted in the hexapeptide Z-Aib-Asn(Trt)-Leu-Aib-Pro-
Ser(tBu)-OMe (23) in 95% yield. Crystallization from CH2Cl2/hexane/EtOH provided
single crystals suitable for X-ray analysis (Fig. 3). The five-membered ring of the
proline residue is disordered in that C(35) equally occupies two sites corresponding to
alternate directions for the flap of an envelope conformation. The crystal lattice
contains large regions filled with solvent molecules, which appear to be disordered and
to only partially occupy their sites (see Exper. Part).
Similar to the C-terminal octapeptide 17, the N-terminal hexapeptide 23 shows a
helical conformation stabilized by intramolecular H-bonds (Table 2, Fig. 4). While
each NH group of the molecule acts as a donor for H-bonds, four of the interactions are
intramolecular H-bonds. Each NH group, from N(4)ÀH to N(13)ÀH, interacts with
the amide O-atom seven atoms further along the peptide backbone, and each of these
interactions has the graph-set motif [35] of S(10). Two b-turns of type I are found,
which are formed by the sequences Aib1-Asn2-Leu3-Aib4 and Asn2-Leu3-Aib4-Pro5,
followed by a b-turn of type II with the sequence Leu3-Aib4-Pro5-Ser(tBu)6 (sequence
numbering according to Trichovirin I). The N(16)ÀH forms an intramolecular H-bond
6
)
The corresponding coupling of 18 and H-Ala-OMe with EDC/HOBt gave 83% of Z-Leu-Aib-Pro-Ala-
OMe [28].

4100
Helvetica Chimica Acta Vol. 86 (2003)
Table 2. Intra- and Intermolecular H-Bonds in the Crystal Structure of 23 (D ˆ donor, A ˆ acceptor)
DÀH
Aa)
DÀH [ä]
H ¥¥¥ A [ä]
D ¥¥¥ A [ä]
DÀH ¥¥¥ A [8]
N(4) ÀH(4)
N(10)ÀH(10)
N(13)ÀH(13)
N(16)ÀH(16)
N(19)ÀH(19)
N(44)ÀH(44)
N(4)ÀH(4)
O(11)
O(17)
O(20)
O(44)
O(8')
O(5'')
N(7)
N(13)
N(16)
N(19)
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
0.88
2.05
2.19
2.12
2.20
1.99
2.08
2.41
2.36
2.39
2.42
2.899(4)
3.050(4)
2.960(4)
2.827(4)
2.856(4)
2.808(4)
2.795(4)
2.760(4)
2.766(4)
2.766(4)
161
167
160
128
169
140
107
108
106
104
N(10)ÀH(10)
N(13)ÀH(13)
N(16)ÀH(16)
1
2
1
2
a) Symmetry operators for singly (') and doubly ('') primed atoms: ‡ x, À À y, À z; and 1 À x, À y, z, resp.
Fig. 4. Crystal packing of 23 showing H-bonding interactions (uninvolved H-atoms omitted for clarity)
with the amide O-atom of the asparagine side chain (graph set: S(6)). The remaining
NH groups, N(19)ÀH and N(44)ÀH, which are unable to form intramolecular
interactions because of their position in the backbone, form intermolecular H-bonds

Helvetica Chimica Acta Vol. 86 (2003)
4101
with the amide O-atoms at the opposite end of different neighboring molecules (graph
sets: C(14) and C(16), resp.). The intermolecular interactions link the molecules into
infinite two-dimensional networks that lie parallel to the xy-plane (Fig. 4). Since all
NH groups are involved in H-bonds, it is unlikely that there are any donor interactions
with solvent molecules in the crystal. However, acceptor interactions cannot be excluded.
2.3. Synthesis of Z-Aib-Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-Val-Aib-Pro-Aib-Leu-Aib-
Pro-Leuol (11b). The base-catalyzed hydrolysis of the hexapeptide 23 gave Z-Aib-
Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-OH (24) in 97% yield, and the free amine H-Val-Aib-
Pro-Aib-Leu-Aib-Pro-Leuol (25) was obtained quantitatively from 17 by hydro-
genolysis under standard conditions. The final coupling of the two segments was carried
out with TBTU/HOBt in MeCN and gave the tetradecapeptide 11b in 41% yield
(Scheme 6).
Scheme 6
The protected tetradecapeptide 11b was thoroughly characterized by NMR
1
spectroscopy. H-NMR TOCSY and COSY Experiments allowed us to correlate all
H-atoms. However, it was not possible to determine the positions of the multiple amino
acids Aib, Leu, and Pro. On the basis of the (13C,1H)-correlated HSQC and HMBC 2D-
spectra, it was also possible to correlate all 13C-NMR signals. Unfortunately, additional
experiments, such as NOE and ROESY, did not lead to further information and,
therefore, the three-dimensional structure of 11a in solution could not be determined.
2.4. Synthesis of Z-Ala-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (26). Following the
strategy described in [6], the nonapeptide 26, which is the C-terminal nonapeptide of
Trichovirin I 4A, was synthesized (Scheme 7). The hydrolysis of Z-Ala-Val-OMe,
subsequent azirine coupling with 3a, and hydrolysis to Z-Ala-Val-Aib-Pro-OH (28)
were performed in an overall yield of 83%. The pentapeptide 29 was obtained in
quantitative yield by coupling 28 with the azirine 1a. Unexpectedly, the acid-catalyzed
hydrolysis of the C-terminal amide group under standard conditions was not successful.
The reaction could not be completed, and it was not possible to separate the starting
material from the product. Therefore, the mixture was used in the next step, the
coupling with H-Leu-Aib-Pro-Leuol (16) to obtain the nonapeptide 26 (33% yield). By
crystallization from EtOH/hexane, single crystals suitable for X-ray crystallography
were obtained (Fig. 5).

4102
Helvetica Chimica Acta Vol. 86 (2003)
Scheme 7
Fig. 5. ORTEP Plot [33] of the molecular structure of 26 (50% probability ellipsoids; arbitrary atom
numbering; thin lines represent intramolecular H-bonds)
The crystal structure of 26 could be solved, but the results were very poor. In
particular, the refinement of the terminal residues (Ala and Leuol) produced poor
results, and the atoms of these groups, as well as some of the Me groups, gave rise to
extremely large anisotropic displacement parameters. However, the overall backbone
conformation of the peptide was clearly defined. The conformations of the end groups
were less well-defined and, in particular, the positions of all atoms from the OH group

Helvetica Chimica Acta Vol. 86 (2003)
4103
to the first 5-membered ring should be taken as being very approximate only. Disorder
might be responsible for these problems, but no disordered model could be developed
successfully. It might be that the molecule is extremely flexible in the crystal lattice,
particularly at the ends of the chain. The nonapeptide 26 also adopts a helical
conformation stabilized by intramolecular H-bonds (Table 3). Each NH group, except
for N(25)ÀH and N(28)ÀH, acts as a donor for intramolecular H-bonds. The acceptor
atoms are the amide O-atoms that are seven atoms further along the peptide chain.
Each of these interactions has the graph-set motif [35] of S(10). N(25)ÀH does not
form any H-bond, while N(28)ÀH forms an intermolecular H-bond with the second
amide O-atom at the opposite end of a neighboring molecule. This is the sole
intermolecular interaction and serves to link the molecules into extended chains
running parallel to the [110] direction (graph-set motif: C(23)). The OH group forms
an intramolecular H-bond with the first amide N-atom to form a closed loop with the
graph set motif of S(6). Because of the uncertainty regarding the true structure of this
section of the molecule, this last interaction might be incorrect.
Table 3. Intra- and Intermolecular H-Bonds in the Crystal Structure of 26 (D ˆ donor, A ˆ acceptor)
DÀH
Aa)
DÀH [ä]
H ¥¥¥ A [ä]
D ¥¥¥ A [ä]
DÀH ¥¥¥ A [8]
O(1)ÀH(1)
N(4)
0.82
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.86
0.86
2.53
2.18
2.18
2.23
2.08
2.27
2.14
2.40
2.42
2.36
2.44
2.33
2.40
2.85(2)
105
156
158
172
160
164
162
107
107
106
107
108
106
N(4)ÀH(4)
O(11)
O(17)
O(20)
O(23)
O(29)
O(8')
N(7)
N(13)
N(16)
N(19)
N(25)
N(28)
2.990(8)
2.996(5)
3.085(6)
2.901(7)
3.104(7)
2.975(7)
2.776(9)
2.797(7)
2.733(6)
2.818(6)
2.726(7)
2.768(8)
N(10)ÀH(10)
N(13)ÀH(13)
N(16)ÀH(16)
N(22)ÀH(22)
N(28)ÀH(28)
N(4)ÀH(4)
N(10)ÀH(10)
N(13)ÀH(13)
N(16)ÀH(16)
N(22)ÀH(22)
N(25)ÀH(25)
a) Symmetry operator for primed atom: 1 ‡ x, 1 ‡ y, z.
3. Conclusions. We have prepared Trichovirin I 1B by an alternative method,
demonstrating the use of the −azirine/oxazolone method×. It has been shown that the
incorporation of a,a-disubstituted a-amino acids into peptide chains with the reactions
described above is very convenient. The combination of this method with the classical
TBTU/HOBt coupling protocol allowed the synthesis of the tetradecapeptide 11b
bearing protecting goups in the side chain of serine and asparagine, as well as the C-
terminal nonapeptide of Trichovirin I 4A. The X-ray crystal structures of all prepared
segments (cf. also [6][34]) showed helical structures formed by a sequence of b-turns.
The results from 2D-NMR measurements gave only some hints regarding the three-
dimensional structure in solution, but it seems very likely that these molecules possess
comparable secondary structures in the crystal and in solution, i.e., b-turns giving rise to
an overall helical structures.

4104
Helvetica Chimica Acta Vol. 86 (2003)
The backbones of the C-terminal nonapeptide of Trichovirin I 1B and Trichovirin I
4A have almost identical shapes. We expect that all of the seven different members of
the Trichovirin I family have very similar helical structures and are able to aggregate to
membrane-channel helix bundles. The chain length of the Trichovirins with only 14
residues is too short to break through the membrane of a cell. Usually, a peptide chain
with 18 20 amino acids is required [36]. Assuming that Trichovirins are helical, two
molecules in line could reach the length required to open up membrane channels.
We thank the analytical sections of our institute for recording various spectra. Financial support of the Swiss
National Science Foundation, F. Hoffmann-La Roche AG, Basel, and the Stiftung f¸r wissenschaftliche Forschung
an der Universit‰t Z¸rich is gratefully acknowledged.
Experimental Part
1. General. Ethyl acetate (AcOEt), distilled over K2CO3; hexane, distilled; CH2Cl2, distilled over CaCl2,
K2CO3, or CaH2; acetonitrile (MeCN), Fluka (puriss.); methanol (MeOH), Fluka (puriss.); ethanol (EtOH),
Kunzmann (99.8%); tetrahydrofurane (THF), distilled over Na/benzophenone; isobutyric acid chloride,
distilled; triethylamine (Et3N), Fluka (purum); amino acids, from Bachem and Fluka; 1-hydroxy-1H-
benzotriazole (HOBt), Fluka (purum); O-(1H-benzotriazol-1-yl)-N,N,N',N'-tetramethyluronium tetrafluoro-
borate (TBTU), Nova Biochem or Bachem. TLC on TLC-Alufoil Kieselgel 60 F254 (Merck). Column
chromatography (CC) on Kieselgel 60, 40 60 mm (Merck). M.p.: Mettler FP-5 or B¸chi B-540 apparatus;
uncorrected. IR Spectra: Perkin-Elmer 781 instrument, in KBr; absorption bands in cmÀ1 (s ˆ strong, m ˆ
middle, w ˆ weak, v ˆ very, sh ˆ shoulder). 1H-NMR Spectra (300, 500, or 600 MHz): Bruker ARX-300, Bruker
DRX-500, and Bruker AMX-600 instruments; chemical shifts d in ppm rel. to SiMe4 (ˆ0.0 ppm), coupling
constants J in Hz. 13C-NMR Spectra (75.5 or 150.9 MHz): Bruker ARX-300 and Bruker AMX-600 instruments;
chemical shifts d in ppm rel. to CDCl3 (ˆ 77.0 ppm); multiplicity from DEPT-spectra. ESI-MS: Finnigan
MAT TSQ-700 instrument.
2. General Procedures. Procedure A (Peptide coupling; GPA). One equiv. of N-protected amino acid or
N-protected peptide, and one equiv. of C-protected amino acid or C-protected peptide were dissolved in MeCN
(0.1 0.2m soln.). To this soln. were added TBTU and HOBt, one equiv. of each, and three equiv. of Et3N, and
the mixture was stirred at r.t. The progress of the reaction was followed by TLC. When the reaction was
complete, the soln. was extracted with a sat. aq. soln. of NaCl (3Â). Then, the solvent was evaporated in vacuo,
and the crude product was purified by CC.
Procedure B (Azirine coupling; GPB). To a soln. of 1 equiv. of N-protected amino acid or peptide in
CH2Cl2 was added one equiv. of a 2H-azirin-3-amine diluted in CH2Cl2. When the reaction was complete
(TLC), the solvent was removed in vacuo, and the crude product was purified by CC or recrystallization.
Procedure C (Base-catalyzed hydrolysis; GPC). A C-protected peptide was dissolved in THF/H2O/MeOH
3 :1:1, and three equiv. of LiOH ¥ H2O were added. When the reaction was complete, 1n HCl was added to adjust
the pH to 1, and the soln. was extracted with CH2Cl2. The org. phase was dried (MgSO4), and the solvent
removed. The crude product was dried in vacuo and used in the next step without further purification.
Procedure D (Hydrogenolysis; GPD). To a soln. of N-protected peptide in MeOH was added a small
amount of catalyst (Pd/C), and the mixture was stirred under H2-atmosphere. When the reaction was complete
(TLC), the soln. was filtered over Celite, and the solvent was evaporated. The crude product was dried in vacuo
and used without further purification.
Procedure E (Acid-catalyzed hydrolysis of peptide amides; GPE). To a soln. of the peptide amide in THF,
the same volume of an aq. 6n HCl soln. was added at 08. After stirring at r.t., the soln. was extracted with
CH2Cl2, and the org. solvent was evaporated. The crude product was dried in vacuo and used without further
purification.
3. Synthesis of Peptide Segments. 3.1. Synthesis of H-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (25). Z-Val-Aib-
Pro-OMe (12) [28]. According to GPB, Z-l-valine (111.2 mg, 0.44 mmol) and methyl N-(2,2-dimethyl-2H-
azirin-3-yl)-l-prolinate (3a, 84 mg, 0.44 mmol) were dissolved in CH2Cl2 (10 ml) and stirred at r.t. for 15 h. The
product was purified by CC (AcOEt/hexane 2 :1, Rf ˆ 0.24): 165 mg (86%) of 12. Colorless foam. IR: 3310m,
3064w, 2964m, 2876w, 1744vs, 1661vs, 1620vs, 1534vs, 1469m, 1455m, 1411s, 1363m, 1288m, 1241s, 1170s, 1094w,
1026m, 741w, 698w. 1H-NMR (300 MHz): 7.35 7.3 (m, 5 arom. H); 6.99 (s, NH(Aib)); 5.40 (d, J ˆ 9, NH(Val));

Helvetica Chimica Acta Vol. 86 (2003)
4105
5.12 (s, PhCH2); 4.55 4.5 (m, CH(a)(Pro)); 3.95 3.9 (m, CH(a)(Val)); 3.72 (s, MeO); 3.7 3.5 (m,
CH2(d)(Pro)); 2.15 2.0, 1.95 1.8(2 m, CH2(b,g)(Pro)); 1.65, 1.61 (2s, 2 Me(Aib)); 0.95, 0.91 (2d, J ˆ 6.8, 2
Me(Val)). 13C-NMR (75.5 MHz): 172.8, 172.1, 169.5 (3s, 3 CˆO); 156.4 (s, OÀCˆO); 128.5, 128.1, 128.0 (3d, 5
arom. CH); 66.9 (t, PhCH2); 60.9, 60.5 (2d, CH(a)(Val), CH(a)(Pro)); 57.1 (s, C(a)(Aib)); 52.2 (q, MeO); 48.0
(t, CH2(d)(Pro)); 31.4 (d, CH(b)(Val)); 27.8( t, CH2(b)Pro)); 25.8( t, CH2(g)(Pro)); 23.5, 23.0, 19.1, 17.8(4 q, 2
Me(Val), 2 Me(Aib)); one arom. C-atom could not be detected.
Z-Val-Aib-Pro-OH (13) [28]. According to GPC, 12 (155 mg, 0.34 mmol) and LiOH ¥ H2O (58mg,
1.38mmol) were dissolved in THF (5 ml), and the mixture was stirred at r.t. for 14 h: 143 mg (95%) of 13.
Colorless foam. IR: 3307m, 3065w, 2967s, 2877w, 1715vs, 1620vs, 1539s, 1470m, 1454m, 1418m, 1366w, 1343w,
1
1292m, 1237s, 1177m, 1027w, 739w, 698w. H-NMR (300 MHz): 7.35 7.30 (m, 5 arom. H); 6.93 (s, NH(Aib));
5.54 (d, J ˆ 8.2, NH(Val)); 5.15 5.1 (m, PhCH2); 4.6 4.5 (m, CH(a)(Val)); 3.96 (dd, J ˆ 6.7, 2.0, CH(a)(Pro));
3.8 3.7 ( m, 1 H of CH2(d)(Pro)); 3.52 3.4 (m, 1 H of CH2(d)(Pro), OH); 2.15 1.7 (m, CH2(b,g)(Pro),
CH(b)(Val)); 1.55, 1.53 (2s, 2 Me(Aib)); 0.96, 0.92 (2d, J ˆ 6.8, 2 Me(Val)). 13C-NMR (75.5 MHz): 174.1, 173.0,
170.9 (3s, 3 CˆO); 156.7 (s, OÀCˆO); 128.5, 128.3, 127.9 (3d, 5 arom. CH); 67.1 (t, PhCH2); 61.4, 60.4 (2d,
CH(a)(Val), CH(a)(Pro)); 57.0 (s, C(a)(Aib)); 48.2 (t, CH2(d)(Pro)); 30.7 (d, CH(b)(Val)); 27.3 (t,
CH2(b)(Pro)); 25.8( t, CH2(g)(Pro)); 24.4, 24.2, 19.2, 17.9 (4q, 2 Me(Val); 2 Me(Aib)); one arom. C-atom
could not be detected.
Z-Val-Aib-Pro-Aib-N(Me)Ph (14). According to GPB, 13 (127 mg, 0.29 mmol) was dissolved in CH2Cl2
(5 ml) and a soln. of 2,2-dimethyl-2H-azirin-3-(N-methyl-N-phenylamine) (1a, 51 mg, 0.29 mmol) in CH2Cl2
(5 ml) was added. The soln. was stirred at r.t. overnight, and the product was purified by CC (AcOEt/MeOH
21:1; Rf 0.3): 164 mg (92%) of 14. Colorless foam. 1H-NMR (300 MHz): 7.45 (s, NH(Aib)); 7.35 7.2 (m, 10
arom. H); 6.88 (s, NH(Aib)); 5.43 (d, J ˆ 8.5, NH(Val)); 5.15, 5.08 (AB, J ˆ 12.1, PhCH2); 4.49 (t, J ˆ 6.4,
CH(a)(Val)); 3.92 (dd, J ˆ 6.4, 2.1, CH(a)(Pro)); 3.51 3.3 (m, CH2(d)(Pro)); 3.33 (s, MeN); 2.15 1.5 (m,
CH2(b,g)(Pro), CH(b)(Val)); 1.50, 1.47, 1.43, 1.40 (4s, 4 Me(Aib)); 0.96, 0.91 (2d, J ˆ 6.8, 2 Me(Val)). 13C-NMR
(75.5 MHz): 173.6, 171.8, 170.7 (3s, 4 CˆO); 156.5 (s, OÀCˆO); 145.4, 136.1 (2s, 2 arom. C); 129.0, 128.6, 128.3,
128.1, 127.4, 126.8 (6d, 10 arom. CH); 67.2 (t, PhCH2); 62.0, 60.7 (2 d, CH(a)(Pro), CH(a)(Val)); 57.3, 57.0 (2s, 2
C(a)(Aib)); 48.0 (t, CH2(d)(Pro)); 40.2 (q, MeN); 30.0 (d, CH(b)(Val)); 27.9 (t, CH2(b)(Pro); 26.3 (t,
CH2(g)(Pro)); 25.6, 25.5, 25.4, 24.1 (4q, 4 Me(Aib)); 19.4, 17.8(2 q, 2 Me(Val)). ESI-MS (MeOH ‡ NaI): 630
‡
([M ‡ Na] ).
Z-Val-Aib-Pro-Aib-OH (15) [28]. According to GPE, a soln. of 14 (721 mg, 1.39 mmol) in THF (10 ml)
was cooled in an ice bath, treated with aq. 6n HCl (10 ml) and stirred at r.t. for 3 d: 552 mg (89%) of 15.
Colorless foam. 1H-NMR (300 MHz): 7.72 (s, NH); 7.4 7.3 (m, 5 arom. H); 7.14 (br. s, NH); 5.56 (br. s, NH);
5.16, 5.09 (AB, J ˆ 12.2, PhCH2); 4.50 (t, J ˆ 7.0, CH(a)(Val)); 4.1 3.9 (m, CH(a)(Pro)); 3.8 3.0 ( m,
CH2(d)(Pro)); 2.2 1.6 (m, CH2(b,g)(Pro), CH(b)(Val)); 1.6 1.4 (m, 4 Me(Aib)); 0.96, 0.91 (2d, J ˆ 6.8, 2
Me(Val)). 13C-NMR (75.5 MHz): 175.9, 172.7, 172.4, 171.3 (4s, 4 CˆO); 156.7 (s, OÀCˆO); 136.3 (s, 1 arom.
C); 128.4, 128.2, 127.9 (3d, 5 arom. CH); 67.8( t, PhCH2); 62.4 (d, CH(a)(Pro), CH(a)(Val)); 61.8( t,
CH2(d)(Pro)); 56.9 (s, 2 C(a)(Aib)); 25.8, 25.5 (2t, CH2(b,g)(Pro)); 24.9, 24.6 (2q, 4 Me(Aib)); 19.3 (q, 2
Me(Val)).
Z-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (17) [28]. According to GPA, 15 (239 mg, 0.46 mmol), H-Leu-Aib-
Pro-Leuol (16) [6][28] (190 mg, 0.46 mmol), TBTU (148 mg), HOBt (71 mg), and Et3N (0.25 ml) were
dissolved in MeCN (25 ml), and the mixture was stirred at r.t. for 1 d. After workup, the crude product was
purified by CC (AcOEt/MeOH 9 :1; Rf 0.31): 278mg (66%) of 17. Crystallization from AcOEt/MeOH gave
single crystals suitable for an X-ray crystal-structure determination. M.p.: dec. above 1158. 1H-NMR (600 MHz,
2D): 7.74, 7.72, 7.66 (3s, 3 NH(Aib)); 7.46 (d, J ˆ 9.1, NH(Leuol)); 7.40 (br. s, NH(Val)); 7.4 7.0 (m, 5 arom. H);
6.60 (d, J ˆ 7.1, NH(Leu)); 5.16, 5.08( AB, J ˆ 12.3, PhCH2); 4.35 4.3 (m, CH(a)(Val), CH(a)(Pro)); 4.2 4.0
(m, CH(a)(Pro), CH(a)(Leuol)); 3.96 (t, J ˆ 7.3, CH(a)(Leu)); 3.8 3.7 ( m, 1 H of CH2(d)(Pro)); 3.6 3.5 (m,
CH2O(Leuol), 2 H of CH2(d)(Pro)); 3.4 3.3 (m, 1 H of CH2(d)(Pro)); 2.3 2.2 (m, 2 CH2(g)(Pro)); 2.2 2.1
(m, CH2(b)(Leu)); 2.0 1.9 (m, 1 H of CH2(b)(Pro)); 1.9 1.8( m, 1 H of CH2(b)(Pro)); 1.8 1.6 ( m,
CH2(b)(Pro), CH(b)(Val)); 1.6 1.5 (m, CH2(b)(Leuol)); 1.55, 1.51, 1.48, 1.46, 1.45, 1.43 (6s, 6 Me(Aib)); 1.2
1.1 (m, CH(g)(Leuol)); 1.1 1.0 (m, CH(g)(Leu)); 0.98, 0.97, 0.90, 0.87 (4d, J ˆ 6.6, 2 Me(Leu), 2 Me(Leuol));
0.88, 0.84 (2d, 2 Me(Val)). 13C-NMR (75.5 MHz): 175.3, 173.5, 173.3, 173.2, 172.9, 172.8, 172.3 (7s, 7 CˆO);
156.9 (s, OÀCˆO); 136.6 (s, 1 arom. C); 128.4, 128.0, 127.8 (3d, 5 arom. CH); 66.8( t, CH2O); 65.3 (t, PhCH2));
64.4, 63.4, 61.7 (3d, CH(a)(Val), 2 CH(a)(Pro)); 56.9, 56.5, 56.4 (3s, 3 C(a)(Aib)); 51.8, 49.9 (2d, CH(a)(Leu),
CH(a)(Leuol)); 48.7, 48.4 (2t, 2 CH2(d)(Pro)); 40.0, 39.0 (2t, CH2(b)(Leu), CH2(b)(Leuol)); 29.0, 28.6 (2t, 2
CH2(b)(Pro)); 27.1 (q, Me); 26.1 (t, 2 CH2(g)(Pro)); 25.7, 25.6 (2q, 2 Me); 24.8, 24.7 (2d, CH(g)(Leu),

4106
Helvetica Chimica Acta Vol. 86 (2003)
CH(g)(Leuol)); 23.8, 23.6, 23.1, 23.0, 22.1, 20.5, 19.2, 18.4 (8q, 8Me). ESI-MS (MeOH ‡ NaI): 936 (100, [M ‡
‡
Na] ).
H-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (25). According to GPD, 17 (221 mg, 0.24 mmol) was dissolved in
MeOH (17 ml), Pd/C was added, and the mixture was stirred under H2-atmosphere at r.t. overnight. After
1
workup, 25 was obtained in quant. yield (188 mg) as a sticky foam. H-NMR (300 MHz): 8.05 (s, NH); 7.98 (s,
NH); 7.68( s, NH); 7.4 7.3 (m, NH); 4.45 4.3 (m, 2 CH(a)); 4.52 (dd, J ˆ 7.9, 1.6, CH(a)); 4.1 4.0 (m, CH(a));
3.85 3.7 (m, CH(a), CH2(d)(Pro)); 3.55 3.4 (m, CH2(d)(Pro)); 3.4 3.3, 3.3 3.2 (2m, CH2O(Leuol)); 2.5
2.2, 2.1 1.5 (2m, CH2(b,g)(Pro), CH2(b)(Leu), CH2(b)(Leuol), CH(g)(Leu), CH(g)(Leuol)); 1.55, 1.51, 1.47,
1.46, 1.45 (5s, 6 Me(Aib)); 1.3 1.1 (m, CH(b)(Val)); 1.0 0.8( m, 2 Me(Leu), 2 Me(Leuol), 2 Me(Val)).
3.2. Synthesis of Z-Aib-Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-OH (24). Z-Leu-Aib-Pro-Ser(tBu)-OMe (19).
According to GPA, Z-Leu-Aib-Pro-OH (18, 853 mg, 1.90 mmol), H-Ser(tBu)-OMe ¥ HCl (400 mg, 1.9 mmol),
TBTU (321 mg), HOBt (153 mg), and Et3N (1 ml) were dissolved in MeCN (15 ml), and the soln. was stirred at
r.t. for 1 d. The crude product was purified by CC (AcOEt/MeOH 25 :1; Rf 0.3): 730 mg (63%) of 19. Colorless
foam. IR: 3294m, 2972s, 2874w, 1720vs, 1654vs, 1534vs, 1470s, 1405s, 1364s, 1246s, 1098m, 1049m, 1027m, 740w,
698w. 1H-NMR (300 MHz): 7.35 7.3 (m, 5 arom. H); 6.98(br. s, 2 NH); 5.2 5.1 (m, PhCH2); 4.65 4.5 (m,
CH(a)(Pro), CH(a)(Ser)); 4.2 4.1 (m, CH(a)(Leu)); 3.8 3.75 ( m, 1 H of CH2(d)(Pro)); 3.71 (s, MeO);
3.65 3.5 (m, 1 H of CH2(d)(Pro), CH2(b)(Ser)); 2.05 1.6 (m, CH2(b,g)(Pro), CH2(b)(Leu), CH(g)(Leu));
1.56 (s, 2 Me(Aib)); 1.13 (s, Me3C); 0.95 0.9 (m, 2 Me(Leu)). 13C-NMR (75.5 MHz): 172.1, 171.6, 170.8(3 s, 4
CˆO); 156.1 (s, OÀCˆO); 136.1 (s, 1 arom. C); 128.4, 128.1, 127.9 (3d, 5 arom. CH); 73.2 (s, Me3C); 66.9 (t,
CH2(b)(Ser)); 61.9 (d, CH(a)(Ser)); 61.5 (t, CH2(d)(Pro)); 57.0 (s, C(a)(Aib)); 53.5 (d, CH(a)(Pro)); 52.9 (d,
CH(a)(Leu)); 52.0 (q, MeO); 48.0 (t, CH2(g)(Pro)); 41.2 (t, CH2(b)(Leu)); 28.5 (q, 1 Me(Aib)); 27.2 (q, Me3C);
24.5 (d, CH(g)(Leu)); 24.1 (q, 1 Me(Aib)); 22.8, 21.8 (2q, 2 Me(Leu)). ESI-MS (MeOH ‡ NaI): 627 ([M ‡
‡
Na] ).
H-Leu-Aib-Pro-Ser(tBu)-OMe (20). According to GPD, 19 (675 mg, 1.11 mmol) was dissolved in MeOH
(10 ml), and a small amount of Pd/C was added. The mixture was stirred under H2 at r.t. overnight: 552 mg
1
(quant.) of 20. Colorless foam. H-NMR (300 MHz): 7.83 (s, NH); 7.12 (br. s, NH); 4.6 4.5 (m, CH(a)(Ser),
CH(a)(Pro)); 3.64 (s, MeO); 3.7 3.5, 3.3 3.2 (2m, CH2(d)(Pro), CH2(b)(Ser), CH(a)(Leu)); 2.1 1.6 (m,
CH2(b,g)(Pro), CH2(b)(Leu)); 1.52, 1.49 (2s, 2 Me(Aib)); 1.3 1.2 (m, CH(g)(Leu)); 1.07 (s, Me3C); 0.89, 0.84
(2d, J ˆ 6.2, 2 Me(Leu)). 13C-NMR (75.5 MHz): 174.4, 172.4, 171.8, 170.8 (4s, 4 CˆO); 73.1 (s, Me3C)); 61.8( d,
CH(a)(Ser)); 61.5 (t, CH2(b)(Ser)); 56.3 (s, C(a)(Aib)); 53.4, 53.0 (2d, CH(a)(Pro), CH(a)(Leu)); 52.0 (q,
MeO); 47.9 (t, CH2(d)(Pro)); 43.9 (t, CH2(b)(Leu)); 27.2 (q, Me3C); 25.5 (t, CH2(b,g)(Pro)); 24.7 (d,
CH(g)(Leu)); 24.8, 24.5, 23.2, 21.2 (4q, 2 Me(Aib), 2 Me(Leu)).
Z-Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-OMe (21). According to GPA, 20 (525 mg, 1.12 mmol), Z-Asn(Trt)-
OH (568mg, 1.12 mmol), TBTU (359 mg), HOBt (171 mg), and Et 3N (0.6 ml) were dissolved in MeCN
(50 ml). The soln. was stirred at r.t. for 1 d. After workup, the crude product was purified by CC (AcOEt/MeOH
25 :1; Rf 0.4): 828 mg (80%) of 21. Colorless foam. IR: 3302m, 2971m, 1658vs, 1514s, 1448m, 1407m, 1365m,
1233s, 1084m, 752w, 700s. 1H-NMR (300 MHz): 7.3 7.0 (m, 20 arom. H); 6.38, 6.18 (2 br. s, 2 NH); 4.98( s,
PhCH2); 4.55 4.4 (m, 2 CH(a)); 4.3 4.2 (m, 2 CH(a)); 3.6 3.5 (m, MeO, CH2(d)(Pro)); 3.33 (br. s, 1 H of
CH2(b)(Asn)); 2.83 (d, J ˆ 5.1, 1 H of CH2(b)(Asn)); 2.0 1.3 (m, CH2(b,g)(Pro), CH2(b)Leu), CH(g)(Leu));
1.27, 1.07 (2s, 2 Me(Aib)); 1.06 (s, Me3C); 0.80, 0.74 (2d, J ˆ 6.4, 2 Me(Leu)). 13C-NMR (75.5 MHz): 172.3, 171.0,
170.5, 169.5 (4s, 6 CˆO); 155.9 (s, OÀCˆO); 144.0 (s, 3 arom. C); 135.9 (s, 1 arom. C); 128.5, 128.4, 128.1, 127.8,
127.0 (5d, 20 arom. CH); 73.1 (s, Me3C)); 70.8( s, Ph3C); 67.0 (t, PhCH2); 61.9 (d, CH(a)(Ser)); 61.5 (t,
CH2(b)(Ser), CH2(b)(Asn)); 56.7 (s, C(a)(Aib)); 53.1, 52.1 (2d, CH(a)(Pro), CH(a)(Leu), CH(a)(Asn)); 51.9
(q, MeO); 47.8( t, CH2(d)(Pro)); 39.9 (t, CH2(b)(Leu)); 38.6 (t, CH2(b,g)(Pro)); 27.3 (q, Me3C); 25.2 (q, 1
Me(Aib)); 24.6 (d, CH(g)(Leu)); 24.2, 22.9, 21.3 (3q, 1 Me(Aib), 2 Me(Leu)). ESI-MS (MeOH ‡ NaI): 983
‡
(100, [M ‡ Na] ), 519 (41, [M ‡ 2Na]2‡).
H-Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-OMe (22). According to GPD, to a soln. of 21 (298mg, 0.31 mmol) in
MeOH (10 ml) was added a small amount of Pd/C. The mixture was stirred under H2-atmosphere at r.t.
overnight: 256 mg (quant.) of 22. Colorless foam. 1H-NMR (300 MHz, CD3OD): 7.35 7.2 (m, 15 arom. H);
4.6 4.3 (m, 3 CH(a)); 4.2 4.1 (m, CH(a)); 3.8 3.3 ( m, CH2(d)(Pro), CH2(b)(Ser)); 3.72 (s, MeO); 2.9 2.6
(m, CH2(b)(Asn)); 2.25 1.5 (m, CH2(b,g)(Pro), CH2(b)(Leu), CH2(g)(Leu)); 1.45, 1.27 (2s, 2 Me(Aib)); 1.16
(s, Me3C); 1.0 0.8( m, 2 Me(Leu)).
Z-Aib-Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-OMe (23). According to GPA,
a mixture of 22 (344 mg,
0.42 mmol), Z-Aib-OH (99 mg, 0.42 mmol), TBTU (134 mg), HOBt (64 mg), and Et3N (0.17 ml) in MeCN
(15 ml) was stirred at r.t. for 1 d. After workup, the crude product was purified by CC (AcOEt/MeOH 35 :1; Rf
0.26): 413mg (95%) of 23. Colorless crystals. Recrystallization from CH2Cl2/EtOH/hexane gave single crystals

Helvetica Chimica Acta Vol. 86 (2003)
4107
suitable for an X-ray crystal-structure determination. M.p.: dec. above 988. IR: 3308s, 3058w, 3032w, 2973m,
2873w, 1742w, 1654vs, 1516vs, 1470m, 1449m, 1385m, 1364m, 1266s, 1196m, 1147w, 1089m, 1026w, 1002w, 914w,
750w, 700m. 1H-NMR (500 MHz, 2D, TOCSY, COSY): 7.88 (d, J ˆ 4, NH(Asn)); 7.62 (d, J ˆ 7.8, NH(Ser)); 7.58
(d, J ˆ 8.3, NH(Leu)); 7.3 7.1 (m, 15 arom. H); 7.1 7.0 (m, 5 arom. H); 5.60 (s, NH(Aib)); 4.95, 4.84 (AB, J ˆ
12.2, PhCH2); 4.55 4.4 (m, CH(a)(Pro), CH(a)(Ser)); 4.35 4.3 (m, CH(a)(Leu), CH(a)(Asn)); 3.7 3.6 (m,
CH2(b)(Ser)); 3.55 3.5, 3.4 3.3 (2m, CH2(d)(Pro)); 2.9 2.8( dd, J ˆ 14.4, 4.0, 1 H of CH2(b)(Asn)); 2.7 2.6
(dd, J ˆ 14.4, 9.2, 1 H of CH2(b)(Asn)); 2.05 1.9 (m, 1 H of CH2(b)(Pro)); 1.9 1.8( m, 1 H of CH2(b)(Pro),
1 H of CH2(g)(Pro)); 1.75 1.7 (m, 1 H of CH2(b)(Leu)); 1.7 1.5 (m, 1 H of CH2(g)(Pro), 1 H of
CH2(b)(Leu), CH(g)(Leu)); 1.42, 1.41, 1.38, 1.21 (4s, 4 Me(Aib)); 1.08( s, Me3C)); 0.79, 0.74 (2d, J ˆ 6.5,
Me(Leu)). 13C-NMR (125.7 MHz): 175.5, 172.3, 172.1, 171.5, 171.0, 169.4 (6s, 7 CˆO); 156.0 (s, OÀCˆO); 144.0
(s, 3 arom. C); 135.4 (s, 1 arom. C); 128.5, 128.2, 127.9, 127.6, 127.0 (5d, 20 arom. CH); 73.0, 70.9 (2s, Me3C,
Ph3C); 67.0 (t, PhCH2); 61.9 (d, CH(a)(Pro or Ser)); 61.5 (t, CH2(b)(Asn)); 56.7, 56.6 (2s, 2 C(a)(Aib)); 53.2 (d,
CH(a)(Ser or Pro)); 52.4 (d, CH(a)(Asn or Leu)); 51.8( d and q, CH(a)(Leu or Asn), MeO); 47.9 (t,
CH2(d)(Pro)); 39.5 (t, CH2(b)(Leu)); 37.3 (t, CH2(b)(Asn)); 28.2 (t, CH2(b)(Pro)); 27.3 (q, Me3C); 26.4, 25.8
(2q, 2 Me(Aib)); 25.7 (t, CH2(g)(Pro)); 24.7 (d, CH(g)(Leu)); 24.9, 23.7 (2q, 2 Me(Aib)); 23.2, 20.7 (2q, 2
‡
Me(Leu)). ESI-MS (MeOH ‡ NaI): 1068(100, [ M ‡ Na] ), 546 (17, [M ‡ 2Na]2‡).
Z-Aib-Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-OH (24). According to GPC, a soln. of 23 (184 mg, 0.18 mmol) and
LiOH ¥ H2O (16 mg, 0.52 mmol) in THF/MeOH/H2O 3 :1 :1 (10 ml) was stirred at r.t. for 2 h: 177 mg (97%) of
24. Colorless foam. IR: 3301s, 2973s, 1650vs, 1525vs, 1194s, 1090m, 914w, 752m, 700s. ESI-MS (MeOH/H2O ‡
‡
‡
‡
AcOH): 964 (22, [M À Z ‡ Ac ‡ Na] ), 942 (38, [M À Z ‡ Ac ‡ 1] ), 684 (100, [M À Z À Trt ‡ 1] ).
3.3. Synthesis of Z-Aib-Asn(Trt)-Leu-Aib-Pro-Ser(tBu)-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (11b): Ac-
cording to GPA, a mixture of 24 (200 mg, 0.19 mmol), 25 (151 mg, 0.19 mmol), TBTU (63 mg), HOBt (30 mg),
and Et3N (0.08ml) in MeCN (10 ml) was stirred at r.t. for 2 d. The crude product was purified by CC (AcOEt/
MeOH/hexane 7:1:1.5): 143 mg (41%) of 11b. Pale yellow foam. IR: 3305vs, 2957s, 2872m, 1650vs, 1536vs,
1470s, 1412s, 1364s, 1287w, 1172m, 1098w, 8 77w, 754w, 701w. 1H-NMR (600 MHz, CD3OD): 7.3 7.1 (m, 20 arom.
H); 4.5 4.3 (m, CH(a)(Pro), 2 CH(a)(Leu), CH(a)(Ser), CH(a)(Asn)); 4.27 (t, J ˆ 8.0, CH(a)(Pro)); 4.20 (t,
J ˆ 8.5, CH(a)(Pro)); 4.11 (dd, J ˆ 4.5, 8.7, CH(a)(Val)); 4.0 3.9 (m, CH(a)(Leuol)); 3.9 3.8( m, 2 H of 3
CH2(d)(Pro)); 3.8 3.7 ( m, CH2(b)(Ser)); 3.65 3.4 (m, 3 H of 3 CH2(d)(Pro), CH2O(Leuol)); 3.4 3.3 (m, 1 H
of 3 CH2(d)(Pro)); 2.96 (dd, J ˆ 14.9, 6.7, 1 H of CH2(b)(Asn)); 2.77 (dd, J ˆ 14.9, 6.2, 1 H of CH2(b)(Asn));
2.4 2.2 (m, 3 H of 3 CH2(b)(Pro)); 2.15 2.0 (m, 1 H of 3 CH2(b)(Pro), CH(b)(Val)); 2.0 1.5 (m, 3
CH2(b)(Pro), 3 CH2(g)(Pro), 2 CH2(b)(Leu), 2 CH(g)(Leu), CH2(b)(Leuol)); 1.55 1.4 (m, 10 Me(Aib));
1.4 1.3 (m, CH(g)(Leuol)); 1.22 (s, Me3C); 1.0 0.8( m, 2 Me(Val), 4 Me(Leu), 2 Me(Leuol)). 13C-NMR
(151 MHz, CD3OD): 177.9, 175.8, 175.6, 175.6, 175.6, 175.2, 175.1, 174.9, 174.9, 174.5, 173.3, 173.1, 171.9 (13s, 15
CˆO); 159.7 (s, OÀCˆO); 146.0 (s, 3 arom. C); 136.2 (s, 1 arom. C); 130.2, 129.0, 128.9, 128.8, 128.0, 127.9 (6d,
20 arom. CH); 71.8( s, Me3C); 66.2 (d, CH(a)(Pro)); 65.9 (t, CH2O(Leuol), PhCH2); 65.3, 64.4 (2d, 2
CH(a)(Pro)); 62.4 (t, CH2(b)(Ser)); 61.7 (d, CH(a)(Val)); 58.0 (d, CH(a)(Ser)); 53.4, 53.2 (2d, 2 CH(a)(Leu),
CH(a)(Asn)); 51.2 (d, CH(a)(Leuol)); 50.8, 50.3, 50.0 (3t, 3 CH2(d)(Pro)); 41.3, 41.0 (2t, 2 CH2(b)(Leu)); 40.6
(t, CH2(b)(Leuol)); 40.0 (t, CH2(b)(Asn)); 31.9 (d, CH(b)(Val)); 30.4 (t, CH2(b)(Pro)); 28.1 (q, Me3C); 27.7 (q,
1 Me(Aib)); 26.8, 26.1 (2t, CH2(b)(Pro)); 26.3, 26.2, 26.1 (3t, CH2(g)(Pro)); 24.4, 24.2, 24.1, 24.0, 23.9, 23.8, 23.7,
23.6 (8q, 9 Me(Aib)); 22.2, 21.6, 21.0, 20.2, 19.9 (5q, 2 Me(Val), 4 Me(Leu) 2 Me(Leuol)). Some signals could
‡
not be detected. ESI-MS (MeOH ‡ NaI): 1757 (8, [M À Me3C ‡ Na] ), 879 (100, [M À Me3C ‡ Na ‡ 1]2‡).
3.4. Synthesis of Z-Ala-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (26). Z-Ala-Val-Aib-Pro-OMe (27). Accord-
ing to GPB, Z-Ala-Val-OH (192 mg, 0.60 mmol) and 3a (117 mg, 0.60 mmol) were dissolved in CH2Cl2 (10 ml),
and the mixture was stirred at r.t. overnight. Evaporation of the solvent gave crude 27 as a white foam, which was
used for the next reaction without purification.
Z-Ala-Val-Aib-Pro-OH (28). According to GPC, a mixture of 27 (308mg, 0.59 mmol) and LiOH ¥ H 2O
(99 mg, 2.38mmol) was stirred at r.t. overnight: 250 mg (83%) of 28. Colorless crystals. M.p.: 79.5 80.08. IR:
3296s, 3066m, 2970s, 1715vs, 1651vs, 1539s, 1469m, 1454m, 1418s, 1365m, 1342m, 1241s, 1181m, 1116w, 1071w,
1027w, 928w, 777w, 740w, 698w. 1H-NMR (300 MHz): 7.4 7.3 (m, 5 arom. H); 6.93 (s, NH(Aib)); 6.78( d, J ˆ 7.3,
NH(Val)); 5.51 (br. s, NH(Ala)); 5.17, 5.09 (AB, J ˆ 12.4, PhCH2); 4.58( t, CH(a)(Val)); 4.25 4.1 (m,
CH(a)(Ala), CH(a)(Pro)); 3.7 3.5 (m, CH2(d)(Pro), OH); 2.3 1.8( m, CH2(b,g)(Pro), CH(b)(Val)); 1.51,
1.47 (2s, 2 Me(Aib)); 1.40 (d, J ˆ 7.3, Me(Ala)); 0.92, 0.82 (2d, J ˆ 6.8, 2 Me(Val)).
Z-Ala-Val-Aib-Pro-Aib-N(Me)Ph (29). According to GPB, a soln. of 28 (164 mg, 0.33 mmol) and 1a
(57 mg, 0.33 mmol) in CH2Cl2 (10 ml) was stirred at r.t. for 20 h: 220 mg (quant.) of 29, which was used in the
1
next step without purification. Colorless solid. H-NMR (300 MHz): 7.65 (br. s, NH); 7.4 7.1 (m, 10 arom. H,
NH(Aib)); 6.59 (d, J ˆ 7.7, NH(Val)); 5.75 (d, J ˆ 4, NH(Ala)); 5.19, 5.08( AB, J ˆ 12.4, PhCH2); 4.55 4.5 (m,

4108
Helvetica Chimica Acta Vol. 86 (2003)
CH(a)(Val)); 4.2 4.0 (m, CH(a)(Ala), CH(a)(Pro)); 3.55 3.3 (m, CH2(d)(Pro)); 3.36 (s, MeN); 2.35 1.7 (m,
CH2(b,g)(Pro), CH(b)(Val)); 1.54, 1.50, 1.43, 1.41 (4s, 4 Me(Aib)); 1.40 (d, J ˆ 7.3, Me(Ala)); 0.93, 0.83 (2d, J ˆ
6.7, 2 Me(Val)). 13C-NMR (75.5 MHz): 173.8, 173.0, 171.9, 171.4, 170.7 (5s, 5 CˆO); 156.8( s, OÀCˆO); 145.5,
136.1 (2s, 2 arom. C); 129.0, 128.7, 128.5, 128.4, 127.6, 127.3 (6d, 10 arom. CH); 67.1 (t, PhCH2); 62.1, 58.2 (2d,
CH(a)(Pro), CH(a)(Ala)); 57.2, 56.9 (2s, 2 C(a)(Aib)); 52.1 (d, CH(a)(Val)); 48.2 (t, CH2(d)(Pro)); 48.1 (t,
CH2(b)(Pro)); 40.1 (q, MeN); 40.0 (t, CH2(b)(Asn)); 29.4 (d, CH(b)(Val)); 28.1 (t, CH2(g)(Pro)); 26.5, 25.7
(2q, 2 Me(Aib)); 25.5 (q, Me(Ala)); 24.0, 19.4 (2q, 2 Me(Aib)); 17.8, 17.2 (2q, Me(Val)). ESI-MS (MeOH ‡
‡
NaI): 701 ([M ‡ Na] ).
Z-Ala-Val-Aib-Pro-Aib-OH (30). According to GPE, 29 (134 mg, 0.20 mmol) was dissolved in THF
(1.5 ml), 6n HCl (1.5 ml) was added at 08, and the mixture was stirred at r.t. overnight. Unexpectedly, the
reaction was not complete, and the 1H-NMR spectrum showed the presence of a ca. 1:1 mixture of 29 and 30.
‡
‡
ESI-MS (MeOH ‡ NaI): 701 (100, [M(29) ‡ Na] ), 612 (91, [M(30) ‡ Na] ). The separation of the mixture
proved to be very difficult and, therefore, the mixture was used in the next coupling step.
Z-Ala-Val-Aib-Pro-Aib-Leu-Aib-Pro-Leuol (26). According to GPA, the mixture of 29/30 (ca. 100 mg),
H-Leu-Aib-Pro-Leuol (16) [6][28] (ca. 50 mg), TBTU (40 mg), HOBt (25 mg), and Et3N (0.1 ml) were
dissolved in MeCN (10 ml), and the mixture was stirred at r.t. for 1 d. After workup, the crude product was
purified by CC (AcOEt/MeOH 9 :1): 28mg of 26. Crystallization from EtOH/hexane gave single crystals
suitable for an X-ray crystal-structure determination. M.p.: dec. above 134.58. IR: 3300s, 2958m, 2872w, 1643vs,
1620vs, 1541vs, 1469m, 1412m, 1385m, 1342m, 1364w, 1296w, 1251m, 1193w, 1171w, 1072w, 1026w, 928w, 733w,
1
697w. H-NMR (300 MHz): 7.75 7.7 (m, NH); 7.50 (br. s, NH); 7.4 7.2 (m, 5 arom. H, NH); 7.06 (d, J ˆ 7.5,
NH); 6.73 (br. s, NH); 5.22, 5.09 (AB, J ˆ 12.6, PhCH2); 4.4 4.0 (m, CH(a)(Val), 2 CH(a)(Pro), CH(a)(Ala,
CH(a)(Leu), CH(a)(Leuol)); 3.79 3.4 (m, CH2(d)(Pro), CH2O(Leuol)); 2.35 1.6 (m, 2 CH2(b,g)(Pro),
CH2(b)(Leu), CH2(b)(Leuol)); 1.6 1.1 (m, 6 Me(Aib), Me(Ala), CH(g)(Leu), CH(g)(Leuol), CH(b)(Val));
0.95 0.8( m, 2 Me(Val), 2 Me(Leu), 2 Me(Leuol)). 13C-NMR (75.5 MHz): 173.5, 173.4, 173.1, 172.6, 171.4 (5s, 8
CˆO); 157.4 (s, OÀCˆO); 136.3 (s, 1 arom. C); 128.7, 128.2, 127.2 (3d, 5 arom. CH); 66.9, 65.3 (2t, PhCH2,
CH2OH); 64.6 (d, CH(a)(Pro)); 63.6 (d, CH(a)(Leuol)); 58.3 (d, CH(a)(Pro)); 57.0, 56.6 (2s, 3 C(a)(Aib));
52.9 (2d, CH(a)(Leu)); 51.9 (d, CH(a)(Val)); 49.8( d, CH(a)(Ala)); 48.9 (t, 2 CH2(d)(Pro)); 40.1, 39.1 (2t,
CH2(b)(Leu), CH2(b)(Leuol)); 29.6 (d, CH(b)(Val)); 29.1, 28.8 (2t, 2 CH2(b)(Pro)); 27.2 (q, 1 Me); 26.3, 26.2
(2t, 2 CH2(g)(Pro)); 25.8( q, Me); 24.9, 24.8(2 d, CH(g)(Leu), CH(g)(Leuol)); 23.9, 23.5, 23.2, 23.1, 20.7, 19.0,
17.4 (7q, 11 Me).
4. Crystal-Structure Determination of 17, 23, and 26 (see Table 4 and Figs. 1 5)7). All measurements were
made on a Rigaku AFC5R diffractometer mounted on a 12 kW rotating-anode generator. Graphite-
monochromated MoKa radiation (l 0.71069 ä) was used for 17 and 23, while graphite-monochromated CuKa
radiation (l 1.54178ä) was employed for 26. The intensities were corrected for Lorentz and polarization effects,
but not for absorption. Data collection and refinement parameters are given in Table 4, and views of the
molecules and packing diagrams are shown in Figs. 1 5.
In the case of 17, the structure was solved by direct methods using SnB [37], which revealed the positions of
ca. 90% of the non-H atoms. The remaining non-H-atoms were located by using the Fourier-expansion routines
of DIRDIF 92 [38]. The non-H-atoms were refined anisotropically. All of the NH and OH H-atoms were placed
in the positions indicated by a difference-electron-density map, and their positions were allowed to refine
together with individual isotropic temperature factors. All remaining H-atoms were fixed in geometrically
calculated positions, and each was assigned a fixed isotropic displacement parameter with a value equal to 1.2Ueq
of its parent C-atom. Refinement of the structure was carried out on F using full-matrix least-squares
procedures, which minimized the function Sw( j Fo jÀj Fc j)2. A correction for secondary extinction was not
applied.
In the case of 23, the structure was solved by direct methods using SHELXS86 [39], which revealed the
positions of all non-H-atoms. The crystal lattice contains large regions filled with solvent molecules, which
appear to be disordered, diffuse, and only partially occupy their sites. It was not possible to adequately model
their contribution to the overall structure. An analysis of the residual electron-density peaks suggests that one or
more partially occupied sites for CH2Cl2 molecules and additional solvent, possibly EtOH and/or H2O are
7
)
CCDC 120342 120344 contain the supplementary crystallographic data for this paper. These data can be
obtained, free of charge, via www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB21EZ, U.K. (fax: ‡441223336033; e-mail:
deposit@ccdc.cam.ac.uk).

Helvetica Chimica Acta Vol. 86 (2003)
4109
present. The solvent molecules occupy a total volume of 1429.5 ä3 per unit cell, divided into two symmetry-
related regions. The best R factor obtained by attempting to model the solvent was ca. 0.10. Therefore, the
SQUEEZE routine [40] of the PLATON program [41] was used. This procedure, which allows the disordered
solvent molecules to be omitted entirely from the subsequent refinement model, significantly improved the
agreement indices and the accuracy of the geometric parameters of the peptide molecule, and there were no
significant peaks of residual electron density to be found in the voids of the structure. The electron count in the
solvent region was calculated to be 96 e per unit cell. This corresponds to the presence of approximately one
CH2Cl2 and two EtOH molecules (94 e), or one CH2Cl2, one EtOH, and three H2O molecules (98e) per unit
cell. Other combinations are possible if one considers that these molecules appear to only partially occupy their
sites. For the purposes of the calculation of the formula weight, density, F(000), and the linear absorption
coefficient, it was assumed that the ratio of peptide/CH2Cl2/EtOH is 4 :1:2. The five-membered ring of the
peptide molecule is disordered in that C(35) equally occupies two sites corresponding to alternate directions for
the flap of an envelope conformation. This disorder was modelled successfully. Enlarged atomic-displacement
ellipsoids for the i-Pr group suggest that there may also be some disorder in this region, but a disordered model
Table 4. Crystallographic Data of 17, 23, and 26
17
26
23
Crystallized from
AcOEt/MeOH EtOH/hexane
CH2Cl2/hexane/EtOH
C58H75N7O11 ¥ 0.25 CH2Cl2 ¥ 0.5 C2H6O
1090.53
Empirical formula
Formula weight [g molÀ1
Crystal color, habit
C47H76N8O10
913.16
C50H81N9O11
984.24
]
colorless, prism colorless, prism colorless, prism
Crystal dimensions [mm]
Temperature [K]
Crystal system
Space group
0.28 Â 0.33 Â 0.50 0.15 Â 0.25 Â 0.35 0.32 Â 0.45 Â 0.50
173(1)
orthorhombic
P21212
4
297(1)
monoclinic
P21
190(1)
orthorhombic
P21212
4
Z
2
Reflections for cell determination 25
23
25
2q Range for cell determination [8] 21 25
84 93
15.652(1)
9.963(1)
19.693(1)
112.781(4)
2831.5(4)
1.154
23 28
21.410(5)
26.392 (5)
11.954 (7)
90
6755(4)
1.072
0.0935
w
Unit-cell parameters a [ä]
21.861(3)
b [ä]
c [ä]
b [8]
V [ä3]
29.791(7)
8.325(6)
90
5421(4)
1.119
0.0789
w
Dx [g cmÀ3
m [mmÀ1
]
]
0.669
w/2q
Scan type
2q(max) [8]
55
121
55
Total reflections measured
8118
4670
9637
Symmetry-independent reflections 7930
4479
9454
Reflections with I > 2s(I)
Reflections used in refinement
Parameters refined
5356
5356
614
3557
4479
645
5368
9448
699
Restraints
0
854
4
R [on F; I > 2s(I) reflections]
wR [on F; I > 2s(I) reflections]
wR [on F2; all indept. reflections]
Weighting parameter [p]a)
Weighting parameters [a; b]b)
Goodness-of-fit
0.0454
0.0365
0.06780.0661
0.2046
0.1804
0.005
0.153; 0.19
1.051
0.1011; 0
0.932
1.557
Final Dmax/s
D1 (max; min) [e äÀ3
0.001
0.19; À0.20
0.005
0.90; À0.41
0.001
0.27; À0.23
]
a) wÀ1 ˆ s2(Fo) ‡ (pFo)2; b) wÀ1 ˆ s2(Fo2 ) ‡ (aP)2 ‡ bP], where P ˆ (Fo2 ‡ 2F2c )/3.

4110
Helvetica Chimica Acta Vol. 86 (2003)
could not be developed successfully for this group. The non-H-atoms were refined anisotropically. All of the H-
atoms were placed in geometrically calculated positions and refined using a riding model, where each H-atom
was assigned a fixed isotropic displacement parameter with a value equal to 1.2Ueq of its parent atom (1.5Ueq for
the Me groups). Refinement of the structure was carried out on F2 using full-matrix least-squares procedures,
which minimized the function Sw(F2o À Fc2 )2. Six reflections, whose intensities were considered to be extreme
outliers, were omitted from the final refinement. A correction for secondary extinction was not applied.
For 26, the structure was also solved by direct methods with SHELXS86 [39], which revealed the positions
of most non-H atoms. All remaining non-H atoms were located in a subsequent difference-electron-density
map. Although all of the non-H atoms could be located, it was very difficult to satisfactorily refine some of the
atoms at each end of the peptide chain, in particular those of the Ph group and the initial chain atoms up to C(5).
Disorder might be responsible for these problems, but no disordered model could be developed successfully. It
seems likely that the electron density is smeared at the ends of the peptide chain, possibly due to the flexibility of
these regions of the molecule. The non-H-atoms were refined anisotropically, although the atoms at the ends of
the chain and many of the Me groups produced extremely large and unrealistic displacement parameters.
Pseudo-isotropic and similarity restraints were applied to the atomic displacement parameters of all
anisotropically refined atoms. A bond-length restraint was also applied to the O(1)ÀC(2) bond to prevent
the bond from becoming excessively long. The H-atoms were defined analogously to 23, and the orientation of
the hydroxy OÀH vector was positioned to chose the direction most likely to result in a H-bond. Refinement of
the structure was carried out on F2 analogously to 23. A correction for secondary extinction was applied.
Neutral atom scattering factors for non-H atoms were taken from [42a], and the scattering factors for H-
atoms were taken from [43]. Anomalous dispersion effects were included in Fc [44]; the values for f' and f'' were
those of [42b]. Structure refinement was performed with the TEXSAN crystallographic software package [45]
for 17, and SHELXL97 [46] for both 23 and 26.
REFERENCES
[1] R. T. N. Luykx, A. Linden, H. Heimgartner, Chimia 1999, 53, 368.
[2] H. Heimgartner, Angew. Chem., Int. Ed. 1991, 30, 238.
[3] D. Obrecht, H. Heimgartner, Helv. Chim. Acta 1981, 64, 48 2;1987, 70, 102.
[4] P. Wipf, H. Heimgartner, Helv. Chim. Acta 1986, 69, 1153; 1987, 70, 354; 1988, 71, 258.
[5] M. Sahebi, P. Wipf, H. Heimgartner, Tetrahedron 1989, 45, 2999.
[6] R. Luykx, C. B. Bucher, A. Linden, H. Heimgartner, Helv. Chim. Acta 1996, 79, 527.
[7] G. Suter, S. A. Stoykova, A. Linden, H. Heimgartner, Helv. Chim. Acta 2000, 83, 2961.
[8] N. Pradeille, H. Heimgartner, J. Pept. Sci. 2003, 9, 827.
[9] S. A. Stoykova, A. Linden, H. Heimgartner, Chimia 2001, 55, 627.
[10] R. A. Breitenmoser, T. Hirt, R. T. N. Luykx, H. Heimgartner, Helv. Chim. Acta 2001, 84, 972.
[11] R. A. Breitenmoser, H. Heimgartner, Helv. Chim. Acta 2002, 85, 885.
[12] S. Stamm, A. Linden, H. Heimgartner, Helv. Chim. Acta 2003, 86, 1317.
[13] M. K. Das, S. Raghothamma, P. Balaram, Biochemistry 1986, 25, 7110; C. Auvin-Guette, S. Rebuffat, Y.
Prigent, B. Bodo, J. Am. Chem. Soc. 1992, 114, 2170.
[14] P. Meyer, F. Reusser, Experientia 1967, 23, 85; P. M¸ller, O. Rudin, Nature (London) 1968, 217, 713; M. K.
Mathew, R. Nagaraj, P. Balaram, J. Biol. Chem. 1982, 257, 2170; G. Jung, H. Br¸ckner, H. Schmitt, in
−Structure and Activity of Natural Peptides×, Eds. W. Voelter, G. Weitzel, de Gruyter, Berlin, 1981, p. 75.
[15] C. Toniolo, E. Benedetti, Trends Biochem. Sci. 1991, 16, 350.
[16] J. M. Humphrey, A. R. Chamberlin, Chem. Rev. 1997, 97, 2243.
[17] L. Whitmore, J. K. Chugh, C. F. Snook, B. A. Wallace, −Workshop Peptaibols: Biosynthesis, Structural
Diversity, Bioactivity, and Mode of Action×, Jena, 2002, Abstracts, p. 9; −The Peptaibol Database×, http://
www. cryst.bbk.ac.uk/peptaibol.
[18] E. Mossel, F. Formaggio, M. Crisma, C. Toniolo, Q. B. Broxterman, W. H. J. Boesten, J. Kamphuis,
P. J. L. M. Quaedflieg, P. Temussi, Tetrahedron: Asymmetry 1997, 8, 1305; M. Horikawa, Y. Shigeri, N.
Yumoto, S. Yoshikawa, T. Nakajima, Y. Ohfune, Bioorg. Med. Chem. Lett. 1998, 8, 2027; B. A. Wallace,
Bioessays 2000, 22, 227; P. A. Grigoriev, A. Berg, B. Schlegel, S. Heinz, U. Gr‰fe, J. Antibiot. 2002, 55, 826.
[19] G. Jung, H. Br¸ckner, R. Bosch, W. Winter, H. Schaal, J. Str‰hle, Liebigs Ann. Chem. 1983, 1096; R. Bosch,
G. Jung, K.-P. Voges, W. Winter, Liebigs Ann. Chem. 1984, 1117.

Helvetica Chimica Acta Vol. 86 (2003)
4111
[20] R. Nagarai, N. Shamala, P. Balaram, J. Am. Chem. Soc. 1979, 101, 16; I. L. Karle, P. Balaram, Biochemistry
1990, 29, 6747.
[21] M. Gatos, F. Formaggio, M. Crisma, C. Toniolo, G. M. Bonora, Z. Benedetti, B. D. Blasio, R. Iacovino, A.
Santini, M. Saviano, J. Kamphuis, J. Pept. Res. 1997, 3, 110; D. Obrecht, M. Altdorfer, U. Bohdal, J. Daly, W.
Huber, A. Labhardt, C. Lehmann, K. M¸ller, R. Ruffieux, P. Schˆnholzer, C. Spiegler, C. Zumbrunn,
Biopolymers 1997, 42, 575; M. Tanaka, M. Oba, N. Imawaka, Y. Tanaka, M. Kurihara, H. Suemune, Helv.
Chim. Acta 2001, 84, 32; M. Oba, M. Tanaka, M. Kurihara, H. Suemune, Helv. Chim. Acta 2002, 85, 3197; C.
Peggion, M. Crisma, F. Formaggio, C. Toniolo, K. Wright, M. Wakselman, J.-P. Mazaleyrat, Biopolymers
2002, 63, 314.
[22] P. Wipf, R. W. Kunz, R. Prewo, H. Heimgartner, Helv. Chim. Acta 1988, 71, 268.
[23] C. Str‰ssler, A. Linden, H. Heimgartner, Helv. Chim. Acta 1997, 80, 1528.
[24] H. Schmitt, G. Jung, Liebigs Ann. Chem. 1985, 321.
[25] J. R. Spencer, V. V. Antonenko, N. G. J. Delaet, M. Goodman, Int. J. Pept. Protein Res. 1992, 40, 28 2; C.
¬
Auvin-Guette, E. Frerot, J. Coste, S. Rebuffat, P. Jouin, B. Bodo, Tetrahedron Lett. 1993, 34, 2481; A. Ogrel,
W. Bloemhoff, J. Lugtenburg, J. Raap, Liebigs Ann. Recl. 1997, 41; C. Piazza, F. Formaggio, M. Crisma, C.
Toniolo, J. Kamphuis, B. Kaptein, Q. B. Broxterman, J. Peptide Sci. 1999, 5, 96.
[26] U. Slomczynska, D. D. Bensen, M. T. Leplawy, G. R. Marshall, J. Am. Chem. Soc. 1992, 114, 4095; U.
Slomczynska, J. Zabrocki, K. Kaczmarek, M. T. Leplawy, D. D. Bensen, G. R. Marshall, Biopoylmers 1992,
32, 1461.
[27] H. Wenschuh, M. Beyermann, E. Krause, M. Brudel, R. Winter, M. Sch¸rmann, L. A. Carpino, M. Bienert,
J. Org. Chem. 1994, 59, 3275; H. Wenschuh, M. Beyermann, H. Haber, J. K. Seydel, E. Krause, M. Bienert,
J. Org. Chem. 1995, 60, 405; L. A. Carpino, M. Beyermannn, H. Wenschuh, M. Bienert, Acc. Chem. Res.
1996, 29, 268.
[28] H. Br¸ckener, A. Koza, Amino Acids 2003, 24, 311.
[29] P. Wipf, H. Heimgartner, Helv. Chim. Acta 1990, 73, 13.
[30] W. Altherr, Ph.D. thesis, University of Zurich, 1994; W. Altherr, H. Heimgartner, in −Peptides 1990×, Eds. E.
Giralt, D. Andreu, ESCOM, Leiden, 1991, p. 107; W. Altherr, H. Heimgartner, in −Peptides 1992×, Eds.
C. H. Schneider, A.N. Eberle, ESCOM, Leiden, 1993, p. 387.
[31] P. Blaser, Diploma thesis, University of Zurich, 1990.
[32] R. T. N. Luykx, Ph.D. thesis, University of Zurich, 2000.
[33] C. K. Johnson, ORTEP II, Report ORNL-5138, Oak Ridge National Laboratory, Oak Ridge, Tennessee,
1976.
[34] R. Gessmannn, P. Benos, H. Br¸ckner, M. Kokkinidis, J. Pept. Sci. 1999, 5, 8 3.
[35] J. Bernstein, R. E. Davis, L. Shimoni, N.-L. Chang, Angew. Chem., Int. Ed. 1995, 34, 1555.
[36] S. Rebuffat, C. Goulard, B. Bodo, J. Chem. Soc., Perkin Trans. 1995, 1849.
[37] R. Miller, S. M. Gallo, H. G. Khalak, C. M. Weeks, J. Appl. Crystallogr. 1994, 27, 613.
[38] P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, S. GarcÌa-Granda, J. M. M. Smits, C. Smykalla,
DIRDIF 92: The DIRDIF program system, Technical Report of the Crystallography Laboratory,
University of Nijmegen, The Netherlands, 1992.
[39] G. M. Sheldrick, SHELXS86, Acta Crystallogr., Sect. A 1990, 46, 467.
[40] P. van der Sluis, A. L. Spek, Acta Crystallogr., Sect. A 1990, 46, 194.
[41] A. L. Spek, PLATON, Program for the Analysis of Molecular Geometry, Version of December 1998,
University of Utrecht, The Netherlands.
[42] a) E. N. Maslen, A. G. Fox, M. A. O×Keefe, in −International Tables for Crystallography×, Ed. A. J. C.
Wilson, Kluwer Academic Publishers, Dordrecht, 1992, Vol. C, Table 6.1.1.1, p. 477; b) D. C. Creagh, W. J.
McAuley, in −International Tables for Crystallography×, Ed. A. J. C. Wilson, Kluwer Academic Publishers,
Dordrecht, 1992, Vol. C, Table 4.2.6.8, p. 219.
[43] R. F. Stewart, E. R. Davidson, W. T. Simpson, J. Chem. Phys. 1965, 42, 3175.
[44] J. A. Ibers, W. C. Hamilton, Acta Crystallogr. 1964, 17, 78 1.
[45] TEXSAN: Single Crystal Structure Analysis Software, Version 5.0, Molecular Structure Corporation, The
Woodlands, Texas, 1989.
[46] G. M. Sheldrick, SHELXL97, Program for Crystal Structure Refinement, University of Gˆttingen, 1997.
Received October 1, 2003