Preparation and structure of β-peptides consisting of geminally disubstituted β2,2- and β3,3-amino acids: A turn motif for β- peptides

Article abstract of DOI:10.1002/(SICI)1522-2675(19981216)81:12<2218::AID-HLCA2218>3.0.CO;2-0

We report on the synthesis of new and previously described β-peptides (1-6), consisting of up to twelve β^2,2- or β^3,3-geminally disubstituted β-amino acids which do not fit into any of the secondary structural patterns of β-peptides, hitherto disclosed. The required 2,2- and 3,3-dimethyl derivatives of 3-aminopropanoic acid are readily obtained from 3-methylbut-2-enoic acid and ammonia (Scheme 1) and from Boc-protected methyl 3-aminopropanoate by enolate methylation (Scheme 2). Protected (Boc for solution-, Fmoc for solid-phase syntheses) 1- (aminomethyl)cycloalkanecarboxylic-acid derivatives (with cyclopropane, cyclobutane, cyclopentane, and cyclohexane rings) are obtained from 1- cyanocycloalkanecarboxylates and the corresponding dihaloalkanes (Scheme 3). Fully ¹³C- and ¹⁵N-labeled 3-amino-2,2-dimethylpropanoic-acid derivatives were prepared from the corresponding labeled precursors (see asterixed formula numbers and Scheme 4). Coupling of these amino acids was achieved by methods which we had previously employed for other β-peptide syntheses (intermediates 18-23). Crystal structures of Boc-protected geminally disubstituted amino acids (16a-d) and of the corresponding tripeptide (23a), as well as NMR and IR spectra of an isotopically labeled β-hexapeptide (2a*) are presented (Figs. 1-4) and discussed. The tripeptide structure contains a ten-membered H-bonded ring which is proposed to be a turn-forming motif for β-peptides (Fig. 2).

Full text of DOI:10.1002/(SICI)1522-2675(19981216)81:12<2218::AID-HLCA2218>3.0.CO;2-0

2218
Helvetica Chimica Acta ± Vol. 81 (1998)
Preparation and Structure of b-Peptides Consisting of Geminally
Disubstituted b2,2- and b3,3-Amino Acids: A Turn Motif for b-Peptides
1
2
3
4
by Dieter Seebach*, Stefan Abele ), Thierry Sifferlen ), Martin Hänggi ), Sibylle Gruner ), and Paul Seiler
Laboratorium für Organische Chemie der Eidgenössischen Technischen Hochschule, ETH-Zentrum,
Universitätstrasse 16, CH-8092 Zürich
We report on the synthesis of new and previously described b-peptides (1 ± 6), consisting of up to twelve
b2,2- or b3,3-geminally disubstituted b-amino acids which do not fit into any of the secondary structural patterns of
b-peptides, hitherto disclosed. The required 2,2- and 3,3-dimethyl derivatives of 3-aminopropanoic acid are
readily obtained from 3-methylbut-2-enoic acid and ammonia (Scheme 1) and from Boc-protected methyl 3-
aminopropanoate by enolate methylation (Scheme 2). Protected (Boc for solution-, Fmoc for solid-phase
syntheses) 1-(aminomethyl)cycloalkanecarboxylic-acid derivatives (with cyclopropane, cyclobutane, cyclo-
pentane, and cyclohexane rings) are obtained from 1-cyanocycloalkanecarboxylates and the corresponding
dihaloalkanes (Scheme 3). Fully 13C- and 15N-labeled 3-amino-2,2-dimethylpropanoic-acid derivatives were
prepared from the corresponding labeled precursors (see asterixed formula numbers and Scheme 4). Coupling
of these amino acids was achieved by methods which we had previously employed for other b-peptide syntheses
(intermediates 18 ± 23). Crystal structures of Boc-protected geminally disubstituted amino acids (16a ± d) and of
the corresponding tripeptide (23a), as well as NMR and IR spectra of an isotopically labeled b-hexapeptide
(2a*) are presented (Figs. 1 ± 4) and discussed. The tripeptide structure contains a ten-membered H-bonded ring
which is proposed to be a turn-forming motif for b-peptides (Fig. 2).
1. Introduction. ± The discovery that short-chain b-peptides form much more stable
secondary structures in solution than do their natural counterparts, the a-peptides, has
brought about a spate of interest in the field of b-peptides [1] (reviews: [2]). Three
types of b-peptide helices and two turn motifs [3 ± 6], an antiparallel [7] and a parallel
[3][8] sheet structure, and tubular arrangements of cyclic b-peptides [9][10] have been
identified so far by CD and 2D-NMR spectroscopy, and by X-ray crystal-structure
analysis. The high stability of the helical structure of b-heptapeptides has been further
supported by molecular-dynamics calculations [11]. The greater structural variability of
b-amino acids accounts for the multitude of possible b-peptide secondary structures.
These findings allow the design of b-peptide secondary structures with well-defined
folding propensities by choosing the ꢀcorrectlyꢁ substituted b-amino acids. The b-
peptides are, therefore, valuable objects for the ultimate design of synthetic enzymes.
However, there do remain substitution patterns (Table 1 in [5]) of b-amino acids,
particularly the b2,2- and b3,3-disubstituted ones, which do not fit in the secondary
1
)
)
Part of the projected Ph.D. thesis of S.A., ETH-Zürich.
Postdoctoral research at ETH-Zürich as part of the Swiss National Science Foundation Project No. 20-
50674.97/1.
2
3
4
)
)
Part of the Master Thesis (Diplomarbeit) of M.H., ETH-Zürich, 1998.
Â
Á
Â
Part of the Master Thesis (Diplomarbeit) of S.G., Ecole Europeenne de Chimie Polymeres et Materiaux,
Strasbourg, carried out at ETH-Zürich, 1998.

Helvetica Chimica Acta ± Vol. 81 (1998)
2219
structures found to date (geminally disubstituted b-amino acids are helix- and pleated-
sheet breaking residues in b-peptides; see the analysis presented in [5])5). Hence, we
chose to synthesize the b-peptides 1 ± 6 in order to study their structure6).
We have also prepared b-peptides consisting of alternating (R)- and (S)-b3-amino acids [12b]. These b-
peptides probably form a new type of secondary structure as indicated by their CD spectra [8].
By conformational analysis of these homopeptides, it should be possible to determine the minimum
number of residues necessary for the formation of stable secondary structures.
5
)
)
6

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Helvetica Chimica Acta ± Vol. 81 (1998)
Their design was encouraged by the high crystallinity of polypeptides from a-amino
acids disubstituted at the C(a)-atom7) [13]. In addition, it was interesting to compare
the b-analogues of Aib peptides with Aib peptides which are known adopt the 310 helix
(fully developed at the pentamer level); Aib is the strongest b-bend and helix
promoter, when incorporated into peptides [15][16]. The 1-(aminomethyl)cyclohex-
anecarboxylic-acid moiety8) was supposed to increase the crystallization tendency by
introducing restraints that could reduce the flexibility of the peptide backbone [17].
Many years ago, Drey and co-workers synthesized a tripeptide consisting of 3-
amino-3-methylbutanoic acid (b-aminoisovaleric acid, b3-HAib) [18] and a hexapep-
tide9) consisting of 3-amino-2,2-dimethylpropanoic acid (b-aminopivalic acid, b2-
HAib) [19]. Their primary goal was to test coupling of sterically demanding amino
acids which could not be efficiently coupled by conventional methods10) [18b][20][21].
However, the structures of these compounds remained unknown.
In the present paper, we describe the synthesis and structural investigations of b-
homopeptides 1 ± 6 consisting of geminally dimethylated b-amino acids (1 ± 3) or of 1-
(aminomethyl)cyclohexanecarboxylic acid (4 ± 6). For solid-state NMR experiments,
the fully 13C- and 15N-labeled b2,2-hexapeptide 2a* was also synthesized. Furthermore,
1-(aminomethyl)cyclopropane-, -butane-, and -pentanecarboxylic-acid derivatives
have been prepared, and their crystal structures determined.
2. Preparation of the b2,2- and b3,3-Amino-Acid Building Blocks. ± Among several
methods for the preparation of b-amino acids [22], only a few are suitable for the
generation of a tertiary center at the b-position: The Michael addition of ammonia to 3-
methylbut-2-enoic-acid derivatives [23], the cycloaddition of alkenes with chlorosul-
fonyl isocyanate, followed by protection of the b-lactam N-atom and saponification
[18b][24], the hydrolysis of 6,6-disubstituted dihydrouracils [25], the three-component
Mannich reaction of a ketone, ammonia, and a malonic-acid derivative [26], and the
reaction of ethyl 3-hydroxy carboxylates with nitriles to give the corresponding N-acyl-
b-amino acids [27].
In view of a straightforward multi-gram scale synthesis of the monomeric building
blocks for peptide coupling, we chose the first-mentioned method for the preparation
of 3-amino-3-methylbutanoic-acid derivatives (b3-HAib residue; Scheme 1). Saponi-
fication of the amide 7, obtained by reaction of ammonia with 3-methylbut-2-enoic
acid11) and Boc-protection afforded the b3,3-amino-acid derivative 8 [4][24b], the
methyl ester 9 of which (required for peptide coupling) was prepared by methylation of
the Cs salt.
7
)
)
The number of solved structures of Aib homopeptides is rapidly increasing, much more so than for any
other amino-acid derivatives [13]; see also the structures of oligo-lva-peptides consisting of enantiomeri-
cally pure 2-amino-2-methylbutanoic-acid residues [14].
The X-ray structures of protected dimers, trimers, and tetramers of this a-amino acid have been reported
(review: [13]).
8
9
)
)
Cf. compound 2d in the present paper.
10
Eventually the synthesis of the trimer of b-aminoisovaleric acid (half the chain length of compound 1
described in the present paper) required the aminolysis of the corresponding dihydro-1,3-oxazinones
[18b], whereas the synthesis of the hexamer of b-aminopivalic acid could be achieved with DCC and
In contrast to the results of our experiments, the corresponding carboxylic acid was reported [23a] as sole
reaction product under the employed conditions.
11
)

Helvetica Chimica Acta ± Vol. 81 (1998)
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Scheme 1. Preparation of b3,3-Amino-Acid Building Blocks
For the preparation of a,a-disubstituted b-amino acids, several methods are known:
the aminomethylation of silyl ketene acetals with N,N-bis[(trimethylsilyl)methoxy]-
methylamine [28], a one-pot Mannich-type condensation of aldehydes, amines, and
silyl ketene acetals in H2O in the presence of InCl3 [29], the Reformatzky reaction of an
appropriate benzotriazol derivative with 2-bromoalkanoates [30], and the dimethyla-
tion of ethyl cyanoacetate with subsequent reduction of the CN to an aminomethyl
group [31]. For the preparation of 3-amino-2,2-dimethylpropanoic-acid derivatives
(b2-HAib residue; Scheme 2), we chose to use a method developed by us [5][32]:
methylation of the Boc-protected methyl 3-aminopropanoate 10 via doubly lithiated
species. The monomethylated ester 11 underwent a second methylation to give the N-
Boc-methyl ester 12 (80% from 10), saponification of which with NaOH yielded the
Boc-protected b2,2-amino acid 13.
The preparation of 1-(aminomethyl)cycloalkanecarboxylic-acid derivatives 15 ± 17
begins with the dialkylation of the corresponding methyl 1-cyanocycloalkanecarbox-
ylates [33][34] by various dibromides12) (Scheme 3). The cyanoesters 14 [36] thus
obtained were hydrogenated with Raney-Ni, and subsequent Boc-protection afforded
the esters 1513). The corresponding Boc-protected amino acids 16 were obtained by
saponification of the amino esters 15 and subsequent Boc-protection14). Again, special
conditions (Scheme 3 and Exper. Part) had to be chosen for the preparation of
cyclopropanecarboxylic acid 16a. Cyano ester 14d was also converted to the Fmoc
derivative 17, required for solid-phase b-oligopeptide syntheses.
12
)
)
After the present work was completed, a similar dialkylation was reported with enantiomerically pure 2,2'-
bis(bromomethyl)-1,1'-binaphthyl as alkylating reagent [35].
13
14
It is interesting to note the mild conditions for the Raney-Ni reduction of cyclopropane derivative 14a.
Higher pressure or temperature resulted in an increased amount of side products. The cobalt-boride
method [35][37], checked also for this reduction, led to lower yields and side products.
Acid 16d was independently prepared by saponification of ester 15d. Whereas saponification of the a,a-
dimethylated ester 12 was accomplished at room temperature, deprotection of the cyclohexane derivative
15d required heating at reflux.
)

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Helvetica Chimica Acta ± Vol. 81 (1998)
Scheme 2. Preparation of b2,2-Amino-Acid Building Blocks. Asterix indicates fully 15N- and 13C-labeled
compounds 10 ± 13 (except protecting groups)a).
a) For specification of the asterixed compound numbers, see Formula 2 above.
Scheme 3. Preparation of 1-(Aminomethyl)cycloalkanecarboxylic-Acid Derivatives
a) H2, Raney-Ni, MeOH, 1 bar, r.t., 4 h. b) Boc2O, MeCN, Et3N, 08, 2 h. c) H2, Raney-Ni, MeOH, 4 bar, 408,
22 h. d) Boc2O, dioxane, CH2Cl2, r.t., 16 h. e) LiOH, MeOH, r.t., 3 d. f) NaOH, MeOH, reflux, 5 h. g) Boc2O,
H2O/dioxane.
3. Synthesis of the b-2,2- and b-3,3-Peptides. ± The b-peptide derivatives 1 ± 6 were
synthesized by both, solution- and solid-phase synthetic procedures. The classical
coupling in solution [3 ± 5] was to supply b-peptides 1, 2, 3, and 4b in quantities, large
enough for crystallization experiments, whereas the flexible solid-phase method,
successfully developed in our laboratory for the synthesis of b-peptides [12]15), was
chosen for the synthesis of b-peptides 4a, 4c, 5, and 6 with varying C- and N-terminal
protecting groups.
15
)
Cf. also independent work [38].

Helvetica Chimica Acta ± Vol. 81 (1998)
2223
3.1. Synthesis in Solution. The a,a- or b,b-disubstituted b-amino acids could be
coupled by conventional methods (EDC/HOBt), without encountering the type of
complications, known for sterically congested a,a-disubstituted a-amino acids [39].
Monomers 9, 12, and 15d were Boc-deprotected with CF3COOH and coupled with the
Boc-protected amino acids 8, 13, or 16d to yield the dipeptide esters 18a ± 20a; N-
deprotection of 18 and 19 with CF3COOH and coupling with the Boc-protected acids 8
or 13 provided the tripeptides 21 and 22, respectively. The CF3COOH salt 20c was
obtained in the same way, and its free amino group was coupled with the acid 16d to
yield the tripeptide ester 23a. The C-termini of the tripeptides were deprotected by
saponification (NaOH/H2O/MeOH) to yield the free acids 21b ± 23b which were
coupled with the corresponding N-deprotected tripeptide esters to give the fully
protected hexapeptides 1a, 2a, and 4b. It is interesting to note that the yields of the
coupling steps involving b2,2-peptides (ca. 80%) always16) exceeded those obtained
with the corresponding b3,3-peptides (ca. 60%). This behavior reminds of the difficulties
encountered in acylations of a-aminoisobutyric acid (Aib) and other a,a-dialkylated
glycine derivatives [40]17). The CF3COOH salts 1c and 2c and the free acids 1b and 2b
were obtained by Boc deprotection and saponification of the fully protected
hexapeptides. Boc Deprotection of the acids 1b and 2b finally led to the hexapeptide
salts 1d and 2d. The N-deprotected hexapeptide derivative 2c was coupled with b2,2-
tripeptide acid 22b and b2,2-hexapeptide acid 2b to give the fully protected b2,2-
nonapeptide 3a (colorless powder, 80%) and the b2,2-dodecapeptide 3b (colorless glass,
62%), respectively. Like all our geminally disubstituted, protected hexapeptides, the
rather hydrophobic compound 3b (Mr 1321.8) is surprisingly well soluble18) in protic
and aprotic solvents (MeOH, CHCl3, CH2Cl2). All fully protected peptides were white
powders, but only the cyclohexylidene derivative 23a could be obtained as crystalline
material19) (see Sect. 4).
3.2. Synthesis on Solid Support. The target oligopeptides 4a, 4c, 5, and 6 with three
to six b-amino acids were synthesized on solid support. The hexapeptide 4c was
prepared according to the reported procedure on ortho-chlorotrityl-chloride resin
[12a][41] by activation of the b2,2-amino acids with BOP/HOBt/Etn)i-Pr)2. Anchoring
yield (55%) and HPLC purity (65 ± 95%) of the crude peptide were comparable to the
reported values [12a ± b]; however, the cleavage yield (47%) was substantially lower for
this sterically more demanding b2,2-hexapeptide. The Rink amide resin [42] was chosen
for the solid-phase synthesis of peptide amides 4a, 5, and 6. Anchoring of the first b2,2-
amino acid to the Rink amide resin was achieved by deprotection of the Fmoc group on
the resin and coupling with Fmoc-protected acid 17, by the usual coupling procedure
(BOP/HOBt/EtN(i-Pr)2). Elongation of the peptide chain was achieved in the same
way as decribed for our previous syntheses on ortho-chlorotrityl-chloride resin [12].
Acetylation of the N-terminus of these b-peptides was expected to increase their
crystallinity; the free amino group of the peptide resin was thus acetylated with Ac2O/
16
)
)
An exception was the fragment coupling to give the hexapeptide 4b which gave only a 36% yield.
Coupling of a,a-diphenylglycine (Dph) with alanine by the EDC/HOBt method proceeded with high
yields at the C-terminus but with very low yields at the N-terminus of Dph [40a].
Normally, b2- and b3-peptides with alkyl side chains become more and more insoluble in common organic
solvents with growing chain lengths [3 ± 5].
17
18
19
)
)
All attempts to crystallize the dimethylated peptides have so far led to amorphous powders.

2224
Helvetica Chimica Acta ± Vol. 81 (1998)
a) For specification of the asterixed compound numbers, see Formula 2, and 10 ± 13 above.
EtN(i-Pr)2 in 10 min. Cleavage from the resin gave satisfactory yields only if the flow
method was applied (see Exper. Part). The crude peptide amides were recovered in
good yields and with purities of 68 ± 83%, as determined by reversed-phase (RP)
HPLC. It is noteworthy that these a,a-disubstituted b-amino acids could be effectively
coupled on both resin types under essentially the same conditions which had been used
for less crowded b-amino acids, as indicated by both, the necessary coupling periods20
)
(15 ± 60 min) and the purity of the crude products.
4. Structural Analyses. ± 4.1. X-Ray Crystal Structures. Single crystals suitable for X-
ray diffraction were obtained of Boc-protected b2,2-amino acids 16a ± d, and of the fully
protected b2,2-tripeptide 23a. Their high crystallinity parallels the behavior of the a-
amino-acid analogues [13]. The conformation of these building blocks is of interest
for studying the effect induced by the b2,2-amino acids when incorporated into b-pep-
tides. The crystal structures of the Boc-protected 1-(aminomethyl)cyclopropane-,
-cyclobutane-, -cyclopentane-, and -cyclohexane-carboxylic acids, 16a ± d, respectively,
are shown in Fig. 1; in the structures of 16c and 16d the carboxy groups occupy axial
positions21) in the envelope and in the chair conformation, respectively.
The crystal structure of b-tripeptide 23a (Fig. 2,a) is characterized by a ten-
membered H-bonded ring between the N-terminal carboxy group and the amide NH of
the second amino acid. This ring is quite similar to the central ten-membered H-bonded
ring of the 12/10/12 helix of a b2/b3-hexapeptide [5], and it actually provides a turn
(Fig. 2,b). In the crystal packing (Fig. 2,c), there are intermolecular H-bonds between
20
)
)
Coupling reactions were monitored using 2,4,6-trinitrobenzenesulfonic acid (TNBS) [43].
In almost all crystal structures containing the a-analogues of 16c [44] and 16d [45], the amino group
occupies the axial position [46].
21

Helvetica Chimica Acta ± Vol. 81 (1998)
2225
Fig. 1. X-Ray crystal structures of 1-(aminomethyl)cycloalkanecarboxylic-acid derivatives 16a ± d. Interestingly,
the conformation of the urethane C N bond of compounds 16a, 16b, and 16d is cis (angle O CO C(b) is
sp). Compound 16c adopts the trans conformation (angle O CO C(b) is ap). As expected for the three-
N
N
membered ring in 16a, the endocyclic angles have values close to 608, but the CO C(a) C(b) angle (t ˆ
117.18) is ± expectedly so ± greater than the standard tetrahedral angle. The angle t is 106.58, 107.08, and 111.18
in the structures of compounds 16d, 16c, and 16b, respectively. For structure determination of compound 16c we
thank Dr. W. B. Schweizer.

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Helvetica Chimica Acta ± Vol. 81 (1998)
Fig. 2. X-Ray structure of 23a. a) Structure of the repeating unit of 23a. b) Central ten-membered H-bonded ring
of the 12/10/12 helix [5]. c) Crystal packing of b2,2-tripeptide 23a, showing intramolecular (2.1 Š) and
intermolecular (2.0 Š) H-bonds. Only the amide protons are drawn.
the carbonyl O-atom of the Boc group of one molecule and the amide NH of the C-
terminal residue of the neighboring molecule.
4.2. 13C,15N-Labeled Peptide 2a*. The restricted mobility of the peptide backbone of
b-peptide 2a in solution is demonstrated by the magnetic nonequivalence of the amide
protons in the 1H-NMR spectra (Fig. 3,a). From this nonequivalence, it may be
concluded that at least one stable secondary structure is populated in CDCl3 at room
temperature. It is not possible to elucidate the 3D structure of this b-homopeptide with
highly overlapping signals by standard 2D-NMR methods. Given the amorphous
nature of the b2,2-hexapeptides 1 ± 3, we decided to try to determine the secondary
structure of the least strained a,a-dimethyl b-hexapeptide 2a by combined liquid- and
solid-state NMR studies of the corresponding 13C,15N-labeled hexapeptide 2a* [47]
(completely 13C- and 15N-labeled, except for the terminal protecting groups Boc and
MeO; see labeled intermediates in Scheme 2, and compounds 19 and 22).
The synthesis of 2a* (Scheme 4) started with the commercially available labeled
reagents b-alanine 15NH2-13CH2-13CH2-13CO2H (99% 13C and 99% 15N) and 13CH3I
(99% 13C), and was accomplished according to the procedures described for the
1
corresponding unlabeled hexapeptide 2a22). The H-NMR spectra (Fig. 3) of the
For each alkylation, only 2 equiv. of 13CH3I were used, instead of 4 equiv. as for the synthesis of the
unlabeled compounds. This modification did not alter the yields of the alkylation steps (97% for the first
and 75% for the second alkylation step).
22
)

Helvetica Chimica Acta ± Vol. 81 (1998)
2227
Fig. 3. 400-MHz 1H-NMR Spectra of a) the unlabeled hexapeptide 2a and b) the 13C,15N-labeled hexapeptide 2a.
100-MHz 13C-NMR Spectra of c) the unlabeled hexapeptide 2a, and d) the 13C,15N-labeled hexapeptide 2a*. All
spectra were measured in CDCl3.

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Helvetica Chimica Acta ± Vol. 81 (1998)
labeled compounds present doublets of multiplets for the a-Me groups, for the CH2
1
protons, and for the NH protons (see, e.g., the H-NMR spectrum of the labeled
hexapeptide 2a* in Fig. 3,b). For all the labeled products the 1J(H,C) coupling
1
constants can easily be determined. They vary from 127 to 140 Hz, classical J(H,C)
values for sp3-hybridized C-atoms. The 1J(H,N) coupling constants are all very similar
(between 90 and 92 Hz) for the various labeled products. The signal of the MeO group
3
of the C-terminal protecting group is always a doublet, due to a J(H,C) coupling of
4 Hz. The 13C-NMR spectra of the labeled compounds are more complicated, due to
the additional 13C,13C coupling (see, e.g., the 13C-NMR spectrum of the labeled
hexapeptide 2a* in Fig. 3,d).
Scheme 4a)
a) See asterixed compounds in previous Schemes and Formulae.
The IR spectrum of labeled 2a* shows the expected differences23) as compared to
1
the spectrum of unlabeled 2a (Fig. 4). Below 1800 cm , the vibrational frequencies of
labeled peptide 2a* are smaller than those of unlabeled 2a; this ratio is inversed in the
CH and NH stretching region.
5. Conclusion and Outlook. ± We have reported the preparation of geminally
dimethylated b3,3-amino acid (Scheme 1) and b2,2-amino-acid building blocks
(Scheme 2), and of the 1-(aminomethyl)cycloalkanecarboxylic-acid derivatives 16a ±
d (Scheme 3). We have demonstrated that, in contrast to the a-analogues, geminally
disubstituted b-amino acids can be coupled to give the corresponding b-hexa-, b-nona-,
and b-dodecapeptides 1 ± 6 in high yields by standard peptide-coupling procedures,
both in solution and on solid phase.
The crystal structures of the 1-(aminomethyl)cycloalkanecarboxylic-acid deriva-
tives 16a ± d (Fig. 1) and of the fully protected b2,2-tripeptide 23a (Fig. 2) underline the
tendency of these geminally disubstituted derivatives to crystallize. b2,2-Tripeptide 23a
(Fig. 2) adopts a ten-membered H-bonded turn, comparable to the central turn that we
have already found in the 12/10/12 helix of a b2/b3-hexapeptide [5].
Ongoing solid-state NMR experiments with the labeled b-hexapeptide 2a* will
elucidate its structure and will eventually demonstrate the potential of a new technique
for structural analysis in the solid state in the absence of single crystals [47].
The following questions and ideas are considered important for future b-peptide
research projects: i) Is there yet another helix formed by b2,2-homopeptides, besides the
known 31 [3 ± 5] and 2.51 [6b] helices? ii) Inspection of the structure of the turn-forming
b2,2-tripeptide 23a (Fig. 2) suggests that b2,2-amino acids can serve as bend promotors, if
they are incorporated into b-peptides. iii) The structure of higher homologs of 23a in
this series still remains unknown (e.g., 4b); from models and classical conformational
analysis, it is evident that a b-peptide consisting of b2,2-amino acids does not fit into the
known secondary structures of b-peptides (see Table 1 in [5]); the ten-membered H-
23
)
The maximum frequency difference is 3.22%, as compared to the maximum theoretical difference of
4.08%, due to the isotope effect.

Helvetica Chimica Acta ± Vol. 81 (1998)
2229
Fig. 4. IR Spectra of a) the unlabeled hexapeptide 2a and b) the 13C,15N-labeled hexapeptide 2a*. The spectra
were measured in CHCl3 (concentration: 10 mg in 1 ml CHCl3).

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Helvetica Chimica Acta ± Vol. 81 (1998)
bonded turn, formed by the b2,2-tripeptide 23a, might be the incipent moiety of a
meander-like or string-of-pearls-type structure of a higher oligomer, stabilized by
intramolecular H-bonds in the successive ten-membered H-bonded rings24). iv) It will
be intriguing to determine the structure of b2,2-homopeptides built from the cyclo-
propanecarboxylic-acid derivative 16a which could impart a considerable restriction to
the peptide backbone due to an exocyclic bond angle C(b) C(a) CO that is greater
than the standard tetrahedral angle. The outcome of the corresponding structural
investigations will be essential for the design and synthesis of more complex b-peptides,
including synthetic b-enzymes and molecular scaffolds with predictable folding
patterns.
Experimental Part
1. General. Abbreviations: Boc2O: di(tert-butyl) dicarbonate, DCC: N,N'-dicyclohexylcarbodiimide,
DMAP: 4-(dimethylamino)pyridine, EDC: 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride,
FC: flash chromatography, Fmoc-OSu: N-[(9H-fluoren-9-ylmethoxy)carbonyloxy]succinimide, HOBt: 1-
hydroxy-1H-benzotriazole, h.v.: high vacuum, 0.01 ± 0.1 Torr, b-HXxx: b-homoamino acid [3 ± 5] [48], TBNS:
2,5,6-trinitrobenzenesulfonic acid. Flasks and stirring bars for the alkylations were dried for ca. 12 h at 1208 and
allowed to cool in a desiccator over silica gel (Blaugel). THF was freshly distilled over Na/benzophenone under
Ar before use. CHCl3 employed for the coupling reactions was filtered over Al2O3 (Alumina Woelm N, act. I) to
remove EtOH. Et3N was distilled from CaH2 and stored under Ar. (i-Pr)2NH was freshly distilled over CaH2.
The BuLi employed was titrated according to the method of Juaristi et al. [49]. MeI was filtrated over Al2O3
(Alumina Woelm N, act. I) before use. Raney-Ni was activated according to [50]. Solvents for chromatography
and workup were distilled from Sikkon (anh. CaSO4; Fluka). ortho-Chlorotrityl chloride and Rink amide resins
were purchased from Novabiochem. The labeled reagents, b-alanine (99% 13C, 99% 15N) and 13CH3I (99% 13C),
were purchased from Campro Scientific. All other reagents were used as received from Fluka. TLC: Merck silica
gel 60 F254 plates; detection with UV, I2 (30 g of I2, 20 g of Kl, 200 ml of EtOH, 200 ml of H2O), anisaldehyde
(9.2 ml of anisaldehyde, 3.75 ml of AcOH, 12.5 ml of conc. H2SO4, 350 ml of EtOH); or ninhydrine (0.6 g of
ninhydrine, 2 ml of AcOH, 13 ml of H2O, 285 ml of BuOH). FC: Fluka silica gel 60 (40 ± 63 mm); at ca. 0.3 bar.
Anal. HPLC: a) For peptide 4c: Knauer HPLC system (pump type 64, EuroChrom 2000 integration package,
degaser, UV detector (variable-wavelength monitor)); b) for compounds 4a, 5, and 6: Waters HPLC system
(pump type 515, automated gradient controller type 680, data module type 746, tunable absorbance detector
type 484); Macherey-Nagel C8 column (Nucleosil 100-5 C8 (250 Â 4 mm)). Prep. HPLC: Knauer HPLC system
(pump type 64, programmer 50, UV detector (variable-wavelength monitor)), Macherey-Nagel C8 column
(Nucleosil 100-7 C8 (250 Â 21 mm)). All indicated temp. were monitored with an internal thermometer (Ebro-
TTX-690 digital thermometer). M.p.: Büchi-510 apparatus; uncorrected. IR Spectra: Perkin-Elmer-782
spectrophotometer. NMR Spectra: Bruker AMX 500 (1H: 500 MHz, 13C: 125 MHz), AMX 400 (1H: 400 MHz,
13C: 100 MHz), ARX 300 (1H: 300 MHz), Varian Gemini 300 (1H: 300 MHz, 13C: 75 MHz), or Varian
Gemini 200 (1H: 200 MHz, 13C: 50 MHz); chemical shifts d in ppm downfield from internal Me4Si (ˆ 0 ppm); J
values in Hz; some compounds show the presence of rotamers which are indicated. MS: VG Tribrid (EI),
Hitachi Perkin-Elmer RHU-6M (FAB, in a 3-nitrobenzyl-alcohol matrix), or LDI-1700 (MALDI) spectrom-
eter; in m/z (% of basis peak). Elemental analyses were performed by the Microanalytical Laboratory of the
Laboratorium für Organische Chemie, ETH-Zürich.
2. Alkylation of b-Alanine Derivatives: General Procedure 1 (GP 1). BuLi (1 equiv.) was added to a soln. of
(i-Pr)2NH in THF (0.5m) at 788. After 20 min at 788, a soln. of the b-alanine derivative in THF (0.8 mm) was
added during 10 min and the mixture stirred for 20 min at 788. MeI (4 equiv.) was then added slowly (temp.
<
658), and the mixture was stirred for 15 min at this temp., subsequently hydrolyzed with sat. NH4Cl soln.,
diluted with Et2O, and washed with sat NaHCO3, NH4Cl, and NaCl solns. The org. layer was dried (MgSO4) and
evaporated. FC yielded the pure product.
3. Boc Deprotection: General Procedure 2 (GP 2). GP 2a: Similarly to the procedure reported in [3 ± 5], the
Boc-protected amino acid was dissolved in CH2Cl2 (0.5m) and cooled to 08. An equal volume of CF3COOH was
24
)
The result will also help to understand the dependence of the secondary structure from the chain length in
this series of b-homopeptides.

Helvetica Chimica Acta ± Vol. 81 (1998)
2231
added, and the mixture was allowed to warm slowly to r.t. and stirred for further 1.5 h. Concentration under
reduced pressure, co-evaporation with CH2Cl2, and drying of the residue under h.v. yielded the crude
CF3COOH salt, which was identified by NMR and used without further purification.
GP 2b: Similarly to the procedure reported in [3 ± 5], the Boc-protected amino acid was dissolved in cold
(48) CF3COOH (0.15m). After stirring at r.t. for 2 h, concentration under reduced pressure and drying of the
residue under h.v. yielded the crude CF3COOH salt, which was lyophilized from dioxane.
4. Ester Hydrolysis: General Procedure 3 (GP 3). GP 3a: Similarly to the procedure reported in [51], a soln.
of the fully protected amino acid or peptide in MeOH (1.2m) was treated with 1n NaOH (1.2 equiv.) at r.t. After
stirring for 16 h, the mixture was diluted with H2O (in the case of small-scale reactions) and extracted with
pentane (2Â). The soln. was adjusted to pH 2 with 1n HCl and extracted with AcOEt (3Â). The org. phase was
washed with sat. aq. NaCl soln., dried (MgSO4), and concentrated under reduced pressure. The acid was either
recrystallized for anal. purpose or used in the next step without further purification.
GP 3b: As GP 3a, except that the reaction mixture (1.5 equiv. NaOH) was refluxed for 2 ± 4 h.
5. Esterification of Boc-Protected Amino Acids: General Procedure 4 (GP 4). Similarly to the procedure
reported in [52], the Boc-protected amino acid was dissolved in EtOH/H2O (0.25m). The pH was adjusted to 7
with aq. 10% Cs2CO3 soln. The mixture was evaporated and dried under h.v. The residue was dissolved in DMF
(0.5m) and MeI (4 equiv.) was added under Ar at 08. After stirring for 24 h at r.t., excess MeI was destroyed with
a few ml 1n NaOH. Upon removal of the solvent, the residue was taken up in AcOEt, washed with sat. aq.
NaHCO3 and NaCl solns., and evaporated.
6. Reduction of CN Derivatives 14b ± d and Subsequent Transformations: General Procedure 5 (GP 5).
GP 5a: Freshly prepared Raney-Ni (ca. 100 g/mol) [50] was added to a soln. of 14 in MeOH (0.25m). This
mixture was stirred for 24 ± 36 h at 408 under H2 (4 bar) in a glass autoclave. Excess H2 was removed by bubbling
Ar through the mixture. After filtration through Celite and evaporation (308, 150 mbar), the crude methyl
1
amino ester was obtained as yellowish oil in quant. yield. This crude product was identified by H-NMR and
immediately used for the Boc-protection step: To a stirred soln. of the free amine in CH2Cl2 (0.5m) a soln. of
Boc2O (1.1 equiv.) in dioxane (0.5m) was added at 08. The mixture was alowed to warm to r.t., and stirring was
continued for 2 ± 12 h. After evaporation, the residue was dissolved in AcOEt. The org. phase was washed with
sat. aq. NH4Cl, NaHCO3, and NaCl solns., dried (MgSO4), and evaporated. FC yielded the pure product.
GP 5b: Compound 14 was reduced as described in GP 5a. The resulting amino ester was saponified by
refluxing in MeOH (1m) with 1n NaOH (1.5 equiv.) for 5 h. After evaporation, the free amino acid was dissolved
in H2O (0.5m) and treated with a soln. of Boc2O (1.1 equiv.) in dioxane (0.6m). The mixture was stirred for 24 h
at r.t. and extracted with pentane. The aq. phase was adjusted to pH 2 with 1n HCl and extracted with AcOEt
(3Â). The org. phase was washed with sat. aq. NaCl soln., dried (MgSO4), and concentrated under reduced
pressure. The acid was recrystallized after refluxing the AcOEt soln. with charcoal and filtration of the hot soln.
through Celite.
7. Peptide Coupling with EDC: General Procedure 6 (GP 6). According to [53], the appropriate CF3COOH
salt was dissolved in CHCl3 (0.5m) and cooled to 08. This soln. was treated successively with Et3N (4 equiv.),
HOBt (1.2 equiv.), a soln. of the Boc-protected fragment (1 equiv.) in CHCl3 (0.25m), and EDC (1.2 equiv.).
The mixture was allowed to warm to r.t. and stirred for 15 h. Subsequent dilution with CHCl3 was followed by
thorough washing with 1n HCl and sat. aq. NaHCO3 (3Â) and NaCl solns. (1Â). The org. phase
was dried (MgSO4) and then concentrated under reduced pressure. FC or recrystallization yielded the pure
peptide.
8. Anchoring of N-Fmoc-Protected b-Amino Acids on the Resin: General Procedure 7 (GP 7). GP 7a:
Esterification of acid 17 with the ortho-chlorotrityl-chloride resin was performed according to [41]. The resin
(initial loading: 1.00 mmol Cl/g) was dried under h.v. for 20 min and swelled in CH2Cl2 (20 ml/mmol) for 10 min.
A soln. of acid 17 (0.8 equiv.) in CH2Cl2 (10 ml/mmol) and (i-Pr)2EtN (2.8 equiv.) were then added successively,
and the suspension was mixed by Ar bubbling for 4 h. Subsequently, the resin was filtered, washed (20 ml/mmol)
with CH2Cl2/MeOH/(i-Pr)2EtN 17:2 :1 (3 Â 3 min), CHCl2 (3 Â 3 min), DMF (2 Â 3 min), CH2Cl2 (3 Â 3 min),
MeOH (2 Â 3 min), and finally dried under h.v. for 12 h. The loading of the resin was then determined on a 5 ± 9-
mg sample (after treatment with 20% piperidine in DMF for 20 min) by measuring the absorbance of the
dibenzofulvene-piperidine adduct (formed during deprotection) at 300, 289, and 266 nm (e ˆ 7800, 5800, and
17500, resp.), taking the average absorbance from the 3 measurements.
GP 7b: Rink amide resin [42] (loading 0.45 mmol/g) was swelled in DMF/CH2Cl2 1:1 (20 ml/mmol) for
30 min and Fmoc deprotected using 20% piperidine in DMF (30 ml/mmol; 2 Â 15 min) under Ar bubbling. A
soln. of acid 17 (3 equiv.), BOP (3 equiv.), and HOBt (3 equiv.) in DMF (2 ml) and (i-Pr)2EtN (9 equiv.) were
added successively to the resin, and the suspension was mixed for 12 ± 60 min by Ar bubbling. Monitoring of the

2232
Helvetica Chimica Acta ± Vol. 81 (1998)
coupling was performed with TNBS [43]. The resin was then filtered and washed (60 ml/mmol) with DMF/
CH2Cl2 1 : 1 (3 Â 3 min) prior to the following Fmoc-deprotection step.
9. b-Peptide on Solid Support: General Procedure 8 (GP 8). GP 8a: The Fmoc group of the first amino acid
attached to the ortho-chlorotrityl-chloride resin was removed using 20% piperidine in DMF (30 ml/mmol, 2 Â
15 min) under Ar bubbling. The resin was then filtered and washed with DMF (30 ml/mmol, 6 Â 3 min). For
each coupling step, a soln. of the Fmoc-b-amino acid (3 equiv.), BOP (3 equiv.) and HOBt (3 equiv.) in DMF
(2 ml), and (i-Pr)2EtN (9 equiv.) were added successively to the resin, and the suspension was mixed by Ar
bubbling for 15 ± 60 min. Monitoring of the coupling reaction was performed with TNBS [43]. In case of a
positive TNBS test (indicating incomplete coupling), the suspension was allowed to react further for 15 ±
60 min. The resin was then filtered and washed (30 ml/mmol) with DMF (3 Â 3 min) prior to the following
Fmoc-deprotection step. After the removal of the last Fmoc protecting group, the resin was washed (30 ml/
mmol) with DMF (6 Â 3 min), CH2Cl2 (3 Â 3 min), Et2O (5 Â 1 min), and dried under h.v. for 12 h.
GP 8b: The Fmoc group of the first amino acid attached to the Rink amide resin was removed using 20%
piperidine in DMF (30 ml/mmol, 2 Â 15 min) under Ar bubbling. The resin was then filtered and washed with
DMF/CH2Cl2 1 : 1 (50 ml/mmol, 6 Â 3 min). For each coupling step, a soln. of the Fmoc-b-amino acid (3 equiv.),
BOP (3 equiv.) and HOBt (3 equiv.) in DMF (2 ml), and (i-Pr)2EtN (9 equiv.) were added successively to the
resin and the suspension was mixed by Ar bubbling for 15 ± 60 min. Monitoring of the coupling reaction was
performed with TNBS [43]. In case of a positive TNBS test (indicating incomplete coupling), the suspension
was allowed to react further for 15 ± 60 min with an additional equiv. of Fmoc-b-amino acid and coupling
reagents. The resin was then filtered and washed (50 ml/mmol) with DMF/CH2Cl2 1 : 1 (3 Â 3 min) prior to the
following Fmoc deprotection step. After the removal of the last Fmoc protecting group, the resin was acetylated
at the N-terminus according to GP 9 prior to the cleaveage procedure.
10. Acetylation of Peptides on Solid Support: General Procedure 9 (GP 9). The Fmoc-deprotected peptide-
resin was washed (30 ml/mmol) with DMF/CH2Cl2 1 :1 (5 Â 3 min) and treated successively with (i-Pr)2EtN
(20 equiv.), Ac2O (10 equiv.) in DMF/CH2Cl2 1 : 1 (2 ml) under Ar bubbling for 10 ± 15 min. Monitoring of the
acetylation was performed with TNBS [43]. The resin was then washed (30 ml/mmol) with DMF (6 Â 3 min),
CH2Cl2 (3 Â 3 min), Et2O (5 Â 1 min), and dried under h.v. for 12 h.
11. Resin Cleavage and Final Deprotection: General Procedure 10 (GP 10). GP 10a: The dry Fmoc-
deprotected ortho-chlorotrityl-chloride resin was treated with CF3COOH (20 ml/mmol, 5 Â 15 min) under Ar
bubbling. The resin was removed by filtration washed with CF3COOH, and the org. phases containing the
peptide were evaporated and co-evaporated with CH2Cl2. The precipitate which formed upon addition of cold
Et2O to the oily residue was collected by filtration or centrifugation. The solid was then dissolved (at least
partially) in H2O (containing 5% AcOH in the case of insoluble material) and lyophilized to afford a crude
product which was analyzed and purified by RP-HPLC.
GP 10b. The dry Fmoc-deprotected Rink amide peptide-resin was first treated with a mixture of CH2Cl2/
CF3COOH/(i-Pr)3SiH 90 :9 :1 (20 ml/mmol, 3 Â 2 ml), then with a mixture of CH2Cl2/CF3COOH/(i-Pr)3SiH
95 : 4 :1 (20 ml/mmol, 3 Â 2 ml), allowing the solvent to pass through the resin bed slowly. Excess CF3COOH/
CH2Cl2 was evaporated, and deprotection was completed by stirring the oily residue in 95% CF3COOH in
CH2Cl2 for 1 h. The solvent was evaporated, co-evaporated with CH2Cl2, dried under h.v., and the oily residue
treated with Et2O as described in GP 10a. Repeated treatment of the resin as described above yielded an
additional fraction of the crude peptide.
12. Reversed-Phase (RP) HPLC Analysis and Purification of b-Peptides: General Procedure 1 (GP 11). RP-
HPLC Analysis was performed on a Macherey-Nagel C8 column/Nucleosil 100-5 C8 (250 Â 4 mm) or Macherey-
Nagel C18 column/Nucleosil 100-5 C18 (250 Â 4 mm) by using a linear gradient of A (0.1% CF3COOH in H2O)
and B (MeCN) at a flow rate of 1 ml/min with UV detection at 220 nm; tR in min. Crude products were purified
by prep. RP-HPLC (Macherey-Nagel C18 column/Nucleosil 100-7 C18 (250 Â 21 mm), gradient of A and B at a
flow rate of 4 ml/min with UV detection at 220 nm) and then lyophilized.
13. b3,3-Hexapeptides 1. 3-Amino-3-methylbutanamide (7). Similarly to [23a], 3,3-dimethylacrylic acid
(50.0 g, 0.50 mol) in aq. NH3 soln. (24%, 550 ml) was heated at 1508 for 18 h in an autoclave. After cooling to
r.t., the green soln. was refluxed for 3.5 h with Ba(OH)2 (15.0 g, 97.2 mmol). The pH of the cooled suspension
was adjusted to 3 ± 4 with conc. H2SO4. This suspension was refluxed in the presence of charcoal for 15 min. The
filtrate was concentrated to dryness and dried under h.v. The crude product was washed with cold EtOH to yield
7 (58.0 g, quant.). White powder. M.p. 2308. Rf 0.54 (EtOH/NH3/H2O 7 :1:1). 1H-NMR (200 MHz, D2O): 1.45
(s, 2 Me); 2.75 (s, CH2). 13C-NMR (50 MHz, D2O): 28.03 (Me); 45.84 (CH2); 55.20, 177.36 (C).
3-{[(tert-Butoxy)carbonyl]amino}-3-methylbutanoic Acid (Boc-b3,3-HAib-OH; 8). Similarly to [50][51], 7
(17.86 g, 154.0 mmol) was refluxed in aq. NaOH (25%, 35 ml) for 24 h. The mixture was cooled to r.t. and diluted

Helvetica Chimica Acta ± Vol. 81 (1998)
2233
with H2O (235 ml) and dioxane (280 ml). At 08, Boc2O (33.6 g, 0.154 mol) was added. After stirring at r.t. for
12 h, dioxane was evaporated. The basic aq. soln. was extracted with pentane (1Â) and adjusted to pH 2 with aq.
HCl (10%). The aq. phase was extracted with AcOEt (3Â). The AcOEt phases were washed with sat. aq. NaCl
soln., dried (MgSO4), and evaporated. Recrystallization (AcOEt/pentane) yielded 8 (16.92 g, 51%). White
powder. M.p. 98 ± 998. Rf 0.09 (Et2O/pentane 1 : 2). 1H-NMR: in agreement with [24b].
Methyl 3-{[(tert-Butoxy)carbonyl]amino}-3-methylbutanoate (Boc-b3,3-HAib-OMe; 9). The acid 8 (6.50 g,
29.9 mmol) was esterified according to GP 4. FC (Et2O/pentane 1:4) yielded 9 (6.20 g, 90%). Colorless oil.
Rf 0.27 (Et2O/pentane 1:4). IR (CHCl3): 3444w, 2978m, 1716s, 1502s, 1454m, 1438m, 1392m, 1368s, 1289m,
1166s, 1081m, 1013w, 864w. 1H-NMR (400 MHz, CDCl3): 1.38 (s, 2 Me); 1.43 (s, t-Bu); 2.70 (s, CH2); 3.67
(s, MeO); 4.87 (br. s, NH). 13C-NMR (100 MHz, CDCl3): 27.50, 28.42 (Me); 44.15 (CH2); 51.11 (C); 51.43 (Me);
‡
79.04, 154.63, 171.82 (C). EI-MS: 231 (0.1, M ), 216(16.3), 158(34.5), 144(24.9), 116(62.0), 109(13.1),
102(51.3), 84(85.9), 73(27.7), 57(100.0), 41(25.2). Anal. calc. for C11H21NO4 (231.29): C 57.12, H 9.15, N 6.06,
O 27.67; found: C 57.05, H 9.06, N 6.05, O 27.70.
Methyl 3-[(3-{[(tert-Butoxy)carbonyl]amino}-3-methylbutanoyl)amino]-3-methylbutanoate (Boc-b3,3
-
HAib-b3,3-HAib-OMe; 18a). Compound 9 (4.50 g, 19.5 mmol) was saponified according to GP 3a and treated
with 8 (4.23 g, 19.5 mmol) according to GP 6. FC (Et2O/pentane 1:1 ! Et2O) yielded 18a (3.87 g, 60%). White
powder. M.p. 76 ± 778. Rf 0.14 (Et2O/pentane 1 : 1). IR (CHCl3): 3441w, 2978m, 1705s, 1666s, 1501s, 1454m,
1391m, 1368m, 1165s, 1081m, 1011w, 866w. 1H-NMR (400 MHz, CDCl3): 1.37 (s, 2 Me); 1.41 (s, 2 Me); 1.44 (s, t-
Bu); 2.45 (s, CH2); 2.79 (s, CH2); 3.66 (s, MeO); 5.17 (s, OC(O)NH); 5.94 (s, NH). 13C-NMR (100 MHz,
CDCl3): 27.19, 27.74, 28.52 (Me); 43.45, 47.45 (CH2); 51.45 (C); 51.76 (Me); 52.00, 79.02, 155.06, 170.57, 171.75
‡
‡
(C). EI-MS: 331 (1.6, [M ‡ 1] ), 330 (5.8, M ), 315(6.5), 257(23.9), 225(19.6), 215(36.2), 183(25.4),
173(33.9), 158(51.8), 143(45.4), 132(21.1), 116(100.0), 102(50.2), 83(21.5), 73(19.0), 58(33.0). Anal. calc.
for C16H30N2O5 (330.42): C 58.16, H 9.15, N 8.48; found: C 58.03, H 9.22, N 8.45.
Boc-b3,3-HAib-b3,3-HAib-b3,3-HAib-OMe (21a). Ester 18a (3.50 g, 10.6 mmol) was Boc-deprotected
according to GP 2a and coupled with 8 (2.30 g, 10.6 mmol) according to GP 6. Recrystallization (AcOEt/
pentane) and FC (AcOEt/pentane 1 :1) yielded 21a (2.69 g, 59%). White powder. M.p. 105 ± 1068. Rf 0.15
(AcOEt/pentane 1:1). IR (CHCl3): 3439w, 3005m, 2974m, 1707s, 1665s, 1501s, 1453m, 1390m, 1368m, 1165s,
1080m, 1047w. 1H-NMR (400 MHz, CDCl3): 1.38 (s, Me); 1.40 (s, Me); 1.41 (s, Me); 1.44 (s, t-Bu); 2.42 (s, CH2);
2.47 (s, CH2); 2.76 (s, CH2); 3.66 (s, MeO); 5.38 (s, OC(O)NH); 6.03 (s, NH); 6.54 (s, NH). 13C-NMR
(100 MHz, CDCl3): 27.13, 27.21, 27.56, 28.52 (Me); 43.55, 47.42, 47.88 (CH2); 51.48 (Me); 51.76, 52.09, 52.77,
‡
‡
78.95, 155.08, 170.54, 171.12, 171.75 (C). FAB-MS: 452 (1.2, [M ‡ Na] ), 430 (100.0, M ), 330(72.9), 299(8.5),
272(13.5), 257(5.3), 231(10.8), 182(5.7), 142(6.9), 132(7.2), 115(7.3), 102(5.8). Anal. calc. for C21H39N3O6
(429.56): C 58.71, H 9.15, N 9.78; found: C 58.78, H 9.05, N 9.82.
Boc-b3,3-HAib-b3,3-HAib-b3,3-HAib-OH (21b). Ester 21a (1.10 g, 2.6 mmol) was saponified according to
GP 3a to yield 21b (1.06 g, quant.). White powder. Acid 21b was used in the next step without further
1
purification. H-NMR (200 MHz, CDCl3): 1.36 (s, Me); 1.40 (s, Me); 1.42 (s, Me); 1.45 (s, t-Bu); 2.50 (s, CH2);
2.54 (s, CH2); 2.81 (s, CH2); 5.29 (br. s, OC(O)NH); 6.20 (s, NH); 6.42 (s, NH).
Boc-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-OMe (1a). Tripeptide 21a (1.10 g,
2.6 mmol) was Boc-deprotected according to GP 2a and coupled with 21b (1.06 g, 2.6 mmol) according to
GP 6. The cloudy viscous mass obtained after 15 h was dissolved in a few ml of CHCl3. After addition of further
HOBt (0.18 g, 1.3 mmol) and EDC (0.25 g, 1.3 mmol), the mixture was stirred for 24 h. FC (MeOH/CH2Cl2
1:18) yielded 1a (2.69 g, 59%). White powder. M.p. 199 ± 2008. Rf 0.11 (MeOH/CH2Cl2 1:18). IR (CHCl3):
3439w, 3336w, 3006m, 2974m, 1706m, 1659s, 1506s, 1453m, 1390w, 1367m, 1261m, 1168m, 1080w, 1015w.
1H-NMR (400 MHz, CDCl3): 1.38 (s, 2 Me); 1.42 ± 1.43 (m, 2 Me, t-Bu); 2.30 (s, CH2); 2.31 (s, CH2); 2.33
(s, CH2); 2.34 (s, CH2); 2.35 (s, CH2); 2.73 (s, CH2); 3.67 (s, MeO); 5.74 (s, OC(O)NH); 6.39 (s, NH); 7.13
(s, NH); 7.19 (s, NH); 7.32 (s, NH); 7.36 (s, NH). 13C-NMR (100 MHz, CDCl3): 26.44, 26.58, 26.63, 27.02, 27.15,
28.53 (Me); 43.68, 47.96, 48.34, 48.42, 48.46 (CH2); 51.51 (Me); 51.72, 52.41, 52.82, 53.93, 53.01, 53.03, 78.69,
‡
‡
155.07, 170.75, 170.84, 170.86, 170.91, 170.94, 171.84 (C). FAB-MS: 749 (6.4, [M ‡ Na] ), 727 (100.0, M ),
627(31.9), 429(6.4), 330(5.4), 297(4.9), 231(5.2), 198(11.0), 182(16.1), 154(8.9), 142(8.7), 136(9.3),
115(7.6). Anal. calc. for C36H66N6O9 (726.95): C 59.48, H 9.15, N 11.56; found: C 59.46, H 8.98, N 11.31.
Boc-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-OH (1b). Compound 1a (400 mg,
0.55 mmol) was dissolved in MeOH (5 ml), and 1n NaOH (0.66 ml) was added. After 30 min, a cloudy mass
was formed that could be solubilized with MeOH (1 ml). The clear soln. was stirred for 14 h at r.t. The viscous
mass was treated with MeOH (10 ml) and 1n NaOH (2.75 ml), and heated at 508 for 3 h. After evaporation of
MeOH, the peptide acid was precipitated with 1n HCl, filtered, and washed with pentane and H2O to yield 1b
(301 mg, 75%). White powder. M.p. 203 ± 2048. Rf 0.06 (MeOH/CH2Cl2 1:18). IR (CHCl3): 3437w, 3345w,

2234
Helvetica Chimica Acta ± Vol. 81 (1998)
3004m, 2975m, 2027w, 1708m, 1655s, 1522s, 1453m, 1390w, 1367m, 1168m, 1082w. 1H-NMR (400 MHz, CDCl3):
1.38 (s, 2 Me); 1.37 (s, 2 Me); 1.40 (s, 4 Me); 1.42 (s, 4 Me); 1.43 (s, t-Bu); 2.31 (s, CH2); 2.37 (s, CH2); 2.38
(s, CH2); 2.39 (s, CH2); 2.47 (s, CH2); 2.66 (s, CH2); 5.57 (br. s, OC(O)NH); 6.45 (s, NH); 7.02 (br. s, NH); 7.07
(s, NH); 7.11 (s, NH); 7.24 (s, NH). 13C-NMR (100 MHz, CDCl3): 26.43, 26.77, 26.82, 26.89, 27.71, 28.52 (Me);
44.20, 46.95, 48.07, 48.38, 48.49 (CH2); 51.76, 52.39, 52.92, 52.93, 52.97, 53.02, 79.06, 155.40, 170.76, 170.82, 170.83,
‡
‡
171.17, 171.60, 173.59 (C). FAB-MS: 736 (12.1, [M ‡ Na] ), 714 (100.0, [M ‡ 1] ), 614(29.3), 415(7.2),
316(5.9), 198(8.7), 182(14.4), 154(26.7), 142(9.5), 136(17.9), 107(5.7).
CF3CO2H ´ H2N-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-OMe (1c). Compound 1a
(150 mg, 0.21 mmol) was Boc-deprotected according to GP 2b to yield 1c (197 mg, quant.). Colorless oil. IR
(CHCl3): 3441w, 3006m, 2970m, 2955w, 2862w, 1778w, 1717m, 1647s, 1509s, 1474m, 1369w, 1312w, 1255w, 1177s,
1
1122m, 1048w, 874w. H-NMR (400 MHz, CDCl3): 1.35 (s, 2 Me); 1.37 (s, 2 Me); 1.42 (s,6 Me); 1.46 (s, 2 Me);
2.31 ± 2.34 (m, 3 CH2); 2.52 (s, CH2); 2.62 (s, CH2); 2.73 (s, CH2); 3.67 (s, MeO); 6.28 (s, NH); 7.35 (s, NH); 7.39
(s, NH); 7.45 (s, NH); 7.98 (s, NH); 8.49 (br. s, NH2). 13C-NMR (100 MHz, CDCl3): 26.25, 26.35, 26.48, 26.65,
26.99, 27.22 (Me); 43.66, 44.56, 46.65, 47.89, 48.02, 48.41 (CH2); 51.64 (Me); 52.54, 52.90, 53.14, 53.17, 53.21,
‡
53.67, 168.83, 170.96, 171.00, 171.20, 171.72, 171.90 (C). FAB-MS: 780(11.6), 650 (15.5, [M ‡ Na] ), 628 (100.0,
‡
[M ‡ 1] ), 397(5.0), 298(5.7), 199(6.9), 170(16.6).
CF3CO2H ´ H2N-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-b3,3-HAib-OH (1d). Compound 1b
(150 mg, 0.21 mmol) was Boc-deprotected according to GP 2b to yield 1d (200mg, quant.). White powder.
RP-HPLC (5 ± 65% B in 20 min): tR 13.3 min, purity >99%. M.p. 50 ± 628. IR (KBr): 3334m, 3081m, 2978m,
1654s, 1540s, 1457m, 1368m, 1252m, 1202s, 1138s, 928w, 874m, 834w, 800m, 722m, 706w, 595w. 1H-NMR
(400 MHz, CD3OD): 1.39 ± 1.41 (m, 12 Me); 2.46 (s, CH2); 2.49 (s, CH2); 2.50 (s, 2 CH2); 2.60 (s, CH2); 2.79
(s, CH2). 13C-NMR (100 MHz, CD3OD): 26.29, 27.29, 27.33, 27.37, 27.52, 27.60 (Me); 44.20, 45.24, 46.69, 47.45,
47.77, 47.79 (CH2); 53.01, 53.87, 53.89, 53.91, 54.07, 54.10, 171.69, 172.80, 172.83, 172.85, 172.87, 174.65 (C).
‡
‡
‡
FAB-MS: 1416(1.9), 1286 (1.3, [2M ‡ Na ‡ K 2] ), 803(3.0), 6.52 (36.8, [M ‡ K] ), 636 (50.4, [M ‡ Na] ),
‡
614 (100, [M ‡ 1] ), 182(5.8), 142(3.9).
14. b-2,2-Hexapeptides 2 and Dodecapeptide 3. Labeled Methyl 3-{[(tert-Butoxy)carbonyl]amino}propa-
noate (Boc-b-HGly-OMe; 10*). Similarly to [51], a soln. of labeled b-HAla-OH (2.11 g, 22.73 mmol) in a
mixture of dioxane (45 ml), H2O (22 ml), and 1n NaOH (23 ml) was stirred and cooled in an ice-water bath. At
08, Boc2O (5.70 g, 26.15 mmol) was added, and stirring was continued at r.t. for 1 h. The soln. was concentrated,
cooled in an ice-water bath, covered with a layer of AcOEt (70 ml), and acidified with a 10% aq. soln. of
NaHSO4 (42 ml) to pH 2. The aq. phase was extracted with AcOEt (5 Â 30 ml), then these AcOEt extracts were
washed with H2O (2 Â 50 ml), dried (MgSO4), evaporated, dried under h.v. to yield Boc-b-HAla-OH (4.03 g,
92%). White powder. Without further purification, Boc-b-HAla-OH (4.03 g, 20.86 mmol) was esterified
according to GP 4. FC (Et2O/pentane 1 : 2) yielded 10* (4.00 g, 93%). Slightly yellowish oil. Rf 0.33 (Et2O/
pentane 1 : 2). 1H-NMR (400 MHz, CDCl3): 1.44 (s, t-Bu); 2.34 ± 2.72 (dm, 1J(H,C) ˆ 127, CH2); 3.16 ± 3.61
(dm, 1J(H,C) ˆ 139, CH2); 3.70 (d, 3J(H,C) ˆ 4, MeO); 4.88 ± 5.18 (dm, 1J(H,N) ˆ 91, NH). 13C-NMR
(100 MHz, CDCl3): 28.35 (Me); 33.90 ± 37.22 (m, CH2); 51.72 (Me); 79.34 (C); 172.93 (d, 1J(C,C) ˆ 57, C).
Methyl 3-{[(tert-Butoxy)carbonyl]amino}-2-methylpropanoate (Boc-b-HGly(a-Me)-OMe; 11). Ester 10
[54] (12.00 g, 58.9 mmol) was alkylated according to GP 1 to yield 11 (12.66 g, 98%) as a clear orange oil which
1
was used in the following step without further purification. Rf 0.29 (Et2O/pentane 1:2). H-NMR (200 MHz,
CDCl3): 1.17 (d, J ˆ 7.1, Me); 1.44 (s, t-Bu); 2.63 ± 2.73 (m, CH); 3.20 ± 3.38 (m, CH2); 3.70 (s, MeO); 4.98
(br. s, NH).
Labeled Methyl 3-{[(tert-Butoxy)carbonyl]amino}-2-methylpropanoate (Boc-b-HGly(a-Me)-OMe; 11*).
Ester 10* (3.52 g, 16.99 mmol) was alkylated according to GP 1 except that only 2 equiv. of labeled MeI
(2.13 ml, 33.99 mmol) were used to yield 11* (3.68 g, 97%) as an orange oil which was used in the following
step without further purification. Rf 0.29 (Et2O/pentane 1:2). 1H-NMR (400 MHz, CDCl3): 0.98 ± 1.36
(dm, 1J(H,C) ˆ 128, Me); 1.43 (s, t-Bu); 2.45 ± 2.90 (dm, 1J(H,C) ˆ 131, CH); 3.01 ± 3.56 (m, CH2); 3.70
(d, 3J(H,C) ˆ 4, MeO); 4.79 ± 5.09 (dm, 1J(H,N) ˆ 91, NH). 13C-NMR (100 MHz, CDCl3): 14.72
(d, 1J(C,C) ˆ 34, Me); 28.38 (Me); 39.32 ± 43.19 (m, CH, CH2); 51.85 (Me); 79.34 (C); 175.87 (d, 1J(C,C) ˆ
57, C).
Methyl 3-{[(tert-Butoxy)carbonyl]amino}-2,2-dimethylpropanoate (Boc-b2,2-HAib-OMe; 12). Ester 11
(12.66 g, 58.1 mmol) was alkylated according to GP 1. FC (Et2O/pentane 1:3) yielded 12 (10.84 g, 81%).
Yellowish oil. Rf 0.31 (Et2O/pentane 1 :3). IR (CHCl3): 3456m, 2981m, 1715s, 1509s, 1474m, 1453m, 1393m,
1368m, 1313m, 1155s, 1048w, 984w, 932w, 856w. 1H-NMR (400 MHz, CDCl3): 1.19 (s, 2 Me); 1.43 (s, t-Bu); 3.23
(d, J ˆ 6.6, CH2); 3.69 (s, MeO); 4.96 (br. s, NH). 13C-NMR (100 MHz, CDCl3): 23.02, 28.38 (Me); 43.69 (C);
‡
‡
48.32 (CH2); 52.00 (Me); 79.17, 156.18, 177.65 (C). EI-MS: 253 (3.3, [M ‡ Na] ), 231 (0.7, M ), 199(6.5),

Helvetica Chimica Acta ± Vol. 81 (1998)
2235
175(7.0), 158(19.4), 149(8.3), 144(33.3), 130(45.7), 126(12.1), 116(18.5), 102(100.0), 98(23.5), 57(56.5).
Anal. calc. for C11H21NO4 (231.29): C 57.12, H 9.15, N 6.06; found: C 57.02, H 9.15, N 6.08.
Labeled Methyl 3-{[(tert-Butoxy)carbonyl]amino}-2,2-dimethylpropanoate (Boc-b2,2-HAib-OMe; 12*).
Ester 11* (3.68 g, 16.57 mmol) was alkylated according to GP 1 except that only 2 equiv. of labeled MeI
(2.07 ml, 33.14 mmol) were used. FC (Et2O/pentane 1 :5) yielded 12* (2.92 g, 75%). Yellowish oil. Rf 0.31
(Et2O/pentane 1 :3). 1H-NMR (300 MHz, CDCl3): 0.93 ± 1.43 (dm, 1J(H,C) ˆ 127, 6 H, Me); 1.43 (s, t-Bu);
2.93 ± 3.51 (dm, 1J(H,C) ˆ 138, CH2); 3.68 (d, 3J(H,C) ˆ 32, MeO); 4.76 ± 5.15 (dm, 1J(H,N) ˆ 91, NH).
3-{[(tert-Butoxy)carbonyl]amino}-2,2-methylpropanoic Acid (Boc-b2,2-HAib-OH; 13). Ester 12 (2.00 g,
8.6 mmol) was saponified according to GP 3a. Recrystallization (AcOEt/pentane) yielded 13 (1.28 g, 69%).
White powder. M.p. 114 ± 1158. Rf 0.34 (Et2O/pentane 1:2). IR (CHCl3): 3456w, 2984m, 2933w, 1708s, 1508s,
1
1476m, 1456w, 1410w, 1395w, 1369m, 1308w, 1169s, 1041w, 933w, 856w. H-NMR (400 MHz, CDCl3; signals of
rotamers in italics): 1.23 (s, 2 Me); 1.44 (s, t-Bu); 3.22 ± 3.27 (m, CH2); 5.02, 6.37 (br., NH). 13C-NMR (100 MHz,
CDCl3): 22.90, 28.37 (Me); 43.61 (C); 47.98 (CH2); 79.40, 156.27, 183.12 (C). EI-MS: 161(9.1), 144(7.2),
130(4.6), 116(5.6), 98(11.4), 88(42.0), 70(24.3), 57(100.0), 41(30.7), 30(30.8). Anal. calc. for C10H19NO4
(217.26): C 55.28, H 8.81, N 6.45; found: C 55.07, H 8.91, N 6.51.
Labeled 3-{[(tert-Butoxy)carbonyl]amino}-2,2-dimethylpropanoic Acid (Boc-b2,2-HAib-OH; 13*). Ester
12* (1.97 g, 8.30 mmol) was saponified according to GP 3a to yield 13* (1.81 g, 98%) as a white powder which
was used in the following step without further purification. M.p. 117 ± 1198. 1H-NMR (400 MHz, CDCl3): 1.01 ±
1.41 (dm, 1J(H,C) ˆ 129, 2 Me); 1.44 (s, t-Bu); 2.99 ± 3.47 (dm, 1J(H,C) ˆ 139, CH2); 4.89 ± 5.18 (dm, 1J(H,N) ˆ
91, NH); 6.27 ± 6.57 (dm, 1J(H,N) ˆ 92, NH, rotamer). 13C-NMR (100 MHz, CDCl3): 22.85 (d, 1J(C,C) ˆ 35,
Me); 22.96 (d, 1J(C,C) ˆ 35, Me); 28.34 (Me); 42.72 ± 44.76 (m, C); 47.67 ± 49-73 (m, CH2); 79.38 (C); 156.10
(C); 181.30 (d, 1J(C,C) ˆ 55, C); 183.07 (d, 1J(C,C) ˆ 54, C).
Methyl
3-[(3-{[(tert-Butoxy)carbonyl]amino}-2,2-dimethylpropanoyl)amino]-2,2-dimethylpropanoate
(Boc-b2,2-HAib-b2,2-HAib-OMe; 19a). Compound 12 (3.60 g, 15.5 mmol) was Boc-deprotected according to
GP 2a and coupled with 13 (3.37 g, 15.5 mmol) according to GP 6. FC (Et2O/pentane 3 :2) yielded 19a (3.98 g,
78%). White powder. M.p. 66 ± 678. Rf 0.33 (Et2O/pentane 3 :2). IR (CHCl3): 3453m, 3006m, 2975m, 1710s,
1656m, 1506s, 1474m, 1392w, 1367m, 1312m, 1158s, 1046w, 930w, 863w. 1H-NMR (400 MHz, CDCl3): 1.18
(s, 2 Me); 1.19 (s, 2 Me); 1.42 (s, t-Bu); 3.21 (d, J ˆ 6.5, CH2); 3.35 (d, J ˆ 6.3, CH2); 3.71 (s, MeO); 5.20
(br., NH); 6.40 (br., NH). 13C-NMR (100 MHz, CDCl3): 23.14, 23.65, 28.39 (Me); 43.25, 43.40 (C); 46.71, 48.92
‡
(CH2); 79.01, 156.42, 177.00, 177.88 (C). EI-MS: 330 (0.3, M ), 257(7.8), 225(16.6), 201(61.9), 169(30.8),
155(52.7), 144(8.3), 126(34.3), 98(100.0), 87(4.1), 70(18.45), 59(20.8), 41(13.2), 30(5.0). Anal. calc. for
C16H30N2O5 (330.42): C 58.16, H 9.15, N 8.48; found: C 58.06, H 9.27, N 8.45.
Labeled Methyl 3-[(3-{[(tert-Butoxy)carbonyl]amino}-2,2-dimethylpropanoyl)amino]-2,2-dimethylpropa-
noate (Boc-b2,2-HAib-b2,2-HAib-OMe; 19a*). Compound 12* (957 mg, 4.03 mmol) was Boc-deprotected
according to GP 2a and coupled with 13* (901 mg, 4.03 mmol) according to GP 6. FC (Et2O/pentane 3 :2)
yielded 19a* (1.26 g, 91%). White powder. Rf 0.33 (Et2O/pentane 3 :2). 1H-NMR (400 MHz, CDCl3): 0.98 ± 1.37
(dm, 1J(H,C) ˆ 127, 4 Me); 1.42 (s, t-Bu); 3.00 ± 3.42 (dm, 1J(H,C) ˆ 139, CH2); 3.13 ± 3.55 (dm, 1J(H,C) ˆ 140,
CH2); 3.71 (d, 3J(H,C) ˆ 4, MeO); 5.05 ± 5.35 (dm, 1J(H,N) ˆ 91, NH); 6.26 ± 6.56 (dm, 1J(H,N) ˆ 91, NH).
13C-NMR (100 MHz, CDCl3): 22.95 ± 23.81 (m, Me); 28.38 (Me); 42.38 ± 44.14 (m, C); 46.44 ± 49.12 (m, CH2);
79.03 (C); 156.28 (C); 156.55 (C); 177.00 (dd, 1J(C,C) ˆ 49, 1J(C,N) ˆ 14, C); 177.90 (d, 1J(C,C) ˆ 56, C).
(Boc-b2,2-HAib-b2,2-HAib-b2,2-HAib-OMe (22a). Dipeptide 19a (3.40 g, 10.3 mmol) was Boc-deprotected
according to GP 2a and coupled with 13 (2.24 g, 10.3 mmol) according to GP 6. FC (AcOEt/pentane 3 :2)
yielded 22a (3.55 g, 80%). White powder. M.p. 107 ± 1088. Rf 0.22 (AcOEt/pentane 3 :2). IR (CHCl3): 3451m,
3006m, 2972m, 1711s, 1652s, 1505s, 1474m, 1392w, 1368m, 1312m, 1158s, 932w, 860w. 1H-NMR (400 MHz,
CDCl3): 1.17 (s, 2 Me); 1.18 (s, 2 Me); 1.19 (s, 2 Me); 1.42 (s, t-Bu); 3.21 (d, J ˆ 6.4, CH2); 3.32 (d, J ˆ 5.9, CH2);
3.35 (d, J ˆ 6.3, CH2); 3.71 (s, MeO); 5.26 (br., NH); 6.46 (br., NH); 6.88 (br., NH). 13C-NMR (100 MHz,
CDCl3): 23.11, 23.62, 23.80, 28.41 (Me); 42.55, 43.21 (C); 46.71, 47.36, 48.97 (CH2); 52.21 (Me); 78.88, 156.42,
‡
‡
177.26, 177.46, 177.90 (C). FAB-MS: 452 (13.9, [M ‡ Na] ), 430 (100,0, M ), 374(6.6), 330(75.9), 300(8.6),
243(5.9), 199(7.0), 170(6.6). Anal. calc. for C21H39N3O6 (429.56): C 58.72, H 9.15, N 9.78; found: C 58.52,
H 9.05, N 9.76.
Labeled Boc-b2,2-HAib-b2,2-HAib-b2,2-HAib-OMe (22a*). Dipeptide 19a* (625 mg, 1.82 mmol) was Boc-
deprotected according to GP 2a and coupled with 13* (408 mg, 1.82 mmol) according to GP 6. FC (AcOEt/
pentane 3 :2) yielded 22a* (772 mg, 94%). White powder. Rf 0.22 (AcOEt/pentane 3 :2). 1H-NMR (400 MHz,
CDCl3): 0.98 ± 1.38 (dm, 1J(H,C) ˆ 129, 6 Me); 1.42 (s, t-Bu); 3.00 ± 3.41 (dm, 1J(H,C) ˆ 139, CH2); 3.11 ± 3.56
(dm, 1J(H,C) ˆ 140, 2 CH2); 3.71 (d, 3J(H,C) ˆ 4, MeO); 5.11 ± 5.41 (dm, 1J(H,N) ˆ 91, NH); 6.32 ± 6.62
(dm, 1J(H,N) ˆ 91, NH); 6.75 ± 7.04 (dm, 1J(H,N) ˆ 91, NH). 13C-NMR (100 MHz, CDCl3): 22.91 ± 23.95

2236
Helvetica Chimica Acta ± Vol. 81 (1998)
(m, Me); 28.39 (Me); 41.68 ± 43.94 (m, C): 46.43 ± 49.16 (m, CH2); 78.89 (C); 176.95 ± 177.77 (m, C); 177.92
(d, 1J(C,C) ˆ 56, C).
Boc-b2,2-HAib-b2,2-HAib-b2,2-HAib-OH (22b). Compound 22a (1.61 g, 3.8 mmol) was saponified according
to GP 3a to yield 22b (1.53 g, 98%). White powder. 22b was used in the next step without further purification.
Rf 0.29 (MeOH/CH2Cl2 1:9). 1H-NMR (200 MHz, CD3OD): 1.15 (s, 2 Me); 1.17 (s, 4 Me); 1.43 (s, t-Bu); 3.18
(s, CH2); 3.29 ± 3.35 (m, 2 CH2).
Labeled Boc-b2,2-HAib-b2,2-HAib-b2,2-HAib-OH (22b*). Compound 22a* (755 mg, 1.68 mmol) was
saponified according to GP 3a to yield 22b* (684 mg, 94%). White powder. 22b* was used in the next step
without further purification. 1H-NMR (400 MHz, CDCl3): 0.98 ± 1.41 (dm, 1J(H,C) ˆ 127, 6 Me); 1.42 (s, t-Bu);
3.01 ± 3.44 (dm, 1J(H,C) ˆ 140, CH2); 3.11 ± 3.59 (dm, 1J(H,C) ˆ 140, 2 CH2); 5.10 ± 5.39 (dm, 1J(H,N) ˆ 91,
NH); 6.08 ± 6.38 (dm, 1J(H,N) ˆ 90, NH); 6.38 ± 6.68 (dm, 1J(H,N) ˆ 90, NH); 6.71 ± 7.02 (dm, 1J(H,N) ˆ 91,
NH). 13C-NMR (100 MHz, CDCl3): 22.97 ± 23.89 (m, Me); 28.38 (Me); 42.08 ± 44.03 (m, C); 46.53 ± 50.62
(m, CH2); 79.36 (C); 176.07 ± 177.68 (m, C); 180.11 (d, 1J(C,C) ˆ 54, C).
Boc-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-OMe (2a). Compound 22a (1.61 g,
3.8 mmol) was Boc-deprotected according to GP 2a and coupled with 22b (1.53 g, 3.7 mmol) according to
GP 6. 2 Â FC (MeOH/CH2Cl2 1 : 18) yielded 2a (2.07 g, 79%). White powder. M.p. 168 ± 1708. Rf 0.23 (MeOH/
CH2Cl2 1 : 18). IR (CHCl3): 3436w, 3323w, 3005m, 2975m, 2923w, 2872w, 1713m, 1656s, 1503s, 1451m, 1390w,
1369m, 1169m, 1077w. 1H-NMR (400 MHz, CDCl3): 1.17 (s, 2 Me); 1.18 (s, 4 Me); 1.19 (s, 6 Me); 1.41 (s, t-Bu);
3.20 (d, J ˆ 6.4, CH2); 3.29 ± 3.32 (m, 4 CH2); 3.34 (d, J ˆ 6.2, CH2); 3.71 (s, MeO); 5.30 (br., NH); 6.49
(br., NH); 7.05 (br., NH); 7.10 (br. t, J ˆ 5.6, NH); 7.20 (br., 2 NH). 13C-NMR (100 MHz, CDCl3): 23.10,
23.64, 23.74, 23.78, 28.41 (Me); 42.16, 42.18, 42.27, 42.30, 43.15 (C); 46.74, 47.47, 49.00 (CH2); 52.25 (Me); 78.80,
‡
‡
156.43, 177.22, 177.51, 177.62, 177.68, 177.93 (C). FAB-MS: 749 (28.8, [M ‡ Na] ), 727 (51.4, M ), 627(100.0),
199(5.6), 170(12.3), 154(6.1), 140(5.2). Anal. calc. for C36H66N6O9 (726.95): C 59.48, H 9.15, N 11.56; found:
C 59.20, H 8.92, N 11.40.
Labeled Boc-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-OMe (2a*). Compound 22a*
(344 mg, 0.76 mmol) was Boc-deprotected according to GP 2a and coupled with 22b* (333 mg, 0.76 mmol)
according to GP 6. 2 Â FC (MeOH/CH2Cl2 1 : 18) yielded 2a* (492 mg, 84%). White powder. M.p. 167 ± 1698.
Rf 0.23 (MeOH/CH2Cl2 1 : 18). IR (CHCl3): 3442m, 3008m, 2960m, 1700m, 1605s, 1483s, 1360w, 1292m, 1158m,
972w. 1H-NMR (400 MHz, CDCl3): 0.98 ± 1.38 (dm, 1J(H,C) ˆ 128, 12 Me); 1.41 (s, t-Bu); 2.99 ± 3.40
(dm, 1J(H,C) ˆ 139, CH2); 3.08 ± 3.55 (dm, 1J(H,C) ˆ 140, 5, CH2); 3.71 (d, 3J(H,C) ˆ 4, MeO); 5.16 ± 5.45
(dm, 1J(H,N) ˆ 91, NH); 6.34 ± 6.64 (dm, 1J(H,N) ˆ 91, NH); 6.91 ± 7.19 (dm, 1J(H,N) ˆ 91, NH); 6.96 ± 7.24
(dm, 1J(H,N) ˆ 91, NH); 7.04 ± 7.35 (dm, 1J(H,N) ˆ 91, 2 NH). 13C-NMR (100 MHz, CDCl3): 22.88 ± 23.87
(m, Me); 28.38 (Me); 41.26 ± 43.86 (m, C); 46.44 ± 49.16 (m, CH2); 176.86 ± 178.20 (m, C). FAB-MS: 785 (15.0,
‡
‡
[M ‡ Na] ), 763 (96.7, M ), 663(100.0), 526(8.7), 421(11.7), 316(11.1), 211(18.2), 180(17.1), 106(13.4).
Boc-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-OH (2b). Compound 2a (520 mg,
0.72 mmol) was dissolved in MeOH (4 ml), treated with 1n NaOH (3.6 ml), and stirred for 15 h at r.t. The
cloudy mixture was solubilized with MeOH (8 ml) and 1n NaOH (3.6 ml) which was further heated at 508 for
5 h. MeOH was evaporated, and the pH was adjusted to 1n HCl. The crude product was extracted with CHCl3
(4Â). The combined org. phases were washed with sat. aq. NaCl soln. (1Â), dried (MgSO4), and evaporated. 2b
(509 mg, quant.). Yellowish powder. M.p. 64 ± 658. Rf 0.06 (MeOH/CH2Cl2 1 : 9). IR (CHCl3): 3447w, 3008m,
1
2972m, 2936w, 1706m, 1648s, 1509s, 1747m, 1394w, 1368m, 1310w, 1170m, 989w. H-NMR (400 MHz, CDCl3):
1.17 (s, 6 Me); 1.18 (s, 4 Me); 1.21 (s, 2 Me); 1.42 (s, t-Bu); 3.22 (d, J ˆ 6.5, CH2); 3.27 ± 3.35 (m, 5 CH2); 5.33
(br., NH); 6.55 (br., NH); 6.85 (br., NH); 7.05 (br., 2 NH); 7.15 (br., NH). 13C-NMR (100 MHz, CDCl3):
23.22, 23.52, 23.67, 23.77, 28.42 (Me); 42.29, 42.53, 42.60, 42.81, 42.86, 43.34 (C); 46.77, 47.38, 47.44, 47.55, 47.65,
‡
48.88 (CH2); 79.00, 156.55, 177.31, 177.51, 177.73, 179.58 (C). FAB-MS: 736 (18.7, [M ‡ Na] ), 714 (83.8, [M ‡
‡
1] ), 640(6.0), 614(100.0), 584(5.6), 170(8.3), 154(9.4), 136(8.3).
CF3COOH ´ H2N-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-OMe (2c). Compound 2a
(320 mg, 0.44 mmol) was Boc-deprotected according to GP 2b to yield 2c (335 mg, quant.). White solid. M.p.
42 ± 508. IR (CHCl3): 3436w, 3323w, 3005m, 2977m, 1727w, 1663s, 1509s, 1453m, 1383w, 1367m, 1256m, 1177s,
1140m, 1085w, 1039w, 874w, 836w. 1H-NMR (400 MHz, CDCl3): 1.19 (s, 10 Me); 1.31 (s, 2 Me); 3.28 ± 3.35
(m, 6 CH2); 3.71 (s, MeO); 6.55 (br. t, J ˆ 5.8, NH); 7.16 (br. t, J ˆ 5.6, NH); 7.25 (br. t, NH); 7.27 (br. t, NH);
7.48 (br., NH); 8.42 (br., NH2). 13C-NMR (100 MHz, CDCl3): 23.09, 23.54, 23.68, 23.84 (Me); 39.35, 42.07,
42.26, 42.34, 43.11 (C); 46.82, 47.52, 47.58, 47.63, 47.69, 48.08 (CH2); 52.27 (Me); 177.62, 177.73, 177.85, 177.94
‡
‡
(C). FAB-MS: 650 (7.6, [M ‡ Na] ), 628 (100.0, [M ‡ 1] ), 330(3.6), 232(3.3), 198(4.0), 182(6.4), 154(4.1),
142(4.8), 136(3.8).

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CF3COOH ´ H2N-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-OH (2d). Compound 2b
(200 mg, 0.28 mmol) was Boc-deprotected according to GP 2b to give 2d (251 mg, quant.). Clear colorless oil.
RP-HPLC (5 ± 65% A in 20 min): tR 12.6 min, purity 98.5%. IR (KBr): 3375m, 2973m, 2936m, 1654s, 1522s,
1475m, 1399w, 1368w, 1313w, 1202s, 1181s, 1139s, 1025w, 991w, 874w, 798w, 721w. 1H-NMR (400 MHz, CD3OD):
1.17 (s, 10 Me); 1.31 (s, 2 Me); 3.02 (s, CH2); 3.30 ± 3.34 (m, 5 CH2). 13C-NMR (100 MHz, CD3OD): 23.72,
23.94, 23.98 (Me); 41.54, 44.26, 44.44, 44.48, 44.51, 44.53 (C); 48.03, 48.56, 48.67 (CH2); 178.26, 179.29, 179.30,
‡
‡
179.35, 179.40, 180.42 (C). FAB-MS: 1416(1.5), 670(2.8), 652(55.6, [M ‡ K] ), 636 (12.1, [M ‡ Na] ), 614
‡
(100.0, [M ‡ 1] ), 298(5.1), 270(4.7), 170(7.7), 154(5.1), 136(4.3).
Boc-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-OMe (3a).
Compound 2a (235 mg, 0.32 mmol) was Boc-deprotected according to GP 2a and coupled with 22b (135 mg,
0.32 mmol) according to GP 6. FC (MeOH/CH2Cl2 1:14) yielded 3a (265 mg, 80%). White powder. M.p. 173 ±
1758. Rf 0.16 (MeOH/CH2Cl2 1:14). IR (CHCl3): 3447m, 3004m, 2970m, 2872w, 1711m, 1646s, 1506s, 1474m,
1392w, 1368w, 1312w, 1159m. 1H-NMR (400 MHz, CDCl3): 1.16 ± 1.21 (m, 18 Me); 1.41 (s, t-Bu); 3.20 (d, J ˆ 6.3,
CH2); 3.26 ± 3.32 (m, 7 CH2); 3.34 (d, J ˆ 6.2, CH2); 3.71 (s, MeO); 5.30 (br., NH); 6.48 (br. t, J ˆ 5.7, NH); 7.05
(br. t, J ˆ 5.7, NH); 7.11 (br. t, J ˆ 5.7, NH); 7.16 ± 7.28 (m, 5 NH). 13C-NMR (100 MHz, CDCl3): 23.10, 23.75,
23.79, 28.41 (Me); 46.74, 47.46, 47.48, 47.52, 48.98 (CH2); 52.25 (Me); 78.80, 156.44, 177.23, 177.52, 177.64,
‡
‡
177.68, 177.94 (C). FAB-MS: 1047 (8.8, [M ‡ Na] ), 1025 (90.0, M ), 925(100.0), 726(4.8), 307(4.7),
298(5.0), 200(11.7), 170(19.5), 154(27.8).
Boc-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-b2,2-HAib-
b2,2-HAib-b2,2-HAib-OMe (3b). A soln. of 2c (126 mg, 0.17 mmol) in CHCl3 (1 ml) was treated with Et3N
(100 ml, 0.73 mmol), HOBt (27 mg, 0.20 mmol), a soln. of 2b (120 mg, 0.17 mmol) in DMF (2 ml), and EDC
(38 mg, 0.20 mmol) under Ar at 08. The mixture was stirred for 18 h at r.t. The solvent was evaporated and the
residue dried under h.v. The oily residue was dissolved in CHCl3 and washed with 1n HCl, sat. aq. NaHCO3
(3Â), and NaCl (1Â) solns. The org. phase was dried (MgSO4) and evaporated. FC (MeOH/CH2Cl2 1 : 14) gave
3b (135 mg, 62%). Glass. Rf 0.19 (MeOH/CH2Cl2 1 : 14). IR (CHCl3): 3440w, 3336w, 3007m, 2972m, 2931w,
2870w, 1711w, 1649s, 1506s, 1475m, 1452m, 1388w, 1367m, 1314w, 1260m, 1174m, 987w, 911w. 1H-NMR
(400 MHz, CDCl3): 1.16 ± 1.19 (m, 12 Me); 1.40 ± 1.43 (m, 12 Me, t-Bu); 2.23 ± 2.31 (m, 6 CH2); 3.20 (d, J ˆ 6.4,
CH2); 3.27 ± 3.30 (m, 5 CH2); 3.67 (s, MeO); 5.30 (br., NH); 6.16 (s, NH); 7.06 (m, 2 NH); 7.21 (br. t, J ˆ 5.7,
NH); 7.25 (br., NH); 7.30 (br. t, J ˆ 5.7, NH); 7.36 (s, NH); 7.44 (s, NH); 7.61 (s, NH); 7.69 (s, NH); 7.78
(s, NH). 13C-NMR (100 MHz, CDCl3): 20.95, 23.65, 23.72, 23.76, 23.79, 26.03, 26.12, 26.21, 26.38, 26.42, 27.01,
28.42 (Me); 42.08, 42.12, 42.25, 42.28, 43.09, 43.17 (C); 47.47, 47.50, 47.54, 47.73, 48.13, 48.17, 48.62, 48.71, 48.92,
48.99 (CH2); 51.64 (Me); 52.50, 53.08, 53.10, 53.12, 78.78, 128.92, 134.68, 156.44, 170.80, 170.84, 170.91, 170.95,
‡
171.85, 176.93, 177.22, 177.64, 177.69 (C). FAB-MS: 1322 (100.0, M ), 1222(68.8), 695(6.9), 594(7.1),
567(7.2), 429(6.0), 397 (7.0), 369(6.2), 298(11.8), 225(20.0), 199(18.2), 182(32.2), 170(25.3).
15. Cyclic b2,2-Amino Acids and b2,2-Peptides 4, 5, and 6. Methyl 1-Cyanocyclohexane-1-carboxylate (14d).
Nitrile 14d was prepared according to [33] in 52% yield. Rf 0.56 (Et2O/pentane 1:1). nD ˆ 1.4580 (in agreement
with [36]). 1H-NMR (200 MHz, CDCl3): 1.24 ± 1.31 (m, CH2); 1.56 ± 1.89 (m, 3 CH2); 2.08 ± 2.14 (m, CH2); 3.82
‡
(m, MeO). EI-MS: 167 (1.0, M ), 152(0.7), 135(4.7), 122(62.0), 112(64.4), 108(91.3), 95(40.3), 81(85.1),
67(100.0), 59(28.9), 55(29.0), 42(37.1), 28(14.8).
Methyl 1-({[(tert-Butoxy)carbonyl]amino}methyl)cyclopropane-1-carboxylate (Boc-b2,2-HAc3c-OMe;
15a). Freshly prepared Raney-Ni (2.0 g) was added to a soln. of 14a (0.828 g, 6.62 mmol) [33], obtained from
the corresponding cyano acid [34] by esterification (MeOH, DCC, DMAP), in MeOH (26.0 ml). This mixture
was stirred for 4 h at r.t. under H2 (balloon). Excess H2 was removed by bubbling Ar through the mixture.
Filtration through Celite and evaporation (308, 100 mbar) gave the crude amine with varying amounts of side
products. An aliquot of this crude primary amine intermediate (0.239 g, 1.89 mmol) was dissolved in MeCN
(4.0 ml) at 08. Et3N (0.26 ml, 1.85 mmol) and Boc2O (0.44 g, 2.0 mmol) were added, and the soln. was stirred for
2 h. After evaporation, the oil was dissolved in Et2O. The Et2O phase was washed with aq. sat. NH4Cl soln., dried
(MgSO4), and evaporated. FC (Et2O/pentane 1:2) yielded 15a (0.34 g, 80%). Colorless oil. Rf 0.32 (Et2O/
pentane 1:2). IR (CHCl3): 3450w, 3008m, 2980w, 1709s, 1507m, 1439m, 1392w, 1367m, 1160s, 939w, 854w.
1H-NMR (400 MHz, CDCl3): 0.95 ± 0.98 (m, 2 CH); 1.21 ± 1.27 (m, 2 CH); 1.44 (s, t-Bu); 3.28 (d, J ˆ 6.4, CH2N);
3.68 (s, MeO); 5.18 (br., NH). 13C-NMR (100 MHz, CDCl3): 14.70 (CH2); 24.96 (C); 28.38 (Me); 43.97 (CH2);
‡
‡
‡
51.98 (Me); 79.22, 156.17, 175.32 (C). FAB-MS: 459 (12.1, [2 M ‡ 1] ), 230 (69.4, [M ‡ 1] ), 229 (3.1, M ),
174(100), 130(56.9).
Methyl 1-({[(tert-Butoxy)carbonyl]amino}methyl)cyclobutane-1-carboxylate (Boc-b2,2-HAc4c-OMe; 15b).
Compound 14b [33][36] (2.20 g, 15.8 mmol) was transformed according to GP 5a. FC (Et2O/pentane 1:3)
yielded 15b (2.10 g, 55%). Colorless oil. Rf 0.33 (Et2O/pentane 1:3). IR (CHCl3): 3453m, 3026m, 3016m,

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Helvetica Chimica Acta ± Vol. 81 (1998)
2981m, 2954m, 2874w, 1712s, 1507s, 1436m, 1393m, 1368m, 1333m, 1250s, 1236m, 1167s, 1128s, 1006w, 981w,
946w, 860w. 1H-NMR (400 MHz, CDCl3): 1.44 (s, t-Bu); 1.84 ± 2.12 (m, 4 CH); 2.34 ± 2.44 (m, 2CH); 3.50 (d, J ˆ
6.3, CH2N); 3.73 (s, CO2Me); 4.94 (br., NH). 13C-NMR (100 MHz, CDCl3): 15.65, 27.64 (CH2); 28.37 (Me);
‡
45.36 (CH2); 47.36 (C); 52.06 (Me); 79.26, 156.41, 176.58 (C). EI-MS: 244 (3.2, [M ‡ 1] ), 188(43.8),
170(34.8), 156(38.1), 144(21.1), 142(12.8), 138 (32.8), 127(14.2), 126(61.1), 115(10.1), 114(100), 110(13.7),
99(18.9), 83(17.3), 82(20.6), 67(34.7), 59(26.2), 57(95.8), 55(10.3), 41(17.5). Anal. calc. for C12H21NO4
(243.30): C 59.24, H 8.70, N 5.76; found: C 59.33, H 8.62, N 5.70.
Methyl 1-({[(tert-Butoxy)carbonyl]amino}methyl)cyclopentane-1-carboxylate (Boc-b2,2-HAc5c-OMe;
15c). Compound 14c [33][36] (4.11 g, 26.8 mmol) was transformed according to GP 5a. FC (Et2O/pentane
1:5) yielded 15c (3.93 g, 57%). Colorless oil. Rf 0.33 (Et2O/pentane 1:5). IR (CHCl3): 3452w, 3146 (br.),
2982m, 2933m, 2862w, 1705s, 1507s, 1455w, 1393w, 1368m, 1326w, 1166s, 1128m, 1039w, 955w, 863w. 1H-NMR
(400 MHz, CDCl3): 1.43 (s, t-Bu); 1.55 ± 1.79 (m, 6 CH); 1.91 ± 2.00 (m, 2 CH); 3.27 (d, J ˆ 6.5, CH2N); 3.70
(s, CO2Me); 5.04 (br., NH). 13C-NMR (100 MHz, CDCl3): 25.57 (CH2); 28.40 (Me); 34.45, 46.13 (CH2); 52.05
‡
‡
(Me); 54.37, 79.17, 156.34, 178.24 (C). EI-MS: 279 (< 1, [M ‡ Na] ), 257 (< 1, M ), 170(11.0), 128(100),
57(37.3). Anal. calc. for C13H23NO4 (257.33): C 60.68, H 9.01, N 5.44; found: C 60.79, H 9.08, N 5.38.
Methyl 1-({[(tert-Butoxy)carbonyl]amino}methyl)cyclohexane-1-carboxylate (Boc-b2,2-HAc6c-OMe; 15d).
Compound 14d (14.7 g, 88.0 mmol) was transformed according to GP 5a. FC (Et2O/pentane 1 :7 ! 2 : 7) yielded
15d (13.59 g, 57%). Colorless oil. Rf 0.21 (AcOEt/pentane 1 : 12). IR (CHCl3): 3451w, 2983w, 2936m, 2861w,
1711s, 1509s, 1454m, 1393w, 1368m, 1165s, 1137m, 1102w, 1022w, 967w, 859w. 1H-NMR (400 MHz, CDCl3): 1.27
(t, J ˆ 7.1, Me); 1.42 (s, t-Bu); 1.25 ± 1.64 (m, 4 CH2); 1.96 ± 2.01 (m, CH2); 3.27 (d, J ˆ 6.4, CH2N); 4.16 (q, J ˆ
7.1, CH2O); 4.76 (br. s, NH). 13C-NMR (100 MHz, CDCl3): 14.24 (Me); 22.51, 25.69 (CH2); 28.37 (Me); 31.46,
‡
47.37 (CH2); 47.70 (C); 60.60 (CH2); 79.18, 155.99, 175.99 (C). EI-MS: 286 (3.6, [M ‡ 1] ), 230(31.7), 212
(11.7), 184(39.9), 156(100.0), 138(19.0), 128(29.4), 110(10.5), 95(26.1), 81(28.5), 74(8.9), 67(14.0),
57(80.4), 41(24.4), 30(16.1). Anal. calc. for C15H27NO4 (285.38): C 63.13, H 9.54, N 4.91; found: C 63.34,
H 9.52, N 5.07.
1-({[(tert-Butoxy)carbonyl]amino}methyl)cyclopropane-1-carboxylic Acid (Boc-b2,2-HAc3c-OH; 16a). To
a soln. of 15a (132 mg, 0.57 mmol) in MeOH (0.6 ml) was added a soln. of LiOH (33 mg, 1.36 mmol) in H2O
(0.3 ml). This soln. was stirred for 3 d. MeOH was evaporated and the residue extracted with Et2O. The aq.
phase was acidified (pH 1 ± 2) with 10% HCl and extracted with Et2O (3Â). The Et2O phase was washed with
H2O, dried (MgSO4), and concentrated under reduced pressure to yield the crude oil. FC (CH2Cl2/MeOH 15 : 1)
and recrystallization (AcOEt/pentane) yielded 16a (58 mg, 48%). Colorless needles. M.p. 121 ± 1228. Rf 0.18
(CH2Cl2/MeOH 15 : 1). IR (CHCl3): 3454w, 2974 (br.), 1705s, 1508m, 1451w, 1395w, 1367w, 1169w, 1046w, 939w,
850w. 1H-NMR (400 MHz, CDCl3; signals of rotamers in italics): 0.88 ± 1.02 (m, 2 CH); 1.24 ± 1.30 (m, 2 CH);
1.44 (s, t-Bu); 3.28 (br. d, J ˆ 5.6, CH2N); 5.26, 6.11 (br., NH). 13C-NMR (100 MHz, CDCl3): 15.44 (CH2); 24.99
‡
(C); 28.40 (Me); 43.69 (CH2); 79.43, 156.32, 181.34 (C). FAB-MS: 216 (42.7, [M ‡ 1] ), 160(100), 116(35.6).
Anal. calc. for C10H17NO4 (215.25): C 55.80, H 7.96, N 6.51; found: C 55.32, H 7.52, N 6.27.
1-({[(tert-Butoxy)carbonyl]amino}methyl)cyclobutane-1-carboxylic Acid (Boc-b2,2-HAc4c-OH; 16b).
Compound 14b [33][36] (4.29 g, 30.8 mmol) was transformed according to GP 5b. Recrystallization (AcOEt/
pentane) yielded 16b (2.57 g, 36%). Colorless crystals. M.p. 94.2 ± 95.28. Rf 0.67 (MeOH/CH2Cl2 1:9). IR
(CHCl3): 3453w, 2978m, 2922m, 1706s, 1506s, 1450w, 1394w, 1361m, 1322w, 1250m, 1167m, 1128w, 1039w, 1006w,
956w, 917w, 861w. 1H-NMR (400 MHz, CDCl3; signals of rotamers in italics): 1.44, 1.49 (s, t-Bu); 1.96 ± 2.18
(m, 4 CH); 2.41 ± 2.47 (m, 2 CH); 3.53 (br. d, J ˆ 6.3, CH2N); 5.02, 6.22 (br., NH). 13C-NMR (100 MHz, CDCl3;
signals of rotamers in italics): 14.13, 15.66, 22.66, 27.59 (CH2); 28.36, 31.60 (Me); 45.03, 46.50 (CH2); 47.25,
‡
‡
47.81, 79.46, 80.88, 156.53, 157.71, 180.41, 182.01 (C). FAB-MS: 459 (11.3, [2M ‡ 1] ), 230 (49.6, [M ‡ 1] ),
174(100). Anal. calc. for C11H19NO4 (229.28): C 57.63, H 8.35, N 6.11; found: C 57.73, H 8.33, N 6.18.
1-({[(tert-Butoxy)carbonyl]amino}methyl)cyclopentane-1-carboxylic Acid (Boc-b2,2-HAc5c-OH; 16c).
Compound 14c [33][36] (13.79 g, 90.0 mmol) was transformed according to GP 5b. Recrystallization
(AcOEt/hexane) yielded 16c (7.03 g, 32%). Colorless crystals, suitable for X-ray analysis. M.p. 124.5 ± 125.58.
Rf 0.45 (MeOH/CH2Cl2 (1:9). IR (CHCl3): 3444w, 2967m, 2867w, 1700s, 1506s, 1450w, 1394m, 1367m, 1167s,
1039w, 906w, 856w. 1H-NMR (400 MHz, CDCl3; signals of rotamers in italics): 1.44, 1.47 (s, t-Bu); 1.60 ± 1.76
(m, 6 CH); 2.00 ± 2.11 (m, 2 CH); 3.28 (d, J ˆ 6.5, CH2N); 5.11, 6.33 (br. t, NH). 13C-NMR (100 MHz, CDCl3;
signals of rotamers in italics): 25.33, 25.70 (CH2); 28.38 (Me); 34.15, 34.62, 45.75, 46.88 (CH2); 54.24, 54.70 (C);
‡
‡
79.36, 80.77, 156.43, 157.74, 181.86, 183.97 (C). FAB-MS: 768 (8.7, [3M ‡ K] ), 525 (12.3, [2M ‡ K] ), 509
‡
‡
‡
‡
(10.8, [2M ‡ Na] ), 487 (16.3, [2M ‡ 1] ), 266 (25.5, [M ‡ Na] ), 244 (11.6, [M ‡ 1] ), 188(61.0), 170(100),
142(50.0). Anal. calc. for C12H21NO4 (243.30): C 59.24, H 8.70, N 5.76; found: C 59.16, H 8.60, N 5.79.

Helvetica Chimica Acta ± Vol. 81 (1998)
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1-({[(tert-Butoxy)carbonyl]amino}methyl)cyclohexane-1-carboxylic Acid (Boc-b2,2-HAc6c-OH; 16d).
Compound 15d (2.92 g, 10.2 mmol) was saponified according to GP 3b. Recrystallization (CH2Cl2/AcOEt/
hexane) yielded 16d (2.35 g, 89%). Colorless crystals. M.p. 156 ± 1588. Rf 0.29 (MeOH/CH2Cl2 1:20). IR (KBr):
3318m, 3260m, 3107w, 2982m, 2954m, 2867m, 2550w, 1894w, 1706s, 1656s, 1483m, 1449s, 1411s, 1367s, 1330m,
1319m, 1237m, 1204m, 1151s, 1140s, 1102m, 1082w, 1047m, 1027m, 980m, 950w, 934w, 921w, 881w, 849w, 822w,
784w, 767w, 749w, 687w, 659w, 590w, 554w, 536w, 446w, 410w. 1H-NMR (400 MHz, (CD3)2NCDO; signals of
rotamers in italics): 1.19 ± 1.39 (m, t-Bu, 5 CH); 1.51 ± 1.59 (m, 3 CH); 1.91 ± 1.99 (m, 2 CH); 3.21 (d, J ˆ 6.5,
CH2N); 6.21, 6.51 (br., NH). 13C-NMR (100 MHz, (CD3)2NCDO, signals of rotamers in italics): 23.41, 26.19
‡
(CH2); 28.46 (Me); 32.00, 48.56, 48.76 (CH2); 78.62, 157.04, 177.77 (C). FAB-MS: 537 (7.1, [2M ‡ Na] ), 515
‡
‡
‡
(13.5, [2M ‡ 1] ), 280 (21.4, [M ‡ Na] ), 258 (33.3, [M ‡ 1] ), 202(100), 184(87.0), 156(60.3). Anal. calc. for
C13H23NO4 (257.33): C 60.68, H 9.01, N 5.44; found: C 60.41, H 8.96, N 5.42.
1-({[(9H-Fluoren-9-ylmethoxy)carbonyl]amino}methyl)cyclohexane-1-carboxylic Acid (Fmoc-b2,2-HAc6c-
OH; 17). Compound 14d (3.0 g, 18.1 mmol) was reduced and saponified as described in GP 3b. The crude free
amino acid was dissolved in H2O (0.2m) and NaHCO3 (ca. 2 equiv.) was added until a pH of 8 ± 9 was reached. A
soln. (0.2m) of Fmoc-OSu (1.1 equiv.) in acetone was added slowly. After 2 h, the mixture was evaporated,
diluted with H2O, and extracted with pentane. The pH was carefully adjusted to 1 ± 2 with 1n HCl and the aq.
phase extracted with AcOEt. The org. phase was washed with H2O, dried (MgSO4), evaporated, and dried
under h.v. Recrystallization (CH2Cl2) gave 17 (5.00 g, 73%). Crystalline solid (needles). M.p. 175 ± 1858. Rf 0.56
(CH2Cl2/MeOH 9 :1). IR (CHCl3): 3677w, 3448w, 3032w, 3012m, 2938m, 2860w, 1717s, 1517s, 1451m, 1228s,
1220s, 1204s, 1137w, 1106w, 1040w, 880w. 1H-NMR (400 MHz, CDCl3): 0.88 ± 2.17 (m, 10 CH); 3.13 (br. s, 0.2 H,
CH2N, rotamer); 3.36 (d, J ˆ 6.6, 1.8 H, CH2N, rotamer); 4.08 ± 4.23 (m, 1 CH); 4.31 (d, J ˆ 7.2, 1.8 H, CH2O,
rotamer); 4.47 (br. s, 0.2 H, CH2O); 5.78 (br. s, 0.1 H, NH, rotamer); 6.41 (s, 0.9 H, NH, rotamer); 7.32 (d, J ˆ
7.5, 2 arom. H); 7.39 (d, J ˆ 7.5, 2 arom. H); 7.69 (d, J ˆ 7.7, 2 arom. H); 7.85 (d, J ˆ 7.6, 2 arom. H); 10.79
(br. s, COOH). 13C-NMR (100 MHz, CDCl3): 23.55, 26.46, 29.83, 32.23 (CH2); 48.08 (CH); 48.74 (CH2); 49.24
(C); 66.98 (CH2); 120.77, 126.15, 127.90, 128.47 (CH); 142.08, 145.14, 157.43, 177.02 (C). FAB-MS: 759 (3.0,
‡
‡
[2M ‡ 1] ), 380 (50.6, [M ‡ 1] ), 338(7.4), 289(43.4), 273(11.9), 244(6.0), 184(10.4), 178(30.4), 166(8.9).
Anal. calc. for C23H25NO4 (379.45): C 72.80, H 6.64, N 3.69; found: C 72.64, H 6.69, N 3.66.
Boc-b2,2-HAc6c-b2,2-HAc6c-OMe (20a). Compound 15d (1.31 g, 4.6 mmol) was Boc-deprotected according
to GP 2a and coupled with 16d (1.19 g, 4.6 mmol) according to GP 6 except that 16d was dissolved in DMF
(0.25m) instead of CHCl3. The crude product 20a was purified by FC (AcOEt/pentane 1 :2) yielding 20a (1.169 g,
62%). Colorles oil. Rf 0.19 (Et2O/pentane 1:1). IR (CHCl3): 3457w, 3364w, 3007m, 2934s, 2859m, 1707s, 1660m,
1
1510s, 1454m, 1392w, 1367m, 1323w, 1246m, 1163s, 1140m, 1105w, 1045w, 964w. H-NMR (400 MHz, CDCl3):
1.16 ± 1.65 (m, t-Bu, 16 CH); 1.79 ± 1.83 (m, 2 CH); 2.04 ± 2.08 (m, 2 CH); 3.19 (d, J ˆ 6.1, CH2N); 3.40 (d, J ˆ
6.2, CH2N); 3.72 (s, CO2Me); 5.30 (s, NH); 6.14 (t, J ˆ 5.7, NH). 13C-NMR (100 MHz, CDCl3): 22.49, 25.65
(CH2); 28.41 (Me); 31.83 (CH2); 46.40 (CH2); 47.60 (C); 48.00 (CH2); 52.25 (Me); 77.04 (C); 156.38 (C);
‡
‡
‡
175.00, 176.70 (C). FAB-MS: 843 (1.4, [2M ‡ Na] ), 821 (1.2, 2M ), 433 (11.9, [M ‡ Na] ), 411 (67.5, [M ‡
‡
1] ), 355(24.7), 311(100), 281(17.0). Anal. calc. for C22H38N2O5 (410.55): C 64.36, H 9.33, N 6.82; found:
C 64.34, H 9.11, N 6.60.
CF3COOH ´ H-b2,2-HAc6c-b2,2-HAc6c-OMe (20c). According to GP 2a, 20a (1.145 g, 2.79 mmol) was Boc-
deprotected yielding the crude CF3COOH salt 20c (1.844 g) which was used in the next step without further
purification. Yellowish oil. Rf 0.09 (AcOEt/pentane 1 : 1). IR (CHCl3): 3011m, 2941s, 2862m, 2568w, 1781s,
1529m, 1454m, 1174s, 1039m, 873w, 818w. 1H-NMR (400 MHz, CDCl3): 1.20 ± 2.39 (m, 20 CH); 3.12 (br. s,
‡
‡
CH2NH3 ); 3.38 (d, J ˆ 6.1, CH2N); 3.71 (s, CO2Me); 6.90 (t, J ˆ 5.9, NH); 7.26 (br. s, NH3 ); 10.65 (br. s,
COOH). 13C-NMR (100 MHz, CDCl3): 22.52, 25.43, 28.63, 31.23, 41.46, 44.23, 47.26 (CH2); 52.61 (CH3); 111.09,
‡
‡
113.94, 116.81, 119.17, 161.26, 169.15, 175.40, 177.56 (C). FAB-MS: 931 (3.6, [3M ‡ 1] ), 643 (5.5, [2M ‡ Na] ),
‡
‡
‡
621 (79.8, [2M ‡ 1] ), 333 (4.4, [M ‡ Na] ), 311 (100, [M ‡ 1] ), 172(9.6), 140(6.3), 112(14.0).
Boc-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-OMe (23a). Compound 16d (0.719 g, 2.79 mmol) was coupled with
the CF3COOH salt 20c (1.145 g, 2.79 mmol) according to GP 6. Recrystallization (CH2Cl2/hexane) afforded 23a
(1.163 g, 76%). Crystalline solid. M.p. 98 ± 1018. Rf 0.20 (AcOEt/pentane 1:2). IR (CHCl3): 3453m, 3007m,
2935s, 2859m, 1707s, 1646s, 1511s, 1455m, 1392w, 1367m, 1248m, 1166m, 1140w, 1047w, 874w. 1H-NMR
(400 MHz, CDCl3): 1.26 ± 1.64 (m, t-Bu, 24 CH); 1.78 ± 1.86 (m, 4 CH); 2.2 ± 2.05 (m, 2 CH); 3.24 (d, J ˆ 6.1,
CH2N); 3.37 (d, J ˆ 5.8, CH2N); 3.40 (d, J ˆ 6.1, CH2N); 3.70 (s, CO2Me); 5.20 (s, NH); 6.28 (s, NH); 6.68
(s, NH). 13C-NMR (100 MHz, CDCl3): 22.35, 22.43, 22.54, 22.66, 25.60, 25.77, 25.81 (CH2); 28.42 (Me); 31.71,
31.84, 31.92 (CH2); 46.10, 46.18, 46.55, 47.00 (CH2); 47.39 (C); 47.51 (CH2); 52.22 (C); 53.43 (Me); 76.78, 77.03,
‡
‡
77.28 (C); 78.88 (C). FAB-MS: 1010 (7.9, [2M ‡ 1] ), 550 (100, [M ‡ 1] ), 450(84.8), 420(27.8), 323(14.4),

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Helvetica Chimica Acta ± Vol. 81 (1998)
281(12.2), 250(14.4), 184(8.3), 172(8.9). Anal. calc. for C30H51N3O6 (549.76): C 65.54, H 9.35, N 7.64; found:
C 65.55, H 9.39, N 7.28.
Boc-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-OH (23b). Compound 23a (1.10 g, 2 mmol) was saponified according
to GP 3b to afford 23b (1.056 g, 99%). White powder. M.p. 172 ± 1748. Rf 0.18 (AcOEt/pentane 1:2). IR
(CHCl3): 3454m, 3383m, 3007m, 2932s, 2861m, 1706s, 1652s, 1515s, 1455s, 1404m, 13368m, 1318w, 1252m, 1165s,
1
1045w, 1024w, 959w, 908m, 849w, 652w. H-NMR (400 MHz, CDCl3): 1.20 ± 2.25 (m, t-Bu, 30 CH); 3.17 (d, J ˆ
6.8, 1.4 H, CH2N, rotamer); 3.21 (d, J ˆ 6.1, 0.6 H, CH2N, rotamer); 3.33 ± 3.37 (m, CH2N); 3.41 ± 3.43
(m, CH2N); 5.23 (br. s, 0.3 H, NH, rotamer); 6.01 (t, J ˆ 6.4, 0.7 H, NH, rotamer); 6.34 (br. s, 0.3 H, NH,
rotamer); 6.45 (t, J ˆ 6.6, NH); 6.65 (br. s, 0.7 H, NH, rotamer). 13C-NMR (100 MHz, CDCl3): 22.49, 22.64,
22.76, 22.97, 23.05, 25.79, 25.86, 26.08 (CH2); 32.14 (Me); 31.58, 31.83, 31.97, 32.14, 46.46, 46.64, 47.04 (CH2);
47.17 (C); 47.27, 47.36, 47.47, 47.85 (CH2); 48.21 (C); 48.41, 49.84 (CH2); 76.72, 77.04, 77.24, 77.35, 79.75, 81.38,
‡
‡
156.68, 158.31, 174.97, 175.40, 175.75, 177.95, 178.40 (C). FAB-MS: 1094 (1.1, [2M ‡ Na] ), 1072 (0.9, [2M ‡ 1] ),
‡
‡
‡
575 (1.1, [M ‡ K] ), 559 (8.0, [M ‡ Na] ), 537 (100.0, [M ‡ 1] ), 437(96.7), 407(22.6), 323(10.4), 267(6.0),
250(6.6), 112(7.1).
CF3COOH ´ H-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-OMe (23c). According to GP 2a, 23a (1.150 g, 2.1 mmol)
was Boc-deprotected to yield 23c (1.162 g), which was used in the next step without further purification.
Yellowish oil. Rf 0.08 (AcOEt/pentane 1 : 1). IR (CHCl3): 3674w, 3446w, 3005m, 2936s, 2861s, 1780m, 1708m,
1643s, 1526s, 1454m, 1434w, 1362w, 1325w, 1296w, 1252w, 1173s, 1167s, 1041m, 986w, 874m, 836m. 1H-NMR
(400 MHz, CDCl3): 1.26 ± 1.59 (m, 24 CH); 1.87 ± 1.88 (m, 2 CH); 2.02 ± 2.03 (m, 4 CH); 3.12 (br. s, CH2N);
‡
3.33 ± 3.36 (m, 2 CH2N); 3.71 (s, CO2Me); 6.63 (t, J ˆ 5.8, NH); 6.93 (br. s, NH3 ); 7.39 (br. s, NH); 7.89 (br. s,
COOH). 13C-NMR (100 MHz, CDCl3): 22.48, 25.47, 31.85, 44.37, 47.24, (CH2); 52.35 (Me); 67.04 (CH2); 77.03,
‡
‡
‡
113.88, 160.41, 176.89 (C). FAB-MS: 922 (2.9, [2M ‡ Na] ), 900 (26.4, [2M ‡ 1] ), 4.88 (0.4, [M ‡ K] ), 472
‡
‡
(4.1, [M ‡ Na] ), 451 (100.0, [M ‡ 1] ), 250(6.6), 140(8.1), 112(11.7).
Boc-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-OMe (4b). Compound 23c
(0.998 g, 1.87 mmol) was coupled with 23b (1.00 g, 1.87 mmol) according to GP 6. FC (AcOEt/pentane 3 :2),
followed by recrystallization (Et2O/pentane 5 :95), yielded 4b (0.651 g, 36%). White powder. M.p. 94 ± 968.
Rf 0.29 (AcOEt/pentane 3 :2). IR (CHCl3): 3446w, 3378w, 3005m, 2933s, 2859m, 1708m, 1641s, 1509s, 1455m,
1252w, 1166w, 654w, 600w. 1H-NMR (400 MHz, CDCl3): 1.16 ± 1.54 (m, t-Bu, 48 CH); 1.84 ± 1.94 (m, 10 CH);
2.03 ± 2.13 (m, 2 CH); 3.22 (d, J ˆ 6.2, CH2N); 3.32 ± 3.39 (m, 5 CH2N); 5.46 (t, J ˆ 5.8, NH); 6.44 (t, J ˆ 6.0,
NH); 6.84 (br. s, NH); 6.95 (t, J ˆ 5.7, NH); 7.03 (br. s, NH); 7.08 (br. s, NH). 13C-NMR (100 MHz, CDCl3):
22.34, 22.38, 22.45, 22.60, 23.73, 25.61, 25.70, 25.75, 25.83 (CH2); 28.43 (Me); 30.65, 31.68, 31.89, 31.92, 31.98,
34.13, 45.89, 46.09 (CH2); 46.15 (C); 46.20, 46.27, 46.50, 46.59, 46.68, 47.07, 47.39 (CH2); 47.57 (C); 52.27 (Me);
‡
78.78, 156.40, 175.76, 175.80, 176.09, 176.25, 176.31, 176.64 (C). FAB-MS: 990 (15.0, [M ‡ Na] ), 968 (72.3,
‡
[M ‡ 1] ), 868 (100.0), 837(9.2), 559(5.9).
Ac-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-NH2 (5). The Rink amide resin (691.6 mg, 0.45 mmol/g) was loaded
with 17 (354 mg, 934 mmol) according to GP 7b. Synthesis according to GP 8b afforded the deprotected
tripeptide on the resin. The resin was washed and filtered successively with DMF (5 Â 3 min), CH2Cl2 (3 Â
3 min), and Et2O (3 Â 3 min), and dried overnight under h.v., before being split in two equal parts. On the first
aliquot, N-acetylation according to GP 9 was accomplished. The rest was used to synthesize tetrapeptide 6.
Cleavage from the resin according to GP 10b afforded the crude tripeptide 5 (62.4 mg, 88%), purity 83% (RP-
HPLC). Purification by RP-HPLC (20 ± 80% B in 20 min) according to GP 11 yielded 5 (25.6 mg, 36%). RP-
HPLC (20 ± 80% B in 20 min; C8): tR 11.97. White fluffy solid. M.p. 2198 (dec., sintering at 2008). IR (CHCl3):
3684w, 3453w, 3008m, 2936s, 2862m, 1658s, 1511s, 1454m, 1252w, 1167w, 939w, 816w. 1H-NMR (400 MHz,
CD3OD): 1.19 ± 2.12 (m, COMe, 30 CH); 3.26 ± 3.32 (m, 2 CH2N); 3.38 (d, J ˆ 6.5, CH2N); 7.28 (br. s, NH);
8.01 (t, J ˆ 6.4, NH). 13C-NMR (100 MHz, CD3OD): 22.75 (Me); 23.89, 24.02, 26.31, 26.93, 26.94, 30.94, 32.74,
33.30, 34.83 (CH2); 42.72 (C); 49.05 (CH2); 124.03, 173.49, 177.26, 178.10, 178.17 (C). FAB-MS: 516 (1.8, [M ‡
‡
‡
‡
K] ), 499 (24.6, [M ‡ Na] ), 477 (100.0, [M ‡ 1] ), 460(38.4), 443(7.5), 338(12.8), 329(10.8).
Ac-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-NH2 (6). According to GP 8b, one more coupling/depro-
tection sequence was performed on the Rink amide resin, loaded with the corresponding deprotected tripeptide
(see synthesis of 5). N-Acetylation according to GP 9 and cleavage according to GP 10b yielded the crude
tetrapeptide 6 (22.5 mg, 24%), purity 68% (RP-HPLC). Purification by RP-HPLC (20 ± 80% B in 20 min)
according to GP 11 yielded 6 (7.3 mg, 10%). White fluffy solid. RP-HPLC (20 ± 80% B in 20 min; C8): tR 11.84.
M.p. 103 ± 1058 (sintering at 938). IR (CHCl3): 3685w, 3340m, 3008m, 2934s, 2859m, 1649s, 1520s, 1458w, 1167w,
932w, 650w. 1H-NMR (500 MHz, CDCl3): 1.16 ± 1.98 (m, 40 CH); 2.02 (s, COMe); 3.28 ± 3.32 (m, 3 CH2N); 3.39
(d, J ˆ 5.8, CH2N); 5.91 (br. s, 0.3 H, NH2); 6.49 (s, NH); 6.50 (br. s, 0.7 H, NH2); 6.60 (s, NH); 6.81 (s, NH);
7.22 (s, NH). 13C-NMR (125 MHz, CDCl3): 22.48, 22.56 (CH2); 22.99 (Me); 25.72, 25.74, 25.76, 29.71, 30.35,

Helvetica Chimica Acta ± Vol. 81 (1998)
2241
31.73, 32.17, 32.25, 32.38, 46.40 (CH2); 46.61, 46.76 (C); 46.90 (CH2); 47.17 (C); 47.61 (CH2); 76.77, 77.02, 77.28,
‡
‡
170.92, 175.57, 175.72, 175.95, 178.71 (C). FAB-MS: 639 (2.4, [M ‡ Na] ), 617 (100.0, [M ‡ 1] ), 550(8.1),
443(6.1), 338(18.1).
Ac-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-NH2 (4a). According to GP 7b, the
Rink amide resin (195.6 mg, 0.45 mmol/g) was loaded with 17 (105 mg, 277 mmol). Synthesis according to GP 8b,
acetylation according to GP 9, and cleavage from the resin according to GP 10b afforded the crude peptide 4a
(87.6 mg, 105%), purity 77% (RP-HPLC). The peptide was purified by RP-HPLC (35 ± 80% B in 20 min)
according to GP 1: 4a (52 mg, 63%). White fluffy solid. RP-HPLC (75% B isocratic in 20 min; C8): tR 4.87. M.p.
99 ± 1028. IR (CHCl3): 3353w, 3008m, 2933s, 2859m, 1645s, 1516s, 1464w, 1448s, 1252w. 1H-NMR (400 MHz,
CDCl3): 1.26 ± 1.60 (m, 48 CH); 1.80 ± 1.85 (m, 2 CH); 1.92-2.10 (m, 10 CH); 2.11 (s, COMe); 3.26 ± 3.41
(m, 6 CH2N); 3.90 (br. s, NH2); 6.57 ± 6.60 (m, 2 NH); 6.91 (s, NH); 7.16 (s, NH); 7.29 (s, NH); 7.62 (s, NH).
13C-NMR (100 MHz, CDCl3): 22.33, 22.38 (CH2); 22.49 (Me); 22.53, 25.67, 31.67, 31.90, 32.12, 32.36 (CH2); 46.69
(C); 46.83, 46.88, 47.04, 47.15, 47.42, 47.72 (CH2); 76.70, 77.22, 77.34, 172.70, 175.46, 176.03, 176.13, 180.34 (C).
‡
‡
FAB-MS: 916 (2.34, [M ‡ Na] ), 894 (100.0, [M ‡ 1] ), 766(7.8), 596(5.1).
CF3COOH ´ H-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-b2,2-HAc6c-OH (4c). According to
GP 7a, the ortho-chlorotrityl-choride resin (381 mg, 1 mmol/g) was esterified with 17 (116 mg, 305 mmol).
Loading 0.35 mmol/g (55%), corresponding to 132 mmol of anchored 17. Synthesis according to GP 8a and
cleavage from the resin according to GP 10a afforded the crude peptide 4c (55.6 mg, 47%), purity 65 ± 95%
(RP-HPLC), depending on the cleavage fraction (purity of first cleavage fraction: 65%; purity of the following
cleavage fractions up to 95% (HPLC)). Prep. RP-HPLC (20 ± 90% B in 45 min) according to GP 11 yielded 4c
(30.7 mg, 26%). White powder. Colorless crystals were obtained from CDCl3 by slow evaporation at r.t. RP-
HPLC (30 ± 90% B in 20 min; C18): tR 14.26. M.p. 1408 (dec., sintering at 1008). Rf 0.03 (AcOEt). IR (CHCl3):
3456w, 3364w, 3056w, 2935s, 2851m, 2236w, 1780w, 1643s, 1523s, 1251w, 1168m. 1H-NMR (400 MHz, CDCl3):
1.20 ± 1.38 (m, 48 CH); 1.51 ± 1.92 (m, 8 CH); 2.07 ± 2.23 (m, 4 CH); 3.05 (br. s, CH2N); 3.28 ± 3.40
‡
(m, 5 CH2N); 5.15 (br. s, NH3 ); 6.44 (s, NH); 6.67 (s, NH); 6.90 (s, NH); 7.04 (s, NH); 7.52 (s, NH); 8.16
(br. s, COOH). 13C-NMR (100 MHz, CDCl3): 22.11, 22.47, 22.78, 25.30, 25.61, 29.71, 31.41, 32.03 (CH2); 44.85
‡
(C); 46.91, 47.26, 47.68 (CH2); 77.02, 174.57, 175.96, 176.11, 176.43, 178.27 (C). FAB-MS: 892 (1.4, [M ‡ K] ),
‡
‡
876 (8.5, [M ‡ Na] ), 854 (100.0, [M ‡ 1] ), 715(5.2), 329(9.7).
16. X-Ray Crystal-Structure Analyses. Compound 23b (C30H51N3O6). Crystals were grown from a
supersaturated CH2Cl2 soln. Crystal size 0.3 Â 0.3 Â 0.4 mm. Crystal data at 193 K for (C30H51N3O6 ´ CH2Cl2,
Mr 634.7): Triclinic, space group P1 (No. 2), 1calc. ˆ 1.26 g cm 3, Z ˆ 2, a ˆ 10.246(4) Š, b ˆ 10.959(2) Š, c ˆ
Å
17.501(6) Š, a ˆ 103.10(2)8, b ˆ 95.12(3)8, g ˆ 116.47(2)8, Vˆ 1672(1) Š3. Nonius CAD4 diffractometer, MoKa
radiation, l ˆ 0.7107 Š, 4918 unique reflections measured in the range 0 < q < 24.038, 4300 observed reflections
with I > 2s(I). The structure was solved by direct methods (SIR92 [55]), and refined by full-matrix least-
squares analysis (SHELXL93 [56]), using an isotropic extinction correction and w ˆ 1/[s2(Fo2) ‡ (0.0377P)2 ‡
1.13P], where P ˆ (Fo2 ‡ 2Fc2)/3 (heavy atoms anisotropic, H-atoms isotropic, whereby H-positions are based on
stereochemical considerations). Final R(F) ˆ 0.037, wR(F2) ˆ 0.095 for 428 variables and 4300 observations.
Further details of the structure analysis are available on request from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB12 1EZ (UK), on quoting the full journal citation.
Compound 16a (C10H17NO4). Crystal data at 263 K. Monoclinic, space group P21/a (No. 14), 1calc. ˆ 1.24 g
cm 3, Z ˆ 4, a ˆ 9.983(2) Š, b ˆ 10.312(3) Š, c ˆ 11.207(3) Š, b ˆ 93.49(2)8, Vˆ 1151.6(5) Š3. Nonius CAD4
diffractometer, CuKa radiation, l ˆ 1.5418 Š, 1820 unique reflections measured in the range 0 < q < 64.84.
Single crystals were obtained by slow evaporation of a CH2Cl2/hexane soln. The structure was solved by direct
methods (SIR92 [55]), and refined by full-matrix least-squares analysis (SHELXL93 [56]), using w ˆ 1/
[s2(Fo2) ‡ (0.069P)2 ‡ 0.335P], where P ˆ (Fo2 ‡ 2Fc2)/3 (heavy atoms anisotropic, H-atoms isotropic, whereby
H-positions are based on stereochemical considerations). Final R(F) ˆ 0.042, wR(F2) ˆ 0.108 for 156 variables
and 1404 observations with I > 2s(I).
Compound 16b (C11H19NO4). Crystal data at 263 K. Monoclinic, space group P21/c (No. 14), 1calc. ˆ 1.21 g
cm 3, Z ˆ 4, a ˆ 11.242(1) Š, b ˆ 11.516(1) Š, c ˆ 11.228(1) Š, b ˆ 108.26(1)8, Vˆ 1257.6(2) Š3. Nonius CAD4
diffractometer, CuKa radiation, l ˆ 1.5418 Š, 2236 unique reflections measured in the range 0 < q < 66.98.
Single crystals were obtained by slow evaporation of a AcOEt/hexane soln. The structure was solved by direct
methods (SIR92 [55]), and refined by full-matrix least-squares analysis (SHELXL93 [56]), using an isotropic
extinction correction and w ˆ 1/[s2(F02) ‡ (0.056P)2 ‡ 0.493P], where P ˆ (Fo2 ‡ 2Fc2)/3 (heavy atoms aniso-
tropic H-atoms isotropic, whereby H-positions are based on stereochemical considerations). Final R(F) ˆ 0.037,
wR(F2) ˆ 0.105 for 168 variables and 1930 observations with I > 2s(I).

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Helvetica Chimica Acta ± Vol. 81 (1998)
Compound 16c (C12H21NO4). Crystals were grown from AcOEt/hexane by slow evaporation at r.t. From a
crystal of size 0.20 Â 0.15 Â 0.05 mm, 2628 reflections were measured on an Enraf Nonius CAD-4 diffractometer
wtih CuKa radiation (graphite monochromator, l ˆ 1.54184 Š). The structure was solved by direct method with
SHELXS-96 [56]. The non-H-atoms were refined anisotropically with SHELXL-97. H-atoms were obtained
from a difference Fourier map and refined with constrained isotropic displacement parameters.
Compound 16d (C13H23NO4). Crystals were grown from a AcOEt soln. by slow evaporation at r.t. Crystal
size 0.20  0.20  0.20 mm. Monoclinic, space group P21/c, 1calc. ˆ 1.223 g cm 3, Z ˆ 4, a ˆ 11.026(2) Š, b ˆ
10.698(3) Š, c ˆ 12.127(4) Š, b ˆ 102.39(2)8, Vˆ 1397.1(7) Š3. Nonius CAD4 diffractometer, CuKa radiation,
l ˆ 1.5418 Š, 2367 unique reflections measured in the range 4.10 < q < 64.978. The structure was solved by direct
methods (SIR92 [55]), and refined by full-matrix least-squares analysis (SHELXL93 [56]), using an isotropic
extinction correction and w ˆ 1/[s2(Fo2) ‡ (0.0434P)2 ‡ 0.5375P], where P ˆ (Fo2 ‡ 2Fc2)/3 (heavy atoms
anisotropic, H-atoms isotropic, whereby H-positions are based on stereochemical considerations). Final
R(F) ˆ 0.0353, wR(F2) ˆ 0.0931 for 190 variables and 2367 observations. Further details of the structure
analysis are available on request from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB12 1EZ(UK), on quoting the full journal citation.
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