Synthesis of β3-homophenylalanine-derived amino acids and peptides by Suzuki coupling in solution and on solid support

Article abstract of DOI:10.1002/hlca.200690143

β-Peptides and, to a certain extent, also mixed α,β- peptides, are resistant to degradation by a variety of proteolytic enzymes that rapidly degrade natural α-peptides. This is one of many characteristics that make β-peptides an attractive class of compou

Full text of DOI:10.1002/hlca.200690143

Helvetica Chimica Acta – Vol. 89 (2006)
1427
Synthesis of b3-Homophenylalanine-Derived Amino Acids and Peptides by
Suzuki Coupling in Solution and on Solid Support
by Michael Limbach1), Markus Löweneck2), Jürg V. Schreiber3), Jens Frackenpohl4), and
Dieter Seebach*
Laboratorium für Organische Chemie, Departement Chemie und Angewandte Biowissenschaften,
ETH-Zürich, HCI Hönggerberg, Wolfgang-Pauli-Strasse 10, CH-8093 Zürich
(phone: +41446322990; fax: +41446321144; e-mail: seebach@org.chem.ethz.ch)
and
Andreas Billich
Novartis Institute for Biomedical Research Vienna, Brunnerstrasse 59, A-1235 Vienna
b-Peptides and, to a certain extent, also mixed a,b-peptides, are resistant to degradation by a variety
of proteolytic enzymes that rapidly degrade natural a-peptides. This is one of many characteristics that
make b-peptides an attractive class of compounds for drug-discovery studies. On the other hand, modern
organometallic reactions such as the Suzuki–Miyaura cross-coupling have become standard tools in
industry laboratories to derivatize side chains of a-peptidic compounds to build up libraries of unnatural
peptides. Combining both features, we prepared (4-bromo)-b3-homophenylalanine derivatives 3–5 and
12 as precursors for Suzuki–Miyaura couplings. From these bromo compounds, we synthesized biaryl-
substituted b-homoamino acids 6, and analogs 13 and 15 of the anti-AIDS drug Saquinavir.
Introduction. – The Suzuki–Miyaura cross-coupling reaction is a powerful synthetic
tool for C,C-coupling reactions of borono derivatives with aryl or vinyl halides or tri-
flates5), as the reaction is unaffected by the presence of H2O, tolerates a broad range
of functional groups, proceeds regio- and stereoselectively in good yields, and nontoxic
inorganic side-products are easily removed. The reaction has been used with (4-bor-
ono)- [2], (4-bromo)- [3], and (4-iodo)phenylalanine [4][5], serine-derived alkylbor-
onic acids [6], and tyrosine triflates [3][7] for the preparation of phenylalanine deriva-
tives, which are modified in the 4-aryl position. These unnatural amino acids have been
incorporated into small peptides to increase both their pharmacological potential and
metabolic stability [8].
1
)
Postdoctoral Research Fellow (2005/2006), financed by Novartis Institute of Biomedical Research,
CH-Basel.
Postdoctoral Research Fellow at ETH-Zürich, financed by Swiss National Science Foundation, Pro-
2
)
A
3
4
)
)
Part of the ETH Dissertation No. 14298, Zürich 2001.
Postdoctoral Research Fellow at ETH-Zürich (2000/2001) financed by the Deutscher Akademischer
Austauschdienst (Hochschulsonderprogramm III).
5
)
For recent reviews, see [1].
© 2006 Verlag Helvetica Chimica Acta AG, Zürich

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Helvetica Chimica Acta – Vol. 89 (2006)
Whereas short a-peptides [5] or even a-proteins [9] bearing the above mentioned
(4-aryl) phenylalanine precursors have been modified by Suzuki–Miyaura cross-cou-
pling, no such operation is known for b- or mixed a,b-peptides. While a-peptides are
rapidly degraded in vivo and in vitro by a multitude of peptidases, substrates with incor-
porated homologated a-amino acids (i.e., b-amino acids) exhibit a superior stability
profile [10]. To avoid a lack of recognition/inhibitory activity of these unnatural pepti-
des resulting from the different spatial arrangements of side chains and to augment sta-
bility, peptides containing both a- and b-amino acid residues may be an ideal compro-
mise.
Herein, we describe the preparation of 4-aryl-substituted b3-homophenylalanine
derivatives6) by Suzuki–Miyaura coupling in solution, starting from 4-bromo-b3-homo-
phenylalanine, which has been incorporated into a resin-bound tetrapeptide and subse-
quently cross-coupled on solid support to give analogs of the HIV-protease inhibitor
and anti-AIDS drug Saquinavir. This ‘late’ functionalization of a peptidic substrate,
combined with the advantages of solid-phase synthesis, is ideally suited for library syn-
thesis in medicinal chemistry to create molecular diversity [11].
Results and Discussion. – 1. Preparation of (S)-Boc-b3h(4-Br)Phe-OMe and (S)-
Fmoc-b3h(4-Br)Phe-OH. To test the Suzuki cross-coupling in solution, the appropri-
ately protected b3-homoamino acid 3 was prepared by Arndt–Eistert homologation
[12], starting from commercially available (S)-Boc-(4-bromo)phenylalanine (1): its
mixed anhydride, formed by treatment with isobutyl chloroformate, was allowed to
react with CH2N2 to give diazo ketone 2 and subsequent Wolff rearrangement in
MeOH/H2O 9 :1 provided the Boc-protected b3-homoamino acid methyl ester 3
(Scheme 1).
Scheme 1. Preparation of the Boc- and Fmoc-Protected b3-Homo-(4-Br)phenylalanine Derivatives 3
and 5 (NMM=N-methylmorpholine, Su=N-succinimido)
6
)
So far, our group has studied b-amino acids and b-peptides exclusively with proteinogenic side
chains! See the extensive review article [10d].

Helvetica Chimica Acta – Vol. 89 (2006)
1429
Although the preparation of the Fmoc-protected derivative 5 should also be possi-
ble from the commercially available Fmoc-protected a-amino acid, we have, instead,
saponified the methyl ester 3 to give the Boc-protected b3-homoamino acid 4, which
was subjected to a protective-group interchange to give the desired Fmoc-protected
b-amino acid 5, suitable for the synthesis of peptides on solid support.
2. Suzuki Cross-Coupling Reaction in Solution. To test the Suzuki cross-coupling
reaction in solution, we chose phenylboronic acid, and its 4-fluoro-, 4-methoxy-, 3-(tri-
fluoromethyl)-, and 3-chloro derivatives, as well as naphthalen-2-ylboronic acid. It
turned out that only 5 mol-% of the commercial Pd0 source tetrakis(triphenylphosphi-
ne)palladium [Pd(PPh3)4] was necessary to give the Boc-b3hPhe-OMe derivatives
6a–6f in yields of 82–97% (Scheme 2). Such enantiomerically pure protected b3-homo-
amino acids have previously been obtained by employing organozinc reagents [13],
which require the preparation and handling of sensitive organometallic reagents.
Scheme 2. Suzuki Cross-Coupling Reaction with the Protected b3-Homoamino Acid Ester 3 in Solu-
tiona) (2-naphth :naphthalen-2-yl)
a) The 13C-NMR spectra of compounds 6a–6f in CDCl3 at 208, as well as in C6
D6 at 658, showed
G
extensive signal broadening, due to the presence of rotamers.
For high conversions, an elevated temperature was required (908, 16 h), and, there-
fore, the reactions were carried out in sealed tubes. The progress of the reactions was
monitored either by TLC or, more advantageously for the reaction on solid support,
by following the evolution of CO2. Both the Boc and the methyl-ester group turned
out to be stable to the relatively harsh reaction conditions. All products were solid,
and their solutions in CHCl3 were levorotatory, except for the naphthyl derivative
6b, which was dextrorotatory.
3. Synthesis of a b-Tripeptide Analog of Saquinavir in Solution. In recent years, a
major research effort has been the development of pharmacologically active inhibitors
of the human immunodeficiency virus protease (HIVp) [14]. In 1990, Roche UK
reported [15][16] the subnanomolar active (Ki =0.12 nM) HIVp-inhibitor Saquinavir,
which contains a novel decahydroisoquinoline–hydroxyethylamine isostere replace-
ment of the Phe-Pro cleavage site of the natural-substrate binding motif Leu165-Asn-
Phe-Pro-Ile169 (Fig. 1).

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Helvetica Chimica Acta – Vol. 89 (2006)
Fig. 1. The HIVp-inhibitor Saquinavir and HIVps natural substrate
Substitution of all a-amino acids by their b2/b3-homologs with the same absolute
configuration and replacement of decahydroisochinoline by proline led to the b-tripep-
tidic Saquinavir analog7) 11 as a synthetic target molecule (Scheme 3).
Scheme 3. Preparation of the b-Peptidic Saquinavir Analog 11 in Solution (EDC: 1-[3-(dimethylami-
no)propyl]-3-ethylcarbodiimide hydrochloride, BtOH: 1-hydroxy-1H-benzotriazole)
7
)
According to our experience, this compound will be absolutely stable towards peptidases [10d–g].

Helvetica Chimica Acta – Vol. 89 (2006)
1431
t
In a first step, BuNH2 was coupled with Boc-(S)-b3hPro-OH (7) [17] under stan-
dard conditions (EDC, BtOH, Et3N) to give the carboxamide 8. Boc Deprotection
AHCTREUNG
and subsequent coupling with Boc-(S)-b3hPhe-OH gave the dipeptide 9, which was
deprotected and coupled with Boc-(R)-b2hVal-OH [18] to afford the b-tripeptide 10.
Final deprotection and coupling with quinaldic acid (=quinoline-2-carboxylic acid) fur-
nished, after purification by normal-phase HPLC, the desired b-peptide derivative 11,
which was fully characterized (cf. Scheme 3, Fig. 2, and see Exper. Part). Compound 11
did, however, not inhibit HIVp at concentrations of up to 60 mM [19].
Fig. 2. CD Spectrum of b-tripeptide derivative 11 at 258 (c=0.2 mM in MeOH). Due to the strong UV
absorption of the quinaldyl group (e=1.0·104 –1.3·104 at 200–250 nM for quinaldine in MeOH soln.)
[20], the CD spectrum of 11 cannot be compared with the spectra of b-peptides bearing only protei-
nogenic side chains [10d].
4. Suzuki Cross-Coupling Reaction on Solid Support. As another Saquinavir analog
suitable for modification by Suzuki coupling, we envisaged the resin-bound tetrapep-
tide 12, in which the phenylalanine residue of the natural substrate-binding motif
Leu165-Asn-Phe-Pro-Ile169 is replaced by (S)-b3h(4-Br)phenylalanine. Resin-bound
peptide 12 was synthesized on Rink amide AM resin [21], as the ester bond of amino
acids/peptides attached to a Wang resin has been reported not to be completely stable
under the conditions of the Suzuki coupling [22]. This strategy has previously been
reported for cross-coupling of resin-bound aromatic halides [23].
The Fmoc-protected amino acids were coupled in threefold excess by activation
with HATU/DIPEA 0.97:2 in DMF for 1 h (Scheme 4). For the coupling of the valua-
ble Fmoc-(S)-b3h(4-Br)-Phe-OH (5) only a twofold excess was used, and the coupling
time was prolonged to 2 h. Each coupling and deprotection step was monitored by the
TNBS test [24] (or by the chloranil test for the secondary amino group of Pro) [25], and,
if necessary, an additional coupling was performed (see Exper. Part).
Cleavage from the resin and deprotection were carried out with TFA/H2O/TIPS,
and the crude peptide was purified by RP-HPLC to give peptide 13 in a total yield

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Helvetica Chimica Acta – Vol. 89 (2006)
Scheme 4. Solid-Phase Synthesis of the Mixed a,b-Tetrapeptide 13, Containing H-(S)-b3h(4-Br)-Phe-
OH (DIPEA: EtN(i-Pr)2, HATU: O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluoro-
phosphate, TFA: CF3COOH, TIPS: (i-Pr)3SiH)
G
of 43% over eleven steps. This compound is by itself a Saquinavir analog, and it is ready
for preparing further derivatives of this type by Suzuki coupling.
With the resin-bound tetrapeptide 12, Suzuki coupling with 3-(trifluoromethyl)phe-
nylboronic acid (under conditions similar to those established in solution) was actually
performed in the presence of 10 mol-% Pd catalyst and EtOH as co-solvent (908, 5 h) to
give the resin-bound coupling product 14. Subsequent cleavage from the resin and
deprotection were carried out as described for the Br derivative 13, and the crude prod-
uct was purified by RP-HPLC to give the desired Saquinavir analog 15 in 59% yield
(Scheme 5). As the completeness of the reaction was monitored by following the evo-
lution of CO2, the crude peptide turned out to be essentially pure (>95% by HPLC).
Unfortunately, neither 13 nor 15 did inhibit HIVp at concentrations of up to 60 mM [19].
In summary, we have shown that the Suzuki coupling with a series of boronic acids
in solution (six examples) as well as on solid support (one example) is a versatile tool
for the derivatization of 4-(4-bromophenyl)-b3-homophenylalanine-containing amino
acids and a,b-mixed peptides. The corresponding cross-coupled products are obtained
in high yields (up to 97%) and with excellent purities. This methodology was success-
fully applied to the synthesis of an analog of the HIV-protease inhibitor Saquinavir.
M. Limbach gratefully acknowledges a fellowships from the German Merit Foundation (Studienstif-
tung des Deutschen Volkes) during undergraduate (2000–2002) and graduate studies (2002–2004).
Novartis Pharma AG, CH-Basel, is acknowledged for financial support. The authors are grateful to M.
Badine and A. K. Beck, ETH-Zürich, for proofreading the manuscript.
Experimental Part
1. General. Solvents for chromatography and workup procedures were distilled from Sikkon (anh.
CaSO4; Fluka) and from KOH (Et2
basic Al2O3 (Alumina, Woelm N, actvity I) to remove EtOH. Et3
CaH2 and stored over KOH. Protected Fmoc-amino acids were purchased from Fluka. Rink amide
ACHTREUNG
AHCTREUNG

Helvetica Chimica Acta – Vol. 89 (2006)
1433
Scheme 5. Suzuki Cross-Coupling with the Mixed a,b-Tetrapeptide 12 Anchored to Rink-Amide Resin
AM resin was purchased from Novabiochem. Caution: The generation and handling of CH2N2 requires
special precautions [26][27]. Abbreviations: Boc: (tert-butoxy)carbonyl, DIPEA: EtN(i-Pr)2, DMAP: 4-
(dimethylamino)pyridine, EDC: 1-[3-(dimethylamino)propyl]-3-ethylcarbodiimide hydrochloride,
Fmoc: [(9H-fluoren-9-yl)methoxy]carbonyl, Fmoc-OSu: (9H-fluoren-9-yl)methyl N-succinimidyl car-
bonate, h.v.: high vacuum (0.01–0.1 Torr), HATU: O-(7-Azabenzotriazol-1-yl)-1,1,3,3-tetramethyluro-
nium hexafluorophosphate, BtOH: 1-hydroxy-1H-benzotriazole, bhXaa: b-homoamino acid
[12][28–30], NMM: N-methylmorpholine, TFA: CF3COOH, TIPS: (i-Pr)3SiH, TNBS: 2,4,6-trinitroben-
G
zenesulfonic acid. TLC: Merck silica-gel 60 F254 plates; detection under UV light at 254 nm and monitor-
ing by solns. of ninhydrine (300 mg of ninhydrine, 3 ml of AcOH, and 100 ml of BuOH) or ‘Mo-stain’ (25
g of phosphomolybdic acid, 10 g of Ce(SO4)2 ·H2O, 60 ml of conc. H2SO4, 940 ml of H2O), followed by
heating with a heat gun. FC: Fluka silica gel 60 (40–63 mesh). The dimensions of the columns are
given as ‘diameter×height of the silica column’. IR Spectra: Measured as 1–2% CHCl3 soln. or KBr pel-
lets on a Perkin-Elmer-782 spectrophotometer, or neat on a Perkin-Elmer 1600 FT-IR spectrophotome-
ter. NMR Spectra: 1H-NMR Spectra were recorded on a Bruker AMX 400 (400 MHz) or Varian Gemini
300 (300 MHz). 13C-NMR Spectra were recorded on a Bruker AMX 400 (100 MHz) or Varian Gemini 300
(75 MHz). Chemical shifts d are given in ppm relative to resonances of solvent (1H: 7.26 ppm for CDCl3,
3.31 ppm for CD3
A
G
N

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Helvetica Chimica Acta – Vol. 89 (2006)
C6D6), coupling constants J are given in Hz; some compounds show the presence of rotamers: the chem-
ACHTREUNG
ical shifts are reported, and the intensities of rotamers were calculated where the signal of rotamers could
be assigned unequivocally. The multiplicities of signals were determined by the DEPT technique:
DEPT:+=primary or tertiary (positive DEPT signal), ꢀ=secondary (negative DEPT signal),
Cq =quaternary C-atoms. Mass spectra: VG Tribrid (EI), Bruker Reflex (MALDI), or IonSpec Ultima
4.7 T FT Ion Cyclotron Resonance (ICR, HR-MALDI, in a 2,5-dihydroxybenzoic acid matrix) mass
spectrometer in m/z (% of basis peak). Anal. HPLC: Analysis was performed on a Merck HPLC system
(LaChrom, pump type L-7150, UV detector L-7400, Interface D-7000, HPLC manager D-7000) with a
Macherey-Nagel C18 column (Nucleosil 100–5 C18 (250×4 mm)). Linear gradient of A: 0.1% TFA in
H2O and B: MeCN at a flow rate of 1 ml/min. TFA for anal. HPLC was used as UV-grade quality
(>99% GC). Prep. HPLC: Merck HPLC system (LaChrom, pump type L-7150, UV detector L-7400,
interface D-7000, HPLC manager D-7000) with Macherey-Nagel C18 column (Nucleosil 100–7 C18
(250×21 mm)). Gradient of A and B at a flow rate of 12 ml/min. TFA for prep. HPLC was used as
UV-grade quality (>99% GC). Normal-phase HPLC analysis was performed on a Lichrosolv Si-60, 7-
mm column (250×4 mm) by using an isocratic mixture or a linear gradient of i-PrOH and hexane at a
flow rate of 1 ml/min with UV detection at 220 nm. Crude products were purified by prep. HPLC on a
Lichrosolv Si-60, 7-mm column (250×21 mm) using an isocratic mixture or a linear gradient of i-
PrOH and hexane at a flow rate of 4 ml/min with UV detection at 220 nm and then evaporated under
reduced pressure. Lyophilization: Hetosicc cooling condenser with h.v. pump. or GAMMA I-20 equipped
with a controller LDC-2 M (Christ Gefriertrocknungsanlagen). M.p.: measured in open-end glass capil-
lary tubes on a Büchi 510 apparatus; uncorrected. Optical rotations [a]rD:t: were measured on a Perkin-
Elmer 241 polarimeter (10 cm, 1-ml cell). The solvents and concentrations (in g/100 ml) are indicated.
CD Spectra: Jasco J-710 spectropolarimeter from 190 to 250 nm with a Jasco PTC-348 WI peltier system
at 258 in 1-mm rectangular cells. The optical system was flushed with N2 at a flow rate of ca. 10 l/min.
Parameters: band width 1.0 nm, resolution 0.2–1 nm, sensitivity 100 mdeg, response 0.5 s, speed 50
nm/min, 5 accumulations. All spectra were corrected for the corresponding solvent spectrum. Peptide
concentration 0.2 mM. The molar ellipticity [q] in 10 deg·cm2 ·molꢀ1 (l in nm) is calculated for the cor-
responding peptide (not normalized), taking into account the mass of TFA for each free amino group.
Smoothing was done by Jasco software. Elemental analyses were performed by the Microanalytical Lab-
oratory of the Laboratorium für Organische Chemie, ETH-Zürich. Peptides with free amino groups form
TFA salts. The molecular mass (MS) corresponds to the peptides without TFA.
2. Synthesis of the Compounds. (S)-3-{[(tert-Butoxy)carbonyl]amino}-4-(4-bromophenyl)-1-diazo-
butan-2-one (2). A soln. of (S)-Boc-(4-bromo)phenylalanine (1; 2.10 g, 6.10 mmol) in THF (35 ml) was
i
cooled to ꢀ208. After addition of ClCO2 Bu (838 ml, 6.41 mmol) and NMM (705 ml, 6.41 mmol), the mix-
ture was stirred at ꢀ208 for 20 min. The resulting white suspension was allowed to warm to ꢀ58, and a
freshly distilled soln. of CH2N2 in Et2O was added until the rich yellow color persisted, and the mixture
G
was stirred under rewarming to r. t. for 16 h. Excess CH2N2 was destroyed by the addition of three drops
of AcOH. The mixture was diluted with AcOEt (200 ml), washed with sat. NH4Cl soln. (50 ml), NaHCO3
(50 ml), and brine (50 ml). Drying of the org. phase (MgSO4), removal of the solvent under reduced pres-
sure, and FC (50 g of SiO2, 3×20 cm; AcOEt/pentane 1:1, Rf 0.32) gave 674 mg (30%) of 2. Pale yellow
solid. M.p. 116–1188. [a]rD:t: =+13.0 (c=5.50, CHCl3). IR (neat): 3336, 3076, 2985, 2933, 2864, 2169, 2113,
1902, 1739, 1688, 1626, 1519, 1485, 1445, 1383, 1329, 1249, 1199, 1120, 1068, 1024, 1010, 975, 943, 897, 857,
812, 789, 765, 734, 704, 660. 1H-NMR (300 MHz, CDCl3): 1.40 (s, tBu); 2.76–3.13 (m, CH2); 4.27–4.49 (m,
CH); 5.00–5.19 (m, CH); 5.19–5.36 (m, CH); 7.06 (d, 3J=8.24, 2 arom. H); 7.41 (d, 3J=8.25, 2 arom. H).
13C-NMR (75 MHz, CDCl3, DEPT): 28.2 (+, tBu); 37.8 (ꢀ, CH2); 54.6 (ꢀ, CH2); 58.1 (+, CH); 80.1 (Cq,
tBu); 120.9 (Cq, arom. C); 131.1 (+, 2 arom. C); 131.6 (+, 2 arom. C); 135.3 (Cq, arom. C); 155.0 (Cq, C=
O); 192.8 (Cq, C=O). MALDI-MS: 392 (98), 390 (100), 370 (16, [M+H]+), 368 (15, M+, C15H18BrN3O3þ ;
calc. 368.2257), 286 (34), 284 (37).
(S)-Methyl 4-(4-Bromophenyl)-3-{[(tert-butoxy)carbonyl]amino}butanoate (3). Compound 2 (633
mg, 1.72 mmol) was dissolved in MeOH/H2O 9 :1 (10 ml). Under exclusion of light, a soln. of
AgCO2CF3 (50 mg, 172 mmol) in Et3N (726 ml, 4.37 mmol) was added at 08, and the resulting mixture
G
was stirred for 18 h under rewarming to r.t. Volatiles were removed under reduced pressure, and the res-
idue was taken up in AcOEt (200 ml). Washing of the org. phase with sat. NH4Cl (50 ml), NaHCO3 (50

Helvetica Chimica Acta – Vol. 89 (2006)
1435
ml), drying (MgSO4), and FC (50 g SiO2, 3×20 cm, AcOEt/pentane 1:1, Rf 0.48) yielded 525 mg of 3
(82%). Colorless solid. M.p. 102–1038. [a]rD:t: =ꢀ190.1 (c=4.30, CHCl3). IR (neat): 3352m, 3087w,
2982w, 2946w, 2868w, 2650w, 2319w, 2078w, 1988w, 1901w, 1729s, 1678s, 1516s, 1442m, 1372m, 1319m,
1
1280w, 1255m, 1211m, 1147s, 1022m, 987w, 889w, 842w, 791w, 756w, 715w, 656m. H-NMR (300 MHz,
C
6D6, 608): 1.38 (s, tBu); 2.07–2.20 (m, CH2); 2.55 (dd, 3J=7.6, 2J=2.4, CH2); 3.26 (s, MeO);
N
3
3
4.00–4.18 (m, CH); 4.62–4.78 (br. s, NH); 6.73 (d, J=8.1, 2 arom. H); 7.18 (d, J=8.1, 2 arom. H).
t
13C-NMR (75 MHz, C6
G
50.8 (+, MeO); 78.8 (Cq, Bu); 120.5 (Cq, arom. C); 131.1 (+, 2 arom. C); 131.6 (+, 2 arom. C); 137.1
(Cq, arom. C); 160.1 (Cq, C=O); 171.2 (Cq, C=O). MALDI-MS: 341 (7), 328 (8), 233 (9), 217 (10),
189 (9), 140 (12), 96 (9). MALDI-HR-MS: 394.0624 ([M+Na]+, C16H22BrNaNO4þ ; calc. 394.0618).
Anal. calc. for C16H22BrNO4 (372.3): C 51.26, H 5.96, N 3.76; found: C 51.67, H 6.20, N 3.84.
(S)-4-(4-Bromophenyl)-3-{[(tert-butoxy)carbonyl]amino}butanoic Acid (4). To a soln. of 3 (1.24 g,
3.33 mmol) in MeOH/H2O 1:1 (50 ml) was added NaOH (666 mg, 16.7 mmol), and the soln. was stirred
at r.t. for 14 h. MeOH was removed under reduced pressure, the aq. phase was acidified to pH 2 with 1N
HCl at 08, and the mixture was extracted with AcOEt (3×100 ml). Drying of the org. phase (MgSO4) and
removal of the solvent under reduced pressure yielded 1.11 g (93%) of 4. Colorless solid. M.p. 140–1428.
[a]rD:t: =ꢀ6.65 (c=6.50, CHCl3). IR (neat): 3355m, 2983w, 1686s, 1520s, 1488m, 1446w, 1366w, 1298m,
1
t
1251m, 1162s, 1054m, 1027w, 917w, 892m, 802m. H-NMR (300 MHz, CDCl3): 1.39 (s, Bu); 2.50–2.67
3
3
(m, CH2); 2.77–3.04 (m, CH2); 3.82–4.20 (m, CH); 5.06 (d, J=7.51, NH); 7.08 (d, J=8.24, 2 arom.
H); 7.42 (d, J=8.24, 2 arom. H); 9.59–10.5 (br. s, CO2H). 13C-NMR (75 MHz, CDCl3, DEPT): 28.2
3
t
t
(+, Bu); 37.5 (ꢀ, CH2); 39.6 (ꢀ, CH2); 48.4 (+, CH); 79.7 (Cq, Bu); 120.5 (Cq, arom. C); 131.0 (+, 2
arom. C); 131.5 (+, 2 arom. C); 136.5 (Cq, arom. C); 155.1 (Cq, C=O); 176.6 (Cq, C=O). MALDI-
MS: 404 (16), 402 (17), 398 (21), 398 (10), 396 (10), 382 (40), 380 (40), 282 (9), 261 (7), 256 (8), 255
(8), 191 (11). MALDI-HR-MS: 380.0468 ([M+Na]+, C15H20BrNNaO4þ ; calc. 380.0462).
(S)-4-(4-Bromophenyl)-3-({[(9H-fluoren-9-yl)methoxy]carbonyl}amino)butanoic Acid (5). To
a
soln. of 4 (1.13 g, 3.16 mmol) in CH2Cl2 (5 ml) at r.t. was added TFA (5 ml), and the soln. was stirred
for 5 h. Volatiles were removed under reduced pressure, and the residue was dried in the h.v. The residue
was dissolved in acetone (20 ml), and Na2CO3 (786 mg, 9.47 mmol), H2O (5 ml), and Fmoc-OSu (1.17 g,
3.47 mmol) were added, and the reaction was stirred for 10 h. Dilution with H2O (40 ml), extraction with
AcOEt (5 ml), acidification of the aq. phase to pH 1 with 1N HCl at 08, extraction of the aq. phase with
AcOEt (3×100 ml), drying of the org. phase (MgSO4), and FC (100 g of SiO2, 3×40 cm; AcOEt/pentane/
AcOH 1:2 :0.02, Rf 0.42) yielded 990 mg (65%) of 5. Colorless solid. M.p. 177–1788. [a]rD:t: =ꢀ16.5
(c=1.00, CHCl3). IR (neat): 2480w, 2360s, 2342s, 2167w, 2051w, 1981w, 1718m, 1677s, 1560w, 1542w,
1508w, 1489w, 1468w, 1451m, 1430m, 1334m, 1225w, 1173w, 1149w, 1105w, 1085w, 1038m, 1010w, 831w,
1
3
798m, 770m, 760m, 731m, 742w, 668w, 643w. H-NMR (300 MHz, CDCl3): 2.48 (d, J=6.86, CH2); 2.71
(dd, J=6.73, 11.0, CH); 2.84 (dd, J=6.73, 9.48, CH); 4.09–4.31 (m, 2 CH2); 7.09–7.15 (m, 2 arom. H);
3
3
7.25–7.40 (m, 6 arom. H); 7.57 (d, J=6.87, 2 arom. H); 7.78 (d, J=7.42, 2 arom. H). 13C-NMR (75
MHz, CDCl3, DEPT): 40.0 (ꢀ, CH2); 40.9 (ꢀ, CH2); 51.0 (+, CH); 67.6 (ꢀ, CH2); 120.9 (+, 2 arom.
C); 121.3 (Cq, arom. C); 126.3 (+, 2 arom. C); 128.2 (+, 2 arom. C); 128.8 (+, 2 arom. C); 132.5 (+, 4
arom. C); 139.0 (Cq, arom. C); 142.7 (Cq, 2 arom. C); 145.3 (Cq, 2 arom. C); 158.1 (Cq, C=O); 174.0
(Cq, C=O). MALDI-MS: 506 (15), 505 (8), 504 (26), 503 (7), 502 (24), 500 (11), 494 (14), 491 (8), 489
(13), 480 (9), 478 (17), 464 (13), 442 (23), 440 (10), 428 (10), 423 (17), 410 (8), 398 (37). MALDI-HR-
MS: 502.0627 ([M+Na]+, C25H22BrNNaOþ4 ; calc. 502.0624).
General Procedure for the Suzuki Coupling in Solution (GP). To a suspension of Na2CO3 (2 equiv., 86
mg, 806 mmol), the corresponding boronic acid derivative (1.5 equiv., 605 mmol), and 3 (1 equiv., 150 mg,
403 mmol) in a degassed mixture of toluene (2 ml) and H2O (250 ml) was added [Pd(PPh3)4] (0.05 equiv.,
24 mg, 20 mmol), and the suspension was heated in a Schlenck tube, sealed with a rubber septum, at 908
for 15 h. The course of the reaction was followed by penetrating the rubber septum with a fine needle to
see whether there is still CO2 formation (foaming of the reaction mixture!). Dilution of the mixture with
AcOEt (150 ml), extraction with sat. NaHCO3 soln. (50 ml) and brine (50 ml), drying of the org. phase
(Na2SO4), and FC on SiO2 (Et2
solids.
A

1436
Helvetica Chimica Acta – Vol. 89 (2006)
(S)-Methyl 4-[1,1’-Biphenyl-4-yl]-3-{[(tert-butoxy)carbonyl]amino}butanoate (6a). Yield: 144 mg
(97%). Colorless solid. M.p. 97–988. Rf (Et2
O/pentane 1:1) 0.52. [a]rD:t: =ꢀ11.8 (c=2.50, CHCl3). IR
(neat): 3367w, 2980w, 1727s, 1682s, 1515s, 1486m, 1438m, 1391w, 1305w, 1266w, 1250m, 1076w, 918w,
837m. 1H-NMR (300 MHz, CDCl3; rotamers A/B ca. 5 :3): 1.41 (s, tBu, rotamer A); 1.42 (s, tBu, rotamer
B); 2.32–2.70 (m, 2 CH2, rotamer A+B); 2.73–3.15 (m, 2 CH2, rotamer A+B); 3.70 (s, MeO, rotamer
A); 3.71 (s, MeO, rotamer B); 4.04–4.38 (m, 2 CH, rotamer A+B); 4.92–5.28 (br. s, 2 CH, rotamer
A+B); 7.11–7.77 (m, 10 arom. H, rotamer A+B). 13C-NMR (75 MHz, CDCl3, DEPT; rotamers A/B
t
t
ca. 5 :3): 28.4 (+, Bu); 36.8 (ꢀ, CH2); 40.0 (ꢀ, CH2); 49.5 (+, CH); 51.8 (+, MeO); 78.8 (Cq, Bu);
126.8 (+, arom. C); 126.9 (+, 2 arom. C); 127.1 (+, 2 arom. C); 128.6 (+, 2 arom. C); 129.7 (+, 2
arom. C); 133.3 (Cq, arom. C); 136.6 (Cq, arom. C); 161.7 (Cq, C=O); 172.0 (Cq, C=O). MALDI-MS:
444 (8), 442 (8), 409 (16), 408 (70), 393 (21), 392 (88), 370 (13), 340 (8), 339 (29), 336 (10), 331 (7), 328
(7), 315 (7), 314 (11), 314 (35), 303 (26), 301 (7), 271 (9), 270 (58), 261 (11), 217 (30), 216 (9), 215 (8),
200 (8), 189 (12), 183 (12), 162 (14), 140 (15), 96 (29). MALDI-HR-MS: 392.1832 ([M+Na]+,
C22H27NNaOþ4 ; calc. 392.1825).
(S)-Methyl 3-{[(tert-Butoxy)carbonyl]amino}-4-[4-(naphthalen-2-yl)phenyl]butanoate (6b). Yield:
154 mg (91%). Colorless solid. M.p. 115–1178. Rf (Et2
O/pentane 1:1) 0.48. [a]rD:t: =+107.9 (c=5.35,
G
CHCl3). IR (neat): 3371m, 2941m, 2861w, 2750w, 2651w, 2585w, 2328w, 2041w, 1990w, 1916w, 1733s,
1686s, 1608w, 1515s, 1443m, 1364m, 1315w, 1248m, 1213w, 1158s, 1044m, 1013m, 958w, 889w, 811s,
t
752m, 614m. 1H-NMR (300 MHz, C6
A
2J=2.6, CH2); 3.31 (s, MeO); 4.20–4.45 (m, CH); 4.72–4.98 (br. s, CH); 7.12–7.19 (m, 2 arom. H);
7.20–7.38 (m, 2 arom. H); 7.43–7.56 (m, 2 arom. H); 7.56–7.80 (m, 4 arom. H); 7.86–7.99 (m, 1
t
arom. H). 13C-NMR (75 MHz, C6
A
t
(+, CH); 50.8 (+, MeO); 78.8 (Cq, Bu); 125.6 (Cq, arom. C); 125.8 (+, arom. C); 126.1 (+, arom. C);
127.6 (+, 2 arom. C); 127.7 (+, arom. C); 128.0 (+, arom. C); 128.3, 128.5, 129.9, 133.0 (Cq, arom. C);
134.2 (Cq, arom. C); 137.3 (+, arom. C); 138.6 (Cq, arom. C); 139.7 (Cq, arom. C); 155.0 (Cq, C=O);
171.4 (Cq, C=O). MALDI-MS: 458 (13), 443 (14), 442 (51), 420 (17), 398 (8), 365 (16), 364 (68), 328
(14), 321 (22), 320 (100), 314 (7), 303 (17), 302 (9), 284 (11), 242 (8), 242 (8), 234 (11), 233 (14), 217
(14), 189 (14), 140 (12), 96 (16). MALDI-HR-MS: 442.1970 ([M+Na]+, C22H29NNaO4þ ; calc. 442.1989).
(S)-Methyl 3-{[(tert-Butoxy)carbonyl]amino}-4-(4’-fluoro[1,1’–biphenyl]-4-yl)butanoate (6c). Yield:
128 mg (82%). Colorless solid. M.p. 104–1068. Rf (Et2
O/pentane 1:1) 0.45. [a]rD:t: =ꢀ92.0 (c=4.70,
CHCl3). IR (neat): 3361m, 2942w, 2859w, 2180w, 2108w, 1904w, 1734s, 1682s, 1605w, 1522s, 1444m,
1362w, 1324w, 1245m, 1214w, 1164s, 1098w, 1043w, 1017w, 949w, 813s, 677w, 631m. 1H-NMR (300
t
MHz, C6D6, 658): 1.40 (s, Bu); 2.25 (d, J=7.5, CH2); 2.76 (dd, J=7.8, J=2.3, CH2); 3.30 (s, MeO);
A
4.20–4.35 (m, CH); 4.58–4.82 (br. s, NH); 6.83 (t, J=8.1, 2 arom. H); 7.02–7.35 (m, 6 arom. H). 13C-
NMR (75 MHz, C
6D6, 658, DEPT): 28.2 (+, Bu); 37.8 (ꢀ, CH2); 40.1 (ꢀ, CH2); 49.3 (+, CH); 50.8
t
(+, MeO); 78.8 (Cq, Bu); 115.3 (Cq, arom. C); 115.6 (Cq, arom. C); 127.1 (+, 2 arom. C); 128.5 (+, 2
arom. C); 128.6 (+, 2 arom. C); 129.8 (+, 2 arom. C); 137.3 (Cq, arom. C); 138.7 (Cq, arom. C); 154.9
(Cq, arom. C); 164.3 (Cq, C=O); 171.3 (Cq, C=O). MALDI-MS: 410 (12), 357 (8), 339 (19), 332 (13),
328 (8), 234 (8), 233 (8), 217 (10), 189 (8), 140 (11), 96 (13). MALDI-HR-MS: 410.1728 ([M+Na]+,
C22H26FNNaOþ4 ; calc. 410.1758).
(S)-Methyl 3-{[(tert-Butoxy)carbonyl]amino}-4-(3’-chloro[1,1’-biphenyl]-4-yl)butanoate (6d). Yield:
143 mg (88%). Colorless solid. M.p. 92–948. Rf (Et2
IR (neat): 3365m, 2955w, 2928w, 2862w, 2360w, 2076w, 1990w, 1931w, 1726s, 1687s, 1590w, 1553w, 1515s,
G
1467m, 1436m, 1362m, 1255s, 1162s, 1090w, 1039s, 961w, 902w, 869w, 833w, 776s, 749w, 674w, 642m. H-
t
NMR (300 MHz, CDCl3): 1.49 (s, Bu); 2.44–2.59 (m, CH2); 2.85 (dd, 3J=7.4, 7.4, CH); 2.98 (dd,
3J=6.3, 5.8, CH); 3.69 (s, MeO); 4.16 (quint., 3J=7.3, CH); 5.09 (d, 3J=7.1, NH); 7.25–7.55 (m, 8
arom. H). 13C-NMR (75 MHz, CDCl3, DEPT): 28.3 (+, Bu); 37.5 (ꢀ, CH2); 39.9 (ꢀ, CH2); 48.7 (+,
t
CH); 51.7 (+, MeO); 79.4 (Cq, Bu); 125.1 (+, arom. C); 127.1 (+, arom. C, 2 C); 127.2 (+, 2 arom.
C); 129.90 (+, 2 arom. C); 129.94 (+, arom. C); 134.6 (Cq, arom. C); 137.5 (Cq, arom. C); 138.1 (Cq,
arom. C); 142.7 (Cq, arom. C); 155.1 (Cq, C=O); 172.1 (Cq, C=O). MALDI-MS: 442 (11), 428 (12),
427 (10), 426 (41), 404 (12), 350 (10), 348 (35), 328 (12), 306 (14), 304 (44), 303 (10), 284 (10), 279 (7),
264 (7), 242 (19), 236 (7), 234 (16), 233 (18), 184 (7). MALDI-HR-MS: 426.1428 ([M+Na]+,
C22H26ClNNaOþ4 ; calc. 426.1443).

Helvetica Chimica Acta – Vol. 89 (2006)
1437
(S)-Methyl 3-{[(tert-Butoxy)carbonyl]amino}-4-(3’-methyl[1,1’-biphenyl]-4-yl)butanoate (6e). Yield:
152 mg (86%). Colorless solid. M.p. 88–908. Rf (Et2
O/pentane 1:1) 0.45. [a]rD:t: =ꢀ10.8 (c=4.50, CHCl3).
IR (neat): 3367m, 3064w, 3028w, 2983w, 2962w, 2851w, 2359w, 1725s, 1687s, 1672w, 1595m, 1518s, 1446m,
1395m, 1364m, 1334s, 1265w, 1155s, 1115s, 1076m, 1035m, 1046m, 998m, 196m, 827m. 1H-NMR (300
t
MHz, CDCl3): 1.32 (s, Bu); 2.50 (dd, 3J=5.77, 2J=3.03, CH2); 2.63–3.01 (m, CH2); 3.62 (s, MeO);
3
4.11 (m, CH); 7.33 (d, J=8.24, 2 arom. H); 7.60–7.65 (m, 4 arom. H); 7.86–7.91 (m, 2 arom. H). 13C-
t
NMR (75 MHz, CDCl3, DEPT): 28.2 (+, Bu); 39.7 (ꢀ, CH2); 40.6 (ꢀ, CH2); 50.1 (+, CH); 51.9
t
(MeO); 79.0 (Cq, Bu); 124.5 (+, 2 arom. C); 127.7 (+, 2 arom. C); 130.4 (+, 2 arom. C); 130.8 (+, 2
arom. C); 131.3 (Cq, arom. C); 138.0 (Cq, arom. C); 139.6 (Cq, arom. C); 142.4 (Cq, arom. C); 154.9
(Cq, C=O); 171.3 (Cq, C=O). MALDI-MS: 477 (8), 476 (32), 461 (25), 460 (100), 438 (17), 404 (9),
383 (14), 382 (69), 345 (7), 339 (7), 338 (33), 303 (9), 235 (27), 234 (10), 233 (11), 217 (7), 184 (11), 162
(11), 140 (13), 96 (8). MALDI-HR-MS: 460.1698 ([M+Na]+, C23H26F3NNaO4þ ; calc. 460.1706).
(S)-Methyl 3-{[(tert-Butoxy)carbonyl]amino}-4-(4’-methoxy[1,1’-biphenyl]-4-yl)butanoate (6f).
Yield: 153 mg (95%). Colorless solid. M.p. 136–1378. Rf (Et2
A
(c=4.55, CHCl3). IR (neat): 3385m, 2981w, 2942w, 2849w, 2643w, 2357w, 2077w, 2043w, 1990w, 1914w,
1734s, 1684s, 1609w, 1503s, 1444m, 1364w, 1251s, 1217w, 1160s, 1120w, 1033s, 807s, 756w, 721w, 679w,
1
t
639w. H-NMR (300 MHz, CDCl3): 1.41 (s, Bu); 2.40–2.62 (m, CH2); 2.91 (dd, J=7.9, J=2.2, CH2);
3.70 (s, MeO); 3.85 (s, MeO); 4.15–4.25 (m, CH); 5.00–5.15 (br. s, NH); 6.99 (d, J=8.0, 2 arom. H);
3
3
7.22 (d, J=8.0, 2 arom. H); 7.492 (d, J=7.5, 2 arom. H); 7.495 (d, J=7.5, 2 arom. H). 13C-NMR (75
t
MHz, CDCl3, DEPT): 28.3 (+, Bu); 37.8 (ꢀ, CH2); 40.2 (ꢀ, CH2); 49.4 (+, CH); 50.7 (+, MeO); 54.7
t
(+, MeO); 78.7 (Cq, Bu); 114.5 (+, 2 arom. C); 126.9 (+, 2 arom. C); 129.8 (+, 2 arom. C); 133.8 (Cq,
arom. C); 136.5 (Cq, arom. C); 139.5 (Cq, arom. C); 159.6 (Cq, C=O); 171.4 (Cq, C=O). MALDI-MS:
438 (10), 423 (10), 422 (45), 400 (19), 369 (14), 345 (11), 344 (58), 328 (11), 301 (18), 300 (100), 284
(10), 282 (9), 263 (45), 242 (9), 241 (97), 235 (32), 234 (8), 233 (11), 217 (10), 189 (17), 140 (13), 96
(11). MALDI-HR-MS: 422.1928 ([M+Na]+, C23H29NNaO5þ ; calc. 422.1938).
Boc-(S)-b3hPro-NHtBu (8). To a soln. of Boc-(S)-b3hPro-OH (57 mg, 440 mmol) in CHCl3 (1.8 ml) at
08 were added tBuNH2 (47 ml, 440 mmol), Et3
N (61 ml, 440 mmol), BtOH (81 mg, 530 mmol), and EDC (10
A
mg, 540 mmol). The mixture was allowed to warm to r.t., and stirring was continued for 4 d. Subsequent
dilution with CHCl3, washing with 1N HCl, aq. sat. NaHCO3, and brine, drying of the org. phase (MgSO4),
removal of the solvent under reduced pressure, and FC (SiO2; MeOH/CH2Cl2 2 :98, Rf 0.23) yielded 22
mg (17%) of 8. Colorless solid. M.p. 74.5–75.58. [a]rD:t: =ꢀ44.9 (c=1.0, CHCl3). IR (CHCl3): 3436w,
1
3007m, 2982m, 1673s, 1511m, 1477w, 1454m, 1403s, 1366s, 1306w, 1169m, 1123m. H-NMR (400 MHz,
t
t
CDCl3): 1.33 (s, Bu); 1.47 (s, Bu); 1.76–1.88 (m, 2 CH); 1.95–2.05 (m, 2 CH); 2.21–2.32 (m, CH);
2.54 (dd, J=13.6, 3.4, 1 H, CH2); 3.31–3.34 (m, 2 CH); 3.98–4.04 (m, CHN); 5.21 (br. s, NH, rotamer);
5.94 (br. s, NH, rotamer). 13C-NMR (100 MHz, CDCl3): 23.5 (CH2); 28.6, 28.8 (Me); 30.7, 42.1, 46.8
(CH2); 51.0 (C); 55.3 (CH); 79.5 (C); 170.4 (C). EI-MS: 183 (27), 127 (29), 115 (15), 110 (34), 100
(10), 84 (23), 83 (32), 82 (12), 70 (100), 69 (14), 68 (16), 58 (49), 57 (25), 56 (29), 55 (12), 44 (17), 42
(11), 41 (38), 39 (14), 28 (12). Anal. calc. for C15H28N2O3 (284.40): C 63.35, H 9.92, N 9.85; found: C
63.37, H 9.73, N 9.72.
Boc-(S)-b3hPhe-(S)-b3hPro-NHtBu (9). To a soln. of 8 (48 mg, 170 mmol) in CH2Cl2 (500 ml) at 08 was
added TFA (500 ml), and the soln. was stirred at r.t. for 1.5 h. The solvent was removed under reduced
pressure, and the residue was dissolved in CHCl3 (1 ml). At 08 were added Et3N (90 ml, 650 mmol),
G
Boc-(S)-b3hPhe-OH (71 mg, 250 mmol), BtOH (38 mg, 250 mmol), and EDC (48 mg, 250 mmol), and
the soln. was stirred under rewarming to r.t. for 23 h. Dilution with CHCl3, washing with 1N HCl, aq.
sat. NaHCO3, and brine, drying of the org. phase (MgSO4), removal of the solvent under reduced pres-
sure, and twofold FC (SiO2; MeOH/CH2Cl2 3 :97 then AcOEt, Rf 0.45) and recrystallization from
CH2Cl2/hexane yielded 38 mg (51%) of 9. Colorless solid. M.p. 151–1528. [a]rD:t: =ꢀ19.3 (c=1.1,
CHCl3). IR (CHCl3): 3433m, 3007s, 2979s, 2877w, 1701s, 1671s, 1625s, 1496s, 1453s, 1392m, 1366s,
1308m, 1286m, 1168s, 1050w, 1029w, 928w, 850w, 639w, 619w, 607w. 1H-NMR (400 MHz, CDCl3; rotamers
t
t
t
t
A/B ca. 8 :1): 1.33 (s, Bu, rotamer B); 1.35 (s, Bu, rotamer A); 1.40 (s, Bu, rotamer B); 1.41 (s, Bu,
rotamer A); 1.81–2.07 (m, 4 CH); 2.14 (dd, J=13.7, 9.0, 1 H, CH2); 2.38 (d, J=5.0, CH2); 2.71 (dd,
J=13.7, 3.3, 1 H, CH2); 2.86 (dd, J=13.4, 8.3, 1 H, CH2); 3.00–3.08 (m, CH); 3.18–3.26 (m, 2 CH);
4.08–4.16 (m, CHN); 4.24–4.30 (m, CHN); 5.48 (br. s, NH, rotamer B); 5.71 (br. s, NH, rotamer A);

1438
Helvetica Chimica Acta – Vol. 89 (2006)
6.05 (br. s, NH); 7.17–7.23 (m, 3 arom. H); 7.27–7.31 (m, 2 arom. H). 13C-NMR (100 MHz, CDCl3; sig-
nals of rotamers in italics): 21.6, 23.7 (CH2); 28.4, 28.8 (Me), 30.1 (CH2); 31.0, 36.8, 40.2, 41.8, 42.3, 45.4,
47.1 (CH2); 49.3 (CH); 51.1 (C); 55.5 (CH); 79.1 (C); 126.5, 128.5, 129.2 (CH); 138.6, 155.4, 168.9, 170.1
(C). MALDI-HR-MS: 368.2307 (100, [MꢀBoc+Na]+, C20
A
G
G
[MꢀBoc+H]+, C20
U
G
G
Boc-(R)-b2hVal-(S)-b3hPhe-(S)-b3hPro-NHtBu (10). To a soln. of 9 (39 mg, 870 mmol) in CH2Cl2 (1
ml), at 08 was added TFA (1 ml), and the soln. was stirred at r.t. for 1.5 h. The solvent was removed under
reduced pressure, the residue was dissolved in CHCl3 (860 ml), and NMM (27 ml, 240 mmol), Boc-(R)-
b2hVal-OH (41 mg, 180 mmol), BtOH (25 mg, 190 mmol), and EDC (33 mg, 170 mmol) were added at
08. The soln. was stirred for 1 h at 08, then for 26 h at r.t. Dilution with CHCl3, washing with 1N HCl,
aq. sat. NaHCO3, and brine, drying of the org. phase (MgSO4), removal of the solvent under reduced
pressure and FC (SiO2; MeOH/CH2Cl2 2 :98, Rf 0.43) yielded 35 mg (72%) of 10. Colorless, highly viscous
1
oil. H-NMR (400 MHz, CDCl3; rotamers A/B ca. 3 :1; all other rotamers are given in italics): 0.88 (d,
J=6.7, Me, rotamer A); 0.94 (d, J=6.7, Me, rotamer A); 1.25 (s, tBu, rotamer B); 1.28 (s, Bu, rotamer
t
A); 1.31 (s, tBu, rotamer B); 1.33 (s, tBu, rotamer A); 1.36 (s, tBu); 1.37 (s, tBu); 1.43 (s, tBu); 1.85–2.23
(m, 7 CH); 2.38–2.46 (m, 2 CH); 2.71–2.79 (m, CH); 2.84 (dd, J=13.5, 8.4, 1 H, CH2); 3.02 (dd, J=13.5,
7.1, 1 H, CH2); 3.13–3.23 (m, 2 CH); 3.31–3.40 (m, CH); 4.21–4.32 (m, CH); 4.40–4.58 (m, CH);
4.90–4.98 (m, CH); 5.46 (s, NH); 5.97 (s, NH); 7.04 (d, J=8.6, NH); 7.17–7.25 (m, 3 arom. H);
7.28–7.32 (m, 2 arom. H); 8.59 (d, NH). 13C-NMR (100 MHz, CDCl3; signals of rotamers in italics):
20.1, 20.8 (Me); 23.7 (CH2); 28.4, 28.5, 28.6, 28.8 (Me); 29.7, 30.1, 30.2, 34.5, 36.5, 39.3, 40.1, 40.6, 41.6,
47.2 (CH2); 47.4, 47.6, 48.7 (CH); 51.2 (C); 54.1, 55.4, 55.5, 55.6 (CH); 62.8 (CH2); 79.1, 126.7, 126.8,
127.0, 128.6, 128.8, 129.1, 129.1, 129.6, 129.8 (CH); 138.2, 156.1, 170.0, 170.2, 173.5 (C). MALDI-HR-
MS: 582.3716 (17), 581.3680 (49, [M+Na]+, C31
A
N4
ACHTREUNG
(100, [MꢀBoc+Na]+, C26
A
N4
A
U
G
ACHTREUNG
calc. 459.3335), 386.2446 (11), 185.1657 (22).
(Quinoline-2-carbonyl)-(R)-b2hVal-(S)-b3hPhe-(S)-b3hPro-NHtBu (11). To a soln. of 10 (35 mg, 620
mmol) in CH2Cl2 (1 ml) at 08 was added TFA (1 ml), and the soln. was stirred at r.t. for 1.5 h. The solvent
was removed under reduced pressure, the residue was dissolved in CHCl3 (0.6 ml) and Et3N (35 ml, 250
A
mmol), quinaldic acid (=quinoline-2-carboxylic acid; 27 mg, 160 mmol), BtOH (26 mg, 190 mmol), and
EDC (36 mg, 190 mmol) were added at 08, and the soln. was stirred under rewarming to r.t. for 17 h. Dilu-
tion with CHCl3, washing with 1N HCl, aq. sat. NaHCO3, and brine, drying of the org. phase (MgSO4),
removal of the solvent under reduced pressure and purification by normal-phase HPLC (Merck system:
the column was first loaded using an isocratic mixture of 2% iPrOH and was then washed with a linear
i
gradient of 2–50% PrOH within 15 min) yielded 30 mg (78%) of 11. White powder. M.p. 79.08 (sint.).
HPLC (Merck system, iPrOH/hexane 25 :75): tR 4.2 min, purity>99%. CD (0.2 mM in
MeOH):+2.1·104 (237 nm), 0 (220 nm), ꢀ3.2·104 (209 nm), 0 (200 nm). IR (KBr): 3314m, 3063w,
2965m, 1647s, 1534s, 1500s, 1452s, 1390m, 1364m, 1310w, 1226m, 1172w, 848w, 777m, 742w, 701m,
625w. 1H-NMR (400 MHz, CD3
OD; rotamer A/B 3 :2): 0.92 (d, J=6.7, Me, rotamer B); 0.94 (d,
A
t
J=6.7, Me, rotamer A); 1.02 (d, J=6.7, Me, rotamer B); 1.02 (d, J=6.7, Me, rotamer A); 1.26 (s, Bu,
t
t
t
rotamer A); 1.28 (s, Bu, rotamer B); 1.29 (s, Bu, rotamer B); 1.30 (s, Bu, rotamer A); 1.77–1.95 (m,
4 CH); 1.99 (dd, J=13.5, 10.0, 1 H, CH2); 2.17–2.40 (m, 2 CH); 2.43 (d, J=6.7, CH); 2.65 (dd,
J=13.4, 3.8, 1 H, CH2); 2.74 (dd, J=13.7, 6.8, 1 H, CH2); 2.86 (dd, J=13.6, 7.2, 1 H, CH2); 3.28–3.39
(m, 2 CH); 3.54–3.67 (m, 2 CH); 4.15–4.27 (m, CHN); 4.53–4.59 (m, CHN, rotamer A); 4.62–4.69
(m, CHN, rotamer B); 6.81–6.87 (m, 1 H); 6.95–7.01 (m, 2 H); 7.08–7.12 (m, 2 H); 7.20–7.25 (m, 1
H); 7.63–7.69 (m, 1 H); 7.71–7.78 (m, 1 H); 7.98 (d, J=8.1, 0.5 H, rotamer A); 7.98 (d, J=8.1, 0.5 H,
rotamer B); 8.07 (d, J=8.5, 1 H); 8.18 (d, J=8.5, 1 H); 8.46 (d, J=8.1, 1 H). 13C-NMR (100 MHz,
CD3OD; signals of rotamers in italics): 20.6, 21.4 (Me); 22.4, 24.5 (CH2); 28.9 (Me); 30.1 (CH); 30.1,
A
31.6, 39.5, 40.8, 40.9, 41.4, 42.4, 46.8, 48.0, 48.3 (CH2); 49.0 (CH); 51.9 (C); 54.5, 54.7, 56.7, 56.9 (CH);
119.5, 127.3, 129.0, 129.2, 129.3, 129.4, 130.3, 130.3, 130.9, 131.5 (CH); 139.0 (C); 139.5 (CH); 148.0,
150.7, 166.6, 171.3, 172.0, 172.7, 175.9 (C). MALDI-HR-MS: 636.3520 (100, [M+Na]+, C36
A
G
N
calc. 636.3526), 614.3716 (4, [M+H]+, C36
A
G
N
(Quinoline-2-carbonyl)-Asn-(S)-(4-Br)-b3hPhe-Pro-Ile-NH2 (13). In a reactor, Rink amide AM resin
(280 mg, 199 mmol, 200–400 mesh, 0.71 mmol/g) was swollen in DMF (5 ml) for 30 min. After filtration,

Helvetica Chimica Acta – Vol. 89 (2006)
1439
the resin was deprotected with piperidine/DMF (20%, 5 ml, 3×10 min) and washed with DMF (5×5 ml,
1 min). The free resin was coupled with the corresponding amino acid (3 equiv.), HATU (2.9 equiv.) and
DIPEA (6 equiv.) for 1 h, i.e., Fmoc-Ile-OH (212 mg, 600 mmol, 1×60 min and 1×45 min), Fmoc-Pro-OH
(202 mg, 600 mmol), Fmoc-b3hPhe-OH (212 mg, 400 mmol), Fmoc-Asn(Trt)-OH (358 mg, 600 mmol), and
quinoline-2-carboxylic acid (173 mg, 1.0 mmol). The resin was washed with DMF (5×5 ml, 1 min) and
CH2Cl2 (5×5 ml, 1 min), and dried in the h.v. The resin was split into three parts, and 144 mg (200
mmol) were cleaved with TFA/H2O/TIPS 95 :2.5 :2.5 for 3 h. Removal of volatiles under reduced pres-
sure, precipitation of the peptides with Et2O, centrifugation, and purification by prep. RP-HPLC
G
(30–40% B in 5 min, 40–95% B in 40 min, 95–99% B in 10 min, tR 47.61 min) yielded 21 mg of 13
(43%). Colorless solid. Anal. RP-HPLC (5–8% B in 9 min, 8–60% B in 41 min, 60–99% B in 10
min): tR 42.21 min, purity>97%. 1H-NMR (400 MHz, CD3
OD; signals of rotamers in italics):
U
0.84–0.96 (m, 6 H); 1.10–1.37 (m, 1 H); 1.49–1.59 (m, 1 H); 1.76–2.06 (m, 4 H); 2.12–2.34 (m, 1 H);
2.52–2.67 (m, 2 H); 2.73–2.93 (m, 4 H); 3.44–3.63 (m, 2 H); 4.20–4.25 (m, 1 H); 4.37–4.51 (m, 2 H);
3
3
3
4.84–4.91 (m, 1 H); 7.08 (d, J=8.5, 2 arom. H); 7.15 (d, J=8.5, 2 arom. H); 7.19 (d, J=8.5, 2 arom.
H); 7.24 (d, J=8.5, 2 arom. H); 7.68–7.72 (m, 1 arom. H); 7.82–7.86 (m, 1 arom. H); 8.00 (d, 3J=8.5,
3
1 arom. H); 8.17–8.20 (m, 2 arom. H); 8.49 (d, 3J=8.6, 1 arom. H). 13C-NMR (100 MHz, CD3
OD;
A
DEPT; signals of rotamers in italics): 11.4 (+, Me); 11.6 (+, Me); 16.0 (+, Me); 16.1 (+, Me); 23.7 (ꢀ,
CH2); 25.8 (ꢀ, CH2); 26.0 (ꢀ, CH2); 26.1 (ꢀ, CH2); 30.8 (ꢀ, CH2); 33.2 (ꢀ, CH2); 37.7 (ꢀ, CH2); 38.0
(ꢀ, CH2); 38.1 (+, CH); 39.1 (ꢀ, CH2); 39.3 (ꢀ, CH2); 40.1 (ꢀ, CH2); 40.2 (ꢀ, CH2); 48.1 (ꢀ, CH2);
48.4, 49.7 (ꢀ, CH2); 50.0, 50.4, 51.7 (+, CH); 52.1 (+, CH); 59.0 (+, CH); 61.6 (+, CH); 61.9 (+, CH);
119.6 (+, arom. C); 119.7 (+, arom. C); 121.2 (Cq, arom. C); 129.1 (+, arom. C); 129.5 (+, arom. C);
130.9 (Cq, arom. C); 131.0 (+, arom. C); 131.6 (+, arom. C); 131.7 (+, arom. C); 132.4 (+, 2 arom.
C); 132.5 (+, arom. C); 132.6 (+, 2 arom. C); 138.7 (+, arom. C); 139.1 (Cq, arom. C); 148.1 (Cq,
arom. C); 150.4 (Cq, arom. C); 166.4 (Cq, C=O); 172.0 (Cq, C=O); 172.1 (Cq, C=O); 172.3 (Cq, C=
O); 172.5 (Cq, C=O); 174.2 (Cq, C=O); 174.6 (Cq, C=O); 175.2 (Cq, C=O); 176.4 (Cq, C=O).
MALDI-MS: 776 (22, 775 (8), 774 (20), 762 (8), 761 (40), 760 (100), 759 (39), 758 (95), 738 (7), 722
(21), 721 (57), 720 (22), 719 (55), 703 (8), 693 (19), 691 (18), 676 (12), 674 (10), 608 (13), 606 (15), 591
(11), 589 (11). MALDI-HR-MS: 758.2259 ([M+Na]+, C35H42BrN7NaO6þ ; calc. 758.2272).
(Quinoline-2-carbonyl)-Asn-(S)-4-(3-trifluoromethyl)-b3hPhe-Pro-Ile-NH2 (15). To a suspension of
12 (144 mg, 70 mmol) in degassed EtOH (300 ml) were added 3-(trifluoromethyl)phenylboronic acid
(25 mg, 130 mmol), Na2CO3 (28 mg, 260 mmol), [Pd(PPh3)4] (8 mg, 7.00 mmol), degassed H2O (130 ml),
and degassed toluene/EtOH 7:3 (1 ml). The tube was sealed and heated under shaking to 908 for 5 h.
The cold suspension was filtered, the resin was washed with EtOH/H2O (5×5 ml, 1 min), DMF (5×5
ml, 1 min), and CH2Cl2 (5×5 ml, 1 min), and dried in h.v. Cleavage with TFA/H2O/TIPS 95 :2.5 :2.5
for 3 h, removal of volatiles under reduced pressure, precipitation of the peptide with cold Et2O, centri-
G
fugation, and purification by prep. RP-HPLC (30–40% B in 5 min, 40–95% B in 35 min, 95–99% B in 10
min, tR 27.03 min) yielded 33 mg of 15 (59%). Colorless solid. Anal. RP-HPLC (5–8% B in 8 min,
1
8–60% B in 52 min, 60–99% B in 10 min): tR 48.55 min, purity>97%. H-NMR (300 MHz, CD3
A
0.85–1.10 (m, 6 H); 1.16–1.28 (m, 1 H); 1.51–1.60 (m, 1 H); 1.77–2.37 (m, 5 H); 2.56–3.07 (m, 5 H);
3.48–3.68 (m, 2 H); 4.20–4.28 (m, 1 H); 4.41–4.54 (m, 2 H); 4.81–4.94 (m, 2 H); 7.29–7.51 (m, 5
arom. H); 7.51–7.79 (m, 3 arom. H); 7.90–8.19 (m, 4 arom. H); 8.35–8.41 (m, 2 arom. H). 13C-NMR
(75 MHz, CD
A
38.2 (+, CH); 39.2 (ꢀ, CH2); 40.2 (ꢀ, CH2); 48.8 (ꢀ, CH2); 52.3 (+, CH); 59.2 (+, CH); 61.7 (+,
CH); 119.7 (+, arom. C); 123.2 (Cq, arom. C); 124.0 (Cq, arom. C); 127.8 (+, 2 arom. C); 129.0 (+,
arom. C); 129.6 (+, arom. C); 130.6 (Cq, arom. C); 130.7 (+, arom. C); 131.4 (+, 2 arom. C); 131.5
(+, arom. C); 131.7 (+, arom. C); 132.4 (+, arom. C); 132.5 (+, arom. C); 132.6 (+, arom. C); 139.2
(+, arom. C); 139.3 (Cq, arom. C); 143.1 (Cq, arom. C); 148.1 (Cq, arom. C); 150.4 (Cq, arom. C);
166.3 (Cq, C=O); 172.3 (Cq, C=O); 174.6 (Cq, C=O); 175.3 (Cq, C=O); 176.6 (Cq, C=O). MALDI-
MS: 841 (10). 840 (23), 826 (13), 825 (47), 824 (100), 803 (20), 802 (38), 787 (8), 786 (31), 785 (67), 757
(9), 633 (8), 533 (8), 456 (14), 235 (11). MALDI-HR-MS: 824.3366 ([M+Na]+, C42H46F3N7NaO6þ ; calc.
824.3354).

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Helvetica Chimica Acta – Vol. 89 (2006)
REFERENCES
[1] N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95, 2457; A. Suzuki, J. Organomet. Chem. 1999, 576, 147; S.
Kotha, K. Lahiri, D. Kashinath, Tetrahedron 2002, 58, 9633; A. Suzuki, ‘Organoboranes in Organic
Synthesis’, Hokkaido University, Sapporo, 2004.
[2] F. Firooznia, C. Gude, K. Chan, Y. Satoh, Tetrahedron Lett. 1998, 39, 3985; Y. Gong, W. He, Org.
Lett. 2002, 4, 3803.
[3] W. Wang, J. Zhang, C. Xiong, V. J. Hruby, Tetrahedron Lett. 2002, 43, 2137; W. Wang, C. Xiong, J.
Zhang, V. J. Hruby, Tetrahedron 2002, 58, 3101.
[4] G. X. Yang, L. L. Chang, Q. Truong, G. A. Doherty, P. A. Magriotis, S. E. de Lazlo, B. Li, M. Mac-
Coss, U. Kidambi, L. A. Egger, E. McCauley, G. Van Riper, R. A. Mumford, J. A. Schmidt, W. K.
Hagmann, Bioorg. Med. Chem. Lett. 2002, 12, 1497.
[5] S. Kotha, K. Lahiri, Bioorg. Med. Chem. Lett. 2001, 11, 2887; S. Kotha, K. Lahiri, Biopolymers 2003,
69, 517; J. C. Yoburn, D. L. van Vranken, Org. Lett. 2003, 5, 2817.
[6] J. E. Harvey, M. N. Kenworthy, R. J. K. Taylor, Tetrahedron Lett. 2004, 45, 2467.
[7] F. Firooznia, C. Gude, K. Chan, C. A. Fink, Y. Qiao, Y. Satoh, N. Marcopoulos, P. Savage, M. E. Beil,
C. W. Bruseo, A. J. Trapani, A. Y. Jeng, Bioorg. Med. Chem. Lett. 2001, 11, 375.
[8] W. Wang, M. Cai, C. Xiong, J. Zhang, D. Trivedi, V. J. Hruby, Tetrahedron 2002, 58, 7365.
[9] A. Ojida, H. Tsutsumi, N. Kasagi, I. Hamachi, Tetrahedron Lett. 2005, 46, 3301.
[10] a) E. Abderhalden, F. Reich, Fermentforsch. 1928, 10, 173; b) E. Abderhalden, R. Fleischmann, Fer-
mentforsch. 1928, 10, 195; c) D. L. Steer, R. A. Lew, P. Perlmutter, A. I. Smith, M.-I. Aguilar, Curr.
Med. Chem. 2002, 9, 811; d) D. Seebach, A. K. Beck, D. J. Bierbaum, Chem. Biodivers. 2004, 1, 1111;
e) D. F. Hook, F. Gessier, C. Noti, P. Kast, D. Seebach, ChemBioChem 2004, 5, 691; f) P. I. Arvids-
son, N. S. Ryder, H. M. Weiss, D. F. Hook, J. Escalante, D. Seebach, Chem. Biodivers. 2005, 2, 401; g)
D. F. Hook, P. Bindschädler, Y. R. Mahajan, R. Sebesta, P. Kast, D. Seebach, Chem. Biodivers. 2005,
2, 591.
[11] J. S. Koh, J. A. Ellman, J. Org. Chem. 1996, 61, 4494; J. Tois, R. Franzén, O. Aitio, I. Laakso, J. Huus-
konen, J. Taskinen, Comb. Chem. High Throughput Screen. 2001, 4, 521; S. L. Schreiber, Science
2000, 287, 1964.
[12] J. Podlech, D. Seebach, Angew. Chem. 1995, 107, 507; Angew. Chem., Int. Ed. 1995, 34, 471; J. Pod-
lech, D. Seebach, Liebigs Ann. Chem. 1995, 1217; D. Seebach, M. Overhand, F. N. M. Kühnle, B.
Martinoni, L. Oberer, U. Hommel, H. Widmer, Helv. Chim. Acta 1996, 79, 913.
[13] C. S. Dexter, R. F. W. Jackson, Chem. Commun. 1998, 75; C. S. Dexter, R. F. W. Jackson, J. Org.
Chem. 1999, 64, 7579; R. W. F. Jackson, I. Rilatt, P. J. Murray, Chem. Commun. 2003, 1241.
[14] J. R. Huff, J. Med. Chem. 1991, 34, 2305; A. G. Tomasselli, W. J. Howe, T. K. Sawyer, A. Wlodawer,
R. L. Heinrikson, Chim. Oggi 1991, 9, 6; K. Appelt, Perspect. Drug Discov. Des. 1993, 1, 23; A. Wlo-
dawer, J. W. Erickson, Annu. Rev. Biochem. 1993, 62, 543; R. E. Babine, S. L. Bender, Chem. Rev.
1997, 97, 1359.
[15] N. A. Roberts, J. A. Martin, D. Kinchington, A. V. Broadhurst, J. C. Craig, I. B. Duncan, S. A. Gal-
pin, B. K. Handa, J. Kray, A. Krohn, R. W. Lambert, J. H. Merrett, J. S. Mills, K. E. B. Parkes, S.
Redshaw, A. J. Ritchie, D. L. Taylor, G. J. Thomas, P. J. Machin, Science 1990, 248, 348.
[16] A. Krohn, S. Redshaw, J. C. Ritchie, B. J. Graves, M. H. Hatada, J. Med. Chem. 1991, 34, 3340.
[17] S. Abele, K. Vögtli, D. Seebach, Helv. Chim. Acta 1999, 82, 1539.
[18] T. Hintermann, D. Seebach, Helv. Chim. Acta 1998, 81, 2093.
[19] D. Scholz, A. Billich, B. Charpiot, P. Ettmayer, P. Lehr, B. Rosenwirth, E. Schreiner, H. Gstach, J.
Med. Chem. 1994, 37, 3079.
[20] E. Pretsch, T. Clerc, J. Seibl, W. Simon, ‘Tabellen zur Strukturaufklärung organischer Verbindungen
mit spektroskopischen Methoden’, 3rd edn., Springer-Verlag, Berlin, 1990, p. U140.
[21] A. Colombo, J.-C. Fernàndez, N. de la Figuera, D. Fernández-Forner, P. Forns, F. Albericio, QSAR
Comb. Sci. 2005, 24, 913.
[22] J. W. Gulles, S. G. Johnson, W. V. Murray, J. Org. Chem. 1996, 61, 5169.
[23] Y. D. Ward, V. Farina, Tetrahedron Lett. 1996, 37, 6993; M. Larhead, G. Lindeberg, A. Hallberg, Tet-
rahedron Lett. 1996, 45, 8219.

Helvetica Chimica Acta – Vol. 89 (2006)
1441
[24] W. S. Hancock, J. E. Batterby, Ann. Biochem. 1976, 71, 261.
[25] T. Vojkovsky, Pept. Res. 1995, 8, 236.
[26] P. Lombardi, Chem. Ind. (London) 1990, 708.
[27] S. Moss, Chem. Ind. (London) 1994, 122.
[28] D. Seebach, P. E. Ciceri, M. Overhand, B. Jaun, D. Rigo, L. Oberer, U. Hommel, R. Amstutz, H.
Widmer, Helv. Chim. Acta 1996, 79, 2043.
[29] T. Hintermann, D. Seebach, Synlett 1997, 437.
[30] D. Seebach, S. Abele, K. Gademann, G. Guichard, T. Hintermann, B. Jaun, J. L. Matthews, J. V.
Schreiber, L. Oberer, U. Hommel, H. Widmer, Helv. Chim. Acta 1998, 81, 932.
Received 24 May, 2006