Solid-phase synthesis of 5-arylhistidines via a microwave-assisted Suzuki-Miyaura cross-coupling

Article abstract of DOI:10.1016/j.tet.2008.08.077

Microwave irradiation efficiently promoted the solid-phase Suzuki-Miyaura reaction of a 5-bromohistidine with various arylboronic acids in the presence of a palladium catalyst. This methodology allowed the synthesis of peptides bearing a histidine residue substituted at position 5 of the imidazole ring with a phenyl, a substituted phenyl, a pyridyl, or a thienyl ring, as well as with the benzene ring of a tyrosine residue.

Full text of DOI:10.1016/j.tet.2008.08.077

Tetrahedron 64 (2008) 10538–10545
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Solid-phase synthesis of 5-arylhistidines via a microwave-assisted
Suzuki–Miyaura cross-coupling
a
b
b
b
a
a,
*
´
Vanessa Cerezo , Muriel Amblard , Jean Martinez , Pascal Verdie , Marta Planas , Lidia Feliu
a
`
´
Laboratori d’Innovacio´ en Processos i Productes de S´ıntesi Organica (LIPPSO), Departament de Quımica, Universitat de Girona, Campus de Montilivi, 17071 Girona, Spain
b Institut des Biomole´cules Max Mousseron (IBMM), UMR 5247 CNRS, Universite´ Montpellier 1, Universite´ Montpellier 2, Faculte´ de Pharmacie, 15 Av. C. Flahault, BP 14 491, 34093
Montpellier Cedex 5, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 6 August 2008
Received in revised form 26 August 2008
Accepted 27 August 2008
Available online 3 September 2008
Microwave irradiation efficiently promoted the solid-phase Suzuki–Miyaura reaction of a 5-bromohis-
tidine with various arylboronic acids in the presence of a palladium catalyst. This methodology allowed
the synthesis of peptides bearing a histidine residue substituted at position 5 of the imidazole ring with
a phenyl, a substituted phenyl, a pyridyl, or a thienyl ring, as well as with the benzene ring of a tyrosine
residue.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Histidine
Arylation
Suzuki–Miyaura cross-coupling
Microwave
Biaryl peptides
Solid-phase synthesis
1. Introduction
a microwave-assisted Suzuki–Miyaura cross-coupling of a 5-bro-
mohistidine with various arylboronic acids.4
Imidazoles bearing an aryl substituent at position 4(5) have
shown a number of biological properties of potential interest
In recent years, a large number of diverse chemical trans-
formations have been transferred from solution to solid-phase,
enabling the design and synthesis of combinatorial libraries for
drug discovery research.5 In particular, the Suzuki–Miyaura cou-
pling has been adapted to solid-phase and has found several ap-
plications to the discovery of novel compounds.6 In this line of view,
we envisioned that the solid-phase Suzuki–Miyaura reaction would
emerge as a successful approach toward peptides bearing 5-aryl-
histidine residues.
including antifungal activity, potent
b-glucosidase or activin
receptor-like kinase 5 (ALK5) inhibition, as well as neuropeptide Y 5
(NPY5) receptor antagonist activity.1 4(5)-Arylimidazoles also occur
naturally in the form of 5-arylhistidines. For example, 5-arylhisti-
dines are central structures of compounds such as the active site of
the heme-copper oxidases, and cytotoxic and antifungal marine
peptides.2
Despite their interest, the preparation of 5-arylhistidines still
remains an important synthetic challenge, including as key step the
arylation of the position 4(5) of the imidazole ring. So far, several
methods for the synthesis of 4(5)-arylimidazoles have been
reported in the literature, but they have never been applied to the
derivatization of a histidine residue.3 Most of these protocols pro-
vide moderate yields and/or require drastic conditions.
In the present study, we describe an efficient solid-phase syn-
thesis of 5-arylhistidines by arylation of the position 4(5) of the
imidazole ring via a microwave-assisted Suzuki–Miyaura cross-
coupling.
2. Results and discussion
With this in mind, we focused our current studies on biaryl
amino acids in the development of an efficient methodology to-
ward the preparation of 5-arylhistidines. Recently, we have repor-
ted the synthesis of these amino acid derivatives in solution via
2.1. Synthesis of SEM-protected 5-bromohistidines
Bromohistidines 1 were selected to study the solid-phase ary-
lation of the position 4(5) of the imidazole ring. The Boc group was
chosen as Na-amino protection because this group was expected to
be stable to the basic conditions and the high temperature of the
Suzuki–Miyaura reaction. To avoid racemization of the histidine
residue during the coupling step to the solid support, the imidazole
* Corresponding author. Tel.: þ34 972 418274; fax: þ34 972 418150.
E-mail address: lidia.feliu@udg.edu (L. Feliu).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.08.077

V. Cerezo et al. / Tetrahedron 64 (2008) 10538–10545
10539
ring was protected with the 2-(trimethylsilyl)ethoxymethyl (SEM)
group. This group can be easily introduced, is stable to the standard
Fmoc/t-Bu solid-phase peptide synthesis protocol and can be re-
moved during the acidic cleavage step. Bromohistidines 1 were
prepared following the protocol previously described in our group
(Scheme 1).4 Commercially available Boc–His–OMe was bromi-
nated by treatment with N-bromosuccinimide (NBS) and alkylated
with SEMCl affording a regioisomeric mixture of the SEM-protected
5-bromohistidines 2 with a 73% overall yield. To be able to compare
the Suzuki–Miyaura arylation of either the mixture or a single
isomer, 2a and 2b were separated by reverse-phase preparative
HPLC. Final hydrolysis of the methyl ester of 2 with LiOH led to
histidines 1 with a 93% yield.
Leu-Leu–NH2 (4) was obtained with 99% purity and was fully
characterized by 1H and 13C NMR, and mass spectrometry.
Arylation of the histidine residue of resin 3 via a microwave-
assisted Suzuki–Miyaura cross-coupling was then studied. The re-
action conditions were optimized by using phenylboronic acid
(Table 1). For each experiment, aliquots of resin were subjected to
the corresponding reaction conditions and, after acidolytic cleav-
age, the obtained crude product mixture was analyzed by LC/MS. To
promote the SEM group removal during the cleavage step, stirring
and time exposure to TFA were found to be essential. When the
cleavage was performed with TFA/CH2Cl2 (95:5) for 2 h under
stirring (entries 6 and 7), the percentage of SEM-protected com-
pounds decreased compared to those experiments carried out
without stirring (entries 1–5). Using TFA/CH2Cl2 (95:5) and stirring
the resin for 3 h afforded only 2% of SEM-protected tripeptide
(entry 8).
The Suzuki–Miyaura reaction was initially attempted using the
catalyst, base, solvent, and temperature that gave the best results
for the synthesis of 5-arylhistidines in solution.4 Thus, the cou-
pling was conducted by using PhB(OH)2 (2 equiv), Pd2(dba)3
(0.1 equiv), and KF (2 equiv) in toluene/EtOH/H2O (9:1:0.2) under
microwave irradiation at 110 ꢀC for 10 min (Table 1, entry 1). Un-
der these conditions only the starting material was recovered.
Using DME/EtOH/H2O (9:9:2) and performing the reaction at
170 ꢀC, traces of the 5-phenylhistidine tripeptide 5 were detected
(entry 2). Taking into account that the reagent concentration is
a key parameter in solid-phase reactions,7 the solvent volume was
reduced ca. threefold (entry 3). In this case, a significant increase
of the arylated product was observed (37% including 5 and SEM-
protected 5). To improve this result, reactions were run using
a larger excess of reagents (entries 4–6). The use of PhB(OH)2
(4 equiv), Pd2(dba)3 (0.2 equiv), and KF (4 equiv) led to the highest
percentage of arylated product (81% including 5 and SEM-pro-
tected 5) and no starting material was detected (entry 5). An in-
crease of the amount of reagents did not improve the results
(entry 6). A prolonged microwave irradiation exposure (15 min)
slightly enhanced the formation of the arylated peptide (84%
including 5 and SEM-protected 5, entry 7). Even though the cross-
coupling reaction of either resin 3a or 3b yields the same 5-phe-
nylhistidine tripeptide after the cleavage step, we examined this
reaction using a single isomer of 3. Notably, the obtained result
was similar to that of the mixture.
O
O
O
BocHN
BocHN
BocHN
OCH3
NH
OCH3
NSEM
OCH3
N
i,ii
N
N
N
Br
Br
SEM
2a
2b
O
O
BocHN
BocHN
OH
NSEM
OH
N
iii
N
N
Br
Br
SEM
1a
1b
Scheme 1. Reagents and conditions: (i) NBS, CH3CN, 0 ꢀC; (ii) (a) DBU, DMF, 0 ꢀC, (b)
SEMCl, 0 ꢀC to rt, 73% (two steps); (iii) LiOH, THF, MeOH, H2O, 25 ꢀC, 93%.
2.2. Synthesis of H–His(5-Ph)-Leu-Leu–NH2
To investigate the feasibility of the solid-phase arylation of the
imidazole ring of a histidine residue, a model peptide incorporating
the SEM-protected 5-bromohistidines 1 was constructed onto
a Rink amide resin. The tripeptidyl resin 3 was prepared following
Fmoc/t-Bu strategy by sequential coupling and deprotection steps
using standard conditions (Scheme 2). Fmoc–Leu–OH couplings
O
O
O
H
N
BocHN
N
H
N
H
SEM
O
N
O
O
O
H
N
H
N
N
v,vi
vii
i,ii,iii,iv
Fmoc-Rink-MBHA resin
H2N
N
Br
Br
3a
Fmoc
N
H
NH2
FmocHN
N
H
O
O
O
H
N
BocHN
N
HN
N
H
N
H
4
Br
O
SEMN
3b
Scheme 2. Reagents and conditions: (i), (iii), and (v) piperidine/DMF, MWI, 75 ꢀC; (ii) and (iv) Fmoc–Leu–OH, HBTU, DIEA, DMF, MWI, 75 ꢀC; (vi) 1a and 1b, HBTU, DIEA, DMF, 25 ꢀC;
(vii) TFA/CH2Cl2, 3 h, stirring.
and Fmoc removal were performed under microwave irradiation.
The mixture of histidine derivatives 1a and 1b was coupled at room
temperature affording the regioisomeric tripeptidyl resins 3a and
3b. An aliquot of this mixture was treated with TFA/CH2Cl2 under
stirring for 3 h at room temperature. The resulting H–His(5-Br)-
It should be noted that the above crude product mixtures
proved to contain a byproduct (4–12%) derived from the reductive
dehalogenation of bromohistidines 3 (Scheme 3). This side-reaction
has been proposed to proceed by the protonation of arylpalladium
intermediates with water or other protic sources and has also been

10540
V. Cerezo et al. / Tetrahedron 64 (2008) 10538–10545
Table 1
Microwave-assisted arylation of 5-bromohistidine tripeptidyl resins 3a and 3b with PhB(OH)2
1. PhB(OH)2
KF, Pd2(dba)3, P(o-tolyl)3
DME/EtOH/H2O, MWI
2. TFA/CH2Cl2
O
O
O
O
O
O
H
N
H
N
H
N
H2N
BocHN
BocHN
N
N
H
NH2
N
H
N
H
N
H
N
H
H
SEM
O
O
O
N
N
N
N
SEMN
Ph
3a
3b
5
Br
Br
Solvent
Entrya
PhB(OH)2
(equiv)
Pd2(dba)3
(equiv)
P(o-tolyl)3
(equiv)
KF
(equiv)
Volume
(mL)
T (ꢀC)
t (min)
6b (%)
4 (%)
4-SEMc (%)
5 (%)
5-SEMd (%)
1e
2e
3e
4e
5e
6i
2
2
2
3
4
5
4
4
0.1
0.1
0.1
0.15
0.2
0.25
0.2
0.2
0.2
0.2
0.2
0.3
0.4
0.5
0.4
0.4
2
2
2
3
4
5
4
4
Tol/EtOH/H2Of
DME/EtOH/H2Oh
DME/EtOH/H2O
DME/EtOH/H2O
DME/EtOH/H2O
DME/EtOH/H2O
DME/EtOH/H2O
DME/EtOH/H2O
4
4
110
170
170
170
170
170
170
170
10
10
10
10
10
10
15
15
dg
d
4
35
21
3
34
17
12
13
d
d
d
d
d
3
8
20
52
63
67
83
d
4
1.2
1.2
1.2
1.2
1.2
1.2
29
45
29
16
17
2
6
9
12
11
11
10
d
d
d
d
7i
8j
a
Each experiment was carried out with 50 mg of resins 3a and 3b (0.94 mmol/g).
Percentage determined by HPLC at 214 nm from the crude reaction mixture.
Compound 4-SEM corresponds to SEM-protected 5-bromohistidine tripeptide 4.
Compound 5-SEM corresponds to SEM-protected 5-phenylhistidine tripeptide 5.
Cleavage was carried out for 2 h without stirring.
Toluene/EtOH/H2O (9:1:0.2).
Not detected.
DME/EtOH/H2O (9:9:2).
Cleavage was carried out for 2 h under stirring.
Cleavage was carried out for 3 h under stirring.
b
c
d
e
f
g
h
i
j
1. KF, Pd2(dba)3, P(o-tolyl)3
DME/EtOH/H2O
MWI, 170 ºC, 15 min
2. TFA/CH2Cl2
O
O
O
O
O
O
H
N
H
N
H
N
BocHN
BocHN
N
H2N
N
H
N
H
N
H
N
H
N
H
NH2
+
4
SEM
H
O
O
O
N
N
N
SEMN
N
3a
3b
6
Br
Br
Scheme 3. Obtention of the dehalogenated compound 6.
reported for Suzuki–Miyaura cross-couplings involving bromo-
imidazole derivatives.3b,8 Nolan and co-workers have recently de-
scribed the dehalogenation of aryl chlorides using a palladium
catalyst and a base in isopropanol under microwave heating.8a
Based on this report, resin 3 was subjected to the Suzuki–Miyaura
conditions but in the absence of PhB(OH)2, leading to 58% of
dehalogenated compound 6 (Scheme 3). Attempts to reduce the
amount of 6 were performed by carrying out the cross-coupling
reaction in DME as solvent. Unexpectedly, despite the absence of
protic solvents, the formation of the dehalogenated product (10%)
was not avoided. In addition, only a 47% of arylated product 5 was
formed and a 39% of starting material 4 was recovered.
(2–15%), all the crude reaction mixtures contained also compound
6 (4–36%). Similar to previous studies, coupling of 3-pyridylboronic
acid with 3 was found to be difficult.9 It was necessary to repeat the
reaction three times to obtain 12 with 52% purity (Table 2). The
5-arylhistidine tripeptides were purified by reverse-phase pre-
parative HPLC and fully characterized by mass spectrometry, and 1H
and 13C NMR. 2D-COSY and HMQC experiments were carried out to
completely assign all the proton and carbon signals. In the case of
compounds 7 and 10, each proton of the amide groups gave rise to
two separate NMR peaks at room temperature. This was attributed
to the presence of two different conformers, a hypothesis that was
confirmed by performing high temperature 1H NMR experiments in
DMSO-d6. As expected, raising the temperature resulted in the
coalescence of the amide protons.
2.3. Synthesis of H–His(5-Ar)-Leu-Leu–NH2
We explored the applicability of the Suzuki–Miyaura reaction to
the coupling of 5-bromohistidine tripeptidyl resin 3 with other
arylboronic acids possessing substituted benzene, pyridine, and
thiophene rings (Table 2). All reactions, which were carried out
using 150 mg of resin 3, afforded the desired 5-arylhistidine tri-
peptides 7–12 in purities ranging from 52 to 83% (including the
corresponding SEM-protected peptide derivatives). Couplings with
5-methyl-2-methoxyphenylboronic acid and 3-nitrophenylboronic
acid gave the highest percentages of arylated product (83 and 80%,
respectively, calculated from the non-protected and SEM-protected
biaryl tripeptide). In addition to the presence of starting material
2.4. Synthesis of peptides containing 5-(3-tyrosyl)histidines
Extension of the above methodology to the synthesis of peptides
containing tyrosine–histidine cross-links was studied. For this
purpose, the tyrosine-3-boronic acid derivative 13 was prepared
from protected 3-iodo-L
-tyrosine10 by Miyaura borylation with
bis(pinacolato)diboron11 and hydrolysis (Scheme 4). Next, Suzuki–
Miyaura coupling of 5-bromohistidine tripeptidyl resin 3 with 13
was investigated (Fig. 1). When resin 3 was treated with tyrosine
boronic acid 13 (4 equiv), Pd2(dba)3 (0.2 equiv), and KF (4 equiv) in
DME/EtOH/H2O (9:9:2) under microwave irradiation at 170 ꢀC for

V. Cerezo et al. / Tetrahedron 64 (2008) 10538–10545
10541
Table 2
Microwave-assisted arylation of 5-bromohistidine tripeptidyl resins 3a and 3b with
various arylboronic acids
O
O
O
O
H
N
H
N
H2N
N
H2N
N
1. ArB(OH)2
KF, Pd2(dba)3, P(o-tolyl)3
DME/EtOH/H2O
N
H
NH2
N
H
NH2
O
O
OCH3
OCH3
HN
HN
MWI, 170 °C, 15 min
2. TFA/CH2Cl2, 3 h, stirring
O
O
H
N
14
15
H2N
3a + 3b
N
H
NH2
H
OCH3
OH
O
N
O
O
N
H2N
H2N
Ar
7-12
Figure 1. Structure of 5-(3-tyrosyl)histidines 14 and 15.
ArB(OH)2
B(OH)2
6a (%) 4 (%) 4-SEMb (%) Peptide Pept (%) Pept-SEMc (%)
was exposed to LiOH (5 equiv) in THF/H2O (7:1) leading to 67% of
the 5-(3-tyrosyl)histidine acid derivative 15.
10
4
15
4
dd
7e
8e
61
75
4
8
OMe
B(OH)2
3. Conclusion
7
OMe
In conclusion, we have developed a straightforward solid-phase
synthesis of 5-arylhistidines. The imidazole ring of a histidine
residue has been derivatized with phenyl, substituted phenyl,
pyridyl, thienyl groups but also with a tyrosine derivative. This
study constitutes the first solid-phase Suzuki–Miyaura coupling
involving the imidazole ring of a histidine. The application of this
methodology to the generation of combinatorial libraries is under
investigation.
B(OH)2
13
20
2
d
d
9e
65
73
7
7
OH
B(OH)2
d
10e
NO2
B(OH)2
4. Experimental section
4.1. General methods
9
d
d
d
d
11e
12f
70
52
d
d
S
N
B(OH)2
36
Commercially available reagents were used throughout without
purification. Solvents were purified and dried by passing through
an activated alumina purification system (MBraun SPS-800) or by
conventional distillation techniques. Melting points (capillary tube)
were measured with an Electrothermal digital melting point ap-
paratus IA 91000 and were uncorrected. IR spectra were recorded
on a Mattson-Galaxy Satellite FT-IR using a single reflection ATR
system as a sampling accessory.
a
b
c
Percentage determined by HPLC at 214 nm from the crude reaction mixture.
Compound 4-SEM corresponds to SEM-protected 5-bromohistidine tripeptide 4.
Pept-SEM corresponds to SEM-protected 5-arylhistidine tripeptides 7–12.
Not detected.
d
e
Reaction conditions: ArB(OH)2 (4 equiv), Pd2(dba)3 (0.2 equiv), P(o-tolyl)3
(0.4 equiv), KF (4 equiv) in DME/EtOH/H2O (9:9:2).
f
Reaction conditions: the resin was treated three times with conditions specified
in footnote e.
Analytical thin layer chromatography (TLC) was performed on
precoated TLC plates, silica gel 60 F254 (Merck). The spots on the TLC
plates were visualized with UV/vis light (254 nm) and/or stained
with a solution of potassium permanganate (1,5 g/100 mL H2O).
Flash chromatography (FC) purifications were performed on silica
gel 60 (230–400 mesh, Merck).
All compounds were analyzed under standard analytical HPLC
conditions on a Beckman Gold apparatus composed of the 126
solvent module, the 168 detector, and the 32 Karat software, on
O
O
O
H2N
BocHN
BocHN
OH
I
OCH3
OCH3
i, ii
iii, iv
OH
OCH3
OCH3
OH
I
B
HO
a
C18 reverse-phase column (VWR chromolith column,
13
50ꢁ3.9 mm): 0–100% B linear gradient over 3 min at a flow rate of
5 mL/min. Solvent A was 0.1% aqueous TFA and solvent B was 0.1%
TFA in CH3CN. Detection was performed at 214 nm and 254 nm.
Preparative HPLC (Waters 4000 apparatus) was carried out on a C18
Scheme 4. Reagents and conditions: (i) Boc2O, Et3N, MeOH, rt; (ii) (a) Cs2CO3, DMF, (b)
CH3I, rt, 79% (two steps); (iii) bis(pinacolato)diboron, PdCl2(dppf), KOAc, DMSO, 80 ꢀC;
(iv) CH3CN/H2O, 75 ꢀC, 63% (two steps).
reverse-phase column (Deltapak column, 100ꢁ40 mm, 15
mm,
30 min, 22% of 5-(3-tyrosyl)histidine peptide was formed but 39%
of starting material 4 was recovered. A second treatment for other
30 min at 170 ꢀC resulted in the decomposition of the tripeptidyl
resin. Next, experiments were performed at 140 ꢀC. Resin 3 was
exposed to the above reagents under microwave irradiation for two
periods of 30 min and 1 h, respectively. Under these conditions, 5-
(3-tyrosyl)histidine peptide 14 was formed (18%) together with its
acid derivative 15 (49%). Similar to previous reactions, it was also
observed the formation of the dehalogenated product 6 (19%) and
the presence of starting material 4 (12%). To promote complete
hydrolysis, the resin obtained from the Suzuki–Miyaura reaction
100 Å) at a flow rate of 50 mL/min. Linear gradients of 0.1% aqueous
TFA and 0.1% TFA in CH3CN were run from 100:0 to 70:30 over
30 min (condition A); from 100:0 to 90:10 over 10 min and from
90:10 to 70:30 over 40 min (condition B); from 100:0 to 85:15 over
15 min and from 85:15 to 70:30 over 30 min (condition C) with UV
detection at 214 nm.
LC/MS analysis was performed on a LC/MS system consisting of
a Waters Alliance 2690 HPLC, coupled to a Micromass Quatro Micro
spectrometer (electrospray ionization mode, ESIþ). All analyses
were carried out using Onyx Monolithic C18 25ꢁ4.6 mm reverse-
phase column (Phenomenex). A flow rate of 1.5 ml/min and

10542
V. Cerezo et al. / Tetrahedron 64 (2008) 10538–10545
a gradient of 0–100% B over 4 min were used (solvent A was 0.1%
HCOOH in water and solvent B 0.1% HCOOH in acetonitrile). Positive
ion electrospray mass spectra were acquired at a solvent flow rate
(CH2-b), 28.92, 29.02 ((CH3)3C), 52.89, 53.36 (CH-a and OCH3),
67.15, 68.04 (CH2O), 75.46, 75.59 (NCH2O), 80.27, 80.79 ((CH3)3C),
102.76, 118.38 (C-5imid), 125.06, 137.28 (C-4imid), 137.76, 138.82 (CH-
of 100
m
L/min. Nitrogen was used for both the nebulizing gas and
2imid), 155.60, 155.63 (CONH), 172.43, 172.87 (COO); HRMS (ESI) m/z
the drying gas. Data were acquired in the scan mode from m/z 200
to 1200 in 0.1 s intervals, 10 scans were summed to produce the
final spectrum. High resolution mass spectra (HRMS) were de-
termined under conditions of ESI on a Bruker MicroTof-Q in-
strument using a lock-spray source.
The microwave-assisted synthesis was performed by using the
Discover SPPS Microwave Manual Peptide Synthesizer (CEM Cor-
poration). 1H and 13C NMR spectra were measured on a Bruker 200,
300, or 400 MHz NMR spectrometer. Chemical shifts were reported
calcd for C18H33BrN3O5Si [M]þ 478.1367, 480.1345, found 478.1375,
480.1352; calcd for C18H32BrN3NaO5Si [M]þ 500.1187, 502.1190,
found 500.1199, 502.1197.
4.2.3. 5-Bromo-N(
(trimethylsilyl)ethoxymethyl]-
tert-butoxycarbonyl-N(
a
)-tert-butoxycarbonyl-N(
-histidine (1a) and 5-bromo-N(
)-[2-(trimethylsilyl)ethoxymethyl]-
p)-[2-
L
a)-
s
L-
histidine (1b)
An aqueous solution of LiOH (1.6 M, 10.5 mmol) was added to
a solution of 2 (1.8 g, 3.8 mmol) in THF/MeOH (1:1, 14.6 mL). The
reaction mixture was stirred at room temperature for 1.5 h. After
this time, the organic solvents were evaporated under reduced
pressure. The resulting residue was adjusted to pH 5–6 by addition
of 1 N HCl followed by extraction with EtOAc (3ꢁ50 mL). The or-
ganic layers were combined, washed with brine (50 mL), and dried
over anhydrous magnesium sulfate. Removal of the solvent affor-
ded histidine derivatives 1 as a white solid (1.7 g, 93%). tR 1.65 and
1.70 min; mp 107–109 ꢀC; IR (neat) 3317, 2954, 1711, 1681, 1366,
as
d values (ppm) directly referenced to the solvent signal.
4.2. Synthesis of SEM-protected 5-bromohistidines 1
4.2.1. Methyl 5-bromo-N(
N-Bromosuccinimide (2.6 g, 14.8 mmol) was added to a solution
of methyl N( )-tert-butoxycarbonyl- -histidinate (4.0 g, 14.8 mmol)
in dry acetonitrile (80 mL) at 0 ꢀC. The reaction mixture was stirred
at this temperature under Ar for 30 min. Then, pyridine (32 L) was
added and the mixture was concentrated in vacuo. Triethylamine
(3.2 mL) was added to the concentrated solution. Removal of the
solvent gave a residue, which was purified by column chromatog-
raphy. Elution with EtOAc/hexane (2:1) afforded methyl 5-bromo-
a)-tert-butoxycarbonyl-L-histidinate
a
L
m
1247, 1159, 1100, 835 cmꢂ1; 1H NMR (200 MHz, CDCl3)
(CH3)3Si), 0.91–1.02 (m, 2H, CH2Si), 1.43 (s, 6.8H, (CH3)3C), 1.48 (s,
2.2H, (CH3)3C), 3.05–3.32 (m, 2H, CH2- ), 3.52–3.62 (m, 2H, CH2O),
4.55–4.65 (m, 1H, CH- ), 5.32 (s, 1.5H, NCH2O), 5.36 (s, 0.5H,
d 0.03 (s, 9H,
b
a
N(
tR 1.02 min; mp 156–158 ꢀC; IR (neat) 3359, 3079, 1693, 1526, 1434,
1323, 1244, 1166 cmꢂ1 1H NMR (200 MHz, CDCl3)
1.37 (s, 9H,
(CH3)3C), 2.94 (dd, J¼6.6 and 15.4 Hz,1H, CH2- ), 3.13 (dd, J¼5.4 and
15.4 Hz, 1H, CH2- ), 3.71 (s, 3H, OCH3), 4.43–4.46 (m, 1H, CH- ),
5.28 (br s, 1H, NH–Boc), 7.43 (s, 1H, CH-2imid), 10.43 (br s, 1H,
NHimid); 13C NMR (50 MHz, CDCl3)
28.94 ((CH3)3C), 30.31 (CH2- ),
53.52 (OCH3), 53.66 (CH- ), 81.56 (C(CH3)3), 115.62 (C-5imid), 124.03
a
)-tert-butoxycarbonyl-
L
-histidinate as a white solid (4.9 g, 95%).
NCH2O), 5.50 (d, J¼7.4 Hz, 1H, CONH), 7.71 (s, 0.8H, CH-2imid), 8.06
(s, 0.2H, CH-2imid), 11.05 (br s, 1H, COOH); 13C NMR (50 MHz, CDCl3)
;
d
d
ꢂ0.74 ((CH3)3Si), 18.33 (CH2Si), 27.53, 29.76 (CH2-
b), 28.95, 29.11
b
((CH3)3C), 53.36, 53.78 (CH- ), 67.45, 67.87 (CH2O), 76.04, 76.47
a
b
a
(NCH2O), 80.28, 80.96 ((CH3)3C), 104.66, 116.20 (C-5imid), 126.10,
135.20 (C-4imid), 137.87, 138.45 (CH-2imid), 155.82, 156.01 (CONH),
173.87,174.31 (COO); HRMS (ESI) m/z calcd for C17H31BrN3O5Si [M]þ
464.1211, 466.1197, found 464.1219, 466.1201.
d
b
a
(C-4imid), 135.55 (CH-2imid), 156.30 (CONH), 172.67 (COO); MS (ESI)
m/z (%) 348.0 (100), 350.0 (96) [MþH]þ, 695.1 (4), 697.1(9), 699.1 (4)
[2MþH]þ, 248.0 (18), 250.0 (16) [MꢂBocþH]þ; HRMS (ESI) m/z
calcd for C12H19BrN3O4 [M]þ 348.0553, 350.0522, found 348.0544,
350.0521; calcd for C12H18BrN3NaO4 [M]þ 370.0373, 372.0352,
found 370.0362, 372.0351.
4.3. Synthesis of methyl N(
a)-tert-butoxycarbonyl-3-borono-
4-methoxy- -tyrosinate (13)
L
4.3.1. Methyl N(a)-tert-butoxycarbonyl-3-iodo-4-methoxy-L-
tyrosinate
Triethylamine (2.2 mL, 24.3 mmol) was added to a solution of 3-
iodo-
-tyrosine10 (4.5 g, 22.1 mmol) in anhydrous methanol
4.2.2. Methyl 5-bromo-N(
(trimethylsilyl)ethoxymethyl]-
bromo-N( )-tert-butoxycarbonyl-N(
silyl)ethoxymethyl]- -histidinate (2b)
DBU (1.5 mL, 9.9 mmol) was added to a solution of methyl
5-bromo-N( )-tert-butoxycarbonyl- -histidinate (2.3 g, 6.6 mmol)
in dry DMF (17 mL) at 0 ꢀC. The reaction mixture was stirred at this
temperature under Ar for 1.5 h. After this time, 2-(trimethylsilyl)-
ethoxymethyl chloride was added (1.3 mL, 9.9 mmol) and the
mixture was stirred for 3 h while the temperature was allowed to
warm to room temperature. The reaction mixture was then poured
into water (150 mL) and the product was extracted with toluene/
EtOAc (1:1, 3ꢁ40 mL). The organic layers were combined, washed
with brine (50 mL), and dried over anhydrous magnesium sulfate.
Removal of the solvent afforded a pale yellow oil, which was pu-
rified by column chromatography. Elution with hexane/EtOAc
(60:40) gave 2 as a colorless oil (2.4 g, 77%). tR 1.75 and 1.90 min; IR
(neat) 2953, 1713, 1489, 1364, 1248, 1207, 1164, 1090, 834 cmꢂ1; 1H
a
)-tert-butoxycarbonyl-N(
-histidinate (2a) and methyl 5-
)-[2-(trimethyl-
p
)-[2-
L
L
(43 mL) at 0 ꢀC. A solution of Boc2O (5.91 g, 26.2 mmol) in anhy-
drous methanol (35 mL) was then added dropwise. The reaction
mixture was stirred for 1.5 h while the temperature was allowed to
warm to room temperature. After this time, the mixture was con-
centrated in vacuo and then adjusted to pH 4 by addition of 1 N HCl.
The solution was transferred to a separatory funnel and extracted
with EtOAc (50 mL). The organic layer was washed with brine
(50 mL), dried over anhydrous magnesium sulfate, and evaporated
a
s
L
a
L
in vacuo to yield N(
(neat) 3355, 3319, 1681, 1503, 1367, 1250, 1157 cmꢂ1
(200 MHz, CDCl3) 1.45 (s, 9H, (CH3)3C), 2.95–3.20 (m, 2H, CH2-
4.51–4.53 (m, 1H, CH-
), 5.20 (br s,1H, CONH), 6.86 (d, J¼8.2 Hz,1H,
CHarom-5), 7.06 (d, J¼8.2 Hz, 1H, CHarom-6), 7.52 (s, 1H, CHarom-2);
13C NMR (50 MHz, CDCl3)
29.01 ((CH3)3C), 37.28 (CH2- ), 55.38
(CH- ), 81.32 (C(CH3)3), 85.98 (Carom-3), 115.84 (CHarom-5), 130.75
a
)-tert-butoxycarbonyl-3-iodo-
L
-tyrosine. IR
1H NMR
),
;
d
b
a
d
b
a
(Carom-1), 131.70 (CHarom-6), 139.86 (CHarom-2), 154.93 (Carom-4),
156.32 (CONH), 176.53 (COO); HRMS (ESI) m/z calcd for C14H18IN-
NaO5 [M]þ 430.0122, found 430.0137.
NMR (200 MHz, CDCl3)
CH2Si), 1.42 (s, 6.8H, (CH3)3C), 1.46 (s, 2.2H, (CH3)3C), 3.04 (dd, J¼8.4
and 15.2 Hz, 1H, CH2- ),
), 3.18 (dd, J¼6.2 and 15.2 Hz, 1H, CH2-
3.50–3.58 (m, 2H, CH2O), 3.74 (s, 0.8H, OCH3), 3.78 (s, 2.2H, OCH3),
4.57–4.68 (m, 1H, CH- ), 5.26–5.32 (m, 2H, NCH2O), 5.86 (br s, 1H,
CONH), 7.48 (s, 0.8H, CH-2imid), 7.65 (s, 0.2H, CH-2imid); 13C NMR
(50 MHz, CDCl3)
ꢂ0.79 ((CH3)3Si), 18.26 (CH2Si), 27.74, 29.94
d 0.03 (s, 9H, (CH3)3Si), 0.89–0.97 (m, 2H,
N(a)-tert-Butoxycarbonyl-3-iodo-L-tyrosine was added to a so-
b
b
lution of Cs2CO3 (8.35 g, 25.4 mmol) in anhydrous DMF (76 mL) and
the reaction mixture was stirred at room temperature for 15 min.
Methyl iodide (3.3 mL, 53.0 mmol) was then added and the mixture
was stirred at room temperature overnight. After this time, the
reaction mixture was concentrated under reduced pressure and
a
d

V. Cerezo et al. / Tetrahedron 64 (2008) 10538–10545
10543
acidified to pH 5–6 by addition of 1 N HCl. The resulting solution
was extracted with EtOAc (100 mL), the organic layer was washed
with brine (100 mL), dried over anhydrous magnesium sulfate,
and concentrated in vacuo. The residue was purified by column
chromatography. Elution with hexane/EtOAc (80:20) gave methyl
microwave irradiation (3 min, 75 ꢀC, 50 W). After each coupling
and deprotection step, the resin was washed with DMF (ꢁ3), MeOH
(ꢁ1), and CH2Cl2 (ꢁ3), and air dried.
4.4.1.1. H–His(5-Br)-Leu-Leu–NH2 (4). An aliquot of the bromo-
tripeptidyl resin 3 was cleaved with TFA/CH2Cl2 (95:5) under stir-
ring for 3 h at room temperature. Following TFA evaporation and
diethyl ether extraction, the crude peptide was dissolved in H2O/
CH3CN (50:50 v/v containing 0.1% TFA), lyophilized, and purified by
reverse-phase preparative HPLC using condition A to afford H–
His(5-Br)-Leu-Leu–NH2 (4). tR 0.92 min; 1H NMR (400 MHz,
N(
yellow solid (7.60 g, 79% calculated from 3-iodo-
1.87 min; mp 63–65 ꢀC; IR (neat) 3333, 1736, 1702, 1510, 1495, 1366,
1255, 1223, 1151, 1048, 1016, 822 cmꢂ1 1H NMR (200 MHz, CDCl3)
), 3.08
), 3.76 (s, 3H, OCH3), 3.88 (s, 3H,
), 5.02 (br s, 1H, CONH), 6.77
a
)-tert-butoxycarbonyl-3-iodo-4-methoxy-L-tyrosinate as a pale
L-tyrosine). tR
;
d
1.46 (s, 9H, (CH3)3C), 2.96 (dd, J¼5.8 and 13.8 Hz, 1H, CH2-
b
(dd, J¼5.6 and 13.8 Hz, 1H, CH2-
b
CO2CH3), 4.53–4.56 (m, 1H, CH2-
a
CD3CN)
2ꢁCH(
and 15.8 Hz, 1H, CH2(
CH2(
)–His), 4.30–4.40 (m, 3H, 3ꢁCH(
(s, 1H, CONH2), 7.62 (d, J¼7.3 Hz, 1H, CONH), 7.80 (d, J¼6.0 Hz, 1H,
CONH), 8.19 (s, 1H, CHimid-2); 13C NMR (100 MHz, CD3CN)
20.45,
20.73, 22.29, 22.37 (4ꢁCH3( )–Leu), 24.46, 24.58 (2ꢁCH( )–Leu),
25.96 (CH2( )–Leu), 52.13, 52.17, 52.91
)–His), 40.07, 40.14 (2ꢁCH2(
d
0.92–0.97 (m, 12H, 4ꢁCH3(
)–Leu), 1.69–1.75 (m, 4H, 2ꢁCH2(
)–His), 3.34 (dd, J¼5.2 and 15.8 Hz, 1H,
)), 6.15 (s, 1H, CONH2), 6.63
d
)–Leu), 1.53–1.66 (m, 2H,
(d, J¼8.4 Hz,1H, CHarom-5), 7.10 (dd, J¼2.2 and 8.4 Hz,1H, CHarom-6),
g
b)–Leu), 3.27 (dd, J¼6.7
7.57 (d, J¼2.2 Hz, CHarom-2); 13C NMR (50 MHz, CDCl3)
d
28.97
b
((CH3)3C), 37.64 (CH2-
b
), 52.92 (CO2CH3), 55.17 (CH-
a
), 57.03
b
a
(OCH3), 80.71 (C(CH3)3), 86.61 (Carom-3), 111.54 (CHarom-5), 130.92
(Carom-1), 130.95 (CHarom-6), 140.91 (CHarom-2), 155.65 (CONH),
157.94 (Carom-4), 172.77 (COO); HRMS (ESI) m/z calcd for
d
d
g
C
16H22INNaO5 [M]þ 458.0435, found 458.0425.
b
b
(3ꢁCH(
a)),124.25 (Cimid-4),136.02 (CHimid-2),167.36,173.43,174.63
4.3.2. Methyl N(
a)-tert-butoxycarbonyl-3-borono-4-methoxy-
L
-
(3ꢁCO); MS (ESI) m/z (%) 459.13 (100), 461.13 (96) [MþH]þ.
tyrosinate (13)
A
solution of N(
a
)-tert-butoxycarbonyl-3-iodo-4-methoxy-
L
-
4.4.2. General method for the microwave-assisted Suzuki–Miyaura
cross-coupling
tyrosinate (0.99 g, 2.28 mmol) in degassed DMSO (10 mL) was
added to a solution of bis(pinacolato)diboron (1.18 g, 4.56 mmol),
PdCl2(dppf) (0.11 g, 0.13 mmol), and KOAc (0.90 g, 9.13 mmol) in
degassed DMSO (5 mL). The mixture was stirred under nitrogen at
80 ꢀC for 7 h. After this time, brine (50 mL) was added and the
resulting solution was extracted with EtOAc (3ꢁ100 mL). The
combined organic extracts were washed with brine (3ꢁ50 mL) and
dried over anhydrous magnesium sulfate. Removal of the solvent
gave a dark brown oil, which was purified by column chromatog-
raphy. Elution with hexane/EtOAc (83:17) afforded a mixture of the
expected boronate together with the boronic acid derivative as
a pale yellow oil. This mixture was dissolved in acetonitrile/H2O
(1:1, 40 mL) and was stirred at 75 ꢀC for 4 h. The resulting solution
was lyophilized to afford a white solid, which was purified by col-
umn chromatography. Elution with hexane/EtOAc (3:1) afforded
the pinacol boronic ester (104 mg, 10%) and elution with hexane/
A 25 mL reaction vessel containing a magnetic stir bar was
charged with bromotripeptidyl resin 3 (300 mg), which was first
swelled in a degassed mixture of DME/EtOH/H2O (9:9:2, 1.2 mL) for
15 min under argon atmosphere. Then, Pd2(dba)3 (0.2 equiv), P(o-
tolyl)3 (0.4 equiv), KF (4 equiv), and the corresponding boronic acid
(4 equiv) were added. The mixture was irradiated at 170 ꢀC for
15 min at 300 W. After the reaction time, upon cooling, the solvent
was removed and the resin was washed with DMF (ꢁ3), EtOH (ꢁ3),
and CH2Cl2 (ꢁ3). The aryltripeptides were released from the solid
support by treatment with TFA/CH2Cl2 (95:5) under stirring for 3 h
at room temperature. Following TFA evaporation and diethyl ether
extraction, the crude peptides were dissolved in H2O/CH3CN (50:50
v/v containing 0.1% TFA), lyophilized, and purified by reverse-phase
preparative HPLC.
EtOAc (1:1) gave methyl N(
methoxy- -tyrosinate (13) (540 mg, 63%). tR 1.45 min; mp 126–
127 ꢀC; IR (neat) 3352, 1737, 1686, 1518, 1494, 1362, 1344, 1234,
1153, 1043, 1015 cmꢂ1 1H NMR (300 MHz, CDCl3)
1.41 (s, 9H,
(CH3)3C), 3.02 (dd, J¼5.6 and 13.8 Hz, 1H, CH2- ), 3.07 (dd, J¼5.5
and 13.8 Hz, 1H, CH2- ), 3.73 (s, 3H, OCH3), 3.89 (s, 3H, CO2CH3),
4.52–4.55 (m, 1H, CH- ), 5.03 (br s, 1H, CONH), 5.54 (br s, 2H,
a
)-tert-butoxycarbonyl-3-borono-4-
4.4.2.1. H–His(5-Ph)–Leu-Leu–NH2 (5). This peptide was purified
L
using condition A. tR 1.02 min; 1H NMR (400 MHz, CD3CN)
d
0.92–
)–Leu,
0.97 (m, 12H, 4ꢁCH3(
2ꢁCH2( )–Leu), 3.43 (d, J¼6.9 Hz, 2H, CH2(
2H, 2ꢁCH(
d
)–Leu), 1.49–1.68 (m, 6H, 2ꢁCH(
g
;
d
b
b
)–His), 4.22–4.34 (m,
)–His), 5.99 (s, 1H,
b
a
)–Leu), 4.49 (t, J¼6.9 Hz, 1H, CH(
a
b
CONH2), 6.67 (s,1H, CONH2), 7.44 (d, J¼7.6 Hz,1H, CONH), 7.48–7.55
(m, 3H, CHarom-3, CHarom-4, CHarom-5), 7.59 (dd, J¼1.8 and 8.1 Hz,
2H, CHarom-2, CHarom-6), 8.20 (d, J¼6.0 Hz, 1H, CONH), 8.55 (s, 1H,
a
B(OH)2), 6.85 (d, J¼8.4 Hz, 1H, CHarom-5), 7.23 (dd, J¼2.2 and 8.4 Hz,
1H, CHarom-6), 7.58 (d, J¼2.2 Hz, 1H, CHarom-2); 13C NMR (75 MHz,
CHimid-2); 13C NMR (100 MHz, CD3CN)
d
20.63, 20.86, 22.31
)–Leu), 26.27 (CH2( )–His),
)–Leu), 51.87, 52.39, 52.91 (3ꢁCH( )), 123.19
(Cimid), 126.96 (Cimid), 128.39 (CHarom-2, CHarom-6), 129.16 (CHarom
CDCl3)
d
28.41 ((CH3)3C), 37.60 (CH2-
b
), 52.41 (CO2CH3), 54.73 (CH-
a),
(4ꢁCH3(
d
)–Leu), 24.37, 24.51 (2ꢁCH(
g
b
55.95 (OCH3), 80.30 (C(CH3)3), 110.34 (Carom-3), 110.84 (CHarom-5),
128.64 (Carom-1), 133.75 (CHarom-6), 137.87 (CHarom-2), 155.37
(CONH), 163.85 (Carom-4), 172.53 (COO); HRMS (ESI) m/z calcd for
40.22, 40.45 (2ꢁCH2(
b
a
-
3, CHarom-5), 129.45 (CHarom-4), 131.67 (Carom-1), 134.24 (CHimid-2),
167.65,172.84,174.47 (3ꢁCO); MS (ESI) m/z (%) 456.9 (100) [MþH]þ,
479.1 (10) [MþNa]þ.
C
16H24BNNaO7 [M]þ 376.1538, found 376.1538.
4.4. Synthesis of H–His(5-Ar)-Leu-Leu–NH2
4.4.2.2. Biarylic peptide 7. This peptide was purified using condi-
4.4.1. Synthesis of bromotripeptidyl resin 3
tion C. tR 1.05 min; 1H NMR (300 MHz, CD3CN)
d
0.84–0.93 (m, 12H,
)–Leu, 2ꢁCH( )–Leu),
)–His), 3.85 (s, 3H, OCH3), 4.13–4.33 (m,
a)–His), 5.79 (s, 0.6H, CONH2), 5.87 (s, 0.4H,
The bromotripeptidyl resin 3 was synthesized manually by the
solid-phase method using standard Fmoc chemistry. Fmoc–Rink–
MBHA resin (0.94 mmol/g) was used as solid support. Couplings of
Fmoc–Leu–OH (3 equiv) were performed using HBTU (3 equiv) and
DIEA (3 equiv) in DMF under microwave irradiation (5 min, 75 ꢀC,
50 W). SEM-protected 5-bromohistidines 1 were coupled using the
same conditions but at room temperature. The completion of the
reactions was checked by the Kaiser and the TNBS tests. Fmoc group
removal was achieved with 20% piperidine in DMF under
4ꢁCH3(
3.28 (d, J¼6.6 Hz, 1H, CH2(
3H, 2ꢁCH( )–Leu, CH(
d
)–Leu), 1.51–1.57 (m, 6H, 2ꢁCH2(
b
g
b
a
CONH2), 6.43 (s, 0.4H, CONH2), 6.53 (s, 0.6H, CONH2), 6.75 (d, J¼
5.5 Hz, 0.6H, CONH), 6.86 (d, J¼7.1 Hz, 0.6H, CONH), 7.09 (t,
J¼7.9 Hz,1H, CHarom-5), 7.15 (d, J¼7.9 Hz,1H, CHarom-3), 7.31 (d,
J¼7.5 Hz, 0.4H, CONH), 7.38 (dd, J¼1.6 and 7.9 Hz, 1H, CHarom-6),
7.52 (td, J¼1.6 and 7.9 Hz, 1H, CHarom-4), 7.85 (d, J¼6.0 Hz, 0.4H,
CONH), 8.56 (s, 1H, CHimid-2); 13C NMR (100 MHz, CD3CN)
d 20.07,

10544
V. Cerezo et al. / Tetrahedron 64 (2008) 10538–10545
20.16, 20.44, 20.51, 21.91, 21.96, 22.02, 22.09 (CH3(
24.16, 24.22 (CH( )–Leu), 25.98 (CH2( )–His), 39.65, 39.83, 40.11
(CH2( )–Leu), 51.04, 51.63, 51.79, 52.55, 52.61 (CH( )–Leu, CH( )–
His), 55.19 (OCH3), 111.51 (CHarom-3), 114.06 (Carom-1), 120.67
(CHarom-5), 123.28 (Cimid), 128.13 (Cimid), 130.89 (CHarom-6), 131.67
(CHarom-4), 133.42 (CHimid-2), 156.67 (Carom-2), 171.17, 172.25,
173.22 (CO); MS (ESI) m/z (%) 487.3 (100) [MþH]þ.
d
)–Leu), 24.12,
4ꢁCH3(
3.42 (dd, J¼5.6 and 16.2 Hz, 1H, CH2(
16.2 Hz, 1H, CH2( )–His), 4.20–4.26 (m, 1H, CH(
(m, 1H, CH( )–His), 6.06 (s, 1H,
)–Leu), 4.47 (t, J¼5.4 Hz, 1H, CH(
d
)–Leu), 1.48–1.68 (m, 6H, 2ꢁCH(
)–His), 3.48 (dd, J¼5.3 and
)–Leu), 4.28–4.34
g
)–Leu, 2ꢁCH2(
b)–Leu),
g
b
b
b
a
a
b
a
a
a
CONH2), 6.71 (s, 1H, CONH2), 7.44 (dd, J¼1.2 and 5.0 Hz, 1H,
CHthienyl-5), 7.52 (d, J¼7.4 Hz, 1H, CONH), 7.58 (dd, J¼2.9 and 5.0 Hz,
1H, CHthienyl-4), 7.83 (dd, J¼1.2 and 2.9 Hz, 1H, CHthienyl-2), 8.17 (d,
J¼6.2 Hz, 1H, CONH), 8.45 (s, 1H, CHimid-2); 13C NMR (100 MHz,
4.4.2.3. Biarylic peptide 8. This peptide was purified using condi-
CD3CN)
d
20.31, 20.54, 21.92, 21.97 (4ꢁCH3(
)–Leu), 26.20 (CH2(
)), 115.04 (Carom), 122.73 (Carom), 124.66
d
)–Leu), 24.02, 24.19
tion A. tR 1.13 min; 1H NMR (400 MHz, CD3CN)
d
0.87–0.96 (m, 12H,
)–Leu, 2ꢁCH2( )–Leu),
2.34 (s, 3H, CH3), 3.30 (dd, J¼6.6 and 14.8 Hz, 1H, CH2( )–His), 3.35
(dd, J¼6.6 and 14.8 Hz, 1H, CH2( )–His), 3.85 (s, 3H, OCH3), 4.26–
4.34 (m, 2H, 2ꢁCH( )–His), 5.94
(2ꢁCH(
g
b
)–His), 39.89, 40.06 (2ꢁCH2( )–Leu),
b
4ꢁCH3(d)–Leu), 1.52–1.71 (m, 6H, 2ꢁCH(
g
b
51.57, 51.98, 52.65 (3ꢁCH(
a
b
(Carom), 126.39 (CHthienyl-2), 127.10 (CHthienyl-4), 127.16 (CHthienyl-5),
133.63 (CHimid-2), 167.50, 172.51, 174.26 (3ꢁCO); MS (ESI) m/z (%)
463.2 (100) [MþH]þ, 485.0 (20) [MþNa]þ.
b
a)–Leu), 4.40 (t, J¼6.6 Hz, 1H, CH(
a
(s, 1H, CONH2), 6.54 (s, 1H, CONH2), 7.06 (d, J¼8.5 Hz, 1H, CHarom-3),
7.22 (d, J¼1.8 Hz,1H, CHarom-6), 7.34 (dd, J¼1.8 and 8.5 Hz, 1H,
CHarom-4), 7.41 (d, J¼7.8 Hz, 1H, CONH), 8.05 (d, J¼6.0 Hz, 1H,
4.4.2.7. Biarylic peptide 12. Following the general procedure, the
Suzuki–Miyaura reaction was carried out by treating resin 3 three
times with using Pd2(dba)3 (0.2 equiv), P(o-tolyl)3 (0.4 equiv), KF
(4 equiv), and 3-pyridylboronic acid (4 equiv). This peptide was
purified using condition B. tR 0.90 min; 1H NMR (300 MHz, CD3CN)
CONH), 8.56 (s, 1H, CHimid-2); 13C NMR (100 MHz, CD3CN)
d 19.37
(CH3), 20.51, 20.78 (2ꢁCH(
(4ꢁCH3( )–Leu), 26.48 (CH2(
51.88, 52.21, 52.94 (3ꢁCH(
g
b
a
)–Leu), 22.28, 22.34, 24.41, 24.51
)–His), 40.08, 40.42 (2ꢁCH2( )–Leu),
)), 55.51 (OCH3), 111.78 (CHarom-3),
d
b
d
0.84–0.92 (m, 12H, 4ꢁCH3(
Leu, 2ꢁCH2( )–Leu), 3.42–3.46 (m, 2H, CH2(
2H, 2ꢁCH( )–Leu), 4.39–4.44 (m, 1H, CH(
d
)–Leu), 1.52–1.638 (m, 6H, 2ꢁCH(
)–His), 4.25–4.29 (m,
)–His), 5.95 (s, 1H,
g)–
114.44 (Carom-1), 123.89 (Cimid), 128.42 (Cimid), 130.41 (CHarom-4),
131.50 (Carom-5), 132.13 (CHarom-6), 133.72 (CHimid-2), 154.86
(Carom-2),167.59,173.09,174.17 (3ꢁCO); MS (ESI) m/z (%) 501.3 (100)
[MþH]þ.
b
a
b
a
CONH2), 6.47 (s, 1H, CONH2), 7.27 (d, J¼7.6 Hz,1H, CONH), 7.78 (br s,
1H, CHpy-5), 7.97 (d, J¼5.6 Hz, 1H, CONH), 8.34 (d, J¼7.8 Hz, 1H,
CHpy-6), 8.42 (s, 1H, CHimid-2), 8.72 (br s,1H, CHpyr-4), 9.00 (br s, 1H,
CHpyr-2); MS (ESI) m/z (%) 458.2 (35) [MþH]þ.
4.4.2.4. Biarylic peptide 9. This peptide was purified using condi-
tion B. tR 0.96 min; 1H NMR (400 MHz, CD3CN)
d
0.89–0.95 (m, 12H,
)–Leu), 1.62–1.70 (m, 2H,
)–Leu), 3.37 (dd, J¼8.4 and 15.5 Hz, 1H, CH2( )–His), 3.48
(dd, J¼5.2 and 15.5 Hz, 1H, CH2( )–His), 4.30–4.36 (m, 2H,
2ꢁCH( )–His), 6.03 (s,
4ꢁCH3(
d
g
)–Leu), 1.51–1.61 (m, 4H, 2ꢁCH2(
b
4.4.2.8. Biarylic peptide 15. Following the general procedure, the
Suzuki–Miyaura reaction was carried out by treating resin 3 with
Pd2(dba)3 (0.2 equiv), P(o-tolyl)3 (0.4 equiv), KF (4 equiv), and ty-
rosine-3-boronic acid 13 (4 equiv) at 140 ꢀC for two periods of
30 min and 1 h, respectively. After the corresponding washes, the
resin was treated with LiOH (5 equiv) in THF/H2O (7:1) at room
temperature for 24 h. After the reaction time, the solvent was re-
moved and the resin was washed with DMF (ꢁ3), MeOH (ꢁ2), H2O
(ꢁ2), DMF (ꢁ3), and CH2Cl2 (ꢁ3). The biarylic peptide 15 was re-
leased from the solid support by treatment with TFA/CH2Cl2 (95:5)
under stirring for 3 h at room temperature. Following TFA evapo-
ration and diethyl ether extraction, the crude peptide was dissolved
in H2O/CH3CN (50:50 v/v containing 0.1% TFA), and lyophilized. tR
14.72 min; MS (ESI) m/z (%) 574.4 (33) [MþH]þ.
2ꢁCH(
b
b
a
)–Leu), 4.42 (dd, J¼5.2 and 8.4 Hz, 1H, CH(
a
1H, CONH2), 6.59 (s, 1H, CONH2), 6.96 (ddd, J¼0.8, 2.0, and
8.0 Hz,1H, CHarom-4), 7.05 (ddd, J¼0.8, 2.0, and 8.0 Hz,1H, CHarom-6),
7.14 (t, J¼2.0 Hz, 1H, CHarom-2), 7.38 (t, J¼8.0 Hz, 1H, CHarom-5), 7.44
(d, J¼7.6 Hz, 1H, CONH), 7.84 (d, J¼5.8 Hz, 1H, CONH), 8.60 (s, 1H,
CHimid-2); 13C NMR (100 MHz, CD3CN)
d 20.41, 20.79, 22.30, 22.34
(4ꢁCH3(
39.89, 40.44 (2ꢁCH2(
53.04 (CH(
d
)–Leu), 24.45, 24.52 (2ꢁCH(
g
)–Leu), 26.23 (CH2(
b
)–His),
)–His),
b
)–Leu), 52.11 (CH(
a)–Leu), 52.16 (CH(
a
a
)–Leu), 115.05 (CHarom-2), 117.35 (CHarom-4), 119.29
(CHarom-6), 122.28 (Cimid), 127.40 (Cimid), 130.61 (CHarom-5), 131.88
(Carom-1), 134.04 (CHimid-2), 157.86 (Carom-3), 167.40, 173.76, 174.36
(3ꢁCO); MS (ESI) m/z (%) 473.3 (100) [MþH]þ.
Acknowledgements
4.4.2.5. Biarylic peptide 10. This peptide was purified using condi-
tion A. tR 1.07 min; 1H NMR (400 MHz, CD3CN)
d 0.86–0.96 (m, 12H,
V.C. is the recipient of a predoctoral fellowship from the Uni-
versity of Girona. This work was supported by grant AGL2006-
13564-C02-02/AGR from MEC of Spain.
4ꢁCH3(
3.48 (d, J¼6.6 Hz, 2H, CH2(
4.24–4.28 (m, 1H, CH( )–Leu), 4.30–4.35 (m, 0.63H, CH(
4.46 (t, J¼6.6 Hz, 1H, CH(
d
)–Leu), 1.51–1.73 (m, 6H, 2ꢁCH2(
b
)–Leu, 2ꢁCH(
g
a
a
)–Leu),
)–Leu),
)–Leu),
b
)–His), 4.16–4.22 (m, 0.37H, CH(
a
a
)–His), 5.92 (s, 0.37H, CONH2), 6.03 (s,
0.63H, CONH2), 6.60 (s, 1H, CONH2), 6.86 (d, J¼6.1 Hz, 0.37H,
CONH), 6.96 (d, J¼7.9 Hz, 0.37H, CONH), 7.44 (d, J¼7.4 Hz, 0.63H,
CONH), 7.78 (t, J¼8.0 Hz, 1H, CHarom-5), 7.99 (ddd, J¼1.0, 2.1, and
8.0 Hz, 1H, CHarom-6), 8.12 (d, J¼6.1 Hz, 0.63H, CONH), 8.35 (ddd,
J¼1.0, 2.1, and 8.0 Hz, 1H, CHarom-4), 8.43 (t, J¼2.1 Hz, 1H, CHarom-2),
References and notes
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d
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`
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d
0.86–0.92 (m, 12H,
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