Parallel synthesis of bis-oxazole peptidomimetics

Article abstract of DOI:10.1016/j.tetlet.2013.10.078

The parallel synthesis of bis-oxazole peptidomimetics starting from Boc-aminoacids and Serine-methyl ester is described. This work presents the synthesis of oxazole aminoacid building blocks in solution phase and their utilization for the solid phase pept

Full text of DOI:10.1016/j.tetlet.2013.10.078

Tetrahedron Letters 54 (2013) 7062–7064
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Parallel synthesis of bis-oxazole peptidomimetics
⇑
Siva Murru, Colette T. Dooley, Adel Nefzi
Torrey Pines Institute for Molecular Studies, 11350 SW Village Parkway, Port St Lucie, FL 34987, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The parallel synthesis of bis-oxazole peptidomimetics starting from Boc-aminoacids and Serine-methyl
ester is described. This work presents the synthesis of oxazole aminoacid building blocks in solution
phase and their utilization for the solid phase peptide synthesis of a library of diverse bis-oxazole pepti-
domimetics in good overall yields.
Received 13 September 2013
Revised 11 October 2013
Accepted 16 October 2013
Available online 24 October 2013
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Oxazole aminoacids
Bis-oxazoles
Peptidomimetics
Parallel solid-phase synthesis
Tea-bag approach
9
Proteins and peptides play an essential role in almost all phys-
iological processes, which makes them an attractive starting point
for drug discovery programs. The potential utilities of peptides as
compounds include muscoride A, diazonamide A,10 and hennox-
1
1
azole A. Considering the potential of the oxazole moiety, there
has been a rising interest in the use of oxazole precursors in pre-
paring bioactive molecules and peptidomimetic structures.12
Small molecules containing an oxazole moiety have been dem-
onstrated to possess drug like properties against varieties of dis-
eases resulting in many FDA-approved drugs containing the
oxazole ring. Known drugs include the anti-inflammatory drug
‘‘Oxaprozin’’ and the streptogramin antibiotic ‘‘Dalfopristin’’.
A few methods have been reported for the preparation of oxa-
1
therapeutics are limited because of their inherent instability to-
ward proteolytic cleavage by peptidases in the gastrointestinal
tract, lack of oral bioavailability, inability to cross the blood–brain
barrier due to their high molecular weight, and rapid excretion
through the liver and kidneys.2 These limitations have generated
an intensive search for peptidomimetics. The alteration of peptides
to peptidomimetics has included peptide side chain manipulation,
amino acid extensions, deletions, substitutions and most recently
backbone modification. A wide variety of peptide mimetic strate-
gies have been described in recent years,2c,d ranging in complexity
from single atom substitution in the amide group to motivated de
novo design of multiple-residue mimetics.3 Continuing with our
work toward the synthesis of peptidomimetics and small molecule
compounds from modified peptides,4 we report the synthesis of
bis-oxazole peptidomimetics starting from Boc-aminoacids and
Serine-methyl ester.
1
3
zole aminoacid precursors; however, their utilization is limited
1
4
to macrocyclic peptide syntheses. Herein, we report the oxazole
aminoacids as building blocks for the solid phase synthesis of di-
verse bis-oxazole peptides. Similar to the biosynthetic pathway,15
our approach to oxazole aminoacids is based on the cyclodehydra-
tion of b-hydroxy peptides to oxazolines followed by ring oxidation
and ester hydrolysis (Scheme 1). Our approach toward the parallel
synthesis of bis-oxazole peptidomimetics is outlined in Scheme 2.
Starting from five oxazole aminoacid building blocks (5A–E), we
performed the parallel synthesis of 25 (5 ꢀ 5) bis-oxazole pepti-
domimetics. The Serine methylester 1 was coupled with different
amino acids 2 to give the corresponding b-hydroxy peptides 3.
The hydroxyl peptide was then converted into oxazoline using
DAST, followed by ring oxidation using BTCM/DBU to yield the de-
sired oxazole aminoacid esters 4. Base mediated hydrolysis of the
oxazole ester gave the corresponding oxazole aminoacids (5A–E)
in good overall yields (Scheme 1).
5
A large number of oxazole-containing natural products, partic-
ularly from the marine environment, have been isolated and found
to exhibit a wide range of biological activities6 such as antibacte-
rial, antiviral, and cytotoxicity. Other oxazole derivatives are also
found to be associated with antifungal, antitubercular, and anti-
7
inflammatory activities. In addition to a number of complex mol-
ecules containing a single oxazole, many natural products contain
0
two or more oxazole rings that are either directly linked (2,4 -bis-
8
oxazoles) or separated by at least three atoms. Known bis-oxazole
Having oxazole aminoacid building blocks in hand, a library of
bis-oxazole peptides was synthesized. The parallel solid phase or-
ganic synthesis (SPOS) of the diverse bis-oxazole dipeptidomi-
1
6
⇑
Corresponding author. Tel./fax: +1 772 345 4739.
E-mail addresses: adeln@tpims.org, anefzi@tpims.org (A. Nefzi).
1
6c,17
metics was performed using the tea-bag approach
(Fig. 1).
0
040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.10.078

S. Murru et al. / Tetrahedron Letters 54 (2013) 7062–7064
7063
OH
Table 1
O
O
O
Synthesis of bis-oxazole peptidomimetics from oxazole aminoacid building blocksa
NH
2
+
NHBoc (i)
MeO
MeO
NHBoc
MeO
HO
N
H
Yieldb (%)
Purityc (%)
Product
Corresponding aminoacid
CH
2
OH
R
O
O
R
(
1)
(2)
(3)
R1
R2
(
ii, iii)
8
8
8
a
b
c
Leu
Leu
Leu
Leu
Leu
Pro
Pro
Pro
Pro
Pro
Val
Val
Val
Val
Val
Phe
Phe
Phe
Phe
Phe
Tyr
Tyr
Tyr
Tyr
Tyr
Leu
Pro
Val
Phe
Tyr
Leu
Pro
Val
Phe
Tyr
Leu
Pro
Val
Phe
Tyr
Leu
Pro
Val
Phe
Tyr
Leu
Pro
Val
Phe
Tyr
78
86
92
83
87
75
68
72
76
67
85
76
74
81
78
82
75
80
83
89
77
73
82
86
91
92
96
88
91
89
92
94
96
87
93
95
93
92
89
91
86
93
95
92
96
87
94
91
93
88
O
O
NHBoc
(iv)
NHBoc
HO
N
N
8d
8e
O
R
R
(
5)
(
4)
8f
Product (5):
Side chain (R): Leu
Yield (%):
58
5A
5B
Pro
61
5C
5D
Phe
56
5E
Tyr
48
8g
8
8
8
8
h
i
j
Val
64
k
Scheme 1. Synthesis of chiral oxazole amino acid building blocks starting from
serine methylester to amino acid. Reaction conditions: (i) HOBt, DIC, DIEA, DMF,
°C ? RT; (ii) DAST, DCM, ꢁ78 °C; (iii) BTCM, DBU, DCM, 0 °C; (iv) 1 N aq NaOH,
8l
8m
8n
8o
0
dioxane/H
2
O.
8
8
8
8
p
q
r
O
O
H
N
s
HO
NHBoc (i)
NHBoc
N
N
8t
8u
O
(5)
R
O
(6)
R
8
8
v
w
R, R' = Leu, Pro, Val, Phe, Tyr
(
ii)
O
O
(i)
8x
8y
H
H
N
N
NHBoc
N
N
a
b
c
O
R
O
R'
Obtained via solid-phase parallel synthesis approach.
Based on the product obtained after resin cleavage.
Based on integration of LC–MS chromatogram.
(
7)
(ii)
(
iii)
O
O
H
H
2
N
N
NH
2
N
N
It should be mentioned here that we obtained a set of distinct
heterocyclic peptides (all B-series), from proline oxazole (B), con-
O
R
O
R'
(
8a - 8y))
taining two oxazoles and one pyrrolidine similar to another bioac-
9
tive peptide Muscoride A. Some of the bis-oxazole peptides from
Scheme 2. Solid phase parallel synthesis of bis-oxazole peptidomimetics from
oxazole aminoacid building blocks. Reaction conditions: (i) DIC (1 equiv), HOBt
proline oxazole, containing a pyrrolidine side chain, exist as cis–
trans isomers which were confirmed by H NMR.
1
(
1 equiv), DMF, 8–12 h. (ii) 55% TFA in DCM, 30 min. (iii) HF-cleavage.
The synthesized bis-oxazole peptides were screened in com-
petitive radioligand receptor binding assays for opioid affinity
for mu and kappa receptors. Compounds with moderate affinity
for mu receptors were identified (Fig. 2). These initial screening
results indicate the high probability of finding new class of
analgesics.
We have developed an approach for the parallel synthesis of
pharmacologically relevant bis-oxazole peptides. This methodol-
ogy can be extended to incorporate bis-oxazole moiety into pep-
tides and peptidomimetics to generate combinatorial libraries for
the purpose of drug discovery. We are in the process of preparing
a large library of bis-oxazole peptidomimetics to screen for their
biological activities. The synthesis of the libraries and results from
their screening for the identification of biologically active com-
pounds will be reported elsewhere.
Starting from p-methyl benzhydrylamine (MBHA), the Boc pro-
tected oxazole aminoacid 5, was coupled using standard coupling
conditions (DIC and HOBt in DMF). The completion of the coupling
reaction was confirmed by the Kaiser test.1 Following Boc-depro-
tection with 55% TFA in DCM and neutralization with 5% DIEA in
DCM, the second aminoacid 5 was coupled using the same condi-
tions as mentioned above.
8
After Boc-deprotection, compounds were cleaved from the resin
and the desired 25 diverse bis-oxazole peptides were obtained in
good yields and high purity (Scheme 2, Table 1). The integrity of
all compounds was confirmed by LC–MS and NMR.
O
HO
NHBoc
N
(5)
O
R
(
Leu)
(Pro)
5B
(Val)
5C
(Phe)
5D
(Tyr)
5E
5
A
AA
AB
AC
AD
AE
BA
BB
BC
BD
BE
CA
CB
CC
CD
CE
DA
DB
DC
DD
DE
EA
EB
EC
ED
EE
5
x 5 = 25 compounds (8a - 8y)
Figure 1. Parallel synthesis of bis-oxazole peptidomimetics (8a–y) from oxazole
aminoacid building blocks (5A–E).
Figure 2. Opioid activity of oxazole dipeptides.

7
064
S. Murru et al. / Tetrahedron Letters 54 (2013) 7062–7064
9.
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Acknowledgment
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Supplementary data
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17. General procedure for the solid-phase parallel synthesis of bis-oxazole peptides
(8a–y): A set of twenty 50 mg bags containing p-methyl benzhydrylamine
3
4
.
.
hydrochloride salt (MBHA) resin (1.15 m equiv/g, 100–200 mesh) was
prepared.1
6c
Reactions were carried out by placing all the bags in
polyethylene bottles. Following neutralization of the resin with 5%
diisopropylethylamine (DIEA) in dichloromethane (DCM), Boc-Oxazole amino
acid (5) (2 equiv) was coupled using the conventional reagents
hydroxybenzotriazole (HOBt, 2 equiv) and diisopropylcarbodiimide (DIC,
2 equiv) in anhydrous DMF shaken overnight. Completion of the coupling
was monitored by the ninhydrin test.18
Following the removal of the Boc group with 55% TFA/DCM for 30 min, the bags
were neutralized with 5% DIEA/DCM. The resin-bound amino acid was treated
with another Boc-oxazole amino acid (5) to yield Boc-protected bis-oxazole
peptides connected to resin (7). Following the Boc-deprotection with 55% TFA/
DCM, the product was cleaved from resin under HF conditions to provide the
desired bis-oxazole peptides. All the products were confirmed by LC–MS and
NMR analysis.
2
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2-((S)-1-Amino-2-(4-hydroxyphenyl)ethyl)-N-((S)-1-(4-carbamoyl oxazol-2-yl)-
3-methylbutyl) oxazole-4-carboxamide (8e): 1H NMR (400 MHz, CDCl ): d 0.97
3
(d, 7H, J = 6.4 Hz), 1.66–1.71 (m, 1H), 1.82–1.87 (m, 2H), 2.15 (br s, 2H), 3.03–
2.97 (m, 1H), 3.15–3.11 (m, 1H), 4.27–4.34 (m, 1H), 5.43 (q, 1H, J = 8.0 Hz),
5.77–5.86 (m, 1H), 6.72 (d, 2H, J = 8.4 Hz), 6.80 (br s, 1H), 6.94 (d, 2H,
J = 8.4 Hz), 8.15 (s, 1H), 8.17 (s, 1H). 13C NMR (100 MHz, CDCl ): d 21.9, 22.6,
3
24.7, 41.5, 42.5, 45.3, 51.6, 115.6, 128.1, 130.3, 135.2, 141.5, 141.8, 155.0,
160.2, 162.6, 163.8, 166.4. LC–MS (ESI): 450 (M+Na).
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