Synthesis of tripeptide mimetics based on dihydroquinolinone and benzoxazinone scaffolds

Article abstract of DOI:10.1002/chem.201102732

In the image: The design and synthesis of peptidomimetics that maintain the configuration of the triad Asp-Thr-Gly found in the catalytic site of the HIV-1 protease (see scheme) are described. By using regioselective nitration and reductive lactamisation,

Full text of DOI:10.1002/chem.201102732

COMMUNICATION
DOI: 10.1002/chem.201102732
Synthesis of Tripeptide Mimetics Based on Dihydroquinolinone and
Benzoxazinone Scaffolds
Aline Dantas de Araujo,[a] Caspar Christensen,[a] Jens Buchardt,[a]
Stephen B. H. Kent,[b] and Paul F. Alewood*[a]
Design and synthesis of novel and structurally diverse het-
erocyclic compounds as peptidomimetics is an area of inten-
sive research in the field of peptide, combinatorial and me-
dicinal chemistry.[1] One common strategy in peptide mimet-
ic design is to restrict the conformational freedom available
to the peptide by replacement of the native peptidyl struc-
ture by rigid mimetic scaffolds. In many instances, stabilisa-
tion of a biologically active conformer by this means can
lead to improved selectivity, potency and metabolic stability.
In other cases the unnatural structure is strategically de-
signed to generate new chemical or biological properties not
found in the original peptide molecule.
Modern chemical ligation methods have enabled the in-
corporation of peptidomimetics into protein molecules by
total chemical synthesis.[2] This capability is particularly
powerful for the systematic dissection of the chemical basis
of protein function, especially enzyme catalysis. In the
course of our studies in this area, we have been interested in
the synthesis of a sterically constrained non-peptide moiety
that mimics the geometry of the tripeptide Asp25-Thr26-
Gly27 found in the active site of the human immunodefi-
ciency virus type 1 (HIV-1) protease.[3] The Asp25 side chain
in each monomer of the homodimeric protein molecule
À
bears the catalytic COOH functionality crucial to the activ-
ity of this aspartyl proteinase. Our ultimate goal is to re-
À
place the COOH functionality with a phenol/phenolate
moiety; this will enable us to systemically tune the electron-
À
ic properties of the phenolic OH and investigate the effect
on the catalysis of peptide bond hydrolysis in the enzyme–
substrate complex. As a first step towards this goal, we de-
signed the bicyclic mimetic molecule 1 that could adopt the
Figure 1. Structure of the designed tripeptide mimetics and related
necessary configuration according to molecular modelling
known dihydroquinolinone and benzoxazinone scaffolds. Bottom: energy
minimized model generated by using the modelling system Sculpt[4] show-
ing the superposition of the mimetic 1 (blue, here connected to a leucine
residue at the amine position) on the 3D configuration of the sequence
Leu24-Asp25-Thr26-Gly27 (yellow) as found in the crystal structure of
the HIV-1 protease.
[a] Dr. A. Dantas de Araujo, Dr. C. Christensen,+ Dr. J. Buchardt,+
Prof. P. F. Alewood
Institute for Molecular Bioscience
The University of Queensland, St. Lucia QLD 4072 (Australia)
Fax : (+61)7-33462101
and energy minimization studies (Figure 1).[4] At first, we
decided to work with constructs 2 and 3, which are simpli-
fied versions of 1 without the Thr side chain. These target
scaffolds are, in fact, related to two known classes of com-
pounds, the 3,4-dihydro-2(1H)-quinolinones 4 (dihydroqui-
nolinone) and 2H-1,4-benzoxazin-3-(4H)-ones 5 (benzoxazi-
none).
E-mail: p.alewood@imb.uq.edu.au
[b] Prof. S. B. H. Kent
Department of Chemistry, University of Chicago
Chicago, Illinois 60637 (USA)
[+] Current address: Protein Technology Novo Nordisk
2720 Maaloev (Denmark)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102732.
Chem. Eur. J. 2011, 17, 13983 – 13986
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13983

Benzoxazinone compounds are found in certain plants,
for instance maize, rye and corn, where they play an impor-
tant role in many chemo-ecological botanical interactions,
and act as phytotoxic, antimicrobial or antifungal media-
tors.[5,6] Such array of bioactivities has raised great interest
in their development as herbicides and fungicides.[6] The di-
hydroquinolinone motif is rarely found in nature,[7] but is
present in the core of commercial drugs, such as cilostazol,[8]
aripiprazole[9] and carteolol.[10] Over the years, both dihydro-
quinolinone[8–11] and benzoxazinone[6,12] heterocycles have
been intensively exploited in the development of new drug
leads. Potential therapeutic applications of these templates
are very broad, including their use as analgesic, antidepres-
sant, antipsychotic, vasodilator, antifungal and anticonvul-
sant agents. Furthermore, compounds possessing the dihy-
droquinolinone skeleton have been employed as intermedi-
ates in the synthesis of antineoplastic alkaloids, like maka-
luvamines[13] and acronycine.[14]
Here, we describe the synthesis of the dihydroquinoline 2
and benzoxaninone 3 peptidomimetics. Although the synthe-
sis of many molecules containing the heterobicycle skeleton
has been reported,[11,15,16] the particular substitution pattern
À
À
found in molecules 2 and 3 (in which the OH and NH2
groups are in meta position to the NH moiety of the hetero-
cycle) is novel and, in the case of 2, imposes an unprece-
dented synthesis challenge. The key strategy to furnish these
compounds is the regioselective incorporation of nitro
groups at the phenol precursor prior to reductive ring clos-
ing lactamisation.[17] Because these compounds are to be em-
ployed in the total chemical synthesis of a HIV-1 protease
analogue, the final products were equipped with protecting
groups suitable for use in solid-phase peptide synthesis
(SPPS).
Scheme 1. Synthesis of the dihydroquinolinone mimetic. Reagents and
conditions: a) H2, Pd/C, MeOH, 100%; b) i. HNO3, AcOH, 128C;
ii. H2SO4, EtOH, reflux, 65%; c) 5-chloro-1-phenyl-1H-tetrazole, Ag2O,
CHCl3/DMSO (10:1), reflux, 52%; d) Tf2O, NEt3, DCM, 95%; e) 9, H2,
Pd/C, dioxane, 97%; f) 10, H2, Pd/C, Mg, NH4OAc, 51%; g) BBr3, DCM,
93%; h) Boc2O, THF, 79%; i) (2,6-Cl2)Bzl-Br, K2CO3, MeCN, 90%;
j) i. BrCH2CO2Et, LiHMDS, THF, 50%; ii. LiOH (aq.), THF, 33%.
For the synthesis of dihydroquinolinone mimetic, 3-hy-
droxy-4-methoxycinnamic acid (6) was chosen as the starting
material, since it contains the correct substitution pattern
for the formation of heterocycle 2, including the presence of
an OH group that was used to direct nitration at positions 2
and 6. As depicted in Scheme 1, compound 6 was first trans-
formed to 7 by catalytic hydrogenation.[18] Regioselective ni-
tration of 7 with nitric acid in acetic acid followed by
esterification gave intermediate dinitrophenol 8.[19] In order
to remove the auxiliary OH group by reductive deoxygena-
tion in a later step, phenol 8 was converted to the 5-phenyl-
tetrazolyl ether 9 or triflate 10. At first, activation of the hy-
droxyl group by this means proved to be difficult, due to the
high acidity of the phenol moiety in 8. Original conditions
for the 5-phenyltetrazolylation[20] by using several bases,
such as K2CO3 and tBuOK, gave no trace of the correspond-
ing ether leaving the parent phenol completely intact. How-
ever, by using Ag2O as the base and refluxing over several
days, the desired product 9 could be obtained in moderate
yield. On the other hand, triflate 10 was promptly prepared
from treatment of 8 with triflic anhydride in good yield, but
showed less deoxygenation efficiency, as discussed next. Pal-
ladium-assisted hydrogenation of the 5-phenyltetrazolyl
ether 9 in dioxane resulted in simultaneous deoxygenation,
reduction of the two nitro groups and ring closing lactamisa-
tion to give dihydroquinolinone 11 in high yield. Under the
same conditions, formation of 11 from triflate 10 was prob-
lematic with formation of several by-products. Addition of
triethylamine to the hydrogenation reaction improved the
rate of bicycle formation, with 35% of the desired product
being formed plus 45% of hydroxylated bicycle. By per-
forming the hydrogenation under the deoxygenation condi-
tions reported by Sajiki et al.,[21] in which Mg metal and
NH4OAc are used as additives, deoxygenation of the triflate
10 was improved, and 11 was prepared in reasonable yield.
Treatment of 11 with boron tribromide in DCM, which was
superior to other Lewis acids, afforded the desired dihydro-
quinolinone 12. Prior to the incorporation of the glycine res-
idue, the bicycle 12 was protected successively with a Boc
group at the amine position (compound 13) and a 2,6-di-
chlorobenzyl group at the phenol position (compound 14).
Finally, the N-amide position of heterocycle 14 was alkylat-
ed with ethyl bromoacetate by using LiHMDS as base, fol-
lowed by saponification, to give the dihydroquinoline build-
ing block 15.
Two synthesis approaches give access to benzoxazinones
and efforts towards this class of compounds have been de-
scribed in a review by Ilas et al.[15] Starting from an appro-
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Chem. Eur. J. 2011, 17, 13983 – 13986

Synthesis of Tripeptide Mimetics
COMMUNICATION
priately substituted nitrophenol, the bicyclic construct can
be prepared by alkylation of the phenol and subsequent re-
ductive cyclisation or by reduction of the nitro group fol-
lowed by ring closure with 2-halo acetyl halides. By varying
the nature of the nitrophenol precursor and alkylating re-
agents, several benzoxazinone derivatives have been made,
most of them possessing substitutions at position C2, N4 and
C6 at the bicyclic core. In the present study, the dinitrophe-
nol precursor 16 was synthesised by nitration of 5-methoxy-
salicylic acid with HNO3 (Scheme 2).[22] Alkylation of 16
regioselective nitration of a phenol, followed by one-pot de-
hydroxylation and ring-closing lactamisation. In addition to
the utility for the synthesis of our target tripeptide mimetics,
the present synthesis methods can also be exploited as new
derivatisation routes for pharmacologically relevant dihy-
droquinolinone and benzoxazinone compounds.
Acknowledgement
This work was supported by the Australian Research Council.
Keywords: benzoxazinones
·
dihydroquinolinones
·
heterocycles · HIV-1 protease · peptidomimetics
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Chem. Eur. J. 2011, 17, 13983 – 13986
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Received: September 1, 2011
Published online: November 14, 2011
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Chem. Eur. J. 2011, 17, 13983 – 13986