C.-B. Zheng et al.
Bioorganic & Medicinal Chemistry Letters 36 (2021) 127785
Scheme 1. General synthetic approach for the final products.
bisbenzylisoquinoline that was isolated from the stem bark of Litsea
lancifolia [7], Curine, a bisbenzylisoquinoline alkaloid [8], Rutae-
carpine, an indolopyridoquinazolinone alkaloid [9], and so forth
The myograph assay was also performed on the mouse aortas to
examine whether these five compounds could reverse the vascular
+
contraction elicited by high extracellular K . The results showed that
[
10–12]. Moreover, efforts from Li’s group have revealed several first-
they induced dose-dependent relaxation of mouse aortas when pre-
contracted with 60 mM KCl at concentrations ranging from 3 to 300
in-class N-bearing heterocyclic derivatives, exemplified by quinoline
derivate (A) and its isoelectronic and isostructural quinoxaline ana-
logues (B) & (C), to be effective vasodilators owing to their noticeable
vasorelaxation effects, thereby designating them as good templates for
discovering more potent vasodilators [13].
μ
M (Fig. 2A and B). In addition, deletion of endothelium could not
significantly alter the effect of the compounds on the relaxation re-
sponses (Fig. 2C and D). These results further supported the vasorelaxant
functions of these five compounds (7s, 7t, 7v, 7w, 7γ) exerted by acting
on vascular smooth muscle cells.
Enlightened by these findings, our newly designed heterocyclic
molecules were optimized as follows. The quinoxaline framework of (C)
was kept unchanged, owing to the fact that the quinoxaline nucleus,
acting as a bioisostere of isoquinoline and a well-tolerated structure in
humans [14], has been proven to be a privileged platform that can be
used as an effective building block for constructing numerous drug-like
chemotypes with potent therapeutic applicability for the treatment of
different pathophysiological conditions [15–17]. Further chemical
diversification was expanded by introducing various substituents at the
Further examination of the vasorelaxant effects of the five effective
compounds was performed using the myograph method. As exhibited in
Fig. 3A-D, phe pre-contracted arteries were statically relaxed by three
compounds (7v, 7w and 7γ) in a dose-dependent (3 ~ 300
μM) and
endothelium-independent pattern.
Given that compounds 7v, 7w and 7γ exhibited dose-dependent and
endothelium-independent vasorelaxant effects on vessel tone, notably
stronger than the others in all the 7-methoxyl-2,3-disubstituted qui-
noxaline derivatives, we consequently examined whether such vaso-
relaxation was derived from the activation of eNOS. To this end, these
three compounds were pre-treated with a nitric oxide (NO) synthase
2
,3-positions of the quinoxaline backbone and by examining their in-
fluence to vasorelaxant activities. Meanwhile, 6-nitro group of (C) was
replaced by an electron-donating methoxyl fragment, due to the po-
tential genotoxic threat of the nitro group [18]. Overall, a novel class of
ω
inhibitor N -nitro-l-arginine methyl ester (L-NAME). In the presence of L-
7
-methoxyl quinoxaline derivatives was proposed, their vasorelaxant
NAME, all the compounds decreased contractile responses of vessel tone
to Phe or high KCl considerably (Fig. 4A-D), suggesting that compounds
7v, 7w and 7γ induced vasorelaxation without activating the eNOS.
Our present study revealed that in relation to commonly used acute
vasodilation drugs, such as acetylcholine (ACH), the 7-methoxyl-2,3-
disubstituted quinoxaline derivatives, exemplified by compounds 7v,
7w and 7γ, demonstrated slow, albeit effective vasorelaxant effects on
Phe induced pre-contracted mice aortas. In particular, treatment of phe
activities on vascular tone, along with underlying mechanisms of action
were also investigated, with the expectation of identifying more prom-
ising small molecular blood pressure-lowering agents.
The target 7-methoxyl-2,3-disubstituted quinoxaline derivatives in
our synthetic protocol were readily prepared from the commercially
available 5-methoxy-2-nitroaniline (1), following the processes listed in
Scheme 1. Reduction of (1) led to 4-methoxybenzene-1,2-diamine (2),
which was subjected to condensation with oxalic acid under acidic
conditions to assemble the quinoxaline nucleus (3). This compound was
pre-contracted arteries with compound 7γ at 300
μ
M for about 40 min
led to approximately 72.7% decrease in maximal contractility (Fig. 5A
and B), whereas others (7v, 7w) decreased maximal contractility by
50% Fig. 6.
3
then submitted to chlorination in the presence of POCl to furnish the di-
chlorinated product (4), which sequentially underwent the sequences of
nucleophilic substitution referenced by literature method with modifi-
cations [19,20], deprotection of N-Boc group and another nucleophilic
substitution to afford the final product (7a-7γ) with relatively satisfac-
tory yields. To conveniently elucidate the target compounds, the general
molecular formula, molecular weight (MW), melting point (m.p.), yield
and calculated logP (clogP) value of target compounds were shown in
gridlines (Table 1).
To sum up, the present results suggested that in our newly synthe-
sized 7-methoxyl-2,3-disubstituted quinoxaline derivatives, some com-
pounds, 7v, 7w and 7γ in particular, induced slow but effective
vasorelaxation dose-dependent and endothelium-independent effects in
+
high extracellular K - and phe-precontracted C57 mice aortic rings. At
present stage, although we can’t reach a clear SAR conclusion since
there were no common structural characteristics among these active
derivatives, it is reasonable to conclude that these three quinoxaline
derivatives might act as potential leads for further structural modifica-
tions and offer therapeutic potential for the treatment of hypertension.
Future efforts will concentrate on structural optimizations of these leads
to find out the clear structure–activity relationships (SARs) by adding
more chemical varieties.
Next, we proceeded to screen the modulatory effects of these qui-
noxaline derivatives on vessel tone using the myograph assay [21].
Aortic segments from mice were exposed to
α-adrenergic receptor
agonist phenylephrine (phe) at 1 M to induce contraction. Among these
μ
compounds, five compounds (7s, 7t, 7v, 7w, 7γ) were observed to relax
vascular contraction in the mouse aorta by completely reversing phe-
induced pre-contraction, while the remaining compounds had no ef-
fects (Fig. 1A and B). For the sake of clarity, only a part of the com-
pounds that had no modulatory effects on vessel tone were shown in
Fig. 1A, and the phe SEM results containing the endothelium were
presented in Fig. 1B.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
2
Gao, Wen-Cong
