Relay tricyclic Pd(ii)/Ag(i) catalysis: Design of a four-component reaction driven by nitrene-transfer on isocyanide yields inhibitors of EGFR

Article abstract of DOI:10.1039/c8cc05845h

Synthesis of pyrazolo[1,5-c]quinazolines from four easily available precursors is presented through a one-pot tricyclic Pd(ii)/Ag(i) relay catalysis. The bimetallic relay cascade forges five new chemical bonds by concatenating six discrete chemical steps. The relay catalysis enables four-component assembly of pyrazolo[1,5-c]quinazolines that selectively inhibit EGFR, exhibit apoptosis through the ROS-induced mitochondrial-mediated pathway, and arrest the cell cycle at the G1 phase.

Full text of DOI:10.1039/c8cc05845h

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Kumar and R. T. Pardasani, Chem. Commun., 2018, DOI: 10.1039/C8CC05845H.
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Relay Tricyclic Pd(II)/Ag(I) Catalysis: Design of a Four-Component
Reaction Driven by Nitrene-Transfer on Isocyanide Yields
Inhibitors of EGFR
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
One-pot synthesis of pyrazolo[1,5-c]quinazolines from four easily employed as a redox7 or photoredox8 partner in the majority of
available precursors is presented through a one-pot tricyclic these reports, which doesn’t expand the overall range of
Pd(II)/Ag(I) relay catalysis. The bimetallic relay cascade forges five transformations. In 2011, Lautens reported relay catalysis of
new chemical bonds by concatenating six discrete chemical steps. Rh-catalyzed alkyne arylation and Pd-catalyzed N-arylation
The relay catalysis enables four-component assembly of steps for the synthesis of dihydroquinolines [Eq.(1)].9,10 Tu
pyrazolo[1,5-c]quinazolines that selectively inhibit EGFR, exhibit employed Co(II)/Ag(I) relay catalysis in 2015 for isocyanide
apoptosis through the ROS-induced mitochondrial-mediated insertion/cycloaddition cascade to access a new pyrrolo[2,3-
pathway, and arrest the cell cycle at G1 phase.
b]indole heterocycle [Eq.(2)].11 Ji reported Rh(II)/Pd(0) dual
catalysis for selective construction of cyclic quarternary carbon
center [Eq.(3)]12 and Ramasastry developed trimetallic relay
catalysis for the synthesis of β-carbolines in 2016 [Eq.(4)].13 In
2017, Wang successfully applied Rh(III)/Ag(I) relay catalysis in
the total synthesis of aristolactam BII14 [Eq.(5)] and Xu
One of the important objectives of synthetic organic chemistry
is to design rapid, efficient and sustainable strategies to access
valuable molecular constructs. To this end, chemists have
designed tandem or cascade reactions that generate complex
molecular frameworks in one-pot without the need for any
isolation of an intermediate.1 Recently, there is an upsurge in
the field of organic synthesis for the development of catalytic
multistep processes, whereby two or more distinct chemical
transformations are promoted by more than one catalysts in a
single flask. Classified as cooperative-,2 synergistic-,3 relay-4 or
sequential-catalysis5 depending on the precise experimental
details offers the potential for the advancement in the
synthesis by removing dependency over iterative processes.6
Of these, orthogonal relay catalyses of transition-metals,
where all metal catalysts are present at the outset, are
somewhat difficult to design owing to the redox
incompatibilities among various metals and a challenging time-
resolution of such methods (Fig. 1). Besides, among various
methods reported so far, the second transition metal is often
Ms
Cl
N
Pd
Lautens, 2011
Rh
(1)
(2)
H
N
Ph
Ms
Ar
N
Ar
N
R
CN
CN
R
R
Ag
Tu, 2015
Co
N
R
NH2
+
O
O
O
O
O
Pd
Ji, 2016
Rh
(3)
(4)
OBz
Ar
OEt
Ar
N2
Me
N
HO
Bi
Ramasastry, 2016
Ag
Pd
R
Ph
N
NHPG
PG
O
O
F
F
Ag
Wang, 2017
Rh
R
NTs
(5)
(6)
N
+
H
OTs
TsO
X
O
O
Pd
Xu, 2017
Ar1
Ar2
Rh
2
+
Ar B(OH)2
Ar1
R4
O
O
R4
Ag
This work R1
Pd
CHO
N3
N
(7)
R3NHNH2
R2 NC
R1
N
N
NH
R2
Fig 1. Various strategies reported for relay catalysis promoted by two or more
transition metal catalysts.
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reported
rhodium/palladium
relay
catalysis
for With an assortment of precursors at hand, a broad diversity of
could be constructed from
enantioselective synthesis of gem-diaryl benzofuran-3(2H)- pyrazolo[1,5-c]quinazolines
5
ones [Eq.(6)].15 In spite of their utility, all methodologies of alkynes (Scheme 2, refer Supporting Information for details).
bimetallic relay catalysis relied on the requirement of The scope of 4-CR was further extended for electron-deficient
prefabricated substrates synthesized by convergent multistep alkenes, such as acrylates and acrylonitrile, to generate
routes. However, the cutting-edge application of multicatalytic tetrahydropyrazolo[1,5-c]quinazolines 2618 of four-point
relay catalysis lies in promoting multicomponent reactions, diversity exhibiting excellent atom economy. The investigation
since it imitates a biological processe where several enzymes underscores high tolerance of the 4-CR reaction to steric and
coordinate to generate complex molecular frameworks from electronic effect. In all cases, the reaction proceeds with high
multiple, simple and easily available starting materials.16 To regiochemical fidelity. Interestingly, the formation of a single
our surprise, multicomponent relay catalysis of two or more diastereomer was observed in 4-CR with alkenes.
transition metals received scant synthetic attention.11 As a
consequence, the scope of the bimetallic relay catalysts is
limited, and its true potential for promoting the
multicomponent reaction has not been explored to date.
We now report a four component reaction (4-CR) employing
four versatile privileged synthons: 2-azidobenzaldehyde,
isocyanide, sulfonyl hydrazides and alkynes promoted by two
different transition metal catalyst in a time-resolved manner,
thus leading to an efficient preparation of pharmacologically
active pyrazolo[1,5-c]quinazolines.17 The corresponding
focused compound collection of pyrazolo[1,5-c]quinazolines
exhibited selective cytotoxicity on cancer cells, inhibited the
Epidermal Growth Factor Receptor (EGFR), increased the
intracellular Reactive Oxygen Species (ROS) levels and altered
the mitochondrial membrane potential (ꢀψm) leading to
apoptosis, cell cycle arrest and cancer cell deaths.
Upon treatment of Pd(OAc)2 (7.5 mol%) and AgOTf (10 mol%)
with
a mixture of 2-azidobenzaldehyde 1aa, tert-butyl
isocyanide 2aa, tosyl hydrazide 3aa, phenylacetylene 4aa and
K3PO4 in toluene, a four-component adduct, pyrazolo[1,5-
c]quinazoline 5aa, was formed (Scheme 1). The structure of
5
was established by X-ray crystallography18 and involved the
formation of five new chemical bonds in a single operation.
Addition of 4 Ǻ molecular sieves was essential to suppress a
major side product, urea 7aa (Scheme 1).19 Both silver and
copper salts worked with equal efficiency in this
transformation. Other Pd-sources, bases, and solvents were
also screened (for details see section S2 of the Supporting
Information), but these turned out to be inferior to the
optimal condition. The 4-CR worked well at room temperature,
and any further increase in the temperature to 60 oC led to the
formation of 9aa18 as the major product.
Scheme 2. Substrate Scope of 4-CR.
Scheme 1. Reaction condition for four-component reaction.
2 | J. Name., 2012, 00, 1-3
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The successful isolation of reaction intermediates, 26ga were subsequently scrutinized for inhibition of ATP
carbodiimide 6aa, and azomethine imine 8aa18 (Fig. 2), dependent phosphorylation of EGFR21 (Table S8). To our
provided meaningful insights into the mechanism of the 4-CR. delight, 5fb was found to be the most potent even with
6aa was readily generated from 2-azidobenzaldehyde 1aa and respect to the standard drug, erlotinib, while 5dd and 26ga
tert-butyl isocyanide 2aa was converted to 8aa upon reacting showed comparable activity in the nanomolar range. This
with the tosyl hydrazide 3aa (refer Section S5.2 of Supporting observation was further supported by molecular docking
Information for details). Interestingly, both steps required studies, which revealed that 26ea and 26ga bind to the ATP
Pd(OAc)2 for catalysis. Conversion of 8aa to pyrazolo[1,5- binding site of EGFR, and exhibit hydrogen bonding
c
]quinazoline 5aa was catalyzed by silver. Although, these interactions with MET769, carboxylate group of 5fb, and
results lend strong support in favor of path A (Fig. 2, via 6aa); carboxylate group of THR830 and THR766 (Fig. 3 and Section
an alternate mechanism via 10aa can also be proposed (Path S7).
B). To probe whether the formation of 8aa could proceed
through both path A and path B, or whether one of these investigational compounds 5dd
A
further biological investigation revealed that the
5fb 26ea and 26ga increased
,
,
pathways was dominating, kinetic studies of 4CR were carried the ROS level (section S6.5) and altered the mitochondrial
out by monitoring the reaction progress with 1H NMR membrane potential (ꢀψm) (section S6.6) of pretreated lung
spectroscopy (section S5.3). The study unequivocally ruled out cancer cell lines A549. Both these biochemical events play a
path B for the formation of 8aa. Moreover, the sluggish rate of key role in inducing the mitochondrial apoptosis and lead to
reaction of the competing concurrent catalytic processes, such lung cancer cell deaths through the ROS-induced
as path B, ensured the time-resolution of the relay catalytic mitochondrial-mediated pathway that involves the release of
process. The regiochemical outcome of 4-CR and H/D the cytochrome c from the mitochondria into the cytosol.
scrambling experiment (Section S5.4) revealed that a possible Anticancer drugs are known to delay the progression of cell
route for the formation of 5aa is base-mediated direct cycle and inhibit the cell proliferation.22 Analysis of the cell
alkynylation of azomethine imine 8aa. Further, no deuterium cycle of control and A549 cell lines treated with 5fb and 26ga
incorporation in 5aa was observed, when the deuterated by flow cytometry showed a substantial rise in the G1 phase of
substrate (ethynyl-d)-benzene was reacted with 8aa (Section cells and the overall drop in the percentage of cycling cells in S
S5.5). This observation supports the formation of acetylide phase and G2-M phase (Fig. 4). The G1 phase arrest by
during the reaction. Based on experimental studies and investigational compounds may be attributed to the fact that
literature precedence,20 it is evident that the 4-CR is mediated EGFR is a mitogen that helps in cell cycle progression from G1
by three independent catalytic cycles of Pd- and Ag-metals to G2/M. The CDK4/6 gets inhibited and causes the cell cycle
(refer section S5.6 for details).
arrest at the G1 phase.23
To explore if the diversity-rich compound collection generated
by the bimetallic orthogonal relay catalysis translates into the
Conclusions
biological activity, pyrazolo[1,5-c]quinazolines
5 and 26 were
screened in the MTT based cell viability assay (section S6.2) on
cancer cell lines of the lung (A549), colon (HCT-116) and
In summary, we have developed a novel four-component
reaction of 2-azidobenzaldehydes, isocyanides, sulfonyl
hydrazides and alkynes/alkenes promoted by orthogonal relay
catalysis of Pd(II)/Ag(I). The 4-CR produced a diverse and
human glioblastoma (U-87 MG) (Table S7). Of these, 5dd, 5fb,
26ea and 26ga exhibited an excellent cytotoxic effect in all
three cancer cells lines. Pleasingly, these compounds also
presented negligible toxicity to Human Peripheral Blood
Mononuclear Cells (HPBMC) implicating a pattern of selectivity
to cancer cells (Section S6.3). Owing to their structural
resemblance to the quinazoline scaffold of gefitinib and
complex heterocycle, pyrazolo[1,5-c]quinazolines with the
formation of five new chemical bonds in a single operation.
The 4-CR exhibited a broad substrate scope with excellent
erlotinib, the investigational compounds 5dd
,
5fb
,
26ea and
CHO TsNHNH2
CNtBu
2aa
3aa
Pd
Pd
N
C
Path A
6aa
NtBu
Ph
4aa
NTs
CHO
N3
t
AgOTf
N
CN Bu, TsNHNH
2
5aa
K3PO4
NHtBu
Pd(OAc)2, 4A MS
toluene, rt, 1 h
N
8aa
toluene, rt, 15 min
1aa
H
N
Path B
N
Ts
Pd
Pd
t
CN Bu
TsNHNH2
3aa
N3
10aa
2aa
8aa
Fig. 2: Control experiments for scrutinizing plausible intermediates.
Fig. 3: Binding pattern representation of 5fb (blue color).
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18 Characterized by X-ray crystallographic analysis with CCDC
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regio-and stereocontrol. A focused compound collection of
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times potent inhibitor of EGFR than an existing drug, erlotinib.
Pyrazolo[1,5-c]quinazolines led to the mitochondrial apoptosis
through the ROS-induced mitochondrial-mediated pathway
that involves the release of the cytochrome c from the
mitochondria into the cytosol leading to the cytotoxic activity
on lung cancer cells (A549) and the cell cycle arrest in G1
phase. The work described herein would pave way for the
development of MCR driven design and assembly of bioactive
heterocycles.
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Conflicts of interest
There are no conflicts to declare.
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Relay Tricyclic Pd(II)/Ag(I) Catalysis: Design of a Four-Component Reaction
Driven by Nitrene-Transfer on Isocyanide Yields Inhibitors of EGFR