S. Kumbhakonam, et al.
Bioorganic&MedicinalChemistryLetters30(2020)127594
O
N3
N3
O
H2N
OAc O
Cl
H2N
O2N
O
O
O2N
2
Pd/C, H2
MeOH
O
N
N
NMP, K2CO3
rt, 3h, 32%
4
1
3
NH2 NH2
N3 N3
K2PtCl4
i. PPh3, THF
ii. dil. HCl
H2O, rt
46%
O
O
O
O2N
H2N
O2N
O
O
i. NaHCO3
O
X-ray Structure of 3
N
N
N
ii. K2PtCl4
H2O, rt
37%
2HCl
NH2 NH2
H2N
Cl
NH2
Pt
H2N
Cl
NH2
Cl
Pt
Cl
6
7
5
Scheme 1. Syntheses of tetrahydro- and dihydroquinoline-based Pt-conjugates 5 and 7; X-ray structure of the intermediate 3 is also shown in the inset.
confirmed their ability to induce redox imbalance in cancer and bac-
terial cells.34 This motivated us to consider dihydroquinoline unit as the
template for attaching diamine ligands for Pt-complexation, and the
results of our investigations involving these new hybrids are outlined
here.
(THQ-diPt). The structure of this di-Pt conjugate was confirmed by ESI-
MS, that gave peak at m/z 908.0768 corresponding to
[M−Cl
+
DMSO]+ and at 986.0879 corresponding to
[M−Cl + 2DMSO]+ (ESI-Fig. 11). HPLC analysis confirmed the purity
of this complex to be ~ 92%. Alternatively, reduction of the tetraazide
9 under PPh3/THF condition gave the tetraamine derivative of dihy-
droquinoline 12 (Scheme 2c). Conversion of this tetraamine to DHQ-
as per the protocol discussed earlier. Its structure was confirmed by ESI-
MS spectrum that had peaks at m/z = 1014.0620 corresponding to
Results and discussion
The reaction sequence adopted for the synthesis of DHQ-Pt con-
jugate 7 is shown in Scheme 1. The key intermediate 3 was first pre-
equivalent 2 in presence of K2CO3 in NMP (ESI).34,35 The structure of 3
was confirmed by NMR, ESI-MS, IR spectroscopy, as well as by X-ray
diffraction analysis of crystals secured by slow evaporation of its ethyl
acetate/hexane solution (ORTEP diagram included in the inset of
Scheme 1). When this diazide (3) was subjected to reduction using Pd/
C, H2, the product 4 arising through complete reduction of azide, nitro-
and alkene functionalities was obtained in quantitative yield. Since this
skeleton could serve as a saturated analog of dihydroquinoline deri-
vative originally envisaged, it was reacted with 1 equiv. of K2PtCl4 in
water to get tetrahydroquinoline-Pt conjugate (THQ-Pt, 5) in 46%
yield.
[M−Cl
+
2DMSO]+ and at 1085.1064 corresponding to
[M−3Cl + 4DMSO]+ (ESI-Fig. 12). HPLC chromatogram of this di-Pt
complex showed a purity of ~95% (ESI-Fig. 13).
Biological studies
Assessment of the ability of 5, 7, 11 and 13 to interact with DNA
was made by ethidium bromide (EB) displacement assay. EB is a ca-
tionic dye having emission at ~ 600 nm (λex 526 nm) in the DNA-
other molecules due to quenching in the aqueous environment.36,37,38
This is generally used to assess the affinity of intercalators and binders
towards DNA. As evident from Fig. 1, increasing the concentration of
these Pt-conjugates resulted in the reduction in emission intensity of
EB. Relatively, the diPt derivatives 11 and 13 were better than the
mono platinum conjugates 5 and 7 in EB displacement. Structural
perturbation in DNA after complexation with these compounds is likely
causing the release of EB molecules from the intercalation sites.
Based on these results, cytotoxicity of these complexes against a
human lung cancer cell line (A549) was evaluated by Alamar Blue
assay.39 After incubation for 48 h, the concentration required for 50%
reduction in cell viability (IC50) was estimated, and the results are
presented in Table 1. In general, their activities were in the same range
to show higher anticancer effect because of multiple cross-linking
possibilities, their activities were slightly lower than the mono-pla-
tinum systems. Overall, the order of activity was found to be DHQ-Pt
(7) > THQ-Pt (5) > THQ-diPt (11) > DHQ-diPt (13). In order to
understand the selectivity profile, they were also tested against mouse
fibroblast NIH3T3 cell lines following the same protocol. Interestingly,
all these new analogs showed better selectivity towards A549 cells
compared to that of cisplatin. The best conjugate in terms of potency
and selectivity was the THQ-Pt complex 5.
Formation of this complex was confirmed through ESI-MS spectrum,
which gave peaks at m/z 544.0820 and 622.0876 corresponding to
[M + H]+ and [M + DMSO + H]+ respectively (ESI-Fig. 4). Purity of
the complex was confirmed by HPLC analysis (ESI-Fig. 5). Subsequent
efforts identified Staudinger reaction condition as suitable for getting
the desired diamine 6 in quantitative yield. Pt-complexation of this
intermediate using K2PtCl4 in water afforded the desired dihy-
droquinoline-Pt conjugate (DHQ-Pt, 7) as a yellow solid in 37% yield.
Its formation was also confirmed by using ESI-MS spectrometric tech-
nique, where it gave peak at m/z 614.0793 corresponding to
[M−Cl + DMSO]+ (ESI-Fig. 6). Purity of this complex was confirmed
by reverse phase HPLC analysis and was found to be ~95% pure (ESI-
Fig. 7).
The reaction sequence used for the synthesis of diplatinum con-
jugates in this series is presented in Scheme 2. The diazide 3 was first
subjected to ester hydrolysis using aq. LiOH to get the free acid 8
(Scheme 2a). It was then coupled with 2-amino-1,3-diazidopropane (2)
yield, which was characterized by NMR, IR and HRMS techniques. As
done for the preparation of mono-Pt conjugate 5, the compound 9 was
reduced using Pd/C, H2 to get tetrahydroquinoline derivative 10
quantitatively (Scheme 2b), which on reaction with two equivalents of
Structurally, compounds 5, 7, 11 and 13 represent fusion of two
different bioactive entities, and hence an attempt was made to get a
2