2
Tetrahedron
reacting with t-butyl nitrite and diiodomethane (step-iii). To our
facile straight forward nine step synthesis of C5ʹ-N-cyclopropyl
carboxamido-C6-amino-C2-alkynylated purine nucleoside
analogues as well as their anticancer activities.18,19
great delight, the envisaged nucleophilic aromatic substitution
with NH3 at C6 position of purine in compound 4 took place
easily and this was combined with cleavage of acetate protecting
groups in compound 5 (step-iv). Regio selective protection of the
C2ʹ and C3ʹ diol moiety of compound 5 (step-v) with 2,2-
dimethoxy propane in PTSA (p-Toluene sulfonic acid) yielded
compound 6. The C-5ʹ hydroxyl group in sugar moiety of 6 was
subsequently oxidized (step-vi)with KMnO4 (Potassium
permanganate) to carboxylic acid derivative 7. In step-vii,
commercially available HOBt (1-Hydroxybenzotriazole) was
used for the conversion of C5ʹ carboxylic group to amide
derivative 8. Deprotection (step-viii) of C2ʹ and C3ʹ diol moiety
was carried out by treatment with formic acid to afford the
compound 9.
The structures of the products were confirmed by 1H NMR, 13
NMR, IR and Mass spectral data. In the 1H NMR spectra of
compounds 2, 3 and 4 signals for the acetyl protons in sugar
moiety were observed at 2.16-2.25 ppm. The protons of C2
NH2 of compounds 3 and 4 appeared as singlets at 6.52 and
5.13 ppm. Broad singlets of OH groups in sugar moiety were
occurred in compound 5. The signals of geminal CH3 protons of
compounds 6, 7 and 8 were appeared as singlets at 1.4-1.6 ppm.
C-5ʹ carboxylic proton of compound 7 was observed at 11.0 ppm
as singlet. The amide protons of compounds 8 and 9 were
observed at 7.2-8.2 ppm as singlets. The 13C NMR spectra
further confirmed the structures of reaction intermediates 2-9 and
title analogues 11a-g. The IR spectra of compounds 2-9 and 11a-
g were in good agreement with the proposed structures: the
absorption bands were in the regions 1615.73 (COOMe str),
3163.09 (N=CH str), 873.86 (C-Cl str), 789.72 (C-I str), 3426.78
(OH str), 3433.03 (COOH str), 1600 (CONH str) and 1203.51
(C-N str) cm-1.
C
The synthesized/modified purine analogues 11a-g are
evaluated for in vitro anti-proliferative activity against human
breast cancer cell line MDA-MBA-231 and human colon cancer
cell line Caco-2 using MTT assay taking doxorubicin (DXN) a
known anticancer drug as a reference compound. Table 1
contains the activity of modified purine analogues expressed as
IC50 values. It is interesting to find that compounds that 11b (IC50
= 7.5 µg/mL) and 11e (IC50 = 6.8 µg/mL) displayed superior
activity than DXN against Caco-2 and MDA-MB-231 cell lines
respectively indicating their selectiveness towards these cell
lines. Compounds 11c and 11d have displayed a moderate
inhibitory effect against MDA-MB-231 cell lines and 11d and
11g exhibited a similar effect against Caco-2 cell lines.
Table 1
Anti-cancer activity of compounds 11a-g.
Compound
IC50 (µg/mL)a
MDA-MB-231
18.8 ± 0.62
7.9 ± 0.54
Caco-2
11a
20.9 ± 1.04
7.5 ± 0.17
19.4 ± 1.23
12.6 ± 0.16
8.3 ± 0.81
19.2 ± 1.81
13.2 ± 0.94
9.1 ± 1.0
11b
Scheme: Reagents and conditions: (i) Ac2O, dry DMF, dry pyridine,
75 °C, 4.15 h, 70%; (ii) POCl3, N,N-Dimethylaniline, Tetra methyl
ammonium chloride, Acetonitrile, 0 °C to rt, 16 h, 64.1%; (iii) t-
Butyl nitrite, CH2I2, Acetonitrile, 80 0C, 4.50 h, 44%; (iv) Liquid
NH3, -78 °C to rt, 18 h, 80%; (v) PTSA, 2,2-Dimethoxypropane,
Acetone, rt, 4 h, 70%; (vi) KMnO4, KOH, H2O, rt, 72 h, 78.6%; (vii)
EDCl, HOBT, DMAP, Cyclopropylamine, DMF, TEA, rt,
Overnight, 70.3%; (viii) 50% Formic acid, 80 °C, 1.5 h, 73.4%; (ix)
Pd(PPh3)2Cl2, Cuprous iodide, dry DMF, dry acetonitrile, dry
triethylamine, rt, 24 h.
11c
12.1 ± 2.01
11.7 ± 0.8
11d
11e
6.8 ± 0.53
11f
21.1 ± 2.83
23.4 ± 0.48
8.9 ± 0.9
11g
Doxorubicin
aThe IC50 value was expressed as the mean ± standard deviation of 50%
inhibitory concentrations of triplicate experiments.
On a general note, Pd catalyzed cross-coupling reactions14,15
have proven for the efficient alkynylations of many iodo or
bromo substituted nucleoside derivatives with terminal
acetylenes. Regardful of this, the synthesis (step-ix) of different
C2 alkynylated nucleoside analogues 11a-g was performed using
Sonogashira coupling16 on the corresponding C2 iodo derivative
9 with terminal alkyne derivatives 10a-g. The resulting
To conclude, we have described a convenient and efficient
multi-step protocol for the synthesis of target compounds 11 by
utilizing Sonogashira reaction. The final derivatives were tested
for anti-cancer efficacy against MDA-MB-231, Caco-2 cell lines.
Among the screened, products 11b and 11e exhibited promising
cytotoxic activity than doxorubicin (standard anticancer agent).
compound was purified by silica gel column chromatography
and/or reverse-phase HPLC. Further, the C2 alkynyl functional
group was interpreted as a synthon and be exploited with click
chemistry to furnish diverse heterocyclic derivatives17 which are
amenable for further modifications. In this work, we report a
Acknowledgments
One of the authors (MSR) is thankful to University Grants
Commission (UGC), New Delhi, Govt. of India for financial
assistance in the form of a senior research fellowship. Authors
Malthum, Shankaraiah
