Synthesis and pharmacophore modelling of 2,6,9-trisubstituted purine derivatives and their potential role as apoptosis-inducing agents in cancer cell lines

Article abstract of DOI:10.3390/molecules20046808

Twelve compounds were obtained by a three step synthetic procedure using microwave irradiation in a pivotal step. All compounds were evaluated in vitro to determine their potential effect on cell toxicity by the MTT method and flow cytometry analysis on four cancer cells lines and Vero cells. Three out of twelve compounds were found to be promising agents compared to a known and effective anticancer drug, etoposide, in three out of four cancer cell lines assayed with considerable selectivity. Preliminary flow cytometry data suggests that compounds mentioned above induce apoptosis on these cells. The main structural requirements for their activity for each cancer cell line were characterized with a preliminary pharmacophore model, which identified aromatic centers, hydrogen acceptor/donor center and a hydrophobic area. These features were consistent with the cytotoxic activity of the assayed compounds.

Full text of DOI:10.3390/molecules20046808

Molecules 2015, 20, 6808-6826; doi:10.3390/molecules20046808
OPEN ACCESS
molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis and Pharmacophore Modelling of 2,6,9-Trisubstituted
Purine Derivatives and Their Potential Role as Apoptosis-Inducing
Agents in Cancer Cell Lines
Jeannette Calderón-Arancibia 1, Christian Espinosa-Bustos 1, Álvaro Cañete-Molina 1,
Ricardo A. Tapia 1, Mario Faúndez 2, Maria Jose Torres 2, Adam Aguirre 2, Margot Paulino 3
and Cristian O. Salas 1,*
1
Departamento de Química Orgánica, Facultad de Química, Pontificia Universidad Católica de Chile,
702843 Santiago de Chile, Chile; E-Mails: jeannettecalderon.a@gmail.com (J.C.-A.);
ccespino@uc.cl (C.E.-B.); acanetem@uc.cl (A.C.-M.); rtapia@uc.cl (R.A.T.)
2
Departamento de Farmacia, Facultad de Química, Pontificia Universidad Católica de Chile,
702843 Santiago de Chile, Chile; E-Mails: mfaundeza@uc.cl (M.F.); mjtorres@uc.cl (M.J.T.);
aaguirred@uc.cl (A.A.)
3
Centro de Bioinformática Estructural-DETEMA, Facultad de Química, Universidad de la
República, C.C. 1157 Montevideo, Uruguay; E-Mail: margot.paulino@gmail.com
* Author to whom correspondence should be addressed; E-Mail: cosalas@uc.cl;
Tel.: +56-2-2354-4427; Fax: +56-2-2354-4744.
Academic Editors: Jean Jacques Vanden Eynde, Annie Mayence and Tien L. Huang
Received: 4 March 2015 / Accepted: 10 April 2015 / Published: 15 April 2015
Abstract: A series of 2,6,9-trisubstituted purine derivatives have been synthesized and
investigated for their potential role as antitumor agents. Twelve compounds were obtained
by a three step synthetic procedure using microwave irradiation in a pivotal step. All
compounds were evaluated in vitro to determine their potential effect on cell toxicity by the
MTT method and flow cytometry analysis on four cancer cells lines and Vero cells. Three
out of twelve compounds were found to be promising agents compared to a known and
effective anticancer drug, etoposide, in three out of four cancer cell lines assayed with
considerable selectivity. Preliminary flow cytometry data suggests that compounds
mentioned above induce apoptosis on these cells. The main structural requirements for their
activity for each cancer cell line were characterized with a preliminary pharmacophore
model, which identified aromatic centers, hydrogen acceptor/donor center and a hydrophobic
area. These features were consistent with the cytotoxic activity of the assayed compounds.

Molecules 2015, 20
Keywords: antitumor; purine derivatives; apoptosis; pharmacophoric model
6809
1. Introduction
The purine framework is present in biological molecules that display key roles in signal pathways in
all living organisms. Therefore, this heterocyclic structure represents an example of denominated
“privilege scaffold”, a term that has been used to refer to the notion of multiple molecules of the same
scaffold having bioactivity [1]. The purine scaffold is one the most abundant N-based heterocycle in
nature and has been involved in a vast array of metabolic and other cellular processes [2] and the
screening of purine libraries against a wide variety of biological targets has contributed to opening new
applications as therapeutics agents [3]. Therefore, the purine scaffold is an interesting structural fragment
that has been incorporated into new drugs. Some examples of major purine-based drugs that are currently
being used are: anticancer agents (6-mercaptopurine, thioguanine) [4,5], antiviral agents to overcome
infections such as herpes or AIDS (acyclovir, ganciclovir, carbovir, abavavir, ddI, etc.) [6–9] and agents
to avoid organ rejection (azathioprine) [10].
In recent years, several di- and tri-substituted purine derivatives have been synthesized and tested on
cancer cells. Schultz et al., synthetized libraries of purine-based compounds that could modulate the
activity of cyclin dependent kinases (CDKs). Given the essential role of CDKs in regulating the cell
cycle, these compounds were very novel and promising as anticancer agents [11]. On the other hand, the
same group has identified a 2,6,9-trisubstituted purine denominated myoseverin (Figure 1), a
microtubule assembly inhibitor with moderate cytostatic activity, which may induce apoptosis [12].
Some pyrazolo[1,5-a]-1,3,5-triazine myoseverin derivatives have been synthesized and evaluated as
antiproliferative agents [13]. Furthermore, other purine’s 2,6,9-substitution pattern has been designed
and screened as CDK inhibitors, such as roscovitine (Figure 1) and related compounds [14–16].
Specifically, the pure R-enantiomer of roscovitine is under study as an oncology drug candidate in
patients diagnosed with non-small cell lung cancer or other malignancies [17,18]. Finally, olomoucine
(Figure 1), is another example of 2,6,9-trisubstituted purines, with antiproliferative effects on cancer cell
lines [19,20].
Figure 1. Chemical structures of 2,6,9-trisubstituted purines with biological properties.

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Therefore, the search for new compounds with selective antitumor activity as an important driving
force for the development and implementation of novel anticancer therapies, where the drug should exert
a cytotoxic effect on malignant cells with a minimal effect on normal cells [21,22]. According to all the
previous findings in the literature, a purine heterocycle scaffold is an interesting starting point to the
identification of new antitumor compounds.
Because there is little data analyzing the influence of lipophilic properties of purine derivatives and
their cytotoxic activity on normal and cancer cells, or their ability to induce apoptosis as part of a
mechanism of action for the most active compounds; we decided to develop a new series of analogues
to myoseverin and analyze the influence of the N-alkyl substitutions on the purine core. The hydrophobic
moiety involved different chain lengths and special distribution, to correlate these molecular patterns
with a potential antitumor activity. Compounds were tested on a panel of five cell lines, including Vero
cells and four human-derived tumor cell lines, namely H1975 (lung), HL-60 (leukemia), HCT116
(colorectal) and HeLa. In addition, our study includes the elucidation of pharmacophore query for active
molecules from a total of 12 compounds that led us to identify the principal structural features that might
be responsible for their cytotoxicity.
2. Results and Discussion
2.1. Synthesis
The preparation of the 2,6,9-trisubstituted purines listed in Table 1 is described in Scheme 1. This
three step-sequence using 2,6-dichloropurine (1) as substrate is shown in Scheme 1 [23–25] and is a
useful and direct approach for the synthesis of required compounds 4a–l in the study. The first step was
the alkylation of 1 with several alkyl halides under basic conditions, in DMF at room temperature to
give a mixture of N9- and N7-alkylated purines regioisomers 2a–f: 2a'–f' in a proportion 4:1 in most
cases. The unequivocal structure of each regioisomer was established by HMBC analysis (For selected
compounds see supporting information) [26]. For example, for compound 2b, the proton signal at 4.18
ppm (H-8) has correlations with the carbon signals at 145.88 ppm (C-5) and 153.16 ppm (C-3a),
meanwhile the compound 2b',the proton signal at 4.42 ppm (H-8) has correlations with carbon signals
at 121.59 ppm (C-6a) and 150.40 ppm (C-5), confirming the formation of both regioisomers (Figure 2).
The regiospecific SNAr on C-6, was efficiently achieved using benzylamine or
p-methoxybenzylamine as nucleophile, using 1-butanol as solvent and in presence of DIPEA by reflux
by 12 h (yields 73%–99%) [27]. Finally, for the last SNAr on C-2, the most unreactive site, we chose to
use microwave irradiations (MW); since the displacement of this site needs harsh conditions (135 °C,
24 to 40 h) [16,28,29]. Therefore, the intermediates 3a–l were substituted by benzylamine in 1-butanol
and DIPEA as a base, under MW for 1 h, afforded 2,6,9-trisubstituted purines 4a–l with medium to high
yields (43%–95%). The structures of the new synthesized compounds were established on the basis of
their spectral properties (IR, MS, 1H-NMR and 13C-NMR, see Supplementary Material).

Molecules 2015, 20
Table 1. In vitro cytotoxicity of compounds 4a–4l on cancer cell lines and Vero cells.
6811
IC50 (µM) a
HCT116
Entry
R1
R2
H1975
14.0 ± 0.26
28.0 ± 0.08
36.0 ± 0.27
12.0 ± 0.19
8.6 ± 0.18
9.9 ± 0.18
>50.0
HL-60
26.0 ± 0.13
>50.0
HeLa
Vero
4a
i-Propyl
Butyl
H
H
13.0 ± 0.12
7.2 ± 0.16
>50.0
32.0 ± 0.13
18.0 ± 0.14
>50.0
20.0 ± 0.11
5.6 ± 0.15
22.0 ± 0.23
2.5 ± 0.20
2.5 ± 0.14
26.0 ± 0.12
15.0 ± 0.23
17.0 ± 0.20
1.9 ± 0.24
26.0 ± 0.18
34.0 ± 0.13
15.0 ± 0.06
>25.0
4b
4c
i-Butyl
Pentyl
i-Pentyl
Hexyl
i-Propyl
Butyl
H
>50.0
4d
H
>50.0
11.0 ± 0.11
5.1 ± 0.14
3.0 ± 0.12
7.1 ± 0.11
7.0 ± 0.16
1.6 ± 0.12
8.0 ± 0.21
11.0 ± 0.26
6.8 ± 0.14
2.8 ± 0.24
10.1 ± 0.14
2.7 ± 0.13
5.3 ± 0.13
10.7 ± 0.18
6.3 ± 0.17
4.9 ± 0.16
9.5 ± 0.14
5.4 ± 0.32
9.0 ± 0.13
8.2 ± 0.33
4e
H
>50.0
4f
H
>50.0
4g
OCH3
OCH3
OCH3
OCH3
OCH3
OCH3
-
22.0 ± 0.13
>50.0
4h
11.0 ± 0.25
3.4 ± 0.28
2.3 ± 0.13
5.6 ± 0.23
>50.0
4i
i-Butyl
Pentyl
i-Pentyl
Hexyl
-
>50.0
4j
4k
>50.0
>50.0
4l
>50.0
Etoposide
8.0 ± 0.33
6.2 ± 0.18
a IC50 values were determined in triplicate in the range of 0.05 to 50 µM.
Reagents and conditions: (i) alkyl halide, K2CO3, DMF, 6 h, rt, 51%–90% for 2a–f and 15%–24% for 2a'–f';
(ii) benzylamine or 4-methoxybenzylamine, DIPEA, 1-BuOH, 12 h, 110 °C, 73%–99%; (iii) benzylamine,
DIPEA, 1-BuOH, 1 h, 150 °C, MW, 43%–95%. For compounds 4a–l, the substituents R1 and R2 are indicated
in Table 1.
Scheme 1. Synthesis of target compounds 4a–l.
Figure 2. Structures of compounds 2b and 2b' and key HMBC correlations.

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2.2. Cytotoxic Studies
The potential in vitro antitumor activity of compounds 4a–l was initially tested for their cytotoxic
effects on H1975, HL-60, HCT116 and HeLa cancer cell lines and Vero cells. A conventional
colorimetric assay was set up to estimate the IC50 values, which represent the concentration of a drug
that is required for 50% inhibition in vitro after 72 h of continuous exposure to compounds. Four serial
dilutions (from 12.5 to 100 µM) for each sample were evaluated in triplicate and etoposide was used as
a positive control and reference drug.
Table 1 shows the IC50 values for cytotoxicity of compounds 4a–l on Vero cells and cancer cell lines.
In general, 2,6,9-trisubstituted purines activity was quite heterogeneous: For example, HL-60 cells
seemed to be more resistant (all compounds tested with IC50 > 20 µM), while H1975, HCT116, Hela
and Vero cells exhibited variable sensitivity. However, a preliminary analysis about the cytotoxicity
indicates that: (i) Compound 4c had no significant cytotoxic effects on four cancer cells (IC50 > 36 µM).
(ii) Compounds 4b, 4d, 4e and 4i had no selectivity, affecting more Vero cells than cancer cells, although
4e was the most potent compound against Hela cells (IC50 = 2.7 µM). (iii) Compounds 4a, 4g, 4h and 4l
showed little activity, compared to etoposide, on the most cancer cells (except 4h in Hela cells with IC50
= 6.3 µM). (iv) Compounds 4f, 4j and 4k, were the most active exhibiting single-digit micromolar IC50
values and with the highest Selectivity Index (SI) values, against three cancer lines, as shown in the
Table 2. It is important to consider that a potential antitumor drug must show low toxicity in mammalian
host cells, and because of that, those more selective compounds are very promising for the development
of new antitumor agents. These results are in agreement with the National Cancer Institute (NCI)
protocols, where compounds exhibiting IC50 values <10 μM or 15 μM are considered active [30]. To
establish a structure–relationship with these compounds, there are some preliminary conclusions that
can be derived from these results.
A quick look at the IC50 values for each cancer cell line suggests a positive correlation with the
lipophilicity of these compounds, which are in agreement with the aim to evaluate the effect on the
cytotoxicity by the alkyl moiety or the methoxy group in the phenyl ring, on the purine scaffold. The
lipophilicity of compounds 4a–l could be estimated through the logarithm form (logP) the
octanol-water partition coefficient (Table 2). However, no correlation was observed between compound
cytotoxicity and their logP values in every cancer cell line. Although of no significance is the trend is
that the compounds with low lipophilicity (logP > 5.3) exhibit the best antitumor activity.
The lack of correlation between lipophilicity and antitumor activity indicates that this parameter is
not decisive in the cytotoxicity, and though is related with the membrane permeability, it does not always
shows a quantitative correlation with this activity [26]. In fact, many other factors need to be considered
in the relationship of structural pattern and cytotoxicity activity. Therefore, it is necessary to search for
other tools to understand the antitumor activity in silico and to explore the structural requirements
determining the observed biological properties [31,32].

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Table 2. Selectivity Index and logP of compounds 4a–4l.
Selectivity Index (SI) a
Compound
LogP b
(Vero/H1975) (Vero/HCT116) (Vero/HeLa)
4a
4b
4c
4d
4e
4f
1.43
0.20
0.61
0.21
0.29
2.63
0.30
1.55
0.56
11.30
6.07
0.30
3.10
1.54
0.78
0.44
0.23
0.49
8.67
2.11
2.43
1.19
3.25
3.09
2.21
4.03
0.63
0.31
0.44
0.25
0.93
4.91
1.40
2.70
0.39
2.74
6.30
1.67
3.05
4.35
4.94
4.86
5.38
5.30
5.83
4.31
4.90
4.81
5.34
5.26
5.78
-
4g
4h
4i
4j
4k
4l
Etoposide
a
b
Selectivity Index: expressed as the ratio IC50 (Vero)/IC50 (cancer cell line); logP values
calculated using MOE program.
2.3. Pharmacophore Elucidation
In order to generate a pharmacophore model (hypothesis) related with cytotoxicity (IC50) of
compounds 4a–l on H1975, HCT116 and Hela cells, the compound with the highest IC50 in every cancer
cell line was chosen as a structural template. These compounds were 4e, 4j and 4i for HeLa, H1975 and
HCT116, respectively. The tridimensional structure of the selected molecule was created and then the
pharmacophore hypothesis was generated using the polar-charged-hydrophobic (PHC) scheme of MOE
program. Figure 3 shows the numbering used to identify pharmacophoric features in each model.
Figure 3. 2D representation of 4e and numbering used to identify the pharmacophoric features.
Each initial hypothesis was evaluated by scoring both active and inactive compounds. The inactive
compounds were not involved in model generation, they only were used to discard hypothesis that did

Molecules 2015, 20
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not distinguish between active or inactive compounds. This fact is especially useful when active
compounds share similar structural pattern. For HeLa and H1975 cells, all molecules with IC50 lower
than 17.9 µM were accepted as actives and all those with IC50 lower than 12.9 µM were accepted as
actives for HTC116. Several structural particularities were identified in each model: aromatic features
(Aro, centered in the purine and phenyl rings), hydrophobic features (Hyd, due that alkyl moiety bonded
to N9 and the benzyl radicals), acceptor features (Acc/Acc2, generated by the presence of N1, N3 and
N7 nitrogen atoms), and donor features (Don/Don2) centered on the N10 and N11 nitrogen atoms.
The results for the complete elucidation strategy for each cancer cell line are summarized in Table 3
and Figure 4, where the essential features of each compound used in this study is seen. The most active
compounds are also identified in every pharmacophoric model. For example: (i) For cytotoxic
compounds on Hela cells, all features manually detected, five resulted associated with the most active
compounds (4d and 4f–l, Figure 4a), which is interpreted as a 91% of recovering and could be accepted
by the pharmacophore as valid, selective and with the higher probability to recognize active and non-
active molecules. (ii) For cytotoxic compounds on H1975 cells, five features resulted associated to the
activity of these compounds and with the elucidated pharmacophore query, eight active compounds (4a,
4d–f, 4h–k) were recognized (Figure 4b), a similar behavior than HeLa cells. (iii) For cytotoxic
compounds on HTC116 cells, five features resulted associated to the most active compounds, three
common features with HeLa but, two new features were detected: an H-bond donor characteristic
centered in the N11 nitrogen atom, and a hydrophobic region centered in the hydrocarbon moiety
adjacent to the N9 nitrogen atom (Figure 4c). With this set of pharmacophoric features, a search was
done within the purine database and the ten most active from the eleven molecules were selected. In this
case, the 4a analogue was rejected. This result proved the discriminatory capacity of the elucidated
pharmacophore. However, these three models are preliminary, because a larger number of compounds
will allow improved pharmacophore models.
Table 3. Pharmacophoric and structure features of the training set for each purine derivatives
on cancer cell lines.
Pharmacophoric Features
Structure Feature
Hela
H1975
HTC116
Purine N1
Purine N3
* Acc
* Acc
-
-
* Acc
Acc
Purine N7
Acc
-
Acc
Alkyl moiety on N9
N10
-
-
Aro/Hyd
-
-
* Don
-
N11
-
Don
Benzyl moiety on C2
Benzyl moiety on C2
Aro/Hyd
Aro/Hyd
Aro/Hyd
Aro/Hyd
Aro/Hyd
Aro/Hyd
* These are the named “projected features” and correspond to regions in which the lone pairs could be located,
making possible the hydrogen bonding.

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(a)
(b)
(c)
Figure 4. Elucidated pharmacophoric model for 2,6,9-trisubstituted purines against each
cancer cell lines: (a) Hela; (b) H1975 and (c) HTC116.
2.4. Apoptosis Assay
Finally, to study the mechanism involved on the antitumor activity of the most active compounds (4f,
4j and 4k), the effects on the apoptosis were evaluated by flow cytometry (FACS) by staining with
propidium iodide (PI). To evaluate sensitivity to 4f, 4j and 4k, Vero, HTC116, H1975 and Hela were
treated with concentrations of 50 µM for 16 h. Fifty µM etoposide was used as positive control. All cells
were completely sensitive to all compounds at the concentration used and the results are shown in Figure 5.
As it can be seen in Figure 5, 4f, 4j and 4k induced apoptosis as a prevalent type of cell death, in high
percentages (>70%), in every cancer cell line. Moreover, 4f, 4j, and 4k induced an apoptotic cell
response greater than etoposide.
VERO
HTC116
Viable
100
80
60
40
20
0
100
80
60
40
20
0
Viable
Apoptotic
Necrotic
Apoptotic
Necrotic
H1975
Viable
Viable
HeLa
Apoptotic
Necrotic
100
80
60
40
20
0
Apoptotic
Necrotic
100
80
60
40
20
0
Figure 5. Viability assays of compounds 4f, 4j and 4k on cancer cell lines.

Molecules 2015, 20
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3. Experimental Section
3.1. General Information
2,6-Dichloropurine, benzyl amines and the alkyl halides, were purchased from Sigma-Aldrich (St. Louis,
MO, USA). Cell culture medium, fetal bovine serum and penicillin-streptomycin were purchased from
Biological Industries (Kibbutz Beit Haemek, Israel). Etoposide, 3-(4,5-dimethylthiazol-2-yl)-2,5-
diphenyl tetrazolium bromide (MTT) propidium iodide and all other chemicals were obtained from
Sigma-Aldrich. Microwave-assisted reactions were carried out in a Microwave Synthesis Reactor
Monowave 300 (Anton Paar GmbH, Graz, Austria), in 10 mL sealed vials. Melting points were
determined on a Thermogeräte Kofler apparatus (Reichert, Werke A.G., Wien, Austria) and were
uncorrected. Infrared spectra were recorded on a Vector 22 spectrophotometer (Bruker Optik GmbH,
Bremen, Germany) using KBr discs. Nuclear magnetic resonance spectra were recorded on a Bruker
AM-400 apparatus using CDCl3 solutions containing TMS as internal standard. HPLC-MS experiments
were performed on an Exactive Plus Orbitrap MS (Bremen, Germany). Mass spectra were obtained on
an HP 5988A spectrometer (Hewlett-Packard, Palo Alto, CA, USA). Thin layer chromatography (TLC)
was performed using Merck GF-254 type 60 silica gel. Column chromatography was carried out using
Merck silica gel 60 (70–230 mesh).
3.2. General Synthetic Procedure to Obtain N-Alkyl Purines 2a–f and 2a'–f'
A mixture of 2,6-dichloropurine 1 (1.0 mmol), the respective alkyl halide (1.5 mmol), and potassium
carbonate (3.0 mmol) in DMF (5 mL) was stirred for 6 h, then the mixture was filtered and evaporated
under vacuum. The products were separated by flash chromatography on silica gel eluting with
EtOAc/CH2Cl2 (2:3).
2,6-Dichloro-9-isopropyl-9H-purine (2a): White solid, yield (67%), mp 100–102 °C. 1H-NMR (CDCl3)
δ 8.17 (s, 1H), 4.97–4.86 (m, 1H), 1.65 (d, J = 6.8 Hz, 6H). 13C-NMR (CDCl3) δ 152.73, 151.66, 146.93,
143.58, 131.05, 48.36, 22.51 (2C). IR (KBr, cm−1): 2985, 1588, 1555, 1358, 1216, 872. HRMS for
(C8H8Cl2N4 [M+H]+). Calcd: 231.0204. Found: 231.0195.
2,6-Dichloro-7-isopropyl-9H-purine (2a'): White solid, yield (20%), mp 106–108 °C. 1H-NMR (CDCl3)
δ 8.17 (s, 1H), 4.97–4.86 (m, 1H), 1.65 (d, J = 6.8 Hz, 6H). 13C-NMR (CDCl3) δ 152.73, 151.66, 146.93,
143.58, 131.05, 48.36, 22.51 (2C). IR (KBr, cm−1): 2990, 1584, 1557, 1364. HRMS for (C8H8Cl2N4
[M+H]+). Calcd: 231.0204. Found: 231.0180.
9-Butyl-2,6-dichloro-9H-purine (2b): White solid, yield (52%), mp 61–63 °C. 1H-NMR (CDCl3) δ 8.10
(s, 1H), 4.27 (t, J = 7.3 Hz, 2H), 2.00–1.80 (m, 2H), 1.51–1.27 (m, 2H), 0.98 (t, J = 7.4 Hz, 3H). 13C-NMR
(CDCl3) δ 153.20, 152.89, 151.71, 145.74, 130.74, 44.42, 31.73, 19.81, 13.42. IR (KBr, cm−1): 2954,
1596, 1556, 1347, 1228, 878. HRMS for (C9H10Cl2N4 [M+H]+). Calcd: 245.0361. Found: 245.0352.
7-Butyl-2,6-dichloro-7H-purine (2b') White solid, yield (18%), mp 78–79 °C. 1H-NMR (CDCl3) δ 8.20
(s, 1H), 4.40 (t, J = 7.3 Hz, 2H), 1.85–1.80 (m, 2H), 1.35–1.27 (m, 2H), 0.89 (t, J = 7.1 Hz, 3H). 13C-NMR

Molecules 2015, 20
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(CDCl3) δ 163.57, 151.49, 145.95, 130.69, 121.57, 47.45, 33.48, 19.57, 13.44. IR (KBr, cm−1): 2952, 1601,
1550, 1229. HRMS for (C9H10Cl2N4 [M+H]+). Calcd: 245.0361. Found: 245.0335.
1
2,6-Dichloro-9-isobutyl-9H-purine (2c): White solid, yield (61%), mp 40–43 °C. H-NMR (CDCl3) δ
8.08 (s, 1H), 4.08 (d, J = 7.4 Hz, 2H), 2.34–2.24 (m, 1H), 0.98 (d, J = 6.7 Hz, 6H). 13C-NMR (CDCl3) δ
153.39, 152.95, 151.75, 146.10, 130.65, 51.78, 28.98, 19.84 (2C). IR (KBr, cm−1): 2954, 1594, 1558,
1352, 1232, 879. HRMS for (C9H10Cl2N4 [M+H]+). Calcd: 245.0361. Found: 245.0351.
2,6-Dichloro-7-isobutyl-7H-purine (2c'): White solid, yield (22%), mp 94–96 °C. 1H-NMR (CDCl3) δ
8.19 (s, 1H), 4.20 (d, J = 7.4 Hz, 2H), 2.19–2.08 (m, 1H), 0.91 (d, J = 6.7 Hz, 6H). 13C-NMR (CDCl3) δ
163.59, 152.89, 150.81, 143.73, 121.68, 54.69, 30.50, 19.55. IR (KBr, cm−1): 2955, 1596, 1558, 1349,
1231. HRMS for (C9H10Cl2N4 [M+H]+). Calcd: 245.0361. Found: 245.0336.
2,6-Dichloro-9-pentyl-9H-purine (2d): White solid, yield (51%), mp 37–41 °C. 1H-NMR (CDCl3) δ 8.10
(s, 1H), 4.26 (t, J = 7.3 Hz, 2H), 1.99–1.83 (m, 2H), 1.44–1.20 (m, 4H), 0.91 (t, J = 7.1 Hz, 3H). 13C-NMR
(CDCl3) δ 153.17, 152.77, 151.58, 145.83, 130.70, 44.66, 29.41, 28.60, 22.01, 13.77. IR (KBr, cm−1):
2958, 1596, 1554, 1351, 1235, 876. HRMS for (C10H12Cl2N4 [M+H]+). Calcd: 259.0517. Found: 259.0506.
2,6-Dichloro-7-pentyl-9H-purine (2d'): Light yellow oil (24%). 1H-NMR (CDCl3) δ 8.25 (s, 1H), 4.40
13
(t, J = 7.4 Hz, 2H), 1.87–1.79 (m, 2H), 1.28–1.20 (m, 4H), 0.86–0.73 (m, 3H). C-NMR (CDCl3) δ
163.52, 152.71, 150.57, 143.63, 121.57, 47.69, 31.22, 28.38, 22.03, 13.78. IR (KBr, cm−1): 2955, 1598,
1554, 1350, 1236. HRMS for (C10H12Cl2N4 [M+H]+). Calcd: 259.0517. Found: 259.0490.
2,6-Dichloro-9-isopentyl-9H-purine (2e): White solid, yield (56%), mp 77–79 °C. 1H-NMR (CDCl3) δ
8.11 (s, 1H), 4.31–4.26 (m, 2H), 1.82 (dd, J = 15.0, 7.1 Hz, 2H), 1.60 (td, J = 13.4, 6.7 Hz, 1H), 1.00 (d,
J = 6.6 Hz, 6H). 13C-NMR (CDCl3) δ 153.18, 152.85, 151.66, 145.64, 130.71, 43.00, 38.46, 25.63, 22.17
(2C). IR (KBr, cm−1): 2956, 1596, 1555, 1347, 874. HRMS for (C10H12Cl2N4 [M+H]+). Calcd: 259.0517.
Found: 259.0508.
1
2,6-Dichloro-7-isopentyl-7H-purine (2e'): White oil, yield (15%). H-NMR (CDCl3) δ 8.23 (s, 1H),
4.44–4.39 (m, 2H), 1.74 (q, 7.1 Hz, 2H), 1.68–1.52 (m, 1H), 0.92 (d, J = 6.7 Hz, 6H). 13C-NMR (CDCl3)
δ 163.55, 152.79, 150.37, 143.59, 121.55, 46.14, 40.43, 25.72, 22.24 (2C). IR (KBr, cm−1): 2962, 1596,
1558, 1351, 1237. HRMS for (C10H12Cl2N4 [M+H]+). Calcd: 259.0517. Found: 259.0490.
2,6-Dichloro-9-hexyl-9H-purine (2f): White solid, yield (58%), mp 35–37 °C. 1H-NMR (CDCl3) δ 8.10
(s, 1H), 4.29–4.23 (m, 2H), 1.92–1.76 (m, 2H), 1.41–1.24 (m, 6H), 0.89 (t, J = 7.0 Hz, 3H). 13C-NMR
(CDCl3) δ 153.18, 152.84, 151.65, 145.79, 130.73, 44.68, 31.05, 29.71, 26.20, 22.38, 13.88. IR
(KBr, cm−1): 2953, 2922, 1599, 1556, 1347, 1217, 878. HRMS for (C11H14Cl2N4 [M+H]+). Calcd:
273.0674. Found: 273.0659.
2,6-Dichloro-7-hexyl-9H-purine (2f'): Light yellow oil, yield (23%). 1H-NMR (CDCl3) δ 8.24 (s, 1H),
13
4.41 (t, J = 7.4 Hz, 2H), 1.93–1.71 (m, 2H), 1.40–1.10 (m, 6H), 0.78 (t, J = 7.0 Hz, 3H). C-NMR
(CDCl3) δ 163.53, 152.83, 150.47, 143.65, 121.57, 47.73, 31.50, 31.05, 25.98, 22.36, 13.85. IR (KBr, cm−1):
2954, 1597, 1554, 1350, 1228. HRMS for (C11H14Cl2N4 [M+H]+). Calcd: 273.0674. Found: 273.0646.

Molecules 2015, 20
6818
3.3. General Synthetic Procedure to Obtain N-Benzyl-9-Alkyl-2-Chloro-9H-Purin-6-Amines 3a–l
DIPEA (1.8 mmol) and the respective benzylamine (0.9 mmol) were added to a solution of the
corresponding 9-alkyl-2,6-dichloro-9H-purine (2a–2f, 1.00 mmol) in 1-butanol (5 mL) and the reaction
mixture was heated at 110 °C overnight. Then, the solvent was removed under vacuum and the crude
product was purified by column chromatographic on silica gel using as eluent a mixture 4:1 of
chloroform/acetone for all derivatives.
N-Benzyl-2-chloro-9-isopropyl-9H-purin-6-amine (3a): White solid, yield (95%), mp 167–170 °C.
1H-NMR (CDCl3) δ 7.66 (s, 1H), 7.40–7.30 (m, 5H), 6.53 (s, 1H), 4.84–4.79 (m, 3H), 1.56 (d, J = 6.8 Hz,
13
6H). C-NMR (CDCl3) δ 154.93, 137.57 (2C), 128.66 (2C), 127.88, 127.86, 127,54, 127.56, 127.08,
118.48, 46.92, 44.59, 22.74 (2C). IR (KBr, cm−1): 3265, 1625, 1571, 1311. HRMS for (C15H16ClN5
[M+H]+). Calcd: 302.1172. Found: 302.1164.
N-Benzyl-9-butyl-2-chloro-9H-purin-6-amine (3b): White solid, yield (90%), mp 160–162 °C. 1H-NMR
(CDCl3) δ 7.48 (s, 1H), 7.42–7.17 (m, 5H), 6.66 (s, 1H), 4.82 (br, 2H), 4.11 (t, J = 7.2 Hz, 2H), 1.87–1.77
(m, 2H), 1.38–1.30 (m, 2H), 0.95 (t, J = 7.4 Hz, 3H). 13C-NMR (CDCl3) δ 155.57, 140.52 (2C), 138.42,
131.32, 129.25, 129.11(2C), 128.35 (2C), 128.04, 44.06 (2C), 32.37, 20.20, 13.89. IR (KBr, cm−1): 3262,
1627, 1352. HRMS for (C16H18ClN5 [M+H]+). Calcd: 316.1329. Found: 316.1316.
N-Benzyl-2-chloro-9-isobutyl-9H-purin-6-amine (3c): White solid, yield (81%), mp 155–157 °C. 1H-NMR
(CDCl3) δ 7.39–7.24 (m, 6H), 6.93 (s, 1H), 4.82 (s, 2H), 3.90 (d, J = 3.4 Hz, 2H), 2.30–2.06 (m, 1H),
0.92 (d, J = 6.5 Hz, 6H). 13C-NMR (CDCl3) δ 155.58, 140.97 (2C), 138.46, 129.09 (2C), 128.33 (2C),
128.02 (2C), 118.91, 51.51, 45.13, 29.33, 20.25 (2C). IR (KBr, cm−1): 3262, 1631, 1352. HRMS for
(C16H18ClN5 [M+H]+). Calcd: 316.1329. Found: 316.1318.
N-Benzyl-2-chloro-9-pentyl-9H-purin-6-amine (3d): White solid, yield (92%), mp 110–113 °C. 1H-NMR
(CDCl3) δ 7.42–7.20 (m, 6H), 7.00 (br, 1H), 4.82 (br, 2H), 4.08 (br, 2H), 1.88–1.71 (m, 2H), 1.40–1.18
(m, 4H), 0.89 (t, J = 7.1 Hz, 3H). 13C-NMR (CDCl3) δ 155.58, 140.54 (2C), 138.53, 129.06 (2C), 128.31
(2C), 127.98 (2C), 118.98, 45.05, 44.29, 30.03, 29.07, 22.52, 14.26. IR (KBr, cm−1): 3264, 1628, 1351.
HRMS for (C17H20ClN5 [M+H]+). Calcd: 330.1485. Found: 330.1472.
N-Benzyl-2-chloro-9-isopentyl-9H-purin-6-amine (3e): White solid, yield (93%), mp 154–155 °C. 1H-NMR
(CDCl3) δ 7.57–7.20 (m, 6H), 7.00 (br, 1H), 4.82 (s, 2H), 4.11 (d, J = 5.2 Hz, 2H), 1.72 (br, 2H), 1.63–1.50
(m, 1H), 0.96 (dd, J = 6.5, 1.8 Hz, 6H). 13C-NMR (CDCl3) δ 155.62, 140.38 (2C), 138.54, 129.10, 129.08,
128.34 (2C), 128.04, 128.00, 118.98, 45.08, 42.61, 39.05, 25.96, 22.65 (2C). IR (KBr, cm−1): 3264, 1629,
1306. HRMS for (C17H20ClN5 [M+H]+). Calcd: 330.1485. Found: 330.1473.
N-Benzyl-2-chloro-9-hexyl-9H-purin-6-amine (3f): White solid, yield (85%), mp 98–100 °C. 1H-NMR
(CDCl3) δ 7.40–7.23 (m, 6H), 6.95 (s, 1H), 4.82 (s, 2H), 4.26–3.97 (m, 2H), 1.95–1.73 (m, 2H), 1.28
13
(br, 6H), 0.88 (t, J = 6.1 Hz, 3H). C-NMR (CDCl3) δ 155.57, 140.52 (2C), 138.51, 129.09, 129.07,
128.33, 128.32, 128.04, 128.00, 118.99, 45.06, 44.32, 31.58, 30.32, 26.64, 22.84, 14.33. IR (KBr, cm−1):
3265, 1630, 1306. HRMS for (C18H22ClN5 [M+H]+). Calcd: 344.1642. Found: 344.1631.

Molecules 2015, 20
6819
2-Chloro-9-isopropyl-N-(4-methoxybenzyl)-9H-purin-6-amine (3g): White solid, yield (99%), mp
140–141 °C. 1H-NMR (CDCl3) δ 7.37 (s, 1H), 7.25 (d, J = 7.3 Hz, 2H), 6.83 (d, J = 7.5 Hz, 2H), 4.85–
4.61 (m, 3H), 3.78 (s, 3H), 1.50 (d, J = 5.5 Hz, 6H). 13C-NMR (CDCl3) δ 159.47, 155.56, 138.02 (2C),
130.66 (2c), 129.60 (2C), 119.17, 114.41 (2C), 55.66, 47.26, 44.44, 23.10 (2C). IR (KBr, cm−1): 3268,
2972, 1643, 1513, 1291, 1250. HRMS for (C16H18ClN5O [M+H]+). Calcd: 332.1278. Found: 332.1263.
9-Butyl-2-chloro-N-(4-methoxybenzyl)-9H-purin-6-amine (3h): White solid, yield (82%), mp 149–150 °C.
1H-NMR (CDCl3) δ 7.55 (s, 1H), 7.28 (dd, J = 7.1, 5.2 Hz, 2H), 6.89–6.82 (m, 2H), 6.63 (s, 1H), 4.73
(s, 2H), 4.12 (t, J = 7.2 Hz, 2H), 3.80 (s, 3H), 1.88–1.76 (m, 2H), 1.42–1.19 (m, 2H), 0.95 (t, J = 7.4 Hz,
13
3H). C-NMR (CDCl3) δ 159.57, 155.44, 140.39 (2C), 130.39, 129.81, 129.77 (2C), 129,73, 114.48
(2C), 55.69, 44.06 (2C), 32.37, 20.20, 13.88. IR (KBr, cm−1): 3262, 2953, 1625, 1513, 1305, 1257.
HRMS for (C17H20ClN5O [M+H]+). Calcd: 346.1435. Found: 346.1424.
2-Chloro-9-isobutyl-N-(4-methoxybenzyl)-9H-purin-6-amine (3i): White solid, yield (81%), mp
115–120 °C. 1H-NMR (CDCl3) δ 7.47 (s, 1H), 7.34–7.22 (m, 2H), 6.91–6.79 (m, 2H), 6.66 (s, 1H), 4.73
(s, 2H), 3.92 (s, 2H), 3.80 (s, 3H), 1.42–1.11 (m, 1H), 0.94–0.91 (m, 6H). 13C-NMR (CDCl3) δ 159.57,
155.48, 140.87 (2C), 130.42, 130.39, 129.76 (2C), 114,49, 114.48 (2C), 55.69, 51.50 (2C), 29.34, 20.25
(2C). IR (KBr, cm−1): 3265, 2956, 1625, 1513, 1305, 1253. HRMS for (C17H20ClN5O [M+H]+). Calcd:
346.1435. Found: 346.1424.
2-Chloro-N-(4-methoxybenzyl)-9-pentyl-9H-purin-6-amine (3j): White solid, yield (83%), mp 124–127 °C.
1H-NMR (CDCl3) δ 7.47 (s, 1H), 7.30–7.23 (m, 2H), 6.85 (d, J = 8.6 Hz, 2H), 6.72 (s, 1H), 4.73 (s, 2H),
4.10 (t, J = 7.2 Hz, 2H), 3.80 (s, 3H), 1.90–1.76 (m, 2H), 1.43–1.16 (m, 4H), 0.89 (t, J = 7.1 Hz, 3H).
13C-NMR (CDCl3) δ 159.56, 155.45, 140.45 (2C), 130.40, 129.81, 129.74 (2C), 129.66, 114.45 (2C),
55.69, 44.29 (2C), 30.05, 29.08, 22.52, 14.25. IR (KBr, cm−1): 3262, 2919, 1626, 1513, 1304, 1243.
HRMS for (C18H22ClN5O [M+H]+). Calcd: 360.1591. Found: 360.1559.
2-Chloro-9-isopentyl-N-(4-methoxybenzyl)-9H-purin-6-amine (3k): White solid, yield (73%), mp
106–110 °C. 1H-NMR (CDCl3) δ 7.53 (s, 1H), 7.34–7.23 (m, 2H), 6.88–6.82 (m, 2H), 6.66 (s, 1H), 4.73
(s, 2H), 4.18–4.08 (m, 2H), 3.80 (s, 3H), 1.79–1.67 (m, 2H), 1.64–1.51 (m, 1H), 0.97 (d, J = 6.6 Hz,
13
6H). C-NMR (CDCl3) δ 159.56, 155.46, 140.26 (2C), 130.40, 129.83, 129.74 (2C), 118.93, 114.47
(2C), 55.69, 42.62 (2C), 39.07, 25.97, 22.64 (2C). IR (KBr, cm−1): 3264, 2953, 1625, 1513, 1306, 1242.
HRMS for (C18H22ClN5O [M+H]+). Calcd: 360.1591. Found: 360.1559.
2-Chloro-9-hexyl-N-(4-methoxybenzyl)-9H-purin-6-amine (3l): White solid, yield (74%), mp 83–86 °C.
1H-NMR (CDCl3) δ 7.53 (s, 1H), 7.28 (dd, J = 8.9, 1.9 Hz, 2H), 6.90–6.79 (m, 2H), 6.59 (s, 1H), 4.73
(s, 2H), 4.11 (t, J = 7.2 Hz, 2H), 3.83–3.75 (m, 3H), 1.83 (d, J = 5.8 Hz, 2H), 1.38–1.23 (m, 6H), 0.88
(t, J = 6.8 Hz, 3H). 13C-NMR (CDCl3) δ 159.56, 155.45, 140.42 (2C), 130.39, 129.84, 129.76, 129.70,
118.95, 114.47 (2C), 55.69, 44.32 (2C), 31.58, 30.34, 26.64, 22.84, 14.32. IR (KBr, cm−1): 3266, 2955,
1625, 1513, 1306, 1256. HRMS for (C19H24ClN5O [M+H]+). Calcd: 374.1478. Found: 374.1714.

Molecules 2015, 20
6820
3.4. General Synthetic Procedure to Obtain N2,N6-Dibenzyl-9-Alkyl-9H-Purine-2,6-Diamine
Derivatives 4a–l
The N-benzyl-9-alkyl-2-chloro-9H-purin-6-amine 3a–l (1.0 mmol), benzylamine (0.9 mmol), DIPEA
(2.0 mmol) and 1-butanol (3 mL) were added to a microwave reaction flask and the reaction mixture
was irradiated for 1 hour at 150 °C. Then the solvent was evaporated under vacuum and the crude product
was purified by column chromatographic on silica gel using chloroform as eluent.
N2,N6-Dibenzyl-9-isopropyl-9H-purine-2,6-diamine (4a): White solid, yield (95%), mp 74–76 °C. 1H-NMR
(CDCl3) δ 7.39–7.22 (m, 11H), 6.35 (br, 1H), 6.08 (br, 1H), 4.62–4.57 (m, 3H), 4.40 (d, J = 5.5 Hz, 2H),
1.48 (d, J = 6.8 Hz, 6H). 13C-NMR (CDCl3) δ 170.34, 159.82, 155.32, 140.88, 139.82, 138.76, 134.94,
129.09, 128.89, 128.75, 128.25, 128.10, 128.06, 127.90, 127.51, 127.22, 115.13, 46.62, 46.44, 44.12,
22.93 (2C). IR (KBr, cm−1): 3289, 2922, 1629, 1548, 1352. HRMS for (C22H24N6 [M+H]+). Calcd:
373.2141. Found: 373.2127.
N2,N6-Dibenzyl-9-butyl-9H-purine-2,6-diamine (4b): White solid, yield (65%), mp 113–115 °C. 1H-NMR
(CDCl3) δ 7.42–7.24 (m, 11H), 6.11 (br, 1H), 5.88 (br, 1H), 4.88–4.53 (m, 4H), 3.98 (t, J = 7.1 Hz, 2H),
13
1.86–1.66 (m, 2H), 1.41–1.16 (m, 2H), 0.92 (t, J = 7.3 Hz, 3H). C-NMR (CDCl3) δ 160.02, 155.30,
140.84, 139.71, 137.38, 129.49, 129.12, 128.90, 128.75, 128.27, 128.15, 128.06, 127.54, 127.23, 114.81,
114.31, 46.43, 44.17, 43.34, 32.31, 20.23, 13.94. IR (KBr, cm−1): 3373, 2929, 1632, 1521, 1351. HRMS
for (C23H26N6 [M+H]+). Calcd: 387.2297. Found: 387.2261.
N2,N6-Dibenzyl-9-isobutyl-9H-purine-2,6-diamine (4c): White solid, yield (74%), mp 111–113 °C.
1H-NMR (CDCl3) δ 7.42–7.24 (m, 3H), 6.02 (br, 1H), 5.86 (br, 1H), 4.62 (d, J = 6.0 Hz, 1H), 4.42 (d,
J = 5.7 Hz, 1H), 3.79 (d, J = 7.2 Hz, 1H), 2.18 (m, 1H), 0.90 (d, J = 6.7 Hz, 1H). 13C-NMR (CDCl3) δ
160.01, 155.26, 140.83, 139.64, 138.67, 137.85, 129.12, 128.91, 128.81, 128.74, 128.58, 128.27, 128.17,
128.08, 127.95, 127.58, 127.23, 114.72, 51.08, 46.45, 44.17, 29.27, 20.41 (2C). IR (KBr, cm−1): 3362,
2922, 1630, 1522, 1353. HRMS for (C23H26N6 [M+H]+). Calcd: 387.2297. Found: 387.2260.
N2,N6-Dibenzyl-9-pentyl-9H-purine-2,6-diamine (4d): White solid, yield (85%), mp 83–86 °C. 1H-NMR
(CDCl3) δ 7.39–7.22 (m, 11H), 6.14 (br, 1H), 5.89 (br, 1H), 4.62 (d, J = 5.9 Hz, 2H), 4.49–4.31 (m, 2H),
13
3.97 (t, J = 7.0 Hz, 2H), 1.88–1.69 (m, 2H), 1.40–1.17 (m, 2H), 0.88 (t, J = 7.1 Hz, 3H). C- NMR
(CDCl3) δ 159.98, 155.26, 140.82, 139.68, 138.70, 137.40, 129.12, 128.90, 128.75 (2C), 128.27, 128.15,
128.06 (2C), 127.95, 127.56, 127.24, 46.43, 44.17, 43.64, 29.94, 29.16, 22.56, 14.30. IR (KBr, cm−1):
3269, 2956, 1632, 1528, 1351. HRMS for (C24H28N6 [M+H]+). Calcd: 401.2454. Found: 401.2418.
N2,N6-Dibenzyl-9-isopentyl-9H-purine-2,6-diamine (4e): White solid, yield (43%), mp 69–70 °C. 1H-NMR
(CDCl3) δ 7.37–7.19 (m, 11H), 6.41 (br, 1H), 5.93 (br, 1H), 4.84–4.66 (m, 2H), 4.61 (d, J = 6.0 Hz, 2H),
3.97–3.95 (m, 2H), 1.73–1.59 (m, 2H), 1.59–1.44 (m, 1H), 0.92 (d, J = 6.6 Hz, 6H). 13C-NMR (CDCl3)
δ 160.02, 155.31, 151.62, 140.84, 140.31, 139.77, 137.20, 129.12, 129.06, 128.88, 128.75, 128.27,
128.11, 128.05, 127.94, 127.50, 127.23, 114.71, 46.40, 41.88, 39.15, 25.86, 22.71 (2C). IR (KBr, cm−1):
3267, 2955, 1630, 1526, 1352. HRMS for (C24H28N6 [M+H]+). Calcd: 401.2454. Found: 401.2417.

Molecules 2015, 20
6821
N2,N6-Dibenzyl-9-hexyl-9H-purine-2,6-diamine (4f): White solid, yield (93%), mp 79–81 °C. 1H-NMR
(CDCl3) δ 7.46–7.26 (m, 11H), 6.06 (br, 1H), 5.81 (br, 1H), 4.79 (br, 2H), 4.63 (s, 2H), 3.98 (s, 2H),
1.74 (br, 2H), 1.28 (br, 6H), 0.80 (br, 3H). 13C-NMR (CDCl3) δ 160.01, 155.28, 140.83, 139.68, 137.40,
129.14, 128.91 (2C), 128.76 (2C), 128.17 (2C), 128.07 (2C), 127.57, 127.25, 114.84, 46.44, 44.19,
43.68, 31.66, 30.21, 26.72, 22.88, 14.39. IR (KBr, cm−1): 3371, 2954, 1630, 1523, 1351. HRMS for
(C25H30N6 [M+H]+). Calcd: 415.2610. Found: 415.2571.
N2-Benzyl-9-isopropyl-N6-(4-methoxybenzyl)-9H-purine-2,6-diamine (4g): White solid, yield (83%),
mp 69–70 °C. 1H-NMR (CDCl3) δ 7.36 (d, J = 7.6 Hz, 2H), 7.34–7.21 (m, 6H), 6.80 (d, J = 8.2 Hz, 2H),
6.09 (br, 2H), 4.67–4.59 (m, 4H), 4.40 (br, 1H), 3.76 (s, 3H), 1.49 (d, J = 6.4 Hz, 6H). 13C-NMR (CDCl3)
δ 159.40, 158.78, 154.83, 140.48, 138.32, 134.47, 131.42, 129.06, 128.71 (2C), 128.36, 127.86, 127.65
(2C), 127.54, 127.51, 126.83, 113.89, 55.27, 46.03, 43.73, 22.55 (2C). IR (KBr, cm−1): 3291, 2927,
1628, 1541, 1354, 1249. HRMS for (C23H26N6O [M+H]+). Calcd: 403.2246. Found: 403.2231.
N2-Benzyl-9-butyl-N6-(4-methoxybenzyl)-9H-purine-2,6-diamine (4h): White solid, yield (91%), mp
92–94 °C. 1H-NMR (CDCl3) δ 7.36 (d, J = 7.4 Hz, 2H), 7.34–7.20 (m, 6H), 6.79 (d, J = 7.6 Hz, 2H),
6.06 (br, 2H), 4.64 (t, J = 8.4 Hz, 4H), 4.40 (d, J = 2.8 Hz, 2H), 3.76 (s, 3H), 1.81–1.72 (m, 2H), 1.34–1.26
(m, 2H), 0.92 (t, J = 7.4 Hz, 3H). 13C-NMR (CDCl3) δ 160.02, 159.22, 155.25, 140.88, 137.33, 131.80,
129.47, 129.09, 128.74 (2C), 128.24, 128.05 (2C), 127.90, 127.89, 127.22, 114.30, 55.66, 46.42, 44.12,
43.32, 32.30, 20.22, 13.94. IR (KBr, cm−1): 3290, 2928, 1632, 1519, 1351, 1251. HRMS for (C24H28N6O
[M+H]+). Calcd: 417.2403. Found: 417.2367.
N2-Benzyl-9-isobutyl-N6-(4-methoxybenzyl)-9H-purine-2,6-diamine (4i): White solid, yield (85%), mp
110–112 °C. 1H-NMR (CDCl3) δ 7.35 (d, J = 7.3 Hz, 2H), 7.32–7.17 (m, 6H), 6.79 (d, J = 7.3 Hz, 2H),
6.22 (br, 2H), 4.71–4.52 (m, 4H), 4.41 (br, 2H), 3.76 (s, 3H), 2.14 (br, 1H), 0.89 (d, J = 6.6 Hz, 6H).
13C-NMR (CDCl3) δ 160.05, 159.19, 155.27, 140.91, 138.80, 137.81, 131.84, 129.46, 129.07, 128.73
(2C), 128.22, 128.07 (2C), 127.89, 127.86, 127.21, 114.29, 55.65, 46.43, 44.08, 29.25, 20.40 (2C). IR
(KBr, cm−1): 3291, 2926, 1619, 1512, 1354, 1253. HRMS for (C24H28N6O [M+H]+). Calcd: 417.2403.
Found: 417.2364.
N2-Benzyl-N6-(4-methoxybenzyl)-9-pentyl-9H-purine-2,6-diamine (4j): White solid, yield (79%), mp
86–88 °C. 1H-NMR (CDCl3) δ 7.36 (d, J = 7.2 Hz, 2H), 7.28–7.21 (m, 6H), 6.80 (d, J = 8.3 Hz, 2H),
6.08 (br, 2H), 4.63 (d, J = 5.8 Hz, 2H), 4.40 (d, J = 4.0 Hz, 2H), 3.95 (d, J = 6.7 Hz, 2H), 3.76 (s, 3H),
13
1.84–1.75 (m, 2H), 1.35–1.24 (m, 4H), 0.87 (t, J = 6.9 Hz, 3H). C-NMR (CDCl3) δ 160.01, 159.25,
155.23, 140.87, 138.78, 137.34, 131.78, 129.47, 129.09, 128.74 (2C), 128.23, 128.04 (2C), 127.89,
127.22, 114.33, 55.66, 46.42, 44.12, 43.63, 29.92, 29.15, 22.54, 14.28. IR (KBr, cm−1): 3293, 2925,
1629, 1527, 1355, 1252. HRMS for (C25H30N6O [M+H]+). Calcd: 431.2559. Found: 431.2518.
N2-Benzyl-9-isopentyl-N6-(4-methoxybenzyl)-9H-purine-2,6-diamine (4k): White solid, yield (74%), mp
1
86–90 °C. H-NMR (CDCl3) δ 7.39–7.22 (m, 8H), 6.80 (d, J = 8.4 Hz, 2H), 6.01 (br, 2H), 4.63 (d,
J = 5.9 Hz, 2H), 4.40 (d, J = 5.6 Hz, 2H), 3.99 (t, J = 7.3 Hz, 2H), 3.76 (s, 3H), 1.68 (q, J = 7.0 Hz, 2H),
1.60–1.50 (m, 1H), 0.93 (d, J = 6.5 Hz, 6H). 13C-NMR (CDCl3) δ 159.59, 158.80, 154.80, 140.44,
138.29, 136.77, 131.32, 129.08, 128.70 (2C), 128.36, 127.85 (2C), 127.64, 127.52, 126.84, 113.90,

Molecules 2015, 20
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55.27, 45.99, 43.73, 41.48, 38.75, 25.46, 22.31 (2C). IR (KBr, cm−1): 3290, 2926, 1620, 1526, 1351,
1252. HRMS for (C25H30N6O [M+H]+). Calcd: 431.2559. Found: 431.2521.
N2-Benzyl-9-hexyl-N6-(4-methoxybenzyl)-9H-purine-2,6-diamine (4l): White solid, yield (98%), mp
108–111 °C. 1H-NMR (CDCl3) δ 7.35–7.21 (m, 8H), 6.79 (d, J = 8.6 Hz, 2H), 6.08 (br, 2H), 4.66–4.62
(m, 4H), 3.95 (t, J = 7.2 Hz, 2H), 3.76 (s, 3H), 1.83–1.73 (m, 2H), 1.27 (br, 6H), 0.86 (t, J = 6.7 Hz, 3H).
13C-NMR (CDCl3) δ 160.02, 159.22, 155.24, 140.88, 138.74, 137.33, 131.78, 129.47, 129.10, 128.74
(2C), 128.25, 128.04 (2C), 127.91, 127.22, 114.30, 55.66, 46.42, 44.14, 43.65, 31.65, 30.20, 26.71,
22.88, 14.38. IR (KBr, cm−1): 3363, 2927, 1632, 1524, 1351, 1242. HRMS for (C26H32N6O [M+H]+).
Calcd: 445.2716. Found: 445.2675.
3.5. Anticancer Activity
3.5.1. Cell Lines Culture
Cancer cell lines were grown at 37 °C in humidified atmosphere and 5% CO2. H1975, HL60, HTC116
and Vero cells were grown in RPMI 1640 culture medium supplemented with 10% fetal bovine serum
and (100 UI/mL) penicillin-streptomicyn. HeLa cells were grown in EMEM culture medium,
supplemented with 10% fetal bovine serum and (100 UI/mL) penicillin-streptomicyn.
3.5.2. Cytotoxicity Study
Cytotoxicity assays were performed by using the MTT reduction method as described
previously [23]. Briefly, cancer cell lines were plated in a flat-bottom 96-wells plate at 10,000 cells per
well density. Then, the cells were incubated with synthetized compounds at different concentrations
(from 0.1 to 50 µM) in 200 µL of 10% fetal bovine serum-RPMI or EMEM culture medium at 37 °C for
72 h. Ten µL of MTT was added at a final concentration of 0.5 mg/mL, incubated at 37 °C for 4 h, and
then solubilized with 10% sodium dodecyl sulfate (SDS) in 0.1 mM HCl and incubated overnight at 37 °C.
Formazan formation was measured at 570 nm in a multiwell reader (StatFax 4200, Awareness
Technology, Inc. Palm City, FL, USA).
3.5.3. Viability Assays with Propidium Iodide
Cell viability was analyzed by FACS as previously described [24–26]. In this assay, after setting the
baseline to exclude cell debris, cells impermeable to propidium iodide (PI negative) are considered as
viable. Two populations of PI-permeable dead cells are distinguished based on fluorescence intensity,
corresponding to either hypodiploid apoptotic cells or necrotic cells with intact DNA. Here, H1975,
HTC116, and HeLa cells were incubated with concentrations of 50 µM of each compound for 16 h at
37 °C. Cells were harvested and stained with 10 µg/mL of propidium iodide to determine cell viability.
Samples containing roughly 1 × 104 cells were evaluated by flow cytometry (FACScanto II; Becton
Dickinson, Mountain View, CA, USA) and analyzed using the software program FCS Express v5.

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3.5.4. Statical Analysis
Values are expressed as means ± standard deviations from three independent experiments. IC50
values, (Compound concentration necessary to decrease at 50% the MTT reduction) and the two-way
analysis of variance (ANOVA) test were performed when necessary with Prism GraphPad 6.0f software
(GraphPad Software Inc., San Diego, CA, USA).
3.6. Pharmacophore Elucidation
The 3D-structure of the pharmacophore hypothesis was carried out using Molecular Operating
Environment (MOE) software version 2013–2014, Chemical Computing Group Inc. (Montreal, QC,
Canada).
4. Conclusions
A study of new 2,6,9-trisubstituted purines on four cancer cell lines and Vero cells, showed that
certain chemical modifications on the purine moiety increased the cytotoxicity and selective effects.
Some of these compounds were more potent than the reference drug etoposide and exhibited selectivity
against three cancer cell lines in comparison with VERO cells, most notably 4f, 4j and 4k. A preliminary
analysis confirms the earlier conclusion that the purine core is a privilege scaffold, but attempts to reveal
general structure-activity relationships using lipophilicity properties were unsuccessful, although
pharmacophoric features have been recognized. Nevertheless, the main mechanism of action observed
for the most active compounds in each cancer cell lines is the induction of apoptosis. These results
showed that 4f, 4j and 4k are promising leads for the development of new antitumor drugs.
Supplementary Materials
Supplementary materials can be accessed at: http://www.mdpi.com/1420-3049/20/04/6808/s1.
Acknowledgments
Financial support from FONDECYT (Research Grant) N° 1120128.
Author Contributions
J.C-A. and C.E-B synthesized and characterized all compounds. A.C-M, C.E-B and R.A.T. analyzed
the data and help to write the paper. M.F, M.J.T and A.A performed the antitumor and FACS analysis.
M.P. performed the pharmacophore elucidation. C.O.S. designed, analyzed and supervised the research
and wrote the paper. All authors read and approved the final manuscript.
Conflicts of Interest
The authors declare no conflict of interest.

Molecules 2015, 20
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Sample Availability: Samples of all compounds are available from the authors.
© 2015 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
distributed under the terms and conditions of the Creative Commons Attribution license
(http://creativecommons.org/licenses/by/4.0/).