Synthesis, cyclopolymerization and cyclo-copolymerization of 9-(2-Diallylaminoethyl)adenine and Its Hydrochloride Salt

Article abstract of DOI:10.3390/molecules171113290

We report herein the synthesis and characterization of 9-(2- diallylaminoethyl) adenine. We evaluated two different synthetic routes starting with adenine where the optimal route was achieved through coupling of 9-(2-chloroethyl)adenine with diallylamine.

Full text of DOI:10.3390/molecules171113290

Molecules 2012, 17, 13290-13306; doi:10.3390/molecules171113290
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molecules
ISSN 1420-3049
www.mdpi.com/journal/molecules
Article
Synthesis, Cyclopolymerization and Cyclo-Copolymerization of
9-(2-Diallylaminoethyl)adenine and Its Hydrochloride Salt
Kamal H. Bouhadir 1,*, Lara Abramian 1, Alaa Ezzeddine 1, Karyn Usher 2 and
Nikolay Vladimirov 3
1
Department of Chemistry, American University of Beirut, Box 11-0236, Beirut, Lebanon
2
Department of Chemistry, West Chester University, West Chester, PA 19383, USA
3
Ashland Research Center, 500 Hercules Road, Wilmington, DE 19808, USA
* Author to whom correspondence should be addressed; E-Mail: kb05@aub.edu.lb;
Tel.: +961-01-350-000; Fax: +961-01-365-217.
Received: 11 September 2012; in revised form: 26 September 2012 / Accepted: 2 November 2012 /
Published: 8 November 2012
Abstract: We report herein the synthesis and characterization of 9-(2-diallylaminoethyl)
adenine. We evaluated two different synthetic routes starting with adenine where the
optimal route was achieved through coupling of 9-(2-chloroethyl)adenine with diallylamine.
The cyclopolymerization and cyclo-copolymerization of 9-(2-diallylaminoethyl)adenine
hydrochloride salt resulted in low molecular weight oligomers in low yields. In contrast,
9-(2-diallylaminoethyl)adenine failed to cyclopolymerize, however, it formed a copolymer
with SO2 in relatively good yields. The molecular weights of the cyclopolymers were
around 1,700–6,000 g/mol, as estimated by SEC. The cyclo-copolymer was stable up to
226 °C. To the best of our knowledge, this is the first example of a free-radical
cyclo-copolymerization of a neutral alkyldiallylamine derivative with SO2. These polymers
represent a novel class of carbocyclic polynucleotides.
Keywords: polynucleotide analogs; adenine; cyclopolymerization
1. Introduction
Novel methods for the preparation of modified oligodeoxynucleotides (ODNs) have been actively
pursued in the last two decades [1–3] due to their potential use in therapeutic and diagnostic
applications [4]. An important prerequisite of synthetic ODNs is their stability against biological

Molecules 2012, 17
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nucleases that result in the cleavage of the phosphodiester backbone in RNA and DNA [5,6]. Extensive
work has been conducted to modify or replace the phosphodiester backbone, furanose ring, nucleic
base or a combination of two or more. A convenient route to form homopolymers resembling
modified ODNs is through the cyclopolymerization of diallylamine derivatives thus replacing the
phosphodiester and ribose moieties simultaneously. The cyclopolymerizations of diallyl quaternary
ammonium salts have been thoroughly investigated during recent years [7,8]. The main interest in this
research is the potential utility of the resulting polymers in industrial and pharmaceutical applications [9]
such as layer-by-layer assembly [10–12], quantum dots, nanoparticle stabilization, paper industry [13],
water treatment [14], metal electroplating, corrosion inhibition [15], cosmetic and hair treatments,
antiperspirants, anion-exchange resins, antistatic agents, protein encapsulation, hydrogel formation [16],
antibacterial properties [17,18] and drug delivery applications. The best studied of these compounds
are the diallyldimethylammonium salts. In contrast, little work has been performed on the
cyclopolymerization of the alkyldiallylammonium derivatives [19–21]. One route to prepare modified
ODNs is based on the cyclopolymerization of quaternary diallylammonium salts with nucleic bases
attached [22]. Poly(diallylquaternary ammonium salts) contain permanent positive groups that render
them insoluble in nonpolar organic solvents and hence limit their utility in such applications. In
contrast, polymers prepared from alkyldiallylammonium salts could be deprotonated to yield the
corresponding neutral polymers. This characteristic of alkyldiallylammonium salts makes them
attractive synthetic precursors in the preparation of neutral as well as cationic modified ODNs.
We report herein the synthesis and polymerization of 9-(2-diallylaminoethyl)adenine and its
hydrochloride salt to form a new class of carbocyclic modified ODNs with ethylene or sulfone groups
replacing the phosphodiester backbone and pyrrolidine rings replacing the furanose units of nucleic acids.
2. Results and Discussion
Two synthetic routes (Scheme 1) have been followed in the preparation of
1-(2-diallylaminoethyl)adenine [23]. In the first route, adenine was heated and maintained at reflux
with ethylene carbonate in dry DMF to afford compound 2 that was chlorinated utilizing thionyl
chloride in dry dioxane to yield the chloro derivative 3 [23,24]. Heating a mixture of 3 and an excess
of diallylamine in dioxane at reflux conditions formed 4 in 60% overall yield from adenine [23].
Compound 7 was prepared in quantitative yield by passing HCl gas through a solution of 4 in
anhydrous ethanol.
In an attempt to increase the overall yield of compound 4, we evaluated an alternative synthetic
route utilizing the Mitsunobu reaction (Scheme 1) [22–24]. Adenine was protected with isobutyric
anhydride and coupled to bromoethanol via the Mitsunobu reaction with triphenylphosphine (Ph3P)
and diisopropylazodicarboxylate (DIAD) to yield 6-isobutyryl-9-(2-bromoethyl)adenine (not isolated)
that was heated and maintained at reflux with excess of diallylamine in dry dioxane to afford 6.
Hydrolysis of the isobutyryl group with sodium methoxide formed 9-(2-diallylaminoethyl)adenine (4)
in 19% overall yield from adenine. These results clearly demonstrate the efficiency of the first route in
comparison to the Mitsunobu route with more than three-fold increase in the overall yield.

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Scheme 1. Synthesis of compounds 4 and 7.
Reagents and Conditions: (i) Ethylene carbonate, NaOH, DMF, reflux, 24 h; (ii) SOCl2, pyridine,
dry dioxane, reflux, 2 h; (iii) diallylamine, dioxane, reflux, 4 d; (iv) isobutyric anhydride, DMF,
reflux, 2 h; (v) DIAD, Ph3P, BrCH2CH2OH, dioxane, r.t., 4 days; (vi) diallylamine, dioxane, reflux,
5 days; (vii) NaOMe, MeOH; (viii) HCl(g), EtOH.
The cyclopolymerization and cyclo-copolymerization of diallylammonium chloride (DAAC)
have been reported to yield poly(diallylammonium chloride) (PDAAC) and poly(diallylammonium
chloride-co-sulfur dioxide) (PDAAC-SO2), respectively [25–27]. Homopolymerizations of
methacrylate-type monomers containing nucleic acid bases have also been reported to yield
modified nucleosides [22,28–31]. This compelled us to investigate the homopolymerization and
copolymerization of monomers 4 and 7 with SO2. The pioneering work of Butler and co-workers has
set the foundations for the cyclopolymerization of diallyl monomers [32]. The presently accepted
mechanism for the cyclopolymerization of diallylammonium salts is depicted in scheme 2 [33]. The
initiator attacks C-1 forming the 5-hexenyl radical that cyclizes via the favored 5-exo-trig mode to
yield a highly reactive and nucleophilic primary radical (Scheme 2). The primary radical attacks
another monomer (intermolecular propagation pathway a) or abstract an allylic hydrogen (degradative
chain transfer pathway b) to yield a stable allylic radical.
The cyclopolymerization of monomer 7 was initiated with ammonium persulfate, tert-butyl hydro-
peroxide or V-50 initiator (Scheme 3). Compound 7 was cyclopolymerized with ammonium persulfate
in water to yield polymer 9 in 17% yield whereas lower yields of 5% and 3.7% were obtained with V-50
and tert-butyl hydroperoxide respectively (Table 1). In contrast, the cyclopolymerization of DAAC in
our laboratory was more efficient using V-50 in water to yield PDAAC in 67% yield. The low
polymerization efficiency of monomer 7 in this study is consistent with reported cyclopolymerizations
of related alkyldiallylammonium chlorides [34]. This difficulty in the polymerization is attributed to an
increase in the rate of the degradative chain transfer reaction (pathway b, Scheme 2) through the
abstraction of the α-hydrogen from the allylic position of the monomer resulting in a stable allylic

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radical [35]. A similar conclusion was derived for the relatively low degree of cyclopolymerization of
monomer 4 (Entry 11, Table 1) since neutral diallyl monomers are known to have a more effective
degradative chain transfer [36,37] than their charged counterparts.
Scheme 2. Mechanism of the cyclopolymerization of quaternary diallylammonium salts.
Scheme 3. Cyclopolymerization of compounds 4 and 7.
Reagents and Conditions: (i) SO2, V-50, DMSO, 70 °C; (ii) ammonium persulfate, H2O, 85 °C;
(iii) HCl(g), Et2O; (iv) (a) SO2, V-50, MeOH, 70 °C; (b) aq. HCl; (v) V-50, DMSO, 70 °C.

Molecules 2012, 17
Table 1. Reaction conditions for the cyclopolymerization of DAAC, 7 and 4 a.
13294
Monomer
Monomer
T
Yield
Mn d
Mw
Mz
dn/dc
Entry
Initiator b
Solvent
PDI
1
2
-
(°C) (%) c (kDa) (kDa) (kDa)
(mL/g)
1
2
V-50
TBHP
V-50
DAAC
H2O
H2O
70
80
70
85
70
70
70
70
70
70
70
67
3.7
5
14.1
28.8
54.5
1.98
0.173
7
-
-
-
-
-
-
3
7
H2O
-
-
-
-
-
-
4
APS
7
H2O
-
17
2
2.5
3
1.25
0.183
5
V-50
DAAC
MeOH
MeOH
DMSO
MeOH
MeOH
DMSO
DMSO
SO2
SO2
SO2
SO2
SO2
SO2
-
92
1350
1700
2190
1.32
0.181
6
V-50
7
7
4
4
4
4
trace
7
-
-
-
-
-
-
-
-
7
V-50
-
-
8
V-50
35
6
-
6.2
-
6.3
-
1.03
0.183
9
VPE-0201
V-50
10.6
trace
trace
-
-
-
-
-
-
10
11
-
-
-
V-50
-
-
-
a
All reactions were conducted in sealed tubes after degassing with nitrogen gas for 10 min (for aqueous
solutions) or freeze-thaw degased (3 cycles) for methanol and DMSO. Reactions were heated for 3 days;
b V-50: 2,2'-azobis(2-methylpropionamidine)dihydrochloride, TBHP: tert-butyl hydroperoxide, APS: ammonium
c
persulfate; VPE-0210: macro azo initiator; Isolated yield after dialysis (for aqueous solutions) or after
trituration in methanol followed by filtration (3 cycles); d The separations were carried out on a PSS Novema
pre-column connected in series to three PSS Novema columns (30 Å, 1000 Å, 10,000 Å). Aqueous oxalic
acid (0.22 M) was used as the mobile phase at 40 °C with nominal flow rate of 0.8 mL/min.
The copolymerizations of olefins with SO2 typically result in high molecular weight polymers in
good yields [38]. This is due to the introduction of a flexible sulfonyl radical into the propagating
polymer chain that reduces its rigidity and increases its solubility [39]. The cyclo-copolymerization of
DAAC with SO2 was initiated with V-50 in methanol to result in the formation of PDAAC-SO2 in high
yields (Entry 5, Table 1). In contrast, compound 7 underwent cyclo-copolymerization with SO2 when
initiated with V-50 in DMSO to form polymer 8 in 7% yield, whereas only traces of the polymer were
detected when the reaction was conducted in methanol (Entries 6 and 7, Table 1). The yield of the
copolymerization of 8 was lower than usual, which suggests that the rate of the degradative chain
transfer (pathway b, Scheme 2) has increased further in this case. This increase may be ascribed to the
cyclization of the allylic radical (Scheme 4) through an intermolecular attack on the C-8 carbon of the
adenine ring to form a thermodynamically stable 6-membered ring as well as a resonance-stabilized
radical specie. A relatively high yield was achieved for the copolymerization of the neutral monomer 4
and SO2 to form polymer 8. V-50 was the optimal initiator resulting in a 35% yield in comparison to
10.6% with the macroinitiator VPE-0201 (Entries 8 and 9, Table 1). The copolymerization of neutral
allylated monomers with SO2 have been recently reported to yield low molecular weight oligomers in
good yields [40,41]. The polymerization efficiency was attributed to the formation of a complex
between SO2 and the diallyl groups of the monomer. This complex presumably facilitates the addition
of the primary radical to SO2 forming a stable sulfonyl radical which attacks an unreacted monomer to
yield a propagating polymer chain.

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Scheme 4. Cyclization of the allylic radical.
We evaluated the solubility of PDAAC-SO2 and 8 in a variety of solvents as seen in Table 2. Both
polymers, PDAAC-SO2 and 8, displayed similar solubility in most solvents investigated, except
DMSO, where 8 was soluble at room temperature and PDAAC-SO2 was not soluble at 2 wt%. This
difference in solubility could be attributed to the hydrophobic nature of the purine ring in 8. The UV-VIS
absorption spectra display a minor shift in the λmax of compound 7 from 263 nm to 260 nm for polymer
8 (Figure 1). This absorption is absence in polymers PDAAC and PDAAC-SO2 due to the absence of
the purine ring. The FTIR spectrum of 8 reveals a signal at 1691 cm−1 that is present in the
corresponding monomer 7 (Figure 2) while the spectra of PDAAC and PDAAC-SO2 show only the
presence of a broad peak at 1635 cm−1. In addition, FTIR revealed the presence of two peaks at 1350 cm−1
and 1157 cm−1 assigned to the symmetric and asymmetric vibrations of the sulfone groups in both
PDAAC and PDAAC-SO2.
Table 2. Solubility (2% w/w) of PDAAC-SO2 and 8 in selected solvents.
PDAAC-SO2
Cold b Hot c
8
Solvent
Formamide
Water
Є a
111
78.4
58.5
47.2
38.3
37.3
32.3
24.3
20.7
7
bp (°C)
210
Cold b
Hot c
+
+
+
+
+
−
±
−
−
−
−
−
+
+
+
±
−
+
−
−
−
−
−
+
+
+
−
−
±
−
−
−
−
−
100
+
Formic Acid
DMSO
100–101
189
+
±
DMF
153
−
Ethylene glycol
Methanol
Ethanol
196–198
65
+
−
78
−
Acetone
56
−
Diglyme
Dioxane
162
−
2.2
101
−
a
b
c
Є: dielectric constant; at room temperature; at the boiling point; +: soluble; −: insoluble;
±: partially soluble.

Molecules 2012, 17
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Figure 1. UV-VIS spectra of 7, PDAAC, PDAAC-SO2 and 8.
263 nm
7
PDAAC
PDAAC-SO2
260 nm
8
(nm)
Figure 2. FTIR spectra of 7, PDAAC, PDAAC-SO2 and 8.
NH
2
N
N
N
N
N
H
7
PDAAC
N
H
H
SO
2
PDAAC-SO2
N
H
H
SO
2
N
8
H
1517
N
N
N
1350
1175
N
1635
NH
2
The 1H-NMR spectrum of compound 8 (Figure 3) showed the aromatic C-H signals of the purine at
8.47 and 8.49 ppm in comparison to the same signals at 8.3 and 7.9 ppm in the corresponding
monomer 7. In addition, the vinylic protons at 6 and 5.6 ppm for the monomer have disappeared in
polymer 8. A similar trend was seen in the 13C-NMR (Figure 4) where the signals of the purine ring are
seen at 151, 152.2, 147.5, 147.2 and 120.7 ppm. In addition, the signal for the vinylic carbons in the
130–140 ppm region of 7 disappeared in polymer 8.

Molecules 2012, 17
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Figure 3. 1H-NMR spectra of 7, PDAAC, PDAAC-SO2 and 8.
NH2
N
N
N
N
N
H
7
N
H
PDAAC
H
SO2
N
PDAAC-SO2
H
H
SO
2
N
H
8
N
N
N
N
NH
2
ppm
Figure 4. 13C-NMR spectra of 7, PDAAC, PDAAC-SO2 and 8.
NH2
N
N
N
N
N
H
7
PDAAC
N
H
H
SO2
N
H
PDAAC-SO2
H
SO2
N
H
8
N
N
N
N
NH2
ppm
Thermogravimetric analysis (TGA) and Differential Scanning Calorimetry (DSC) were used for the
thermal analysis of the cyclo-copolymers. The TGA thermograms are shown in Figure 5. The TGA of
polymer 8 exhibited an onset temperature of 226 °C with dm/dTmax = 261 °C whereas PDAAC-SO2
has an onset temperature of 251 °C with a dm/dTmax = 287 °C. Copolymer 8 was thermally stable up

Molecules 2012, 17
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to a temperature of 226 °C below that of PDAAC-SO2. Around 79% of the polymer weight was lost
during the first transition for 8 in comparison to only 49% weight loss for PDAAC-SO2. The weight
loss for PDAAC-SO2 is attributed to depolymerization and release of HCl and SO2. In contrast, the
degradation of 8 is initiated at a lower temperature presumably due to the breakdown of the thermally
sensitive adenine moiety. Figure 6 shows the DSC thermograms of PDAAC-SO2 and polymer 8. No
discernible peaks are found for PDAAC-SO2 in the temperature range of 0–240 °C . In contrast, a
distinct glass transition state is seen for 8 at 166 °C. The molecular weight distributions of polymers 8
and 9 are shown in Table 1. The average molecular weight of PDAAC was 28.8 kDa whereas the
copolymer PDAAC-SO2 was around 1,700 kDa (Entry 1, Table 1). This is consistent with reported
increase in molecular weight distributions of related copolymers with SO2. A similar trend (but to a
lesser extent) was observed for the cyclopolymerization of 4 and 7 that resulted in a 2-fold increase in
the molecular weight of copolymer 8 in comparison to 9.
Figure 5. TGA thermograms for the thermal degradation of PDAAC-SO2 (top) and 8
(bottom) at a heating rate of 10 °C min−1 under a flow of nitrogen gas.
Figure 6. DSC thermograms of PDAAC-SO2 (left) and 8 (right) for the first cooling cycle
and second heating cycle at a heating rate of 10 °C min−1 under a flow of nitrogen gas.
Glass Transition: 166.0 ˚C

Molecules 2012, 17
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3. Experimental
3.1. General
Reagents used in the syntheses were purchased from the Aldrich Chemical Company (Milwaukee,
WI, USA), ACROS Chemicals (Loughborough, UK), Fisher Scientific Company (Fair Lawn, NJ,
USA) and were used as received. Dioxane was dried over sodium metal and distilled directly before
use. Disposable dialysers (MWCO 1000 Da), cellulose acetate/cellulose nitrate mixed esters
membranes, were purchased from the Sigma Aldrich Chemical Company (Steinheim, Germany).
Slide-A-Lyzer 3.5K dialysis cassettes (MWCO 3500) were purchased from Pierce (USA). The macro azo
initiators VPE-0201 and the water-soluble initiator 2,2'-azobis(2-methylpropionamidine)dihydrochloride
(V-50) were obtained from Wako Pure Chemical Industries (Richmond, VA, USA) and were used
as received.
3.2. Measurements
Melting points were determined on a Mettler Toledo FP62 apparatus and are uncorrected. NMR
spectra were determined in deuterated solvents with tetramethylsilane (TMS) or sodium 2,2,-dimethyl-
2-silapentane-5-sulfonate (DSS) as the internal standards on a Bruker AV 300 NMR spectrometer.
Chemical shifts are reported in ppm (δ) downfield relative to TMS or DSS. Infrared spectra were
recorded as KBr pellets using a Nicolet 4700 FTIR spectrometer with a Hewlett Packard Desk jet
840C plotter. The IR bands are reported in wave numbers (cm−1). The UV-VIS absorption spectrum
was measured using Jasco V-570 UV/VIS/NIR spectrometer. SEC analysis was performed on a liquid
chromatograph consisting of a Waters Breeze solvent delivery system and Waters M717 autosampler
(Waters Corporation, Milford, MA, USA), a DAWN EOS light scattering photometer and an
OPTILAB rEX differential refractive index detector (Wyatt Technology Corporation, Santa Barbara,
CA, USA). Aqueous oxalic acid (0.22 M) at 40 °C with nominal flow rate of 0.8 mL/min was used as
the mobile phase. The separations were carried out on a PSS Novema pre-column connected in series
to three PSS Novema columns (30 Å, 1,000 Å, 10,000 Å) from Polymer Standard Service (Amherst,
MA, USA) (8.0 mm × 300 mm, 10 μm). All samples were prepared by stirring overnight in the mobile
phase at a concentration of 1–2 mg/mL and filtered through 0.45 μm PVDF membrane filter. The TGA
data was obtained using TGA2050 Thermogravimetric Analyzer (TA Instruments, New Castle, DE,
USA) and Thermal Advantage software, v1.1A. The samples were equilibrated at 120 °C and the
temperature was increased to 1,000 °C at a rate of 10 °C /min under a flow of nitrogen gas. The DSC
data was obtained using a Mettler Toledo DSC821 and Star Software v 8.10. The samples were
weighed directly into an aluminium pan with lid using a microbalance. Samples were preheated to 220 °C ,
cooled to −50 °C at a rate of −20 °C /min then heated to 220 °C at a rate of 20 °C /min.
9-(2-Hydroxyethyl)adenine (2). A one liter round-bottom flask was charged with adenine (14.9 g,
0.11 mol), ethylene carbonate (10.6 g, 0.12 mol), sodium hydroxide (0.5 g, 12.5 mmol), and dry DMF
(450 mL) and the mixture was heated and maintained at reflux overnight. The solvent was removed
under reduced pressure and the residue was crystallized from ethanol to yield a white solid (19.71 g,
1
94%). Mp 238–239 °C ; H-NMR (300 MHz, DMSO-d6) δ 3.5 (t, 2H, J = 5.45 Hz), 4.0 (t, 2H,

Molecules 2012, 17
13300
J = 5.32 Hz), 5.1 (br s, 1H), 7.0 (br s, 2H), 7.9 (s, 1H), 7.95 (s, 1H); 13C-NMR (75 MHz, DMSO-d6) δ
45.6 (CH2), 59.1 (CH2), 118.6 (C), 141.3 (CH), 149.5 (CH), 152.2 (C), 155.8 (C); UV-VIS λmax 265
nm; IR (KBr) 3311, 3246, 3056, 2930, 2865, 1687, 1607, 1574, 1067, 1018 cm−1. Spectroscopic data
are consistent with those reported in the literature [23–25].
9-(2-Chloroethyl)adenine (3). A 250 mL round-bottom flask was charged with 2 (10.63 g, 59.3 mmol),
dry dioxane (330 mL), freshly distilled pyridine (4 mL), and thionyl chloride (83 mL, 114 mmol). The
reaction mixture was heated and maintained at reflux for two hours. The solvent was removed under
reduced pressure and the residue was triturated with carbon tetrachloride, filtered, and the solid was
1
recrystallized from ethanol (11.6 g, 99%). Mp 204–205 °C ; H-NMR (300 MHz, D2O) δ 4.0 (t, 2H,
J = 5.71 Hz), 4.7 (t, 2H, J = 5.72 Hz), 8.1 (s, 1H), 8.4 (s, 1H); 13C-NMR (75 MHz, D2O) δ 45.4 (CH2),
48.7 (CH2), 120.9 (C), 147.2 (CH), 147.8 (CH), 151.3 (C), 152.7 (C); 1H-NMR (300 MHz, DMSO-d6)
δ 3.75 (t, 2H, J = 5.43), 4.2 (t, 2H, J = 5.43 ), 8.1 (s, 1H), 8.15 (s, 1H); 13C-NMR (75 MHz, DMSO-d6)
δ 42.9 (CH2), 45.3 (CH2), 117.8 (C), 144.1 (CH), 145.0 (CH), 148.5 (C), 150.2 (C); UV-VIS λmax 261 nm;
IR (KBr) 3342, 3062, 2083, 1701, 1614, 1559, 1518, 1411 cm−1. Spectroscopic data are consistent with
those reported in the literature [23–25].
9-(2-Diallylaminoethyl)adenine (4). (From compound 3): A 100 mL round bottom flask was charged
with 3 (5 g, 25.3 mmol), dry dioxane (100 mL) and diallylamine (14.7 g, 151.8 mmol). The reaction
mixture was heated and maintained at reflux for four days. The solvent was removed under reduced
pressure to yield a solid residue that was dissolved in aqueous HCl (10%, 30 mL), washed with CHCl3
(4 × 40 mL), rendered basic with sodium hydroxide pellets, and extracted with CHCl3 (4 × 40 mL).
The organic layers were combined, dried over anhydrous magnesium sulfate, filtered and the solvent
was removed under reduced pressure. The residue was crystallized from acetone-hexane and dried
under reduced pressure to yield a pale beige powder (4.26 g, 65%). Mp 142–144 °C ; 1H-NMR (300 MHz,
CDCl3) 2.80 (t, 2H, J = 5.9 Hz), 3.05 (d, 2H, J = 6.4 Hz), 4.18 (d, 1H, J = 5.8 Hz), 5.04 (m, 2H), 5.60
13
(m, 1H), 5.90 (br s, 2H), δ 7.85 (s, 1H), 8.28 (s, 1H); C-NMR (75 MHz, CDCl3) 57.1 (CH2), 41.1
(CH2), 52.1 (CH2), δ 118.1 (CH2), 119.3 (C), 134.7 (CH), 141.4 (CH), 150.0 (C), 152.6 (CH), 155.3
(C); UV-VIS λmax 264 nm; IR (KBr) 3290, 3119, 2979, 2801, 2679, 1670, 1644, 1602, 1574, 1514,
1478, 1414, 1352, 1322, 1205, 909 cm−1. HRMS (EI) calcd for C13H18N6 (M+), 258.15929, found
258.15860.
9-(2-Diallylaminoethyl)adenine (4). (From compound 6): A 100 mL three-necked round-bottom flask
immersed in an ice-water bath was charged with methanol (10 mL) and sodium (0.1 g, 4.3 mmol). A
solution of 6 (0.5 g, 1.52 mmol) in methanol (10 mL) was added drop-wise under a flow of nitrogen
gas. The ice-water bath was removed and the mixture was heated and maintained at reflux for four
hours then stirred overnight at ambient temperature. The reaction flask was immersed in an ice-water
bath and distilled water (10 mL) was added. The volume was then reduced to 1/10 of its initial volume
under reduced pressure. The mixture was acidified with aqueous HCl (10%, 5 mL) and washed with
chloroform (3 × 20 mL). The aqueous layer was separated, neutralized with aqueous NaOH (10%, 5 mL)
and the product was extracted with chloroform (3 × 25 mL). The organic layers were combined, dried

Molecules 2012, 17
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over anhydrous sodium sulfate and filtered. The solvent was removed under reduced pressure to yield
a pale yellow solid (0.3 g, 77%).
N6-Isobutyryladenine (5). A 50 mL round-bottom flask was charged with adenine (1 g, 7.4 mmol),
isobutyric anhydride (3.617 g, 22.2 mmol), and dry DMF (25 mL). The mixture was heated and
maintained at reflux for two hours resulting in a clear yellow solution. The solvent was removed under
reduced pressure and the crude solid was crystallized from a mixture of ethanol and water (30 mL, 1:1)
1
to yield white crystals (1 g, 66%). Mp 228–230 °C ; H-NMR (300 MHz, DMSO-d6) δ 1.18 (d, 6H,
J = 6.8 Hz,), 2.96 (hep, 1H, J = 6.8 Hz), 8.40 (s, 1H), 8.60 (s, 1H); 13C-NMR (75 MHz, DMSO-d6) δ
19.1 (CH3), 34.0 (CH), 113.6 (C), 144.3 (C), 145.5 (CH), 151.1 (CH), 161.3 (C), 177.0 (C); IR (KBr)
3280, 3068, 2974, 2825, 1687, 1655, 1626, 1554, 1513, 1466, 1432, 1390, 1369, 1328, 1307, 1219 cm−1.
Spectroscopic data are consistent with those reported in the literature [22–25].
N6-Isobutyryl-9-(2-diallylaminoethyl)adenine (6). A 250 mL three-necked round-bottom flask was
charged with compound 5 (0.5 g, 2.43 mmol), bromoethanol (0.375 g, 2.92 mmol), triphenyl phosphine
(1.286 g, 4.8 mmol), and dry dioxane (100 mL). The flask was partially immersed in an ice-water bath and
a solution of diisopropylazodicarboxylate (1.039 g, 4.8 mmol) in dry dioxane (50 mL) was added
drop-wise under an atmosphere of nitrogen. The solution turned clear halfway through the addition.
The ice-water bath was removed and the reaction was stirred at room temperature for four days under
nitrogen. The solid precipitate was removed by filtration and the filtrate was evaporated under reduced
pressure. Diallylamine (0.487 g, 4.86 mmol), and dry dioxane (50 mL) were added to the oily residue
and the mixture was heated and maintained at reflux for five days. The solvent was evaporated under
reduced pressure and the residue was acidified with aqueous HCl (10%, 10 mL), washed with
dichloromethane (3 × 10 mL), neutralized with aqueous NaOH (10%, 10 mL), and extracted with
dichloromethane (3 × 20 mL). The organic layers were combined, dried over anhydrous sodium sulfate
and filtered. The solvent was removed under reduced pressure and the oily residue was triturated with
hexane. The white solid was filtered and dried under reduced pressure to yield the product (0.3 g,
37.5%). Mp 88–90 °C; 1H-NMR (300 MHz, CDCl3) δ 1.2 (d, 6H, J = 6.8 Hz), 2.8 (d, 1H, J = 5.7 Hz),
2.8 (d, 1H, J = 5.8 Hz), 3.1 (d, 2H, J = 6.3 Hz), 3.2 (hep, 1H, J = 6.8 Hz), 4.3 (d, 1H, J = 5.5 Hz), 4.3
(d, 1H, J = 6.0 Hz), 3.1 (d, 2H, J = 6.3 Hz), 5.1(m, 2H), 5.6 (m, 1H), 8.2 (s, 1H), 8.7 (s, 1H), 10.0 (br s,
1H); 13C-NMR (75 MHz, CDCl3) δ 117.8 (CH2), 56.9 (CH2), 134.7 (CH), 42.1 (CH2), 51.8 (CH2),
151.6 (C), 143.7 (CH), 121.9 (C), 149.3 (C), 152.0 (CH), 176.6 (C), 35.5 (CH), 19.1 (CH3); IR (KBr)
3544, 3304, 3172, 3090, 3034, 2970, 2925, 2806, 1709, 1675, 1611, 1579, 1542, 1489, 1458, 1436,
1401, 1349, 1316, 1275, 1216, 921 cm−1. HRMS (ESI) calcd for C17H25N6O (M+1)+, 329.208436,
found 329.2091.
9-(2-Diallylaminoethyl)adenine.HCl (7). Dry HCl gas, generated by the drop-wise addition of
concentrated H2SO4 to NaCl, was bubbled through a clear solution of 9-(2-diallylaminoethyl)adenine
(3.5 g, 13.5 mmol) in ethanol (400 mL). The solid precipitate was filtered, washed with ethanol and
dried under reduced pressure (3.91 g, 98%); Mp 202–204 °C ; 1H-NMR (300 MHz, D2O) δ 3.8 (t, 2H,
J = 6.62 Hz), 3.9 (d, 4H, J = 7.27 Hz), 4.9 (t, 2H, J = 6.62 Hz), 5.6 (d, 4H, J = 5.65 Hz), 5.7 (s, 1H),
5.9 (m, 2H, J = 5.92 Hz), 8.4 (s, 1H), 8.47 (s, 1H); 13C-NMR (75 MHz, D2O) δ 41.8 (CH2), 53.1

Molecules 2012, 17
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(CH2), 58.2 (CH2), 121.1(C), 127.6 (CH2), 130.2 (CH), 147.1 (CH), 147.7 (CH), 151.6 (C), 152.9 (C);
UV-VIS λmax 263 nm; IR (KBr) 3396, 3080, 2980, 2939, 1691, 1616, 1649, 1595, 1514, 1449, 1436,
1416, 1214, 971 cm−1; GC: I = 949, retention time 18 min; MS (EI) m/z (relative intensity) 258.2
(M−HCl, 0.05), 217 (32), 136 (25), 110 (100), 108 (21), 41 (57); HRMS (EI) m/e calcd for C13H18N6
(M−HCl)+ 258.15929, found 258.16146.
3.3. Procedure for Cyclopolymerization of DAAC with V-50
A glass tube (8 mm i.d., 11 mm o.d.) was charged with DAAC (2 g, 14.96 mmol) and distilled
water (8 mL) and placed in a preheated oil bath at 70 °C . The tube was sealed with a septum and the
mixture was purged with nitrogen gas for twenty minutes. V-50 (30 mg, 0.11 mmol) was added and
the mixture was purged with nitrogen gas for an additional ten minutes, stirred and heated at 70 °C for
three days. The polymer was precipitated with the addition of EtOH. The gelatinous solid was dried
under reduced pressure. The solid was triturated in EtOH, filtered and dried under reduced pressure
(1.35 g, 67%): M.p. decomposed at 305 °C ; 1H-NMR (300 MHz, D2O) δ 0.9 (br s), 1.1 (br s), 1.3 (br s), 1.5
(br s), 1.6 (br s), 1.9 (br s), 2.3 (br s), 2.9 (br s), 3.1 (br s), 3.4 (br s), 3.5 (br s); 13C-NMR (75 MHz, D2O) δ
26.9 (CH2), 31.6 (CH2), 42.6 (CH), 45.2 (CH), 50.7 (CH2), 52.2 (CH2); FTIR 3363, 2922, 1635, 1456 cm−1.
3.4. Procedure for Cyclo-Copolymerization of DAAC with V-50
A glass tube (8 mm i.d., 11 mm o.d.) was charged with DAAC ( 1 g, 7.48 mmol), a solution of
MeOH (1.65 mL) containing SO2 (0.48 g, 7.48 mmol) and V-50 (15 mg, 55.3 μmol). The mixture was
freeze-thawed degassed (three cycles) and the tube was sealed under reduced pressure. The mixture
was heated at 65–70 °C for three days. The white precipitate was triturated with MeOH, filtered and
dried under reduced pressure to yield a white solid (1.36 g, 92%): M.p. decomposed above 240 °C ;
13
1H-NMR (300 MHz, D2O) δ 3.3 (br s), 3.5 (br s), 3.6 (br s), 3.8 (br s); C-NMR (75 MHz, D2O) δ
36.8 (CH), 38.7 (CH), 50.7 (CH2), 51.2 (CH2), 53.0 (CH2), 56.5 (CH2); FTIR 3410, 2920, 2743, 1635,
1405, 1303, 1125 cm−1.
3.5. Procedure for Cyclopolymerization of 7 with V-50
A 10 mL test tube was charged with 7 (0.47 g, 1.59 mmol), distilled water (1 mL), V-50 initiator
(9 mg, 33.2 μmol). The tube was sealed with a septum and the solution was purged with nitrogen gas
for ten minutes. The mixture was stirred and heated at 90 °C for four days. The solvent was removed
under reduced pressure and the solid residue was dissolved in distilled water and dialyzed for three
days (MWCO 1000, against distilled water). The solvent was removed under reduced pressure to yield
polymer 9 (24 mg, 5%). Mp decomposed above 220 °C ; 1H-NMR (300 MHz, D2O) δ 1.1 (br s), 2.6 (br s),
2.7 (br s), 2.9 (br s), 3.1(br s), 3.2 (br s), 3.4 (br s), 3.6 (br s), 8.5 (s, 1H), 8.52 (s, 1H); 13C-NMR (75 MHz,
D2O) δ 27.2 (CH2), 29.1 (CH2), 42.4 (CH), 43.0 (CH2), 43.3 (CH2), 56.6 (CH2), 57.0 (CH2), 60.4
(CH2), 120.4 (C), 147.2 (CH), 151.7 (C), 152.5 (C); UV-VIS λmax 260 nm; IR (KBr) 3362, 2922, 2851,
1661, 1632, 1468, 1419 cm−1.

Molecules 2012, 17
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3.6. Procedure for Cyclopolymerization of 7 with Ammonium Persulfate
A 10 mL test tube was charged with 7 (257.7 mg, 0.82 mmol), distilled water (1 mL) and
ammonium persulfate (3.68 mg, 16.5 μmol). The tube was sealed with a septum and the mixture was
purged with nitrogen gas for ten minutes. The tube was heated at 90 °C with stirring for four days. The
polymer phased out with the addition of acetone to yield a viscous-orange oily precipitate. The oily
layer was separated, dissolved in distilled water and dialyzed for four days (MWCO 3500) against
distilled water. The solvent was then evaporated under reduced pressure to yield polymer 9 (44 mg, 17%):
Mp decomposed above 220 °C ; 1H-NMR (300 MHz, D2O) δ 1.1 (br s), 2.6 (br s), 2.7 (br s), 2.89 (br s),
13
3.1 (br s), 3.2 (br s), 3.4 (br s), 3.6 (br s), 8.5 (s, 1H), 8.52 (s, 1H); C-NMR (75 MHz, D2O) δ 27.2
(CH2), 29.1 (CH2), 42.4 (CH), 43.0 (CH2), 43.3 (CH2), 56.6 (CH2), 57.0 (CH2), 60.4 (CH2), 120.4 (C),
147.2 (CH), 151.7 (C), 152.5 (C); UV-VIS: λmax 260 nm; IR (KBr) 3362, 2922, 2851, 1661, 1632,
1468, 1419 cm−1.
3.7. Procedure for the Cyclo-Copolymerization of 7 with V-50
A glass tube (8 mm i.d., 11 mm o.d.) was charged with 7 (109.8 mg, 0.372 mmol), a solution of
DMSO (1.138 mL) containing SO2 (24 mg, 0.372 mmol) and V-50 (15 mg, 55.3 μmol). The mixture
was degassed (freeze-thawed) three times, frozen and the tube was sealed under reduced pressure,
allowed to warm up to room temperature and placed in a preheated sand bath at 70 °C for three days.
The polymer was precipitated with ethanol, filtered, washed repeatedly with ethanol and dried under
1
reduced pressure to form polymer 8 (8.9 mg, 7%): Mp decomposed above 250 °C ; H-NMR (300 MHz,
13
D2O) δ 1.37 (br s), 3.0 (br s), 3.36 (br s), 3.58 (br s), 3.66 (br s), 8.47 (s, 1H), 8.49 (s, 1H); C-NMR
(75 MHz, D2O) δ 35.4 (CH), 36.9 (CH), 43.0 (CH2), 53.9 (CH2), 54.1 (CH2), 56.8 (CH2), 59.4 (CH2),
120.7 (C), 147.2 (CH), 147.5 (CH), 152.2 (C), 151.7 (C); UV-VIS: λmax 260 nm; IR (KBr) 3385, 1684,
1517, 1419, 1350, 1306, 1173, 1127 cm−1.
3.8. Procedure for Cyclo-Copolymerization of 4 with V-50
A glass tube (8 mm i.d., 11 mm o.d.) was charged with 4 (0.5 g, 1.94 mmol), a solution of methanol
(3.76 mL) containing SO2 (0.124 g, 1.94 mmol) and V-50 (15 mg, 55.3 μmol). The mixture was
degassed (freeze-thawed) three times, frozen and the tube was sealed under reduced pressure, allowed
to warm up to room temperature and placed overnight in a preheated sand bath at 70 °C to yield a
gelatinous orange precipitate. The solvent was decanted, the gelatinous precipitate was washed with
MeOH, dissolved in aqueous HCl (10%) and dialyzed for one day (MWCO 3500) against distilled
water, during which a gelatinous layer formed in the dialysis tube with the liquid layer on top. The top
layer was decanted, the gelatinous layer was dissolved in aqueous HCl (10%) and the solvent was
evaporated under reduced pressure to yield polymer 8 as a white solid (0.24 g, 35%). Mp decomposed
1
above 250 °C ; H-NMR (300 MHz, D2O) δ 1.3 (br s), 3.0 (br s), 3.3 (br s), 3.5 (br s), 3.6 (br s), 8.47
(s, 1H), 8.49 (s, 1H); 13C-NMR (75 MHz, D2O) δ 35.4 (CH), 36.9 (CH), 43.0 (CH2), 53.9 (CH2), 54.1
(CH2), 56.8 (CH2), 59.4 (CH2), 120.7 (C), 147.2 (CH), 147.5 (CH), 152.2 (C), 151.7 (s); UV-VIS λmax
260 nm; IR (KBr) 3385, 1684, 1517, 1419, 1350, 1306, 1173, 1127 cm−1.

Molecules 2012, 17
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4. Conclusions
The synthesis of 9-(2-diallylaminoethyl)adenine hydrochloride salt from adenine in four steps was
reported. The free-radical homopolymerization and copolymerization of this salt with sulfur dioxide
exhibited relatively low degree of polymerization under the conditions investigated in this study. In
contrast, the cyclo-coplymerization of 9-(2-diallylaminoethyl)adenine with SO2 resulted in novel
low molecular weight oligomers in good yields. Further studies are currently underway to determine
if related purine and pyrimidine-substituted 2-diallylaminoethyl derivatives display similar
free-radical polymerizations.
Acknowledgements
The authors would like to thank James E. Brady at the Ashland Research Center for his help in
determining the polymer molecular weight distribution. This work was supported by the Lebanese
National Council for Scientific Research (LNCSR), the Fulbright Scholar program and the Arab
Fund Fellowship.
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Sample Availability: Samples of compounds 2–9 are available from the authors.
© 2012 by the authors; licensee MDPI, Basel, Switzerland. This article is an open access article
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