Molecules 2012, 17
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CDCl3) ppm: δ 2.07 (s, 3H), 3.78 (s, 2H), 5.27 (s, 2H). 13C-NMR (125 MHz, CDCl3) ppm: δ 21.0
(2CH3), 69.3 (2CH2), 89.2 (2CH2), 170.5 (2C).
N-{9-[2-(2-Acetylamino-6-oxo-1,6-dihydropurin-9-ylmethoxy)-ethoxymethyl]-6-oxo-6,9-dihydro-1H-
purin-2-yl}acetamide (11) [16]. Compound 10 (1 g, 4.325 mmol), p-toluenesulfonic acid (0.02 g),
tetrabutylammonium bromide (0.03 g) and Dowtherm Q (13 mL) as solvent, were placed in a
three-necked flask equipped with a distillation system and a N2 inlet. The mixture was heated under an
inert atmosphere at 110–120 °C and a solution of 9 (0.97 g, 4.7 mmol) in Dowtherm Q (1 mL) was
added in three portions over 1 h. Vacuum was applied (100 mmHg) and the flask was heated at 150 °C
to distil off acetic anhydride. The reaction mixture became orange and, after 15 h heating, compound 9
had been consumed. The mixture was allowed to cool to room temperature and acetone (20 mL) was
added. The mixture was stirred and cooled in an ice-water bath for 1 h 30 min. A solid appeared that
was filtered and washed with acetone (2 × 15 mL) to give 11 as a white solid. The solid was suspended
in DMF (3 mL) and the mixture was heated to 100 °C for 30 s [31]. The solid was filtered and dried
(1.05 g, 52% yield; 92% purity by HPLC; mp 301.9 °C). 1H-NMR (500 MHz, DMSO-d6) ppm: δ 2.16
(s, 6H), 3.56 (s, 4H), 5.40 (s, 4H), 8.08 (s, 2H), 11.71 (s, 2H). 13C-NMR (125 MHz, DMSO-d6) ppm: δ
23.1 (2CH3) + 30.0 (2CH2), 67.2 (2CH2), 72.0 (2CH2), 119.6 (2C), 139.4 (2CH), 147.5 + 148.2 (2C),
154.3 (2C), 172.9 (2C), 205.7 (2C). HRMS (FAB+): Calc. for C18H20N10O6 [M++1] = 473.1646; found
[M++1] = 473.1647.
9,9'-[1,2-Ethanediylbis(oxymethylene)]bis-2-amino-1,9-dihydro-6H-purin-6-one (3). Compound 11
(0.8 g, 1.7 mmol) was suspended in distilled water (8 mL) and ethanolamine (0.8 mL,
0.08 mol) was added. The mixture was heated under reflux for 10 h and then allowed to cool to room
temperature. Aqueous HCl (10%) was added to give pH 7 and the mixture was heated at 90 °C for
15 min. The solid was filtered off and dried in a vacuum oven at 40 °C. The solid was dissolved in
DMSO (8 mL) and heated to 80–90 °C for 15 min. The solution was filtered and water (10 mL) was
added dropwise. The mixture was stirred at room temperature for 30 min. A white solid formed and the
suspension was filtered, the residue suspended in DMSO (8 mL) and heated to 80–90 °C for 15 min.
Then, the solution was filtered and water was added dropwise (~10 mL) until the liquid became
cloudy. The mixture was then stirred at room temperature for 30 min. The resulting white solid was
filtered and dried in a vacuum oven at 100 °C. Compound 3 was obtained as a white powder (0.51 g,
77% yield; 95% purity by HPLC, Mp > 400 °C). 1H-NMR (500 MHz, DMSO-d6) ppm: δ 3.54 (s, 4H),
13
5.30 (s, 4H), 6.51 (s, 4H), 7.78 (s, 2H), 10.63 (s, 2H). C-NMR (125 MHz, DMSO-d6) ppm: δ 40.6
(CH2), 56.9 (CH2), 67.0 (2CH2), 71.4 (2CH2), 116.0 (2C), 137.1 (2CH), 150.9 (2C), 153.3 (2CH),
156.2 (2C). HRMS (FAB+): Calc. for C14H16N10O4 [M+1] = 389.3464; found [M++1] = 389.3466.
Higher purity can be obtained by repeating the DMSO/water crystallisation process.
4. Conclusions
Validation of analytical methods of active pharmaceutical ingredients (API) requires the availability
of reference samples of impurities eventually formed in the synthetic processes of the corresponding
API, to be used as standards. In this case, a rare impurity of acyclovir has been obtained through an
acid catalysed PTC process which ensures regioselective alkylation of 2,9-diacetylguanine (10).