Molecules 2019, 24, 2447
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Design of experiments and statistical data analysis was performed using Design-Expert® v. 9 (Stat-Ease,
Inc., Minneapolis, MN, USA). Calibrated HPLC analyses were performed on a Shimadzu (Kyoto,
Japan) LC-20A Prominence equipped with a CBM-20A communication bus module, two LC-20AD
dual piston pumps, a SPD-M20A photodiode array detector, and a Rheodyne 7725i injector (Rheodyne
Inc., Cotati, CA, USA) with a 20-
µL stainless steel loop. An Ultra II aqueous RP18 column (estek
Belle-fonte, PA, USA, 250 mm 4.6 mm ID, 5
×
µ
m, 100 Å) was used as the analytical column. The HPLC
analyses were performed using MeOH/H2O (25:75, v/v) + 0.1% diethylamine as the mobile phase
at a 1.0 mL min−1 eluent flow rate, after previous conditioning by passing through the column the
mobile phase for at least 30 minutes at the same eluent velocity. Before use, the mobile phase was
filtered through a 0.22-mm Millipore filter (Bedford, MA, USA) and then degassed with 20 minutes of
sonication. The column temperature was controlled through a Grace (Sedriano, Italy) heather/chiller
(Model 7956 R) thermostat. All the analyses were performed at a 25 ◦C column temperature. Ultrapure
water (
%
= 18.3 MΩ × cm) for HPLC analysis was obtained through New Human machine Power I
Scholar (Human Corporation, Seoul, Korea) purification system.
3.2. Protocol and Flow Set-Up for DoE Optimisation
The flow setup employed is depicted in Figure 2. A solution of o-phenylenediamine (2, 108 mg,
1 mmol, 1 M) in THF and a solution of CDI (1.1–5.0 mmol, 1.1–5.0 M) in THF/PEG300 (7:3, v/v) were
injected into the loops and pumped with a flow rate of 0.05–0.5 mL min−1 for each pump. After the
injection and switching of the valves into the loops, the solutions were mixed together in a T-piece
mixing element and flowed through the thermocouple-controlled coil reactor (10 mL, τ = 100–10 min
heated at the desired temperature (110–210 ◦C). The reactor output was monitored by UV detector
and readily collected in a fraction collector. The reaction yield was determined by calibrated HPLC
analysis of the crude reaction mixture.
)
3.3. Protocol for the Gram-Scale Flow Synthesis of N-(2-chlorobenzyl)-5-cyano-benzimidazol-2-one (3)
The flow set-up employed is depicted in Figure 4.
A
solution of
3-amino-4-((2-chlorobenzyl)amino)benzonitrile ( , 2.58 g, 10 mmol, 10 mL, 1 M) in THF and
4
a solution of CDI (6.8 g, 4.2 equiv., 10 mL, 4.2 M) in THF/PEG300 (7:3, v/v) were continuously pumped
through the HPLC pumps with a flow rate of 0.075 mL min−1 for each pump. The streams were
mixed in a T-piece mixing element and flowed into a 10-mL stainless-steel thermocoupled reaction
coil (10 mL, τ = 67 min) heated at 210 ◦C. The reactor output was detected by an in-line UV detector,
collected, taken with Et2O into a gravity separatory funnel, and washed with 3N of HCl to remove
any trace of unreacted starting material. Then, the collected organic layer was concentrated in vacuo,
affording the desired N-(2-chlorobenzyl)-5-cyano-benzimidazol-2-one (
>95% HPLC purity, 15 g d−1 productivity).
3) (2.78 g, 9.86 mmol, 99% yield,
3.4. Compounds Characterisation
Benzimidazol-2-one (
d6-DMSO): δ 6.89 (m, 4H), 10.57 (s, 2H). 13C-NMR (100.6 MHz, d6-DMSO): δ 108.3, 120.3, 129.5, 155.1.
N-(2-chlorobenzyl)-5-cyano-benzimidazol-2-one (
) [26]: White solid (m.p.: 234–235 ◦C, lit. [26]:
233.9–234.9 ◦C). 1H-NMR (400 MHz, CDCl3):
5.24 (s, 2H), 6.93 (d, J = 8.2 Hz, 1H), 7.08 (dd, J1 = 8.0 Hz
1
) [36]: White solid (m.p.: > 200 ◦C, lit. [38]: >250 ◦C). 1H-NMR (400 MHz,
3
δ
,
J2 = 1.6 Hz, 1H), 7.17–7.22 (m, 1H), 7.24-7.28 (m, 1H), 7.33 (dd, J1 = 8.2 Hz, J2 = 1.4 Hz, 1H), 7.39 (d, J
= 1.4 Hz, 1H), 7.43 (dd, J1 = 8.0 Hz, J2 = 1.6 Hz, 1H), 8.09 (s, 1H). 13C-NMR (100.6 MHz, d6-DMSO):
δ 42.0, 103.4, 109.1, 112.3, 120.0, 126.4, 128.0, 128.5, 129.1, 129.7, 130.0, 132.2, 133.8, 134.1, 154.5.
4. Conclusions
In this communication, we developed a novel flow method for the preparation of
benzimidazol-2-one (1) core, which is a well-established privileged scaffold in medicinal chemistry
and a common structural framework of numerous biologically active compounds endowed with