M. Gao et al. / Bioorg. Med. Chem. Lett. 24 (2014) 254–257
257
7.51 (d, J = 7.5 Hz, 1H, Ph-H), 7.89 (d, J = 7.5 Hz, 1H, Ph-H), 8.78 (d, J = 8.0 Hz,
1H, Ar-H), 12.57 (s, OH).
(e). 2-Bromobenzo[4,5]imidazo[1,2-a]pyrimidine (5): Phosphorus oxybromide
(17.8 g, 62.0 mmol) was added to suspension of compound
4 (5.74 g,
31.0 mmol) in 1,2-dichloroethane (130 mL) and DMF (1 mL). The reaction
mixture was heated to reflux for 3 h. The reaction mixture was concentrated in
vacuo, diluted with ice water and quenched with ammonium hydroxide to
adjust pH to 8. The resulting residue was filtered and washed with water
followed by ethyl ether, and dried under vacuum to obtain 5 (7.46 g, 97%) as a
yellow solid, Rf = 0.22 (2% MeOH/CH2Cl2), mp 287–289 °C. 1H NMR (DMSO-d6):
d 6.57 (d, J = 7.5 Hz, 1H, Ar-H), 7.53 (t, J = 7.5 Hz, 1H, Ph-H), 7.49 (t, J = 7.5 Hz,
1H, Ph-H), 7.63 (d, J = 8.0 Hz, 1H, Ph-H), 8.12 (d, J = 8.0 Hz, 1H, Ph-H), 9.15 (d,
J = 7.5 Hz, 1H, Ar-H). MS (ESI+): 248 ([M+H]+, 40%).
(f). 2-(1-(Benzo[4,5]imidazo[1,2-a]pyrimidin-2-yl)piperidin-4-yl)ethanol (7): A
mixture of compound 5 (1.49 g, 6.0 mmol), compound 6 (970 mg, 7.5 mmol)
and K2CO3 (1.66 g, 12.0 mmol) in DMA (50 mL) was stirred at 130 °C for 6 h.
The reaction mixture was cooled to RT, filtered, and washed with DMA. The
combined the organic phase was evaporated in vacuo. The resulting residue
was purified by column chromatography (2–15% MeOH/CH2Cl2) on silica gel to
afford 7 (1.42 g, 80%) as an off white solid, Rf = 0.17 (10% MeOH/CH2Cl2), mp
178–180 °C. 1H NMR (DMSO-d6): d 1.15–1.17 (m, 2H, CH2), 1.37–1.41 (m, 2H,
CH2), 1.72–1.80 (m, 3H, CH and CH2), 2.98–3.00 (m, 2H, CH2), 3.46–3.49 (m, 2H,
CH2), 4.56 (br s, 1H, OH), 6.87 (d, J = 8.0 Hz, 1H, Ar-H), 7.14 (dt, J = 1.0, 8.0 Hz,
1H, Ph-H), 7.26 (dt, J = 1.0, 8.0 Hz, 1H, Ph-H), 7.49 (d, J = 8.0 Hz, 1H, Ph-H), 7.93
(d, J = 8.0 Hz, 1H, Ph-H), 8.93 (d, J = 8.0 Hz, 1H, Ar-H). MS (ESI+): 297 ([M+H]+,
100%).
(g). 2-(4-(2-Fluoroethyl)piperidin-1-yl)benzo[4,5]imidazo[1,2-a]pyrimidine (8,
T808): To a solution of compound 7 (237 mg, 0.80 mmol) in CH2Cl2 (30 mL)
was slowly added DAST (774 mg, 4.8 mmol) at 0 °C. The reaction mixture was
continued to stir at 0 °C for 1 h, and then at RT for 30 min. The solvent was
evaporated and the resulting residue was purified by column chromatography
(1–4% MeOH/CH2Cl2) on silica gel to obtain 8 (122 mg, 51%) as a white solid,
Rf = 0.43 (10% MeOH/CH2Cl2), mp 176–178 °C. 1H NMR (CDCl3): d 1.25–1.33 (m,
2H, CH2), 1.63–1.72 (m, 2H, CH2), 1.82–1.89 (m, 3H, CH and CH2), 3.01 (t,
J = 12.3 Hz, 2H, CH2), 4.49 (t, J = 6.0 Hz, 2H, CH2), 4.58 (t, J = 6.0 Hz, 2H, CH2),
6.42 (d, J = 8.0 Hz, 1H, Ar-H), 7.20 (dt, J = 1.0, 8.0 Hz, 1H, Ph-H), 7.35 (dt, J = 1.0,
8.0 Hz, 1H, Ph-H), 7.55 (d, J = 8.0 Hz, 1H, Ph-H), 7.71 (d, J = 8.0 Hz, 1H, Ph-H),
8.23 (d, J = 8.0 Hz, 1H, Ar-H). MS (ESI+): 299 ([M+H]+, 100%).
27. (a). General: All commercial reagents and solvents were purchased from
Sigma–Aldrich and Fisher Scientific, and used without further purification.
Melting points were determined on a MEL-TEMP II capillary tube apparatus
and were uncorrected. 1H NMR spectra were recorded on a Bruker Avance II
500 MHz NMR spectrometer using tetramethylsilane (TMS) as an internal
standard. Chemical shift data for the proton resonances were reported in parts
per million (ppm, d scale) relative to internal standard TMS (d 0.0), and
coupling constants (J) were reported in hertz (Hz). Liquid chromatography–
mass spectra (LC–MS) analysis was performed on an Agilent system, consisting
of an 1100 series HPLC connected to a diode array detector and a 1946D mass
spectrometer configured for positive-ion/negative-ion electrospray ionization.
Chromatographic solvent proportions are indicated as volume: volume ratio.
Thin-layer chromatography (TLC) was run using Analtech silica gel GF
uniplates (5 Â 10 cm2). Plates were visualized under UV light. Normal phase
flash column chromatography was carried out on EM Science silica gel 60
(230–400 mesh) with a forced flow of the indicated solvent system in the
proportions described below. All moisture- and air-sensitive reactions were
performed under a positive pressure of nitrogen maintained by a direct line
from a nitrogen source. Analytical RP HPLC was performed using a Prodigy
(Phenomenex) 5
H2O; flow rate 1.0 mL/min; and UV (254 nm) and
detectors. Semi-preparative RP HPLC was performed using
(Phenomenex) 5
m C-18 column, 12 nm, 10 Â 250 mm; mobile phase 50%
CH3CN/50% H2O; 4.0 mL/min flow rate; UV (254 nm) and -ray (PIN diode)
flow detectors. C18 Plus Sep-Pak cartridges were obtained from Waters
l
m C-18 column, 4.6 Â 250 mm; mobile phase 20% EtOH/80%
-ray (PIN diode) flow
Prodigy
c
(h).
2-(1-(Benzo[4,5]imidazo[1,2-a]pyrimidin-2-yl)piperidin-4-yl)ethyl
a
methanesulfonate (9, T808P): A solution of methanesulfonyl chloride (149 mg,
1.3 mmol) in CH2Cl2 (10 mL) was added dropwise to a mixture of compound 7
(296 mg, 1.0 mmol) and Et3N (303 mg, 3.0 mmol) in CH2Cl2 (40 mL) at 0 °C. The
reaction mixture was stirred at 0 °C for 1 h, and then at RT for 1 h. The reaction
mixture was evaporated in vacuo, and the resulting residue was purified by
column chromatography (1–4% MeOH/CH2Cl2) on silica gel to afford 9 (281 mg,
75%) as a white solid, Rf = 0.40 (10% MeOH/CH2Cl2), mp 237–239 °C. 1H NMR
(CDCl3): d 1.25–1.30 (m, 2H, CH2), 1.71–1.75 (m, 2H, CH2), 1.86–1.88 (m, 5H,
CH and 2Â CH2), 3.01–3.05 (m, 5H, CH2 and CH3), 6.44 (d, J = 8.0 Hz, 1H, Ar-H),
7.17 (dt, J = 1.0, 8.0 Hz, 1H, Ph-H), 7.35 (dt, J = 1.0, 8.0 Hz, 1H, Ph-H), 7.56 (d,
J = 8.0 Hz, 1H, Ph-H), 7.71 (d, J = 8.0 Hz, 1H, Ph-H), 8.26 (d, J = 8.0 Hz, 1H, Ar-H).
MS (ESI+): 375 ([M+H]+, 100%).
l
c
Corporation (Milford, MA). Sterile Millex-FG 0.2
from Millipore Corporation (Bedford, MA).
lm filter units were obtained
(b). (E)-1,1,1-Trichloro-4-ethoxybut-3-en-2-one (1): A 250 mL round-bottomed
flask was charged with trichloroacetyl chloride (35.3 g, 194 mmol). Under
nitrogen the flask was cooled with an ice bath to 0 °C and ethyl vinyl ether
(41.9 g, 582 mmol) was added dropwise. The mixture was kept on stirring at
0 °C for 5 h and then at RT for 12 h. The dropping funnel was replaced by a
short Vigreux column and excess ethyl vinyl ether was removed at 25 °C under
reduced pressure (20 mmHg). The bath temperature was raised to 130 °C
under reduced pressure (20 mmHg) to start elimination of gas (hydrogen
chloride), which was accompanied by formation of a deep black color solution
and required 1 h or so until no gas came out. Distillation of the residue under
reduced pressure afforded 1 (38.8 g, 92%) as a yellowish oil, bp 75–77 °C/
2 mmHg (lit.15 bp 116–118 °C/13 mmHg). 1H NMR (CDCl3): d 1.41 (t, J = 7.0 Hz,
3H, CH3), 4.10 (q, J = 7.0 Hz, 2H, OCH2), 6.15 (d, J = 12.5 Hz, 1H, 3-H), 7.86 (d,
J = 12.5 Hz, 1H, 4-H).
(i).
2-(4-(2-[18F]Fluoroethyl)piperidin-1-yl)benzo[4,5]imidazo[1,2-a]pyrimidine
([18F]8, [18F]-T808): No-carrier-added (NCA) aqueous H[18F]F was produced
by 18O(p,n)18F nuclear reaction using a Siemens Eclipse RDS-111 cyclotron by
irradiation of H218O (2.5 mL). H[18F]F (7.4-18.5 GBq) in [18O]water plus 0.1 mL
K2CO3 solution (1.7 mg) and Kryptofix 2.2.2 (10 mg) in 1.0 mL CH3CN with
additional 1 mL CH3CN were placed in the fluorination reaction vial (10-mL V-
vial) and repeated azeotropic distillation (17 min) was performed at 110 °C to
remove water and to form the anhydrous K[18F]F-Kryptofix 2.2.2 complex. The
precursor T808P (9, 1 mg) dissolved in DMSO (1.0 mL) was introduced to the
reaction vessel and heated at 140 °C for 8 min to affect radiofluorination. After
cooling to 100 °C, the crude reaction mixture was treated with 3 N NaOH
(1 mL). The reaction was heated at 100 °C for 8 min. The contents of the
reaction vial were cooled down and diluted with 0.1 M NaHCO3 (1 mL), and
injected onto the semi-preparative HPLC column with 3 mL injection loop for
purification. The product fraction was collected in a recovery vial containing
30 mL water. The diluted tracer solution was then passed through a C-18 Sep-
Pak Plus cartridge, and washed with water (5 mL Â4). The cartridge was eluted
with EtOH (1 mL Â2) to release [18F]8, followed by saline (10 mL). The eluted
(c). 2-(Trichloromethyl)benzo[4,5]imidazo[1,2-a]pyrimidine (3). Compound
1
(13.47 g, 62.0 mmol) was added to mixture of compound (7.99 g,
a
2
60.0 mmol) and triethylamine (6.26 g, 62.0 mmol) in toluene (250 mL). The
reaction mixture was heated to reflux for 2 h, concentrated in vacuo, filtered,
and dried in air to give 3 (16.8 g, 98%) as a yellow solid, Rf = 0.31 (2% MeOH/
CH2Cl2), mp 226 °C (decomposed). 1H NMR (DMSO-d6): d 7.51 (dt, J = 1.0,
8.0 Hz, 1H, Ph-H), 7.62 (dt, J = 1.0, 8.0 Hz, 1H, Ph-H), 7.76 (d, J = 7.0 Hz, 1H, Ar-
H), 7.94 (d, J = 8.0 Hz, 1H, Ph-H), 8.42 (d, J = 8.0 Hz, 1H, Ph-H), 9.78 (d,
J = 7.0 Hz, 1H, Ar-H). MS (ESI+): 286 ([M+H]+, 100%).
(d). Benzo[4,5]imidazo[1,2-a]pyrimidin-2-ol (4): NaOH (1 N, 42 mL, 42 mmol)
was added to
a
mixture containing compound
3
(9.16 g, 32.0 mmol) in
product was then sterile-filtered through a Millex-FG 0.2 lm membrane into a
acetonitrile (160 mL). The reaction was heated to reflux for 30 min, and then
the reaction was cooled and concentrated. Added ice to the resulting residue,
followed by HCl (1 N, 30 mL) to adjust pH of the solution to 8. Filtered the solid
and dried in air to afford 4 (5.87 g, 99%) as a white solid, Rf = 0.16 (10% MeOH/
CH2Cl2), mp >280 °C (decomposed). 1H NMR (DMSO-d6): d 6.09 (d, J = 7.5 Hz,
1H, Ar-H), 7.23 (dt, J = 1.0, 8.0 Hz, 1H, Ph-H), 7.29 (dt, J = 1.0, 8.0 Hz, 1H, Ph-H),
sterile vial. Total radioactivity was assayed and total volume was noted for
tracer dose dispensing. Retention times in the semi-preparative HPLC system
were: tR 8 = 11.52 min, tR 9 = 18.87 min, tR
in the analytical HPLC system were: tR 8 = 5.64 min, tR 9 = 9.02 min, tR
18F]8 = 5.64 min. The decay corrected radiochemical yields of [18F]8 were
35–45%.
[
18F]8 = 11.52 min. Retention times
[