Synthesis and Purification of Fluorine-18 Labeled Maleimide Derivative Probe

The precursor 4-formyl-N,N,N-trimethylanilinium trifluoromethanesulfonate (1) was purchased from ABX advanced biochemical compounds (Radeberg, Germany). The non-radioactive cold standard 6-fluoropyridinealdehyde was obtained from Sigma Aldrich (St. Louis, MO, USA). The precursor 6-(N,N,N-trimethylamino)nicotinaldehyde trifluoromethanesulfonate (2),^{19 (link)} 1-(6-(aminooxy)hexyl)- 1H-pyrrole-2,5-dione^{13 (link),20} and non-radioactive cold standard of fluorine-18 labeled maleimide derivative, fluoronicotinaldehyde O-(6-(2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)hexyl) oxime, FPyMHO (6),^{13 (link)} were synthesized by literature methods. All other chemicals and solvents were received from Sigma Aldrich (St. Louis, MO, USA) and used without further purification. Anhydrous solvents were used for all radiolabeling reactions. Fluorine-18 was received from National Institutes of Health cyclotron facility (Bethesda, MD, USA). Chromafix 30-PS-HCO₃ anion-exchange Sep-Pak cartridges were purchased from Macherey-Nagel (Düren, Germany) and the packing material was reduced to half (~20 mg). Luna (2) C18 column (10 × 250 mm, 5 μm) was obtained from Phenomenex (Torrance, CA, USA). Other columns and all other Sep-Pak cartridges used in this synthesis were obtained from Agilent Technologies (Santa Clara, CA, USA) and Waters (Milford, MA, USA), respectively. Oasis MCX Plus cartridge was conditioned by passing 5-mL acetonitrile. Sep-Pak plus C18 cartridges were conditioned with 5-mL ethanol, 10-mL air, and 10-mL water. Low resolution mass spectra (MS) were recorded on a 6130 Quadrupole LC/MS, Agilent Technologies instrument equipped with a diode array detector. ¹H, ¹³C, and ¹⁹F NMR spectra were recorded on a 400-MHz Bruker spectrometer. Chemical shifts (ppm) are reported relative to the solvent residual peaks of acetonitrile (δ ¹H, 2.50 ppm; ¹³C 118.26, 1.79), methanol (δ ¹H, ¹H, 3.34 ppm), and chloroform (δ ¹H, 7.26 ppm). ¹⁹F NMR spectra are reported with reference to the trifluoroacetic acid (δ ¹⁹F, −76.72 ppm). High performance liquid chromatography (HPLC) purification and analytical HPLC analyses for radiochemical work were performed on an Agilent 1200 Series instrument equipped with multi-wavelength detectors along with a flow count radiodetector (Eckert & Ziegler, B-FC-3500 diode).
HPLC conditions: