CC BY ND NC 4.0 · SynOpen 2017; 01(01): 0173-0179
DOI: 10.1055/s-0036-1591863
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Synthesis and Radiosynthesis of Prospective 2-Nitroimidazole Hypoxia­ PET Tracers via Thiazolidine Ligation with 5-Fluorodeoxyribose (FDR)

Institute of Medical Sciences and Kosterlitz Centre for Therapeutics, School of Medicine, Medical Sciences and Nutrition, University of Aberdeen, Foresterhill, Aberdeen, AB25 2ZD, Scotland, UK   Email: m.zanda@abdn.ac.uk   Email: s.dallangelo@abdn.ac.uk
,
Institute of Medical Sciences and Kosterlitz Centre for Therapeutics, School of Medicine, Medical Sciences and Nutrition, University of Aberdeen, Foresterhill, Aberdeen, AB25 2ZD, Scotland, UK   Email: m.zanda@abdn.ac.uk   Email: s.dallangelo@abdn.ac.uk
,
M. Zanda*
Institute of Medical Sciences and Kosterlitz Centre for Therapeutics, School of Medicine, Medical Sciences and Nutrition, University of Aberdeen, Foresterhill, Aberdeen, AB25 2ZD, Scotland, UK   Email: m.zanda@abdn.ac.uk   Email: s.dallangelo@abdn.ac.uk
C.N.R. – I.C.R.M., via Mancinelli 7, 20131 Milan, Italy
› Author Affiliations
M.M. gratefully acknowledges SULSA (http://www.sulsa.ac.uk/) for a PhD studentship
Further Information

Publication History

Received: 19 October 2017

Accepted: 21 November 2017

Publication Date:
12 December 2017 (online)

Abstract

The first prospective fluorinated PET tracers for imaging hypoxia obtained via thiazolidine-ligation are reported. Three 1,2-thiol-amine linkers were combined with four different 2-nitroimidazole spacers via amide or urea bond formation. The resulting compounds were submitted to thiazolidine-ring-forming ligation reaction with the fluorinated carbohydrate l-5-fluoro-5-deoxy-ribose (FDR), affording the desired candidate PET tracers in variable yields. The same ligation reactions performed on l-ribose – a by-product of [18F]FDR radiosynthesis – under conditions mimicking a radiochemical production showed that the fluorinated adducts can be efficiently purified and isolated by HPLC. Finally, one of the prospective hypoxia tracers was successfully produced in radiolabelled form in 29.2% radiochemical yield from [18F]FDR.

Supporting Information

 
  • References and Notes

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  • 22 Synthesis of 4c: Hydrazine monohydrate (124 μL, 2.52 mmol) was added to a solution of 13 (340 mg, 0.84 mmol) in EtOH (5 mL) and the reaction mixture was heated at reflux for 3 h. After cooling to 0 °C the resulting white precipitate was filtered off and the filtrate was concentrated under reduced pressure. The residue was dissolved with Et2O (5 mL) and the resulting white precipitate was filtered, then the filtrate was concentrated under reduced pressure to afford 4c (448 mg, 98.4%) as a yellow oil. 1H NMR (CDCl3, 400 MHz): δ = 4.16 (br, 1 H), 3.00 (dd, J = 11.6, 6.1 Hz, 1 H), 2.65–2.54 (m, 2 H), 2.47 (d, J = 11.6 Hz, 1 H), 1.73–1.49 (m, 4 H), 1.60 (s, 6 H), 1.44–1.41 (m, 2 H), 1.33 (s, 9 H). 13C NMR (CDCl3, 100 MHz): δ = 152.3, 79.8, 69.4, 64.1, 41.8, 31.3, 30.9, 30.2, 29.6 (2C), 28.4 (3C). MS (ESI): m/z calcd for C13H26N2O2S: 275.1 [M+H]+, 297.2 [M+Na]+ 303.2 [M+K]+; found: 275.2 [M+H]+, 297.2 [M+Na]+, 303.1 [M+K]+
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  • 27 A shorter synthesis of 14d has been reported, see: Joyard Y., Azzouz R., Bischoff L., Papamicaël C., Labar D., Bol A., Bol V., Vera P., Grégoire V., Levacher V., Bohn P.; Bioorg. Med. Chem.; 2013, 21: 3680; and references therein. However, imidazolyl carbinol 24 was already being used in our labs for a related project, therefore it was used as an intermediate for 14d
  • 28 Synthesis of 25a: DIPEA (156 μL, 0.92 mmol) and HATU (350 mg, 0.92 mmol) were added to a solution of 4a (200 mg, 0.77 mmol) in anhydrous CH2Cl2 (10 mL) at 0 °C and the mixture was allowed to react at r.t. for 1 h. Then the amino derivative 14a (260 mg, 1.53 mmol) dissolved in CH2Cl2 (2 mL) was added to the mixture. After 16 h under stirring, the mixture was washed with a 0.5 M aq. NaOH solution (3 × 6 mL) and then with a 0.1 M aq. HCl solution (3 × 6 mL), dried over Na2SO4 and concentrated under reduced pressure. The crude material was purified by flash chromatography (Hex/EtOAc, from 8:2 to 7:3) to afford 25a (229 mg, 72.3%) as a yellow oil. 1H NMR (CDCl3, 400 MHz, mixture of rotamers): δ = 7.32 (s, 1 H), 7.08 (s, 1 H), 6.55 (br, 1 H), 4.72 (br, 1 H), 3.42–3.11 (m, 4 H), 1.82 (s, 3 H), 1.73 (s, 3 H), 1.42 (s, 9 H). 13C NMR (CDCl3, 100 MHz): δ = 171.7, 153.3, 144.7, 128.4, 127.0, 81.8, 71.4, 67.5, 47.4, 36.0, 31.0, 29.3, 28.9, 28.4 (3C). MS (ESI): m/z calcd for C17H27N5O5S: 436.2 [M+Na]+, 452.0 [M+K]+; found: 436.1 [M+Na]+, 452.0 [M+K]+
  • 29 Synthesis of 25f: A solution of 4c (152 mg, 0.56 mmol) in CH2Cl2 (2 mL) was added dropwise to a solution of CDI (90 mg, 0.56 mmol) in anhydrous CH2Cl2 (3 mL), at 0 °C under N2 atmosphere, then the mixture was allowed to react at r.t. for 1 h. After 16 h under stirring, the mixture was added via syringe to a solution of 14a (226 mg, 1.33 mmol) in CH2Cl2 (3 mL) under N2 atmosphere. After 16 h under stirring the mixture was concentrated under recued pressure. Purification by FC on silica gel (Hex/EtOAc, from 3:7 to 7:3) gave 25f (148 mg, 56.7%) as a yellow oil. 1H NMR (CDCl3, 400 MHz, 2 rotamers): δ = 7.35 (br, 1 H), 7.02 (br, 1 H), 5.55 (br, 2 H), 4.40 (t, J = 6.9 Hz, 2 H), 4.19 (br, 1 H), 3.24–2.94 (m, 5 H), 2.49 (d, J = 11.8 Hz, 1 H), 2.05–1.86 (m, 2 H), 1.81–1.66 (m, 2 H), 1.63 (s, 3 H), 1.61 (s, 3 H), 1.48–1.26 (m, 11 H). 13C NMR (CDCl3, 100 MHz, 2 rotamers): δ = 159.0, 152.7, 144.6, 128.2, 127.1, 80.3, 69.4, 64.0, 47.8, 39.8, 36.6, 31.6, 31.4, 30.6, 30.1, 29.6, 28.4 (3C), 27.3. MS (ESI): m/z calcd for C20H34N6O5S: 471.2 [M+H]+, 493.2 [M+Na]+; found: 471.2 [M+H]+, 493.2 [M+Na]+
  • 30 Synthesis of 3a: Compound 25a was dissolved in a TFA/H2O/MeOH 3:2:1 mixture and heated to 65 °C for 2 h. Solvents were then concentrated under reduced pressure at 60 °C, then the residue was dissolved in ethanol and passed through a SiliaBond ® carbonate pad to give the crude 3a as trifluoroacetate salt (55.7 mg, 74.8%). The compound was used in the next reaction without any further purification. 1H NMR (CD3OD, 400 MHz, in mixture with the dimer): δ = 7.62–7.54 (m, 1 H), 7.20–7.15 (m, 1 H), 4.60–4.42 (m, 2 H), 4.31–4.19 (m, 1 H), 3.52–3.33 (m, 2 H), 3.24–3.09 (m, 2 H), 2.19–2.00 (m, 2 H). 13C NMR (CD3OD, 100 MHz, in mixture with the dimer): δ = 167.4, 144.6, 127.4, 127.2, 51.7, 47.4, 37.8, 36.3, 29.7. MS (ESI): m/z calcd for C9H15N5O3S: 274.1 [M+H]+, 296.1 [M+Na]+; found: 274.0 [M+H]+, 296.0 [M+Na]+
  • 31 Synthesis of 1a: [19F]FDR ([19F]2) (5 mg, 0.033 mmol) was added to a solution of 3a (32.0 mg, 0.083 mmol) and DTT (12.8 mg, 0.083 mmol) in a 1 M sodium acetate buffer solution (pH 4.5), then the mixture was allowed to react at 30 °C for 20 min. Purification by RP-HPLC (Column: Phenomenex Luna C18 250 × 10.00 mm, 5 μm; mobile phase: A (H2O + 0.05% TFA), B (ACN + 0.05% TFA); gradient: from 5% B to 6% B in 15 min; flow: 5 mL min−1; tR : 12.5 min) gave 1a as trifluoroacetate salt (10.6 mg, 61.3 %). NMR analyses were performed after treatment of 1a with SiliaBond ® carbonate (10% w/w) in EtOH, under gentle stirring for 1 h in order to freebase trifluoroacetate salt. 1H NMR (CD3OD, 400 MHz, – four diastereoisomers – two major isomer in ~3:2 ratio were identified): δ = 7.55 (d, J = 1.2 Hz, 1 H), 7.16 (d, J = 1.2 Hz, 1 H), 4.89–4.83 (m, 1 H), 4.62–4.42 (m, 4 H), 4.25 (dd, J = 7.0, 6.8 Hz, 1 H), 4.09–4.04 (m, 1 H), 3.91 (dd, J = 7.4, 4.6 Hz, 1 H), 3.66 (dd, J = 7.4, 5.8 Hz, 1 H), 3.39–3.23 (m, 3 H), 3.02–2.91 (m, 1 H), 2.16–2.05 (m, 2 H); δ (second isomer) = 7.57 (d, J = 1.2 Hz, 1 H), 7.17 (d, J = 1.2 Hz, 1 H), 4.92 (d, J = 2.6 Hz, 1 H), 4.67 (dd, J = 9.8, 3.0 Hz, 1 H), 4.63–4.42 (m, 3 H), 4.25 (dd, J = 7.0, 6.8 Hz, 1 H), 4.04–3.95 (m, 1 H), 3.85–3.75 (m, 2 H), 3.39–3.14 (m, 3 H), 3.02–2.91 (m, 1 H), 2.16–2.05 (m, 2 H). 13C NMR (CD3OD, 100 MHz, – four diastereoisomers – two major isomers in ~3:2 ratio were identified): δ (first isomer) = 172.5, 144.7, 127.2, 127.1, 84.1 (d, J CF = 167 Hz), 73.6 (d, J CF = 7 Hz), 72.3, 72.1, 71.8 (d, J CF = 18 Hz), 71.4, 65.8, 35.8, 34.9, 30.0; δ (second isomer) = 172.4, 144.7, 127.2, 127.1, 84.3 (d, J CF = 167 Hz), 73.6 (d, J CF = 7 Hz), 72.3, 72.0, 71.9, 71.9 (d, J CF = 18 Hz), 70.2, 66.2, 36.6, 34.9. 29.9. 19F NMR (376 MHz, CD3OD): δ (first isomer) = –233.0 (dt, J = 48.0, 22.7 Hz); δ (second isomer) = –233.6 (dt, J 1 = 48.0 Hz, J 2 = 22.3 Hz); MS (ESI): m/z calcd for: C14H22FN5O6S: 408.1 [M+H]+, 430.1 [M+Na]+; found: 408.0 [M+H]+, 430.0 [M+Na]+
  • 32 Optimised radiosynthesis of [18F]1a: A solution of sodium acetate buffer (6 M, pH 4.5) was added to a solution of 3a (2.5 mg, 6.4 μmol), DTT (1 mg, 6.4 μmol) and [18F]FDR ([18F]2) (2.5–10 MBq) in 0.5–1.0 mL of H2O to form a 70% v/v sodium acetate buffer solution (final concentration 4.2 M). After ~20 min the mixture was purified by RP-HPLC (Column: Phenomenex Luna C18 250 × 10.00 mm, 5 μm) to give [18F]1a in 29% RCY (decay corrected)