Introduction

Multidentate complexing compounds containing hydroxypyridinone (HOPO) moieties as binding motif shown exemplarily in Scheme1 have been studied in the past for their ability to coordinate hard metallic cations (Santos 2002). In particular, they have been considered as tools for the complexation of Fe3+ for the treatment of iron overload (Turcot et al. 2000; Abergel and Raymond 2006). Sequestering agents bearing the HOPO residue were developed, e.g. for decontamination or decorporation applications due to the electronic properties of actinide cations being similar to Fe3+ (Gorden et al. 2003). Furthermore, the stable complexation of Gd3+ was proven using HOPO-based chelators associated with an improved relaxometry and sensitivity of Gd-based contrast agents for magnetic resonance imaging (MRI) (Raymond and Pierre 2005; Werner et al. 2008; Datta and Raymond 2009).

Scheme 1
scheme 1

Known used HOPO-acid scaffolds

Full size image

In the field of radiopharmacy, HOPO compounds have been also applied as ligands for the stable cation complexation of radionuclides. Examples are known for both isotopes 67Ga (γ emitter) and 68Ga (β+ emitter) (Clevette et al. 1990; Chaves et al. 2011; Ma et al. 2016) or for the β+ emitter 89Zr (Deri et al. 2014; Deri et al. 2015; Guérad et al. 2017; Roy et al. 2021). They are in use for nuclear imaging being subjects for a safe radionuclide chelation using HOPO ligands. Even other cations from radionuclides like 43/44/47Sc (Phipps et al. 2021), 149/152/155/161Tb (Mishiro et al. 2019), 86Y (Carter et al. 2020) or 227Th (Ramdahl et al. 2016; Hammer et al. 2017, 2020) as therapeutic radionuclide especially for targeted alpha or beta therapies use multidentate HOPO chelators for a stable complexation (Zhou et al. 2021). Interestingly, the majority of multidentate HOPO ligands used for radiopharmaceutical applications is based on open-chain molecule backbones, while only little is known about the combination of aza-crown ethers containing HOPO binding residues. In this paper, we present the synthetic access to five new HOPO-based aza-crown ethers using Kryptofix K22 (1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) as basic chemical scaffold.

Experimental

All chemicals were purchased from commercial suppliers and used without further purification unless otherwise specified. Anhydrous THF was purchased from Sigma-Aldrich (Schnelldorf, Germany), and deuterated solvents were purchased from deutero GmbH (Kastellaun, Germany). NMR spectra of all compounds were recorded on an Agilent DD2-400 MHz NMR or an Agilent DD2-600 MHz NMR spectrometer with ProbeOne. Chemical shifts of the 1H and 13C spectra were reported in parts per million (ppm) using TMS as internal standard for 1H and 13C spectra. Mass spectrometric (MS) data were obtained on an Advion Expression CMS by electron spray ionization (ESI). TLC detections were performed using silica gel 60 F254 sheets from Merck (Darmstadt, Germany). TLCs were developed by visualization under UV light (λ = 254 nm). Chromatographic separations were accomplished by using an automated silica gel column chromatography system Biotage Isolera Four and appropriate columns (Biotage, Sfär Silica HC D). A reversed phase HPLC system (Knauer Azura) with Zorbax 300SB-C18 (250 × 4.6 mm) semi-preparative column and acetonitrile/water (0.1% TFA each) as mobile phase was used for final HPLC purification (10–40% acetonitrile in H2O within 35 min).

Syntheses

N,N′-[(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(ethane-2,1-diyl)]bis[1-(benzyloxy)-6-oxo-1,6-dihydropyridine-2-carboxamide] (3a)

Compound 1a (161 mg, 0.46 mmol), 1,2-HOPO-acid 2 (283 mg, 1.15 mmol), EDC∙HCl (221 mg, 1.15 mmol), and Oxyma (164 mg, 1.15 mmol were dissolved in anhydrous acetonitrile (15 mL) and stirred overnight at 50 °C. After TLC control, the solvent was removed and the crude product mixture dissolved in chloroform (20 mL). The organic phase was washed with saturated hydrogencarbonate solution (3 × 20 mL) and afterwards dried over Na2SO4. After removal of the solvent, purification was done with using automated column chromatography (eluent: ethyl acetate/EtOH 0 → 100%) to obtain 3a as yellow oil (76 mg, 20%). Rf: 0.05 (ethyl acetate/EtOH, 2/3); 1H NMR (400 MHz, CDCl3): δ = 2.43–2.57 (m, 12H, NCH2 + OCH2), 3.20–3.29 (m, 16H, NCH2 + OCH2), 3.30–3.39 (m, 4H, NCH2), 5.37 (s, 4H, CH2Ar), 6.25 (d, 2H, 3J = 6.8 Hz, Ar–H), 6.66 (d, 2H, 3J = 9.2 Hz, Ar–H), 7.26 (dd, 2H, 3J = 6.8 Hz, 3J = 9.2 Hz, Ar–H), 7.32–7.36 (m, 6H, Bn), 7.54–7.58 (m, 4H, Bn), 8.11 (br. s, 2H, NH); 13C NMR (101 MHz, CDCl3): δ = 38.5 (br. s, CH2), 53.6 (br. s, CH2), 54.4 (CH2), 69.1 (br. s, CH2), 69.9 (CH2), 79.3 (CH2Ar), 105.0, 123.6, 128.6, 129.3, 130.7 (5 × CHAr), 133.9 (CAr), 138.1 (CHAr), 158.8, 160.7 (2 × C=O); MS (ESI +): m/z = 402 [M + 2H]2+. Anal. Calcd. for C42H54N6O10: C, 62.83; H, 6.78; N, 10.47; O, 19.93; Found: C, 62.81; H, 6.75; N, 10.50.

N,N′-[(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(propane-3,1-diyl)]bis[1-(benzyloxy)-6-oxo-1,6-dihydropyridine-2-carboxamide] (3b)

Compound 2 (200 mg, 0.82 mmol) was suspended in anhydrous chloroform (10 mL), oxalyl chloride (120 µL, 1.41 mmol) and a drop of DMF were added. The reaction mixture was stirred at 40 °C for 4 h. Then, the solvent and the remaining oxalyl chloride were removed in vacuum to obtain the acid chloride. Compound 1b (100 mg, 0.27 mmol) and NaHCO3 (50 mg, 0.60 mmol) were dissolved in anhydrous THF (10 mL) in another flask and cooled to 0 °C. The acid chloride, dissolved in anhydrous THF (2 mL), was added dropwise at 0 °C to the solution containing compound 1b and the reaction mixture was stirred at rt overnight. Next, the solvent was changed to chloroform (20 mL) and washed with hydrogen carbonate (3 × 20 mL). The organic phase was dried over Na2SO4, the solvent was removed and the crude product was purified via automated column chromatography (eluent: ethyl acetate/methanol 0% → 100%) to obtain compound 3b as yellowish oil (71 mg, 32%). 1H NMR (400 MHz, CDCl3): δ = 1.53–1.63 (m, 4H, NCH2), 2.28–2.55 (m, 12H, NCH2 + OCH2), 3.26 (s, 8H, OCH2), 3.30–3.48 (m, 16H, NCH2 + OCH2), 5.34 (s, 4H, CH2Ar), 6.23 (d, 2H, 3J = 6.9 Hz, Ar–H), 6.66 (d, 2H, 3J = 9.2 Hz, Ar–H), 7.27 (dd, 2H, 3J = 6.9 Hz, 3J = 9.2 Hz, Ar–H), 7.30–7.38 (m, 6H, Bn), 7.49–7.56 (m, 4H, Bn), 8.02 (br. s, 2H, NH); 13C NMR (101 MHz, CDCl3): δ = 25.6, 38.9, 52.9, 53.6, 69.2, 70.2 (6 × CH2), 79.4 (CH2Ar), 104.7, 123.5, 128.6, 129.4, 130.6 (5 × CHAr), 133.7 (CAr), 138.2 (CHAr), 158.7, 160.8 (2 × C=O); MS (ESI +): m/z = 831 [M + H]+, 853 [M + Na]+. Anal. Calcd. for C44H58N6O10: C, 63.60; H, 7.04; N, 10.11; O, 19.25; Found: C, 63.41; H, 7.05; N, 10.05.

N,N′-[(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(ethane-2,1-diyl)]bis(1-hydroxy-6-oxo-1,6-dihydropyridine-2-carboxamide) (4a)

Under an argon atmosphere, compound 3a (208 mg, 0.26 mmol) was dissolved in anhydrous dichloromethane (5 mL) and cooled to 0 °C. Afterwards, BBr3 (49 µl, 0.54 mmol) was added and the reaction mixture was stirred at ambient temperature overnight. Next, the solvent and remaining BBr3 were removed. The crude product was then cooled with liquid nitrogen and MeOH was added under stirring. After warming to rt, the solvent was removed and re-dissolved in a minimum amount of MeOH. Ice-cold diethyl ether was added to precipitate the final product. The diethyl ether was decanted, the product was washed with cold diethyl ether and dried to obtain compound 4a (130 mg, 80%) as yellow–brown oil. Final purification was done using semipreparative HPLC. 1H NMR (400 MHz, D2O): δ = 3.54–3.65 (m, 12H, NCH2 + OCH2), 3.74 (s, 8H, OCH2), 3.82–3.95 (m, 12H, NCH2 + OCH2), 6.82 (d, 2H, 3J = 7.1 Hz, Ar–H), 6.89 (d, 2H, 3J = 9.2 Hz, Ar–H), 7.63 (dd, 2H, 3J = 7.1 Hz, 3J = 9.2 Hz, Ar–H); 13C NMR (101 MHz, D2O): δ = 34.6, 52.5, 53.5, 63.5, 69.7 (5 × CH2), 109.0, 121.3, 139.1 (3 × CHAr), 139.4 (CAr), 159.9, 163.0 (2 × C=O); MS (ESI +): m/z = 623 [M + H]+, 645 [M + Na]+; Anal. Calcd. for C32H44F6N6O14 (as TFA salt): C, 45.18; H, 5.21; N, 9.88; O, 26.33; Found: C, 45.15; H, 5.23; N, 9.90.

N,N′-[(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(propane-2,1-diyl)]bis(1-hydroxy-6-oxo-1,6-dihydropyridine-2-carboxamide) (4b)

Under an argon atmosphere, compound 3b (67 mg, 0.08 mmol) was dissolved in anhydrous dichloromethane (5 mL) and cooled to 0 °C. Afterwards, BBr3 (29 µl, 0.32 mmol) was added and the reaction mixture was stirred at ambient temperature overnight. Next, the solvent and remaining BBr3 were removed. The crude product was then cooled with liquid nitrogen and MeOH was added under stirring. After warming to rt, the solvent was removed and re-dissolved in a minimum amount of MeOH. Ice-cold diethyl ether was added to precipitate the final product. The diethyl ether was decanted, the product was washed with cold diethyl ether and dried to obtain compound 4b as yellow–brown oil. Final purification was done using semipreparative HPLC (11.8 mg, 52%). 1H NMR (400 MHz, D2O): δ = 2.05–2.18 (m, 4H, CH2), 3.33–3.43 (m, 4H, NCH2), 3.47–3.62 (m, 12H, NCH2 + OCH2), 3.77 (s, 8H, OCH2), 3.85–3.97 (m, 8H, NCH2 + OCH2), 6.47 (d, 2H, 3J = 6.8 Hz, Ar–H), 6.86 (d, 2H, 3J = 9.1 Hz, Ar–H), 7.63 (dd, 2H, 3J = 6.8 Hz, 3J = 9.1 Hz, Ar–H); 13C NMR (101 MHz, D2O): δ = 22.3, 36.6, 50.8, 52.8, 63.6, 69.7 (6 × CH2), 108.1, 120.9, 139.1 (3 × CHAr), 140.3 (CAr), 160.1, 162.7 (2 × C=O); MS (ESI +): m/z = 651 [M + H]+, 673 [M + Na]+. Anal. Calcd. for C34H48F6N6O14 (as TFA salt): C, 46.47; H, 5.51; N, 9.56; O, 25.49; Found: C, 46.40; H, 5:66; N, 9.46.

N,N′-[(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(ethane-2,1-diyl)]bis[1-(benzyloxy)-2-oxo-1,2-dihydropyridine-3-carboxamide] (7a)

Compound 5 (114.6 mg, 0.47 mmol) was suspended in anhydrous toluene (10 mL), oxalyl chloride (40 µL, 0.47 mmol) and a drop of DMF were added. The reaction mixture was stirred at 40 °C for 4 h. Then, the solvent and the remaining oxalyl chloride were removed in vacuum to obtain 6. Compound 1a (74 mg, 0.21 mmol) and triethylamine (73 µL, 52 mmol) were dissolved in anhydrous THF (10 mL) in another flask and cooled to 0 °C. Compound 6, dissolved in anhydrous THF (2 mL), was added dropwise at 0 °C to the solution containing compound 1a and the reaction mixture was stirred at rt overnight. Next, the solvent was changed to chloroform (20 mL) and washed with hydrogen carbonate (3 × 20 mL). The organic phase was dried over Na2SO4, the solvent was removed and the crude product was purified via automated column chromatography (eluent: ethyl acetate/ethanol 0% → 100%) to obtain compound 7a as yellowish oil (69 mg, 40%).1H NMR (400 MHz, CDCl3): δ = 2.78 (t, 4H, 3J = 6.6 Hz, CH2N), 2.88 (t, 8H, 3J = 5.8 Hz, CH2N), 3.48–3.55 (m, 4H, CH2N), 3.60 (s, 8H, CH2O), 3.64 (t, 8H, 3J = 5.8 Hz, CH2O), 5.27 (s, 4H, CH2Ar), 6.12 (t, 2H, 3J = 7.2 Hz, Ar–H), 7.29 (dd, 2H, 3J = 6.9 Hz, 4 J = 2.2 Hz, Ar–H), 7.33–7.39 (m, 10H, Bn), 8.40 (dd, 2H, 3J = 7.3 Hz, 4J = 2.2 Hz, Ar–H), 9.64 (t, 2H, 3J = 5.4 Hz, NH); 13C NMR (151 MHz, CDCl3): δ = 37.9 (br. s, CH2), 54.2 (CH2), 54.6 (br. s, CH2), 70.2 (br. s, CH2), 70.8 (CH2), 79.0 (CH2Ar), 104.6 (CHAr), 123.7 (CAr), 129.0 (Bn), 129.8 (Bn), 130.2 (Bn), 133.3 (CHAr), 139.6 (CAr), 142.4 (CHAr), 158.8 (CHAr), 163.5 (C=O); MS (ESI +): m/z = 402 [M + 2H]2+. Anal. Calcd. for C42H54N6O10: C, 62.83; H, 6.78; N, 10.47; O, 19.93; Found: C, 62.79; H, 6.81; N, 10.49.

N,N′-[(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(propane-3,1-diyl)]bis[1-(benzyloxy)-2-oxo-1,2-dihydropyridine-3-carboxamide] (7b)

Compound 5 (200 mg, 0.82 mmol) was suspended in anhydrous chloroform (10 mL), oxalyl chloride (120 µL, 1.41 mmol) and a drop of DMF were added. The reaction mixture was stirred at 40 °C for 4 h. Then, the solvent and the remaining oxalyl chloride were removed in vacuum to obtain the acid chloride. Compound 1b (100 mg, 0.27 mmol) and NaHCO3 (50 mg, 0.60 mmol) were dissolved in anhydrous THF (10 mL) in another flask and cooled to 0 °C. The acid chloride, dissolved in anhydrous THF (2 mL), was added dropwise at 0 °C to the solution containing compound 1b and the reaction mixture was stirred at rt overnight. Next, the solvent was changed to chloroform (20 mL) and washed with hydrogen carbonate (3 × 20 mL). The organic phase was dried over Na2SO4, the solvent was removed and the crude product was purified via automated column chromatography (eluent: ethyl acetate/methanol 0% → 100%) to obtain compound 3b as yellowish oil (66 mg, 30%). 1H NMR (400 MHz, CDCl3): δ = 1.71–1.81 (m, 4H, NCH2), 2.60 (t, 3J = 7.1 Hz, 4H, NCH2), 2.78 (t, 3J = 5.8 Hz, 8H, OCH2), 3.26 (s, 8H, OCH2), 3.39–3.66 (m, 20H, NCH2 + OCH2), 5.27 (s, 4H, CH2Ar), 6.13 (t, 2H, 3J = 7.0 Hz, Ar–H), 7.29 (dd, 2H, 4J = 2.0 Hz, 3J = 7.0 Hz, 2H, Ar–H), 7.27 (dd, 2H, 3J = 6.9 Hz, 3 J = 9.2 Hz, 2H, Ar–H), 7.32–7.42 (m, 10H, Bn), 8.41 (dd, 2H, 4J = 2.0 Hz, 3J = 7.5 Hz, 2H, Ar–H), 9.58 (br. s, 2H, NH); 13C NMR (101 MHz, CDCl3): δ = 27.4, 37.9, 53.4, 54.0, 70.0, 70.8 (6 × CH2), 79.0 (CH2Ar), 104.7, 123.7, 129.0, 129.8, 130.1 (5 × CHAr), 133.2 (CAr), 139.4 (CAr), 142.4 (CHAr), 158.8, 163.3 (2 × C=O); MS (ESI +): m/z = 831 [M + H]+, 853 [M + Na]+. Anal. Calcd. for C44H58N6O10: C, 63.60; H, 7.04; N, 10.11; O, 19.25; Found: C, 63.55; H, 6.99; N, 10.14.

N,N′-[(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(ethane-2,1-diyl)]bis(1-hydroxy-2-oxo-1,2-dihydropyridine-3-carboxamide) (8a)

Under an argon atmosphere, compound 7a (128 mg, 0.16 mmol) was dissolved in anhydrous dichloromethane (5 mL) and cooled to 0 °C. Afterwards, BBr3 (49 µL, 0.54 mmol) was added and the reaction mixture was stirred at ambient temperature overnight. Next, the solvent and remaining BBr3 were removed. The crude product was then cooled with liquid nitrogen and MeOH was added under stirring. After warming to rt, the solvent was removed and redissolved in a minimum amount of MeOH. Ice-cold diethyl ether was added to precipitate the final product. The diethyl ether was decanted, the product was washed with cold diethyl ether and dried to obtain compound 8a (98 mg, 99%) as yellow–brown oil. Final purification was done using semipreparative HPLC. 1H NMR (400 MHz, D2O): δ = 3.51–3.71 (m, 20H, NCH2 + OCH2), 3.81–3.94 (m, 12H, NCH2 + OCH2), 6.68 (t, 2H, 3J = 7.1 Hz, Ar–H), 8.20 (d, 2H, 3J = 9.2 Hz, Ar–H), 8.36 (dd, 2H, 3J = 7.1 Hz, 3J = 9.2 Hz, Ar–H), 10.03 (t, 2H, 3J = 5.6 Hz, NH); 13C NMR (101 MHz, D2O): δ = 34.5, 53.6, 53.7, 63.6, 69.7 (5 × CH2), 106.7 (CHAr), 119.1 (CAr), 140.4, 142.1 (2 × CHAr), 158.9, 166.8 (2 × C=O); MS (ESI +): m/z = 623 [M + H]+, 645 [M + Na]+. Anal. Calcd. for C32H44F6N6O14 (as TFA salt): C, 45.18; H, 5.21; N, 9.88; O, 26.33; Found: C, 45.13; H, 5.20; N, 9.87.

N,N′-[(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(ethane-2,1-diyl)]bis(1-hydroxy-2-oxo-1,2-dihydropyridine-3-carboxamide) (8b)

Under an argon atmosphere, compound 7b (66 mg, 0.08 mmol) was dissolved in anhydrous dichloromethane (5 mL) and cooled to 0 °C. Afterwards, BBr3 (49 µL, 0.54 mmol) was added and the reaction mixture was stirred at ambient temperature overnight. Next, the solvent and remaining BBr3 were removed. The crude product was then cooled with liquid nitrogen and MeOH was added under stirring. After warming to rt, the solvent was removed and redissolved in a minimum amount of MeOH. Ice-cold diethyl ether was added to precipitate the final product. The diethyl ether was decanted, the product was washed with cold diethyl ether and dried to obtain compound 8b (71 mg, > 99%) as yellow–brown oil. Final purification was done using semipreparative HPLC. 1H NMR (400 MHz, D2O): δ = 2.04–2.16 (m, 4H, CH2), 3.30–3.39 (m, 4H, NCH2), 3.42–3.60 (m, 12H, NCH2 + OCH2), 3.68 (s, 8H, OCH2), 3.79–3.92 (m, 8H, NCH2 + OCH2), 6.66 (t, 2H, 3J = 7.2 Hz, Ar–H), 8.16 (d, 2H, 3J = 6.5 Hz, Ar–H), 8.36 (d, 2H, 3J = 7.2 Hz, Ar–H), 9.89 (t, 2H, 3J = 4.9 Hz, NH); 13C NMR (101 MHz, D2O): δ = 22.7, 36.2, 50.5, 52.7, 63.5, 69.6 (6 × CH2), 106.8 (CHAr), 119.5 (CAr), 139.9, 141.7 (2 × CHAr), 158.9, 165.9 (2 × C=O); MS (ESI +): m/z = 651 [M + H]+, 673 [M + Na]+, 689 [M + K]+. Anal. Calcd. for C34H48F6N6O14 (as TFA salt): C, 46.47; H, 5.51; N, 9.56; O, 25.49; Found: C, 46.55; H, 5:59; N, 9.67.

N,N′-[(1,4,10,13-Tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(ethane-2,1-diyl)]bis(3-methoxy-1-methyl-2-oxo-1,2-dihydropyridine-4-carboxamide) (11)

Compound 9 (249.4 mg, 1.36 mmol) was suspended in anhydrous toluene (10 mL), oxalyl chloride (1.1 mL, 15.1 mmol), and a drop of DMF were added. The reaction mixture was stirred at 40 °C for 4 h. Then, the solvent and the remaining oxalyl chloride were removed in vacuum to obtain 10. Compound 1 (200 mg, 0.57 mmol) and triethylamine (174 mg, 1.72 mmol) were dissolved in anhydrous dichloromethane (10 mL) in another flask and cooled to 0 °C. Compound 10, dissolved in anhydrous dichloromethane (2 mL), was added dropwise at 0 °C to the solution containing compound 1 and the reaction mixture was stirred at rt overnight. Next, the solvent was changed to chloroform (30 mL) and washed with hydrogen carbonate (3 × 30 mL). The organic phase was dried over Na2SO4, the solvent was removed and the crude product was purified via automated column chromatography (eluent: ethyl acetate/methanol 50% → 100%) to obtain compound 11 as yellowish oil (125 mg, 0.18 mmol, 32%).1H NMR (400 MHz, CDCl3): δ = 2.74 (t, 4H, 3J = 6.0 Hz, CH2N), 2.84 (t, 8H, 3J = 5.8 Hz, CH2N), 3.44–3.50 (m, 4H, CH2N), 3.53–3.57 (m, 14H, CH2O + CH3), 3.59 (t, 8H, 3J = 5.8 Hz, CH2O), 4.06 (s, 6H, CH3), 6.76 (d, 2H, 3J = 7.2 Hz, Ar–H), 7.08 (d, 2H, 3J = 7.2 Hz, Ar–H), 8.43 (t, 2H, 3J = 4.5 Hz, NH); 13C NMR (151 MHz, CDCl3): δ = 37.7 (CH3), 37.8, 53.8, 54.0 (3 × CH2), 60.2 (CH3), 70.1, 70.8 (2 × CH2), 104.9 (CHAr), 130.5 (CAr), 132.1 (CHAr), 147.8 (CAr), 159.7, 163.3 (2 × C=O); MS (ESI +): m/z = 340 [M + 2H]2+, 679 [M + H]+. Anal. Calcd. for C32H50N6O10: C, 56.62; H, 7.43; N, 12.38; O, 23.57; Found: C, 56.67; H, 7.41; N, 12.35.

N,N′-((1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(ethane-2,1-diyl))bis(3-hydroxy-1-methyl-2-oxo-1,2-dihydropyridine-4-carboxamide) (12)

Under an argon atmosphere, compound 11 (115 mg, 0.17 mmol) was dissolved in anhydrous dichloromethane (5 mL) and cooled to 0 °C. Afterwards, BBr3 (100 µL, 292 mmol) was added and the reaction mixture was stirred at ambient temperature overnight. Next, the solvent and remaining BBr3 were removed. The crude product was then cooled with liquid nitrogen and MeOH was added under stirring. After warming to rt, the solvent was removed and redissolved in a minimum amount of MeOH. Ice-cold diethyl ether was added to precipitate the final product. The diethyl ether was decanted, the product was washed with cold diethyl ether and dried to obtain compound 8b (108 mg, 98%) as yellow–brown oil. Final purification was done using semipreparative HPLC. 1H NMR (400 MHz, DMSO-d6): δ = 3.29–3.88 (m, 38H, CH2N + CH2O + CH3), 6.50 (d, 2H, 3 J = 7.4 Hz, Ar–H), 7.21 (d, 2H, 3 J = 7.4 Hz, Ar–H), 8.64 (t, 2H, 3 J = 5.3 Hz, NH), 9.50 (br. s, 2H, OH); 13C NMR (151 MHz, CDCl3): δ = 36.9 (CH3), 34.4, 52.4, 52.8, 64.4, 69.4 (4 × CH2), 60.2 (CH3), 70.1, 70.8 (2 × CH2), 103.1 (CHAr), 117.4 (CAr), 127.9 (CHAr), 146.8 (CAr), 158.2, 165.6 (2 × C=O); MS (ESI +): m/z = 651 [M + H]+, 673 [M + Na]+. Anal. Calcd. for C34H48F6N6O14 (as TFA salt): C, 46.47; H, 5.51; N, 9.56; O, 25.49; Found: C, 46.65; H, 5:76; N, 9.85.

Results and discussion

Diaza-crown ethers are subjected to function as basic skeleton to prepare multidentate cyclic chelators. To introduce the respective HOPO functions, two primary diazacrown ethers 1a,b were prepared according to the literature in two steps starting from diaza-18-crown-6 ether, which was treated with N-(2-bromoethyl)phthalimide (Lukyanenko et al. 2004) or N-(3-bromopropyl)phthalimide (Quici et al. 1999), respectively, according to published procedures. The second step comprises the removal of the phthalimide moiety with hydrazine to obtain N,N′-bis(aminoethylene) compound 1a and N,N′-bis(aminopropylene) compound 1b, both containing two free primary amino functions to introduce the HOPO groups. The reaction path is shown in the Supporting Information.

Synthesis of the HOPO-functionalized crown ethers 4a,b, 8a,b, and 12

To avoid side reactions, the 1,2-HOPO-core is introduced into multidentate amine skeletons in its O-benzyl-protected form as activated ester (succinimidyl ester, see: Huang et al. 2019; TFP ester, see: Workman et al. 2020), using peptide coupling conditions (Daumann et al. 2016) or as acid chloride (see e.g. Phipps et al. 2023). In our case, the O-benzyl protected 1,2-HOPO-acid 2 was used as well, which was prepared from the respective acid and benzyl bromide (Deri et al. 2014). Bn-1,2-HOPO-acid 2 was reacted with the basic macrocycle 1a using EDC∙HCl (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) and Oxyma (ethyl cyanohydroxyiminoacetate) to yield the O-benzyl-HOPO-functionalized diazacrown ether 3a (20% yield). In contrast, 3b was prepared from 2 in 32% yield, which was converted into the acid chloride with oxalyl chloride beforehand and then reacted with 1b. Finally, the benzyl protecting groups of 3a,b were cleaved with BBr3 under anhydrous conditions to obtain the final HOPO-ligands 4a,b in 80 and 52% yield, respectively. The synthesis procedure to HOPO derivative 4a,b is shown in Scheme 2.

Scheme 2
scheme 2

Synthesis of 1,2-HOPO-based lariat ethers 4a and 4b

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Little is known about ligand formed by the 1,2,3-HOPO-acid moiety. For the preparation of the 1,2,3-HOPO-ligands 8a,b, it is also necessary to protect the hydroxy function of the starting HOPO derivative. Thus, the O-benzyl protected 1,2,3-HOPO-acid 5 is used, which was prepared from the HOPO acid by O-alkylation with benzyl bromide (Workman et al. 2020). They used activated esters based on TFP or mercaptothiazoline for the connection of the 1,2,3-HOPO moiety to the amine. In our case, carbodiimides such as EDC were used to directly react HOPO derivative 5 with the macrocycles 1a,b without using an activated ester. Notably, both HOPO-functionalized macrocycles 7a,b were not obtained. Thus, Bn-1,2,3-HOPO-acid 5 was converted into the corresponding acid chloride 6 using oxalyl chloride (Workman et al. 2020) and then subsequently reacted with crown ethers 1a,b under mild conditions using triethyl amine as base to obtain 7a in 40% yield and 7b in 30% yield. Finally, the benzyl groups were quantitatively cleaved with BBr3 obtaining the final derivatives 8a and 8b (see Scheme 3).

Scheme 3
scheme 3

Synthesis of 1,2,3-HOPO-based lariat ethers 8a and 8b

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The 2,3-Me-HOPO-acid was mainly used as intermediate in the synthesis of pharmacologically active compounds (Sweeney et al. 2008). In this case, the synthesis route starts from the di-N,O-methyl protected 2,3-HOPO acid ethyl ester. The saponification under basic conditions delivered the free acid 9 (Sweeney et al. 2008), which was converted into its respective acid chloride 10. Compound 10 was then subsequently reacted with 1a to obtain the desired dimethyl-protected derivative 11 in 32% yield. Finally, the methyl groups were cleaved with BBr3. An excess of BBr3 combined with a longer reaction time is necessary. Otherwise the partly deprotected compound will be obtained (data not shown). The whole reaction path is shown in Scheme 4.

Scheme 4
scheme 4

Synthesis of the 2,3-Me-HOPO-based lariat ether 12

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Conclusions

The combination of macrocyclic compounds with HOPO-functions delivers new multidentate ligands for a stable complexation of metal ions. For this purpose, five new HOPO-functionalized diazacrown ethers were prepared using a convenient synthesis procedure. Their structures were confirmed by NMR and ESI MS.