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Synlett 2018; 29(09): 1239-1243
DOI: 10.1055/s-0036-1591950
letter
© Georg Thieme Verlag Stuttgart · New York

A Convenient Synthesis for HBED-CC-tris(tert-butyl ester)

Ata Makarem*
German Cancer Research Center (DKFZ) Heidelberg, Division of Radiopharmaceutical Chemistry, INF 280, 69120 Heidelberg, Germany   Email: a.makarem@dkfz.de
,
Moritz Konrad
German Cancer Research Center (DKFZ) Heidelberg, Division of Radiopharmaceutical Chemistry, INF 280, 69120 Heidelberg, Germany   Email: a.makarem@dkfz.de
,
Christos Liolios
German Cancer Research Center (DKFZ) Heidelberg, Division of Radiopharmaceutical Chemistry, INF 280, 69120 Heidelberg, Germany   Email: a.makarem@dkfz.de
,
Klaus Kopka
German Cancer Research Center (DKFZ) Heidelberg, Division of Radiopharmaceutical Chemistry, INF 280, 69120 Heidelberg, Germany   Email: a.makarem@dkfz.de
› Author AffiliationsThis work was partly funded by a grant of the German Cancer Aid (Deutsche Krebshilfe), project number 70112043.
Further Information

Publication History

Received: 28 January 2017

Accepted after revision: 11 February 2018

Publication Date:
07 March 2018 (online)

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Abstract

HBED-CC is a bifunctional complexing agent that, at ambient temperature, tightly chelates the trivalent radiometal 68Ga (T1/2 = 68 min). This complexing agent has attracted a lot of interest in tumor imaging applications. Depending on the chemical structure, different HBED-CC variants may be employed as radiolabeling precursor for the synthesis of desired radiopharmaceuticals. In this context, HBED-CC-tris(tert-butyl ester) is the only known monovalent variant of HBED-CC which is used for the synthesis of non-symmetric HBED-CC-based radiopharmaceuticals. Commercial HBED-CC-tris(tert-butyl ester) is very ­expensive, with limited availability. Nevertheless, no synthetic procedure for this useful product has been reported to date. This work introduces a convenient and comparatively cost-efficient method for the preparation of HBED-CC-tris(tert-butyl ester).

Key words

HBED-CC - bifunctional chelator - hexadentate ligand - synthesis - radiotracer

Supporting Information

    Supporting information for this article is available online at https://doi.org/10.1055/s-0036-1591950.
  • Supporting Information
 

  • References and Notes

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  • 28 Synthesis of Methyl 3-{3-[({2-[(tert- Butoxycarbonyl)amino] ethyl}imino)methyl]-4-hydroxyphenyl}propanoate (4) To a solution of methyl 3-(3-formyl-4-hydroxyphenyl)propanoate (2) (3.00 g, 14.41 mmol, 1.0 equiv) in anhydrous dichloromethane (10 mL) was added tert-butyl (2-aminoethyl)carbamate (3) (2.54 g, 15.85 mmol, 1.1 equiv). The mixture was stirred at room temperature for 1 h, and then diluted with dichloromethane (40 mL). The reaction mixture was then washed with aq. sodium bisulfite (0.5 M, 50 mL). The product residue in the aqueous phase was recovered by washing four times with dichloromethane. Then, the combined organic phases were dried over sodium sulfate, filtered and the solvent was removed under reduced pressure to give product 4 as an orange oil (quantitative yield). 1H NMR (400.13 MHz, CDCl3, 25 °C): δ = 8.32 (s, 1 H, CCHN), 7.15 (dd, 3 J H–H = 8.4 Hz, 4 J H–H = 2.2 Hz, 1 H, CHCHCCH), 7.09 (d, 4 J H–H = 2.2 Hz, 1 H, CCHC), 6.90 (d, 3 J H–H = 8.4 Hz, 1 H, CHCHCCH), 4.72 (br s, 1 H, NH), 3.70 (t, 3 J H–H = 5.7 Hz, 2 H, CH 2CH2NHBoc), 3.66 (s, 3 H, OCH3), 3.45 (m, 2 H, CH2CH 2NHBoc), 2.90 (t, 3 J H–H = 7.7 Hz, 2 H, CH 2CH2CO2), 2.60 (t, 3 J H–H = 7.7 Hz, 2 H, CH2CH 2CO2), 1.43 (s, 9 H, C(CH3)3) ppm. 13C{1H} NMR (100.61 MHz, CDCl3, 25 °C): δ = 173.40 (CO2CH3), 166.59 (COH), 159.57 (CCHN), 155.98 (Boc CO2), 132.65 (CHCHCCH), 131.10 (CHCHCCH), 130.81 (CCHC), 118.60 (CCHN), 117.24 (CHCHCCH), 79.71 (BocCO2 CCH3), 59.57 (CH2CH2NHBoc), 51.78 (CO2 CH3), 41.37 (CH2 CH2NHBoc), 36.00 (CH2 CH2CO2), 30.07 (CH2CH2CO2), 28.51 (C(CH3)3). HRMS (ESI+, CH2Cl2/MeOH): m/z calcd for C18H27N2O5: 351.1914; found: 351.1914. HRMS (ESI-, CH2Cl2/MeOH): m/z calcd for C18H25N2O5: 349.1769; found: 349.1769. Synthesis of Methyl 3-{3-[({2-[(tert-Butoxycarbonyl)amino] ethyl}amino)methyl]-4-hydroxyphenyl}propanoate (5) A solution of compound 4 (4.90 g, 14.00 mmol, 1.0 equiv) in 2,2,2-trifluoroethanol (60 mL) was cooled in an ice-bath. Sodium borohydride (1.32 g, 35 mmol, 2.5 equiv) was added in portions and then the reaction mixture was allowed to warm to room temperature. After being stirred under an inert atmosphere at room temperature for one hour, the reaction was quenched with water (50 mL). The product was extracted with dichloromethane and the organic layer was dried over sodium sulfate and filtered. By removal of the solvent under reduced pressure compound 5 was obtained as a colorless oil in quantitative yield. This product was almost pure and used in the next step without further purification. However, for spectroscopic characterization it was purified through HPLC. 1H NMR (400.13 MHz, CDCl3, 25 °C): δ = 6.98 (dd, 3 J H–H = 8.2 Hz, 4 J H–H = 2.2 Hz, 1 H, CHCHCCH), 6.81 (d, 4 J H–H = 2.2 Hz, 1 H, CCHC), 6.74 (d, 3 J H–H = 8.4 Hz, 1 H, CHCHCCH), 4.75 (br s, 1 H, CH2NHCH2), 3.97 (s, 2 H, CCH 2NH), 3.66 (s, 3 H, OCH3), 3.28 (m, 2 H, CH 2NHBoc), 2.83 (t, 3 J H–H = 7.7 Hz, 2 H, CH 2CH2CO2), 2.78 (t, 3 J H–H = 5.7 Hz, 2 H, CH 2CH2NHBoc), 2.56 (t, 3 J H–H = 7.7 Hz, 2 H, CH2CH 2CO2), 1.44 (s, 9 H, C(CH 3)3). 13C{1H} NMR (100.61 MHz, CDCl3, 25 °C): δ = 173.62 (CO2CH3), 156.51 (Boc CO2 ), 156.33 (COH), 131.16 (CHCHCCH), 128.63 (CCHC), 128.47 (CHCHCCH), 122.27 (CCH2NH), 116.52 (CHCHCCH), 79.78 (BocCO2 CCH3), 52.48 (CCH2NH), 51.70 (CO2 CH3), 48.55 (CH2CH2NHBoc), 40.16 (CH2NHBoc), 36.20 (CH2 CH2CO2), 30.24 (CH2CH2CO2), 28.50 (C(CH3 )3). HRMS (ESI+, CH2Cl2/MeOH): m/z calcd for C18H29N2O5: 353.2071; found: 353.2071. HRMS (ESI-, CH2Cl2/MeOH): m/z calcd for C18H27N2O5: 351.1925; found: 351.1925. Synthesis of [Methyl 3-(3-{[(2- Aminoethyl)amino]methyl}-4-hydroxyphenyl)propanoate] Dihydrochloride (6) Compound 5 (4.90 g, 13.90 mmol) was dissolved in 4 M HCl in dioxane (20 mL). After being stirred at room temperature for 15 min, diethyl ether (40 mL) was added and the reaction mixture stirred for another 5 min. The white precipitate was filtered, washed with diethyl ether and dried under vacuum to give salt 6 (4.04 g, 12.37 mmol, 89%). Mp 190–191 °C. 1H NMR (400.13 MHz, D2O, 25 °C): δ = 7.24 (m, 2 H, CHCHCCH), 6.94 (d, 3 J H–H = 9.0 Hz, 1 H, CHCHCCH), 4.31 (s, 2 H, BnCH 2), 3.66 (s, 3 H, OCH 3), 3.45 (m, 4 H, CH 2CH 2NH), 2.89 (t, 3 J H–H = 7.3 Hz, 2 H, CH 2CH2CO2), 2.70 (t, 3 J H–H = 7.3 Hz, 2 H, CH2CH 2CO2). 13C{1H} NMR (100.61 MHz, D2O, 25 °C): δ = 176.50 (CO2CH3), 153.48 (COH), 132.68 (CHCHCCH), 131.49 (CCHC), 131.42 (CHCHCCH), 116.80 (CCH2N), 115.70 (CHCHCCH), 52.16 (CO2 CH3),47.42 (Bn CH2), 43.48 (ArCH2NHCH2), 35.45 (CH2NH2), 35.40 (CH2 CH2CO2), 29.25 (CH2CH2CO2) ppm. Anal. Calcd. (included 3.56% water; hydroscopic salt) C: 46.30, H: 6.97, N: 8.31; found C: 46.46, H: 6.88, N: 8.13. Synthesis of tert-Butyl 3-(4-Hydroxy-3-{[(2-{[2-hydroxy-5-(3-methoxy-3-oxopropyl)benzyl]amino}ethyl)imino] methyl}phenyl)propanoate (9) Dry triethylamine (1.51 g, 14.88 mmol, 3 equiv) was added to a solution of compound 6 (1.77 g, 5.46 mmol, 1.1 equiv) in absolute methanol (25 mL). To this mixture was added aldehyde 8 (1.24 g, 4.96 mmol, 1 equiv) and the reaction mixture was stirred for 1 h at room temperature. The reaction mixture was diluted with dichloromethane (50 mL) and washed with sodium bicarbonate solution (0.5 M, 50 mL). The product residue was recovered from the aqueous phase by washing four times with dichloromethane. The combined organic phase was dried over sodium sulfate, filtered and the solvent was removed under reduced pressure to give product 9 as orange solid in quantitative yield. Mp 77–79 °C. 1H NMR (400.13 MHz, CDCl3, 25 °C): δ = 8.35 (s, 1 H, CCHN), 7.15 (dd, 3 J H–H = 8.4 Hz, 4 J H–H = 2.2 Hz, 1 H, CHCHC-(CH2)2CO2Me), 7.08 (d, 4 J H–H = 2.2 Hz, 1 H, CCHC-(CH2)2CO2Me), 6.98 (dd, 3 J H–H = 8.2 Hz, 4 J H–H = 2.1 Hz, 1 H, CHCHC-(CH2)2CO2 t-Bu), 6.88 (d, 3 J H–H = 8.4 Hz, 1 H, CHCHC-(CH2)2CO2Me), 6.81 (d, 4 J H–H = 2.1 Hz, 1 H, CCHC-(CH2)2CO2 t-Bu), 6.74 (d, 3 J H–H = 8.2 Hz, 1 H, CHCHC-(CH2)2CO2 t-Bu), 3.98 (s, 2 H, BnCH 2), 3.75 (t, 3 J H–H = 5.3 Hz, 2 H, CH 2CH2NH), 3.65 (s, 3 H, OCH 3), 3.00 (t, 3 J H–H = 5.3 Hz, 2 H, CH2CH 2NH), 2.83 (m, 4 H, CH 2CH2CO2Me and CH 2CH2CO2 t-Bu), 2.56 (t, 3 J H–H = 7.8 Hz, 2 H, CH2CH 2CO2Me), 2.50 (t, 3 J H–H = 7.6 Hz, 2 H, CH2CH 2CO2 t-Bu), 1.40 (s, 9 H, C(CH3 )3). 13C{1H} NMR (100.61 MHz, CDCl3, 25 °C): δ = 173.58 (CO2CH3), 172.28 (CO2t-Bu), 166.76 (CCHN), 159.31 (C(OH)CCH(imine)), 156.53 (C(OH)CCH2), 132.82 (CHCHC-(CH2)2CO2Me), 131.17 (C-(CH2)2CO2Me and C-(CH2)2CO2 t-Bu), 131.15 (CCHC-(CH2)2CO2Me), 128.60 (CCHC-(CH2)2CO2 t-Bu), 128.45 (CHCHC-(CH2)2CO2 t-Bu), 122.17 (CCH2NH), 118.4 (CCHN), 117.24 (CHCHC-(CH2)2-CO2 t-Bu), 116.51 (CHCHC-(CH2)2CO2Me), 80.52 (CO2 CCH3), 59.31 (CH2CH2NH), 52.53 (Bn CH2), 51.67 (CO2 CH3), 48.60 (CH2 CH2NH), 37.33 (CH2 CH2CO2 t-Bu), 36.18 (CH2CH2CO2Me), 30.22 (CH2CH2CO2Me), 30.19 (CH2CH2CO2 t-Bu), 28.18 (C(CH3 )3). HRMS (ESI+, CH2Cl2/MeOH): m/z calcd for C27H37N2O6: 485.2646; found: 485.2649; C27H36N2NaO6: 507.2471; found: 507.2474. HRMS (ESI-, CH2Cl2/MeOH): m/z calcd for C27H35N2O6: 483.2501; found: 483.2500. Synthesis of tert-Butyl 3-(4-hydroxy-3-{[(2-{[2-hydroxy-5-(3-methoxy-3-oxopropyl)benzyl]amino}ethyl)amino]methyl} phenyl)propanoate (10) A solution of compound 9 (2.33 g, 4.80 mmol, 1.0 equiv) in 2,2,2-trifluoroethanol (50 mL) was cooled in an ice-bath. Sodium borohydride (0.45 g, 12 mmol, 2.5 equiv) was added in portions to this solution, and then the reaction mixture was allowed to warm to room temperature. After being stirred under inert gas atmosphere at room temperature for one hour, the reaction was quenched with water (50 mL). The product was extracted with dichloromethane and the organic layer was dried over sodium sulfate and filtered. By removal of the solvent under reduced pressure white solid 10 was obtained in quantitative yield. Mp 74–76 °C. 1H NMR (400.13 MHz, CDCl3, 25 °C): δ = 6.98 (d, 3 J H–H = 8.2 Hz, 2 H, CHCHC-(CH2)2CO2Me and CHCHC-(CH2)2CO2 t-Bu), 6.80 (br s, 2 H, CCHC-(CH2)2CO2Me and CCHC-(CH2)2CO2 t-Bu), 6.73 (dd, 3 J H–H = 8.2 Hz, 4 J H–H = 2.2 Hz, 2 H, CHCHC-(CH2)2CO2Me and CHCHC-(CH2)2CO2 t-Bu), 3.94 (s, 2 H, BnCH 2), 3.65 (s, 3 H, OCH 3), 2.81 (m, 8 H, NH(CH 2)2NH, CH 2CH2CO2Me and CH 2CH2CO2 t-Bu), 2.56 (t, 3 J H–H = 7.5 Hz, 2 H, CH2CH 2CO2Me), 2.50 (t, 3 J H–H = 7.8 Hz, 2 H, CH2CH 2CO2 t-Bu), 1.41 (s, 9 H, C(CH 3)3). 13C{1H} NMR (100.61 MHz, CDCl3, 25 °C): δ = 173.60 (CO2CH3), 172.56 (CO2 t-Bu), 156.38 (COH), 156.53 (COH), 131.49 (C-(CH2)2CO2Me), 131.20 (C-(CH2)2CO2 t-Bu), 128.70 (CCHC-(CH2)2CO2Me), 128.62 (CCHC-(CH2)2CO2 t-Bu), 128.48 (CHCHC-(CH2)2CO2Me), 128.44 (CHCHC-(CH2)2CO2 t-Bu), 122.22 (CCHC-(CH2)2CO2Me), 122.06 (CCHC-(CH2)2CO2 t-Bu), 116.46 (CHCHC-(CH2)2CO2Me), 116.34 (CHCHC-(CH2)2CO2 t-Bu), 80.37 (CO2 CCH3), 52.70 (Bn CH2), 51.68 (CO2 CH3), 47.97 (CH2 CH2NH-ArCO2Me), 47.93 (CH2 CH2NH-ArCO2 t-Bu), 37.55 (CH2 CH2CO2 t-Bu), 36.16 (CH2 CH2CO2Me), 30.38 (CH2CH2CO2 t-Bu), 30.20 (CH2CH2CO2Me), 28.18 (C(CH3 )3). HRMS (ESI+, CH2Cl2/MeOH): m/z calcd for C27H39N2O6: 487.2803; found: 487.2816. HRMS (ESI-, CH2Cl2/MeOH): m/z calcd for C27H37N2O6: 485.2657; found: 485.2657. Synthesis of 3-(3-{[(2-{[5-(2-tert-Butoxycarbonylethyl)-2-hydroxybenzyl]-tert-butoxycarbonylmethylamino}ethyl)-tert-butoxycarbonylmethylamino]methyl}-4-hydroxyphenyl)propionic Acid [HBED-CC-tris(tert-butyl ester)] Compound 10 (1.10 g, 2.26 mmol, 1 equiv) and anhydrous sodium carbonate (0.96 g, 9.06 mmol, 4 equiv) were suspended in anhydrous acetonitrile (25 mL). To this mixture was added tert-butyl 2-bromoacetate (0.93 g, 4.76 mmol, 2.1 equiv). The reaction mixture stirred for 3.5 h under reflux conditions. It was cooled to room temperature, filtered and the solvent was removed under reduced pressure. Then the residue was dissolved in methanol (15 mL), followed by dilution with water (10 mL). To this mixture was added sodium hydroxide solution (4 M, 5 mL) slowly. After being stirred for 1 h, the reaction mixture was cooled in an ice-bath and the pH was adjusted to 5–6 with HCl (0.5 M, ca. 40 mL). The crude product was extracted with ethyl acetate and the organic phase was dried over sodium sulfate, filtered, concentrated under reduced pressure and purified by flash column chromatography (ethyl acetate–hexane, 3:2) to give HBED-CC-tris(tert-butyl ester) as a colorless solid (0.53 g, 0.76 mmol, 33.4%). Note: The overall product yield was 28%. Mp 43–45 °C. 1H NMR (400.13 MHz, CDCl3, 25 °C): δ = 7.00 (sept, 2 H, CHCHC-(CH2)2CO2H and CHCHC-(CH2)2CO2 t-Bu), 6.76 (m, 4 H, CCHC-(CH2)2CO2H, CCHC-(CH2)2CO2 t-Bu, CHCHC-(CH2)2CO2H and CHCHC-(CH2)2CO2 t-Bu), 3.68 (d, 4 H, BnCH 2), 3.16 (d, 4 H, t-BuO2CH 2N), 2.84 (t, 3 J H–H = 7.6 Hz, 2 H, CH 2CH2CO2H), 2.78 (t, 3 J H–H = 7.8 Hz, 2 H, CH 2CH2CO2 t-Bu), 2.66 (s br, 4 H, N(CH 2)2N), 2.61 (t, 3 J H–H = 7.6 Hz, 2 H, CH2CH 2CO2H), 2.46 (t, 3 J H–H = 7.8 Hz, 2 H, CH2CH 2CO2t-Bu), 1.45 (d, 18 H, NCH2CO2C(CH 3)3), 1.41 (s, 9 H, CH2CH2CO2C(CH 3)3). 13C{1H} NMR (100.61 MHz, CDCl3, 25 °C): δ = 176.93 (CO2H), 172.72 (CH2CH2 CO2 t-Bu), 170.30 ([NCH2 CO2 t-Bu]-ArCO2H), 170.27 ([NCH2 CO2 t-Bu]-ArCO2 t-Bu), 155.94 (COH), 155.71 (COH), 131.52 (C-(CH2)2CO2H), 130.87 (C-(CH2)2CO2 t-Bu), 129.44 (CCHC-(CH2)2CO2H), 129.16 (CCHC-(CH2)2CO2 t-Bu), 129.11 (CHCHC-(CH2)2CO2H), 129.08 (CHCHC-(CH2)2CO2 t-Bu), 121.71 (CCHC-(CH2)2CO2H), 121.47 (CCHC-(CH2)2CO2 t-Bu), 116.63 (CHCHC-(CH2)2CO2H), 116.46 (CHCHC-(CH2)2CO2 t-Bu), 82.38 ([NCH2CO2 CCH3]-ArCO2H), 82.30 ([NCH2CO2 CCH3]-ArCO2 t-Bu), 80.49 (-(CH2)2CO2 CCH3), 58.13 (Bn C-ArCO2H), 58.06 (Bn C-ArCO2 t-Bu), 55.87 ([NCH2CO2]-ArCO2H), 55.69 ([NCH2CO2]-ArCO2 t-Bu), 50.32 (NCH2 CH2N), 37.59 (CH2 CH2CO2 t-Bu), 35.88 (CH2 CH2CO2H), 30.39 (CH2CH2CO2 t-Bu), 30.01 (CH2CH2CO2H), 28.20 (C(CH3 )). HRMS (ESI+, MeOH): m/z calcd for C38H57N2O10: 701.4008; found: 701.4027. HRMS (ESI–, MeOH): m/z calcd for C38H55N2O10: 699.3862; found: 699.3861. Anal. Calcd. (%) C: 65.12, H: 8.05, N: 4.00; found C: 65.35, H: 7.95, N: 3.91.