Highly efficient synthesis of polyfluorinated dendrons suitable for click chemistry

Abstract

A highly efficient convergent synthesis of C_2v-symmetric dendrons is presented. The generation build-up was achieved by copper(I)-catalyzed azide–alkyne cycloadditions. The resulting G2-dendron consists of 72 magnetically equivalent fluorine atoms and displays a single sharp resonance in its ¹⁹F NMR spectrum.

---

Nowadays, magnetic resonance imaging (MRI) is an established and very versatile, non-invasive, non-destructive imaging technique for diagnosis in human medicine.¹ MRI provides high resolution between soft tissues (few millimeters for human scanners and up to 50 μm for ultra high-field animal scanners), but without the need for ionizing and thus damaging radiation.² Besides ¹H, other NMR active heteronuclei could be used as well for MRI, which allows for adding a second “color” to anatomical scans obtained by classical ¹H MRI.³ However, most nuclei are unfit for MRI applications, because of their inherent physical, chemical and biological properties. But ¹⁹F has been shown to be exceptionally suitable for MRI as it is the second most sensitive, stable nucleus for MR spectroscopy (directly after ¹H) and has 100% natural abundance.^4,5 Furthermore, the resonance frequency of ¹⁹F differs by only 6% from the frequency of ¹H, which allows creating ¹⁹F MR images on common ¹H MRI devices.³

The conventional ¹H MRI makes use of the nuclear spin of the ubiquitous water molecules within organic tissues to enable visualization. On the contrary, fluorine is virtually absent in the biosphere.⁵ The external addition of a suitable, fluorinated compound (called probe or tracer) is thus mandatory for ¹⁹F MRI.⁶ Only teeth and the bone matrix contain considerable amounts of endogenous fluorine. However, immobilized fluorine exhibits a very short spin–spin relaxation time and is therefore not detectable by ¹⁹F MRI.⁵ Consequently, ¹⁹F MR images of biological samples do not have an intrinsic background signal, which results in an extremely high contrast.⁵ In summary, the information obtained from ¹H and ¹⁹F MRI is complementary: ¹H MRI collects morphological data (anatomy) and ¹⁹F MRI enables tracking of a fluorinated, exogenous compound (e.g. drugs).⁷

Suitable probes for ¹⁹F MRI should preferably fulfill several requirements. Of course, the probe must be chemically inert and biologically compatible.³ Most organofluorine compounds easily match these requirements because of the extraordinary strength of the C–F bond.⁸ The probe should also have a large number of fluorine atoms to provide a high spin density.³ This is often accomplished by using perfluorinated compounds, like perfluorooctyl bromide (PFOB).⁹ But perfluorinated molecules typically have the drawback of displaying multiple, split ¹⁹F signals, which reduces signal intensity and leads to image artefacts.¹⁰ This problem can be overcome by applying ¹⁹F MRI probes with high symmetry, like perfluoro[15]crown-5 ether (PF15C5).¹¹ Finally, it would be advantageous, if the probe is water-soluble to allow for easy application without the need for complex formulation.¹² This is challenging since highly fluorinated molecules are in general very hydrophobic.¹³ As a consequence, many probes have to be applied as stable emulsions, e.g. by mixing with a surfactant like lecithin.^9,11 In some cases, amphiphilic polyfluorinated polymers have also been shown to self-assemble into micelles in aqueous solution, which allowed for direct application without the need for stabilizing additives.¹⁴ Nevertheless, a more elegant approach would be to acquire the requested water solubility via conjugation with hydrophilic compounds.¹² Hence, ¹⁹F MRI probes that follow a rational design, should also provide a conjugation site for tuning their water solubility.

Inspired by the spherically symmetric ¹⁹F MRI tracers from Yu et al.^12,15 and owing our own interest in fluorine chemistry,¹⁶ we set out to find an easy access to dendrimeric molecules, which can meet the aforementioned requirements. Herein, we present an efficient three-step procedure for the preparation of C_2v-symmetric polyfluorinated dendrons by applying the copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC),¹⁷ which is a common reaction for the preparation of various dendrimers and MRI probes,¹⁸ as a key reaction.

Since perfluoro-tert-butanol already has nine magnetically equivalent fluorine atoms and displays neither interfering ¹H–¹⁹F nor ¹⁹F–¹⁹F coupling,^15a this molecule was chosen as a fluorinated precursor. Furthermore, Horváth et al. have shown that the sodium salt of this perfluorinated alcohol reacts with various benzyl bromides via nucleophilic substitution in moderate to good yields.¹⁹ After a twofold substitution of dibromide 1-Br²⁰ the polyfluorinated aryl iodide 1-OC(CF₃)₃ was accessible in excellent yield (Scheme 1). Next, silylalkyne 2-SiMe₃ was synthesized via Sonogashira coupling²¹ and upon subsequent deprotection with K₂CO₃ in methanol terminal alkyne 2-H was obtained. Notably, both silane 2-SiMe₃ and alkyne 2-H were poorly soluble in the reaction mixture, although methanol is known to be a good fluorophilic solvent.¹³ However, the deprotection proceeded quantitatively and alkyne 2-H could be isolated by simple filtration without the need for further purification. The first-generation dendron (G1-dendron) 3 was build up by twofold CuAAC of alkyne 2-H with diazide 1-N₃.²⁰ The three-step generation build-up (Sonogashira reaction, deprotection, CuAAC) for the G1-dendron 3 had a total yield of 82% and could be done easily on a gram scale.

Scheme 1 Synthesis of G1-dendron 3. Reagents and conditions: (i) NaOC(CF₃)₃, DMF, rt, 18 h, 88%; (ii) trimethylsilylacetylene, Pd(PPh₃)₂Cl₂, CuI, NEt₃, THF, 45 °C, 4 h, 89%; (iii) K₂CO₃, MeOH, rt, 1.5 day, >99%; (iv) 1-N₃, Cu(CH₃CN)₄PF₆, 2,6-lutidine, THF, 40 °C, 2 days, 92%.

During this sequence the ¹⁹F NMR spectrum of every compound showed just one sharp singlet. This proved that all fluorine atoms are indeed magnetically equivalent. Moreover, iodide 1-OC(CF₃)₃ and alkyne 2-H could be crystallized from perfluorohexane or cyclohexane, respectively, to obtain single crystals suitable for X-ray crystallography. The molecular structures of those perfluoro-tert-butyl ethers are depicted in Fig. 1.

Fig. 1 Molecular structure of iodide 1-OC(CF₃)₃ (left side) and alkyne 2-H (right side) with displacement ellipsoids drawn at 50% probability level.²²

The generation build-up sequence was repeated once more to afford the second-generation dendron 5 (Scheme 2). Expectedly, the overall yield for this second cycle decreased slightly (67%), which can be explained by increased steric hindrance.^15b Nevertheless the yields for each step were still quite high (77–94%). The 72 fluorine atoms of dendron 5 resulted in just one sharp singlet in the respective ¹⁹F NMR spectrum. Hence, dendron 5 has considerably more magnetically equivalent fluorine atoms than typical, commercially available ¹⁹F MRI probes (e.g. PF15C5).

Scheme 2 Synthesis of G2-dendron 5. Reagents and conditions: (i) trimethylsilylacetylene, Pd(PPh₃)₂Cl₂, CuI, NEt₃, THF, 45 °C, overnight, 94%; (ii) K₂CO₃, MeOH, rt, 1.5 day, 93%; (iii) 1-N₃, Cu(CH₃CN)₄PF₆, 2,6-lutidine, THF, 40 °C, 2 days, 77%.

To provide dendron 5 with an appropriate conjugation site, the sequence of Sonogashira coupling with trimethylsilylacetylene and subsequent desilylation with K₂CO₃ was repeated one last time (Scheme 3). Alkyne 6-H was obtained in 37% overall yield after 9 steps from dibromide 1-Br. Finally, we tried to convert dendron 6-H into a more hydrophilic form via conjugation with an azide-functionalized undecaethylene glycol derivative. This strategy of tethering unpolar molecules to hydrophilic ethylene glycols is commonly used in medicine to improve the water solubility.²³ In addition, poly(ethylene glycol) moieties have also been used to improve the biocompatibility of highly fluorinated hyperbranched polymers.²⁴ However, conjugate 7 was still too hydrophobic to dissolve in water. But overall this is just a minor drawback as ¹⁹F MRI probes can also be applied in emulsified form.^9,11,14

Scheme 3 Synthesis of conjugate 7. Reagents and conditions: (i) trimethylsilylacetylene, Pd(PPh₃)₂Cl₂, CuI, NEt₃, THF, 45 °C, overnight, 82%; (ii) K₂CO₃, MeOH, rt, 1.5 day, 92%; (iii) O-(2-azidoethyl)-O′-methylundecaethylene glycol, Cu(CH₃CN)₄PF₆, 2,6-lutidine, THF, rt, 2.5 days, 31%.

The ¹⁹F NMR spectrum of conjugate 7 expectedly showed a sharp singlet (Fig. 2). At a magnetic flux density of 9.4 T, the resonance signal had a narrow full width at half maximum of 7 Hz. This clearly demonstrates the potential applicability of the synthesized dendron as ¹⁹F MRI probe.

Fig. 2 ¹⁹F NMR spectrum (377 MHz) of dendron 7 in CDCl₃ (trifluoroacetic acid was used as internal standard).

In summary, conjugate 7 could overcome most of the challenges that should be met by appropriate ¹⁹F MRI probes. It exhibits 72 fluorine atoms and has just one single, sharp resonance in the ¹⁹F NMR spectrum. As a result conjugate 7 features a high spin density without the disadvantage of causing chemical shift artefacts. The only remaining challenge would be the lack of water solubility. To address this issue, dendron 6-H could be conjugated with other more hydrophilic (bio)molecules.²⁵ Hence, alkyne 6-H is a versatile precursor for the generation of various ¹⁹F MRI agents.

Conclusions

We described a facile and highly efficient procedure for the preparation of C_2v-symmetric dendrons that are well suited for ¹⁹F MRI. This dendron synthesis is comprised of three steps that are repeated iteratively: Sonogashira coupling, alkyne deprotection and CuAAC for the generation build-up. After 9 steps the second-generation dendron 6-H was obtained in excellent overall yield. This molecule displays 72 magnetically equivalent fluorine atoms and exhibits a terminal alkyne moiety, which serves as a tether. The latter is an important feature, because it could be used for adjusting the water solubility of a potential ¹⁹F MRI probe via conjugation with hydrophilic molecules. Since conjugate 7 is still too unpolar to be water-soluble, other hydrophilic compounds will be tested in the near future to create polyfluorinated probes that are even more suitable for ¹⁹F MRI.

Acknowledgements

This work was supported by the Chemical Industry Fund and the Karlsruhe School of Optics & Photonics (KSOP).

Notes and references

1. (a) A. M. Blamire, Br. J. Radiol., 2008, 81, 601; (b) N. Mistry, A. Rosenkrantz, S. Roys, J. Papadimtiriou, K. Siddiqui, M. Naslund, J. Borin, W. D'Souza and R. P. Gullapalli, Mol. Cell. Pharmacol., 2013, 5, 5; (c) D. Bartusik, D. Aebisher and B. Tomanek, J. Mol. Imaging Dyn., 2013, 2, 112.
2. J. C. Knight, P. G. Edwards and S. J. Paisey, RSC Adv., 2011, 1, 1415.
3. M. Srinivas, A. Heerschap, E. T. Ahrens, C. G. Figdor and I. J. M. de Vries, Trends Biotechnol., 2010, 27, 363.
4. (a) Y. B. Yu, Wiley Interdiscip. Rev.: Nanomed. Nanobiotechnol., 2013, 5, 646; (b) Z. X. Jiang, Y. Feng and Y. B. Yu, Chem. Commun., 2011, 47, 7233.
5. (a) J. Ruiz-Cabello, B. P. Barnett, P. A. Bottomley and J. W. M. Bulte, NMR Biomed., 2011, 24, 114; (b) K. K. J. Chan and D. O'Hagan, Methods Enzymol., 2012, 516, 219.
6. M. Srinivas, P. Boehm-Sturm, C. G. Figdor, I. J. de Vries and M. Hoehn, Biomaterials, 2012, 33, 8830.
7. (a) M. Higuchi, N. Iwata, Y. Matsuba, K. Sato, K. Sasamoto and T. C. Saido, Nat. Neurosci., 2008, 8, 527; (b) M. Srinivas, P. A. Morel, L. A. Ernst, D. H. Laidlaw and E. T. Ahrens, Magn. Reson. Med., 2007, 58, 725; (c) Y. B. Yu and Z.-X. Jiang, J. Pharm. Drug Deliv. Res., 2012, 1, 1.
8. (a) D. O'Hagan, Chem. Soc. Rev., 2008, 37, 308; (b) S. F. Flaim, Artif. Cells, Blood Substitutes, Immobilization Biotechnol., 1994, 22, 1043.
9. (a) D. M. Freeman, H. H. Muller, R. E. Hurd and S. W. Young, Magn. Reson. Imaging, 1988, 6, 61; (b) C. Giraudeau, B. Djemaï, M. A. Ghaly, F. Boumezbeur, S. Mériaux, P. Robert, M. Port, C. Robic, D. L. Bihan, F. Lethimonnier and J. Valette, NMR Biomed., 2012, 25, 654; (c) O. Diou, N. Tsapis, C. Giraudeau, J. Valette, C. Gueutin, F. Bourasset, S. Zanna, C. Vauthier and E. Fattal, Biomaterials, 2012, 33, 5593.
10. (a) H. K. Lee, O. Nalcioglu and R. B. Buxton, Magn. Reson. Med., 1991, 21, 21; (b) H. K. Lee, O. Nalcioglu and R. B. Buxton, Magn. Reson. Med., 1992, 23, 254.
11. (a) J. M. Janjic and E. T. Ahrens, Wiley Interdiscip. Rev.: Nanomed. Nanobiotechnol., 2009, 1, 492; (b) R. B. Heeswijk, Y. Pilloud, U. Flögel, J. Schwitter and M. Stuber, PLoS One, 2012, 7, e42236; (c) M. Srinivas, L. J. Cruz, F. Bonetto, A. Heerschap, C. G. Figdor and I. J. M. de Vries, Biomaterials, 2010, 31, 7070.
12. (a) Z.-X. Jiang, X. Liu, E.-K. Jeong and Y. B. Yu, Angew. Chem., 2009, 121, 4849 (Angew. Chem., Int. Ed., 2009, 48, 4755); (b) M. B. Taraban, L. Yu, Y. Feng, E. V. Jouravleva, M. A. Anisimov, Z.-X. Jiang and Y. B. Yu, RSC Adv., 2014, 4, 54565.
13. I. T. Horváth, D. P. Curran and J. A. Gladysz, Handbook of Fluorous Chemistry, Wiley-VCH, Weinheim, 2004.
14. W. Du, A. M. Nyström, L. Zhang, K. T. Powell, Y. Li, C. Cheng, S. A. Wickline and K. L. Wooley, Biomacromolecules, 2008, 9, 2826.
15. (a) Z.-X. Jiang and Y. B. Yu, Tetrahedron, 2007, 63, 3982; (b) X. Yue, M. B. Taraban, L. L. Hyland and Y. B. Yu, J. Org. Chem., 2012, 77, 8879.
16. (a) M. S. Wiehn, S. D. Lindell and S. Bräse, J. Comb. Chem., 2009, 11, 960; (b) M. S. Wiehn, D. Fürniss and S. Bräse, J. Comb. Chem., 2009, 11, 982; (c) M. Döbele, S. Vanderheiden, N. Jung and S. Bräse, Angew. Chem., 2010, 122, 6122 (Angew. Chem., Int. Ed., 2010, 49, 5986); (d) M. Döbele, M. S. Wiehn and S. Bräse, Angew. Chem., 2011, 123, 11737 (Angew. Chem., Int. Ed., 2011, 50, 11533); (e) A. Hafner and S. Bräse, Adv. Synth. Catal., 2011, 353, 3044; (f) A. Hafner and S. Bräse, Angew. Chem., 2012, 124, 3773 (Angew. Chem., Int. Ed., 2012, 51, 3713); (g) A. Hafner and S. Bräse, Adv. Synth. Catal., 2013, 355, 996; (h) A. Hafner, A. Bihlmeier, M. Nieger, W. Klopper and S. Bräse, J. Org. Chem., 2013, 78, 7938; (i) A. Hafner, T. J. Feuerstein and S. Bräse, Org. Lett., 2013, 15, 3468; (j) A. Hafner, T. S. Fischer and S. Bräse, Eur. J. Org. Chem., 2013, 7996; (k) D. K. Kölmel, B. Rudat, D. M. Braun, C. Bednarek, U. Schepers and S. Bräse, Org. Biomol. Chem., 2013, 11, 3954; (l) S. Zhong, A. Hafner, C. Hussal, M. Nieger and S. Bräse, RSC Adv., 2015, 5, 6255.
17. For a review: (a) E. F. V. Scriven and K. Turnbull, Chem. Rev., 1988, 88, 297; (b) S. Bräse, C. Gil, K. Knepper and V. Zimmermann, Angew. Chem., 2005, 117, 5320 (Angew. Chem., Int. Ed., 2005, 44, 5188); (c) S. Bräse and K. Banert, Organic Azides – Syntheses and Applications, Wiley, Chichester, 2009; (d) J. E. Hein and V. V. Fokin, Chem. Soc. Rev., 2010, 39, 1302; (e) B. T. Worrell, J. A. Malik and V. V. Fokin, Science, 2013, 340, 457; (f) R. Berg and B. F. Straub, Beilstein J. Org. Chem., 2013, 9, 2715; (g) N. V. Sokolova and V. G. Nenajdenko, RSC Adv., 2013, 3, 16212; (h) D. K. Kölmel, N. Jung and S. Bräse, Aust. J. Chem., 2014, 67, 328.
18. (a) G. Franc and A. Kakkar, Chem. Commun., 2008, 5267; (b) M. Suchý, R. Bartha and R. H. E. Hudson, RSC Adv., 2103, 3, 3249; (c) A. Thirunarayanan, S. Raja, G. Mohanraj and P. Rajakumar, RSC Adv., 2014, 4, 41778.
19. X. Zhao, W. Y. Ng, K.-C. Lau, A. E. C. Collis and I. T. Horváth, Phys. Chem. Chem. Phys., 2012, 14, 3909.
20. B. Sookcharoenpinyo, E. Klein, Y. Ferrand, D. B. Walker, P. R. Brotherhood, C. Ke, M. P. Crump and A. P. Davis, Angew. Chem., 2012, 124, 4664 (Angew. Chem., Int. Ed., 2012, 51, 4586).
21. For a review: (a) R. Chinchilla and C. Nájera, Chem. Soc. Rev., 2011, 40, 5084; (b) M. Schilz and H. Plenio, J. Org. Chem., 2012, 77, 2798; (c) S. Bräse, in Organometallics in Synthesis, Third Manual, ed. M. Schlosser, Wiley, Hoboken, 2013, p. 777.
22. ESI.†.
23. G. Pasut and F. M. Veronese, Adv. Drug Delivery Rev., 2009, 61, 1177.
24. (a) K. J. Thurecht, I. Blakey, H. Peng, O. Squires, S. Hsu, C. Alexander and A. K. Whittaker, J. Am. Chem. Soc., 2010, 132, 5336; (b) B. E. Rolfe, I. Blakey, O. Squires, H. Peng, N. R. B. Boase, C. Alexander, P. G. Parsons, G. M. Boyle, A. K. Whittaker and K. J. Thurecht, J. Am. Chem. Soc., 2014, 136, 2413.
25. (a) S. B. L. Vollrath, D. Fürniss, U. Schepers and S. Bräse, Org. Biomol. Chem., 2013, 11, 8197; (b) D. Fürniss, T. Mack, F. Hahn, S. B. L. Vollrath, K. Koroniak, U. Schepers and S. Bräse, Beilstein J. Org. Chem., 2013, 9, 56; (c) D. K. Kölmel, A. Hörner, F. Rönicke, M. Nieger, U. Schepers and S. Bräse, Eur. J. Med. Chem., 2014, 79, 231; (d) D. Althuon, F. Rönicke, D. Fürniss, J. Quan, I. Wellhöfer, N. Jung, U. Schepers and S. Bräse, Org. Biomol. Chem., 2015, 13, 4226.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, analytical data, ¹H and ¹³C NMR spectra of the prepared compounds. CCDC 981372 (1-OC(CF₃)₃ and 981373 (2-H). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra02804c

---