Synthesis of perfluorinated analogs of DOTA and NOTA: Bifunctional chelating groups with potential applications in hybrid molecular imaging

Article abstract of DOI:10.1016/j.tetlet.2013.08.035

The synthesis of novel NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) and DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) chelating groups bearing perfluorinated appendages is described. DOTA and NOTA groups are used in the production

Full text of DOI:10.1016/j.tetlet.2013.08.035

Tetrahedron Letters 54 (2013) 5755–5757
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of perfluorinated analogs of DOTA and NOTA: bifunctional
chelating groups with potential applications in hybrid molecular
imaging
Raphaël Hoareau a, Peter J. H. Scott a,b,
⇑
a Department of Radiology, University of Michigan, Ann Arbor, MI, USA
b The Interdepartmental Program in Medicinal Chemistry, University of Michigan, Ann Arbor, MI, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of novel NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) and DOTA (1,4,7,10-tetraaza-
cyclododecane-1,4,7,10-tetraacetic acid) chelating groups bearing perfluorinated appendages is
described. DOTA and NOTA groups are used in the production of radiopharmaceutical agents for PET
and SPECT imaging (by chelation of radioactive metal ions), as well as MRI contrast agents (by chelation
of lanthanide Ln3+ ions). The novel perfluorinated variants disclosed herein will enhance the synthesis
and purification of such agents, as they are compatible with fluorous purification strategies. Moreover,
the perfluorous tag is anticipated to be detectable by 19F-MRI, suggesting future applications in hybrid
molecular imaging such as PET–MRI.
Received 12 July 2013
Revised 7 August 2013
Accepted 10 August 2013
Available online 16 August 2013
Keywords:
PET–MRI
Multimodality imaging
Chelation
Ó 2013 Elsevier Ltd. All rights reserved.
Radioactive metal ions
Fluorous chemistry
The synthesis of novel NOTA (1,4,7-triazacyclononane-1,4,7-tri-
acetic acid) and DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-
tetraacetic acid) compounds is an important area of research
because of their widespread use as chelating groups in, for exam-
ple, molecular imaging.1 Such chelating groups can be attached
to bioactive molecules and then used to complex lanthanide ions
such as Gd3+ to generate contrast agents for magnetic resonance
imaging (MRI).2 Alternatively, if radioactive metal ions are com-
plexed then radiopharmaceuticals for positron emission tomogra-
phy (PET) or single photon emission computed tomography
(SPECT) imaging are readily accessible.3 In the latter case, the
short-lived radionuclides typically employed in diagnostic PET
and SPECT imaging studies require rapid and efficient methods
for the synthesis and purification of radiopharmaceuticals. With
this goal in mind, we were interested in exploiting fluorous tech-
niques in our radiopharmaceutical manufacturing program, and
the objective of this research was to synthesize chelating groups
bearing perfluorinated tags. However, as there is growing interest
in the use of fluorinated compounds in 19F MRI and magnetic res-
onance spectroscopy (MRS),4 introduction of the proposed perflu-
orous tags could serve a dual purpose as they should also be
detectable by 19F MRI. 19F is the only stable isotope of fluorine
and therefore 100% naturally abundant. Whilst fluorine occurs in
the body, it is localized in bones and teeth and has been shown
to have a very short T2 relaxation time.5 The combination of these
factors means that exogenously administered fluorine containing
compounds can be seen clearly in [19F]MRI scans and there is no
significant interference from background fluorine signals. There-
fore we believe that bioactive molecules tagged with perfluori-
nated chelating groups have the potential to function as a new
class of bimodal agents for PET/19F-MRI or SPECT/19F-MRI depend-
ing upon the choice of radioactive metal ion (Fig. 1).6 In this Letter
we report the initial chemical synthesis of NOTA and DOTA analogs
functionalized with perfluorooctane side chains (C8F17– = Rf), and
⇑
Corresponding author. Tel.: +1 (734)615 1756; fax: +1 (734)615 2557.
E-mail address: pjhscott@umich.edu (P.J.H. Scott).
Figure 1. Concept of perfluorinated-chelating groups.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.08.035

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R. Hoareau, P. J. H. Scott / Tetrahedron Letters 54 (2013) 5755–5757
Scheme 1. Reagents and conditions: (a) (COCl)2, DMF (Cat.), DCM; (b) tBuOH, 2,6-lutidine, DCM, ꢀ40 °C; (c) (1) KOH, THF; (2) BnBr, DMF; (d) (1). MsCl, NEt3, DCM, 0 °C ꢀ rt,
2 h; 2. Cyclen (2 equiv), DCM, rt, ꢀ50 °C; (e) tButyl 2-bromoacetate (3 equiv), K2CO3, MeCN, rt; (f) H2, Pd/C, MeOH (20% from 1); (g) (1). C8F17(CH2)3NH2, HATU, DIPEA, DMF, rt,
24 h; (2) F-SPE (88%).
introduction of a perfluorous group along with a point of attach-
ment for a bioactive molecule. In the first approach, chelating
groups were prepared initially and the perfluorous functionality
was introduced as the last step using commercially available 3-
(perfluorooctyl)propylamine (4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-
heptadecafluoroundecylamine, Sigma–Aldrich) (Scheme 1). Thus,
R-DOTA-AMP (2) was prepared in 6-steps and 20% overall yield
from (S)-5-oxotetrahydrofuran-2-carboxylic acid (1) as previously
described by Levy et al.7 With gram quantities of R-DOTA-AMP
(2) in hand, we initially attempted to couple it with 3-(perfluoro-
Scheme 2. Reagents and conditions: (a) (1). C8F17(CH2)3NH2, HATU, DIPEA, DMF, rt,
24 h; (2). F-SPE (92%).
octyl)propylamine under standard peptide coupling conditions
(N,N0-dicyclohexylcarbodiimide or 1-ethyl-3-(3-dimethylamino-
propyl)carbodiimide), but the reaction proved sluggish and difficult
to purify. A much faster and cleaner coupling reaction proceeded
when
demonstrate their compatibility with fluorous chemistry. Evalua-
tion of bioactive molecules tagged with the new chelating groups
as bimodal agents for hybrid molecular imaging is ongoing, and
will be reported in due course.
Potential chelating groups must have a site for attaching a bioac-
tive molecule as well as a site for attachment of the perfluorous tag
(Fig. 1). Initial investigations focused upon two strategies for the
O-(7-azabenzotriazol-1-yl)-N,N,N0,N0-tetramethyluronium
hexafluorophosphate (HATU) was used as the activating agent.
Purification of the crude reaction mixture by fluorous solid-phase
extraction (F-SPE) provided the perfluorinated analog of DOTA (3)
in 88% yield.8 In the case of the corresponding NOTA-analog,
commercially available starting material 4 (ChemTek) was treated
Scheme 3. Reagents and conditions: (a) K2CO3, MeCN, 50 °C, overnight (48%); (b) BF3ꢁEt2O, MeCN, 50 °C, 22 h (65%); (c)
a-bromo ester, K2CO3, MeCN, rt, overnight (67%,
R = tBu; 51%, R = Bn); (d) bromo acetic acid, NaOH, THF/H2O, rt, overnight (43%); (e) 1M NaOH (aq), THF, 55 °C, 6 h (100%).
Scheme 4. Reagents and conditions: (a) K2CO3, MeCN, 50 °C, overnight (60%).

R. Hoareau, P. J. H. Scott / Tetrahedron Letters 54 (2013) 5755–5757
5757
in an analogous fashion to yield a perfluorinated analog of NOTA (5)
References and notes
in 92% yield (Scheme 2).
1. For recent reviews see: (a) León-Rodríguez, L. M.; Kovacs, Z. Bioconjugate Chem.
2008, 19, 391–402; (b) Brücher, E.; Baranyai, Z. S.; Tircsó, G. Y. In Biomedical
Imaging: the Chemistry of Labels, Probes and Contrast Agents; Braddock, M., Ed.;
Royal Society of Chemistry: Cambridge, 2013.
2. For recent reviews see: (a) Caravan, P.; Ellison, J. J.; McMurry, T. J.; Lauffer, R. B.
Chem. Rev. 1999, 99, 2293–2352; (b) Yang, C.-T.; Chuang, K.-H. Med Chem.
Commun. 2012, 552–565.
3. For recent reviews see: (a) Hubers, D.; Scott, P. J. H. In Biomedical Imaging: the
Chemistry of Labels, Probes and Contrast Agents; Braddock, M., Ed.; Royal Society
of Chemistry: Cambridge, 2013; (b) Brechbiel, M. W. Q. J. Nucl. Med. Mol.
Imaging 2008, 52, 166–173; (c) Weiner, R. E.; Thakur, M. L. In Handbook of
Radiopharmaceuticals: Radiochemistry and Applications by M; Welch, J.,
Redvanly, C. S., Eds.; Wiley: Chichester, 2003.
4. For recent reviews see: (a) Yu, J. X.; Kodibagkar, V. D.; Cui, W.; Mason, R. P. Curr.
Med. Chem. 2005, 12, 819–848; (b) Wolf, W.; Presant, C. A.; Waluch, V. Drug
Delivery Rev. 2000, 41, 55–74.
5. Code, R. F.; Harrison, J. E.; McNeill, K. G.; Szyjkowski, M. Magn. Reson. Med.
1990, 13, 358–369.
6. (a) Frullano, L.; Catana, C.; Benner, T.; Sherry, A. D.; Caravan, P. Angew. Chem.,
Int. Ed. 2010, 49, 2382–2384; For a recent review see: (b) Lee, S.; Chen, X. Mol.
Imag. 2009, 8, 87–100.
7. Levy, S. G.; Jacques, V.; Zhou, K. L.; Kalogeropoulos, S.; Schumacher, K.; Amedio,
J. C.; Scherer, J. E.; Witowski, S. R.; Lombardy, R.; Koppetsch, K. Org. Process Res.
Dev. 2009, 13, 535–542.
8. Carboxylic acid 2 (98 mg, 0.14 mmol, 1.2 equiv) and HATU (55 mg, 0.14 mmol,
1.2 equiv) were dissolved in DMF (1.5 mL). To the homogenous yellow solution
The second approach utilized perfluorous epoxide
7 to
introduce the perfluorous tag at the start of the synthesis. Thus,
treatment of commercially available cyclen (6) with perfluorinated
epoxide 7 in the presence of potassium carbonate or boron
trifluoride etherate yielded DOTA intermediate 8 in 48% or 65%
yield, respectively, (Scheme 3).9 DOTA derivative 8 was allowed
to react with a-bromo esters to generate the tert-butyl- and ben-
zyl-protected perfluorinated analogs of DOTA 9a and 9b in 67%
and 51% yield, respectively. Alternatively, 8 could be coupled
directly with bromo acetic acid to provide the fully deprotected
chelating group 10 in 43% yield.10 These compounds were all frag-
ile white powders most readily purified by fluorous solid-phase
extraction (F-SPE) and isolated by lyophilization. Treating NOTA
11 with epoxide 7 also yielded the expected perfluorinated-NOTA
intermediate 12 in 60% yield. Surprisingly however, analogous
coupling reactions between NOTA-intermediate 12 and
a-bromo
esters did not yield the desired products, although there is no obvi-
ous explanation for the decomposition products formed instead
(Scheme 4).
Finally, with a range of protected perfluorinated-chelating
groups in hand, conditions were explored for removal of the ester
protecting groups as the final step. Initial efforts focused upon
acid-mediated hydrolysis but this was quickly found to be ineffec-
tive and resulted in elimination of the perfluorous tag (and other
decomposition products) in the case of trifluoroacetic acid (TFA),
or recovery of starting material in the case of HCl. Therefore atten-
tion was turned to ester saponification using sodium hydroxide
which, in the case of protected-intermediates 9a and 9b, provided
perfluorinated-DOTA analog 10 in 67–100% yield (Scheme 3).
In conclusion, syntheses of novel NOTA and DOTA chelating
groups bearing perfluorinated appendages have been developed.
All compounds were characterized by NMR spectroscopy, elemen-
tal analysis, and/or high-resolution mass spectrometry, and gram
quantities are available for evaluation in hybrid molecular imaging
applications.
was
added
DIPEA
(37
l
L,
0.21 mmol,
1.8 equiv)
and
3-
(perfluorooctyl)propylamine (57 mg, 0.12 mmol). The reaction was stirred
under nitrogen at rt for 24 h. After this time the reaction mixture was poured
into a mixture of CH2Cl2/H2O (50/50 mL). The organic layer was washed with
brine, dried (MgSO4) and concentrated. Perfluorous SPE (2 g-cartridge) gave
compound 3 as a light yellow amorphous solid (143 mg, 88%). 1H NMR (CDCl3,
400 MHz) @ in ppm: 6.73 (1H, t, J = 8.0 Hz), 3.43–3.26 (7H, m), 2.80–1.81 (26H,
m), 1.45 (36H, s); HRMS: M+Na+ measured 1182.4810; calculated 1182.4794.
9.
A mixture of cyclen 6 (364 mg, 2.11 mmol, 2.1 equiv), K2CO3 (708 mg,
5.12 mmol, 5.0 equiv) and CH3CN (2 mL) was heated to 50 °C. To this was
added a solution of perfluorous epoxide 7 (488 mg, 1.02 mmol) in CH3CN, and
the reaction was stirred overnight at 50 °C. After this time the solvent was
evaporated under vacuum, and the resulting yellow residue was dissolved in
DMF/H2O (9/1 v/v, 2 mL) and purified by Fluorous-SPE (2 g, elution with
acetone 50 mL) to give 8 as a white powder (318 mg, 48%). 1H NMR (CDCl3,
400 MHz) @ in ppm: 4.21–4.14 (m, 1H), 3.56–2.50 (m, overlay, 20H), 2.38–2.07
(m, 4H); 1HRMS: M+H+ measured 649.1816; calculated 649.1835.
10. Perfluorinated cyclen
8 (100 mg, 0.15 mmol), 2-bromoacetic acid (85 mg,
0.61 mmol, 4.1 equiv) and sodium hydroxide (57 mg, 1.42 mmol, 9.2 equiv)
were dissolved in H2O/THF (1/3 v/v, 8 mL) and stirred for 20 h at rt. The
solution was diluted with aq. ammonium formate (20 mM, 50 mL) and passed
through a fluorous SPE cartridge (2 g). The cartridge was washed twice with aq
ammonium formate (20 mM, 12 mL). Compound 10 was then eluted off with
acetone (40 mL) and methanol (40 mL). After concentration under vacuum, the
residue was dissolved with water (70 mL), frozen and lyophilized to give 10 as
a white fluffy powder (61 mg, 43%). 1H NMR (CDCl3, 400 MHz) @ in ppm: 4.60
(s, 8H), 3.80–3.30 (m, 1H), 3.06–2.60 (m, 10H), 2.65–1.90 (m, 12 H); HRMS:
M+H+ measured 823.1984; calculated 823.1994.
Acknowledgments
Financial support of this research from the Office of Biological
and Environmental Research (BER) of the Office of Science (SC),
at the US Department of Energy (DE-FG02-08ER64645), is
gratefully acknowledged.