A mild method for regioselective labeling of aromatics with radioactive iodine

Article abstract of DOI:10.1002/ejoc.201300419

A novel technique to label ortho-, meta-, and para-trimethylsilyl- substituted aryl substituents with radioactive iodide is described. The method takes advantage of the ipso-directing and activating properties of trimethylsilyl substituents on the arenes.

Full text of DOI:10.1002/ejoc.201300419

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300419
A Mild Method for Regioselective Labeling of Aromatics with Radioactive
Iodine
Mads H. Rønnest,[a,b] Felix Nissen,[c] Palle J. Pedersen,[a] Thomas O. Larsen,[b]
Walter Mier,[c] and Mads H. Clausen*[a]
Keywords: Isotopic labeling / Radiochemistry / Medicinal chemistry / Iodine / Silanes / Thallium
A novel technique to label ortho-, meta-, and para-trimethyl-
silyl-substituted aryl substituents with radioactive iodide is
described. The method takes advantage of the ipso-directing
and activating properties of trimethylsilyl substituents on the
arenes. The method was demonstrated on a griseofulvin ana-
logue with promising anticancer properties and on lidocaine,
a widely used local anesthetic drug. Treatment of a trimethyl-
silyl precursor with Tl(OCOCF3)3 followed by Na125I consist-
ently afforded radioactive purities over 95% in all cases.
more, many iodination methods are only applicable to acti-
vated aromatics, such as tyrosine in labeling of peptides and
proteins.[5]
Introduction
Methods for labeling of organic molecules with radio-
active isotopes of iodine, such as 123I, 125I, and 131I, are
important for medical applications. Iodinated drug candi-
dates are used for screening antibodies in radioimmuno-
assays (RIAs)[1] and in preclinical ADME (absorption, dis-
tribution, metabolism, and excretion) studies.[2] Owing to
emission of gamma radiation, iodinated radiopharmaceuti-
cals are also used in scintigraphic imaging for disease diag-
nostics,[3] for example, in the localization of tumors by use
of meta-iodobenzylguanidine (MIBG) labeled with 123I.[4]
The iodination of aromatics can be achieved by different
methods,[5] and the choice of labeling method depends very
much on the substrate. Many of the most widely used label-
ing techniques involve the use of oxidizing agents such as
chloramine T,[6] iodogen,[7] and lactoperoxidase;[8] however,
these methods suffer from low selectivity and are restricted
to activated aromatics. The regioselectivity (labeling posi-
tion) can be hard to control and to some extent so can the
degree of labeling (the number of iodine atoms introduced).
Depending on the substrate, this can result in the formation
of numerous side products,[5b] which then have to be puri-
fied, often by tedious chromatographic methods.[9] Further-
Therefore, there is a need for a broader collection of se-
lective iodination techniques that can be performed under
mild conditions. Our interest in iodination techniques arose
during our search for potent analogues of the natural prod-
uct griseofulvin (1)[10] that can inhibit centrosomal cluster-
ing in cancer cells.[11] GF-15 (compound 2, Figure 1) was
identified as one of the most active analogues,[12] and for
in vivo biodistribution and half-life studies we became in-
terested in 125I labeling of 2.[13] As the activities of the three
iodinated isomers of 2 (i.e., 8–10, see Scheme 1) in a pheno-
typic whole-cell assay for multipolarity[11] are markedly dif-
ferent (data not shown), we found it crucial to apply a
method that would afford a single regioisomer and thus en-
able us to access the ortho- (i.e., 8), meta- (i.e., 9), and para-
iodo (i.e., 10) isomers.
[a] Department of Chemistry, Center for Nanomedicine and
Theranostics,
Figure 1. Structures of griseofulvin (1), its analogue GF-15 (2),
griseofulvic acid (3), and lidocaine (4).
Kemitorvet 207, 2800 Kgs. Lyngby, Denmark
Fax: +45-4593-3968
E-mail: mhc@kemi.dtu.dk
McKillop et al.[14] have introduced a method for the iod-
ination of small aryl-containing organic molecules by the
use of Tl(OCOCF3)3 and iodide. Mechanistically, the thall-
ated aromatics are formed by direct electrophilic met-
alation of the aryl ring with Tl(OCOCF3)3, after which li-
gand exchange with iodide followed by ipso substitution
creates the aryl–iodine bond. Whereas the reaction of the
Homepage: http://www.organic.kemi.dtu.dk/Research/mhcl
[b] Department of Systems Biology, Center for Microbial
Biotechnology,
Søltofts Plads, Building 221, 2800 Kgs. Lyngby, Denmark
[c] Department of Nuclear Medicine, University Hospital
Heidelberg,
69120 Heidelberg, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300419.
3970
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2013, 3970–3973

Regioselective Labeling of Aromatics with Radioactive Iodine
Initially, the labeling procedure was tested with nonradi-
oactive iodide. The three mono-TMS-analogues of griseo-
fulvin (i.e., 5–7) were dissolved in MeCN/TFA (7:3) and then
subjected to Tl(OCOCF3)3 (2 equiv.) for 30 min after which
NaI (4 equiv.) was added, and the reaction mixtures were
stirred for 2 min and then quenched by the addition of a
saturated aqueous solution of Na2CO3. Gratifyingly,
LCMS analysis of the reaction mixtures showed that the
corresponding iodides were formed as the only product,
whereas some unreacted starting material was also detected.
Iodinated products 8, 9, and 10 were isolated by column
chromatography in 44, 8, and 80% yield, respectively. The
low isolated yield of 9 is due to low conversion of starting
material 6 into the thallated intermediate, which presum-
ably is a consequence of the meta-substitution pattern in 6.
However, for use in labeling with radioactive iodide the low
conversion in the electrophilic aromatic thallation is not
critical, as radioactive iodide is used in substoichiometric
amounts, whereby it becomes the limiting reagent anyway
(vide infra). The formation of only the ipso-substituted
product was confirmed by comparison of the NMR spectra
and the LC–MS data for iodides 8–10, which were made
through a conventional route from the corresponding iodo-
benzyl alcohols (see the Supporting Information). Further-
more, the identity and uniformity of the products were veri-
fied by their retention times in HPLC analyses.
Scheme 1. Labeling of griseofulvin analogues with 127I and 125I.
Reagents: (a) Tl(OCOCF3)3, NaI, MeCN/TFA (7:3); (b) Tl(OC-
OCF3)3, Na125I, MeCN/TFA (7:3). RCP = radiochemical purity.
aryl thallium(III) bis(trifluoroacetate) intermediate with
iodide affords solely the ipso-substituted product,[15] the ini-
tial thallation can yield mixtures of the three possible re-
gioisomers, depending on the substrate and conditions.[16]
The labeling is typically performed in pure trifluoroacetic
acid (TFA).[16,17] In the case of substrates that cannot toler-
ate these conditions, addition of a cosolvent, such as
MeCN, can usually prevent substrate degradation.[16] How-
ever, addition of a cosolvent also lowers the reactivity,
which means that deactivated aromatics can be challenging
to label. When we exposed griseofulvin analogue 2 to the
reaction conditions [Tl(OCOCF3)3 in neat TFA], hydrolysis
to form griseofulvic acid (3, Figure 1) took place within
minutes, but when milder conditions [Tl(OCOCF3)3 in
MeCN/TFA, 7:3] were used, no conversion of 2 was ob-
served.
Additionally, the iodination of 7 (para-TMS) was tested
with a lower amount of TFA (10 and 20% TFA in MeCN)
but still with the use of 2 equiv. Tl(OCOCF3)3 and an excess
amount of NaI. At 10% TFA, the iodo product was only
observed in trace amounts (LC–MS analysis), whereas at
20% TFA, the conversion to the iodo product was Ͻ20%,
and this demonstrates the necessity of TFA for the reaction
to occur. It was also investigated how the reaction was af-
Results and Discussion
In continuation of the work by McKillop et al., a regiose- fected by using lower amounts of Tl(OCOCF3)3 (0.9 equiv.)
lective variant of the iodination method was published by in MeCN/TFA (7:3), and under these conditions, the con-
Bell et al.[18] They discovered that trimethylsilyl (TMS)-sub- version of 5, 6, and 7 into the iodo products was 1, 5, and
stituted aryls, such as 3-trimethylsilyltoluene, undergo ipso 20%, respectively. In complete absence of Tl(OCOCF3)3, no
substitution when treated with Tl(OCOCF3)3, whereas none conversion of the starting material took place, and this
of the other regioisomers are formed above trace levels. underlines that protodesilylation does not occur under the
Furthermore, the introduction of the TMS group increases conditions.
the reactivity of the aromatic system, whereby less activated
With the promising results from the iodination with 127I
aryls can be thallated and iodinated. The application of this in hand, we went on with 125I labeling, in which the radio-
iodination technique has, to the best of our knowledge, only active iodine was the limiting reagent. The labeling was per-
been used scarcely and only with nonradioactive iodide. formed by dissolving TMS precursors 5, 6, and 7 in MeCN/
Therefore, motivated by the work of Bell et al.[18] we hy- TFA (7:3) after which the thallation was initiated by ad-
pothesized that by use of TMS-aryls 5, 6, and 7 (see dition of Tl(OCOCF3)3 (0.5 equiv.). The reaction mixture
Scheme 1), incorporation of radioactive iodide into griseof- was stirred for 2 min at 20 °C, Na125I (1.7–3.4ϫ10–4 equiv.)
ulvin analogue 2 would be possible via the corresponding was added, and HPLC analysis was performed after 30 s.
thallated intermediates. To further elucidate the scope of As evident from the chromatograms in Figure 2, the radio-
the technique, iodination of lidocaine (4, Figure 1) was also chemical purity (RCP) by HPLC for all three radioiodin-
investigated. Lidocaine (4) is a widely used local anesthetic ations was Ͼ95%. The identity of the labeled compounds
drug, and iodination of this drug has previously been ex- was confirmed by coelution with nonradioactive reference
plored by El-Moselhy et al.[19]
The TMS-benzyl analogues of griseofulvin (i.e., 5–7, channel).
compounds in the HPLC analyses (detected in the UV
Scheme 1) were synthesized from the corresponding TMS-
TMS analogue 11 (see Scheme 2) of lidocaine (4) was
benzyl alcohols by using known procedures (see the Sup- synthesized in three steps from commercially available 4-
porting Information).[12]
bromo-2,6-dimethylaniline. For this substrate, we took ad-
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M. H. Clausen et al.
SHORT COMMUNICATION
Figure 3. HPLC analysis (radio channel) of the 125I labeling of lido-
caine analogue 11.
Figure 2. HPLC analysis (radio channel) of the 125I labeling of
griseofulvin analogues 5, 6, and 7 into corresponding iodides 8, 9,
and 10, respectively.
mation, Scheme S1, for details).[22] Another popular
method for iodination of aromatics through ipso substitu-
tion of an activator group is by use of arylstannanes,[5,23]
which are typically converted into the corresponding aryl
iodide by treatment with NaI and chloramine T (or another
oxidant such as peracetic acid) in an acidic aqueous envi-
ronment. Hence, the use of TMS-aryls and Tl(OCOCF3)3
is an attractive alternative to arylstannanes, especially for
molecules with low water solubility and for substrates that
are sensitive to highly acidic environments (such as neat
TFA) or oxidizing reagents. It is also noteworthy that the
radioiodination with thallium trifluoroacetate is performed
for only 30 s, which minimizes the time in which the mole-
cules are subjected to the acidic reaction conditions.
vantage of one of the mild methods available for the intro-
duction of a TMS group: the palladium-catalyzed cross-
coupling of an aryl halide with hexamethyldisilane[20] (see
the Supporting Information). The test iodination with
Na127I (0.5 equiv.) with the use of 0.5 equiv. Tl(OCOCF3)3
in MeCN/TFA (7:3) gave desired iodinated lidocaine 12
(full conversion by LC–MS, see Supporting Information).
The iodination also occurred if MeCN/TFA/H2O (6:3:1)
was used as the solvent mixture, which demonstrates that
H2O is tolerated in the reaction (see the Supporting Infor-
mation for LC–MS analysis). Subjection of lidocaine (4)
with no TMS group to the iodination conditions gave no
iodide incorporation, whereas when neat TFA was used as
the solvent, iodination was observed, and this verifies that
under mild reaction conditions the TMS group is crucial
for the reactivity of the aromatic ring. Performing the reac-
tion with 11 and Na125I gave the desired incorporation of
125I with high radiochemical purity as shown for two inde-
pendent experiments in Figure 3.
Conclusions
We were successful in applying the electrophilic aromatic
thallation for activation of TMS-aryls in the radioiodin-
ation of a potent griseofulvin analogue and lidocaine. Ra-
diochemical purities Ͼ95% were achieved in all cases. The
conditions used are superior to those previously reported,
as the iodination proceeds at ambient temperature and in a
less acidic environment. Overall, this regioselective iodin-
ation method was found to be very convenient, and we en-
vision that this procedure will be a strong alternative to the
use of other labeling strategies.
Scheme 2. Iodination of TMS-lidocaine analogue 11. Reagents:
(a) Tl(OCOCF3)3, NaI, MeCN/TFA (7:3); (b) Tl(OCOCF3)3,
Na125I, MeCN/TFA (7:3).
Experimental Section
Typical Procedure for 125I Labeling: The TMS-analogue (70 nmol)
was dissolved in MeCN/TFA (7:3, 35 μL), after which a solution of
Tl(OCOCF3)3 in MeCN/TFA (7:3, 15 μL, 2.33 mm, 35 nmol) was
added. The reaction mixture was stirred for 2 min at 20 °C and
then an aqueous solution of Na125I (ca. 0.1 μL, 1–2 MBq, 12–
24 pmol) was added. After 2 min of stirring at 20 °C, HPLC analy-
sis was performed.
The use of TMS-aryls as precursors for radioiodination
has been reported previously.[5a,21] However the reported re-
action conditions are rather harsh (e.g., NaI and tert-butyl
hypochlorite or H2O2 in acetic acid at 60 °C) and long reac-
tion times (Ͼ30 min) are needed to obtain reasonable con-
version.[21] The need for new procedures that can transform
TMS-aryls into their corresponding iodine analogue was re-
cently demonstrated in the attempted iodination of a tom-
ography tracer for the histamine H3 receptor, for which the
use of standard reagents such as chloramine T and peracid
Safety Note: Thallium compounds are extremely toxic to inha-
lation, skin contact, and ingestion. Toxicity is cumulative, and
Tl(OCOCF3)3 should be handled with care in a well-ventilated
fume hood.
Supporting Information (see footnote on the first page of this arti-
resulted in less than 10% labeling (see the Supporting Infor- cle): Experimental procedures, LC–MS data, and NMR spectra.
3972 Eur. J. Org. Chem. 2013, 3970–3973
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Regioselective Labeling of Aromatics with Radioactive Iodine
[10] A. E. Oxford, H. Raistrick, P. Simonart, Biochem. J. 1939, 33,
240–248.
Acknowledgments
[11] B. Rebacz, T. O. Larsen, M. H. Clausen, M. H. Rønnest, H.
Löffler, A. D. Ho, A. Krämer, Cancer Res. 2007, 67, 6342–
6350.
[12] a) M. H. Rønnest, B. Rebacz, L. Markworth, A. H. Terp, T. O.
Larsen, A. Krämer, M. H. Clausen, J. Med. Chem. 2009, 52,
3342–3347; b) M. H. Rønnest, M. S. Raab, S. Anderhub, S.
Boesen, A. Krämer, T. O. Larsen, M. H. Clausen, J. Med.
Chem. 2012, 55, 652–660.
This research was supported by the Danish Research Council (ref.
274-07-0561, support to M. H. R., T. O. L., and M. H. C.), the
Danish Cancer Society and Karen Krieger Fonden (support to
P. J. P. and M. H. C.), and the German Federal Ministry of Educa-
tion and Research (BMBF, MoBiTech, support to F. N.)
[13] M. S. Raab, I. Breitkreutz, M. H. Rønnest, B. Leber, T. O.
Larsen, L. Wagner, S. Anderhub, G. Konotop, P. J. Hayden, K.
Podar, J. Fruehaul, F. Nissen, W. Mier, U. Haberkorn, A. D.
Ho, H. Goldschmidt, K. C. Anderson, M. H. Clausen, A.
Krämer, Cancer Res. 2012, 72, 5374–5385.
[14] a) A. McKillop, J. S. Fowler, M. J. Zelesko, J. D. Hunt, E. C.
Taylor, G. McGillivray, Tetrahedron Lett. 1969, 10, 2423–2426;
b) A. McKillop, J. S. Fowler, M. J. Zelesko, J. D. Hunt, E. C.
Taylor, G. McGillivray, Tetrahedron Lett. 1969, 10, 2427–2430;
c) A. McKillop, J. D. Hunt, M. J. Zelesko, J. S. Fowler, E. C.
Taylor, G. McGillivray, F. Kienzle, J. Am. Chem. Soc. 1971, 93,
4841–4844.
[1]
[2]
a) S. J. Goldsmith, Semin. Nucl. Med. 1975, 5, 125–152; b) S. A.
Berson, R. S. Yalow, A. Bauman, M. A. Rothschild, K. New-
erly, J. Clin. Invest. 1956, 35, 170–190.
a) T. A. Baille, Chem. Res. Toxicol. 2008, 21, 129–137; b) S.
Summerfield, P. Jeffrey, Expert Opin. Drug Discovery 2009, 4,
207–218.
J. L. H. Eersels, M. J. Travis, J. D. M. Herscheid, J. Labelled
Compd. Radiopharm. 2005, 48, 241–257.
a) G. A. Kaltsas, P. Putignano, J. J. Mukherjee, M. A. Satta,
D. G. Lowe, K. E. Britton, J. P. Monson, A. B. Grossman,
G. M. Besser, Clin. Endocrinol. 1998, 49, 685–689; b) B. G.
Taal, C. A. Hoefnagel, R. A. Valdes Olmos, H. Boot, Eur. J.
Cancer 1996, 32, 1924–1932.
[3]
[4]
[15] A. McKillop, J. D. Hunt, J. Organomet. Chem. 1970, 24, 77–
88.
[16] E. C. Taylor, F. Kienzle, R. L. Robey, A. McKillop, J. D. Hunt,
J. Am. Chem. Soc. 1971, 93, 4845–4850.
[17] a) M. Eisenhut, W. E. Hull, A. Mohammed, W. Mier, D. Lay,
W. Just, K. Gorgas, W. D. Lehrmann, U. Haberkorn, J. Med.
Chem. 2000, 43, 3913–3922; b) C. Nicholl, A. Mohammed,
W. E. Hull, B. Bubeck, M. Eisenhut, J. Nucl. Med. 1997, 38,
127–133.
[18] H. C. Bell, J. R. Kalman, J. T. Pinhey, S. Sternhell, Tetrahedron
Lett. 1974, 15, 3391–3394.
[19] T. F. El-Moselhy, M. A. El-Kersh, F. Nyberg, R. Hede, M. A.
Eldawy, Med. Chem. Res. 1998, 8, 163–170.
[20] a) E. Shirakawa, T. Kurahashi, H. Yoshida, T. Hiyama, Chem.
Commun. 2000, 1895–1896; b) E. McNeill, T. E. Barder, S. L.
Buchwald, Org. Lett. 2007, 9, 3785–3788; c) S. E. Denmark,
J. M. Kallemeyn, Org. Lett. 2003, 5, 3483–3486.
[21] a) D. S. Wilbur, K. W. Anderson, W. E. Stone, A. H.
O’Brien Jr., J. Labelled Compd. Radiopharm. 1982, 19, 1171–
1188; b) S. M. Moerlein, H. H. Coenen, J. Chem. Soc. Perkin
Trans. 1 1985, 9, 1941–1947.
[22] S. Champion, J. Gross, A. J. Robichaud, S. Pimlott, J. Labelled
Compd. Radiopharm. 2011, 54, 674–677.
[23] a) D. E. Seitz, G. L. Tonneson, S. Hellman, R. N. Hanson, J. S.
Adelstein, J. Organomet. Chem. 1980, 186, C33–C36; b) A. C.
Donovan, J. F. Valliant, J. Labelled Compd. Radiopharm. 2011,
54, 65–71.
[5] a) R. H. Seevers, R. F. Counsell, Chem. Rev. 1982, 82, 575–590;
b) V. Vergote, S. Bodé, K. Peremans, H. Vanbree, B. Baert, G.
Slegers, C. Burvenich, B. De Spiegeleer, J. Chromatogr. B 2007,
850, 213–220; c) R. Finn, in: Handbook of Radiopharmaceuti-
cals: Radiochemistry and Applications (Eds.: M. J. Welch, C. S.
Redvanly), Wiley, Chichester, UK, 2003; d) H. Coenen, J. Mer-
tens, B. Mazieere, Radioiodination Reactions for Radiopharma-
ceuticals, Springer, Dordrecht, The Netherlands, 2006; e) D.
Hubers, P. J. H. Scott, in: RSC Drug Discovery Series No. 15
Biomedical Imaging: The Chemistry of Labels, Probes and Con-
trast Agents (Ed.: M. Braddock), Royal Society of Chemistry,
Cambridge, 2012; f) L. Koehler, K. Gagnon, S. McQuarrie, F.
Wuest, Molecules 2010, 15, 2686–2718; g) M. Bourdoiseau, Int.
J. Radiat. Appl. Instrum. B 1986, 13, 83–88; h) M. Eisenhut,
W. Mier, in: Handbook of Nuclear Chemistry, Springer, Berlin,
2012.
[6] a) W. M. Hunter, F. C. Greenwood, Nature 1962, 194, 495–496;
b) F. C. Greenwood, W. M. Hunter, J. S. Glover, Biochem. J.
1963, 89, 114–123.
[7] a) P. J. Fraker, J. C. Speck Jr., Biochem. Biophys. Res. Commun.
1978, 80, 849–857; b) W. Nieuwenhuizen, J. J. Emeis, A. Ver-
mond, P. Kurver, D. Van der Heide, Biochem. Biophys. Res.
Commun. 1980, 97, 49–55.
[8] J. J. Marchalonis, Biochem. J. 1969, 113, 299–305.
[9] D. D. Dischino, R. J. Mattson, M. D. Ogan, M. F. Parker, J. P.
Yevich, J. Labelled Compd. Radiopharm. 2002, 45, 987–994.
Received: March 20, 2013
Published Online: May 14, 2013
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