Metal-mediated retro Diels-Alder of dicyclopentadiene derivatives: A convenient synthesis of [(Cp-R)M(CO)3] (M = 99mTc, Re) complexes

Article abstract of DOI:10.1021/ja077741l

The synthesis of piano-stool type complexes [(Cp-R)^99mTc(CO)₃] from water and, if possible, directly from [^99mTcO₄]^- is a persisting challenge. Such complexes are very convenient for labeling biomolecules since they are small in size and innocent with respect to noncovalent interactions in biological systems. We found that [(Cp-R)^99mTc(CO)₃] can be prepared directly from the dimerized Diels-Alder precursors and from [^99mTcO₄]^-. Since the concentration of the respective monomers in the reaction solutions was undetectable, the formation of [(Cp-R)^99mTc(CO)₃] must be based on a metal-mediated retro Diels-Alder reaction with concerted Cp coordination. The authenticity of the formed ^99mTc complexes was confirmed by comparing their HPLC retention times with those of the structurally characterized rhenium surrogates. Besides the principle access to piano-stool complexes for radiopharmaceutical studies, a metal-mediated retro Diels-Alder reaction under concomitant formation of the corresponding piano-stool complex has, to our knowledge, not been observed before and might apply to other metal cations as wel. Copyright

Full text of DOI:10.1021/ja077741l

Published on Web 01/11/2008
Metal-Mediated Retro Diels-Alder of Dicyclopentadiene Derivatives: A
Convenient Synthesis of [(Cp-R)M(CO)3] (M ) 99mTc, Re) Complexes
Yu Liu, Bernhard Spingler, Paul Schmutz, and Roger Alberto*
UniVersity of Zu¨rich, Institute of Inorganic Chemistry, Winterthurerstrasse 190, 8057 Zu¨rich, Switzerland
Received October 16, 2007; E-mail: ariel@aci.unizh.ch
Site-specific targeting of receptors and overexpressed genes as
related to cancer or neurodegenerative diseases is the basic concept
of molecular imaging with radionuclides or contrast agents.1
Achieving a high target/nontarget ratio within a short period of
time is crucial for a precise diagnosis. In molecular radioimaging,
99mTc plays still a major role, but the design of a 99mTc bioconjugate
fulfilling the “conditio sine qua non” mentioned before is very
challenging. The size of the complex and chelator functionalities
may govern the biological behavior of the vector, rather than the
vector itself.2 To optimize these parameters, the cyclopentadienyl
(Cp) ligand is an obvious choice. Cp is one of the smallest
“innocent” ligands and stably binds to the [99mTc(CO)3]+ core.
Notably, it mimics arene rings in pharmaceuticals or biomolecules
such as amino acids. This structural analogy was successful in
ferrocifen, the ferrocene (Fc) analogue of tamoxifen, as pioneered
by Jaouen et al.3-6 or steroid hormones combined with [(Cp)Re-
(CO)3].7,8 Recent results from our group indicated that amino acids
derivatized with Fc or [(Cp)Re(CO)3] are surrogates for natural
amino acids.9
yield. Since alkenes do not coordinate to 1 in water (i.e., no reaction
with HCp at all) and [HCp-COO-] did not ring-deprotonate, the
formation of 2 must consist in initial σ-binding to carboxylate with
subsequent or concerted coordination to cyclopentadiene (path A
in Scheme 1).
Scheme 1. Stepwise Reaction of 1 with HCp-R and Thiele’s Acid
3, Respectively
Whereas biomolecules with pendent [(Cp)Re(CO)3] or Fc are
accessible via classical organometallic methods, the 99mTc analogues
must be prepared directly from aqueous buffer, apparently incom-
patible conditions for HCp which rapidly decomposes in water.
Efforts by Katzenellenbogen et al. to prepare [(Cp-R)99mTc(CO)3]
by the so-called double ligand transfer reaction gave the desired
piano-stool complexes but from organic solvents in autoclaves.10,11
Alternatively, the [Fe(Cp)]+ moiety in Fc-derivatized biomolecules
could be replaced by [Tc(CO)3]+ but still at T > 100 °C and in the
presence of DMSO.12 We reported an aqueous synthesis at T <
100 °C with biomolecules bound to the cyclopentadiene ring via
an R-keto group. Still, all these Cp-derivatized compounds are
sensitive and limit types of biomolecules and chemistry.13,14
It would be highly desirable to find a general approach to [(Cp-
R)99mTc(CO)3]-type complexes from stable precursors, conjugated
to biomolecules and generating the active cyclopentadiene in situ.
We present in this paper the unexpected formation of [(Cp-R)99mTc-
(CO)3]-type complexes from their Diels-Alder dimerized precur-
sors (HCp-R)2. Since a thermal retro Diels-Alder reaction, (HCp-
R)2 f HCp-R, did not occur to a measurable extent under our
conditions, monomerization and coordination to [99mTc(CO)3]+ are
concerted; hence, cleavage is metal-mediated.
The formation of Cp complexes is a thermodynamic driving
force. After carboxylate coordination, the Lewis acid [99mTc(CO)3]+
exerts a strong electronic interaction with the adjacent cyclopen-
tadiene. This distinct affinity for the formation of {(η5-Cp)Tc}
implied the use of a “pre-cyclopentadiene” instead of the compa-
rably reactive HCp-R itself. Evidently, the Diels-Alder dimer
(HCp-COOH)2, 3 (Thiele’s acid), is such a “protected” cyclopen-
tadiene. Although thermal cracking of 3 requires T > 160 °C,
reaction of 3 with 1 at 95 °C for 30 min in buffer resulted in the
quantitative formation of 2 (path B).
No free HCp-COOH was detected, excluding an in situ thermal
retro Diels-Alder reaction and subsequent entry into path A.
Complex 2 was also prepared directly from [99mTcO4]- in the
presence of 3 and under the same conditions as used for the
preparation of 1 (Supporting Information [SI]).16 To our knowledge,
this is the first evidence of a metal-mediated retro Diels-Alder
reaction with concerted formation of the corresponding Cp complex.
It can be hypothesized that other metal centers will behave similarly
and new mixed Cp-H2O complexes will become accessible.
A metal-mediated retro Diels-Alder reaction implies a general
approach to [(Cp-R)99mTc(CO)3] with more complex functionalities,
“R”, bound to the dicyclopentadiene building block. Synthetically,
biomolecules can be conjugated to precursors such as 3 via, for
example, amide formation as exemplified with 4 (Scheme 2).
Alternatively, NaCp is first reacted with, for example phenyl-acetic
acid ethyl ester, and then dimerized to yield 5.
Originally, we supposed that the mechanism of the aqueous
synthesis of [(Cp-COCH3)99mTc(CO)3] involved deprotonation at
the cyclopentadiene ring followed by direct coordination to
[
99mTc(OH2)3(CO)3]+, 1.13 Initial coordination to the enolate fol-
lowed by an intramolecular recoordination to η5-Cp seemed less
likely. On a mechanistic base, no complex formation was expected
with cyclopentadiene carboxylic acid (HCp-COOH) since acidity
of C-H is too low in aqueous solution.15 We found that 1 still
reacted with HCp-COOH in aqueous buffer at almost the same rate
as with HCp-COCH3 to yield [(Cp-COOH)99mTc(CO)3], 2, in high
Compounds 4 and 5 reacted directly with [99mTcO4]- or 1 to
give 6 and 7 after 30-120 min at 95 °C in quantitative yield. We
emphasize that no measurable amounts of the respective monomers
of 4 and 5 were observed. Further derivatives behaved comparably
(SI). For all reactions, one HCp-R per 99mTc must be released but
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J. AM. CHEM. SOC. 2008, 130, 1554-1555
10.1021/ja077741l CCC: $40.75 © 2008 American Chemical Society

C O M M U N I C A T I O N S
a
Scheme 2. Reaction of 1 or [99mTcO4]- (Isolink Kit) with (HCp-R)2
metal-mediated retro Diels-Alder reactions. Furthermore, a strong
leaving group effect has been demonstrated in thermal retro Diels-
Alder reactions with HCp.18 To probe this effect qualitatively, we
prepared the heterodimers 10 from C5Cl6 and HCp-COOH. Again,
the desired product was not received. We conclude that potentially
coordinating functionalities such as keto-, amide-, or carboxylate
groups should be present on the former diene and the dienophile
in the Diels-Alder product. Since concentrations are extremely low,
the presence of two biologically active groups is not impairing
labeling reactions or applicability.
We have shown that [99mTc(OH2)3(CO)3]+ mediates the retro
Diels-Alder reaction of dicyclopentadiene derivatives with con-
comitant formation of corresponding piano-stool complexes. A
weakly coordinating, Cp pendent group binds to the metal and
initiates a retro Diels-Alder reaction and coordinative shift. This
opens an avenue to the versatile preparation of [(Cp-R)99mTc(CO)3]
complexes with attached targeting molecules “R”, avoiding thereby
reactive organic precursors. Solid-phase bound Diels-Alder prod-
ucts will yield no carrier-added complexes.
a Na[H3BCO2H] (4 mg), Na2B4O7‚10H2O (7 mg), Na2tartrate‚2H2O (7
mg) and, e.g., 4 (10-3 M), 30-60 min, 95 °C.
In conclusion, labeling of biomolecules with piano-stool-like
99mTc and Re complexes from water is now to become an option
in the development toward novel radiopharmaceuticals.
Figure 1. ORTEP presentations of complexes 6(ethyl ester) and 2.
Acknowledgment. This research was supported in part by a
Eureka project and Mallinckrodt Med. B.V.
Supporting Information Available: Labeling and synthetic pro-
cedures, HPLC conditions, and crystallographic data for 2, 6 and 10 in
CIF format. This material is available free of charge via the Internet at
http://pubs.acs.org.
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