Dendronized Cryptophanes as Water-Soluble Xenon Hosts for 129 Xe Magnetic Resonance Imaging Rahul Tyagi, †,§ Christopher Witte, ‡,§ Rainer Haag,* ,† and Leif Schrö der* ,‡ † Institut fü r Chemie und Biochemie, Freie Universitä t Berlin, Takustrasse 3, Berlin 14195, Germany ‡ ERC Project BiosensorImaging, Leibniz-Institut fü r Molekulare Pharmakologie (FMP), Robert-Roessle-Strasse 10, Berlin, Germany * S Supporting Information ABSTRACT: Cryptophane cages are very promising for 129 Xe-MRI. These molecular cages are extremely hydrophobic, which currently limits their use for diagnostic applications. To overcome this, the synthesis of water-soluble dendronized cryptophanes with surface groups for further functionalization is reported here. These molecules retained all the “core properties of cryptophane” that are crucial for biosensor applications as analyzed by Hyper-CEST imaging and spectroscopy. This approach is promising for developing new generations of xenon-cryptophane-based biosensors. S ince cryptophane cages were first synthesized by Collet and co-workers in 1981, their ability to host molecules and atoms has been well explored. In particular, these molecular cages exhibit a high affinity for xenon. Although xenon is a chemically inert gas, these interactions with cryptophanes have been exploited to “functionalize” xenon. 1 In addition, it can be hyperpolarized using spin-exchange optical pumping, 2 thereby increasing its detectable signal by 4-5 orders of magnitude in nuclear magnetic resonance (NMR) experiments. Altogether, these properties have led to the development of xenon- cryptophane (Xe@Cry) based biosensor NMR and nuclear magnetic imaging (MRI). Since this concept was first introduced by Pines and co-workers in 2001, extensive research has been performed in this field. 3 One particularly important advance in the field was the development of the Hyper-CEST (chemical exchange saturation transfer with hyperpolarized nuclei) detection technique. 4 This indirect detection technique relies on the exchange of xenon in and out of the host to amplify the Xe@Cry biosensor signal. A selective saturation pulse can only depolarize those xenon nuclei bound in the Xe@ Cry complex, thus making them effectively invisible to magnetic resonance detection. With an exchange rate in the range of 10- 100/s, a saturation pulse of only a few seconds allows each cage to potentially depolarize thousands of xenon nuclei. This depolarization accumulates in the pool of free xenon nuclei in solution, and it is the depolarization of this pool that is measured after taking a reference measurement without selective saturation. For the development of Cry-Xe biosensors, cryptophanes and their derivatives have been functionalized with ligands that can bind to a specific target molecule. 1 To increase the capability of Xe biosensors, there have been numerous efforts to make them an effective tool for diagnostic applications. 3 Although cryptophanes are attractive candidates for hosting xenon atoms for biosensors, their use has been limited for in vitro or in vivo diagnostic applications because of their high hydrophobicity, which leads to unspecific anchoring into the cell membrane. 5 In this context, several approaches have been taken to enhance the water solubility of cryptophanes. Cryptophane can be modified/functionalized in a variety of ways to achieve this goal such as metal complexation of cryptophanes, 6 the core synthesis of cryptophanes possessing a number of polar groups, 7 and also conjugation of water solubilizing linkers. 8 Rousseau and co-workers have recently reported the synthesis of a cryptophane core molecule possessing poly(ethylene glycol)s as a water-solubilizing unit and free alkyne groups 9 that provide room for further functionalization. Although this route clearly provides safe access to this promising cryptophane derivative, the compound was produced in five steps in only 3% overall yield and is therefore lacking in practical applicability. This is also the major drawback of most of the reported routes for producing such molecules. We realized that the grafting of a water-solubilizing linker to cryptophane is a more convenient and promising methodology to enhance its water solubility in order to avoid metal contamination, tedious protection-deprotection multi- step synthesis, etc. Furthermore, there are not many examples of functionalizable cryptophane derivatives that have been produced with the grafting approach and also been evaluated for Xe binding affinity and their suitability for 129 Xe Hyper- CEST signal sensitivity. Hence, there is a need for a more convenient and versatile approach for producing such functionalizable, water-soluble cryptophane molecules and their applicability for developing a new generation of Xe@ Cry biosensors. Moreover, retaining the core binding and Received: July 4, 2014 Letter pubs.acs.org/OrgLett © XXXX American Chemical Society A dx.doi.org/10.1021/ol501951z | Org. Lett. XXXX, XXX, XXX-XXX