Synthesis of a [2.2.2] Cryptand Containing Reactive Functional Groups

The functional group-containing potassium ionophore 195,245-dibromo-4,7,13,16,20,23-hexaoxa-1,10-diaza-19(1,2),24(1,2)-dibenzabicyclo[8.8.6]tetracosaphane has been synthesized.


Introduction
Potassium (K + ) is a major cationic constituent of living cells; its concentration in intercellular living cells is around 150 mmol·dm -3 , whilst its extracellular levels are typically around 4 mmol·dm -3 . Potassium level measurements of are importance in the fields of cell physiology and clinical medicine [1,2]. Fluoroionophores based on a fluorescent coumarin group probe and the ionophore 1,10-diaza-18crown-6 chelating group (abbreviated by the acronym CD18), which is selective for K + ions, and capable of measuring K + in an aqueous physiological environment have been reported [3].
Fluoroionophore sensors possess a guest binding site (ionophore) having a heteroatom with nonbonding electron pairs such as nitrogen, and a photon-interaction site (fluorophore) capable of accepting electrons. In the absence of a cation that binds selectively in the cavity of ionophore, when the fluorophore is excited by a photon, electron transfer from the ionophore to the fluorophore results in fluorescence quenching. In the presence of a cation which selectively binds to the ionophore and hence inhibits the electron transfer process, the fluorescence of fluorophore is switched on.
In order to attain high, yet selective binding of a potassium ion chelator some rigidity in the system was considered necessary compared to the selective binder CD18. Hence, the 1,10-diaza-18-crown-6 OPEN ACCESS chelating group in CD18 was replaced by the bicyclic ionophore "cryptand [2.2.2]" which resulted in a selective, high affinity fluoresecent probe for K + abbreviated CD222 [4].
Cryptands are cavities containing macromolecules which form stable complexes with alkali metal ions. For a given cation, the stability constant is largest for the cation which fits best into the cavity of the ligand. Thus stability maxima are found for Li [ [6]. Other potassium fluoroionophores based on triazocryptands have been reported [7,8]. The cavity size of the cryptand [2.2.2] (2.8 Å in diameter) closely matches the size of potassium cation (2.66 Å). This paper describes the synthesis of a bromo-derivative of the ionophore dibenzo-cryptand [2.2.2] which serves as a precursor of potential fluoroionophores for potassium ions [9]. The fluorophore can be introduced by, for example, reacting the bromodibenzocryptand [2.2.2] with t-butyl lithium, followed by reaction with a 2,7-dichloroxanth-9-one derivative to give a fluoroscein type fluoroionophore.
Treatment of two equivalents of (I) with 1,2-dibromoethane and potassium carbonate in dimethyl formamide (DMF) afforded 1,2-bis(2-nitrophenoxy)ethane (II). Reduction of II with 10% palladiumon-charcoal as the catalyst produced the diamino derivative III. The diamine III was next reacted in tetrahydrofuran under high dilution conditions [10] with 3,6-dioxaoctanedioyl dichloride (1,2ethylene-O,O-diglycolic acid chloride) to gave the lactam IV. The lactam IV was reduced with lithium aluminum hydride (LiAlH 4 ) in THF to give the azacrown V [11]. Figure 1 shows the complete 1 H-NMR spectrum of V and the insert shows the 6.4-7.2 ppm region, and the assignment of spectral data are given in the Experimental section.
19 5 ,7,13,16,20,2)24(1,2)-dibenzabicyclo [8.8.6]tetracosaphane-2,9-dione (XI): To a stirred solution of triethylamine (1.45 g, 14.3 mmol) in toluene (300 mL) maintained at 0 ο C were added the bisamine X (1.4 g, 2.7 mmol) in THF (100 mL), and 3,6-dioxa-octanedioic acid chloride (0.6 g, 2.8 mmol) in toluene (100 mL), simultaneously at a rate of 0.3 mL/min with the aid of an infusion pump. After the addition was completed, the mixture was stirred at room temperature overnight. The mixture was filtered and removal of solvent in vacuo gave an oil which was chromatographed using silica and CH 2 Cl 2 and methanol (10%) to yield a white solid Conversion of XI to VIII: To a solution of diamide XI (200 mg, 0.3 mmol) in THF (4 mL) was added borane in THF (1 M, 3 mL), and the mixture heated under reflux for 2 hrs. The reaction mixture was worked up as described above to yield white solid, which showed IR (KBr) and 1 H-NMR data similar to those of VIII reported above.

Conclusions
In summary, the dibromdibenzocryptand [2.2.2] VIII, a potential potassium ionophore, has been synthesized. The bromo derivatives of the benzocryptand are likely be linked to a fluorophore, such as those described by McGimpsey in [16].