Cryptands lipophilic

Although the cryptands are powerful cation complexing agents, there has been a need felt for increasing the lipophilicity of these materials. In particular, Montanari and his coworkers have utilized the lipophilic cryptands in phase transfer catalytic proces-sesi5,40 Lehn and his group. In all of this work, the principal structural varia- 

Lehn s approach is slightly more complex than that illustrated above in that the diol is chloromethylated and then treated with cyanide. Hydrolysis then affords the diacid which may be carried through as shown. It should also be noted that once the bis-acyl halide is in hand, it may be treated directly with an open-chained amine to yield a lipophilic diazacrown, after reduction  

Landini, Montanari and Rolla ° were able to incorporate cyclohexyl residues in the strands and thereby increase lipophilicity. As with the compounds referred to above, these were effective phase transfer catalysts. 

Vdgtle and his coworkers have reported the synthesis of cryptands containing lipophilic structural elements like o-, m- andp-phenylene, biphenyl and pyridine nuclei.  

By incorporating trisubstituted aromatic rings, Vogtle has prepared what he calls endo-lipophilic cryptands . Such a compound is illustrated below as 8. The synthetic approach to these systems parallels those outlined in previous discussion . 

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The lipophilic cryptands shown in Figure 2.17 were prepared by analogous synthetic routes (53) to that described in Scheme 4. 

Nucleophilic substitution of octyl methane sulphonate by anions in biphasic chlorobenzene-water is catalysed by a lipophilic cryptand containing a ,4 alkyl side chain. Lipophilic cryptates exist as monomeric species in low-polarity solvents... 

Alkali metal ions are involved in numerous important biological processes, such as transmission of nerve impulses, nervous control of secretion and muscle functions, protein synthesis, and enzymatic regeneration of metabolism. Crown compounds, being complex formers, are naturally apt to interfere in such processes. For instance, it was found that lipophilic cryptands of structural formula... 

Montanari and coworkers have been particularly active in this area. They have generally utilized crowns or cryptands having long arms attached to them. These lipophilic arms are typically terminated in a primary or secondary amino function which may serve as a nucleophile in the reaction with a chloromethylated polystyrene residue. 

Phase transfer catalysis. As well as their use in homogeneous reactions of the type just described, polyethers (crowns and cryptands) may be used to catalyse reactions between reagents contained in two different phases (either liquid/liquid or solid/liquid). For these, the polyether is present in only catalytic amounts and the process is termed phase transfer catalysis . The efficiency of such a process depends upon a number of factors. Two important ones are the stability constant of the polyether complex being transported and the lipophilicity of the polyether catalyst used. 

Both quaternary onium salts and cation complexes of lipophilic multidentate ligands (crown-ethers and cryptands) have been used as catalysts in two-phase systems in the presence of base (OH, F, etc.). However, under these conditions, the lack of chemical stability of quaternary salts and the very low complexation constants of multidentate ligands (especially crown-ethers) make all these systems barely effective in the activation of such anions. 

Sr was more stable in vivo but Ag was more lipophilic. These results suggest that generator-produced isotopes such as Rb-82 (T% = 75 sec) sequestered inside cryptands may be useful freely diffusible tracers for measuring blood flow by positron emission tomography. It would be more convenient to make this measurement with generator-produced isotopes than with water from cyclotron-produced oxygen-15 (Th = 122 sec). 

Results with Sr ] in Mice. While the results with Ag cryp-tate were encouraging, we sought further preliminary evidence of the potential value of labeled cryptates as blood-flow radiopharmaceuticals. There were several reasons for these studies the monovalent silver ion is very polarizable and thus may not be a general model for monovalent cations (5,17). In contrast, divalent cations form stronger inclusive cryptates than monovalent cations of the same ionic radii. On the other hand, the added charge of the divalent ion would require that the cryptand shield more charge if it is to result in an equally lipophilic complex. 

Chromium complexes with crown ethers as ligands 90MI43. Cryptands as lipophilic cell ligands 86PAC1503. 

In their protonated form polyamines are often attractive receptors for a variety of anions, as exemplified by many examples now known in nature, industry and model systems [34, 36]. The characteristic extraction properties of a range of such ligands toward halide, pertechnetate and perrhenate ions are strongly influenced by the ligand s protonation behavior and lipophilicity. Extraction therefore shows a pH-dependence, as well as being influenced by all manner of structural factors [35, 37, 38]. Some results for the perrhenate extraction by three different substituted tripodal tetraamines 7a-c and two structure-related octaamino cryptands 7d,e (Fig. 4.11) are shown in Fig. 4.12 [35, 37]. 

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