Polyether bicyclic

Urethane gels and ultrasoft elastomers are a more recent development.18 They are made primarily by reacting high-molecular-weight polyether polyols with a stoichiometric deficiency of isocyanate. The low NCO-to-OH ratio allows for a wide latitude in hardness adjustment. These low-hardness elastomers are used for seating applications (such as gel bicycle seats), shoe inserts, and soft padding for orthopedic devices. 

Lehn and Sauvage (34) suggest that bicyclic ligands should be better suited than non-cyclic polyethers to selectively complex cations. Their data (Table 8) show this to be valid for the series of bicyclic amines represented by the structures in Fig. 3. 

As stated by Pedersen [1] at the very beginning, systematic names of polyethers are too cumbersome for repeated use. Thus, he coined an efficient system of their trivial names that is in common use. The polyethers were dubbed crown ethers or crowns . The total number of atoms in the macrocycle preceded this term whilst the number ofoxygen atoms followed it. The presence ofaromatic 44 or saturated 60 rings in the systems is marked at the beginning. In Lehn s system for bicyclic molecules called cryptands the number of OCH2CH2 units in each branch ofthe bicyclic hosts 61-63 ([1.1.1]-, [2.1.1]- and [2.2.2]cryptands... 

Sodium or potassium ions can also participate in the phase-transfer process when they are converted to lipophilic cations by complexation or by strong specific solvation. A variety of neutral organic compounds are able to form reasonably stable complexes with K+ or Na + and can act as catalysts in typical phase-transfer processes. Such compounds include monocyclic polyethers, or crown ethers (1), and bicyclic aminopolyethers (cryptates) (2). They can solubilize inorganic salts in nonpolar solvents and are particularly recommended for reactions of naked anions. Applications of these compounds have been studied.12,21-31... 

Usually, the ion exchange rate between a solvated metal ion and the ion fixed in the complex compoimd is very fast. This is the reason why most of the cation complexes with cyclic polyethers have high exchange rates which are in the order of 10 s >20) Although the exchange rate for metal ions with a bicyclic cryptand is significantly lower than with monocyclic crown ethers the kinetic isotopic... 

Polymer structure-thermodynamic control. The polymers described above, containing dloxolane and dloxane rings, are the klnetlcally controlled products In these polymerizations. Bailey reported the polymerization of several bicyclic [2.2.2]orthoesters and also splro orthoesters under much harsher conditions (24). Simultaneous opening of both rings Is achieved In this way, leading to linear polyethers with ester branching. This Is usually accompanied by volume expansion. 

Cycloadditions only proceeding after electron transfer activation via the radical cation of one partner are illustrated by the final examples. According to K. Mizono various bis-enolethers tethered by long chains (polyether or alkyl) can be cyclisized to bicyclic cyclobutanes using electron transfer sensitizer like dicyanonaphthalene or dicyano-anthracene. Note that this type of dimerization starting from enol ethers are not possible under triplet sensitization or by direct irradiation. Only the intramolecular cyclization ci the silane-bridged 2>. s-styrene can be carried out under direct photolysis. E. Steckhan made use of this procedure to perform an intermolecular [4+2] cycloaddition of indole to a chiral 1,3-cyclohexadiene. He has used successfully the sensitizer triphenylpyrylium salt in many examples. Here, the reaction follows a general course which has been developed Bauld and which may be called "hole catalyzed Diels-Alder reaction". 

Although they have been developed primarily for use with alkali and alkaline-earth ions (see page 200), it is appropriate to mention here the polyether macrocycles. These are of two main types (1) monocyclic ones,7 such as (21-XXV1), and (2) the bicyclic molecules,8 such as (21-XXVII). These compounds are remarkably powerful complexing agents for alkali and alkaline-earth ions, giving unusual coordination geometries.9 There is as yet no detailed report of their complexes with other metal ions. 

Pinnatoxins are the cyclic imines most closely related in structure to spirolides (Figure 26.4). They differ slightly in the polyether ring system (6-5-6) and in an additional bicyclic ether moiety in the macrocycle, which is absent in spirolides. Pinnatoxin variants differ in the length of their cyclohexenyl side chains pinnatoxin-A has just a Cl carboxylic group, the enantiomeric diastereoisomers pinnatoxin-B and -C possess a C2 entity consisting of a 2-amino acetic acid function, and pinnatoxin-D includes a C4 y-ketobutyric acid moiety. 

More recent work has pressed towards even more selective sensors for calcium, stimulated by a 2 1 ligand calcium stoichiometry found by Petranek and Ryba (129) for a calcium crown ether which exhibited high calcium ion selectivity over magnesium. Thus, Shono and co-workers (130) have studied a bicyclic polyether amide derivative with enhanced selectivity for calcium, while Simon and co-workers (131) have reported a non-crown neutral carrier for calcium which forms a 3 1 ligand calcium complex with very high selectivity for calcium over sodium and potassium, that is, by factors of 10 and 10, respectively. This indicates that macrocyclic systems are not a prerequisite for high selectivity. 

Nucleosides and nucleotides can also be analyzed together. Lamb and Ye [149], for example, used a neutral divinylbenzene polymer (Dionex lonPac NSl), which they coated with an -decyl-substituted bicyclic polyether such as l,10-diaza-4,7,13,16,21,24-hexaoxabicyclo[8.8.8]hexacosane (D2.2.2). This coating results in a chromatographic matrix, on which both ionic and hydro-phobic interactions can occur. The so-called cryptands [150] (like their mono-cyclic analogs, bicyclic polyethers) form rather stable complexes with alkali metals. These metals are included in the cavity of the ring system their presence results in weak covalent bonds between the cation and the two N atoms. 

Ort/io-selectivity is generally observed in the reactions of 2,4-dichloro- and 2,4-difluoro-nitrobenzene with alkoxide and thiophenoxide ions [199]. Also in less activated systems, nucleophiles generated from phenols and thiophenols with potassium fluoride-alumina and 18-crown-6-polyether will react in DMSO with cyano- or nitro-substituted fluoro- or chloro-benzenes [200]. Interestingly, the reaction of difluorobenzenes with two diffoent alcohols can occur sequentially. Introduction of the first etho" function deactivates the ring, and use of more forcing conditions allows substitution of the second fluorine [201]. Consecutive displacements of fluorine and nitro groups have been observed in the reaction of ort/io-fluoronitrobenzene with chiral acyl bicyclic lactones in a highly enantioselective synthesis of spirooxindoles [202]. 

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