Cyclic polyethers ethers

Ethers form Lewis acid Lewis base complexes with metal ions Certain cyclic polyethers called crown ethers, are particularly effective m coor dinatmg with Na" and K" and salts of these cations can be dissolved m nonpolar solvents when crown ethers are present Under these conditions the rates of many reactions that involve anions are accelerated... 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Crown ether (Section 16.4) A cyclic polyether that, via ion-dipole attractive forces, forms stable complexes with metal ions. Such complexes, along with their accompanying anion, are soluble in nonpolar solvents. 

Crown ethers are cyclic polyethers. Larger crown ethers contain a cavity that can partially engulf atomic ions. 18-crown-6 actually binds so tightly that it can extract this ion into benzene from water, driving counterions, like MnOc, into the benzene layer, i.e. 

Aliphatic carboxylic acids Alkyl ethyl ethers Cyclic polyethers Phosphorous compounds Rearrangement peak in dioxanes... 

Clark s group also reported on ring-closing enyne metathesis for the preparation of six- and seven-membered cyclic enol ethers 428 n= 1,2) as potential building blocks for the synthesis of marine polyether natural compounds such as brevetoxins and ciguatoxins. Metathesis products 428 were obtained from ene-ynes 427 in 72-98% yield when the NHC-bearing catalyst C was used (Scheme 84) [179]. 

There are two other approaches to enhancing reactivity in nucleophilic substitutions by exploiting solvation effects on reactivity the use of crown ethers as catalysts and the utilization of phase transfer conditions. The crown ethers are a family of cyclic polyethers, three examples of which are shown below. 

Tandem carbonyl olefmation—olefm metathesis utilizing the Tebbe reagent or dimethyl-titanocene is employed for the direct conversion of olefmic esters to six- and seven-mem-bered cyclic enol ethers. Titanocene-methylidene initially reacts with the ester carbonyl of 11 to form the vinyl ether 12. The ensuing productive olefm metathesis between titano-cene methylidene and the cis-1,2 -disubstituted double bond in the same molecule produces the alkylidene-titanocene 13. Ring-closing olefin metathesis (RCM) of the latter affords the cyclic vinyl ether 14 (Scheme 14.8) [18]. This sequence of reactions is useful for the construction of the complex cyclic polyether frameworks of maitotoxin [19]. 

Crown ethers are cyclic polyethers designated [n]crown-m where n is the ring size and m the number of oxygen atoms, for instance [18] crown-6 1. They show a high affinity for cationic guest molecules, especially alkali metal cations, where the cation is commonly complexed within the cavity of macrocycle or sand-... 

Some particularities of the extraction of ions from an aqueous organic phase, and of the phase catalyzed polyetherification will be summarized. These will represent the fundamentals of our work on the synthesis of some novel classes of functional polymers and sequential copolymers. Examples will be provided for the synthesis of functional polymers containing only cyclic imino ethers or both cyclic imino ethers as well as their own cationic initiator attached to the same polymer backbone ABA triblock copolymers and (AB)n alternating block copolymers and a novel class of main chain thermotropic liquid crystalline polymers containing functional chain ends, i.e., polyethers. 

Three major topics of research which are based on phase transfer catalyzed reactions will be presented with examples. These refer to the synthesis of functional polymers containing functional groups (i.e., cyclic imino ethers) sensitive both to electrophilic and nucleophilic reagents a novel method for the preparation of regular, segmented, ABA triblock and (A-B)n alternating block copolymers, and the development of a novel class of main chain thermotropic liquid-crystalline polymers, i.e., polyethers. 

In 1967, C. J. Pederson of DuPont deNemours Co. synthesized the cyclic polyethers ( ) These cyclic polyethers are commonly referred to as "crown ethers" (see Figure 3). In solution, crown ethers are extremely effective ligands for a wide range of metal ions. The size of the ring cavity and the ionic radius of the metal affect the stability of the complex. Tables I and II list the cavity diameters for the crown ethers and the ionic radii of a number of metal ions (6-11). 

The crown ethers were originally discovered by Pedersen at duPont in 1967 <67JA7017>. Although cyclic polyethers had previously been reported <55JOC1147,56MI52100, 57BRP585229,59JCS3767), Pedersen was the first person to develop a systematic synthetic... 

Cyclic polyethers, or crown ethers, are cyclic structures containing ethylene glycol units as 2 2 with n > 2 and generally between 4... 

Poly ethers and Cyclic Poly ethers. During the past several decades, linear and cyclic polyethers have gained considerable prominence. This is largely due to their remarkable ability to complex metallic and organic cations. The linear poly ethers of the polyethylene... 

Early work in the area of crown ether biological mimics was reported by Vogtle and coworkers (Tummler, 1977). These early compounds were made in an effort to duplicate natural ion carriers in the sense that they could complex biologically important ions for transport. The early, two-dimensional crowns (such as 5) and non-cyclic polyethers gave way to the three-dimensional ciyptands (6), which generally complexed ions more tightly but which lack the dynamics of podands, crown ethers, or lariat ethers. 

The examples given here of the use of MM in synthesis are taken from the review by Lipkowitz and Peterson [28]. In attempts to simulate the metal-binding ability of biological acyclic polyethers, the tricyclic 1 (Fig. 3.12) and a tetracyclic analogue were synthesized, using as a guide the indication from MM that these molecules resemble the cyclic polyether 18-crown-6, which binds the potassium ion the acyclic compounds were found to be indeed comparable to the crown ether in metal-binding ability. 

Cyclic ethers with larger rings than epoxides include tetrahydrofuran (THF), tetrahydropyran (THP), and dioxane. Large-ring cyclic polyethers, called crown ethers, can selectively bind metal ions, depending on the ring size. 

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