Macrocycles polyether

The strength of this bonding depends on the kind of ether Simple ethers form relatively weak complexes with metal ions but Charles J Pedersen of Du Pont discovered that cer tain polyethers form much more stable complexes with metal ions than do simple ethers Pedersen prepared a series of macrocyclic polyethers cyclic compounds contain mg four or more oxygens m a ring of 12 or more atoms He called these compounds crown ethers, because their molecular models resemble crowns Systematic nomencla ture of crown ethers is somewhat cumbersome and so Pedersen devised a shorthand description whereby the word crown is preceded by the total number of atoms m the ring and is followed by the number of oxygen atoms... 

The compound known as 18-crown-6 is one of the simplest and most useful of the macrocyclic polyethers. Its synthesis in low yield was first reported by Pedersen. Greene and Dale and Kristiansen" have reported syntheses of the title compound from triethylene glycol and triethylene glycol di-p-toluenesulfonate. Both of these procedures use strong base and anhydrous conditions and achieve purification by more or leas classical methods. The combination of distillation and formation of the acetonitrile complex affords crown of high purity without lengthy chromatography or sublimation. ... 

In Pedersen s first full paper on the synthesis of macrocyclic polyethers, he reported the synthesis of thirty-three macrocycles having a variety of ring sizes and substituents. These compounds were prepared by a variety of routes and Pedersen offered a scheme in which he generalized the synthetic approaches he used. He designated the four principal methods V , W , X , and Y . The methods are diseussed individually. 

Macrocyclic polyethers containing the 2.2-paracyclophane unit are interesting structures and several such compounds have been prepared . Despite the diverse structural possibilities, the syntheses of these molecules have generally been accomplished by straightforward Williamson ether syntheses. The only unusual aspect of the syntheses appears to be a novel approach to certain paracyclophanes developed by Helgeson (see footnote 7a in Ref. 91). The first step of Eq. (3.28) illustrates the formation of the required tetrol, which is then treated with base (KOH or KO-t-Bu) and the appropriate diol dito-sylate to afford the macrocycle. 

Almost as soon as Pedersen announced his discovery of the crown ethers (see Chaps. 2 and 3) it was recognized by many that these species were similar to those prepared by Busch and coworkers for binding coinage and transition metals (see Sect. 2.1). The latter compounds contained all or a predominance of nitrogen and sulfur (see also Chap. 6) in accordance with their intended use. The crown ethers and the polyazamacrocycles represented two extremes in cation binding ability and preparation of the intermediate compounds quickly ensued. In the conceptual sense, monoazacrowns are the simplest variants of the macrocyclic polyethers and these will be discussed first. 

In the course of experiments directed at the synthesis of open-chained equivalents of macrocyclic polyethers (see Chap. 7), Rasshofer and Vogtle found that 9 or its derivatives were formed rather than the desired compounds . Although the yields obtained by this method are modest, the facts that the reaction is conducted without the use of high dilution and in one-pot recommend it. 

In light of the considerable recent Interest in the macrocyclic polyether-polyester antibiotics, Samat and coworkers have prepared a large-ring macrocycle as a model for the... 

From their structures, it appears that the hydrolytic stability of macrocyclic lactones must necessarily be inferior to macrocyclic polyethers. Ease of synthesis of the cyclic esters is therefore one of the aspects which commend them to interest. It is probably for this reason that such lactones have not been made more often by the interesting approach of Kdgel and Schroder . These workers report the ozonolysis of dibenzo-18-crown-6 in a mixture of methanol and dichloromethane at —20°. Reduction of the ozon-ide at —75° using dimethylsulfide followed by warming and addition of acetone led to formation of 6 in 14% yield. The bis-oxalate had mp 164—165° from acetone, very similar to that of the starting crown. The transformation is illustrated below in Eq. (5.9). 

The first elaboration of crowns deliberately into sulfur-containing analogs seems to have been conducted by Pedersen . The first of these efforts was directed at incorporating sulfur in complexes of macrocyclic polyethers. Inclusion of sulfur as a portion of ring system 5 was also accomplished as shown in Eq. (6.4) and is reported in a patent which issued before the latter, but was actually filed after it. We note... 

We have not attempted to cover all or even most aspects of crown chemistry and some may say that the inclusions are eclectic. We felt that anyone approaching the field would need an appreciation for the jargon currently abounding and for the so-called template effect since the latter has a considerable bearing on the synthetic methodology. We have, therefore, included brief discussions of these topics in the first two chapters. In chapters 3—8, we have tried to present an overview of the macrocyclic polyethers which have been prepared. We have taken a decidedly organic tack in this attempting to be comprehensive in our inclusion of alkali and alkaline earth cation binders rather than the compounds of use in transition metal chemistry. Nevertheless, many of the latter are included in concert with their overall importance. 

Self-assembly of secondary dialkylammonium ions with macrocyclic polyethers and formation of pseudorotaxanes 96CC1483. 

Structure-chemical aspects of complex formation in metal halide-macrocyclic polyether systems 99UK136. 

Derived from Macrocyclic Polyethers and the Related Compounds... 

Gokel, G. W., Korzeniowski, S. H. Macrocyclic Polyether Syntheses, Berlin Heidelberg New York, Springer-Verlag 1982... 

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