Polyether ring

Cation Ionic diam/pm Cation Ionic diam/pm Polyether ring Hole size /pm... 

Additional macrocyclic polyether groups show no co-operative effect when the polyether rings diverge. For instance, the bis(crown ethers) [58] and [59]... 

Crown ethers [327] in which (L)-cysteinyl residues are connected with the polyether ring via amide bonds in the 2, 3, 11, and 12-positions exhibit... 

This residue is a mixture of stereoisomerio dicyclohexyl-18-crown-6 polyethers which may be contaminated with some unchanged dibenzo-18-crown-6 polyether and with alcohols arising from hydrogenolysis of the polyether ring. The submitter reports that this residue is sufficiently pure for many purposes such as the preparation of complexes with potassium hydroxide which are soluble in aromatic hydrocarbons. 

With Secondaiy and Tertiary Amine Functions in Chiral Macrocyclic Polyether Rings. 264... 

A number of chiral crown compounds containing more than one macrocyclic polyether ring have been described in the literature. They have been derived... 

Cation Cation Diameter Polyether Ring Hole Diameter (A)... 

The size of the ion must be smaller than that of the hole in the polyether ring. Thus, a stable complex was not formed if the ion was too large to fit into the polyether ring. 

Steric hindrance in the polyether ring prevents the formation of the complex. 

The slightly less negative value found for the entropy term can be explained by the preorganization of the ligand in the 1,3-alternate conformation, where only a small part of the crown ether moiety is rather flexible. This flexibility is lost with the large cesium cation, which fits very well into the cavity created by the polyether ring and the aromatic nuclei.34... 

The preparation of fused, polyether rings via RCM of enol ethers was reported by Rainier and co-workers <02T1997, 01TL179> (Scheme 38). The enol ether was prepared from the corresponding acetate using Takai s reduced-Ti protocol <94JOC2668>. Seven-membered, cyclic enol ethers could also be formed by this methodology. 

To confirm the inclusion mode for a sec. amine, the 1 1 piperidine 4 (n = 1) complex was isolated and its molecular structure was determined by X-ray analysis (Fig. 10) [22], The chromophore is planar (except the o-nitro group) within 0.1 A, to which the cyclic polyether ring extends perpendicularly. The protonated piperidine having a chair form structure is placed on the crown ring and stabilized by very short N+ —H—O- (2.65 A) and N+ —H—O (2.93 A) hydrogen bonds, and two N + —O ion-dipole interactions (3.17 and 3.16 A). The remaining ether oxygen, 0(2), seems to be ineffective on the complexation. 

Two types of BTX have been characterised, designated type A and type B type A consists of a flexible spine of 10-fused polyether rings type B contains a rigid ladder of 11 rings. Additional analogues have been isolated from shellfish and these are postulated to be bioconversion products, the result of metabolism in the mollusc. In addition to the BTX, several phosphorous containing molecules were isolated from K. brevis that exhibit high ichthyotoxicity (Koley et al. 1995 Husain et al. 1996 Mazunder et al. 1997). 

Brevetoxins are methylated cyclic polyether toxins. There are two main types type A comprised of a flexible backbone of 10 fused polyether rings, and type B with a rigid backbone of 11 poly ether... 

Figure 2.1. Brevetoxins are based on two different structural backbones, based on what are perceived to be the two parent molecules, PbTx-2 (brevetoxin B) and PbTx-1 (brevetoxin A). All other known derivatives are based on alteration of the R-side chain, epoxidation across the double bond in the H-ring of PbTx-2, or derivatization at the C-37 hydroxyl in PbTx-2. PbTx-8, the chloromethyl ketone derivative of PbTx-2, is an artifact of chloroform extraction and subsequent phosgene conversion of PbTx-2. Common features include trans-fused polyether ring systems consisting of five- to nine-membered rings. denotes likely chemical artifact from extraction (Baden et al. 2005).

Figure 3.25. Catenane synthesis from a polyether ring and a C-shaped cyclophane precursor...

Figure 4 (a) Maitotoxin. (b) Common structure feature of marine polyether ring junctions (3). (c) Brevetoxin A biosynthesis as proposed by 1)... 

Gallimore AR, Stark CBW, Bhatt A, Harvey BM, Demyd-chuck Y, Bolanos-Garcia V, Fowler DJ, Staunton J, Leadlay PF, Spencer JB. Evidence for the role of the monB genes in polyether ring formation during monensin biosynthesis. Chem. Biol. 2006 13 453-460. 

In another study, success was achieved in clipping a phenanthroline-polyether ring of the usual type on to each of three arms of a much larger cryptand-like skeleton that incorporated a phenantroline group in each arm (Figure 6.10). The prod-... 

Crown ethers are syntheHc macrocyclic polyethers that can form selective complexes with various cations. Anl8-crown-6 ether is illustrated in Fig. 8, where 18 indicates the total number of atoms in the polyether ring and 6 the number of oxygen atoms. Chiral crown ethers have been synthesized by... 

---