Macrobicyclic polyethers

Concerning the efficacy of this approach, the authors state that compared with the methyl ester of 4 -carboxybenzo-15-crown-5, the macrobicyclic polyethers are considerably more effective in binding and NH4 cations . ... 

The covalent assembly of guest-encapsulating host species has long been an area of research interest. The first examples of host species binding their guests within a three-dimensional array of interactions were the class of compounds known as cryptands [26,27]. The cryptands were designed as hosts for alkali metal cations and are based on macrobicyclic-polyethers (Fig. 2). Typically, cryptands are synthesised by the addition of a diacyl-chloride to an azacrownether (Scheme 1). 

The synthesis of a new 6/s(phosphotriester) macrobicyclic polyether cryptand, 0=P[0(CH2)20(CH2)20(CH2)20]3P=0, called phosphocrypt, has been described <94JOC7695>. The synthesis of four 1,1 -binaphthyl-based macrocyclic 6/sphosphane ligands (e.g., 13), in both racemic and optically forms, has been reported <94CB1411> their square planar cfr-P2-Metal (Ni Pd) complexes have also been established <94CB1411,94HCA409>. 

Under LL-PTC conditions, the anions associated with bulky lipophilic cations (quaternary onium salts, macro-cyclic and macrobicyclic polyethers, etc.) are extracted into the organic medium specifically solvated by a limited number (1-11 moles per mol of Q Y") of water molecules. Furthermore, two additional molecules of water are found to accompany the cyclic polyethers 5, 10, as both free and complexed ligands. This hydration sphere, characteristic of each anion, always reduces, more or less remarkably, anionic reactivity. As shown in Table 4, when changing from LL-PTC to anhydrous homogeneous... 

An established area of application of macrocyclic polyethers is the stereoselective complexation of chiral guest primary alkylammonium salts by optically active host macrocycles. Full details of the resolution, optical stability, and inclusion into chiral hosts of the binaphthol (84), and also of the chiral recognition properties of crowns [e.g. (85)] based on simple carbohydrate precursors, have been reported this year. An extension of the latter work" utilizes derivatives of the more complex carbohydrates D-glucose and D-galactose. The macrobicyclic polyethers (86) derived from D-glycerol or pentaerythritol have been suggested as potential chiral (at the bridgeheads) cryptands. 

The introduction of dipolar aprotic solvents " and the discovery of macrocyclic (crown ethers and macrobicyclic polyethers (cryptands represent some of the more significant steps. Dating from the late sixties, a new general technique was developed phase-transfer catalysis (PTC). PTC has the advantages of being extremely simple and economical and so met with immediate success in industrial applications. 

Classic PTC requires the presence of an aqueous-organic two-phase solution. As discussed previously, the catalyst (a quaternary salt, a macrocyclic or macrobicyclic polyether as a complex, and in general whatever kind of PTC catalyst, see Sect. 5) extracts some water into the apolar organic phase, mostly as specific solvation sphere of the anion. The amount of water transported, though scare, may affect the reaction to give undesired side effects. 

Macrocyclic and macrobicyclic polyethers bonded to polymers have been used mainly in ionic chromatography or in reverse osmosis membranes Recently,... 

Macrobicyclic ligands with carbon bridgeheads. The two carbon bridgehead macrobicyclic polyethers (70) and (71) synthesized by Parsons (Figure 2.23) are found to exhibit very high stability constants (64) (see Table 2.5) with (70) displaying the rare high selectivity of K /Rb. ... 

Macrocyclic polyethers (or crown ethers) also possess attractive cation complexing abilities resulting in the formation of lipophilic species. Not surprisingly, such ionophores have been studied with respect to calcium sensors (32-34), and two examples are shown in Table 3.12. The macrobicyclic polyether-amide (7) provides a better calcium ISE than its monocyclic analogue (8). 

Sodium, potassium and caesium have been used as the metal cation templates for the preparation of the bis(2-hydroxyethyl)diazacrown ethers L1518-L1520 and the macrobicyclic polyethers L1521-L1532 [175, 176], following the transformations depicted in Scheme 6-11. 

Scheme 6-11. Template synthesis of crown ethers L1518-L1520 and macrobicyclic polyethers L1521-L1532.

The aza-macrobicyclic polyethers are efficient catalysts in phase transfer reactions. 

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