Chiral crown ethers properties

Only Cram (36) has published a rationale for the very high (99%) enantiomeric excess achieved in the reaction of methyl vinyl ketone and the hydrindanone in the presence of the chiral crown ether. This mechanism envisions a bimolecular complex comprising the potassium cation and chiral host as one entity and the enolate anion of the hydrindanone as the counterion. Methyl vinyl ketone lies outside this complex. The quinine-catalyzed reaction appears to have a termo-lecular character, since the hydroxyl of the alkaloid probably hydrogen bonds with the methyl vinyl ketone—enhancing its acceptor properties—while the quin-uclidine nitrogen functions as the base forming the hydrindanone—alkaloid ion pair. 

The functionality that is often necessary for a chiral crown ether to serve a particular purpose can usually be introduced by the synthetic chemist without too much difficulty. The practice here, however, can be very much more demanding on account of the promiscuous receptor properties of the compounds that have to be separated and isolated pure from reaction mixtures containing many components. 

The properties of chiral crown ethers in solution both in their free state and as their complexes with a wide range of substrates have been reviewed exten-... 

By choosing to survey the field of chiral crown ethers at the level of their chemical production, it has been possible to expose the wide range of stmctural types that have been prepared and characterized to date. Clearly, the properties of some have been investigated and exploited by their makers to a much larger... 

The enantioselective binding properties of certain chiral crown ethers have been employed in the resolution of amino add racemates. The racemic amino ester is adsorbed onto silica gel as its ammonium salt and eluted by a chloroform solution of the chiral crown ether. An excellent separation of the two enantiomers is achieved by this method (74JA7100). 

After many trials in the molecular design of chiral crown ethers, a 1,1 -binaphthyl unit incorporated in a macroring by substitution in the 2,2 -positions proves to possess the desirable properties. The naphthalene-containing system, chosen for practical and strategic reasons, imparts rigidity and lipophilicity to conventional cyclic polyethers. The synthesis of such a host is presented in Fig. 5.2. 

The property of chiral recognition shown by asymmetric crown ethers towards chiral substrates clearly mirrors the asymmetry of natural interactions and processes. This field is covered in Section 5.21.3.2.2. 

Crown ethers of the type discussed in this section have been used as sensors, membranes, or materials for chromatography. Shinkai used cholesterol-substituted crown ether 10 as a sensor for chirality in chiral ammonium compounds (Scheme 16). It was found that the pitch of the cholesteric phase exhibited by 10 was changed upon addition of the chiral salt. As the wavelength of reflection for incident light depends on the pitch, a color change was observed that was visible to the naked eye [45, 46]. Such chirality sensing systems were known before but chromophores had to be bound to the crown ether in order to observe color changes [47]. This problem could be overcome by 10, which uses intrinsic properties of the chiral nematic phase. 

Shinkai and coworkers prepared numerous novel amphiphilic crowns (Shinkai, 1990) and incorporated them into membranes, formed membranes from them, or used them in liquid crystalline assemblies to control properties (He et al., 1990). Interest in this area continues. Four chiral amphiphilic crown ethers were recently reported that recognize enantiomeric amino acids when examined as Langmuir films (Badis et al., 2004). Finally, it is interesting to note that liposomes formed from amphiphiles (e.g., crown ethers) having neutral headgroups (i. e., niosomes) have been studied as drug delivery vehicles (Uchegbu and Vyas, 1998). 

Carbohydrate crown ethers were obtained with ethylene spacers from a crown ether point of view, the carbohydrate vicinal diols are replacing one ethylene glycol unit [181,182], Cyclic compounds synthesized include bis-gluco-15-crown-5 82, bis-gluco-21-crown-7, and tetra-gluco-24-crown-8 (O Scheme 14). These chiral macrocycles could serve as catalysts in the asymmetric Michael addition of methyl a-phenylacetate to methyl acrylate. With the goal to study molecular interactions, P,P, and Q, Q -bis-maltosides with aliphatic two-, three-, or four-earbon spacers were synthesized [183]. Spaced cyclodextrins were prepared to study their supramolecular properties [184,185]. 

---