Crown ether, hosts

Host-guest inclusion complexes, 262—263 antibiotic hosts, 231—233 cahxarene hosts, 228—231 chiral crown ether hosts, 213—218 cyclic oligosaccharide hosts, 218—222 cyclodextrin host selectivities, 223/ host molecular size, 221 hnear ohgosaccharide hosts, 222—228 ir- TT stacking interactions, 217 proteic hosts, 231 Human 15-hpoxygenase, 52/... 

M. Sawada et al., Enantioselective complexation of carbohydrate or crown ether hosts with organic ammonium ion guests detected by FAB mass spectrometry. J. Am. Chem. Soc. 115, 7381-7388 (1993)... 

HOST-GUEST INCLUSION COMPLEXES Chiral crown ether hosts... 

In the context of crown ether hosts, non-covalent bonds of pole-pole, pole-dipole, and dipole-dipole types can all be employed [3-6] in the formation of host-guest complexes. Where the guest species is an alkali metal (i.e. Li, Na", K", Rb, Cs ), alkaline earth metal (i.e. Mg, Ca, Sr, Ba ), or harder transition or post-transition metal (e.g. Ag", TT, Hg, Pb, La, Ce ) cation [3-6,14], an electrostatic (M" O) pole-dipole interaction binds the guest to the host whilst the (M" X ) pole-pole interaction with the counterion (X ) is often retained. The features are exemplified by the X-ray crystal structure [15] shown in Fig. la for the 1 2 complex (1) (NaPF jj formed between dibenzo-36-crown-12 (1) and NaPF. Molecular complexes involving metal cations have considerable strengths even in aqueous solution and a template effect involving the metal cation is often observed during the synthesis of crown ether derivatives. 

A number of other crown ether hosts with more than one primary binding site have been synthesised [17,63]. CPK molecular models of the chiral bismacrocycle (5)-(42) reveal that this host possesses a jaws-like receptor site in which the binaphthyl unit can serve as a hinge. The X-ray crystal structure [64] of the 1 1 complex ((S)-(42)-H3N(CH2)4NHj2PFj) shown in Fig. 5 possesses the expected structure. The host (S)-(42) has been used [5] to bind other bifunctional guests. 

Fig. 1 Crown ether hosts used as second-sphere ligands for transition metal complex guests...

Fig. 3 The second-sphere adduct between a ferrioxammine B guest and an ionophore or crown ether host.

Complexation of neutral hydrophobic organic guests and hydrophilic metal cations is usually characterized by different solvent dependencies. In this context, it is of interest to examine the binding behavior of the cyclophano-crown-ether host (shown in Fig. 6) in solvents of varying... 

Fig. 6 Interaction of potassium ion and 6-methoxy-2-naphtho-nitrile with cyclophane-crown ether host various water-methanol mixtures. View this art in color at www.dekker.com.)...

Fullerenes can also act as hosts. The reactive surface of the Cgo shell can be utilised to attach a plethora of functional groups. An elegant example uses a fullerene crown ether host as an electrochemical sensor for K+. Compound 5.31 incorporates a dibenzo[18]crown-6 in close proximity to the Cgg surface, acfing as fhe... 

FIGURE 9.9 Enantioselectivity (IRIS) for enantiomeric chiral crown ether host selectors discriminating enantiomeric isoto-pomers of amino acid methyl esters in competitive binding experiments by FAB-MS. Qnantitative cross-chiral relationships... 

Cyclophano-crown ether host (Scheme 8), possessing cyclic polyether and cyclophane moieties in a single molecule and ability to simultaneously bind a potassium cation and 6-methoxy-2-naphthonitrile in a water-methanol mixture, is of particular interest, since the contrasting solvent effects can be examined in the same host. Indeed, neutral 6-methoxy-2-naphthonitrile guest forms a 160-fold more stable complex with the cyclophane moiety in more hydrophilic 60 40 water-methanol solvent than in pnre methanol. In contrast, the affinity toward potassium ion is enhanced by a factor of 120 in pure methanol rather than in 60 40 water-methanol. Scheme 8... 

Smith and coworkers have also harnessed related anunonium ion/crown ether interactions to template the assembly of two Boc-protected L-lysine dendrons (Scheme 9)." It was observed that the addition of potassium ions, a competitive gnest of the crown ether hosts, triggers disassembly of the snpramolecniar dendrimer (41) to its component parts (42 and 43). This example of stimuli-responsive dendritic architectures that release small molecules on demand clearly demonstrates advantages inherent in non-covalent dendrimer synthesis over conventional, covalent approaches. 

Huang et al. reported apseudo[l]catenane produced by a combination of pillar[5]arene and crown ether host-guest chemistry. A novel co-pillar[5]-arene, bearing two benzo-18-crown-6 units (8.48), was synthesized by a CuAAC reaction (Figure 8.8). 1,8-Diaminooctane was added as the guest molecule to thread into the pillar[5]arene cavity to form the... 

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