Acyclic hosts

Acyclic hosts with pendant binding sites are termed podands. The simplest podands are simply acyclic analogues of the crown ethers such as pentaethyleneglycol dimethylether (3.11), analogous to [18] crown-6, or its di-ol analogue (3.12). 

On the other hand, host-guest systems being composed of preorganized macrocyclic, cage-type or rigid acyclic host molecules... 

Figure 2.23 shows one example of an acyclic host that can recognize a guest molecule through the cooperative effects of different types of molecular interaction. The recognized guest in this case is L-tryptophan, and three different... 

Longer helicates with a central metal ion wire can be obtained from oligomeric bipyridines and similar ligands. 01igo-l,2-bipyridines have the potential to behave as bipyridine or terpyridine derivatives. These acyclic hosts undergo self-organization in the presence of metals with tetrahedral or octahedral coordination numbers (Figure 5.35). Double helices with several metal ions in close contact are thus formed". ... 

This section discusses some classical and contemporary examples of structural preorganization based either on the formation of macrocychc hosts or on the use of acyclic hosts comprising multiple small rings fused together. Subsequently, we discuss the importance of desolvation on the degree of preorganization of the host which impacts the overall affinity of host toward guest. 

Johnson and coworkers designed an acyclic host made more rigid by acetylene groups inserted adjacent to a central pyridine unit. The result was the highly conju-... 

Bile salts are chiral rigid molecules. They serve as important building blocks in the synthesis of both cyclic and acyclic hosts." " Natural bile acids are employed for enantioseparation of racemates of various classes of organic compounds by enantioselective inclusion complex-ation in the solid state." " " Crystals of bile acids contain... 

Noncyclic molecules are another class of hosts, having a chainlike or branching backbone with a number of binding sites on the acyclic molecules. Compared to macrocycles, acyclic hosts have more flexible structures, causing faster sensing responses but lower binding constants. In this section, a couple of typical examples of sensors based on noncyclic molecules are presented. 

Podand An acyclic host bearing binding sites that can encircle a suitable guest, e.g. tetraglyme. 

Kuokkanen evaluated a series of constants. Kpeg for substituted diazonium ions with PEG 1000 and found a reaction constant (p = 1.12) comparable to those for complexation with the three crown ethers investigated by Nakazumi et al. (1983), p = 1.18-1.38). It is therefore likely that the host-guest interaction of diazonium ions with acyclic polyethers is basically similar to that with crown ethers. A dual substituent parameter analysis (DSP, see Sec. 8.3) for (Nakazumi et al., 1987)... 

Extensive molecular dissection of the morphine molecule over the past several decades led to a host of molecules which showed narcotic analgesic activity even though they possessed but faint suggestion of the structural features present in morphine itself. Thus, both cyclic molecules such as meperidine (70) and alphaprodine (71), and acyclic Compounds such as methadone (72) were found to be effective analgesics. Common features of these compounds were formalized by the Beckett-Casy rule, which states as minimal required structural features (a) an aromatic ring attached to... 

To test this hypothesis further, simple acyclic cobaltocenium derivatives (Fig. 38) containing tertiary amide groups ([72] and [73]) were prepared and complexation with Cl- and Br anions was investigated in solution by nmr spectroscopy. No shifts of the host s proton resonances were observed, emphasizing again the importance of the amide N—H group in anion binding. 

While earlier it was generally thought that the acyclic monoterpene alcohols 242-245 are derived from the host tree s oleoresin component, myreene [453], more recent results clearly show that at least in some species they are produced denovo [454-456]. 

Table 15 Chiral discrimination ability (t/Is-Dn values) of permethylated cyclic and acyclic oligosaccharide hosts toward amino acid 2-propyl ester salts by FAB-MS...

Cidofovir (Vistide) is an acyclic phosphonate cytosine analogue with activity against herpesviruses including CMV, HSV-1, HSV-2, EBV, and VZV. It also inhibits adenoviruses, papillomaviruses, polyomaviruses, and poxviruses. Activation of cidofovir requires metabolism to a diphosphate by host cellular enzymes. Because this activation does not depend upon viral enzymes, similar levels of cidofovir diphosphate are seen in infected and uninfected cells. Cidofovir diphosphate competes with deoxycytidine triphosphate (dCTP) for access to viral... 

Figure 20. Potentiometric responses to anionic guest 78 by PVC matrix liquid membranes containing cyclophanes (74, 76,77) or acyclic reference compound (75). (a) Membrane potential as a function of pH in the presence ( ) or absence (o) of 1.00 X 10 M guest, (b) Membrane potential as a function of guest concentration at pH 5.0 (0.01 M AcONa-AcOH buffer) and room temperature (ca. 20 °C). Membrane composition DOS (95 mg) and PVC (44 mg) containing 6.2 x 10 mmol host.

Even before the discovery of the katapinands vide supra), work upon bidentate Lewis acid hosts suggested the possibility of chelation of anionic sjjecies by acyclic-boron-containing ligands such as BF2CH2CH2BF2. In many ways mul-... 

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