Bridged calixarenes intramolecular

As in the case of carbon-bridged calixarenes, conformational behaviors of azacalixarenes in solution have been investigated by means of temperature dependent H NMR spectroscopy. It was reported that azacalixarenes 4a-f, 7b-d, 7f, 7g, 8a, 20b, and 20f were conformationally flexible in solution [17-19,24,25]. Gong et al. pointed out from their NMR studies of compounds 7b, 7f, 20b, and 20f that the lack of steric hindrance and intramolecular hydrogen bonds were responsible for their high conformational mobihty in solution, as compared to carbon-bridged calixarenes [24]. 

The possible number of inherently chiral structures and conformers further increases if the calixarene contains both different phenolic units and different bridges in the macrocyclic skeleton. For example, two chiral monoethers 88a,b are available from dihomooxacalix[4]arenes (one -CH2-0-CH2- bridge instead of -CH2-).17188b is the preferred product of the mono-O-alkylation, since the negative charge of the respective phenoxide anion is better stabilized by intramolecular hydrogen bonds due to the smaller distance between the phenoxide anion and the hydroxy groups. Tetraketone derivatives (Y = CH,-C(0)-R) in the two possible partial cone conformations, have been prepared in moderate yields. 

As polyhydroxy compounds, calix[w]arenes can be reacted with multifunctional electrophilic reagents not only inter- but also intramolecularily. Some di- and multicalixarenes as examples of intermolecular reactions are reported in Section VUI. Intramolecular (i.e. bridging) reactions have been used to rigidify the calixarene skeleton, to protect hydroxy groups and to create ligands for metal cations, which in many cases show an unprecedented selectivity. 

A simple calixarene built up with four phenol units linked via methylene bridges is shown in Fig. 10 (substitution of the methylene junctions with S atoms gives rise to the class of thiocalixarenes). Cooperative networks of intramolecular hydrogen bonds (circular array of hydrogen bonds, observed also in cyclodextrins) play a capital role in the cavity shape as well as in the conformational features of the macrocyclic skeleton [291-293]. Encapsulation of other molecules in the cavity is also controlled by hydrogen bonds [294]. 

Another way to synthesize novolacs is under weak acidic conditions. Typical reaction conditions are a pH-range between 4 and 7 and the use of bivalent metal salts (e.g. Ca, Zn, Mg, Pb, Co, Cu, Ni) [56,57]. These conditions are highly selective for ortho,ortho -novolacs that are of increasing interest These compounds possess only a vacant para position for further reactions. Such materials have increased importance as molding compounds and foundry resins because ortho-linked novolacs can only react at the para position and they possess a hyperacidity [58]. The hyperacidity can be easily explained by the formation of intramolecular hydrogen bondings that stabilize the phenoxy anion. Complete methylene bridging at the ortho position of phenolic compounds can lead to calixarenes. 

Scheme 10.5 Distal bridging of formyl calixarenes via intramolecular McMurry reductive coupling...

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