Ethers, crown hemispherands

Oxygen atoms usually are less strong nucleophiles compared to sulfur atoms. Thus the formation of ethers belongs to the group of reactions using weakly reactive starting materials [12]. Many of the compounds which are important in su-pramolecular chemistry were cyclized by formation of an ether bond, e.g., several crown ethers [31], hemispherands [32], host molecules of host/guest chemistry [9i, 33], catenanes [34], and several natural products. 

As stated above, systematic names of macrocyclic host molecules were absurdly complicated for routine discussions [22]. Therefore Vogtle proposed the name coronand for crown ethers, and that of coronates for their complexes while cryptand complexes were called cryptates . The corresponding noncyclic analogues are podands such as 64 [23] and podates, respectively. The cumbersome name podando-coronands (and correspondingly podando-coronates ) was proposed for lariat ethers [24] having at least one sidearm like 65. Examples of hemispherands 66 [25], cavitands 25 [26] and those of some other hosts are discussed in Chapter 7 in some detail, whilst the exceptional stability of fragile guests 4 [2a] and 67 [27] in the hemicarcerand 5 cavity are discussed in Chapters 1 and Section 7.3. 

Hemispherands like 66 [37] and 239 [38] are macrocycles in which at least halfofthe ligating heteroatoms is preorganized prior to complexation (another part being crown ether) while in cryptahemispherands like 240 [39] there are two crown type bridges. The latter are very strong binders of alkali metal ions. 

Cross-fertilisation between the crown ethers (or, more generally, corands), cryptands, spherands and podands has produced an enormous range of hybrid hosts such as cryptaspherands and hemispherands, many exhibiting all the useful features of the parent materials (Figure 3.16). 

Other molecules include cryptophanes, for example, 24, hemispherands (an example is 25 " ), and podands The last-named are host compounds in which two or more arms come out of a central structure. Examples are 26 and 27 " and the latter molecule binds simple cations, such as Na, K, and Ca. Lariat ethers are compounds containing a crown ether ring with one or more side chains... 

Fig. 4 Representatives of typical cyclophane host families calixarene (21), resorcinarene (22), cavitands (23, 24), cdrcerand (25). hemicarcerand (26), crown ether (27), cryptand (28), spherand (29). and hemispherand (30)...

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