Huge macrocycles

Tetraloop compounds 9 contain a huge macrocycle (formed by fourfold metathesis), which is covalently connected to the calix[4]arene skeleton via four urea linkages. Cleavage of these connections is easily possible in boiling acetic add (Scheme 5.13). Thus, flexible, macrocyclic molecules 11 are available in 50-75% yield [53], which would be difficult to obtain by non-templated macrocydization reactions under high-dilution conditions. 

Scheme 5.14 Reaction sequence (schematic) forthe synthesis of huge macrocycles using a calix[4]arene twice as template. The single steps are discussed in the text.

A huge group of macrocycles which contain 2,6-disubstituted phenols are the calixarenes [51]-[53]. Their conformation has been investigated intensively (Gutsche, 1989, 1991). In most conformations, however, the phenolic rings are oriented almost vertically in relation to the plane of the macrocydic ring. Therefore the OH functions are not oriented in an intra-annular fashion. Nevertheless the pK values of calixarenes [51] differ from those of other comparable phenols. The reason for this is the... 

Figure 2.1.47 Huge channel structure (including hydrogen bonds) of cyclohexa-amylose potassiumacetate hydrate (62) (P2i2i2) [93] along [001] (slight rotation around Y to visualize three layers) with potassium acetate and water in the channels between the macrocycles.

In 1937, van Alphen described the synthesis of the aza-cycloalkane (saturated azamacrocycle) 1,4,8,11-tetraazacy-clotetradecane (6), more commonly known as cyclam, obtained by reaction of 1,3-dibromopropane and 1,4,8,11-tetraazaundecane in the presence of an alkali. Twenty-four years later, Stetter and Mayer synthesized the ligand by a different route and showed that the material described by van Alphen contained only a small amount of the claimed ligand, but it took another five years for the first metal complex with cyclam to be reported. After these early works, a huge number of saturated azamacrocycles were synthesized, making the azacycloalkanes one of the most studied class of macrocyclic ligands. " ... 

Since the pioneering work of Charles J. Pedersen, Donald J. Cram and Jean-Marie Lehn, macrocyclic chemistry attracted a considerable attention from chemists all over the world [1-4]. An ability of macrocycles to be highly selective receptors (host molecules) for a number of metal and organic cations or anions, as well as for the small or even huge (e.g. fiillerenes) neutral organic substances is the reason of that interest [5-8]. The design of molecular devices such as molecular containers and reactors on one hand, and molecular switches (based on catenanes and molecular knots) on the other hand is another area of the macrocyclic chemistry application. So, the macrocyclic chemistry plays an important role in the material science, especially in the constmction of nano-sized materials on the bottom-up principles [5, 6, 9-11]. 

---