Natural cyclophanes

Because of totally synthetic nature, cyclophanes are one of the most important class of hosts which will contribute to understanding of the detailed aspects of... 

Following the biogenetic classification, we first address macrolactones, divided into the main family groups that have received synthetic attention via RCM (re-sorcinylic, salycilates, other antibiotic macrolides, macrocyclic musks, epothilones, amphidinolides, other polyketides, and natural cyclophanes), then the terpenoids, followed by the macrocycles obtained from the amino acid metabolism (lactams, depsipeptides, alkaloids), and finally the glycolipids. 

While the previous receptors are typically used in organic solvents, except for the cyclodextrins, there are special cases of cyclophane receptors supphed with peripheral charges (ammonium units) (107—12) or ionizable groups (carboxylate functions) (113,114) (Fig. 17) to allow substrate recognition, as in nature, in an aqueous medium, profiting from the solvophobic effects of water (115). 

Cyclophanes are naturally suited for MMPI (15b) calculations. The results ofsuch calculations regarding the structures and electronic spectra of the [m] paracyclophanes (n = 5-10) agreed well with the experimental data (169). Attempted X-ray analyses of [2.4]- and [2.5](9,10)-anthracenophanes (46) encountered serious disorder in the ahphatic bridges. MMPI calculations of all possible conformers of these molecules revealed four and six energy minima for 46a and 46b, respectively. Comparison of the calculated C10 C10 distances and bridge conformations with X-ray information unambiguously identified two conformations each for 46a and 46b as the final solutions. These and the calculated structures of photoisomer 47 were highly useful in the interpretation of fluorescence spectra and photoisomerization processes of 46 (170). 

The properties of natural macrocycles, the cyclodextrins, has stimulated interest in the preparation of synthetic macrocycles. Three basic types have been made macrocyclic amines, cyclophanes, and cyclic peptides. Hersh-field and Bender (33) prepared a bicyclic amine with hydroxamate sub-... 

While nature uses coenzyme-dependent enzymes to influence the inherent reactivity of the coenzyme, in principle, any chemical microenvironment could modulate the chemical properties of coenzymes to achieve novel functional properties. In some cases even simple changes in solvent, pH, and ionic strength can alter the coenzyme reactivity. Early attempts to present coenzymes with a more complex chemical environment focused on incorporating coenzymes into small molecule scaffolds or synthetic host molecules such as cyclophanes and cyclo-dextrins [1,2]. While some notable successes have been reported, these strategies have been less successful for constructing more complex coenzyme microenvironments and have suffered from difficulties in readily manipulating the structure of the coenzyme microenvironment. 

There are several types of natural and synthetic molecular hosts, such as cyclodextrin and cyclophane, that are shaped to accommodate neutral and charged organic molecules in the three-dimensional cavity. The inclusion complexation by molecular hosts is driven by various weak forces like van der Waals, hydrophobic, hydrogen bonding, ion-dipole, and dipole-dipole interactions, and therefore the molecular recognition process seems much more complicated. In expanding the scope of the present theory, it is intriguing and inevitable to perform the extrather-... 

In contrast to cyclodextrins, there is no parent cyclophane structure available in nature. Therefore, the design of the host molecule starts from the... 

Schneider and coworkers investigated the capability of the tetraazonia cyclophane 51 to bind nucleosides and nucleotides in water [72]. Its two diphenylmethane units border a lipophilic cavity for the uptake of likewise lipophilic molecules or subunits. As evidenced by HNMR studies of the complexes, only in the case of the purine derived nucleosides and nucleotides (A, G, AXP, GMP) the base moiety is included inside the host cavity but not in the case of the pyrimidine analogues (U, C, UMP, CMP, TMP). Due to their hydrophilic nature, the sugar and phosphate groups remain outside the niche. 

Cyclodextrins and calixarenes are members of a general class of macrocycles commonly referred to as cyclophanes, although the naturally occurring CDs are usually considered separately from the synthetic members of the cyclophane family of compounds. The properties and guest-host complexes of cyclophanes... 

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