Concave structure

Acid-Base Behaviour in Macrocycles and Other Concave Structures... 

In the previous Sections, the properties of acids and bases in macrocycles and other concave structures have been compared. A number of factors have been recognized which influence the acidity or basicity of an acid or base (i) hydrogen bonds, (ii) hindered solvation (exclusion of solvent), (iii) formation of tight ion pairs (high microacidity but low overall acidity), and (iv) Coulomb forces when poly anions are formed. A fifth influence, (v) steric hindrance, still has to be discussed. 

Acid-base behaviour macrocycles and other concave structures, 30, 63... 

Macrocycles and other concave structures, acid-base behaviour in, 30,63 Macromolecular systems in biochemical interest, 13C NMR spectroscopy in, 13,279 Magnetic field and magnetic isotope effects on the products of organic reactions, 20,1 Mass spectrometry, mechanisms and structure in a comparison with other chemical processes, 8, 152... 

Inside the cell, numerous chemical processes take place at the same time. The cell solves the problem of generating a large number of different molecules at the same time with a high selectivity by the use of enzymes. The selectivity of these proteins is largely determined by their geometry. Also the selectivity of another class of proteins, the receptors, is influenced by geometrical features. Receptors and enzymes have in common that they are equipped with concave structures such as clefts or cavities [1] in which substrate molecules are bound or chemically modified. 

Whereas the identification of a [1 + 1] or a [2 + 2] cyclization product was unequivocal by MS, it was more difficult to distinguish between the concave bimacrocycles and the bis-cyclophanes. But a combination of NOE investigations with complexation studies followed by NMR, and finally X-ray analyses proved the bimacrocyclic concave structure of the concave acids and bases 21,29 and 38 [15, 20, 27a]. 

The addition of ethanol to diphenylketene (59a) shows that a standard catalyst like pyridine (50) may be incorporated into a concave structure and that... 

In the thiol 79 and the sulfinic acid 80, two new acidic functionalities are now present in the 2 -position of a m-terphenyl system and should be incorporable into concave structures by a double bridging of the m-terphenyl. For the acetyl-protected thiol 78, the corresponding concave thiol acetate 42 (Scheme 7) could be obtained after tetrabromination of the four methyl groups and bridging with m-phenylenedithiol (see Sect. 2.3). 

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