Stabilization chromonics

Bond angles and lengths for 3-bromoflavone have been determined <82AX(B)983>. Whilst the two rings of the chromone system are essentially coplanar, the 2-phenyl group is twisted by 45.9° relative to the rest of the molecule apparently the steric interaction between the bromine atom at C-3 and H-2 outweighs the resonance stabilization which would be associated with a completely planar structure. 

An unusual oxidative breakdown of a p-tolyl group to a carboxylic acid (688) is accomplished in moderate yield without degrading the chroman system (51JCS76) and the stability of this system to oxidation (cf. chromone, Section 2.23.9.3) is further illustrated by the conversion of an ethyl to a carboxyl group in the formation of 2,4,4-trimethylchroman-2,7-dicarboxylic acid (689) (57JCS3060). 

Even though various antileukotriene drugs have been synthesized, none has reached clinical acceptability. Inhibition of histamine release by an apparent mast cell stabilizing mechanism (in lung tissue, but probably not elsewhere) is achieved with the carboxy-chromone derivative cromolyn sodium. The mechanism is believed to involve inhibition of histamine release from pulmonary mast cells by blocking Ca2+ movement through membrane channels. 

The synthesis of a series of triazolobenzopyranones carrying piperazinoalkoxy substituents on the phenyl ring demonstrated the feasibility of integrating the mast-cell-stabilizing properties of the chromone (benzopyranone) moiety and H( antihistaminic properties into one molecule. The most effective compound of a small series was BR-28390. Its antihistaminic potency (guinea pig ileum) was the same as mepyramine histamine release inhibition effectiveness (tested by rat passive peritoneal anaphylaxis) was somewhat less than cromolyn. 

Nanohybridization of B67 with silica network was recently attempted by Hara et al. (2007). The chromonic LC of B67 was mixed with silica precursor of tetraethoxysilane (TEOS) together with an anionic surfactant (EmalC20) and 2-(2-aminoethoxyl)ethanol (AEE). Here AEE plays an important role in stabilizing the columnar structure of B67. This molecule works as the mediator of the interface between the anionic B67 aggregate and silica. Without AEE, the columnar structure is transformed to a lamella structure in the process of drying (Hara et al., 2007). 

Furthermore, the yields of such transformations were generally improved due to the greater stability of the resulting rearrangement products. These studies represented a novel approach toward the synthesis of thio- and amino-chromones. 

A simple view of the state of a chromonic system is that the molecules are effectively insoluble in one dimension. The energy involved in their aggregation is more or less comparable to kT for each aromatic ring and only at extremely low dilution are there individual, unaggregated molecules. For ionic systems, the columns attract counterions, forming electrical double layers. This results in column-column repulsion, which causes the columns to behave as if they were separated by compressed springs. This effect presumably stabilizes the hexagonal structure of the M phase and causes the columns to move apart equally when the mesophase is diluted. 

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