Chromonic materials

The ease of intercalation of other chro-monic and potentially chromonic materials. A number of biochemical reagents, such as acridines and ethidium bromide, intercalate readily between the stacked bases in DNA and RNA. Anticancer drugs, such as the square planar platinum complexes, intercalate avidly (Fig. 14) and naturally occurring antibiotics, such as actinomycin, similarly act by intercalation into the stack of bases [67]. They act as tailor-made spanners in the works and prevent the reading and replication of DNA. 

Khellin is a natural product closely related to the psoralens in which a chromone ring has been substituted for the cou-marin. The plant material has been used since ancient times as a folk remedy modern pharmacologic work has confirmed the bronchio-dilating and antispasmodic activity of khellin. The synthesis outlined below, it should be noted, is selected from a half-dozen or so reported within the last quarter century. 

Syntheses of 2- and 3-substituted chromones normally start from 2-hydroxyphenyl ketones. In the first of two examples, a route to flavone is shown in Scheme 5,10 using 2-hydroxyacetophenone (2-hydrox-yphenylethanone) and benzoyl chloride as starting materials. Initially, the phenolic group of the acetophenone is O-acylated by benzoyl chloride, using pyridine as a base (a Schotten-Baumann-type reaction). 

The problems of configurational assignment are principally the same for the cases discussed in Sections 4.3.3.2.2. (Formation of Chiral, Racemic Products) and 4.3.3.2.3.1. [Formation of Nonracemic Products with Known Configuration at (at least) One Chiral Unit]. This can be illustrated by the example of the conjugate addition of a methyl group to a chromone (9 on p 411) which has been performed both with racemic and with enantiomerically pure material. Thus the (relative) configuration of the reaction products was determined making use of both the racemic and the nonracemic series (see pp 472 and 480)108. 

The vigorous conditions which are necessary serve to detract from the value of the Perkin synthesis, leading to the production of tarry material which adversely affects the yield of coumarin. Difficulties encountered in the synthesis of substituted o-hydroxybenzaldehydes also limit the application of this route. The obvious advantages of the method are that there are no doubts about the orientation of the product and that, unlike the Pechmann reaction, formation of the isomeric chromones is not possible. 

Naturally, less scope is available with the ester component and the C-2 substituent in the chromone is commonly alkyl or ethoxycarbonyl, or is absent. The use of diethyl oxalate offers an added attraction the intermediate 1,3-diketo ester (436) readily undergoes transesterification, thereby allowing the synthesis of various chromone-2-carboxylic esters to be achieved from the one starting material (Scheme 148) (74JCS(P1)2570). 

The pyranobenzopyran 1 is readily available and has proved to be a useful starting material for the preparation of a number of 3-substituted chromones. Thus, reaction of 1 with a large excess of an enolisable ketone RCOCH3 in the presence of aqueous acid resulted in smooth conversion into the chromones 2. 

The reaction between the acid chloride of chromone-2-carboxylic acid and ethyl ethoxymagnesioacetoacetate probably leads to the expected fi-diketone which enolizes and cyclizes spontaneously to spirofuranone(52).127 A different approach was made by Hungarian workers in their synthesis of tachrosin (53), an unusual kind of flavone isolated from Tephrosia poly-stachyoides and one of the earliest natural furanones to be isolated. They subjected an unsaturated ketone (Scheme 32) to oxidative rearrangement by thallium(III) salts, a reaction well known in chalcone chemistry, and eliminated methanol from the product to obtain the necessary starting material.128... 

The furyl chromone, which is a nonfluorescent compound, is smoothly converted by irradiation to the isomeric 1-arylfuro [3,4-b]chromone-3-acrolein. The starting material absorbs strongly at 295 nm where the reaction product does not absorb. As the reaction proceeds, the absorption in this region decreases. At longer wavelengths the product absorbs more than the precursor and shows two maxima at 368 nm and at 430 nm. It has an orange color and fluoresces Intensively at 527 nm (Xmax 7 nm ... 

Fluoroalkyl chromone derivatives can be obtained from the base catalysed reaetion between 2,2-dihydropolyfluoroalkanoates and phenols. Initial loss of HF allows Michael addition of the phenol and subsequent cyclisation of the enol ether yields the heterocycle. m-Substituted phenols give the expected mixture of 5- and 7-substituted chromones, whilst dihydroxy aromatic compounds give polycyclic materials (94JFC263). In a more conventional approach Z-3-(aryloxy)-polyfluoroalkenoic adds, derived from Michael addition of phenols to polyfluoro-2-alkynoic acids, undergo intramolecular Friedel-Crafts acylation to 2-polyfluoro-alkylchromones (94JFC25). 

Reversed-phase HPLC is the method recommended for the screening of plant materiaL in which chromone alkaloids can be present. Mostly C-18 columns were applied with aqueous mobile phases containing high concentrations of buffer at pH 4.5-6.5, modified with methanol or acetonitrile. Polyhydroxy alkaloids of the pyrrohdine, piperidine, pyrro-lizidine, indolizidine, and tropane classes, because of their lack of a suitable chromophore, have rarely been analyzed... 

The Simonis chromone synthesis has also been applied to phenol-like systems. The reaction of 40b (in equilibrium with 40a) with 6 and P2O5 provided a mixture of the Pechmann and Simonis products 41 and 42 when anhydride 40a,b was substituted (R = Me). When the material was unsubstituted (R = H) only 43 was isolated. The authors suggested that the formation of the 1,4-pyran was followed by rearrangement to the more stable product. While the yields were low, entry was established into these highly oxidized bis-pyranyl systems. 

In all the cases, the desired products were obtained with high selectivity, almost free of secondary products, and the unchanged starting materials were recovered nearly quantitatively. The yields of flavones were similar to those of chromones, however, the experiments performed in solvent-free conditions showed a substantial reduction of the reaction times. No stereoelectronic effects owing to the substituent were observed on the yield for any of the catalysts (Table 1.6). Both recycled catalysts showed almost constant activity when used in two consecutive batches after the first one. 

The total synthesis of the natural chromone pestalotiopsone A includes a microwave-promoted aldol condensation and oxa-Michael cyclization to construct the chromanone core, followed by 2-iodoxybenzoic acid (IBX)-induced dehydrogenation to afford the desired chromone. The synthesis of pestalotiopsones B, C, and F is also accomplished using similar approaches (13OBC1109). 1,3,5-Trimethoxybenzene is the starting material for the... 

What is the discotics for the thermotropic liquid crystals that is the lyotropic chromonic liquid crystals (LCLCs) for the lyotropic materials. LCLC molecules are plank-like rather than rod-like, rigid rather than flexible, aromatic rather than aliphatic. Typical LCLC molecules and their aggregation in... 

Since very little is known about LCLC structures and properties, the recognition of these materials is not widespread. However, as one of the pioneers in the field, J. Lydon, writes "A single commercial application, however, could change this picture overnight. The liquid crystal literature will be as full of chromonic studies as it now is with ferroelectric chevron structures. Watch this space."... 

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