Chiral chromonics

There appear to be no reports of small molecule chiral chromonic mesogens, but it is easy to convert the nonchiral N phase into a cholesteric phase by adding a small water-soluble chiral solute such as an amino acid. Lee and Labes carried out an extensive study of the relationship between the pitch of the induced chiral phase and the concentration of various dopants [55]. They found that the twist varies linearly with concentration over large concentration ranges, and the helical twisting power is characteristic of the particular dopant. This approach offers a novel form of assay for small quantities of water-soluble chiral compounds. 

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. 

A structurally unusual 3-blocker that uses a second molecule of itself as the substituent on nitrogen is included here in spite of the ubiquity of this class of compounds. Exhaustive hydrogenation of the chromone (13-1) leads to a reduction of both the double bond and the carbonyl group, as in the case of (11-2). The car-boxyhc acid is then reduced to an aldehyde (13-2) by means of diisobutylaluminum hydride. Reaction of that intermediate with the ylide from trimethylsulfonium iodide gives the oxirane (13-3) via the addition-displacement process discussed earlier (see Chapters 3 and 8). Treatment of an excess of that epoxide with benzylamine leads to the addition of two equivalents of that compound with each basic nitrogen (13-4). The product is then debenzylated by catalytic reduction over palladium to afford nebivolol (13-5) [16]. The presence of four chiral centers in the product predicts the existence of 16 chiral pairs. 

The cycloaddition of ketene acetals to 3-formylchromone exhibits good diastereoselectivity and meihanolysis of the pyrano[4,3- ] -pyran affords the chromanone ester (21) without racemisation. The chiral auxiliary, the diol (22), can be recyclised and the overall process represents an asymmetric conjugate addition to the chromone (95 JCS(P1 )2293). 

The reactivity of 47/-benzopyran-4-ones in Diels-Alder reactions is well documented <1987T3075>, and recently high asymmetric induction has been achieved in the reaction of 3-alkoxycarbonyl-substituted chromones with chiral auxiliaries and Danishefsky s diene <1991JOC2058>. It should be noted that 3-formylchromones can react as heterodienes in the stereoselective inverse electron Diels-Alder reaction with enol ethers <1994T11755> to provide a route to pyrano[4,3-A][l]benzopyrans a heterocyclic nucleus which occurs naturally in the fungal metabolite fulvic acid <1984CC1565>. The thermal Diels-Alder reaction of 477-pyran-4-one 405 in the presence of an excess of Danishefsky s diene 404 provided cycloadduct 406 (Equation 32) <1996H(43)745>. 

A chiral ruthenium(II)—NHC complex catalyzes the asymmetric hydrogenation of chromones and flavones to prepare enantiomerically enriched chromanols and flavanols 49, chromanones, and flavanones 50 (Scheme 76) (13AGE8454). Chiral 2-alkylchromanones are prepared in good yields and... 

Chromonic liquid crystals are a special class of lyotropic liquid crystals. Flat aromatic systems with polar substituents form columnar aggregates by n-n interactions in solution. The mesophases of concentrated DNA solutions or of copper-tetracarboxyphthalocyanine are examples for this. Special types of chiral mesophases have been observed [29]. 

The fingerprint texture of a chirally doped chromonic drug N phase. This sample shows the N -t I phase of a 5% solution of arginine in a 22% sample of DSCG at room temperature. The separation of the striations corresponds to 1/2 the pitch of the helicoidal structure. (x300, crossed polars.)... 

---