Chromone epoxidation

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. 

C4C1im][BF4] OH h2o2 Epoxidation of chromones and flavonoids product extracted with Et20 very low [22]... 

Flavones and isoflavones (3-aryl-chromones) are quantitatively converted into 2,3-epoxides by exposure to dimethyl dioxirane flavone oxides are quantitatively converted by acid into 3-hydroxy-flavones, which are naturally occurring. ... 

Epoxidation of enone can be carried out in the absence of a metal catalyst in ionic liquid. A basic aqueous solution of hydrogen peroxide behaves as an efficient oxidant both in [bmim][PF6], which is immiscible in water, and in hydrophilic [bmim] [BF4]. Quantitative yields of the corresponding epoxides were obtained after very short reaction times (Scheme 27). The use of H2O2 in ionic liquids is a viable alternative to using water as the solvent. There is a significant rate acceleration when these salts are used as the reaction medium. This strategy has been successfully applied to the epoxidation of chromones and flavonoids in all cases, the reaction was dramatically improved in [bmim] [BF4] compared with standard classical organic sol vents. 

Typical of juvenile hormone antagonists are the precocenes, e.g. (4.80) which are chromones occurring in garden plants. Even in low concentrations, they produce precocious moulting to give a sterile adult (Bowers etal., 1976). Their site of action is the gland that synthesizes juvenile hormone, which activates the precocene, apparently by epoxidation, and is destroyed by the product. (Brooks, Pratt and Jennings, 1979). 

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