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Chalcones borohydrides

The stereoselective epoxidation of chalcones, followed by acid-catalysed ring closure and concomitant cleavage of the epoxide ring, provides a very efficient route to chiral flavon-3-ols and, subsequently, by borohydride reduction to produce flavan-3,4-diols [13, 14], It has been shown that diastereoselective reduction of the chiral flavon-3-ols by sodium borohydride in methanol yields the trans-2,3-dihydroxy compounds, whereas borohydride reduction in dioxan produces the cis-isomers [14] the synthetic procedure confirms the cis configuration of the 2,3-hydroxy groups of naturally occurring leucodelphinidins [14]. [Pg.538]

A study of the influence of the nature of the solid support showed that silica, celite, cellulose or magnesium sulphate in combination with borohydride can also be used successfully in the microwave-assisted reduction of carbonyl compounds. The choice of the solid support has been reported to influence the chemoselectivity of the reduction of chalcone. Under optimised conditions the reduction of the alkene can be suppressed using borohydride on silica, whereas the use of cellulose as solid support seems to favour C=C reduction (Scheme 4.11 )27. [Pg.81]

The use of numerous polymer-supported optically active phase transfer catalysts was further extended by Kelly and Sherrington11351 in a range of phase transfer reactions including a variety of displacement reactions, such as sodium borohydride reductions of prochiral ketones, epoxidation of chalcone, addition of nitromethane to chalcone and the addition of thiophenol to cyclohexanone. Except in the chalcone epoxidation, all the examined resin catalysts proved to be very effective. However, with none of the chiral catalyst system examined was any significant ee achieved. The absence of chiral induction is a matter of debate, in particular over the possible reversibility of a step and the minimal interaction within an ion pair capable of acting as chiral entities in the transition state and/or the possible degradation of catalysts and leaching. [Pg.188]

A niamber of reactions have been examined using these phase transfer catalytic species. These include displacement reactions of solid potassium phenoxide, solid sodium azide and aqueous sodium azide on racemic ethyl-2-bromopropionate in toluene (similar to reaction 5), sodium borohydride (solid) reductions of acetophenone and octan-2-one, chalcone epoxidations using aqueous H2O2/OH" (reaction 4) and Michael additions of nitromethane to chalcone in the presence of anhydrous potassium fluoride (reaction 3). Details of these results will be published in due course and at the moment it is possible to record only a few representative examples of the displacements and reductions. [Pg.257]


See other pages where Chalcones borohydrides is mentioned: [Pg.176]    [Pg.75]    [Pg.285]    [Pg.1302]    [Pg.334]    [Pg.169]    [Pg.417]    [Pg.136]   
See also in sourсe #XX -- [ Pg.538 ]

See also in sourсe #XX -- [ Pg.8 , Pg.538 ]

See also in sourсe #XX -- [ Pg.8 , Pg.538 ]




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Chalcone

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