Chalcones formation

Finally, in analogy to the chalcone formation by CIR, the use of Ai-tosyl propargyl amines 20 leads to the formation of A/ -tosyl enimines 21 in moderate to excellent yields (Scheme 12) [85]. 

It has recently been found that chalcone formation may be carried out under the influence of copper(II) 2,2 -bipyridyl or a complex of cobalt(II) acetate and a 4-vinylpyridine-styrene-divinylbenzene copolymer, chalcone is obtained in high yield and the catalyst may be recovered and reused. ... 

The synthesis of an analogue, a pyranochalcone isolated from a related species Millettia pachycarpa, which is described (ref.84) as an insect anti-feedant, commenced with the prenylation of 2,4-dihydroxyacetophenone with prenyl bromide in a basic medium to afford several products but primarily 2,4-dihydroxy-3-C-prenylacetophenone in 50% yield. Formic acid cyclisation gave a pyrone which was condensed with anisaldehyde. The resulting chalcone was oxidised in which reaction the non hydrogen-bonded hydroxyl group participated to obtain the final product. This was also be derived by the alternative of chalcone formation from 2,4-dihydroxyacetophenone followed by prenylation (non-specific) and finally DDQ oxidation. 

To confirm that the imprinted recognition site was indeed the reactive center, reactions were conducted in the presence of the imprinting template, 28, to determine its ability to inhibit the polymer-catalyzed reaction. A series of aldol reactions were conducted with increasing concentrations of 28. Figure 6 shows a Line weaver-Burk plot (a) and a Dixon plot (b) illustrating the increase in concentration of 28 leads to the decrease in efficiency of the MIP P-17 for the catalysis of chalcone formation. The concentration-dependent inhibition of chalcone production by 28 implies the presence of a specific reaction center in the polymer matrix. 

Using 4-courmaroyl-CoA (in most species) and three molecules of malonyl-CoA, chalcone synthase (CHS) carries out a series of sequential decarboxylation and condensation reactions, to produce a polyketide intermediate that then undergoes cyclization and aromatization reactions that form the A-ring and the resultant chalcone structure. The chalcone formed from 4-courmaroyl-CoA is naringenin chalcone. In a few species, caffeoyl-CoA and feruloyl-CoA may also be used as substrates for chalcone formation. Malonyl-CoA is formed from acetyl-CoA by acetyl-CoA carboxylase (ACC). Acetyl-CoA may be produced in mitochondria, plastids, peroxisomes, and the cytosol by a variety of routes. It is the cytosolic acetyl-CoA that is used for flavonoid biosynthesis, and it is produced by the multiple subunit enzyme ATP-citrate lyase that converts citrate, ATP, and CoA to acetyl-CoA, oxaloacetate, ADP, and inorganic phosphate [15]. 

The hydroxylation pattern of the chalcone is discriminating those with hydroxy-groups as in 5.71) are converted into 5,7-dihydroxyflavonoids [as (5.75)], those with hydroxy-groups at C-2 and C-4 only [i.e. one oxygen lost from C-6 during chalcone formation, see (5.7/)] afford selectively 7-hydroxy-flavonoids [see (5.75)]. The intact incorporation of chalcones into flavonoids has been proved using doubly labelled precursors [57] (for further discussion of this technique see Section 2.2.1). 

In solutions of pH 1.0 and lower, anthocyanins exist solely as red-coloured flavylium salts. When increasing pH, the equilibrium shifts in favour of colourless carbinol pseudo base and the red colour fades. Around the range of pH values of 4.0 to 4.5, anthocyanins are completely colourless. Another increase in pH is manifested by the purplish-red colour, which is caused by formation of a neutral quinoid base that requires the presence of free hydroxyl groups on one of C-5, C-7 or C-4 carbons. In solutions of pH 7 a blue coloured quinoid base is formed. After some time or following an increase in pH value, a gradual decrease of blue colour intensity occurs as a result of yellow chalcone formation. If the solution is acidified to around pH 1.0, the blue quinoid and colourless carbinol bases are converted back into red flavyhum cations. The transformation of chalcones is slower and not quantitative. 

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