Development of a Concise Synthesis of the Flavonoid Chrysin

Biotransformation of 5,7-Methoxyflavones by Selected Entomopathogenic Filamentous Fungi

5,7-Dimethoxyflavone, a chrysin derivative, occurs in many plants and shows very low toxicity, even at high doses. On the basis of this phenomenon, we biotransformed a series of methoxy-derivatives of chrysin, apigenin, and tricetin obtained by chemical synthesis. We used entomopathogenic fungal strains with the confirmed ability of simultaneous hydroxylation/demethylation and glycosylation of flavonoid compounds. Both the amount and the place of attachment of the methoxy group influenced the biotransformation rate and the product's amount nascent. Based on product and semi-product structures, it can be concluded that they are the result of cascading transformations. Only in the case of 5,7,3',4',5'-pentamethoxyflavone, the strains were able to attach a sugar molecule in place of the methoxy substituent to give 3'-O-β-d-(4″-O-methylglucopyranosyl)-5,7,4',5'-tetramethoxyflavone. However, we observed the tested strains' ability to selectively demethylate/hydroxylate the carbon C-3' and C-4' of ring B of the substrates used. The structures of four hydroxyl-derivatives were determined: 4'-hydroxy-5,7-dimethoxyflavone, 3'-hydroxy-5,7-dimethoxyflavone, 3'-hydroxy-5,7,4',5'-tetramethoxyflavone, and 5,7-dimethoxy-3',4'-dihydroxyflavone (5,7-dimethoxy-luteolin).

Modified Syntheses of the Dietary Flavonoid Luteolin

Two novel syntheses of the flavone luteolin are described. In the first, 3,5-dimethoxyphenol was converted to 2-hydroxy-4,6-dimethoxyacetophenone and then by condensation with 3,4-dimethoxybenzaldehyde to 2′-hydroxy-3,4,4′,6′-tetramethoxychalcone. In the second, the chalcone step was prepared in which 3,5-dimethoxyphenol was acylated with 3,4-dimethoxycinnamoyl chloride. The chalcone was then cyclised with iodine and demethylated with pyridine hydrochloride to form luteolin in 47% and 40% overall yield, respectively. Several disadvantages of previous syntheses like long reaction time, harsh reaction conditions and low overall yield have been overcome.

Practical Synthesis of Naringenin

Two routes for the synthesis of the flavanone naringenin are described. In the first, 3,5-dimethoxyphenol is converted to 2-hydroxy-4,6-dimethoxyacetophenone and then by condensation with anisaldehyde to 2′-hydroxy-4,4′,6′-trimethoxychalcone. The chalcone is then cyclised with aqueous hydrochloric acid and demethylated with pyridine hydrochloride to form naringenin in 45% overall yield. The condensation of 2-hydroxy-4,6-dimethoxyacetophenone with anisaldehyde could also directly produce 4′,5,7-trimethoxyflavanone, which was then converted into naringenin in 60% overall yield. In the second route, a single step for the preparation of the chalcone is used in which 1,3,5-trimethoxybenzene is acylated with p-methoxycinnamic acid. Although the synthesis of naringenin is achieved in a lower overall yield of 29%, the process is simpler.