Synthesis of Dihydroflavonols and Flavonols

Abstract

Dihydroflavonols (flavonolones) are 3-hydroxy flavanones and are found to occur in nature, e.g. Fustin in the young fustic and the yellow cedar, Ampelopsin in the Ampelopsio melioefolia, etc. From the point of view of biogenesis dihydroflavonols seem to occupy an important place in the evolution of anthoxanthins.A number of methods have been evolved for the synthesis of dihydroflavonols: (a) from an o-hydroxy chalkone or flavanone by the action of alkaline hydrogen peroxide [Murakami (1) 1935], (b) by converting a chalkone dibromide into its diacetate and further cyclization with hydrochloric acid [Oyamada (2) 1939], (c) by the action of lead tetraacetate on flavonones and subsequent deacetylation by HC1 gas in a nitrogen atmosphere, [Oyamada (3) 1943], and (d) recently by reduction of flavonols to dihydroflavonols with sodium hydrosulphide as a reducing agent [Geissman (4) 1952],Dihydroflavonols have now been synthesized from o-hydroxy-or o-acetoxychalkone dibromides of the type RCOCHBrCHBrR', where R' contains a methoxy or benzyloxy group in the para position, by the action of aqueous acetone and sodium carbonate. Formation of a dihydroflavonol was proposed to take place through a 0-hydroxy derivative formed by the action of water on the 0-bromine atom [Marathey (5) 1952; Marathey (6) 1953]. These 0-hydroxy derivatives have been isolated in a few cases and the postulation made in the earlier work has been found to be correct [Marathey (7) 1954], Thus the reaction could be represented as follows:rX\0H Br Br R Rf J\

Synthesis of methoxybenzoflavones and assignments of their NMR data

“…Among the 14 compounds, 6 , 9 , 13 , and 14 were novel. The 1 H NMR data of four derivatives 2 , 8 , 10 , and 12 , the 13 C NMR data of two derivatives 7 and 11 , and both the 1 H and 13 C NMR data of derivative 1 have been previously reported . However, NMR data for three derivatives 3 , 4 , and 5 have not yet been reported …”

The Reaction of N‐Bromoacetamide with α, β‐Unsaturated Carbonyl Compounds. Part II

“…To our knowledge, this is the first case in which a chalcone epoxide could be isolated from a reaction of this type. The formation of epoxides as intermediates in similar reactions has been suggested by Marathey [4,5] but their isolation from the reaction mixture in the presence of a strong base has never been r epor-ted.« Treatment of the epoxide (IV) with sodium carbonate in acetone gave the hydroxyflavanone (Va).…”

“…• We reported in part I that epoxide (IV) was easily available from bromohydrin (I) by using the milder base, sodium acetate to effect cyclisation, according to the method of Marathey [4].…”

Asymmetric Cyclization of 2′‐Hydroxychalcones to Flavanones: Catalysis by Chiral Brønsted Acids and Bases

“…p-Methoxycinnamoyl chloride (17) was prepared by a literature procedure. [66] The chalcones and flavanones in this work are known substances which were prepared according to literature references and/or identified by comparison of NMR spectra and melting points to literature data: 2Ј,4Ј-dihydroxychalcone, [67] 7-hydroxyflavanone (9), [8j] 3,4-methylenedioxy-2Ј-hydroxychalcone, [68] 3Ј,4Ј-methylenedioxyflavanone (10), [34,9b,69] 5Ј,6Ј-benzo-2Ј-hydroxychalcone (28) and 5,6-benzoflavanone 31, [8k] 4-methoxy-5Ј,6Ј-benzochalcone (29) and 4Ј-methoxy-5,6-benzoflavanone 32, [70] naringenin 4Ј,7-dimethyl ether chalcone (27) [8i] and naringenin 4Ј,7-dimethyl ether (30). [60,71] General Procedure for the Cinchona-Catalyzed Cyclization of 2Ј-Hydroxychalcones to Flavanones: The 2Ј-hydroxychalcone (5 mg, ca 0.017 mmol) and the alkaloid (2.5 mg for CN and CD; 2.7 mg for QN and QD; 0.0085 mmol, 50 mol-%) are stirred in the solvent (0.5 or 1 mL; filtered through neutral Al 2 O 3 ) at room temperature.…”