Flavanones dehydrogenation

Although flavanones [25] and thioflavanones [26] gave normally under these conditions the expected a-hydroxy dimethyl acetals, their aza analogues, 2-aryl-tetrahydroquinolones, afforded dehydrogenation products [27] ... 

Flavanones undergo a variety of transformations by HTI or other hypervalent iodine reagents. In methanol, at room temperature, they were dehydrogenated to the corresponding flavones [40], A similar dehydrogenation occurred in chromanones [41]. Dramatic solvent effects occurred when methanol was replaced by other solvents. Thus, in trimethyl orthoformate ring-contraction was the main pathway and methyl 2-aryl-2,3-dihydrobenzofuran-3-carboxylates the major products, accompanied by substantial amounts of 3-methoxyflavones [42] ... 

Photosensitized dehydrogenation.1 Irradiation of flavanones with this sensitizer provides flavones in moderate (—60%) yield. This dehydrogenation has been carried out with CAN or DDQ. 

Thallium trinitrate has been shown to be an efficient reagent for the dehydrogenation of chroma-nones and flavanones (Table 10, entry 5). the reaction being carried out in methanol at room temperature. The addition of perchloric acid to chromanone oxidations enhances the yields and reaction rates by promotion of enolization, but qiparently was without effect on flavanones. The ease with which chimanones and similar compound may be dehydrogenated has also permitted unusual procedures such as trimethylsilyl chloride/acetic anhydride and photolysis to be used (Table 10, entries 6 and... 

The system also effects dehydrogenation of flavanones (1) to flavones (2) in high yield. Presumably an intermediate 3-iodoflavanone is formed, which undergoes dehydrohalogenation. ... 

Three reactions of flavanones may be used to represent the broad classes of biotransformation reactions which have been observed in oxaarenes dehydrogenation, hydroxylation, and ring scission (Figure 6.57) (Ibrahim and Abul-Hajj 1990). 

Hydroxychalcone is readily cyclized with hot aqueous alkali this is the basis of a widely applicable synthesis of flavanones [B-4]. In addition to the examples of dehydrogenative cyclization of chalcones described in Part 1 (pp. 191, 192), that produced by DDQ should be mentioned. In boiling dioxan, the reaction takes 20-72 h and produces flavones together with smaller amounts of flavanones and aurones. 

Several reagents have been used to dehydrogenate flavanones ... 

It involves an intramolecular, possibly acid-catalysed, Michael addition of the phenolic OH group resulting in a cyclization to the flavanone 14 which undergoes dehydrogenation to give a flavone. 

The dehydrogenation of 2 -hydroxychalcones and flavanones to flavones, of l-(2-hydroxyaryl)alk-2-en-l-ones and chroman-4-ones to chromones and of thiochroman-4-ones to thiochromones can be accomplished using iodine in hot DMSO <97HCM223>. E-3-Styrylchromones result from the oxidative rearrangement of 5-aryl-l-(2-hydroxyphenyl)penta-2,4-dien-l-ones with thallium(lll) nitrate <97LA2065>. Dimethyldioxirane converts flavanones into flavones by way of the 2-hydroxyflavanone. This approach enables flavans to be converted to a variety of flavonoids <97TL4651>. 

Dehydrogenation of flavanones. Flavanones are oxidized to flavones in quantitative yield by DDQ. The reaction is slow in refluxing benzene ca. 60 hours), but is complete within 5--12 hours when conducted in dioxane at reflux.°... 

However, the flavanone naringenin (10) [but not dihydro-kaempferol (13)] is the substrate for flavone formation in snapdragons. Antirrhinum majus (Scrophulariaceae) (flavone synthase II) (Fig. 11.10). In this plant, flavones arise from dehydrogenation of flavanones and not from dehydration of dihydroflavonols (Britsch et al., 1981). A similar enzyme system converts dihydroflavonols to flavonols (Britsch et al., 1981). In other work, the enzyme responsible for oxidation of flavanones to flavones in snapdragon Antirrhinum majus) was isolated from a microsomal fraction and shown to require NADPH and molecular oxygen (Britsch et al., 1981 Dewick, 1989 Forkmann and Stotz, 1981). The system appears to be a cytochrome P-450-dependent monooxygenase. This system also is known from Glycine max... 

Various interconversion reactions are used for the synthesis of analogs. Partial methylation and demethylation (3, 16, 130, 249-251, 263, 264), rearrangement of C-8 C-8 to C-6 C-8 biphenyl isomers, and dehydrogenation reactions converting flavones to flavanones have been employed (3, 16, 227, 251, 264, 293). Many biflavonoids have now been synthesized and references are compiled in Locksley s (232) and Geiger and Quinn s (125, 126) reviews. 

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