Flavanones, rearrangement

T3037). 3-Aroylflavanones (640) offer a choice of sites for attack by hydroxylamine, but the endocyclic carbonyl is preferentially attacked to give the isoxazoline (641) (79IJC(B)5lo). Oximes (642) of flavanones rearrange in acid solution to the isoxazoline (643) (80H(14)1319). 

Hashim, M.F. et al.. Reaction mechanism of oxidative rearrangement of flavanone in isofiavone biosynthesis. FEBS Lett., 271, 219, 1990. 

Tetrahydroquinolones can be transformed also by (diacetoxyiodo)benzene 3 to the aromatic arylquinolines, a structure found in various alkaloids [101]. Depending on the reagent, it is possible to oxidize flavanones 50 either into flavones 51 or into rearranged isoflavones 52 [102, 103]. (Diacetoxyiodo)-benzene 3 or the polymer-supported reagent 18 were also efficient reagents for the oxidation of 1,4-dihydropyridines 53 to the corresponding pyridine derivatives 54, Scheme 23 [104]. 

Baeyer-Villiger rearrangement of flavanone derivatives with MTO/H2O2 was first reported in 2001 by Saladino and coworkers and has successfully been applied for various substituted flavanones (Scheme 14). In the case where R1, R2, and R3 are methoxy groups (naringenin) or when all R-groups are methoxy groups (hesperetin), oxidation toward the benzoquinone product is the major product... 

HAGMANN, M.-L., GRISEBACH, H., Enzymatic rearrangement of flavanone to isoflavone. FEBS Lett., 1984,175,199-202. 

Wessely-Moser rearrangement. Rearrangement of flavones and flavanones possessing a 5-hy-droxyl group, through fission of the heterocyclic ring and reclosure of the intermediate diaroylmeth-anes in the alternate direction. 

The final main group of structures is composed of spirobiflavonoids (442-447) spirobiflavonoids made up of two C15 units (448-449) phenolic spiro derivatives derived from C15 and C14 units (450) a C15 moiety of flavonoid origin and a Ci4-stilbene substructure linked via a y-lactone ring (attachment of the stilbene derivative to the carbocation intermediate of the oxidation of flavanone to flavanol, and subsequent rearrangement of this intermediate) (451-452) and a C15 moiety of flavonoid origin and a Cu-stilbene substructure linked via a y-laetone ring (453-455). 

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>. 

When the cinnamoyl esters 123 (R = Me or OMe), 126, and 128 were supported with a AlCls-ZnCE mixture on silica gel and exposed to MWI for 7 min, the resulting ortho-hydroxy rearranged products were spontaneously cyclized by intramolecular conjugate addition of the phenolic hydroxyl group to the a,/l-unsaturated system to afford the flavanones 124, 125, 127, and 129, respectively, in excellent yields (73-87%) (Scheme 25) (99JCR(S)574). 

Flavanones undergo a Baeyer-Villiger rearrangement to dihydro-l,5-benzodioxepin-2-ones on treatment with H2O2 and methyltrioxorhenium <01TL5401>. The synthesis of benzoxazepines from chromanones forms part of a review of the 7-membered ring compounds <01JHC1011>. 

Since its discovery, Wessely-Moser rearrangement has been widely used in the structural elucidation of flavanones, chromones, xanthones, and their derivatives. Its popularity should be largely attributed to the fact that differentiation between a 5,7,8-arranged benzopyranone and its 5,6,7-isomer is often difficult, and sometimes impossible, by the classical techniques such as NMR studies (even with shift reagents). ... 

The precursors of flavonoid biosynthesis include shikimic acid, phenylalanine, cinnamic acid, and p-coumaric acid. Shikimic acid acts as an intermediate in the biosynthesis of aromatic acid. The basic pathways to the core isoflavonoid skeletons have been established both enzymatically and genetically [16]. The synthesis of isoflavones can be broadly divided into three main synthetic pathways the formylation of deoxybenzoins, the oxidative rearrangement of chalcones and flavanones, and the arylation of a preformed chromanone ring. In leguminous plants, the major isoflavonoids are produced via two branches of the isoflavonoid biosynthetic pathway, and the different branches share a majority of common reactions [1]. Unlike the common flavonoid compotmds, which have a 2-phenyl-benzopyrone core structure, isoflavones, such as daidzein and genistein, are 3-phenyl-benzopyrone compounds. Biochemically, the synthesis of isoflavones is an offshoot of the flavonoids biosynthesis pathway. Several attempts have aimed to increase... 

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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