Bruno Gentili

Flavonoids of citrus—VI : The structure of neohesperidose

Abstract The flavanone glycosides naringin, poncirin and neohesperidin all contain the disaccharide neohesperidose, since, on treatment with alkali, they yield the same degradation product, phloracetophenone 4′-neohesperidoside. The structure of neohesperidose is 2-O-α- l -rhamnopyranosyl- d -glucopyranose, as shown by methylation studies and optical rotations. Citrus flavanones that contain neohesperidose are bitter, while the corresponding flavanones that contain the isomeric disaccharide, rutinose (6-O-α- l -rhamnopyranosyl- d -glucopyranose), are tasteless.

C-2 stereochemistry of naringin and its relation to taste and biosynthesis in maturing grapefruit

Abstract Circular dichroism and proton magnetic resonance spectroscopy can be used to determine the C-2 chirality of naringin, the main bitter principle of grapefruit. Thus, the aglycone chirality of naringin has been studied as a function of grapefruit maturity. The amount of (2 S ) isomer is 85–92% in immature grapefruit but only 55–60% in mature grapefruit. These results are discussed in relation to the current postulates relating to flavanone biosynthesis. A naringin sample predominant in the (2 R ) isomer has been prepared and found by some tasters to be more bitter than the (2 S ) naringin. It is concluded that the naringin isomerization in ripening grapefruit is not responsible for debittering.

Flavonoids of citrus—VII : Limocitrol and isolimocitrol

Abstract The flavonols limocitrol and isolimocitrol, isolated from Citrus limon , are shown to be 3,5,7,4′-tetrahydroxy-6,8,3′-trimethoxyflavone and 3,5,7,3′-tetrahydroxy-6,8,4′-trimethoxyflavone, respectively. Each occurs as a 3-β- d -glucoside, as does the previously described flavonol, limocitrin (3,5,7,4′-tetrahydroxy-8,3′-dimethoxyflavone).

Circular dichroism of c-glycosylflavones : A chiroptical method for differentiating 6-C-,8-C- and 6,8-Di-C-β-glycosyl isomers

Abstract Examination of a wide variety of C-glycosylflavones has shown that the sign of the CD band at 250–275 nm is diagnostic of the point of attachment of the glycosyl residue to the phenolic moiety. A positive CD band at 250–275 nm indicates a 6-C-linkage, as in isovitexin, whereas a negative CO band in this region indicates an 8-C- linkage, as in vitexin. 6,8-Di-C-β-glycosylflavones generally show two CD bands at 250–275 nm, a positive one at 263–275 nm and a negative one at 250–262 nm. Preliminary studies on C-α-glycosylflavones indicate that both 6-C- and 8-C- isomers show a negative CD band at 250–275 nm. The chiroptical properties of C-β-glycosylflavones are explained by proposing a quadrant rule based upon the substituted benzoyl chromophore present in these molecules.