Robert M. Horowitz

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).

Flavonoids of citrus—VIII : Synthesis of Limocitrol, limocitrin and spinacetin

Abstract Allan-Robinson syntheses of limocitrol (3,5,7,4′-tetrahydroxy-6,8,3′-trimethoxyflavone), limocitrin (3,5,7,4′-tetrahydroxy-8,3′-dimethoxyflavone) and spinacetin (3,5,7,4′-tetrahydroxy-6-3′- dimethoxyflavone) are described. The direction of ring closure to give limocitrin or spinacetin is determined by the blocking group (benzyl or benzoyl) at the 4-position of the B-ring component.

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.

Conformational isomerism and its relation to the mutarotation of isocolchicine

The structures of two atropisomers of isocolchicine that are present mutarotated solutions have been determined utilizing 500 MHz 1H n.m.r. data.

Chiroptical differentiation of 6- and 8-C-glycosylflavones

C.d. studies of C-glycosylflavones have shown that a positive Cotton effect at 250–275 nm indicates that the glycosyl residue is linked to C-6 (e.g. isovitexin) while a negative Cotton effect at 250–275 nm indicates it is linked to C-8 (e.g. vitexin).