B ring substitutions

Fig. 22.—Structure-Sweetness Relationship (With Regard to B-Ring Substitution) in Dihy-drochalcone Sweeteners.

The B-ring substitution pattern influences the absorbance. Pure prodelphinidin polymers show much lower specific absorption coefficients (El% = 62) than pure procyanidin polymers (E 0/o = 130) [141]. The molar absorption coefficients of dimeric and higher oligomeric procyanidins are similar (see Tab. (8)). The overall UV absorbance of an oligomer approximately amounts to the sum of the absorbances of the monomer units [226,254], However, UV detection of individual procyanidins after chromatographic separation implies that - in this case - molar absorbances depend on structure. In the series (+)-catechin, procyanidin B2, Cl and a tetramer molar absorbances decreased [262], Treutter et al. [154] found lower molar absorbances for dimers with a 2,3-cis stereochemistry (procyanidin B2 and B5) compared to (-)-epicatechin and (+)-catechin which nearly showed identical molar calibration plots. The molar absorbance of the doubly-linked procyanidin A2 was much higher than the ones of the monomers. 

Together with two isoflavones, genistein (8) and prunetin (9), the relative activity of each compound was determined using the particle method. The results for the various flavones lacking B-ring substitution are... 

UV spectra of flavanones do not supply conclusive information on the pattern of B-ring substitution. Removal of the 5-hydroxyl group causes a hypsochromic shift of Band II while increasing oxygenation in the B-ring has no noticeable effect on the UV spectra. 

Naturally Occurring Intermediates.—The possibility that o-hydroxydibenzoyl-methanes are intermediates in flavone synthesis, particularly of those flavones which lack B-ring substitution, was suggested by the earlier discovery of a compound formulated as 2,4,6-trihydroxydibenzoylmethane 4-glucoside in... 

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