Circular dichroism, optical rotatory dispersion and absolute configuration of flavanones, 3-hydroxyflavanones and their glycosides : Determination of aglycone chirality in flavanone glycosides

Abstract

Flavanones of 2 S configuration and 3-hydroxyflavanones of 2 R,3 R configuration, having equatorial 2-aryl substituents in the former or diequatorial 2,3-substituents in the latter, exhibit a positive Cotton effect due to the n → π * transition (∼ 330 nm) and a negative Cotton effect in the n α π * region (∼ 280–290 nm). Flavanone glycosides possessing chiral aglycones show Cotton effects quite similar to their optically active aglycones while flavanone glycosides having racemic aglycones show only weak Cotton effects at 250–350 nm. The π → π * Cotton effect is more suitable for determining aglycone chirality in flavanone glycosides. In addition to the two high wavelength bands, 2 S-flavanones generally showed a positive Cotton effect at 245–270 nm and a negative Cotton effect at 225–240 nm. trans 2 R,3 R-3-Hydroxyflavanones gave two Cotton effects below 270 nm both of which were positive.