Substitution effect of hydroxyl groups on the 1H and 13C chemical shifts in hydroxyflavonols

Flavonoids belong to the secondary metabolites naturally occurring in plants. They have C6-C3-C6 framework and can be classified into several subclasses based on the oxidation level of the central pyran ring. 1 Their diverse structures are prepared by oxidation, alkylation, glycosyl-ation, and so on. The variety of their biological activities such as anti-cancer, anti-viral, anti-inflammatory, and anti-oxidative activities is related with their structures. 2,3 Most flavonoids are known to show anti-oxidative effects. Of them, scavenging effects of flavonol derivatives were report-ed previously. 4 While the scavenging effect of 2'-hydroxy-flavonol was 84%, that of 3'-hydroxyflavonol was 39%. Vitamin C showed the effect of 89% in the same experi-mental condition. That is, the anti-oxidative effects of flavo-nol derivatives are dependent upon the number of hydroxyl groups and their positions. Many flavonol derivatives are being isolated from natural sources. Nuclear magnetic resonance (NMR) spectroscopy is widely used for their identification. Because the position and the number of substituted hydroxyl groups cause the changes of the 1 H and 13 C chemical shifts, it is important to elucidate the substitution effect of hydroxyl groups. The substitution effect on flavonol derivatives can be used to identify the structures of the newly isolated hydroxyflavonol derivatives. In order to elucidate the substitution effects of hydroxyl groups on 1 H and 13 C chemical shifts in hydroxy-flavonols, 18 derivatives were chosen. Their structures and nomenclatures are shown in Figure 1. Of eighteen hydroxy-flavonol derivatives, the NMR data of eleven hydroxy-flavonol derivatives (1-6, 8-11, 17) have been previously reported. 5-7 Because the 1 H and 13 C chemical shifts of the remaining seven derivatives (7, 12-16, 18) were not reported yet, we carried out their complete assignments.