Basic structure of flavonoids. Flavonoids are divided into classes based on the structure of ring C. Basic structure corresponds to flavan which are named flavanols (catechins) if hydroxylated at position C3. Flavanones have keto group on position C4. If the double bond C2 = C3 is present in structure flavanols and flavanones are named flavones and flavanonols, respectively. Isoflavonoids have B-ring at the position C3.
Bojić et al. Nutrition Journal 2011 10:73 doi:10.1186/1475-2891-10-73