Typical flavonoids, such as flavanone (1.29), have a six-member heterocycle. Flavonoids have an A-, B-, and C-ring, and are typically depicted with the A-ring on the left-hand side. The A-ring originates from the condensation of three malonyl-CoA molecules, and the B-ring originates from p-coumaroyl-CoA. These origins explain why the A-ring of most flavonoids is either meta-dihydroxylated or meta-trihydroxylated.
In typical flavonoids one of the meta-hydroxyl groups of the A-ring contributes the oxygen to the six atom-heterocycle. The six member oxygen heterocycle of typical flavonoids may be a pyran (1.30), pyrylium (1.31), or pyrone ring (1.32). The B-ring is typically mono-hydroxylated, ortho-dihydroxylated, or vic-trihydroxylated. The B-ring may also have methyl-ethers as substituents.
Isoflavones, isoflavanones and neoflavonoids are also members of the flavonoid group. They all have the C6-C3-C6 structure but the B-ring is in a different position on the oxygen heterocycle. Examples are isoflavone (1.33) and the neoflavonoid dalbergin (1.34).
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