ABSTRACT
1-Benzopyryliums, coumarins, and chromones are very widely distributed throughout the plant kingdom where many secondary metabolites contain them. The isomeric 2-benzopyrylium system does not occur naturally and only a few isocoumarins occur as natural products: as a consequence much less work on these has been described. Coumarins, chromones, and isocoumarins react with both nucleophiles and electrophiles in much the same way as do quinolones and isoquinolones. Anthocyanidins are generally bound to sugars, and these glycosides are known as anthocyanins. Ring-opened products from chromones and secondary amines can be obtained where again the nucleophile has attacked at C-2. Subject to the restrictions set out below, phenols react with 1,3-dicarbonyl compounds to produce 1-benzopyryliums or coumarins depending on the oxidation level of the 1,3-dicarbonyl component. Alternatively, the process is conducted in two steps: first, acylation of the phenolic hydroxyl, and secondly, an intramolecular base-catalysed Claisen condensation, known as the Baker-Venkataraman rearrangement: a synthesis of flavone is illustrative of the latter.
