Six-membered ring heterocycles containing one oxygen atom

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This chapter describes six-membered ring heterocycles containing one oxygen atom. The pyrilium cation, 2-pyrone, 4-pyrone, and their benzo-fused analogues, the benzopyrilium cation, coumarin, and chromone, are the parent structures of a series of six-membered ring heterocycles containing one oxygen atom. The impetus for research in this area comes from the enormous number of plant-derived natural products based on the benzopyrilium, coumarin, and chromone structures. The pyrylium cation is the oxygen analogue of pyridine and is a six π-electron aromatic system. Nevertheless, being a cation, it is reactive towards nucleophiles and is readily hydrolysed to give dialdehyde. In contrast, 2- and 4-pyrones are considered to have relatively little aromatic character. The chapter then looks at the synthesis of a pyrylium salt, a coumarin, and a chromone.