Six-membered ring heterocycles containing one oxygen atom

The pyrilium cation 9.1, 2-pyrone 9.2, 4-pyrone 9.3, and their benzo-fused analogues the benzopyrilium cation 9.4, coumarin 9.5, chromone 9.6, 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 red, violet, and blue pigments of flower petals are called anthocyanins, and are glycosides of various benzopyrilium cations. Delphinidin chloride 9.7, for example, is a blue pigment. Khellin 9.8 is a natural product which has found clinical application in the treatment of bronchial asthma and has been the starting point for the design of many totally synthetic chromones with improved biological properties. 

Coumarin 9.5 is itself a natural product which been found in over sixty species of plants. 

In natural product chemistry, the acetal formed between an aliphatic or aromatic alcohol and a sugar is termed a glycoside. 

The carbonyl groups of 4-pyrone and 4-pyridone absorb at approximately 1650 cm and 

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Six-membered ring heterocycles containing one oxygen atom... 

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