Heterocycles containing two or more oxygen atoms

HETEROCYCLES CONTAINING TWO OR MORE OXYGEN ATOMS 643.1 Dioxins... 

The present review includes data on the synthesis and chemical properties of the fluorine containing derivatives of furan, thiophene, selenophene, phospholene, and hve-membered heterocycles with two or more oxygen and sulfur atoms pubhshed prior 2007. Every section starts with discussion on the synthetic methods for unsaturated heterocycles (furan, thiophene etc.) and is followed by synthesis and transformations of dihydro and tetrahydro derivatives. 

A heterocycle is a cyclic compound that contains atoms of two or more elements in its ring, usually carbon along with nitrogen, oxygen, or sulfur. Heterocyclic amines are particularly common, and many have important biological properties. Pyridoxal phosphate, a coenzyme sildenafil (Viagra),... 

Heterocycles containing an Oxygen Atom in each of Two or More Rings... 

We have looked at the five-membered aromatic heterocycles pyrrole, furan and thiophene in Section 11.5. Introduction of a second heteroatom creates azoles. This name immediately suggests that nitrogen is one of the heteroatoms. As soon as we consider valencies, we discover that in order to draw a five-membered aromatic heterocycle with two heteroatoms, it must contain nitrogen A neutral oxygen or sulfm atom can have only two bonds, and we cannot, therefore, have more than one of these atoms in any aromatic heterocycle. On the other hand, there is potential for having as many nitrogens as we like in an aromatic ring. 

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