Cyclic nitrogen-oxygen ring

Part 2 (Volumes 2 and 3) deals with mono- and poly-cyclic compounds with one or more six-membered heterocyclic ring, with nitrogen, oxygen and sulfur as the heteroatoms. Volume 2 contains the six-membered rings with one nitrogen atom (Part 2A) and Volume... 

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),... 

Cyclic compounds in which one or more of the ring atoms is not carbon are termed heterocycles the noncarbon atoms are referred to as heteroatoms. We shall limit our discussions to compounds in which the heteroatoms are nitrogen, oxygen, or sulfur. For the purposes of studying and understanding their properties, heterocycles are conveniently grouped into two classes, i.e. non-aromatic and aromatic. 

Current work is focused on the benzoheteropines with the fused pyrrole (or indole), thiophene or furan rings, i.e., ortho-fused 6 + 7 + 5 ring systems with carbons only on the six-membered ring, one heteroatom on the five-membered ring and one or more heteroatoms on the seven-membered ring. The variety of heteroatoms is limited to nitrogen, oxygen and sulfur. Several examples of the related cyclic systems with the other heteroatom distribution or peri-fusion are briefly summarized in Section 4.3. The current first specialized review covers synthetic, reactivity and structural aspects reported from the late 1989 until 2007. 

In most of the cyclic compounds that we have studied so far—benzene, naphthalene, cyclohexanol, cyclopentadiene—the rings are made up only of carbon atoms such compounds are called homocyclic or alicyclic compounds. But there are also rings containing, in addition to carbon, other kinds of atoms, most commonly nitrogen, oxygen, or sulfur. For example ... 

Cyclic organic compounds are classified as carbocycles or as heterocycles. CarbocycJic rings contain only carbon atoms, but heterocyclic rings contain one or more different atoms in addition to carbon. Nitrogen, oxygen, and sulfur are the most common heteroatoms. 

Interspersed among the carbon atoms of these molecular chains and rings there are often atoms of other elements, especially nitrogen, oxygen, and sulphur, in that order of importance. This heterogeneity is particularly marked in the ring-systems of cyclic compounds. 

Heterocyclic Compounds A class of cyclic compounds containing rings with some carbon atoms replaced by other atoms such as oxygen, sulfur, and nitrogen. 

Heterocyclic compounds are cyclic compounds in which one or more atoms other than carbon (usually nitrogen, oxygen, or sulfur) are present in the ring. An example is pyridine, C5H5N, a colorless liquid with an unpleasant odor, which is among the products of distilling coal. The electronic structure of pyridine can be described as a hybrid of several valence-bond structures ... 

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