Heterocyclic Compounds: An Introduction
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Abstract
The IUPAC Gold Book describes heterocyclic compounds as: Cyclic compounds having as ring members atoms of at least two different elements, e.g. quinoline, 1,2-thiazole, bicyclo[3.3.1]tetrasiloxane [1]. Usually they are indicated as counterparts of carbocyclic compounds, which have only ring atoms from the same element. Another classical reference book, the Encyclopaedia Britannica, describes a heterocyclic compound, also called a heterocycle, as: Any of a class of organic compounds whose molecules contain one or more rings of atoms with at least one atom (the heteroatom) being an element other than carbon, most frequently oxygen, nitrogen, or sulfur [2]. Although heterocyclic compounds may be inorganic, most contain within the ring structure at least one atom of carbon, and one or more elements such as sulfur, oxygen, or nitrogen [3]. Since non-carbons are usually considered to have replaced carbon atoms, they are called heteroatoms. The structures may consist of either aromatic or non-aromatic rings. Heterocyclic chemistry is the branch of chemistry dealing with the synthesis, properties, and applications of heterocycles. Heterocyclic derivatives, seen as a group, can be divided into two broad areas: aromatic and non-aromatic. In Figure 1.1, five-membered rings are shown in the first row, and the derivative 1 corresponds to the aromatic derivative, furan, while tetrahydrofuran (2), dihydrofuran-2-one (3), and dihydrofuran-2,5-dione (4) are not aromatic, and their reactivity would be not unlike that expected of an ether, an ester, or