Rings that Contain One Heteroatom

This chapter deals with heteromacrocycles rather than smaller-ring heterocycles. Traditionally, heterocycles are three- to eight- or nine-membered rings that contain one or more noncarbon heteroatoms. Many, if not most, of the known heterocycles are aromatic compounds in which the noncarbon atoms contribute electrons to meet Huckel rule requirements. The crown ether situation contrasts with that of traditional heterocycles. The bulk of crown ether compounds contain 12 or more members and are aliphatic rather than aromatic. Those heterocycles that have cation binding properties usually contribute one or at most two donors to a cation s primary solvation... 

Not all cyclic compounds are hydrocarbons Many substances include an atom other than carbon called a heteroatom (Section 1 7) as part of a ring A ring that contains at least one heteroatom is called a heterocycle, and a substance based on a heterocyclic ring is... 

Substances that contain one or more atoms other than carbon as part of a nng are called heterocyclic compounds Rings m which the heteroatom IS oxygen nitrogen or sulfur rank as both the most common and the most important... 

As shown in Scheme 2, two heteroatom-carbon bonds are constructed in such a way that one component provides both heteroatoms for the resultant heterocycle. By variation of X and Z entry is readily obtained into thiazoles, oxazoles, imidazoles, etc. and by the use of the appropriate oxidation level in the carbonyl-containing component, further oxidized derivatives of these ring systems result. These processes are analogous to those utilized in the formation of five-membered heterocycles containing one heteroatom, involving cyclocondensation utilizing enols, enamines, etc. 

Heterocyclic compounds, or heterocycles, are molecules with rings that contain more than one type of atom they are to be distinguished from carbocycles, which are molecules with rings that contain only carbon atoms. The heterocyclic compounds of greatest interest to medicinal chemists have carbon rings containing one or two heteroatoms— atoms other than carbon. 

Recall from Section 9.3 that a heterocycle is a ring that contains at least one heteroatom. 

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