Rings that Contain Three or More Heteroatoms

RINGS THAT CONTAIN THREE OR MORE HETEROATOMS... 

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... 

Hantzsch-Widman names are used for one-ring heterocyclic systems that do not have trivial names. The names are applied to monocyclic compounds containing one or more heteroatoms in three- to ten-membered rings. The names are derived by combining the appropriate prefix or prefixes for the heteroatoms with a stem denoting the size of the ring (see below). The state of hydrogenation is indicated either in the stem or by the prefixes dihydro-, tetrahydro-, etc. 

Many molecules that are not aromatic contain one or more heteroatoms. These are very common and include ring sizes of three to six containing N, O, or S. There are also derivatives containing two or more heteroatoms. 

In this chapter, synthetic methods are classified by the ring that is being formed. We deal successively with rings containing two, three and four or more heteroatoms. Within each category, classification is first by the relative orientation of the heteroatoms, and then by the ring size. 

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