Larger Ring Heterocycles

As far as we are aware no larger ring heterocyclic phosphorus ylides have yet been described. 

A recent concise review on the photochemistry of azepines and larger ring heterocycles is also available (B-76PH288). 

Nucleophilic substitutions at the azepine nucleus are confined mainly to derivatives of hydroazepines such as lactim ethers, imidoyl chlorides and amidines, and to hydride ion reductions of carbonyl and imine groups (see Section 5.16.3.5.2). In addition some transan-nular nucleophilic displacements have been described, but such reactions are not as common with azepines as with larger ring heterocycles. 

Every heterocyclic compound can be referred back to a parent ring system. These systems have only H-atoms attached to the ring atoms. The lUPAC rules allow two nomenclatures. The Hantzsch-Widman nomenclature is recommended for three- to ten-membered heterocycles. For larger ring heterocycles, replacement nomenclature should be used. 

In this chapter, eight-membered (e.g. azocines 1), nine-membered (e.g. oxonin, thionin and azonine 2-4) and larger ring heterocycles, as well as porphyrins and other tetrapyrrole systems, will be discussed. 

Crown ethers and cryptands [15] also belong to the group of larger ring heterocycles, but will not be discussed further in this book. 

Since perfluorinated heterocycles have distinct and often unique chemistry, this group was considered as a separate category and data on perfluorinated aromatic and nonaromatic compounds are given in Chapters 8 and 9, respectively. The information on seven-membered and larger ring heterocycles, including perfluorinated crown ethers and polyfluorinated macrocycles, is provided in Chapter 10, which concludes Part I of the book. 

For the same reason naming of perfluorinated, nonaromatic derivatives may be difficult and in this case, replacement nomenclature is used, despite the fact that this it is recommended by lUPAC for use with larger ring heterocycles. The use of replacement nomenclature allows simplifying the process and shortening some names of heterocycles. For example, in case of two compounds shown in Fig. 0.4, perfluoro-1 -azacyclopentene-1 and perfluoro-1 -azacyclohexene-1 names are often used instead of perfluoro-3,4-dihydro-2//-pyrrole and perfluoro-2,3,4,5-tetrahydropyridine in scientific literature. 

In what may foreshadow a versatile and general route to heavily functionalized 1,2-diazocines, as well as to many other larger-ring heterocyclic compounds, oxidative cleavage by periodate of the carbon-carbon double bond of a bicyclic pyrazolone was reported to yield 1,2,6,7-tetrahydro-l-methyl-2-phenyl-l,2-diazocin-3,4,8(5. -trione (29) (85%) (Equation (4)) cleavage of larger-ring... 

The Hantzsch-Widman nomenclature is recommended for 3- to 10-membered heterocycles. For larger ring heterocycles, replacement nomenclature should be used. 

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