Heterocycle nomenclature

Cyclic ethers are named either as heterocyclic compounds or by specialist rules of heterocyclic nomenclature. Radicofunctional names are formed by citing the names of the radicals R and R followed by the word ether. Thus methoxyethane becomes ethyl methyl ether and ethoxyethane becomes diethyl ether. 

Another type of anion, confined for practical purposes to boron compounds, has no unshared electrons at the anionic site, and must be thought of as being formed by addition of hydride to a boron atom (or other atom with an incomplete valence shell). Such structures were not anticipated at the time general heterocyclic nomenclature was developed, and they are only recently being fitted into systematic nomenclature (lUPAC Provisional Recommendation 83.2). Proposals for a suffix to indicate such structures are under consideration (1982). 

Thiamine is present in cells as the free form 1, as the diphosphate 2, and as the diphosphate of the hydroxyethyl derivative 3 (Scheme 1) in variable ratio. The component heterocyclic moieties, 4-amino-5-hydroxymethyl-2-methylpyrimidine (4) and 4-methyl-5-(2-hydroxyethyl)thiazole (5) are also presented in Scheme 1, with the atom numbering. This numbering follows the rules of nomenclature of heterocyclic compounds for the ring atoms, and is arbitrary for the substituents. To avoid the use of acronyms, compound 5 is termed as the thiazole of thiamine or more simply the thiazole. This does not raise any ambiguity because unsubstituted thiazole is encountered in this chapter. Other thiazoles are named after the rules of heterocyclic nomenclature. Pyrimidine 4 is called pyramine, a well established name in the field. A detailed account of the present status of knowledge on the biosynthesis of thiamine diphosphate from its heterocyclic moieties can be found in a review by the authors.1 This report provides only the minimal information necessary for understanding the main part of this chapter (Scheme 2). 

Alternative names are shown in some cases this should emphasize that there is often no unique correct name. Sometimes, it can be advantageous to bend the rules a little so as to provide a neat name rather than a fully systematic one. Typically, this might mean adopting a lower priority functional group as the suffix name. It is important to view nomenclature as a means of conveying an acceptable unambiguous stmc-ture rather than a rather meaningless scholastic exercise. Other examples will occur in subsequent chapters, and specialized aspects, e.g. heterocyclic nomenclature, will be treated in more detail at the appropriate time (see Chapter 11). Stereochemical descriptors are omitted here, but will be discussed under stereochemistry (see Sections 3.4.2 and 3.4.3). 

Lactams are named in several ways. They are named as alkanolactams by the IUPAC substitutive system, such as 3-propanolactam, 4-butanolactam, 5-pentanolactam, and 6-hexano-lactam, respectively, for the 4-, 5-, 6-, and 7-membered rings, respectively. An alternate IUPAC method, the specialist heterocyclic nomenclature system, names these lactams as 2-azetidinone, 2-pyrrolidinone, 2-piperidinone, and hexahydro-2f/-azepi n-2-one, respectively. These lactams are also known by the trivial names fl-propiolactam, a-pyrrolidone (y-butyrolactam), a-piperidone (8-valerolactam), and e-caprolactam, respectively. 

SPECIALIST HETEROCYCLIC NOMENCLATURE Rule B-l. Extension of Hantzsch-Widman System... 

In the remaining two chapters in the present volume, L. B. Clapp deals with 1,2,4-oxadiazoles, and A. D. McNaught has tackled the daunting task of explaining the intricacies of heterocyclic nomenclature. 

It could be more correct to form peri-heterocyclic nomenclature in accordance with the unified principle based on the names of the corresponding peri-cyclic hydrocarbons for example, 1-azaacenaphthylene instead of benzo[cc/]indole and 1-oxaphenalene instead of naphtho[bc]py-rane. However, we tried to use as a rule the adopted names. Moreover, for... 

The system is clumsy and obscure, however, in handling compounds in which the divalent carbon is located in a ring. Kirmse s suggestion of using the prefix carbena- in a fashion similar to the use of aza-, oxa-, etc. in organic heterocyclic nomenclature has much to recommend it and will be used where necessary in the subsequent discussion. On this basis, 6, for example, would be named l-carbena-2-methylcyelopentane. 

The numbering of the skeleton in a ring system is generally a consequence of the way in which the skeleton is named. In order to avoid confusion as to the peripheral numbering sequence and locations of substituents in the fused systems, structural formulae are drawn as in Chemical Abstracts in accordance with the rules of heterocyclic nomenclature. Thereby the reader is not required to possess detailed knowledge of the elaborate series of priorities used in the structural presentation and numbering of ring systems. 

The systematic endings for naming heterocyclic rings with up to 10 atoms are summarized in Section 2.1.3. A more detailed account of heterocyclic nomenclature can be found in CHEC-I . 

Some bicyclic BN heterocycles are mentioned to explain the rules of nomenclature. Owing to the lack of appropriate fundamental compounds with trivial names, the general heterocycle nomenclature is used ((23)- 25)). 

Systematic heterocyclic nomenclature is used throughout the review except where the use of another type of nomenclature is more practical. 

Due to overlap between perfluoro- and heterocycle nomenclatures, some names may be rather complicated and contradictory, such as perfluoro tetrahydrofatant), for the compound that has no single hydrogen in the molecule ... 

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