Heterocyclic Hantzsch-Widman names

Oxepin is the Hantzsch-Widman name for a seven-membered unsaturated heterocycle with one oxygen atom and the numbering follows the convention for monocyclic heterocycles. However, the isomeric benzene oxide has different numbering in agreement with the 7-oxabi-cyclo[4.1.0]hepta-2,4-diene structure, position 1 now corresponds to position 2 in the oxepin. 

In one method, the non-detachable prefixes hydro- and dehydro- (in the case of mancudes only) can be used to indicate the addition or subtraction of one hydrogen atom. As even numbers of hydrogen atoms are involved when a carbon-carbon single bond becomes a double or triple bond, the multiplicative prefixes di-, tetra-, etc., as well as the appropriate locants, are used. The prefix dehydro- is always used to indicate the subtraction of hydrogen atoms from saturated heterocycles having Hantzsch-Widman names. 

IUPAC/Chemical Abstracts System (IUPAC Rule B-3). The fusion operations described in Section II, A, 3, a are applied to trivial and/or Hantzsch-Widman names of heterocyclic (and, if present, car-bocyclic) components see examples 25-27. In addition a unit consisting of a benzene ring fused to a one-ring heterocycle can be considered as a single component. If this one-ring heterocycle contains 3-10 members it... 

Hantzsch-Widman name a name devised by Hantzsch and Widman for describing heterocyclic systems, in which the prefix denotes a hetero atom(s) and the suffix denotes the ring size and degree of saturation e.g., oxirene, aziridine. 

Heterocyclic systems can be named by various methods, depending on ring size, the presence or absence of metal atoms, the availability of trivial names, etc. For this index the naming of the heterocyclic parents has generally followed the practices of IUPAC and Chemical Abstracts. Thus Hantzsch-Widman names have been employed for all monocyclic systems and for the other systems when the smallest ring size is five atoms or more, but replacement nomenclature has usually been followed for bicyclic and higher systems when they contain three- and four-membered rings. 

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. 

Provided that no retained trivial name is applicable, fused heterocyclic systems are given Hantzsch-Widman names. Fusion of components follows the same pattern as established for corresponding hydrocarbons and an intricate series of seniority rules must be run through for selecting the parent component. Traditional abbreviated terms are retained for the following fusion components furo, imidazo, (iso)quino, pyrimido, thieno. 

The two most important methods for naming heterocyclic parent hydrides are the extended Hantzsch-Widman system and replacement nomenclature. 

This method is useful when dealing with simple heterocycles, but it can become clumsy with more complex ones. An alternative is the Hantzsch-Widman system, which uses the same prefixes, but adds a stem name designed to indicate not only the ring size but also the state of unsaturation or saturation (note when the stem name begins with a vowel the last letter, a, of the prefix is dropped). The stem names for rings containing up to 10 atoms are shown in Table 1.1. 

Table 1.1 Hantzsch-Widman stem names for heterocycles with 3-10 ring atoms...

Another example of the prevailing use of "unnecessary" affixes in IUPAC inorganic nomenclature occurs in the extended Hantzsch-Widman system. Here a Greek capital delta (A) is inserted to denote a double bond in a heterocyclic ring. For example, the four-member ring which IUPAC would name as A3-l, 2-Azarsetine (Figure 27) in the proposed system is simply ... 

Abstracts, only the Hantzsch-Widman nomenclature has been used. Occasionally, one encounters names not strictly in accordance with prescribed terminology, e.g., azocane or azacyclooctane to denote the saturated, eight-membered heterocycle containing one nitrogen. In this article, we are more concerned with clarity, than with the consistent use of IUPAC nomenclature. 

The parent isoxazolidine (1) can be named as 1,2-oxazolidine according to the extension of the Hantzsch-Widman system. However, since the corresponding heterocycle (2) with the greatest number of non-cumulative double bonds is conventionally called isoxazole, 1 is most frequently referred to as isoxazolidine, which is used in Chemical Abstracts (CA). The hydro-isoxazole or -isoxazoline nomenclature is used solely in connection with the condensed ring for instance, compound 3 may be called either octahydro-l,2-benzisoxazole (CA nomenclature) or 4,5-tetramethyleneisoxazolidine (as a derivative of the isoxazolidine), or 7-oxa-8-azabicyclol4.3.0]nonane (3 ) (Stelzner method). A bis-isoxazolidine is called tetrahydro-2//-isoxazolo[2,3-6]-isoxazole (4) (CA method), or conventionally isoxazolizidine (4 ). 

There are three types of nomenclature used for heterocyclic compounds. Many heterocycles have trivial names, which are based on their occurrence, special properties, or historical reasons such as discovery of particular material. Systematic names of heterocyclic compounds derived from the structure of the compound are governed by lUPAC rules, which are divided into two groups the Hantzsch-Widman and replacement nomenclatures. In this book, we were trying to follow the guidelines for naming the heterocyclic compounds, which are summarized in Chapter 2 of the excellent book The Chemistry of Heterocycles Structures, Reactions, and Applications ... 

It should be pointed out that currently both trivial and systematic names are commonly used for naming the heterocyclic compounds. For example, an organic chemist will recognize without any difficulty the structures connected to names such as furane, pyrrole, pyrrolidine, pyrazole, imidazole, pyridine, or piperidine, despite the fact that all these names are trivial. On the other hand, the complex heterocycles require more sophisticated approaches in order to avoid ambiguity and correctly translate the chemical strucmre into the name. For these, compound names are often made using either trivial name (e.g., indazole for benzopyrazole, benzimidazole, indole, and isoindole) or the Hantzsch-Widman nomenclature, for example, 1,2,3- or 1,2,5-oxadiazoles, 1,3-dioxolane, 1,2- or 1,3-dithiolane, and 1,3- or 1,4-dioxane. It should be noted that the Hantzsch-Widman nomenclature treats the unsaturated heterocycle with maximum number of conjugated double bonds as parent compound. This adds another layer of complexity, giving rise to names such as... 

The molecules of Chapters 1 and 2 were species that defy one of the basic principles of the structural theoiy of organic chemistiy (or at least the theory in its simplest form, based on experience, without invoking electronic theory) the valences of tetracoordinate carbon are directed toward the comers of a tetrahedron. They are molecules that, within the confines of the theory, should not exist. The subject of this chapter, in contrast, appears to defy no rules of the stmctural theory oxirene (this is the usual name it is based on the clumsy Hantzsch-Widman system, in which heterocyclic rings of 3, 4, etc. atoms are given special names like oxirane, oxirene, oxetane, etc. An alternative system utilizes the currently neglected logical and convenient method of replacement nomenclature oxacyclopropane, oxacyclo-propene, oxacyclobutane, etc. ), 1, looks at first glance like a normal molecule. No unusual stereochemical constraints are imposed on the molecule, all atoms have... 

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