Hantzsch-Widman names numbering

The prefixes to be attached to the name of the base component are formed by changing the terminal e of the trivial or Hantzsch-Widman names of the components (in the form containing the maximum number of non-cumulative double bonds) into o . Some prefixes are further abbreviated, as exemplified in the following list ... 

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. 

The Hantzsch-Widman names are preferred for rings with up to 10 members, in organic nomenclature. For saturated rings and mancude rings (rings with the maximum number of... 

Systematic names of the Hantzsch-Widman type, as well as fusion names, imply unsaturation to the extent of the maximum number of noncumulative double bonds. Suitable suffixes (Table 3) serve to identify the fully saturated derivatives. Partially unsaturated rings may be designated by hydro prefixes with locants and an even-numbered multiplier, such as 2,3-dihydro , applied to a fully unsaturated name, or by dehydro prefixes applied to the saturated name, e.g. (116). 

Numbering of the ring starts with the hetero atom and proceeds around the ring so as to give substituents (or other hetero atoms) the lowest numbered positions. When two or more different hetero atoms are present, oxygen takes precedence over sulfur and sulfur over nitrogen for the number one position. Examples follow to illustrate both the heterocycloalkane and the Hantzsch-Widman systems. Trivial names also are included. 

Last part of endings of a number of parent names of saturated heteromonocycles in Hantzsch-Widman nomenclature, i.e. of irane , etane , olane , ane , inane , epane , ocane , onane and ecane (see Section IR-6.2.4.3). 

General ending of parent names of three-membered heteromonocycles with the maximum number of non-cumulative double bonds (i.e. one double bond) in Hantzsch-Widman nomenclature, cf. Section IR-6.2.4.3. See also irine . 

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 ). 

Sequence and numbering of the heteroatoms follow the rules given in 2.1. The two compounds chosen as examples could also be named according to the Hantzsch-Widman system silole, 1,4,2-thiazasilane. 

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... 

At this point, we can introduce an entirely different system of nomenclature that is nevertheless accepted by lUPAC and is extremely valuable in multicyclic and bridged saturated systems. This is the replacement system, where the hydrocarbon name that would correspond to the entire ring structure, as if no heteroatom were present, is stated, and then given a Hantzsch-Widman prefix and number for the heteroatom(s). Thus, phenanthridine shown previously has the ring framework of the hydrocarbon phenanthrene, with N at position 5. The replacement name would be 5-azaphenanthrene. 

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