Heterocyclic Systems

Rule B-1. Extension of Hantzsch-Widman System

1.1 - Monocyclic compounds containing one or more hetero atoms in a three- to ten-membered ring are named by combining the appropriate prefix or prefixes from Table B-I (eliding "a" where necessary) with a stem from Table B-II. The state of hydrogenation is indicated either in the stem, as shown in Table B-II , or by the prefixes "dihydro-", "tetrahydro-'', etc., according to Rule B-1.2.

Examples to Rule B-1.1

1.2 - Heterocyclic systems whose unsaturation is less than the one corresponding to the maximum number of non-cumulative double bonds are named by using the prefixes "dihydro-", "tetrahydro-", etc.

In the case of 4- and 5-membered rings, a special termination is used for the structures containing one double bond, when there can be more than one non-cumulative double bond.

No. of members
of the partly
saturated rings
no nitrogen
4 -etine -etene
5 -oline -olene

Examples to Rule B-1.2

1.3 - Multiplicity of the same hetero atom is indicated by a prefix "di-", "tri-", etc., placed before the appropriate "a" term (Table B-I).

Example to Rule B-1.3

1.4 - If two or more kinds of "a" terms occur in the same name, their order of citation is in order of their appearance in Table B-I of Rule B-1.1.

Examples to Rule B-1.4

1.51 - The position of a single hetero atom determines the numbering in a monocyclic compound.

Example to Rule B-1.51

1.52 - When the same hetero atom occurs more than once in a ring, the numbering is chosen to give the lowest locants to the hetero atoms.

Example to Rule B-1.52

1.53 - When hetero atoms of different kinds are present, the locant 1 is given to a hetero atom which is as high as possible in Table B-I. The numbering is then chosen to give the lowest locants to the hetero atoms.

Examples to Rule B-1.53

The numbering must begin with the sulfur atom. This condition eliminates 2,1,4-thiadiazine. Then the nitrogen atoms receive the lowest possible locant, which eliminates 1,3,6-thiadiazine.

The numbering has to begin with the sulfur atom. The choice of this atom is determined by the set of locants which can be attributed to the remaining hetero atoms of any kind.

As the set 1,2,5 is lower than 1,3,4 or 1,5,4 in the usual sense, the name is 1,5,2-dithiazine.

See Recommendations'93 R-2.3.3

Trivial and Semi-trivial Names
Fused Heterocyclic Systems
Replacement Nomenclature
Cationic Hetero Atoms

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