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Heterocyclic Systems

Rule B-3. Fused Heterocyclic Systems

3.1 - "Ortho-fused" and "ortho- and peri-fused" ring compounds containing hetero atoms are named according to the fusion principle described in Rule A-21 for hydrocarbons. The components are named according to Rules A-21, B-1 and B-2. When the name of a component in a fusion name contains locants (numerals or letters) that do not apply also to the numbering of the fused system, these locants are placed in square brackets (as are also the locants for fusion positions required by Rule A-21.5). The base component should be a heterocyclic system. If there is a choice, the base component should be, by order of preference:

(a) A nitrogen-containing component.

Example to Rule B-3.1(a)


not Pyrido[2,3-b]naphthalene

(b) A component containing a heteroatom (in the absence of nitrogen) as high as possible in Table B-I.

Example to Rule B-3.1(b)


not Furo[2,3-b]thiophene

(c) A component containing the greatest number of rings.

Example to Rule B-3.1(c)


not 7H-Indolo[3,2-f]quinoxaline

(d) A component containing the largest possible individual ring.

Example to Rule B-3.1(d)


not 2H-pyrano[3,2-b]furan

(e) A component containing the greatest number of hetero atoms of any kind.

Example to Rule B-3.1(e)


not o-Oxazino[4,5-b]pyridine

(f) A component containing the greatest variety of hetero atoms.

Examples to Rule B-3.1(f)


not 1H-Oxazolo[5,4-c]pyrazole


not 4H-Thiazolo[4,5-d]imidazole

(g) A component containing the greatest number of hetero atoms first listed in Table B-I.

Example to Rule B-3.1(g)


not Thiazolo[5,4-f]benzoselenazole

(h) If there is a choice between components of the same size containing the same number and kind of hetero atoms choose as the base component that one with the lower numbers for the hetero atoms before fusion.

Example to Rule B-3.1(h)


3.2 - If a position of fusion is occupied by a hetero atom, the names of the component rings to be fused are so chosen as both to contain the hetero atom.

Example to Rule B-3.2


3.3 - The following contracted fusion prefixes may be used: furo, imidazo, isoquino, pyrido, pyrimido, quino and thieno.

Examples to Rule B-3.3



3.4 - In peripheral numbering of the complete fused systems, the ring system is oriented and numbered according to the principles of Rule A-22. When there is a choice of orientations, it is made in the following sequence in order to:

(a) Give low numbers to hetero atoms;



(b) Give low numbers to hetero atoms in order of Table B-I, thus:


(c) Allow carbon atoms common to two or more rings to follow the lowest possible numbers (see Rules A-22.2 and A-22.3). [A hetero atom common to two rings is numbered according to Rule B-3.4(e)], thus:

not or
In a compound name for a fusion prefix (i.e., when more than one pair of square brackets is required), the points of fusion in the compound prefix are indicated by the use of unprimed and primed numbers, the unprimed numbers being assigned to the ring attached directly to the base component, thus:

(d) Give hydrogen atoms lowest numbers possible:


(e) The ring is numbered as for hydrocarbons but numbers are given to all hetero atoms even when common to two or more rings. Interior hetero atoms are numbered last following the shortest path from the highest previous number.

3.5 - As exceptions, two-ring systems in which a benzene ring is fused to a hetero ring may be named by prefixing numbers indicating the positions of the hetero atoms to benzo followed by the name of the heterocyclic component. Numbering is assigned by the principles set forth in Rule B-3.4 (a), (b), and (d)The names provided by this rule may also be used for components of more complex fused systems.

Examples to Rule B-3.5


not Benz[d]oxepin


not 4H-Benz[d][1,3]oxazine


not 1H-Benzo[e]pyrrolo[1,2-a]azepine

See Recommendations'93 R-2.4.1

Replacement Nomenclature
Cationic Hetero Atoms


This HTML reproduction of Sections A, B and C of IUPAC "Blue Book" is as close as possible to the published version [see Nomenclature of Organic Chemistry, Sections A, B, C, D, E, F, and H, Pergamon Press, Oxford, 1979. Copyright 1979 IUPAC.] If you need to cite these rules please quote this reference as their source.

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