Contents of Section
FR-1. Definitions
FR-1.1 Fusion
FR-1.1.1 Ortho-fused
FR-1.1.2 Ortho- and peri-fused
FR-1.1.3 Fusion atom
FR-1.1.4 Peripheral atom
FR-1.1.5 Bridgehead atom
FR-1.1.6 Interior atom
FR-1.2 Fused ring system
FR-1.2.1 Bridged fused ring system
FR-1.2.2 Multiparent name
FR-1.2.3 Multiplicative prefix name
FR-1.3 Components of a fused ring system
FR-1.3.1 Parent component
FR-1.3.2 Attached component(s)
FR-1.3.3 Interparent component(s)
FR-1.4 Bridges
FR-1.4.1 Simple bridge prefix
FR-1.4.2 Composite bridge prefix
FR-1.4.3 Bivalent bridge
FR-1.4.4 Polyvalent bridge
FR-1.4.5 Independent bridge
FR-1.4.6 Dependent bridge
Reference for this Section
Fusion nomenclature is concerned with a two dimensional representation of a polycyclic ring system with the maximum number of non-cumulative double bonds. In addition this system may be bridged (see FR-8), or involved in assemblies or spiro systems (not covered by these rules). For ring systems any ring fused to other rings on all sides must be itself named (i.e. it is not treated as a hole). For nomenclature purposes two rings which have two atoms and one bond in common may be regarded as being derived from the two rings as separate entities. The process of joining rings in this way is termed fusion.
Two rings that have only two atoms and one bond in common are said to be ortho-fused.
Example:
The two benzene rings of naphthalene are ortho-fused together
FR-1.1.2 Ortho- and peri-fused
In a polycyclic compound, a ring ortho-fused to different sides of two other rings that are themselves ortho-fused together (i.e. there are three common atoms between the first ring and the other two) is said to be ortho- and peri-fused to the other two rings.
Example:
Phenalene is considered as being composed of three benzene rings, each of which is ortho- and peri-fused to the other two.
Any atom of a fused ring system which is common to two or more rings is termed a fusion atom.
An atom that forms part of the outer perimeter of a fused ring system is called a peripheral atom.
An atom of a fused ring system to which a bridge (see FR-1.4) is attached is called a bridgehead atom.
Any fusion atom which is not a peripheral atom is termed an interior atom.
A system where each bond is part of a ring; where each ring is ortho-fused (FR-1.1.1), or ortho- and peri-fused (FR-1.1.2), to at least one other ring; and where no bond is common to more than two rings is termed a fused ring system.
FR-1.2.1 Bridged fused ring system
A ring system where some of the rings constitute a fused ring system and the remaining rings are created by one or more bridges (FR-1.4) is termed a bridged fused ring system.
A fused ring system name which is constructed using an interparent component (FR-1.3.3) and two or more identical parent components (FR-1.3.1) is called a multiparent name.
FR-1.2.3 Multiplicative prefix name
A fused ring system name which is constructed using one or more multiplicative prefix(es) (di-, tri-, etc. or bis-, tris-, etc.) to indicate multiple occurrences of the attached component(s) (FR-1.3.2) is called a multiplicative prefix name.
FR-1.3 Components of a fused ring system
Allowed fusion components are rings or ring systems which can be named without the application of any fusion nomenclature principles (see Appendices 1 and 2). Fused ring systems which do not have such a name are named by joining together appropriately selected fusion components.
The parent component (referred to as the base or principle component in previous versions of these rules) is the one with highest seniority according to the criteria given in FR-2.3 and is represented in the fusion name by that ring or ring system which is cited last in the name. It may be mono- or poly-cyclic (see FR-1.3.2 for an example).
FR-1.3.2 Attached component(s)
The components of a fused ring system which are not covered by the parent component are called attached component(s) and are expressed by fusion prefixes. Prefixes denoting components directly fused to the parent component are called first-order fusion prefixes. The rest are called second-order, third-order, etc. fusion prefixes which means they correspond to the second, third, etc. component reached when moving away from the parent component across fusion sites. All attached components may be mono- or poly-cyclic.
Example:
pyrano[2',3':4,5]cyclohepta[1,2-g]quinoline
Parent component - quinoline
First-order fusion prefix - cyclohepta
Second-order fusion prefix - pyrano
Note
Details of the method for naming fused ring systems are not given until FR-4.
FR-1.3.3 Interparent component(s)
In a system that consists of two (or more) parent components ortho-, or ortho- and peri-fused to the same attached component the latter is called the first-order interparent component. Likewise if two (or more) parent components are linked by three appropriately attached components there will be two identical first-order interparent components and a second-order interparent component. Systems with five or more interparent components are regarded in a similar way (see FR-6 for the special rules used to name systems with interparent components).
Examples:
benzo[1,2-d:4,5-d']bisazepine
Parent component - azepine
Interparent component - benzo
benzo[1",2":3,4;4",5":3',4']dicyclobuta[1,2-d:1',2'-d']bisazepine
Parent component - azepine
First-order interparent component - cyclobuta
Second-order interparent component - benzo
An atom or group of atoms is named as a bridge by means of a prefix if it fulfils one or more of the following criteria:
a. If it connects two or more non-adjacent positions of the same ring in a fused ring system.
b. If it connects two or more positions of different rings of a fused ring system and does not thereby form a new ortho- and peri-fused ring.
c. If it connects positions in a ring of a fused ring system to a previously described bridge but cannot be included as part of that bridge (an independent bridge: see FR-1.4.6).
d. If it connects the atoms at the end of a bond common to two rings of a fused ring system.
e. If it is necessary to describe a system with only ortho- or ortho- and peri-fusions but which cannot be completely named by fusion principles (see FR-7).
Examples:
Notes
1. See rule A-31.1 (ref 3) for a different definition of a bridge in a von Baeyer system.2. Beilstein uses cyclo to indicate a direct bond between two non-adjacent positions of a fused ring system [cf. rule F-4.1 (ref 3) for use with natural products]. For example, the skeleton below (see also note 3) is named by Beilstein as 1,5-cycloindene instead of as a bridged pentalene. The preferred bridged pentalene has a bivalent bridge (see FR-8.2.h).
Example:
With complex systems cyclo may facilitate generation of names. For example FR-8.2.i is illustrated by a system which could be called 3,4-cyclo-2,5-(metheno)dipentaleno-
[2,1-f:2',1'-f']cyclopenta[,1-a:3,4-a']dipentalene (see FR-5.4.c for numbering)
3. For the purposes of naming polycyclic fused and bridged fused ring systems the parent hydride is considered as a two-dimensional skeleton with normally the maximum number of non-cumulative double bonds in the fused ring system [i.e. excluding the bridge(s)]. The exceptions to this situation are when there is a double bond between a bridge and a fused ring system in the structure under consideration (i.e. requiring the use of the ending '-ylidene', see FR-8.3.2), or when a bridge terminates at an existing fusion atom or a trivalent atom, (see FR-8.1.2). In considering alternative ways of naming a bridged fused ring system the number and location of the double bonds in alternative parent structures may vary. Also the parent structure so described may not exist as such, but only in a hydrogenated or partially hydrogenated form. For example although all three structures in note 2 above have the same skeleton the double bonds of the preferred structure are located differently from the other two, since the maximum number of non-cumulative double bonds is always assigned in the ortho- or ortho- and peri-fused portion (see FR-9.1).
A simple bridging prefix describes an atom, or group of atoms that may be considered as a single unit e.g. epoxy, butano, benzeno.
FR-1.4.2 Composite bridge prefix
A composite bridge prefix describes a group of atoms that can only be considered as a contiguous sequence of simple bridges e.g. (epoxymethano) = epoxy + methano = -O-CH2- . It has also been called a compound bridge prefix.
A bivalent bridge is one which is connected by single bonds to two different positions of a fused ring system or a bridged fused ring system.
Examples:
A polyvalent bridge is one that is connected to a fused ring system by three or more single bonds or their multiple bond equivalents. A polyvalent bridge may often be considered as a combination of two bivalent simple bridges - one first-order and the other dependent.
Polyvalent bridges may be further classified as bipodal, tripodal etc. where the bridge is attached at two positions, three positions etc.
Examples:
tripodal bridge
bipodal bridge
A bridge which only connects two or more positions of a fused ring system is called an indepenent bridge. See example with FR-1.4.6.
A bridge which connects one or more positions of a fused ring system to one or more positions on a simple or composite independent bridge, and cannot be expressed as part of a larger composite bridge is called a dependent bridge.
Example:
4,5,12-(methanetriyl)-2,9,7-(propane[1,2,3]triyl)anthracene
the methanetriyl group C-14 is a dependent bridge
the propane[1,2,3]triyl group at C-11 to C-13 is an independent bridge
3. IUPAC Nomenclature of Organic Chemistry, Sections A and B, 1st edition, 1958; 2nd edition, 1966; 3rd edition (combined with section C), 1971; 4th edition (combined with sections C, D, E, F and H), 1979.
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