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The Graphic Representation of Chemical Formulae in the Publications of International Nonproprietary Names (INN) for Pharmaceutical Substances
(1995; 53 pages) View the PDF document
Table of Contents
View the documentAcknowledgements
View the document1. INTRODUCTION
View the document2. ACYCLIC STRUCTURES
View the document3. CYCLIC STRUCTURES
View the document4. IONIC STRUCTURES
View the document5. ISOTOPICALLY MODIFIED COMPOUNDS
View the document6. COORDINATION COMPOUNDS
View the document7. STEREOCHEMISTRY
View the document8. CARBOHYDRATES
View the document9. STEROIDS
View the document10. TERPENOIDS
View the document11. PROSTANOIDS
View the document12. ALKALOIDS
View the document13. ANTIBIOTICS
View the document14. POLYPEPTIDES
View the document15. POLYMERS
View the documentAcknowledgements
View the documentReferences
 

3. CYCLIC STRUCTURES

3.1 Rings are shown in full as polygons. The symbols of the carbon atoms that form the ring are not shown, but are represented by the vertices of the rings. The hydrogen atoms attached to them are not represented unless they are needed to show stereochemistry. The symbols of atoms other than carbon are shown with all the hydrogen atoms attached to them but without linking dashes. Single, double or triple bonds are indicated thus:

3.2 In aromatic systems a circle should not be used to depict delocalized electrons; instead, alternating single and double bonds are shown (Kekulé representation):

In monocyclic aromatic compounds, double bonds should be arranged to have the lowest possible numbering:

3.3 In fused polycyclic systems a double bond should form the fusion bond nearest to the right-hand side:

3.4 Six-membered rings should be represented with a vertex at the base rather than a horizontal bond when the chains linked to them are represented in the form of lines at an angle to one another (as is preferred for acyclic chains - see section 2.3):

3.5 Depiction of the ring with a horizontal bond at the base is, however, preferred when the chains are shown in the compact form:

Not used by the INN Programme

3.6 Rings are shown as regular polygons when they consist of up to eight atoms:

3.7 Wherever possible, the regularity of the polygons is maintained in the drawing of fused cyclic compounds:

3.8 However, in fused polycyclic systems the polygons may often be distorted in order to maintain the symmetry of the structure:

3.9 Rings with more than eight vertices are often shown with re-entrant angles. It is recommended by Chemical Abstracts Service (CAS) that they should be drawn like amalgamated rings with five, six or seven vertices:

3.10 These recommendations need not always be followed. In particular, the shape of such large rings as those of macrolide antibiotics is often determined by the presence of more or less bulky substituents and the need to indicate stereochemical conformations:

3.11 When a substituent is attached to an atom occupying a position in a ring (carbon or heteroatom), the direction to be taken by the dash linking it to that atom can be found by extending the line bisecting the cycle:

3.12 Where two substituents are attached to the same ring atom, they should generally both be at the same angle to the bisector, and preferably at a right angle to the adjacent side:

However, for the graphic representation of certain structures, such as steroids, other considerations may have priority.

3.13 Substituents are normally placed outside rings, except in steroids, terpenes and alkaloids (see sections 9, 10 and 12) and where substituents attached at bridgeheads can only be displayed inside the rings of polycyclic structures:

3.14 In bridged structures, a non-atomic bridge (direct bond) is represented by a straight line, an atomic bridge by lines at an angle to one another. The symbols for carbon atoms are not shown; however, if the bridge contains one or more heteroatoms, the atomic symbols for those atoms are shown. To give some perspective to the figure or to represent stereochemical features, wedges, thickened lines or broken lines can be used:

3.15 Sometimes a three-dimensional approach is possible, if a planar representation is considered not clear enough:

Tridimensional structures are rarely used by the INN Programme

 

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