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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
 

7. STEREOCHEMISTRY

7.1 As already mentioned, a broken line denotes a bond projecting behind the plane of the paper and a filled wedge one projecting in front of that plane. A line of normal thickness denotes a bond lying in the plane of the paper. Hatched lines are sometimes used instead of broken lines. The practice of using a reversed wedge instead of a broken line for a bond projecting behind the plane of the paper is not recommended. In complicated structures, the dashes can be lengthened, shortened or displaced if necessary.

7.2 Hydrogen is represented by its symbol “H” whenever a configuration has to be shown.

Geometric isomerism

7.3 For compounds containing double bonds it is customary to draw the formula so that the reference plane of the double bond is perpendicular to that of the paper; the bonds whereby atoms are attached directly to the doubly bonded atoms lie in the plane of the paper and are depicted with lines of normal thickness.

7.4 Isomers are shown with the two sequence-rule-preferred atoms or groups (each attached to one atom of the double bond) placed on the same side of the reference plane for the (Z)-isomer and on the opposite side of this for the (E)-isomer:

7.5 In simplified carbon chains depicted by lines at an angle to one another, the hydrogen, if any, may be omitted (see sections 10 and 11):

Examples of (Z)-compounds:


zimeldine


zuclopenthixol

Examples of (E)-compounds:


baxitozine


terbinafine

(Note that the two bonds attached to the carbons of the triple bond are drawn in a straight line.)

7.6 The same conventions are used for the isomers of oximes:


cefdaloxime (Z)

7.7 If the stereochemistry relative to the double bond is not specified a linear representation may be useful:


ethchlorvynol

7.8 The same conventions are used for compounds with several double bonds:


sorbic acid (E,E)

Compounds with one centre of asymmetry

7.9 In acyclic compounds with one centre of asymmetry, the general conventions can be used to represent each isomer either as a linear structure or with lines at an angle to each other (if possible, the larger “condensed” groups should be on the right, for aesthetic reasons).


D-alanine (R)


L-alanine (S)

(International Nonproprietary Names apply, by definition, to the L-form.)

7.10 The racemate can be represented by showing both isomers side by side or, more simply, showing only the (R)-isomer followed by the legend “and enantiomer”.


DL-alanine (RS)


ibuprofen

7.11 Similar representations are used for cyclic compounds with one centre of asymmetry:


D-proline (R)


L-proline (S)


DL-proline (RS)

(International Nonproprietary Names apply, by definition, to the L-form.)

7.12 If the chirality of the centre is unknown or not specified, the bonds joining atoms or groups to the chiral atom are shown as lines of “normal” thickness. The use of a star or asterisk to identify the chiral centre may be useful:


ethoheptazine (non specified)

Compounds with several chiral centres

7.13 In compounds containing several centres of asymmetry, the same conventions apply to each of these centres:


ephedrine (1R,2S)


levomenthol

7.14 The racemates (racephedrine and racementhol respectively) are depicted by the same structures followed by the legend “and enantiomer”, rather than by showing the two isomers side by side.

7.15 The same conventions are used for cis-trans isomerism relative to a planar (or approximately planar) ring:


pemedolac (±)-cis


spiradoline (±)-(5R*,7S*,8S*)

7.16 Mixtures of epimers are often shown by using the “normal” dashes at the epimeric centre (see also section II):


englitazone

However, the substance is preferably represented by showing the (R)-isomer at the epimeric centre, placing an asterisk near this C atom and adding the legend “and epimer at C*”:

7.17 In more complicated cases, it is better to draw each component of the mixture so as to show all the pecularities of the structure:


crilvastatin: [(±)-cis-] only for the cyclohexane ring

Isomerism of fused rings

7.18 In polycyclic compounds, the atoms or groups attached at saturated bridgeheads common to two rings are shown by their symbols so as to indicate the stereochemistry resulting from the way that the cycles are fused.

The cis-isomer is depicted with the bonds shown either both as wedges or both as broken lines:


tandospirone

The trans-isomer is depicted with one of the bonds as a wedge and the other as a broken line:


isomolpan

 

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