Only a very elementary account is given here. When one heterocyclic ring is present, this is chosen as the parent compound. If more than one heterocyclic ring is present, the order of preference is given to the nitrogen containing component (nitrogen rings are the most common). For a component containing a hetero-atom other than nitrogen, the order of preference is that in (i) above (0 before S, etc.). When the parent compound has been chosen, its name is prefixed by the name of the fused ring attached, e.g., benz(o), naphth(o). Also, the parent compound chosen is the component containing the largest number of rings and has a simple name. For the purpose of numbering, the structure is written with the greatest number of rings in a horizontal position and a maximum number of rings above and to the right of the horizontal row. Numbering is then carried out (usually) in a clockwise direction starting with the uppermost ring farthest to right and omitting atoms at ring junctions. To distinguish isomers, the peripheral sides of the parent compound' are lettered a, b, c, etc., beginning with a for the side 1,2, b fOr 2,3, etc. To the letter as early in the alphabet as possible, denoting the side where fusion occurs, are prefixed, if necessary, the numbers indicating the positions of fusion of the other component; their order conform- to the direction of lettering of the base component. It should be noted that, these numbers apply tc :tie prefixed component (as a separate entity) and not to the combined system (which is numbered according to the usual rules).
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