When using general IUPAC rules for locant number minimization, one might be tempted to use the compound locant for some rather simple cases like multi-substituted cyclohexenes, like
‘cyclohex-1(6)-ene-1,2,4-triol’ instead of
cyclohex-6-ene-1,2,4-triol
This (x)y type of locant is called compound locant in IUPAC Nomenclature of Organic Chemistry (Preferred names (2013)) terminology:
P-14.3 LOCANTS
P-14.3.1 Types of locants
Traditional types of locants are arabic numbers, for example, 1, 2, 3; primed locants, for example, 1′, 1′′′, 2′′; locants including a lower case Roman letter, for example, 3a, 3b; italicized Roman letters, for example, O, N, P; Greek letters, for example, ‘α-, β-, γ-’ and compound locants, for example, ‘1(10)’ and ‘5(17)’.
(…)
Why ‘cyclohex-1(6)-ene-1,2,4-triol’ is most likely wrong might be concluded from the following rule
P-31 MODIFICATION OF THE DEGREE OF HYDROGENATION OF PARENTHYDRIDES
(…)
P-31.1 THE ENDINGS ‘ENE’ OR ‘YNE
(…)
P-31.1.4 Bi- and polycyclic von Baeyer parent hydrides
(…)
P-31.1.4.2 If there is a choice of names and numbering, the following criteria are considered in order until a decision is reached:
(1) a minimum number of compound locants. A compound locant is used for a double bond if the locants of the atoms at each end of the bond do not differ by a value of one. When a compound locant is required, the higher locant is cited in parentheses. A benzene ring is shown and described as a cyclohexatriene corresponding to the <PERSON> structure. Other aromatic rings are treated similarly, When required;
Note: The seniority of single locants over compound locants was established in 1989 for numbering steroids [see S3-2.5(2), ref. 16] and extended to von Baeyer systems in the 1999 publication on von Baeyer nomenclature (see VB-8.3.1, ref. 7).
Examples:
bicyclo[4.2.0]-oct-6-ene
[not bicyclo[4.2.0]-oct-1(8)-ene]
(…)
which however applies for polycyclic rings.
The mentioned ref 16 (IUPAC Nomenclature of steroids) section:
3S-2 Fundamental carbocycles, unsaturation and alkyl substitution at C-17
(…)
3S-2.5 Unsaturation
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2) If there is a choice of locants, single ones are preferred to compound locants, e.g. 576
estra-5,7,9-triene is preferred to estra-5(10),6,8-triene.
So, IUPAC organic nomenclature rules tend to avoid these compound locants where it's possible. But, as <PERSON><IP_ADDRESS> If there is a choice of names and numbering, the following criteria are considered in order until a decision is reached:
(1) a minimum number of compound locants. A compound locant is used for a double bond if the locants of the atoms at each end of the bond do not differ by a value of one. When a compound locant is required, the higher locant is cited in parentheses. A benzene ring is shown and described as a cyclohexatriene corresponding to the Kekulé structure. Other aromatic rings are treated similarly, When required;
Note: The seniority of single locants over compound locants was established in 1989 for numbering steroids [see S3-2.5(2), ref. 16] and extended to von Baeyer systems in the 1999 publication on von Baeyer nomenclature (see VB-8.3.1, ref. 7).
Examples:
bicyclo[4.2.0]-oct-6-ene
[not bicyclo[4.2.0]-oct-1(8)-ene]
(…)
which however applies for polycyclic rings. 576