Continued from 3S-5. Stereochemical modifications
3S-6. Lengthening and shortening of side chains and elimination of methylgroups
6.1 Use of the prefix homo-
6.2 Use of the prefix nor-
6.3 Preference between homo- and nor-
3S-6.1. Use of the prefix homo-
Lengthening of a side chain by insertion of one (or more) methylene groups is indicated by the prefix homo- (dihomo- etc.). The prefix is preceded by the (complex) locant(s) of the carbon atom(s) inserted. Examples:
(1) If the methylene group is inserted into a methyl C-H bond or next to a methylene group the letter a (b etc.) is added to the locant of the highest numbered atom (e.g. example 69 is 27a-homo- not 26a-homo-).
(2) If the methylene group is inserted between two side-chain branch points that are directly linked, or between C-17 and a branch point at C-20 (i.e. Table 1 except pregnane) both locants are used with the letter a (b etc.) and the higher numbered locant in parenthesis. The higher number may be omitted in a structural formula but must be used in the name of the homo-steroid (e.g. 17a in compound 71).
3S-6.2. Use of the prefix nor-
Elimination of a methylene group (-CH2-) from a steroid side chain is indicated by the prefix nor-, which in all cases is preceded by the number of the carbon atom that disappears. When alternatives are possible, the number attached to nor- is the highest permissible. Elimination of two methylene groups is indicated by the prefix dinor-. The remainder of the original steroid numbering is retained (see 73). Examples:
Exceptions: By recommendations 3S-2.1 and 3S-2.2 the names gonane (for 18,19-dinorandrostane) and estrane (for 19-norandrostane) constitute exceptions to the above Recommendation 3S-6.2. The names gonane and estrane are used also as parent names for their derivatives.
However, 18-nor- and 19-nor- are used with other parent and trivial names, as in 19-norpregnane, 18,19-dinorspirostan, and 18-norestrone.
The compound produced by shortening the side chain of pregnane is named 17-methylandrostane rather than 21-norpregnane. See also Recommendation 3S-6.3. Examples:
3S-6.3. Preference between homo- and nor-
A compound with a skeleton that differs from the fundamental parent systems (see 3S-2.1 to 3S-2.4 and 3S-3.1 to 3S-3.4) by changes in the side chain(s) can often be named by several different methods involving either side chain lengthening (3S-6.1) or shortening (3S-6.2) and/or alkylation (3S-4.0) and/or addition of carbon atoms associated with a functional group [3S-4.1(b), 3S-4.2(a), 3S-4.4(a)]. The preferred name is selected by the following recommendations applied in order:
(a) A name should be derived by the fewest number of modifications of the fundamental parent system. Both detachable (e.g. alkyl) and non-detachable (e.g. homo or nor) prefixes are considered as modifications. Dihomo, dinor, etc., are counted as two modifications each.
(b) Non-detachable prefixes are preferred to detachable prefixes. Examples:
(1) As an exception to recommendation (b) above, 17-methyl-5-androstane is preferred to 21-nor-5-pregnane and 5-androstane-17-carboxylic acid is preferred to 21-nor-5-pregnan-20-oic acid.
(2) Compound 77 is 13-ethyl-18-nor-5-estrane in current Chemical Abstracts Service index nomenclature.
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