Continued from 3S-4. Functional groups (continued) (3S-4.4 to 3S-4.10)
3S-5. Stereochemical modifications
5.1 Use of the prefix ent-References for this section
5.2 Use of and for bridgeheads
5.3 Use of / and ent-
5.4 Use of the prefix rac-
5.5 Use of R* and S*
3S-5.1. Use of the prefix ent-
If, as for instance in a synthetic compound, there is stereochemical inversion at all the asymmetric centres whose configurations need not be specified in a name, the italicized prefix ent- (a contracted form of enantio-) is placed in front of the complete name of the compound. This prefix denotes inversion at all asymmetric centres (including those due to named substituents) whether these are cited separately or are implied in the name. Examples:
Note When numerals are used to enumerate formulae, the prefix ent- may be used to indicate the enantiomer. Thus, e.g. compound 64 above may be designated ent-63.
3S-5.2. Use of and for bridgeheads
If there is stereochemical inversion at not more than half of the asymmetric centres whose configurations need not be specified in a name, the configurations of the hydrogen atoms or substituents at the affected bridgeheads, or the carbon chain (if any) at position 17, are stated by means of a prefix or prefixes or , each with its appropriate locant, placed before the stem name laid down in 3S-2 and 3S-3. Examples:
Note The prefix retro, indicating 9,10 configuration, should not be used.
3S-5.3. Use of / and ent-
The enantiomer of a compound designated as in Recommendation 3S-5.2 is given the same name preceded by ent.
Note This Recommendation covers the compounds in which there is inversion at a majority, but not all, of the asymmetric centres that need not be specified in the name. Examples:
3S-5.4. Use of the prefix rac-
Racemates, as for instance obtained by synthesis, are named by use of an italicized prefix rac- (an abbreviation of racemo-), placed before the complete name of the compound, the enantiomer chosen for naming being that required by Recommendations 3S-5.1 to 3S-5.3.
Example: A racemate composed of compounds 63 and 64 (= ent-63) is named rac-17-hydroxyandrost-4-en-3-one or rac-testosterone.
Note The designation of and to the substituents refers to the normal series, not to the ent-series; the prefix ent- indicates that their absolute spatial orientation is inverted.
3S-5.5. Use of R* and S*
When the relative, but not the absolute, configuration of two or more asymmetric centres in a steroid derivative is known, this may be indicated by R* or S* according to Recommendations E-4.10 and F-6.7 of .
2. International Union of Biochemistry (1978) Biochemical nomenclature and related documents, The Biochemical Society, London.
3. International Union of Pure and Applied Chemistry, Nomenclature of organic chemistry, Sections A, B, C, D, E, F and H, 1979 Edition, Pergamon Press, Oxford, 1979. Section E appeared also in pp. 1-18 of , and section F in pp. 19-26 of  and in Eur. J. Biochem. 86, 1-8 (1978).
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