Contrinued from 3S-2. Fundamental carbocycles, unsaturation and alkyl substitution at C-17 (3S-2.1 to 3S-2.4)
3S-2. Fundamental carbocycles, unsaturation and alkyl substitution at C-17 (continued)
2.5 UnsaturationReferences for this section
2.6 Stereochemistry of double bonds
2.7 Steroids with a side chain as part of the parent carbocycle and an alkylsubstituent at C-17
2.8 Steroids with two alkyl substituents at C-17
2.9 Replacement of carbon atoms by hetero atoms
2.10 Additional rings formed within the steroid skeleton
Unsaturation is indicated by changing -ane to -ene, -adiene, -yne etc., or -an- to -en-, -adien-, -yn- etc. Examples:
(1) It is now recommended that the locant of a double bond is always adjacent to the syllable designating the unsaturation.
(2) If there is a choice of locants, single ones are preferred to compound locants, e.g. estra-5,7,9-triene is preferred to estra-5(10),6,8-triene.
(3) The use of Δ (Greek capital delta) is not recommended to designate unsaturation in individual names. It may be used, however, in generic terms, like 'Δ">5-steroids'.
(4) When an unsubstituted 24(25)-double bond is present the two methyl groups attached at position 25 are numbered 26 and 27 with the lower number for the methyl group trans to position 23.
(5) See addendum for pronunciation of estrene and pregnene.
3S-2.6. Stereochemistry of double bonds
The stereochemistry of double bonds in the side chain should be indicated using the E,Z convention (cf. Recommendation E-2.2 in ). The same applies to the seco compounds of the vitamin D series (see 3S-9). Example:
3S-2.7. Steroids with a side chain as part of the parent carbocycle and an alkyl substituent at C-17
If a steroid has two carbon chains attached at position 17 and one of them is included in Table 1, the compound is named as a 17-alkyl derivative of that steroid. If both side chains at C-17 are in Table 1, then the larger one is used, or the one in the normal configuration, e.g. 17-ethyl-5α-cholestane, 17-ethyl-5α-pregnane (not 17-ethyl-5α,17α-pregnane). The carbon atoms of the alkyl substituent may be numbered by a superscript number added to the number of the atom to which it is attached, e.g. 171, 172... etc. (cf. example 26). Examples:
Note Locants with superscript numbers are intended for the identification of the atoms e.g. in 13C-nmr spectral assignments, not as locants for further substitution e.g. oxymetholone [an international non-proprietary name (INN) see 3S-4.10 and Appendix] is 17β-hydroxy-2-(hydroxymethylene)-17α-methyl-5α-androstan-3-one, not 21,17β-dihydroxy-17α-methyl-2-methylene-5α-androstan-3-one. (See also addendum)
3S-2.8. Steroids with two alkyl substituents at C-17
If a steroid has two carbon chains attached at position 17, neither of which is included in Table 1, the compound is named as a 17,17-disubstituted androstane. Examples:
(1) Some steroids of this class can be related to a fundamental structure listed in Table 1 by applying the rules of chain lengthening and shortening (3S-6). Thus structure 28 could be called 17-methyl-21,24-dinor-5α,17α-cholane. This procedure is not recommended as it often yields cumbersome names.
(2) The carbon atoms of the alkyl substituents may be numbered by a superscript number added to the number of the steroid skeletal atom to which it is attached. The shorter of the two substituent chains receives primed superscript numbers. If the two substituent chains are of equal length and attached to the steroid ring system the substituent on the β-face has primed superscript numbers. If the two substituent chains are attached to the side chain of the steroid and are of equal length but one is further substituted the other chain has primed superscript numbers. In the absence of further substitution the pro-S group has primed superscript numbers.
(3) Locants with superscript numbers are intended for the identification of the atoms e.g. in 13C-nmr spectral assignments, not as locants for further substitution (see note to 3S-2.7 and also addendum).
3S-2.9. Replacement of carbon atoms by hetero atoms
When in the carbon skeleton of a steroid a carbon atom is replaced by a hetero atom, the replacement (oxa-aza) system of nomenclature is used with steroid names and numbering (cf. Recommendation B-4 in ). Example:
3S-2.10 Additional rings formed within the steroid skeleton
When an additional ring is formed by means of a direct link between any two carbon atoms of the steroid ring system or the attached side chain, the name of the steroid is prefixed by cyclo; this prefix is preceded by the numbers of the positions joined by the new bond and Greek letter (α, β, or ξ) denoting the configuration associated with the new bond, unless that designation is already implicit in the name. Examples:
Note Compound 30 is also known as i-cholesterol; likewise the 3,5-cyclosteroids are often called i-steroids.
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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