Continued from 3S-2. Fundamental carbocycles, unsaturation and alkyl substitution at C-17 (continued) (3S-2.5 to 3S-2.10)
3S-3. Steroids with heterocyclic rings in the side chain
3.0 General definitionsReferences for this section
3.5 Steroid alkaloids
3S-3.0. General definitions
Many important naturally occurring steroids contain one or more additional heterocyclic ring(s), fused or attached to ring D, formed by modifications of the side chain. These steroids can be grouped into the following families: (a) cardanolides, (b) bufanolides, (c) spirostans, (d) furostans, (e) steroid alkaloids.
The name cardanolide is used for the compound of structure 33, i.e. the parent compound of digitaloid lactones with the configuration illustrated. The 14-configuration as well as the 20R-configuration are implied in the name. Examples:
Names such as card-20(22)-enolide are used for the naturally occurring unsaturated lactones of this type. The names 14,21- and 16,21-epoxycardanolide are used for the compounds containing a 14,21- or a 16,21-oxygen bridge respectively, as shown in 36.
Note The former recommendation that the configuration at C-14 must always be stated as an affix is abandoned; 14 is implied unless otherwise stated. See also note 1 to 3S-1.5.
The name bufanolide is used for the compound of structure 37, the parent compound of the squill-toad poison group of lactones, with the configurations 14,20R as shown implied in the name. Unsaturated derivatives are named by replacing the suffix -anolide by -enolide, -adienolide, etc.; thus the name bufa-20,22-dienolide is used for the naturally occurring doubly unsaturated lactones.
Note As with cardanolides, the recommendation to always state the configuration at C-14 is abandoned. See also note (1) to 3S-1.5. Examples:
The name spirostan is used for compounds of structure 40 (this is a 16,22:22,26-diepoxycholestane); this name specifies the configuration shown for all the asymmetric centres except positions 5 and 25. A prefix 5- or 5- is added in the usual way (see Recommendation 3S-1.5). Configurations at C-16 and C-17, if different from those shown in formula 40, are designated by 16(H) and 17(H). Configurations at C-20 and C-22, if different from those shown in formula 40, are designated by the sequence-rule procedure (see Section E in ) or, if unknown, by . Steric relations of substituents at C-23, C-24 or C-26 are in all cases designated by the sequence-rule procedure or, if unknown, by . Examples:
(1) Although ring E, like rings , , C and D, can conveniently be shown by projection on to the plane of the paper, ring F cannot be adequately represented in this way since C-23, C-24 and C-26 and the oxygen atom lie in a plane that is perpendicular to the plane of the paper. Ring F is conveniently drawn as in formulae 40-43; in formula 40, for instance, the broken line from C-22 to oxygen denotes that the oxygen atom and C-26 of ring F lie behind the plane of the paper and that consequently C-23 and C-24 lie in front of the plane of the paper (configuration R at C-22). In formula 41 the configuration at C-22 is reversed and must be stated in the name as 22S. It is conventional to draw ring F as a chair, but this conformation is not implied in the name spirostan. Whatever the conformation of ring F, C-27 and the 25-hydrogen atom may be considered to lie in the plane of the paper and so cannot be denoted by broken or thickened lines. In 42 the methyl group is axial (above the general plane of ring F), and in 41 it is equatorial (in the general plane of ring F).
(2) The R,S specification may also be affected by substituents attached to ring F or C-27, as in compound 43 (cf. 42).
The name furostan is used for the compound of structure 44 (16,22-epoxycholestane); this name specifies the configurations at all the asymmetric centres except position 5, 22 and (if position 26 is substituted) also 25. Configuration at C-5 is designated by use of or in the usual way (see Recommendation 3S-1.5), and configurations at C-22 and, if necessary, C-25 by the sequence-rule procedure, or in all these cases by if unknown.
Substituents are indicated in the usual way, as in example 45.
3S-3.5. Steroid alkaloids
If systematic names for steroid alkaloids are desired, they should be derived from pregnane, cholestane or some other parent name given in Table 1.
Trivial names for the parent saturated structures of steroid alkaloids are so chosen that they end in -anine. In the case of unsaturated compounds, this ending is changed to -enine, -adienine, etc. as appropriate.
An extensive treatment of the nomenclature of steroid alkaloids is beyond the scope of these recommendations. A special appendix to section F of ref. 3 dealing with these problems is in preparation.
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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