Continued from 3S-6. Lengthening and shortening of side chains and elimination of methylgroups
3S-7. Ring contraction and expansion
7.1 Use of the prefix nor-References for this section
7.2 Use of the prefix des-
7.3 Use of the prefix homo-
7.4 Use of both nor- and homo- in one name
7.5 Use of the prefix abeo-
3S-7.1. Use of the prefix nor-
Ring contraction by loss of an unsubstituted methylene group is indicated by the prefix nor-. For loss of two methylene groups, dinor- is used. The methylene group(s) lost is considered to include the highest numbered unsubstituted atom of the ring involved, and the locant of that lost methylene group precedes the prefix nor- in the nor-compound. The remainder of the original numbering is retained. Example:
In 4-nor-5-androstane (78) C-4 is missing.
3S-7.2. Use of the prefix des-
The removal of a terminal ring, with addition of a hydrogen atom at each junction atom with the adjacent ring, is indicated by the prefix des-, followed by the italic capital letter designating that ring (3S-1.1); substituents and stereochemistry implied in the trivial name remain unless otherwise stated. Example:
Note The prefix des- is used here, not de-, because of the ease of confusing de- with the letter D in speech.
3S-7.3. Use of the prefix homo-
Ring expansion by inclusion of one methylene group is indicated by the prefix homo-, by two methylene groups by dihomo-. The prefix is preceded by the (complex) locant(s) of the carbon atom(s) inserted. These are chosen in the following way:
(a) If the methylene group is not inserted between directly linked bridgeheads or between C-13 and C-17 carrying a side chain, the letter a, b etc. is added to the locant of the highest numbered atom of the ring that is not a bridgehead. Examples:
(b) If the methylene group is inserted between directly linked bridgeheads or between C-13 and C-17 carrying a side chain, the letter a (or a and b) is added to the pair of locants indicating the atoms on either side, and the higher numbered locant is placed in parentheses. Examples:
The locant(s) with the prefix homo- is placed after the prefixes required for functional groups (cf. 3S-4), before the stereodescriptor (e.g. 5) of the stem name, or if such is not required, directly before the stem name.
(1) Recommendation 3S-7.3 deviates from the former convention in 2S-8; the present recommendation is in line with the general Recommendation F-4.5 (cf. Section F in ).
(2) Although complex locants are necessary in the names of homo-steroids, simple locants may be used with structural formulae, e.g. 8a in compound 82 or 13a in compound 85 with the lower-numbered carbon atom as parent locant.
3S-7.4. Use of both nor- and homo- in one name
The above recommendations for ring expansion and contraction may be used for modifications of two rings in the same molecule, as illustrated in example 86.
(1) Names incorporating homo- and nor- are normally preferred to alternatives incorporating cyclo- and seco- (cf. examples 84 and 86).
(2) A compound with excessive modification of the steroid skeleton usually is best described by a systematic name. It is recommended that no more than two of the steroid rings be altered by any combination of the prefixes homo- and nor-.
3S-7.5. Use of the prefix abeo-
As an alternative to the application of nor- and homo- to the same molecule, the abeo system may sometimes be used. A compound that may be considered to arise from a steroid by bond migration may be given the name laid down in the preceding Recommendations for the steroid in question, to which is attached a prefix of the form x(yz)abeo-. This prefix is compiled as follows: a numeral denoting the stationary (unchanged) end of the migrating bond (x) is followed by parentheses enclosing (i) the number denoting the original position (y) from which the other end of this bond has migrated, (ii) an arrow, and (iii) the number (z) denoting the new position to which the bond has moved (see formulae 87a and 87b).
The original numbering is retained for the new compound and is used for the numbers x, y and z. It is always necessary to specify the resulting stereochemistry. Examples:
(1) The abeo nomenclature described in this Recommendation is permissive, not compulsory. It is most suitable for use in discussions of reaction mechanism and biogenesis. For registration in a general (non-steroidal) compendium, the general systematic names, or names assigned by the homo- or nor- method, may be preferable. Thus 88 is identical with 86.
(2) The abeo system has the advantage that the normal numbering of the steroid skeleton is retained. An example is 89, the parent structure of some steroid alkaloids. Also, 84 may be named 9(1019)abeo-5,10a(H)-pregnane, C-9a remaining C-19.
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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