Nomenclature of Steroids

Rule 2S-7 Ring Contraction or Expansion

7.1 - Ring contraction and ring expansion (other than insertion of atoms betwwen directly linked bridgeheads or, when a steroid sidechain is present, between C-13 and C-17) are indicated by prefixes "nor" and "homo", respectively, preceded by an italic capital letter indicating the ring affected. For loss or insertion of two methylene groups, "dinor" and "dihomo" are used. "Homo" and "nor", when occurring in the same name, are cited in alphabetical order.

Examples to Rule 2S-7.1





Notes: (a) By too extended use, this nomenclature can be applied to compounds whose steroid character is excessively modified. It is recommended that it be confined to steroids containing at least one angular methyl group, or a steriod 17-sidechain, or a steroidal group on ring D (e.g. a spirostan); also that no more than two of the steroid fings may be altered by any combination of the operations denoted by "nor" and "homo". When these conditions are not met, general systematic nomenclature should be used.
(b) Names incorporating "homo" and "nor" are normally preferred to alternatives incorporating "cyclo" and "seco".

7.3 - On ring expansion (other than insertion of atoms between directly linked bridgeheads or, when a 17-sidechain is present, between C-13 and C-17), the letter a (and b etc. as necessary) is added to the highest number in the ring enlarged exclusive of ring junctions, and this letter and number are assigned to the last peripheral carbon atom in the order of numbering of the ring affected.

Examples to Rule 2S-7.3



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