Exceptions: By Rules 2S-2.1 and 2S-2.2 the names gonane (for 18,19-dinorandrostane) and estrane (for 19-norandrostane) constitute exceptions to the above Rule 2S-6.1. The names gonane and estrane are used also as parent Names for their Derivatives.
Examples to Rule 2S-6.1
However, 18-nor- and 19-nor- are used with other trivial names, as in 19-norpregnane, 18,19-dinorspirostan, 18-norestrone.
The compound produced by shortening the 17-sidechain of pregnane is named 17-methylandrostane rather than 21-norpregnane. See also Note to Rule 2S-2.2.
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