Gonane
Gonane, also known as perhydrocyclopentaphenanthrene, is a tetracyclic hydrocarbon ring structure and the fundamental steroid nucleus. It consists of a phenanthrene ring fused with a cyclopentane ring. Unlike steroid hormones, gonane has no methyl groups at the C10 and C13 positions and no side chain at the C17 position. Because gonane has six centers of chirality, it has 64 theoretically possible stereoisomers. However, only a few of these stereoisomers are actually encountered with steroids. The most common stereoisomers of gonane are 5α-gonane and 5β-gonane.
Estrane is the 13β-methyl variant of gonane, androstane is the 10β,13β-dimethyl variant of gonane, and pregnane is the 10β,13β-dimethyl, 17β-ethyl variant of gonane.
The term gonane is also used to refer to a group of progestins that are carbon 18-homologated 19-nortestosterone derivatives including levonorgestrel and its analogues. The term is used to distinguish them from the estranes.
