Gonane

Gonane (C17), also known as perhydrocyclopenta[a]phenanthrene, 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 (2⁶) 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 (C18) is the 13β-methyl variant of gonane, androstane (C19) is the 10β,13β-dimethyl variant of gonane, and pregnane (C21) is the 10β,13β-dimethyl, 17β-ethyl variant of gonane.