Cody J Cubbage

Finasteride belongs to the subclass of steroids called azasteroids. All steroids contain a core structure called gonane, a structure composed of 3 cyclohexane rings and 1 cyclopentane ring "fused" together. Although gonane has 6 chirality centers, giving it 64 possible steroisomers, most steroids found in organisms contain the 5α-gonane stereoisomer. The function and naming of steroids are determined by what functional groups are attached to the rings of gonane and by the substitution of carbon atoms with in the rings with different atoms.  

In the case of Finasteride, two methyl groups have been attached to carbons 10 and 13, placing it in the androstane classification of steroids. The presence of a double bond in the A-ring and a ketone at carbon 3 make Finasteride are traits that it shares with the male sex hormone Testosterone, another androstane . However, the structures of Finasteride and Testosterone diverge at this point. Carbon 4 in Finasteride has been substituted with a nitrogen atom, which makes Finasteride a 4-azasteroid.