Identify the structure of testosterone

Testosterone is a steroid with the fused four-ring system. Its structure is commonly written as: $17\beta\text{-hydroxyandrost-4-en-3-one}$

We need to count the number of asymmetric (chiral) carbon atoms.

Rule for asymmetric carbon

A carbon is asymmetric if:

• it is sp$^3$ hybridised, and
• it is attached to four different groups.

So we exclude:

• carbonyl carbon $($at $C_3)$
• alkene carbons $($at $C_4$ and $C_5)$
• any carbon having two identical substituents.

Examine the steroid skeleton

In testosterone, the stereogenic centers occur at the ring-junction and substituted carbons.

The chiral carbons are:

• $C_8$
• $C_9$
• $C_{10}$
• $C_{13}$
• $C_{14}$
• $C_{17}$

Why these are chiral

• $C_8, C_9, C_{10}, C_{13}, C_{14}$ are all $sp^3$ carbons in the fused ring system with four different environments because the two ring paths attached to each are not identical.
• $C_{17}$ is attached to:
  • $OH$
  • $H$
  • one side of ring D
  • the side chain leading effectively to a methyl-substituted framework Hence it also has four different groups.

Why others are not chiral

• $C_3$ is carbonyl carbon, so not chiral.
• $C_4$ and $C_5$ are double-bonded, so not chiral.
• methyl carbons are not chiral.
• methylene carbons have two hydrogens, so not chiral.

Total count

Therefore, total number of asymmetric carbon atoms in testosterone is: $6$

Comparison with stored answer

Stored correct answer = $6$

Our derived answer also = $6$.