Step 1: Understanding the Concept:
Cholestane is the fully saturated parent skeleton of cholesterol and other steroids. It consists of four fused rings, labeled A, B, C (six-membered each) and D (five-membered), joined together into one rigid tetracyclic system.
Step 2: Key Formula or Approach:
To find the fusion carbons, trace where each pair of neighboring rings shares atoms: A/B share two carbons, B/C share two carbons, and the two angular methyl-bearing carbons sit at the A/B and C/D junctions.
Step 3: Detailed Explanation:
Ring A and ring B share carbons 5 and 6. Ring B and ring C share carbons 8 and 9. Carbon 10 is the angular carbon at the A/B junction, carrying the C19 methyl group. Carbon 13 is the angular carbon at the C/D junction, carrying the C18 methyl group. Listing all of these together gives 5, 6, 8, 9, 10 and 13 as the complete set of ring-fusion carbons in the steroid nucleus.
Step 4: Final Answer:
The cholestane ring system is fused at carbons 5, 6, 8, 9, 10 and 13.