Question

Look at the following structure of the female hormone estrone, and tell whether each of the two indicated ring-fusions is cis or trans.

Short answer

Both structures have trans-ring fusions.

Step-by-step solution

Cis and trans conformation of decalin

In cis-decalin, the hydrogen atoms at the bridgehead carbons are on the same face of the rings; in trans-decalin, the bridgehead hydrogens are on opposite faces.

Axial and equatorial positions

Axial bonds are the bonds that are perpendicular or vertical to the ring plane. Equatorial bonds are approximately horizontal in the plane of the ring.

Conformation

In both, it can be seen that the bridgehead hydrogens are on opposite faces. So, both the structures are trans.