So far, we’ve only talked about cyclic molecules containing one ring. But, of course, molecules with multiple rings are very common in nature. A prime example is the steroid structure, exemplified by the common oral contraceptive ethinyl estradiol.

estradiol

Here, we’re just going to talk about two very simple cases of molecules containing two rings. We’re going to focus our efforts on cyclohexane, and examine in close detail the situation where two cyclohexane rings are bonded to each other in the most common way – with the two ring junctions (“bridgeheads”) on adjacent carbons, a situation we refer to as fused rings.

The parent molecule is called “decalin” since there are ten carbons in total. However, remember that any time we have two substituents on a cyclohexane ring (as we do here), it is essential to draw in the stereochemistry in order to avoid ambiguity! Two stereoisomers are possible here: one where the hydrogens at both ring junctions are “cis”, and the other where they are “trans”. …show more content…
What could be simpler than two hexagons joined together?

Well, just like with cyclohexane itself, the complexity comes when we examine their most stable three-dimensional structures. Each six membered ring will adopt a chair conformation. Believe it or not, the cis and trans stereoisomers of decalin have remarkably different shapes.

There’s no better way to see this than by making a model. I love Vine for making movies of what it’s like to “fly-over” these