Observe the structure (a). Doesn’t it look like our old friend housane after a tornado? It kept its roof but only just.
 |
| (a) |
|---|
|
Let us number it in the following fashion:
 |
| (a) |
|---|
The atom 3 is a quaternary carbon while the rest are secondary carbons. Or, using the graph theory language, we can say that in the graph (a) the degree of vertex 3 is 4 and the degrees of the rest of vertices are 2. The atom 3 is also known as a spiro atom [1, SP-0] while the whole structure is an example of spiro union.
