Cholecalciferol (vitamin D3), an example of a 9,10-secosteroid. The hydroxyl group (HO-) is in position C3 of the parent steroid A-ring. The triene substructure attached to the ring bearing the hydroxyl group is a result of the ring scission (cleavage) giving rise to this secosteroid.
The parent steroid skeleton. The B-ring of the parent steroid is broken between C9 and C10 to yield the vitamins D.
Secosteroids (sec·o·ster·oid, sek'ō-stēr'oyd) are a subclass of the tetracyclic steroid class that are structural relatives bearing a cleavage of one of the four rings of that parent class. The subclass nomenclature derives from the verb Latin: secare meaning "to cut", and Latin: stere of steroid, meaning "solid, three-dimensional". Subclasses are defined by the carbon atoms of the parent steroid skeleton where the ring cleavage has taken place, e.g., 9,10-secosteroids for derivatives deriving from cleavage between carbon atoms C9 and C10 of the steroid B-ring (similarly 5,6-secosteroids, 13,14-steroids, etc.).