A secosteroid is a type of steroid with a "broken" ring. The word secosteroid derives from the Latin verb secare meaning "to cut",[1] and 'steroid'. Secosteroids are described as a subclass of steroids under the IUPAC nomenclature.[2] Some sources instead describe them as compounds derived from steroids.[3]
Types or subclasses of secosteroids are defined by the carbon atoms of the parent steroid skeleton where the ring cleavage has taken place. For example, 9,10-secosteroids are derived from cleavage of the bond between carbon atoms C9 and C10 of the steroid B-ring (similarly 5,6-secosteroids, 13,14-secosteroids, etc.).
The prototypical secosteroid is cholecalciferol (vitamin D3).[4] Its IUPAC systematic is "(5Z,7E)-(3S)-9,10-secocholestra-5,7,10(19)-trien-3-ol".
Some nonsteroidal estrogens, like doisynolic acid (cleaved on the D ring)[5] and allenolic acid, are also secosteroids or secosteroid-like compounds.