Secosteroid

A secosteroid (/ˈsɛkoʊˌstɛrɔɪd/) is a type of steroid with a "broken" ring. The word secosteroid derives from the Latin verb secare meaning "to cut",^[2]: 241 and 'steroid'. Secosteroids are described as a subclass of steroids under the IUPAC nomenclature.^{[1][1]: §3S-1 [3]} Some sources instead describe them as compounds derived from steroids.^[4]

^[1]

Cholecalciferol, an example of a 9,10-secosteroid. IUPAC-approved carbon numbering and ring labeling is shown in the picture. Since secosteroids are derived from steroids, they retain the same labeling system as steroids. ^{[1][1]: §3S-8}

The parent steroid skeleton. The B-ring of the parent steroid is broken between C9 and C10 to yield D vitamins.

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.).^{[1]: §3S-8}

The prototypical secosteroid is cholecalciferol (vitamin D₃).^[5] Its IUPAC systematic is "(5Z,7E)-(3S)-9,10-secocholestra-5,7,10(19)-trien-3-ol".^{[1]: §3S-8}

Some nonsteroidal estrogens, like doisynolic acid (cleaved on the D ring)^[6] and allenolic acid,^{[citation needed]} are also secosteroids or secosteroid-like compounds.