3. Steroids

The cyclopentanoperhydrophenantrenic skeleton is the building block of many naturally occurring steroidal compounds, including cholesterol, genital hormones, corticoids, digitalis, certain curare derivatives and fusidic acid, an antibiotic obtained by fermentation from Fusidium coccineum cultures (figure 5 ). Added to these steroidal substances are D vitamins such as ergocalciferol, an antirachitic secosteroid whose structure results from the cleavage between carbons 9 and 10 of ergosterol (figure 6 ). This cleavage is most often achieved by ultraviolet irradiation of the precursor provitamin.