3. Steroids

The cyclopentanoperhydrophenantrenic skeleton is the building block of many naturally occurring steroidal compounds: 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.