Ergosterol ring opening

Irradiation of steroidal 3,7-dienes with ultraviolet light may result in ring opening and formation of various trienes. The most important reaction of this type is the conversion of ergosterol to previtamin Dj. 

On the positive side, there are photochemical reactions that are essential for human health. One of these is the formation of vitamin D (the antirachitic vitamin) by irradiation of ergosterol. This photochemical reaction is an electro-cyclic ring opening of the cyclohexadiene ring of ergosterol of the type described in Section 28-2D. The product, previtamin D2, subsequently rearranges thermally to vitamin D2 ... 

We can now go back to the reaction that introduced this section—the photochemical electrocyclic ring opening of ergosterol to give provitamin D2. By looking at the starting material and product we can deduce whether the reaction is conrotatory or disrotatory. 

There are also examples of electrocyclic reactions that follow the stereochemical outcomes (conrotatory vs. disrotatory) expected for reactions under orbital symmetry control. For example, the photochemical ring opening of Eq. 16.24 should be a six-electron, conrotatory process, and indeed the product has the predicted trans double bond. An important biological example of such a process is the photochemical conversion of ergosterol to pre-vitamin D (Eq. 16,25), a key event in the synthesis of vitamin D. 

Irradiation of ergosterol gives provitamin D2, a precursor of vitamin D2 (a deficiency of which causes softening of the bones, especially in children). Is the ring opening conrotatory or disrotatory (Caution The product is written in a more stable conformation than that obtained upon ling opening.)... 

Ergosterol occurs in plants and yeast and is important as a precursor of vitamin D (Figure 14—18). When irradiated with ultraviolet Hght, it acquires antirachitic properties consequent to the opening of ring B. 

Ergosterol undergoes similar changes although with the 7,8 unsaturation already present ring B opening occurs facilely with the result that in addition to vitamin 03, lumisterol, tachysterol and the 5,6-trans vitamin are formed. 

Vitamin D is unique in that it can be either obtained from the diet (as vitamin D2 or D3) or synthesized from a cholesterol precursor, a process that requires reactions in the skin, liver, and intestine. The calciferols, including several forms of vitamin D, are a family of steroids that affect calcium homeostasis (Fig. 34.26). Cholecalciferol (vitamin D3) requires ultraviolet light for its production from 7-dehydrocholesterol present in cutaneous tissues (skin) in animals and from ergosterol in plants. This irradiation cleaves the carbon-carbon bond at C9-C10 to open the B ring to form cholecalciferol, an inactive precursor of l,25-(OH)2-cholecalciferol (calcitriol). Calcitriol is the most potent biologically active form of vitamin D (see Fig. 34.26). 

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