(a) In the skin of animals exposed to sunlight, 7-dehydrocholesterol is converted ehydrocholesterol into the hormone cholecalciferol, the so-called "vitamia" \(D_{3}\) that plays a vital role in the development of bones. In the laboratory, the following sequence was observed: What processes are actually taking place in these two reactions? Show details. (b) An exactly analogous reaction sequence is used to convert the plant steroid ergosterol into ergocalciferol, the "vitamin" \(\mathrm{D}_{2}\) that is added to milk: Fo ergosterol \(\mathrm{h} \mathrm{U} \rightarrow\) pre-ergocalciferol warm \(\rightarrow\) ergocalciferol What is the structure of pre-ergocalciferol ? Of ergo calciferol? (c) On heating at \(190^{\circ}\), pre-ergocalciferol is converted into IX and \(\mathrm{X}\), stereoisomers of ergosterol. What reaction is taking place, and what are the structures of IX and \(\mathrm{X}\) ? (d) Still another stereoisomer of ergosterol, \(\mathrm{XI}\), can be converted by ultraviolet light into pre-ergocalciferol. What must XI be ?

In this exercise, we analyze the conversion processes of certain compounds into hormones, the reactions involved, and the structures of these compounds. a) The conversion of 7-dehydrocholesterol to cholecalciferol (vitamin D3) is a UVB radiation-initiated photochemical process that takes place in skin cells exposed to sunlight. b) The conversion of ergosterol to ergocalciferol (vitamin D2) is an analogous reaction to the conversion of 7-dehydrocholesterol to cholecalciferol. Pre-ergocalciferol is formed by breaking the bond between carbons 9 and 10 and forming a new bond between carbons 10 and 19. Ergocalciferol is formed from pre-ergocalciferol through a heat-induced process that rearranges the double bonds. c) When pre-ergocalciferol is heated at 190°C, it undergoes an elimination reaction, forming two stereoisomers, IX and X, of ergosterol. IX has a trans configuration, while X has a cis configuration, with both having a double bond between carbons 5 and 6. d) The XI stereoisomer of ergosterol has a different configuration around the carbon-carbon double bond at positions 5 and 6 compared to ergosterol, pre-ergocalciferol, IX, and X species, most likely a cis configuration. This allows for ultraviolet light-induced conversion into pre-ergocalciferol.