Question

(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 ?

Short answer

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.

Step-by-step solution

Identify the conversion process

The conversion of 7-dehydrocholesterol to cholecalciferol (vitamin D3) is a photochemical process that takes place when an organism is exposed to sunlight. The ultraviolet B (UVB) radiation in sunlight initiates this process. The 7-dehydrocholesterol absorbs the UVB radiation in the skin cells and undergoes a transformation that leads to the synthesis of cholecalciferol (vitamin D3). This hormone plays a vital role in bone development and overall health. #b) Determining the structure of pre-ergocalciferol and ergocalciferol#

Analogy with Cholecalciferol Synthesis

The conversion of ergosterol to ergocalciferol (vitamin D2) is an analogous reaction to the conversion of 7-dehydrocholesterol to cholecalciferol. This means that ergosterol absorbs ultraviolet light and undergoes a transformation, resulting in the production of pre-ergocalciferol and eventually ergocalciferol (vitamin D2).

Pre-ergocalciferol structure

In the pre-ergocalciferol molecule, the UV-induced photochemical reaction results in the breakage of the bond between carbons 9 and 10, forming a new bond between carbons 10 and 19. Thus, pre-ergocalciferol has a similar structure to ergosterol, with the change being this new bond between carbons 10 and 19.

Ergocalciferol structure

Ergocalciferol (vitamin D2) is formed from pre-ergocalciferol through a heat-induced process that rearranges the double bonds. These rearranged double bonds include moving one of the double bonds between the carbons 5 and 6 to a position between carbons 1 and 3. The structure of ergocalciferol is similar to that of cholecalciferol (vitamin D3), with a different side chain at carbon 17. #c) Reaction taking place and structures of IX and X#

Reaction taking place

When pre-ergocalciferol is heated at 190°C, it undergoes an elimination reaction, leading to the formation of two stereoisomers, IX and X, of ergosterol.

Structures of IX and X

These stereoisomers of ergosterol (IX and X) have similar structures to ergosterol, but with different configurations around the carbon-carbon double bond. The IX stereoisomer has a trans configuration, while the X stereoisomer has a cis configuration. Both configurations have the double bond between carbons 5 and 6, which differentiates them from the other end-products of this process. #d) Determining the structure of XI#

Identify the stereoisomer of ergosterol

Another stereoisomer of ergosterol, XI, can also be converted into pre-ergocalciferol by ultraviolet light. The most likely difference in the structure of XI compared to ergosterol (the starting compound) would be around the carbon-carbon double bond between carbons 5 and 6.

Structure of XI

The XI stereoisomer would have a different configuration around the carbon-carbon double bond at positions 5 and 6 compared to ergosterol, pre-ergocalciferol, IX, and X species. Among the configurations, the most suitable option would be a cis configuration around the double bond for the XI species. This would make XI structurally and conformationally distinct, allowing for ultraviolet light-induced conversion into pre-ergocalciferol.

Key concepts

Vitamin D Synthesis

Vitamin D synthesis is a fascinating photochemical process. It involves the transformation of substances found in living organisms into a vital nutrient, specifically vitamin D. This synthesis primarily occurs in the skin and is activated by sunlight, more specifically by ultraviolet B (UVB) radiation.

Here's how it works:

• When skin is exposed to UVB radiation, it converts 7-dehydrocholesterol, a type of cholesterol naturally found in the skin, into cholecalciferol (also known as vitamin D3).
• This transformation is crucial for your health because vitamin D is essential for maintenance of healthy bones and a robust immune system.
• Similarly, in plants, ergosterol is converted into ergocalciferol (vitamin D2), following an analogous process when exposed to UV light.

Both forms of vitamin D, whether D2 or D3, are important parts of maintaining a healthy lifestyle, showcasing nature’s brilliant method of providing this essential nutrient.

Cholecalciferol

Cholecalciferol, or vitamin D3, plays a pivotal role in our overall health. It assists in absorbing calcium and phosphorus, thereby supporting the growth and maintenance of healthy bones.

Cholecalciferol is produced primarily through skin exposure to sunlight. During this process:

• UVB rays penetrate the skin, transforming 7-dehydrocholesterol into cholecalciferol.
• Once formed, it is then transported to the liver and kidneys, where it is converted into its active form, calcitriol, that the body can use.

This compound is more than just a vitamin; it acts as a hormone, sending messages to various parts of the body to regulate calcium absorption and bone health. As a result, adequate levels of cholecalciferol ensure that bones remain strong and structurally sound, warding off diseases like osteoporosis.

Ergocalciferol

Ergocalciferol, also referred to as vitamin D2, is another form of vitamin D critical to human health. Unlike cholecalciferol which is synthesized in animal skin, ergocalciferol is primarily derived from plants and fungi.

Here’s the transformation process:

• Exposure to sunlight results in the conversion of ergosterol, a steroid present in plants, to ergocalciferol.
• Following exposure to UV light, it goes through a series of transformations, first becoming pre-ergocalciferol and then converting into fully active ergocalciferol.

Though both D2 and D3 forms of vitamin D are beneficial, ergocalciferol primarily contributes to human nutrition through dietary sources such as fortified foods and supplements, thereby helping maintain adequate vitamin D levels especially in regions with limited sunlight.

Stereoisomers

Stereoisomers are molecules that have the same molecular formula but differ in the spatial arrangement of their atoms. This subtle difference can result in substantial variations in physical and chemical properties.

In the context of vitamin D synthesis:

• Pre-ergocalciferol heated at conditions like 190°C can lead to the creation of stereoisomers such as IX and X, which are forms of ergosterol.
• These stereoisomers exhibit either a trans or cis configuration around specific carbon-carbon double bonds. For instance, IX has a trans configuration, whereas X takes on a cis configuration.
• An additional stereoisomer, labeled XI, with a variation similar to a cis configuration across the bonds, can revert to pre-ergocalciferol when exposed to UV light.

These subtle changes can have profound effects on the biological activity and functionality of these compounds, showcasing the importance of stereochemistry in biochemistry.