Pre-vitamin

The i j -configuration of the 6,7-double bond in pre-vitamin D is critical to its subsequent thermal rearrangement to the active vitamin. A photochemical isomerization of pre-vitamin D to yield the inactive trans-isoTnen occurs under conditions of synthesis, and is especially detrimental if there is a significant short wavelength component, eg, 254 nm, to the radiation continuum used to effect the synthesis. This side reaction reduces overall yield of the process and limits conversion yields to ca 60% (71). Photochemical reconversion of the inactive side product, tachysterol, to pre-vitamin D allows recovery of the product which would otherwise be lost, and improves economics of the overall process (70). 

An especially important case is the thermal equilibrium between precalciferol (pre-vitamin Dj, 22) and calciferol (vitamin D2, 23). ... 

UV light (280-320 nm) causes the photoconversion of 7-dehydrocholesterol to pre-vitamin D3. This pre-vitamin can undergo further photoconversion to tachysterol and lumisterol or can undergo a temperature-dependent isomerization to cholecalciferol (vitamin D3, 6.8). At body temperature, this... 

Conical intersections with ring opening configurations do not occur only in 5-membered rings. In the case of cyclohexadiene [73] and related systems, such as chromenes [92] and pre-vitamin D [93], the puckering process can be rationalized... 

The biosynthesis of vitamin D3 (8.24) involves the thermal [1,7]-sigmatropic rearrangement of pre-vitamin 8.23, which is obtained by conrotatory electrocyclic ring opening of 7-dehydrocholesterol (8.22) (Scheme 8.3). 

Semihydrogenation of 9 with the same catalyst completes a total synthesis of pre-vitamine... 

Dehydrocholesterol is converted (by the action of sunlight) to vitamin D3 which, by regulating calcium metabolism, prevents the bone disease termed rickets. A laboratory analysis of the sequence indicates that an intermediate, pre-vitamin D3, is the compound actually formed by the action of light. (a) Provide a mechanism for its formation. 

The conversion of pre-vitamin D3 to vitamin D3 can be "explained" with the aid of a "no mechanism" mechanism, (b) Suggest a "mechanism" for this interconversion that lacks any of the classical intermediates of organic chemistry. 

Vitamin D Analogues - The analogues (151) of pre-vitamin D3 have been synthesised and their irradiation in THF solution at wavelengths around 3(X) nm brings about cisArans-isomerism Cyclization to (152) is also observed and in this process there is a wavelength and excited state dependence. Thus with... 

The quantum yields for the Interconversion of E-and Z-hexatrlene are remarkably low when compared to the values for E/Z-lsomerizatlon In the pre-vitamin D/tachysterol system (Figure 1). 

In the preceding sections we have presented a survey of recent experimental data on the photochemistry of (pre)vitamin D and Its Isomers as well as of some related simple trlenes. 

Subsequent investigations showed that D vitamins were involved, and eventually it became known that one of several D vitamins, called vitamin D3, is the curative factor. Vitamin D3 is formed in the skin from 7-dehydrocholesterol by two reactions. In the first reaction (below), ultraviolet light in the UV-B range (280-320 nm, which can penetrate the epidermal layer) brings about a 6-electron conrotatory electrocyclic reaction (see Special Topic H, WileyPLUS) to produce pre-vitamin D3. Following this event, a spontaneous isomerization (by way of a [1,7] sigmatropic hydride shift) produces vitamin D3 itself. 

Vitamin D is a pro-hormone that comes in two forms, cholecalciferol (vitamin D3) and ergocalciferol (vitamin Dj) (Haddad and Hahn 1973). Most of vitamin D is obtained as vitamin D3, through synthesis in the skin after exposure to UVB radiation (typically from sunlight) of wavelengths 290-315 nm (Holick 1995). The synthesis converts 7-dehydrocholesterol (DHC-7) to pre-vitamin D3, which is then quickly converted by heat induction to vitamin D3. Overexposure to UVB radiation does not cause vitamin D toxicity as excess pre-vitamin D3 and vitamin D3 is inactivated by radiation (Holick et al. 1981, Webb et al. 1989). Vitamin D3 can also be obtained from animal-based food products, with oily fish (e.g., salmon, sardines, and mackerel) being the best natural source, and egg yolk and meat containing smaller quantities. Vitamin D2 is the plant form of vitamin D and can occur naturally in some types of mushrooms (Lamberg-Allardt 2006). 

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. 

Windaus had assumed a series of intermediate steps, but Velluz could show that the photochemical reaction produces precalciferol (pre-vitamin D ), which is in equilibrium with Vitamin D2 through a thermal rearrangement reaction vitamin D2 is the major component of the equilibrium. Both lumisterol and tachysterol are side products and not intermediates. As can be seen from the formulas, the transition to precalciferol entails the opening of ring B with the consequent formation of a new double bond. The rearrangement to vitamin D2 consists in a shift of the double bonds by one place. 

Possible photoisomerization of compounds in the vitamin D series in organic glass at liquid nitrogen temperature was first reported by Havinga more than 40 years agoP Stereospecific photoisomerizations of pre-vitamin D (4a, b) in an organic glass were demonstrated only recently. The results are consistent with HT at the vinylic centers shown below. 

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