Photochemistry of previtamin

Figure 10. Wavelength-dependent photochemistry of previtamin D3. (Reprinted from Ref. 73 with permission of copyright owner the American Chemical Society.)...

Photochemistry of previtamin D (41 see also Special Topic 6.4) is an example of E Z isomerization in trienes. Thermal (A) interconversion of its conformers 41a,b is suppressed in a cold, rigid matrix of a solidified medium (92 K) (Scheme 6.8). Upon irradiation, 41a isomerizes to the thermodynamically more stable tachysterol conformer 42a, whereas 42b is formed exclusively from the conformer 41a. 

Intriguingly, the conical intersection model also suggests that E,Z-isomerization of acyclic dienes might be accompanied by conformational interconversion about the central bond, reminiscent of the so-called Hula-Twist mechanism for the efficient ,Z-photo-isomerization of the visual pigment rhodopsin in its rigid, natural protein environment101. A study of the photochemistry of deuterium-labelled 2,3-dimethyl-l,3-butadiene (23-d2) in low temperature matrices (vide infra) found no evidence for such a mechanism in aliphatic diene E,Z -photoisomerizations102. On the other hand, Fuss and coworkers have recently reported results consistent with the operation of this mechanism in the E,Z-photoisomerization of previtamin D3 (vide infra)103. 

The photochemistry of vitamin D (see also Special Topic 6.4 above and Scheme 6.8) has also played a central role in the development of modern organic photochem-istry.564,598,617 618 The concept of non-equilibration of excited rotamers (NEER Section 6.1.1) has been used to explain the excitation-wavelength dependence of E Z isomerization (Section 6.1.1) of previtamin D3 (41).619 Whereas the quantum yield for E Z isomerization decreases with increasing wavelength, the formation efficiencies of the 6jt-electron conrotatory ring-closure products, diastereomeric 7-dehydrocholester-ol (provitamin D) (64) and lumisterol (65) (Scheme 6.22), increase dramatically. This was found to occur on the basis of a participation of both the Si and S2 excited states in the photoreaction.620 For example, the quantum yields of 64 and 65 formation were [Pg.244]

Under these conditions, the photochemical transformation of cyclohexa-2,4-dienones into doubly unsaturated carboxylic acids or their derivatives is an ideal photoreaction for the synthetic chemist, since the products created absorb at significantly shorter wavelengths than the starting materials employed. What this implies does not require further laboring, in the light of the previtamin-D (5) photochemistry outlined earlier (Sect. LI). In-depth study of the addition of protic nucleophiles to photochemically generated dieneketenes was to result in insights into reaction mechanisms that enable the syn-... 

In recent years, it has been suggested that not all the excitation wavelength dependence of quantum yields in the vitamin D field can be accounted for by the NEER principle. We consider below the competing mechanisms that have been proposed to account for photochemical observations and describe some of the strategies that have been employed to improve the photochemical production of the previtamins from the provitamins. Optimization of the previtamin yields improves vitamin yields as the latter are formed thermally from the previtamins via 1,7-suprafacial hydrogen shifts. Readers interested in the rich photochemistry leading to overirradiation products should consult previous reviews. ... 

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