Provitamin photolysis

The conversion of ergosterol (provitamin Da) (la) or 7-dehydro-cholesterol (provitamin Da) (lb) into the corresponding vitamins by sequential photolysis and thermolysis is an important commercial process and has received considerable study. Also, the nature of the steroid molecule, its conformational restrictions and rigid backbone, makes it... 

As mentioned above, the initial photoconversion of the provitamin (1) into the previtamin (171) (Scheme VII) is followed by a facile interconversion of the previtamin with tachysterol (173), and to a lesser extent with lumisterol (172) and the starting provitamin. These secondary interconversions are difficult to avoid as the ultraviolet absorption spec-trxim of the previtamin completely overlaps that of the starting (1). Optimization of previtamin formation can be attained by short irradiation times at low temperature (55), by operating at a wavelength close to 295 nm (i) and by a second irradiation in the presence of a triplet sensitizer which will catalyze a more favorable ratio of previtamin to tachysterol to be formed (52). If the photolysis is carried out on a... 

Cholecalciferol (vitamin D3, I) is formed from cholesterol in the skin through photolysis of 7-dehydrocholesterol (provitamin D3) by ultraviolet light ( sunshine vitamin cf. 3.8.2.2.2). Similarly, vitamin D2 (ergocalciferol, II cf. Formula 6.2) is formed from ergosterol. 

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