Vitamin D3 (cholecalciferol) is produced through the action of ultraviolet irradiation (UV) on its precursor 7-dehydrocholesterol. Our skin makes vitamin D3 and supplies about 90 percent of our vitamin D. This molecule occurs naturally in the skin of animals and in milk. Vitamin D3 can be made by exposure of the skin to UV, or by exposing milk directly to UV (one commercial method). Vitamin D3 is also found in oily fish and cod liver oil.
The transformation that converts 7-dehydrocholesterol to Vitamin D3 (cholecalciferol) occurs in two steps. First, 7-dehydrocholesterol, is photolyzed by ultraviolet light in a 6-electron conrotatory ring-opening electrocyclic reaction. The product is previtaminD3. Second, previtaminD3 spontaneously isomerizes to vitaminD3 (cholecalciferol) in an antarafacial sigmatropic [1,7] hydride shift. At room temperature, the transformation of previtaminD3 to vitamin D3 takes about 12 days to complete.




isomerization of previtamin D to vitamin D.