Vitamin semi-synthesis

The anti-rachitic factors resulting from the ultraviolet irradiation of cholesterol (40) and ergosterol (41) namely vitamin Dj (42) cholecalciferol and Dj, ergocalciferol (43) respectively are important dietary materials the chemistry of which was only elucidated by the investigations of many chemists (ref.41). Vitamin Dj is most easily derived by semi-synthesis from cholesterol through formation firstly of 7-dehydrocholesterol by reaction with N-bromosuccinimide followed by dehydrobromination with collidine. Ultraviolet light irradiation affords previtamin Dj which is thermally isomerised to the endo compound shown and thence to the exo... 

Lipid chemistry contains many instances of semi-synthesis in the chemistry of the fatty vitamins, prostaglandins and leukotrienes, the phospholipids, and the glycerides with their structural relatives. 

These ylids are classified as semi-stabilised or of intermediate reactivity, and their stereoselectivity may be poor.43 If the stereochemistry of the double bond in the ylid (from 167) is E, this is generally retained in the product, but if it is Z, as in the ylid derived from 170, low temperatures are needed to stop rotation at the allyl ylid stage. At -25 °C less than 5% -171 is formed in the synthesis of vitamin D metabolites.44 The stereochemistry of the new double bond is generally not well controlled, 1 1 ratios of E Z are not uncommon, but Vedejs finds that phosphonium salts such as 172 with two phenyl and two allyl groups give good yields of T-dicncs45 such as 173. 

The selective semi-hydrogenation of alkynes, is a particularly important reaction in the context of fine chemieals manufaeture. The acetylenic group, RC CR, readily participates in substitution reactions enabling the formation of new carbon-carbon bonds, for example, and selective hydrogenation, leading to alkene or alkane species, further enhances the synthetic utility of the alkynes and has been exploited in the synthesis of biologically active compounds, e.g. insect sex pheromones (pest control) and vitamins [1-3]. 

---