Vitamin production from Wittig reaction

In their industrial production, the Wittig reaction is again a preferred process step for linking selected building blocks which are identical to those for the vitamin A synthesis or are logically derived from these. The advantages of the building block principle become particularly clear in the carotenoid syntheses 88,9 c,35). 

The industrial development of the vitamin A synthesis from the C15 and C5 building blocks was characterized by the special features of the Wittig reaction and the sensitivity of the starting materials as well as of the end products to chemical and thermal effects. 

If E,E dienes like 163 are wanted, then such Wittig reactions are ideal as the mixed products can be equilibrated to the E,E diene by addition of small amounts of radical generators such as iodine or PhSH. The commercial (BASF) synthesis of Vitamin A involves all trans retinol 174 that can be made from two different allylic ylids derived from 175 and 178 with the appropriate aldehydes 176 and 177. In both cases E,Z mixtures are formed, but equilibration with iodine gives 174 with an all E side chain.46 A different synthesis of such compounds appeared in chapter 11. 

Triphenylphosphine oxide from the commercial production of vitamin A by a Wittig reaction is recycled by reaction with phosgene to give the phosphine dichloride and reduction of the dichloride to triphenylphosphine with elemental phosphorus (461. Polymeric phosphine oxides have been converted to phosphine dichlorides with oxalyl chloride and then reduced to phosphine with diisobutylaluminum hydride (471. 

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