Investigations in the vitamin A series. III. Rearrangement of the<i>retro</i>‐system to the<i>normal</i>system of conjugated double bonds in the vitamin A series

Abstract

Abstract A new synthesis of vitamin A essentially based on the almost quantitative rearrangement of the retro ‐system to the normal system of conjugated double bonds is described. This rearrangement is carried out by converting the retro C 15 acid and the retro C 20 ‐acid respectively, by means of phosphorus trichloride in suitable solvents, into the corresponding retro ‐acid chlorides, which thereupon rearrange smoothly to the acid chlorides with the normal system of conjugated double bonds, β‐ionylidene acetic acid chloride and vitamin A acid chloride respectively. The acid chlorides are reduced with LiAlH 4 to the corresponding alcohols β‐ionylidene ethanol and vitamin A. The physical and chemical properties of the intermediates and geometric isomers obtained in the various reaction steps are described.