A new color reaction of steroid with anhydrous aluminum chloride and anisaldehyde. IV. Studies on the reaction mechanism.

Abstract

The mechanism of the new color reaction of steroid has been studied on cholesterol. In this reaction, cholesterol is changed at first by anhydrous aluminum chloride into cholesteryl chloride and then into cholest-3, 5-diene. Colorimetric and chromatographic investigations reveal that they are intermediates from cholesterol to the next step of the reaction. A colorless species obtained from cholesteryl chloride and anisaldehyde gives a characteristic color with anhydrous aluminum chloride or other Lewis acid. For this colorless reaction product, a benzal type structure conjugated with double bonds in rings of the steroid is postulated, and a mechanism based on the Prins reaction is proposed, for which a support is obtained from the spectroscopic data in comparison with a reaction product of cholest-5-ene. It is concluded that the coloration may be due to a coupling of the reaction product with Lewis acid which gives rise to resonance structures.