The mechanism of the oxidation of narceine by means of selenous acid -sulphuric acid

Abstract

The chemistry of selenous acid was misunderstood for many years, being explained by electron back donation. In this communication we disclose the reaction mechanism of the interaction of narceine with selenous acid in sulphuric acid. Narceine is a complex molecule that displays a carboxylic acid, a tertiary amine, a ketone, three methoxy groups and a benzodioxole; so, it was necessary select the principal reaction site. Although a methoxy group can be demethylated with the employed reagent since codeine gives the same violet colour obtained with morphine. The chemical deportment of the methylenedioxy ring in the presence of methoxy groups needed revision. It was found that the five-member ring is cleaved preferentially. Thus, a sustained reaction mechanism could be advanced. The reactive species is protonated selenous acid, which reacts with an oxygen of the cyclic group, water being eliminated. The oxonium ion is neutralized by ring opening in a concerted mechanism involving water reaction at the methylene. An organometallic intermediate is obtained, a selenite, and a hemiacetal. Finally, protonation of the selenium-oxygen double bond produces an acidolysis via a synchronous mechanism involving five electron shifts. This way selenium (II) hydroxide, an ortho-quinone and formaldehyde are obtained.