THE MICHAEL ADDUCT OF L-ASCORBIC-ACID TO METHYL VINYL KETONE - ITS REMARKABLE SELF-CONDENSATION AND OTHER REACTIONS

The structure of the Michael adduct of L-ascorbic acid (1) to methyl vinyl ketone was established as 2-(3-oxobutyl)-3- oxo-L-gulonolactone(3,6)hemiketal (3). In aqueous solution 3 is prone to undergo base- or acid-promoted degradations. 1-(2-Furyl)-1 ,4-pentanedione was isolated from the acidic product mixture. Under strictly anhydrous acid-catalyzed conditions, if a suitable nucleophile like ethanol is present, the tricyclic ethyl ketal 6b forms. In the absence of a nucleophile, the unexpected self-condensation of 3 occurs leading to cyclo-trimer 7, the structure of which was established by X-ray crystallography. Trimer 7, held together via ketal bridges, is converted quantitatively into the tricyclic methyl ketal 6a in methanolic hydrogen chloride. Mechanisms are proposed for the formation of 6a and b or 7.