Abstract
The present invention relates to a process for preparing a product of formula I: characterized in that the reaction is catalyzed both by thiamine or a thiamine salt and by ascorbic acid in a form which is free or salified or an organic acid salt of an alkaline metal, preferably sodium acetate, potassium tartrate, sodium succinate, or a reductone, preferably 2-hydroxypropanedial or 2,3-dihydroxycyclopent-2-ene-1-one in an organic solvent.