| abstract |
Abstract of the Disclosure: A process for preparing R/S-.gamma.-lipoic acid of the formula I or R/S-.alpha.-lipoic acid of the formula II (I) (II) entails A. reacting cyclohexanone in the presence of a suitable free radical initiator with a vinyl alkyl ether of the formula III , (III) where R1 is C1-C3-alkyl, B. converting the resulting 2-alkoxyethylcyclohexanone of the formula IV (IV) by Baeyer-Villiger oxidation with an organic peracid or one of the salts thereof or an inorganic peroxo compound into a mixture of the lactone of 8-alkoxy-6-hydroxyoctanoic acid and of 8-alkoxy-6-hydroxy-octanoic acid, or else, when performic acid is used, into a mixture essentially composed of 8-alkoxy-6-formyloxyoctanoic acid, 8-alkoxy-6-hydroxyoctanoic acid and its lactone, C. reacting the resulting mixture in a conventional manner in the presence of hydrobromic acid or hydriodic acid with thiourea to give R/S-.gamma.-lipoic acid of the formula I, and isolating the latter, or D. converting the resulting crude R/S-.gamma.-lipoic acid by aerial oxidation in the presence of catalytic amounts of iron(III) compounds into R/S-.alpha.-lipoic acid of the formula II, E. subjecting the latter to a continuous distillation in a thin film evaporator, and F. crystallizing the resulting R/S-.alpha.-lipoic acid distillate. |