An interesting sidetrack in the tofisopam synthesis: Lithium variant of a stereospecific Oppenauer oxidation

Abstract

In the course of the elaboration of a new manufacturing process of anxiolytic drug tofisopam, a stereospecific lithium-catalyzed Oppenauer-type oxidation was observed. Bromine-lithium exchange on the ethylene ketal of 3-(2-bromo-4,5-dimethoxyphenyl)pentan-2-one followed by quenching with 1 equivalent of 3,4-dimethoxybenzaldahyde gave the corresponding alcohols as a mixture of two diastereomeric racemates. On the other hand, when 2.5 equivalents of the aldehyde were applied, only one of the two alcohol diastereomers could be isolated. The lithium alkoxide precursor of the other diastereomeric alcohol, due to the presence of the excess of benzaldehyde as the oxidant, underwent an Oppenauer-type oxidation to give the corresponding ketone.