Improved chemoenzymatic asymmetric synthesis of (S)-Rivastigmine

(S)-Rivastigmine [(S)-1] was obtained via a four-step synthesis using an asymmetric enzymatic transamination protocol as the key step. An early introduction of the carbamate pharmacophore side chain avoided the use of protective group strategies and hence led to a considerable shortcut. This strategy required a novel ω-transaminase from Paracoccus denitrificans, which could transform the highly polar key substrate 3-acetylphenyl ethyl(methyl)carbamate (4) to the corresponding amine (S)-5 in 99% ee and >80% conversion.