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The discovery synthesis of asenapine that was used for the manufacture of drug substance batches up to 10 kg contained two chemical steps that were major bottlenecks for scale-up. One of these steps involved a magnesium/methanol reduction of an enamide moiety that was severely hampered by safety and efficiency problems. The other step was a laborious chromatography and isomerization cycle that was marked by a poor yield and extremely low throughput. The safety issues of the magnesium/methanol reduction could be solved by adding portions of magnesium to a solution of the enamide. In addition, an alternative process for the conversion of the mixture of cis- and trans-lactam into the desired trans-isomer was developed, circumventing the chromatographic separation.

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