A DMAP-Catalyzed Approach to the Industrial-Scale Preparation of <i>N</i>‑6-Demethylated 9,10-Dihydrolysergic Acid Methyl Ester: A Key Cabergoline and Pergolide Precursor

2015-02-20T00:00:00Z (GMT) by Zdenko Časar Tomaž Mesar
A scalable new approach for the preparation of <i>N</i>-6-demethylated 9,10-dihydrolysergic acid methyl ester using 2,2,2-trichloroethyl chloroformate was developed. A key discovery that enabled the efficient and industrial-scalable process is linked to the rigorous extrusion of water in the reaction system and application of an organic catalyst such as 4-(<i>N</i>,<i>N</i>-dimethylamino)­pyridine (DMAP) instead of the alkali metal bicarbonate additives. Namely, in the previously known process, the concomitant presence of bicarbonates and traces of water triggers side reaction cycles that produce and accumulate hydrochloric acid and water. The former slows down the reaction. Moreover, these cycles cause the formation of multiple carbonate and alcohol-type side products to a significant extent that provide a low-quality <i>N</i>-6-demethylated product. All of these shortcomings are circumvented by the application of DMAP as a catalyst and the use of a reaction medium free of water. This approach allows operation on an industrial scale (51 kg batch) with higher yields, shorter reaction times, and improved product quality.