Total Syntheses of (−)-Galanthamine and (−)-Lycoramine

Total Syntheses of (−)-Galanthamine and (−)-Lycoramine

Author: Sarah Maier

The alkaloid (–)-galanthamine, produced by snowdrop flowers, is as an inhibitor of the enzyme acetylcholinesterase. Due to this effect, it can be used to treat Alzheimer’s disease and has been approved as a drug. Since it is a pharmacologically active molecule with an interesting chemical structure, galanthamine has been the target of different total synthesis approaches.

Fu-Min Zhang, Lanzhou University, Yong-Qiang Tu, Shanghai Jiao Tong University, China and colleagues have performed total syntheses of (–)-galanthamine and the structurally related (–)-lycoramine (general structure pictured, dashed bond only in galanthamine). The syntheses proceed via a direct, catalytic, and enantioselective assembly of the cis-hydrodibenzofuran core of the molecule (rings A, B, and C). This structure was prepared by a six-step route starting from commercially available 3-butyn-1-ol, using a Pd-catalyzed Suzuki coupling and a spirocyclic pyrrolidine (SPD)-catalyzed enantioselective Robinson annulation reaction. This core was functionalized by a series of oxidation and reduction reactions and the product was converted to galanthamine using a Pictet-Spengler cyclization for the final ring-closing step. If the double bond in the ABC  intermediate is reduced first, an analogous reaction sequence gives lycoramine.

The work shows that the spirocyclic pyrrolidine (SPD)-catalyzed enantioselective Robinson annulation, previously used for the synthesis of morphine and codeine, has the potential to allow easier access to a variety of natural products. According to the researchers, the method also provides an alternative path to galanthamine-type alkaloids and their analogues, which could be used for syntheses of lead compounds for pharmaceutical screening.


 

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