Application of Gold(I) Catalysis in the Synthesis of Bridged Carbocycles, (±)-Magellanine and (±)-Salvinorin A

• Main Author: McGee, Philippe
• Other Authors: Barriault, Louis
• Format: Others
• Language: en
• Published: Université d'Ottawa / University of Ottawa 2018
• Subjects: Gold catalysis; Natural product; Total synthesis
• Online Access: http://hdl.handle.net/10393/38485; http://dx.doi.org/10.20381/ruor-22738

Gold was considered for a long time to be an inert metal and was only in 1986 that the first homogeneous gold-catalyzed transformation was reported. In our laboratory, we isolated a surprisingly stable vinyl complex that resulted from an unexpected 1,2-silyl migration while working on a gold(I)-catalyzed reaction for the synthesis of polyprenylated polycyclic acylphloroglucinols (PPAPs). We herein report the isolation of a variety of organogold species where we could control the silyl migration based on the nature of the silyl group installed on the terminal alkyne. Silyl groups bearing an aromatic ring inhibited the silyl migration while the aliphatic silyl group afforded the 1,2-silyl migrated adduct. After mechanistic investigation of this intriguing migration, we believe that this process goes through a relatively rare gold vinylidene intermediate. More than 15 organogold complexes were isolated in good yield and characterized by x-ray crystallography. Investigation of their reactivity led to the formation of C(sp3)-C(sp2) bonds using electrophilic reagents without the use of Pd-based catalysts. We have also developed a new gold(I)-catalyzed dehydro Diels-Alder reaction using a simple monocyclic silyl enol ether. This methodology proceeds effectively with a wide scope by the use of [JackiephosAu(NCMe)]SbF6 in toluene. This methodology was then applied to the synthesis of magellanine, an architecturally complexed angular natural product isolated in 1976 from the club moss Lycopodium Magellanicum. The key step precursor was rapidly constructed via a Mitsunobu/Diels-Alder reaction that generated the requisite carboxaldehyde. The dehydro Diels-Alder reaction afforded the molecular skeleton of magellanine diastereoselectively in 91% yield. The synthesis was successfully accomplished in 11 steps demonstrating the ability of the gold(I) salt to rapidly construct complex molecules. Since the discovery of salvinorin A, a lot of efforts were exerted in order to optimize the biological activity for treatment of central nervous system disorders. Development of a new synthetic routes to salvinorins are essential to afford novel functionalized analogues. The decalin framework of salvinorin A was assembled with a Diels-Alder reaction with Et2AlCl followed by a gold(I)-catalyzed 6-endo-dig carbocyclization with [JohnphosAu(NCMe)]SbF6. Further functionalization afforded an elaborated intermediate which possesses the correct stereochemistry of the natural product. Following these promising results, efforts are currently in progress for the completion of the total synthesis.