Syntheses of heterocyclic compounds CCCXCVI. Alternative total synthesis of (+-)-galanthamine

Dearomative logic in natural product total synthesis

Covering: 2011 to 2022The natural world is a prolific source of some of the most interesting, rare, and complex molecules known, harnessing sophisticated biosynthetic machinery evolved over billions of years for their production. Many of these natural products represent high-value targets of total synthesis, either for their desirable biological activities or for their beautiful structures outright; yet, the high sp³-character often present in nature's molecules imparts significant topological complexity that pushes the limits of contemporary synthetic technology. Dearomatization is a foundational strategy for generating such intricacy from simple materials that has undergone considerable maturation in recent years. This review highlights the recent achievements in the field of dearomative methodology, with a focus on natural product total synthesis and retrosynthetic analysis. Disconnection guidelines and a three-phase dearomative logic are described, and a spotlight is given to nature's use of dearomatization in the biosynthesis of various classes of natural products. Synthetic studies from 2011 to 2021 are reviewed, and 425 references are cited.

Total Syntheses of Galanthamine and Lycoramine via a Palladium-Catalyzed Cascade Cyclization and Late-Stage Reorganization of the Cyclized Skeleton

Herein, we report the highly efficient total syntheses of galanthamine and lycoramine from a common tetracyclic intermediate. This concise synthetic route features a two-phase strategy, which includes the early-stage rapid construction of a tetracyclic skeleton followed by the late-stage selective reorganization of the tetracyclic skeleton. Key to the success of this strategy are a palladium-catalyzed carbonylative cascade annulation, a DDQ-mediated intramolecular regioselective oxidative lactamization, as well as a BF₃·Et₂O-promoted reorganization of the bridged tetracyclic skeleton.

Concise syntheses of (-)-galanthamine and (+/-)-codeine via intramolecular alkylation of a phenol derivative

Suzuki coupling of 7 to 8 gave the biphenyl derivative 9. Reaction of 9 with ethyl vinyl ether/bromine/base gave 10, which on treatment with CsF/DMF at 130 degrees C resulted in the cross-conjugated 2,5-cyclohexadienone 6. Acid hydrolysis of 6 gave 11, which was reductively aminated to give (+/-)-narwedine 2. Since 2 has been converted into (-)-galanthamine 1 in two steps, this synthesis proceeds in eight steps with an overall yield of 63%. Also treatment of the cross-conjugated cyclohexadienone 6 with nitromethane/base gave 12, which was reduced to provide 13. Reduction of the nitro group in 13 to an amine, followed by reductive amination under acidic conditions, arrives at the codeine skeleton 15. Elaboration of 15 into (+/-)-codeine proceeds via the previously unknown alpha-epoxide derivative 18. This is the shortest synthesis of codeine (13 steps, 20% overall yield) and, for the first time, allows access to codeine without having to reduce codeinone.

Divergent enantioselective synthesis of (-)-galanthamine and (-)-morphine

An efficient divergent synthetic strategy for the synthesis of the opiate and amaryllidaceae alkaloids emerges by employing a Pd-catalyzed asymmetric allylic alkylation (AAA) to set the stereochemistry. Three generations of syntheses of galanthamine are discussed in detail with particular focus on the scope of the palladium-catalyzed AAA reactions and intramolecular Heck reactions. The pivotal tricyclic intermediate is available in six steps from 2-bromovanillin and the monoester of methyl 6-hydroxycyclohexene-1-carboxylate. This intermediate requires only two steps to convert to (-)-galanthamine. Using a Heck vinylation, we found that the fourth ring of codeine/morphine could be formed. The final ring formation involves a novel visible light-promoted hydroamination. Thus, six steps are required to convert the pivotal tricyclic intermediate into codeine, which has been demethylated in high yield to morphine.

Biomimetic Synthesis of (.+-.)-Galanthamine and Asymmetric Synthesis of (-)-Galanthamine Using Remote Asymmetric Induction

(+/-)-Galanthamine (1) was synthesized in excellent yield by applying PIFA-mediated oxidative phenol coupling of N-(4-hydroxy)phenethyl-N-(3',4',5'-trialkoxy)benzyl formamide (15b) as a key step. Because of the symmetrical characteristics of the pyrogallol moiety in the substrate (15b), the phenol coupling resulted in a sole coupling product except for volatile components from the oxidizing agent. On the basis of the successful results of the above strategy, (-)-galanthamine (1) was synthesized by employing a novel remote asymmetric induction, where conformation of the seven-membered ring in the product of the phenol coupling was restricted by forming a fused-chiral imidazolidinone ring with D-phenylalanine on the benzylic C-N bond of the tri-O-alkylated gallyl amino moiety. The conformational restriction and successive debenzylation of the protected hydroxyl groups on the pyrogallol ring caused diastereoselective cyclization to yield a cyclic ether having the desired stereochemistry for the synthesis of (-)-1.