Recent advances in the total synthesis of galantamine, a natural medicine for Alzheimer's disease

Covering: 2006 to 2023(-)-Galantamine is a natural product with distinctive structural features and potent inhibitory activity against acetylcholine esterase (AChE). It is clinically approved for the treatment of Alzheimer's disease. The clinical significance and scarcity of this natural product have prompted extensive and ongoing efforts towards the chemical synthesis of this challenging tetracyclic structure. The objective of this review is to summarize and discuss recent progress in the total synthesis of galantamine from 2006 to 2023. The contents are organized according to the synthetic strategies for the construction of the quaternary center. Key features of each synthesis have been highlighted, followed by a summary and outlook at the end.

Easy Access to Phenanthridinones via Metal-Free Cascade Benzannulation and C-N Bond Formation

A concise route for the synthesis of dihydrobenzo[j]phenanthridinones has been disclosed through an aryne annulation strategy under metal-free reaction conditions. The reaction involves multiple C-C and C-N bond cleavages/formations via Diels-Alder reaction, aromatization-driven C-N bond cleavage, and amide formation.

Biomimetic total synthesis of (-)-galanthamine via intramolecular anodic aryl-phenol coupling

(-)-Galanthamine as a drug for the treatment of Alzheimer's disease has attracted synthetic chemists for decades. However, previous total synthetic and biomimetic approaches often use stoichiometric oxidants (metal oxidants or hypervalent iodine) to prepare the target product. Anodic oxidative coupling offers a sustainable alternative method which is, for the first time, successfully applied to the total synthesis of (-)-galanthamine. We report a new asymmetric total synthesis of (-)-galanthamine by using an anodic aryl-phenol coupling as the key synthetic step.

Asymmetric Total Syntheses of (-)-Lycoramine, (-)-Lycoraminone, (-)-Narwedine, and (-)-Galanthamine

A concise asymmetric total synthesis of naturally occurring Amaryllidaceae alkaloids sharing dihydrobenzofuran scaffolds, (-)-galanthamine (1a), (-)-lycoramine (1b), (-)-narwedine (2a), and (-)-lycoraminone (2b), is reported. Orthoester Johnson-Claisen rearrangement of allyl alcohol (+)-9 (98% ee) in diisopropylethylamine furnished enantioenriched cyclohexene (+)-8 (97.4% ee) with a quaternary stereogenic center.

Cascade aryne insertion/vinylogous aldol reaction of vinyl-substituted β-keto/enol carbonyls

Cyclic and acyclic vinyl substituted β-keto/enol carbonyl substrates, on reaction with arynes, result in differentially substituted naphthyl carbocycles, hitherto difficult to synthesize with existing protocols. While the substitutions on the arynes have no role, the ring size of the cyclic β-keto/enol esters has a profound influence on the product formation.

Transition-Metal-Mediated Chemo- and Stereoselective Total Synthesis of (-)-Galanthamine

An asymmetric synthetic route to (-)-galanthamine (1), a pharmacologically active Amaryllidaceae alkaloid used for the symptomatic treatment of early onset Alzheimer's disease, was successfully established with very high levels of stereocontrol. The key to achieving high chemo- and stereo-selectivity in this approach was the use of transition-metal-mediated reactions, namely, enyne ring-closing metathesis, Heck coupling, and titanium-based asymmetric allylation.

Concise Synthesis of Chromene/Chromane-Type Aryne Precursors and Their Applications

The gram-scale synthesis of 5,6-, 6,7-, and 7,8-chromene/chromane-type aryne precursors and their applications in regioselective transformation to other functional derivatives is reported. Chromene/chromane-type arynes are generated under mild conditions, which can further undergo [2 + 2], [3 + 2], and [4 + 2] cycloaddition reactions, nucleophilic addition reactions, and σ-insertion reactions to produce structurally novel substituted chromenes and chromanes. The excellent regioselectivity of the reaction is facilitated by the oxygen-containing guiding groups at the ortho-position of the triple bond, which can be removed or switched to other functional groups including alkenyl, aryl, heteroaryl, and arylamino groups.

Aromaticity-Driven Access to Cycloalkyl-Fused Naphthalenes

We report the efficient synthesis of cycloalkyl-fused naphthalenes through the [4 + 2]-cycloaddtion/decarboxylative aromatization of alkyne-tethered aryne insertion adducts. These scaffolds were difficult to synthesize using conventional reactions. The reaction proceeds via the formation of a benzopyrylium intermediate followed by intramolecular [4 + 2] cycloaddition and a subsequent decarboxylation pathway. This method is also compatible with allene-tethered substrates to afford similar products. In addition, the one-pot synthesis of polysubstituted naphthalenes via aryne insertion/benzannulation has also been developed in good yield.

sp3 -Rich Glycyrrhetinic Acid Analogues Using Late-Stage Functionalization as Potential Breast Tumor Regressing Agents

Late-stage functionalization (LSF) aids drug discovery efforts by introducing functional groups onto C-H bonds on pre-existing skeletons. We adopted the LSF strategy to synthesize analogues of the abundantly available triterpenoid, glycyrrhetinic acid (GA), by introducing aryl groups in the A-ring, expanding the A-ring and selectively activating one methyl group of the gem-dimethyl groups. Intriguingly, two compounds were found to preferentially accumulate in the mitochondrial compartment of MDA-MB-231 breast cancer cells, to cause depolarization of mitochondrial membrane potential and to induce antiproliferative and anti-invasive effects through enhanced mitochondrial superoxide production with parallel depletion of GSH levels. Furthermore, intraperitoneal administration of these two compounds, in comparison with GA, greatly regressed breast tumor growth and metastasis in a SCID mouse model bearing labeled MDA-MB-231 cells.

Isolation, Identification, and Optimization of γ-Aminobutyric Acid (GABA)-Producing Bacillus cereus Strain KBC from a Commercial Soy Sauce moromi in Submerged-Liquid Fermentation

A new high γ-aminobutyric acid (GABA) producing strain of Bacillus cereus was successfully isolated from soy sauce moromi. This B. cereus strain named KBC shared similar morphological characteristics (Gram-positive, rod-shaped) with the reference B. cereus. 16S rRNA sequence of B. cereus KBC was found to be 99% similar with B. cereus strain OPWW1 under phylogenetic tree analysis. B. cereus KBC cultivated in unoptimized conditions using De Man, Rogosa, Sharpe (MRS) broth was capable of producing 523.74 mg L−1 of GABA within five days of the cultivation period. By using response surface methodology (RSM), pH level, monosodium glutamate (MSG) concentration and temperature were optimized for a high concentration of GABA production. The pH level significantly influenced the GABA production by B. cereus KBC with p-value = 0.0023. GABA production by B. cereus KBC under the optimized condition of pH 7, MSG concentration of 5 g L−1 and temperature of 40 °C resulted in GABA production of 3393.02 mg L−1, which is 6.37-fold higher than under unoptimized conditions. Overall, this study has shown that B. cereus KBC isolated from soy sauce moromi is capable of producing a high concentration of GABA together with the optimal fermentation conditions that have been statistically analysed using RSM.

Total Synthesis of Galanthamine and Lycoramine Featuring an Early-Stage C-C and a Late-Stage Dehydrogenation via C-H Activation

Herein, we report a novel strategy toward galanthamine and lycoramine. The concise synthesis was enabled by a Rh-catalyzed gram-scale C-C activation for the tetracyclic carbon framework and a regioselective Pd-catalyzed C-H activation for double-bond introduction. An aqueous-phase Beckmann rearrangement was performed for nitrogen atom insertion. Galanthamine and lycoramine were completed in 11 and 10 steps, respectively.