Synthetic Studies on Tetracyclic Diquinane Lycopodium Alkaloids Magellanine, Magellaninone and Paniculatine

(-)-Magellanine, (+)-magellaninone, and (+)-paniculatine are three natural products isolated from the Lycopodium family that share a unique 6-5-5-6-fused tetracyclic diquinane core skeleton. Several members of this family have potent s anti-inflammatory and acetylcholinesterase-inhibitory properties and are under development for the treatment of Alzheimer's and other neurodegenerative diseases. Several research groups have undertaken the formal and total syntheses of this class of natural products. This review highlights over 20 reported total syntheses of these three alkaloids and the development of synthetic methods for the assembly of their core skeletons.

Unified Total Synthesis of Tetracyclic Diquinane Lycopodium Alkaloids (+)-Paniculatine, (-)-Magellanine, and (+)-Magellaninone

A unified route for the total synthesis of three tetracyclic diquinane Lycopodium alkaloids (+)-paniculatine, (-)-magellanine, and (+)-magellaninone has been accomplished in 13-14 overall steps based on late-stage diverse transformations from an advanced tetracyclic common intermediate. In the established synthesis, quick formation of the two five-membered rings was efficiently achieved by an intramolecular reductive coupling of ketone-carbonyl and ester-carbonyl and an organocatalytic intramolecular Michael addition of aldehyde-derived enamine to an internal enone functionality with satisfactory redox and step economies and excellent stereoselectivities, providing the requisite tricyclic carbo-framework possessing multiple dense stereogenic centers, and an intramolecular reductive amination finally furnished the essential piperidine ring.

Photochemical Approaches to Complex Chemotypes: Applications in Natural Product Synthesis

The use of photochemical transformations is a powerful strategy that allows for the formation of a high degree of molecular complexity from relatively simple building blocks in a single step. A central feature of all light-promoted transformations is the involvement of electronically excited states, generated upon absorption of photons. This produces transient reactive intermediates and significantly alters the reactivity of a chemical compound. The input of energy provided by light thus offers a means to produce strained and unique target compounds that cannot be assembled using thermal protocols. This review aims at highlighting photochemical transformations as a tool for rapidly accessing structurally and stereochemically diverse scaffolds. Synthetic designs based on photochemical transformations have the potential to afford complex polycyclic carbon skeletons with impressive efficiency, which are of high value in total synthesis.

Exceedingly Concise and Elegant Synthesis of (+)-Paniculatine, (-)-Magellanine, and (+)-Magellaninone

Starting from inexpensive (+)-pulegone as the chiral building block, a highly convergent synthesis of the unusual diquinane-based structures of (+)-paniculatine (1), (-)-magellanine (2), and (+)-magellaninone (3), has been achieved. This approach is based upon a tandem acylation-alkylation of ketoester 8 and palladium-catalyzed olefin insertion, oxidation, and hydrogenation for construction of the tetracyclic framework. This exceedingly concise strategy, requiring only 12-14 steps, is the shortest to date.

Collective total synthesis of tetracyclic diquinane Lycopodium alkaloids (+)-paniculatine, (-)-magellanine, (+)-magellaninone and analogues thereof

The collective total synthesis of tetracyclic diquinane Lycopodium alkaloids, (+)-paniculatine, (-)-magellanine, (+)-magellaninone, and two analogues (-)-13-epi-paniculatine and (+)-3-hydroxyl-13-dehydro-paniculatine, has been accomplished. By logic-guided addition of a strategically useful hydroxyl group at C-3 of paniculatine, the formidable tetracyclic core was rapidly synthesized utilizing a site-specific and stereoselective aldol cyclization, thus making the ABD → ABCD tetracyclic approach to diquinane Lycopodium alkaloids attainable for the first time.

Stereoselective total syntheses of three Lycopodium alkaloids, (-)-magellanine, (+)-magellaninone, and (+)-paniculatine, based on two Pauson-Khand reactions

The total syntheses of (-)-magellanine, (+)-magellaninone, and (+)-paniculatine were completed from diethyl l-tartrate via the common intermediate in a stereoselective manner. The crucial steps in these syntheses involved two intramolecular Pauson-Khand reactions of enynes: the first Pauson-Khand reaction constructed the bicyclo[4.3.0] carbon framework, the corresponding A and B rings of these alkaloids in a highly stereoselective manner, whereas the second Pauson-Khand reaction stereoselectively produced the bicyclo[3.3.0]skeleton, which could be converted into the C and D rings of the target natural products.

Strategic use of pinacol-terminated Prins cyclizations in target-oriented total synthesis

An important objective of contemporary synthesis endeavors is the development of new transformations that rapidly evolve molecular complexity in a stereocontrolled fashion. One approach toward this goal is to combine two or more distinct reactions into a single transformation, producing a process often referred to as a sequential, tandem, cascade, or domino reaction. In this Perspective, we discuss the development of one such tandem reaction, the acid-promoted (or catalyzed) Prins-pinacol rearrangement, with particular emphasis on its implementation as the key strategic element in the total synthesis of heterocyclic and carbocyclic natural products.