Protecting Group-Free Total Synthesis of (−)-Lannotinidine B

Deprotective Functionalization: A Direct Conversion of Nms-Amides to Carboxamides Using Carboxylic Acids

The nature of protecting group chemistry necessitates a deprotection step to restore the initially blocked functionality prior to further transformation. As this aspect of protecting group manipulation inevitably adds to the step count of any synthetic sequence, the development of methods enabling simultaneous deprotection and functionalization ("deprotective functionalization"-distinct from "deprotection followed by functionalization") is appealing, as it has the potential to improve efficiency and streamline synthetic routes. Herein, we report a deprotective functionalization of the newly introduced Nms-amides guided by density functional theory (DFT) analysis, which exploits the inherent Nms reactivity. Mechanistic studies further substantiate and help rationalize the exquisite reactivity of Nms-amides, as other commonly used protecting groups are shown not to exhibit the same reactivity patterns. The practicality of this approach was ultimately demonstrated in selected case studies.

Synthetic Efforts toward Lannotinidine G Based on an Aziridinium-Mediated Ring Contraction and Dienyne Metathesis

Lannotinidine G is a unique Lycopodium alkaloid that features a tricyclic [6/6/6] core with 3 contiguous stereocenters and a 1,3-diene moiety in addition to a 7-membered lactone. Herein, we disclose our efforts toward the synthesis of this natural product, which achieved the construction of the aza-tricyclic core with the correct configuration at its three stereocenters. Key features of our strategy include a highly diastereoselective Fráter-Seebach alkylation and Corey-Chaykovsky type epoxide formation, an unusual aziridinium-mediated ring contraction for the formation of the piperidine moiety, and a regioselective dienyne metathesis.

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.

Enantioselective Total Synthesis of (+)-Sieboldine A and Analogues Thereof

An 11-step enantioselective total synthesis of (+)-sieboldine A (1) has been accomplished from (5R)-methylcyclohex-2-en-1-one (16), in which an intramolecular ketone/ester reductive coupling followed by one-pot acidic treatment to quickly construct the unique oxa-spiroacetal and a TsOH-catalyzed displacement to directly form the characteristic N-hydroxyazacyclononane ring successfully served as the key methodologies. Moreover, several full-skeleton analogues of 1 were also synthesized on the basis of the advanced intermediates, and their inhibitory effects on electric eel acetylcholinesterase were examined.

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.

Multicomponent double Mannich alkylamination involving C(sp2)–H and benzylic C(sp3)–H bonds

Alkylamines are ubiquitous in pharmaceuticals, materials and agrochemicals. The Mannich reaction is a well-known three-component reaction for preparing alkylamines and has been widely used in academic research and industry. However, the nucleophilic components in this process rely on C(sp²)−H and activated C(sp³)−H bonds while the unactivated C(sp³)−H bonds involved Mannich alkylamination is a long-standing challenge. Here, we report an unprecedented multicomponent double Mannich alkylamination for both C(sp²)−H and unactivated benzylic C(sp³)−H bonds. In this process, various 3-alkylbenzofurans, formaldehyde and alkylamine hydrochlorides assemble efficiently to furnish benzofuran-fused piperidines. Mechanistic studies and density functional theory (DFT) calculations revealed a distinctive pathway that a multiple Mannich reaction and retro-Mannich reaction of benzofuran and dehydrogenation of benzylic C(sp³)−H bonds were key steps to constitute the alkylamination. This protocol furnishes a Mannich alkylamine synthesis from unusual C–H inputs to access benzofuran-fused piperidines with exceptional structural diversity, molecular complexity and drug-likeness. Therefore, this work opens a distinctive vision for the alkylamination of unactivated C(sp³)−H bonds, and provides a powerful tool in diversity-oriented synthesis (DOS) and drug discovery.

The Mannich reaction is a three-component reaction for preparing alkylamines, but the nucleophilic components rely on C(sp²)−H and activated C(sp³)−H bonds. Here, the authors report an unprecedented multicomponent double Mannich alkylamination for both C(sp²)−H and unactivated benzylic C(sp³)−H bonds.

Concise Unified Access to (-)-8-Deoxy-13-dehydroserratinine, (+)-Fawcettimine, (+)-Fawcettidine, and (-)-8-Deoxyserratinine Using a Direct Intramolecular Reductive Coupling

A short, scalable, and collective total synthesis of four fawcettimine-type Lycopodium alkaloids in eight or nine steps is disclosed. A dense multi-small-ring spiro-α-aminocyclopentanone successfully served as the key intermediate, which was directly accessed by a LiDBB-mediated intramolecular reductive coupling of the aliphatic imine and an ester-carbonyl. Compared to those that employ classical Heathcock intermediate(s) containing a nine-membered ring, the new strategy shows the significant improvement of the synthetic step and redox economies as well as excellent stereochemical control.

Copper Catalyzed Decarboxylative Functionalization of Ketoacids

Selective copper catalyzed activation of ketoacids and notably bio-sourced 1,3-acetonedicarboxylic acid, represents an attractive strategy to solve key synthetic challenges. Condensation with aldehydes under exceedingly mild conditions can create more rapidly known natural products scaffolds such as 1,3 polyols. In this account, the recent progress in this field, notably through multicatalytic combination with organocatalysis is described. In addition to the rapid preparation of natural product fragments, cascade incorporation of fluorine also provided new type of synthetic analogues of improved properties in a broad range of applications.

Total Synthesis of Lyconesidine B, a Lycopodium Alkaloid with an Oxygenated, Amine-Type Fawcettimine Core

This report describes the total synthesis of the complex, oxygenated tetracyclic alkaloid, lyconesidine B. The key synthetic challenge involves diastereoselective generation of a decahydroquinoline ring with a quaternary carbon at the angular position via domino cyclopropanation, ring-opening, and reduction. Another crucial step is the domino ene-yne metathesis involving a quaternary ammonium ion, leading to the construction of a decahydroazaazulen framework.

Formal Total Synthesis of (±)-Lycojaponicumin C

The formal total synthesis of (±)-lycojaponicumin C has been accomplished. Key transformations include a Rh-catalyzed formal [3 + 2] cycloaddition reaction to construct the bicyclic [3.3.0] scaffold bearing two vicinal quaternary carbon centers, a stereoselective γ-hydroxyl directed Michael addition to introduce the vinyl group at a bulky position, and a late-stage ring-closing metathesis reaction to form the cyclohexanone ring.

Recent advances on the total synthesis of alkaloids in mainland China

Alkaloids are a large family of natural products that mostly contain basic nitrogen atoms. Because of their intriguing structures and important functions, they have long been popular targets for synthetic organic chemists. China's chemists have made significant progress in the area of alkaloid synthesis over past decades. In this article, selected total syntheses of alkaloids from research groups in mainland China during the period 2011–16 are highlighted.

Lyconadins G and H, Two Rare Lyconadin-Type Lycopodium Alkaloids from Lycopodium complanatum

Two rare lyconadin-type Lycopodium alkaloids, lyconadins G (1) and H (2), together with four known ones (3-6), were isolated from Lycopodium complanatum. The structures were determined on the basis of their spectroscopic analyses, and the absolute configuration of 1 was established by an X-ray crystallographic analysis. It is the first time to establish the absolute configuration of lyconadin-type Lycopodium alkaloid by an X-ray diffraction experiment. In addition, these findings may provide more information for the biosynthesis of lyconadins.

Ruthenium-Catalyzed Metathesis Cascade Reactions in Natural Products Synthesis

In this account, we provide a brief summary of recent developments in ruthenium-catalyzed metathesis cascade reactions towards the total synthesis of natural products. We also highlight recent progress from our own laboratory regarding the synthesis of securinega alkaloids and humulanolides, which has resulted in the development of novel ruthenium-catalyzed metathesis cascade reactions. Inspired and guided by the pioneering and elegant research conducted in this area, we developed a regio-controlled relay dienyne metathesis cascade reaction and a cyclobutene-promoted RCM/ROM/RCM cascade reaction for the synthesis of securinega alkaloids and humulanolides, respectively.

Catalytic Asymmetric Total Synthesis of (-)-Actinophyllic Acid

Described herein is a catalytic asymmetric total synthesis of (-)-actinophyllic acid, with the key step being a chiral phosphine-catalyzed [3 + 2] annulation between an imine and an allenoate to form a pyrroline intermediate in 99% yield and 94% ee. The synthesis also features CuI-catalyzed coupling between a ketoester and a 2-iodoindole to shape the tetrahydroazocine ring; intramolecular alkylative lactonization; SmI2-mediated intramolecular pinacol coupling between ketone and lactone subunits to assemble the complex skeleton of (-)-actinophyllic acid; and an unprecedented regioselective dehydroxylation.

Expedient Synthesis of (+)-Lycopalhine A

Two amino acids play a key role in the first total synthesis of lycopalhine A. L-glutamic acid serves as a convenient chiral starting material for the 13-step synthesis, and l-proline promotes an unusual 5-endo-trig Mannich cyclization that generates the central pyrrolidine ring of the Lycopodium alkaloid. The bicyclo[3.3.0]octanol moiety of the molecule is formed through an intramolecular aldol addition that may occur spontaneously in nature.

A new method testing the orthogonality of different protecting groups

A new test was elaborated to identify a new set of orthogonal protecting groups. With the developed method eight different protecting groups were tested under various deprotection conditions and the complex reaction mixtures were analysed by HPLC. The developed method allows for quick identification of orthogonality using simple model structures.

Synergistic Cu–amine catalysis for the enantioselective synthesis of chiral cyclohexenones

An unprecedented utilization of 1,3-acetonedicarboxylic acid as a 1,3-bis-pro-nucleophile and a reactive acetone surrogate in enantioselective catalysis has been reported.

The first asymmetric total synthesis of lycoposerramine-R

The first asymmetric total synthesis of lycoposerramine-R was accomplished by a strategy featuring the stereoselective intramolecular aldol cyclization giving a cis-fused 5/6 bicyclic skeleton and a new method for the construction of the pyridone ring via the aza-Wittig reaction, thereby establishing the absolute configuration of the natural product.

Total synthesis of aplykurodinone-1

The concise total synthesis of aplykurodinone-1 with an unusual cis-fused hydrindane moiety has been accomplished without the need for any protecting group chemistry using a unique SmI2 mediated reductive cascade cyclization reaction and a direct cuprate mediated 1,4-addition. This work represents the first example of the use of a SmI2-mediated intramolecular cascade cyclization reaction between "halide, alkene and aldehyde" groups.

Divergent total syntheses of (-)-lycopladine D, (+)-fawcettidine, and (+)-lycoposerramine Q

Enantioselective total syntheses of (+)-fawcettidine and (+)-lycoposerramine Q as well as the first total synthesis of (-)-lycopladine D from a common intermediate have been accomplished by a divergent path. The common intermediate was derived from a Hajos-Parrish-like diketone by a stereoselective Birch reduction and a Suzuki coupling. The synthesis of (-)-lycopladine D featured an allylic oxidation and a biomimetic aminoketalization while the route to (+)-fawcettidine and (+)-lycoposerramine Q highlighted an oxidative rearrangement.