Total Syntheses of Pyroclavine, Festuclavine, Lysergol, and Isolysergol via a Catalytic Asymmetric Nitro-Michael Reaction

Expanding the squaramide library as mycobacterial ATP synthase inhibitors: Innovative synthetic pathway and biological evaluation

Mycobacterial ATP synthase is a validated therapeutic target for combating drug-resistant tuberculosis. Inhibition of this enzyme has been featured as an efficient strategy for the development of new antimycobacterial agents against drug-resistant pathogens. In this study, we synthesised and explored two distinct series of squaric acid analogues designed to inhibit mycobacterial ATP synthase. Among the extensive array of compounds investigated, members of the phenyl-substituted sub-library emerged as primary hits. To gain deeper insights into their mechanisms of action, we conducted advanced biological studies, focusing on the compounds displaying a direct binding of a nitrogen heteroatom to the phenyl ring, resulting in the highest potency. Our investigations into spontaneous mutants led to the validation of a single point mutation within the atpB gene (Rv1304), responsible for encoding the ATP synthase subunit a. This genetic alteration sheds light on the molecular basis of resistance to squaramides. Furthermore, we explored the possibility of synergy between squaramides and the reference drug clofazimine using a checkerboard assay, highlighting the promising avenue for enhancing the effectiveness of existing treatments through combined therapeutic approaches. This study contributes to the expansion of investigating squaramides as promising drug candidates in the ongoing battle against drug-resistant tuberculosis.

Total Synthesis of (+)-Isolysergol

The enantioselective synthesis of (+)-isolysergol was completed in 18 steps, and an overall yield of 11% was obtained from (2R)-(+)-phenyloxirane as a chiral pool. Key features of the synthesis include a stereoselective intramolecular 1,3-dipolar addition of nitrone with terminal olefin and a Cope elimination to furnish the D ring. A rhodium-catalyzed intramolecular [3 + 2] annulation of a benzene ring with α-imino carbenoid was designed to afford the 3,4-fused indole scaffold at the late stage of the synthesis.

A Short Synthesis of Ergot Alkaloids and Evaluation of the 5-HT1/2 Receptor Selectivity of Lysergols and Isolysergols

Key transformations in a four-step synthesis of the ergot alkaloid scaffold include a novel cesium carbonate-mediated hydrogen autotransfer alkylation to generate the C(3)-C(4) bond and an intramolecular Heck reaction that directly establishes the C(9)-C(10) alkene of methyl lysergate. An ester reduction and a streamlined experimental procedure establish a readily scalable, expedient total synthesis of all four stereoisomers of lysergol and isolysergol, including the previously unknown (-)-lysergol, for pharmacological evaluation at 5-HT_1A and 5HT_2A,B,C receptors. A bicyclic scaffold is also characterized for the first time in the intramolecular Heck coupling.

Enantioselective Total Synthesis of [3]-Ladderanol through Late-Stage Organocatalytic Desymmetrization

Ladderane phospholipids, with their unusual ladder-like arrangement of concatenated cyclobutane rings, represent an architecturally unique class of natural products. However, despite their fascinating structure and other necessary impetus, only a few synthetic studies of these molecules have been reported so far. We have now devised a concise total synthesis of [3]-ladderanol, a component of natural ladderane phospholipids, using an organocatalytic enantioselective desymmetrizing formal C(sp² )-H alkylation. Our synthetic strategy rests on the late-stage introduction of chirality, thus allowing facile access to both enantiomers of [3]-ladderanol as well as an analogue. This is the first time a desymmetrization strategy is applied to the synthesis of [3]-ladderanol. The scope of this desymmetrizing C(sp² )-H alkylation of meso-cyclobutane-fused cyclohexenediones is also presented.

Catalysts’ evolution in the asymmetric conjugate addition of nitroalkanes to electron-poor alkenes

This review provides a journey of the catalyst usage for the enantioselective conjugate addition of nitroalkanes to electron-poor olefins from the early attempts to the latest achievements. Selected applications are also reported.

Nonbiomimetic total synthesis of indole alkaloids using alkyne-based strategies

Biomimetic natural product synthesis is generally straightforward and efficient because of its established feasibility in nature and utility in comprehensive synthesis, and the cost-effectiveness of naturally derived starting materials. On the other hand, nonbiomimetic strategies can be an important option in natural product synthesis since (1) nonbiomimetic synthesis offers more flexibility and can demonstrate the originality of chemists, and (2) the structures of derivatives accessible by nonbiomimetic synthesis can be considerably different from those that are synthesised in nature. This review summarises nonbiomimetic total syntheses of indole alkaloids using alkyne chemistry for constructing core structures, including ergot alkaloids, monoterpene indole alkaloids (mainly corynanthe, aspidosperma, strychnos, and akuammiline), and pyrroloindole and related alkaloids. To clarify the differences between alkyne-based strategies and biosynthesis, the alkynes in nature and the biosyntheses of indole alkaloids are also outlined.

Organocatalytic Asymmetric Synthesis of Aza-Spirooxindoles via Michael/Friedel-Crafts Cascade Reaction of 1,3-Nitroenynes and 3-Pyrrolyloxindoles

An asymmetric [3+3] cyclization of nitroenynes and 3-pyrrolyloxindoles has been realized with a chiral bifunctional squaramide catalyst. This Michael/Friedel-Crafts cascade strategy provides a facile and efficient access to enantioenriched polycyclic aza-spirooxindoles with 32-95% isolated yields and excellent stereocontrol under mild reaction conditions.

Biomimetic Total Syntheses of Ergot Alkaloids via Decarboxylative Giese Coupling

Biomimetic total syntheses of Festuclavine and Pyroclavine were achieved by a sequential radical coupling. The key steps include intramolecular decarboxylative Giese reaction to form the central C ring and 4-nitrobenzenesulfonyl (Ns)-directed indole C4-H olefination to introduce the indole C4 component. In addition, D-ring formation was completed by decarboxylative alkenylation and intramolecular S_N2 reaction.

Recent applications of the Wittig reaction in alkaloid synthesis

The Wittig reaction is the chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide (the Wittig reagent) to afford an alkene and triphenylphosphine oxide. Noteworthy, this reaction results in the synthesis of alkenes in a selective and predictable fashion. Thus, it became as one of the keystone of synthetic organic chemistry, especially in the total synthesis of natural products, where the selectivity of a reaction is paramount of importance. A literature survey disclosed the existence of vast numbers of related reports and comprehensive reviews on the applications of this important name reaction in the total synthesis of natural products. However, the aim of this chapter is to underscore, the applications of the Wittig reaction in the total synthesis of one the most important and prevalent classes of natural products, the alkaloids, especially those showing important and diverse biological activities.

The endo-aza-Michael addition in the synthesis of piperidines and pyrrolidines

Intramolecular endo-aza-Michael additions are categorised in various ways.

Thiourea-Catalyzed Enantioselective Malonate Addition onto 3-Sulfonyl-3'-indolyl-2-oxindoles: Formal Total Syntheses of (-)-Chimonanthine, (-)-Folicanthine, and (+)-Calycanthine

A general approach to bispyrroloindolines via a key thiourea-catalyzed addition of malonates to 3-sulfonyl-3'-indolyl-2-oxindoles is reported. The enantioselelective process is found to be highly effective (up to 94% ee), where a C-C bond formation leads to the synthesis of a number of 2-oxindoles with an all-carbon quaternary stereocenter.

An expeditious route to the synthesis of the enantioenriched tetracyclic core of ergot alkaloids via an organocatalytic aldol reaction

The synthesis of the tetracyclic skeleton of ergot alkaloids has been developed via a key organocatalytic enantioselective aldol reaction using paraformaldehyde as the C1-unit in the presence of thiourea catalyst followed by a key Pd-catalyzed directed coupling accelerated by the DavePhos ligand. Utilizing the aforementioned strategy, we have synthesized a key tetracyclic intermediate in up to 95% ee with high yield.