Total Synthesis of Chaetominine

p-Toluenesulfonic acid-promoted organic transformations for the generation of molecular complexity

Since the beginning of this century, p-toluenesulfonic acid (p-TSA) catalysed organic transformations have been an active area of research for developing efficient synthetic methodologies. Often, catalysis using p-TSA is associated with many advantages, such as operational simplicity, high selectivity, excellent yields, and ease of product isolation, which make organic synthesis convenient and versatile. Notably, p-TSA is a non-toxic, commercially available, inexpensive solid organic compound that is soluble in water, alcohols, and other polar organic solvents. p-TSA is a strong acid compared to many protic or mineral acids and its high acidity helps activate different organic functional groups. p-TSA-promoted conversions are fast, have a high atom and pot economy, and feature a multiple bond-forming index. Therefore, the utilization of p-TSA enables the synthesis of many important structural scaffolds without any hazardous metals, making it desirable in numerous applications of sustainable and green chemistry. Recently, this emerging area of research has become one of the pillars of synthetic organic chemistry to synthesise biologically relevant, complex carbocycles and heterocycles. This study provides a comprehensive summary of methods, applications, and mechanistic insights into p-TSA-catalysed organic transformations, covering the literature reports that have appeared since 2012.

Potassium tert-Butoxide-Mediated Cascade Synthesis of Rutaecarpine Alkaloid Analogues: Access to Molecular Complexity on Multigram Scales

In this study, we present a novel and cost-effective approach for synthesizing biologically significant analogues of rutaecarpine alkaloid through a one-step cascade reaction. The pentacyclic core of rutaecarpine alkaloid analogues is efficiently constructed using 2-aminobenzonitriles and substituted indole-2-carbaldehydes in the presence of the affordable base KOtBu. The salient feature of this approach is the promotion of a sequential cascade process within a single reaction vessel including the formation of a dihydroquinazolinone ring, oxidation, and cyclization. This method can be successfully applied on a larger scale, making it economically viable.

Metal-free sp2 -C7-H Borylation of Tryptophan Containing Peptides and Late-stage Modification

The discovery of milder and robust strategies to enable the introduction of organoboronates in peptides remains conspicuously underdeveloped. Herein, we demonstrate an efficient method for the site-selective sp2 -C7-H borylation of tryptophan under metal-free condition using BBr3 directed by pivaloyl group. The versatility of this approach is that gram scale synthesis and C7-borylated N-Phth-Trp(N-Piv)(C7-BPin)-OMe was modified into various C7-substituted derivatives. Moreover, the strategy enables for the peptide elongation and late-stage borylation of peptides, natural product Brevianamide F and drug Oglufanide.

C-H radiocyanation of bioactive molecules via sequential iodination/copper-mediated cross-coupling

This report describes a net C-H radiocyanation reaction for the transformation of electron rich (hetero)aromatic substrates into 11CN-labeled products. Electrophilic C(sp2)-H iodination of the (hetero)arene with N-iodosuccinimide is followed by Cu-mediated radiocyanation with K11CN. This sequence is applied to a variety of substrates, including the nucleobases uracil and cytosine, the amino acids tyrosine and tryptophan, and the peptide LYRAGWRAFS, which undergoes selective C-H radiocyanation at the tryptophan (W) residue.

Organocatalytic Asymmetric [3 + 2]-Annulations of γ-Sulfonamido/γ-Hydroxy-α,β-Unsaturated Ketones with Cyclic N-Sulfimines: Synthesis of Chiral Polyheterotricyclic Imidazolidines and Oxazolidines

The first organocatalytic asymmetric [3 + 2]-annulation of γ-sulfonamido-α,β-unsaturated ketones with cyclic N-sulfimines has been developed, and enantioenriched functionalized polyheterotricyclic imidazolidines were obtained in good yields and with excellent enantioselectivities. This approach was also extended to the asymmetric [3 + 2]-annulation of γ-hydroxy-α,β-unsaturated ketones, affording enantioenriched polyheterotricyclic oxazolidines. In addition, base-catalyzed [3 + 2]-annulations of γ-sulfonamido/γ-hydroxy-α,β-unsaturated ketones with cyclic N-sulfimines were re-investigated under mild reaction conditions for the synthesis of racemic polyheterotricyclic imidazolidines and oxazolidines with excellent diastereoselectivities.

Stereoselective Syntheses of Highly Functionalized Imidazolidines and Oxazolidines via Ring-Opening Cyclization of Activated Aziridines and Epoxides with Amines and Aldehydes

A mild one-pot stereospecific synthetic route to highly functionalized imidazolidines and oxazolidines via S_N2-type ring-opening of the corresponding activated aziridines and epoxides with amines followed by p-toluenesulfonic acid (PTSA)-catalyzed intramolecular cyclization with aldehydes has been developed. The methodology tolerates a variety of functional groups and furnishes the desired products in high yields (up to 92%) with excellent stereoselectivities (de, ee > 99%). Interestingly, imidazolidines were formed as the cis-isomers, whereas oxazolidines were produced as trans-isomers exclusively.

Total Synthesis of Conulothiazole A

An efficient total synthesis of the chlorinated thiazole-containing natural product conulothiazole A is reported. Key features of this synthesis include a novel rhodium-catalyzed enantioselective hydrogenation of a 2-enamido-thiazole and a vinylic Finkelstein reaction that could be implemented at all stages of the synthesis to install the chlorinated alkene.

Rapid Generation of Molecular Complexity by Chemical Synthesis: Highly Efficient Total Synthesis of Hexacyclic Alkaloid (-)-Chaetominine and Its Biosynthetic Implications

The efficiency becomes a key issue in today's natural product total synthesis. While biomimetic synthesis is one of the most elegant strategies to achieve synthetic efficiency and thus to approach the ideal synthesis, most biogenetic pathways are unknown or unconfirmed. In this account, we demonstrate, through the shortest and also the most efficient asymmetric total syntheses of the hexacyclic alkaloid (-)-chaetominine to date, that on the basis of biogenetic thinking, one can develop quite efficient bio-inspired total synthesis, which in turn serves to suggest and chemically validate plausible biosynthetic routes for the natural product. The synthetic strategy thus developed is also inspiring for the development of other synthetic methods and efficient total synthesis of other natural products.

Copper-Catalyzed Alkenylation of Cyanamides

An efficient procedure for the copper-catalyzed cross-coupling between a broad range of cyanamides and iodoalkenes is reported. Upon reaction with catalytic amounts of copper(I) iodide and 2,2'-bisimidazole in the presence of cesium carbonate in DMF at 80 °C, a fast, regioselective, and stereoretentive cross-coupling occurs. This reaction, which was found to have a broad substrate scope, provides the first general entry to N-alkenylcyanamides, building blocks that hold great synthetic potential.

Synthesis of Polycyclic Imidazolidinones via Amine Redox-Annulation

α-Ketoamides undergo redox-annulations with cyclic secondary amines, such as 1,2,3,4-tetrahydroisoquinoline, pyrrolidine, piperidine, and morpholine. Catalytic amounts of benzoic acid significantly accelerate these transformations. This approach provides polycyclic imidazolidinone derivatives in typically good yields.

Copper-catalyzed direct alkylation of heteroarenes

An efficient and broadly applicable process is reported for the direct alkylation of C-H bonds in heteroarenes, privileged scaffolds in many areas of science. This reaction is based on the copper-catalyzed addition of alkyl radicals generated from activated secondary and tertiary alkyl bromides to a wide range of arenes, including furans, thiophenes, pyrroles, and their benzo-fused derivatives, as well as coumarins and quinolinones.

Bioinspired Collective Syntheses of Iboga-Type Indole Alkaloids

We present the application of a bioinspired collective synthesis strategy in the total syntheses of seven iboga-type indole alkaloids: (±)-tabertinggine, (±)-ibogamine, (±)-ibogaine, (±)-ibogaine hydroxyindolenine, (±)-3-oxoibogaine hydroxyindolenine, (±)-iboluteine, and (±)-ervaoffines D. In particular, tabertinggine and its congeners serve as iboga precursors for the subsequent biomimetic transformations into other iboga-type alkaloids.

Stereodivergent and enantioselective total syntheses of isochaetominines A–C and four pairs of isochaetominine C enantiomers: a six-step approach

We report the first total syntheses of (−)-isochaetominines A–C and eight 2,3-cis-stereoisomers of isochaetominine C, and structural revisions of (−)-pseudofischerine and (−)-aniquinazoline D.

Recent developments in the chemistry of quinazolinone alkaloids

This review focuses on the recent advances in the chemistry of quinazolinone alkaloids which covers the newly isolated quinazolinone alkaloids with their biological activities and the recently reported total syntheses of quinazolinone alkaloids from 2006 to 2015.

Photoassisted synthesis of enantiopure alkaloid mimics possessing unprecedented polyheterocyclic cores

Enantiopure alkaloid mimics are synthesized via high yielding intramolecular cycloadditions of photogenerated azaxylylenes tethered to pyrroles, with further growth of molecular complexity via post-photochemical transformations of primary photoproducts. This expeditious access to structurally unprecedented polyheterocyclic cores is being developed in the context of diversity-oriented synthesis, as the modular design allows for rapid "pre-assembly" of diverse photoprecursors from simple building blocks/diversity inputs.

Why do some Fischer indolizations fail?

The mechanisms of the Fischer indole synthesis and competing cleavage pathways were explored with SCS-MP2/6-31G(d) and aqueous solvation calculations. Electron-donating substituents divert the reaction pathway to heterolytic N-N bond cleavage and preclude the acid-promoted [3,3]-sigmatropic rearrangement.