Direct annulation and alkylation of indoles with 2-aminobenzyl alcohols catalyzed by TFA

Synthesis of a Natural Product-Based 5H-Thiazolo[5',4':5,6]pyrido[2,3-b]indole Derivative via Solid-Phase Synthesis

The solid-phase synthesis method is optimized for building chemical libraries. Furthermore, chemical libraries are essential tools in drug discovery used to identify hit compounds. We constructed a 5H-thiazolo[5',4':5,6]pyrido[2,3-b]indole derivative library using solid-phase synthesis. The indole insertion reaction at the benzylic position using a Lewis acid and the oxidative cyclization reaction using iodine were used for synthesis. Using optimized solution-phase reaction conditions, a solid-phase synthesis method comprising a total of eight steps was employed to build a 5H-thiazolo[5',4':5,6]pyrido[2,3-b]indole derivative library. In addition, we found an efficient compound library synthesis route with each synthetic step having a yield of 62-82%.

Syntheses of Novel Spirobenzazepinoindole Derivatives via Lewis-Acid Catalyzed Pictet-Spengler Cyclization

The syntheses of novel spirobenzazepinoindole derivatives has been achieved through a highly efficient and synthetic route. The approach involves a two-step reaction, utilizing indole derivatives, 2-amino benzyl alcohol, and ninhydrin as key starting materials under mild reaction conditions. The reaction proceeds via a sequential cascade process involving cyclization, condensation and spiro-annulation, leading to the formation of the spirobenzazepinoindole core structure in good to excellent yields. The method offers broad substrate scope, high atom economy, and operational simplicity. The synthesized spirobenzazepinoindoles were fully characterized by spectroscopic techniques, including NMR (1H & 13C), IR and mass spectrometry. The methodology provides a valuable tool for the rapid generation of structurally complex spirobenzazepinoindoles, which could serve as scaffolds for the development of new therapeutic agents.

Synthesis of Functionalized Tetrahydroquinoline Containing Indole Scaffold via Chemoselective Annulation of Aza-ortho-quinone Methide Precursor

The chemoselective annulation of aza-ortho-quinone methide generated by in situ o-chloromethyl sulfonamide has been achieved with bifunctional acyclic olefin. This efficient approach provides access to the diastereoselective synthesis of functionalized tetrahydroquinoline derivatives containing indole scaffolds through the inverse-electron-demand aza-Diels-Alder reaction under mild reaction conditions with excellent results (up to 93% yield, > 20:1 dr). Moreover, this article realized the cyclization of α-halogeno hydrazone with electron-deficient alkene affording the tetrahydropyridazine derivatives, which had never been reported.

Diastereoselective synthesis of polycyclic indolines via dearomative [4 + 2] cycloaddition of 3-nitroindoles with ortho-aminophenyl p-quinone methides

A formal [4 + 2] cycloaddition of 3-nitroindoles with ortho-aminophenyl p-quinone methides via a dearomatization process was developed. This method provides a facile approach for preparing tetrahydro-5H-indolo[2,3-b]quinolones with good results. With the bifunctional Cinchona alkaloid-squaramide as the catalyst, the asymmetric version of the reaction successfully afforded the corresponding chiral products with moderate to good enantioselectivities. This work represents the first dearomative cycloaddition of electron-deficient heteroarenes triggered by aza-Michael addition from p-QMs.

Synthesis of Tetrahydro-5H-indolo[2,3-b]quinolines through Copper-Catalyzed Cascade Reactions of Aza-o-quinone Methides with Indoles

A variety of tetrahydro-5H-indolo[2,3-b]quinolines were prepared in 40-97% yields through a copper(II)-catalyzed cascade reaction of aza-o-quinone methides generated in situ from 2-(chloromethyl)anilines and indoles. Experimental results showed that the reaction underwent double 1,4-additions and sequential intramolecular cyclization. The present method features broad substrate scope, good functional group tolerance, and easy gram scalable preparation of indolo[2,3-b]quinolines.

Palladium-Catalyzed Dearomative [4 + 2]-Cycloaddition toward Hydrocarbazoles

An efficient method for the construction of hydrocarbazoles bearing three continuous sterically hindered stereocenters, two quaternary and one tertiary, via a highly diastereoselective palladium-catalyzed [4 + 2]-cycloaddition/dearomatization of 3-nitroindoles has been developed. The cycloaddition of 3-nitroindoles occurs at ambient conditions with a 1,4-zwitterionic intermediate, in situ generated from γ-methylidene-δ-valerolactones. The further synthetic utility of this method is demonstrated by the multifaceted transformations possible from the products. The catalytic asymmetric aspect of this transformation has also been explored.

Palladium-catalyzed cascade cyclization of allenamide with 2-iodoanilines to access functionalized indoloquinolines

A novel and efficient palladium-catalyzed cascade cyclization to indoloquinoline derivatives in one pot has been developed by using allenamide derivatives and 2-iodoanilines as the key building blocks. The process involved two cyclizations: intramolecular cyclization/π-allylic substitution and intramolecular 6-endo Heck cyclization. Furthermore, dihydrobenzofuro[2,3-b]quinoline derivatives could also be achieved via this strategy using allenyl ethers instead of allenamides. The readily available substrates, mild conditions, high efficiency and step economy make this strategy a promising method in the synthesis of polycyclic motifs.

Recent advances in the application of Diels–Alder reactions involving o-quinodimethanes, aza-o-quinone methides and o-quinone methides in natural product total synthesis

This review summarizes recent advances in Diels–Alder reactions involving o-QDMs, o-QMs and aza-o-QMs. The power and potential of this strategy in organic synthesis and natural product total synthesis is highlighted.

Accessing benzooxadiazepinesviaformal [4 + 3] cycloadditions of aza-o-quinone methides with nitrones

Seven membered N-heterocycles were constructed through a formal [4 + 3] cycloaddition of nitrones within situgenerated aza-o-quinone methides.

Recent Advances on the Total Syntheses of Communesin Alkaloids and Perophoramidine

The communesin alkaloids are a diverse family of Penicillium-derived alkaloids. Their caged-polycyclic structure and intriguing biological profiles have made these natural products attractive targets for total synthesis. Similarly, the ascidian-derived alkaloid, perophoramidine, is structurally related to the communesins and has also become a popular target for total synthesis. This review serves to summarize the many elegant approaches that have been developed to access the communesin alkaloids and perophoramidine. Likewise, strategies to access the communesin ring system are reviewed.

Metal-free α-amination of secondary amines: computational and experimental evidence for azaquinone methide and azomethine ylide intermediates

We have performed a combined computational and experimental study to elucidate the mechanism of a metal-free α-amination of secondary amines. Calculations predicted azaquinone methides and azomethine ylides as the reactive intermediates and showed that iminium ions are unlikely to participate in these transformations. These results were confirmed by experimental deuterium-labeling studies and the successful trapping of the postulated azomethine ylide and azaquinone methide intermediates. In addition, computed barrier heights for the rate-limiting step correlate qualitatively with experimental findings.