Formation of Cyclohepta[b]indole Scaffolds via Heck Cyclization: A Strategy for Structural Analogues of Ervatamine Group of Indole Alkaloid

Enantioselective Total Syntheses of (-)-Silicine and (-)-20-Episilicine

The enantioselective total syntheses of (-)-silicine and (-)-20-episilicine, which contain a chiral piperidine with three contiguous chiral centers (D-ring) and a strained seven-membered ring (C-ring) attached to an indole, were achieved. The key steps of these syntheses included a chiral secondary amine-catalyzed formal aza-[3 + 3] cycloaddition reaction and Lewis acid-mediated irreversible ring-closing reaction. In addition, the stereochemistry at C20 was controlled at a later stage in the syntheses.

BF3-OEt2 Catalyzed C3-Alkylation of Indole: Synthesis of Indolylsuccinimidesand Their Cytotoxicity Studies

A simple and efficient BF₃-OEt₂ promoted C3-alkylation of indole has been developed to obtain3-indolylsuccinimidesfrom commercially available indoles and maleimides, with excellent yields under mild reaction conditions. Furthermore, anti-proliferative activity of these conjugates was evaluated against HT-29 (Colorectal), Hepg2 (Liver) and A549 (Lung) human cancer cell lines. One of the compounds, 3w, having N,N-Dimethylatedindolylsuccinimide is a potent congener amongst the series with IC₅₀ value 0.02 µM and 0.8 µM against HT-29 and Hepg2 cell lines, respectively, and compound 3i was most active amongst the series with IC₅₀ value 1.5 µM against A549 cells. Molecular docking study and mechanism of reaction have briefly beendiscussed. This method is better than previous reports in view of yield and substrate scope including electron deficient indoles.

Phosphine- and water-promoted pentannulative aldol reaction

An intramolecular aldol reaction promoted by tributylphosphine and water is described. Thereby, metal-free access to highly functionalised cyclopenta-fused arenes and heteroarenes is achieved.

Cobalt-catalysed reductive C-H alkylation of indoles using carboxylic acids and molecular hydrogen

The direct CH-alkylation of indoles using carboxylic acids is presented for the first time. The catalytic system based on the combination of Co(acac)3 and 1,1,1-tris(diphenylphosphinomethyl)-ethane (Triphos, L1), in the presence of Al(OTf)3 as co-catalyst, is able to perform the reductive alkylation of 2-methyl-1H-indole with a wide range of carboxylic acids. The utility of the protocol was further demonstrated through the C3 alkylation of several substituted indole derivatives using acetic, phenylacetic or diphenylacetic acids. In addition, a careful selection of the reaction conditions allowed to perform the selective C3 alkenylation of some indole derivatives. Moreover, the alkenylation of C2 position of 3-methyl-1H-indole was also possible. Control experiments indicate that the aldehyde, in situ formed from the carboxylic acid hydrogenation, plays a central role in the overall process. This new protocol enables the direct functionalization of indoles with readily available and stable carboxylic acids using a non-precious metal based catalyst and hydrogen as reductant.

Cyclohepta[b]indoles: A Privileged Structure Motif in Natural Products and Drug Design

Seven-membered rings fused with an indole are termed cyclohepta[b]indoles. Compounds exhibiting this structure motif display a broad spectrum of biological activities, ranging from inhibition of adipocyte fatty-acid-binding protein (A-FABP), deacetylation of histones, inhibition of leukotriene production p53, antituberculosis activities, and anti-HIV activities. These biological profiles are found in natural products containing the cyclohepta[b]indole motif, as well as in pharmaceuticals that contain this structure motif. Therefore, the biology of molecules derived from the skeleton of cyclohepta[b]indoles, as well as cyclopenta- and cyclohexa[b]indoles, has attracted considerable interest from the pharmaceutical industry as potential therapeutics in recent years. This is reflected by more than two dozen patents that have been issued in the past decade, solely based on the cyclohepta[b]indole structure motif. The efficient preparation of highly functionalized and unsymmetrically substituted cyclohepta[b]indoles has therefore become of central interest for synthetic organic chemists. Historically, this structure motif most often has been prepared by means of a Fischer indole synthesis. Although very robust and useful, this reaction poses certain limitations. Especially unsymmetrically functionalized cyclohepta[b]indoles are not suitable for a Fischer indole type synthesis, since product mixtures are inevitable. Therefore, novel methodologies to overcome these synthetic obstacles have been developed in recent years. This Account introduces all natural products and pharmaceutical compounds exhibiting the cyclohepta[b]indole motif. The structural variability within cyclohepta[b]indole alkaloids in combination with the broad range of organisms where these alkaloids have been isolated from, strongly suggests that the cyclohepta[b]indole is somehow a "privileged" structure motif. The organisms producing these compounds range from evergreen trees (actinophyllic acid) to cyanobacteria (ambiguinines). The synthetic methodologies to construct these molecular scaffolds (natural and unnatural in origin) are in turn highlighted and discussed with regard to their potential to access highly functionalized and unsymmetrical cyclohepta[b]indoles, for which they specifically have been designed. The methods are classified with respect to reaction type and whether or not they are enantioselective. Finally, the syntheses of cyclohepta[b]indole natural products are presented, thereby in each case, focusing on the construction of this structure motif in the course of the respective total synthesis. As a conclusion, we end by contrasting the methodological progress in the field with the actual successful application of the newly developed methods to the synthesis of complex structures to pinpoint the urgent requirement for further synthetic development for efficient synthetic design of this "privileged" structure motif.

Palladium mediated regioselective intramolecular Heck reaction: synthesis of 1,3,4-trisubstituted pyrazolo[3,4-b]pyridines, 3H-pyrazolo[3,4-c]isoquinolines and 3H-pyrazolo[4,3-f][1,7]naphthyridines

A new approach for the synthesis of 1,3,4-trisubstituted pyrazolo[3,4-b]pyridines, 3H-pyrazolo[3,4-c]isoquinolines and 3H-pyrazolo[4,3-f][1,7]naphthyridines via an intramolecular Heck reaction is described.