Transition metal complexes in organic synthesis, part 36. Cyclization of tricarbonyliron complexes by oxygen to 4a,9a-dihydro-9H-carbazoles: Application to the synthesis of mukonine, mukonidine, and pyrido[3,2,1-jk]carbazoles

Insights from Binding on Quadruplex Selective Carbazole Ligands

Polymorphic G-quadruplex (G4) secondary DNA structures have received increasing attention in medicinal chemistry owing to their key involvement in the regulation of the maintenance of genomic stability, telomere length homeostasis and transcription of important proto-oncogenes. Different classes of G4 ligands have been developed for the potential treatment of several human diseases. Among them, the carbazole scaffold with appropriate side chain appendages has attracted much interest for designing G4 ligands. Because of its large and rigid π-conjugation system and ease of functionalization at three different positions, a variety of carbazole derivatives have been synthesized from various natural or synthetic sources for potential applications in G4-based therapeutics and biosensors. Herein, we provide an updated close-up of the literatures on carbazole-based G4 ligands with particular focus given on their detailed binding insights studied by NMR spectroscopy. The structure-activity relationships and the opportunities and challenges of their potential applications as biosensors and therapeutics are also discussed. This review will provide an overall picture of carbazole ligands with remarkable G4 topological preference, fluorescence properties and significant bioactivity; portraying carbazole as a very promising scaffold for assembling G4 ligands with a range of novel functional applications.

Anti-tuberculosis activity and structure-activity relationships of oxygenated tricyclic carbazole alkaloids and synthetic derivatives

A series of 49 oxygenated tricyclic carbazole derivatives has been tested for inhibition of the growth of Mycobacterium tuberculosis and a mammalian cell line (vero cells). From this series, twelve carbazoles showed a significant anti-TB activity. The four most active compounds were the naturally occurring carbazole alkaloids clauszoline-M (45), murrayaline-C (41), carbalexin-C (27), and the synthetic carbazole derivative 22 with MIC₉₀ values ranging from 1.5 to 3.7μM. The active compounds were virtually nontoxic for the mammalian cell line in the concentration range up to 50μM.

First total syntheses of chrestifoline-B and (±)-chrestifoline-C, and improved synthetic routes to bismurrayafoline-A, bismurrayafolinol and chrestifoline-D

We describe an efficient synthesis of the methylene-bridged biscarbazole alkaloids bismurrayafoline-A, bismurrayafolinol and chrestifoline B–D using an Ullmann-type coupling at the benzylic position.

An efficient Au-catalyzed synthesis of isomukonidine, clausine L, mukonidine, glycosinine, mukonal, and clausine V from propadienyl methyl ether

Naturally occurring carbazole alkaloids such as isomukonidine, clausine L, mukonidine, glycosinine, mukonal, and clausine V have been synthesized through a gold-catalyzed cyclization reaction of 1-(indol-2-yl)-2-methoxy-2,3-allenols, which are readily available from indolecarbaldehydes and methoxypropadiene. Compared to the reported procedures, our approach is general, atom economic, highly selective, and the starting materials are easily available.

Progress in the synthesis of canthine alkaloids and ring-truncated congeners

The canthines represent a fairly large subclass of β-carboline alkaloids, with the first members described 75 years ago. Over the last 60 years, many members of the parent compound, canthin-6-one (1), have been isolated from various plant sources, principally the Rutaceae and Simaroubaceae families, and recently from fungi. Structures isolated from these sources have been the subject of total synthesis, which continues to the present day. This review gives a broad overview of synthetic approaches to canthines over a 30-year period from 1982 to 2012 and summarizes recent reports on the synthesis of less well known ring-truncated congeners. These include C-ring-truncated ("ABD", 2) and A-ring-truncated ("BCD", 3) ring systems, which are providing new scaffolds for potentially useful therapeutic applications.

Practical large scale synthesis of half-esters of malonic acid

A practical large-scale synthesis of monomethyl malonate and monoethyl malonate, which are among the most commonly applied half-esters in organic synthesis, is described, applying the highly efficient selective monohydrolysis of symmetric diesters we reported before. The optimal conditions with regard to the type of base, equivalent, co-solvents, and the reaction time have been examined for large-scale reactions. Monomethyl malonate and monoethyl malonate were obtained in high yields with near 100% purity within only half a day. The conditions of this selective monohydrolysis reaction are environmentally benign and straightforward, as it requires only water, a small proportion of a volatile co-solvent, and inexpensive reagents, and produces no hazardous by-products, and therefore the synthetic utility of this reaction in process chemistry is expected.