A Palladium-Catalyzed Ullmann Cross-Coupling/Reductive Cyclization Route to the Carbazole Natural Products 3-Methyl-9H-carbazole, Glycoborine, Glycozoline, Clauszoline K, Mukonine, and Karapinchamine A

Abstract

The title natural products 2-7 have been prepared by reductive cyclization of the relevant 2-arylcyclohex-2-en-1-one (e.g. 20) to the corresponding tetrahydrocarbazole and dehydrogenation (aromatization) of this to give the target carbazole (e.g. 4). Compounds such as 20 were prepared using a palladium-catalyzed Ullmann cross-coupling reaction between the appropriate 2-iodocyclohex-2-en-1-one and o-halonitrobenzene.

Total Syntheses of the Amaryllidaceae Alkaloids Zephycandidine III and Lycosinine A and Their Evaluation as Inhibitors of Acetylcholinesterase

“…The palladium‐catalyzed Ullmann cross‐coupling reaction has proven to be a useful but under‐utilized method for cross‐coupling various aryl halides with other species, most notably α‐iodoenones . Given our familiarity with this process we sought to apply it in the present context.…”

Synthesis Strategies for α‐, β‐, γ‐ and δ‐Carbolines

“…Apparently, replacing the initial benzene with a pyridine, α-carboline was firstly synthesized by modified Graebe-Ullmann method (Scheme 2A). [24] Through heating o-phenylenediamine (21) and 2-chloropyridine (22) in ethanol, the coupled compound 23 was obtained. Then, 23 was diazotized to afford the diazo compound 25 through the intermediate 24, which spontaneously converted to 1,2,3triazole 26.…”

Synthesis of Pentacyclic Framework of Herquline A

“…[19] Eventually, we could transform 18 to 20 in 58% yield under conditions attempted for Pd-catalyzed Ullmann-type reaction (Scheme 3, table B, entry 4). [20] It is important to note that a single diastereomer (either 19 or 20) with an energetically more stable cis-fused 5/6 bicyclic moiety was obtained in all cases. Serendipitous access to tetracyclic ketone 20 prompted us to explore the C3À C3' bond formation via Pd catalyzed αarylation of ketone.…”