Palladium-Catalyzed Synthesis of Natural and Unnatural 2-, 5-, and 7-Oxygenated Carbazole Alkaloids from N-Arylcyclohexane Enaminones

Abstract

Abstract:A palladium-catalyzed synthesis of the carbazole framework is described, including the preparation of 2-, 5-, and 7-oxygenated natural and unnatural carbazole alkaloids. A series of N-arylcyclohexane enaminones, generated by condensation of cyclohexane-1,3-dione with diverse anilines, were aromatized by a Pd(0)-catalyzed thermal treatment to afford the corresponding diarylamines. The latter were submitted to a Pd(II)-catalyzed cyclization and methylation processes to provide the desired carbazoles, including clausine V. Following an inverse strategy, a new and short total synthesis of glycoborine is also reported.

I2‐Catalyzed Total Synthesis of Collective 7‐Oxygenated Carbazole Alkaloids: Clauszoline‐K, ‐L, Clausine‐M, ‐N and Formal Synthesis of Siamenol

“…Many synthetic approaches have been reported for the synthesis of oxygenated carbazole alkaloids by employing transition metal catalysts. However, among them Pd‐catalyzed reactions such as aromatization and cyclization, Suzuki–Miyaura cross coupling, N‐ arylation and oxidative C–H activation, intramolecular oxidative C−H amination, linear‐selective Negishi cross‐coupling, and amination followed by oxidative cyclization . Moreover, includes Cu‐catalyzed N‐ arylation and subsequent Pd‐catalyzed intramolecular C–H arylation, Au‐catalyzed benzannulation, and Diels‐Alder approach followed by Suzuki coupling …”

Condensation of Diacetyl with Alkyl Amines: Synthesis and Reactivity of p-Iminobenzoquinones and p-Diiminobenzoquinones

“…Diarylamines have become important synthetic targets as fine chemicals and precursors of a variety of N -containing pharmacological and natural products [38,39,40], such as carbazoles and ellipticines [41,42,43,44,45]. Due to the importance of diarylamines, a great number of synthetic approaches have been designed for their preparation [46,47,48].…”

Highly selective Diels–Alder and Heck arylation reactions in a divergent synthesis of isoindolo- and pyrrolo-fused polycyclic indoles from 2-formylpyrrole

“…This synthetic approach allows for the preparation of widely polysubstituted octahydropyrrolo [3,4e]indole-1,3-dione derivatives. However, the conversion into their aromatic skeleton either by using manganese oxide or DDQ as the oxidizing reagents, or even including Pd/C at high temperatures (250 °C) [45], failed to obtain the series of indoles 23. It is likely that the electron withdrawing effect of the formyl group at the C-7 position counterbalance the delocalization direction of the electronic density provided by the nitrogen lone-pair, which plausibly stabilizes the cationic or radical species formed by the oxidant reagent during the aromatization process [46], as efficiently occurred with derivatives 9.…”