Palladium -catalyzed synthesis of carbazole derivatives and formal total syntheses of several naturally occurring carbazole alkaloids

A mild and efficient route to substituted carbazolones has been developed. This novel procedure consists of two consecutive palladium-catalyzed reactions, an intermolecular Stille coupling followed by an intramolecular reductive N-heteroannulation. For example, 1,2-dihydro-4(3H)-carbazolone was prepared in good isolated yield (74%) by the reductive cyclization of 2-(2-nitrophenyl)-2-cyclohexen-1-one using Pd(dba)2 (6 mol%), 1,3-bis(diphenylphosphino)propane (6 mol%), 1,10-phenanthroline monohydrate (12 mol%), and carbon monoxide (90 psi) in DMF at 80°C. 2-(2-Nitrophenyl)-2-cyclohexen-1-one was prepared via a Stille coupling of 2-iodo-2-cyclohexen-1-one and 2-(tri-n-butylstannyl)-1-nitrobenzene. Many functional groups and ring sizes were tolerated in these reactions.;This novel approach to carbazolones was successfully applied to the formal total syntheses of several naturally occurring carbazole alkaloids including murrayaquinone A, murrayafoline A, koenigine-quinone A, murrayanine, dimeric O-demethylmurrayafoline A, and (+)-aspidospermidine. These new syntheses are generally more efficient and higher yielding compared to the previously reported syntheses of these natural products.;In addition, reductive cyclizations of 2-(2-nitrophenyl)-2-cycloalkenones using 10% Pd/C and 1 atm of hydrogen gas in methanol at ambient temperature yielded carbazole derivatives in excellent yields. For example, reduction of 2-(2-nitrophenyl)-2-cyclohexen-1-one gave 1,2,3,4-tetrahydrocarbazole in 95% yield. Methyl-substitution on the cyclohexenone ring regioselectively produced methyl-substituted tetrahydrocarbazoles, however substitution on the benzene ring led to mixtures of carbazole products