Palladium approaches to beta- and gamma-carbolines

A variety of substituted β- and γ-carbolines containing aryl, alkyl, hydroxymethyl, ester and silyl functionality at the 3- and 4-positions have been prepared by the palladium-catalyzed iminoannulation of internal and terminal alkynes. This methodology has been successfully employed to the synthesis of two biologically active β-carboline alkaloids, ZK93423 and abecarnil.; A variety of 3-substituted β- and γ-carbolines have been synthesized from N-substituted 3-iodo-1H-indole-2-carboxaldehydes and 2-bromo-1H-indole-3-carboxaldehydes, respectively. The coupling of these aldehydes with various terminal acetylenes using cat. PdCl ₂(PPh₃)₂/CuI readily affords the corresponding alkynylindole carboxaldehydes, which have subsequently been converted to the corresponding tert-butylimines and cyclized to β- and γ-carbolines by either copper-catalyzed or thermal processes.; A variety of N-substituted 2-bromo-1H-indole-3-carboxaldehydes incorporating an alkyne-containing tether on the indole nitrogen have been converted to the corresponding tert-butylimines, which have been subjected to palladium-catalyzed intramolecular iminoannulation, affording various γ-carboline derivatives with an additional ring fused across the 4- and 5-positions in good to excellent yields. This “intramolecular” concept has been extended to other palladium-catalyzed annulations to synthesize various complex heteropolycycles.; A novel 1,4-palladium migration has been observed by trapping the palladium intermediate generated from o-iodobiaryls by way of a Suzuki coupling reaction. The choice of reaction conditions is important in turning “on” and “off” the migration. There are important electronic effects controlling the distribution of the palladium intermediates, which is reflected in the isomer distribution of the Suzuki products.