Synthetic studies of cis-3a-aryloctahydroindole derivatives by copper- catalyzed cyclization of N-allyltrichloroacetamides: Facile construction of benzylic quaternary carbons by carbon-carbon bond-forming reactions

Cyclization of N-(2-aryl-2-cyclohexen-1-yl)trichloroacetamides by a copper catalyst was investigated. It is crucially important for successful cyclization under mild conditions that alkoxycarbonyl groups are introduced to the nitrogen atom of the N-allyltrichloroacetamides as well as that CuCl- (bipyridine) is used as the catalyst. Three compounds, N-(2-phenyl-2- cyclohexen-1-yl)-, N-[2-(3,4-dimethoxyphenyl)-2-cyclohexen-l-yl]-, and N-[2- (3,4-methylenedioxyphenyl)-2-cyclohexen-1-yl]trichloroacetamides, where the Cbz or MeO₂C- group was attached to the nitrogen atom, were instantly converted to the corresponding trichlorinated cis-3a-aryloctahydroindol-2- ones in high yields at room temperature. The reactions offer a facile access to alkaloid skeletons such as mesembrines and crinines; as the simplest examples, total synthesis of (±)-mesembrane and (±)-crinane was accomplished. The effect of alkoxycarbonyl substituents in the amide group was compared with that of the methyl substituent. N-Methyl-N-(2-phenyl-2- cyclohexen-1-yl)trichloro-acetamide quickly reacted in the presence of CuCl(bipyridine) catalyst; however, the products were a mixture of complicated materials including a small amount of the desired lactam. The role of the alkoxycarbonyl group was discussed in terms of rate of rotation of the N-C bond in the N-allyltrichloroacetamides; variable-temperature NMR studies and X-ray structure determination of related compounds were carried out as supporting evidence.