Cobalt-catalyzed carbon-carbon bond formation: Synthesis and applications of enantiopure pyrrolidine derivatives^[1]

In the presence of cobalt catalysts and tetramethylethylenediamine (TMEDA), the iodine atom in (S)-2-(iodomethyl)pyrrolidines was replaced by an aryl or an alkynyl group from the corresponding Grignard reagent, and the coupling products were obtained in good to excellent yields (16 examples; 75-94% yields). The scope and limitations of this protocol were examined. The stereochemistry of the pyrrolidines was unaffected by the reaction conditions. The coupling products are important building blocks of phenanthroindolizidine alkaloids. Palladium-catalyzed formal [4+2] cycloaddition of 2,2′-diiodobiphenyl with the thus-generated (S)-2-(3-trimethylsilyl-2-propynyl)pyrrolidine gave a good yield of the desilylated phenanthrene, which was then converted into unnatural (+)-(S)-tylophorine by the Pictet-Spengler cyclization.