Pyrrolidine synthesis via ring contraction of pyridines

A ring contraction of easily available cyclic compounds to smaller cycles that are valuable but difficult to synthetically access is one of important skeletal editing strategies. Pyrrolidine synthesis via a ring contraction of pyridines, which are abundant, cheap, and readily available bulk chemicals in chemical industry, is highly promising to accelerate drug discovery and development research due to the great demand of pyrrolidine skeletons in medicinal molecules. Herein we report a photo-promoted ring contraction of pyridines with silylborane to afford pyrrolidine derivatives bearing a 2-azabicyclo[3.1.0]hex-3-ene skeleton. The reaction demonstrates broad substrate scope with high functional group compatibility, realizing facile access to 6-silyl-2-azabicyclo[3.1.0]hex-3-ene derivatives that work as powerful synthons for the synthesis of functionalized pyrrolidines and nitrogen-containing compounds. The reaction mechanism is clarified to proceed via 2-silyl-1,2-dihydropyridine and vinylazomethine ylide as intermediates, which are connected via photochemical or thermal silyl migration.