Catalytic Synthesis of Cyclic Guanidines via Hydrogen Atom Transfer and Radical-Polar Crossover
Shunya Ohuchi, Hiroki Koyama, Hiroki Shigehisa
Abstract
Cyclic guanidines are found in many biologically active compounds and natural products. Further, the formation of the atypical seven-membered ring of cyclic guanidine remains challenging because of a lack of efficient preparation strategies and low yield. Herein, a catalytic synthetic method for cyclic guanidines was developed via transition-metal hydrogen atom transfer and radical-polar crossover. This mild and functional-group-tolerant process enabled the cyclization of alkenyl guanidines bearing common protective groups, such as Cbz and Boc groups. This powerful method provided not only typical five- and six-membered rings but also the atypical seven-membered ring. The derivatization of the products afforded various heterocycles. We also investigated the selective cyclization of monoprotected or heteroprotected (TFA and Boc) alkenyl guanidines and their further derivatizations.