Direct excitation of carbonyl cyclopropanes: From divergent photo-isomerization and annulation to unified reductive C-C cleavage

We report herein our studies on the direct photoactivation of carbonyl cyclopropanes to give biradical intermediates, leading to selective cleavage of the more substituted carbon-carbon bond. Depending on the substrate structure, extended alkenes were isolated or directly reacted in a photo-Nazarov process to give bicyclic products. Based on these results, a unified reductive ring-opening reaction was developed by using diphenyl disulfide as a HAT reagent. By performing a sequential cyclopropanation/selective ring opening reaction, we achieved a CH2 insertion into the alpha,beta- bond of both acyclic and cyclic unsaturated carbonyl compounds. Our protocol therefore provides a further tool for framework-editing of carbocycles, complementing the recent progress in "skeletal editing" strategies.