Photochemical Synthesis of Lactones, Cyclopropanes and ATRA Products: Revealing the Role of Sodium Ascorbate

Light-mediated processes have received significant attention, since they have re-surfaced unconventional reactivity platforms, complementary to conventional polar chemistry. gamma-Lactones and cyclopropanes are prevalent moieties, found in numerous natural products and pharmaceuticals. Among various methods for their synthesis, light-mediated protocols are coming to the spotlight, although these are contingent upon the use of photoorgano- or metal-based catalysts. Herein, we introduce a novel photochemical activation of iodo-reagents via the use of cheap sodium ascorbate or ascorbic acid to enable their homolytic scission and addition onto double bonds. The developed protocol was applied successfully to the formal [3+2] cycloaddition for the synthesis of gamma-lactones, traditional atom transfer radical addition (ATRA) reactions and the one-pot two-step conversion of alkenes to cyclopropanes. In all cases, the desired products were obtained in good to high yields, while the reaction mechanism was thoroughly investigated. Depending on the nature of the iodo-reagent, a halogen or a hydrogen-bonded complex is formed, which initiates the process. Photochemistry: A direct and mild photochemical synthesis of lactones, cyclopropanes and ATRA products, utilizing sodium ascorbate or ascorbic acid as the halogen/hydrogen bonding mediator and irradiation at 370 nm, 390 nm or 427 nm LED as the irradiation source. A variety of iodo-reagents were activated via halogen or hydrogen bonding and reacted successfully with a number of alkenes without the use of an external photocatalyst, leading to products in good to excellent yields (up to 95 % yield).** image