Photooxidative Rearrangement of Furans to Oxaspirolactones by Metal–Organic Framework Catalysis

The oxaspirolactone motif remains underrepresented among naturally occurring dioxaspirocyclic motifs in studies of synthetic methodology development. In this study, we report the use of PCN-222, a water-stable, porphyrinic, zirconium-based metal–organic framework (MOF), as a multifunctional, bioinspired heterogeneous catalytic material for the selective photooxidative synthesis of oxaspirolactones from hydroxyalkylfurans. Preliminary mechanistic studies suggest that PCN-222 produces multiple reactive oxygen species (ROS) concurrently via energy-transfer (EnT) and charge-transfer (CT) processes. These ROS could oxidize furan and elicit cascading reactions that ultimately lead to the formation of oxaspirolactones. Recycling studies of PCN-222 reveal superlative retention of its catalytic activity despite usage of up to five (5) consecutive cycles. The catalytic reaction can also be performed under sunlight irradiation or solvent-free, using chromatography-grade silica gel as the medium, demonstrating alternative, greener procedures for the synthesis of oxaspirolactones. The formal total synthesis of crassalactone D and its 4-epimer was accomplished using our established reaction conditions, highlighting the potential of reticular materials as catalysts for more sophisticated organic reactions, including natural product synthesis. Lastly, green metrics from the CHEM21 project were used to evaluate the sustainability of the reported protocol.