Photogenerated Oxetanes as a Gateway to Uphill Cyclopropanation.

Despite their privileged status in pharmaceutical chemistry, the sustainable synthesis of cyclopropanes remains a fundamental challenge. Conventional methods rely on hazardous diazo precursors or pre-functionalized substrates that generate stoichiometric byproducts. Here, we report an overall endergonic, catalytic route to cyclopropanes from abundant, renewable starting materials. Furan oxetanes, readily formed by photochemical [2 + 2] cycloaddition of aldehydes and furan, undergo an intramolecular SN-type rearrangement under Lewis acid catalysis, producing cyclopropanes with ideal atom economy. The strategy enables access to meso-cyclopropane dicarbaldehydes, which are versatile intermediates that streamline the preparation of otherwise challenging bioactive motifs. By harnessing photogenerated oxetanes as cyclopropane precursors, this work offers a sustainable way to densely functionalized cyclopropanes from simple feedstocks under mild conditions.