Synthesis of Enantioenriched Borylated Cyclopropanols from Photoexcited Acylsilanes

Cyclopropanes have played an important role in helping drug candidates achieve clinical success. Incorporation of cyclopropanes in bioactive compounds can lead to improved physicochemical properties or bolstered binding affinities due to conformational restrictions that favor protein target engagement. Tremendous effort has focused on the enantioselective synthesis of functionalized cyclopropanes; however, limitations regarding functional group variety and their relative configurations on the ring call for the exploration of alternative synthetic approaches. Here, we present the synthesis of enantioenriched 1,1-siloxycyclopropylboronic esters from photoexcited β-boryl acylsilanes using a ring-contractive 1,2-boronate rearrangement strategy. Triplet energy transfer catalysis is effective in triggering the desired cyclization via α-siloxycarbene formation; however, an unexpected stereoablative energy transfer process was observed and investigated. An alternative approach using direct photoexcitation was thereby developed and enabled stereochemical information to be relayed with complete fidelity through the transformation. The functional utility of 1,1-siloxycyclopropylboronic esters is demonstrated through a series of orthogonal transformations at the oxygen and boronic ester handles.