There are a limited number of ring‐contraction methodologies which convert readily available five‐membered rings into strained four‐membered rings. Here we report a photo‐induced radical‐mediated ring contraction of five‐membered‐ring alkenyl boronate complexes into cyclobutanes. The process involves the addition of an electrophilic radical to the electron‐rich alkenyl boronate complex, leading to an α‐boryl radical. Upon one‐electron oxidation, ring‐contractive 1,2‐metalate rearrangement occurs to give a cyclobutyl boronic ester. A range of radical precursors and vinyl boronates can be employed, and chiral cyclobutanes can be accessed with high levels of stereocontrol. The process was extended to the preparation of benzofused cyclobutenes and the versatility of the boronic ester was demonstrated by conversion to other functional groups.
There are a limited number of ring‐contraction methodologies which convert readily available five‐membered rings into strained four‐membered rings. Here we report a photo‐induced radical‐mediated ring contraction of five‐membered‐ring alkenyl boronate complexes into cyclobutanes. The process involves the addition of an electrophilic radical to the electron‐rich alkenyl boronate complex, leading to an α‐boryl radical. Upon one‐electron oxidation, ring‐contractive 1,2‐metalate rearrangement occurs to give a cyclobutyl boronic ester. A range of radical precursors and vinyl boronates can be employed, and chiral cyclobutanes can be accessed with high levels of stereocontrol. The process was extended to the preparation of benzofused cyclobutenes and the versatility of the boronic ester was demonstrated by conversion to other functional groups.
Like a tightly pulled loop in a rope , cyclic vinyl boronate complexes can undergo a ring contraction concomitant with an increase in strain when reacting with electrophilic radicals generated by visible light. As V. K. Aggarwal and co‐workers show in their Communication on page 6525, a variety of cyclobutyl boronic esters were obtained in high yields and with excellent stereocontrol. This includes compounds containing contiguous quaternary stereocenters and also benzofused cyclobutenes, which are substantially more strained.
Wie eine zusammengezogene Seilschleife können cyclische Vinylboronatester unter Zunahme der Ringspannung kontrahieren, wenn sie mit elektrophilen Radikalen reagieren, die durch Bestrahlung mit sichtbarem Licht entstehen. Wie V. K. Aggarwal et al. in ihrer Zuschrift auf S. 6587 zeigen, können so verschiedene Cyclobutylboronsäureester in hohen Ausbeuten und mit ausgezeichneter Stereokontrolle synthetisiert werden. Dazu gehören sowohl benzoanellierte Produkte als auch solche mit benachbarten quartären Stereozentren.
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