Synthesis of Enantiomerically Pure Cyclopropyl Boronic Esters

The synthesis of enantiomerically pure cyclopropyl boronic esters 12a−d and 13a−d has been achieved. The high‐yielding process involves the condensation of readily available alkenyl boronic acids 1a−d with (2R,3R)‐1,4‐dimethoxy‐1,1,4,4‐tetraphenyl‐2,3‐butanediol (10), Pd(II) acetate catalyzed cyclopropanation with diazomethane, and chromatographic separation of the diastereoisomers. The absolute configuration has been determined by converting 12a−d and 13a−d to the corresponding cyclopropanols (1R,2R)‐4a+b/(1R,2S)‐4c+d and (1S,2S)‐4a+b/(1S,2R)‐4c+d, respectively, compounds that allowed correlations to be made with previously published data.