Enantiomerically pure cyclopropylboronic esters: auxiliary- versus substrate-control

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Abstract
Stable, enantiomerically pure cyclopropylboronic esters are synthesized from alkynes by a hydroboration–cyclopropanation sequence. The direct hydroboration—utilizing 1,3,2-dioxaborolane 4—is most convenient, however, with more functionalized side-chains it failed to give the desired intermediates. Using the more reactive dicyclohexylborane, followed by oxidation and transesterification, is a good alternative one-pot conversion. Cyclopropanations were performed either following a Simmons–Smith protocol or with diazomethane–palladium(II) acetate. The influence on the diastereoselectivity of the auxiliary 1 is compared with the influence of an additional stereogenic center in the side-chain.

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