Enantiomerically pure cyclopropylboronic esters:: auxiliary- versus substrate-control

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.