Enantiomerically pure cyclopropylboronic esters: auxiliary- versus substrate-control

Stable, enantiomerically pure cyclopropylboronic esters are synthesized fro m alkynes by a hydroboration-cyclopropanation sequence. The direct hydrobor ation-utilizing 1,3,2-dioxaborolane 4-is most convenient, however, with mor e functionalized side-chains it failed to give the desired intermediates. U sing the more reactive dicyclohexylborane, followed by oxidation and transe sterification, is a good alternative one-pot conversion. Cyclopropanations were performed either following a Simmons-Smith protocol or with diazometha ne-palladium(II) acetate. The influence on the diastereoselectivity of the auxiliary 1 is compared with the influence of an additional stereogenic cen ter in the side-chain.