Synthesis of Enantiomerically Pure Cyclopropanes from Cyclopropylboronic Acids.

A general method for the stereocontrolled synthesis of cyclopropanes is described. Various, highly stable, enantiomerically pure alkenylboronic esters 13 have been conveniently synthesized by the direct hydroboration of alkynes 11 using the new chiral 1,3,2-dioxaborolane 15. The high stability was also demonstrated by the selective deprotection of a tert-butyldimethylsilyl protecting group without hydrolyzing the boronic ester.