Differentially Protected Glycols from α‐Halo Boronic Esters and Lithiated Benzoates

Abstract

In the presence of sparteine 2,4,6‐triisopropylbenzoates were lithiated enantioselectively in order to generate chiral d1 reagents. Their reaction with α‐halo boronic esters, resembling chiral a1 reagents, was explored. Enantiomerically enriched α‐halo boronic esters are readily available by Matteson Homologation (MH). A model reaction with α‐bromopentyl pinandiol boronate was optimized to an acceptable yield but proved to be quite sensitive. Generation of the d1 reagent from the corresponding stannane stabilized the protocol and delivered moderate yields for β‐unsubstituted α‐halo boronic esters. Good to excellent yields were obtained for β‐substituted α‐halo boronic esters, which can be generated by iterative MH, thus establishing this method as a useful terminus for an iterative Matteson sequence.