Stereoselective synthesis of alcohols, XXX: E‐ and Z‐pentenylboronates, reagents for simple diastereoselection on addition to aldehydes

Pentenylboronates, either >90% Z ( 4 ) or >95% E ( 3 ), can be obtained by reaction of the pentenyl Grignard reagent with different borates. The Z ‐pentenylboronates 4 add to aldehydes under high simple diastereoselection to give the diastereomerically pure syn ‐homoallyl alcohols 10 . The corresponding addition of the E ‐pentenylboronates leads to the anti ‐homoallyl alcohols as E ‐( 15 )/ Z ‐( 17 ) mixtures.