Preparation of 5-hydroxy-1-alkenyboronates from 1-alkynylboronates, Cp2ZrCl2/2EtMgBr, and aldehydes.

1-Alkynylboronates form five-membered zirconacycles with Cp(2)ZrCl(2)/2-EtMgBr as indicated by deuterium labeling. The zirconacycles add aldehydes to form seven-membered zirconacycles. Hydrolysis of the latter provides 5-hydroxy-1-alkenylboronates in fair to good isolated yields. Both aliphatic and aromatic aldehydes undergo insertion.