Substituted Oxocanes by Intramolecular Allylboration Reactions. Entry to an Efficient Synthesis of (+)-Laurencin

Δ5-Oxocenes, such as cis-3-hydroxy-2-vinyl-Δ5-oxocenes, can be generated by an intramolecular aldehyde−allylboration reaction. This allowed a rapid and stereoselective access to the trisubstituted oxocene 24 which had been transformed by the Holmes group into (+)-laurencin. The key feature of this effective intramolecular allylboration reaction is that both the aldehyde and the allylboronate functionality have been generated in a masked form in situ. They are liberated by an aqueous workup which initiates the ring closure reaction.