Stereoselective construction of an all-carbon quaternary stereocenter by Barbier-type allylation in aqueous media is described. Sugar-derived aldehydes, 2,3-O-isopropylidene-D-glyceraldehyde and 2,4-O-benzylidene-D-threose, were reacted with geranyl chloride in the presence of zinc powder to provide gamma-adducts bearing quaternary stereocenters with good stereoselectivity.
Two stereocontrolled routes to the tricyclic core of (-)-callophycoic acid A are described. Our synthetic strategy relied on stereoselective allylboration using a new allylboronate reagent to construct the all-carbon quaternary stereocenter in the core, followed by efficient radicalcyclization or palladium-catalyzed reductive cyclization to form its multisubstituted cyclohexane ring. The tetrahydrooxepin
Stereoselective construction of all-carbon quaternary stereocenters by allylboration of chiral aldehydes is described. Sugar-derived aldehydes were allylated with geranylboronate or nerylboronate to provide γ-adducts possessing quaternary stereocenters with high diastereoselectivity. The reaction was applied to the synthesis of a fragment of (+)-vibsanin A.