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 radical
cyclization or palladium-catalyzed reductive cyclization to form its
multisubstituted cyclohexane ring. The tetrahydrooxepin ring was constructed
by intramolecular etheration. This study provides the first method
for the stereoselective synthesis of the characteristic tricyclic
skeleton of callophycoic acids.
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