Stereoselective Total Synthesis of Hainanolidol and Harringtonolide via Oxidopyrylium-Based [5 + 2] Cycloaddition

The tetracyclic carbon skeleton of hainanolidol and harringtonolide was efficiently constructed by an intramolecular oxidopyrylium-based [5 + 2] cycloaddition. An anionic ring-opening strategy was developed for the cleavage of the ether bridge in 8-oxabicyclo[3.2.1]octenes derived from the [5 + 2] cycloaddition. Conversion of cycloheptadiene to tropone was realized by a sequential [4 + 2] cycloaddition, Kornblum–DeLaMare rearrangement, and double elimination. The biomimetic synthesis of harringtonolide from hainanolidol was also confirmed.