6-Methylenebicyclo[3.2.1]oct-1-en-3-one: A Twisted Olefin as Diels-Alder Dienophile for Expedited Syntheses of Four Kaurane Diterpenoids.

6-Methylenebicyclo[3.2.1]oct-1-en-3-one, a twisted and highly reactive enone, was prepared for the first time by elimination of its bromide precursor. Its reactions as a dienophile with several dienes in Diels-Alder reactions proceeded smoothly to provide tricyclic and tetracyclic adducts, which allowed short syntheses (10-11 steps) of four kurane diterpenoids including 11β-hydroxy-16-kaurene, 11α-hydroxy-16-kaurene, liangshanin G, and gesneroidin B.