Total Synthesis of (±)-Hedychenone:  Trimethyldecalin Terpene Systems via Stepwise Allenoate Diene Cycloaddition

The total synthesis of hedychenone <b>1</b> is described. The cycloaddition of the hindered diene <b>2</b> and the allenecarboxylate <b>3</b> has been shown conclusively to proceed via the [2+2] cycloadduct <b>5</b> to give a 2:1 mixture of the desired formal Diels−Alder adducts, the exo and endo isomers <b>4xn</b> and is thus a stepwise [4+2] cycloaddition. The exo isomer <b>4x</b> was converted in four steps (reduction, oxidation, olefination, and desilylation) into hedychenone <b>1</b>.