Total Synthesis of the Sesquiterpene (?)&cuparenone: Use of Three-carbon Annulation+ in Synthesis

x_~xxxx_ A total synthesis of B-cuparenone, which highlights the advantages and disadvantages of the three-carbon annulation process, is described. The cyclopentane ring system is found in numerous naturally occurring products, including such diverse compounds as steroids, prostaglandins, and terpenes. We have recently published a new method for the formation of cyclopentanones from olefins, a process termed three-carbon annulation. 2 We now wish to describe the advantages and limitations of this annulation method and to report its use in the total synthesis of (')a-cuparenone 1. Isolated3 from the essential oil of the wood of "mayur pankhi," S-cuparenone 1 has been the target of several synthetic efforts. This interest is no doubt due to the two contiguous quarternary centers in the cyclopentanone ring, a reasonable synthetic challenge. To date five total syntheses have been described, each of which forms the cyclopentane system in a different way. 4 A very simple approach to 1 would involve the three-carbon annulation of the tetrasubstituted olefin 4. This process must begin with the Diels-Alder reaction of the diene 3 with the olefin 4 to afford _ the adduct 2, which would then be transformed into 1 by the usual four-step procedure. However, under all conditions tried (e.g., heating 3 and 2 to > 200°C in a sealed tube for several days) Me 1) Reduce 2) Oxidize 3) Hydrolyze Ar Me 4) Decarboxylate 1 Ar = e-tolyl 4 Ar = p-tolyl