A Concise Synthetic Route to the Stereotetrad Core of the Briarane Diterpenoids

A concise synthesis (under 10 steps) of the stereotetrad core of the briarane diterpenoids is reported. This approach harnesses the unique reactivity of salicylate ester derived 2,5-cyclohexadienones to quickly build complexity. In particular, a highly diastereoselective acetylide conjugate addition/beta-ketoester alkylation sequence was used to set the relative configuration of the C1 (quaternary) and C10 (tertiary) vicinal stereocenters. The sterochemical outcome of the beta-ketoester alkylation appears to be governed by torsional steering in the transition state.