(−)-Astrogorgiadiol: a shorter route to A-ring synthon

A new route to the key A-ring/C-ring precursor synthon of (−)-astrogorgiadiol is described, for the strategy based on Robinson annulation. This synthon is also key for the synthesis of other related calicoferols and astrogorgols. The route takes full advantage of the opening of 6-methoxy-1-tetralol with bis(sym-collidine) bromine(I) triflate. It involves a new protection–deprotection sequence of an enone. It is the shortest synthesis to date, consisting of seven steps.