Synthesis of helianane, an unusual marine sesquiterpene employing ring-expansion by flash vacuum thermolysis

A total synthesis of the marine sesquiterpene helianane 1 is described involving the thermal rearrangement of the benzoxabicyclo[4.2.0]octenone 4 to generate the dienone 5 incorporating the benzoxocane ring system of 1. This dienone was converted to the key ketone 11, which on interaction with methylmagnesium iodide followed by hydrogenation of the resulting alkene 18 furnished helianane 1.