A concise synthesis of the ten-membered lactone 26 is described which constitutes the key intermediate of a previous total synthesis of the marine natural product ascidiatrienolide 1 and can also be elaborated into the closely related didemni- lactones 2 ± 4 .T heE/Z-ratio obtained in the ring- closing metathesis (RCM) reaction forging such non- enolide structures is found to be dependent on the relative