The first and highly stereoselective asymmetric total syntheses of eight-membered ring ether marine natural products (+)-3-(E)-pinnatifidenyne and (+)-3-(Z)-pinnatifidenyne have been accomplished. Notable features of our syntheses include a novel and efficient construction of oxocene 5 by a highly stereo- and regioselective internal alkylation and direct ketone synthesis of ketone 16 from the α-alkyloxy amide moiety in oxocene 5.