Spirastrellolide studies. Synthesis of the C(1)–C(25) southern hemispheres of spirastrellolides A and B, exploiting anion relay chemistry

Construction of the C(1)–C(25) southern fragments of both spirastrellolide A and B are described. Highlights of the syntheses include effective use of the three component anion relay chemistry (ARC) tactic recently introduced by our laboratory, a stereoselective spirocyclization via concomitant Ferrier reaction to elaborate the BC spiroketal and use of two dithiane unions to install the A ring as well as C(22)–C(25) fragment. The synthesis proceeded with longest linear sequences of 33 and 32 steps, respectively for spirastrellolide A and spirastrellolide B.