The Spongistatins: Architecturally Complex Natural Products-Part Two: Synthesis of the C(29-51) Subunit, Fragment Assembly, and Final Elaboration to (+)-Spongistatin 2

An advanced ABCD fragment (see picture) constructed on the way to the total synthesis of the potent antitumor agent (+)-spongistatin 2 has also been used in a formal total synthesis of (+)-spongistatin 1. In both cases the critical step was an acid-mediated epimerization in the presence of Ca2+ ions to stabilize the axial–equitorial CD spiroketal.