Short Diastereoselective Synthesis of the C1–C13 (AB Spiroacetal) and C17–C28 Fragments (CD Spiroacetal) of Spongistatin 1 and 2 through Double Chain‐Elongation Reactions

A unique and practical synthetic sequence for rapid access to polyketides and to further the spiroacetals derived from them, which utilizes a bidirectional Hosomi–Sakurai allylation approach around key allylsilanes in the synthesis of the AB and CD ring systems of spongistatin 1 and 2, is reported. The synthesis of the AB spiroacetal 9 requires 13 steps, with a longest linear sequence of seven steps in an overall yield of 27 %. The synthesis of the CD spiroacetal 13 requires 15 steps, with a longest linear sequence of 11 steps in an overall yield of 30 %. Both syntheses start from but‐3‐enol.