Synthesis of the GHIJKL Fragment of Gymnocin-B.

The GHIJKL fragment of gymnocin-B was synthesized using the oxiranyl anion strategy. The first highlight of the synthesis is the bromoketone cyclization reaction on the oxepane ring to construct the fused bisoxepane GH ring. The second key step is the introduction of the trans-4-hydroxy-3-methyloxepane J ring via addition of trimethylaluminum to a conjugated oxonium moiety, followed by diastereoselective epoxidation and regioselective reduction.