Synthesis of 7,6-Spirocyclic Imine with Butenolide Ring Provides Evidence for the Relative Configuration of Marine Toxin 13-desMe Spirolide C.

Efficient synthesis of the partial structure of marine toxin 13-desMe spirolide C was achieved via the selective Diels-Alder reaction and C-C bond formation with the use of a silatrane substituent and the subsequent facile formation of a γ-butenolide ring. The comparison of NMR data between the synthesized diastereomers and the natural product strongly suggested that the relative configuration at the C4 position was S configuration with respect to the 7,6-spirocycle.