Synthesis of the Spirocyclic Imine Fragment of Portimines Using a γ-Hydroxymethyl-αβ-butenolide Dienophile in a Diastereoselective Diels-Alder Reaction.

The synthesis of the spirocyclic imine fragment of the portimine family of marine toxins has been achieved. A densely functionalized key lactone-ester intermediate was assembled via a highly diastereoselective Diels-Alder cycloaddition, involving a novel γ-hydroxymethyl-α,β-butenolide dienophile. A Stille coupling was employed to install the vinyl group. Selective elaboration of the two carbonyl groups successfully afforded the spirocyclic imine fragment.