Biomimetic Macrocycle-Forming Diels−Alder Reaction of an Iminium Dienophile: Synthetic Studies Directed Toward Gymnodimine

The macrocyclic core of gymnodimine has been constructed via an intramolecular Diels−Alder reaction of an α,β-unsaturated iminium dienophile
in water. The cycloaddition furnished a single exo-product, along with two endo-products. Through X-ray analysis of a suitable derivative, the
stereochemistry of the exo-product was established, thereby demonstrating that its stereochemistry matches that of gymnodimine. In contrast,
macrocyclization of an analogous α,β-unsaturated ketone dienophile gave only undesired endo-products. Interestingly, the imine dienophile
shows remarkable stability in water.