Acylammonium Salts as Dienophiles in Diels–Alder/Lactonization Organocascades

α,β-Unsaturated acyl­ammonium
salts, generated in situ from commodity acid chlorides
and a chiral isothio­urea
organo­catalyst, comprise a new and versatile family of chiral
dieno­philes for the venerable Diels–Alder (DA) cycloaddition.
Their reactivity is unveiled through a highly diastereo- and enantio­selective
Diels–Alder/lactonization organo­cascade that generates cis- and trans-fused bicyclic γ-
and δ-lactones bearing up to four contiguous stereo­centers.
Moreover, the first examples of DA-initiated, stereo­divergent
organo­cascades are described delivering complex scaffolds found
in bioactive compounds. The origins of stereo­selectivity are
rationalized through computational studies. In addition, the utility
of this methodology is demonstrated through a concise approach to
the core structure of glaciolide and formal syntheses of fraxinellone,
tri­sporic acids, and tri­sporols.