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Acylammonium Salts as Dienophiles in Diels–Alder/Lactonization Organocascades

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posted on 17.12.2015 by Mikail E. Abbasov, Brandi M. Hudson, Dean J. Tantillo, Daniel Romo
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.