Total Syntheses of <i>ent</i>-Heliespirones A and C

Abstract

Stereodivergent total syntheses of <i>ent</i>-heliespirone A and C were both completed in 11 vessels and ∼24% combined overall yield (A + C). These syntheses employed an identical inverse demand Diels–Alder reaction between a surrogate for an extendedly conjugated γ–δ unsaturated <i>ortho</i>-quinone methide and l-lactic-acid-derived exocyclic enol ether. Novel reactions of special note include a diastereoselective reduction of a chroman spiroketal by combination of borontrifluoride etherate and triethyl silane, along with oxidative rupture of a chroman etherial ring into the corresponding <i>p</i>-quinone by argentic oxide (AgO). In addition, an unusual <i>intramolecular</i> etherification of a 3° alcohol caused by cerium ammonium nitrate was observed