Total Syntheses of ent-Heliespirones A and C

Stereodivergent total syntheses of ent-heliespirone A and C were both completed in 11 vessels and similar to 24% combined overall yield (A + C). These syntheses employed an identical inverse demand Die Is Alder reaction between a surrogate for an extendedly conjugated gamma-delta unsaturated orthoquinone 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 p-quinone by argentic oxide (Ago). In addition, an unusual intramolecular etherification of a 3 degrees alcohol caused by cerium ammonium nitrate was observed.