Epoxides in synthesis. Synthesis of the novel 2,6-dioxabicyclo[3.2.1]octane units in the citreoviridinols and the aurovertins

The 2,6-dioxabicyclo[3.2.1]octane ring systems, viz. systems 24 and 34, present in citreoviridinol and the aurovertins, have been produced in a stereoselective manner by treatment of the corresponding 4-hydroxytetrahydrofuran epoxides 22 and 33, respectively, with toluene-p-sulphonic acid. A number of related epoxy alcohol cyclisations leading to isomeric dioxabicyclooctanes are reported. These studies have led to a greater understanding of the biosynthetic pathway to the citreoviridinols and the aurovertins, and a greater appreciation of the role of epoxide intermediates in the biosynthesis apparatus.