Oxidation of o-isopropylphenol with lead tetraacetate : Synthesis of the unit of celastroidin terpenes by the diels-alder condensation

Lead tetraacetate oxidation of o-isopropyl phenol under various conditions led to the dienone acetate which, when dimerized, represents a central part of the celastroidin penta- and hexa-terpenes (natural products from the Mexican shrub Hippocratea Celastroides). Six oxidation products formed in this reaction (the iso-propyl dienolone acetate, two dimers, benzoquinone, and two phenol acetates) were identified with help of 2D and 3D NMR, GC and LC-MS. From this, we concluded that the dimer skeleton observed for the dienolone acetate in natural products corresponded to the product from the photochemical pathway, and that the synthetic dimer has the opposite geometry; this was verified by molecular modelling. The oxidation of the second compound, o-cresol, leads to a similar profile of products.