Tandem Wessely oxidation and intramolecular Diels–Alder reactions. III. Synthesis of isotwistanes

Abstract

Wessely oxidation of o-(3-alkenyl) phenols (5) with lead tetraacetate gives 6-acetoxy-6-(3-alkenyl)-2,4-cyclohexadien-1-ones (6), which undergo intramolecular Diels–Alder reactions to give isotwistene (hexahydro-1,5-methanoindene) derivatives (7). These, on hydrolysis followed by oxidation with periodic acid, give hexahydro-1-oxo-1H-indene-5-carboxylic acids (41, 46). Compounds 5 were prepared either by Grignard coupling of o-methoxybenzyl chloride with allyl halides followed by de-O-methylation with sodium thioethoxide or via reduction of dihydrocoumarins (18–20) to 2-chromanols (21–23) with diisobutylaluminum hydride, followed by Wittig reaction of these with ethyl (triphenylphosphoranylidene)acetate.