A new construct of the cis-3a-aryloctahydroindole skeleton via the [4+2] cycloaddition of furanyl carbamates

Several 2-methylthio-5-amidofurans containing tethered unsaturation were prepared via the reaction of dimethyl(methylthio)-sulfonium tetrafluoroborate (DMTSF) with β-alkoxy-γ-dithiane amides. Thermolysis of these furans resulted in an intramolecular Diels-Alder reaction (IMDAF). The resulting oxa-bridged cycloadducts underwent a subsequent rearrangement to form bicyclic lactams. Model studies were directed toward mesembrine as well as the core skeleton of the 3,4-benzoerythrinane skeleton. Using this cascade sequence, a formal synthesis of the alkaloid erysotrine was accomplished.