An Improved Synthesis of 3,6-Dihydro-as-indacene

This contribution describes an updated synthetic route to 3,6-dihydro-as-indacene along with full characterization of all inter­mediates. The title compound is prepared by Mannich condensation of 2-methylfuran with formaldehyde and dimethylamine hydrochloride, quaternization of the resulting amine with methyl iodide, and conversion into the ammonium hydroxide salt by treatment with silver oxide in water. Subsequent Hoffmann elimination and [6,6]-cycloaddition through pyrolysis produces a furanocyclophane, which after photooxidation, intramolecular cycloaddition, and dehydration with sodium carbonate affords 2,3,6,7-tetrahydro-1,8-dione-as-indacene. Reduction of this diketone gives a mixture of alcohols, which after dehydration under slightly basic or acidic conditions produces 3,6-dihydro-as-indacene. The structure is confirmed by X-ray diffraction, and all intermediates are characterized by means of 1H and 13C NMR spectroscopy.