“…The title compound 4 is accessible through photoisomerization of Diels-Alder adduct 3, a transformation that has been thoroughly studied (Baggiolini et al, 1967;Klinsmann et al, 1972) since dione 4 -via diol 5 (Amman et al, 1980;Amman & Ganter, 1977;Amman & Ganter, 1981) -is a valuable precursor to an unusual bishomoaromatic dication (Prakash et al, 1987) and to heterodiamantanes and other oxa-cage compounds (Amman et al, 1980;Amman & Ganter, 1977;Amman & Ganter, 1981). We have recently reported the structure of diol derivative 5 (Harris et al, 2008) and herein report the structure of the parent dione 4.…”