1,2,3,4-Tetrahydro-1,2-dimethylidenenaphtlhalene 11 has been derived in three steps from tetralone. In the condensed state and at - 80degrees, it undergoes a highly chemo- and regioselective cyclodimerization to give 3.3',4,4'-tetrahydro-2-methylidenespiro[naphthalene-1 (2H).2(1H)-phenanthrene] (14). the structure of which has been established by single-crystal X-ray-diffraction analysis. Dimer 14 undergoes cycloreversion to diene 11 under flash-pyrolysis conditions. The reaction of diene 11 with SO, occurs without acid promoter at -80degrees and gives it mixture of (+/-)-1,4,5.6-tetrahydronaphth[1,2-d][1,2]oxathin 2-oxide (23 a single sultine). 1.3.4,5-tetrahydronaphtho[1.2-c]thiophene 2,2-dioxide (25), and dimer 14. The high reactivity of diene 1 in its Diels-Alder cyclodimerization and its highly regioselective hetero-Diels-A Ider addition with SO, can be interpreted in terms of the formation of relatively stable diradical intermediates or by concerted processes with transition states that can be represented as diradicaloids.