Irradiation　of　4-aryl-4H-pyrans　in　either　the　solid　state　or　in　solution　gave　rise　to　the　formation　of　novel　oxa-cage　compounds,　3,9-dioxatetraasteranes,　derived　from　a　double　[2　+　2]　cycloaddition　reaction.　Meanwhile,　an　unexpected　oxetane　formed　by　the　Paterno-Buchi　reaction　of　benzophenone　with　the　pyrans,　was　established　by　H-1　NMR　data　as　well　as　by　X-ray　crystallographic　analysis.　Experimental　observations　and　theoretical　calculations　confirmed　that　the　photodimerization　was　effectively　catalyzed　by　the　triplet　excited　state　of　benzophenone,　while　the　Paterno-Buchi　reaction　was　competitive　with　the　process.　(C)　2013　Elsevier　Ltd.　All　rights　reserved.