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Acid\u2010catalysed rearrangements of these cycloadducts, 4a,7a\u2010dihydro\u20104<jats:italic>H<\/jats:italic>\u2010furo[2,3\u2010<jats:italic>e<\/jats:italic>][1,2]oxazines and 1,4,4a,7a\u2010tetrahydrofuro[3,2\u2010<jats:italic>c<\/jats:italic>]pyridazines, were studied. Using 1 equiv. of TFA, the bicyclic heterocycles derived from furan and 2\u2010methylfuran were converted into furans bearing side\u2010chains incorporating oxime and hydrazone groups by ring\u2010opening of the six\u2010membered ring with the concomitant aromatization of the furan ring. The use of TFA as solvent led to rearrangement via spirocyclic intermediates, followed by furan ring\u2010opening to afford functionalized isoxazoles or pyrazoles. Furthermore, furo\u2010oxazines derived from 2,5\u2010dimethylfuran, in which furan aromatization is precluded, were converted efficiently into 6<jats:italic>H<\/jats:italic>\u20101,2\u2010oxazines through a furan ring\u2010opening reaction upon thermolysis in the presence of a catalytic amount of <jats:italic>p<\/jats:italic>\u2010toluenesulfonic acid. The tetrahydrofuro[3,2\u2010<jats:italic>c<\/jats:italic>]pyridazines derived from furan and dimethylfuran underwent acid\u2010catalysed addition reactions, leading to 6\u2010substituted hexahydrofuro[3,2\u2010<jats:italic>c<\/jats:italic>]pyridazines, when in the presence of alcohols or water.<\/jats:p>","DOI":"10.1002\/ejoc.201700453","type":"journal-article","created":{"date-parts":[[2017,5,18]],"date-time":"2017-05-18T14:33:39Z","timestamp":1495118019000},"page":"4011-4025","update-policy":"https:\/\/doi.org\/10.1002\/crossmark_policy","source":"Crossref","is-referenced-by-count":25,"title":["Hetero\u2010Diels\u2013Alder and Ring\u2010Opening Reactions of Furans Applied to the Synthesis of Functionalized Heterocycles"],"prefix":"10.1002","volume":"2017","author":[{"given":"Am\u00e9rico J. S.","family":"Alves","sequence":"first","affiliation":[{"name":"CQC and Department of Chemistry University of Coimbra  3004\u2010535 Coimbra Portugal"}]},{"given":"Susana M. 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