Uncatalyzed reactions of α-(trihaloethylidene)nitroalkanes with push–pull enamines: a new type of ring–ring tautomerism in cyclobutane derivatives and the dramatic effect of the trihalomethyl group on the reaction pathway

A new type of ring–ring isomerism, which consists of the reversible transformation of cyclobutane derivatives into substituted 1,2-oxazine N-oxides was found and studied by NMR spectroscopy and X-ray diffraction analysis. The starting materials were prepared by the stereoselective reaction of α-(trifluoroethylidene)nitroalkanes, which react with ethyl β-morpholino- and β-piperidinocrotonates at the more nucleophilic α-position, whereas the reaction of α-(trichloroethylidene)nitroalkanes proceeds at the β-methyl group to give the corresponding linear products.