Generation of highly strained 2,3-bridged 2H-azirines via cycloaddition reactions of 2-azidobuta-1,3-dienes and photolysis of the resulting cyclic vinyl azides

The 2-azidobuta-1,3-dienes 10a,b were treated with sulfur dioxide to prepare the 3-azido-3-sulfolenes 11a,b, and with electron-poor dienophiles to generate the Diels–Alder products 12a,b, 13a,b, 14a,b, 15a,b, and 16b. Both types of reactions provide an access to cyclic vinyl azides, which lead to short-lived 2,3-bridged 2H-azirines 18a,b and 20a,b on photolysis. Whereas the highly strained heterocycle 18b could be characterized by low temperature NMR spectra, the bridgehead azirines 18a and 20a,b were trapped with cyclopentadiene by stereoselective [4+2]-cycloaddition or with hydrogen cyanide to give bicyclic 2-cyanoaziridines. The latter transformation led to compound 22b that showed temperature-dependent 1H NMR spectra because of rapid equilibration of the configuration at the aziridine N atom. High-level quantum chemical studies, however, indicated that not only both invertomers of 22b (endo, exo) but also two conformers of each (chair, boat) are involved.