2H-Azirines continue to be used as versatile synthetic reagents. In addition to a full report' on the intramolecular cycloaddition reactions of substituted 2H-azirines noted last year,, further preparative uses have been outlined. Photolysis of 3-crotyl-2H-azirine (1) yields the endo-cyclopropanopyrrole system (2), which, on heating, epimerizes to the more stable exo-form. This reaction involves a cycloaddition of the carbenoid mesomer of a nitrile ylide, and is apparently the first example of this type involving a 1,3-dipolar specie^.^(1)(2)But-2-enyl-2H-azirines are thermally decomposed via vinyl nitrenes, e.g. (3; R = CO,Me) which undergoes a novel 1,4-hydrogen transfer to give an @-unsaturated imine (4; R = C0,Me). A 1,5-H shift followed by electrocyclic ring-closure and loss of ammonia yields the biphenyl (5) (12%) or, by intramolecular Michael addition and loss of methyl acetate, 3-methyl-2-phenylpyridine (58%).4Vacuum pyrolysis of 2-aminoazirines provides a useful source of 2-azabutadienes, e.g. CH,=C(Me)N=CHNMe,, which readily enter into Diels-Alder cycloaddition with suitable dienophiles to give di-and tetra-hydropyridines in practicable yields.'
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