Nitrogen-containing heterocycles: 1,3-dipolar cycloaddition of stabilized nitrones with alkynes; primary cycloadducts, first and second generation rearrangement processes
[4.3.0]- and [5.3.0]Bicyclic ring systems containing a nitrogen atom at the bridgehead position were prepared by a [3+2] addition of acetylenic dipolarophiles to the conformationally locked cyclic α-alkoxycarbonylnitrones 1a–c and 18. Reaction proceeded with a high degree of diastereofacial selectivity with cycloaddition taking place to the face of the dipole opposite the C-5 methyl group. Reaction