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
Stereochemical control–-nitrones from oximes by 1,3-azaprotio cyclotransfer or 1,2-prototropy
作者:Colm O'Mahony、Frances Heaney
DOI:10.1039/cc9960000167
日期:——
Oximes 1a,b and 10 are configurationally stable at elevated temperatures, the (E)-oximes react exclusively via a concerted 1,3-azaprotio cyclotransfer reaction to give 6- and 7-membered cyclic dipoles, respectively, whilst their Z-isomers react via a 1,2-prototropy-cycloaddition sequence furnishing fused isoxazolidines.
α-Oximono-esters as precursors to heterocycles – generation of oxazinone N-oxides and cycloaddition to alkene dipolarophiles
undergoes facile thermal cyclisation affording nitrones 1c and 1d in good yield. Oximes E-5a,b with a terminal methyl substituent on the alkene moiety furnish nitrone only under the influence of an external electrophile [PhSeBr/AgBF4]. A terminal Ph substituent on 5c,d prohibits formation of the cyclic dipole irrespective of reaction conditions, and whilst 5d reacts to afford a bicyclic isoxazolofuranone