Nitrogen-containing heterocycles: 1,3-dipolar cycloaddition of stabilized nitrones with alkynes; primary cycloadducts, first and second generation rearrangement processes
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(2001). Nitrogen-containing heterocycles: 1,3-dipolar cycloaddition of stabilized nitrones with alkynes; primary cycloadducts, first and second generation rearrangement processes. Journal of the Chemical Society, Perkin Transactions 1 (24), 3382-3392
[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 alpha -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 with the C-phenyl nitrones, 1b and 1c, was straightforward and the structure of 12b, arising from reaction of 1b with dimethyl acetylenedicarboxylate has been determined by single crystal X-ray diffraction. The identity of the product(s) from reaction of C-methyl nitrones, 1a or 18, with dimethyl acetylenedicarboxylate varies with reaction duration; 12a and 20 are the primary cycloaddition products and the pyrrolooxazinones 14 and 22 appear after prolonged reaction duration. A similar pattern of reactivity is observed when the same dipoles react with methyl propiolate. The structure of 14 has been confirmed following X-ray crystallographic analysis. The primary cycloadducts, 12a, 20, 24a and 30, bearing a C3a-methyl group had poor thermal stability and rearranged to the pyrrolooxazinones 13, 21, 25 and 31 respectively. A mechanistic proposal for the origin of the fused pyrroles is included. A C-6 methyl substituent on the dipole 18 had no determining influence on the stereochemical course or the rate of the cycloaddition reaction established by its unsubstituted analogues 1a and 1b. In addition to its mechanistic findings, this paper reports two significant synthetic advances: access to a range of unusually substituted hetero-fused pyrroles and to isoxazolooxazepinones, a rare bicyclic ring system.