The azepinoindole substructure common to a number of Stemona alkaloids is constructed by employing a 7-endo radical cyclization of a readily available N-alkylated hydroindolone substrate. The indolone precursors are prepared via [1 + 4] cycloaddition between a vinyl isocyanate and either dimethoxycarbene or cyclohexylisocyanide.

The carbonyldicobalt-mediated alkyne/allene/CO cocyclization gives 4-alkylidenecyclopentenones as the major [2+2+1] cycloadducts. The regio-and stereoselectivities depend mainly on the substitution pattern of both the alkyne and the allenic moieties, which can be rationalized using the Magnus mechanism. However, contrary to this model, and in agree-

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