Novel [1,5] sigmatropic rearrangements of cyclohexadienones generated fromfischer carbene complexes. A new strategy for installing the C-20 angular ethyl group in aspidospermidine alkaloids

[GRAPHICS] We report here the first examples of a [1,5] sigmatropic rearrangement in a 4a-alkyl-4a-hydrocarbazol-4-one to yield a 3-alkylcarbazol-4-one with a re -aromatized indole nucleus, The reaction of 1-methyl-3-substituted-indole-2 -carbene complexes 1 with terminal alkynes yields 3,4a-dialkyl-1-methoxy-9- methylcarbazol-4-ones 2. These 4a-substituted carbazolones thermally rearra nge to cleanly give the more highly aromatic 3,3-dialkyl-1-methoxy-9-methyl carbazol-4-ones 3. This reaction provides a convenient entry to the Aspidos perma family of alkaloids, which contain a 3,3-disubstituted carbazole nucl eus.