General Method for the Synthesis of Bridged Indole Alkaloids. Nucleophilic Addition of Indoleacetic Ester Enolates to N-Alkylpyridinium Salts

Article abstract of DOI:10.1021/jo00291a013

A short route to tetracyclic ring substructures of C-mavacurine, Strychnos, and akuammiline-type alkaloids, based on the addition of methyl 1-, 2-, or 3-indoleacetate anions to N-alkylpyridinium salts followed by acid cyclization of the resultant 1,4-dihydropyridines, is reported.Further stereoselective elaboration of the C-20 (E)-ethylidene substituent results in the synthesis of the indole alkaloid vinoxine (7b) and of 4-ethylidene-hexahydro-1,5-methanoazocino<4.3-b>- and -<3,4-b>indoles 14-17, 32, and 35.Some mechanistic aspects concerning the regiochemistry of the nucleophilic addition to the pyridinium ring are discussed.