Stereodivergent reduction of enelactams embedded in hexahydroindoles. Synthesis of trans-3-substituted-cis-3a-methyloctahydroindoles

A stereodivergent synthesis of cis- and trans-octahydroindole derivatives from the ethylene acetal of methyl 1-benzyl-3a-methyl-2,5-dioxo-2,3,3a,5,6-hexahydro-1H-3-indoleacetate is reported. Under ionic reduction conditions the enamide group was reduced to afford a trans-ring fused product, while a hydrogenation process led to the formation of a cis-ring fused lactam, which was transformed into a building block for daphniphyllum alkaloid synthesis after an epimerization at C-3.