A new route to the indolopyridonaphthyridine ring system: synthesis of N-benzyl-13b,14-dihydronauclefine and N-benzyl-13b,14-dihydroangustine

The indolo[2:3,3':4']pyrido[1,2-b]naphthyridine ring system, which is present in several alkaloids, has been prepared in a single reaction between the lithio derivative of a 3-cyano-4-methylpyridine and N-benzyl-3,4-dihydro-β-carboline in the presence of trimethylsilyl trifluoromethanesulfonate. Using appropriately substituted starting materials, N-benzyl-13b,14-dihydronaucléfine and N-benzyl-13b,14-dihydroangustine have been made in this way. A new preparation of the synthetic intermediate 9-benzyl-3,4-dihydro-β-carboline is also described.