Synthesis of homofascaplysin C and indolo[2,3-a]carbazole from ditryptophans

The 2,2'-biindole core of ditryptophan cross-links is prominent in the fasc aplysins and the indolocarbazole glycoside natural products. N-Acyliminium ions derived from the C-terminus of ditryptophan peptides cyclize in one of two modes: N-alkylation or C-alkylation. The surrounding peptide offers so me control over the course of the cyclization and allows the preparation of homofascaplysin C or indolo[2,3-alpha]carbazole. These targets are modest, but they are generated through carbocyclic intermediates rich in stereoche mistry, and decidedly non-peptide in character.