Kapakahine B: A Cyclic Hexapeptide with an α-Carboline Ring System from the Marine Sponge Cribrochalina olemda

摘要

β-Carbolines, biogenetically readily derivable from tryptophan, are a familiar structural feature of marine alkaloids. Conversely, α-carbolines, also derivable from tryptophan, are tare. The grossularines, isolated from a tunicate, constitute the only previous example. We now report the structure of a cyclic hexapeptide with a tryptophan-derived α-carboline which is part of a fused tetracyclic system that also includes phenylalanine-derived (Phe-2) imidazolone. Moreover, another tryptophan (Trp-1) is linked by its indole nitrogen atom to a ring juncture of the α-carboline and forms a peptide bond with a second phenylalanine (Phe-1) which bears a free amino group. Because of this non-peptide feature, a C-N bond to an indole nitrogen, we have called this compound kapakahine.