A simple and practical approach to the synthesis of the marine sponge pigment fascaplysin and related compounds

Fascaplysin 1, an antimicrobial and cytotoxic red pigment of the marine sponge Fascaplys-inopsis sp., has been synthesized in five steps from tryptamine 2 in 44% overall yield. The key steps in the synthesis are: (a) dehydrogenation of the dihydro-β-carboline intermediate 4 simultaneously with its benzylic oxidation on treatment with MnO2 and (b) the thermal cyclization of the resulting β-carboline 7 into a quaternary salt 8. Similarly, indoloisoquinolines 15 and 16, the tetracyclic analogues of 1, were prepared in six steps from α-amino ketone 9 in 59 and 55% overall yields, respectively.