Study on the absolute configuration of (-)-palau'amine

The absolute stereochemistry of the immunosuppressive pyrrole-imidazole alkaloid (-)-palau'amine from the marine sponge Stylotella aurantium is analyzed by CD spectroscopy. With the help of a series of model compounds it is shown that the CD spectrum of (-)-palau'amine can be explained based on the assumption that the pyrrolopyrazinone partial structure is planar in 2,2,2-trifluoroethanol (TFE). Surprisingly, the natural product (-)-dibromophakellin shows the opposite Cotton effect, despite exhibiting the same absolute configuration. (C) 2010 Elsevier Ltd. All rights reserved.