Prenylated Acylphloroglucinols with Leishmanicidal Activity from the Fern Elaphoglossum lindbergii

Purification of a diethyl ether extract of the Argentinian fern Elaphoglossum lindbergii afforded five new prenylated acylphloroglucinols, lindbergins E-I (1-5), of which two showed significant in vitro leishmanicidal activity against promastigotes of Leishmania braziliensis and L. amazonensis. The structures of compounds 1-5 were elucidated based on analysis of their spectroscopic data and comparison with values previously reported for other phloroglucinol derivatives isolated from plant species of the genera Hypericum, Dryopteris, and Elaphoglossum. Fragmentation and rearrangement patterns of prenylated acylphloroglucinols were analyzed, and some mechanisms were proposed to rationalize the peaks observed in the mass spectra of these natural products produced by EI and FAB. Compounds isolated from E. lindbergii show the opposite absolute configuration when compared to those reported from E. crassipes. Empirical evidence indicates that acylphloroglucinols carrying a prenylated acylfilicinic acid residue possess a high-amplitude configuration-dependent Cotton effect centered at 350-360 nm in their CD curves, from which the absolute configuration of the sole chiral center of the prenylated acylfilicinic acid moiety can be deduced.