Published: January 24, 2011 Copyright r 2011 American Chemical Society and American Society of Pharmacognosy 194 dx.doi.org/10.1021/np100620x | J. Nat. Prod. 2011, 74, 194–207 ARTICLE pubs.acs.org/jnp Configurational Assignment of Cyclic Peroxy Metabolites Provides an Insight into Their Biosynthesis: Isolation of Plakortolides, seco-Plakortolides, and Plakortones from the Australian Marine Sponge Plakinastrella clathrata Ken W. L. Yong, † James J. De Voss, † John N. A. Hooper, ‡ and Mary J. Garson* ,† † School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, QLD 4072, Australia ‡ Queensland Museum, P.O. Box 3300, South Brisbane, QLD 4101, Australia b S Supporting Information ABSTRACT: Sixteen new compounds, comprising nine new plakortolides K-S(1-9), four seco-plakortolides (10-13), and three plakortones (14-16), were isolated from the Australian sponge Plakinastrella clathrata. Structural elucidation, including relative configurational assignment, was based on extensive spectroscopic analysis, while the absolute configurations of 1-4 were deduced from 1 H NMR analyses on MPA esters derived from Zn/AcOH reduction products. Diastereomeric sets of plakortolides, e.g., K and L, or M and N, differed in configuration at C-3/C-4 rather than at C-6, a stereochemical result that compromises a biosynthetic pathway involving Diels-Alder cycloaddition of molecular oxygen to a Δ 3,5 -diunsaturated fatty acid precursor. The biosynthesis may plausibly involve cyclization of a 6-hydroperoxydienoic acid intermediate following stereospecific introduction of the hydroperoxy group into a polyketide-derived precursor. Isolated seco-plakortolides converted under mild conditions into plakortones with full retention of configuration, suggesting C-6 hydroxy attack on an R,β-unsaturated lactone intermediate. The NMR data reported for the compound named plakortolide E are inconsistent with the current literature structure and are those of the corresponding seco- plakortolide (19). The reported conversion of the metabolite into a plakortone ether on storage is consistent with this structural revision. C yclic peroxides are a distinctive suite of bioactive metabolites frequently encountered in marine sponges of the genera Plakortis and Plakinastrella. Individual metabolites can be cate- gorized as belonging to the plakortolide, plakinic acid, plakoric acid, plakortone, or plakortide families. 1 In addition to pro- nounced cytotoxic, antifungal, and anti-inflammatory activity, recent screening programs have shown that members of these classes of metabolites possess potent antiparasitic activity and so may be promising drug leads against tropical diseases such as malaria, African sleeping sickness, or leishmaniasis. 1,2 The plakortolide structure is characterized by an aromatic unit connected via a methylene chain to a bicyclic peroxy-lactone ring system. A metabolite with a plakortolide skeleton was briefly described in 1980, 3 but Davidson’s 1991 report of the cytotoxic metabolite named plakortolide included the first detailed de- scription of the assignment of relative configuration and used ROESY data and molecular modeling for this purpose. 4 The various plakortolides isolated since these two initial reports exhibit variation in alkyl chain length (generally C 8 or C 10 , but some C 9 examples are found), methylation and/or unsaturation pattern, and terminal group (either phenyl or p-hydroxyphenyl). 4-11 In this paper we report six new plakortolides (1-6) possessing a C 12 methylene chain, three C 10 homologues (7-9), together with seco derivatives 10-13, and the stereochemically related plakortones 14-16 from Plakinastrella clathrata Kirkpatrick, Received: September 3, 2010